Iron-induced 3D nanoporous iron-cobalt oxyhydroxide on carbon cloth as a highly efficient electrode for oxygen evolution reaction

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.

Ligand Modulation of Active Sites to Promote Electrocatalytic Oxygen Evolution

Rationally designed catalysts hold the key to address the sluggish kinetics of oxygen evolution reaction (OER). However, engineering the active sites of such catalysts still faces grand challenges. This study proposes a feasible ligand modulation strategy to boost the OER catalytic activity of cobalt-iron oxyhydroxide ((Fe,Co)OOH). The 2-methylimidazole (MI) ligand coordination on (Fe,Co)OOH reduces the orbital overlap between the Fe/Co 3d and O 2p, which weakens the adsorption to oxygen-containing intermediates and thus facilitates the unfavorable O₂ desorption. As a result, the MI ligand modulated (Fe,Co)OOH achieves an excellent OER performance with low overpotentials (230/290 mV at 10/100 mA cm^-2 ) and excellent durability (>155 h). This study provides a novel ligand modulation strategy for the design of OER catalysts.

The Effect of Fe Dopant Location in Co(Fe)OOHx Nanoparticles for the Oxygen Evolution Reaction

The addition of iron (Fe) can in certain cases have a strong positive effect on the activity of cobalt and nickel oxide nanoparticles in the electrocatalytic oxygen evolution reaction (OER). The reported optimal Fe dopant concentrations are, however, inconsistent, and the origin of the increased activity due to Fe dopants in mixed oxides has not been identified so far. Here, we combine density functional theory calculations, scanning tunneling microscopy, and OER activity measurements on atomically defined Fe-doped Co oxyhydroxide nanoparticles supported on a gold surface to establish the link between the activity and the Fe distribution and concentration within the oxyhydroxide phase. We find that addition of Fe results in distinct effects depending on its location on edge or basal plane sites of the oxyhydroxide nanoparticles, resulting in a nonlinear OER activity as a function of Fe content. Fe atom substitution itself does not lead to intrinsically more active OER sites than the best Co sites. Instead, the sensitivity to Fe promoter content is explained by the strong preference for Fe to locate on the most active edge sites of oxyhydroxide nanoparticles, which for low Fe concentrations stabilizes the particles but in higher concentrations leads to a shell structure with less active Fe on all edge positions. The optimal Fe content thereby becomes dependent on nanoparticle size. Our findings demonstrate that synthesis strategies that adjust not only the Fe concentration in mixed oxides but also its distribution within a catalyst nanoparticle can lead to enhanced OER performance.

Fe(Co)OOH Dynamically Stable Interface Based on Self-Sacrificial Reconstruction for Long-Term Electrochemical Water Oxidation

FeOOH on the real catalytic interface for the oxygen evolution reaction (OER) is chemically unstable to dissolve in alkaline media. Herein, based on the perspective of the dynamically stable interface, we purposely design the well-dispersed nanorod arrays of CoMoO₄ as a host on activated iron foam (IF) to realize the optimal redeposition of FeOOH, constructing a self-sacrificial template rich in the FeOOH surface. Notably, at long-time oxidation potential, the precatalyst FeOOH-CoMoO₄ can realize MoO₄^2- dissolution and redeposition of Co oxyhydroxides on FeOOH host simultaneously, constructing a dynamically stable Fe(Co)OOH interface. The introduction of CoOOH improves conductivity and provides synergistic effect with FeOOH to lower the energy barrier for OER and maintain long-time stability, eventually exhibiting a low overpotential of 298 mV to reach the current density of 100 mA cm^-2 and high stability over 60 h. This work demonstrates the feasibility of manipulating metal dissolution-redeposition process for a dynamically stable interface.

Fe-Induced electronic optimization of mesoporous Co–Ni oxide nanosheets as an efficient binder-free electrode for the oxygen evolution reaction

An efficient binder-free OER electrode CoNiFeO_x/NF with mesoporous structure was synthesized by a facile strategy of hydrothermal method and post-annealing.

Coupling Co2P/CoSe2 heterostructure nanoarrays for boosting overall water splitting

Exploiting environmentally friendly and robust electrocatalysts for overall water splitting is of utmost importance in order to alleviate the excessive global energy consumption and climate change. Herein, a simple phosphoselenization method was used to prepare Co2P and CoSe2 coupled nanosheet and nanoneedle composite materials on nickel foam (Co2P/CoSe2/NF). Density functional theory calculations showed that Co2P had a higher water adsorption energy compared with CoSe2, indicating that H2O molecules are strongly adsorbed on the active sites of Co2P, which speeds up the kinetic process of water splitting. The Co2P/CoSe2-300 material displayed superior electrocatalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline medium. It's worth noting that the Co2P/CoSe2-300 composite material nanoarrays merely needed an ultralow overpotential of 280 mV to drive a current intensity of 100 mA cm-2 for OER. In addition, when a two-electrode system was constructed for overall water splitting, the current intensity of 20 mA cm-2 could be reached while requiring an ultrasmall cell voltage of 1.52 V, which is one of the best catalytic activities reported up to now. Experimental and density functional theory calculations showed that the superior electrocatalytic performance of Co2P/CoSe2-300 could be attributed to its higher electron-transfer rate, higher water adsorption energy, and the synergistic effect of Co2P and CoSe2. Our work provides a novel approach for the one-step construction of composite materials as environmentally friendly and inexpensive water splitting catalysts.

Porous Fiber Processing and Manufacturing for Energy Storage Applications

The objective of this article is to provide an overview on the current development of micro- and nanoporous fiber processing and manufacturing technologies. Various methods for making micro- and nanoporous fibers including co-electrospinning, melt spinning, dry jet-wet quenching spinning, vapor deposition, template assisted deposition, electrochemical oxidization, and hydrothermal oxidization are presented. Comparison is made in terms of advantages and disadvantages of different routes for porous fiber processing. Characterization of the pore size, porosity, and specific area is introduced as well. Applications of porous fibers in various fields are discussed. The emphasis is put on their uses for energy storage components and devices including rechargeable batteries and supercapacitors.

Controlled phosphating: a novel strategy toward NiP3@CeO2 interface engineering for efficient oxygen evolution electrocatalysis

Although Ni phosphides are efficient for hydrogen evolution reactions, they are unfavorable for oxygen evolution reactions, so their application in alkaline water electrolysis is limited. It is a feasible method for creating a novel Ni phosphide/oxide heterogeneous interface to promote the oxygen evolution kinetics of Ni phosphide materials in an alkaline medium, yet it has been an unprecedented challenge for researchers. In this work, NiP3@CeO2 hybrid nanoparticles are firstly in situ grown on Ni foam (NiP3@CeO2/NF) via a novel controlled phosphating strategy. The NiP3@CeO2/NF catalysts display a fairly small overpotential of 200 mV to achieve a current density of 25 mA cm-2 for the oxygen evolution reaction (OER) under alkaline conditions, 110 mV smaller than that of NiO@CeO2/NF. It is noteworthy that the improved electrocatalytic performance of NiP3@CeO2/NF can be attributed to rapid electron transfer and the synergistic catalytic effect of the hybrid material. Density functional theory results demonstrate that NiP3 shows a stronger water adsorption energy than CeO2. The novel strategy of controlled phosphating to construct transition metal phosphide/oxide interfaces provides new ideas and methods for the development of efficient and practical water splitting catalysts.