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Supporting Information
The effect of in-situ nitrogen doping on oxygen evolution
reaction of MXenes
Yi Tang, Chenhui Yang, Yapeng Tian, Yangyang Luo, Xingtian Yin, and Wenxiu Que *
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International
Center for Dielectric Research, and Shaanxi Engineering Research Center of Advanced Energy
Materials and Devices, School of Electronic & Information Engineering, Xi’an Jiaotong University,
Xi’an 710049, Shaanxi, People’s Republic of China.
* Corresponding authors: wxque@mail.xjtu.edu.cn.
Electronic Supplementary Material (ESI) for Nanoscale Advances.
This journal is © The Royal Society of Chemistry 2020
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Table of Contents
1. Experimental Section...........................................................................................................................3
1.1 Chemicals and Characterization .............................................................................................3
1.2 Synthesis of the Ti3AlC2, Ti3AlC1.8N0.2 and Ti3AlC1.6N0.4 ceramic powders.........................3
1.3 Synthesis of the few-layered Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 flakes...................................4
1.4 Electrode preparation and electrochemical testing ...............................................................4
2. Calculation............................................................................................................................................6
3. Figures...................................................................................................................................................7
Figure S1. Schematic preparation procedure of the Ti3C1.6N0.4 flakes.............................................7
Figure S2. (a) XRD patterns, the corresponding diffraction peaks at around (b) (002) and (c) (104)
of the Ti3AlC2, Ti3AlC1.8N0.2 and Ti3AlC1.6N0.4 powders. ................................................................8
Figure S3. (a) Ti 2p spectra and (b) C 1s spectra of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 samples.
...........................................................................................................................................................9
Figure S4. CV curves of the Ti3C2 (a), Ti3C1.8N0.2 (b), and Ti3C1.6N0.4 (c) electrocatalysts at
various scan rates. ...........................................................................................................................10
Figure S5. Curves of conductivity of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 electrocatalysts. .......11
Figure S6. The SEM images of the Ti3C1.6N0.4 catalysts after cycling at current density of 10 mA
cm-2 for 12 h....................................................................................................................................12
4. Tables ..................................................................................................................................................13
Table S1. XPS results of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 samples. ......................................13
Table S2. XPS results of the contents of different nitrogen species in the Ti3C1.8N0.2 and
Ti3C1.6N0.4 electrocatalysts. .............................................................................................................14
Table S3. Simulated Rs and Rct and CPE values of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4
electrocatalysts. ...............................................................................................................................15
References...............................................................................................................................................16
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1. Experimental Section
1.1 Chemicals and Characterization
TiN (2-10 μm particle size, 99.50% purity, Aladdin), TiC (2-4 μm particle size, 99.00% purity,
Aladdin), Al (1-3 μm particle size, 99.50% purity, Aladdin), Ti (≤ 48 μm particle size, 99.99% purity,
Aladdin) and Nafion solution (5 wt% in deionized water) were purchased from Sigma-Aldrich. LiF
(99.99 %), hydrochloric acid (technical grade) and KOH (99.99%) were purchased from Sinopharm
Chemical Reagent Co., Ltd. Except as otherwise specified, all the chemicals were used without further
purification. The high purity deionized water was purified using an UPH standard ultrapure water
instrument (Sichuan ULUPURE pure science & technology Co., Ltd., China).
Scanning electron microscopy (SEM) images of the samples were observed using JSM-6390 with
energy-dispersive X-ray analysis (EDAX) from JEOL Inc., Japan, and transmission electron microscopy
(TEM) and selected area electron diffraction (SAED) results of the samples were obtained by using JEM-
2010 from JEOL Inc., Japan. X-ray diffraction (XRD) patterns of the samples were performed on a
Rigaku D/max 2200 pc diffractometer with Cu Kα radiation (λ = 0.15406 nm, 60 kV, 60 mA, 5° min-1
from 5 to 70°), and the XRD data were analyzed by using the Jade 6.0 software. Raman spectra
measurements of the samples were performed by using a LabRam Aramis Raman spectrometer with a
He-Ne laser (λ = 633 nm). The electronic state and composition of the samples were recorded by an X-
ray photoelectron spectrometer (XPS, ESCALAB Xi+, Thermo Fisher Scientific, USA) with an exciting
source of Al Kα (400 W, 45 eV pass energy, 650 μm spot size). Contact angles of the films were measured
by using the contact angle measurement instrument JC2000D2 (POWEREACH, Shanghai zhongchen
Digital Technology Apparatus Co., Ltd., China). The thickness of the film was measured by using a high-
accuracy submicrometer digimatic micrometer (293-240, Mitutoyo, Japan) with a resolution of 1 μm.
For electric conductivity test, the current and the potential of the test device were measured by using a
Linear Sweep Voltammetry (LSV) method in a CHI 660E electrochemical workstation.
1.2 Synthesis of the Ti3AlC2, Ti3AlC1.8N0.2 and Ti3AlC1.6N0.4 ceramic powders
Firstly, all powders of TiC (2-4 μm particle size, 99.00% purity, Aladdin), Al (1-3 μm particle size,
99.50% purity, Aladdin) and Ti (≤ 48 μm particle size, 99.99% purity, Aladdin) were mixed in a molar
ratio of 2:1.2:1. The mixed powders were ball-milled with ethyl alcohol for 6 h at a speed of 400 rpm,
and dried in a vacuum oven at 40 oC for 24 h. Then, the dried mixture was annealed in an alundum tube
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in Ar gas at a flow of 40 mL min-1. The sintering process was conducted at 1350 °C for 2 h at a heating
rate of 8 oC min-1. The sintered product was grinded by stainless steel mortar and sieved through a 400
mesh screen for the sake of the initial particle size was controlled at < 38 μm. Thus, the Ti3AlC2 powders
were obtained for further study.
The synthesis processes of the Ti3AlC1.8N0.2 and Ti3AlC1.6N0.4 powders were similar to that of the
Ti3AlC2 powders except for the molar ratios of mixed powders were adjusted to TiN:TiC:Al:Ti =
0.2:1.8:1.2:1 for the Ti3AlC1.8N0.2 powders and TiN:TiC:Al:Ti = 0.4:1.6:1.2:1 for the Ti3AlC1.6N0.4
powders, respectively.
1.3 Synthesis of the few-layered Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 flakes
As shown in the Figure S1, the few-layered Ti3C1.6N0.4 flakes were prepared according to the precious
work with a certain modification 1. Firstly, slowly adding 2 g LiF powders to 20 mL 9 M HCl aqueous
solution with stirring for 30 min to achieve the mixed etching solution. Then, 1 g of the as-prepared
Ti3AlC1.6N0.4 powders were slowly added to the above mixed etching solution, then stirred at a speed of
200 rpm at 35 °C for 24 h. Afterward, the solid residue was repeatedly washed with ultrapure water until
the pH value of the supernatant was larger than 6. Then, the Ti3C1.6N0.4 sediments were dispersed in 200
mL of de-oxygenated ultrapure water and sonicated for 60 min under Ar flow in ice-bath. Finally, the
dark green supernatant was collected by centrifuging for 30 min at 1500 rpm, and named as few-layered
Ti3C1.6N0.4 suspension. The Ti3C1.6N0.4 suspension was restored at 4 °C in the refrigerator before being
used. The synthesis processes of the Ti3C1.8N0.2 flakes and Ti3C2 flakes were similar to that of the
Ti3C1.6N0.4 flakes.
1.4 Electrode preparation and electrochemical testing
Active materials (4.0 mg, e.g. Ti3C2 flakes, Ti3C1.8N0.2 flakes or Ti3C1.6N0.4 flakes) was mixed with
ethanol (500 μL), ultrapure water (485 μL) and Nafion (15 μL, 5.0 wt%), followed in ice-bath
ultrasonication for 40 min to form a uniform suspension. The electrocatalyst ink (12.5 μL) was then
loaded onto a pretreated piece of carbon fiber paper (CFP, 0.25 cm × 0.25 cm) and dried under ambient
condition for 6 h. The average mass loading was calculated to be around 0.2 mg cm-2.
The performance of the electrocatalysts towards OER was executed on a CHI 660E electrochemical
workstation (CH Instruments, China) under room temperature with a standard three-electrode system,
including a working electrode (Ti3C2 flakes, Ti3C1.8N0.2 flakes or Ti3C1.6N0.4 flakes), a counter electrode
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(Pt foil, area of 1.0 cm × 1.0 cm) and a reference electrode (SCE). Before the measurements, the aqueous
electrolyte of 1 M KOH was bubbled with N2 flow (30 mL min-1) for 30 min. During measuring, a slow
gas flow (5 mL min-1) should be maintained over the electrolyte to ensure continuous gas saturation. The
linear sweep voltammetry (LSV) was obtained at a low scan rate of 5 mV s-1. The electrical double-layer
capacitance (Cdl) of the electrocatalyst was measured from cyclic voltammetry (CV) in a small potential
range of 1.17 – 1.27 V vs. RHE without apparent faradic processes occurring. The plot of the current
density difference [Δj = (ja – jc)/2] at 1.22 V vs. SCE against the scan rates (10 – 100 mV s-1) was linearly
fitted, and its slope was the Cdl of the tested electrocatalysts. Electrochemical impedance spectroscopy
(EIS) measurement was conducted at a potential of 1.62 V vs. RHE by applying an AC voltage with
amplitude of 5 mV in a frequency range from 100 kHz to 10 mHz. Chronopotentiometric measurement
was carried out through applying a current density of 10 mA cm-2 for 12 h.
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2. Calculation
All measured potentials vs. SCE are converted to a reversible hydrogen electrode (RHE) potential
based on the Nernst equation as below:
E vs. RHE (V) = E vs. SCE (V) + 0.05916 × pH + 0.2412 (V) (1)
where E vs. RHE is the applied potential vs. RHE; E vs. SCE is the applied potential vs. SCE reference
electrode, pH is the pH value of the electrolyte (1 M KOH, pH=14).
The overpotential is calculated according to the following formula (2):
η (V) = E vs. RHE – 1.23 (V) (2)
Tafel slope is calculated by plotting overpotential η vs. logarithm of current density from polarization
curves according to the following equation.
η (V) = b × log j(3)
where ƞ is the overpotential, b is the Tafel slope, j is the current density.
The calculation of ECSA is based on the measured double-layer capacitance (Cdl) of the synthesized
electrode. The Cdl of the electrocatalyst is measured from CV curves in a small potential range of 1.17 –
1.27 V without apparent faradaic processes occurring. The plot of the current density difference at 1.22
V against the scan rates (10 − 100 mV s-1) is linearly fitted, and its slope is the twice Cdl of the tested
electrocatalyst.
For the electric conductivity test, as shown in Figure S5, the current and the potential of the test device
are measured by using a Linear Sweep Voltammetry (LSV) method in a CHI 660E electrochemical
workstation.
The conductivity κ (S cm-1) of the Ti3C1.6N0.4 film can be calculated according to the following
equations:
R = U / I (4)
ρ=R S / L (5)
κ=1 / ρ = L / (R × S) (6)
κ = (I × L) / (U × S) (7)
where κ (S cm-1) is electrical conductivity, ρ (Ω·cm) is the resistivity, and R (Ω) is resistance. I (A) is the
response current between the Ti3C1.6N0.4 films (remove the current of other components), L (cm) is the
thickness of the film, U (V) is the potential, S (cm2) is the effective contact area between the film and
GCE.
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3. Figures
Figure S1.
Figure S1. Schematic preparation procedure of the Ti3C1.6N0.4 flakes.
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Figure S2.
Figure S2. (a) XRD patterns, the corresponding diffraction peaks at around (b) (002) and (c) (104) of
the Ti3AlC2, Ti3AlC1.8N0.2 and Ti3AlC1.6N0.4 powders.
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Figure S3.
Figure S3. (a) Ti 2p spectra and (b) C 1s spectra of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 samples.
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Figure S4.
Figure S4. CV curves of the Ti3C2 (a), Ti3C1.8N0.2 (b), and Ti3C1.6N0.4 (c) electrocatalysts at various scan
rates.
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Figure S5.
Figure S5. Curves of conductivity of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 electrocatalysts.
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Figure S6.
Figure S6. The SEM images of the Ti3C1.6N0.4 catalysts after cycling at current density of 10 mA cm-2
for 12 h.
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4. Tables
Table S1. XPS results of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 samples.
Sample
Ti 2p
(at.%)
C 1s
(at.%)
N 1s
(at.%)
O 1s
(at.%)
F 1s
(at.%)
Cl 2p
(at.%)
Ti3C2
26.26
44.62
-
12.01
12.15
4.95
Ti3C1.8N0.2
24.07
45.39
3.76
13.63
8.72
4.43
Ti3C1.6N0.4
21.47
44.17
5.72
15.70
9.16
3.79
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Table S2. XPS results of the contents of different nitrogen species in the Ti3C1.8N0.2 and Ti3C1.6N0.4
electrocatalysts.
Catalysts
N-Ti (at.%)
N-5 (at.%)
N-Q (at.%)
N-6 (at.%)
Ti3C1.8N0.2
52.46
41.20
6.33
-
Ti3C1.6N0.4
61.75
21.85
13.52
2.88
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Table S3. Simulated Rs and Rct and CPE values of the Ti3C2, Ti3C1.8N0.2 and Ti3C1.6N0.4 electrocatalysts.
Catalysts
Rs (ohm)
Rct (ohm)
CPE-T (ohm)
CPE-P (ohm)
Ti3C2
5.39
2692
0.00107
0.86828
Ti3C1.8N0.2
4.88
2113
0.0014
0.86554
Ti3C1.6N0.4
3.12
279.2
0.0031
0.96554
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References
1. Y. Tang, C. Yang, Y. Yang, X. Yin, W. Que and J. Zhu, Electrochim. Acta, 2019, 296, 762-770.
