Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review

Abstract

Industrial chlor-alkali electrolysis represents one of the most energy- and resource-intensive
technological applications of electrocatalysis. Improving process efficiency becomes a critical issue
for the sustainable development and for alleviating the energy and environmental crisis. Rational
design in the morphology of RuO2-based anodic electrocatalytic coatings and the control in the
coating microstructure can contribute to massive energy saving compared to the current commercial
Ru0.3Ti0.7O2 coating. This review covers recent developments in the anodic coatings. Performance
enhancement for RuO2-based anodic coatings is achieved by using alternative preparation routes
of sol-gel and electrodeposition. The target control in the coating surface morphologies and the
increase in the utilization of active Ru species are demonstrated.

Full text

Z. Phys. Chem. 227 (2013) 651–666 / DOI 10.1524/zpch.2013.0338
© by Oldenbourg Wissenschaftsverlag, München
Anodic Electrocatalytic Coatings for Electrolytic
Chlorine Production: A Review
By Ruiyong Chen
1
,
∗
,
#
,VinhTrieu
1
,
2
, Bernd Schley
1
, Harald Natter
1
, Jürgen Kintrup
2
,
Andreas Bulan
2
, Rainer Weber
2
, and Rolf Hempelmann
1
1
Physical Chemistry, Saarland University, 66123 Saarbrücken, Germany
2
Bayer MaterialScience AG, 51368 Leverkusen, Germany
(Received August 6, 2012; accepted in revised form October 11, 2012)
(Published online December 3, 2012)
Chlor-Alkali Electrolysis / DSA / Electrocatalyst / RuO
2
/ Coating / Sol-gel /
Electrodeposition
Industrial chlor-alkali electrolysis represents one of the most energy- and resource-intensive
technological applications of electrocatalysis. Improving process efficiency becomes a critical issue
for the sustainable development and for alleviating the energy and environmental crisis. Rational
design in the morphology of RuO
2
-based anodic electrocatalytic coatings and the control in the
coating microstructure can contribute to massive energy saving compared to the current commercial
Ru
0.3
Ti
0.7
O
2
coating. This review covers recent developments in the anodic coatings. Performance
enhancement for RuO
2
-based anodic coatings is achieved by using alternative preparation routes
of sol-gel and electrodeposition. The target control in the coating surface morphologies and the
increase in the utilization of active Ru species are demonstrated.
1. Introduction
Chlorine and its derivative compounds are involved in a wide range of industrial
production branches [1]. Its significant role in modern chemistry has been vividly il-
lustrated by a chlorine tree with rock salt as root [2]. Industrial scale production of
Cl
2
from the electrolysis of aqueous NaCl solution (anode reaction: 2Cl
−
→Cl
2
+2e
−
,
E
Θ
=1.36 V/SHE) dates back to the late 18th century. Cl
2
is produced at the anode by
passing an electric current through the brine solution. The electrolyzers, electrode reac-
tions and electrode materials employed in industrial processes have been innovated over
the years towards energy-efficient and environment-friendly implementation. Other par-
allel industrial routes to produce Cl
2
are HCl electrolysis and Deacon process. In both
cases, molecular chlorine is recycled from the excess supply of HCl [1,3,4]. Some re-
markable breakthroughs in the chlor-alkali electrolysis industry include (i) the develop-
*
Corresponding author. E-mail: ruiyong.chen@kit.edu
#
Present address: Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76021
Karlsruhe, Germany

652 R. Chen et al.
ment of energy-saving membrane cell technology in the 1970s [5], (ii) the invention of
dimensionally stable anode (DSA
R

, an oxide coated titanium electrode) in 1965 [6]and
the subsequent industrialization by De Nora [7,8], and (iii) the impressive development
of oxygen depolarized cathode (ODC, O
2
+2H
2
O+4e
−
→4OH
−
, E
Θ
=0.4V/SHE)
by a joint cooperation of Bayer MaterialScience, De Nora and Uhde in 1998 [9]. The
combination of these innovative techniques has been put into demonstration practice
and becomes the most preferred choice in the design and construction of new chlor-
alkali plants. In 2010, the continuous expansion of the demand for Cl
2
supply reached
about 68 Mtons worldwide. Accordingly, this causes an electrical energy consumption
of about 2.1×10
11
kWh/a, accounting for about 40–50% of the variable manufacture
costs, and indirectly a CO
2
emission amount of about 100 Mtons/a[10,11]. Meanwhile,
chlor-alkali industry is also one of the resource-intensive production processes, which
depends highly on the use of the rare strategic metal ruthenium as catalysts. About
3 tons Ru per year (∼ 12% of the annual production of Ru, Ru is used in the form of
oxide for the electrochemical oxidation of Cl
−
to Cl
2
) is used currently for the fab-
rication of anodic electrocatalytic coatings [12]. Technical innovation to improve the
energy-efficiency and resource-efficiency in the chlor-alkali industry has become a crit-
ical issue in view of the current energy and environmental challenges we are facing and
the increasing shortages in the primary resources [13].
Electrocatalyst is a key factor for the sustainable industrial processes, which enables
the electron transfer reactions at the electrode/electrolyte interface with substantial en-
ergy saving. The total energy consumption in the chlor-alkali process is proportional to
the total cell voltage, including thermodynamic potential of anodic and cathodic reac-
tions, electrode overpotential, ohmic drop from the electrolyte, membrane and bubble
effect, etc.[14] In terms of electrocatalysis, the over-potential related to the electron
transfer reaction at electrode surface could be reduced by proper selection of electrode
materials with the lowest possible ruthenium content and by optimizing the preparation
techniques. Ohmic drop arising from the bubble effect is aslo related to the electrode
materials and the coating morphologies [15,16]. The state-of-the-art Ru
0.3
Ti
0.7
O
2
/Ti
electrode, which is prepared by thermal decomposition route, has achieved great suc-
cess in industrial application over the past four decades. Numerous efforts have been
made to understand the reaction mechanism, kinetics and to modify the performance
by introducing additional component(s) into the RuO
2
−TiO
2
system since 1970’s [17].
However, the scientific research activities turn largely from anodic Cl
2
evolution to elec-
trocatalytic O
2
and H
2
evolution after about 1990’s because the industry was satisfied
with the DSA performance in chlor-alkali cell [14]. Further development to improve
the coating performance in comparison to that of state-of-the-art electrode coating was
sponsored by Bayer MaterialScience in 2006 in the attempt to reduce the electrode po-
tential about 100 mV (about 3–4% energy saving) and to reduce simultaneously the
ruthenium content.
The present paper is aimed at providing an overview for the recent innovative
developments of RuO
2
-based anodic electrocatalytic coatings, including the techni-
cal features of alternative preparation routes and their ability to control the coating
structure, to improve the electrode performance and to reduce the use of Ru. This
review is organized as follows: Sect. 2 describes the general issues and intriguing
properties of RuO
2
-based mixed oxides employed in the DSA. Section 3 compares

Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review 653
several coating preparation techniques, the involved reaction mechnisms and their po-
tential influence on the final coating microstructure. Section 4 addresses the coating
morphology-dependent properties and how the coating surface morphology can be tai-
lored by controlling the preparation processes and by using different preparation routes.
Section 5 deals with the phase structure of mixed oxide coatings and how the phase
composition is correlated to the preparation techniques. Section 6 presents the electro-
catalytic activity of newly developed electrode coatings for Cl
2
evolution. Section 7 is
the summary and outlook.
2. RuO
2
-based mixed oxide electrocatalysts
RuO
2
crystallizes with tetragonal rutile structure (Fig. 1). The atomic radius of Ru
4+
(62×10
−12
m) is close to that of some transition metal ions listed in Table 1,which
makes it easy to form solid solution by doping other transition metal(s) into the rutile
lattice according to the Hume-Rothery rule [18]. Rutile-type benchmark RuO
2
−TiO
2
electrocatalyst shows excellent catalytic activity, electrochemical stability, good selec-
tivity and good electrical conductivity as anode material for the electrochemical oxida-
tion of Cl
−
to Cl
2
. The valve metal oxides (typically TiO
2
,ZrO
2
,SnO
2
,Ta
2
O
5
,Nb
2
O
5
,
etc.) are used to stabilize the ruthenium cations within the crystal lattice. The practi-
cal molar percentage of Ru in DSA, which is 30 mol% for the commercial RuO
2
−TiO
2
electrodes, is a compromise among the activity, stability (a service life about 8–12 years
in industrial application [19]), selectivity and material cost. Employed as electrocata-
lysts, high electronic conductivity is a major prerequisite to be fulfilled in the selection
of candidate materials. RuO
2
and IrO
2
are metallic conductors. The resistivity of RuO
2
and IrO
2
at room temperature are about 3.5×10
−7
Ω ·mand3.2×10
−7
Ω ·m, respec-
tively [20,21], which is close to that of metallic titanium. However, TiO
2
and other
transition metal oxides (their metal ions are listed in Table 1) show generally semi-
conducting or insulating character. The possible lower limit of the Ru content in DSA
is determined by the required apparent activity for Cl
2
evolution and also by the con-
ductivity of mixed oxide matrix, which is described by the percolation theory (certain
amount of Ru or Ir is needed to assure continuous electrically conductive percola-
tion paths) [14]. The electrochemical dissolution rate of Ru species under the harsh
electrolysis conditions can be slowed down by using mixed oxides [22]. Compared
to RuO
2
, IrO
2
is stable for HCl electrolysis and O
2
evolution reactions. Aside from
the economic consideration, high content of Ru (> 50 mol%) in DSA will reduce the
current efficiency due to the side reaction of O
2
evolution (2H
2
O → O
2
+4H
+
+4e
−
,
E
Θ
=1.23 V/SHE) and thus results in a low purity of the produced Cl
2
gas [23].
The RuO
2
-based mixed oxides are usually deposited onto a valve metal substrate
(Ti is commonly used) with a thin film form to maximize the utilization of noble metal
species. The coating preparation will be discussed in Sect. 3. The Ti substrate (usually
with an expanded mesh geometric shape to facilitate the release of evolving Cl
2
bub-
bles) remains its geometric structure and keeps a constant anode/cathode gap upon ex-
posure to the electrolysis environments. Thermal treatment is necessary to reinforce the
adhesion between the oxide coating and the underlying substrate and also to crystallize
the active phase in the fabrication of DSA. The suitable thermal sintering temperature

654 R. Chen et al.
Fig.1. Tetragonal rutile structure (P4
2
/mnm, space group 136) of the active phase for chlorine evolution.
Red balls are ruthenium atoms (this site is shared by other transition metal(s) for the mixed oxide), and
green balls are oxygen atoms.
Table1. Ionic radius of transition metal ions used in the fabrication of electrode coatings.
Ru
4+
Ti
4+
Ir
4+
Sn
4+
V
5+
Ta
5+
Nb
5+
Mo
6+
ionic radius/
×10
−12
m
62 60.5 62.5 83 59 64 69 65
Fig.2. Phase diagram of binary oxide RuO
2
−TiO
2
[26]. The green rectangular region marks the restric-
tion conditions (sintering temperature and ruthenium content) for the fabrication of RuO
2
−TiO
2
electrode
coatings.
for the fabrication of anodic coatings is usually between 400–550
◦
C. High tempera-
tures (> 550
◦
C) will result in the partial oxidation of the Ti substrate [24], which will
increase the ohmic resistance of the oxide film due to the formation of insulating TiO
x
interlayer. Besides, the metastable rutile solid solution Ru
x
Ti
1−x
O
2
starts to decompose
into separated RuO
2
and TiO
2
phases at higher temperatures (> 550
◦
C) [25]. For the
fabrication and optimization of RuO
2
−TiO
2
coatings, the restricted range of Ru content
and thermal treatment temperature is marked in green in Fig. 2.

Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review 655
The commercial Ru
0.3
Ti
0.7
O
2
electrocatalyst has a mixed phase containing little
amount of anatase TiO
2
, Ru-rich and Ru-poor rutile-type solid solution phases [15,19],
in which the rutile-type solid solution structure is responsible for the catalytic activity
and electrochemical stability. The formation of the heterogeneous structure is related to
the thermal decomposition preparation route, as discussed in Sect. 3. The typical sur-
face and cross-sectional morphology and the structural and compositional changes of
DSA electrodes after many years of industrial use have been reported elsewhere [19].
The preparation techniques, which could affect the coating morphology and the phase
structure (see the phase diagram of the binary RuO
2
−TiO
2
system [26]inFig.2)of
mixed oxide coatings, will be discussed in Sects. 4 and 5. Atomic scale surface analy-
sis [27] and density functional theory calculations [28–30]ofRuO
2
revealed that the
coordinatively unsaturated Ru atoms (Ru
cu
) and the surface bridging O atoms (O
br
)de-
termine its catalytic reactivity. Therefore, electrocatalyst coatings should be prepared
with increased exposure of active sites to the reactant species. This can be achieved
by using nanoscale catalysts [31], by creating porous structure and by controlling the
coating microstructure and surface morphologies [15], as discussed below.
RuO
2
-based DSA electrodes are also involved in many technological applications,
such as H
2
O electrolysis, electro-organic synthesis and electrochemical oxidation [32–
34]. In addition, RuO
2
is the key catalytic component for the Deacon process. A de-
tailed discussion for the recent development of stable RuO
2
-based Deacon catalysts by
Sumitomo and Bayer MaterialScience can be found in the literatures [3,4,10]. A com-
prehensive review paper covering the versatile role of RuO
2
as oxidation catalysts in
the fields of heterogeneous catalysis as well as electrocatalysis and the atomic scale
understanding of physicochemical properties of RuO
2
has been published recently by
Over [12].
3. Coating preparation techniques
Wet chemical routes such as thermal decomposition and sol-gel methods are most
widely used techniques. An oxide coating is deposited onto a substrate by wet coating
of precursor solutions such as brushing, dipping and subsequent thermal treatment. The
catalyst particles are thus immobilized onto the conductive substrate. The electrochem-
ical synthesis of metal oxides is achieved by passing an electric current between the
electrodes. The synthesis takes place at the electrode/electrolyte interface. An overview
by Therese and Kamath compared the technical features of various electrodeposition
processes such as anodic deposition, cathodic deposition and their applications in the
synthesis of oxide materials [35]. The reaction mechanisms involved in several prep-
aration routes of thermal decomposition, sol-gel and cathodic electrodeposition in the
fabrication of RuO
2
-based oxide coatings are summarized in this section.
DSA are fabricated traditionally by thermal decomposition technique, in which
oxide is obtained by thermally assisted breaking of chemical bonds of the precursor
salts (Ru−Cl bonds for RuCl
3
, for instance) and the subsequent formation of M−O
bonds in O
2
-containing atmosphere [36]. Structural change from the chloride precur-
sor salts to their respective oxides during the thermal decomposition processes have
been investigated in detail for RuCl
3
and IrCl
3
by extended X-ray absorption fine

656 R. Chen et al.
structure [37]. The coordination environment (from M−Cl to M−O) and coordination
numbers of the Ru and Ir cations change with the variation of sintering temperature.
The structure change occurs at 250–300
◦
C and the final RuO
2
, IrO
2
rutile structure is
formed at about 450
◦
C. In the preparation of mixed oxide, due to the difference in the
temperature for structure change among different precursor salts (i.e., the different ki-
netics of thermal decomposition of various precursors), some salts can be converted
to oxide faster than others and this will result in a heterogeneous microstructure with
mixed clusters of individual composition rather than solid solution [38]. This could ex-
plain the formation of mixed phase for the thermal decomposition prepared commercial
Ru
0.3
Ti
0.7
O
2
coatings (Table 2 in Sect. 5)[15]. In addition, the actual chemical com-
position of mixed oxide may deviate from its nominal composition during the thermal
decomposition in case that some precursors will evaporate at high temperature rather
than decompose [39]. For instance, distinct off-stoichiometry was observed commonly
for the SnO
2
-containing oxide coatings prepared by conventional thermal decompos-
ition route [40]. This fact can restrict the ability of thermal decomposition route to
explore the optimized combination of noble metal oxides and valve metal oxides. We
found that the sol-gel Ru
0.3
Sn
0.7
O
2
coating shows superior performance to the thermal
decomposition Ru
0.3
Ti
0.7
O
2
coating [15], as discussed in Sect. 6.
Sol-gel technique is superior to thermal decomposition technique to obtain coat-
ings with improved stability [41]. In general, sol-gel is particularly suitable to fabricate
mixed oxides because molecular level homogeneity of M−O−M

networks can be
obtained through the controlled hydrolysis (M−OH is formed by ligand exchange reac-
tion) and condensation reactions (the formation of M−O−M

with the release of H
2
O,
ethanol etc.) [42]. For the preparation of mixed oxides, chelating agents can be used to
control the competitive hydrolysis and condensation reaction rates of different precur-
sors [43]. The synthesis of pure phase oxides with the formation of solid solution can
be easily achieved.
For the wet chemical routes, due to the molar concentration of the precursor solu-
tion (the solubility of precursors in solvent) is limited to assure a stable and homoge-
neous solution, multiple coating/thermal treatment cycles are needed (Fig. 3, route A)
to gain a thick oxide coating (a few micrometers in thickness) to meet the industrial
demands of durability for years. It has been reported that the active surface area of
electrode coatings increases with coating thickness until the coating surface reaches to
a constant roughness [44]. Further increase in thickness can only prolong its service life.
As an alternative, for electrodeposited coatings the coating thickness is dependent on
the duration of deposition. Only one-step thermal sintering after electrodeposition is ne-
cessary to finish the preparation procedure (Fig. 3, route B). Note that the resistivity of
the deposits is a factor limiting the development of thick coatings during electrodepo-
sition. Chu et al. [45] reported that under galvanostatic control, the operating voltage
keeps increasing during the electrodeposition due to the low conductive nature of the
deposits in the preparation of mixed RuO
2
−TiO
2
oxide.
To prepare RuO
2
−TiO
2
oxides with the electrodeposition route, the correspond-
ing precursor salts need to be mixed and dissolved into electrochemical bath. Under
galvanostatic or potentiostatic control, quasi-simultaneous electrodeposition of metal
ions, M
n+
, or their complexes occur at the working electrode (Ti substrate) surface
as thin film. For cathodic electrodeposition, it is considered that the electrochemi-

Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review 657
Fig.3. Flowchart of the coating preparation procedure. Route A: wet chemical route; Route B: electrode-
position. Thick coatings can be obtained by reduplicative coating/sintering operation for Route A, or by
prolonging the electrodeposition time with the subsequent one-step thermal sintering for Route B.
cally produced OH
−
(2H
2
O+2e
−
→H
2
+2OH
−
, E
Θ
=−0.83 V/SHE) increases the
local pH value and this is responsible for the formation of oxides or hydroxides
(M
n+
+n(OH)
−
→M(OH)
n
↓) [45]. Intermediate species such as Ti(O
2
)(OH)
(4−n)+
n−2
,
TiO
3
(H
2
O)
x
,Ru,Ru(OH)
4
, (RuTi)O
x
(OH)
y
,RuO
2
·xH
2
O may form depending on the
preparation conditions and parameters [45–47]. These intermediates will be converted
into oxide by the subsequent calcination under O
2
-containing atmosphere. Due to the
difference in the redox potential among various M
n+
, the competitive deposition could
happen and this will results in a heterogeneous microstructure with mixed phases [47].
Zhitomirsky et al. reported that the electrodeposition of Ru and Ti species follows in-
dependent mechanisms [48], which is also responsible for the structural heterogeneity.
In addition, parasitic process such as the H
2
or O
2
evolution may happen, which has
been exploited to prepare materials with special morphologies using H
2
or O
2
bubbles
as dynamic templates [49].
4. Surface morphology of electrode coatings
RuO
2
-based electrocatalysts are used as coating form supported onto a metallic Ti sub-
strate. The coating morphology can affect significantly the electrode performance such
as the available active surface area, electrode deactiviation due to the passivation of the
Ti-substrate [15]andalsoCl
2
gas bubble evolution behavior [16]. DSA has a typical
mud-crack surface morphology, as shown in Fig. 4a. The cracks are formed during ther-
mal treatment due to the development of tensile stress [50](Fig.4c). The crack gaps

658 R. Chen et al.
Fig.4. (a) Representative SEM image of the surface morphology. (b) Sketch of the cross-section. (c) For-
mation mechanism of the mud-crack coating [50]. Electrolyte can penetrate through the gaps and attack
the underlying Ti-substrate, which results in the formation of insulating TiO
x
interlayer between the oxide
coating and the Ti-substrate [15].
Fig.5. SEM images of (a) mud-crack commercial Ru
0.3
Ti
0.7
O
2
coating prepared by thermal decomposition,
(b) mud-crack sol-gel Ru
0.25
Ti
0.75
O
2
coating prepared by drop-coating technique, (c) crack-free sol-gel
Ru
0.25
Ti
0.75
O
2
coating prepared by dip-coating technique [51], (d) crack-free Ru
0.25
Ir
0.10
Ti
0.65
O
2
coating pre-
paredbysol-gel/electrophoretic deposition from 0.45 mol L
−1
starting solution at constant current density
of 3mAcm
−2
for 90 min, (e) mud-crack Ru
0.25
Ir
0.10
Ti
0.65
O
2
coatingpreparedbysol-gel/electrophoretic
deposition from 0.45 molL
−1
starting solution at constant current density of 5 mAcm
−2
for 30 min. (f)
Dependence of voltammetric charges (q) on the potential sweep rates (ν) for mud-crack coating (b) and
crack-free coating (c) [51].
may accommodate electrolyte and therefore may offer more available inner surface
area. However, the penetration of electrolyte through the gaps may attack the underly-
ing Ti substrate and result in the formation of an insulating TiO
x
interlayer between the

Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review 659
Fig.6. (a,b) SEM images of electrodeposited Ru
0.18
Ti
0.82
O
2
. (c) Outer surface to inner surface ratio
(q
outer
/q
inner
)forRu
x
Ti
1−x
O
2
coatings with different Ru contents. CV was recorded in 0.5MH
2
SO
4
at room
temperature.
oxide coating and the substrate (Fig. 4b). The increase in the ohmic resistance of the
oxide film can result in the loss of electrode performance before the complete electro-
chemical dissolution of the active Ru species, which will cause an ineffective use of Ru.
In this section, two different strategies to control the electrode coating surface mor-
phologies and thus to improve the coating performance were described. One way is to
fabricate crack-free sol-gel coatings (Fig. 5),whichareproventobeeffectivetoavoid
the direct contact of electrolyte with Ti-substrate. They are promising to prolong the
electrode service life and to utilize Ru more effectively [51]. Another way to modify the
RuO
2
−TiO
2
coating morphology is practiced by the electrodeposition route. A novel
surface structure with Ru-containing spheres sitting on the top of the mud-crack layer
was obtained (Fig. 6), which has the capability of increasing the outer active surface
area (thus increasing the utilization of Ru) and improving electroactivity with reduced
Ru content [47]. The electrode/electrolyte interface processes for the coatings with
tailored surface morphologies were characterized by the in situ technique of electro-
chemical cyclic voltammetry (CV). The electrochemical contact of electrolyte with the
oxide coating matrix could provide valuable information about the electrochemically
accessible active surface, which is proportional to the integration area of the anodic

660 R. Chen et al.
branches in CV curves (i.e., voltammetric charge, q). Furthermore, this could help es-
timating the inner (such as inner gaps, pores) and outer surface area by changing the
potential sweep rates (ν), since the electrochemical response rates of the inner/outer
surface are ν-dependent [52]. Thus, the penetration character of oxide coatings for elec-
trolyte can be evaluated by using CV measurements.
For the preparation of crack-free sol-gel coatings, repetitive sol-gel dipping-
withdrawing/sintering cycles were performed [51]. In this case, the tensile stress was
relaxed through the plastic deformation for each single thin layer during thermal treat-
ment [50]. By applying thin layers with diluted coating solutions, no cracks were
formed, as observed in Fig. 5c. Thick crack-free coatings can be obtained by increas-
ing the wet-coating/sintering cycle times. The as-obtained crack-free sol-gel coatings
are impermeable for the electrolyte, as confirmed by CV measurements (Fig. 5f). The
crack-free coating shows independent behavior of q on ν. In contrast, for the mud-
crack coating q shows an initial sharp decrease with ν from 5–50 mVs
−1
and becomes
constant when ν exceeds 50 mV s
−1
.
The compact and crack-free coatings can be used as protective innerlayer for the
fabrication of DSA [51]. On top of the innerlayer, a crack oxide layer can be applied
considering the needs of a high apparent activity for Cl
2
evolution. We have also demon-
strated that a crack-free sol-gel RuO
2
−SnO
2
coating can be directly used without mud-
crack toplayer owing to its novel nanopore-nanocatalyst architecture structure [15].
It shows improved overall electrocatalytic activity than the commercial RuO
2
−TiO
2
coating.
Another effective preparation technique to obtain crack-free coating is the sol-
gel/electrophoretic deposition (Fig. 5d,e). Zhitomirsky has given a comparison be-
tween the electrophoretic and electrolytic deposition [53]. As compared in Fig. 5d,e,
by controlling the applied current density and the solution concentration, cracks can be
avoided. By keeping a constant current density, the deposition rate is uniform during
the deposition time. High deposition rate at highly applied current densities results in
a thick and crack coating (Fig. 5e).
For the TiO
2
−RuO
2
coatings prepared by electrodeposition route, cracks with very
broad gaps in the range of 2–8 μm were observed after post-sintering (Fig. 6a). A dis-
tinct surface character of the electrodeposited coatings is the formation of spheres (with
a size of a few hundred nm to about 2 μm) on the top of the mud-crack surface (Fig. 6).
We have reported that the formation of sphere surface was related to the Ru contents
in the electrodeposited coatings [47]. A transition from a smooth surface for pure TiO
2
to a spherical surface with increasing Ru content in Ru
x
Ti
1−x
O
2
is clearly visible. The
coating composition (the Ru content) of the electrodeposited Ru
x
Ti
1−x
O
2
can be easily
controlled by the bath composition. A linear correlation between coating composition
(analysed quantitatively by ICP-OES) and bath composition was observed [47]. CV
measurements showed that the Ru content has an influence on the ratio of the outer
surface to the inner surface (q
outer
/q
inner
,Fig.6c). The formation of sphere surface with
increasing Ru content leads to a considerable increase of the q
outer
/q
inner
ratio, indicating
that the electrochemically active Ru sites are preferentially located at outermost parts
of the coating with increasing Ru content. The direct expose of Ru species to the elec-
trolyte leads to a more efficient utilization of Ru, since the outer surface is the main
working domain during Cl
2
evolution. Thus, with the formation of spheres, a targeted

Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review 661
increase of the outer active surface is achieved. In addition, a surface morphology with
large outer surface should represent an excellent prerequisite for a fast removal of
evolving Cl
2
bubbles, which should bring about a decrease of the bubble-induced ohmic
resistance.
Ternary TiO
2
−RuO
2
−MO
x
coatings (M=Ir, Sb, Mn) were also electrodeposited
in the attempt to reduce further the noble metal content with improved catalytic activ-
ity. The third component could either exhibit a catalytic activity for Cl
2
evolution or
compensate the depressing effect of TiO
2
on the catalytic activity. The special surface
morphology with mud-crack structure is maintained despite the introduction of a third
component [47].
The coating surface morphology has significant influence on the Cl
2
bubble evo-
lution behavior. The coating surface should be fabricated in favor of the release of
evolving bubbles. The micro-structural impact of sol-gel electrode coatings on the bub-
ble evolution has been reported elsewhere [15]. We have also developed an analytical
strategy to assist the evaluation of property of newly designed electrode coatings for
bubble evolution, based on the wavelet transform analysis of bubble-induced electro-
chemical noise signals [16]. The precise experimental determination of the ohmic drop
arising from bubble effect has not been completely clarified.
5. Phase structure
Rutile-type solid solution phase is the active component for Cl
2
evolution reaction. The
dependence of phase composition and crystallite size on the preparation routes is sum-
marized in Table 2. The crystal structure parameters and crystallite sizes were refined
from the X-ray diffraction patterns by the Rietveld method using the TOPAS software
(Bruker AXS). Crystallite sizes are specified as the volume averaged column heights.
Phase composition for rutile solid solution Ru
x
M
1−x
O
2
was calculated based on the
refined lattice parameters using Vegard’s law [54].
For RuO
2
−TiO
2
coatings prepared by thermal decomposition and electrodepo-
sition routes, two rutile solid solution phases (Ru-rich and Ru-poor) were observed
along with an inert phase of anatase TiO
2
. Metallic Ru phase was also present in the
electrodeposited coatings. Jow et al. compared anodic, cathodic and cyclic voltammet-
ric deposition of ruthenium oxide from aqueous RuCl
3
solutions [55]. Metallic Ru
was exclusively observed in the cathodic deposited film through X-ray photoelectron
spectroscopy. This undesired cathodic metal deposition (Ru metal is instable under
electrolysis conditions) can be inhibited partially by applying higher current densities in
the electrodeposition process (Table 2). In this case, the cathodic production of OH
−
is
faster and the deposition of M(OH)
n
is favored. Meanwhile, the competitive deposition
of titanium species at highly applied current density results in the formation of large
amount of separated TiO
2
inert phase. Ru-rich solid solution phase showed a draw-
back in the catalyst selectivity for Cl
2
evolution reactions [23,56]. The formation of the
mixed phase is due to either the competitive reactivity of the precursors or the difference
in the reaction mechanism during the preparation procedure, as described in Sect. 3.
The heterogeneity in the microstructure is also not favorable for the long-term electrode
stability [41].

662 R. Chen et al.
Table2. Phase structure of electrode coatings prepared by different techniques. The data were given from
the Rietveld refinement of the X-ray diffraction patterns.
preparation nominal preparation Ru load- phase crystallite
technique composition conditions ing/gm
−2
composition wt% size
b
/nm
thermal
decomposition
Ru
0.3
Ti
0.7
O
2
a
[15] 12.1 Ru
0.84
Ti
0.16
O
2
Ru
0.16
Ti
0.84
O
2
anatase TiO
2
13.2
80.0
6.8
10
21
–
sol-gel Ru
0.4
Ti
0.6
O
2
[31]
Ru
0.3
Sn
0.7
O
2
[15]
10.3
5.8
Ru
0.34
Ti
0.66
O
2
Ru
0.35
Sn
0.65
O
2
100
100
18
5
sol-gel/
solvothermal
Ru
0.4
Ti
0.6
O
2
[31] 10.3 Ru
0.7
Ti
0.3
O
2
anatase TiO
2
66.3
33.7
3
–
electro-
deposition
Ru
0.18
Ti
0.82
O
2
[47]30mAcm
−2
2 ∼3Ru
0.95
Ti
0.05
O
2
Ru
0.12
Ti
0.88
O
2
Ru
anatase TiO
2
12.2
75.5
11.8
0.5
16
18
–
–
60 mAcm
−2
2 ∼3Ru
0.56
Ti
0.44
O
2
Ru
0.16
Ti
0.84
O
2
Ru
anatase TiO
2
10.1
73.7
0.9
15.3
19
14
–
–
a
Commercial coating supplied from Bayer MaterialScience.
b
Crystallite site was given only for the active phase.
Single rutile solid solution phase has been achieved for the sol-gel derived
RuO
2
−TiO
2
and RuO
2
−SnO
2
coatings (Table 2). The substitution of Ti by Sn is
effective to reduce the crystallite size from about 18 to 5 nm. A different strategy
to obtain nanocatalysts has been demonstrated by a combined sol-gel/solvothermal
route [31,57]. Solvothermal processing of the amorphous RuO
2
−TiO
2
xerogel coating
results in the pre-crystallization of anatase TiO
2
, which acts as support for the sub-
sequent crystallization and growth of rutile Ru
x
Ti
1−x
O
2
nanoparticles during the post-
sintering. These nanocatalysts show superior performance for Cl
2
evolution [15,31], as
discussed in Sect. 6.
6. Chlorine evolution activity
The apparent electrocatalytic activity of electrode coatings depends on the intrinsic
catalytic activity (material dependent), and the geometric contribution (coating surface
morphology and micro-structure dependent). The electrode overpotential and bubble-
induced ohmic resistance (which contribute to the total cell voltage), rely on the design
of electrode coatings, the preparation routes and synthesis parameters. The evaluation
of the electrode performance is usually performed by measuring the electrode poten-
tial at a given constant current density (the galvanostatic polarization technique) or by

Anodic Electrocatalytic Coatings for Electrolytic Chlorine Production: A Review 663
Table3. List of the iR-corrected electrode potentials for Cl
2
evolution of different electrode coatings. The
galvanostatic polarization was measured at 400mA cm
−2
(i.e., 4kA m
−2
)in3.5 M NaCl, pH3 at 80
◦
Cin
a home-made Teflon flow-cell, with forced convention of electrolyte (100 mLmin
−1
).
preparation route nominal composition Ru loading/gm
−2
potential/
V vs. NHE
ΔE
d
/mV
thermal decomposition
a
Ru
0.3
Ti
0.7
O
2
Ru
0.3
Ti
0.7
O
2
12.1
6.1
1.434
1.423
sol-gel Ru
0.3
Sn
0.7
O
b
2
Ru
0.3
Sn
0.7
O
c
2
7.7
5.8
1.374
1.342
54↓
86↓
electrodeposition Ru
0.18
Ti
0.82
O
2
Ru
0.21
Ti
0.79
O
2
Ru
0.15
Ir
0.02
Ti
0.83
O
2
Ru
0.15
Sn
0.11
Ti
0.74
O
2
2.4
3
2.1
1.4
1.382
1.372
1.408
1.446
46↓
56↓
20↓
17↑
a
Commercial coating supplied from Bayer MaterialScience.
b
Mud-crack coating.
c
Crack-free coating.
d
Electrode potential difference between sol-gel, eletrodeposition coatings and the commercial coating,
“↓” means reduction.
measuring the current density at a given potential (the potentiostatic polarization tech-
nique). In this section, we compared the electrocatalytic performance of some repre-
sentative sol-gel coatings, electrodeposition coatings with the commercial Ru
0.3
Ti
0.7
O
2
coatings. An extended comparison and evaluation of electrode performance for various
electrode coatings can be found in [25,58].
The electrodes were polarized galvanostatically (at 4 kAm
−2
) under quasi-
industrial conditions (3.5 M NaCl, pH3, 80
◦
C, with forced convection of electrolyte:
100 mLmin
−1
). The high-throughput test cells were developed to screen the electrode
performance, as reported elsewhere [58]. The measured electrode potential was cor-
rected for ohmic resistance (iR) of electrolyte derived by impedance spectroscopy. The
coating composition and the corresponding preparation technique, Ru loading amount,
and the electrode potential for Cl
2
evolution are summarized in Table 3. The commer-
cial Ru
0.3
Ti
0.7
O
2
coatings with different Ru loading amount have an averaged electrode
potential of about 1.428 V vs. NHE. It shows independent behavior of electrode po-
tential on the Ru loading amount, indicating that both coatings have the same surface
roughness (the same available active sites and active surface area) [44]. Higher Ru
loading amount is then only responsible for a longer coating service life.
For the sol-gel Ru
0.3
Sn
0.7
O
2
coatings with a Ru loading amount of about 6–8 gm
−2
,
a decrease in the electrode potential by about 50–90 mV in comparison to that of
commercial coatings is obtained (Table 3). The remarkable enhancement in the per-
formance of sol-gel Ru
0.3
Sn
0.7
O
2
coatings can be attributed to the extremely small crys-
tallite size (5 nm, Table 2). The crystal growth is inhibited in the binary RuO
2
−SnO
2
coating due to the large difference in the lattice parameters between the RuO
2
(a =
4.4994Å, c = 3.1071Å, V = 62.9Å
3
)andSnO
2
(a = 4.7382 Å, c =3.1871 Å, V =

664 R. Chen et al.
71.6Å
3
)[39], than that between RuO
2
and rutile TiO
2
(a =4.5933Å, c = 2.9592Å,
V =62.4Å
3
)[59–61]. Except for the coating chemical composition and Ru loading
amount, the coating surface morphology has a significant influence on the overall elec-
trode performance. Crack-free Ru
0.3
Sn
0.7
O
2
coatings show a better overall performance
than the mud-crack Ru
0.3
Sn
0.7
O
2
coatings, due to the difference in the bubble evolu-
tion behavior [15]. The dependence of the overall electrode polarization behavior on the
local activity and bubble evolution behavior has been addressed elsewhere [15].
Improvement in the electrode performance is also obtained for the electrodeposited
RuO
2
-based coatings with a lower Ru molar percentage of about 15–20 mol% and
a lower Ru loading amount of about 2–3 gm
−2
(Table 3), arising from their special
surface morphology [47].
The as-achieved results mean a decrease in the electrode potential and simultan-
eously a reduction in the use of Ru. A decrease of electrode potential of by 90 mV
indicates an energy saving of about 3–4% for the NaCl electrolysis cell with ODC cath-
ode (the total cell voltage is about 2.1Vat4kAm
−2
) or with H
2
evolving cathode (the
total cell voltage is about 3.1Vat4kAm
−2
)[9]. A high fraction of energy saving can
be expected for the HCl electrolysis cell due to its low cell voltage [62,63].
7. Summary and outlook
This paper reviews the recent research activities in the purpose of enhancing the anodic
coating performance for industrial electrolytic Cl
2
production, which is one of the most
energy- and resource-intensive industrial technological applications of electrocatalysis.
Optimization in the electrode activity and stability has been achieved through adopt-
ing alternative preparation routes such as sol-gel and electrodeposition. The reaction
mechanisms and technical features of both methods are summarized and compared with
that of the conventional thermal decomposition technique. We show that the preparation
techniques can determine fundamentally the final microstructure, phase composition
and properties of electrode coatings. We report the key innovations in the structure
design of oxide coatings to enhance the active mass utilization of Ru through better mi-
crostructural control of coating surface morphology (crack-free or spherical surface).
A new concept to fabricate coatings with protective crack-free inner layer and cat-
alytically active outer layer is proposed. The crack-free coatings are impermeable for
electrolyte confirmed from the CV measurements, which can avoid the Ti substrate
passivation during electrolysis operation. We demonstrate that a crack-free sol-gel
Ru
0.3
Sn
0.7
O
2
coating is potentially durable and exhibits interestingly improved cata-
lytic activity for Cl
2
evolution in comparison to a commercial Ru
0.3
Ti
0.7
O
2
coating. For
sol-gel electrode coatings the improvement in electrode activity is found to be related
to the use of nano-catalysts. The inhibition of crystal growth is realized when using
SnO
2
as valve metal oxide or by using a novel sol-gel/solverthermal route. Compared
to the thermal decomposition and sol-gel routes, electrodeposition technique exhibits
unique character to obtain special coating surface morphology with an increased uti-
lization of outer surface and accordingly an improved catalytic activity. It is considered
that the outer surface of electrode coatings is the main working part during intensive
Cl
2
gas evolution and the spherical outer surface may facilitate the release of evolving

Anodic Electrocatalytic Coatings forElectrolytic Chlorine Production:A Review 665
Cl
2
bubbles. The critical parameters affecting the coating surface and the phase struc-
ture is the applied current density. Multi-doping can be easily obtained by controlling
the bath composition. The structural and crystal size-controlled preparation of mixed
oxide catalyst coatings presented in this paper can be extended to the design of other
multicomponent heterocatalysts and has promise as electrocatalyst fabrication route for
current chlor-alkali industry. Further improvements of the electrolysis cell rely on de-
creasing the cathodic overpotential and membrane ohmic drop, since both contribute
largely to the total cell voltage.
Acknowledgement
Results reported in this review paper have been achieved with the financial support from
the joint BMBF project “Innovative Technologies for Resource Efficiency – Resource-
Intensive Production Processes: Improving the efficiency of chlorine production” (FKZ:
033R018G, 033R018D) and also from the Bayer MaterialScience AG “Identification
and Characterization of Electrocatalysts for the Electrolytic Chlorine Production”. We
gratefully acknowledge our partners: Prof. Wilhelm F. Maier, Prof. Klaus Stöwe, Prof.
Wolfgang Schuhmann, Dr. Detre Teschner, Prof. Herbert Over, who have participated
in the innovation project and offered valuable comments and discussions.
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