A simple strategy to construct cobalt oxide-based high-efficiency electrocatalysts with oxygen vacancies and heterojunctions

Recent Advances in Co-Based Electrocatalysts for Hydrogen Evolution Reaction

Water splitting is a promising technique in the sustainable "green hydrogen" generation to meet energy demands of modern society. Its industrial application is heavily dependent on the development of novel catalysts with high performance and low cost for hydrogen evolution reaction (HER). As a typical non-precious metal, cobalt-based catalysts have gained tremendous attention in recent years and shown a great prospect of commercialization. However, the complexity of the composition and structure of newly-developed Co-based catalysts make it urgent to comprehensively retrospect and summarize their advance and design strategies. Hence, in this review, the reaction mechanism of HER is first introduced and the possible role of the Co component during electrocatalysis is discussed. Then, various design strategies that could effectively enhance the intrinsic activity are summarized, including surface vacancy engineering, heteroatom doping, phase engineering, facet regulation, heterostructure construction, and the support effect. The recent progress of the advanced Co-based HER electrocatalysts is discussed, emphasizing that the application of the above design strategies can significantly improve performance by regulating the electronic structure and optimizing the binding energy to the crucial intermediates. At last, the prospects and challenges of Co-based catalysts are shown according to the viewpoint from fundamental explorations to industrial applications.

Multifunctional Ni3S2@NF-based electrocatalysts for efficient and durable electrocatalytic water splitting

Transition-metal sulfides (TMSs) have indeed drawn dramatic interest as a potential species of electrocatalysts by virtue of their unique structural features. However, their poor stability and inherent activity have impeded their use in electrocatalytic water splitting. Here, we provide a rational design of a hierarchical nanostructured electrocatalyst containing CeOx-decorated NiCo-layered double hydroxide (LDH) coupled with Ni3S2 protrusions formed on a Ni foam (NF). Specifically, the as-prepared electrocatalyst, denoted as Ni2Co1 LDH-CeOx/Ni3S2@NF, presents only 250 and 300 mV overpotential at ±100 mA cm-2, respectively, along with the Tafel slope values of 92 and 52 mV dec-1, as well remarkable long-term life for water splitting in an alkaline electrolyte. Based on systematic experiments and theoretical analysis, the superior electrocatalytic property in terms of Ni2Co1 LDH-CeOx/Ni3S2@NF can be imputed to the following reasons: the porous framework of Ni3S2@NF provides a largely surface area and high conductivity; the NiCo LDH nanosheets provide enriched active sites and favorable adsorption ability; the oxygen-vacancy-rich CeOx optimizes the electronic configuration. Overall, these factors work synergistically to expedite the catalytic kinetics of splitting water. Our work concentrates on a rational interface to devise efficient, multifunctional, and serviceable electrocatalysts for future applications.

Enhanced Li-Ion Diffusion and Cycling Stability of Ni-Free High-Entropy Spinel Oxide Anodes with High-Concentration Oxygen Vacancies

High-entropy oxide (HEO) is an emerging type of anode material for lithium-ion batteries with excellent properties, where high-concentration oxygen vacancies can effectively enhance the diffusion coefficient of lithium ions. In this study, Ni-free spinel-type HEOs ((FeCoCrMnZn)₃O₄ and (FeCoCrMnMg)₃O₄) were prepared via ball milling, and the effects of zinc and magnesium on the concentration of oxygen vacancy (O_V), lithium-ion diffusion coefficient (D_Li⁺), and electrochemical performance of HEOs were investigated. Ab initio calculations show that the addition of zinc narrows down the band gap and thus improves the electrical conductivity. X-ray photoelectron spectroscopy (XPS) results show that (FeCoCrMnZn)₃O₄ (42.7%) and (FeCoCrMnMg)₃O₄ (42.5%) have high O_V concentration. During charge/discharge, the O_V concentration of (FeCoCrMnZn)₃O₄ is higher than that of (FeCoCrMnMg)₃O₄. The galvanostatic intermittent titration technique (GITT) results show that the D_Li⁺ value of (FeCoCrMnZn)₃O₄ is higher than that of (FeCoCrMnMg)₃O₄ during charge and discharge. All of that can improve its specific discharge capacity and enhance its cycle stability. (FeCoCrMnZn)₃O₄ achieved a discharge capacity of 828.6 mAh g^-1 at 2.0 A g^-1 after 2000 cycles. This work provides a deep understanding of the structure and performance of HEO.

Recent Progress on Bimetallic-Based Spinels as Electrocatalysts for the Oxygen Evolution Reaction

Electrocatalytic water splitting is a promising and viable technology to produce clean, sustainable, and storable hydrogen as an energy carrier. However, to meet the ever-increasing global energy demand, it is imperative to develop high-performance non-precious metal-based electrocatalysts for the oxygen evolution reaction (OER), as the OER is considered the bottleneck for electrocatalytic water splitting. Spinels, in particular, are considered promising OER electrocatalysts due to their unique properties, precise structures, and compositions. Herein, the recent progress on the application of bimetallic-based spinels (AFe₂ O₄ , ACo₂ O₄ , and AMn₂ O₄ ; where A = Ni, Co, Cu, Mn, and Zn) as electrocatalysts for the OER is presented. The fundamental concepts of the OER are highlighted after which the family of spinels, their general formula, and classifications are introduced. This is followed by an overview of the various classifications of bimetallic-based spinels and their recent developments and applications as OER electrocatalysts, with special emphasis on enhancing strategies that have been formulated to improve the OER performance of these spinels. In conclusion, this review summarizes all studies mentioned therein and provides the challenges and future perspectives for bimetallic-based spinel OER electrocatalysts.

Solution Combustion Synthesis of Novel S,B-Codoped CoFe Oxyhydroxides for the Oxygen Evolution Reaction in Saline Water

Green hydrogen presents itself as a clean energy vector, which can be produced by electrolysis of water by utilizing renewable energy such as solar or wind. While current technologies are sufficient to support commercial deployment of fresh water electrolyzers, there remain a few well-defined challenges in the path of commercializing direct seawater electrolyzers, predominantly related to the sluggish oxygen evolution reaction (OER) kinetics and the competing chlorine evolution reaction (CER) at the anode. Herein, we report the facile and swift fabrication of an S,B-codoped CoFe oxyhydroxide via solution combustion synthesis for the OER with apparent CER suppression abilities. The as-prepared S,B-(CoFe)OOH-H attained ultralow overpotentials of 161 and 278 mV for achieving current densities of 10 and 1000 mA cm^-2, respectively, in an alkaline saline (1 M KOH + 0.5 M NaCl) electrolyte, with a low Tafel slope of 46.7 mV dec^-1. Chronoamperometry testing of the codoped bimetallic oxyhydroxides showed very stable behavior in harsh alkaline saline and in neutral pH saline environments. S,B-(CoFe)OOH-H oxyhydroxide showed a notable decrease in CER production in comparison to the other S,B-codoped counterparts. Selectivity measurements through online FE calculations showed high OER selectivity in alkaline (FE ∼ 97%) and neutral (FE ∼ 91%) pH saline conditions under standard 10 mA cm^-2 operation. Moreover, systematic testing in electrolytes at pH values of 14 to 7 yielded promising results, thus bringing direct seawater electrolysis at near-neutral pH conditions closer to realization.

Constructing oxygen vacancy-enriched Fe2O3@NiO heterojunctions for highly efficient electrocatalytic alkaline water splitting

The oxygen vacancy-enriched Fe2O3@NiO heterojunctions assembled by nanoparticles and nanosheets can be used as a highly efficient and stable dual-function electrocatalyst to achieve efficient all-water splitting.

Molybdenum and cobalt co-doped VC nanoparticles encapsulated in nanocarbon as efficient electrocatalysts for the hydrogen evolution reaction

Molybdenum and cobalt co-doped VC nanoparticles encapsulated in nanocarbon (Mo, Co-VC@C) are successfully synthesized via a one-pot calcination route, exhibiting excellent electrocatalytic HER performance.

Promoting the Phosphidation Process using an Oxygen Vacancy Precursor for Efficient Hydrogen Evolution Reaction

Based on previous works, most of the transition metal phosphides (TMPs) were directly prepared by decomposing NaH₂ PO₂ with the precursors at high temperatures, which resulted in different degrees of phosphidation in the final product. Therefore, it is necessary to design an innovative approach to enhance the degree of phosphidation in the material using crystal defects. Here, oxygen-vacancy iron oxide/iron foam (Ov-Fe₂ O₃ /IF) was firstly prepared by generating oxygen vacancy in situ in an iron foam through heating in vacuum conditions. Subsequently, FeP/IF was formed by phosphating Ov-Fe₂ O₃ /IF. Under the effects of oxygen vacancies, oxygen-vacancy iron oxide could be completely phosphatized to produce more active sites on the surface of the material. This, in turn, could result in a catalyst with exceptional hydrogen evolution activity. Thus, the successful fabrication of FeP/IF demonstrated in this work provides an effective and feasible way for the preparation of other high-efficiency catalysts.

Interfacial Engineering of a MoO2-CeF3 Heterostructure as a High-Performance Hydrogen Evolution Reaction Catalyst in Both Alkaline and Acidic Solutions

Exploring an efficient and pollution-free hydrogen evolution reaction (HER) electrocatalyst based on the combination of rare-earth metal and nonnoble metal is of significant importance. However, successfully achieving such a goal remains highly challenging. Herein, a nanosheet comprising a MoO₂-CeF₃ heterojunction (MoO₂-CeF₃/NF) is successfully prepared via a three-step method. (1) Growth of hexahedral nickel hydroxide [Ni(OH)₂] on a 3D nickel foam (NF) as the scaffold. (2) In situ hydrothermal growth of a precursor nanosheet structure on the scaffold. (3) Calcination treatment at 450 °C in the presence of hydrogen. Herein, the electron redistribution at the heterointerface of CeF₃ and MoO₂ is a contributing factor toward enhanced HER activity. Appropriate introduction of CeF₃ can enlarge the size of nanosheets, increase numerous active sites, increase the catalytic durability of the material, and change electron distribution on the MoO₂ interface; all of the above improve HER activity. Because of its interfacial nanosheet structure, MoO₂-CeF₃/NF demonstrates pre-eminent HER capability in both alkaline (1.0 M KOH) and acidic (0.5 M H₂SO₄) electrolytes, with extremely small overpotentials of 18 and 42 mV at 10 mA cm^-2, respectively. This is obviously lower than the overpotential of Pt/C in alkaline media (27 mV), and it is also close to the overpotential of Pt/C in acidic media (41 mV), at the same current density_. More importantly, MoO₂-CeF₃/NF displays a better HER activity than Pt/C at a current density of >112 mA cm^-2 in both alkaline and acidic electrolytes. This work offers a novel strategy toward high-performance hydrogen production by designing a transition metal oxide and rare-earth metal heterojunction.