Enhanced activity promoted by amorphous metal oxyhydroxides on CeO2 for alkaline oxygen evolution reaction

Synergistic engineering of heterostructure and oxygen vacancy in cobalt hydroxide/aluminum oxyhydroxide as bifunctional electrocatalysts for urea-assisted hydrogen production

Designing inexpensive, high-efficiency and durable bifunctional catalysts for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) is an encouraging tactic to produce hydrogen with reduced energy expenditure. Herein, oxygen vacancy-rich cobalt hydroxide/aluminum oxyhydroxide heterostructure on nickel foam (denoted as Co(OH)2/AlOOH/NF-100) has been fabricated using one step hydrothermal process. Theoretical calculation and experimental results indicate the electrons transfer from Co(OH)2 to highly active AlOOH results in the interfacial charge redistribution and optimization of electronic structure. Abundant oxygen vacancies in the heterostructure could improve the conductivity and simultaneously serve as the active sites for catalytic reaction. Consequently, the optimal Co(OH)2/AlOOH/NF-100 demonstrates excellent electrocatalytic performance for HER (62.9 mV@10 mA cm-2) and UOR (1.36 V@10 mA cm-2) due to the synergy between heterointerface and oxygen vacancies. Additionally, the in situ electrochemical impedance spectrum (EIS) for UOR suggests that the heterostructured catalyst exhibits rapid reaction kinetics, mass transfer and current response. Importantly, the urea-assisted electrolysis composed of the Co(OH)2/AlOOH/NF-100 manifests a low cell voltage (1.48 V @ 10 mA cm-2) in 1 M KOH containing 0.5 M urea. This work presents a promising avenue to the development of HER/UOR bifunctional electrocatalysts.

Facile synthesis of CeO2-decorated W@Co-MOF heterostructures as a highly active and durable electrocatalyst for overall water splitting

Rational coupling of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts is extremely important for practical overall water splitting; however, it is still challenging to construct such bifunctional heterostructures. Herein, a CeO2/W@Co-MOF/NF bifunctional electrocatalyst was prepared via a two-step in situ growth method involving an electrodeposition process. The incorporation of the W element enhanced the electronic interaction and enlarged the electrochemical surface area. After the electrodeposition of CeO2, the obtained CeO2/W@Co-MOF/NF possessed abundant heterointerfaces with a modulated local distribution, which promoted water dissociation and rapid electrocatalytic kinetics. In particular, it required very low overpotentials of 239 mV and 87 mV to reach a current density of 10 mA cm-2 in OER and HER, respectively. A corresponding alkaline electrolysis cell afforded a cell voltage of 1.54 V at 10 mA cm-2 to boost overall water splitting. This work provides a feasible strategy to fabricate MOF-based complexes and explores their possible use as bifunctional catalysts toward overall water splitting.

An advanced PdNPs@MoS2 nanocomposite for efficient oxygen evolution reaction in alkaline media

In response to the increasing availability of hydrogen energy and renewable energy sources, molybdenum disulfide (MoS2)-based electrocatalysts are becoming increasingly important for efficient electrochemical water splitting. This study involves the incorporation of palladium nanoparticles (PdNPs) into hydrothermally grown MoS2via a UV light assisted process to afford PdNPs@MoS2 as an alternative electrocatalyst for efficient energy storage and conversion. Various analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS), were used to investigate the morphology, crystal quality, and chemical composition of the samples. Although PdNPs did not alter the MoS2 morphology, oxygen evolution reaction (OER) activity was driven at considerable overpotential. When electrochemical water splitting was performed in 1.0 M KOH aqueous solution with PdNPs@MoS2 (sample-2), an overpotential of 253 mV was observed. Furthermore, OER performance was highly favorable through rapid reaction kinetics and a low Tafel slope of 59 mV dec-1, as well as high durability and stability. In accordance with the electrochemical results, sample-2 showed also a lower charge transfer resistance, which again provided evidence of OER activity. The enhanced OER activity was attributed to a number of factors, including structural, surface chemical compositions, and synergistic effects between MoS2 and PdNPs.

Metal-Oxides- and Metal-Oxyhydroxides-Based Nanocomposites for Water Splitting: An Overview

Water electrolysis is an important alternative technology for large-scale hydrogen production to facilitate the development of green energy technology. As such, many efforts have been devoted over the past three decades to producing novel electrocatalysis with strong electrochemical (EC) performance using inexpensive electrocatalysts. Transition metal oxyhydroxide (OxH)-based electrocatalysts have received substantial interest, and prominent results have been achieved for the hydrogen evolution reaction (HER) under alkaline conditions. Herein, the extensive research focusing on the discussion of OxH-based electrocatalysts is comprehensively highlighted. The general forms of the water-splitting mechanism are described to provide a profound understanding of the mechanism, and their scaling relation activities for OxH electrode materials are given. This paper summarizes the current developments on the EC performance of transition metal OxHs, rare metal OxHs, polymers, and MXene-supported OxH-based electrocatalysts. Additionally, an outline of the suggested HER, OER, and water-splitting processes on transition metal OxH-based electrocatalysts, their primary applications, existing problems, and their EC performance prospects are discussed. Furthermore, this review article discusses the production of energy sources from the proton and electron transfer processes. The highlighted electrocatalysts have received substantial interest to boost the synergetic electrochemical effects to improve the economy of the use of hydrogen, which is one of best ways to fulfill the global energy requirements and address environmental crises. This article also provides useful information regarding the development of OxH electrodes with a hierarchical nanostructure for the water-splitting reaction. Finally, the challenges with the reaction and perspectives for the future development of OxH are elaborated.

Enhancing the bifunctional activity of CoSe2 nanocubes by surface decoration of CeO2 for advanced zinc-air batteries

The coupling of oxygen evolution and reduction reactions (OER and ORR) plays a key role in rechargeable Zn-air batteries (ZABs). However, both OER and ORR still suffer from sluggish kinetics, even when using the mainstream precious metal-based catalysts. Herein, oxygen vacancies-rich CeO₂ decorated CoSe₂ nanocubes are proposed as a novel air electrode to drive OER and ORR for ZABs. The resultant CeO₂ coupled CoSe₂ nanocubes (CeO₂@CoSe₂-NCs) catalyst exhibits a significantly enhanced bifunctional activity relative to the pristine CoSe₂-NCs and the pristine CeO₂. Moreover, an assembled ZABs using this CeO₂@CoSe₂-NCs electrode delivers a high output power density of 153 mW cm^-2 and a long-life stability over 400 cycles, superior to the benchmark Pt/C-IrO₂ electrode. Theoretical calculations reveal that the electronic interaction and oxygen vacancies in CeO₂@CoSe₂-NCs contribute to efficient oxygen electrocatalysis. This protocol provides a promising approach of constructing oxygen vacancies in hybrid catalysts for energy conversion and storage devices.