Hydroxylation-Inducing the Coupled Dual-Centers in Highly Amorphized Ni0.76Mo1.24/Mo2N Nanoarrays with Superior Alkaline Hydrogen Evolution

doi:10.21203/rs.3.rs-2441531/v1

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Hydroxylation-Inducing the Coupled Dual-Centers in Highly Amorphized Ni_0.76Mo_1.24/Mo₂N Nanoarrays with Superior Alkaline Hydrogen Evolution

https://doi.org/10.21203/rs.3.rs-2441531/v1

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The site-blocking effect (SBE) of nonelectro-chemical steps (nonECS) gives the sluggish kinetics in alkaline hydrogen evolution reaction (HER), but remains rather challenging in electrocatalysis. Herein, we explored nonECS on the highly amorphized NiMoN/NC(500-R) nanoarrays, in which Ni_0.76Mo_1.24 and Mo₂N with the crystallinity of only 13.3% were embedded into nitrogen-rich derived carbons (NC) at 500^oC, then followed by electrochemical surface reconstruction. The amorphization and hydroxylation induce the coupled dual-centers of Ni-Mo species to circumvent SBE through separating H*- and H₂-involved elementary steps from nonECS, and deliver efficient transfer of individual species and heavy charge accumulation to actuate the whole HER. Resultantly, the exceptional catalytic activity is demonstrated by ultra-low overpotentials of 5.8 and 200.6mV at 10 and 1000mA cm^-2, respectively. Importantly, NiMoN/NC(500-R) survive in lasting alkaline HER for 50 days at 500mA cm^-2 almost without degeneration. This work presents the key to get rid of SBE in electrocatalysis, and the inspiration to rationally design the amorphized nanoarrays of electrocatalysts.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis

Physical sciences/Engineering/Chemical engineering

Physical sciences/Materials science/Nanoscale materials/Structural properties

The increasing environmental concerns have motivated great interest in hydrogen energy that has high gravimetric energy density, earth abundance and wide applications without emitting pollutants.^1–4 Currently, hydrogen evolution reaction (HER) from water has been recognized as the most promising technique of sustainable production as powered by clean electricity. However, the renewable grid supply-fluctuation of photovoltaics, wave and wind energies is imposed by spatial, temporal, climatic and seasonal factors. Therefore, the real-time conversion of these energies requires HER electrocatalysts that respond sensitively to the dynamical input power, and operate competently over a wide current-density range and long-term durability.^5–7

Acidic HER usually is two to three orders of magnitude faster than alkaline HER,^8,9 but the vulnerable metal-based catalysts and proton exchange membrane in acidic solutions make large-scale hydrogen production to prefer in alkaline media.^10,11 Therefrom, the low-cost transition metal-based catalysts are intensively exploited to minimize HER barriers.^12–18 Unfortunately, sluggish kinetics on the electrocatalysts incurs high overpotentials, and becomes particularly worse at industrial current densities (≥ 500 mA cm^− 2).^19,20 Currently, an effective strategy has been developed by vertically growing transition metal-based nanoarrays (NAs) on metal substrates, and the resultant monolithic catalysts are brought about smooth mass transfer and negligible interface electric-resistances, improving significantly in catalytic efficiency.^21–27 However, the NAs have high crystallinity with mechanical rigidity, industrially, are difficult to bear massive H₂ release. Although amorphization imparts catalysts with rich active sites, flexible architectures, desirable catalytic activity and durability,^28–33 the amorphized NAs incline to geometrically collapse to increase charge and mass transfer barriers, and are not considered in previous literature.

Alkaline HER is launched from Volmer reaction, and followed by Heyrovsky or Tafel reaction (Scheme S1).^34,35 The reaction rate is controlled by electrochemical steps and concomitant nonelectrochemical steps (nonECS). The electrochemical steps involve in electron transfer in H₂O dissociation and concerted proton-electron transfer, and the nonECS embrace adsorption/diffusion of H₂O, H*, H₂ and OH⁻. nonECS are directly related to site-blocking effect (SBE), and play the governed roles as the electron transfers are not rate-determining step (RDS). To circumvent SBE, ideally, we should decouple nonECS through integrating a site to catalyze Volmer reaction with another to catalyze Heyrovsky or Tafel reaction. Moreover, the separated elementary steps lower the whole reaction barrier more on the coupled dual-center electrocatalysts than on single active site, and give higher catalytic efficiencies. In fact, the electrocatalysis are designed rarely following the synergistic dual-center motivation to separately study multiple nonECS, and commonly is measured by HER activity with balancing H*, H₂O and OH⁻ interactions in single site. The key challenge lies in the coupled dual-center electrocatalysts requires enough partitioning spaces and favorable electronic structures to suppress SBE and recombination of H* with OH⁻ in HER.

Here, we report the hydroxylation-conditioning nonECS to affect HER kinetics as revealed by density functional theory (DFT) calculations, pulse voltammetry, and operando electrochemical impedance spectroscopy (EIS). The crystalline NiMoO₄ arrays on nickel foam (NF) were nitrogenized by ionic liquid (IL) of 1-ethyl-3-methylimidazolium tetrazolate ([C₂mim][Tet], Scheme S2) at 500^oC, and the resultant highly amorphized NAs are embedded with Mo₂N and Ni_0.76Mo_1.24 in the skeleton of IL-derived nitrogen-rich carbons (NC). Subsequently, the Ni species are fully hydroxylated by the electrochemical surface reconstruction (ESRC) to harvest NiMoN/NC(500-R) NAs. It is demonstrated that the 3D self-supported architecture gives robust durability, efficient mass and charge transfer. Attractively, the amorphization and hydroxylation induce the super-large electrochemical surface area (ECSA), and create the active dual-centers to separate H*- and H₂-involved elementary steps from H₂O- and OH*-involved nonECS in NiMoN/NC(500-R) NAs. Thereby, the chemical coupling of Mo with Ni species provides the cooperative arenas for different nonECS, and significantly alleviate SBE to achieve the unexpected catalytic activity in alkaline HER.

Microstructural feature and surface hydroxylation.

The highly amorphized NiMoN/NC(500) NAs were synthesized by IL nitridation to split NiMoO₄·xH₂O cuboid arrays (Fig. 1A). NiMoO₄·xH₂O nanorods with smooth surfaces have the average radial size of 0.54µm as shown in scanning electron microscopy (SEM) images (Figure S1). After the aqueous solution of IL was uniformly coated on the hydrophilic surfaces, the thermal nitridation at 500°C converts NiMoO₄·xH₂O to NiMoN/NC(500) NAs. The chemical conversion doesn’t alter the array morphology, but split the nanorods to slenderer ones with an average radial size of 0.18µm (Fig. 1B). Subsequently, NiMoN/NC(500) was activated by ESRC with 100 circles of cyclic voltammetry (CV) between 1.4~-0.4V vs. RHE in 1M KOH, and the resultant NiMoN/NC(500-R) presents a little blurrier appearances owing to the surface hydroxylation (Fig. 1C).³⁶ As shown by scanning transmission electron microscopy (STEM) image, the nanorod from NiMoN/NC(500-R) NAs is built by the closely packed nanoparticles (Fig. 1D), and renders the electrocatalyst with the fully expanded ECSA. The phase interfaces are identified by the lattice spaces of 0.240 and 0.209 nm that are respectively indexed to (111) and (114) planes of Mo₂N and Ni_0.76Mo_1.24 in high-resolution TEM (Fig. 1E). The paragenesis of Mo₂N and Ni_0.76Mo_1.24 is further evidenced by their spots/circles in selected electron area diffraction (SAED) patterns (Fig. 1F). Moreover, the angle annular dark field (HAADF)-STEM image and energy dispersive X-ray spectroscopy (EDS) mapping images verify the homogeneous element distributions over the nanorod, and imply that Mo and Ni species are embedded into NC (Fig. 1G ~ L). Instead of simple chemical digestion, interestingly, the thermal nitridation of IL split nanorods into slimmer ones, and simultaneously the arrays are well maintained in NiMoN/NC(500). The fact is clarified by the morphologic evolutions with IL loadings on NiMoO₄·xH₂O arrays (Figure S2). As calcined at 500°C, IL of 25mg chemically modifies NiMoO₄ nanorods to present rough surfaces. Increasing IL to 50mg, the most of nanorods are axially hollowed out, and some are split into finer ones. If introducing 200mg IL, the nanorods interconnect one another to give the 3D mesoporous network. It is the optimized IL loading of 100mg that renders NiMoN/NC(500) with the finest NAs and the fully exposed active sites.

The crystalline-phases and crystallinity are analyzed by X-ray diffraction (XRD) patterns. Except the incomplete conversion of NiMoO₄·xH₂O at 400°C (Figure S3), as comparison, we synthesized Ni₃Mo₃N/NC(600) and Mo₂C/NC(700) NAs respectively at 600 and 700°C, following the similar procedures for NiMoN/NC(500). Combining the analyses on their powders without NF interferences (Figure S4), the crystalline phases are Mo₂N and Ni_0.76Mo_1.24 in NiMoN/NC(500) (Fig. 2A), Ni₃Mo₃N in Ni₃Mo₃N/NC(600), and Ni₃Mo₃N and β-Mo₂C in Mo₂C/NC(700) (Figure S5). As subjected to ESRC, the XRD patterns are not altered in NiMoN/NC(500-R) and Ni₃Mo₃N/NC(600-R), whereas the Ni₃Mo₃N diffractions disappear in Mo₂C/NC(700-R). The disintegration of crystalline Ni₃Mo₃N is verified by the more amorphous appearances of Mo₂C/NC(700-R) than the other samples (Figure S6). Noticeably, the most broadened diffraction profiles imply the highly dispersed crystalline-phases in NiMoN/NC(500-R). Reference to the (221) diffraction of Ni₃Mo₃N in Ni₃Mo₃N/NC(600), quantitatively, the relative crystallinity of NiMoN/NC (500-R) is evaluated to be only 13.3% by the full width of half maximum of (111) diffraction of Mo₂N (Figure S7). Despite of the very low χ_C, NiMoN/NC(500-R) is most antioxidative in air even under high temperature owing to the profound surface hydroxylation (Figure S8) as shown by linear sweep voltamogram (LSV) of oxygen evolution reaction (OER) and X-ray photoelectron spectroscopy (XPS) analysis later. In Raman spectra, moreover, the disappearances of bending (344.9 cm^− 1) and stretching (940.8 cm^− 1) vibrations of Mo-O^37,38 give the additional proof of NiMoO₄·xH₂O conversions (Fig. 2B, Figure S9), and the D and G bands indicate IL-derived carbons from the pyrolysis of IL.³⁹ The metal element contents hold almost unchanged in these electrocatalysts from the analyses of inductively coupled plasma source mass spectrometer and survey scans of XPS (Figure S10, Table S1). Obviously, the self-supporting NiMoN/NC(500-R) NAs were mainly constructed by integrating NC with highly amorphized Mo₂N and Ni_0.76Mo_1.24 together.

Ni²⁺(Ni(OH)_2−δ)/Ni³⁺(NiOOH) oxidation peak indicates the electrooxidation in Ni-based catalysts in OER,^40,41 and is altered by surface hydroxylation. At 10mA cm^− 2 in LSV, the potential increases from 1.317V for NiMoN/NC(500) to 1.349V for NiMoN/NC(500-R) (Fig. 2C). The increment of 32mV is ascribed to higher barrier of surface oxidation, and implies that the electrooxidation is deactivated by ESRC owing to deepening hydroxylation. Comparatively, the oxidation potential of Ni²⁺ at 10mA cm^− 2 in Ni₃Mo₃N/NC(600-R) is 21mV more than in Ni₃Mo₃N/NC(600), and is almost same in Mo₂C/NC(700) and Mo₂C/NC(700-R) (Figure S11). Hence, the surfaces of NiMoN/NC(500-R) are profoundly hydroxylated by ESRC, Ni₃Mo₃N/NC(600-R) moderately, and Mo₂C/NC(700-R) weakly. The surface hydroxylation is further clarified by the dominant OH species from high-resolution XPS of O 1s in three post-electrcatalysts (Figure S12).⁴² Except C-metal species, the binding energies of each species are same in the deconvoluted O1s, N1s, C1s spectra before and after ESRC (Figure S12-S14), ^43–46 implying that the nonmetal elements situate in similar chemical surroundings and are not the crucial factors to raise distinct catalytic activities.

As active Ni-Mo species, their surface valence states depend heavily on annealing temperature, and evolve with ESRC to achieve favorable electronic structures for HER. As compared with three pre-catalysts (Figure S15), all post-catalysts indicate the enhanced signals of Ni-O species but the weakened ones of Ni-Mo and Ni^{0 37,43,47}owing to ESRC hydroxylation (Fig. 2D). Distinctively, the disintegration of crystalline Ni₃Mo₃N in Mo₂C/NC(700-R), consistent with XRD result, is verified by the almost disappearing peak of Ni⁰/Ni-Mo, and suggests the hydroxylation largely destructs the chemical bonding and coupling of Ni with Mo atoms. Evidently, the three post-catalysts represent the same valence states of Ni⁰/Ni-Mo, but NiMoN/NC(500-R) has the binding energy of Ni-O that are 0.3eV lower than Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R), and are more nucleophilic to support the spilled H*. Furthermore, Mo-N and Mo-C species are derived from the chemical combination of Mo element with NC. Mo-C species is not detected in NiMoN/NC(500-R), but present in Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R) that respectively give the binding energies of 0.5 and 0.8 eV lower than Mo-N (Fig. 2E). As transferred to measuring system, the deconvoluted peaks of Mo⁴⁺ and Mo⁶⁺ indicate that Mo species in catalyst surfaces are readily oxidized by air,^43,48 and the oxidized Mo sites are more electrophilic to adsorb water. Therefore, the catalytic dual-centers from Ni-Mo bonding are constructed by coupling of the partially oxidized Mo species with the fully hydroxylated Ni species. Considering the crystalline-phases, reasonably, the coupled dual-centers of Ni-Mo species are most abundant on the highly amorphized NiMoN/NC(500-R), moderate on the crystalline Ni₃Mo₃N/NC(600-R), and almost absent on Mo₂C/NC(700-R) without Ni-Mo bonding.

Evidently, the surface-hydroxylation is necessary to induce the coupled dual-centers to circumvent SBE of onoECS, and is analyzed by the adsorption thermodynamics from DFT-calculations with the structural models (Figure S16). The nonECS in HER involves mainly with the adsorption of H₂O and H* along with desorption of OH* and H₂ (Fig. 3A). The Gibbs free energy (ΔG_H2O) indicates the Mo site presents the strongest adsorption (Fig. 3B), whereas the adsorbed H₂O on Ni site is undesirable on dual-center HONiMoN (Figure S17). Comparatively, a slightly exothermal process is implied by the similar small ΔG_H2O on Mo and Ni sites of NiMoN. Once the adsorbed H₂O is dissociated by the applied bias, OH* is left behind on Mo site, and the concomitant H* spills over the catalysts. Therein, OH* desorption from HONiMoN into bulk is thermodynamically most favorable as suggested by the positive ΔG_OH* of 0.17eV (Fig. 3B). Moreover, |ΔG_H*| of the spilled H* on Ni site of HONiMoN give the most reasonable value of 0.15 eV close to zero, and the optimal energy barrier for H* adsorption and H₂ desorption (Fig. 3C). By contrast, ΔG_H2O, ΔG_OH* and |ΔG_H*| in HOONiMoN respectively indicate the thermodynamically unfavourable adsorption/desorption in HER (Fig. 3B and Figure S18), and clarify that the NiOOH with high valence state, instead of Ni-OH, impairs the catalytic activity. Consistent with XPS analyses, the nonmetallic O atoms are not dominant factors to affect the catalytic activities. Thereby, the coupled dual-centers are induced by hydroxylation, and each center performs its own functions to circumvent SBE and alleviate recombination of H* with OH^- on HER catalysts.

Electrocatalysis In Hydrogen Evolution

The electrocatalytic activities are evaluated by LSVs with 90% iR-compensation in 1M KOH, and the commercial Pt/C was employed for comparison (Fig. 4A). The overpotential (η) reflects the response of electrocatalysts to alkaline HER at a given current density (j) (Fig. 4B). For NiMoN/NC(500) NAs, η₁₀, η₅₀₀, η₁₀₀₀, and η₁₅₀₀ at 10, 500, 1000, and 1500 mA cm^− 2 respectively are 10.0, 159.7, 271.6, and 352.2mV, much lower than the benchmark Pt/C catalyst. The hydroxylation promotes the catalytic activity, and decreases η₁₀, η₅₀₀, η₁₀₀₀, and η₁₅₀₀ to 5.8, 117.0, 200.6, and 260.6mV, respectively. Furthermore, Δη/Δlog|j| ratios well address the response of j to η for catalytic efficiencies over the wide range of current density (Fig. 4C and Figure S19).^37,49 Δη/Δlog|j| of commercial Pt/C represents the sharp increase with current density. Besides the ultra-low η, Δη/Δlog|j| in NiMoN/NC(500) is as low as 12.9, 113.0, and 201.0mV⋅dec^− 1, and is further lowered by hydroxylation to 7.0, 99.0, and 153.1mV⋅dec^− 1 in NiMoN/NC(500-R) over 4 ~ 10, 250 ~ 500, and 500 ~ 1000 mA⋅cm^− 2, respectively. In contrast, Ni₃Mo₃N/NC(600) and Mo₂C/NC(700) give the larger η and Δη/Δlog|j| than NiMoN/NC(500), showing that the crystallization of catalysts is unfavorable to the catalytic activities. η and Δη/Δlog|j|, instead of the decrease in NiMoN/NC(500), are increased by ESRC in Mo₂C/NC(700-R) but are hardly altered in Ni₃Mo₃N/NC(600-R). Evidently, the decomposition of Ni₃Mo₃N in Mo₂C/NC(700-R) results in the absence of coupled dual-centers and conducts the most sluggish catalysis, which becomes more pronounced at enlarging current densities. It is the highly amorphized NiMoN/NC(500-R) NAs that has the richest coupled dual-centers to catalyze HER, responds sensitively to tiny current densities, and works ideally at industrial current densities.

Kinetically, Tafel slope is an direct parameter to reflect HER mechanism.⁵⁰ NiMoN/NC(500) and NiMoN/NC(500-R) presents the Tafel slopes of 34.52 and 26.63mV dec^− 1, close to 39.4 and 29.6mV dec^− 1 in Heyrovsky and Tafel reactions, respectively (Figure S20 and Scheme S1). Apparently, RDS is Tafel and Heyrovsky reactions on NiMoN/NC(500), and is Tafel reaction on NiMoN/NC(500-R) that is generally identified as RDS from the noble metal-based electrocatalysts in acidic HER.⁵¹ In the absence of Ni-Mo species, by contrast, Mo₂C/NC(700-R) give the Tafel slopes of 65.40mV dec^− 1, and Volmer reaction comes into play as RDS. Intrinsically, Tafel reaction as RDS in alkaline HER implies that HER rate depends mainly on H*- and H₂-involved elementary steps, and NiMoN/NC(500-R) with the abundant coupled dual-centers works in water dissociation more efficient than NiMoN/NC(500) and much more than Mo₂C/NC(700-R). Compared with the state-of-the-art catalytic activity in alkaline HER, therefore, NiMoN/NC(500-R) is superior not only to the transition metal-based electrocatalysts (Fig. 4D and Table S2), but also to the noble metal-based electrocatalysts (Fig. 4E and Table S3).

The stability is a critical metric for commercial feasibility of NiMoN/NC(500-R), particularly, at large current densities.⁵² After the CV was continuously run for 10,000 cycles, the negligible change from LSVs suggests the high cyclability of NiMoN/NC (500-R) (Fig. 4F). As subjected to the chronoamperometry for examining long-term stability, NiMoN/NC(500-R) is run at 500 mA cm^− 2 for 1,200h, and the LSVs show an overpotential increase by only 76.4 mV (Fig. 4G). Quantitatively, C_sta is introduced to evaluate catalyst stability following the equation:⁵³

C_sta = jtΔE^− 1

Where t is whole working time, and ΔE is overpotential difference after and before the test. NiMoN/NC(500-R) gives C_sta of 2.76 × 10⁷ C cm^− 2 V^− 1, and surpasses the state-of-the-art HER catalysts (Fig. 4H and Table S4). After the chronoamperometry, the unchanged array structure and phase composition show the strong mechanical and chemical stabilities of NiMoN/NC(500-R) NAs to tolerate the release of great amount of H₂ bubbles (Figure S21 and 22). It is the amorphization that imparts the robust flexibility to 3D NiMoN/NC(500-R) NAs, and underlies in the intrinsic stability to operate at high current densities. Moreover, the Faradaic efficiencies on NiMoN/NC(500-R) were respectively measured at 100 and 500 mA cm^− 2 to be 99.93 and 98.47% (Figure S23), close to theoretical value, indicating the almost all charge utilization without parasitic-side reactions in H₂ production.^38,54

Bifunctional Mechanism To Circumvent Site-blocking Effect

The hydroxylation significantly improve the catalysis of NiMoN/NC(500-R), but deactivate Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R) in HER. In this regard, the greatest ECSA lends the highly amorphized NiMoN/NC(500-R) with rooms large enough to conduct nonECS. As determined by double-layer capacitance (C_dl) (Figure S24-S25),^28,55 ECSA of NiMoN/NC(500) is 654.7 times larger than NF (Fig. 5A), is extended by 42.0% in NiMoN/NC(500-R) owing to full hydroxylation as analyzed from OER and TGA (Fig. 2C and S8). Because of crystallization phases, instead, ESRC respectively reduces ECSAs by 1.1 and 8.4% in Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R). Averagely, the coupled dual-centers in NiMoN/NC(500-R) are respectively separated by 4.0 and 9.5 folds more disperse than those in Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R) according to the roughly same metal loadings (Table S1). The better spaced active sites bring higher catalytic efficiencies as reflected by the exchange current density (j₀) and turnover frequency (TOF).⁵⁶ NiMoN/NC(500) gives j₀ of 7.56mA⋅cm^− 2, is comparable with Ni₃Mo₃N/NC(600) (6.31 mA⋅cm^− 2), and prominent to Mo₂C/NC(700) (1.01mA⋅cm^− 2) (Figure S20 and S26). Correspondingly, ESRC increases j₀ with 34.8% in NiMoN/NC(500-R), but decreases j₀ with 4.5% and 41.6% respectively in Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R). At the overpotential of 100mV, TOF in NiMoN/NC(500-R) is 1.4, 1.8, 1.9, 11.1 and 14.1 times more than NiMoN/NC(500), Ni₃Mo₃N/NC(600), Ni₃Mo₃N/NC(600-R), Mo₂C/NC(700) and Mo₂C/NC(700-R) (Figure S27), respectively. Demonstrably, the coupled dual-centers of Ni-Mo species have lowest steric hindrances and best accessibility in NiMoN/NC(500-R), and become more activated by the inducement of surface hydroxylation.

The designated electronic structures are necessary for coupled dual-centers to perform respective role in HER. Compared with Ni foil, X-ray absorption near-edge spectroscopy (XANES) of Ni K-edge in NiMoN/NC(500-R) shifts to higher energy, and increases in white-line intensity (Fig. 5B), indicating the unoccupied states in Ni species that can support well to spilled H*. The projected density of states (PDOS) verify that the hydroxylation leads to electron redistribution through interactions of Ni with O (Figure S28). Below Fermi level (E_f), the denser PDOS of Ni atoms in HONiMoN suggest more electron delocalization than in NiMoN (Fig. 5C), and favor H* adsorption and H₂ desorption. The Mo K-edge XANES in NiMoN/NC(500-R) moves to low energy close to adsorption edge of Mo foil (Fig. 5D), and gives a valence state of 0.79 to accelerate water adsorption but not to weaken OH^− 1 desorption (Figure S29). The hydroxylation removes the electrons from E_f of Mo atoms in NiMoN, but their PDOS below E_f remains roughly unchanged in HONiMoN, implying the suitable oxidation state to prefer water adsorption and OH^− 1 desorption (Fig. 5C).

The coupled dual-centers bring about the concerted adsorptions in pre-reaction, and devote themselves to high charge accumulation and pseudocapacitance (C_pseudo) on NiMoN/NC(500-R).^57,58 Although no redox peak is detected from the CV curves (Figure S30), the current responses in the pulse voltammetry protocols at different applied potentials (E) behave as a capacitor owing to the accumulated charge (Q) (Figure S31). To avert bond rupture and formation, the linear fitting of Q against E is implemented in the potential region of 60 ~ 10mV without HER current (Figure S32), and the capacitance is obtained from the slope to be 8288.7mF cm^− 2 for NiMoN/NC(500) and 9741.0mF cm^− 2 for NiMoN/NC(500-R). C_pseudo is calculated by subtracting C_dl from the capacitance, and is 7443.7 mF cm^− 2 for NiMoN/NC(500) and 8541.5 mF cm^− 2 for NiMoN/NC(500-R) (Fig. 5E, Figure S25). Evidently, the amorphization imparts catalysts with the superior stored-charge capacity. After ESRC, the hydroxylation further enhances C_pseudo with ΔC_pseudo (1097.8mF cm^− 2) that is 3 times larger than ΔC_dl (355.0mF cm^− 2). The great C_pseudo suggests the significant contribution of coupled dual-centers to pre-deprotonation, coincident with DFT-calculated strong adsorption of water. Moreover, the reaction order (α) for NiMoN/NC(500-R) (5.10) is more than that for NiMoN/NC(500) (4.21) from the slope of logj versus logQ (Fig. 5E, Figure S33), indicating the hydroxylation to promote reaction rate through charge accumulation. Hence, we conclude that the coupled dual-centers give the heavy charge accumulation to initiate HER current, and the high catalytic activity of NiMoN/NC(500-R).

The whole HER kinetics is expedited by the coupled bifunctional centers for different nonECS in NiMoN/NC(500-R), and unveiled by Operando electrochemical impedance spectroscopy (EIS) (Figure S34). There are two semicircles in a Nyquist plot that is fitted by Armstrong equivalent electric circuit (Figure S35). The semicircle in high frequency (R_ct1) is associated with the mass transfer in nonECS, and the semicircle in low frequency (R_ct2) is related to the electron transfer in electrochemical steps.⁵⁹ Commonly, R_ct2 depends on intrinsic activity, drops monotonically with overpotential (Figure S36), and cannot clarify very different activities of catalysts, particularly at large current densities. Observably, R_ct1 gives the second smallest value to NiMoN/NC(500), further is reduced by hydroxylation to the smallest one in NiMoN/NC(500-R), almost independent on the overpotentials (Fig. 5F). Therefore, the circumvention of SBE gives the most efficient mass transfer through the coupled bifunctional centers to separate H*- and H₂-involved elementary steps from H₂O- and OH*-involved nonECS, and still hold the robust capability at ≥ 500mA cm^− 2 in NiMoN/NC(500-R). Contrarily, Ni₃Mo₃N/NC(600) with crystallization gives larger R_ct1, and Mo₂C/NC(700) without the coupled Ni-Mo centers has the largest R_ct1. After ESRC, R_ct1 increases in Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R), and becomes far more detrimental to HER kinetics at high overpotentials. The evolutions of R_ct1 with overpotentials are roughly similar to the dependences of η and Δη/Δlog|j| on current density, clearly, the construction of coupled dual-centers is critical to suppress recombination of H* with OH⁻ and achieve the desired HER kinetics.

Overall Water-splitting Performances

For water electrolysis, the anode of NiFe-LDH/NiMoN/NC(500) NAs were prepared by electrodeposition of NiFe-layered double hydroxide (NiFe-LDH), using NiMoN/NC(500) as array template. The SEM images demonstrate that the array architecture is still held, and NiFe-LDH nanosheets grow epitaxially along NiMoN/NC nanorods to present the petal-like arrangements (Fig. 6A and Figure S37). The XRD pattern verifies the deposition of NiFe-LDH on NiMoN/NC500 during electrochemical-treatment (Figure S38). The electrocatalysis of NiFe-LDH/NiMoN/NC(500) in alkaline OER is evaluated by LSVs in O₂-saturated 1 M KOH, and the overpotentials of 228 and 300mV respectively deliver current densities of 100 and 500mA cm^− 2, better than NF-supported NiFe-LDH (306 and 392mV), much superior to the benchmark of NF-supported RuO₂ (Fig. 6B). The great catalytic activity in OER profit from the array template of NiMoN/NC(500) to direct NiFe-LDH growth.

Inspired by the excellent HER and OER performances, the water–alkali electrolyzers were assembled using NiFe-LDH/NiMoN/NC(500) or NiMoN/NC(500-R) as anode, NiMoN/NC(500-R), NiMoN/NC(500) or NiFe-LDH/NiMoN/NC(500) as cathode, and 1M KOH solution as electrolyte to estimate the practical overall water splitting. The polarization curves of overall water-splitting show that the NiFe-LDH/NiMoN/NC(500)ǁNiMoN/NC(500-R) electrolyzer respectively exhibits an ultralow cell voltage of 1.530 and 1.607V at 100 and 500mA cm^− 2, lower than NiFe-LDH/NiMoN/NC(500)ǁNiMoN/NC(500) (1.577 and 1.648V), NiMoN/NC(500-R)ǁNiMoN/NC(500-R) (1.624 and 1.835 V) and NiFe-LDH/NiMoN/NC(500)ǁNiFe-LDH/NiMoN/NC (500) (1.781 and 1.975V) (Fig. 6C). These results indicate that the asymmetric electrolyzer of NiFe-LDH/NiMoN/NC(500)ǁNiMoN/NC(500-R) engenders ideal overall-water-splitting efficiency through the combination of catalysts with high HER and OER activities. Moreover, the operational stability of NiFe-LDH-NiMoN/NC(500)ǁNiMoN/NC(500-R) electrolyzer was tested by chronoamperometry at 500mA cm^− 2 and cell voltages of 1.607V (Fig. 6D). After the operation for 200h, this electrolyzer retain outstanding overall water splitting performance with a voltage drop of 15mV. The catalytic activity and stability are superior to most of the ever-reported water electrolyzers in alkaline media (Table S5).

The almost amorphous Ni_0.76Mo_1.24 and Mo₂N are chemically embedded into the nitrogen-rich IL-derived carbons and set up the highly amorphized NiMoN/NC(500-R) NAs, and expand ECSA up to 929.8 times more than that of NF. Compared with Ni₃Mo₃N/NC(600-R) and Mo₂C/NC(700-R), we reveal the coupled dual-centers of Ni-Mo species are induced by the amorphization and surface hydroxylation from ESRC, and the SBE of nonECS is circumvented by the bifunctional mechanism: one is H₂O adsorption and OH^− 1 desorption on the partially oxidized Mo species, the other is the H* spillover and H₂ desorption but undesirable for H₂O adsorption on the fully hydroxylated Ni species. Thereby, the separation of H*- and H₂-involved elementary steps from H₂O- and OH*-involved nonECS impart NiMoN/NC (500-R) with the rapid mass transfer and heavy charge accumulation, and inhibit the recombination of H* with OH* to deliver superior catalytic efficiency in whole HER. Consequently, the ultra-low overpotentials of 5.8, 117.0 and 200.6mV, as well as Δη/Δlog|j| of 7.0, 99.0 and 153.1mV⋅dec^− 1, are respectively fulfilled by NiMoN/NC(500-R) at 10, 500 and 1000 mA cm^− 2, outperforming to Pt-based electrocatalysts. Although the crystallinity in the NAs is as low as 13.3%, attractively, NiMoN/NC(500-R) with the flexible nitrogen-rich skeleton can endure the harsh operation conditions of HER, and only gives an overpotential increment of 76.4 mV after lastingly run for 50 days at 500 mA cm^− 2. The work presents the generalizable paradigms to circumvent the SBE of nonECS through amorphization and surface reconstitution, and to design the highly amorphized NAs using various ionic liquids, meeting the commercialization requirements on electrocatalysts with high activity and durability beyond HER.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21573059 and 22208088).

Author Contributions

Hucheng Zhang and Huanli Jia conceived the project and designed experiments. Huanli Jia carried out the synthesis and performed materials characterization and electrochemical measurements. Hucheng Zhang, Haiyan Wang, Jianji Wang and Huanli Jia discussed the working mechanism. Huanli Jia, Ze Li. and Fangfang Yan analyzed the data. Xiaofang Su performed the DFT calculations. Huanli Jia, Hucheng Zhang, Haiyan Wang and Fangfang Yan wrote and revised the manuscript. All authors discussed the results and contributed to the final version of the manuscript.

Competing interests

There are no conflicts to declare.

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Hydroxylation-Inducing the Coupled Dual-Centers in Highly Amorphized Ni_0.76Mo_1.24/Mo₂N Nanoarrays with Superior Alkaline Hydrogen Evolution

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Electrocatalysis In Hydrogen Evolution

Bifunctional Mechanism To Circumvent Site-blocking Effect

Overall Water-splitting Performances

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