Oxygen-coordinated MOF membrane facilitated construction of supported Co₂P/CoP@C heterostructures for water electrolysis

Abstract

Integration of cobalt phosphides (Co₂P and CoP) in a carbon matrix shows great promise for developing high-performance catalysts for water electrolysis. Nevertheless, the controlled synthesis of these two phases with an effective interface, uniform dispersion, and a simplified synthesis process is still challenging. Herein, we proposed a strategy that involves pre-construction of the Co/CoO@C heterostructure followed by post-conversion by phosphorization to achieve the precise synthesis of the Co₂P/CoP@C heterostructure, which was realized by utilizing a MOF as the self-sacrificial template. The oxygen-coordinated MOF that shows advantages in building the Co/CoO heterostructure was employed in this work and grown into a high-quality membrane via cathodic electrodeposition on the graphite substrate (Gss). The obtained catalyst (Gss-Co₂P/CoP@C-800) requires 146 and 365 mV overpotentials to achieve a current density of 100 mA cm⁻² for the HER and OER, respectively, and 1.54 V to achieve a current density of 10 mA cm⁻² for water electrolysis. Beyond the significantly enhanced conductivity that originates from the robust interaction between the MOF and Gss, the establishment of an effective Co₂P/CoP interface also plays a pivotal role in contributing to the high performance of Gss-Co₂P/CoP@C-800. As revealed by density functional theory (DFT) calculations, the unique d-orbital electron distribution of Co₂P/CoP and the enhanced state density near the Fermi level facilitate its efficient electron transport and render the Co₂P/CoP heterostructure region a crucial active site for water electrolysis. This study will provide new insights into the rational design and construction of heterostructures based on MOFs for efficient and green energy conversions.

---

Introduction

Hydrogen (H₂) has been considered a very promising renewable and clean fuel for mitigating energy crisis and environmental problems.^1–4 Water electrolysis is one of the most efficient techniques for producing hydrogen, which involves two key components: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.^5,6 Currently, the state-of-the-art commercial catalysts for the HER and OER are mainly noble metal-based materials (e.g., Pt, IrO₂, and RuO₂) due to their excellent electrocatalytic activity.^7,8 However, the scarcity and high cost of these materials limit their large-scale industrial applications.^9,10 Therefore, developing efficient non-precious metal-based catalysts, particularly bifunctional catalysts, is crucial for achieving cost-effective and sustainable water electrolysis systems.

In recent years, transition metal phosphides (TMPs, TM = Ni, Co, Fe, Mo, etc.) have attracted much attention as electrocatalysts for the HER and OER due to their low price, abundant reserves, and efficient catalytic activity, especially cobalt phosphides.^11–14 CoP with a high percentage of phosphorus atoms is usually more favorable for the HER, whereas an excess of phosphorus can lead to a low number of free electrons in CoP, which decreases its catalytic performance.^15,16 Co₂P with abundant metal Co–Co bonds can significantly promote the formation of key OER active intermediates with excellent conductivity.¹⁷ Thus, integration of these two cobalt phosphides (Co₂P/CoP) holds promise for achieving high-performance bifunctional catalysts for water electrolysis.¹⁸ Further combination with carbon (CoP/Co₂P@C) can enhance the conductivity and stability of CoP/Co₂P, leading to an enhanced catalytic performance. Despite recent advances in the studies of CoP/Co₂P@C heterostructures, current strategies for their synthesis can be further improved in terms of controlled synthesis with an effective interface, uniform dispersion, and simplification of the synthesis process.

Metal–organic frameworks (MOFs), a rapidly developed class of porous materials, have garnered significant attention across various fields due to their structural diversity, high specific surface area, and designability.^19–22 MOFs can serve as an ideal class of precursor materials for the preparation of transition metal-based carbon electrocatalysts due to their densely dispersed transition metal centers and diverse organic ligands.^23–25 To date, cobalt phosphide-based catalytic materials have been prepared using MOFs, especially the zeolitic imidazolate frameworks (ZIFs). Liu et al. fabricated a carbon based OER electrocatalyst embedded with Co₂P/CoP nanoparticles by utilizing melamine modified ZIF-9 as a precursor, followed by phosphorization at 700 °C.²⁶ Lu's group reported a nitrogen-doped carbon-decorated CoP@FeCoP yolk-shelled micro-polyhedral catalyst prepared from the ZIF-67@Co–Fe Prussian blue analogues, showing both HER and OER properties.²⁷ Lee and coauthors constructed a CoP_x-based catalyst by carbonization and phosphorization of ZIF-67, followed by coating with NiFe-layered double hydroxide and a second phosphatization process for water electrocatalysis.²⁸ Despite these excellent research studies, they mainly focus on the regulation of the composition of MOFs, such as expanding the variety of metal centers to create synergistic effects, combining with other functional molecules for introducing heteroatoms or providing extra active sites to gain bifunctionality. From the perspective of the MOF structure, leveraging their structural characteristics in the absence of extrinsic species to more purposefully and effectively construct the active sites and adjust their catalytic mechanism is a more compelling avenue to augment the catalytic efficiency and subsequently develop catalysts based on MOFs. However, research in this area is relatively scant. MOFs built from nitrogen-coordinated ligands exhibit a pronounced propensity towards forming Co–N_x and metal Co sites during the pyrolysis process, a trend extensively corroborated in the literature.^29–34 These sites are subsequently transformed into metal phosphides during phosphorization. When an oxygen-coordinated MOF is employed, the coordination environment between the metal center and oxygen would be more conducive to the formation of Co oxides, with a possible companion of metallic Co due to the reduction of carbon. Several studies have suggested that the conversion from metal oxides to phosphides is found more favorable experimentally.^35,36 Therefore, utilizing oxygen-coordinated MOFs as precursors seems more promising for highly efficient directional conversion of phosphides.

At this end, to confirm the easier conversion from metal oxides to phosphides than from Co–N_x, we first calculated the energy required for the transformation from Co–N and Co–O bonds to the Co–P bond and the results suggest that the Co–O bond is energetically and geometrically favorable in this process. Therefore, we propose a strategy herein that involves pre-construction of the Co/CoO@C heterostructure followed by post-conversion by phosphorization to achieve the precise synthesis of Co₂P/CoP@C, based on an oxygen-coordinated Co-MOF (Co-squarate) for water electrolysis. The Co-squarate MOF employed was first grown into a high-quality membrane via cathodic electrochemical deposition on the Gss. The coordination between Co and O in the Co-squarate MOF could facilitate the formation of CoO, part of which would be reduced to metal Co in the reducing environment of carbon during pyrolysis, forming a Co/CoO heterostructure. It was subsequently transformed into Co₂P/CoP through low-temperature phosphatization. The obtained Gss-Co₂P/CoP@C-800 catalyst exhibits good activity toward the HER and OER, requiring 146 and 365 mV overpotentials to achieve 100 mA cm⁻² current density, respectively, and 1.54 V to achieve a current density of 10 mA cm⁻² for water electrolysis. The growth of the MOF on the Gss enables their close contact, accelerating the reaction kinetics and durability for water electrolysis. DFT calculations reveal that the Co₂P/CoP heterostructure in Gss-Co₂P/CoP@C-800 serves as a crucial active site due to its unique d-orbital electron distribution and enhanced state density near the Fermi level, facilitating its efficient electron transport during the electrochemical process. The strategy proposed in this study can be referenced in the design and synthesis of other heterostructures based on MOFs for energy conversion applications.

Experimental

Experimental materials

All reagents were used without further purification. Cobalt chloride hexahydrate (CoCl₂·6H₂O, ≥96%), 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid, ≥98%), potassium hydroxide (KOH, ≥85%), and sodium hypophosphite (NaH₂PO₂, ≥98%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Commercial 20 wt% Pt/C, RuO₂ (99 wt%), and Nafion solution (5 wt%) were obtained from Sigma-Aldrich. The Gss was purchased from Jingxi Carbon with a diameter of 18 mm. Deionized (DI) water was used in all experiments.

Preparation of the Co-squarate MOF membrane

First, the Gss was cleaned by ultrasonic vibration in ethanol and deionized water respectively for 10 min and then dried at room temperature. A mixture of 0.71 g of CoCl₂·6H₂O, 0.91 g of squaric acid, and 0.25 g of KOH was dissolved in 100 mL of water and sonicated for a few minutes at room temperature. The electrodeposition process of the Co-squarate MOF membrane was carried out in the above solution by using the Gss as the cathode and a graphite electrode as the anode. After electrodeposition for 90 min at a voltage of 1.6 V, the Co-squarate MOF membrane was washed with water and dried in an 80 °C oven.

Preparation of Gss-Co/CoO@C-X

The above-prepared Co-squarate MOF membrane was placed in a tube furnace and purged with nitrogen for 2 h at room temperature. Then, the temperature was raised to a preset temperature at a rate of 2 °C min⁻¹ and held for 2 h. The obtained sample was noted as Gss-Co/CoO@C-X, where X represents the heating temperature.

Preparation of Gss-Co₂P/CoP@C-800

NaH₂PO₂ (2 g) and the synthesized Gss-Co/CoO@C-800 were placed upstream and downstream of the quartz tube, respectively. Under a stable nitrogen flow, the temperature was raised to 300 °C at a rate of 2 °C min⁻¹ and held for 2 h. The sample prepared was denoted as Gss-Co₂P/CoP@C-800.

Characterization

Powder X-ray diffraction (PXRD) was carried out with Cu Kα radiation (λ = 0.15418 nm) on a Rigaku Ultima IV diffractometer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for morphological and structural characterization on a Hitachi Regulus S8100 scanning electron microscope and a JEM-2100F transmission electron microscope, respectively. The influence of temperature on the structure of the MOF was evaluated by thermogravimetric analysis (TGA) on a Mettler Toledo thermogravimetric analyzer under N₂ with a ramp rate of 10 °C min⁻¹ from 40 to 900 °C. The defective carbon structure was characterized with a Raman microscope spectrometer (LABRAM HR EVO) using helium–neon (532 nm) lasers. A surface area analyzer SSA-4300 was used to collect the N₂ adsorption/desorption isotherms and the corresponding pore size distributions of the catalysts. An inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 710) was used to quantify the Co content of the catalyst. X-ray photoelectron spectra (XPS) were recorded with an ESCA250Xi spectrometer.

Electrochemical measurements and DFT calculations

All details of electrochemical measurements and DFT calculations are presented in the ESI.†

Results and discussion

For the rational design of the Co₂P/CoP@C heterostructure, a pre-constructing and post-converting strategy based on the Co-MOF was proposed. As for the selection of the Co-MOF structure, we started with the DFT calculation on the energy required for the transformation from the Co–N bond in Co–N_x and the Co–O bond in CoO to the Co–P bond. As shown in Fig. 1, the energy needed for the conversion of the Co–O bond into the Co–P bond is 1.11 eV, which is a significant energy advantage over the 4.70 eV required for the Co–N bond. Furthermore, in terms of bond length changes, the conversion of the Co–O bond into the Co–P bond involves a relatively smaller amplitude of bond length variation, further highlighting the superiority of Co oxides for post-conversion into cobalt phosphides, according to which we chose an oxygen-coordinated Co-MOF (Co-squarate) as a prototype to fabricate the target catalyst. A Co-squarate MOF membrane was first grown on the Gss via electrochemical deposition, which was transformed into the final catalyst through two steps. As presented in Fig. 2, in the first pre-constructing step, the Co-squarate MOF was pyrolyzed directionally to construct the Co/CoO@C heterostructure. The intimate association between oxygen and Co ions in Co-squarate facilitates the generation of the CoO phase at high temperatures, which was confirmed by the XRD and TEM characterization studies. In the second post-converting step, the Co/CoO@C heterostructure was converted into the Co₂P/CoP@C heterostructure through a low-temperature phosphorization with preservation of the phase interface, as revealed by the XRD and TEM characterization studies as well.

Fig. 1 The energy calculated for the conversion from the Co–N (left) and Co–O (right) bond to the Co–P bond.

Fig. 2 The schematic illustration of the preparation process of Gss-Co₂P/CoP@C-800.

To fabricate a high-quality Co-squarate MOF membrane, the impact of operating voltage on the membrane's structure and morphology was thoroughly investigated. Fig. S1† presents the XRD spectra of Co-squarate MOF membranes prepared under voltages ranging from 1.4 to 1.8 V. The diffraction peaks observed at 2θ of 26.4°, 42.4°, and 44.3° correspond to the Gss, while the additional peaks align perfectly with the simulated pattern of the Co-squarate MOF, confirming the successful deposition of Co-squarate on the Gss. The morphological characteristics of these prepared membranes were unveiled through SEM characterization, as depicted in Fig. S2.† As illustrated in Fig. S2a and 2b,† the surface of the Gss is uneven, riddled with substantial voids. At a deposition voltage of 1.4 V, only sparse Co-squarate crystals were detected on the Gss surface, and no coherent MOF membrane was formed (Fig. S2c and 2d†). Increasing the deposition voltage to 1.5 V resulted in modest growth of Co-squarate crystals on the Gss, yet significant gaps persisted between the crystals (Fig. S2e and 2f†). Further elevating the voltage to 1.6 V yielded a high-quality, polycrystalline MOF membrane with a smooth surface (Fig. S2g and 2h†). However, escalating the voltage to 1.7 V (Fig. S2i and 2j†) and 1.8 V (Fig. S2k and 2l†) expedited the membrane formation process due to the faster reaction rate, resulting in increased roughness and a higher density of MOF crystals, which were prone to detachment from the substrate. Consequently, the high-quality membrane prepared at 1.6 V was selected for subsequent conversion processes. The energy-dispersive spectroscopy (EDS) mapping of this membrane reveals a uniform distribution of C, O, and Co across its surface, as shown in Fig. S3.†

The effect of increased temperature on the Co-squarate MOF structure was analyzed by the TGA test (Fig. S4†). The observed weight loss prior to 275 °C is attributed to the elimination of both adsorbed and coordinated water. Notably, the Co-squarate framework maintained its stability up to 355 °C, with complete carbonization achieved at approximately 500 °C. As the pyrolysis temperature plays a pivotal role in the synthesis of Co/CoO@C heterostructures, higher temperatures exceeding 500 °C (specifically, 600, 700, 800, and 900 °C) were considered in this work to further investigate their effects on the Gss-Co/CoO@C-X structures. Fig. 3a illustrates the XRD patterns corresponding to the Gss-Co/CoO@C samples carbonized at various temperatures (600, 700, 800, and 900 °C). At 600 °C, diffraction peaks, apart from those attributed to the Gss, emerged at 2θ values of 36.5°, 42.4°, and 61.5°, which are assigned to the (111), (200), and (220) crystal planes of CoO (as referenced by JCPDS 48-1719), the intensity of which gradually decreases with increasing temperature. Conversely, the peaks located at 2θ of 44.2° and 51.5°, representing the (111) and (200) crystal planes of Co (JCPDS 15-0806), became progressively more pronounced as the temperature rose. These findings suggest that higher pyrolysis temperatures lead to a reduction in CoO content and a concurrent increase in Co content, indicating that the Co/CoO ratio can be fine-tuned by adjusting the carbonization temperature. Electrochemical tests further revealed that the optimal Co/CoO ratio was achieved at 800 °C, which is more favorable for obtaining the high performance catalytic materials.

Fig. 3 (a) XRD patterns of Gss-Co/CoO@C-600, 700, 800, and 900 with the localized magnification images; (b) TEM image, (c) HRTEM images, and (d and e) the corresponding spacing of the lattice fringes of Gss-Co/CoO@C-800.

The SEM images of Gss-Co/CoO@C-600, Gss-Co/CoO@C-700, Gss-Co/CoO@C-800, and Gss-Co/CoO@C-900 are depicted in Fig. S5.† After the pyrolysis process, the decomposition of organic linkers and the release of oxygen atoms lead to the disintegration of the Co-squarate crystal structure, increasing the surface roughness of the cube, and creating gaps in the membrane. Overall, the Gss-Co/CoO@C retains the morphological features of the Co-squarate MOF membrane, which might be one of the reasons for the good stability of the catalysts. The detailed microstructure of Gss-Co/CoO@C-800 was further analyzed by transmission electron microscopy (TEM) (Fig. 3b). As shown in Fig. 3c, a distinct boundary between the Co and CoO phases can be observed, confirming the successful construction of the Co/CoO@C heterostructure. The interplanar spacings of 0.211 nm (Fig. 3d) and 0.205 nm (Fig. 3e) are attributed to the (200) and (111) crystal planes of CoO and Co, respectively, in alignment with the XRD results.

After phosphatization, Gss-Co/CoO@C-800 was converted into Gss-Co₂P/CoP@C-800. The XRD pattern of Gss-Co₂P/CoP@C-800 reveals diffraction peaks at 2θ of 40.7° and 48.1° that can be well indexed to the (121) and (211) crystal planes of Co₂P (JCPDS 32-0306) and CoP (JCPDS no. 29-0497), respectively (Fig. 4a), indicating the coexistence of Co₂P and CoP in Gss-Co₂P/CoP@C-800. The morphology of Gss-Co₂P/CoP@C-800 demonstrates no significant changes compared to Gss-Co/CoO@C-800 (Fig. 4b). Further TEM characterization discloses the porous structural features of Gss-Co₂P/CoP@C-800 (Fig. 4c). High-resolution TEM images reveal distinct boundaries between the Co₂P and CoP phases, marked with yellow dashed lines. The lattice fringes with interplanar spacings of 0.189, 0.196, and 0.247 nm belong to the (211), (112) and (111) crystal planes of CoP, respectively. Meanwhile, the lattice stripes with interplanar spacings of 0.205, 0.220, and 0.221 nm are attributed to the (130), (201) and (121) crystal planes of Co₂P (Fig. 4d–i). These results confirm the formation of Co₂P/CoP@C heterostructures. As indicated by the previous studies, the creation of heterogeneous interfaces can furnish additional active sites for catalyzing the HER and OER.^37,38 EDS mappings demonstrate a homogeneous distribution of Co, C, P, and O elements within Gss-Co₂P/CoP@C-800 (Fig. 4j). Fig. S6† displays the Raman spectrum of Gss-Co₂P/CoP@C-800 in comparison with Gss-Co/CoO@C-800. The I_D/I_G value calculated for Gss-Co₂P/CoP@C-800 is 1.02, which is higher than the 0.94 of Gss-Co/CoO@C-800, indicating that phosphatization introduced a greater number of defects into the carbon structure of Gss-Co₂P/CoP@C-800. The content of the Co element in Gss-Co₂P/CoP@C-800 was determined to be 57.04 wt% via ICP analysis (Table S1†), which is lower than the 78.77 wt% found in Gss-Co/CoO@C-800, a result attributable to the weight gain from the incorporation of the P element. Further evidence was provided by N₂ adsorption–desorption measurements, which show a lower Brunauer–Emmett–Teller (BET) surface area of Gss-Co₂P/CoP@C-800 (99.02 m² g⁻¹) compared to Gss-Co/CoO@C-800 (197.63 cm² g⁻¹) (Fig. S7†). Derived from the Co-squarate MOF, which exhibits no N₂ adsorption due to its essentially non-porous structure,³⁹ the porosity of Gss-Co/CoO@C-800 originates from the collapse of the MOF's periodic framework and the release of O-related groups during the pyrolysis of the MOF. Further phosphatization altered the existence form of the Co element and its surface P–O bonding, not only leading to a weight increase but also causing steric hindrance that obstructed part of the porosity, resulting in the reduction of specific N₂ adsorption and the BET surface area. The surface wettability of Gss, Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800 was investigated via water contact angle measurements. As presented in Fig. S8,† the Gss exhibits hydrophobicity with a water contact angle of 135°, while Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800 demonstrate hydrophilicity, with water contact angles of 26° and 8°, respectively. This enhancement in hydrophilicity facilitates interactions between the active sites and the electrolyte, as well as the detachment of generated hydrogen and oxygen on the catalyst surface, thereby promoting the reaction kinetics.

Fig. 4 (a) XRD pattern, (b) SEM image, (c) TEM image, (d and e) HRTEM images and (f–i) the corresponding spacing of the lattice fringes of Gss-Co₂P/CoP@C-800; and (j) elemental mappings of Gss-Co₂P/CoP@C-800.

A deeper understanding of the elemental composition and chemical states in the Gss-Co₂P/CoP@C-800 surface was gained from XPS analysis. The survey spectrum reveals four prominent peaks located at 134.0, 294.7, 532.5, and 782.4 eV (Fig. S9†), which correspond to P 2p, C 1s, O 1s, and Co 2p, respectively. Compared to Gss-Co/CoO@C-800, there is a notable increase in surface oxygen content, coinciding with the emergence of phosphorus (Table S2†), suggesting a potential correlation between the presence of oxygen and phosphorus. High-resolution XPS spectra for these four elements are presented in Fig. 5. Specifically, Fig. 5a illustrates the Co 2p_3/2 spectrum of Gss-Co₂P/CoP@C-800, which has been deconvoluted into four peaks positioned at 779.0, 781.8, 783.3, and 786.2 eV. The peak at 779.0 eV is attributed to the low-oxidation-state Co^δ+ (where 0 < δ < 2) in Co₂P,^40,41 while the peaks located at 781.8 (Co³⁺) and 783.3 eV (Co²⁺) correspond to the Co–P and Co–PO_x species, respectively. The peak appearing at 786.2 eV is identified as a satellite peak. In comparison with Gss-Co/CoO@C-800, the peaks representing Co³⁺ and Co²⁺ exhibit a significant positive shift upon combination with phosphorus atoms, indicating a higher oxidation state of cobalt and suggesting possible electron transfer from cobalt to phosphorus in Gss-Co₂P/CoP@C-800.^42,43 This electron transfer is considered crucial for enhancing both HER and OER performance. In the P 2p spectrum (Fig. 5b), two fitted peaks are visible at 129.7 and 130.49 eV, corresponding to P 2p_3/2 and P 2p_1/2, respectively.^44–46 These peaks are associated with the formation of Co–P bonds in Gss-Co₂P/CoP@C-800.⁴⁷ Additionally, a peak emerging at 134.3 eV is ascribed to high-valence phosphorus species resulting from the oxidation of the Co₂P/CoP surface.^48,49 The C 1s spectra for both Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800 were deconvoluted into four peaks: C–C (284.6 eV), C=C (285.2 eV), O–C (286.0 eV) and O=C–O (289.2 eV) as shown in Fig. 5c.⁵⁰ The larger peak areas corresponding to C–O and O=C–O species in Gss-Co₂P/CoP@C-800 indicate a higher degree of carbon oxidation, which contributes to the observed increase in surface oxygen content. Fitting the O 1s spectrum of Gss-Co₂P/CoP@C-800 shows two peaks at 531.8 and 533.2 eV that are related to –OH groups and adsorbed water, respectively (Fig. 5d).^51,52 Notably, the Co–O peak (located at 529.9 eV) that was observed in the spectrum of Gss-Co/CoO@C-800 is absent in Gss-Co₂P/CoP@C-800, indicating successful transformation from Co/CoO to Co₂P/CoP.

Fig. 5 High-resolution XPS spectra of (a) Co 2p_3/2, (b) P 2p, (c) C 1s, and (d) O 1s of Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800.

The electrocatalytic performance of the prepared catalysts was evaluated through a standard three-electrode setup in 1.0 M KOH solution. Initially, the effect of pyrolysis temperature on HER performance was examined via LSV measurements to determine the optimal Co/CoO ratio for catalyst fabrication. As depicted in Fig. 6a, the catalyst derived from 800 °C exhibited the highest activity among its counterparts, with a smallest overpotential of 256 mV at a current density of 10 mA cm⁻². Consequently, Gss-Co/CoO@C-800 was selected as the precursor for further phosphatization transformation. The resultant Gss-Co₂P/CoP@C-800 demonstrated significantly enhanced HER performance compared to Gss-Co/CoO@C-800 (Fig. 6b), evidenced by the sharply reduced overpotential of 146 mV at a current density of 100 mA cm⁻², which is only 30 mV higher than that of 20 wt% Pt/C under the same conditions. The negligible influence of the Gss on the HER catalytic activity of Gss-Co₂P/CoP@C-800 was confirmed within the measured potential range. Notably, Gss-Co₂P/CoP@C-800 exhibited an overpotential of only 170 mV at a current density of 200 mA cm⁻², outperforming 20 wt% Pt/C (199 mV) (Fig. 6d), indicating its robust potential for high-current HER catalysis. Tafel plots derived from the linearization of LSV curves (Fig. 6c) reveal that Gss-Co₂P/CoP@C-800 possesses a Tafel slope of 64.6 mV dec⁻¹, significantly lower than the 106.3 mV dec⁻¹ observed for Gss-Co/CoO@C-800 and even less than the 73.3 mV dec⁻¹ of Pt/C. This smaller Tafel slope underscores the faster reaction kinetics of Gss-Co₂P/CoP@C-800, attributed to its efficient electron transfer kinetics. Electrochemical impedance spectroscopy (EIS) further elucidated the interfacial electron transfer dynamics of the catalysts. As shown in Fig. 6e, Gss-Co₂P/CoP@C-800 demonstrated a lower charge transfer resistance (R_ct) compared to Gss-Co/CoO@C-800, suggesting a faster charge transfer in the heterogeneous structure of Gss-Co₂P/CoP@C-800. The enhanced electron transfer capability of Gss-Co₂P/CoP@C-800 can be ascribed to partial electron transfer from Co to P atoms, as revealed by XPS analysis. This structural arrangement of Co and P sites not only facilitates electron transfer during electrochemical processes but also lowers the energy barrier for reactant adsorption–desorption, thereby enhancing reaction efficiency. To delve deeper into the intrinsic activity of these catalysts, double-layer capacitance (C_dl) was determined via cyclic voltammetry (CV) at varying scan rates in the non-Faradaic region (Fig. S10†). The calculated C_dl values for Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800 are 9.63 and 10.57 mF cm⁻², respectively (Fig. S11†), based on which their electrochemical active surface areas (ECSA) are estimated to be 240.75 and 264.25 cm². The higher ECSA of Gss-Co₂P/CoP@C-800 suggests a greater number of exposed active sites that contribute to its superior electrocatalytic performance. As an essential criterion for catalyst evaluation, the stability of Gss-Co₂P/CoP@C-800 for the HER was investigated via chronopotentiometric tests at a current density of 10 mA cm⁻² (Fig. 6f). After 48 h, the potential of Gss-Co₂P/CoP@C-800 changes from −0.10 V to −0.13 V, in contrast to the significant voltage degradation observed for Pt/C, indicating a superior HER durability of Gss-Co₂P/CoP@C-800 in alkaline media. These findings demonstrate the exceptional HER performance of Gss-Co₂P/CoP@C-800, particularly at high current densities, highlighting its potential for efficient large-current water electrolysis.

Fig. 6 (a) LSV polarization curves of Gss, Gss-Co/CoO@C-600, 700, 800 and 900 in 1 M KOH; (b) LSV polarization curves and (c) the corresponding Tafel plots of Gss, Gss-Co/CoO@C-800, Gss-Co₂P/CoP@C-800, and Pt/C; (d) overpotentials required to achieve 100 and 200 mA cm⁻² for Gss-Co/CoO@C-800, Gss-Co₂P/CoP@C-800, and Pt/C; (e) Nyquist plots of Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800; and (f) long-time chronopotentiometric tests at a current density of 10 mA cm⁻² for Gss-Co₂P/CoP@C-800 and Pt/C.

The OER activity of the samples was assessed in the same electrolyte used for the HER. As depicted in Fig. 7a, the Gss exhibits virtually negligible OER catalytic activity, demonstrating that the measured OER catalytic performance originates from the supported cobalt-based species. Similar to the HER scenario, the catalyst synthesized at 800 °C demonstrates the highest OER activity among its counterparts, with a minimum overpotential of 375 mV at a current density of 10 mA cm⁻². After phosphatization treatment, a significant enhancement in OER activity was observed for Gss-Co₂P/CoP@C-800 (Fig. 7b), which requires overpotentials of 190 and 365 mV to achieve current densities of 10 and 100 mA cm⁻², respectively, outperforming Gss-Co/CoO@C-800 (375 and 451 mV) and RuO₂ (280 and 444 mV) (Fig. 7d). The superior OER performance of Gss-Co₂P/CoP@C-800 is further substantiated by the Tafel plots. Fig. 7c illustrates the Tafel slopes of various catalysts, showing the following order: Gss-Co₂P/CoP@C-800 (66.7 mV dec⁻¹) < Gss-Co/CoO@C-800 (72.0 mV dec⁻¹) < RuO₂ (86.0 mV dec⁻¹) < Gss (216.4 mV dec⁻¹), suggesting higher reaction kinetics for Gss-Co₂P/CoP@C-800. To investigate their electron transfer capabilities for the OER, Nyquist plots were obtained in 1.0 M KOH. As shown in Fig. 7e, Gss-Co₂P/CoP@C-800 exhibits a smaller R_ct than Gss-Co/CoO@C-800, suggesting its faster charge transfer. This enhanced electron transfer can be attributed to the built-in electric field generated by charge reconfiguration of the heterostructures. The C_dl values for Gss-Co₂P/CoP@C-800 and Gss-Co/CoO@C-800 were found to be 30.6 and 17.7 mF cm⁻², respectively (Fig. S12 and S13†), corresponding to the ECSA values of 442.5 and 765.0 cm². Analogous to HER conditions, Gss-Co₂P/CoP@C-800 shows a larger electrochemically active surface area compared to Gss-Co/CoO@C-800, correlating with its higher OER activity. The durability of Gss-Co₂P/CoP@C-800 in catalyzing the OER was evaluated through chronopotentiometric tests in comparison with RuO₂. At a current density of 10 mA cm⁻², Gss-Co₂P/CoP@C-800 undergoes a minor potential increase of 40 mV (from 1.55 to 1.56 V) after continuous operation for 48 h, while RuO₂'s potential rises to 1.61 V after just 6 h from the same initial point (Fig. 7f). This suggests superior OER durability for Gss-Co₂P/CoP@C-800 over the commercial RuO₂ catalyst. Collectively, these findings underscore the high OER performance of the prepared Gss-Co₂P/CoP@C-800 catalyst, positioning it as a potential candidate for efficient electrocatalytic oxygen production.

Fig. 7 (a) LSV polarization curves of Gss, Gss-Co/CoO@C-600, 700, 800 and 900 in 1 M KOH; (b) LSV polarization curves and (c) the corresponding Tafel plots of Gss, Gss-Co/CoO@C-800, Gss-Co₂P/CoP@C-800, and RuO₂; (d) overpotentials required to achieve current densities of 10 and 100 mA cm⁻² for Gss-Co/CoO@C-800, Gss-Co₂P/CoP@C-800, and RuO₂; (e) Nyquist plots of Gss-Co/CoO@C-800 and Gss-Co₂P/CoP@C-800; and (f) long-time chronopotentiometric tests at a current density of 10 mA cm⁻² for Gss-Co₂P/CoP@C-800 and RuO₂.

Inspired by the promising HER and OER catalytic performance of Gss-Co₂P/CoP@C-800, we delved deeper into its potential as a bifunctional catalyst for water electrolysis. An electrolytic cell was assembled using Gss-Co₂P/CoP@C-800 as both the anode and cathode. As depicted in Fig. 8a, this catalyst exhibited remarkable efficiency, requiring 1.54 V to achieve a current density of 10 mA cm⁻², lower than the 1.58 V required for a similar current density employing commercial RuO₂ and Pt/C as the respective anode and cathode. Furthermore, a current density of 100 mA cm⁻² was achieved at a voltage of 1.62 V for the Gss-Co₂P/CoP@C-800-based cell, which is merely 80 mV higher than the voltage needed to attain a current density of 10 mA cm⁻², demonstrating the great potential of Gss-Co₂P/CoP@C-800 as a bifunctional catalyst for water electrolysis. To assess the performance of Gss-Co₂P/CoP@C-800 in prolonged water electrolysis, a chronopotentiometric test was conducted for continuous water splitting at the current density of 10 mA cm⁻² (Fig. 8b). Over 48 h of operation, a small voltage increase from 1.55 V to 1.58 V was detected, signifying its high durability for practical water electrolysis applications. Additionally, multi-current steps were considered to further evaluate the durability of Gss-Co₂P/CoP@C-800 (Fig. 8c). As the current density increased from 10 mA cm⁻² to a maximum of 150 mA cm⁻² and then returned to 10 mA cm⁻², the potential of the sample remained consistently stable. This excellent bifunctional catalytic performance of Gss-Co₂P/CoP@C-800 categorizes it among the high-performance catalysts based on MOFs for water electrolysis (Fig. 8d and Table S3†).

Fig. 8 (a) LSV curves of water electrolysis for Gss-Co₂P/CoP@C-800 (+//−) and Gss-RuO₂ (+)//Gss-Pt/C (−) cells in 1 M KOH (inset: photograph of the cell using Gss-Co₂P/CoP@C-800 as both the anode and cathode); (b) long-time chronopotentiometric tests at a current density of 10 mA cm⁻² for Gss-Co₂P/CoP@C-800 (+//−) and Gss-RuO₂ (+)//Gss-Pt/C (−) cells; (c) multi-current steps for the Gss-Co₂P/CoP@C-800 (+//−) cell; and (d) voltage of the Gss-Co₂P/CoP@C-800 (+//−) cell at a current density of 10 mA cm⁻² in comparison with recently reported MOF-derived electrocatalysts for water electrolysis (Table S3†).

To delve deeper into the catalytic performance of Gss-Co₂P/CoP@C-800, we leveraged density functional theory (DFT) simulation, zeroing in on the partial density of states (PDOS) analysis of the Co₂P/CoP heterostructure model, as depicted in Fig. 9a. The simulation outcomes reveal that all optimized geometric atomic structures exhibit a typical zero-bandgap metallic characteristic, a feature that vastly facilitates the unhindered migration of electrons within the catalyst. Crucially, the d-orbital electrons exhibit a highly concentrated energy distribution in the vicinity of the Fermi level, indicating their pivotal role in modulating the electronic properties of Gss-Co₂P/CoP@C-800 and driving chemical reactions.⁵³ Upon further comparison, it becomes evident that the Co₂P/CoP heterostructure boasts a more pronounced continuity and a significant enhancement in electron density at the Fermi level. This unique property is recognized as one of the key factors contributing to the high catalytic performance of this heterostructure. As shown in Fig. 9b, we have conducted a detailed analysis of the d-orbital electron distribution and d-band center (ε_d) for the three models (Co₂P/CoP, CoP, and Co₂P). The specific calculations reveal that the d-band center positions for Co₂P/CoP, CoP, and Co₂P are −0.34, 0.83, and 0.54 eV, respectively. As confirmed by previous studies, the closer the d-band center is to the Fermi level, the better the catalytic performance of the material.⁵⁴ The ε_d value of Co₂P/CoP stands out as being particularly close to the Fermi level compared to CoP and Co₂P, implying that it can promote the electron transfer process more efficiently from d-electrons to adsorbates. Consequently, the Co₂P/CoP heterostructure is poised to achieve higher electron transport efficiency, thereby enhancing the rate of catalytic reactions.

Fig. 9 (a) The projected density of states (PDOS), (b) density of states (DOS) and D-band centers, and (c) charge density differences and Bader charge of Co₂P/CoP, (d) Gibbs free energy diagrams of the OER and (e) Gibbs free energy plots of the HER on Co₂P/CoP, CoP and Co₂P.

The state density and Bader charge of the Co₂P/CoP heterostructure are illustrated in Fig. 9c, where the green regions represent electron deficiency, and the yellow regions signify electron accumulation. It reveals that electrons primarily concentrate within the heterojunction formed by the two phases, with significantly more electron transfer from the Co₂P phase compared to the CoP phase. Consequently, the periphery of the heterojunction is likely to serve as an important active center. The lower electron density at the Co₂P site facilitates the adsorption of electron-rich oxygen atoms, while the CoP region proximal to the heterojunction potentially serves as a crucial site for hydrogen (H) adsorption. The unique electron distribution characteristics of the Co₂P/CoP heterostructure are essential for understanding the catalytic mechanism of Gss-Co₂P/CoP@C-800. Moreover, the Gibbs free energies for all steps of the OER and HER were calculated, and the results are presented in Fig. 9d and e, respectively. Fig. S14† showcases the optimized adsorption models corresponding to these free energy calculations. Across all three models (Co₂P/CoP, CoP, and Co₂P), we observed an increased energy barrier during the transition from *OH to *O. Notably, the highest uphill free energy encountered in the final electron transfer step of this process serves as the rate-determining step for the OER. The energy barriers for this slowest step were calculated to be 1.44, 1.65, and 1.82 eV, respectively, on the Co₂P/CoP, CoP, and Co₂P models, unequivocally demonstrating the more favorable conditions for the OER on the Co₂P/CoP heterostructure. Similarly, the HER analysis of the hydrogen adsorption free energies for the three models yielded energy values of −0.08, −0.35, and −0.54 eV, respectively, further corroborating the superior performance of the Co₂P/CoP heterostructure compared with CoP and Co₂P.

Conclusions

In summary, we have proposed a pre-constructing and post-converting strategy to prepare the Gss-Co₂P/CoP@C-800 catalyst for water electrolysis. In this strategy, a Co/CoO@C heterostructure was constructed first by using an oxygen-coordinated Co-squarate MOF as the precursor, utilizing its structural advantages in close proximity to metal centers and oxygen atoms to generate the CoO phase. Parts of the CoO species would be reduced to metallic Co under the influence of the generated carbon, giving a desired Co/CoO@C heterostructure, which was then converted into the target Co₂P/CoP@C by a phosphatization step. This strategy results in a catalyst (Gss-Co₂P/CoP@C-800) with unique features, including Co₂P/CoP heterointerfaces, large surface area and good electronic conductivity, endowing the catalyst with enhanced catalytic activity for the HER and OER in 1.0 M KOH. Gss-Co₂P/CoP@C-800 requires low overpotentials of 146 and 365 mV, respectively, to achieve a current density of 100 mA cm⁻², outperforming the commercial 20 wt% Pt/C and RuO₂. Remarkably, an alkaline electrolyzer employing Gss-Co₂P/CoP@C-800 as both the anode and cathode attains a current density of 10 mA cm⁻² at a low voltage of 1.54 V with demonstrated long-term stability. This work provides valuable insights into the rational design and optimization of transition metal phosphide electrocatalysts for efficient energy conversion applications.

Author contributions

Chongxi Zhang: investigation, data curation, and writing – original draft. Fengting Li: software and formal analysis. Dong Wu: investigation and methodology. Qingmeng Guo: data curation and validation. Zhanning Liu: methodology and formal analysis. Zhikun Wang: software and formal analysis. Zixi Kang: resources and writing – review & editing. Lili Fan: conceptualization, supervision, resources, and writing – review & editing. Daofeng Sun: project administration, supervision, and writing – review & editing.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 22275210 and 22171288), the Natural Science Foundation of Shandong Province (ZR2022MB009 and ZR2024MB128), and the Outstanding Youth Science Fund Projects of Shandong Province (ZR2022YQ15).

References

1. M. S. Dresselhaus and I. Thomas, Alternative energy technologies, Nature, 2001, 414, 332–337.
2. Z. Y. Yu, Y. Duan, X. Y. Feng, X. Yu, M. R. Gao and S. H. Yu, Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects, Adv. Mater., 2021, 33, 2007100.
3. F. Song and X. Hu, Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis, Nat. Commun., 2014, 5, 4477.
4. J. Wang, G. Wang, B. Cheng, J. Yu and J. Fan, Sulfur-doped g-C₃N₄/TiO₂ S-scheme heterojunction photocatalyst for Congo Red photodegradation, Chin. J. Catal., 2021, 42, 56–68.
5. J. Du, F. Li and L. Sun, Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction, Chem. Soc. Rev., 2021, 42, 2663–2695.
6. B. You, M. T. Tang, C. Tsai, F. Abild-Pedersen, X. Zheng and H. Li, Enhancing electrocatalytic water splitting by strain engineering, Adv. Mater., 2019, 31, 1807001.
7. Z. W. Chen, J. Li, P. Ou, J. E. Huang, Z. Wen, L. Chen, X. Yao, G. Cai, C. C. Yang and C. V. Singh, Unusual Sabatier principle on high entropy alloy catalysts for hydrogen evolution reactions, Nat. Commun., 2024, 15, 359.
8. H. J. Kim, E. Hong, Y. Hong, J. Kim, M. K. Kabiraz, Y.-M. Kim, H. Lee, W. S. Seo and S.-I. Choi, Surface distortion of FeRu nanoparticles improves the hydrogen evolution reaction performance in alkaline media, Nanoscale, 2023, 15, 5816–5824.
9. S. Li, T. Wang and Q. Li, Tuning metal-support interaction of Pt-based electrocatalysts for hydrogen energy conversion, Sci. China: Chem., 2023, 66, 3398–3414.
10. M. Sayed, J. Yu, G. Liu and M. Jaroniec, Non-noble plasmonic metal-based photocatalysts, Chem. Rev., 2022, 122, 10484–10537.
11. Y. Wang, X. Li, Z. Huang, H. Wang, Z. Chen, J. Zhang, X. Zheng, Y. Deng and W. Hu, Amorphous Mo-doped NiS0.5Se0.5 nanosheets@crystalline NiS0.5Se0.5 nanorods for high current-density electrocatalytic water splitting in neutral media, Angew. Chem., Int. Ed., 2023, 62, e202215256.
12. H. Yao, S. Wang, Y. Cao, R. Chen, Z. Lu, J. Hu, J. Xie and A. Hao, High-performance bifunctional electrocatalysts of CoFe-LDH/NiCo2O4 heterostructure supported on nickel foam for effective overall water splitting, J. Alloys Compd., 2022, 926, 166846.
13. Z. Zang, Q. Guo, X. Li, Y. Cheng, L. Li, X. Yu, Z. Lu, X. Yang, X. Zhang and H. Liu, Construction of a S and Fe co-regulated metal Ni electrocatalyst for efficient alkaline overall water splitting, J. Mater. Chem. A, 2023, 11, 4661–4671.
14. Z. Pu, T. Liu, I. S. Amiinu, R. Cheng, P. Wang, C. Zhang, P. Ji, W. Hu, J. Liu and S. Mu, Transition-metal phosphides: activity origin, energy-related electrocatalysis applications, and synthetic strategies, Adv. Funct. Mater., 2020, 30, 2004009.
15. Y. Shi, M. Li, Y. Yu and B. Zhang, Recent advances in nanostructured transition metal phosphides: synthesis and energy-related applications, Energy Environ. Sci., 2020, 13, 4564–4582.
16. X. Cui, L. Lin, T. Xu, J. Liu, M. Tang and Z. Wang, In situ fabrication of MOF@CoP hybrid bifunctional electrocatalytic nanofilm on carbon fibrous membrane for efficient overall water splitting, Int. J. Hydrogen Energy, 2024, 49, 1446–1457.
17. Z. Jin, P. Li and D. Xiao, Metallic Co₂P ultrathin nanowires distinguished from CoP as robust electrocatalysts for overall water-splitting, Green Chem., 2016, 18, 1459–1464.
18. W. Gong, H. Zhang, L. Yang, Y. Yang, J. Wang and H. Liang, Core@shell MOFs derived Co₂P/CoP@NPGC as a highly-active bifunctional electrocatalyst for ORR/OER, J. Ind. Eng. Chem., 2022, 106, 492–502.
19. I. E. Khalil, J. Fonseca, M. R. Reithofer, T. Eder and J. M. Chin, Tackling orientation of metal-organic frameworks (MOFs): The quest to enhance MOF performance, Coord. Chem. Rev., 2023, 481, 215043.
20. Y. Y. Xue, X. Y. Bai, J. Zhang, Y. Wang, S. N. Li, Y. C. Jiang, M. C. Hu and Q. G. Zhai, Precise Pore Space Partitions Combined with High-Density Hydrogen-Bonding Acceptors within Metal-Organic Frameworks for Highly Efficient Acetylene Storage and Separation, Angew. Chem., Int. Ed., 2021, 60, 10122–10128.
21. C. Li, H. Zhang, M. Liu, F.-F. Lang, J. Pang and X.-H. Bu, Recent progress in metal–organic frameworks (MOFs) for electrocatalysis, Ind. Chem. Mater., 2023, 1, 9–38.
22. I. Senkovska, V. Bon, L. Abylgazina, M. Mendt, J. Berger, G. Kieslich, P. Petkov, J. L. Fiorio, J.-O. Joswig, T. Heine, L. Schaper, C. Bachetzky, R. Schmid, R. A. Fischer, A. Pöppl, E. Brunner and S. Kaskel, Understanding MOF Flexibility: An Analysis Focused on Pillared Layer MOFs as a Model System, Angew. Chem., Int. Ed., 2023, 62, e202218076.
23. A. Kumar and S. Bhattacharyya, Porous NiFe-oxide nanocubes as bifunctional electrocatalysts for efficient water-splitting, ACS Appl. Mater. Interfaces, 2017, 9, 41906–41915.
24. R. Deng, M. Guo, C. Wang and Q. Zhang, Recent advances in cobalt phosphide-based materials for electrocatalytic water splitting: From catalytic mechanism and synthesis method to optimization design, Nano Mater. Sci., 2024, 6, 139–173.
25. C. Zhang, Z. Xing, Y. Peng, H. Zhou, L. Zhang and Z.-H. Lu, Inlaying CoP/Ni2P/Fe2P triple heterostructure in MOF-derived carbon nanobox for robust oxygen evolution reaction, Fuel, 2024, 365, 131181.
26. H. Liu, S. Yang, J. Ma, M. Dou and F. Wang, Surface engineering of MOFs as a route to cobalt phosphide electrocatalysts for efficient oxygen evolution reaction, Nano Energy, 2022, 98, 107315.
27. J. Shi, F. Qiu, W. Yuan, M. Guo and Z.-H. Lu, Nitrogen-doped carbon-decorated yolk-shell CoP@FeCoP micro-polyhedra derived from MOF for efficient overall water splitting, Chem. Eng. J., 2021, 403, 126312.
28. E. Vijayakumar, S. Ramakrishnan, C. Sathiskumar, D. J. Yoo, J. Balamurugan, H. S. Noh, D. Kwon, Y. H. Kim and H. Lee, MOF-derived CoP-nitrogen-doped carbon@NiFeP nanoflakes as an efficient and durable electrocatalyst with multiple catalytically active sites for OER, HER, ORR and rechargeable zinc-air batteries, Chem. Eng. J., 2022, 428, 131115.
29. M. Cong, D. Sun, L. Zhang and X. Ding, In situ assembly of metal-organic framework-derived N-doped carbon/Co/CoP catalysts on carbon paper for water splitting in alkaline electrolytes, Chin. J. Catal., 2020, 41, 242–248.
30. J. Lei, Z. Li, C. Wang, H. Yao, X. Wang and J. Hu, Nanocage confined nitrogen-rich MOF-derived Co/CoN heterostructure for overall water splitting, Chem. Eng. Sci., 2024, 293, 120068.
31. A. Li, L. Zhang, F. Wang, L. Zhang, L. Li, H. Chen and Z. Wei, Rational design of porous Ni-Co-Fe ternary metal phosphides nanobricks as bifunctional electrocatalysts for efficient overall water splitting, Appl. Catal., B, 2022, 310, 121353.
32. S. Yu, J. Li, Y. Du, Y. Wang, Y. Zhang and Z. Wu, Sulfur-modified MOFs as efficient electrocatalysts for overall water splitting, Coord. Chem. Rev., 2024, 520, 216144.
33. Y. Pan, K. Sun, S. Liu, X. Cao, K. Wu, W.-C. Cheong, Z. Chen, Y. Wang, Y. Li, Y. Liu, D. Wang, Q. Peng, C. Chen and Y. Li, Core–Shell ZIF-8@ZIF-67-Derived CoP Nanoparticle-Embedded N-Doped Carbon Nanotube Hollow Polyhedron for Efficient Overall Water Splitting, J. Am. Chem. Soc., 2018, 140, 2610–2618.
34. X. Lv, J. Ren, Y. Wang, Y. Liu and Z.-Y. Yuan, Well-Defined Phase-Controlled Cobalt Phosphide Nanoparticles Encapsulated in Nitrogen-Doped Graphitized Carbon Shell with Enhanced Electrocatalytic Activity for Hydrogen Evolution Reaction at All-pH, ACS Sustainable Chem. Eng., 2019, 7, 8993–9001.
35. B. Liu, B. Zhong, F. Li, J. Liu, L. Zhao, P. Zhang and L. Gao, Co₂P/CoP heterostructures with significantly enhanced performance in electrocatalytic hydrogen evolution reaction: Synthesis and electron redistribution mechanism, Nano Res., 2023, 16, 12830–12839.
36. E. Cao, Z. Chen, H. Wu, P. Yu, Y. Wang, F. Xiao, S. Chen, S. Du, Y. Xie, Y. Wu and Z. Ren, Boron-Induced Electronic-Structure Reformation of CoP Nanoparticles Drives Enhanced pH-Universal Hydrogen Evolution, Angew. Chem., Int. Ed., 2020, 59, 4154–4160.
37. G. Huang, M. Hu, X. Xu, A. A. Alothman, M. S. S. Mushab, S. Ma, P. K. Shen, J. Zhu and Y. Yamauchi, Optimizing Heterointerface of Co2P–CoxOy Nanoparticles within a Porous Carbon Network for Deciphering Superior Water Splitting, Small Struct., 2023, 4, 2200235.
38. W. Wu, Y. Huang, X. Wang, P. K. Shen and J. Zhu, Composition-optimized manganese phosphide nanoparticles anchored on porous carbon network for efficiently electrocatalytic hydrogen evolution, Chem. Eng. J., 2023, 469, 143879.
39. L. Li, L. Guo, Z. Zhang, Q. Yang, Y. Yang, Z. Bao, Q. Ren and J. Li, A Robust Squarate-Based Metal-Organic Framework Demonstrates Record-High Affinity and Selectivity for Xenon over Krypton, J. Am. Chem. Soc., 2019, 141, 9358–9364.
40. H. Zhang, K. Zhang, S. Ashraf, Y. Fan, S. Guan, X. Wu, Y. Liu, B. Liu and B. Li, Polar O–Co–P Surface for Bimolecular Activation in Catalytic Hydrogen Generation, Energy Environ. Mater., 2022, 6, e12273.
41. L. Chen, Y. Zhang, H. Wang, Y. Wang, D. Li and C. Duan, Cobalt layered double hydroxides derived CoP/Co₂P hybrids for electrocatalytic overall water splitting, Nanoscale, 2018, 10, 21019–21024.
42. Y. Zhang, H. Liu, R. Ge, J. Yang, S. Li, Y. Liu, L. Feng, Y. Li, M. Zhu and W. Li, Mo-induced in situ architecture of Ni_xCo_yP/Co₂P heterostructure nano-networks on nickel foam as bifunctional electrocatalysts for overall water splitting, Sustainable Mater. Technol., 2022, 33, e00461.
43. J. Liu, Y. Gao, X. Tang, K. Zhan, B. Zhao, B. Y. Xia and Y. Yan, Metal–organic framework-derived hierarchical ultrathin CoP nanosheets for overall water splitting, J. Mater. Chem. A, 2020, 8, 19254–19261.
44. Y. Hao, Y. Xu, W. Liu and X. Sun, Co/CoP embedded in a hairy nitrogen-doped carbon polyhedron as an advanced tri-functional electrocatalyst, Mater. Horiz., 2018, 5, 108–115.
45. Q. Shi, Q. Liu, Y. Zheng, Y. Dong, L. Wang, H. Liu and W. Yang, Controllable Construction of Bifunctional Co_xP@N,P-Doped Carbon Electrocatalysts for Rechargeable Zinc-Air Batteries, Energy Environ. Mater., 2021, 5, 515–523.
46. Z. Lu, Y. Cao, J. Xie, J. Hu, K. Wang and D. Jia, Construction of Co₂P/CoP@Co@NCNT rich-interface to synergistically promote overall water splitting, Chem. Eng. J., 2022, 430, 132877.
47. T. S. Kim, H. J. Song, J. C. Kim, B. Ju and D. W. Kim, 3D Architectures of Co_xP Using Silk Fibroin Scaffolds: An Active and Stable Electrocatalyst for Hydrogen Generation in Acidic and Alkaline Media, Small, 2018, 14, 1801284.
48. K. Zhang, G. Zhang, Q. Ji, J. Qu and H. Liu, Arrayed Cobalt Phosphide Electrocatalyst Achieves Low Energy Consumption and Persistent H₂ Liberation from Anodic Chemical Conversion, Nano-Micro Lett., 2020, 12, 154.
49. L. Bo, W. Liu, R. Liu, Y. Hu, L. Pu, F. Nian, Z. Zhang, P. Li and J. Tong, CoP/Co₂P hollow spheres embedded in porous N-doped carbon as highly efficient multifunctional electrocatalyst for Zn–air battery driving water splitting device, Electrochim. Acta, 2022, 403, 139643.
50. M. Li, S. Guan, L. An, H. Zhang, Y. Chen, J. Shi, Y. Fan and B. Liu, Protection and confinement effect of carbon on Co/Co_xO_y nano-catalyst for efficient NaBH₄ hydrolysis, Int. J. Hydrogen Energy, 2022, 47, 20185–20193.
51. H. Wang, Q. Gao, S. Sun, W. Zhang and S. Yao, CoP@NRGO composite as a high-efficiency water electrolysis catalyst for hydrogen generation, J. Solid State Chem., 2020, 290, 121596.
52. Y. Zeng, X. Zhu, Y. Zhang, S. Chen, W. Zhang and L. Wang, One-step novel synthesis of Co₂P/CoP and its hydrogen evolution reaction performance in alkaline media, Mater. Chem. Phys., 2022, 277, 125419.
53. Y.-N. Zhou, F.-L. Wang, J. Nan, B. Dong, H.-Y. Zhao, F.-G. Wang, N. Yu, R.-N. Luan, D.-P. Liu and Y.-M. Chai, High-density ultrafine RuP₂ with strong catalyst-support interaction driven by dual-ligand and tungsten-oxygen sites for hydrogen evolution at 1 A cm⁻², Appl. Catal., B, 2022, 304, 120917.
54. Y.-N. Zhou, F.-T. Li, B. Dong and Y.-M. Chai, Double self-reinforced coordination modulation constructing stable Ni⁴⁺ for water oxidation, Energy Environ. Sci., 2024, 17, 1468–1481.

---

Footnotes

† Electronic supplementary information (ESI) available: Details of electrochemical measurements, DFT calculations, XRD patterns, SEM images and elemental mapping results, Raman spectra, N₂ adsorption–desorption isotherms, water contact angles, XPS spectra, CV curves, double-layer capacitance and optimized adsorption models for DFT calculations, tables of ICP results, XPS data and comparison with reported electrocatalysts. See DOI: https://doi.org/10.1039/d4qi03103b

‡ These authors contributed equally to this work.

---