Facile Access to an Active γ‐NiOOH Electrocatalyst for Durable Water Oxidation Derived From an Intermetallic Nickel Germanide Precursor

Eccentric Corrosion-Induced Formation of γ-NiFeOOH and γ-NiCoOOH on NiFeCo Alloy for Enhanced OER

An indirect, swift, and easy method of enhancing the oxygen evolution reaction (OER) performance of an economically viable Fe-rich NiFeCo (NFC) alloy has been developed. This approach leverages the anodic potential sweeps applied indirectly to the counter electrode (CE) when one does cathodic hydrogen evolution reaction (HER) on by potential sweeping at the working electrode (WE). In this method, NFC was intentionally corroded indirectly by using it as a CE for the potential sweeping HER experiment done with a Pt WE. The indirectly corroded NFC (NFC_IC) featuring mostly γ-NiFeOOH and γ-NiCoOOH entities on the surface was able to begin the OER at an onset overpotential of 250 mV and reach the benchmark of 10 mA cm-2 at 290 mV, which is 60 and 55 mV lesser than that of the bare NFC and RuO2, respectively, all with exceptionally faster kinetics, as evidenced by a relatively smaller Tafel slope of 30 mV dec-1. These insights into designing a trimetallic alloy-based OER electrocatalyst have opened a previously unknown avenue in the development of advanced self-supported OER electrodes for better and efficient H2 production via water electrolysis.

Electron delocalization-modulated hydroxyl binding for enhanced hydrogen evolution reaction activity

The introduction of foreign metals with a higher oxophilicity represents a promising strategy to promote water dissociation and in turn kinetics of alkaline hydrogen evolution reaction (HER). However, the further improvement of HER activity is limited by the unfavorable interaction of hydroxyl generated by the dissociation of water with active sites. Herein, we propose a strategy of alkaline earth metal cations-driven electron delocalization to elaborately tailor the binding of hydroxyl with the active sites. Taking FeNiMg-layered double hydroxides (FeNiMg-LDH) as a prototypical example, the combined operando spectroscopy analysis and theoretical calculations show that the introduction of Mg cations in solid- solution phase can create a local electronic field and delocalize the electron between Fe and adsorbed hydroxyl, resulting in an optimization of hydroxyl binding strength. Accordingly, FeNiMg-LDH lowers the overpotentials to deliver 10 mA cm-2 in alkaline electrolyte by 39 and 64 mV, compared to FeNi-LDH and Ni-LDH catalysts, respectively. This work sheds new light on the rational design of advanced HER electrocatalyst for alkaline water electrolysis.

Exploring the properties, types, and performance of atomic site catalysts in electrochemical hydrogen evolution reactions

Atomic site catalysts (ASCs) have recently gained prominence for their potential in the electrochemical hydrogen evolution reaction (HER) due to their exceptional activity, selectivity, and stability. ASCs with individual atoms dispersed on a support material, offer expanded surface areas and increased mass efficiency. This is because each atom in these catalysts serves as an active site, which enhances their catalytic activity. This review is focused on providing a detailed analysis of ASCs in the context of the HER. It will delve into their properties, types, and performance to provide a comprehensive understanding of their role in electrochemical HER processes. The introduction part underscores HER's significance in transitioning to sustainable energy sources and emphasizes the need for innovative catalysts like ASCs. The fundamentals of the HER section emphasizes the importance of understanding the HER and highlights the key role that catalysts play in HER. The review also explores the properties of ASCs with a specific emphasis on their atomic structure and categorizes the types based on their composition and structure. Within each category of ASCs, the review discusses their potential as catalysts for the HER. The performance section focuses on a thorough evaluation of ASCs in terms of their activity, selectivity, and stability in HER. The performance section assesses ASCs in terms of activity, selectivity, and stability, delving into reaction mechanisms via experimental and theoretical approaches, including density functional theory (DFT) studies. The review concludes by addressing ASC-related challenges in HER and proposing future research directions, aiming to inspire further innovation in sustainable catalysts for electrochemical HER.

Local Proton-Mediated Synthesis of a High-Entropy Borate Library

High-entropy compounds (HECs) provide extensive possibilities for exploring distinctive properties and potential applications. However, most HECs reported so far are synthesized by an arduous high-temperature treatment and special equipment, which is clearly not scalable for practical application. Here a scalable room-temperature solution synthetic strategy is reported for a library of high-entropy borates with arbitrary metal component numbers from 5 to 12 up to 3302 kinds in total and more than a hundred grams per operation within one minute. In conjunction with theoretical and in situ investigations, it is uncovered that the highly local concentration of protons at ethanol/aqueous interface is favorable to the creation of a stable thermodynamic microenvironment and a desirable kinetic miscibility reservoir, thus enabling a formation of single-phase borates. With the FeCoNiMoCu high-entropy borate, it is further shows that it functions as a highly active catalyst for catalytic oxygen evolution reaction. The work opens up opportunities for the scalable synthesis of HECs for energy storage and conversion applications.

Near-infrared light-driven biomass conversion

Current photocatalytic technologies mainly rely on the input of high-energy ultraviolet-visible (UV-vis) light to obtain the desired excited states with adequate energy to drive redox reactions, precluding the use of low-energy near-infrared (NIR) light that occupies ~50% of the solar spectrum. Here, we report the efficient utilization of NIR light by coupling the low-energy NIR photons with reactive biomass conversion. A unique mechanism of photothermally synergistic photocatalysis was revealed for the selective biomass conversion under NIR light. Using biomass-derived 5-hydroxymethylfurfural (HMF) conversion as a model reaction, it was found that NIR and UV-vis light featured markedly different reaction patterns. 5-Formyl-2-furancarboxylic acid (FFCA) was almost exclusively produced under NIR light, whereas UV-vis light favored the formation of 2,5-diformylfuran (DFF) as the major product. This work provides a paradigm for sustainable and selective chemical synthesis using the Earth's abundant resources, sunlight and biomass.

Crystalline-Amorphous Ni2 P4 O12 /NiMoOx Nanoarrays for Alkaline Water Electrolysis: Enhanced Catalytic Activity via In Situ Surface Reconstruction

Water electrolysis affords a promising approach to large-scale hydrogen yield, but its efficiency is restrained by the sluggish water dissociation kinetics. Here, an efficient bifunctional electrocatalyst of in situ formed crystalline nickel metaphosphate on amorphous NiMoO_x nanoarrays supported on nickel foam (c-Ni₂ P₄ O₁₂ /a-NiMoO_x /NF) for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution is reported. The c-Ni₂ P₄ O₁₂ /a-NiMoO_x /NF can deliver a current density of 10 mA cm^-2 at a low potential of 78 mV for HER, and a current density of 20 mA cm^-2 at an overpotential of 250 mV for OER. Moreover, it only requires a small cell voltage of 1.55 V at 10 mA cm^-2 for robust water splitting with outstanding long-term durability over 84 h. Various spectroscopic studies reveal that in situ surface reconstruction is crucial for the enhanced catalytic activity, where c-Ni₂ P₄ O₁₂ /a-NiMoO_x is transformed into c-Ni₂ P₄ O₁₂ /a-NiMoO₄ during the HER process, and into c-Ni₂ P₄ O₁₂ /a-NiOOH in the OER process. This work may provide a new strategy for uncovering the catalytic mechanism of crystalline-amorphous catalysts.

Porous Indium Tin Oxide-Supported NiFe LDH as a Highly Active Electrocatalyst in the Oxygen Evolution Reaction and Flexible Zinc-Air Batteries

The oxygen evolution reaction (OER) is crucial for hydrogen production from water splitting and rechargeable metal-air batteries. However, the four-electron mechanism results in slow reaction kinetics, which needed to be accelerated by efficient catalysts. Herein, a hybrid catalyst of novel nickel-iron layered double hydroxide (NiFe LDH) on porous indium tin oxide (ITO) is presented to lower the overpotential of the OER. The as-prepared NiFe LDH@ITO catalyst showed superior catalytic activity toward the OER with an overpotential of only 240 mV at a current density of 10 mA/cm². The catalyst also offered high stability with almost no activity decay after more than 200 h of chronopotentiometry test. Furthermore, the applications of NiFe LDH@ITO in (flexible) rechargeable zinc-air batteries exhibited a better performance than commercial RuO₂ and can remain stable in cycling tests. It is supposed that the superior catalytic behavior originates from the ITO conductive framework, which prevents the agglomeration and facilitates the electron transfer during the OER process.

Intermetallic Compounds: Liquid-Phase Synthesis and Electrocatalytic Applications

Characterized by long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure, intermetallics have attracted increasing attention in the area of chemical synthesis and catalytic applications. Liquid-phase synthesis of intermetallics has arisen as the promising methodology due to its precise control over size, shape, and resistance toward sintering compared with the traditional metallurgy. This short review tends to provide perspectives on the liquid-phase synthesis of intermetallics in terms of both thermodynamics and methodology, as well as its applications in various catalytic reactions. Specifically, basic thermodynamics and kinetics in the synthesis of intermetallics will be first discussed, followed by discussing the main factors that will affect the formation of intermetallics during synthesis. The application of intermetallics in electrocatalysis will be demonstrated case by case at last. We conclude the review with perspectives on the future developments with respect to both synthesis and catalytic applications.

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.

Evolving Highly Active Oxidic Iron(III) Phase from Corrosion of Intermetallic Iron Silicide to Master Efficient Electrocatalytic Water Oxidation and Selective Oxygenation of 5-Hydroxymethylfurfural

In a green energy economy, electrocatalysis is essential for chemical energy conversion and to produce value added chemicals from regenerative resources. To be widely applicable, an electrocatalyst should comprise the Earth's crust's most abundant elements. The most abundant 3d metal, iron, with its multiple accessible redox states has been manifold applied in chemocatalytic processes. However, due to the low conductivity of FeIII Ox Hy phases, its applicability for targeted electrocatalytic oxidation reactions such as water oxidation is still limited. Herein, it is shown that iron incorporated in conductive intermetallic iron silicide (FeSi) can be employed to meet this challenge. In contrast to silicon-poor iron-silicon alloys, intermetallic FeSi possesses an ordered structure with a peculiar bonding situation including covalent and ionic contributions together with conducting electrons. Using in situ X-ray absorption and Raman spectroscopy, it could be demonstrated that, under the applied corrosive alkaline conditions, the FeSi partly forms a unique, oxidic iron(III) phase consisting of edge and corner sharing [FeO6 ] octahedra together with oxidized silicon species. This phase is capable of driving the oxyge evolution reaction (OER) at high efficiency under ambient and industrially relevant conditions (500 mA cm-2 at 1.50 ± 0.025 VRHE and 65 °C) and to selectively oxygenate 5-hydroxymethylfurfural (HMF).