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Recent Advances in Electrocatalysts for Halogenated Organic Pollutants Degradation

June 2019
Environmental science. Nano. 6(Pt 2)

June 2019
6(Pt 2)

DOI:10.1039/C9EN00411D

Authors:

Zhijie Chen

UNSW Sydney

Bing-Jie Ni

UNSW Sydney

Electrocatalysis has recently been extensively employed for the degradation of halogenated organic pollutants (HOPs) that normally act as persistent, toxic, and bioaccumulative substances in the environment and pose threats to aquatic species as well as human beings. This review article broadly gives the up-to-date status on promising electrocatalysts for the degradation of HOPs, with particular emphasis on the strategies for promoting the activities of catalysts. Firstly, the catalysts for oxidative mineralization process including metallic oxides- and carbon-based anodes, as well as the oxidative dehalogenation mechanism of these catalysts, are comprehensively presented. Secondly, the catalysts for reductive degradation process, which contains metal- and metal complexes-based cathodes, with their applications and organic transformation pathways, are fully analyzed. Thirdly, recent advances in the integrated techniques are introduced, and the integration of membrane techniques, biological methods, Fenton processes, photocatalysis with electrocatalysis are discussed. Finally, several key directions for further research are exploited, which includes catalysts design, experimental optimization, scientific understanding exploration, and effective coupling techniques.

SEM images of Ti/PbO 2 (a); Ti/SnO 2 -Sb/PbO 2 (b); Ti/TNAs*/PbO 2 (c); lateral view of

…

a) A schematic illustration of the degradation process; the removal ratios (b) and electrosorption kinetics (c) of PFOA and PFOS on CNTs-

…

Mechanistic scheme of oxidative degradation of HOPs on inactive anodes (a) and on active anodes (b).

…

Schematic diagram illustrating the electrochemical oxidation mechanism of PFCs.

…

Schematic representation of synthesis of the free-standing Ag nanohemisphere and Ag nanowire array electrodes (a); Top-view SEM images of Ag electrodes: Ag rod (Agbulk) after mechanical polishing (b); Ag nanohemisphere array electrode (Ag-NHSs) after

…

Figures - uploaded by Zhijie Chen

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Environmental significance

Electrocatalysis is an efficient approach for the degradation of halogenated organic pollutants.

Currently, both electrooxidative and electroreductive dehalogenation processes are widely

applied, and the key challenge lies in the development of high-performance electrocatalysts.

Given the rapid advances in the design and development of efficient catalysts towards

electrochemical dehalogenation, this review aims to summarise recent progress in popular

electrocatalysts and theoretical understanding of dehalogenation processes. Additionally,

several key prospects in this field are presented. This critical review can facilitate the design

and application of next-generation electrocatalysts with ameliorated performances.

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Recent Advances in Electrocatalysts for Halogenated Organic Pollutants Degradation

Zhijie Chen, Yiwen Liu, Wei Wei, Bing-Jie Ni*

Centre for Technology in Water and Wastewater, School of Civil and Environmental

Engineering, University of Technology Sydney, Sydney, NSW 2007, Australia

*Corresponding author:

Tel.: +61 295147401; E-mail: bingjieni@gmail.com (Prof. Bing-Jie Ni)

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Abstract

Electrocatalysis has recently been extensively employed for the degradation of halogenated

organic pollutants (HOPs) that normally act as persistent, toxic, and bioaccumulative

substances in the environment and pose threats to aquatic species as well as human beings.

This review article broadly gives the up-to-date status on promising electrocatalysts for the

degradation of HOPs, with particular emphasis on the strategies for promoting the activities

of catalysts. Firstly, the catalysts for oxidative mineralization process including metallic

oxides- and carbon-based anodes, as well as the oxidative dehalogenation mechanism of

these catalysts, are comprehensively presented. Secondly, the catalysts for reductive

degradation process, which contains metal- and metal complexes-based cathodes, with their

applications and organic transformation pathways, are fully analyzed. Thirdly, recent

advances in the integrated techniques are introduced, and the integration of membrane

techniques, biological methods, Fenton processes, photocatalysis with electrocatalysis are

discussed. Finally, several key directions for further research are exploited, which includes

catalysts design, experimental optimization, scientific understanding exploration, and

effective coupling techniques.

1. Introduction

Halogenated organic compounds, namely organics containing one or more elements of

fluorine (F), chlorine (Cl), bromine (Br) and iodine (I), have aroused increasing concerns as it

poses threat to aquatic species as well as human beings. These organics are widely employed

in many industrial and consumer products such as pesticides, flame retardants, fungicides,

insecticides, dyes, dielectrics, coolants, plant regulators, plasticizers, and drugs.1-4 Due to

their high consumptions during past years, these compounds have been detected in air, water,

and soil, which result in severe environmental pollution owing to their high toxicity,

persistence, bioaccumulation, and wide distribution.4, 5

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Under such circumstance, effective methods to degrade these halogenated organic pollutants

(HOPs) are extremely demanding. In the last few decades, extensive efforts have been

devoted to the degradation of HOPs. Particularly, many processes have been developed to

cleave the strong C-X bonds (X= F, Cl, Br and I) in HOPs, such as microbial degradation,6, 7

ionizing radiation,8, 9 metallic reduction,10-12 photochemistry and photocatalysis,13-15

sonolysis,16, 17 and electrocatalysis18-21. Although it is highly difficult to compare the

efficiency of different degradation methods due to various target pollutants, initial

concentrations, source of pollution, experimental setups and different physicochemical

parameters of these processes applied,22 electrocatalysis distinguishes itself from these

methods for its easy implementation, high energy efficiency, amenability of automation and

safety, versatility, high effectiveness, universal degradation capability, as well as

environmental compatibility.1, 23-26

As a destructive technology for HOPs degradation,27 especially for those persistent

perfluorinated compounds (PFCs), electrocatalysis has attracted much more attention

recently.28 Several previous publications have discussed the application of electrocatalysis in

the dehalogenation process.1, 2, 28-32 For example, Sandra et al.1 reviewed the electroreduction

of organic halides in terms of electrode reaction mechanisms, electrode materials as well as

process operation. Martin et al.2 considered many vital aspects of the electroreduction

process of HOPs, including cell designs, experimental methods, and degradation mechanisms.

In an earlier work by the same group, the electrochemical reduction of halogenated organic

compounds was also discussed regarding the species of pollutants.32 Apart from

electrochemical reduction, Niu and coworkers28 reviewed the electrooxidation of

perfluorinated compounds, and the major focus was on the electrode types and operational

factors, including potential, current density, pH value, plate distance, and etc. These

informative articles have provided an overview of the electrocatalytic degradation processes

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of HOPs related to reduction or oxidation, however, the catalysts applied in this field have

not been comprehensively reviewed. In fact, it is now widely recognized that electrocatalysts

(electrodes) play a key role in the degradation of HOPs,33, 34 and explosive development of

innovative catalysts are pushing the boundaries of HOPs degradation speedily during past

years. Nevertheless, to date, there is no attempt has been made to provide a comprehensive

summary of these recent findings to facilitate the development of new effective catalysts.

Therefore, in this review article, we summarize the recent development in electrochemical

dehalogenation catalysts, focusing on the key strategies for promoting the activities of

catalysts, as well as the connections between the nature of catalysts (such as morphology,

chemical component, conformation) and their catalytic performance. In addition, the key

mechanisms accounting for electrooxidative dehalogenation (EOD) and electroreductive

dehalogenation (ERD) are highlighted, as well as the advanced integrated techniques. Based

on the review, the outlooks on key future research directions are also pointed out.

2. Catalysts for electrooxidative dehalogenation

Electrooxidation has been proved as an effective method to mineralize halogenated organic

pollutants. To date, numerous catalysts have been applied in the electrooxidative

dehalogenation process. This section provides an overview of the main catalysts (anodes) in

EOD, including metal oxides (PbO2, SnO2, and others)-based and carbon (boron-doped

diamond and others)-based electrodes. The diverse modification strategies that have been

applied to upgrade the performance of these electrodes are also highlighted. Furthermore, the

main electrooxidative dehalogenation mechanisms are discussed.

2.1. PbO2-based catalysts

PbO2-based catalysts have been widely explored as the electrode materials in EOD since they

possess many merits, including low cost, high conductivity, chemical resistance, as well as

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high oxygen overvoltage.23, 33, 35 Table 1 summarizes the recently representative studies of the

EOD by various PbO2-based catalytic materials. These results clearly suggested that the

structure and components of the catalysts have a vital influence on their degradation

performance. The effective measures that can upgrade the catalytic potential of PbO2-based

catalysts are detailed in this work.

Pristine PbO2 materials are excellent candidates for EOD due to their high catalytic activity,

good chemical stability, great conductivity, as well as low cost.36 Normally, the traditional

PbO2 coated catalysts can flake from the substrate (e.g., Ti sheets) for their relatively large

interface resistance.28 To address this problem, interlayers have been introduced to increase

the chemical stability and thus the degradation performance of PbO2 electrode.36-38 For

example, Niu et al.38 used SnO2-Sb as an interlayer to construct a Ti/SnO2-Sb/PbO2 anode

with the electrodeposition method for the decay of perfluorooctanoic acid (PFOA). The

degradation of PFOA followed pseudo-first-order kinetics and the degradation efficiency on

Ti/SnO2-Sb/PbO2 anode achieved 91.1%, with the defluorination rate of PFOA being 77.4%.

Recently, the same group found that TiO2 nanotubes arrays (TNAs*) performed better than

SnO2-Sb when worked as a transition layer for the defluorination of fluoxetine (FLX).39 They

noticed that the particles on the Ti/TNAs*/PbO2 anode are smaller and distributed much more

compactly than that of the Ti/SnO2-Sb/PbO2 and Ti/ PbO2 anodes (Fig. 1 a-c). Impressively,

the optimized electronic transport through the “T-shaped” electrode (Fig.1 d) and the

improved hydroxyl radical yield derived from selective PbO2 crystal face growth could

facilitate the degradation process (Fig. 1 e-f). Other than metallic oxides, a graphene

nanosheet was also employed as an interlayer to construct a novel PbO2 electrode (GNS-

PbO2) for degradation of 2-chlorophenol (2-CP).36 The GNS-PbO2 displayed better

electrochemical activities, a higher dehalogenation rate constant and better service lifetime

than those of the conventional PbO2 electrode.

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Fig.1 SEM images of Ti/PbO2 (a); Ti/SnO2-Sb/PbO2 (b); Ti/TNAs*/PbO2 (c); lateral view of

TNAs*(d); Nyquist plot of impedance spectra of Ti/SnO2-Sb/PbO2 and

Ti/TNAs*/PbO2 anodes in 0.5 mol L−1 Na2SO4, inset illustrations are the electronic transfer

pathways through Ti/TNAs*/PbO2 and Ti/SnO2-Sb/PbO2 anodes(e); concentrations of •OH

production on three anodes at the applied current density of 10 mA cm-2 (f). Reproduced with

However, several defects of pure PbO2 catalyst have limited its widespread applications,

including leaching of toxic Pb ions and relatively large interface resistances.28, 40 Furthermore,

the electrocatalytic activity of bare PbO2 electrodes still requires further improvements.38, 41,

42 Therefore, it is crucial to enhance the stability and electrocatalytic activity of PbO2

electrodes, with doping, compositing, and surface modification being reported as efficient

ways to achieve the goal.40, 43-45 To date, many foreign compositions have been doped on

PbO2 surfaces, such as Bi, Fe, Al, F, Co, Ce, Yb, Gd, Er, La, Au, Zr, Ni, and Sn.28, 33, 40, 44, 46-

49 Among them, Ce is the most used dopant because Ce element can offer PbO2 a new

nucleation center that enhances the electron transfer capacity, minimizes the internal stress of

PbO2 crystals, hampers the further growth of PbO2 and decreases its average size.23, 28 Hence,

the Ce-doped PbO2 catalysts display upgraded electrocatalytic performance towards HOPs.

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For example, a well-designed TiO2-NTs/Ce-PbO2 electrode was prepared for the elimination

of Rhodamine B (RhB).23 Compared to TiO2-NTs/PbO2 electrode, the incorporation of Ce

favored the generation of a denser and smoother film, with decreased crystal size, as well as a

higher exposure of preferred crystalline orientation. The small crystals provided more active

sites for electrochemical oxidation, and the dense film hindered the penetration of electrolytes.

The abundant active sites and great durability results from the Ce-doping improve the overall

electrochemical performance. Recently, metals other than Ce have gained more attention as

efficient dopants in the fabrication of PbO2-based electrodes due to their better

dehalogenation capabilities. As a good example, in the mineralization of enrofloxacin (ENR),

Wang and coworkers43 found that Ti/SnO2-Sb/La-PbO2 electrode was much more compact

than Ti/SnO2-Sb/Ce-PbO2 and Ti/SnO2-Sb/PbO2 electrodes (Fig.2 a-c), and La doping

significantly reduced the crystallite size of β-PbO2 (Fig. 2 d). Accordingly, the La-doped

materials may provide more active sites for the generation of radicals. As a result, these

characteristics of La-doped electrode showed better degradation performance (Fig. 2 e-f).

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Fig.2 SEM images of the top of Ti/SnO2-Sb/PbO2 (a); Ti/SnO2-Sb/Ce-PbO2 (b); Ti/SnO2-

Sb/La-PbO2 (c), XRD patterns of Ti/SnO2-Sb/PbO2, Ti/SnO2-Sb/Ce-PbO2 and Ti/SnO2-

Sb/La-PbO2 electrodes (d), electrochemical degradation of 10.0 mg L-1 ENR (e) and the

concentrations of hydroxyl radicals production by using Ti/SnO2-Sb/PbO2, Ti/SnO2-Sb/Ce-

PbO2 and Ti/SnO2-Sb/La-PbO2 electrodes (f). Reproduced with permission from ref. 43,

Composting is another useful strategy to obtain a significant leap in the dehalogenation

performance of catalysts since composite-based electrodes possess a superior lifetime, high

HOP, great chemical stability, as well as excellent degradation power.50 For PbO2, the

commonly employed incorporating metal oxides include CeO2, ZrO2, Co3O4, RuO2, and

TiO2.51 Among these, the PbO2-CeO2 composite electrode is a powerful candidate for

EOD.34, 45, 46 This composite is usually fabricated using a composite electrodeposition method,

which can obtain higher cerium contents than the aforementioned Ce-PbO2 electrode.34, 45

Therefore, the electrocatalytic ability and chemical stability of PbO2-CeO2 composite

electrode are higher than that of Ce-PbO2 electrode.45 A novel three-dimensional PbO2-CeO2

composite electrode was fabricated with a composite electrodeposition method using oxygen

bubbles as a template.34 The 3D/PbO2-CeO2 composite electrode possessed a three-

dimensional interconnected porous structure, and the CeO2 nanoparticles were observed

on pore walls. Moreover, the composite electrode owned noteworthy capability in hydroxyl

radicals generation resulting from the high OEP as well as the high specific surface area, with

thiamethoxam being completely mineralized into CO2 and H2O after electrolysis for 90 min.

Alternatively, PbO2-ZrO2 and Ga2O3-PbO2 have also been employed to remove HOPs

successfully.52, 53

Hydrophobic properties of anodes also have a significant impact on the OEP and the

production of •OH radicals that are the main active components in EOD process. Thus

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regulating the surface property of PbO2 from hydrophilic to hydrophobic is greatly

advisable.41, 54 Consequently, the surfactants and polymers are widely applied as additives to

upgrade the surface properties of anodes. For instance, a PbO2 electrode modified with

hydrophobic polyvinylidene fluoride (PVDF) was employed to degrade PFOA, and

experimental results illustrated that the designed electrode still kept the β-PbO2

crystallographic structure.37 The PVDF modified anode exhibited a rough face and the small

particles were uniformly distributed. The nanostructures could significantly improve the

specific area, and provide abundant reactivity points for the generation of hydroxyl radicals.

The presence of PVDF also enhanced the hydrophobicity and the lifetime of the bare PbO2

electrode. Correspondingly, the Ti/SnO2-Sb2O5/PbO2-PVDF anode showed a higher

degradation efficiency of PFOA than the Ti/SnO2-Sb2O5/PbO2.

The particle size of catalysts also affects the electrocatalytic performance significantly, and a

smaller size is more favorable due to the more active sites for catalytic reactions. Duan and

co-researchers55 fabricated a lauryl benzene sulfonic acid sodium (LAS)-carbon nanotube

(CNT)-modified PbO2 electrode using thermal deposition and electrodeposition methods, and

this modified anode exhibited better degradation performance for 4-chlorophenol than the

PbO2 electrode. It was found that the crystal particles on the LAS-CNT-PbO2 anode were

evidently smaller than those on the PbO2 anode, and its special surface structure consisted of

small “stars with four corners” like crystallites. In addition, many bumps appeared on the

surface of LAS-CNT-PbO2 anode, and such feature resulted in a larger effective surface area

for the production of active radicals. Totally, these characteristics of surface morphology and

coating composition play a vital role in enhancing the capacity of the anode.

Table 1 Summary of representative HOPs electrooxidative dehalogenation using PbO2-based anodes in recent

years.

Anode

Preparation methods

HOPs

CHOP /

mg L−1

electrolyte

current

density

/ mA

cm−2

Other

parameters

HOP

Removal

rate/%

COD

Remov

rate/%

TOC

Removal

rate/%

Ref.

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Ti/TiO2-

NTs/Ag2O/PbO2

electrodeposition

perfluorooctane

sulfonate

0.0929

1.4 g L-1

NaClO4

100 mL; 30 ±

2 °C; 12 cm2

74.87

(180min)

Double sided

Ti-Pt/β-PbO2

electrodeposition

ciprofloxacin

0.1 M

Na2SO4

6.2 cm2 and

8.37 cm2; pH

10; 25 °C

100

(2h)

~75(5h)

Ti/TNAs∗/PbO2

anodic oxidation;

electrodeposition

fluoxetine

0.02 M

Na2SO4

3 cm2;

sequential

experiment

94.3

83.1

GNS-PbO2

electrophoretic

deposition; electro-

deposition

2-chlorophenol

0.05 M

Na2SO4

200 mL; 30 °C

95.42

(120min)

55.09

(120min)

Ti/α-PbO2/β-PbO2

electrodeposition

2-chlorophenol

0.1 M

Na2SO4

35 °C; 8 cm2

100

(180 min)

TiO2-NTs/SnO2-

Sb/PbO2

anodization;

sol-gel technique

flutriafol

5 g L-1

Na2SO4

1 L; 25 °C

80.42

(180min)

45.35

(180min)

3DEM-PbO2

sol-gel method;

sedimentation and

solvent evaporation;

electrodeposition

flutriafol

0.05 M

Na2SO4

electrochemical

filtration

reactor; 200

mL; 19.63 cm2

73.8

(180min)

26.4

(180min)

Ti/TiO2-NTs/SnO2-

Sb/Fe-doped PbO2

anodization process;

thermal deposition;

electrodeposition

methylene blue

120 mL; 15.75

cm2

(30min)

La-Y-PbO2

electrodeposition

levofloxacin

800

0.1 M

Na2SO4

250 mL; 24

cm2; pH 3

F- 93.5

(150min)

Yb-PbO2

electrodeposition

acetamiprid

0.05 M

Na2SO4

250

500 mL; 50 cm2

98.96

(120min)

74.57

(120mi

Ti/SnO2-Sb/Zr-PbO2

sol-gel method;

electrodeposition

Perfluorooctanoic acid

100

10 mM

NaClO4

100 mL;96 cm2;

25 °C;

sequential

reaction system

>70

(90min)

(90

min)

Ti/SnO2-Sb2O5/Bi-

PbO2

thermal

decomposition;

electrodeposition

tebuconazole

12.5

0.01 M

NaClO4

27 °C

(120 min)

Ti/Sn-SbOx/Al-PbO2

thermal deposition;

electrodeposition

chloramphenicol

500

0.2 M

Na2SO4

pH 3; 250 mL

87.30

(2.5 h)

52.06

(2.5 h)

Ti/SnO2-Sb/La-PbO2

sol-gel technique;

electrodeposition

enrofloxacin

20 mM

Na2SO4

30 mL; 25 cm2

95.3 (30

min)

95.1 (30

min)

3D/PbO2-CeO2

composite

electrodeposition

thiamethoxam

0.15 M

Na2SO4

500 mL; 6 cm2;

pH 6

100

(90min)

89.0

(90min

)

PbO2-CeO2

composite

electrodeposition

malachite green

0.2 M

Na2SO4

100mL; pH 5

95.4

(90min)

69.3

(90min

)

Ga2O3-PbO2

sol-gel method;

composite plating

method

bromocresol green

sodium

100

ppm

0.1 M PBS

100 mL; 8 cm2;

40 °C

~70

(60min

)

Ti/RuO2-SnO2-

TiO2/PbO2-CeO2

sol-gel method;

electrodeposition

4-Chlorophenol

100

10 g L−1

Na2SO4

pH 6.7

> 90

(80min)

94.71

(140mi

Ti/SnO2-Sb2O3/

PbO2-ZrO2

thermal

decomposition; pulse

electrodeposition

rhodamine B

0.2 M

Na2SO4

200

100 mL; 25 °C

100

(100min)

82.9

(100min)

PTFE-doped β-PbO2

electrodeposition

norfloxacin

100

0.1 M

Na2SO4

500 mL; 11.76

cm2; 40 °C

100 (12h)

70(12h)

Ti/Sn-SbOx/PbO2-

PDMS

thermal deposition;

electrodeposition

p-chlorophenol

0.5mM

0.05M

Na2SO4

200 mL

99.0(120mi

92.9

(120min)

Ti/SnO2-

Sb2O5/PbO2-PVDF

co-electrodeposition

polyvinylidene

fluoride

100

1.4 g L-1

NaClO4

200 mL; 17.64

cm2; 26 ± 2 °C

92.1

TiO2-based SnO2-

Sb/FR-PbO2

anodization; sol-gel

method;

electrodeposition

ofloxacin

0.05 M

Na2SO4

differential

column batch

reactor; 300

mL; 10 cm2; pH

6.25

>99

(90min)

Ti/PTFE-F-CeO2-

PbO2

electrodeposition

p-chlorophenol

1.0mM

100

200 mL; 25 ±

1 °C

48.4

(150min)

20.1

(150mi

 initial volume of reaction solution;  anode area

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2.2. SnO2-based catalysts

SnO2-based electrocatalysts are frequently used to mineralize HOPs, though SnO2 is an n-

type semiconductor with a band gap of 3.6 eV.65 To fully expand the range of its applications,

doping is the most widely employed strategy to change the intrinsic property of SnO2 and

make it a hotspot in the field of electrode material. Several sophisticated doped SnO2-based

catalysts and their applications are discussed in this work.

Table 2 summarizes the recently representative SnO2-based catalytic anodes used in the

process of HOPs oxidation. It can be found that antimony (Sb) is the most general dopant as

Sb-doped anodes possess high OEP, high stability and favorable electrocatalytic

characteristics.66, 67 For instance, a SnO2-Sb/Ti electrode fabricated with the sol-gel method

was employed for the degradation of ciprofloxacin.68 The coating particles of the anode were

uniform in size, with diameters ranging from 20-100 nm. The nanostructure of the SnO2-

Sb/Ti anode exhibited a large surface area, and thereby provided plentiful active sites for the

electrocatalytic process. In 120 min, the removal ratios of ciprofloxacin, COD, and TOC

were around 99.5%, 86.0%, and 70.0%, respectively.

Components (e.g., F, Ni, Bi, Fe, Ru, Co, Ce, Pd, TiN) often work as co-dopants to improve

the performance of Sb-SnO2 electrode.65, 69-71 Yang et al. 65 analyzed the effect of six

elements (e.g., Fe(III), Ni(II), Co(II), Ru(III), Ce(III), Pd(II)) on the performance of Sb-SnO2

electrode toward mineralization of Eosin Y. In this study, both Ni and Fe could upgrade the

electrocatalytic activities of Sb (5%)-SnO2, especially Ni. Further study implied that the

improvement by Ni doping might be ascribed to the elevated involvement of organic peroxy

radicals. Recently, a well-designed F and Sb co-doped SnO2 anode displayed excellent

capacities in the degradation of perfluorooctane sulfonate (PFOS).69 The co-doped electrode

presented better degradation capacities than that of single doped electrodes, and the main

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reason was that the formed smooth surface could provide more physically adsorbed •OH

radicals and result in relatively high OEP. TiN, as a kind of metal nitride, has been

extensively employed as an electrode material due to its unique features, such as relatively

low costs, high hardness, good chemical stability, great electric conductivity, and superior

catalytic activities.72, 73 Duan et al. 70 prepared a Ti/Sb-SnO2-TiN anode for degradation of

methylene blue. The TiN-doped anode exhibited a smaller crystal grain size, and thereby

boosted the active sites and specific surface area for the electrocatalytic reaction. In addition,

the electrochemical impedance spectroscopy tests suggested that the introduction of TiN

could obviously decrease the charge transfer resistance (150 ohm vs.1334 ohm), and thus

improving the conductivity of the anode. Compared with Ti/Sb-SnO2, Ti/Sb-SnO2-TiN had a

higher adsorption capacity of hydroxyl oxygen species (1.97 times), as well as a higher

decolorization efficiency (1.54 times) and a larger kinetic rate constant (3.24 times).

With good stability and a great electrocatalytic activity, SnO2-based mixed metal oxide

materials are also favourable anodes in the electrocatalytic dehalogenation process.74 For

example, Bai and co-authors 75 designed a Ti/SnO2-RuO2 electrode to decompose

bromocresol green (BCG) in wastewater. The “cracked-mud” structure electrode showed a

high specific surface area, which could provide abundant active sites and enhance the

catalytic performance. Under the optimal experimental conditions, the removal ratio of BCG

reached 91% within 150 min. Vargas et al. 76 fabricated a SnO2-Sb2O5 anode to mineralize

dichlorvos (DDVP) that is a widely used organophosphorus insecticide. The reaction kinetics

for DDVP electrochemical oxidation was higher than other advanced oxidation methods (e.g.,

TiO2/UV and ZnO/UV), and the COD could be completely removed in about 100 min.

Table 2 Summary of representative HOPs electrooxidative dehalogenation using SnO2-based anodes in recent

years.

Anode

Preparation methods

HOPs

CHOP /

mg L−1

electrolyt

current

density

/ mA

cm−2

Other parameters

HOP

Removal

rate/%

COD

Removal

rate/%

TOC

Removal

rate/%

Ref.

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MI-meso SnO2

evaporation induced

self-assembly

2,4-

dichlorophenoxyacetic

acid

10 ppm

0.1 M

Na2SO4

6.34 ±

0.05

100 mL; pH ~7;

applied voltage

2.5V

99.15(3h)

93.2 ±

1.5 (3h)

Ti/TiHx/Ni-Sb-SnO2

spin-coating;

pyrolysis process

methylene blue

0.5 M

H2SO4

100 mL; pH 2; 1

cm2

~100

(10min)

Ti/Sn-Sb/SnO2-F-Sb

electrodeposition;

sol-gel

perfluorooctane

sulfonate

100

10 mM

NaClO4

50 mL; pH 3; 25 ±

3 °C

>99

(120min)

Ti/SnO2-RuO2

thermal decomposition

bromocresol green

100

4 g L-1

Na2SO4

100 mL; 5 cm2;

pH 7; 30 °C

(150min)

SnO2-Sb/Ti

sol-gel method

ciprofloxacin

0.05 M

Na2SO4

250 mL; 15 cm2;

30 °C

99.5

(120min)

86.0

(120min)

70.0

(120min)

Ti/Ta2O5–SnO2

thermal

decomposition; drop

casting

methylene blue

0.1

0.1 M

Na2SO4

2.2 cm2; pH 6.5;

25 ± 2 °C

95(2h)

85(2h)

Ti/SnO2–F

sol-gel method; dip

coating;

perfluorooctanoic acid

100

10 mM

NaClO4

50 mL; 30 cm2; 25

± 3 °C

>99

(30min)

92.6

(30min)

Ti/SnO2–Sb2O5–

Bi2O3

sol-gel method

1H,1H,2H,2H-

perfluorooctane

sulfonic acid

1.4 g L-1

NaClO4

6.8

200 mL; 30 cm2;

32 °C; 0.4%

CH3OH as the co-

solvent

23.8

(3.5h)

SnO2-Sb2O5

2,2-dichlorovinyl

dimethyl phosphate

0.1 M

Na2SO4

50 mL; 1 cm2; 25

± 3 °C

~100

(100min)

Ti/Sb-SnO2-NGNS

sol-gel method; dip

coating

methylene blue

0.25 M

Na2SO4

50 mL; 2 cm2

97.7

(100min)

Ti/Sb–SnO2–TiN

pulse

electro-codeposition

methylene blue

0.25 M

Na2SO4

50 mL; 2 cm2

100

(120min)

SnO2-Sb/CA

sol-gel method;

solvent exchange;

ambient pressure

drying; pyrolysis

perfluorooctanoate

100

0.1 M

Na2SO4

60 mL; 5 cm2

91(5h)

86(5h)

Ti/Ni–Sb–SnO2

dipping-coating

Eosin Y

1 mM

0.1 M

Na2SO4

22.22

20 mL; 4.5 cm2

~48

(25min)

~75(2.5h)

Ti/SnO2-Sb-Bi

sol-gel method

perfluorooctanoate

1.4 g L-1

NaClO4

22.07

25 mL; 11.33 cm2

>99 (2h)

Mp-SnO2/BDD

anodic treatment;

sol-gel technique

clofibric acid

200

0.05 M

Na2SO4

4 cm2; 100 mL; 25

± 2 °C;

(240min)

2.3. Other metal oxide-based catalysts

Like the oxides of Pb and Sn, other metal oxides (e.g., RuO2, Bi2O3, IrO2, Ti4O7) are also

employed as electrodes in recent years owing to their relatively low cost, high chemical

stability and superior electrocatalytic activities.85, 86 Table 3 collects the recently

representative studies involve the electrochemical oxidation of HOPs by metal oxides (except

PbO2 and SnO2)-based catalytic materials. Among these, RuO2- and Ti4O7- based electrodes

are underlined in this work as two typical examples.

RuO2 has emerged as a promising candidate for mineralization of HOPs owing to its high

stability, high OEP, great electrochemical performance and low cost.86, 87 In addition, RuO2

showed high capacity toward the generation of active oxidants such as chlorine, ozone, and

free radicals.88 Kaur and co-authors 86 used the Ti/RuO2 anode to decay ofloxacin antibiotic

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(OFLX), and experimental results suggested that H2O2, HO2•, HOCl, ClO- and chemisorbed

•OH are all active components in the degradation process. As a result, removal ratios of

OFLX and TOC were about 80% (30 min) and 46.3% (240 min), respectively. To enhance

the performance of RuO2 anode through enlarging the electrochemical surface area, Zhang et

al. 89 fabricated a tubular porous Ti-RuO2 electrode and designed a corresponding reactor to

degrade anticancer drugs wastewater containing 5-Fluoro-2-Methoxypyrimidine (5FMP). The

electrode with a porous structure had larger active surface area than the plate electrode with

the same geometrical area. Additionally, the micro-sized pores of Ti-RuO2 electrode achieved

a vertical flow between the anode and cathode in the presence of the pump. As a result, the

designed reactor could not only improve the mass transfer process but also generate a

homogeneous velocity distribution and high turbulent mixing around the electrode. For these

merits, the 5FMP was completely removed and the removal efficiency of COD reached 84.1%,

within 180 min of electrocatalysis.

Ti4O7 that is a Magnéli phase titanium sub-oxide has been suggested as a good candidate for

electrodes owing to its great conductivity (~1000 S cm-1), high OEP (2.6 V/SHE, standard

hydrogen electrode), good chemical stability, and easy preparation.90-92 Therefore, Magnéli

phase Ti4O7 ceramic materials have been widely worked as anodes in the electrochemical

purification of wastewater. Lin et al. 92 found that the macroporous Magnéli phase Ti4O7

ceramic materials possessed interconnecting macropores, with size ranging within 1-8 m

(Fig. 3 a-d). Compared to Ce-PbO2 and BDD anodes, the porous Ti4O7 ceramic electrode had

a greater surface area and the enhanced interphase mass transfer, thus improving the overall

current efficiency. As a result, the porous anode exhibited better performance than Ce-PbO2

and BDD anodes in the mineralization of poly- and perfluoroalkyl substances (Fig. 3 e-f).

Additionally, previous work reported that Co3O4,93 IrO2,94 and Bi2O385, 95 are also excellent

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candidates for the EOD process, owing to their favourable electrochemical activities, good

mechanical and chemical resistance.95

Fig.3 SEM image of the surface (a) and cross-section (b) of the macroporous Magnéli phase

Ti4O7 ceramic plate materials; (c, d) results of Hg intrusion porosimetry analysis of pore size

distribution; concentration change of 0.5 mM PFOA (e) and 0.1 mM PFOS (f)

during electrooxidation process by different anodes. Reproduced with permission from ref.

Table 3 Summary of representative HOPs electrooxidative dehalogenation using metal oxides (except PbO2 and

SnO2)-based anodes in recent years.

Anode

Preparation

methods

HOPs

CHOP / mg

L−1

electrolyte

current

density/

mA cm−2

Other

parameters

HOP

Removal

rate/%

COD

Removal

rate/%

TOC

Removal

rate/%

Ref.

Ti/RuO2

ofloxacin

2 g L-1 NaCl

1.5 L; pH 6.8 ±

0.1; 1 A

~80 (30

min)

46.3

(240min)

Ti/RuO2-IrO2

4-chlorophenol

50 ppm

1 g L-1 NaCl

32 cm2; 1 L; pH

7.38;

57.61

(89 min)

tubular porous Ti-

RuO2

sol–gel

technique;

thermal

decomposition

5-Fluoro-2-

Methoxypyrimidine

61.2

94.2 cm2; 1 L;

pH 5.0; flow

rate 0.31 L min-

100

84.1

RuO2–PdO–TiO2/Ti

sol–gel; dip-

coating

rhodamine B

1.043×

10−5 M

1.709×

10−3 M NaCl

0.5 L; 0.02 A

71.70

(30 min)

RuO2-coated Ti

rhodamine 6G

200

0.2 M NaCl

16.52

300 mL; 115

cm2; 25 °C; pH

>99

>90

Ti/RuO2

thermal

oxytetracycline

2.01× 10-3

0.80 M

2 cm2; pH 5.45

100

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decomposition

hydrochloride

phosphate

buffer

(120

min)

Co3O4

nanowires/carbon

cloth

triclosan

1M KOH

6 cm2

(60 min)

Ti/IrO2

norfloxacin

62.6 μM

5×

10−2 M NaCl

6.53

150 mL; 3.06

cm2; pH 9.0

100

(20 min)

Ti-Bi2O3

galvanostatic

electrodepositio

Reactive Orange 4

0.32 mM

10 mM H2O2;

10 mM

Na2SO4

20±0.5 °C; pH 7

100

(40min)

stainless steel/Bi2O3

electrodepositio

calcination

Methylene blue

0.08 mM

10 mM H2O2;

10 mM

Na2SO4

20±0.5 °C; pH

7±0.2

100

(125min)

stainless steel/Bi2O3

electrodepositio

calcination

Reactive Orange 4

0.08 mM

10 mM H2O2;

10 mM

Na2SO4

20±0.5 °C; pH

7±0.2

100(35mi

Au/Bi2O3

electrodepositio

Crystal violet

10 mM H2O2;

1 mM Na2SO4

pH 7±0.1

100

(20min)

porous Ti4O7 ceramic

high-

temperature

sintering

perfluorooctanoate

0.5 mM

20 mM

NaClO4

50 cm2; 250

mL; 25±1 °C

>99.9

(180min)

>95

(180min)

porous Ti4O7

ceramic

high-

temperature

sintering

perfluorooctanesulfo

nate

0.1 mM

20 mM

NaClO4

50 cm2; 250

mL; 25±1 °C

93.1 ±

3.4

(180min)

90.3 ±

1.8

(180min)

2.4. Carbon-based catalysts

Anode materials derived from carbon have also been extensively employed in the EOD

process since these electrodes own excellent chemical stability together with low resistivity.

The widely reported carbon-based catalysts include BDD,21, 100, 101 graphite,20, 102 activated

carbon fiber,103 carbon nanotubes,104, 105 and granular activated carbon26, 106. The catalytic

performance of representative carbon-based anodes for HOPs degradation is presented in

Table 4. Among these carbon-based anodes, BDD is the most widely explored one due to its

distinct advantages over other anodes, such as high corrosion resistance, favorable

electrochemical stability, high OEP (2.8 V/SHE), a wide electrochemical potential window,

and a low background current.107 Numerous studies suggest that many intrinsic properties of

BDD anodes and electrochemical experimental conditions have a great impact on the

degradation performance of BDD anodes.

BDD films are usually fabricated with the chemical vapor deposition (CVD) method, and a

substrate is necessary to support the BDD film and improve its stability.108-110 The commonly

used substrates include Si, Ta, Ti, Nb, and W, but it is difficult to evaluate the stability of

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BDD films as a function of substrate type due to the non-uniform preparation parameters

among different studies, as well as evident pros and cons of each substrate.108 Apart from the

substrate, another intrinsic property that can heavily influence the electrocatalytic

performance of BDD is the sp3 (diamond)/sp2 (graphite) carbon ratio.111-115 For example, six

p-Si BDD anodes with different sp3/sp2 carbon ratios (165-323) showed various degradation

capacities towards 2, 4-dichlorophenoxyacetic acid (2, 4-D).112 A more speedy and effective

dechlorination of 2, 4-D is due to a higher sp3 carbon content in the BDD anode. However,

the sp3/sp2 ratio has no effect on the type of detected intermediates. On the other hand, Araújo

and co-authors 114 found that more aromatic intermediates are formed at the BDD6 anode

(high sp3 carbon), while less of them are yielded when BDD1 anode (high sp2 carbon) was

used, in the mineralization of RhB. Their further exploration indicated that a higher content

of sp3 carbon resulted in greater TOC and COD decomposition by electrochemical

mineralization with hydroxyl radicals. In contrast, BDD anode with a lower sp3/sp2 ratio

favoured the electrochemical conversion (formation of many intermediates) because of the

strong adsorption of reactants on graphite carbon sites. In other words, sp2 carbon species

behave as “active anode” parts, which mainly follow the direct oxidation process and leads to

a lower RhB combustion ratio. More details about this aspect can be found in previous

reviews.111, 113

To further upgrade the electrochemical oxidation performance of the planar/bare BDD

anodes, the surface modification is highly recommended.116, 117 Nanocrystallizing,118

chemical etching,119 electrochemical anodizing,120, 121 porous structuring,122, 123 and

hybridizing116, 123 are proved as the efficient strategies to achieve better electrochemical

performance of BDD electrodes. The general idea of these methods is to improve the active

surface area, enhance the intrinsic electrochemical activity and boost the durability of

electrodes.123 Urtiaga et al.118 employed an ultrananocrystalline boron doped conductive

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diamond electrode that possessed a thin film coating and nanoscale grain size to degrade

PFOA (100 mg•L-1). Several key experimental parameters were investigated systematically,

including the supporting electrolyte, the applied current density and the hydrodynamic

conditions. The results suggested that PFOA degradation and mineralization ratios were

higher than 90% within 6 hours, at an applied current density of 200 A•m-2. Currently, the

applications of modified BDD in the mineralization of HOPs are rarely reported, and more

attention should be encouraged to make higher use of these great BDD electrodes.

In the EOD process, the supporting electrolyte has a conspicuous influence on the

mineralization performance of BDD anodes. Specifically, the degradation kinetics and

efficiency, as well as the degradation pathways are sensitive to electrolytes.124-126 The

commonly used electrolytes include NaCl, Na2SO4, NaNO3, and NaHCO3. Among these,

NaCl and NaSO4 are the most attractive ones since they can produce highly effective

hydroxyl radicals, such as Cl•, ClOx•, SO4•-, and S2O82-, which are beneficial to the decay of

HOPs.127, 128 Carneiro et al.125 compared the effect of five different electrolytes (Na2SO4,

NaCl, Na2CO3, NaNO3, Na3PO4) on the removal of enrofloxacin, using a BDD anode.

Compared to other electrolytes, NaCl led to an appreciably faster degradation of enrofloxacin,

and the most initial intermediates were chlorinated. Nevertheless, Na2SO4 manifested a better

performance than NaCl in the electrooxidation of losartan with a BDD electrode.126 Using

Na2SO4 as supporting electrolyte, the mineralization ratio (67%) was higher than that of the

NaCl system (56%). Interestingly, Mena et al.127 designed an experiment to clarify the

contribution of hydroxyl radicals and electrogenerated sulfur-based radicals (e.g.,

peroxodisulfate) in the degradation of three different ionic liquids (ILs) using a BDD anode.

Their results suggested that sulfate could improve the efficiency of the electrooxidation

process and prevent the formation of polymers.

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In addition, many studies underscore the importance of the water matrix on the EOD process

with BDD anodes.129-131 Naturally, waters contain various organic and inorganic, non-target

components. These constituents may interact with the oxidizing radicals and the target

pollutants. As a result, the presence of these chemicals can heavily impact the performance of

BDD anodes. The frequently investigated components include chloride, sulfates, carbonates,

and humic acid. As aforementioned, chloride can introduce different active chlorine species

that complement •OH in the mineralization of HOPs. However, a high concentration of Cl-

can improve Cl2 evolution and hence, cause a reduction of the anode performance. What’s

worse, several organochlorinated components, such as chloramines and trihalomethanes, may

be generated in the electrocatalysis process, which is detrimental to the degradation process

owing to their high toxicity and strong recalcitrance.132, 133 In contrast, radicals derived from

sulfates act as good mediators and contribute to the indirect electrocatalysis of pollutants.129

Conversely, the presence of carbonates/bicarbonates can reduce the mineralization efficiency

of BDD anodes, due to the fact that carbonates/bicarbonates are •OH scavengers, namely,

carbonates/bicarbonates can consume •OH and reduce the availability of •OH to target

HOPs.132 Humic acid (HA) is an analogue of natural organics typically detected in waters.

HA has been recognized as a detrimental component in the degradation of HOPs for several

reasons: (1) competition between HA and target HOPs for radicals, (2) HA is a radical

scavenger and (3) binding reactions between HA and pollutants.132

Other than BDD, carbon nanotubes, activated carbon, and graphite have also been employed

in the decay of HOPs. Carbon nanotubes (CNTs) attract enormous attention due to their one-

dimensional conduction pathways, chemical stability, good electrical conductivity as well as

high surface area.104, 134 Combined with graphene, the CNTs-graphene composite electrode

exhibited excellent performance in the mineralization of perfluorinated compounds (PFCs)

owing to the synergy of electrochemical degradation and electrosorption (Fig. 4).135 Granular

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activated carbon (AC) often acts as a particulate electrode in the electrolysis system, which

can enhance the overall active electrode area, conductivity, and mass transfer and improve

the electrochemical degradation kinetics and COD removal ratio.136 Pedersen et al. 137

explored the synergy of AC adsorption and electrochemical degradation in the mineralization

of 2-methyl-4-chlorophenoxy acetic acid (MCPA) and 2-methyl-4-chlorophenoxy propionic

acid (MCPP). Their results indicated that synergies of 121-126% were found for MCPA and

MCPP at w/w AC: the organic ratio of 5:1, with an electric field strength of 375 V•m-1.

Recently, graphite has been employed as an anode for the mineralization of atrazine for the

first time.20 The oxygen-containing functional groups produced by the positively polarized

graphite electrode played a catalytic role. The X-ray photoelectron spectroscopy (XPS)

analysis verified the presence of oxygen-containing functional groups (C-OH, COOH, and

C=O) on the graphite anode surface during the anodic oxidation (water oxidation) process.

Surprisingly, no indicator for the production of abundant hydroxyl radicals at graphite

electrodes was found, and the atrazine was virtually degraded by the chemisorbed reactive

oxygen.

Fig.4 (a) A schematic illustration of the degradation process; the removal ratios (b) and

electrosorption kinetics (c) of PFOA and PFOS on CNTs-

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20% graphene composite electrode fitted by pseudo-first-order equation (- -) and pseudo-

Elsevier.

Table 4 Summary of representative HOPs electrooxidative dehalogenation using carbon-based anodes in recent

years.

Anode

HOPs

CHOP /

mg L−1

electrolyte

current

density/

mA cm−2

Other

parameters

HOP

Removal

rate/%

COD

Removal

rate/%

TOC

Removal

rate/%

Ref.

BDD

norfloxacin

100

2 g·L−1

Na2SO4

250 cm3

(30min)

138

graphite

atrazine

0.05 M NaCl

300 mL; pH

6.8; 25 ± 2 °C

100

(60min)

BDD

(B/C 500 ppm)

5-fluorouracil

6 g·L−1

Na2SO4

200 mL; pH

6.0; 20 °C

100

(6h)

BDD

(B/C 10000 ppm)

Helectine F

(C23H12Cl2N6Na3O10S3

)

6 g·L−1

Na2SO4

8.8

flow reactor;

2.3 cm2; 0.1 L;

pH 3.0; 0.6 L

h−1

94.9

86.3

139

BDD

(B/C 10000 ppm)

Helectine D

(C29H16Cl1N7Na4O13S4

)

6 g·L−1

Na2SO4

8.8

flow reactor;

2.3 cm2; 0.1 L;

pH 3.0; 0.6 L

h−1

94.8

95.7

139

BDD

4-chlorophenol

500

0.1 M Na2SO4

400

400 mL; pH

6.5; 25 °C

(150min)

101

CNTs/AG/ITO

2-chlorophenol

100 mL; pH 2;

bias voltage 4 V

(180min)

104

BDD

(B/C 1000 ppm)

thiamethoxam

0.1 M Na2SO4

12.5 cm2; 150

100

(20min)

(120min)

132

BDD

(B/C 500 ppm)

ciprofloxacin

69 μM

0.1 M K2SO4

7.24

flow reactor; 69

cm2; 1 L; pH

3.0; 360 L h−1;

30 °C

100

(180min)

130

MCD BDD

perfluorooctanoic acid

0.24 mM

70 cm2; 293 ± 2

K;1 L

100(4h)

89(4h)

140

BDD

lindane

0.42

50 mM

Na2SO4

400

230 mL; 24

cm2; pH 7.0;

25 °C

100

(4h)

129

BDD

(B/C 100 ppm)

enrofloxacin

100

0.1 M NaCl

filter-press flow

cell; 23.75 cm2;

1 L; pH 3.0;

420 L h−1

100

(1h)

125

BDD

losartan

0.377mM

50 mM

Na2SO4

100 mL; 24

cm2; pH 7.0;

25 °C

100

(180min)

126

BDD

poly- and

perfluoroalkyl

substances

1.652

70 cm2; 2 L; 3 L

min-1; flow-by

cell

99.7

(10h)

>90

(10h)

141

CNT sponges

perfluorooctanoic acid

0.1

10 mM

Na2SO4

200 mL; 1 cm2;

25 ± 1 °C;

bias voltage 4 V

>90

(180min)

142

BDD

diquat dibromide

100

0.5

300 mL; 100

cm2; pH 6.5;

25°C; 500 mL

min-1

99.17

(5h)

57.64

(5h)

71.31

(5h)

143

Apart from electrochemical activity, the stability of electrocatalysts is another key property in

practice. The deactivation of anodes is a common issue in the electrochemical oxidation

process, especially the PbO2 and SnO2 based catalysts.33, 37, 39, 68, 70, 78, 90, 144 There are many

factors responsible for the anodic deactivation, such as mechanical damages, detachment,

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consumption/dissolution, and passivation, as well as operating conditions (e.g., temperature,

applied current), etc.70, 145, 146

To alleviate the deactivation and prolong the lifetime of catalysts, the efficient strategies (e.g.,

doping, introducing interlayer) which focus on optimizing the catalyst configuration have

exhibited great potentials.78 Recently, Yu et al.33 investigated the influence of Ce doping and

the RuO2-SnO2-TiO2 interlayer on the stability of the PbO2 electrode through the accelerated

life tests. Three samples, namely PbO2-0 (unmodified PbO2/Ti), PbO2-1(Ti/ RuO2-SnO2-

TiO2/PbO2), and PbO2-3 (Ti/RuO2-SnO2-TiO2/PbO2-CeO2, Ce/Pb = 0.01), exhibited

completely different anti-deactivation abilities. The service lifetime of PbO2-3 was 215 h,

which was 0.7 times longer than that of PbO2-1(125 h) and 9.2 times longer than that of

PbO2-0 (21 h). Consequently, both Ce doping and the existence of the RuO2-SnO2-TiO2

interlayer could notably improve the catalysts’ durability. Other than the interlayer, Ce

doping could conduce the formation of compact surface layer, which also regulated the

durability. The compact surface could effectively hinder the penetration of electrolytes into

the Ti substrate and decrease the inner mechanical stress on PbO2, thus improving the

durability. Many similar reports also emphasise the role of doping and interlayer in the

construction of robust anodes.144, 147-149

2.5. Electrooxidative dehalogenation mechanisms

The successful cleavage of C-X bonds through electrochemical oxidation by catalysts mainly

derives from reactive species and/or electrons. Hence, the EOD process can be roughly

divided into two categories, namely, direct anodic oxidation and indirect oxidation. It is

worth noting that many degradation processes involve both pathways,38, 65, 150 and the direct

oxidation reaction is a vital rate-limiting step for the degradation of pollutants that are

unreactive with reactive oxygen species (e.g., fluorinated compounds).108

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The direct anodic oxidation process occurs directly on the surface of anodes and involves

direct electrons transfer between the HOPs and anode surface. Adsorption of organics onto

anode surfaces is an important step in this process as it largely determines the degradation

rate.151 Another important factor is the applied potential, and the wanted potential should be

higher than that of the H2O oxidation reaction. Otherwise, the applied electrode is susceptible

to surface poisoning and lead to a decreased degradation efficiency.152 Although the direct

anodic oxidation generally result in very poor degradation performance,153 there are a few

studies that report favorable HOPs abatement results.154, 155

The indirect oxidation process is based on the reactive species (e.g., •OH, H2O2, O3, •Cl, Cl2,

ClO-, SO4•-, S2O82-) that produced on the surface of anodes. Among these reactive species,

•OH is almost a defining requirement of the indirect oxidation process for its high redox

potential (2.80 V/SHE). In most circumstances, indirect electrooxidation degradation of

HOPs can be explained with •OH. However, the degradation pathway depends heavily on the

adsorption status of •OH on anode surfaces. On one hand, the chemisorbed •OH on active

anodes (e.g., Pt, RuO2, IrO2) can react drastically with anodes and form higher oxides or

superoxides. This strong interaction between anodes and •OH decreases the catalytic

efficiency of reactive species and lead to the partial transformation of HOPs, which called

electrochemical conversion. On the other hand, the physisorbed •OH on inactive anodes (e.g.,

BDD, doped-SnO2, PbO2) presents a weak interaction with inactive anodes and can result in

complete electrochemical degradation, which named electrochemical combustion. The

classification of anodes mainly depends on the •OH production capacity, but not all

electrodes can be clearly divided into the mentioned two types and some anodes manifest

characteristics of both types.108 To present the mentioned degradation process more

intuitively, schemes based on the model proposed by Comninellis156 are displayed in Fig.5.

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Fig.5 Mechanistic scheme of oxidative degradation of HOPs on inactive anodes (a) and on

active anodes (b).

Other radicals such as chlorine- and sulfur-based species also play important roles in the

mineralization of HOPs. Active chlorine species generated from solution begins with the

direct oxidation of chloride ions on anode surface via Eq. (1), and the generated chlorine

diffuses away from the electrode surface and yields Cl- and HClO through disproportionation

(Eq. (2)).

2Cl-  Cl2(aq) + 2e- (1)

Cl2(aq) + H2O  HClO + Cl- + H+ (2)

The HClO in acid-base is in equilibrium with ClO- with pKa of 7.55 (Eq. (3)).157

HClO  ClO- + H+ (3)

It should be pointed out that the existence of these oxidative species highly depends on pH

value (pH< 3, Cl2; pH= 3-8, HClO; pH> 8, ClO-). As a result, the oxidation of pollutants is

more efficient in acidic media as the standard redox potentials of HClO (1.49 V/SHE) and Cl2

(1.36 V/SHE) are higher than that of ClO- (0.89 V/SHE).133 Furthermore, the conversion of

HClO to ClO3- (Eq. (4)) can interfere the degradation performance as ClO3- is less active in

the oxidation reaction.158

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6HClO + 3H2O→ 2ClO3− + 4Cl− + 12H+ + 1.5O2 + 6e− (4)

Likewise, chloride species can react with radicals and form perchlorates by Eqs. (5)- (8).

Cl− + •OH → ClO− + H+ + e− (5)

ClO− + •OH → ClO2− + H+ + e− (6)

ClO2− + •OH → ClO3− + H+ + e− (7)

ClO3− + •OH → ClO4− + H+ + e− (8)

Actually, the evolution of ClO4− is an undesirable access because of its low oxidation

capacity and serious hazardousness for human health.133 Furthermore, those organo-

chlorinated species formed are also detrimental to the mineralization process for their strong

persistence and high toxicity.108

S2O82- with a high redox potential (2.01 V/SHE) is another often mentioned radical because

Na2SO4 is the most widely used electrolyte. The generation of S2O82- is presented with Eq.

(9).159 This radical has a strong oxidation potential and can react with organic pollutants,

which results in a desirable degradation. Interestingly, the potential of persulfate can be

greatly enhanced with different activation methods,160 and the related studies are reviewed by

Matzek and Carter.161 The SO4•- is another sulfur-based radical, which also shows a high

redox potential (2.5-3.1V/SHE). The formation and transformation of sulfate radicals are

presented as Eqs.(10)- (12).160 Previous reports indicated that both persulfate radicals and

sulfate radicals could facilitate the mineralization of organic pollutants due to their high

oxidation ability.160, 162, 163

2SO42- → S2O82- + 2e- (9)

SO42- → SO4•- + e- (10)

SO4•- + H2O → SO42- + •OH + H+ (11)

S2O82- + e- → SO42- + SO4•- (12)

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Other radicals (e.g., H2O2 and O3) are also quite important complements to •OH in the EOD

process. More information can be found in the previous reviews.133, 164

It is necessary to place emphasis on the mineralization pathway of perfluorinated compounds

(PFCs), due to the high persistence of C-F and the terrible toxicity of PFCs. In addition, it is

suggested that the mineralization of PFCs cannot be solely explained by radicals (e.g., •OH),

which is quite different from the degradation process of normal HOPs. Niu et al.165 found that

the degradation of PFCAs began with a direct anodic oxidation process (Eq. (13)), and the

formed radicals (e.g., CnF2n+1•) can react with •OH or other radicals. With a series of

reactions (intramolecular rearrangement and hydrolysis), PFCAs can be totally mineralized

by repeating the CF2-unzipping cycle, and the related reaction is presented in Eqs. (14)-(17).

However, Li and co-authors suggested that the mineralization of PFOA (e.g., C7F15COOH)

was primarily driven by •OH radicals (Eq. (18)) in an MW-Fenton-like process.166 Indeed,

mechanisms related to different PFCs and different degradation systems may vary

significantly. More detailed information can be referred to previous review works,28, 167 and a

classical PFCs degradation process is presented in Fig. 6.28

CnF2n+1COO- → CnF2n+1COO• + e- (13)

CnF2n+1COO• → CnF2n+1• + CO2 (14)

CnF2n+1• + HO• → CnF2n+1OH (15)

CnF2n+1OH → CnF2n-1OF + HF (16)

CnF2n-1OF + H2O → Cn-1F2n-2COO- + HF + H+ (17)

C7F15COO- + HO• → C7F15COO• + OH- (18)

Therefore, due to the complexity of HOPs structure, radical species, and water matrix, the

oxidation degradation pathway should be explored on a case-by-case basis with the assistance

of advanced analysis methods.

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Fig.6 Schematic diagram illustrating the electrochemical oxidation mechanism of PFCs.

3. Catalysts for electroreductive dehalogenation

The electrocatalytic reduction is a highly effective method for the degradation of HOPs.

Different from the electrooxidation process that generally results in complete mineralization

of HOPs, the overall goal of the electroreductive process is either total dehalogenation

(principally hydrodehalogenation) of HOPs to ease the further treatment, or the

transformation into value-added chemicals.30, 168 In this part of the review, commonly

reported catalysts such as Ag-based electrodes, Pd-based electrodes, and metal complexes

(organometallic compounds) are analysed. Subsequently, diverse reduction mechanisms are

reviewed to provide a better insight into the electrolytic scission of C-X bonds.

3.1. Ag-based catalysts

Recently, silver-based cathodes have attracted much attention owing to their strong

electrocatalytic activities toward dehalogenation of HOPs. The high affinity of Ag for halide

ions plays the vital role.169, 170 Compared with the classic glass carbon electrode which

commonly acts as a noncatalytic cathode, the electrochemical breakage of C-X bonds at

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silver cathode occurs at a much less negative potential.171 Several critical examples involve

the ERD process with Ag-based cathodes are summarised in Table 5.

Table 5 Summary of representative HOPs electroreductive dehalogenation using Ag-based cathodes.

electrode

electrolyte

HOPs

Main Products/ Results

Ref.

(0.071 cm2)

DMF + 0.05 M

TMAP

2-bromo-5-chlorothiophene (1)

3-bromo-2-chlorothiophene (2)

2,5-dibromothiophene (3)

2-Chlorothiophene (1)

2-Chlorothiophene (2)

2-bromothiophene (3)

171

Etched Ag-PVP-Ni

0.05 M NaOH

50 ppm Alachlor™

Conversion rate 93 ± 3%;

deschloroalachlor 69 ± 4%;

Cl- 77 ± 2%

169

Ag-ZSM-5/SS

0.1 M TEAP-MeCN

2.6 mM PhCH2Cl

yield of electrocarboxylation 61%

172

(0.071 cm2)

DMF + 0.050 M

TBABF4

10 mM 1-bromo-6-chlorohexane (1)

10 mM 1-chloro-6-iodohexane (2)

1-chlorohexane 63%, n-hexane 18%

6-chloro-1-hexene 10% (1);

1-chlorohexane 43%, 6-chloro-1-

hexene 23%, 1,12-dichlorododecane

22% (2)

173

roughened Ag

0.5 M NaClO4

(anolyte)+0.5 M

NaClO4 (catolyte)

5 mM 3,4,5,6-Tetrachloropicolinic

Acid(TeCP) (pH = 7.0)

3,5,6-T 83.1%, TeCP 8.4%, 3,6-D

4.3%

174

roughened Ag

0.5 M HClO4

(anolyte)+0.5 M

NaClO4 (catholyte)

5 mM TeCP (pH = 4.0)

TeCP 60.0%, 3,5,6-T 35.0%, 3,5-D

4.3%

174

roughened Ag

mesh

0.5 M NaOH

3.33 mM 3,5,6-trichloropicolinic

acid (3,5,6-T)

3,5,6-T 61%, 3,6-D 38%, 3,5-D 4.3%

175

DMF + 0.05 M TMAP

5 mM 1,2-dibromohexane (1)

10 mM 1,6-dibromohexane (2)

1-hexene 107% (1);

1-hexene 22%, n-hexane 39%, 1,5-

hexadiene 18%, 5-hexen-1-ol 7% (2)

176

 dimethylformamide;  tetramethylammonium perchlorate;  tetra-n-butylammonium tetrafluoroborate

Nanosizing is an effective strategy to enhance the performance of Ag cathodes as well as to

reduce the consumption of this precious metal.172, 177 Ag nanoparticles present distinctive

capabilities that are different from the bulk material because of their decreased dimensions

and improved surface to volume ratios.178 For instance, Sui and co-workers172 fabricated an

Ag-ZSM-5/SS zeolite modified electrode with a facile one-step method, and Ag nanoparticles

(average diameter of 9 nm) was formed in-situ with ZSM-5 films grown on the stainless steel.

The addition of well-dispersed Ag nanoparticles highly elevated the conductivity of the

zeolite film, with a low resistance of 4 mΩ•sq-1. Further study suggested that the

electrochemically active surface areas played a key role in determining the catalytic

performance. Compared to the Ag bulk electrode, ZSM-5 provided more sites for dispersing

and stabilizing Ag nanoparticles and promoted Ag nanoparticles’ catalytic activity. As a

result, The Ag-ZSM-5/SS displayed marvelous electrocatalytic activities towards

electrocarboxylation of PhCH2Cl, with the electrocarboxylation yield of 61% was obtained at

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-1.1 V. However, the application of Ag bulk electrode showed a lower yield of 40% at a more

negative potential of -1.5 V. Brzózka et al.178 systematically designed three Ag electrodes,

namely, silver nanohemisphere (Ag-NHS) electrode, nanowire (Ag-NW) array electrode and

silver rod (Ag-bulk) electrode, and the detailed synthesis routes of Ag-NHS electrode and

Ag-NW electrode are presented in Fig. 7 a. The as-prepared two nanomaterials exhibited

quite different morphologies and diameters. Ag-NHSs possessed an average diameter of

126nm and height of 126nm, while the diameter and length of Ag-NWs were 90nm and 3.5

m (Fig. 7 b-d). The smaller size of Ag-NWs could provide more active sites for the

electrochemical reaction. Compared with Ag-bulk electrode, both Ag-NHS and Ag-NW

manifested better electrochemical performance and stability in the degradation of CHCl3.

Furthermore, the Ag-NW showed about 200 mV less over potential than Ag-bulk electrode,

and that of Ag-NHS was about 100 mV.

Fig.7 Schematic representation of synthesis of the free-standing Ag nanohemisphere and

Ag nanowire array electrodes (a); Top-view SEM images of Ag electrodes: Ag rod (Ag-

bulk) after mechanical polishing (b); Ag nanohemisphere array electrode (Ag-NHSs) after

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silver electrodeposition (c); Ag nanowire array electrode (Ag-NWs) after silver

Another encouraging way to boost the electrocatalytic activity of Ag cathodes is based on

changing Ag bulk to multi-metallic electrodes.179, 180 Vanrenterghem et al.179 prepared

bimetallic Ag-Ni glassy carbon supported cathodes (Ag-Ni/GC) with a two-step procedure,

and the designed Ag-Ni/GC electrodes were compared with Ag/Ni foam electrodes for the

reduction of benzyl bromide. Their results suggested that pretreatment such as cathodising in

alkali and scanning in acid had an obvious effect on the surface composition of Ag-Ni/GC

cathodes, with the cathodising and scanning processes leading to a higher atomic ratio of Ag-

Ni as 1/13 and 1/4 respectively in comparison with the 1/17 for the non-treated one. Further

study indicated that Ag-Ni/GC electrode cathodised in alkali was the best one for the

degradation of benzyl bromide, and the yield of the sole product toluene (49 mol%) was

higher than that of bulk Ag cathode (37 mol%). Hence, the presence of Ni has a beneficial

influence on the electrocatalytic capacity of Ag.

Table 5 also suggests that the degradation performance of Ag-based cathodes relies heavily

on HOP structures, apart from the intrinsic properties of electrodes. In particular, the

molecular structures of HOPs have a significant impact on the dissociative electron transfer

(DET) process in the dehalogenation reaction. For example, electroreduction of a series of

aliphatic and aromatic bromides on the silver cathode has been studied by cyclic voltammetry

in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4).170 In the case of aromatic

bromides, the electron transfer process supplies a radical anion intermediate that further

dissociates to R• and Br-, and this reaction called a stepwise mechanism. It is worth noting

that the electrocatalytic activity for aromatic bromides relies highly on the molecular

structure of RBr, decreasing with growing ability of the RBr to delocalize the negative charge

in the radical anion intermediate. However, for the reduction of different aliphatic bromides,

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Ag always displays a great electrocatalytic activity, which has been confirmed by several

studies.181-183

3.2. Pd-based catalysts

Noble metal catalysts (e.g., Pd, Ru, Rh, Pt) that show great electrocatalytic activities have

also been applied to the electrochemical dehalogenation of HOPs. Among these, Pd is the

most frequently used cathode as it possesses an extraordinary ability to intercalate hydrogen

within its lattice.184 Moreover, the formed active chemical species Pd-H normally acts as the

source of atomic hydrogen (H*).1, 185 Quite different from the dissociative electron transfer

(DET) process with Ag-based catalysts, electrocatalytic hydrodehalogenation (ECH) process

using atomic hydrogen is the main HOPs degradation pathway with Pd-based electrodes.

Therefore, enormous efforts have been devoted to the development of high-performance Pd-

based cathodes that can produce sufficient H* and have a long lifetime. A summary of

representative HOPs electroreductive dehalogenation studies with Pd-based cathodes is

presented in Table 6.

Table 6 Summary of representative HOPs electroreductive dehalogenation using Pd-based cathodes in recent

years.

Cathode

Preparation methods

HOPs

CHOP /

mg L−1

Electrolyte

Other parameters

HOP

Removal

rate/%

Ref.

TiC-Pd/Ni

electroless

deposition

2,4-dichlorobenzoic

acid

0.2

10 mM

Na2SO4

10 mA; 80 mL

(catholyte), 40 mL

(anolyte); pH 4;

298 ± 0.3 K

99.8

(90min)

Pd/PANI/Ni

two-step

galvanostatic

electropolymerizatio

n method

2,4-dichlorophenol

0.5

0.05 M Na2SO4

5 mA; 50

mL(catholyte), 50

mL (anolyte);

40 °C

89.1

(120min)

186

Pd/MnO2/Ni

electrodeposition

2,4-dichlorobenzoic

acid

0.2

10 mM

Na2SO4

10 mA; 80 mL

(catholyte), 40 mL

(anolyte); 30 ±

1 °C; pH 4

100

(120min)

187

Pd/CNTs

impregnation

method

4-chlorophenol

H2 10 mL min-1;

313 K

(50min)

188

Pd/PANI-

NiHCF-

CNT/Ti

electrodeposition

4-chlorophenol

100

0.1 M Na2SO4

1 mA cm-2; 100

mL (catholyte),

100 mL (anolyte);

16cm2

98.59

(120min)

189

EDPd/CNx/N

electrodeposition

3,6-

dichloropicolinic

acid

0.226

34.2 mM

NaCl

2.5 mA cm-2;

25 °C; 72 mL

(catholyte), 36 mL

(anolyte)

97.8(4h)

190

Ni/Pd

electrodeposition

Para-

50 mM

10 mA cm-2; pH

191

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chloronitrobenzene

Na2SO4

7; 25 °C; 50 mL

(30min)

Pd/CNTs-

nafion film/Ti

dip-coating method;

electroplating

2,3,5-

trichlorophenol

100

0.05 M Na2SO4

5 mA; pH 2.3;

25 °C; 50 mL

(catholyte), 50 mL

(anolyte)

100

(100min)

192

PE-Pd/Ni

pulsed

electrodeposition

2,4-

dichlorophenoxyacet

ic acid

0.226

34.2 mM

NaCl

1.5 mA cm-2;

25 °C; 72 mL

(catholyte), 36 mL

(anolyte)

(3h)

193

1Rh-0.5Pd/Ni

electrodeposition

4-fluorophenol

0.1

0.1 M H2SO4

(anolyte); 20 mM

aqueous phosphate

buffer(catholyte)

H2 10 mL min-1;

30 °C; 40 mL

(catholyte), 40 mL

(anolyte); pH 3

100

(45min)

194

Pd–

Fe/graphene

photo-induced

reduction

4-bromophenol

100

0.03 M Na2SO4

150 mL; 16 cm2;

25 mA cm-2; pH 7

100

(60min)

195

Pd58Ni42(SDB

S)/PPy/Ti

electrodeposition

pentachlorophenol

0.05 M Na2SO4

5 mA; pH 2

100

(90min)

196

Pd-In/Al2O3

coprecipitation

trichloroacetic acid

0.5

2 mM

Na2SO4

50 mL (catholyte),

50 mL (anolyte);

0.9 mA cm-2; pH 7

(30min)

197

The nature of Pd cathode materials plays a critical role in the dehalogenation process, as well

as some key parameters, including fabrication methods, interlayers, and substrates. Among

these, the fabrication methods and interlayers are emphasized in this work, as well as the

coupling strategy. The fabrication method plays a vital role in determining the catalytic

performance of electrodes. He and co-workers193 fabricated Pd-based electrodes by pulsed

electrodeposition (PE-Pd/foam-Ni) and traditional chemical deposition (CD-Pd/foam-Ni).

Compared with the latter one, PE-Pd/foam-Ni possesses upgraded dechlorination efficiency

and current efficiency for the degradation of 2,4-dichlorophenoxyacetic acid (2, 4-D). The

reason was that PE-Pd/foam-Ni possessed a smaller particle size, a higher percentage of

exposed (111) facets together with a higher number of edge/ corner sites than that of CD-

Pd/foam-Ni. These characteristics favored the production and stabilization of chemisorbed

H*. Furthermore, to exploit the most intrinsic properties of Pd catalysts, a robust interlayer

with a high specific surface area is often required.189, 190 For instance, Li et al.186 found that

the conductive polymer polyaniline (PANI) in the Pd/PANI/Ni electrode existed in the form

of needle-shaped, and Pd0 clusters dispersed quite well in the network on PANI. The

Pd/PANI/Ni electrode owned a higher specific surface area (6.8 m2g-1) than Pd/Ni electrode,

which resulted in a better removal efficiency for 2,4-dichlorophenol than the latter one (89.1%

vs. 71.1%). Moreover, Luo and coworkers187 examined the role of MnO2 in a Pd/MnO2/Ni

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foam electrode, and they found that the introduction of MnO2 could drastically decrease the

consumption of Pd by four times when achieved complete removal of 2,4-dichlorobenzoic

acid (Fig. 8 a). Surprisingly, MnO2 could promote the water dissociation and provide more

atomic H* for dechlorination, which is the main reason for the electrochemical

dehalogenation improvement of Pd/MnO2/Ni foam electrode (Fig. 8 b). Coupling with other

metal(s) is another way to decrease the dosage of Pd and boost its electrocatalytic properties.

The synergistic effect of metals can upgrade the activity and selectivity of catalysts and

relieve catalyst aging and loss.197, 198 For example, Wu and co-workers198 developed a

palladium-nickel/multi-walled carbon nanotubes/graphite felt electrode (Pd-Ni/MWCNTs/GF)

for the electrocatalytic hydrodechlorination of 4-chlorophenol (4-CP). Experimental results

suggested that the interaction between Pd and Ni was beneficial for Pd reinforcement, and

thus improving the stability of electrodes. Compared to those cathodes with single-metal Pd,

Pd-Ni/ MWCNTs/GF demonstrated better stability and higher reactivity for the degradation

process, with complete removal of 4-CP being achieved in 30 min.

Fig.8 The removal efficiency of 2,4-DCBA by six different electrodes (a); schematic of 2,4-

DCBA dechlorination process by the Pd/MnO2/Ni foam electrode (b). Reproduced with

Experimental parameters (e.g., pH value, applied current, electrolyte) also impact the

performance of cathodes heavily.185, 198, 199 Take the cathode potential as an example, Jiang et

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al.199 found that the applied potential had a great influence on the evolution of three different

hydrogen species, namely adsorbed atomic hydrogen (H*ads), absorbed atomic hydrogen

(H*abs), and molecular hydrogen (H2). Combined with the degradation efficiency and kinetics

of 2,4-Dichlorophenol (Fig. 9), they confirmed that H*ads is the active species, H*abs is inert,

while H2 bubbles are detrimental to the electrocatalytic hydrodechlorination reaction.

Consequently, to achieve a desired degradation result, a moderate potential is needed to

produce sufficient H*ads and limit the negative effect of H2.

Fig.9 The correlation of EHDC efficiency (denoted as (C−C0)/C0 × 100%) and H*ads

evolution on C-Pd under different cathode potentials from -0.65 to -1.10 V (a); pseudo-first-

order representation of the EHDC reaction of 2,4-DCP versus time under different cathode

potentials (The inset is the relationship between ln kap and ln E.) (b); schematic illustration of

the potential-dependent 2,4-DCP EHDC mechanism (c). Reproduced with permission from

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3.3. Metal complex-based catalysts

In recent years, electrogenerated low-valent metal complexes, known as mediator catalysts,

have attracted immense attention to the catalytic reduction of HOPs. These metal complexes

are generated at a more positive potential than that needed for the direct breakage of C-X

bonds. Surprisingly, these electrogenerated catalysts possess great catalytic activities,

preparative accessibility, as well as structural flexibility.2, 200 To date, nickel together with

cobalt species are the most prominent catalysts for the reduction of HOPs, and Table 7

summarises the catalytic performance of the representative metal complexes.

Nickel(I) salen, usually electrogenerated at glass carbon cathodes in either acetonitrile or

dimethylformamide, has presented an excellent catalytic activity towards the reduction of

different HOPs, including 1,6‑dihalohexanes, 1-bromodecane, 1-iododecane,

α,ω‑dihaloalkanes, 6‑bromo‑1‑hexene, haloalkynes, cyclohexyl bromide, and cyclohexyl

iodide, etc.201, 202 For example, Wagoner and co-workers203 examined the catalytic reduction

of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) with nickel(I) salen in dimethylformamide.

The bulk electrolysis of less than 20.0 mM HBCD underwent in a six-electron process, which

resulted in the conversion of the initial pollutant to 1,5,9-cyclododecatriene (88-95%), with

small amounts of cyclododeca-1, 5-dien-9-yne (3-4%). In addition, it is worth noting that

nickel (I) salen-catalyzed reduction of alkyl halides often leads to alkylation of one or both

imino (C=N) bonds, which seriously impedes the dehalogenation process.2, 202 Under such

circumstance, structurally modified nickel(I) salen species have exhibited a better

performance, mainly due to they possess methyl protecting group on imino (C=N) bonds.201,

204

Cobalt complexes such as cobalt(I) alamin (vitamin B12), cobalt(I) salen, cobalt(I)

polypyridyl, and cobalt (I) phthalocyanine complexes have displayed great catalytic

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performance toward the reduction of organohalides.2, 30 Among these complexes, vitamin B12

that consists of a cobalt atom coordinated by four nitrogen atoms of the corrin ring. Its

derivatives has been widely investigated for its extensive application in synthesis,

bioremediation, biocatalysis, as well as electrocatalysis, especially in the electrocatalytic

reduction process of organic halides.205-207 As reported, vitamin B12 species are able to reduce

arenes, ethanes, ethylenes, higher alkanes, chlorinated methanes, etc.30, 206, 207 More in-depth

information related to the vitamin B12 dehalogenation performance can be referred to a recent

review.206 Similar to nickel(I) salen, cobalt(I) salen is another extensively discussed catalyst

in the scission of C-X bonds. In a typical study, 1,1,1-trichloro-2,2,2-trifluoroethane (CFC-

113a) was reduced by electrogenerated cobalt(I) salen at a glassy carbon cathode.

Surprisingly, CFC-113a (10.0 mM) has been successfully converted, with the main product

distribution of 2,2-dichloro-1,1,1-trifluoroethane (52%), 2-chloro-1,1,1-trifluoroethane and 2-

chloro-1,1-difluoroethene (7%), and 1,1-difluoroethene (1%). In addition, the salen ligand of

cobalt(I) salen was modified through the addition of a CF3CCl2- or CF3CHCl- moiety to the

C=N bond in the catalytic reduction process.208

Table 7 Summary of representative HOPs electroreductive dehalogenation using metal complex cathodes.

Metal complex

electrolyte

HOPs

Main Products

Ref.

1.0 mM nickel(I)

salen

DMF + 0.1

M TBABF4

10.0 mM 1,2,5,6,9,10-

hexabromocyclododecane

isomers of 1,5,9-cyclododecatriene (95%),

isomers of cyclododeca-1,5-dien-9-yne (3%)

203

2.0 mM nickel(I)

salen

DMF + 0.1 M

TBABF4

2.0 mM cyclohexyl bromide (1)

2.0 mM cyclohexyl iodide (2)

Cyclohexane (61%), cyclohexene (40%),

bicyclohexyl (5%) (1);

Cyclohexane (65%), cyclohexene (15%),

bicyclohexyl (7%) (2)

202

2.0 mM cobalt(I)

salen

DMF + 0.1 M

TBABF4

2.0 mM cyclohexyl iodide

Cyclohexane (63%), cyclohexene (22%),

bicyclohexyl (12%)

202

2.0 mM Cobalt(I)

Salen

DMF + 0.1 M

TBABF4

0.202 mM 1,1,1-Trichloro-

2,2,2-trifluoroethane

2,2-dichloro-1,1,1-trifluoroethane (52%), 2-

chloro-1,1,1-trifluoroethane + 2-chloro-1,1-

difluoroethene (7%), 1,1-difluoroethene (1%)

208

1.0 mM

structurally

modified nickel(I)

salen

DMF + 0.1 M

TBABF4

10.0 mM 1‑bromodecane (1)

10.0 mM 1‑iododecane (2)

n-eicosane (70%), n-decane (8%), 1‑decene

(4%), N,N‑dimethylundecanamide (4%) (1);

n-eicosane (63%), n‑decane (12%), 1‑decene

(3%), N,N‑dimethylundecanamide (5%) (2)

201

2.0 mM Ni(II)

Salen

DMF + 0.05 M

TBABF4

20 mM benzyl bromide (1)

20 mM 1-

bromomethylnaphthalene (2)

toluene (48%), toluene (19%), benzyl ether

(18%) (1);

1-methylnaphthalene (52%), 1,2-bis(1-

naphthyl)ethane (30%), bis(1-

naphthylmethyl) ether (2)

209

2.0 mM Ni(II)

Salen

DMF + 0.05

M TBABF4

5.0 mM 4,4’-(2,2-

dichloroethene-1,1-

diyl)bis(chlorobenzene)

1,1-Diphenylethene (14%), 1-Chloro-4-(1-

phenylvinyl)benzene (51%)

210

1.0 mM nickel(I)

salen

DMF + 0.1 M

TBABF4

10.0 mM 1-Iodooctane (1);

10.0 mM 1-Bromooctane (2);

5.0 mM 1-Bromo-5-decyne

Octane (4%), 1-octene (9%), hexadecane

(87%) (1);

Octane (5%), 1-octene (4%), hexadecane

211

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(3);

5.0 mM 1-Iodo-5-decyne (4);

(89%) (2);

Dec-5-yne (11%), dec-1-en-5-yne (3%),

pentylidenecyclopentane (74%) (3);

Dec-5-yne (8%), dec-1-en-5-yne (3%),

pentylidenecyclopentane (88%) (4)

3.4. Electroreductive dehalogenation mechanisms

Mechanisms of the electroreductive scission of C-X bonds have been broadly investigated,

and still are, discussed in many authoritative studies,1, 2, 168 and are also outlined in this work

briefly.

Firstly, the heterogeneous and homogeneous electron transfer (ET) plays an important role in

the reduction process.168 The extensively researched dissociative electron transfer (DET)

theory can account for vast experimental results when Ag-based cathodes are applied, and a

scheme mechanism of DET is provided in Fig. 10 a.212 The scheme a is a stepwise

mechanism, involving the formation of an intermediate radical anion (RX•-) before the

cleavage of the C-X bond, while scheme b that named concerted mechanism represents the

simultaneous electron transfer and the C-X bond breaking. Literature data suggest that

degradation of aromatic halides mainly follows the stepwise mechanism, whereas aliphatic

halides prefer the concerted one,1, 170 but there are several exceptions to this rule.212 Hence, it

is necessary to carry out careful exploration of the reaction system before assigning one or

another mechanism to a typical degradation process. Additionally, the scheme c that is a

variant of the concerted mechanism referred as sticky DET, and two fragments (e.g., R• and

X-) can induce ion-dipole interactions in the solvent cage before they diffuse apart in this

procedure. Generally, many subsequent reactions will occur and produce dimers and

hydrodehalogenated species (Eqs. (19)- (23)), and SH is the solvent.2, 168

R• + e-  R- (19)

R• + SH  RH + S• (20)

R- + SH  RH + S- (21)

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R- + RX  R-R + X- (22)

R• + R•  R-R (23)

Secondly, when electrogenerated low-valent metal complexes are employed as catalysts, the

ET process will occur in homogeneous conditions.168 In a representative work, Chan and co-

workers 213 discussed the use of cobalt porphyrin to catalyze the hydrodehalogenation of aryl

bromides, and the plausible mechanism is illustrated in Fig. 10 b. Initially, CoII(ttp) abstracts

a bromine atom from ArBr to generate CoIII(ttp)Br and Ar•. Then, the Ar• abstracts an H

from solvent to form the ArH, while the CoIII(ttp)Br undergoes ligand substitution with KOH

and generates CoIII(ttp)OH and KBr. Afterwards, CoIII(ttp)OH produces H2O2 and regenerates

CoII(ttp) by reductive elimination. Subsequently, the H2O2 catalyzed by CoII(ttp) or base to

H2O and O2.

Fig.10 The general mechanism of dissociative electron transfer (a). Reproduced with

hydrodehalogenation mechanism of aryl bromides (b). Reproduced with permission.

Finally, the indirect dehalogenation process with the H*ads is another efficient way to degrade

HOPs,168 and this mechanism often associates with the application of Pd-based catalysts. The

reductive dehalogenation process is known as electrocatalytic hydrogenolysis (ECH),214, 215

which can be described as chemisorbed hydrogen atoms first generated on the cathode

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surface by electrolysis of water and then react with adsorbed HOPs. The key steps are listed

in Eqs. (24)- (29).193, 214, 216

2H2O (2H3O+) +2e- + 2M  2(H)adsM + 2OH- (2H2O) (Volmer step) (24)

R-X + M  (R-X)adsM (25)

(R-X)adsM + 2(H)adsM  (R-H)adsM + 2M+ HX (26)

(R-H)adsM  R-H + M (27)

(H)adsM + H2O + e-  H2 + OH- + M (Heyrovsky step) (28)

(H)adsM + (H)adsM  H2 + 2M (Tafel step) (29)

where M represents the metallic surface, and HX denotes HOPs. Actually, the ECH process

is just a combination of Eqs. (21)- (24), whereas the hydrogen evolution reaction (HER)

reaction (Eqs. (25)- (26)) occurs as the competition process. Hence, it is of great importance

to depress the HER reaction in the degradation process.

The HER process starts with the formation of H*ads and then the electrochemical desorption

(Volmer-Heyrovsky), or the recombination of the atomic H*ads (Volmer-Tafel).217-219 Both

HER and ECH process require abundant surface adsorbed hydrogen through the Volmer step,

so the key to suppress the HER process is to block the Heyrovsky step or the Tafel step.

Currently, two strategies are often applied to optimize the degradation process through

effectively vanquish the HER process. The first strategy is using cathode materials with a

favorable ability for the generation and retainment of H*ads. In this regard, Pd distinguishes

itself not only for its high efficiency in taking up protons to generate H* but also due to its

property to retain the H* by adsorption onto the Pd surface (H*ads) and absorption to the Pd

atoms via the formation of Pd hydride (H*abs).220 Therefore, Pd can maintain a high surface

hydrogen concentration for the ECH process. In most of the cases, it is difficult for other

materials (e.g., Pt, Rh) to retain the generated H*ads, since the H*ads are easily transform to

molecular hydrogen through the Heyrovsky step or the Tafel step. Recently, several effective

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methods have been applied to further enhance the selectivity of Pd.190, 220 For example, Liu

and co-workers220 found that the defective sits in Pd nanowires could enhance the stability of

the generated H*ads and suppress the HER process. Due to the magnificent H*ads

generation/provision capacity, the defect-rich Pd nanowires exhibited a higher 2,4-

dichlorophenol removal eﬃciency with a six-times-lower Pd loading than the unmodified Pd

materials. Another strategy to extinguish the HER process is to optimize the operating

parameters, such as the cathode potential,185, 199, 221, 222 pH value,223 and dissolved oxygen217.

Among these factors, the cathode potential is most frequently studied. Commonly, the

kinetics of H*ads production are modulated by the cathode potential, and a more negative

potential will usually hasten H*ads generation, which could offer sufficient H*ads for enhanced

ECH.221 However, as a side reaction, H*ads may evolve into H2 through the Heyrovsky step or

the Tafel step at a more reductive potential. Such side reaction can compete with ECH in

H*ads utilization, and the generated hydrogen bubbles also interfere with the mass transfer of

H+ and HOPs to the cathode surface, resulting in a decay in ECH efficiency.199 As a result, an

optimal cathode potential is critical for the restraint of HER.

However, it should be noted that there is still lacking of fundamental principles for

interpreting the dehalogenation mechanism of a typical HOP without experimental

examination due to that fact that there are many unexpected observations related to the

mechanisms mentioned above.187, 195, 197, 212, 215 For instance, Liu et al.197 found that atomic

H*ads formation function and the electron transfer process both existed in the trichloroacetic

acid (TCAA) removal process, while the enhanced indirect atomic H*ads reduction process

played a principal role in the degradation process (Fig. 11).

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Fig.11 Schematic diagram of reaction mechanism for TCAA reduction over Pd-In/Al2O3.

4. Integrated Technologies

Recently, the number of strategies has grown rapidly with the development of coupling

synergistic processes based on the combination of electrocatalysis.224 The coupling can

upgrade the overall degradation performance toward HOPs due to the synergy between

different sophisticated processes.224, 225 Fenton reaction,87, 226 membrane filtration,227, 228

biological treatment,225, 229 photocatalysis,230, 231 ozonation,225, 232 adsorption,225, 233

ultrasound,230, 234 UV irradiation,230, 235 microwave,236 and thermolysis237, 238 are the widely

reported methods that can assist the electrochemical activity of electrocatalysis. In this part of

the review, the first four methods are detailed due to their wide applications. A list of

representative HOPs dehalogenation with synergistic methods is shown in Table 8.

Table 8 Summary of representative HOPs dehalogenation using synergistic methods.

Method

Experimental conditions

HOPs

Degradation rate (%)

Ref.

Reactive electrochemical filter

system

50 mM Na2SO4; inlet flow rate 1.8

L min-1; 25 ± 1 °C; pH 7; 0.25A;

3.0V

50 mg L−1 rhodamine

97.6(30min)

227

Reactive electrochemical

membranes

100 mM KH2PO4; pH 4.5; flow rate

0.5 mL min−1; 3.0V

10 μM atrazine

>99.9 (100min)

239

Bioelectrode-anaerobic sludge

-660 mV; 400mL; 35 °C

50 mg L−1 2,4-

100(12h)

229

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system

dichloronitrobenzene

Nanofiltration- electrooxidation

14.9V; 20 °C; 50 A m-2

870 mg L−1 PFHxA

~100 (1.47h)

240

O3 -electrochemical coupled process

60 mA cm-2; pH 3; 0.750L; ozone

concentration 5 ± 0.5 mg L-1

20 mg L−1 endosulfan

COD 95 (40min);

TOC 94 (40min)

232

Electro-Fenton/BDD

80 mA cm-2; flow rate 4 L min-1; 3L;

Fe2+ 0.5mM; 25 °C

300 mg L−1 reactive

yellow HF

90(180min)

241

Photo-assisted electrochemical

oxidation

2 g L-1 Na2SO4; pH 4.5 ± 0.2; flow

rate of 1 L min-1; 250 W HPL-N

lamp

0.2 mg L−1

fluoroquinolone

76 (360min)

231

Hydrothermal electrocatalytic

oxidation

50 mM Na2SO4; 0.2M H2O2; 0.55L;

5 mA cm-2; 135 °C

100 mg L−1 2,4-

dichlorophenoxyacetic

acid

93.7(90min);

TOC 99.8(180min)

237

Bioelectrocatalysis system

Volume of single chamber 130mL;

55± 2 °C; mixed anaerobic seed

sludge concentration ~3000 mg L-1;

hydraulic retention time 2 d;

0.4 mM p-

fluoronitrobenzene

0.188 mM d-1

242

Sonoelectrochemical system

0.1 M Na2SO4; frequency 380 kHz;

25±2 °C; 20 mA

0.025mM methylene

blue

100 (1h)

234

nZVI and electrochemical reduction

system

cathode 80 mL, anode 40mL; 2 g

L−1 NaCl; 20 ±0.5 °C; 0.167 mA

cm-2

50 mg L−1 2,4-

dichlorophenoxyacetic

Acid

87.6(4h)

243

The electro-Fenton (EF) process, namely the combination of electrochemical oxidation (EO)

with the Fenton process, which usually electrogenerates Fenton’s reagents (i.e., ferrous ion

and hydrogen peroxide) in situ.244,245 Compared with the traditional Fenton process, EF

consumes fewer reagents and thus produces less iron sludge. Additionally, the EF process

often exhibits higher activity than EO for the mineralization of HOPs. For example, Guzman

and co-workers 241 employed both EO and EF to treat the reactive yellow HF dye, and they

found that the •OH generated by the EF process removed more rapidly organic pollutants

than EO. An interesting chapter by Oturan, N., and Oturan, M. A. keeps a close eye on

important parameters of EF process, including principles, operational parameters, application,

heterogeneous EF, and bioelectro-EF, which may benefit researchers in this field.245

Photocatalysis is another effective approach to eliminate pollutants from wastewater.246 In the

last few years, photocatalysis has shown to enhance the degradation efficiency of

electrocatalysis.231, 247-249 Generally, the following points can explain the synergistic effect.

On the one hand, the external bias lead to a decline in the electron-hole recombination

process and the UV-photons reaching the electrode surface form excited radicals. On the

other hand, the photo irradiation can activate the electrochemically generated reactive

radicals and overcome the mass-transport limitation.249 To mineralize the norfloxacin in

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wastewater, Salatiel and co-authors231 found that the ultraviolet radiation could significantly

improve the degradation performance of electrochemical oxidation. The coupling strategy

possessed a higher TOC abatement (76%) than the direct photolysis (60%) and the

electrochemical oxidation process (26%) in 360 min. In-depth analysis suggested that the

increase in mineralization could be explained by a decrease in the electron-hole

recombination on the TiO2 phase of the electrode, and thus improving the generation of

hydroxyl radicals.

In the traditional flow-by EO process, the diffusion of HOPs to the reaction zone is quite

difficult, which results in a low mass transfer rate constant.227 To overcome this mass transfer

limitation, combining membrane filtration with the EO process in flow-through mode is an

efficient method for HOPs mineralization. In this process, water moves through the pores on

the designed electrode materials, which can minimize the thickness of the boundary layer,

thus significantly upgrading the mass transfer rate of HOPs moving towards the electrode

surface.226, 227, 239 In a recent work, tubular porous Ti/SnO2-Sb filters which possessed

excellent electrochemical activity and penetration flux were worked as anodic reactive

electrochemical membranes in a reactive electrochemical filter system.227 The developed

reactive electrochemical filter system in flow-through mode resulted in a higher RhB

oxidation efficiency than the flow-by mode by 8.6-fold under the same operational conditions.

Bioelectrochemical systems (BESs) which based on the integration of the biological process

and the electrochemical method have attracted increasing attention in the HOPs degradation

process because the BES process possesses high degradation efficiency, low operational cost,

environmental sustainability, high selectivity, ambient operating conditions and reusable

effluents.242, 250 In a typical study, a coupled microbial electrosynthesis-upflow anaerobic

sludge reactor (MES-UASB) was established to mineralize 2,4-dichloronitrobenzene,229 and

the experimental results indicated that the coupled MES-UASB obtained a high

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dechlorination efficiency (100%) at -660 mV in 12h. More information involves this topic

can be found in pervious reviews and chapters.250-253

5. Conclusions and prospects

In this work, the recent development of electrocatalysts for the degradation of HOPs in terms

of the strategies for promoting the catalysts activities is comprehensively reviewed. It can be

concluded that both electrooxidation and electroreduction are effective strategies to

mineralize HOPs, with the application of adequate catalysts.

In the electrooxidative process, metal oxides (e.g., PbO2 and SnO2) and BDD are extensively

applied as anodes due to their high performance in the production of •OH that is the main

reactive radical in the oxidation degradation pathway. Additionally, nanostructuring,

compositing, doping, and surface modification are extensively used stategies to upgrade the

ability of these catalysts. Notably, the oxidation mechanisms that involve •OH rely heavily on

the properties of catalysts. For the active anodes (e.g., Pt, RuO2, IrO2), the degradation

process often leads to electrochemical conversion, while the inactive anodes (e.g., BDD,

doped-SnO2, PbO2) commonly result in electrochemical combustion.

In the electroreductive process, metals (e.g., Ag and Pd) and low-valent metal complexes are

responsible for the reduction of HOPs through the generated •H and electrons. To obtain a

better degradation performance, many sound methods such as nanosizing, structure designing,

alloying, and component modification are extensively introduced. The reduction principles

are closely related to the type of catalysts. The dissociative electron transfer (DET) theory

can account for vast experimental phenomena when Ag-based cathodes are used. The

homogeneous ET process will occur when electrogenerated low-valent metal complexes are

employed as catalysts. In addition, the application of Pd-based catalysts is an indirect

dehalogenation process, with the H*ads acting as the main active radical.

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Identifying the structure-performance relationship of materials is crucial to the design of

desirable electrocatalysts. For the electrochemical dehalogenation process, both activity and

stability of catalysts need to be considered carefully. As aforementioned, nanostructuring,

compositing, component regulation (doping, alloying), inserting interlayer, and surface

modification have shown great potential in designing robust and highly active electrocatalysts.

Nanostructuring catalysts with well- dispersed active components that exhibit a high active

surface area can provide more sites for the degradation reaction. Compositing also leads to a

high specific surface area, specially, when a conductive substrate is involved, the

conductivity and mass transfer process can be significantly elevated, and thus accelerating the

degradation process. Component regulation, such as doping and alloying, usually results in

more abundant active components on catalyst surfaces, and thereby benefits the generation of

more active radicals for the decomposition reaction. In addition, the optimal components

possess a decent interaction strength with radicals (e.g., H*, •OH), and diminish the side

reactions (e.g., oxygen and hydrogen evolution reactions). Interlayer and surface modification

have a significant impact on the lifetime of electrocatalysts by hindering the penetration of

electrolytes, decreasing the inner mechanical stress, and preventing the catalyst damages (e.g.,

detachment, consumption, dissolution). Other strategies that mentioned above also benefit the

durability of catalysts.

However, more innovative catalysts and technological methods are still highly desired for the

further development of electrocatalytic dehalogenation process due to some critical

limitations/challenges related to the practical application of the current electrocatalytic

dehalogenation treatment.1, 28, 254 Firstly, the electricity is required continuously to support the

electrochemical degradation process, and the electrical efficiency should be checked when

evaluating the applicability of the electrocatalysis techniques. To decrease the energy cost,

integrating electrocatalysis with other renewable energy techniques (e.g., solar photovoltaic

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power) and optimizing the operational parameters are highly advisable. Another interesting

factor is the wastewater conductivity, which can be enhanced through the addition of

seawater and inorganic salts (e.g., Na2SO4, phosphate buffer, Na2CO3). Secondly, the

released toxic ions (e.g., Pb2+) of electrodes may decrease the durability of electrodes and

pose toxicity toward organisms, while the fouling and corrosion of electrodes (e.g., BDD)

may also lead to an unstable degradation efficiency. These factors could hinder the industrial

applications of the electrocatalytic degradation method. As a result, the robust and highly

active electrodes are extremely needed. Thirdly, the formation of toxic by-products in the

degradation of HOPs, especially halogen-based compounds (e.g., BrO3-, ClO3-, ClO4-), can

limit the adoption of the electrochemical mineralization techniques. To control the generation

of these products, the optimal operational strategies and the efficient electrode modifications

are necessary. Currently, the electrocatalysis has been extensively studied in the

mineralization of pollutants beyond HOPs, while the degradation of real wastewater is still

limited. Consequently, it is of great significance to explore the potential application of

electrochemical method in degrading complex pollutants in industrial wastewater. As

indicated, the controllable electroreduction of organic pollutants can lead to the

transformation of pollutants into value-added chemicals, for example the generation of n-

hexane from the degradation of 1, 2-dibromohexane. Accordingly, the electrochemical

method is a promising approach to generate valuable chemicals with specific functional

groups. In summary, a combination of hybrid technologies, ideal operating conditions, and

electrode modifications needs to be explored to address these crucial challenges.

Herein, several prospects are listed below for the purpose of upgrading existing

electrochemical degradation techniques.

Developing high-performance catalysts is of great importance and many strategies including

doping, nanostructuring, facet engineering, hybrid engineering, defect engineering, and

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alloying are efficient ways to improve the activity of catalysts. Furthermore, the structure-

performance relationship should be taken seriously with the help of computational methods,

and high-through screening is also vital to select typical catalysts for a determined pollutant.

Another factor should be taken into consideration is the cost. More efforts should be devoted

to using those low-cost materials (e.g., non-precious metals, graphite-based materials) and

simple fabrication methods to obtain cost-effective electrodes.

To maximize the performance of electrocatalysts, it is necessary to optimize the experimental

parameters in the degradation process. Normally, a reactor/system with a high electrode area

to volume ratio, a low cell voltage, and a high mass transport rate is desired.2 As discussed

aforementioned, the role of current, voltage, pH, electrolyte, and concentration of HOPs is

also vital to the activity of catalysts, as well as the degradation pathway. Hence, systematic

investigations should be conducted to achieve better mineralization efficiency.

Advancing the scientific understanding of dehalogenation/electrochemical process is still a

key challenge, and the mechanism should be investigated case by case with the experimental

and theoretical methods, especially those operando characterization techniques. The

applications of advanced in situ characterization techniques have benefited a lot for exploring

the active sites on electrocatalysts for various reactions.255 For instance, the advanced

scanning electrochemical microscopy technique can offer spatially resolved details of

electrode reactivity at the micron to submicron scale, which is highly beneficial for

characterizing the performance of catalysts and identifying mechanisms involves HOPs

transformation.108 This process could also leads to the design of more favorable catalysts.

Additionally, the theoretical study based on density functional theory can provide critical

information to understand the interaction between catalysts/radicals and pollutants, thus

leading to a better understanding of the degradation/mineralization process.220, 256, 257

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Taking advantages of other useful processes for HOPs degradation, coupling electrocatalysis

with those sophisticated methods will significantly upgrade the degradation performance. On

one side, to achieve better degradation efficiencies, integrating those verified effective

techniques such as membrane techniques, biological methods, Fenton processes, and

photocatalysis with electrocatalysis is a promising solution. On the other hand, to decrease

the operational cost, mainly the electricity utilization, it is advisable to investigate the

combination the electrochemical degradation process with renewable energy (e.g., solar

photovoltaic power, wave power, wind energy, geothermal energy) in future research.

Acknowledgments

This work is supported by an Australian Research Council (ARC) Future Fellowship

(FT160100195). Mr Zhijie Chen acknowledges the China Scholarship Council (CSC) for the

scholarship support.

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Table of Contents Entry

Advanced Electrocatalysts for Halogenated Organic Pollutants Degradation

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Article

Feb 2024

Electrocatalytic dehalogenation technology is a promising approach for the synthesis of chemicals (such as pesticides and pharmaceutical intermediates) and the disposal of halogenated organic pollutants. Compared with the traditional chemical reduction technology, the electrocatalytic reduction dehalogenation method has the advantages of high efficiency, controllable operation, and reduced secondary pollution. This review systematically consolidates the recent advances in the electrocatalytic dehalogenation in the application of organic synthesis and environmental degradation, focusing on the involved mechanisms, and influences factors (e.g., electrocatalysts, solution environment, and reaction conditions) on the performance of electrocatalytic dehalogenation. Furthermore, the latest characterization and analytical methods and the industrial application of electrocatalytic dehalogenation technology are summarized. Lastly, the existing challenges and perspectives are proposed for efficient electrocatalytic dehalogenation.

Recent Advances in Transition Metal-Based Electrocatalysts for Alkaline Hydrogen Evolution

Article

Full-text available

May 2019

With the increasing demands in energy consumption and raising environmental concerns, it is of vital significance for developing renewable and clean energy sources to substitute traditional fossil fuels. As an outstanding candidate, hydrogen is recognized as a green energy carrier for its high gravimetric energy density, zero carbon footprints, and earth-abundance. Currently, water splitting in alkaline electrolytes represents one of the most promising methods for sustainable hydrogen production, and the key challenge lies in the development of high-performance electrocatalysts for hydrogen evolution reaction (HER). Given the rapid advances in the design and development of efficient catalysts towards alkaline HER, especially the capable transition metal (TM)-based materials, this review aims to summarise recent progress in theoretical understanding of alkaline HER and the TM-based electrocatalysts. TM-based catalysts classified by their different anionic compositions (metals, alloys, oxides, hydroxides, sulfides, selenides, tellurides, nitrides, phosphides, carbides, and borides) are comprehensively showcased. Special attention is given to the mainstream strategies that can upgrade the catalytic properties of each category, as well as the underlying structure-activity regimes. Additionally, the challenges for the future development of novel catalysts are also analyzed.

Boron Doped Diamond: Current Progress and Challenges in view of Electroanalytical Applications

Article

Full-text available

Dec 2018

This review examines the current status (from 2016 to December 2018) of the electroanalytical application of boron-doped diamond (BDD), in view of its advantages and challenges for electroanalytical applications. The effect of the boron-doping level, sp3/sp2 ratio, BDD surface termination on electrochemical properties of BDD electrode are discussed. The most interesting and significant developments concerning the electrochemical oxidation and reduction of chemical substances onto BDD in a single and multi-analyte modes, beneficial role of surfactants in BDD electrode-based electroanalysis, surface modifications, modification electrode surfaces with metal nanoparticles, porous BDD surfaces, and the specificity of BDDE in the microfluidic context are considered. Basic information on the electroanalysis of organic substances in single- and multi-analyte detection mode is given in the tables. Others non common electroanalytical applications of BDD, such as BDD as an electrode material for electrogenerated electrochemiluminescence; BDD for in vivo on-line determination of analytes; detecting the bio-potential changes of plants; pH and gas sensing are presented.

Comparative Degradation of Atrazine by Anodic Oxidation at Graphite and Platinum Electrodes and Insights into Electrochemical Behavior of Graphite Anode

Article

Full-text available

Jan 2019

Anodic oxidation capacities of graphite and platinum (Pt) electrodes were investigated for the degradation of the herbicide atrazine in synthetic aqueous medium. The electro-oxidation of atrazine under various supporting electrolytes and applied currents was conducted in an undivided reactor with different electrode materials. The electrochemical behavior between anode material and supporting electrolyte was investigated by means of cyclic voltammetry. Graphite anode exhibited good oxidation capacity for degradation of 10 mg L−1 atrazine in near-neutral pH condition, achieving over 90% removal efficiency in all cases within 60 min. Atrazine removal efficiency of 100% was obtained with graphite anode in 60 min of reaction time at applied current 60 mA, NaCl concentration 0.05 mol L−1, and initial pH 6.8; nevertheless, less than 60% of removal was achieved with Pt anode under the same conditions. Five times consecutive runs demonstrated that the atrazine removal efficiency was obviously increased with the positive polarized graphite anode. Positive polarization of graphite anode during anodic oxidation process functionalizes the surface, generating oxygen-containing functional groups; these functional groups played a catalytic role during the oxidation of substrates. An oxidation pathway was proposed in which atrazine was virtually destroyed by chemisorbed reactive oxygen at the graphite anode. Anodic oxidation capacities of graphite and platinum (Pt) anodes were investigated for the degradation of atrazine. Experimental results show that graphite anode was more powerful than Pt to destroy atrazine in all cases. Oxygen-containing functional groups generated from positive polarization of graphite played catalytic roles during the oxidation process.

A Co–Fe Prussian blue analogue for efficient Fenton-like catalysis: the effect of high-spin cobalt

Article

May 2019

we first report the high spin Co-Fe Prussian blue analogue (Co-Fe PBA) as highly efficient Fenton-like catalyst for sulfate radical (SO4•-) production. Our experiments and density functional theory (DFT) calculations...

Recent developments and advances in boron-doped diamond electrodes for electrochemical oxidation of organic pollutants

Article

Nov 2018
SEP PURIF TECHNOL

Boron-doped diamond (BDD) electrode has been considered as an optimal electrode material for electrochemical oxidation of organic contaminants in the aquatic environment due to its good physical and chemical properties. The fundamental research and practical application of BDD electrode in the mineralization of organic pollutants have been well developed up to now. In this review, the preparation and electrochemical properties of BDD film electrode are focused first. Then, we investigate the electrocatalytic process and degradation mechanisms on BDD electrodes based on the electrochemical oxidation of refractory pollutants in recent years. The enhancement approaches towards electrochemical oxidation kinetics are further highlighted, which include the establishment of high activity BDD electrodes and enhancement of mass transfer process during the electrochemical oxidation. Other combination technologies based on electrochemical oxidation with BDD electrode are also described to expand the application of BDD electrode in wastewater treatment. Further, the opportunity and prospective of BDD electrode are forecasted according to the existing problems in wastewater treatment.

Electrochemical oxidation of acetamiprid using Yb-doped PbO2 electrodes: Electrode characterization, influencing factors and degradation pathways

Article

Oct 2018
SEP PURIF TECHNOL

The Yb-doped PbO2 electrodes were fabricated by electrodeposition method. SEM and XRD experiments revealed that the adulteration of Yb can decrease the crystal grain size and make the electrodes more compact. Electrochemical measurements, accelerated service lifetime and bulk electrolysis experiments proved that Yb-doped PbO2 electrodes had higher oxygen evolution overpotential, stability and acetamiprid removal ratio than pure PbO2 electrodes. The effect of operating parameters, such as current density, initial acetamiprid concentration, and pH value, on the electrocatalytic degradation of acetamiprid were systematically studied. The electrocatalytic oxidation reaction of acetamiprid on Yb-doped PbO2 electrodes was highly effective, which obeyed the pseudo-first-order kinetics model. Intermediates analysis and electrocatalytic degradation mechanism of acetamiprid by Yb-doped PbO2 electrodes were implemented, and the degradation pathways were divided into three different sub-routes due to three function groups on acetamiprid initially attacked by hydroxyl radicals. Finally, all the byproducts were decontaminated into CO2 and H2O. These results proved that Yb-doped PbO2 electrodes have more promising application potential for the electrocatalytic degradation of refractory organic pollutants.

Degradation of p-chloroaniline using an electrochemical ceramic microfiltration membrane with built-in electrodes

Article

Oct 2018
ELECTROCHIM ACTA

An electrochemical ceramic microfiltration membrane with built-in cathode (Ti mesh) and Ti/RuO2 anode, which had dual functions of separation and electrochemical oxidation, was developed for p-chloroaniline (PCA) removal from contaminated waters. Results showed that the degradation of PCA followed the pseudo-first-order kinetics in all conditions. PCA degradation efficiency increased with the increase of applied voltages in the range of 0–3.0 V. The optimum solution pH for PCA decay was 7.0. An initial PCA concentration higher than 30 μM had no significant influence (p > 0.05) on the degradation efficiency of PCA. At an applied voltage of 2.0 V and an electrolysis time/hydraulic retention time of 2 h, the removal efficiency of PCA under flow-through mode was found to be 3.6 times that of flow-by mode, due to the better contact and reaction of contaminants with the oxidants generated in the vicinity of membrane surface. It also showed that [rad]OH arisen from anodic water oxidation reaction played a key role in PCA degradation. Benzoquinone, aniline, p-aminophenol, hydroquinone, malonic acid, succinic acid, oxamic acid, maleic acid, oxalic acid, α-ketoglutaric acid and formic acid were identified as the main PCA decay products, leading to a lower biological toxicity of the effluent. The system also demonstrated a favorable performance for contaminant elimination after a long-term operation. These results highlight the potential of this electrochemical microfiltration membrane system for efficient PCA degradation.

Electrochemically reductive dechlorination of 3,6-dichloropicolinic acid on a palladium/nitrogen-doped carbon/nickel foam electrode

Article

Oct 2018
ELECTROCHIM ACTA

Pd was electro-deposited on Ni foam using nitrogen-doped carbon as an inter-layer (EDPd/CNx/Ni). The as-prepared material was employed as the cathode for the electrochemical dechlorination of 3,6-dichloropicolinic acid (3,6-D) in an aqueous solution. When compared to the electrodes that direct the electrodeposition or chemical deposition of Pd on Ni foam, EDPd/CNx/Ni exhibited higher activity and selectivity during the electrocatalytic hydrogenation of 3,6-D to form the intermediates 3-chloropicolinic acid and 6-chloropicolinic acid, as well as the final product, 2-picolinic acid. Furthermore, the modified electrode maintained its high dechlorinating activity after continuous operation for three batches. On the basis of physical-chemical characterization and electrochemical measurements, it is proposed that the superior performance of the EDPd/CNx/Ni electrode benefits sufficiently from the accelerated generation of adsorbed hydrogen ([H]ads). The electrodeposition method facilitates the generation of Pd with a higher percentage of exposed {111} facets. The CNx functions as an efficient support for the enlargement of the electrochemically active surface area, the reduction of Pd particle size, and improvement of the Pd dispersion. Moreover, CNx may also act as an electrocatalyst for the water-splitting reaction and favors the stabilization of chemisorbed active hydrogen.

Application of Ionizing Radiation in Decomposition of Perfluorooctanoate (PFOA) in Waters

Article

Sep 2018
Chem Eng J

It has been demonstrated through the use of both gamma-rays and beam of accelerated electrons (EB) for the radiolytic decomposition of PFOA in diluted aqueous solutions that the most reactive species among the products of water radiolysis are hydrated electrons eaq. The reaction of PFOA with the hydrated electrons takes place with the largest yield in strongly alkaline deaerated solutions. The PFOA decomposition performed at an initial concentration level of 1 mg/L by γ-irradiation requires the use of absorbed doses below 10 kGy, which, depending on the activity of employed ⁶⁰Co source, requires 2–3 h irradiation time. Similar results were obtained at pH 12 in the presence and also in the absence of t-butanol in the treated solutions. In the case of EB irradiation in a wide range of the absorbed doses (up to 100 kGy), a very pronounced effect of the dose-rate was observed, which can be attributed to various recombination reactions of the products of water radiolysis. A significant improvement in the yield of the PFOA decomposition by EB irradiation can be observed in the presence of trace amounts of nitrate or 100 mM of a formate in the treated solutions. In optimized conditions, about 70–80% decomposition of the initial level of 1 ppm of PFOA can be obtained with an irradiation using 100 kGy absorbed dose, which corresponds to about 5 s treatment with the use of an employed electron accelerator. The acute toxicity measurements and a kinetic modeling are presented, together with a suggestion of the mechanism of radiolytic decomposition of PFOA.

Electrochemical Oxidation of Atrazine and Clothianidin on Bi-doped SnO2-TinO2n-1 Electrocatalytic Reactive Electrochemical Membranes

Article

Sep 2018

This research focused on improving mineralization rates during the advanced electrochemical oxidation treatment of agricultural water contaminants. For the first time, bismuth-doped tin oxide (BDTO) catalysts were deposited on Magnéli phase (TinO2n-1, n = 4-6 ) reactive electrochemical membranes (REMs). Terephthalic acid (TA) was used as OH● probe, whereas atrazine (ATZ) and clothianidin (CDN) were chosen as model agricultural water pollutants. The BDTO-deposited REMs (REM/BDTO) showed higher compound removal than the REM, due to enhanced OH● production. At 3.5 V/SHE complete mineralization of TA, ATZ, and CDN was achieved for the REM/BDTO upon a single pass in the reactor (residence time ~ 3.6 s). The energy consumption for REM/BDTO was as much as 31-fold lower than the REM, with minimal energy consumption per log removal of < 0.53 kWh m-3 for TA (3.5 V/SHE), < 0.42 kWh m-3-3 for CDN (3.0 V/SHE) with REM/BDTO/EDT. Density functional theory simulations provided potential dependent activation energy profiles for ATZ, CDN, and various oxidation products. Efficient mass transfer and a reaction mechanism involving direct electron transfer and reaction with OH● were responsible for the rapid and complete mineralization of ATZ and CDN at very short residence times.