Turning Ni-based hydroxide into an efficient hydrogen evolution electrocatalyst by fluoride incorporation

Elucidating the performance of hexamethylene tetra-amine interlinked bimetallic NiCo-MOF for efficient electrochemical hydrogen and oxygen evolution

Bimetallic metal-organic frameworks (MOFs) play a significant role in the electrocatalysis of water due to their large surface area and availability of increased numbers of pores. For the inaugural time, we examine the effectiveness of a hexamethylene tetra-amine (HMT)-induced 3D NiCo-MOF-based nanostructure as a potent bifunctional electrocatalyst with superior performance for overall water splitting in alkaline environments. The structural, morphological, and electrochemical properties of the as-synthesized bifunctional catalyst were examined thoroughly before analyzing its behavior towards electrochemical water splitting. The HMT-based NiCo-MOF demonstrated small overpotential values of 274 mV and 330 mV in reaching a maximum current density of 30 mA cm-2 for hydrogen and oxygen evolution mechanisms, respectively. The Tafel parameter also showed favorable HER/OER reaction kinetics, with slopes of 78 mV dec-1 and 86 mV dec-1 determined during the electrochemical evaluation. Remarkably, the NiCo-HMT electrode exhibited a double-layer capacitance of 4 mF cm-2 for hydrogen evolution and 23 mF cm-2 for oxygen evolution, while maintaining remarkable stability even after continuous operation for 20 hours. This research offers a valuable blueprint for implementing a cost-effective and durable MOF-based bifunctional catalytic system that has proven to be effective for complete water splitting. Decomposition of water under higher current densities is crucial for effective long-term generation and commercial consumption of hydrogen.

Fluoride-incorporated cobalt-based electrocatalyst towards enhanced hydrogen evolution reaction

We report an electrocatalyst, Co bases (metallic Co and Co(OH)₂) with fluoride-incorporated CoO coating on the surface of (CoO-F/Co), was synthesized by the electro-deposition method. The porous network architecture of CoO-F/Co on the glassy carbon electrode exhibited an ultra-low overpotential of 15 mV, achieving the geometric current density of 10 mA cm^-2 in 1.0 M KOH, which were comparable with the HER performance of numerous reported noble metal electrocatalysts. It is demonstrated that fluoride incorporation improved the electrodeposition particle size, electronic density, conductivity and hydrophilicity of CoO-F/Co the HER performance.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Strategies and Perspectives to Catch the Missing Pieces in Energy-Efficient Hydrogen Evolution Reaction in Alkaline Media

Transition metal hydroxides (M-OH) and their heterostructures (X|M-OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy-efficient alkaline HER.

Nb-Doped nickel nitride-derived catalysts for electrochemical water splitting

An alkaline electrolyzer using Nb-doped nickel nitride-derived catalysts as electrodes for overall water splitting delivers 10 mA cm−2 at a low cell voltage of 1.61 V.

Interfacial synergy between dispersed Ru sub-nanoclusters and porous NiFe layered double hydroxide on accelerated overall water splitting by intermediate modulation

Construction of an efficient bifunctional electrocatalyst through a rational interface-engineering strategy to optimize the adsorption energy of H* and OH* species at the atomic/molecular level is of great importance for water splitting. Although conventional NiFe layered double hydroxide (LDH) shows excellent performance for alkaline oxygen evolution reactions (OERs), it shows extremely poor activity toward hydrogen evolution reactions (HERs) due to weak hydrogen adsorption and sluggish kinetics. In this work, integration of sub-nanoscale Ru species with NiFe LDH can dramatically enhance the adsorption energy of H* and improve their HER kinetics. Besides, benefitting from the desired potential-induced strategy, the Ru-NiFe LDH interfaces will convert to RuO2-NiFe(OOH)x interfaces to optimize the adsorption energy of OH* to meet the requirement of strengthening the OER performance. Strikingly, the Ru-NiFe LDH-F/NF sample (NF: Ni foam) shows an excellent OER and HER performance with an overpotential of 230.0 mV and 115.6 mV at a current density of 10 mA cm-2, respectively, as well as outstanding durability. The overall water splitting device was fabricated by using Ru/NiFe LDH-F/NF as both the HER and OER electrode with a potential of 1.53 V to achieve a current density of 10 mA cm-2. In addition, the theoretical calculations demonstrated that the Ru-NiFe LDH interfaces could optimize the adsorption energy of H* and OH*. This study provides a new insight into the development of highly efficient bifunctional electrocatalysts for water electrolysis.

Integrating Rh Species with NiFe-Layered Double Hydroxide for Overall Water Splitting

NiFe-layered double hydroxide (LDH) is thought of as a promising bifunctional water-splitting catalyst, owing to its excellent performances for alkaline oxygen evolution reactions (OERs). However, it shows extremely poor activity toward hydrogen evolution reactions (HERs) due to the weak hydrogen adsorption. We demonstrated that the integration of Rh species and NiFe-LDH can dramatically improve its HER kinetics without sacrificing the OER performance. The Rh species were initially integrated into NiFe-LDH as oxidized dopants and metallic clusters (< 1 nm). In 1 M KOH electrolyte, an overpotential of 58 mV is needed to catalyze 10 mA cm^-2 HER current density. Furthermore, this catalyst only requires 1.46 V to drive an electrolyzer at 10 mA cm^-2. A strong interaction between metallic Rh clusters and NiFe hydroxide during the HER process is revealed. The theoretical calculation shows the Rh ions replace Fe ions as the major active sites that are responsible for OERs.

Introduction of Mn(iii) to regulate the electronic structure of fluorine-doped nickel hydroxide for efficient water oxidation

OER is the key step to increase the rate of water-splitting reaction. Design and construction of appropriate defects is an effective strategy to enhance catalytic activity. Mn has stronger e^--e^- repulsion by the local influence of its 3d orbital electrons. When Mn(iii) was successfully introduced into two dimensional F-doped Ni(OH)₂, it can tune the surface electronic structure of the F-doped Ni(OH)₂ to increase its oxygen deficiency content. In this work, the as-synthesized Mn and F co-doped Ni(OH)₂-NF on Ni foam (Mn-F/Ni(OH)₂-NF) shows remarkable oxygen evolution performance, exhibiting 233 mV overpotential at 20 mA cm^-2, and the Tafel slope is 56.9 mV dec^-1 in 1 M KOH. The performance is better than that of the same loading of IrO₂ on Ni foam. Density functional theory (DFT) calculations further show that the introduction of oxygen defects can significantly improve the OER catalytic performance of Mn-F/Ni(OH)₂-NF.

Fluoride-Induced Dynamic Surface Self-Reconstruction Produces Unexpectedly Efficient Oxygen-Evolution Catalyst

The oxygen-evolution reaction (OER) is a key process in water-splitting systems, fuel cells, and metal-air batteries, but the development of highly active and robust OER catalyst by simple methods is a great challenge. Here, we report an in situ dynamic surface self-reconstruction that can dramatically improve the catalytic activity of electrocatalysts. A fluoride (F^-)-incorporating NiFe hydroxide (NiFe-OH-F) nanosheet array was initially grown on Ni foam by a one-step hydrothermal method, which requires a 243 mV over-potential (η) to achieve a 10 mA cm^-2 current density with a Tafel slope of 42.9 mV dec^-1 in alkaline media. After the surface self-reconstruction induced by fluoride leaching under OER conditions, the surface of NiFe-OH-F was converted into highly mesoporous and amorphous NiFe oxide hierarchical structure, and the OER activity at η = 220 mV increases over 58-fold. The corresponding η at 10 mA cm^-2 decreases to 176 mV with an extreme low Tafel slope of 22.6 mV dec^-1; this performance is superior to that of the state-of-the-art OER electrocatalysts.

Hierarchical FeNiP@Ultrathin Carbon Nanoflakes as Alkaline Oxygen Evolution and Acidic Hydrogen Evolution Catalyst for Efficient Water Electrolysis and Organic Decomposition

Efficiency of hydrogen evolution via water electrolysis is mainly impeded by the kinetically sluggish oxygen evolution reaction (OER). Thus, it is of great significance to develop highly active and stable OER catalyst for alkaline water electrolysis or to substitute the more kinetically demanding acidic OER with a facile electron-donating reaction such that OER is no longer the bottleneck half-reaction for either acidic or alkaline water electrolysis. Herein, the hierarchical Fe-Ni phosphide shelled with ultrathin carbon networks on Ni foam (FeNiP@C) is reported and shows exceptional OER activity and enhanced chemical stability in 1 M KOH. This unique electrode provides large active sites, facile electron transport pathways, and rapid gas release, resulting in a remarkable OER activity that delivers a current density of 100 mA/cm² at an overpotential of 182 mV with a Tafel slope of 56 mV/dec. Combining the hydrogen evolution reaction with organic pollutant (methylene blue) oxidation, a multifunctional electrolyzer for simultaneous cost-effective hydrogen generation and organic pollutant decomposition in acid wastewater is proposed. Our strategies in this work provide attractive opportunities in energy- and environment-related fields.