Synergistic Tuning of CoO/CoP Heterojunction Nanowire Arrays as Efficient Bifunctional Catalysts for Alkaline Overall Water Splitting

Bifunctional electrocatalysts with superior activity and durability are of great importance for electrocatalytic water splitting. Herein, hierarchical structured CoO/CoP heterojunctions are successfully designed as highly efficient and freestanding bifunctional electrocatalysts toward overall water splitting. The unique microstructure combining two‐dimensional nanosheets with one‐dimensional nanowires enables numerous exposed active sites, shortened ion‐diffusion pathways, and enhanced conductivity, significantly improving performance. Such freestanding electrodes achieve high current density over 400 mA cm−2 at low overpotentials and have exceptional electrocatalytic activity as well as long‐term durability for both hydrogen and oxygen evolution reactions under alkaline conditions. Remarkably, a high current density of 20 mA cm−2 is generated at a low cell voltage of 1.56 V in an alkaline water electrolyzer, originating from synergistic interactions between CoO and CoP exposing active sites and facilitating charge transfer and enhancing kinetics. This work provides new insight into designing low‐cost but high‐performance bifunctional electrocatalysts for practical water splitting.


Introduction
Electrochemical water splitting is believed to be the most efficient and promising strategy for the generation of high-purity hydrogen (H 2 ) as a green fuel and an alternative energy carrier. [1][2][3][4] Its large-scale practical implementation is noticeably impeded by a low efficiency where a large amount of extra energy is required to overcome the overpotential. [5,6] Precious metal-based DOI: 10.1002/smtd.202300071 catalysts are usually adopted to promote hydrogen evolution reaction (HER) at the anode and oxygen evolution reaction (OER) at the cathode, respectively. [7,8] However, their global scarcity, exorbitant price, and particle agglomerationinduced devitalization make them uncompetitive in future markets. [9][10][11] Moreover, no single precious metal-based electrocatalyst can efficiently promote both HER and OER simultaneously, while each electrocatalyst is often incompatible with the other reaction system. The development of low-cost bifunctional electrocatalysts with high performance comparable to their precious metal-based counterparts is of great importance but quite challenging.
Transition metals and their compounds (oxides, phosphides, nitrides, etc.) have received extensive research as electrocatalysts for HER or OER processes. [12][13][14][15][16][17][18] Compared to precious metals, transition metals are relatively cheap and have multiple valences which can be manipulated to show different catalytic behaviors. For example, cobalt oxides (e.g., Co 3 O 4 , [12] CoO [14] ) exhibit high catalytic activity towards OER due to the fast formation of metal-oxyhydroxide in alkaline systems, while nickel and molybdenum phosphides (e.g., Ni 2 P, [16] MoP, [17] NiMoP 2 [18] ) show superior performance for HER because of the moderate free energy for H 2 adsorption and highly active d electrons. [19][20][21] Although extensive progress has been made in utilizing these materials in water splitting, the performance improvement is mainly achieved at the lab scale with relatively low reaction currents below 100 mA cm −2 .
Powdery catalysts are usually used for HER and OER electrocatalysts by adding adhesive (e.g., Nafion) to firmly attach particles to current collectors (e.g., glassy carbon, nickel foam, carbon cloth, etc.). The binder can impede the transfer of charge within the electrode as well as the release of generated bubbles. [22] Furthermore, the powdery electrocatalyst may be peeled off from the electrode by the rapidly formed bubbles at high current densities, thus limiting their activity and stability and rendering the powdery catalyst-based electrode unsuitable for practical applications. It is of great significance to develop self-supported hierarchical electrodes for water splitting in practical applications. In comparison to powder catalysts, the self-supported electrode is capable of operating at high current densities of over 100 mA cm −2 for an extended period of time while maintaining superior performance and stability, reducing the gap between lab screening and industrial application. What's more, the specific surface area of the catalyst can be improved by the uniform growth on the support, as well as enhanced stability by inhibiting active site agglomeration through anchoring effects, and improved electronic conductivity by promoting the charge transfer between the catalyst and the support. [23][24][25][26][27] Despite significant progress in the study of transition metal-based self-supported catalysts, little progress has been made in the study of heterojunction self-supported catalysts composed of transition metal oxides and phosphides.
Hence, we design a novel self-supported hierarchical catalyst consisting of CoO/CoP heterojunction nanowire arrays on nickel foam (NF) for efficient water splitting under alkaline media. Such a unique structure of a disordered nanosheet array composed of nanowires is constructed through the hydrothermal generation of precursors followed by calcination and phosphorization at low temperatures. The self-supported catalyst establishes powerful electronic interactions by incorporating CoO and CoP interfaces, adjusting the reaction kinetics of HER and OER on the catalyst surface. Moreover, the 1D nanowire structure enables more exposed catalytic active sites, while the disordered 2D nanosheet structure helps rapid ion and electron transport. Benefiting from the distinguished crystal structure and morphology, the as-synthesized NF/O-CoP catalyst achieves high current density over 400 mA cm −2 at low overpotentials and exhibits exceptional electrocatalytic performance towards both HER and OER as well as long-term durability in 1 M KOH solution, which even surpasses precious metal-based counterparts. Furthermore, when the NF/O-CoP electrode is used as both an anode and cathode towards water splitting, the water electrolyzer requires only 1.51 V to reach the current density of 10 mA cm −2 , demonstrating the considerable potential for practical application.

Results and Discussion
The NF/O-CoP catalyst has been successfully fabricated on nickel foam by a simple hydrothermal-calcination-phosphorization three-step method and the synthetic procedure is depicted in

Characterization of the Electrocatalysts
The composition and structure of the crystal are analyzed by _Xray diffraction (XRD). As plotted in Figure S2, Supporting Information, the XRD patterns of the as-prepared self-supported samples depict three typical diffraction peaks of NF (PDF# 04-0850) at 44.5°, 51.9°, and 76.3°that are assignable to the (111), (200) and (220) planes, respectively. [28] On account of the excessively strong intensity of NF signals, it is not conducive to analyzing and observing the components of the synthesized catalysts by the self-supported materials. In order to get reliable results, the powder samples collected from NF are used to perform XRD tests. As observed in Figure 2a, catalysts after hydrothermal reaction and further oxidation show typical peaks of Co(OH)CO 3 (PDF# 48-0083) [29] and Co 3 O 4 (PDF# 43-1003), [30] indicating the formation of NF/CoCOH and NF/Co 3 O 4 , respectively. For NF/O-CoP and NF/CoP samples, the three peaks at 31.6°, 36.6°, and 48.1°are quite consistent with (011), (102), and (211) planes of CoP (PDF# 29-0497), [31] indicating the successful phosphorization of samples. The peaks of the NF/O-CoP sample located at 36.6°, 42.4°, and 61.6°agree well with the planes of (111), (200), and (220) of CoO (PDF# 43-1004), [14] while the typical diffraction peak of CoO is not found in the NF/CoP sample. The above results confirm that the hybrid phase of CoO and CoP was successfully constructed in NF/O-CoP samples on NF.
The X-ray photoelectron spectroscopy (XPS) survey spectra of NF/O-CoP and NF/CoP samples show the presence of C, O, Co, and P ( Figure 2b). In Co 2p spectra of NF/O-CoP (Figure 2c), the peaks at ≈793.8/797.9 eV and 778.7/781.7 eV are respectively fitted to Co 2p 1/2 and Co 2p 3/2 [13,32,33] The peaks at the binding energies of 778.7 and 793.8 eV are associated with the Co-P bond in the CoP crystalline phase, and those peaks at 781.7 and 797.9 eV are assigned to the Co-O bond in CoO. [34][35][36] The binding energy of Co 2p in the NF/O-CoP sample shows a positive shift of 0.4 eV relative to that of the NF/CoP sample, confirming the www.advancedsciencenews.com www.small-methods.com down-shifted d-band center of Co and the electronic interaction in the CoO/CoP heterojunction. Such electronic interaction could lead to the electron redistribution between CoO and CoP active components and alter the reaction kinetics which is likely to favor enhanced electrocatalysis. [37] The electronic interaction in NF/O-CoP was further confirmed by the core-level P 2p spectra. As shown in Figure 2d, the two peaks at 130.0 eV and 129.1 eV are assigned to P 2p 1/2 and P 2p 3/2 respectively, [33,34] where a positive shift of 0.1 eV can be observed in NF/O-CoP sample, consistent with the results in Co 2p spectra. The peaks at 133.1 and 133.9 eV in NF/O-CoP are attributed to the phosphide P-O species, while the P-O bond in NF/CoP is from superficial oxidation of CoP because of air contact, which is also the reason for Co-O signal in Co 2p spectra. [38,39] High-resolution O 1s spectra in Figure 2e also prove this phenomenon. The peak at 531.5 eV in both NF/ CoP and NF/O-CoP is fitted to the metal-hydroxy group, [40] and the peak located at 533.1 eV corresponds to absorbed water. [33] As observed in Figure 3a, the low-magnified scanning electron microscope (SEM) image shows the 2D nanosheet morphology of the NF/O-CoP catalyst on the NF. However, high-magnified images reveal that 2D nanosheets consist of well-aligned nanowires. This hierarchical 1D/2D morphology could expose the surface area and provide interspace for mass transport (Figure 3b,c). By contrast, the bare NF presents a clean and smooth surface ( Figure  S3

Electrocatalytic HER Performance
The HER performance of the NF/O-CoP, NF/CoP, NF/CoCOH, NF/Co 3 O 4 , bare NF, and NF-supported 20 wt.% Pt/C electrodes is evaluated in an alkaline solution. As shown in Figure 4a,b, NF/O-CoP exhibits the overpotential of 124 mV at 100 mA cm −2 , which is considerably less than that of NF/CoP (176.1 mV), NF/CoCOH (351 mV), and NF/Co 3 O 4 (413.9 mV), indicating the advantage of heterojunction construction in NF/O-CoP. Impressively, a low overpotential of 174.9 mV is sufficient for the NF/O-CoP to obtain a high current density of 400 mA cm −2 , even surpassing Pt/C (188.3 mV). Such superior performance may be due to the strong interaction between catalyst and NF support, enabling high affinity and accelerated electron transfer at high current density. By contrast, the Pt/C catalyst on nickel foam will fall off at high current density due to the formation of a significant number of hydrogen bubbles, resulting in worse performance and stability. It indicates the critical importance of developing self-supported hierarchical electrodes for high current densities and the potential of NF/O-CoP for utilization in practical applications.
In order to study the intrinsic activity of HER, Tafel slopes are calculated from LSV data to evaluate the reaction mechanism and their value can determine the rate-controlling step of www.advancedsciencenews.com www.small-methods.com the electrocatalyst. [41][42][43] The Tafel slope of NF/O-CoP is measured as 57.2 mV dec −1 (Figure 4c), which is larger than that of Pt/C (39.6 mV dec −1 ), but superior to that of NF/CoP (87.6 mV dec −1 ), NF/CoCOH (88.4 mV dec −1 ), NF/Co 3 O 4 (94.5 mV dec −1 ) and bare NF (135.5 mV dec −1 ), indicating its much faster HER kinetics and it improves the reaction kinetic after calcination. The Tafel slope value of 57.2 mV dec −1 indicates that HER operates under Volmer-Heyrovsky mechanism (H 2 O(l)+e − →H ads +OH − (aq) and H ads +H 2 O(l) +e − →H 2 (g)+OH − (aq)), [44,45] and adsorption and dissociation of water molecules are the ratecontrolling step. The decreased Tafel slopes of NF/O-CoP compared with its counterparts illustrate that the synergistic interac-tion between CoO and CoP and the heterojunction interface improve the HER reaction kinetics and facilitate the electron transfer process. The ECSA is estimated by calculating the C dl through CV under various scan rates ( Figure S9, Supporting Information). Benefiting from the well-constructed nanowire morphology, NF/O-CoP shows the highest C dl of 77.7 mF cm −2 , which is slightly higher than that of NF/CoP (76.8 mF cm −2 ), but significantly higher than NF/CoCOH (3.73 mF cm −2 ), NF/Co 3 O 4 (2.2 mF cm −2 ), and bare NF (1.2 mF cm −2 ). The largest ECSA of CoO/CoP heterojunction nanowire arrays leads to a significant increase in active sites, thus contributing to superior HER performance. Besides, the catalyst activity and electrode kinetics analysis are carried out using EIS. The resistance to charge transfer (R ct ) is associated with electrocatalytic kinetics, with a lower R ct value indicating a faster reaction rate. [15,16,46] The R ct value (Figure 4e) of NF/O-CoP (0.22 Ω) is smaller than that of NF/CoP (0.49 Ω), NF/CoCOH (2.34 Ω), NF/Co 3 O 4 (2.28 Ω), and bare NF (5.05 Ω), which discloses the improved electrons transport efficiency. The smallest resistance in the NF/O-CoP is mainly due to the synergistic interaction between CoO and CoP, as well as its freestanding hierarchical structure, its seamless contact catalyst-support interface, and its binder-free property, which not only substantially accelerates interfacial charge transfer, but also mass transfer. The results of C dl , ECSA, and R ct for the as-prepared catalyst are shown in Table S1, Supporting Information.
It is of great importance to evaluate whether the catalysts have practical application prospects through stability tests. The stability of NF/O-CoP is evaluated by multistep chronopotentiometric (CP) tests. It is obvious that the potential of NF/O-CoP re-mains stable with no significant attenuation under different current densities, indicating its extraordinary mass transport ability and mechanical strength of NF/O-CoP during the HER process (Figure 4f). [47] Furthermore, NF/O-CoP reveals remarkable durability during 24 h of continual measurements in Figure 4h. The remarkable stability is further demonstrated after 10 000 cycle CV testing, which reveals that the LSV curve only exhibits little deterioration at high current rates (Figure 4h inset). It's important to note that the as-prepared NF/O-CoP exhibits better HER activity than a majority of the previously reported catalysts with exceptional HER performance (Figure 4g and Table S2, Supporting Information).

Electrocatalytic OER Performance
NF/O-CoP also shows outstanding OER activity in alkaline media. As presented in Figure 5a,b, NF/O-CoP shows the lowest overpotential compared to the NF/CoP, NF/CoCOH, NF/Co 3 O 4 , NF/RuO 2 , and bare NF at the same current density. More importantly, the NF/O-CoP only requires 312 mV at a high current density of 400 mA cm −2 . In addition, the Tafel slope of NF/O-CoP is 84.6 mV dec −1 (Figure 5c), which is superior to that of NF/CoP (91.2 mV dec −1 ), NF/CoCOH (93.6 mV dec −1 ), NF/Co 3 O 4 (105.8 mV dec −1 ), NF/RuO 2 (132.5 mV dec −1 ) and bare NF (158.7 mV dec −1 ), indicating that NF/O-CoP exhibit better OER kinetics by improved adsorption of OH − and the binding energy oxygen-containing intermediates. Meanwhile, the C dl and R ct values further indicate that the NF/O-CoP can improve the electrocatalytic performance and increase the active sites for OER. The NF/O-CoP shows the highest C dl of 97.5 mF cm −2 in Figure 5e, which is higher than those of NF/CoP (90.5 mF cm −2 ), but significantly higher than NF/CoCOH (12.9 mF cm −2 ), NF/Co 3 O 4 (7.2 mF cm −2 ), and bare NF (3.3 mF cm −2 ). The R ct value (Figure 5f) of NF/O-CoP (1.38 Ω) is smaller than those of NF/CoP (3.07 Ω), NF/CoCOH (5.15 Ω), NF/Co 3 O 4 (6.25 Ω), and bare NF (27.25 Ω). The multistep CP curve of NF/O-CoP towards OER is also tested and the potential responses were rapid and steady at the current density ranging from 10 to 100 mA cm −2 (Figure 5g). Additionally, NF/O-CoP shows excellent OER durability in Figure 5h,i in alkaline media. The turnover frequency (TOF) values of the synthesized samples are given in Figure S10, Supporting Information. The NF/O-CoP catalyst demonstrates the highest TOF value in comparison with NF/CoP, NF/CoCOH, and NF/Co 3 O 4 , further implying intrinsically more active sites.
With a view to a better understanding of the reaction mechanism, the structure-property NF/O-CoP electrode is studied after CP tests. As demonstrated in Figure S11, Supporting Information, the morphology of nanosheets and nanowires remained basically unchanged after HER and OER stability tests, proving excellent structural stability. The TEM images of NF/O-CoP after CP tests also confirm the well-maintained 1D nanowire nanos- XRD ( Figure S13, Supporting Information) characterization suggests that the main crystalline phase of NF/O-CoP remained nearly identical as before after long-term HER and OER measurements. XPS spectra of NF/O-CoP after CP tests are analyzed in detail to further study the surface chemical status of elements, and O, Co, and P elements can also be found in survey spectra ( Figure 6c). For NF/O-CoP after CP tests, at the low binding energies of Co 2p (778.7 and 793.8 eV) and P 2p spectra (129.2 eV) disappeared as shown in Figure 6d,e. Transitional metal phosphide structures are composed of covalent and ionic bonds, with ionic bonds having much longer bonding lengths than covalent bonds. [48] The disappearance of the Co-P bonds peaks after CP tests indicates that Co-P covalent bonds on the catalyst's surface are rapidly converted to ionic bonds. For P 2p spectra, the peaks of phosphide P-O species (133.2 and 134.1 eV) shift positively after HER stability tests, while the peaks of phosphide P-O species (132.5 and 133.6 eV) shift negatively after OER stability tests. For O 1s spectra, the peaks of the metal-hydroxy group (531.4 eV) shift to a lower binding energy after the OER stability measurement, and it can be obviously observed that group concentration intensively increases after OER stability measurement. Partial metal phosphides undergo an apparent and irreversible phase transformation into metal oxide/oxyhydroxide when operating in the OER potential range, unlike HER in a reducing potential environment. As shown in Figure 6f, the OH − ions are absorbed to the Co site to form the Co-OH intermediate, simultaneously releasing an electron, which is subsequently oxidized by Co-OH to form Co-O, and when another OH − ion attacks the Co-O bond, the CoOOH intermediates are formed in alkaline conditions. Finally, the electrocatalytic cycle is completed by -OOH oxidation and oxygen release. The formation of CoOOH species, which has been claimed to be the catalytic active sites for OER, has been validated by TEM and XPS investigations. The above results confirm the retention of the nanowire and nanosheet structures, as well as the formation of the CoOOH species after stability tests, which explains the reason for the excellent OER and HER long-term durability of NF/O-CoP catalyst under an alkaline condition.

Overall Water Electrolysis
The NF/O-CoP catalyst is directly used as both the cathode and anode under alkaline solution to investigate the overall watersplitting performance owing to the outstanding electrochemical performance of both HER and OER. The NF/O-CoP catalyst was powered by a 1.53 V battery and plenty of gas can be detected on both electrodes (Figure 7b). As observed in Figure 7c, the twoelectrode system comprised of NF/O-CoP|| NF/O-CoP only requires 1.51 V and 1.56 V to achieve 10 mA cm −2 and 20 mA cm −2 , respectively, which is superior to the vast majority of previously reported transition metal-based bifunctional catalysts (Figure 7e  and Table S3, Supporting Information). The long-term stability of NF/O-CoP|| NF/O-CoP was determined by continuous operation at 20 mA cm −2 in Figure 7d. The results show that the activity remains stable with a slight increase, indicating outstanding durability and enormous potential for practical applications.

Conclusion
A bifunctional 3D hierarchical heterostructure electrode composed of NF/O-CoP nanowires without binder has been successfully fabricated by a hydrothermal-calcination-phosphorization three-step method for efficient overall water splitting. The hierarchical structure with 1D nanowires and 2D nanosheets as well as the synergistic interaction between CoO and CoP heterojunctions enable strong electronic interactions in NF/O-CoP catalysts, facilitating charge transfer and enhanced reaction kinetics. Therefore, the hierarchical heterojunction structure is able to fully exploit the advantages of both components, leading to a bifunctional and excellent electrocatalyst toward overall water splitting. In alkaline solutions, the hierarchical NF/O-CoP electrodes show high electrocatalytic activity as well as long-term durability in comparison to earth-abundant electrocatalysts. For the water splitting, NF/O-CoP electrodes showed excellent stability and require a voltage of 1.56 V to achieve 20 mA cm −2 . The NF/O-CoP electrodes are promising candidates for overall water splitting based on these results. It is also possible to prepare other www.advancedsciencenews.com www.small-methods.com bifunctional electrocatalysts by utilizing the synthetic strategies and interface engineering of the heterostructure. This work provides a new insight into designing low-cost but high-performance bifunctional electrocatalysts for practical water splitting.

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