Recently, the depletion of fossil fuels and the associated exponential rise in global warming have stimulated considerable interest in alternative sources of renewable energy production and conversion [1, 2]. The oxygen evolution reaction (OER), a half reaction involving electrochemical water splitting, has attracted considerable attention due to its critical role in water splitting and metal–air batteries and for hydrogen production and metal regeneration [3, 4]. High electrochemical overpotential typically results from the kinetically sluggish four-electron oxidation reaction at the anode [5]. Ruthenium and iridium oxides have been developed as the most efficient electrocatalysts for the OER to date due to their low overpotentials and high proton mobility efficiency. However, their high cost, scarce reserves, and poor durability considerably prohibit more extensive applications [6]. To reduce the reaction overpotentials and obtain good reaction rates with reduced input energy, the development of effective non-precious metal electrocatalysts with high activity and low cost is required [7]. To date, some efficient alternatives OER catalysts in alkaline electrolytes using noble metal-based non-precious electrocatalysts including layered double hydroxides [8, 9], sulfides [10, 11], transition-metal oxides [12], hydroxides [13], metal free catalysts [14] and phosphate [15] have been reported [16].

Among different electrocatalysts for OER, metal–organic frameworks (MOFs), and in particular zeolitic imidazolate frameworks (ZIFs), have gained attention as novel sacrificial templates to construct efficient porous carbon-based electrocatalysts [17]. These materials have been widely used in gas storage or separation [18], catalysis [19], CO₂ reduction [20], and water-splitting [21] because of their excellent chemical and physical properties including high specific surface area, large number of active sites (an active site means the point where catalysis reaction can happen) [22], and tunability of both metals and ligands in the MOFs [23]. MOFs have been used as precursors to synthesize OER catalysts because of their controllable porous structure and nearly infinite design possibilities. In addition, various MOF-derived carbon-based porous metal compounds can be obtained through direct carbonization without further processing or templating. Among cobalt-based MOF materials, ZIF-67 is one of the most investigated MOFs and offers a large number of active cobalt sites. For example, carbonization of ZIF-67 results in a porous Co@N-doped carbon composite (named N/Co-doped PCP//NRGO), which is very active toward the water splitting reaction [24]. In this study, we demonstrate a facile MOF-derived approach for the effective synthesis of OER electrocatalysts in 1.0 mol/L of KOH, producing CoNiP/NC derived from ZIF-67 and doped with nickel through a two-step carbonization and phosphorization calcination precedure in Ar atmosphere. Compared to a previous report of the phosphatization of sodium hypophosphite [25], the strategy described herein of simultaneous carbonization and phosphorization calcinations of Ni-doped ZIF-67 can more easily yield CoNiP/NC hybrid composites. These results suggest that the MOF-derived CoNiP/NC is a promising electrocatalysts for the OER for water splitting in alkaline electrolytes. The N-doped cobalt nickel phosphorus porous carbon electrocatalysts CoNiP/NC, and CoNiP/NC700, where 700 referrs to the calcination temperature (℃), exhibited superior activity towards oxygen evolution with an onset overpotential of approximately 220 mV and an overpotential of approximately 300 mV in alkaline solution at a current density of 10 mA/cm². The better electrocatalytic activity of the hybrid material can be attributed to the superior synergistic effect of Co, Ni, P and C due to their strong electron coupling interactions. The interconnected amorphous carbon anchored the active Co compounds preventing aggregation and afforded conducting channels for electron transfer.

In total, 0.5 g of ZIF-67 particles were dispersed in 200 mL of ethanol containing 1 g of nickel nitrate hexahydrate. After stirring for 30 min, the ZIF-67Ni yolk-shelled particles were formed, collected by centrifugation (at 9000 r/min for 10 min), and dried at 50 ℃ for 5 h. Subsequently, the CoNi/NCx, which x refers to the calcination temperature (℃), were obtained by annealing the as-obtained yolk-shelled particles at temperatures of 600, 700, 800, 900, and 1000 ℃ for 3 h at a ramp rate of 5 ℃/min under an Ar atmosphere. To obtain CoNiP/NCx, 50 mg CoNix and 1.0 g sodium hypophosphite were placed at two separate positions in a combustion boat and subjected to phosphidation at 300 ℃ for 2 h at a ramp rate of 5 ℃/min under an Ar atmosphere.

The obtained samples were characterized using powder X-ray diffractometry (XRD, Bruker-AxsD8) with Cu K_α radiation (λ = 0.15406 nm) at 2θ = 5°‒70°, operating at a voltage of 40 kV and current of 40 mA. The morphologies of the samples were characterized by field emission-scanning electron microscope (FE-SEM, Hitachi SU-70) operating at 10 kV. The samples used for SEM imaging were gold sputtered prior to analysis. X-ray photoelectron spectroscopy (XPS) data were acquired on an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Scientific) using Al K_α radiation.

Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed on an Autolab electrochemical workstation (NOVA 1.9). To prepare the electrodes for OER testing, 5 mg of active material was added to a mixture of 1.9 mL ethanol and 0.1 mL Nafion solution with sonication for 60 min. Subsequently, the catalyst (20 μL) was pipetted out and dropped onto a glassy carbon electrode with a diameter of 5 mm and fully dried at room temperature for 12 h before measurements were taken (loading of ∼0.255 mg/cm²). The electrochemical cell was assembled as a conventional three-electrode system with electrochemical workstation in KOH solution 1.0 mol/L. The counter and reference electrodes were composed of a Pt foil and an Ag/AgCl-saturated KCl electrode, respectively, and a glassy carbon electrode (GCE, 5 mm in diameter) was used as the working electrode. All measured potentials were normalized to the reversible hydrogen electrode (RHE) by adding a value of (0.197 + 0.059 pH) V. A scan rate of 10 mV/s was used in the linear sweep voltammetry to obtain the polarization curves, 1.0 V‒1.8 V.

Fig. S1 (Supporting Information) shows the XRD patterns of ZIF-67-Simulated, ZIF-67, ZIF-67Ni, Co/NC700, CoNi/NC700, and CoNiP/NC700 samples. Compared to the simulated data of ZIF-67, the ZIF-67 precursor has similar diffraction peaks, indicating the successful synthesis of ZIF-67 [26]. The XRD patterns of ZIF-67Ni is similar to that of ZIF-67 [27]. The standard spectral data of Co (PDF #01-1255) and Ni (PDF #71-2336) exhibit the same diffraction peaks as these of Co/NC700 and CoNi/NC700. According to the CoNiP standard (PDF #71-2336) spectrum shown in Fig. 2(f), the CoNiP/NC700 was successfully synthesized. However, it is difficult to ascertain the exact stoichiometry of CoNiP/NC because the polymorphs of cobalt nickel phosphides contains several types; Co_1.49Ni_0.51P (ICDD No. 04-019-5182), Co_1.4Ni_0.6P (ICDD No. 04-005-5616), and CoNiP (ICDD No. 04-001-4562). Furthermore, the various XRD patterns obtained under different conditions are shown in Fig. S1.

The SEM image in Fig. 2(a) indicates that ZIF-67 is a regular polyhedron with an obvious edge and diameter of approximately 700 nm. Fig. 2(c) shows that ZIF-67Ni conserves polyhedron shape, but with a rougher surface due to the reaction with Ni(NO₃)₂ in ethanol solution for 30 min. The CoNi/NC700 obtained by the calcination of ZIF-67Ni in Ar for 3 h at 700 ℃ was shown Fig. 2(d). The size and shape of the final product, CoNiP/NC700, was largely unchanged after subsequent phosphidation step for 300 ℃ (Fig. 2(e)). With higher calcination temperatures, more serious ZIF-67 deformation occurs. Furthermore, the various SEM images obtained under different conditions are shown in Fig. S2.

To determine the detailed elemental composition, valence state, and OER activity of CoNi/NC and CoNiP/NC materials, XPS tests were performed (Fig. S4 and Fig. 3). Fig. 4(a) indicates the presence of Co, Ni, P, C, N and O in CoNiP/NC700. In addition, a small amount of N was detected in the CoNiP/NC700 samples, likely originating from the organic ligands, 2MI, in the MOFs. In the high-resolution Co 2p XPS spectra (Fig. S4 and Fig. 3(c)), the CoNiP/NC700 samples show two main peaks at 781.6 and 793.7 eV [5, 17]. The Ni 2p spectrum of CoNi/NC800 exhibits two major peaks at 853.8 and 869.9 eV (Fig. S4), which can be attributed to Ni^δ+ in the Ni–P compound [4, 28]. The Ni 2p spectrum of CoNiP/NC700 exhibits two major peaks at 856.2 and 874.3 eV. For Ni 2p_1/2, the corresponding peak attributed to Ni^δ+ in the Ni–P compound was observed at 869.7 eV and peaks assigned to Ni²⁺ are located at 874.3 eV. A high-resolution spectrum of the P 2p region shows a peak at 133.4 eV, reflecting the binding energy of P 2p_3/2 [5]. The XPS data demonstrate the successful chemical conversion of CoNi/NC to CoNiP/NC via the low-temperature gas phosphidation process.

The OER activity of the samples with the same mass loading of 0.255 mg/cm² was evaluated in KOH solution 1.0 mol/L at room temperature. Fig. 4(a) shows the polarization curves obtained by LSV measurements. For comparison, a bare GCE, ZIF-67, and ZIF-67Ni, were also studied (Fig. S5.1). The bare GCE, ZIF-67, and ZIF-67Ni had onset overpotentials of approximately 370, 310, and 330 mV vs RHE, respectively, and an overpotentials of approximate 550, 450, and 440 mV, respectively, at a current density of 10 mA/cm², indicting negligible electrocatalytic activity and a low OER response. Fig. 4(a) shows that the polarization curve of CoNiP/NC700 exhibits the superior activity toward the OER compared to these of CoNi/NC700 and Co/NC700, and shows an onset overpotential of approximate 220 mV vs RHE and an overpotential of approximate 300 mV at a current density of 10 mA/cm².

Meanwhile, the polarization curve of CoNiP/NC also exhibits the superior electrocatalytic activity toward the OER compared to these of CoNi/NC and Co/NC (Fig. S6), suggesting that the overpotential of CoNiP/NC is relatively lower than these of CoNi/NC and Co/NC. The better electrocatalytic activity of the hybrid material can be attributed to the superior synergistic effect between Co, Ni, P and C due to their strong electron coupling interactions. The interconnected amorphous carbon anchored the active Co compounds preventing aggregation and afforded conducting channels for electron transfer. It should be noted that the overpotential for the OER activity of CoNiP/NC700 is much lower than that of most reported metal-free and transition metal oxides OER catalysts (Table S1, Supporting Information). Among reported electrocatalytic materials, the CoNiP/NC700 showed the highest electrocatalytic activity (Fig. 4(d)), due to its MOF-derived porous carbon structure and N/Co/Ni-doping effect [27, 29]. The Tafel plots of Co/NC700, CoNi/NC700 in Fig. 4(b) were derived from the polarization curves in Fig. 4(a). A lower Tafel slope value indicates more rapid OER kinetics and significant chemical and electronic coupling [30]. In comparison to Co/NC700 and CoNi/NC700, the CoNiP/NC700 electrocatalyst exhibits a lower onset potential and higher current density for the OER. It is clear that CoNiP/NC700 shows the lowest Tafel slope (Fig. 4(b)) among the materials tested. The superior electrocatalytic activity of the hybrid material can be attributed to the synergistic effect between Co, Ni, P and C due to their strong electron coupling interactions. The interconnected amorphous carbon anchored the active Co compounds preventing aggregation and afforded conducting channels for electron transfer. Furthermore, Figs. S6.1–S6.4 show the various polarization curves and Tafel plots obtained under different conditions. The excellent stability of the electrode was confirmed by the polarization curve stability after continuous potential sweeps (Fig. 5).

A negligible difference was observed in the polarization curves after 10 h of continuous testing in 1.0 mol/L KOH. These results indicate that CoNiP/NC700 exhibits the excellent stability in alkaline solution. The stability of CoNiP/NC700 catalysts for the OER was measured by the i-t tests at a constant potential of 1.53 V vs. RHE. It is clear that the CoNiP/NC700 catalysts annealed at 700 ℃ exhibit superior durability, with negiligible decay (4.0 %) in OER activity during 10 h of continuous operation (Fig. 5).

CoNiP/NC materials were successfully developed as efficient electrocatalysts for the OER by carbonization and phosphorization calcinations of preformed Co-based MOFs. The CoNiP/NC700 electrocatalyst exhibited excellent catalytic performance with a low overpotential of 300 mV at a current density of 10 mA/cm² and Tafel slopes of 66 mV/dec in alkaline electrolytes, due to its MOF-derived porous carbon structure and the N/Co/Ni-doping effect. The CoNiP/NC electrocatalyst obtained at different temperatures exhibited better electrocatalytic activity than these of the Co/NC and CoNi/NC materials. In addition, the facile synthesis method based on MOF templates to for the preparation of CoNiP/NC is low cost and uses abundant resources, providing a synthetic method for practical applications. The catalysts developed herein are expected to find use in clean energy devices, and the synthesis methods could be to extended to other materials with different structures and properties, especially for bimetallic or trimetallic hybrid composites.