N/P-doped NiFeV oxide nanosheets with oxygen vacancies as an efficient electrocatalyst for the oxygen evolution reaction

Abstract

Plasma treatment as an effective strategy can simultaneously achieve surface modification and heteroatom doping. Here, an N/P-doped NiFeV oxide nanosheet catalyst (N/P-NiFeVO) constructed by Ar/PH₃ plasma treatment is used to drive the oxygen evolution reaction (OER). The introduction of V species leads to the formation of an ultrathin ordered nanostructure and exposure of more active sites. Compared to the 2D NiFeV LDH, the prepared N/P-NiFeVO by plasma treatment possesses multiple-valence Fe, V and Ni species, which regulate the intrinsic electronic structure and enable a superior catalytic activity for the OER in alkaline media. Specifically, the N/P-NiFeVO only require an overpotential of 273 mV to drive the current density of 100 mA cm⁻². What's more, the electrode can maintain a stable current density in a long-term oxygen evolution reaction (∼120 h) under alkaline conditions. This work provides new insight for the rational design of mixed metal oxides for OER electrocatalysts.

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Introduction

Hydrogen is considered to be a clean energy source with high energy density that can solve the problem of depletion of fossil resources and rapid consumption of energy.^1–4 Electrochemical water splitting is considered to be one of the most efficient and sustainable strategies to produce hydrogen.^5,6 Unfortunately, the sluggish kinetics during the OER process seriously affects the energy conversion efficiency.^7,8 Noble metal Ru-based materials with best performance suffer from high cost and poor durability, which hinders development of renewable energy technologies.^9,10 Thus, it is of great importance to develop OER electrocatalysts with low cost and high activity.^11,12

In the last few decades, a great variety of transition metal-based catalysts with low cost and high activities have been widely investigated, especially layered double hydroxides (LDHs).^13–16 These materials are distinct 2D layer materials with excellent physicochemical characteristics, which makes it appropriate to design various hybrid materials with ion doping to improve the electrochemical activities. However, the OER activity of LDHs is limited by the low number of active sites at the edges and weak conductivity. Plasma treatment has been considered an efficient strategy to accelerate the OER process.^17–19 It is found that surface etching and heteroatom doping can be achieved simultaneously under certain conditions, which can not only increase the active surface area, but also regulate the energy barrier during the OER process. Wang et al. reported a CoFe LDH and Co₃O₄ nanosheets with oxygen vacancies, which were created using plasma in Ar/water vapour. It is observed that the CoFe LDH and Co₃O₄ nanosheet catalysts exhibit lower overpotential values, meanwhile the Tafel value is only 68 mV dec⁻¹.²⁰ These results provide strong explanation for the successful improvement of OER performance. Liang et al. fabricated NiCoP nanosheets through Ar/PH₃ plasma.²¹ The prepared nanosheets showed a lower potential value of 273 mV at a current density of 100 mA cm⁻².

Herein, a novel approach is designed to fabricate an N/P-decorated Ni–Fe–V oxide electrocatalyst (N/P-NiFeVO). Facile preparation was realized by plasma treatment, which provided a greatly distinctive nanosheet structure, which was obtained from NiFeV-LDH precursors. The obtained NiFeV LDH with plasma treatment shows N/P doping and the formation of oxygen vacancies, which can largely enhance the electrochemical performance by providing a larger electrochemically active surface area and increased active sites. As an OER electrode, N/P-NiFeVO exhibited a low overpotential (273 mV at 100 mA cm⁻²) and an ultra-small Tafel value (34 mV dec⁻¹) in 1 M KOH aqueous electrolyte. And it showed a stable catalytic activity with a high current density of 100 mA cm⁻² for 120 h. This work sheds light on the preparation of high efficiency electrocatalysts.

Results and discussion

Scheme 1 is a schematic diagram of the synthesis procedure of N/P-NiFeVO. A one-step hydrothermal method is first carried out to synthesize a V-doped NiFe LDH on Ni foam (denoted as NiFeV LDH). Then, a nanostructure consisting of N/P-NiFeVO nanosheets with rich O vacancies is constructed via the following N₂/PH₃ plasma reduction step. As revealed in Fig. S2,† an optimum hydrothermal reaction time can result in the best OER performance. The NiFeV LDH-3 h precursor presents a smaller overpotential, lower Tafel slope and optimum charge transfer resistance than the other as-prepared samples. Consequently, the precursor prepared through a hydrothermal reaction of 3 h was chosen as the optimum sample.

Scheme 1 Schematic illustration of the synthesis procedure of N/P-NiFeVO@NF.

X-ray diffraction (XRD) spectroscopy was then employed to investigate the phase structure of the NiFeV LDH precursor and N/P-NiFeVO. The diffraction peaks in Fig. 1a can be well assigned to the NiFe LDH (JCPDS no. 26-1286). After N₂ plasma treatment, all peaks in Fig. 1b from the NiFeV LDH vanish and the new diffraction peaks can be ascribed to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of cubic NiFe₂O₄ (JCPDS no. 54-0964), demonstrating successful conversion from the LDH to the NiFe₂O₄ phase. The peaks appearing at 36.9° can be attributed to the (111) plane of VP₂ (JCPDS: 30-1426). Additionally, the additional peak appearing at 40.7° can be attributed to the (111) plane of Fe₂P (JCPDS: 51-0943) and Ni₂P (JCPDS: 03-0953).

Fig. 1 XRD patterns of NiFeV-LDH (a) and N/P-NiFeVO (b).

The initial NiFeV LDH and N/P-NiFeVO nanosheets mentioned above were synthesized and the surface morphology was characterized by scanning electron microscopy (SEM) as shown in Fig. 2a and b and transmission electron microscopy (TEM) as shown in Fig. 2c–g. The N/P-NiFeVO sample shows a similar nanosheet morphology, and plasma etching was employed to introduce more oxygen vacancies into the precursor. The TEM images revealed a relatively rough surface with small irregular pores on the ultrathin nanosheets, relative to the smooth surface of the pristine NiFeV LDH. The high-resolution TEM (HRTEM, Fig. 2d–g) images indicate the lattice fringe spacing of 0.15 nm and 0.25 nm consistent with the (440) and (311) planes characteristic of NiFe₂O₄ nanosheets. However, the 0.22 nm and 0.24 nm lattice fringes at the edges are attributed to the (111) and (111) planes of Ni₂P/Fe₂P (Fig. 2f and g), implying that the plasma etching preserves the P species. Furthermore, the selected area electron diffraction (SAED) pattern of N/P-NiFeVO can be well indexed to all the planes of Ni₂P and Fe₂P (Fig. 2h). The energy-dispersive X-ray spectroscopy (EDX) mapping images of N/P-NiFeVO show an atomic-level distribution of Ni, Fe, V, O, N and P in the whole nanosheet, manifesting the successful fabrication of N/P-NiFeVO.

Fig. 2 SEM (a) and HRTEM (c) images of the NiFeV LDH; (b) SEM and HRTEM (d–g) images of N/P-NiFeVO; (h) SAED pattern and the corresponding EDS elemental mapping of N/P-NiFeVO.

X-ray photoelectron spectroscopy (XPS) was performed to further investigate the chemical environment and valence states of the NiFeV LDH and N/P-NiFeVO. For the NiFeV LDH, the XPS spectra of Ni 2p can be fitted with two distinct doublets (2p_3/2 and 2p_1/2). The peaks at 852.3 and 869.2 eV can be ascribed to the metal Ni, while the peaks at about 855.6 and 873.7 eV correspond to Ni²⁺.^22,23 Regarding N/P-NiFeVO, the characteristic peaks at 856 and 870.3 eV in the Ni 2p spectrum indicate the presence of Ni⁺,²⁴ as shown in Fig. 3a. In addition, after plasma treatment, the appearance of Fe²⁺ and Fe⁰ peaks confirms the partial reduction of the sample by PH₃ plasma²⁵ (Fig. 3b). In Fig. 3c, in comparison with the NiFeV LDH, the binding energy of V 2p of N/P-NiFeVO shifts to a lower binding energy, and the content of V³⁺ notably increases, which results from the reduction effect of PH₃ plasma.²⁶ In Fig. 3d, the peaks at 529.8, 531.3 and 532.4 eV are assigned to the M–O, oxygen vacancies and oxygen of adsorbed water, respectively.²⁷ The intensities of oxygen vacancies and M–O of N/P-NiFeVO are enhanced compared to those of the NiFe LDH, indicating that the plasma etching employed introduced more oxygen vacancies into N/P-NiFeVO. The spectra of N 1s in Fig. 3e reveal the information about the valence states and chemical compositions. The N 1s spectra can be fitted with three peaks at 397.3, 398.8 and 401.1 eV, which are ascribed to V–N, Ni (Fe)–N, and N–O, respectively.^28,29 In the P 2p spectrum (Fig. 3f), the peaks at 129.1 and 129.9 eV correspond to P 2p_3/2 and P 2p_1/2 of metal phosphide, and the peak at 133.4 eV is associated with P–O.³⁰

Fig. 3 High-resolution XPS spectra of N/P-NiFeVO and NiFeV LDH: (a) Ni 2p, (b) Fe 2p, (c) V 2p, (d) O 1s, (e) N 1s and (f) P 2p.

Subsequently, the OER performances of N/P-NiFeVO and derived samples were evaluated in 1 M KOH aqueous electrolyte using a standard three-electrode configuration (Fig. 4a). All potentials were iR-compensated and converted to a reversible hydrogen electrode (RHE) scale. As expected, compared with P-NiFeVO, N/P-VO, N/P–Ni Fe₂O₃, NiFeV LDH and IrO₂, N/P-NiFeVO exhibited the lowest overpotential at the same current density (273 mV at 100 mA cm⁻²) and an ultra-small Tafel value (34 mV dec⁻¹), suggesting that the plasma etched electrocatalyst with ultrathin nanosheets can serve as a good electrocatalytically active species for the OER (Fig. 4b). Noteworthily, such a low overpotential is superior to those of many OER heterostructured catalysts (Table S1†). In addition, the electrochemically active area (ECSA) was calculated from cyclic voltammetry (Fig. 5) at different scan rates from 20 to 120 mV s⁻¹ within the non-faradaic potential range. It is clear that the ECSA of N/P-NiFeVO nanosheets with plasma treatment is larger than that of the NiFeV LDH material. In other words, the surface reconstruction by plasma treatment does not compromise the high specific surface of the original catalyst. As shown in Fig. 5f, the C_dl of N/P-NiFeVO is 3 mF cm⁻², noticeably larger than that of the NiFeV LDH and its derivatives (1.5 mF cm⁻²), which indicates that more active sites were generated. To gain more insights into the intrinsic activity for the OER, the LSV polarization curves were normalized with ECSA. As depicted in Fig. S4,† N/P-NiFeVO possesses superior intrinsic catalytic activity compared to the other catalysts.^31,32 It can be inferred that N/P-NiFeVO constructed by plasma treatment has more exposed active sites, which enable its remarkable catalytic activities.^33,34

Fig. 4 OER electrocatalytic performance of various electrodes in 1 M KOH. (a) LSV curves (with iR correction) at a scan rate of 2 mV s⁻¹; (b) Tafel slopes; (c) Nyquist plots; inset: the equivalent circuit used for data analyses. (d) The chronoamperometric curve of N/P-NiFeVO at a constant current density of 100 mA cm⁻² over 120 h.

Fig. 5 Cyclic voltammograms at different scan rates (20–200 mV s⁻¹) in the potential range of 0.9–1.0 V vs. RHE for the various electrocatalysts: (a) N/P-NiFeVO, (b) P-NiFeVO, (c) N/P-VO, (d) N/P–Ni-Fe₂O₃, and (e) NiFeV LDH. (f) Difference in current density plotted against the scan rate for various samples showing the extraction of the double-layer capacitances.

At the same time, its stability is also outstanding, showing good stability up to 120 h with little change at a constant current density of 100 mA cm⁻² (Fig. 4d). Further SEM images of the post-OER catalyst show a rougher structure with irregular clusters on nanosheets (Fig. S5†). In the surface oxidation under anodic voltage, the morphology of the catalyst was largely retained compared to that of the as-prepared sample. After the OER test, the high-resolution XPS spectra of Ni 2p (Fig. S6a†) show two peaks ascribed to Ni²⁺, while the Ni⁰ peak completely disappeared. In addition, the only remaining Fe³⁺ peak confirms the complete oxidation of Fe⁰ and Fe²⁺ (Fig. S6b†). In Fig. S6c,† the peaks at 530.3, 531.3 and 532.6 eV are assigned to the M–O, oxygen vacancies and oxygen of adsorbed water, respectively.^35,36 Fig. S6d† shows peaks at 515.4, 516.2 and 517.1 eV, which are assigned to V³⁺, V⁴⁺and V⁵⁺. Compared to those of N/P-NiFeVO, the intensities of V–N, Ni (Fe)–N and N–O of the post-OER catalyst weakened and the P 2p peak completely disappeared, which is due to the intense oxidation process (Fig. S6e and f†).

To evaluate the durability of N/P-NiFeVO in alkaline media, we assembled a two-electrode system simulating a water splitting cell using Pt/C as a hydrogen evolution catalyst and N/P-NiFeVO as an oxygen evolution catalyst. As shown in Fig. 6a, the cell voltages in alkaline media of Pt/C∥N/P-NiFeVO and Pt/C∥IrO₂ are 1.56 V@10 mA cm⁻² and 1.62 V@10 mA cm⁻², respectively. Moreover, for the Pt/C∥N/P-NiFeVO, the cell voltage can remain at a similar level even after continuous working for 120 h. Such a performance for water electrolysis is better than that of most recently reported NiFe-based electrocatalysts (Table S2†). Overall, these results clearly indicate that N/P-NiFeVO is indeed a competent and persistent OER catalyst.

Fig. 6 (a) Polarization curves (without iR correction) of Pt/C@NF//N/P-NiFeVO@NF and Pt/C@NF//IrO₂@NF at a scan rate of 2 mV s⁻¹ for overall water splitting; the inset shows the photograph of the electrolyzer powered by a battery of 1.56 V. (b) The chronoamperometric curve of the Pt/C@NF//N/P-NiFeVO@NF cell at a constant current density of 10 mA cm⁻² over 120 h.

Conclusion

In summary, a well-aligned N/P-doped NiFeVO oxide nanosheet catalyst was prepared by Ar/PH₃ plasma treatment of the NiFe LDH. Plasma treatment not only leads to the transformation of the NiFe LDH to NiFeVO nanosheets, but also induces the formation of oxygen vacancies. Accordingly, the N/P-doped NiFeVO nanosheets with a high specific surface area and plentiful pores provide unobstructed channels for electrolyte ion diffusion and boosted charge transfer kinetics. These advantages endow the N/P-doped NiFeVO electrode with outstanding electrochemical properties. This work shows that tuning the synthesis conditions and composition of mixed metal oxides is an effective strategy to enhance the catalytic performance for the OER.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support from the Natural Science Foundation of Shanghai (21ZR1445700) and the Science and Technology Commission of Shanghai Municipality (23440790400) is gratefully acknowledged.

References

1. L. Xu, S. Wu, X. He, H. Wang, D. Deng, J. Wu and H. Li, Chem. Eng. J., 2022, 437, 135291.
2. P. Erickson, M. Lazarus and G. Piggot, Nat. Clim. Change, 2018, 8, 1037–1043.
3. F. P. Perera, Environ. Health Perspect., 2017, 125, 141–148.
4. R. Gao and D. P. Yan, Adv. Energy Mater., 2020, 10, 1900954.
5. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. G. Nocera, Chem. Rev., 2010, 110, 6474–6502.
6. R. Gao, H. Zhang and D. P. Yan, Nano Energy, 2017, 31, 90–95.
7. N. T. Suen, S. F. Hung, Q. Quan, N. Zhang, Y. J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365.
8. M. Arif, G. Yasin, M. Shakeel, M. A. Mushtaq, W. Ye, X. Y. Fang, S. F. Ji and D. P. Yan, Mater. Chem. Front., 2019, 3, 520–531.
9. D. Chen, R. Lu, Z. Pu, J. Zhu, H. W. Li, F. Liu, S. Hu, X. Luo, J. Wu, Y. Zhao and S. Mu, Appl. Catal., B, 2020, 279, 119396.
10. Y. Lee, J. Suntivich, K. J. May, E. E. Perry and Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 3, 399–404.
11. Q. Q. Ji, Y. Kong, H. Tan, H. L. Duan, N. Li, B. Tang, Y. Wang, S. H. Feng, L. Y. Lv, C. Wang, F. C. Hu, W. H. Zhang, L. Cai and W. S. Yan, ACS Catal., 2022, 12, 4318–4326.
12. S. Jin, ACS Energy Lett., 2017, 2, 1937.
13. G. F. Chen, T. Y. Ma, Z. Q. Liu, N. Li, Y. Z. Su, K. Davey and S. Z. Qiao, Adv. Funct. Mater., 2016, 26, 3314–3323.
14. J. Yu, T. A. Le, N. Q. Tran and H. Lee, Chem. – Eur. J., 2020, 26, 6423–6436.
15. C. Hu, L. Zhang and J. Gong, Energy Environ. Sci., 2019, 12, 2620–2645.
16. S. Dresp, F. Dionigi, M. Klingenhof, T. Merzdorf, H. Schmies, J. Drnec, A. Poulain and P. Strasser, ACS Catal., 2021, 11, 6800–6809.
17. F. Song and X. Hu, Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis, Nat. Commun., 2014, 5, 4477.
18. L. Ma, I. Jia, X. Guo and L. Xiang, Chin. J. Catal., 2014, 35, 108–119.
19. A. U. Haq, S. Askari, A. McLister, S. Rawlinson, J. Davis, S. Chakrabarti, V. Svrcek, P. Maguire, P. Papakonstantinou and D. Mariotti, Nat. Commun., 2019, 10, 1–8.
20. R. Liu, Y. Wang, D. Liu, Y. Zou and S. Wang, Adv. Mater., 2017, 29, 1–7.
21. H. Liang, A. N. Gandi, D. H. Anjum, X. Wang, U. Schwingenschlögl and H. N. Alshareef, Nano Lett., 2016, 16, 7718–7725.
22. A. L. Wang, H. Xu and G. R. Li, ACS Energy Lett., 2016, 1, 445–453.
23. H. J. Zhang, X. P. Li, A. Hähnel, V. Naumann, C. Lin, S. Azimi, S. L. Schweizer, A. W. Maijenburg and R. B. Wehrspohn, Adv. Funct. Mater., 2018, 28, 1706847.
24. P. Zhou, D. N. Xing, Y. Y. Liu, Z. Y. Wang, P. Wang, Z. K. Zheng, X. Y. Qin, X. Y. Zhang, Y. Dai and B. B. Huang, J. Mater. Chem. A, 2019, 7, 5513–5521.
25. X. L. Liu, X. S. Lv, P. Wang, Q. Q. Zhang, B. B. Huang, Z. Y. Wang, Y. Y. Liu, Z. K. Zheng and Y. Dai, Electrochim. Acta, 2020, 333, 135488.
26. Y. M. Hu, Z. L. Wang, W. J. Liu, L. Xu, M. L. Guan, Y. P. Huang, Y. Zhao, J. Bao and H. M. Li, ACS Sustainable Chem. Eng., 2019, 7, 16828–16834.
27. H. H. Lv, X. B. Zhang, F. M. Wang, G. J. Lv, T. T. Yu, M. L. Lv, J. W. Wang, Y. Zhai and J. Q. Hu, J. Mater. Chem. A, 2020, 8, 14287–14298.
28. M. L. Yan, K. Mao, P. X. Cui, C. Chen, J. Zhao, X. Z. Wang, L. J. Yang, H. Yang, Q. Wu and Z. Hu, Nano Res., 2020, 13, 328–334.
29. S. S. Tan, Y. H. Dai, Y. L. Jiang, Q. L. Wei, G. B. Zhang, F. Y. Xiong, X. Q. Zhu, Z. Y. Hu, L. Zhou, Y. C. Jin, K. Kanamura, Q. Y. An and L. Q. Mai, Adv. Funct. Mater., 2020, 2008034.
30. M. J. Guo, S. Y. Song, S. S. Zhang, Y. Yan, K. Zhan, J. H. Yang and B. Zhao, ACS Sustainable Chem. Eng., 2020, 8, 7436–7444.
31. N. Li, L. Cai, C. Wang, Y. Lin, J. Z. Huang, H. Y. Sheng, H. B. Pan, W. Zhang, Q. Q. Ji, H. L. Duan, W. Hu, W. H. Zhang, F. C. Hu, H. Tan, Z. H. Sun, B. Song, S. Jin and W. S. Yan, J. Am. Chem. Soc., 2021, 143, 18001–18009.
32. N. Li, L. Cai, G. P. Gao, Y. Lin, C. Wang, H. J. Liu, Y. Y. Liu, H. L. Duan, Q. Q. Ji, W. Hu, H. Tan, Z. M. Qi, L. W. Wang and W. S. Yan, Nano Lett., 2022, 22, 6988–6996.
33. Y. Yan, C. Liu, H. Jian, X. Cheng, T. Hu, D. Wang, L. Shang, G. Chen, P. Schaaf, X. Wang, E. Kan and T. Zhang, Adv. Funct. Mater., 2021, 31, 2009610.
34. Z. Wang, B. Xiao, Z. Lin, Y. Xu, Y. Lin, F. Meng, Q. Zhang, L. Gu, B. Fang, S. Guo and W. Zhong, Angew. Chem., Int. Ed., 2021, 60, 23388–23393.
35. Z. Xu, Y. Ying, G. Zhang, K. Li, Y. Liu, N. Fu, X. Guo, F. Yu and H. Huang, J. Mater. Chem. A, 2020, 8, 26130.
36. S. Klaus, Y. Cai, M. W. Louie, L. Trotochaud and A. T. Bell, J. Phys. Chem. C, 2015, 119, 7243.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt00943f

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