Dominating Role of Ni0 on the Interface of Ni/NiO for Enhanced Hydrogen Evolution Reaction

Steering elementary steps towards efficient alkaline hydrogen evolution via size-dependent Ni/NiO nanoscale heterosurfaces

Alkaline hydrogen evolution reaction (HER), consisting of Volmer and Heyrovsky/Tafel steps, requires extra energy for water dissociation, leading to more sluggish kinetics than acidic HER. Despite the advances in electrocatalysts, how to combine active sites to synergistically promote both steps and understand the underlying mechanism remain largely unexplored. Here, Density Functional Theory (DFT) calculations predict that NiO accelerates the Volmer step while metallic Ni facilitates the Heyrovsky/Tafel step. A facile strategy is thus developed to control Ni/NiO heterosurfaces in uniform and well-dispersed Ni-based nanocrystals, targeting both reaction steps synergistically. By systematically modulating the surface composition, we find that steering the elementary steps through tuning the Ni/NiO ratio can significantly enhance alkaline HER activity, and Ni/NiO nanocrystals with a Ni/NiO ratio of 23.7% deliver the best activity, outperforming other state-of-the-art analogues. The results suggest that integrating bicomponent active sites for elementary steps is effective for promoting alkaline HER, but they have to be balanced.

Enhanced Electrocatalytic Hydrogen Oxidation on Ni/NiO/C Derived from a Nickel-Based Metal-Organic Framework

The sluggish hydrogen oxidation reaction (HOR) under alkaline conditions has hindered the commercialization of hydroxide-exchange membrane hydrogen fuel cells. A low-cost Ni/NiO/C catalyst with abundant Ni/NiO interfacial sites was developed as a competent HOR electrocatalyst in alkaline media. Ni/NiO/C exhibits an HOR activity one order of magnitude higher than that of its parent Ni/C counterpart. Moreover, Ni/NiO/C also shows better stability and CO tolerance than commercial Pt/C in alkaline media, which renders it a very promising HOR electrocatalyst for hydrogen fuel cell applications. Density functional theory (DFT) calculations were also performed to shed light on the enhanced HOR performance of Ni/NiO/C; the DFT results indicate that both hydrogen and hydroxide achieve optimal binding energies at the Ni/NiO interface, resulting from the balanced electronic and oxophilic effects at the Ni/NiO interface.

Integrating Catalysis of Methane Decomposition and Electrocatalytic Hydrogen Evolution with Ni/CeO2 for Improved Hydrogen Production Efficiency

Ni/CeO₂ enables either methane decomposition or water electrolysis for pure hydrogen production. Ni/CeO₂ , prepared by a sol-gel method with only one heat treatment step, was used to catalyze methane decomposition for the generation of H₂ . The solid byproduct, Ni/CeO₂ /carbon nanotube (CNT), was further employed as an electrocatalyst in the hydrogen evolution reaction (HER) for H₂ production. The Ni/CeO₂ catalyst exhibits excellent activity for methane decomposition because CeO₂ prevents carbon encapsulation of Ni nanoparticles during the preparation process and forms a special metal-support interface with Ni. The derived CNTs act as antenna to improve conductivity and promote the dispersion of agglomerated Ni/CeO₂ . In addition, they provide H₂ diffusion paths and prevent Ni/CeO₂ from peeling off the HER electrode. Although long-term methane decomposition reduces the HER activity of Ni/CeO₂ /CNTs (owing to degradation of the delicate Ni/CeO₂ interface), the tunable nature of the synthesis makes this an attractive sustainable approach to synthesize future high-performance materials.

The Co2+/Ni2+ ion-mediated formation of a topochemically converted copper coordination polymer: structure-dependent electrocatalytic activity

The presence of Co²⁺/Ni²⁺ ions strongly influenced the formation of copper coordination polymers that showed a structure-dependent hydrogen evolution reaction catalytic activity.

First-Principles Determination of Active Sites of Ni Metal-Based Electrocatalysts for Hydrogen Evolution Reaction

The determination of active sites of materials is essential for the molecular design of high-performance catalysts. In this study, the first-principles method is applied to investigate the active sites of low-cost Ni metal-based electrocatalysts for hydrogen evolution reactions (HER), which is a promising alternative to expensive Pt metal-based catalysts. The adsorption of hydrogen on different sites of pristine and partially oxidized Ni(111) surface is investigated. All of the possible configurations have been systematically investigated here with the consideration of their Boltzmann distribution. Using the Gibbs free energy of intermediate H atoms (Δ G_H*) as a descriptor, it is found that the Δ G_H* increases with the increase of the coverage of oxygen atoms. The slightly oxidized surface Ni atoms are theoretically identified to be the best catalytic centers for the electrocatalytic HERs when the coverage of oxygen is considerably low. On the basis of the analyses of Bader charge distribution and density of states, our results reveal that the superior performance of the slightly oxidized surface Ni atoms can be ascribed to the optimal electronic properties.

Bimetallic NiMoN Nanowires with a Preferential Reactive Facet: An Ultraefficient Bifunctional Electrocatalyst for Overall Water Splitting

Faceted nanomaterials with highly reactive exposed facets have been the target of intense researches owing to their significantly enhanced catalytic performance. NiMoN nanowires with the (100) facet preferentially exposed were prepared by an in situ N/O exchange and the morphology tuned by using a rationally designed NiMoO₄ precursor. The facet-tuned NiMoN nanowires exhibited excellent electrocatalytic activity for the hydrogen evolution reaction (HER) under both alkaline and acidic conditions that was comparable to that of noble metal platinum. DFT calculations further revealed that the catalytic activity of NiMoN nanowires towards HER on the (100) reactive facet is significantly greater than that on the (001) or (101) facets, owing to the low adsorption free energy of H* (ΔG_H* ) on the (100) facet. The NiMoN nanowires also demonstrated outstanding activity towards the alkaline oxygen evolution reaction and an excellent durable activity for overall water splitting, with a cell potential as low as 1.498 V at 20 mA cm^-2 . This work provides insights into improving electrocatalytic activity and developing advanced non-noble metal bifunctional electrocatalysts.

Ni@NiO Nanowires on Nickel Foam Prepared via "Acid Hungry" Strategy: High Supercapacitor Performance and Robust Electrocatalysts for Water Splitting Reaction

Ni/NiO core-shell nanowires on nickel foam (NF) are successfully synthesized using an "acid-hungry" strategy. The 3D electrode with large accessible active sites and improved conductivity, possesses an optimized ionic and electronic transport path during electrochemical processes. High areal capacitance of 1.65 F cm-2 is obtained at an ultrahigh current density of 100 mA cm-2 , which is 19.88 times higher than pristine NF. The direct growth of nanowires makes the present supercapacitor electrode robust for long-term cycling test. By virtue of the favorable hydrogen adsorption energies on Ni0 and OHads energy on NiO or NiOOH, the 3D electrode exhibits high performance in hydrogen evolution reaction with 146 mV at η10 mA cm-2 and Tafel value of 72 mV dec-1 , and oxygen evolution reaction with 382 mV at η10 mA cm-2 and Tafel value of 103 mV dec-1 in 1 m KOH. An electrolyzer using 3D electrodes as both anode and cathode can yield a current density of 10 mA cm-2 at 1.71 V, and possesses superior long-term stability to an electrolyzer consisting of Pt/C||Ir/C. The present work develops an effective and low-cost method for the large-scale fabrication of Ni/NiO core-shell nanowires on commercial NF, providing a promising candidate for supercapacitors, fuel cells, and electrocatalysis.

Phase-pure pentlandite Ni4.3Co4.7S8 binary sulfide as an efficient bifunctional electrocatalyst for oxygen evolution and hydrogen evolution

Developing an efficient non-noble bifunctional electrocatalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in the same electrolyte is significant for lowering the cost of electrochemical water splitting. Herein, a phase-pure pentlandite Ni4.3Co4.7S8 bifunctional electrocatalyst was synthesized via a hydrothermal process using a commercial nickel foam as the nickel source. The active metallic nickel source and the chelating agent ethylenediamine play important roles in the formation of phase-pure pentlandite Ni4.3Co4.7S8 binary sulfide. Physicochemical characterizations, electrochemical measurements and density functional theory (DFT) computations illustrate that the material has an exposed high-indexed (022) surface with a biomimetic hydrogenase-like structure, and that the pentlandite phase has metallic characteristics, with next-nearest neighbor metal-metal bonds, as well as there being a high overlap of density of state (DOS) at the Fermi-level due to the synergistic effect between Ni and Co ions. In addition, there is an elevation of the d-state center (from -2.84 to -1.52 eV) with high occupation of the anti-bonding eg (dx2-y2 and dz2) d-orbitals. These properties endow the Ni4.3Co4.7S8 bifunctional electrocatalyst with higher catalytic activity for OER than RuO2, with comparative activity for HER to commercial Pt/C and with a low over-potential for all water splitting in an alkaline electrolyte. The studies here provide a novel strategy to synthesise phase-pure pentlandite nickel cobalt binary sulfides and boost their applications in electrochemical water splitting.

Metallic Ni3S2 Films Grown by Atomic Layer Deposition as an Efficient and Stable Electrocatalyst for Overall Water Splitting

We describe the direct preparation of crystalline Ni₃S₂ thin films via atomic layer deposition (ALD) techniques at temperatures as low as 250 °C without postthermal treatments. A new ALD chemistry is proposed using bis(1-dimethylamino-2-methyl-2-butoxy) nickel(II) [Ni(dmamb)₂] and H₂S as precursors. Homogeneous and conformal depositions of Ni₃S₂ films were achieved on 4 in. wafers (both metal and oxide substrates, including Au and SiO₂). The resulting crystalline Ni₃S₂ layers exhibited highly efficient and stable performance as electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline solutions, with a low overpotential of 300 mV and a high turnover frequency for HER and an overpotential of 400 mV for OER (at a current density of 10 mA/cm²). Using our Ni₃S₂ films as both the cathode and the anode, two-electrode full-cell electrolyzers were constructed, which showed stable operation for 100 h at a current density of 10 mA/cm². The proposed ALD electrocatalysts on planar surfaces exhibited the best performance among Ni₃S₂ materials for overall water splitting recorded to date.

NiO as a Bifunctional Promoter for RuO2 toward Superior Overall Water Splitting

Conventional development of nanomaterials for efficient electrocatalysis is largely based on performance-oriented trial-and-error/iterative approaches, while a rational design approach at the atomic/molecular level is yet to be found. Here, inspired by a fundamental understanding of the mechanism for both oxygen and hydrogen evolution half reactions (OER/HER), a unique strategy is presented to engineer RuO₂ for superior alkaline water electrolysis through coupling with NiO as an efficient bifunctional promoter. Benefitting from desired potential-induced interfacial synergies, NiO-derived NiOOH improves the oxygen binding energy of RuO₂ for enhanced OER, and NiO also promotes water dissociation for enhanced HER on RuO₂ -derived Ru. The resulting hybrid material exhibits remarkable bifunctional activities, affording 2.6 times higher OER activity than that of RuO₂ and an HER activity comparable to Pt/C. As a result, the simple system requires only 1.5 V to deliver 10 mA cm^-2 for overall alkaline water splitting, outperforming the benchmark PtC/NF||IrO₂ /NF couple with high mass loading. Comprehensive electrochemical investigation reveals the unique and critical role of NiO on the optimized RuO₂ /NiO interface for synergistically enhanced activities, which may be extended to broader (electro)catalytic systems.

Role of non-metallic atoms in enhancing the catalytic activity of nickel-based compounds for hydrogen evolution reaction

The transition-metal compounds (MX) have gained wide attention as hydrogen evolution reaction (HER) electrocatalysts; however, the interaction between the non-metallic atom (X) and the metal atom (M) in MX, and the role of X in the enhanced catalytic activity of MX, are still ambiguous. In this work, we constructed a simple model [X/Ni(100)] to decipher the contribution of X towards enhancing the catalytic activity of NiX, which allows us to accurately predict the trend in HER catalytic activity of NiX based on the easily accessible physico-chemical characteristics of X. Theoretical calculations showed that the electronegativity (χX) and the principle quantum number (nX) of X are two important descriptors for evaluating and predicting the HER catalytic activity of NiX catalysts effectively. X atoms in the VIA group can enhance the HER activity of X/Ni(100) more significantly than those in the second period due to the large χX or nX. At a relatively low X coverage, the S/Ni(100) possesses the best HER activity among all of the discussed X/Ni(100) models, and the optimum surface S : Ni atomic ratio is about 22-33%. Further experiments demonstrated that the Ni-Ni3S2 catalyst with a surface S : Ni atomic ratio of 28.9% exhibits the best catalytic activity and lowest charge transfer resistance. The trend in catalytic activity of NiX with differing X offers a new possible strategy to exploit MX materials and design new active catalysts rationally.

Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions

Hydrogen evolution reaction (HER) in alkaline medium is currently a point of focus for sustainable development of hydrogen as an alternative clean fuel for various energy systems, but suffers from sluggish reaction kinetics due to additional water dissociation step. So, the state-of-the-art catalysts performing well in acidic media lose considerable catalytic performance in alkaline media. This review summarizes the recent developments to overcome the kinetics issues of alkaline HER, synthesis of materials with modified morphologies, and electronic structures to tune the active sites and their applications as efficient catalysts for HER. It first explains the fundamentals and electrochemistry of HER and then outlines the requirements for an efficient and stable catalyst in alkaline medium. The challenges with alkaline HER and limitation with the electrocatalysts along with prospective solutions are then highlighted. It further describes the synthesis methods of advanced nanostructures based on carbon, noble, and inexpensive metals and their heterogeneous structures. These heterogeneous structures provide some ideal systems for analyzing the role of structure and synergy on alkaline HER catalysis. At the end, it provides the concluding remarks and future perspectives that can be helpful for tuning the catalysts active-sites with improved electrochemical efficiencies in future.

Dominating Role of Aligned MoS2/Ni3S2 Nanoarrays Supported on Three-Dimensional Ni Foam with Hydrophilic Interface for Highly Enhanced Hydrogen Evolution Reaction

When using water splitting to achieve sustainable hydrogen production, low-cost, stable, and naturally abundant electrocatalysts are required to replace Pt-based ones for the hydrogen evolution reaction (HER). Herein, for the first time, a novel nanostructure with one-dimensional (1D) MoS₂/Ni₃S₂ nanoarrays directly grow on a three-dimensional (3D) Ni foam is developed for this purpose, showing excellent catalytic activity and stability. The as-prepared 3D MoS₂/Ni₃S₂/Ni composite has an onset overpotential as low as 13 mV in 1 M KOH, which is comparable to Pt-based electrocatalyst for HER. According to the classical theory, the Tafel slope of the new composite is relatively low, as it goes through a combined Volmer-Heyrovsky mechanism during hydrogen evolution. All the results attribute the excellent electrocatalytic activity of the nanostructure to the electrical coupling among Ni, Ni₃S₂, and MoS₂, the super hydrophilic interface, the synergistic catalytic effects produced by the MoS₂/Ni₃S₂ nanoarrays, and abundant exposed active edge sites. These unique and previously undeveloped characteristics of the 3D MoS₂/Ni₃S₂/Ni composite make it a very promising earth-abundant electrocatalyst for HER.

Nanoporous Sulfur-Doped Copper Oxide (Cu2OxS1-x) for Overall Water Splitting

Developing active and bifunctional noble metal-free electrocatalysts is crucial for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in the full water splitting process. A ternary nanoporous sulfur-doped copper oxide (Cu₂O_xS_1-x) was successfully synthesized on Cu foam. The obtained Cu₂O_xS_1-x/Cu shows robust electrocatalytic activity toward HER with a low overpotential of 40 mV at 10 mA cm^-2 and a Tafel slope of 68 mV dec^-1 and exhibits long-term stability in acid solution. Moreover, Cu₂O_xS_1-x shows excellent electrocatalytic activity for OER, HER, and overall water splitting as a bifunctional catalyst in 1.0 M KOH electrolyte. The sulfur doping strategy implemented here can greatly improve the catalytic performance and stability in both acidic and alkaline water electrolyzers and presents an efficient catalyst for overall water splitting.

Fabrication of strong bifunctional electrocatalytically active hybrid Cu–Cu2O nanoparticles in a carbon matrix

Earth-abundant copper-based hybrid Cu–Cu₂ONPs@C in the carbon matrix exhibited enhanced OER and HER catalytic activity compared to pure Cu₂O and CuNPs@C.