Material characterizations
Prior studies have shown that Co species tend to form lamellar structures, whereas V species are more inclined towards one-dimensional growth. Based on these bases, we have developed coral-like nanoarrays composed of nanosheets by utilizing the distinct behaviors of Co and V. As illustrated in Fig. 1a, a liquid phase self-assembly method was employed to prepare coral-like interlaced cobalt vanadate nanosheet arrays precursor with the Co3V2O8 phase supported on nickel foam substrate (CoVO/NF) (Supplementary Fig. 1). Indeed, the layered structure can be seen in the absence of V, while fibrous structure is obtained without using Co (Supplementary Fig. 2). It further proposed that Co is responsible for the stratification during catalyst growth, while V facilitates epitaxial growth. Subsequently, it underwent nitridation and phosphidation treatments to prepare CoN/VN@NF and P-CoVO@NF, respectively. To eliminate the interference from NF signals, the X-ray diffraction (XRD) characterizations of CoN/VN@NF and P-CoVO@NF are scraped from the NF substrates. As shown in Fig. 1b, CoN/VN@NF primarily exhibits the hexagonal Co2N0.67 phase and cubic VN phase. For P-CoVO@NF, the peaks still correspond to the cubic Co3V2O8 phase as that of CoVO@NF, suggesting that phosphorus is incorporated in a doped form. The scanning electron microscopy (SEM) images in Fig. 1c, d show that both the nitridation and phosphidation treatments maintain the coral-like nanosheet array structure, despite causing a rough surface. Such unique 3D micro-nano structures assembled by low-dimensional structures not only prevents the self-agglomeration during reaction processes to promote stability, but also furnishes an abundance of active sites to accelerate the electron transfer for enhancing electrocatalytic activity. Transmission electron microscopy (TEM) (Fig. 1e) indicates that the nanosheet in CoN/VN@NF is composed of nanoparticles. It can increase the accessibility of the surface to allow the internal Co2N0.67-VN heterojunction directly contact with the reactants. High-resolution TEM (HRTEM) image shows clear lattice fringes and close contact between Co2N0.67 (101) and VN (200) (Fig. 1f), confirming the formation of heterojunction. The elemental mapping displays a uniform distribution of Co, V and N elements in CoN/VN@NF (Fig. 1g). The obtained P-CoVO@NF represents overlapping nanosheets with the lattice fringes of Co3V2O8 (122) plane (Fig. 1h, i), while the uniform distribution of Co, V, O and P elements can be observed (Fig. 1j). Additionally, an amorphous layer with a thickness of about 20 nm covers on the surface, and Fourier-transform infrared spectroscopy (FTIR) further proves the presence of P-O group (Supplementary Fig. 3).25 Since phase transformation during OER process always involves bond-breaking, so the amorphous structure can more readily evolve than the crystalline structure.26
The X-ray photoelectron spectroscopy (XPS) survey spectra indicate the presence of Co, V, N and O elements in CoN/VN@NF, while Co, V, P and O elements exist in P-CoVO@NF (Supplementary Fig. 4). As depicted in Fig. 2a, the binding energies of Co 2p in CoN/VN@NF show a negative shift of 0.69 eV compared to CoVO@NF. Specially, the peaks at 779.73 and 795.95 eV of CoN/VN@NF respectively correspond to the Co3+ in the Co 2p3/2 and Co 2p1/2 orbitals, while the peaks at 781.55 and 797.41 eV are assigned to Co2+.27 The V 2p spectrum reveal two extra peaks after nitridation of CoVO@NF (Fig. 2b). The peaks at 514.66 and 516.33 eV respectively attribute to V‒N‒O and V‒O bonds, indicating that the introduction of nitrogen anions results in the coupling between the electronic orbitals of Co and V with N atoms.28 Besides, the V‒N bond located at 513.05 eV demonstrates the formation of metal nitride, which is further confirmed by the M‒N bond in N 1s spectrum (Fig. 2c top). 29 For P-CoVO@NF, the Co 2p spectrum also exhibit the presence of Co3+ at 780.02 and 795.95 eV, Co2+ at 781.88 and 797.76 eV, while the peak at 516.3 eV in the V 2p spectrum ascribes to the V‒O bond. Compared to CoVO@NF, significant negative shifts in the Co 2p and V 2p peaks can be observed for P-CoVO@NF. Notably, the XPS spectrum reveals that the P predominantly exist in the form of P‒O bond at 133.71 eV, accompanying by a small amount of metal phosphide (Fig. 2c bottom).30 The P-O group with strong nucleophilicity could act as proton acceptor to activate surface oxygen during OER process. In addition to the oxygen vacancies (VO) and metal‒oxygen (M‒O) peaks, an extra peak at 532.1 eV in the O 1s spectrum is attributed to the P-O bond (Supplementary Fig. 5).31
X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) could provide accurate information of the valence states and coordination structures. As shown in Fig. 2d, the position of the pre-edge absorption peak indicates the valence state order of Co as follows: CoN/VN@NF < P-CoVO@NF < CoVO@NF. The Fourier Transform (FT) EXAFS (FT-EXAFS) spectra for CoN/VN@NF and P-CoVO@NF implies that the introduction of nitrogen and P-O group respectively form the Co‒N and a small number of Co‒P coordinated structures (Fig. 2e). The dominant peaks at 1.46 and 2.20 Å for CoN/VN@NF can be assigned to the first coordination shell of Co‒N/O bond and the inner coordination shell of Co‒Co bond (Supplementary Fig. 6 and Table 1), respectively. Compared with CoVO@NF, the decreased intensity of Co‒O bond at around 1.52 Å for P-CoVO@NF suggests a reduced average coordination number, indicating the formation of VO, which could enhance the electron transfer capability. Additionally, the small bulge at around 2.06 Å corresponds to the Co‒P bond.32 Wavelet Transform (WT) is further recovered the coordination information of the first shell in both k-space and R space. After optimizing based on the Morlet wavelet, the Co K-edge WT-EXAFS of CoN/VN@NF and P-CoVO@NF respectively exhibit oscillations at 5.5 and 5.8 Å–1 correspond to Co‒N/O and Co‒O bonds (Fig. 2f), whereas the minimal peak of Co‒P bond is difficult to be reflected.
As shown in Fig. 2g, the peak located at 5470 eV is designated as the 1s→3d transition of tetravalent V.33 The weak pre-edge absorption peak in both CoN/VN@NF and P-CoVO@NF indicate that the introduction of anions transforms the low-coordinate VO4 tetrahedra in CoVO@NF into high-coordinate V(N,O)6 octahedra with higher symmetry. The pre-edge absorption peak gradually moves in the low energy direction from CoVO@NF to P-CoVO@NF and then to CoN/VN@NF, indicating that the oxidation state of V decreases due to the reduced oxygen content. Furthermore, the FT-EXAFS spectra of CoN/VN@NF exhibit four principal features below 6 Å (Fig. 2h). Specially, the peaks at 1.63 and 2.42 Å correspond to the inner shell composed of six symmetric V‒N and V‒V bond, while the peaks near 3.70 and 4.45 Å are ascribed to the higher V‒N and V‒V shells, essentially consistent with the standard rock-salt structure of VN (Supplementary Fig. 7 and Table 2).34 For P-CoVO@NF, the strongest V‒O bond is located at 1.62 Å rather than the 1.12 Å for CoVO@NF, attributing to the P incorporation causes the stretching of V‒O bond within Co3V2O8 lattice. Such local structural variant makes the V‒O bond easily to break, resulting in the dissolution of V during OER process, thus promoting the reconstruction of Co species. As the WT-EXAFS illustrated in Fig. 2i, the highest intensity peaks for CoN/VN@NF and P-CoVO@NF in the first coordination shell are ascribed to the V‒N and V‒O bonds at 7.50 and 6.20 Å–1, respectively. The k-value for the V‒O bond in P-CoVO@NF is much higher than the CoVO@NF (6.20 vs. 4.81 Å–1), revealing that the V‒O bond is stretched by the P incorporation.
Electrochemical performance
HER performance. The HER activity was evaluated in 1.0 M KOH by a three-electrode system, and the commercial 20% Pt/C catalyst loaded on NF (denoted as Pt/C@NF) was also tested. As displayed in Fig. 3a, CoN/VN@NF requires overpotentials of only 55 mV and 164 mV at the current densities of 10 mA cm–2 (η10) and 100 mA cm–2 (η100), respectively. It is significantly better than CoVO@NF (93 and 204 mV) and P-CoVO@NF (84 and 197 mV), approaching commercial 20% Pt/C (17 and 129 mV) (Fig. 3b). Notably, CoN/VN@NF also exhibits outstanding HER activity over the recently reported non-precious metal electrocatalysts (Supplementary Table 3). The highest current response of CoN/VN@NF is attributed to the introduced nitrogen atom can increase the d-electron density of metal, resulting in a redistribution of the density of states near the Fermi level (EF) and generating a similar electronic structure as that of precious metal. The poor HER activity of nitride NF substrate indicates a negligible contribution to electrocatalytic activity (Supplementary Fig. 8).
Compared to CoVO@NF and P-CoVO@NF, the lower Tafel slope value of 53 mV dec–1 for CoN/VN@NF inidicates a faster kinetics (Fig. 3c), which also suggests the Volmer-Heyrovsky mechanism of CoN/VN@NF towards HER.35 The Nyquist plots collected by electrochemical impedance spectroscopy (EIS) and the double-layer capacitance (Cdl) values show that CoN/VN@NF displays the smallest charge transfer resistance and much more active sites (Fig. 3d,e and Supplementary Fig. 9). After normalizing by the ECSA values, CoN/VN@NF still exhibit largest current density (JECSA) (Supplementary Fig. 10), further confirming the outstanding intrinsic activity. Moreover, CoN/VN@NF shows a turnover frequency (TOF) value of 1.16 s–1 at 0.2 V, which is significantly higher than CoVO@NF (0.50 s–1) and P-CoVO@NF (0.54 s–1) (Supplementary Fig. 11). It maintains the higher level across the entire range of applied potentials, further provinging the rapid kinetics of CoN/VN@NF.
Durability is another crucial criterion for assessing the potential application of electrocatalyst. As depicted in Fig. 3f, the CoN/VN@NF maintains its initial activity without significant degradation after 10,000 continuous cyclic voltammetry (CV) cycles. It also can stably operate for over 100 h at a current density of 100 mA cm–2, further confirming the superior stability of CoN/VN@NF (inset of Fig. 3f). After stability test, it preserves the original coral-like nanoarray structure, while the binding energies of Co and V in the XPS spectra show slightly variation (Supplementary Fig. 12). It reveals that the stable surface chemical state of CoN/VN@NF is conductive to long-term operation.
OER performance. The OER activity was also investigated in alkaline electrolyte, and the commercial RuO2 catalyst supported on NF (denoted as RuO2@NF) was studied for comparison. As presented in Fig. 3g, the P-CoVO@NF performs much better OER activity than the compared catalysts. The P-CoVO@NF only requires the overpotentials of 296 and 317 mV to deliver the current densities of 50 and 100 mA cm–2, respectively, much lower than CoVO@NF (341 and 373 mV), CoN/VN@NF (389 and 425 mV) and RuO2@NF (376 and 420 mV) (Fig. 3h). Significantly, the activity exceeds most of the reported transition metal based electrocatalysts (Supplementary Table 4). As displayed in Fig. 3i, the P-CoVO@NF possesses the lowest Tafel slope value of 77 mV dec–1 than CoVO@NF (152 mV dec–1), CoN/VN@NF (133 mV dec–1) and RuO2@NF (138 mV dec–1). Generally, the Tafel slope value for OER increases with the enhanced coverage of *O under the site-isolation model.36 Consequently, the enhanced activity of P-CoVO@NF, suggested by the lowest Tafel slope value, may be due to the accelerated kinetics of *O transformation. Additionally, the higher conductivity of P-CoVO@NF facilitates charge transfer, thereby enhancing OER activity (Fig. 3j). Moreover, the OER activity of NF substrate after phosphating is not well, so the activity of the substrate can be negligible (Supplementary Fig. 13).
P-CoVO@NF exhibits outstanding intrinsic OER activity with a moderate ECSA value and a JECSA value about 9 times higher than that of CoN/VN@NF at 1.55 V (Fig. 3k and Supplementary Fig. 14–15). The TOF value of P-CoVO@NF is 0.17 s–1 at an overpotential of 300 mV, approximately 5.7 times as that of CoN/VN@NF (Supplementary Fig. 16), further proving the higher intrinsic activity of P-CoVO@NF. Durability test shows that no noticeable activity decay is observed after 100 h at 100 mA cm–2 (Fig. 3l). Furthermore, only a slight decay is noted after 10,000 continuous CV cycles, while the intensity of the oxidation peak increases because extra metal species could be oxidized during OER process. After the stability test, the surface of P-CoVO@NF were covered with thinner nanosheets as confirmed by SEM images (Supplementary Fig. 17).
OER Mechanism
Generally, the metal species on the surface of catalysts undergo in-situ reconstruction during OER process. The operando Bode plots indicate that the charge transfer signals at different characteristic frequencies can be divided into two areas (Fig. 4a-c). Specially, the electron transfer response in the inner layer of the catalyst occurs at the high-frequency range of 102−104 Hz, whereas the charge transfer response at the catalyst/electrolyte interface is observed at the low-frequency range of 10− 2−102 Hz.37 Compared with CoVO@NF and CoN/VN@NF, the P-CoVO@NF exhibits lower peaks in both high and low frequency ranges, and these peaks decrease more rapidly with the increasing potential. It indicates a faster oxidation of intermediates and deprotonation of *OOH for P-CoVO@NF, revealing the much better OER activity.38 The Nyquist plots in Supplementary Fig. 18 are fitted based on a hypothetical equivalent circuit model composed of three parts (Fig. 4d). In detail, the Rs stands for the electrolyte resistance. The first parallel circuit involves constant phase element (CPE1) and resistance (RCEOR) related to the electron transfer from the inner layer of catalyst to the reaction interface, and the CEOR represents the surface reconstruction of catalyst during OER. The second parallel circuit includes a constant phase element (CPE2) and resistance (ROER) associated with the charge transfer of interface reaction.39 Relatively, the smallest RCEOR value of P-CoVO@NF implies the easier oxidization and reconstruction (Supplementary Table 5). At the low potential range of 1.20 ~ 1.45 V, the ROER values of P-CoVO@NF and CoVO@NF drop sharply but still large, manifesting that the catalysts only occur rapid structural changes. The ROER value of P-CoVO@NF approaches zero when the potential exceeds 1.45 V indicates the occurrence of OER process, whereas it starts at above 1.5 V for CoN/VN@NF (Fig. 4e). It further uncovers the superior OER kinetics and intrinsic activity of P-CoVO@NF. Hence, the regulation of P-O can improve the utilization rate of *OH, endowing P-CoVO@NF with faster charge transfer and deprotonation capability, while the self-reconstructed interface facilitates to OER acitivity.
Operando XAFS was further employed to investigate the structure change of Co sites during OER. As depicted in Fig. 4f, the Co K-edge absorption of P-CoVO@NF is situated between CoO and CoOOH at an open-circuit potential (OCP), indicating that the oxidation state of Co is between + 2 and + 3. When the potential promotes from 1.20 to 1.50 V, the Co K-edge gradually shifts towards higher energy, suggesting the increase of Co valence state. The valence states of Co under different applied potentials are further quantified by analyzing the first derivative of absorption edge.40,41 As displayed in Fig. 4g, the oxidation state of Co gradually increases from + 2.31 at OCP to + 2.69 at 1.20 V, then to + 2.71 at 1.30 V and + 2.76 at 1.40 V. When the voltage exceeds the oxygen evolution potential, the charge on the catalyst surface is insufficient to further oxidize Co, which causes electrons to escape from the surface oxygen and results in the formation of O2. Simultaneously, cobalt species undergo surface reconstruction, and its oxidation state eventually rises to + 3.31 at 1.5 V. It can be inferred that the OER occurs at the potential range of 1.4‒1.5 V, consistent with the above Bode phase angle analyses. Moreover, the Co sites exhibit stronger attraction to O 2p electrons at a high potential, benefiting to the deprotonation of M‒OH.42 Consequently, Co sites with higher oxidation states enhance the adsorption of OH‒ ions to form Co‒OH, thereby reducing the required potential for deprotonation and generating more reactive oxygen species to promote OER activity. Figure 4h and Supplementary Fig. 19 displays the changes of coordination structures for the surface Co species. At OCP, the peak at 1.5 Å is associated with the single scattering path of Co‒O bond, as well as the Co‒Co (d-Oh) from distorted octahedral (tetrahedral-like) coordination structure is located at 2.8 Å. Upon increasing the potential to 1.2 V, a new peak of Co‒Co (r-Oh) at 2.4 Å originated from regular octahedral coordination structure (Supplementary Table 6). This generates a shift from high-spin Co2+ to low-spin Co3+ and the decrease of band gap from 0.615 eV to 0 eV, resulting in a high conductivity and well OER activity43 (Fig. 4i). Furthermore, the V 2p XPS spectra show almost no change except for the decreased intensity, implying the partial V dissolution during OER (Fig. 4j and Supplementary Table 7). The positive shift of Co 2p spectrum along with the increased area of Co3+ peak again indicates the elevated oxidation state for cobalt species (Supplementary Fig. 20). The solution after OER was analyzed by performing Inductively coupled plasma-mass spectrometry (ICP-MS). The content of Co are negligible, whereas V is detected with content of 7.5 µg mL− 1 (Supplementary Table 8), further ascertaining that a small amount of V leaches into solution during OER. The dissolution of V creates vacancies in the Co3V2O8 lattice, which could provide sufficient space to promote such a configurational transformation. The evoluted oxygen-bridged octahedral center is similar to the local environment of Co‒(O)OH, making the Co site benefits to the adsorption of oxygen species, and then oxidation and structural reconfiguration. As the potential further increases, the peak intensity of Co‒O begins to decline, which possibly ascribes to the electrons removal from CoOOH, leading to the protons generation and subsequent O2 release. Additionally, the decline in the peak height of the Co-O bond indicates the formation of VO during the OER process, which in turn enhances the adsorption of intermediate species and the rate of electron conduction in the materials.
Furthermore, the relative intensity of P 2p spectrum decreases, accompanied by the disappearance of metal phosphides. The O 1s spectrum presented shows an increase in VO after stability test, which maybe facilitates the adsorption and desorption of intermediates (Supplementary Fig. 21). The presence of O‒H bond is attributed to the in-situ reconstructed CoOOH, originating from the surface metal phosphides decompose into POx and CoOOH. Besides, a portion of V‒O bond will break at a high potential, and the POx ions dissolve from Co‒POx, thus creating pathway to allow more metal sites contact with the OH‒ ions in the electrolyte to form CoOOH. The evoluted CoOOH can provide effective active sites to lower the reaction barrier and accelerate the reaction process. Raman spectroscopy was conducted to reveal the variation of the chemical composition after OER test. As shown in Fig. 4k, P-CoVO@NF displays O − V−O bond at 816 cm‒1 and V − O bond at 988 cm‒1.44 Moreover, the stretching vibrations of Co − O−V bond originated from the presence of VO, can be split into the doublet peaks at 280 and 334 cm–1. The peaks at 660 and 943 cm–1 are respectively attributed to the A1g vibrational modes of Co-O and O − P−O bonds.45 After CV cycles test, the characteristic peaks of Co3V2O8 disappear, while the new peaks emerge at 468 and 548 cm‒1 respectively belong to the Eg and F2g vibrational modes of CoOOH.
Theory calculations
The electronic charge distribution of CoVO@NF, CoN/VN@NF and P-CoVO@NF models based on DFT calculations are shown in Fig. 5a-c and Supplementary Table 9. It can be concluded that the charge of both Co and V atoms follows this order: CoVO@NF > P-CoVO@NF > CoN/VN@NF. Therefore, both nitrogen and phosphate anions regulation promote the electron density around Co and V atoms. Additionally, the nitrogen anion could generate a stronger electron coupling interaction with metals compared to P-O group, resulting in a lower charge distribution, which matches well with the above XPS and XANES analyses. For CoN/VN@NF, the electron accumulation at the nitrogen site is more conducive to the capture of hydrogen protons, thereby promoting HER activity. In contrast, the phosphorus site in P-CoVO@NF exhibits higher positive charge, which is benefitical to adsorb hydroxyl groups. Based on these results, we will consider nitrogen site as the adsorption site for *H in CoN/VN@NF, while the phosphorus site as the adsorption site for *OH in P-CoVO@NF in the subsequent computational processes of HER. Moreover, the CoVO@NF, CoN/VN@NF and P-CoVO@NF models exhibit a continuous density of states (DOS) near the EF (Fig. 5d), suggesting the narrow band gaps and excellent conductivity. The ɛd values of Co in CoVO@NF, CoN/VN@NF and P-CoVO@NF are ‒1.23, ‒1.09 and ‒1.43 eV, respectively, while the corresponding values of V are ‒0.08, ‒0.02 and ‒0.17 eV (Fig. 5e,f). Relatively, the Co and V in CoN/VN@NF model show the highest electron density and the ɛd values are much closer to the EF, leading to stronger ntermediates adsorption and efficient electron transfer. The Co and V in P-CoVO@NF model exhibit a moderate electron density with the furthest ɛd values from EF, suggesting a higher occupancy of antibonding states, which weakens the intermediates adsorption and facilitates the subsequent reaction steps.46 Based on the hydrogen binding energy (HBE) theory, the*H Gibbs free energy (ΔG*H) for CoN/VN@NF (‒0.23 eV) is much closer to zero indicates the promoted HER activity (Supplementary Fig. 22‒24). Moreover, the water dissociation serves as the rate-determining step (RDS) for both CoVO@NF and P-CoVO@NF models, whereas the water adsorption is the RDS for CoN/VN@NF model (Fig. 5g and Supplementary Fig. 25). The energy barrier values of RDS demonstrate that the Co site is the active site for CoVO@NF and P-CoVO@NF models, while V site is beneficial to the *OH adsorption for CoN/VN@NF model. Besides, the exothermic water dissociation of CoN/VN@NF model suggests its excellent HER activity.
To identify the active sites for OER, the P, V and Co sites in P-CoVO@NF model are respectively selected as the active sites, in which the Co site exhibits the lowest energy barrier for the RDS (Supplementary Fig. 26–29). Specially, the RDS of P-CoVO@NF model is the formation of *OOH with an energy barrier of 1.76 eV (Fig. 5h), significantly lower than CoVO@NF of 2.62 eV and CoN/VN@NF of 1.92 eV (*O formation step is the RDS). It is concluded that the formation of strongly nucleophilic P-O group greatly enhance the interfacial proton transfer, thereby reducing the energy barrier for the formation of *O and promoting the conversion rate of intermediates. The weaker electron coupling effect of P-O group can lower the adsorption strength of oxygen-containing intermediates and accelerate the deprotonation of *OH, thus enhancing the OER kinetics. Since the cobalt species in P-CoVO@NF gradually reconstructs into cobalt hydroxide during OER, so the P-CoOOH model was also built (inset of Fig. 5i). The Co site in P-CoOOH model has a much more positive charge of 0.92 e, and the ɛd value of ‒1.38 eV is closer to the EF compared to P-CoVO@NF (Supplementary Fig. 30), indicating the stronger adsorption of oxygen-containing intermediates. Furthermore, the ΔG of oxygen-containing intermediates at the Co site in P-CoOOH is obviously decreased (Fig. 5i and Supplementary Fig. 31), while the formation of *OOH is identified as the RDS. Specifically, the difference between ΔG*O and ΔG*OOH at the Co site in P-CoOOH is 1.68 eV, which is lower than P-CoVO@NF, indicating a thermodynamically favorable OER process of P-CoOOH. The charge configuration analyses indicate that the increase of Co valence state after reconstructing induces a pseudo-electrophobic effect, thereby reducing the positive charge on the O atom as well as the negative charge on the H atom in *OH. It reveals that the O‒H bond in *OH is easy to cleavage under the attack of OH‒ ions, benefitting to the formation of *OOH. Therefore, such a structural reconfiguration could promote OER kinectics.
We further utilize CoN/VN@NF as cathode and P-CoVO@NF as anode for overall water splitting by an H‒type electrolytic cell separated with an exchange membrane. The P-CoVO@NF(+)||CoN/VN@NF(‒) system only requires a voltage of 1.43 V to achieve a current density of 10 mA cm‒2, whereas a higher voltage of 1.62 V is needed for RuO2@NF(+)||PtC@NF(‒) system (Supplementary Fig. 32). Importantly, a 1.75 V voltage for P-CoVO@NF(+)||CoN/VN@NF(‒) system could drive a large current of 100 mA cm‒2. The outstanding performance is comparable to the reported advanced electrocatalysts (Supplementary Table 10). It also displays a prominent durability without obvious attenuation current density for 100 h (Supplementary Fig. 33). The Faradaic efficiency was measured by the H–type electrolytic cell with collection device of the evolved H2 and O2 gases (Supplementary Fig. 34). The obtained volume ratio of H2:O2 is about 2:1 at a current density of 40 mA cm− 2, and the P-CoVO@NF(+)||CoN/VN@NF(‒) system exhibits a Faraday efficiency of near 100% (Supplementary Fig. 35). The P-CoVO@NF//CoN/VN@NF system also can be driven by a 1.45 V solar cell (Supplementary Fig. 36). The H2 and O2 gases are conspicuously generated on the cathode and anode, respectively, further manifesting the potential for converting low-voltage electrical energy originated from solar energy into chemical energy. Furthermore, we also assembled an AEM electrolyzer to evaluate the practice application of catalysts. The P-CoVO@NF and CoN/VN@NF were respectively used as the anode and cathode with the AEM made of industrial fluorocarbon acid resin film (Fig. 6a). The electrolyzer was tested within the temperature range of 25 to 70°C using 1 M KOH electrolyte, and it could be concluded that the voltage and total cell resistance decreased with the working temperature increased (Fig. 6b,c). At 70°C, a voltage of 1.88 V was able to drive a current density of 500 mA cm− 2, while a voltage of 1.98 V achieved a current density of 1000 mA cm–2, outperforming most of the advanced non-precious metal-based electrocatalysts.47,48 The WDC was calculated to be 4.31 kWh Nm–3@250 mA cm–2, approaching the international forefront in terms of efficiency. Additionally, the electrolyzer attaining an high energy conversion efficiency of 78.7% under an operational current density of 500 mA cm–2, as well as excellent stability over a span of 200 hours (Fig. 6d). This can be attributed to the construction of 3D micro-nano structures and the occurrence of surface restructuring. The results suggest that the optimized nanostructure design of P-CoVO@NF and CoN/VN@NF could serve as potential applications is commercial AEM systems.