Hierarchical CoNiO2 Microflowers Assembled by Mesoporous Nanosheets as Efficient Electrocatalysts for Hydrogen Evolution Reaction

In order to alleviate the energy crisis and propel a low-carbon economy, hydrogen (H2) plays an important role as a renewable cleaning resource. To break the hydrogen evolution reaction (HER) bottleneck, we need high-efficiency electrocatalysts. Based on the synergistic effect between bimetallic oxides, hierarchical mesoporous CoNiO2 nanosheets can be fabricated. Combining physical representations with electrochemical measurements, the resultant CoNiO2 catalysts present the hierarchical microflowers morphology assembled by mesoporous nanosheets. The ultrathin two-dimensional nanosheets and porous surface characteristics provide the vast channels for electrolyte injection, thus endowing CoNiO2 the outstanding HER performance. The excellent performance with a fewer onset potential of 94 mV, a smaller overpotential at 10 mA cm−2, a lower Tafel slope of 109 mV dec−1 and better stability after 1000 cycles makes CoNiO2 better than that of metallic Co and metallic Ni.


Introduction
Fossil fuels are running out and the planet is getting worse, meaning it is really important to develop high-power and energy density alternative energy devices. There is no doubt that hydrogen (H 2 ) is a renewable and clean energy that can be obtained from the environment [1][2][3][4]. It is well known that hydrogen is the most abundant element on earth; currently, 90% of H 2 is acquired from fossil fuels, leading to the aggravation of the energy crisis [5]. Electrocatalytic water (H 2 O) splitting, using H 2 O as the hydrogen source and powered by renewable electricity, is regarded as a potential H 2 synthesis strategy to minimize carbon emissions [6][7][8][9][10]. The hydrogen evolution reaction (HER) is, however, still limited by the large overpotential that is found in splitting H 2 O, which makes the reaction performance hover at a significantly lower level. It has been shown that certain noble metals with the low overpotential, such as platinum-based materials, are expected to serve as excellent catalysts for hydrogen production in practical application, though the scarcity of these materials and the high price make them difficult to use in practice [11][12][13][14].
To date, non-precious metals as the substitute materials have been extensively explored for HER to replace precious metals. By virtue of their structure design and synergistic effect, catalysts are capable of introducing hydrogen into a system at a higher performance level. There is a possibility of increasing the electrochemical HER performance by increasing the number of active sites in nanosheet structures. For example, Ni 0.85 Se nanosheet on graphene owns many active sites and well HER catalytic property [21]. It may also be advantageous to improve performance by taking advantage of the synergistic effects of Materials 2023, 16,2204 2 of 12 bimetallic electrocatalysts. Singh et al. reported that the B-TiZr-2.5 catalyst has improved HER activity [22]. It is recognized that the single metal catalyst is not enough to make the HER catalyst active. Therefore, other metal oxide phases need to be added to obtain higher HER performance.
It is well known that most transition monometal oxides have a very good catalytic activity, but due to their low active site and specific surface area, their further applications are limited. Due to this, many efforts have been made in order to further enhance the catalytic properties of transition monometal oxides. Among them, the introduction of bimetallic oxides is a strategy. Wang et al. reported that NiCo 2 O 4 @ Ni(OH) 2 /NiOOH catalyst has the improved HER activity [23]. In view of the synergistic effect, developing the efficient Co-Ni oxide catalysts by a simple and cost-effective method is attractive and necessary for HER [24]. Herein, hierarchical CoNiO 2 microflowers assembled by mesoporous nanosheets are synthesized successfully by a simple hydrothermal method together with an annealing process. Benefitting from the large specific surface area and synergistic effect between Ni-Co metals, this resultant CoNiO 2 shows stunning catalytic properties for HER, such as the low Tafel slope of 109 mV dec −1 , small onset potential of 94 mV, low charge transfer resistance and a long-term stable behavior regarding catalysis.

Synthesis of CoNiO 2
The hierarchical CoNiO 2 microflower was prepared [25,26]; concretely, an ethanolwater (2:1 v/v) mixture containing 0.73 g Ni(NO 3 ) 2 ·6H 2 O, 1.45 g Co(NO 3 ) 2 ·6H 2 O and 1.4 g hexamethylenetetramine (HMT). This was then transferred into a 50 mL autoclave lined with Teflon after it had been stirred for 30 min. Upon sealing the autoclave, it was placed into an electric oven which had been set at 95 • C for 8 h. We first cooled down the as-obtained Co-Ni precursors under a vacuum to room temperature, washed with water and ethanol several times to remove the unreacted residues, followed by drying at 60 • C for 8 h under vacuum to ensure the precursors were completely dry. Lastly, CoNiO 2 was synthesized following annealing at 450 degrees Celsius in an Ar atmosphere for two hours at 1 • C per minute. For comparison, the individual Ni and Co catalysts were procured by the same method without adding Cobaltous nitrate hexahydrate and Nickel nitrate hexahydrate, respectively.

Characterizations
In terms of surface analysis, X-ray Photoelectron Spectroscopy (XPS, ESCALAB250) is a practical method that can be used for both qualitative and quantitative analyses of solid surfaces, as well as structural identification. X-ray diffraction (XRD, Rigaku TTR III) is a research method to obtain information such as the composition of materials, the structure or morphology of molecules in materials. Scanning electron microscope (FESEM, FEI Quanta 200 FEG) can not only observe the surface morphology, but also analyze the components and elements. Transmission Electron Microscopy (TEM) and High-Resolution TEM (HRTEM) are instruments for imaging and analyzing the microstructure of materials (FEI Tecnai G2 F20).

Electrochemical Measurements
It took 550 µL of 6/5 (v/v) water/ethanol solution, 5 mg of CoNiO 2 powder and 20 µL of Nafion (5 wt %) to make the mixed solution and this was sonicated mildly for 30 min. Then, 5 µL of the prepared catalyst solution was evenly coated on carbon fiber cloth with an area of 1 cm 2 . Typical electrochemical measurements were carried out using a three-electrode setup involving the working electrode (the loaded carbon cloth), as well as the reference electrode (the Hg/HgO solution) and the counter electrode (the carbon rod). According to the equation below, the potentials reported in this study have been converted to the reversible hydrogen electrode (RHE) scale and are now expressed as follows:

Results and Discussion
The synthetic method can be demonstrated visually in Figure 1, in which a two-step method had been used to prepare the typical mesoporous CoNiO 2 . Hydrothermal processes are characterized by the hydrolysis of ethanol and hexamethylenetetramine, forming carbonate ion and hydroxide ion, which are then reacted with Ni 2+ and Co 2+ to form precursors of nickel and cobalt hydroxy-carbonates during the reaction. Secondly, under the Ar atmosphere, the as-prepared Ni-Co precursor was transformed into mesoporous CoNiO 2 with a hierarchical structure. Furthermore, individual Co and Ni catalysts were prepared so that the synergistic effect of these components could be evaluated. reference electrode (the Hg/HgO solution) and the counter electrode (the carbon rod cording to the equation below, the potentials reported in this study have been conv to the reversible hydrogen electrode (RHE) scale and are now expressed as fo E(RHE) = E(Hg/HgO) + 0.098 + 0.0592 × pH. The electrocatalytic experiments were c out on an Autolab PGSTAT 302 (Eco Chemie) workstation.

Results and Discussion
The synthetic method can be demonstrated visually in Figure 1, in which a tw method had been used to prepare the typical mesoporous CoNiO2. Hydrotherma cesses are characterized by the hydrolysis of ethanol and hexamethylenetetramine, ing carbonate ion and hydroxide ion, which are then reacted with Ni 2+ and Co 2+ to precursors of nickel and cobalt hydroxy-carbonates during the reaction. Secondly, the Ar atmosphere, the as-prepared Ni-Co precursor was transformed into mesop CoNiO2 with a hierarchical structure. Furthermore, individual Co and Ni catalysts prepared so that the synergistic effect of these components could be evaluated. To investigate the crystal structure of the electrocatalyst, an XRD analysis ha carried out. The characteristic peaks of CoNiO2 in Figure 2  It was observed that there were no significant peaks of impurities, which indicate the products were phase pure. Particularly, compared with the single metallic Ni specimen, CoNiO2 presents the weaker peak intensities. The relatively poor crysta means that CoNiO2 exhibits the thinner thickness and higher defects concentration, is expected to show better activity in the hydrogen evolution reaction. To investigate the crystal structure of the electrocatalyst, an XRD analysis has been carried out. The characteristic peaks of CoNiO 2 in Figure 2 [28]. It was observed that there were no significant peaks of impurities, which indicated that the products were phase pure. Particularly, compared with the single metallic Ni or Co specimen, CoNiO 2 presents the weaker peak intensities. The relatively poor crystallinity means that CoNiO 2 exhibits the thinner thickness and higher defects concentration, which is expected to show better activity in the hydrogen evolution reaction. Electrocatalysts were characterized morphologically using scanning electron microscopy (SEM). As shown in the low-magnification SEM image (Figure 3a,b), the as-prepared CoNiO2 material presents the flower-like microsphere structure with an average diameter of about 9 μm. According to the high-resolution SEM images, it is also evident that the microflowers are a result of the two-dimensional thin nanosheets that are assembled into the microflowers (Figure 3c,d). For comparison, the morphologies of the metallic Ni and metallic Co were also provided, as shown in Figure 3e,f. It is worth noting that the individual Ni and Co samples are formed by spherical particles along with the severe agglomeration simultaneously. Having a regular and open hierarchical structure gives CoNiO2 a more specific surface area, makes electrolyte injection efficient and helps it be more catalytic. The conductivity of CoNiO2 material obtained after calcination is increased, which makes hydrogen evolution easier. The above results show that the prepared CoNiO2 material has a large specific surface area, exposed more active sites and good conductivity.   Electrocatalysts were characterized morphologically using scanning electron microscopy (SEM). As shown in the low-magnification SEM image (Figure 3a,b), the asprepared CoNiO 2 material presents the flower-like microsphere structure with an average diameter of about 9 µm. According to the high-resolution SEM images, it is also evident that the microflowers are a result of the two-dimensional thin nanosheets that are assembled into the microflowers (Figure 3c,d). For comparison, the morphologies of the metallic Ni and metallic Co were also provided, as shown in Figure 3e,f. It is worth noting that the individual Ni and Co samples are formed by spherical particles along with the severe agglomeration simultaneously. Having a regular and open hierarchical structure gives CoNiO 2 a more specific surface area, makes electrolyte injection efficient and helps it be more catalytic. The conductivity of CoNiO 2 material obtained after calcination is increased, which makes hydrogen evolution easier. The above results show that the prepared CoNiO 2 material has a large specific surface area, exposed more active sites and good conductivity. Electrocatalysts were characterized morphologically using scanning electron microscopy (SEM). As shown in the low-magnification SEM image (Figure 3a,b), the as-prepared CoNiO2 material presents the flower-like microsphere structure with an average diameter of about 9 μm. According to the high-resolution SEM images, it is also evident that the microflowers are a result of the two-dimensional thin nanosheets that are assembled into the microflowers (Figure 3c,d). For comparison, the morphologies of the metallic Ni and metallic Co were also provided, as shown in Figure 3e,f. It is worth noting that the individual Ni and Co samples are formed by spherical particles along with the severe agglomeration simultaneously. Having a regular and open hierarchical structure gives CoNiO2 a more specific surface area, makes electrolyte injection efficient and helps it be more catalytic. The conductivity of CoNiO2 material obtained after calcination is increased, which makes hydrogen evolution easier. The above results show that the prepared CoNiO2 material has a large specific surface area, exposed more active sites and good conductivity.   We investigated the morphology of the as-prepared CoNiO 2 sample by transmission electron microscopy and high-resolution transmission electron microscopy in a further study. According to the TEM images of CoNiO 2 shown in Figure 4a-d, the thin nanosheets are used to construct the microflowers, and therefore, TEM images are in agreement with SEM observations, where the nanosheets are assembled by thin walls. A distinctive characteristic of CoNiO 2 microflowers is that they are formed by thin nanosheets of CoNiO 2 over a thickness of approximately 100-300 nanometers. In Figure 4e, it is depicted that the lattice fringes with separations of 0.211 nanometers and 0.244 nanometers correspond to the crystallographic planes (200) and (111) for cubic CoNiO 2 , respectively. It is also worth mentioning that the SADE image exhibits a pattern along the zone axis of the [110] zone of cubic CoNiO 2 , which further confirms that CoNiO 2 phase has been successfully generated (Figure 4f).
x FOR PEER REVIEW 5 of 12 We investigated the morphology of the as-prepared CoNiO2 sample by transmission electron microscopy and high-resolution transmission electron microscopy in a further study. According to the TEM images of CoNiO2 shown in Figure 4a-d, the thin nanosheets are used to construct the microflowers, and therefore, TEM images are in agreement with SEM observations, where the nanosheets are assembled by thin walls. A distinctive characteristic of CoNiO2 microflowers is that they are formed by thin nanosheets of CoNiO2 over a thickness of approximately 100-300 nanometers. In Figure 4e , it is depicted that the lattice fringes with separations of 0.211 nanometers and 0.244 nanometers correspond to the crystallographic planes (200) and (111) for cubic CoNiO2, respectively. It is also worth mentioning that the SADE image exhibits a pattern along the zone axis of the [110] zone of cubic CoNiO2, which further confirms that CoNiO2 phase has been successfully generated (Figure 4f). Further confirmations of surface elemental composition were done using X-ray photoelectron spectroscopy ( Figure 5). Figure 5a shows the full scan spectrum of CoNiO2, in which Co, Ni and O elements coexist. According to the Co 2p spectra (Figure 5b), the binding energies at 795.0 eV and 779.8 eV correspond to the peaks of Co 2p1/2 and Co 2p3/2, respectively, while the satellite peaks of 803.3 eV and 785.9 eV are associated with its satellites [29]. A correlation was found between Ni 2p3/2 and Ni 2p1/2, as indicated by the characteristic peaks of Ni 2p in Figure 5c located at 854.0 eV and 872.8 eV, and the relevant satellite peaks were also observed at 861.1 eV and 879.5 eV [30]. There are also two characteristic peaks in the O 1s spectrum (Figure 5d); one at 529.6 eV represents oxygen species in the CoNiO2 phase and the other represents oxygen-containing residuals like H-O-H. Based on the above characterizations, the hierarchical CoNiO2 microspheres are synthesized successfully.
Using a typical three-electrode setup that contained a concentration of 1 M KOH, hierarchical CoNiO2, metallic Ni and metallic Co were investigated for their electrocatalytic activities. Figure 6a illustrates that HER polarization curves were measured at 1 mV s −1 with IR correction for CoNiO2, Ni, Co and bare carbon cloth. The CoNiO2 catalyst has a lower onset overpotential of 94 mV when compared to Ni and Co catalysts, and has a Further confirmations of surface elemental composition were done using X-ray photoelectron spectroscopy ( Figure 5). Figure 5a shows the full scan spectrum of CoNiO 2 , in which Co, Ni and O elements coexist. According to the Co 2p spectra (Figure 5b), the binding energies at 795.0 eV and 779.8 eV correspond to the peaks of Co 2p 1/2 and Co 2p 3/2 , respectively, while the satellite peaks of 803.3 eV and 785.9 eV are associated with its satellites [29]. A correlation was found between Ni 2p 3/2 and Ni 2p 1/2 , as indicated by the characteristic peaks of Ni 2p in Figure 5c located at 854.0 eV and 872.8 eV, and the relevant satellite peaks were also observed at 861.1 eV and 879.5 eV [30]. There are also two characteristic peaks in the O 1s spectrum (Figure 5d); one at 529.6 eV represents oxygen species in the CoNiO 2 phase and the other represents oxygen-containing residuals like H-O-H. Based on the above characterizations, the hierarchical CoNiO 2 microspheres are synthesized successfully. reaction kinetics of HER using Tafel plots as a way. Figure 6b illustrates that the Tafel slope of CoNiO2 is 109 mV dec −1 , a value lower than both Ni (149 mV dec −1 ) and Co catalysts (225 mV dec −1 ). As indicated by the Tafel slope of 109 mV dec −1 , the hydrogen evolution process on CoNiO2 is governed by the Volmer-Heyrovsky mechanism, and the lower value is indicative of the more favorable characteristics of its kinetic behavior toward hydrogen evolution. The stability and durability of the as-synthesized CoNiO2 are also evaluated in alkaline condition. Figure 6c shows that the chronopotentiometry curve shows no obvious degradation after 10 h at 10 mA cm −2 . As can be seen from Figure 6c (inset), the polarization curve starts to overlap the initial curve almost completely, and this maintains the current density of 98.2% even after 1000 continuous sweeping cycles. All the above results prove that CoNiO2 has excellent HER activity, along with great stability.
Using electrochemical impedance spectroscopy (EIS), we conducted an analysis with an overpotential of −300 mV in order to better understand the HER kinetics of CoNiO2, metallic Ni and metallic Co. Figure 6d shows the corresponding Nyquist plots and Figure  6d inset shows the equivalent circuit model. It is clearly observed that CoNiO2 presents the lowest charge transport resistance (Rct), reflecting its better electronic conductivity. Specifically, CoNiO2, Ni and Co possess Rct values of 5.13, 5.27 and 5.35 Ω, respectively. Smaller Rct values indicate that CoNiO2 owns quicker charge transfer than other samples. During the HER process of CoNiO2, the electrons deliver directly and efficiently between the catalyst and the solution, contributing well to activity. Using a typical three-electrode setup that contained a concentration of 1 M KOH, hierarchical CoNiO 2 , metallic Ni and metallic Co were investigated for their electrocatalytic activities. Figure 6a illustrates that HER polarization curves were measured at 1 mV s −1 with IR correction for CoNiO 2 , Ni, Co and bare carbon cloth. The CoNiO 2 catalyst has a lower onset overpotential of 94 mV when compared to Ni and Co catalysts, and has a faster current response when the potential goes up, while pure carbon cloth has negligible electrocatalytic activity. Furthermore, CoNiO 2 has only an overpotential of 170 mV at 10 mA cm −2 , which is lower than that of Ni (188 mV) and Co (244 mV), showing that it has a superior catalytic activity compared to these two metals. The HER performance of CoNiO 2 is also superior to other Co-Ni composite materials reported previously (Table 1), which benefits from the nanoflower structure providing more active sites and the synergistic effect.   An electrochemical surface area with a higher value typically means there are more active sites in the electrochemical layer, which results in higher HER activity. In addition, the electrochemical surface area varies with the capacitance of the double layer, so that it is proportional to the surface area. There are several CV curves that can be seen in Figure  7a-c related to electrocatalysts that have been tested with a scan rate that ranges from 20 to 100 mV s −1 , namely CoNiO2, metallic Ni and metallic Co in an alkaline electrolyte. At 0.77 V, we calculated the current density difference between the anode and cathode (∆j = Janode − Jcathode). It can be obtained twice the value of double-layer capacitance (Cdl) by fitting the scanning rate and ∆j together. As you can see in Figure 7d, CoNiO2 has a large Cdl value (8.05 * 10 −4 mF cm −2 ) compared to metallic Ni (5.10 * 10 −4 mF cm −2 ) and metallic Co (3.15 * 10 −4 mF cm −2 ). There was a noticeable difference between CoNiO2 and all the other We further explored the inherent properties of different catalysts by evaluating the reaction kinetics of HER using Tafel plots as a way. Figure 6b illustrates that the Tafel slope of CoNiO 2 is 109 mV dec −1 , a value lower than both Ni (149 mV dec −1 ) and Co catalysts (225 mV dec −1 ). As indicated by the Tafel slope of 109 mV dec −1 , the hydrogen evolution process on CoNiO 2 is governed by the Volmer-Heyrovsky mechanism, and the lower value is indicative of the more favorable characteristics of its kinetic behavior toward hydrogen evolution.
The stability and durability of the as-synthesized CoNiO 2 are also evaluated in alkaline condition. Figure 6c shows that the chronopotentiometry curve shows no obvious degradation after 10 h at 10 mA cm −2 . As can be seen from Figure 6c (inset), the polarization curve starts to overlap the initial curve almost completely, and this maintains the current density of 98.2% even after 1000 continuous sweeping cycles. All the above results prove that CoNiO 2 has excellent HER activity, along with great stability.
Using electrochemical impedance spectroscopy (EIS), we conducted an analysis with an overpotential of −300 mV in order to better understand the HER kinetics of CoNiO 2 , metallic Ni and metallic Co. Figure 6d shows the corresponding Nyquist plots and Figure 6d inset shows the equivalent circuit model. It is clearly observed that CoNiO 2 presents the lowest charge transport resistance (R ct ), reflecting its better electronic conductivity. Specifically, CoNiO 2 , Ni and Co possess R ct values of 5.13, 5.27 and 5.35 Ω, respectively. Smaller R ct values indicate that CoNiO 2 owns quicker charge transfer than other samples. During the HER process of CoNiO 2 , the electrons deliver directly and efficiently between the catalyst and the solution, contributing well to activity.
An electrochemical surface area with a higher value typically means there are more active sites in the electrochemical layer, which results in higher HER activity. In addition, the electrochemical surface area varies with the capacitance of the double layer, so that it is proportional to the surface area. There are several CV curves that can be seen in Figure 7a-c related to electrocatalysts that have been tested with a scan rate that ranges from 20 to 100 mV s −1 , namely CoNiO 2 , metallic Ni and metallic Co in an alkaline electrolyte. At 0.77 V, we calculated the current density difference between the anode and cathode (∆j = J anode − J cathode ). It can be obtained twice the value of double-layer capacitance (C dl ) by fitting the scanning rate and ∆j together. As you can see in Figure 7d, CoNiO 2 has a large C dl value (8.05 * 10 −4 mF cm −2 ) compared to metallic Ni (5.10 * 10 −4 mF cm −2 ) and metallic Co (3.15 * 10 −4 mF cm −2 ). There was a noticeable difference between CoNiO 2 and all the other materials investigated in respect of their electrochemical surface area and HER activity in the alkaline electrolyte based on the results.  The composition of the CoNiO2 process was examined by XRD following the HER for 10 h. As shown in Figure 8a, the XRD curves before and after 10 h test of CoNiO2 in an alkaline electrolyte have been displayed. The results show that all the peaks after the 10 h test have remained approximately the same as they were before the test, which proves that the CoNiO2 in an alkaline electrolyte is a very stable compound. SEM images ( Figure  8b) were taken of the same samples after 10 h of HER in an alkaline electrolyte, which indicate that the morphologies after the test are nanoparticles agglomerated compared to the ones before the test. As shown in Figure 9a, the characteristic peak of the cobalt-nickel precursor located at 9.5° corresponds to the (001) planes of Co(OH)2 (JCPDS No. 51-1731), and is located at 19.3° (001) planes of Ni(OH)2 (JCPDS No. 14-0117). As shown in the SEM image ( Figure  9a,b), the cobalt-nickel precursor presents the flower-like microsphere structure. The composition of the CoNiO 2 process was examined by XRD following the HER for 10 h. As shown in Figure 8a, the XRD curves before and after 10 h test of CoNiO 2 in an alkaline electrolyte have been displayed. The results show that all the peaks after the 10 h test have remained approximately the same as they were before the test, which proves that the CoNiO 2 in an alkaline electrolyte is a very stable compound. SEM images (Figure 8b) were taken of the same samples after 10 h of HER in an alkaline electrolyte, which indicate that the morphologies after the test are nanoparticles agglomerated compared to the ones before the test.  The composition of the CoNiO2 process was examined by XRD following the HER for 10 h. As shown in Figure 8a, the XRD curves before and after 10 h test of CoNiO2 in an alkaline electrolyte have been displayed. The results show that all the peaks after the 10 h test have remained approximately the same as they were before the test, which proves that the CoNiO2 in an alkaline electrolyte is a very stable compound. SEM images ( Figure  8b) were taken of the same samples after 10 h of HER in an alkaline electrolyte, which indicate that the morphologies after the test are nanoparticles agglomerated compared to the ones before the test. As shown in Figure 9a, the characteristic peak of the cobalt-nickel precursor located at 9.5° corresponds to the (001) planes of Co(OH)2 (JCPDS No. 51-1731), and is located at 19.3° (001) planes of Ni(OH)2 (JCPDS No. 14-0117). As shown in the SEM image ( Figure  9a,b), the cobalt-nickel precursor presents the flower-like microsphere structure. (001) planes of Ni(OH) 2 (JCPDS No. 14-0117). As shown in the SEM image (Figure 9a,b), the cobalt-nickel precursor presents the flower-like microsphere structure. The surface elemental composition of the CoNiO2 process was examined by XPS following the post-HER. As shown in Figure 10, the XPS curves of CoNiO2 of the post-HER in an alkaline electrolyte have been displayed. The results show that the chemical surface composition of post-HER is the same as before and that there is no difference, which proves that the CoNiO2 in an alkaline electrolyte is a very stable compound.  The surface elemental composition of the CoNiO 2 process was examined by XPS following the post-HER. As shown in Figure 10, the XPS curves of CoNiO 2 of the post-HER in an alkaline electrolyte have been displayed. The results show that the chemical surface composition of post-HER is the same as before and that there is no difference, which proves that the CoNiO 2 in an alkaline electrolyte is a very stable compound. The surface elemental composition of the CoNiO2 process was examined by XPS following the post-HER. As shown in Figure 10, the XPS curves of CoNiO2 of the post-HER in an alkaline electrolyte have been displayed. The results show that the chemical surface composition of post-HER is the same as before and that there is no difference, which proves that the CoNiO2 in an alkaline electrolyte is a very stable compound.

Conclusions
In summary, it can be concluded that the hierarchical mesoporous CoNiO 2 has been successfully prepared, and that this CoNiO 2 can be used as a catalyst in water to catalyze the HER reaction. In HER, CoNiO 2 only has an onset potential of 94 mV and an overpotential of 170 mV at 10 mA cm −2 due to the synergistic effect, compared with metallic Co and metallic Ni. Meanwhile, CoNiO 2 presents the improved kinetic characteristics with a Tafel slope (109 mV dec −1 ). The enhanced reaction kinetics of CoNiO 2 can be attributed to the high surface area, and are beneficial for contacting with electrolytes and enhancing the transport of charges. Additionally, CoNiO 2 shows very good long-term stability and can withstand for over 10 h. The excellent electrocatalytic activity with high stability makes CoNiO 2 promising for application in HER.