Edited by: Yu Zhang, Hong Kong University of Science and Technology, Hong Kong
Reviewed by: Xiaoliang Yu, Hong Kong Polytechnic University, Hong Kong; Kun Rui, Nanjing Tech University, China
This article was submitted to Electrochemistry, a section of the journal Frontiers in Chemistry
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It is of significant necessity to explore inexpensive and high-active electrocatalysts toward hydrogen evolution reaction (HER) in both acidic and basic media. In this work, V-doped CoP nanosheet arrays supported on the carbon cloth (V-CoP/CC) are fabricated though a facile water-bath/phosphorization method. The nanoarray structure on the three-dimensional self-supporting electrode can provide a large electrochemical active surface area with more exposed active sites to accelerate the reaction kinetics. Furthermore, V doping is able to tune the electronic properties and thus enhance the intrinsic catalytic activity of CoP. Consequently, the V-CoP/CC electrode exhibits excellent electrocatalytic activities toward HER in both 0.5 M H2SO4 and 1 M KOH solutions with small overpotentials of 88 and 98 mV at a current density of 10 mA cm−2, respectively. The present work will offer a feasible way to tailor the catalytic activity by hetero-atoms doping toward HER.
Hydrogen is an effective energy carrier to solve the contemporary energy crisis and environmental pollution (Luo et al.,
The earth-abundant transitional metal-based materials, such as alloys (Zhang et al.,
Herein, the influence of V doping on the electrocatalytic activity of CoP toward HER is investigated and identified in both acidic and basic media. The V-doped CoP nanosheet arrays grown on carbon cloth (CoP/CC) have been prepared though a facile water bath/phosphorization approach. Owing to the 3D conductive CC substrate, nanosheet array structure, and the optimized electronic structure by V doping, CoP/CC exhibits much enhanced electrocatalytic HER activity with low overpotentials of 88 and 98 mV at 10 mA cm−2 in 0.5 M H2SO4 and 1 M KOH, respectively. The excellent performance indicates that V doping is an efficient strategy to improve the performance of HER electrocatalysts in both acidic and alkaline solutions.
Prior to synthesis, a piece of carbon cloth was treated in 6 M nitric acid at 85°C overnight. For the preparation of cobalt-based metal-organic frameworks on CC (CoMOF/CC), 2 mmol cobalt (II) nitrate hexahydrate [Co(NO3)2·6H2O] and 16 mmol 2-methylimidazole (2-mim) were dispersed in 40 ml deionized water (18.2 MΩ), respectively. Then the homogeneous aqueous solution of 2-mim was added into the Co(NO3)2 solution and a piece of treated CC was put into the above mixture for 2 h at room temperature. For the synthesis of V-Co layered double hydroxide (LDH), the as-prepared CoMOF was immersed into 50 ml water/ethanol solution (4:1 in volume) containing 100 mg sodium orthovanadate (Na3VO4), then it reacted at 50°C for 20 min. Finally, the V-Co LDH grown on the CC and 0.2 g NaH2PO2 placed at the middle of tube furnace, with NaH2PO2 at the upstream side, which were then heated to 350°C with a ramping rate of 2°C min−1 under Ar flow and kept for 2 h. The mass loading of the as-synthesized V-CoP is about 2.4 mg cm−2. For the preparation of hollow CoP, CoMOF was annealed for 1 h at 300°C with a heating rate of 1°C min−1 under air conditions and then the phosphidation procedure was similar to that of V-CoP. The mass loading of CoP is about 2 mg cm−2.
Field-emission scanning electron microscopy (SEM, FEI NanoSEM 450) and transmission electron microscopy (TEM, FEI Talos F200X) were used to study the morphology and structure of the as-synthesized samples. The phased structure was identified by X-ray diffraction (XRD, Shimadzu XRD-7000).
Electrochemical performance of the electrodes was tested on an electrochemical workstation (CHI 660E). The as-received samples and carbon rod were served as the working electrode and counter electrode, respectively. The reference electrodes were Hg/HgO electrode and saturated calomel electrode (SCE) in 1.0 M KOH (pH = 13.8) and 0.5 M H2SO4 (pH = 0.6), respectively. Polarization curves were tested by linear sweep voltammetry (LSV) at 2 mV s−1, and the potential values were calibrated to reversible hydrogen electrodes (RHE) with IR-correction, ERHE= EHg/HgO(SCE) + 0.097 + 0.059 pH. The IR-correction was conducted by ECorrected = ERaw - IRs, here, Rs corresponds to the series resistance, which can be obtained from the electrochemical impedance spectrum (EIS) measurements. The durability of the sample was studied by chronopotentiometry method at a constant current density of 10 mA cm−2 and cyclic voltammetry (CV) at a scan rate of 50 mV s−1 for 2,000 cycles. EIS was measured at −0.1 V vs. RHE in the frequency range between 0.1 Hz and 100 KHz with an AC amplitude of 5 mV. CV measurement was tested in the potential range from 0.15 to 0.25 V vs. RHE at various scan rates to estimate the electrochemical active surface area (ECSA).
The preparation process of the V-CoP/CC electrode is schematically illustrated in
To further investigate the microstructure, the V-CoP nanosheets are characterized by TEM imaging. As displayed in
The results of the above characterization indicate that V-doped CoP nanosheet arrays on the CC are successfully prepared. The electrocatalytic HER performance of the sample in the acidic solution is first investigated. As displayed in
HER performance of V-CoP/CC and CoP/CC in 0.5 M H2SO4.
The V-CoP/CC electrode has shown enhanced electrocatalytic performance for HER in acidic media. Meanwhile, water-alkali electrolyzer needs active and low-cost electrocatalysts that can work well in basic condition. As shown in
HER performance of V-CoP/CC and CoP/CC in 1 M KOH.
To evaluate the stability of V-CoP/CC in both acidic and basic solutions, long-term CV scanning is performed for 2,000 cycles. As displayed in
Polarization curves of V-CoP/CC initially and after 2,000 CV cycles
In summary, a novel V-CoP nanosheet array grown on the CC skeleton has been successfully synthesized though a facile method. The V doping poses a positive effect on the electrocatalytic performance for HER under both acidic and alkaline conditions. The optimized electronic property by V doping is of great benefit in increasing the intrinsic catalytic activity of CoP. In addition, the nanoarray architecture is of great benefit for offering more accessible catalytic active sites. As a result, the V-CoP/CC electrode only requires small overpotentials of 88 and 98 mV to reach the current density of 10 mA cm−2 in acidic and basic media, respectively. The present hetero-atom doping strategy would offer a new window for rationally designing high active catalysts toward HER.
All datasets generated for this study are included in the article/
WH carried out experiments and wrote the paper. HS performed and analyzed experimental results. LR carried out experiments. DN designed experiments and contributed to writing the paper. All authors contributed to the article and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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