Trimetallic MOF-derived CoFeNi/Z-P NC nanocomposites as efficient catalysts for oxygen evolution reaction

Abstract

We used sodium hydroxide-mediated approach and tannic acid etching to prepare hollow structured trimetallic MOF-derived CoFeNi/Z-P NC nanocomposites. Remarkably, the resulting CoFeNi/Z-P NC nanocomposites have large specific surface area and mesoporous structure, making their active sites more accessible and mass transfer more effective. More complex trimetallic components provide greater possibilities for further improving electrocatalytic performance. The CoFeNi/Z-P NC nanocomposites demonstrate notable enhancements for the OER, and 10 mA cm⁻² current density is achieved at a low overpotential of 244 mV, with a low Tafel slope of 66.2 mV dec⁻¹ and have good stability in alkaline solutions. In addition, as a cathode material for overall alkaline water splitting, CoFeNi/Z-P NC is better than RuO₂ with longer cycling stability.

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1. Introduction

Due to its critical role in numerous energy conversion and storage technologies, the oxygen evolution reaction (OER) has attracted a lot of attention as a research topic in recent years.^1–4 However, the OER's slow kinetics and multi-electron transfer process make it challenging to use in practical applications because the reaction requires a significant overpotential to drive it.^5,6 The remarkable catalytic efficacy of noble metal catalysts, such as RuO₂ and IrO₂, in promoting the OER is widely known, but their high cost and limited availability severely restrict their widespread practical usage.^7,8 Therefore, to deal with the OER, it is vital to create efficient and long-lasting alternative electrocatalysts.^9,10

With the aim of resolving this issue, nanostructured transition metal phosphides (TMPs) were recognized as promising candidates for catalysts in the oxygen evolution reaction (OER), and important developments have been achieved in this area.^11–13 However, these catalysts based on transition metal phosphides still need to perform better in the OER. According to studies, controlling the structure and chemical composition of transition metal-based electrocatalysts is necessary to increase their intrinsic OER activity.^14,15 The first control includes changing the structure to promote mass transportation and increase the exposure of active sites.¹⁶ Given this, hollow-structured nanoporous materials have gained a lot of attention including those that have a large specific surface area, lots of active sites, and short charge transfer distances.^17,18 The latter control entails purposeful modification of stoichiometric compositions to regulate the electrical characteristics, thus affecting interactions with reaction intermediates and reaction dynamics.¹⁹ Consequently, internal or phase interface engineering has emerged as a viable strategy for controlling the electrical properties.²⁰ In their bimetallic phosphides made from MOF precursors, Chen et al. showed that a Ni₂P/CoP contact was essential for producing a synergistic effect and boosting the electrocatalytic activity.²¹ However, due to the highly intricate chemical composition of transition metal phosphides, even slight variations in the metal-to-phosphorus stoichiometric ratio can result in substantial alterations in their structures. As a result, studies concentrating on combining hollow structures with various transition metal phosphides are scarce.

Zhang et al.²² synthesized NiCoP/NC PHCs by using bimetal as precursor and combining tannic acid-induced chemical corrosion with subsequent roasting and phosphating. The obtained composite material has a good hollow structure. Hong et al.²³ developed a very simple sodium hydroxide-mediated method to prepare Fe–Co bimetallic MOFs in high yields. However, there are currently few methods for doping two other metals into ZIF-67 to form trimetallic ZIFs, and there are even fewer studies on hollow trimetallic MOFs and their derivatives.

In this study, we synthesized trimetallic CoFeNi-ZIF specimens by incorporating iron and nickel dopants into ZIF-67. Then, we used tannic acid as an etchant to perform in situ chemical etching on the cobalt-based trimetallic zeolite imidazole framework polyhedron (CoFeNi-ZIF), forming hollow-structured CoFeNi-Z. Subsequently, two steps of carbonization and phosphating were carried out. The as-synthesized CoFeNi/Z-P NC nanocomposite exhibits a distinctive hollow structure and benefits from the coordinating impact arising from the uniform integration of metallic Fe and Ni. These nanocomposites display a low overpotential of 244 mV at a current density of 10 mA cm⁻² due to increased electrocatalytic activity for the OER. Additionally, as a cathode material for overall alkaline water electrolysis, CoFeNi/Z-P NC is better than RuO₂ with longer cycling stability.

2. Experimental

2.1 Chemicals

2-Methylimidazole (Adamas, 98%), sodium hydroxide (Aladdin, 97%), cobalt nitrate hexahydrate (Shanghai Lingfeng Chemical Reagent Co., Ltd, ≥99%), iron(III) chloride hexahydrate (Greagent, 99%), nickel nitrate hexahydrate (Greagent, ≥98.5%), and sodium hypo-phosphite monohydrate (Wuxi Yatai United Chemical Co., Ltd, 99%) were acquired. All of these chemicals were used as received.

2.2 Synthesis of CoFeNi/ZIF

By using the bimetallic synthesis technique described in the previous literature, CoFeNi-ZIF is synthesized. In a typical synthesis, 5 mL of 1 M aqueous solution of sodium hydroxide was combined with 5 mL of 3.36 M aqueous solution of 2-methylimidazole under stirring, forming solution A. For the preparation of solution B, 0.446 mL of 0.8 M aqueous solution of cobalt nitrate and 0.223 mL of 0.8 M aqueous solution of nickel nitrate were mixed with 1.78 mL of 0.1 M aqueous solution of iron trichloride, and the mixture volume was adjusted to 6 mL with deionized water. Subsequently, solution B was slowly added to solution A using a pipette. The reaction proceeded for 3 hours at a stirring rate of 400 rpm. The final product was obtained by centrifugation at an acceleration of 8000 rpm, collected, and dried at 60 °C for further use after being washed three times in methanol. CoFe-ZIF and CoNi-ZIF compounds were synthesised using similar techniques.

2.3 Synthesis of CoFeNi/Z

In a typical synthesis, 1.25 g of tannic acid and 250 mL of methanol were combined with 0.5 g of the previously synthesised CoFeNi-ZIF compound. The mixture was stirred for 5 minutes at a speed of 300 rpm. The resulting product was then recovered by centrifugation at an acceleration of 8000 rpm, dried at 60 °C for further use after being cleaned three times in methanol and deionized water.

2.4 Synthesis of CoFeNi/Z-P NC

The dried CoFeNi-Z material was loaded into a ceramic boat and placed into a temperature-controlled furnace. The temperature of the furnace was gradually increased from 20 °C to 700 °C at a heating rate of 10 °C min⁻¹, and this temperature was then maintained for two hours. Subsequently, the furnace was allowed to cool off on its own to room temperature. Throughout the entire process, a continuous pure argon gas flow was employed. The resulting product was designated as CoFeNi/Z NC.

The as-prepared CoFeNi/Z NC and NaH₂PO₂·H₂O at a mass ratio of 1 : 10 were placed downstream and upstream in the tube furnace. The sample was then heated to 400 °C at an average rate of 5 °C min⁻¹ and maintained at this temperature for two hours under Ar atmosphere. After cooling to room temperature, a phosphide product called CoFeNi/Z-P NC was obtained and further utilized.

3. Results and discussion

Fig. 1 shows a schematic representation of the CoFeNi/Z-P NC ternary alloy synthesis process. In a typical synthesis, the raw materials were added to an aqueous sodium hydroxide and 2-methylimidazole mixture, which quickly changed the mixture's colour to purple. This reaction yielded a metal–organic framework (MOF) precursor, referred to hereafter as CoFeNi-ZIF. For comparison, CoFe-ZIF and CoNi-ZIF samples were also prepared (Fig. S1†). The XRD patterns of the CoFe-ZIF and CoNi-ZIF samples exhibited similarities to that of CoFeNi-ZIF (Fig. S2†) and analysis using energy-dispersive X-rays (EDS) confirmed the elemental species of CoFeNi-ZIF (Fig. S3†).

Fig. 1 Schematic diagram of the synthesis of materials.

Tannic acid is used as the etching agent in in situ chemical etching to convert CoFeNi-ZIF into CoFeNi-Z (step 2 in Fig. 1). It takes 5 minutes to complete this process. The etching reaction induced by tannic acid is a surface functionalization-assisted etching process. Tannic acid provides protons to destroy the coordination bonds of CoFeNi-ZIF, causing the disintegration of the MOF. At the same time, tannic acid can be adsorbed on the surface of CoFeNi-ZIF to protect the MOF from being etched.²⁴ SEM and TEM were employed to illustrate the unique structure and morphology of the sample (Fig. S4 and S5†). The color change from CoFeNi-ZIF to CoFeNi-Z also confirms the evolution of the structure (Fig. S6†), which is consistent with our previous findings.²² CoFeNi/ZIF-P can be obtained by the direct carbonization of CoFeNi-ZIF and then phosphating. CoFeNi-Z is first carbonized and then phosphorized to obtain CoFeNi/Z-P NC. CoFeNi/ZIF NC and CoFeNi/Z NC are obtained by the direct carbonization of CoFeNi-ZIF and CoFeNi-Z under an argon atmosphere.

X-ray diffraction, Raman spectroscopy, and BET tests were also carried out to determine the chemical state of the sample and to disclose the synergistic impact of various metal phosphides. The CoFeNi/Z-P NC alloy single-particle X-ray diffraction (XRD) pattern is shown in Fig. 2a. The diffraction peaks of CoP (PDF#29-0497), FeP₂ (PDF#06-0561), and NiP₂ (PDF#13-0213) appeared in CoFeNi/Z-P NC, indicating that the crystalline phases of CoP, FeP₂, and NiP₂ were formed.^23,25,26 The D band and G band are represented by two unique peaks in the Raman spectrum at 1351 and 1596 cm⁻¹, respectively (Fig. 2b). A 2D band peak was also observed at 2703 cm⁻¹.²⁷ While the G bands come from in-plane sp² carbon vibrations, the D bands come from sp² carbon imperfections. Additionally we noticed weaker 2D bands indicating the presence of graphene-like carbon sheets in the material.²⁸ The calculated intensity ratio I_D/I_G is 0.99, 1.08, and 0.95 for CoFeNi/ZIF NC, CoFeNi/Z NC, and CoFeNi/Z-P NC electrocatalysts, respectively. The high I_D/I_G ratio indicates the generation of numerous defects, implying significant doping of the graphene layer with heterogeneous atoms.²⁹ Additionally, the larger and weaker 2D bands suggest that the alloy has thin graphene layers on it. BET analysis confirms some evolution in the material structure (Fig. 2c and d), demonstrating that the CoFeNi-Z material has a specific surface area of 270.138 m² g⁻¹, a typical pore size of 8.885 nm, and a total volume of pores of 0.429 cm³ g⁻¹. Comparatively, CoFeNi-ZIF has a surface area of 361.033 m² g⁻¹, a pore size of 6.022 nm on average, and a total volume of pores of 0.392 cm³ g⁻¹. This further confirms the impact of etching on the material structure.

Fig. 2 (a) The XRD patterns of CoFeNi/Z-P NC are shown. (b) The Raman spectra of CoFeNi/ZIF NC, CoFeNi/Z NC, and CoFeNi/Z-P NC. (c) The nitrogen adsorption–desorption isotherms of CoFeNi-ZIF and CoFeNi-Z. (d) The pore size distribution of CoFeNi-ZIF and CoFeNi-Z.

Fig. 3a shows the SEM image of CoFeNi-ZIF. It can be seen that it is a dodecahedron between 50–100 nm in size. Fig. 3b shows the scanning electron microscope image of CoFeNi-Z; the morphology of the sample after in situ etching has changed; the surface is rough and more irregular than before. Fig. 3c shows the morphology of CoFeNi/Z-P NC. The transmission electron microscope (TEM) image in Fig. 3d confirms the sample's hollow structure, and it also clearly shows the N-doped graphene shell and that the number of layers of graphene is approximately 5 to 10. The ESI† contains higher resolution HRTEM images of the hollow structure (Fig. S7†). The (200), (200), and (310) planes of FeP₂, CoP, and NiP₂ are depicted in the high-resolution TEM (HRTEM) image as lattice fringes with d-spacing values of approximately 0.247 nm, 0.252 nm, and 0.162 nm,³⁰ respectively, which closely match the expected spacing values (Fig. 3e). A polycrystalline structure can be seen in the CoFeNi/Z-P NC selected area electron diffraction (SAED) pattern (Fig. 3f), as is evident from the diffraction rings. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Fig. 3g), which shows differential properties in the inner and outer portions, further confirms a core–shell structure for the nanocrystals, and analysis using energy-dispersive X-rays (EDS) confirms the elemental species of CoFeNi/Z-P (Fig. S8†).

Fig. 3 SEM images of (a) CoFeNi-ZIF, (b) CoFeNi-Z, and (c) CoFeNi/Z-P NC. (d) TEM images of CoFeNi/Z-P NC. (e) HRTEM image of CoFeNi/Z-P NC. (f) SAED pattern of CoFeNi/Z-P NC. (g–n) STEM image and elemental mappings of CoFeNi/Z-P NC.

Analysis of CoFeNi/Z-P NC samples using X-ray photoelectron spectroscopy (XPS) reveals important information about their chemical composition and bonding states. On the surface of CoFeNi/Z-P NC, the whole XPS spectrum in Fig. 4a verifies the existence of C, N, P, and transition metal ions Co, Fe, and Ni.³¹ According to the Co 2p spectra (Fig. 4b), the first pair of peaks identified at 780.18 eV (Co 2p_3/2) and 792.58 eV (Co 2p_1/2) corresponds to Co in the Co²⁺ oxidation state. The second set of peaks, located at 782.38 eV (Co 2p_3/2) and 796.68 eV (Co 2p_1/2), indicates the existence of Co³⁺ species. The peaks at 785.88 eV and 802.48 eV correspond to shakeup satellites. Additionally, the emergence of a unique spin–orbit peak at 777.78 eV provides further evidence that Co–P bonds were produced in the CoFeNi/Z-P NC sample.^32,33 The peaks at 706.18 eV (Fe 2p_2/3) and 718.38 eV (Fe 2p_1/2), in the Fe 2p spectra (Fig. 4c) are indicative of Fe–P bonds, and peaks at 711.88 eV (Fe 2p_2/3) and 723.68 eV (Fe 2p_1/2) represent Fe–O surface bonds.³⁴ The Ni 2p XPS spectrum (Fig. 4d) reveals the detailed electronic states of the Ni₂P compound, exhibiting four spin–orbit peaks along with two satellite peaks labeled as “Sat.”. The existence of both Ni²⁺ and Ni³⁺ species is indicated by peaks at 852.38 eV and 855.38 eV, which are followed by peaks at 869.18 eV and 873.18 eV, respectively.^35,36 A pair of peaks at 128.78 eV and 129.28 eV, attributed to P 2p_3/2 and P 2p_1/2, respectively, in the high-resolution P 2p spectrum (Fig. 4e), suggests the formation of metal phosphides. The peak at 133.08 eV suggests that sample oxidation in the air caused the formation of P–O bonds.^37,38 Pyridinic nitrogen (397.98 eV), pyrrolic nitrogen (399.28 eV), graphitized nitrogen (400.68 eV), and nitrogen oxide (404.68 eV) correspond to the four peaks seen in the N 1s spectrum (Fig. 4f).^39,40 The electrocatalytic activity is significantly impacted by both pyridinic-N and graphitic-N species; this is important to highlight.^41,42 Table S1† shows the chemical compositions of CoFeNi/Z-P NC by XPS measurement.

Fig. 4 XPS spectra of CoFeNi/Z-P NC. (a) Survey scan, (b) Co 2p, (c) Fe 2p, (d) Ni 2p, (e) P 2p, and (f) N 1s peaks of CoFeNi/Z-P NC.

The catalytic performance of the catalyst for the oxygen evolution reaction (OER) was assessed using a typical three-electrode electrochemical cell device in 1 M potassium hydroxide solution. The counter electrode was a graphite rod, while the reference electrode was a saturated Ag/AgCl electrode. LSV was used to assess the OER activity of samples and this was compared to that of a commercial RuO₂ electrocatalyst used as a benchmark. The LSV curves in Fig. 5a demonstrate the excellent performance of the CoFeNi/Z-P NC catalyst, which has an overpotential of 244 mV at a current density of 10 mA cm⁻². Compared to the values presented in Fig. 5b for RuO₂ (344 mV), CoFeNi/ZIF NC (319 mV), CoFeNi/Z NC (299 mV), and CoFeNi/ZIF-P NC (269 mV), this value is much lower. The Tafel slope is then obtained by fitting the linear component of the Tafel diagram with the Tafel equation (η = blog j + a), where b is the Tafel slope and j is the current density. RuO₂ has a value of 97.4 mV dec⁻¹ on the Tafel slopes in Fig. 5c, which is consistent with the theoretical value. Compared to CoFeNi/ZIF NC, CoFeNi/Z, and CoFeNi/ZIF-P NC, which had Tafel slopes of 59.7 mV dec⁻¹, 65 mV dec⁻¹, and 68.4 mV dec⁻¹, respectively, CoFeNi/Z-P NC had a measured Tafel slope of 66.2 mV dec⁻¹. We compared the OER electrocatalytic performance of the obtained catalyst with that of different catalysts (Table S2†). At the same time, we compared the RuO₂ parameter obtained from the experiment with the literature reports (Table S3†). The Nyquist plots in Fig. 5d indicate that CoFeNi/Z-P NC exhibited smaller semicircle diameters compared to CoFeNi/ZIF NC, CoFeNi/Z NC, and CoFeNi/ZIF-P NC, suggesting a decreased charge transfer resistance at the catalyst/electrolyte contact. A decreased charge transfer resistance denotes greater electrode material conductivity, which is generally recognised to be connected to electrocatalysis kinetics.

Fig. 5 (a) Polarization curves and (b) overpotential performance. Tafel slopes (c) and EIS Nyquist plots (d). (e) The non-faradaic region of each CV curve was fitted to obtain information about the double-layer capacitance. (f) The durability test was conducted by measuring the polarization curves of CoFeNi/Z-P NC before and after the test, with the chronoamperometry measurement performed at 1.48 V for 10 hours (the inset).

Furthermore, cyclic voltammetry was used to quantify the electrochemical double-layer capacitance (C_dl), which is thought to be directly proportional to the electrochemically active surface area (ECSA). The CV curves for CoFeNi/ZIF NC, CoFeNi/Z NC, CoFeNi/ZIF-P NC, and CoFeNi/Z-P NC in the non-faradaic area are shown in Fig. S8† at various scan rates (20, 40, 60, 80, and 100 mV s⁻¹). As depicted in Fig. 5e, the calculated C_dl value for CoFeNi/Z-P NC is 5.63 mF cm⁻², surpassing those of CoFeNi/ZIF NC (1.06 mF cm⁻²), CoFeNi/Z NC (2.73 mF cm⁻²), and CoFeNi/ZIF-P NC (3.64 mF cm⁻²). With only a slight decline in current density over a 10-hour period, Fig. 5f and the inset show that CoFeNi/Z-P NC has strong durability. After this step, we collected the sample and performed TEM analysis (Fig. S10†). The graphene shell is clearly visible and the lattice parameters correspond to those before testing (Fig. 3e).

As shown in Fig. 6a, a current density of 10 mA cm⁻² could be achieved by applying a potential of 1.588 V between two electrodes, which is even smaller than that of its Pt-RuO₂ counterpart (1.627 V). The very stable current density at a high potential of 1.59 V as shown in i–t curves (Fig. 6b) further demonstrated the strong stability of CoFeNi/Z-P NC. The negligible activity of carbon paper further proved that the activity came from the catalysts. The excellent activity of the alloy catalyst, accompanied by high stability, made it a potential alternative to precious metal catalysts in practical water splitting.

Fig. 6 (a) Polarization curves of Pt/C-CoFeNi/Z-P NC and Pt/C-RuO₂ catalyst couples for overall water splitting in 1.0 M KOH. (b) Time-dependent current density curves (i–t curve) under static overpotential of 1.59 V in 1.0 M KOH. The inset is an optical photograph showing the generation of H₂ and O₂ bubbles for Pt/C-CoFeNi/Z-P NC on a glassy carbon electrode.

4. Conclusion

In summary, this study presents an efficient catalyst CoFeNi/Z-P NC, derived from CoFeNi trimetallic hollow MOF phosphides through a combination of MOF derivation, in situ etching, and phosphating techniques. CoFeNi/Z-P NC exhibits effective catalytic synergy for the electrochemical OER. The structural transformation of CoFeNi-ZIF into hollow CoFeNi-Z derivatives as well as phosphides was observed by physical characterization. The CoFeNi/Z-P NC sample, which demonstrates increased catalytic activity for the OER, only needs an overpotential of 244 mV to achieve a current density of 10 mA cm⁻² on a glassy carbon electrode. It has a low Tafel slope of 66.2 mV dec⁻¹ and excellent stability, and we have expanded the application of CoFeNi/Z-P NC in total water splitting. This work introduces a new preparation method of trimetallic hollow MOFs and extends their applications in energy conversion and catalytic reactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 62374107), The Talent Program of Shanghai University of Engineering Science (QNTD202104), Shanghai Local Universities Capacity Building Project of Science and Technology Innovation Action Program (21010501700), Class III Peak Discipline of Shanghai-Materials Science and Engineering (High-Energy Beam Intelligent Processing and Green Manufacturing).

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Footnote

† Electronic supplementary information (ESI) available: The characterization instrument model of the sample, the method of electrochemical measurement, and the synthesis process diagram of the sample and the SEM, TEM, XRD, EDS and other information of the comparative samples. See DOI: https://doi.org/10.1039/d3dt02818f

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