CoP₂/Fe-CoP₂ yolk–shell nanoboxes as efficient electrocatalysts for the oxygen evolution reaction

Abstract

The development of an efficient electrocatalyst is an important requirement for water splitting systems to produce clean and sustainable hydrogen fuel. Herein, we synthesized CoP₂/Fe-CoP₂ yolk–shell nanoboxes (YSBs) as efficient electrocatalysts for the oxygen evolution reaction (OER). Initially, zeolitic imidazolate framework-67/CoFe-Prussian blue analogue (ZIF-67/CoFe-PBA) YSBs were prepared by the reaction of ZIF-67 and [Fe(CN)₆]³⁻ ions in the presence of a small amount of water as an etching agent. The size of the CoP₂ yolk depends on the amount of water. The heteronanostructure composed of the CoP₂ yolk and the Fe_xCo_1−xP₂ shell with a cubic shape was obtained by phosphidation of ZIF-67/CoFe-PBA YSBs. Benefiting from the unique structure and chemical composition, the CoP₂/Fe-CoP₂ YSB electrocatalyst has a large specific surface area of 114 m² g⁻¹ and shows superior electrocatalytic performances for the OER such as a low overpotential of 266 mV, a small Tafel slope value of 68.1 mV dec⁻¹, and excellent cyclic stability.

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1. Introduction

Electrochemical water splitting is a sustainable and environmentally friendly approach to producing clean hydrogen fuel.¹ The development of active electrocatalysts for the oxygen evolution reaction (OER) is critical to promoting commercial applications of hydrogen fuel cells. The OER, the half reaction of the water splitting reaction at the anode, is a four-electron transfer reaction, which requires high energy to overcome the kinetic barrier of the reaction. Therefore, in the past several decades, many efforts have been devoted to develop an efficient OER catalyst to improve electrode kinetics and stability in acid and alkaline solutions.^2–4 RuO₂ and IrO₂ catalysts are known to be the most active catalysts for the OER.^5,6 However, their high cost and low natural abundance limit their commercial use.⁷ Alternatively, transition metal oxides, chalcogenides, carbides, nitrides and phosphides have been explored to replace the precious metal based OER catalysts.^8–13

Transition metal phosphides have recently emerged as Earth-abundant catalysts for the overall water splitting process.¹⁴ In particular, cobalt phosphides with various stoichiometric ratios (CoP, Co₂P, CoP₂) have attracted much attention in the research field of the OER.^15–17 According to the recent theoretical and experimental studies, the catalytic activity of CoP₂ is superior to those of CoP and Co₂P.¹⁸ The positively charged cobalt ions serve as hydroxyl receptors and the negatively charged P atoms facilitate the desorption of O₂ molecules. Wang et al. investigated for the first time the electrocatalytic OER performance of CoP₂ nanoparticles on reduced graphene oxide sheets.¹⁹ Furthermore, the use of bimetallic phosphides obtained by the doping of other transition metal atoms improves the electrocatalytic performance by optimizing the electronic structure and surface energy of the catalysts.²⁰

Morphology control is another effective way to further improve the electrochemical performances of the catalysts.^21–25 Among hollow nanostructures, the yolk–shell structure has been found to be one of the most efficient structures for electrocatalysts.²⁶ The large specific surface area of hollow yolk–shell structures can provide more active sites for redox reactions. Yin et al. reported the synthesis of Ni_xCo_1−xP yolk–shell spheres showing high activity and stability for overall water splitting.²⁷ Metal–organic frameworks (MOFs) can provide a variety of possibilities to prepare unique hollow materials with well-defined interior voids, a large surface area, a low density, and abundant active sites.^28,29 He et al. reported MOF-derived synthesis of carbon-incorporated nickel–cobalt mixed metal phosphide (NiCoP/C) nanoboxes by a reaction of ZIF-67 nanocubes with Ni(NO₃)₂.³⁰ Furthermore, Prussian blue analogues (PBAs) have emerged as useful precursors for the synthesis of hollow nanostructures for energy storage and conversion applications.^31–34

Herein, we demonstrate the rational design and synthesis of CoP₂/Fe-CoP₂ yolk–shell nanoboxes by the formation of the cobalt iron ferricyanide (CoFe-PBA) structure on sacrificial ZIF-67. CoP₂/Fe-CoP₂ YSBs exhibited excellent electrocatalytic activity for the OER, in terms of low overpotential and low Tafel slope values with long time cyclic stability.

2. Experimental

2.1. Materials

Co(NO₃)₂·6H₂O (98%), 2-methylimidazole (99%), cetyltrimethylammonium bromide (CTAB) (98%), K₃[Fe(CN)₆] (99%), Nafion solution (5 wt%) and NaH₂PO₂ (99.99%) (Sigma-Aldrich). Ethanol and propanol (Samchun Chemicals).

2.2. Synthesis of ZIF-67/CoFe-PBA YSBs

ZIF-67 nanocube (NC) particles were prepared according to a previous report.³⁵ A solution of 44 mg of ZIF-67 NCs in 30 mL of ethanol was sonicated for 10 minutes and mixed with a solution of 100 mg of K₃[Fe(CN)₆] in 10 mL of water at 50 °C. After stirring for 1 hour, the solid product was separated, washed several times with ethanol, and dried under reduced pressure at room temperature. To prepare ZIF-67/CoFe-PBA NBs, 15 mL of water was used in the reaction mixture.

2.3. Synthesis of CoP₂/Fe-CoP₂ YSBs

The solid sample of ZIF-67/CoFe-PBA YSBs (30 mg) was heated at 300 °C with a heating rate of 2 °C min⁻¹ in the presence of NaH₂PO₂ (150 mg) under an Ar gas flow for 2 h to afford the product. The same process was applied to prepare Fe-CoP₂ nanoboxes (NBs).

2.4. Characterization

X-ray diffraction (XRD) measurements were performed to characterize the crystal structure of the product (Rigaku, X-MAX 2000-PC). The morphology and composition of the synthesized nanocrystals were characterized by field emission scanning electron microscopy (FESEM, Hitachi HF-4800), transmission electron microscopy (TEM), and high-resolution TEM (JEM-ARM200F, JEOL, acceleration voltage: 200 kV). The surface area and pore size distribution were determined using an ASAP-2420 (Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) was conducted to characterize the chemical and valency states of the products.

2.5. Electrochemical measurements

Electrochemical investigation of the products was conducted in a three electrode configuration in 1 M KOH using PGSTAT 302N Autolab (Metrohm) – the working electrode (glassy carbon of 3 mm diameter), reference electrode (Ag/AgCl; saturated KCl) and counter electrode (graphite rod). 2 mg of catalyst was dispersed in 200 μL solution (isopropanol: 190 μL and 5 wt% Nafion: 10 μL) and sonicated for 20 min. Then, 2 μL of the catalyst ink was loaded on the glassy carbon and then dried at ambient temperature. The average loading of the catalyst was about 0.21 mg cm⁻². The stability test was performed at a constant overpotential (vs. RHE). Cyclic voltammetry (CV, scan rate: 10 mV s⁻¹) was conducted to measure the double-layer capacitance (C_dl). Impedance analyses were performed in the frequency range of 0.1 Hz to 100 Hz (ZIVA-MP2A).

3. Results and discussion

The synthesis procedure of CoP₂/Fe-CoP₂ YSBs is schematically illustrated in Scheme 1. ZIF-67 nanocrystals with the average size of 500 nm were synthesized by a surfactant assisted hydrothermal method (Fig. S1†).³⁵ Cobalt ferricyanide (CoFe-PBA) was formed on the sacrificial ZIF-67 NCs by the reaction of Fe(CN)₆³⁻ anions and Co₂⁺ cations released from ZIF-67 NCs to form the Co-ZIF/CoFe-PBA yolk–shell structure. CoP₂/Fe-CoP₂ YSBs were obtained by direct phosphidation under an anaerobic atmosphere.

Scheme 1 Schematic representation for the synthesis of CoP₂/Fe-CoP₂ YSB catalysts.

Fig. 1 shows the FESEM and TEM images of ZIF-67/CoFe-PBA YSBs with the average size of ∼500 nm. Fig. 1b shows the yolk–shell particle composed of a solid core and an outer shell comprising small nanograins. According to the elemental mapping analysis, the Co element is uniformly distributed in both the yolk and shell, whereas Fe is observed only in the outer shell of the nanobox (Fig. 1c). Fig. S2† shows the XRD pattern of ZIF-67/CoFe-PBA YSBs. Along with the XRD peaks of ZIF-67, two characteristic peaks were also observed at 25.0° and 35.5° due to the (002) and (400) planes of Co–Fe PBA, which proves the formation of the hybrid MOF structure.

Fig. 1 (a) SEM and (b) TEM images of ZIF-67/CoFe-PBA YSBs. (c) Corresponding mapping images (scale bar: 200 nm).

Water promotes the ion exchange reaction of ZIF-67 NCs and K₃[Fe(CN)₆]. The morphology of YSBs can be controlled by changing the water/ethanol ratio. In a condition of 5 mL water and 25 mL ethanol, the size of ZIF-67 crystals reduces slightly and the CoFe-PBA shell begins to appear (Fig. 2a). Increasing the amount of water to 10 mL makes the ZIF-67 cube smaller and results in a cubic yolk–shell structure (Fig. 2b). The ZIF-67 yolk disappears by adding more water (Fig. 2c). Water dissolves ZIF-67 crystals and the released Co₂⁺ cations react with [Fe(CN)₆]³⁻ anions to form a cubic shell, i.e., the etching of ZIF-67 crystals and the formation of Co–Fe PBA occur simultaneously. The shrinkage and collapse of the nanobox structure were observed with 20 mL of water (Fig. 2d).

Fig. 2 TEM images observed under different reaction conditions. (a) 5 mL (b) 10 mL (c) 15 mL and (d) 20 mL of water.

CoP₂/Fe-CoP₂ YSBs were synthesized by a direct phosphidation process. Fig. 3 shows the XRD pattern of the product and the main diffraction peaks can be assigned to monoclinic CoP₂ (JCPDS no. #77-0263).

Fig. 3 XRD pattern of CoP₂/Fe-CoP₂ YSBs.

The shape and size of the yolk–shell structure were retained without noticeable changes even after phosphidation with the average size of 500 nm (Fig. 4a). Broken particles clearly show that the outer shell of a particle is comprised of tiny nano-sized particles (Fig. 4b). Fig. 4c and d show the TEM images of yolk–shell particles, which are in full consistency with the SEM results. The lattice fringe spacing of 0.25 nm corresponds to the (200) plane of CoP₂ (Fig. 4e). STEM mapping analysis reveals that Co, Fe, and P elements are evenly distributed. Co and P are found throughout the particle, but Fe is found at the outer shell (Fig. 4f). The elemental composition of Fe/Co/P was observed to be 0.27 : 0.73 : 2, being consistent with the stoichiometry of transition metal diphosphides (MP₂) (Fig. S3†). Furthermore, the relative amounts of CoP₂ and Fe-CoP₂ of CoP₂/Fe-CoP₂ YSBs could be estimated by measuring the dimensions of a single hybrid particle. The calculated mass percentages of Fe-CoP₂ and CoP₂ are about 61% and 39%, respectively, suggesting that the atomic ratio of Fe and Co is about 1 : 1.26.

Fig. 4 (a and b) SEM and (c and d) TEM images of CoP₂/Fe-CoP₂ YSBs. (e) Corresponding HR-TEM image and (f) STEM mapping images (scale bar: 100 nm).

The Brunauer–Emmett–Teller (BET) specific surface area was found to be 114 m² g⁻¹ with a pore size distribution of 11–19 nm, indicating that the yolk–shell particles have a large surface area and are of mesoporous nature (Fig. 5a). The chemical valence states of the elements were characterized by XPS analyses. The spin–orbit doublets of Co 2p_3/2 and Co 2p_1/2 observed at 781.2 and 794.1 eV, respectively, in the Co 2P spectrum (Fig. 5b) can be consistent with those of the spectrum of FeCoP₂ nanowires.³⁶ Additional doublets at 786.2 and 803.8 eV are attributed to the oxidized Co.^36,37 The Fe 2p spectrum (Fig. 5c) exhibits two distinct peaks located at 708.9 and 722.1 eV, reflecting the binding energies of Fe²⁺ 2p_3/2 and Fe²⁺ 2p_1/2, respectively. The second doublets at 711.5 eV and 724.5 eV are due to Fe³⁺.^36,37Fig. 5d shows the P 2p spectrum consisting of two characteristic peaks at 129.1 eV for P 2p_3/2 and a relatively broader peak at 133.7 eV for the P–O species due to surface oxidation.³⁸

Fig. 5 (a) Nitrogen adsorption–desorption isotherm (the inset shows the pore size distribution curve) for CoP₂/Fe-CoP₂ YSBs. (b) High-resolution XPS spectra of Co 2p, (c) Fe 2p and (d) P 2p for CoP₂/Fe-CoP₂ YSBs.

The TEM images of Fe-CoP₂ NBs are shown in Fig. 6. The surface of the nanoboxes is composed of small nanograins. The obtained lattice fringe value of 0.25 nm corresponds to the (2 0 0) plane of CoP₂ (Fig. 6b). The XRD pattern and elemental composition of Fe-CoP₂ NBs are shown in Fig. S4 and S5.† The obtained diffraction peaks are well matched with monoclinic CoP₂ (JCPDS no. #77-0263). The elemental composition of Fe/Co/P of the Fe-CoP₂ nanobox is almost similar to that of CoP₂/Fe-CoP₂ YSBs (Fig. S5†). In addition, the elemental mapping image (Fig. 6c) shows that Co and P elements are relatively evenly distributed on the entire surface of the image, whereas Fe accumulated on the edge of the image. This suggests that the dissolved Co cations from the core part were deposited on the inner surface of the hollow box. Therefore, the total elemental ratio of Fe/Co/P does not seem to depend on the structural change during the reaction. Furthermore, CoP₂ nanocubes were synthesized by the phosphidation of ZIF-67 NCs for the comparison of the eletrocatalytic activity with that of yolk–shell particles. The XRD pattern and FESEM image of CoP₂ nanocubes are provided in Fig. S6.†

Fig. 6 (a) TEM image and (b) HR-TEM image of Fe-CoP₂ NBs. (c) Corresponding mapping images (scale bar: 100 nm).

The polarization curve in Fig. 7a shows that CoP₂/Fe-CoP₂ YSBs exhibit a low overpotential (η₁₀) of 266 mV, whereas Fe-CoP₂ NBs need a slightly high overpotential (η₁₀) of 320 mV. These values are much lower than those of the IrO₂ benchmark catalyst (380 mV), CoP₂ NCs (360 mV), and ZIF-67/CoFe-PBA YSBs (460 mV). However, the yolk–shell nanobox catalyst exhibits a high current density of 50 mA cm⁻² and 100 mA cm⁻² at an overpotential (η₁₀) of 350 and 410 mV, respectively, which are higher than other catalysts. Additionally, the overpotential of CoP₂/Fe-CoP₂ YSBs is also significantly lower than the reported values of other catalysts, including NiCoP/C nanoboxes (300 mV),³⁰ Co_0.6Fe_0.4P (298 mV),³² AlCoP (330 mV),³⁹ CoP/carbon dots (400 mV),⁴⁰ (Co_0.54Fe_0.46)₂P (370 mV),⁴¹ Ni_0.6Co_1.4P nanocages (300 mV),⁴² Fe–Co–P nanoboxes (269 mV),⁴³ and hollow FeCo₂P (320 mV).⁴⁴ As shown in Fig. 7b, the measured Tafel slope of CoP₂/Fe-CoP₂ YSBs (61.8 mV dec⁻¹) is lower than those of Fe-CoP₂ NBs (68.9 mV dec⁻¹), CoP₂ NCs (71.4 mV dec⁻¹), IrO₂ (79.5 mV dec⁻¹) and ZIF-67/CoFe-PBA YSBs (88.2 mV dec⁻¹), implying that the CoP₂/Fe-CoP₂ YSB catalyst has faster OER kinetics. Fig. 7c shows the overpotential and the corresponding Tafel slope values of the catalysts. EIS measurement was carried out to demonstrate the charge transfer resistance (R_ct) of the catalysts (Fig. 7d). CoP₂/Fe-CoP₂ YSBs exhibited a lower R_ct value (18.3 Ω) than Fe-CoP₂ NBs (33.1 Ω), CoP₂ NCs (40.1 Ω), and ZIF-67/CoFe-PBA YSBs (57.8 Ω). The higher electrical conductivity of CoP₂/Fe-CoP₂ YSBs might be due to the hybrid yolk–shell structure, which promotes efficient charge transfer, prevents aggregation during the electrochemical reaction, and generates strong couple effects that result in enhanced electrocatalytic activity.^32,45

Fig. 7 (a) Linear sweep voltammetry curves of different electrocatalysts. (b) Corresponding Tafel slopes. (c) The required overpotential to achieve a current density of 10 mA cm⁻² and Tafel slope values for different catalysts. (d) Corresponding EIS Nyquist plots.

The durability test was performed for CoP₂/Fe-CoP₂ YSBs using CV at a fixed scan rate of 10 mV s⁻¹ (Fig. 8a). A slight overpotential increase (11 mV) was observed for CoP₂/Fe-CoP₂ YSBs, whereas Fe-CoP₂ NBs showed an overpotential increase of 17 mV after 3000 cycles (Fig. S7†). The chronoamperometry test was conducted at a static potential of 1.50 V (vs. RHE) (Fig. 8b). The current density of the CoP₂/Fe-CoP₂ YSB catalyst could be maintained even after electrolysis for 10 hours. The SEM image revealed that the yolk–shell structure was preserved well after the durability test (Fig. S8†), implying the robust nature of the catalysts.

Fig. 8 (a) LSV curves of CoP₂/Fe-CoP₂ YSBs before and after 3000 cycles. (b) Chronoamperometry stability test at a static potential of 1.5 V (vs. RHE) for 10 hours. (c) Cyclic voltammetry curves of CoP₂/Fe-CoP₂ YSBs at different scan rates. (d) The capacitive current density as a function of scan rate for CoP₂/Fe-CoP₂ YSBs, Fe-CoP₂ NBs and CoP₂ NCs. The measured double-layer capacitance of the system is taken as the average of the absolute value of the slope of linear fits to the data.

As shown in Fig. S9,†ex situ XPS measurement was conducted to investigate the surface chemical composition of CoP₂/Fe-CoP₂ after the OER. The high resolution Co 2p XPS spectrum shows that the peaks at 780.3 and 795.6 eV are related to Co³⁺ and high resolution Fe 2p peaks at 709.5 and 722.6 eV are attributed to Fe³⁺.^46–48 The binding energy of P 2p at 129.1 eV disappeared, which suggests the oxidation of the P centre on the surface of the catalyst. Therefore, the surface of the catalyst is oxidized into metal oxides or hydroxides during the OER under strong alkaline conditions, consistent with the previous reports.^43,49

The electrochemically active surface area (ECSA) was determined using CV measurements for CoP₂/Fe-CoP₂ YSBs, Fe-CoP₂ NBs and CoP₂ NCs (Fig. 8c, S10a and S10b†). The anodic current densities measured at a fixed potential of 1.35 V (vs. RHE) are plotted with respect to scan rates in Fig. 8d. The capacitance of 19.43 mF cm⁻² for CoP₂/Fe-CoP₂ YSBs is higher than those of Fe-CoP₂ NBs (15.17 mF cm⁻²) and CoP₂ NCs (10.62 mF cm⁻²), implying that the CoP₂/Fe-CoP₂ YSB catalyst shows a high double-layer capacitance and a high electrochemically active surface area because of the yolk–shell structure.

The superior electrocatalytic activity might be ascribed to the unique yolk–shell structure with a favourable chemical composition. CoP₂/Fe-CoP₂ YSBs composed of CoP₂ particles (core) and Fe-CoP₂ nanoboxes (shell) show a significantly lower overpotential than their ingredients. The presence of Fe, a unique yolk–shell structure, and the hollow nature of the catalyst are believed to contribute to enhancing the electrocatalytic activity for the OER. Particularly, a large electrochemical surface area and abundant electroactive sites of the yolk–shell structure can effectively shorten the diffusion length of ions and expose more active sites than solid counterparts.^50–52 In addition, the integrated nanostructure prevents the aggregation during continuous gas evolution.⁵³

3. Conclusions

In summary, we synthesized CoP₂/Fe-CoP₂ YSBs using hybrid MOFs as a promising electrocatalyst for the OER. The anion-exchange reaction and subsequent phosphidation process resulted in the formation of CoP₂/Fe-CoP₂ YSBs. The formation process of yolk–shell structures was observed systematically by TEM under different reaction conditions. Owing to the large surface area and many active sites, the CoP₂/Fe-CoP₂ YSB catalyst not only provides superior electrocatalytic OER performances such as low overpotential and small Tafel slope values, but also shows superior cyclic stability. This synthetic approach can be extended to produce a variety of metal phosphides with a hollow structure for energy storage and conversion applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF-2019R1F1A1059831).

References

1. H. Wang, H. W. Lee, Y. Dend, Z. Li, P. C. Hsu, Y. Li, D. Lin and Y. Cui, Nat. Commun., 2015, 6, 1.
2. N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365.
3. N. B. Halck, V. Petrykin, P. Krtil and J. Rossmeisl, Phys. Chem. Chem. Phys., 2014, 16, 13682–13688.
4. M. Tahira, L. Pana, F. Idrees, X. Zhang, L. Wang, J.-J. Zou and Z. L. Wang, Nano Energy, 2017, 37, 136–157.
5. Y. Lee, J. Suntivich, K. J. May, E. E. Perry and Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 3, 399–404.
6. T. Reier, M. Oezaslan and P. Strasser, ACS Catal., 2012, 2, 1765–1772.
7. Z. Cai, X. Bu, P. Wang, J. C. Ho, J. Yang and X. Wang, J. Mater. Chem. A, 2019, 7, 5069–5089.
8. E. Fabbri, A. Habereder, K. Waltar, R. Kötz and T. J. Schmidt, Catal. Sci. Technol., 2014, 4, 3800–3821.
9. S. Anantharaj, S. R. Ede, K. Sakthikumar, K. Karthick, S. Mishra and S. Kundu, ACS Catal., 2016, 6, 8069–8097.
10. U. D. Silva, J. Masud, N. Zhang, Y. Hong, W. P. R. Liyanage, M. A. Zaeem and M. Nath, J. Mater. Chem. A, 2018, 6, 7608–7622.
11. Y.-J. Tang, C.-H. Liu, W. Huang, X.-L. Wang, L.-Z. Dong, S.-L. Li and Y.-Q. Lan, ACS Appl. Mater. Interfaces, 2017, 9, 16977–16985.
12. C. Liu, G. Bai, X. Tong, Y. Wang, B. Lv, N. Yang and X.-Y. Guo, Electrochem. Commun., 2019, 98, 87–91.
13. L. Ji, J. Wang, X. Teng, T. J. Meyer and Z. Chen, ACS Catal., 2020, 10, 412–419.
14. A. Parra-Puerto, K. L. Ng, K. Fahy, A. E. Goode, M. P. Ryan and A. Kucernak, ACS Catal., 2019, 9, 11515–11529.
15. H. Li, Q. Li, P. Wen, T. B. Williams, S. Adhikari, C. Dun, C. Lu, D. Itanze, L. Jiang, D. L. Carroll, G. L. Donati, P. M. Lundin, Y. Qiu and S. M. Geyer, Adv. Mater., 2018, 30, 1705796.
16. Y. Bai, H. Zhang, Y. Feng, L. Fang and Y. Wang, J. Mater. Chem. A, 2016, 4, 9072–9079.
17. S. Yang, M. Xie, L. Chen, W. Wei, X. Lv, Y. Xu, N. Ullah, O. C. Judith, Y. B. Adegbemiga and J. Xie, Int. J. Hydrogen Energy, 2019, 44, 4543–4552.
18. H. Li, P. Wen, D. S. Itanze, M. W. Kim, S. Adhikari, C. Lu, L. Jiang, Y. Qiu and S. M. Geyer, Adv. Mater., 2019, 31, 1900813.
19. J. Wang, W. Yangac and J. Liu, J. Mater. Chem. A, 2016, 4, 4686–4690.
20. L. Wang, H. Wu, S. Xi, S. T. Chua, F. Wang, S. J. Pennycook, Z. G. Yu, Y. Du and J. Xue, ACS Appl. Mater. Interfaces, 2019, 11, 17359–17367.
21. H. Mistry, A. S. Varela, S. Kühl, P. Strasser and B. R. Cuenya, Nat. Rev., 2016, 1, 16009.
22. Y. Guo, T. Park, J. W. Yi, J. Henzie, J. Kim, Z. Wang, B. Jiang, Y. Bando, Y. Sugahara, J. Tang and Y. Yamauchi, Adv. Mater., 2019, 31, 1807134.
23. S. Li, X. Hao, A. Abudulac and G. Guan, J. Mater. Chem. A, 2019, 7, 18674–18707.
24. B. Wang, C. Tang, H.-F. Wang, X. Chen, R. Cao and Q. Zhang, Adv. Mater., 2019, 31, 1805658.
25. M. Li, X. Deng, Y. Liang, K. Xiang, D. Wu, B. Zhao, H. Yang, J.-L. Luo and X.-Z. Fu, J. Energy Chem., 2020, 50, 314–323.
26. Z. Li, M. Li, Z. Bian, Y. Kathiraser and S. Kawi, Appl. Catal., B, 2016, 188, 324–341.
27. Z. Yin, C. Zhu, C. Li, S. Zhang, X. Zhang and Y. Chen, Nanoscale, 2016, 8, 19129–19138.
28. X.-C. Xie, K.-J. Huang and X. Wu, J. Mater. Chem. A, 2018, 6, 6754–6771.
29. V. Ganesan, P. Ramasamy and J. Kim, Int. J. Hydrogen Energy, 2017, 42, 5985–59992.
30. P. He, X.-Y. Yu and X. W. Lou, Angew. Chem., Int. Ed., 2017, 56, 3897–3900.
31. C. Xuan, J. Wang, W. Xia, Z. Peng, Z. Wu, W. Lei, K. Xia, H. L. Xin and D. Wang, ACS Appl. Mater. Interfaces, 2017, 9, 26134–26142.
32. Y. Lian, H. Sun, X. Wang, P. Qi, Q. Mu, Y. Chen, J. Ye, X. Zhao, Z. Deng and Y. Peng, Chem. Sci., 2019, 10, 464–474.
33. Y. Fang, X.-Y. Yu and X. W. Lou, Adv. Mater., 2018, 30, 1706668.
34. J.-Y. Xie, Z.-Z. Liu, J. Li, L. Feng, M. Yang, Y. Ma, D.-P. Liu, L. Wang, Y.-M. Chai and B. Dong, J. Energy Chem., 2020, 48, 328–333.
35. V. Ganesan, S. Lim and J. Kim, Chem. – Asian J., 2018, 13, 413–420.
36. C. Tang, L. Gan, R. Zhang, W. Lu, X. Jiang, A. M. Asiri, X. Sun, J. Wang and L. Chen, Nano Lett., 2016, 16, 6617–6621.
37. J. Han, G. Chen, X. Liu, N. Zhang, S. Liang, R. Ma and G. Qiuc, Chem. Commun., 2019, 55, 9212–9215.
38. F. Li, Y. Bu, Z. Lv, J. Mahmood, G.-F. Han, I. Ahmad, G. Kim, Q. Zhong and J.-B. Baek, Small, 2017, 13, 1701167.
39. X. Lv, Z. Hu, J. Ren, Y. Liu, Z. Wang and Z.-Y. Yuan, Inorg. Chem. Front., 2019, 6, 74–81.
40. M. Zhu, Y. Zhou, Y. Sun, C. Zhu, L. Hu, J. Gao, H. Huang, Y. Liu and Z. Kang, Dalton Trans., 2018, 47, 5459–5464.
41. A. Mendoza-Garcia, D. Sub and S. Sun, Nanoscale, 2016, 8, 3244–3247.
42. B. Qiu, L. Cai, Y. Wang, Z. Lin, Y. Zuo, M. Wang and Y. Chai, Adv. Funct. Mater., 2018, 28, 1706008.
43. H. Zhang, W. Zhou, J. Dong, X. F. Lu and X. W. Lou, Energy Environ. Sci., 2019, 12, 3348–3355.
44. J. Wang, J. Wang, M. Zhang, S. Li, R. Liu and Z. Li, J. Alloys Compd., 2020, 821, 153463.
45. X. Wang, J. Feng, Y. Bai, Q. Zhang and Y. Yin, Chem. Rev., 2016, 116, 10983–11060.
46. L.-M. Cao, Y.-W. Hu, S.-F. Tang, A. Iljin, J.-W. Wang, Z.-M. Zhang and T.-B. Lu, Adv. Sci., 2018, 5, 1800949.
47. Y. Lian, H. Sun, X. Wang, P. Qi, Q. Mu, Y. Chen, J. Ye, X. Zhao, Z. Deng and Y. Peng, Chem. Sci., 2019, 10, 464–474.
48. J. Shi, F. Qiu, W. Yuan, M. Guo and Z.-H. Lu, Chem. Eng. J., 2021, 403, 126312.
49. C. Du, L. Yang, F. Yang, G. Cheng and W. Luo, ACS Catal., 2017, 7, 4131–4137.
50. S. Yue, S. Wang, Q. Jiao, X. Feng, K. Zhan, Y. Dai, C. Feng, H. Li, T. Feng and Y. Zhao, ChemSusChem, 2019, 12, 4461–4470.
51. R. Cai, H. Jin, D. Yang, K.-T. Lin, K. Chan, J. Sun, Z. Chen, X. Zhang and W. Tan, Nano Energy, 2020, 17, 104542.
52. T. Yang, L. Yin, M. He, W. Wei, G. Cao, X. Ding, Y. Wang, Z. Zhao, T. Yu, H. Zhaoa and D. Zhang, Chem. Commun., 2019, 55, 14343–14346.
53. G. Mei, H. Liang, B. Wei, H. Shi, F. Ming, X. Xu and Z. Wang, Electrochim. Acta, 2018, 290, 82–89.

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Footnote

† Electronic supplementary information (ESI) available: XRD pattern and SEM image of ZIF-67. The XRD pattern of ZIF-67/CoFe-PBA YSBs. The EDS spectrum of CoP₂/Fe-CoP₂ YSBs and Fe-CoP₂ NBs. The OER stability curves of Fe-CoP₂ NBs and the cyclic voltammetry curves of Fe-CoP₂ NBs and CoP₂ NCs. The SEM images of CoP₂/Fe-CoP₂ YSBs after the durability test. See DOI: 10.1039/d0nr08108f

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