A highly active and durable hybrid Ni/NiOOH catalyst by synergistic high-temperature deposition and electrochemical oxidization for hydrogen evolution

Abstract

Constructing a heterointerface has become a preferred strategy for the hydrogen evolution reaction (HER) due to the synergistical H₂O dissociation and *H adsorption. Ni/Ni(OH)₂ hybrid catalysts with an isogenous heterointerface have exhibited great potential in the alkaline HER. However, designing high performance Ni/Ni(OH)₂ and understanding the catalytic mechanisms still remains challenging. Herein, we demonstrate that the HER performance of Ni/Ni(OH)₂ depends significantly on the interface density and deprotonation. Experimentally, Ni and Ni(OH)₂ grains are refined to enlarge the interface density at elevated temperature, and the activity and stability are rationally tuned by delicately regulating deprotonation at various oxidization potentials. Theoretical calculations reveal that the deprotonation energy decreases with grain refinement, which promotes the interface electron redistribution. The deprotonation lowers the H₂O dissociation energy and alleviates *H adsorption, but the excessive deprotonation leads to strong *OH adsorption, retarding H₂O dissociation, whereas the stability is enhanced. The optimum Ni/Ni(OH)₂ hybrid catalyst reaches an outstanding HER performance with an overpotential of 30 mV@10 mA cm⁻² and stable activity for over 300 hours at an extremely large current density (2.0 A cm⁻²), surpassing most of the reported HER catalysts. This work initiates a new pathway to improving catalytic performance by regulating the interface density and valence state.

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New concepts

The Ni/Ni(OH)₂ isogenous heterojunction catalyst exhibits excellent hydrogen evolution reaction (HER) activity, with its high and low valence Ni species acting as the active sites for H₂O dissociation and *H adsorption, respectively. Theoretically, the active site density and intrinsic activity of each active site determine the overall HER activity. However, it is still challenging to simultaneously improve the interface density and intrinsic activity of Ni/Ni(OH)₂, as well as to understand its catalytic mechanism. In this study, high-temperature electrochemical deposition was employed to simultaneously increase the generation of Ni²⁺ and OH⁻, refining both Ni and Ni(OH)₂ grains and enhancing the density of the heterointerface. The activity and stability are further improved by delicately regulating the deprotonation at the appropriate oxidization potential. In addition, DFT calculations deeply explored the deprotonation mechanism of Ni(OH)₂, finding that refining the grains facilitates deprotonation, while deprotonation reduces the H₂O dissociation energy and alleviates *H adsorption. Although excessive deprotonation can increase the stability of Ni(OH)₂, it will lead to strong *OH adsorption and retard the subsequent H₂O dissociation, which is not conducive to the continuous progress of the HER process. Based on the above discussion, this study perfects the Volmer step of the traditional alkaline HER mechanism by adding the process of H₂O replacing OH*, in order to clarify the HER process.

Introduction

Water electrolysis is an effective route for high-purity hydrogen production and energy storage to stabilize the intermittency of green energy sources such as wind and solar.^1–3 As a primary technology of great prospects, alkaline water electrolysis is still limited by the cathodic hydrogen evolution reaction (HER) owing to the kinetically sluggish Volmer step for water dissociation.^4–6 High-valence metal species are effective in cleaving the H–O bond of water molecules and accelerating the Volmer step.^7,8 Therefore, heterojunction catalysts with dual active sites for H₂O dissociation and *H adsorption have been constructed by combining high-valence metal species and low-valence metal species,⁹ such as Mo–NiS/Ni₃S₂,¹⁰ Mo₂C@BCN,¹¹ BP/CoNiSe₂,¹² Ni₂P/NiMoO_x,¹³ Mo–MoNi₄/NiOOH,¹⁴ Ni/CeO_x,¹⁵ Pt/Ni(OH)₂,^16,17 Pt/FeOOH,¹⁸ Pt/CeO_x,¹⁹ Pt/RuO_x,⁶ Pt/MoO_x,²⁰ Pt/MoS₂,²¹ or Pt–Co/CoO₂,²² and exhibit superior HER activity.

Among these heterojunction catalysts, the pure Ni-based heterojunction catalysts are a special type which comprises different valence Ni species acting as the dual active sites, and they are also named as isogenous heterojunction catalysts.^23,24 The different valence species in Ni-based isogenous heterojunction or hybrid catalysts, such as Ni/Ni(OH)₂, can be produced simultaneously by a one-step process or sequentially by a two-step process.^24–27 Note that for the heterojunction catalysts, the active interface is exposed at the surface directly contacting with the electrolyte. Thus, the HER activity depends heavily on the exposed interface density,^28,29 which is closely related to the synthesis process. It is reasonably speculated that the hybrid catalysts produced by a one-step process, such as electrochemical deposition, should provide a larger interface density or more active sites because of the random and intertwined characteristics of the heterointerface in the catalysts.^30–32

Besides the active site density, the intrinsic activity of each active site also determines the overall HER activity.^33–36 For the Ni-based hybrid catalysts, the higher valence Ni species can further accelerate H₂O dissociation, conducive to the intrinsic activity.^37–41 Thereafter, our group and other reports have found that the HER activity can be improved by regulating the content of the high valence Ni species and the valence. In addition, the high valence Ni species can be further oxidized to a higher valence by interlayer deprotonation accompanying the transformation from Ni(OH)₂ to NiOOH.^42–44 Measures that can shorten the proton transport path and weaken the bonding strength can accelerate deprotonation and benefit the HER activity. Recently, it has been reported that reducing the grain size⁴⁵ and intercalating ionized groups, such as NH⁴⁺,⁴⁶ HPO₄²⁻,⁴⁷ –COOH,⁴⁸ WO₄²⁻ (ref. 49) and POM,⁵⁰ accelerate the deprotonation procedure, but all of these reports focus on improving the oxygen evolution reaction (OER) activity, rather than the HER activity. Moreover, excessive deprotonation leads to over oxidization of Ni species, which strongly enhances the *OH adsorption, leading to *OH blocking and retarding the Volmer step for water dissociation.^28,51–53 Meanwhile, the deprotonation and protonation of transition hydroxides are reversible. The activity of Ni/Ni(OH)₂ hybrid catalysts might degrade with the higher valence species (NiOOH) turning back to Ni(OH)₂ during long-term operation. Thus, these Ni/Ni(OH)₂ hybrid catalysts still have a huge gap to overcome compared with recently reported HER catalysts combining high activity and stability that nearly meet industrial requirements. It is necessary to clarify the relationship of the HER activity to the interface characteristics, including interface density, charge-density distribution, and deprotonation.

In this work, we propose a high-temperature electrochemical deposition and oxidization strategy to regulate the interface characteristics and then optimize the HER performance of Ni/NiOOH hybrid catalysts. High-temperature electrochemical deposition accelerated the nucleation rate of Ni and Ni(OH)₂, refined the grains, and finally enlarged the interface density. Furthermore, the structure and charge-density distribution at the interface were modulated by deprotonation at various oxidizing potentials. The optimal hybrid catalyst Ni/NiOOH-90 shows outstanding HER activity with an overpotential of 30 mV@10 mA cm⁻², comparable to the noble metal catalysts. The high temperature and deprotonation reinforced the adhesion strength of the catalysts and stabilized the interface structure. Finally, the hybrid catalyst Ni/NiOOH-90 operated stably for over 300 hours at extremely large current densities (2.0 A cm⁻²). Moreover, the degraded catalyst could be reactivated through electrochemical oxidization. The mechanisms of the HER activity and stability associated with the interface modulation were revealed by density functional theory (DFT) calculations.

Experimental section

Synthesis of the Ni/Ni(OH)₂ pre-catalyst

Prior to the deposition, Cu or Ni mesh (1 cm × 3 cm) was sonicated in ethanol, DI H₂O, and 3 M H₂SO₄ for 15 minutes each, successively. The electroplating solution contains 0.6 M NiSO₄ and 10 mM Ni(NO₃)₂. Electrochemical deposition was carried out in a three-electrode setup, where the Cu or Ni mesh served as the working electrode, an MMO plate (Ti plate coated with mixed metal oxides IrO₂/RuO₂) as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The Ni/Ni(OH)₂ pre-catalysts were deposited at a constant potential of −2.2 V (vs. Ag/AgCl) for 500 s under varying temperatures (25 °C, 50 °C, and 90 °C). Further, −2.2 V was found to be the optimal deposition potential to balance the production rate of metal Ni and OH⁻. The highest temperature is limited to 90 °C, otherwise the sample becomes ununiform, and 500 s is the shortest deposition duration for the best HER activity, which is almost constant even on prolonging the deposition time.

Synthesis of the Ni/NiOOH Hybrid catalyst

The Ni/Ni(OH)₂ pre-catalyst deposited at 25 °C, 50 °C, or 90 °C is converted into the active Ni/NiOOH catalyst through electrochemical oxidization. Specifically, the pre-catalyst undergoes cyclic voltammetry (CV) in a three-electrode cell within 1 M KOH solution. The CV scans are performed over a potential range of 0.2 V–0.9 V (vs. Hg/HgO reference electrode) for five cycles. In this process, the Ni/Ni(OH)₂ pre-catalyst is used as the working electrode, a graphite plate serves as the counter electrode, and a Hg/HgO (1 M KOH) electrode is used as the reference electrode. For the convenience of subsequent description, the samples of Ni/NiOOH synthesized at 25 °C, 50 °C, and 90 °C are designated as Ni/NiOOH-25, Ni/NiOOH-50, and Ni/NiOOH-90, respectively.

Synthesis of the Ni catalyst

The electroplating aqueous solution is 0.6 M NiSO₄. Using the same three-electrode system as described above, a constant potential of −2.2 V was applied for 500 s.

Synthesis of the Ni(OH)₂ catalyst

The electroplating aqueous solution is 0.6 M Ni(NO₃)₂. Using the same three-electrode system as described above, a constant potential of −2.2 V was applied for 500 s.

Characterization methods

X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (XRD, Rigaku Smart Lab 9 KW with Cu Kα radiation) at 40 kV and 150 mA. The morphology of the samples was observed with a field emission scanning electron microscope (FESEM, Gemini 300, Zeiss) under an accelerating voltage of 2 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+, Thermo) was used to obtain the photoelectron spectra using Al Kα radiation. Transmission electron microscopy (TEM, Talos F200X, Thermofisher) was used to observe the microstructure at an accelerating voltage of 200 kV.

Electrochemical testing

All electrochemical experiments were conducted using a DH7001B electrochemical workstation. A three-electrode setup was employed, with a Hg/HgO electrode as the reference electrode, a graphite plate (2 × 3 cm²) as the counter electrode, and Ni/NiOOH as the working electrode. The hydrogen evolution performance of the catalyst was evaluated in a 1 M KOH solution (pH 13.8). Linear sweep voltammetry (LSV) measurements were performed over a potential range of −0.9 V to −1.25 V at a scan rate of 5 mV s⁻¹, with an IR compensation of 90%. All potentials were converted to the reversible hydrogen electrode (RHE) using the Nernst equation: E(vs. RHE) = E(vs. Hg/HgO) + 0.059 × pH + 0.098 V. Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 10 to 0.1 Hz. The electrochemical active surface area (ECSA) was determined from the double-layer capacitance (C_dl) (ESCA = C_dl/C_s, C_s = 40 µF cm⁻²). Scanning rates of 20 mV s⁻¹ to 100 mV s⁻¹ were used in the non-faradaic region of the 1 M KOH electrolyte. The stability of the catalyst was tested through potential-time chronoamperometry at current densities of 200 mA cm⁻² and 2000 mA cm⁻².

DFT calculation details

All first-principle DFT calculations were performed by using the VASP software package. The exchange and correlation effects of electrons were described by using the Perdew–Burke–Ernzerhof functional of the generalized gradient approximation (GGA-PBE). During structure optimization, the cutoff energy was set to 400 eV and the k-point was set to 2 × 2 × 1. In addition, the force and energy convergence criteria were set to 0.03 and 1 × 10⁻⁵ eV, respectively. In the subsequent calculation of the charge distribution and density of states (electronic properties analysis), the k-point is set to 6 × 6 × 1 to ensure accuracy. The computational hydrogen electrode (CHE) model was applied to simulate the HER pathway and determine the reaction energy barrier for different slab models. Additionally, the adsorption energies (E_ads) were calculated as E_ads = E_ad/sub − E_ad − E_sub, where E_ad/sub, E_ad, and E_sub were the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively. The free energies were obtained using G = E_total + E_ZPE − TS, where E_total, E_ZPE, and TS were the ground-state energy, zero-point energies, and entropy terms, respectively, with the latter two taking vibration frequencies from DFT. The large grain Ni(OH)₂ was modelled with a (4 × 4) supercell to examine the hydrogen desorption process, containing two layers of Ni(OH)₂. For small grain Ni(OH)₂, we used a (2 × 4) supercell. The hydrogen desorption energy (HDE) was calculated as: E_HDE = E_H(n−1) − E_Hn + 1/2 E(H₂), where E_H(n−1) and E_Hn are the total DFT energies for the Ni hydroxide systems. E(H₂) is the total energy of H₂.

Results and discussion

Morphology and structural characterization

The Ni/NiOOH-90 hybrid catalysts were synthesized on a Cu/Ni mesh substrate (Fig. S1) by an electrochemical process at 90 °C (Fig. 1a). First, 10 mM nitrate ions (NO₃⁻) were added to the deposition solution to promote the formation of Ni(OH)₂. During the electrochemical deposition, Ni(OH)₂ is derived from NO₃⁻ reduction and H₂ generation. Therefore, the ratio of Ni to Ni(OH)₂ can be regulated by altering the NO₃⁻ concentration, deposition potential and temperature (Fig. S2). This theoretically provides the possibility to tune the composition and thus the heterointerface density of the Ni/Ni(OH)₂ hybrid precatalysts. The Ni/Ni(OH)₂ hybrid precatalysts were electrochemically oxidized and transformed into Ni/NiOOH hybrid catalysts with a higher content of high-valence Ni. The oxidization peaks in the oxidization CV curves indicate the transformation from Ni²⁺ to Ni³⁺ (Fig. S3). In comparison, the high-temperature sample exhibits a larger redox peak areas, suggesting more transformation from Ni²⁺ to Ni³⁺. The negative shift of the reduction peaks with increasing temperature predicts the enhanced stability of Ni³⁺ species in the high-temperature samples.

Fig. 1 (a) Schematic of the synthesis process of the Ni/NiOOH hybrid catalysts. SEM images of (b) Ni/NiOOH-90, (c) Ni/NiOOH-50, and (d) Ni/NiOOH-25. HRTEM images of (e) Ni/NiOOH-90 and (f) Ni/NiOOH-25.

The samples synthesized at the various temperatures have distinct morphologies. SEM images show that the high-temperature samples Ni/NiOOH-90 and Ni/NiOOH-50 were composed of nanosheets, the typical layer double hydroxide (LDH) shape of Ni(OH)₂ (Fig. 1b and c). Compared to Ni/NiOOH-50, Ni/NiOOH-90 has thinner and denser nanosheets. However, the low-temperature sample Ni/NiOOH-25 displays a protruding structure and not nanosheets, distinctly different from Ni/NiOOH-90 and Ni/NiOOH-50 (Fig. 1d). In comparison, Ni/NiOOH-90 has the highest specific surface area. The HRTEM images in Fig. 1e and f clearly resolve the crystallographic features. As shown by the representative areas marked with blue and yellow dotted lines, each area corresponds to one crystalline grain. The yellow areas exhibit the characteristic lattice spacings of 0.20 nm, corresponding to the (111) crystallographic plane of metal Ni. The blue areas exhibit the characteristic lattice spacings of 0.17 nm and 0.24 nm, corresponding to the (102) and (101) crystallographic planes of metal Ni(OH)₂, respectively. All lattice spacings for Ni and Ni(OH)₂ align well with the literature values.^24,27,31 The heterointerface between Ni and Ni(OH)₂ can be distinguished clearly. Note that the grain size (5 nm) for Ni/NiOOH-90 is less than that (10 nm) for Ni/NiOOH-25, suggesting that high temperature accelerates nucleation, leading to smaller grain sizes and accordingly a high heterointerface density. Further, EDS-mapping indicates the homogeneous distribution of Ni and O in the well-mixed hybrid phase, but the particle shapes are hard to distinguish due to the extremely small sizes (Fig. S4).

Fig. 2a presents the XRD patterns of the Ni/NiOOH-25 and Ni/NiOOH-90. Besides the peaks at 43.2°, 50.4°, and 74.1° corresponding to the Cu mesh substrate (JCPDS#04–0836), the peaks at 44.5°, 51.8°, and 76.3° are assigned to the (111), (200), and (220) crystal planes of metal Ni (JCPDS#04–0850). For the Ni/NiOOH-90, the weak peaks at 22.74°, 33.46°, and 59.98° correspond to the (006), (101) and (110) crystal planes of Ni(OH)₂ (JCPDS#38-0175), confirming the coexistence of Ni and Ni(OH)₂ in the sample. For Ni/NiOOH-25, no corresponding diffraction peaks of Ni(OH)₂ were detected in the XRD pattern, even though the lattice fringes were identified in the HRTEM images and Ni(OH)₂ has a larger grain. In contrast, the diffraction peaks of metal Ni in the XRD pattern for Ni/NiOOH-90 are weaker and wider than in Ni/NiOOH-25. This is opposite to the experimental results on plating metal Ni without NO₃⁻ addition, where Ni is well crystallized with rising temperature (Fig. S5). We attribute the poor crystallization of metal Ni in Ni/NiOOH-90 and Ni(OH)₂ in Ni/NiOOH-25 to the crystallizations of Ni and Ni(OH)₂ disturbing each other. Raising the temperature during the electrochemical deposition can promote H₂ generation, accompanying an improved producing rate of OH⁻. The high Ni(OH)₂ content in Ni/NiOOH-90 facilitates the crystallization of Ni(OH)₂ and suppresses that of metal Ni, and the opposite case is observed for Ni/NiOOH-25, which contains a low content of Ni(OH)₂, suppressing the crystallization of Ni(OH)₂ and facilitating the crystallization of metal Ni. Further, this is proven by the XRD results obtained from samples that were electrochemically deposited from 0.6 M Ni(NO₃)₂ solution with the NiSO₄ concentration increasing gradually. The crystallinity of Ni(OH)₂ becomes weakened with the gradual introduction of metal Ni to the pure Ni(OH)₂ (Fig. 2b).

Fig. 2 (a) XRD patterns of Ni/NiOOH-90 and Ni/NiOOH-25. (b) XRD patterns of the Ni(OH)₂ sample with various amounts of Ni introduced. XPS spectra of (c) precatalyst Ni/Ni(OH)₂ and (d) hybrid catalyst Ni/NiOOH.

To reveal more abundant information on the hybrid structure and composition varying with the deposition temperature and electrochemical oxidation, XPS analysis was conducted, as shown in Fig. 2c. The XPS results indicate that in all samples the element Ni has three valence states, namely Ni⁰, Ni²⁺, and Ni³⁺, assigned to Ni metal (852.39 eV), Ni(OH)₂ (855.35 eV), and NiOOH (856.66 eV), respectively. The content of each valence state varies with the deposition temperature and the electrochemical oxidization. The Ni³⁺ content in all samples increases after electrochemical oxidization (Fig. 2d). The ratio of Ni³⁺ to Ni²⁺ in Ni/NiOOH-25 increases from 0.36 : 1 to 2.5 : 1 while in Ni/NiOOH-90 the ratio increases from 1.45 : 1 to 3.33 : 1. For the high-temperature sample, whether or not undergoing electrochemical oxidization, the component content of high-valence Ni is significantly higher than Ni²⁺. This is further verified by the electrochemical oxidization CV curves (Fig. S3). With the deposition temperature rising, the oxidization peak shifts positively and the peak area increases. Since both the loading amount and the active site density account for the positive peak shift and increased peak area, high-temperature samples of varied loading amounts are prepared with the deposition duration varying from 20 s to 100 s. Actually, increasing the loading amount by prolonging the deposition duration causes a positive peak shift and increased peak area (Fig. S6). However, even if the samples Ni/NiOOH-90 deposited for 20 s and Ni/NiOOH-25 deposited for 300 s have the same thickness, the high-temperature sample has significantly larger redox peak areas, and the oxidization peak shifts positively (Fig. S7). This observation suggests that high-temperature deposition facilitates oxidation of Ni²⁺ to Ni³⁺. The high-resolution spectra of Ni/NiOOH-25 and Ni/NiOOH-90 reveal that for both samples, the Ni⁰ peaks shift to a higher binding energy while the Ni²⁺ and Ni³⁺ peaks shift to lower binding energies, but for the high-temperature samples the peaks shift significantly, indicating the better electron transfer and redistribution between the metal Ni and Ni(OH)₂ or NiOOH.

Electrochemical performance

The electrocatalytic activity was evaluated in 1 M KOH using a typical three-electrode system (Fig. 3a). For all samples Ni/NiOOH-25, Ni/NiOOH-50, and Ni/NiOOH-90, the overpotentials at the current densities of 10 mA cm⁻² are 46 mV, 36 mV, and 30 mV, respectively, while the overpotentials at 100 mA cm⁻² are 166 mV, 136 mV, and 110 mV, respectively. The overpotentials at 10 mA cm⁻² are obviously superior to those of pure metal Ni (245 mV), pure Ni(OH)₂ (186 mV), Ni/Ni(OH)₂-25 (182 mV), Ni/Ni(OH)₂-50 (174 mV), and Ni/Ni(OH)₂-90 (121 mV), and even lower than that of Pt (77 mV) (Fig. 3b). These results suggest that the HER activity is improved with the deposition temperature rising, and the activity of Ni/NiOOH-90 is superior to most of the previous reports (Fig. 3c). Combining the above structural characteristics, the hybrid structure was prepared by one-step electrochemical deposition and comprises a highly intertwined structure of small Ni and Ni(OH)₂ particles, which spontaneously produces a high-density exposed interface toward the outside. The high-density exposed interface provides the active sites for the HER and the HER activity can be improved through tuning the Ni³⁺/Ni²⁺ ratio by subsequent oxidization. In comparison, other heterojunction catalysts are usually synthesized by coating one material on the other through a two-step process and the interface density and the valence ratio are hard to control. To study the HER kinetics, the Tafel slopes were compared according to the polarization curve of the catalysts. As shown in Fig. 3d, the calculated Tafel slopes for the Ni/NiOOH-25, Ni/NiOOH-50, and Ni/NiOOH-90 samples are 57.4 mV dec⁻¹, 36.9 mV dec⁻¹ and 29.5 mV dec⁻¹, respectively, indicating the favorable HER reaction rate for Ni/NiOOH-90. Generally, a Tafel slope close to 120, 40, or 30 mV dec⁻¹ represents that the rate-determining step of the HER process is the Volmer, Heyrovsky, or Tafel step, respectively.^54,55 Moreover, the calculated Tafel slope indicates that during the hydrogen evolution process, the Volmer–Heyrovsky mechanism is converted to the Volmer–Tafel mechanism as the preparation temperature rises from 25 °C to 90 °C.^28,56

Fig. 3 HER performance of the various electrodes in 1 M KOH. (a) LSV with 90% iR-correction. (b) The overpotential at current densities of 10 mA cm⁻², 50 mA cm⁻², and 100 mA cm⁻². (c) HER overpotential comparison with reported catalysts. (d) The Tafel slopes. (e) The electrochemical double-layer capacitance. (f) The EIS results.

To elucidate the origin of the variations in HER activity, the electrochemical active surface areas (ECSAs) of the catalysts were tested. The ECSA for the Ni/NiOOH-25, Ni/NiOOH-50 and Ni/NiOOH-90 samples increases from 7.25 mF cm⁻² to 25.71 mF cm⁻² and 55.28 mF cm⁻², respectively, indicating the gradually increasing active sites within the catalysts as the temperature rises (Fig. 1e and Fig. S8). Electrochemical impedance spectroscopy (EIS) was employed to examine the resistance of the catalyst layer, as well as the kinetics and interfacial behavior of the electrochemical reactions occurring at the catalyst/electrolyte interface. The smaller diameter of the semicircle in the Nyquist plot represents the smaller charge transfer resistance (R_ct) or the faster reaction kinetics. As shown in Fig. 1f, the decreased R_ct of Ni/NiOOH with raising the deposition temperature suggests improved electronic conductivity and electrocatalytic activity. Different from other studies that exhibit a single semicircle, all EISs of the hybrid catalysts in this work have two semicircles, representing the coexistence of two types of interface for electron transfer. One is the general catalyst/electrolyte interface, at which the resistance to charge transfer is assigned to R_ct1. The other might be the internal interface between the metal Ni and NiOOH, across which charge transfer produces resistance R_ct2. Moreover, as the deposition temperature rises, both R_ct1 and R_ct2 decrease, strongly suggesting the increased interface area of the internal Ni/NiOOH interface and the catalyst/electrolyte interface associated with the reduced grain sizes. The significantly decreased R_ct2 compared to R_ct1 could be attributed to the shortened charge transferring length associated with the nanosheet morphology.

Electrochemical stability

Stability is an important criterion for assessing catalyst performance and a prerequisite for future industrial applications. We compared the stability of Ni/NiOOH-90 and Ni/NiOOH-25 (Fig. 4a) and evaluated their electrochemical performance before and after the stability tests (Fig. 4b and c). From the electrochemical performance of the samples before and after the 100-h stability tests at 200 mA cm⁻² in Fig. 4b and c, the overpotential of the Ni/NiOOH-90 catalyst at 10 mA cm⁻² and 100 mA cm⁻² is found to increase only 14 mV and 20 mV, respectively. In contrast, the overpotential of the Ni/NiOOH-25 catalyst significantly increased by 44 mV and 74 mV at the same current densities. Moreover, the Ni/NiOOH-90 catalyst largely maintained its original nanosheet structure (Fig. S9), indicating its excellent morphological stability and enhanced adhesion strength. The low-resolution SEM images further proved the better mechanical strength of the high-temperature sample without cracks due to internal stress (Fig. S10). To further illuminate the stability of the high-temperature sample, we compared the components of the samples before and after the stability tests. The XPS results show that the ratio of Ni³⁺/Ni²⁺ in Ni/NiOOH-90 decreased from 3.33 to 1.67, while that in Ni/NiOOH-25 decreased from 2.5 to 0.38 (Fig. 4d). The Ni/NiOOH-90 catalyst retains a high content of the high-valence Ni. Note that after reactivation of the samples undergoing the stability test, all of the electrochemical characteristics, including the CV curves (Fig. 4b and c), Tafel plots (Fig. S11), and ECSA measurements (Fig. S12 and S13), show that the electrochemical performance of Ni/NiOOH-90 is restored and is even superior to the original performance. XPS analysis of the reactivated samples indicated that the Ni³⁺/Ni²⁺ ratio is increased significantly from 1.67 to 2.72, nearly restoring the initial ratio (Fig. S14). The XRD results also show a similar structural change in the peak intensity (Fig. S15). In contrast, Ni/NiOOH-25 shows worse performance after reactivation. In view of the outstanding stability, a more severe stability test was conducted on Ni/NiOOH-90. Fig. 4e shows the stability of Ni/NiOOH-90 at 2 A cm⁻² for 300 h, which surpasses all previous results (Fig. 4f). The sample was reactivated after every 100-hour test (Fig. S16). The SEM image (insert in Fig. 4e) also shows that the sample maintains the original nanosheet-like structure after the 300-h stability test at a large current density of 2 A cm⁻². XRD analysis showed no significant change in composition after the stability test (Fig. 4g). Ni/NiOOH-90 shows slight degradation in the HER performance and the original activity is nearly restored after reactivation (Fig. 4h and Fig. S17).

Fig. 4 (a) Stability test of Ni/NiOOH-90 and Ni/NiOOH-25 at 200 mA cm⁻² for 100 h. LSV with 90% iR-correction before and after the stability test and reactivation of (b) Ni/NiOOH-90 and (c) Ni/NiOOH-25. (d) XPS spectra of the samples after the stability test. (e) Stability test of Ni/NiOOH-90 at 2000 mA cm⁻² for 300 h. (f) HER stability test comparison with reported catalysts. (g) XRD pattern of Ni/NiOOH-90 after the stability test. (h) LSV of samples with 90% iR-correction after reactivation.

DFT calculation and mechanism analysis

The above results have proved that the high-temperature samples have a refined structure and large interface density, and thus exhibit better activity and stability. Moreover, the degraded activity after the stability test could be restored by reactivation. These behaviors are closely related to the microscopic structural characteristics of the hybrid catalysts. Density functional theory (DFT) calculations were conducted to further investigate the enhanced HER activity and stability of the Ni/NiOOH hybrid electrocatalyst by structural and component modulation. During the calculation, a pure metal Ni model and four distinct Ni/Niⁿ⁺ interface models, including a model of large Ni grains and Ni(OH)₂, named as (Ni/Ni(OH)₂-L) and the oxidized model Ni/NiOOH-L, and a model of small Ni grains and Ni(OH)₂ (Ni/Ni(OH)₂-S) and the oxidized model Ni/NiOOH-S, were constructed (Fig. S18). The calculations employed Ni/Ni(OH)₂ and the oxidized Ni/NiOOH models to simulate the effects of nickel valence states on catalyst activity. The grain sizes of Ni and Ni(OH)₂ were changed to model grain size effects on catalyst activity. Electronic interactions at the interface of the precatalyst Ni/Ni(OH)₂ and oxidized Ni/NiOOH were compared by calculating differential charge density and Bader charge analyses (Fig. S19 and S20). Clearly, after oxidization, more charge transfers from the metal Ni to NiOOH, which increases the valence state of Ni and reduces that of NiOOH. This redistribution of electron density aligns well with the peak shift of Ni, Ni²⁺ and Ni³⁺ in the XPS results owing to oxidization (Fig. 2a and b). As depicted in Fig. S21 and S22, the H₂O dissociation pathway (*H₂O, TS, and *OH + *H) and *H adsorption step were calculated for the five models. The H₂O dissociation energies are 0.92, 0.73, 0.75, 0.69, and 0.5 eV while the *H adsorption energies are −0.683, −0.648, −0.525, −0.321, and −0.221 eV for pure Ni, Ni/Ni(OH)₂-L, Ni/NiOOH-L, Ni/Ni(OH)₂-S, and Ni/NiOOH-S, respectively (Fig. 5a and b). The calculation results suggest that the energy barriers of the H₂O dissociation and *H adsorption of the Ni/NiOOH hybrid are significantly lower than those of pure Ni, pure Ni(OH)₂ and NiOOH (Fig. S23). Compared to Ni/Ni(OH)₂, the Ni/NiOOH catalysts are more favorable in terms of H₂O dissociation and *H adsorption energy. The energy barriers of H₂O dissociation and *H adsorption are reduced for the smaller metal Ni and NiOOH grains (Fig. 5a and b). Moreover, the density of states shows that Ni/NiOOH has good conductivity and the lowest d-band center, indicating that Ni/NiOOH has the most suitable adsorption capacity of the *H intermediate (Fig. S24 and S25). Since the interface contains the HER active sites, the total activity depends heavily on the interface density. The high-temperature samples of small grain size have higher HER activity because of the high interface density.

Fig. 5 (a) Potential energy profile of *H₂O dissociation on different models. (b) The calculated Gibbs free energy of *H at different sites on different models. (c) Calculated H desorption energy as a function of the amount of desorbed H for large and small Ni(OH)₂ grains. (d) Deprotonation process of Ni(OH)₂ to NiOOH. (e) CV curves of Ni/NiOOH-90, Ni/NiOOH-25, pure Ni, and pure Ni(OH)₂. (f) Schematic illustration of the synergistic promotion of the HER for the Ni/NiOOH interface structure. (g) XRD patterns of Ni(OH)₂ oxidized at various potentials.

Since the hybrid catalyst is oxidized through the interlayer deprotonation of Ni(OH)₂, catalytic activity is inevitably related to the deprotonation degree and determined by the oxidization potential. The relationship of activity with the oxidization potential is verified by CV scanning of the Ni/Ni(OH)₂ precatalyst with the oxidization potential ranging from 0.5 V to 1.8 V (Fig. S26). High valence Ni species at the interface not only reduce the H₂O dissociation energy, but are also responsible for *OH adsorption and *H adsorption on the metal Ni. The peaks in the CV curves represent redox procedures at the different deprotonation and protonation degrees. For Ni/NiOOH-90 with smaller grains, two oxidation peaks at 0.25 V and 1.4 V correspond to two deprotonation degrees, which need different oxidization potentials. In comparison, the case is complex for Ni/NiOOH-25 because the large grains provide more possibility for deprotonation, leading to more peaks in the curves. The valence increases with the oxidizing potential shifting positively. The peaks observed at more negative potentials in this work have never been reported previously because the samples have seldom been scanned at so wide a potential range. The DFT results on the relationship of the energy barrier to the deprotonation degree show that the edge sites need lower energy compared to the inner sites (Fig. 5c and d). It is thus confirmed that the deprotonation energy is related to the deprotonation degree, and that the peaks at the various oxidization potentials represent different deprotonation degrees. The peaks at more negative potentials are assigned to deprotonation at edge sites, and the final oxidization peaks at around 1.4 V are complete deprotonation to NiOOH for both samples. It should be noted that the larger grains have higher H desorption energy than the small grains, especially at the edge sites, which agrees well with the negative shift of the oxidization peaks in the CV curves for Ni/NiOOH-90 (Fig. 5e). In addition, the negative H desorption energy after the initial deprotonation in Fig. 5c suggests spontaneous deprotonation at the outermost layer of the Ni(OH)₂ grains, identical to the existence of Ni³⁺ in the pristine Ni/Ni(OH)₂ precatalysts in Fig. 2c and d. The activity dependence of the Ni/NiOOH hybrid catalysts on Ni valence state due to deprotonation at various oxidizing potentials is clearly revealed in Fig. S26. Ni/NiOOH-90 reaches the best activity at the oxidization potential of 0.9 V. Further increasing the oxidation potential to 1.9 V, the overpotential is increased to 76 mV instead of decreasing. These results imply that the hybrid catalysts might be overoxidized at more positive potentials. Theoretically, the hybrid catalyst of higher valence Ni could accelerate H₂O dissociation, which is beneficial to the HER activity. However, the activity of the sample after complete deprotonation at 1.4 V degrades. We attribute the degraded activity to the excessive oxidization, which leads to enhanced *OH adsorption on high valence Ni sites.^28,51–53 This makes the subsequent replacement of *OH with H₂O a sluggish kinetics step and suppresses the next H₂O dissociation cycle. DFT calculation demonstrates that the *OH replacement energy with H₂O is increased with the deprotonation degree (Fig. S27), which causes *OH blockage and retards the kinetics. Based on the above discussion, we perfect the Volmer step of the traditional alkaline HER mechanism to clarify the HER process. The reaction steps during the HER procedure occur as follows, and the HER cycle is shown schematically in Fig. 5f. Furthermore, the complete reaction pathway was calculated, revealing a high energy barrier (0.49 eV) for the step of *OH replacement by H₂O (Fig. S28).

Volmer step:

H₂O → *H₂O_Niⁿ⁺ (I: H₂O adsorption on high valent Niⁿ⁺)

*H₂O_Niⁿ⁺ → *H_Ni⁰ + *OH_Niⁿ⁺ (II: *H adsorption on metal Ni₀)

H₂O + *OH_Niⁿ⁺ + e⁻ → *H₂O_Niⁿ⁺ + OH⁻ (III: *OH replacement by H₂O)

Heyrovsky step:

H₂O + *H_Ni⁰ + e⁻ → OH⁻ + H₂ (H₂ desorption at high oxidizing potential)

or Tafel step:

*H_Ni⁰ + *H_Ni⁰ → H₂ (IV: H₂ desorption at low reduction potential)

According to the DFT results, as the deprotonation ratio reaches 1, the *OH replacement energy with H₂O (Fig. S27) surpasses the H₂O dissociation energy (Fig. 5b) and makes replacement the rate determining step, which is in good agreement with the observation in the CV curves (Fig. S26).

Note that the HER stability is found to depend on the oxidization potentials. The stability becomes worse at more negative oxidization potential (Fig. S29a). The stability is improved with the oxidization potential shifting positively, especially as the oxidization potential surpasses the second oxidation peak (Fig. S29b). This confirms that Ni³⁺ species formed at more positive potentials are stable, but metastable at low potentials. We speculate that the first oxidation peak at 0.3 V corresponds to partial conversion of Ni²⁺ to Ni³⁺ species because of the easy deprotonation (Fig. 5a). The second oxidation peak at 1.5 V is derived from the deep deprotonation.⁵⁷ The two oxidization peaks suggest the coexistence of regions with different deprotonation energy in the sample. The large potential span between the two oxidization peaks indicates the large difference in deprotonation energy between the edge and interior sites. The HER stability after the second oxidization indicates the formation of a more stable structure, suggesting a phase transformation between the two oxidization potentials (Fig. 5f).^45,47,58,59 As shown in Fig. S30, the significantly larger area of the oxidation peak in the first CV cycle compared to the subsequent CV cycles means that a part of the Ni³⁺ is retained even on undergoing reduction at more negative potential. Moreover, the XPS spectra and XRD pattern show that Ni³⁺ still exists in the sample after the stability test (Fig. 4d and f), and its performance is still far superior to that of the precatalyst, which confirms the stability of Ni/NiOOH-90.

Conclusion

In summary, we uncover the mechanisms of Ni/Ni(OH)₂ hybrid catalysts in alkaline water electrolysis and successfully prepare Ni/NiOOH heterointerface catalysts using high-temperature electrochemical technology combined with electrochemical oxidization. The high temperature treatment could enhance the activity due to the enlarged interface density with refined grains of Ni species with different valence. And the reasonable electrochemical oxidization lowers the H₂O dissociation and *H adsorption energy, and meanwhile avoids *OH blockage due to the strong adsorption. Theoretical calculations further demonstrate the relationship of the grain sizes and the deprotonation to the activity and stability. Thus, the optimum Ni/Ni(OH)₂ hybrid catalyst reaches an outstanding HER performance with an overpotential of 30 mV@10 mA cm⁻² and its activity is stable for over 300 hours at an extremely large current density (2.0 A cm⁻²). This synthesis technique for the hybrid Ni/Ni(OH)₂ system is also suitable for preparing other hybrid systems (e.g., Co/CoOOH, Fe/FeOOH) and the tuning mechanisms of the electronic configuration are suitable for other systems with dual active sites, such as heterojunction interfaces, heteroatomic doping and heterovalent states of the same atom.

Author contributions

Kebin Yang: conceptualization; data curation; formal analysis; writing – original draft. Weibing Wu: conceptualization; funding acquisition; writing – review & editing. Yizhong Lu: supervision; writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All data generated or analyzed during this study are included in the published article and its supplementary information (SI) files. Supplementary information is available. See DOI: https://doi.org/10.1039/d5nh00724k.

Acknowledgements

This work was supported by the Shandong Provincial Natural Science Foundation (ZR2019MEM033), the National Natural Science Foundation of China (22172063), the Young Taishan Scholar Program (tsqn201812080), and the Independent Cultivation Program of Innovation Team of Ji’nan City (2021GXRC052).

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