Nickel-Based Electrocatalysts for Water Electrolysis

Nickel-based catalysts for electrolytic decomposition of ammonia towards hydrogen production

Nickel is an attractive metal for electrochemical applications because it is abundant, cheap, chemically resilient, and catalytically active towards many reactions. Nickel-based materials (metallic nickel, its alloys, oxides, hydroxides, and composites) have been also considered as promising electrocatalysts for ammonia oxidation. The electrolysis of ammonia aqueous solution results in evolution of gaseous hydrogen and nitrogen. Up to date studies showed that metallic Ni and Ni (hydro)oxides are not catalytically active unless they are electrochemically converted to NiOOH at ~1.3 V vs. RHE. Then, dehydrogenation of NH3 begins with electron coupled proton transfer to NiOOH resulting in a would-be reversible reduction of the latter to Ni(OH)2. Unlike the water electrolysis process, in which solely oxygen is obtained at the anode, during ammonia electrooxidation apart from release of N2, many undesired oxygenated nitrogen moieties may also turn up. These products appear after at least partial dehydrogenation of ammonia. Studies on NiOOH activity have been conducted for systems containing various modifiers, e.g., Cu, Co, S, P, however, their particular role in catalytic activity has not yet been elucidated. Nowadays research is being conducted in the direction of increasing the activity, selectivity, and stability of NiOOH. In this review, the electroactivity of Ni is analyzed and discussed in accordance with its oxidation states along with the ammonia oxidation mechanism. The main research problems to be solved and challenges for the future industrial use of ammonia are presented.

Phytochemical-assisted green synthesis of CuFeOx nano-rose electrocatalysts for oxygen evolution reaction in alkaline media

This study represents a green synthesis method for fabricating an oxygen evolution reaction (OER) electrode by depositing two-dimensional CuFeO_x on nickel foam (NF). Two-dimensional CuFeO_x was deposited on NF using in situ hydrothermal synthesis in the presence of Aloe vera extract. This phytochemical-assisted synthesis of CuFeO_x resulted in a unique nano-rose-like morphology (petal diameter 30-70 nm), which significantly improved the electrochemical surface area of the electrode. The synthesized electrode was analyzed for its OER electrocatalytic activity and it was observed that using 75% Aloe vera extract in the phytochemical-assisted synthesis of CuFeO_x resulted in improved OER electrocatalytic performance by attaining an overpotential of 310 mV for 50 mA cm^-2 and 410 mV for 100 mA cm^-2. The electrode also sustained robust stability throughout the 50 h of chronopotentiometry studies under alkaline electrolyte conditions, demonstrating its potential as an efficient OER electrode material. This study highlights the promising use of Aloe vera extract as a green and cost-effective way to synthesize efficient OER electrode materials.

NiCoP-nanocubes-decorated CoSe2 nanowire arrays as high-performance electrocatalysts toward oxygen evolution reaction

Designing effective, robust, and low-cost catalysts for oxygen evolution reaction (OER) is an urgent requirement yet challenging task in water electrolysis. In this study, a NiCoP-nanocubes-decorated CoSe₂ nanowires arrays three-dimensional/two-dimensional (3D/2D) electrocatalyst (NiCoP-CoSe₂-2) was developed for catalyzing OER via a combined selenylation, co-precipitation, and phosphorization method. The as-obtained NiCoP-CoSe₂-2 3D/2D electrocatalyst exhibits a low overpotential of 202 mV at 10 mA cm^-2 with a small Tafel slope of 55.6 mV dec^-1, which is superior to most of reported CoSe₂ and NiCoP-based heterogeneous electrocatalysts. Experimental analyses and density functional theory (DFT) calculations proof that the interfacial coupling and synergy between CoSe₂ nanowires and NiCoP nanocubes are not only beneficial to strengthen the charge transfer ability and accelerate reaction kinetics, but also facilitate the optimization of interfacial electronic structure, thereby enhancing the OER property of NiCoP-CoSe₂-2. This study offers insights for the investigation and construction of transition metal phosphide/selenide heterogeneous electrocatalyst toward OER in alkaline media and broadens the prospect of industrial applications in energy storage and conversion fields.

Electrodeposited Ni-Mo coatings as electrocatalytic materials for green hydrogen production

In this work, nanocrystalline nickel and nickel-molybdenum alloys were electrodeposited from electrolytes based on deep eutectic solvents. Eutectic mixtures of choline chloride with ethylene glycol (ethaline) and urea (reline) were used as typical representatives of deep eutectic solvents. The deposited Ni and Ni-Mo films were evaluated as potential electrocatalytic materials for green hydrogen production via electrolysis of alkaline aqueous solutions. The electrodeposited samples were characterized by XRD, SEM and EDX techniques, and the electrochemical behavior was evaluated by means of linear voltammetry and Tafel analysis. It was shown that the deposition of nickel (without molybdenum) from the electrolytes based on ethaline provides a higher electrocatalytic activity of the material with respect to the hydrogen evolution reaction than the material deposited from the reline-based electrolytes. The reline-based plating electrolytes contribute to a greater inclusion of molybdenum in the fabricated Ni-Mo alloys and therefore ensure increased electrocatalytic activity as compared with the ethaline-based electrolytes. The electrocatalytic behavior well correlates with the molybdenum content in the coatings. Ni and Ni-Mo electrodeposits produced from the deep eutectic solvent-mediated plating baths exhibit improved electrocatalytic performance and can be considered as promising catalytic materials for water electrolysis in green hydrogen energy.

RuCo Alloy Nanoparticles Embedded into N-Doped Carbon for High Efficiency Hydrogen Evolution Electrocatalyst

For large-scale and sustainable water electrolysis, it is of great significance to develop cheap and efficient electrocatalysts that can replace platinum. Currently, it is difficult for most catalysts to combine high activity and stability. To solve this problem, we use cobalt to regulate the electronic structure of ruthenium to achieve high activity, and use carbon matrix to protect alloy nanoparticles to achieve high stability. Herein, based on the zeolitic imidazolate frameworks (ZIFs), a novel hybrid composed of RuCo alloy nano-particles and N-doped carbon was prepared via a facile pyrolysis-displacement-sintering strategy. Due to the unique porous structure and multi-component synergy, the optimal RuCo500@NC750 material in both acidic and alkaline media exhibited eminent HER catalytic activity. Notably, the 3-RuCo500@NC750 obtained a current density of 10 mA cm−2 at 22 mV and 31 mV in 0.5 M H2SO4 and 1.0 M KOH, respectively, comparable to that of the reference Pt/C catalyst. Furthermore, the Tafel slopes of the catalyst are 52 mV Dec−1 and 47 mV Dec−1, respectively, under acid and alkali conditions, and the catalyst has good stability, indicating that it has broad application prospects in practical electrolytic systems. This work contributes to understanding the role of carbon-supported polymetallic alloy in the electrocatalytic hydrogen evolution process, and provides some inspiration for the development of a high efficiency hydrogen evolution catalyst.

An efficient and stable iodine-doped nickel hydroxide electrocatalyst for water oxidation: synthesis, electrochemical performance, and stability

The design of oxygen evolution reaction (OER) catalysts with higher stability and activity by economical and convenient methods is considered particularly important for the energy conversion technology. Herein, a simple hydrothermal method was adopted for the synthesis of iodine-doped nickel hydroxide nanoparticles and their OER performance was explored. The electrocatalysts were structurally characterized by powder X-ray diffraction analysis (P-XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), and BET analysis. The electrochemical performance of the electrocatalysts was assessed by cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy. The abundant catalytic active sites, oxygen vacancies, low charge-transfer resistance, and a high pore diameter to pore size ratio of iodine-doped Ni(OH)₂ were responsible for its excellent catalytic activity, whereby OER was initiated even at 1.52 V (vs. RHE) and a 330 mV overpotential was needed to reach a 40 mV cm⁻² current density in 1 M KOH solution. The material also exhibited a low Tafel slope (46 mV dec⁻¹), which suggests faster charge-transfer kinetics as compared to its counterparts tested under the same electrochemical environment. It is worth noting that this facile and effective approach suggests a new way for the fabrication of metal hydroxides rich in oxygen vacancies, thus with the potential to boost the electrochemical performance of energy-related systems.

Oxygen evolution reaction mechanism under alkaline conditions over the iodine-doped Ni(OH)₂ surface.