Optimizing Heterointerface of Co2P–Co
x
O
y
 Nanoparticles within a Porous Carbon Network for Deciphering Superior Water Splitting

It is of great significance to design a bifunctional electrocatalyst for promoting hydrogen (HER) and oxygen (OER) evolution reactions simultaneously. Herein, inspired by the appropriate H atom binding energy on cobalt phosphides and excellent oxygen evolution kinetics on cobalt oxides, the regulative synthesis of a Co2P–Co x O y (Co x O y  = CoO or Co3O4) heterogeneous nanoparticle‐anchored porous carbon network electrocatalyst via one‐pot heat treatment is reported. The as‐synthesized Co2P–Co3O4/C exhibits superior electrochemical activity with low overpotentials of 86 mV for HER and 246 mV for OER at 10 mA cm−2 in an alkaline electrolyte. Moreover, compared to the commercial Pt/C || RuO2/C system, the Co2P–Co3O4/C || Co2P–Co3O4/C system presents outstanding activity toward overall water splitting (1.55 V@10 mA cm−2), which is well maintained over long‐term (120 h) electrocatalysis. Density functional theory calculations show that the rich interfaces between Co2P and Co3O4 offer a synergistic effect, which enables Co2P–Co3O4/C as an excellent electrocatalyst toward both HER and OER.


Introduction
The development of efficient and economical electrocatalysts is of great significance for the production of green hydrogen via water electrolysis. [1] Pt-and Ru-based materials are considered to be the most efficient electrocatalysts toward hydrogen (HER) and oxygen (OER) evolution reactions. However, the scarcity and high price of these noble metals limit their practical deployment. [2] Non-noble and Earth-abundant metals, such as transition metal (Fe, [3] Co, [4] Ni, [5] and Mn [6] )-based materials, have therefore been widely fabricated as efficient electrocatalysts toward water splitting because of their unique electronic structures, low price, and environmental friendliness. [7] As a notable example, cobalt phosphide represents one of the most promising HER catalysts, in which cobalt sites ensure suitable hydrogen adsorption free energy, and the negatively charged P atom can extract electrons from the metal atom as a proton receptor. [8] On the other hand, transition metal (oxy) hydroxides generally exhibit excellent oxygen evolution activity. In particular, cobalt oxides have shown higher intrinsic activity than nickel and ferrite (oxy)hydroxides, [9] with Co 3 O 4 being located at the apex of the oxygen evolution volcano diagram. As a popular OER catalyst, Co 3 O 4 is a typical spinel oxide in which Co 2þ cations and oxygen anions form tetrahedral structures, and Co 3þ cations and oxygen anions form octahedral structures, resulting in good stability. The coexistence of Co 2þ and Co 3þ cations contributes to the excellent redox ability and high catalytic activity of Co 3 O 4 . [10] However, in alkaline solutions, the HER kinetics is limited to the Volmer step, and the OER oxidation process is complex, resulting in poor overall water decomposition efficiency. Therefore, it would be advantageous to have an electrode that could catalyze both OER and HER reactions concurrently. Combining the inherent catalytic properties of cobalt phosphide and cobalt oxide has emerged as a promising strategy. The catalyst structure can be adjusted by designing interface engineering. [11] Different components combine to form a heterostructure interface, which can provide sites for species adsorption/desorption, thereby improving bifunctional catalytic activity and achieving large-scale commercial applications.
In terms of synthesis, transition metal nanoparticles (TMNs) are easily oxidized into the corresponding metal oxides, yet the control of the degree of oxidation remains challenging, which is easily affected by the particle size, morphology, and synthesis conditions involved. [12] Meanwhile, regulating the appearance, morphology, and size of metal phosphide nanomaterials remains a thorny problem. For the synthesis of cobalt phosphide, the asreported methods mainly include solution-phase reactions, [13] hydrothermal reactions, [14] plasma-assisted methods, [7a] and electrodeposition methods. [15] However, these strategies require strict reaction conditions and the processes involved are complex and dangerous. In addition, the agglomeration and accumulation of nanoparticles lead to insufficient catalytic sites and performances. As a reliable strategy, carbon materials with high conductivity, large specific surface areas, and excellent stability have been introduced as supports. In particular, these carbon materials have been included in porous network structures because open pores benefit the uniform distribution of nanoparticles. [16] Hence, directly embedding highly active nanocatalysts into an appropriate carbon matrix provides a simple way to develop a variety of transition metal-based nanomaterials with high dispersity, activity, and stability. [17] With these facts in mind, in this study, we successfully construct Co 2 P-Co x O y /C (Co x O y ¼ CoO or Co 3 O 4 ) by simply heating a mixture of KOH and Co 2þ chelating resin. These heterogeneous Co 2 P-Co x O y nanoparticles are uniformly anchored into a porous carbon network to improve the conductivity, facilitate the penetration of the electrolyte and transmission of active substances, and enhance the reaction kinetics. Notably, the Co 2 P-Co 3 O 4 heterogeneous nanoparticles combine the highly active HER and OER components, and their mutual coupling greatly can boost the catalytic activity. Thus, the as-synthesized catalyst shows superior electrocatalytic activity and excellent stability toward both HER and OER. When this catalyst is used as both a cathodic and anodic electrocatalyst, the corresponding watersplitting device only requires a voltage of 1.55 V to achieve a current density of 10 mA cm À2 and operates for 120 h without noticeable decay, outperforming the commercial Pt/C || RuO 2 / C system. This work inspires the design and synthesis of lowcost, high-performance bifunctional electrocatalysts.

Results and Discussion
The synthetic route of the Co 2 P-Co 3 O 4 heterogeneous structure anchored into the porous carbon network is schematically illustrated in Figure 1A. The presence of Co 2þ chelates and the amino methyl phosphonate group in the resin affords a uniform Co 2þ distribution at the molecular level. [18] In our designed strategy, KOH acts as an activator and reacts with the carbon matrix to produce many pores. The X-ray diffraction (XRD) results confirm the coexistence of Co 3 O 4 and Co 2 P components in Co 2 P-Co 3 O 4 /C without impurities. Figure 1B displays the XRD pattern of Co 2 P-Co 3 O 4 /C, in which the characteristic peaks located at 40.7, 40.9, 43.2, and 44°can be indexed to the (121), (201), (220), and (130) planes of orthorhombic Co 2 P (JCPDS no. 32-0306) and the diffraction peaks at 36.8 and 31.2°to the (311) and (220) planes of cubic-phase Co 3 O 4 (JCPDS no. 42-1467), respectively. As shown in Figure 1C, Co 2 P-Co 3 O 4 /C exhibits a 3D network porous frame structure with bright particles evenly distributed on the surface and/or inside the holes. The pores are composed of open mesopores and macropores, which provide 1) a large active surface area, thereby favoring electrolyte penetration, and 2) rich channels for bubble diffusion to accelerate the HER and OER dynamics. The transition electron microscopy (TEM) image ( Figure 1D) clearly illustrates that numerous nanoparticles (%40 nm in size) are uniformly distributed in the multilayer porous carbon network ( Figure 2B-H). Moreover, the high-resolution TEM (HRTEM) images ( Figure 1E,F) confirm the presence of a carbon layer with a thickness of %10 nm in the outer layer of Co 2 P-Co 3 O 4 /C. This carbon layer does not hinder the transfer of active electrons from the inside to the surface of the carbon layer. The carbon shell coating can not only provide sufficient physical protection for the internal metal particles but also not affect the chemical reaction on the catalyst surface, effectively improving the stability of the catalyst. Figure 1G shows clear diffraction fringes, in which the interplanar spacings of 0.202 and 0.221 nm correspond to the (400) plane of Co 3 O 4 and (121) plane of Co 2 P, respectively. There are abundant interfaces between the Co 2 P and Co 3 O 4 phases, which provide abundant catalytic active sites to promote the rapid transfer of electrons and offer a synergistic role to enhance HER/OER electrocatalysis. [19] In addition, the selected-area electron diffraction (SAED) pattern of Co 2 P-Co 3 O 4 ( Figure 1H) shows multiple rings and scattering dots of Co 2 P and Co 3 O 4 , which prove the existence of Co 2 P-Co 3 O 4 heterostructure. The compositional distribution of Co 2 P-Co 3 O 4 /C was further investigated by energy-dispersive X-ray spectroscopy (EDX) mapping ( Figure 1I). The results reveal that elemental O, Co, and P are uniformly distributed across the whole Co 2 P-Co 3 O 4 nanoparticle, and the surface of the nanoparticle is covered with both an oxide and a carbon layer.
A series of characterizations of the synthesized Co 2 P-CoO/C were also performed ( Figure 2). The XRD pattern ( Figure 2A) of Co 2 P-CoO/C confirms the coexistence of CoO and Co 2 P components in Co 2 P-CoO/C without impurities. SEM and TEM images show that metal particles are evenly distributed in the porous carbon network ( Figure 2B-H). The HRTEM images ( Figure 2F,G) show lattice stripes, in which the crystal plane spacings of 0.221 and 0.213 nm correspond to the (121) plane of Co 2 P (JCPDS no. 32-0306) and (200) plane of CoO (PDF no. 43-1004), respectively. Figure S1, Supporting Information, shows the pure Co 2 P/C phase and a uniform pore carbon network structure. Co 2 P-CoO/C and Co 2 P/C display similar morphologies to that of Co 2 P-Co 3 O 4 /C. These results confirm the controllable oxidation of Co 2 P. By introducing a certain amount of O 2 (mixed gas V Nitrogen /V Oxygen ¼ 95:5) during heat treatment, Co 2 P is partly converted to the cobalt oxides with controlled stoichiometric ratios of Co and O. As a result, Co 2 P-Co 3 O 4 /C is obtained by extending the injection time to ensure sufficient O 2 content.
X-ray photoelectron spectroscopy (XPS) was performed to determine the surface electronic state of Co 2 P-Co 3 O 4 /C.
The survey XPS spectrum shows the presence of elemental C, Co, P, and O, which is consistent with the EDX results. As shown in Figure S2A, Supporting Information, the high-resolution Co 2p spectrum of Co 2 P-Co 3 O 4 /C presents three pairs of peaks at 780.7/796.3, 782.5/797.5, and 786.4/802.7 eV assigned to Co-P, Co-O, and satellite signals, respectively. [20] As shown in the P 2p spectrum ( Figure S2B, Supporting Information), the peaks located at 133.5 and 134.4 eV are assigned to the P─C and P─O bonds, respectively. There are two small peaks at 130.4 and 129.2 eV, which are attributed to P 2p 1/2 and P 2p 3/2 in the Co 2 P phase, respectively. The Co-P peak is relatively weak, which may be because the surface is covered by an oxide layer and carbon layer. [21] The O 1s spectrum of Co 2 P-Co 3 O 4 /C ( Figure S2C, Supporting Information) can be deconvoluted into four peaks, namely, the peak located at 530.5 eV derived from the Co-O lattice oxygen, two peaks located at 531.4 and 532.3 eV corresponding to the oxygen vacancy and OH À species, respectively, and the peak at 533.8 eV corresponding to the physically adsorbed water. [22] The presence of oxygen vacancies is conducive to lattice recombination, which affects surface desorption and intermediate formation. In turn, this improves the catalytic reaction rate and plays an important role in enhancing the OER performance. [23] The C 1s spectrum ( Figure S2D, Supporting Information) displays three main peaks, corresponding to the C─C/C═C (284.7 eV), C─P/C─N (286.1 eV), and C─C═O (289 eV) bonds, respectively. [24] Survey XPS spectrum and high-resolution XPS spectra for Co 2 P─CoO/C and Co 2 P/C are shown in Figure S3 and S4, Supporting Information.
To further explore the structural and compositional features of Co 2 P-Co 3 O 4 /C, Raman spectra were also recorded ( Figure S5, Supporting Information). The Raman spectrum of Co 2 P-Co 3 O 4 /C shows two characteristic carbon peaks at 1350 and 1585 cm À1 , corresponding to the D and G bands (I D /I G ¼ 1.01), respectively. In addition, the peak at 663 cm À1 is ascribed to the Co-O stretching of Co 2 P-Co 3 O 4 /C. [7a] The characteristic peak of Co 2 P-Co 3 O 4 /C shows a slight blueshift compared to that of Co 2 P-CoO/C, indicating the generation of oxygen vacancies. [23a] The N 2 isothermal adsorption-desorption curve ( Figure S6, Supporting Information) of Co 2 P-Co 3 O 4 /C confirms the presence of a porous structure. Co 2 P-Co 3 O 4 /C presents the largest surface area (327.2 m 2 g À1 ) compared to those of Co 2 P-CoO/C (260.2 m 2 g À1 ) and Co 2 P/C (274.3 m 2 g À1 ). In addition, the pore distribution curve of Co 2 P-Co 3 O 4 /C shows that it is mainly   Table S2, Supporting Information.
www.advancedsciencenews.com www.small-structures.com composed of micropores and mesoporous pores, which are mainly derived from the resin and the pore-forming action of KOH. The electrocatalytic HER performance of the as-prepared Co 2 P-Co 3 O 4 /C was first evaluated in 1.0 M KOH solution with a typical three-electrode system. Co 2 P-CoO/C, Co 2 P/C, and Pt/C were also tested for comparison. As shown in Figure 3A, the iR-compensated linear sweep voltammetry (LSV) curves show that Co 2 P-Co 3 O 4 /C exhibits excellent HER activity. A low overpotential of 86 mV (vs reversible hydrogen electrode (RHE)) was required to afford a current density of 10 mA cm À2 , which is comparable to the value reported for Pt/C (40 mV vs RHE) and superior to those of Co 2 P-CoO/C (120 mV vs RHE) and Co 2 P/C (182 mV vs RHE). The Tafel diagrams obtained from the LSV curves further verify that Co 2 P-Co 3 O 4 /C displays favorable reaction kinetics ( Figure 3B). Specifically, the Tafel slope of Co 2 P-Co 3 O 4 /C is only 49.7 mV dec À1 , which is much lower than those of Co 2 P-CoO/C (69.8 mV dec À1 ) and Co 2 P/C (103.7 mV dec À1 ). These results indicate that the HER process on the Co 2 P-Co 3 O 4 /C electrocatalyst follows the Volmer-Heyrovsky mechanism. [25] In addition, electrochemical impedance spectroscopy (EIS) and powder resistivity were tested to explore the charge transfer kinetics during the HER process. Co 2 P-Co 3 O 4 /C presents the lowest charge transfer resistance (R ct ) value of 7.06 Ω ( Figure S7 and Table S1, Supporting Information) and powder resistivity value of 1.086 Ω cm ( Figure S8, Supporting Information) compared to those of Co 2 P-CoO/C and Co 2 P/C, suggesting that Co 2 P-Co 3 O 4 /C has the fastest charge transfer kinetics. To further evaluate the intrinsic activity of the Co 2 P-Co 3 O 4 /C catalyst, cyclic voltammetry (CV) tests were carried out in a non-Faradaic region (0.75-0.8 V vs RHE) with various scanning speeds in 1.0 M KOH solution ( Figure S9, Supporting Information). The double-layer capacitance (C dl ) was calculated to obtain the electrochemically active surface area (ECSA). As shown in Figure S9A, Supporting Information, Co 2 P-Co 3 O 4 /C exhibits the largest C dl value of Figure 4. A) OER performances of Co 2 P-Co 3 O 4 /C, Co 2 P-CoO/C, Co 2 P/C, and RuO 2 /C in 1.0 M KOH, and B) the corresponding Tafel diagrams. C) LSV plots of Co 2 P-Co 3 O 4 /C across 10 000 cycles. D) I-t plot for 100 h of chronoamperometric testing. E) Comparison of the Co 2 P-Co 3 O 4 /C OER performance with those of recently reported OER electrocatalysts, and the detailed values are listed in Table S3, Supporting Information.
www.advancedsciencenews.com www.small-structures.com 105.5 mF cm À2 compared to those of Co 2 P-CoO/C (64.5 mF cm À2 ) and Co 2 P/C (62.4 mF cm À2 ). The large C dl proves that Co 2 P-Co 3 O 4 /C has rich active sites and increased intrinsic activity, which make it show excellent HER activity. These results originate from the porous structure and suitable particle size of Co 2 P-Co 3 O 4 /C, which enhance electrolyte penetration and electron transfer. [26] In addition to the catalytic activity, the durability of the catalyst is equally important for the practical application of water electrolysis. Therefore, the stability of the Co 2 P-Co 3 O 4 /C catalyst was next measured by continuous CV scanning and long-term chronoamperometry testing in 1.0 M KOH. As shown in Figure 3C, the polarization curve of Co 2 P-Co 3 O 4 /C shows negligible variation after 10 000 CV cycles. Moreover, Co 2 P-Co 3 O 4 /C displayed continuous operation for over 100 h with a negligible decrease of current density at 10 mA cm À2 . After 5000 HER CV cycles, the powder on foam nickel was collected to check its stability. The XRD spectrum ( Figure S10, Supporting Information) of Co 2 P-Co 3 O 4 /C shows that the composition did not change after the HER test. Figure S11A,B, Supporting Information, shows the XPS spectra of Co and P elements after the HER test. Their peaks show almost no shift compared with the spectra before the HER test, indicating that the species maintain their initial chemical state, which indicates that there is no obvious surface reconstruction during the HER process. SEM image ( Figure S12, Supporting Information) shows that Co 2 P-Co 3 O 4 /C can still maintain its original structure and morphology after the HER test. These results confirm the high activity and stability toward HER on the Co 2 P-Co 3 O 4 /C electrocatalyst. This electrocatalyst is superior to those of many recently reported noble metal-free HER electrocatalysts ( Figure 3D, Table S2, Supporting Information).
The OER performances of the as-prepared Co 2 P-Co 3 O 4 /C, Co 2 P-CoO/C, and Co 2 P/C were also evaluated in 1.0 M KOH. As shown in Figure 4A, Co 2 P-Co 3 O 4 /C exhibits the best OER activity and only requires an overpotential of 246 mV versus Figure 5. A) Survey XPS spectrum, and high-resolution XPS spectra of B) Co 2p, C) P 2p, and D) O 1s of Co 2 P-Co 3 O 4 /C before and after 5000 CV scanning cycles toward OER. E) XRD pattern of Co 2 P-Co 3 O 4 /C before and after 5000 CV scanning cycles toward OER. F,G) HRTEM, and H,I) HAADF-STEM images and the corresponding elemental distribution mappings. J) Schematic diagram of the OER reaction mechanism. RHE to reach a current density of 10 mA cm À2 , which is lower than those of Co 2 P-CoO/C (274 mV vs RHE) and Co 2 P/C (266 mV vs RHE). Notably, the OER activities of Co 2 P-Co 3 O 4 / C are even better than those of RuO 2 /C at high current densities. In addition, compared with Co 2 P-CoO/C (88.6 mV dec À1 ), Co 2 P/C (85.1 mV dec À1 ), and RuO 2 /C (87.8 mV dec À1 ), Co 2 P-Co 3 O 4 /C shows the lowest Tafel slope of 69.5 mV dec À1 , indicating superior OER kinetics ( Figure 4B). Notably, Co 2 P-Co 3 O 4 /C also displays excellent OER stability. As shown in Figure 4C, the LSV of Co 2 P-Co 3 O 4 /C after 10 000 CV scanning is better than the first LSV curve toward OER. Similarly, after 100 h of I-t chronoamperometric testing, the current density of Co 2 P-Co 3 O 4 /C is slightly increased ( Figure 4D). In fact, in the OER process, the electrolyte gradually penetrates the pores of Co 2 P-Co 3 O 4 /C, and even into some pores occupied and blocked by nanoparticles, and then contacts more active sites. Thus, the interaction between Co 2 P-Co 3 O 4 /C and electrolyte is enhanced, which accelerates the electron transfer and gradually enhances the activity of the catalyst. The OER performance of Co 2 P-Co 3 O 4 /C outperforms many recently reported OER electrocatalyst performances ( Figure 4E, Table S3, Supporting Information). These results demonstrate the improved OER activity and stability of Co 2 P-Co 3 O 4 /C, due to the Co 2 P-Co 3 O 4 heterostructure and porous shells of the oxide and carbonized layers.
To further investigate the OER mechanism of Co 2 P-Co 3 O 4 /C in an alkaline medium (1.0 M KOH), the composition and structure of Co 2 P-Co 3 O 4 /C after 5000 OER CV cycles were systematically characterized by XPS, XRD, SEM, and TEM. The XPS spectra reflect the chemical changes on the surface of Co 2 P-Co 3 O 4 /C during the OER test. Figure 5A shows two strong peaks after the OER test. The peaks at 692 and 852 eV are derived from Nafion and nickel foam, respectively. The decrease in C content is attributed to the erosion of the carbon shell by KOH. The electrolyte becomes fully permeated after the reaction channel is opened and the active substance begins to function. The Co 2p spectra of Co 2 P-Co 3 O 4 /C ( Figure 5B) show that after OER the Co 2p 3/2 and Co 2p 1/2 peaks move toward higher energy levels, and the intensity of the Co─P bond decreases while that of Co─O bond increases. Moreover, the Co 2 P is partially converted to Co oxide, which is conducive to water electrolysis. In the P 2p XPS spectrum ( Figure 5C), the crystallinity of P decreases, confirming that P is dissolved or oxidized. The O 1s XPS spectrum ( Figure 5D) shows that the OH À intensity increased after OER cycling, indicating that many hydroxide free radicals are adsorbed on the material surface, which can increase the adsorption of hydroxides and further enhance the OER activity. The XRD pattern of the Co 2 P─Co 3 O 4 /C powder after OER testing was also recorded ( Figure 5E). The coexistence of the Co 2 P and Co 3 O 4 phases hardly changes, while the peak of the Co 2 P phase decreases slightly, further indicating that Co 2 P is partially oxidized and partially dissolved and permeated into the electrolyte during the OER test. [27] The TEM images ( Figure 5F,G) of the Co 2 P─Co 3 O 4 /C catalyst after OER show that its exterior is coated with a %10 nm Co oxide and an exterior carbon layer of %5 nm Figure 6. Side-view schematic models of A) Co 2 P (002)-Co 3 O 4 (400), B) Co 2 P (002)-CoO (200), and C) Co 2 P (002). D) Hydrogen adsorption free energy diagram of Co 2 P-Co 3 O 4 , Co 2 P-CoO, and Co 2 P with various sites for HER. E) Free energy diagram of Co 2 P-Co 3 O 4 for OER. thickness. The lattice fringe with a crystal plane spacing of 0.244 nm corresponds to the (311) plane of Co 3 O 4 , while the lattice fringe with an inner plane spacing of 0.187 nm corresponds to the (031) plane of Co 2 P. The high-angle-annular dark-field scanning transmission electron microscopy-scanning electron transmission microscopy (HAADF-STEM) diagrams ( Figure 5H,I) and EDX images further confirmed that the post-OER Co 2 P─Co 3 O 4 /C is coated with an exterior cobalt oxide layer. This Co 3 O 4 layer enhances the corrosion resistance in alkaline solutions and provides a superior reaction platform for the internal Co 2 P/Co 3 O 4 . [28] The composition and morphology of Co 2 P─CoO/C and Co 2 P/C after OER testing were also characterized and shown in Figure S13-S15, Supporting Information. The XRD diagram ( Figure S14A, Supporting Information) of Co 2 P-CoO/C after 5000 OER CV cycles testing displays new peaks at 31. 2, 36.8, and 59.3°corresponding to Co 3 O 4 (JCPDS No. 42-1467), in addition to the characteristic Co 2 P and CoO peaks. These results confirm that CoO and Co 2 P were easily oxidized to Co 3 O 4 .
To investigate the origin of the HER and OER activities of the Co 2 P-Co x O y /C electrocatalyst and elucidate the effect of the heterogeneous interfaces between Co 2 P-Co x O y , density functional theory (DFT) calculations were performed. The side-and topview atomic models of Co 2 P-Co 3 O 4 , Co 2 P-CoO, and Co 2 P were therefore established ( Figure 6A-C and S16, Supporting Information), and the atomic configurations of H adsorption on their surfaces built ( Figure S17, Supporting Information). Figure 6D shows the H adsorption Gibbs free energy (ΔG H* ) of Co 2 P-Co 3 O 4, Co 2 P-CoO, and Co 2 P with various sites for HER. The ΔG H* values of the Co 2 P, Co 2 P site in Co 2 P-CoO, CoO site in Co 2 P-CoO, Co 2 P site in Co 2 P-Co 3 O 4 , and Co 3 O 4 site in Co 2 P-Co 3 O 4 are 0.7927, À0.6624, À0.755, À0.1, and À0.1212 eV, respectively. The |ΔG H* | value of Co 2 P is large, but through partial oxidation, CoO and Co 3 O 4 are introduced respectively to form Co 2 P-CoO and Co 2 P-Co 3 O 4 heterostructures, which effectively reduce the |ΔG H* | value. The synergistic effect of heterostructures helps to reduce the energy barrier of the initial water dissociation step and optimize the subsequent H adsorption/desorption in alkaline HER. A catalyst with ΔG H* close to 0 is considered to be an ideal HER catalyst. Compared with Co 2 P-CoO and Co 2 P, |ΔG H* | of Co 2 P-Co 3 O 4 is closer to 0; thus, Co 2 P-Co 3 O 4 has higher HER activity. Side-view schematic models of OH*, O*, OOH*, and O 2 * adsorbed on the surface of Co 2 P-Co 3 O 4 during the OER reaction were also established ( Figure S18, Supporting Information). Figure 6E illustrates the free energy diagram of Co 2 P-Co 3 O 4 for OER. The OER process consists of four elementary steps, of which the transition from *O to *OOH (O* þ OH À ! OOH* þ e À ) has the highest energy barrier, indicating that this step is the rate-determining step (RDS) of the OER. The ΔG value for the RDS of Co 2 P-Co 3 O 4 is 2.054 eV, which is lower than that of the RDS for Co 3 O 4 (2.576 eV), indicating that the synergistic effect of heterostructure is beneficial to enhance the adsorption of oxygen intermediates in OER.
Encouraged by the superior activity and stability of Co 2 P-Co 3 O 4 /C for HER and OER in KOH, we further used Co 2 P-Co 3 O 4 /C as both an anodic and a cathodic electrocatalyst in a two-electrode system to evaluate the overall water-splitting Figure 7. A) Water splitting characteristics of Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C and Pt/C || RuO 2 /C measured in a two-electrode system in 1.0 M KOH. B) Long-term durability testing by chronoamperometry at 1.55 V. C) Polarization curves of the Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C system recorded before and after working for 120 h. D) Overall water splitting photographs of the electrolyzer using Co 2 P-Co 3 O 4 /C as both the cathode and anode.
www.advancedsciencenews.com www.small-structures.com performance. For comparison, commercial Pt/C and RuO 2 /C were tested simultaneously as the cathodic and anodic electrocatalysts, respectively. As shown in Figure 7A, Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C requires a low voltage of 1.55 V to achieve a current density of 10 mA cm À2 , which is superior to that of Pt/C || RuO 2 /C (1.59 V @ 10 mA cm À2 ). In addition, the Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C system showed remarkable long-term stability, as verified by the retained current density after a 120 h chronoamperometry test at 1.55 V ( Figure 7B). Moreover, the overall water-splitting performance of Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C was retested after the stability test. The LSV curve showed no attenuation compared to the initial curve ( Figure 7C). In particular, the Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C electrolyzer can be driven at 1.6 V, whereby numerous bubbles emerge from the electrode surface ( Figure 7D and Video S1, Supporting Information). Notably, this overall water-splitting performance is superior to that of many previously reported non-noble electrocatalysts (Table S4, Supporting Information), indicating the good application prospects for practical overall water splitting.

Conclusion
We have developed a Co 2 P-Co x O y /C heterostructure catalyst using a simple one-step heat treatment method, achieving phase and morphologically controllable oxidation based on Co 2 P nanoparticles. The as-constructed Co 2 P-Co 3 O 4 /C shows an intriguing structure with small particle size and large specific surface area and requires a low overpotential of 86 mV versus RHE for HER and 246 mV versus RHE for OER to afford a current density of 10 mA cm À2 in alkaline electrolyte. More importantly, when assembled into alkaline electrolytic cells for full water splitting, the Co 2 P-Co 3 O 4 /C || Co 2 P-Co 3 O 4 /C system exhibits a low voltage of 1.55 V to achieve a current density of 10 mA cm À2 , along with superior long-term durability (operating at 1.55 V for 120 h). These experimental results and DFT calculations indicate that the synergistic effect between Co 2 P and Co 3 O 4 plays an important role in boosting water electrolysis. This study guides the rational design and synthesis of Co-based bifunctional water electrolysis electrocatalysts via heterostructure engineering.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.