Nickel-Based Electrocatalysts for Water Electrolysis

Ni/Fe Fluorides (Hydroxide) Nanocomposite as Efficient OER Catalyst

The synthesis of efficient oxygen evolution reaction (OER) catalysts that markedly reduce the overpotential over an extended period is crucial for electrolytic water splitting toward hydrogen production. A kind of Ni/Fe fluoride (hydroxide) nanocomposite OER catalyst is designed and prepared by a two-step method for the first time. The nanocomposite with the optimal OER performance (Ni : Fe precursor ratio of 9 : 1) is observed to possess a nanoparticle morphology with size of about 100 nm. Each nanoparticle hosts extensive nanoregions of Ni4OHF7, NiFeF5 ⋅ 2H2O and Fe1.9F4.75 ⋅ 0.95H2O phases. The optimal nanocomposite (Ni : Fe precursor ratio of 9 : 1) exhibits OER overpotential of merely 208 mV and 349 mV at 10 mA cm-2 and 100 mA cm-2 respectively, tafel slope of 53.1, and outstanding stability for 10 h duration at 100 mA cm-2. The superior OER catalytic performance of the optimal nanocomposite after CV activation is mainly ascribed to the comprehensive catalytic effect of multiple Ni, Fe active sites from three phases, the smaller charge transfer resistance achieved at this particular Ni : Fe precursor ratio. The abundant resources of Ni, Fe, F elements and the superior OER properties of the Ni/Fe fluorides (hydroxide) nanocomposite, make it a good OER catalyst candidate for electrolytic water splitting toward hydrogen production.

Review of Ni-Based Materials for Industrial Alkaline Hydrogen Production

Hydrogen has been recognized as a green energy carrier, which can relieve energy shortage and environmental pollution. Currently, alkaline water electrolysis (AWE) driven by renewable energy to produce large-scale green hydrogen is a mainstream technology. However, tardy cathodic hydrogen evolution reaction (HER) and stability issue of catalysts make it challenging to meet the industrial requirements. Ni-based materials have attracted wide attention, thanks to their low cost and rich tuning possibilities, and many efforts have focused on their activity and stability. However, due to the significant discrepancy between laboratory and industrial conditions, these catalysts have not been widely deployed in industrial AWE. In this review, we first introduce the differences between laboratory and industrial stage, especially concerning equipment, protocols and evaluation metrics. To shorten these gaps, some strategies are proposed to improve the activity and stability of the Ni-based catalysts. Besides, some key issues related to the catalysts in industrial AWE device are also emphasized, including reverse-current and foreign ions in the electrolyte. Finally, the challenges and outlooks on the industrial alkaline AWE are discussed.

Expanding the frontiers of electrocatalysis: advanced theoretical methods for water splitting

Electrochemical water splitting, which encompasses the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), offers a promising route for sustainable hydrogen production. The development of efficient and cost-effective electrocatalysts is crucial for advancing this technology, especially given the reliance on expensive transition metals, such as Pt and Ir, in traditional catalysts. This review highlights recent advances in the design and optimization of electrocatalysts, focusing on density functional theory (DFT) as a key tool for understanding and improving catalytic performance in the HER and OER. We begin by exploring DFT-based approaches for evaluating catalytic activity under both acidic and alkaline conditions. The review then shifts to a material-oriented perspective, showcasing key catalyst materials and the theoretical strategies employed to enhance their performance. In addition, we discuss scaling relationships that exist between binding energies and electronic structures through the use of charge-density analysis and d-band theory. Advanced concepts, such as the effects of adsorbate coverage, solvation, and applied potential on catalytic behavior, are also discussed. We finally focus on integrating machine learning (ML) with DFT to enable high-throughput screening and accelerate the discovery of novel water-splitting catalysts. This comprehensive review underscores the pivotal role that DFT plays in advancing electrocatalyst design and highlights its potential for shaping the future of sustainable hydrogen production.

Effect of Ni-Doping on the Optical, Structural, and Electrochemical Properties of Ag29 Nanoclusters

Atomically precise metal nanoclusters (NCs) can be compositionally controlled at the single-atom level, but understanding structure-property correlations is required for tailoring specific optical properties. Here, the impact of Ni atom doping on the optical, structural, and electrochemical properties of atomically precise 1,3-benzene dithiol (BDT) protected Ag29 NCs is studied. The Ni-doped Ag29 (NiAg28(BDT)12) NCs, are synthesized using a co-reduction method and characterized using electrospray ionization mass spectrometry (ESI MS), ion mobility spectrometry (IMS), and X-ray photoelectron spectroscopy (XPS). Only a single Ni atom doping can be achieved despite changing the precursor concentration. Ni doping in Ag29 NCs exhibits enhanced thermal stability, and electrocatalytic oxygen evolution reaction (OER) compared to the parent NCs. Density functional theory (DFT) calculations predict the geometry and optical properties of the parent and NiAg28(BDT)12 NCs. DFT is also used to study the systematic single-atom doping effect of metals such as Au, Cu, and Pt into Ag29 NCs and suggests that with Ni and Pt, the d atomic orbitals contribute to creating superatomic orbitals, which is not seen with other dopants or the parent cluster. The emission mechanism is dominated by a charge transfer from the ligands into the Ag core cluster regardless of the dopant.

Biology-Based Synthesis of Nickel Single Atoms on the Surface of Geobacter sulfurreducens as an Efficient Electrocatalyst for Alkaline Water Electrolysis

Single-atom metal catalysts are promising electrocatalysts for water electrolysis. Nickel-based electrocatalysts have shown attractive application prospects for water electrolysis. However, synthesizing stable Ni single atoms using chemical and physical approaches remains a practical challenge. Here, a facile and precise method for synthesizing stable nickel single atoms on the surface of Geobacter sulfurreducens using a microbial-mediated extracellular electron transfer (EET) process is demonstrated. It is shown that G. sulfurreducens can effectively anchor nickel single atoms on their surface. X-ray absorption near-edge structure and Fourier-transformed extended X-ray absorption fine structure spectroscopy confirm that the nickel single atom is coordinated to nitrogen in the cytochromes. The as-synthesized nickel single atoms on G. sulfurreducens exhibit excellent bifunctional catalytic properties for alkaline water electrolysis with low overpotential (η) to achieve current density (10 mA cm-2) for both hydrogen evolution reactions (η = 80 mV) and oxygen evolution reaction (η = 330 mV) with minimal catalyst loading of 0.0015 mg Ni cm-2. The nickel single-atom catalyst shows long-term stability at a constant electrode potential. This synthesis method based on the EET capability of electroactive bacteria provides a simple and scalable approach for producing low-cost and highly efficient nonnoble transition metal single-atom catalysts for practical applications.

Quasi-1D Oxides as Electrocatalysts for Water-Splitting: Case Study of Sr9M2Mn5O21 (M = Co, Ni, Cu, Zn)

We have demonstrated that multimetal oxides with a quasi-1D structure can be effective electrocatalysts for water electrolysis. Four materials have been systematically investigated, namely, Sr9Co2Mn5O21, Sr9Ni2Mn5O21, Sr9Cu2Mn5O21, and Sr9Zn2Mn5O21, comprising 1D chains of face-sharing MnO6 octahedra and MO6 trigonal prisms (M = Co, Ni, Cu or Zn), which are held together by strontium ions located between the chains. These materials show a consistent trend in electrocatalytic properties for both half-reactions of water-splitting, i.e., oxygen evolution and hydrogen evolution reactions (OER and HER). In particular, Sr9Ni2Mn5O21 has an outstanding performance for OER, with an overpotential of 0.37 V, which is lower than those of many 3D and 2D oxides, and rivals the activity of noble metal catalysts, such as RuO2. This is important given that the OER is considered the bottleneck of the water-slitting process. Chronopotentiometry studies, combined with X-ray photoelectron spectroscopy (XPS) and X-ray diffraction experiments pre- and post-reaction, indicate that Sr9Ni2Mn5O21 is highly stable and retains its structural integrity upon electrocatalytic reactions. This study highlights the potential of quasi-1D oxides as active electrocatalysts for water-splitting.

Synthesis of spinel Nickel Ferrite (NiFe2O4)/CNT electrocatalyst for ethylene glycol oxidation in alkaline medium

Nickel-iron-based spinel oxide was prepared and supported on multi-walled carbon nanotubes to enhance the electrochemical oxidation of ethylene glycol in an alkaline medium. NiFe2O4 was prepared using facile sol-gel techniques. Then the prepared material was characterized using different bulk and surface techniques like powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmitted electron microscope (TEM). Different electrodes of NiFe2O4/CNT ratios were prepared to find out the optimum spinel oxide/CNT ratio. The activity of the metal spinel oxides composite was characterized toward ethylene glycol conversion by different electrochemical techniques like cyclic voltammetry (CV), Chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The modified electrode reached an oxidation current of 43 mA cm-2 in a solution of 1.0 M ethylene glycol and 1.0 M NaOH. Furthermore, some kinetics parameters (like diffusion coefficient, and rate constant) were calculated to evaluate the catalytic performance. Additionally, the electrode showed extreme stability for long-term ethylene glycol oxidation.

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.