Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction

Gold nanodot assembly within a cobalt chalcogenide nanoshell: Promotion of electrocatalytic activity

The assembly of functional nanoparticles within materials with unique architectures can improve the interfacial surfaces, defects, and active sites, which are key factors for the designing novel nanocatalysts. Nano metal-organic framework (NMOF) can be employed to fabricate nanodots-confined nanohybrids for use in electrocatalytic processes. Herein, we report a controlled synthesis of gold nanodot assembly within cobalt chalcogenide nanoshell (dots-in-shell Au/Co_xS_y nanohybrids). A cobalt-based NMOF (the cobalt-based zeolite imidazole framework, ZIF-67) is used as a versatile sacrificial template to yield dots-in-shell Au/Co_xS_y nanohybrids. Due to the synergistic effect of the well-dispersed Au nanodots and the thin Co_xS_y nanoshell, the obtained dots-in-shell Au/Co_xS_y nanohybrids exhibit enhanced performance for the oxygen evolution reaction (OER) with low overpotential values at a current density of 10 mA cm^-2 and a small Tafel slope (343 mV and 62 mV dec^-1, respectively).

Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction

Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large‐scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half‐reaction. Co‐, Ni‐, and Fe‐based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co‐, Ni‐, and Fe‐based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis.

NixCu1−x/CuO/Ni(OH)2 as highly active and stable electrocatalysts for oxygen evolution reaction

Ni–Cu alloy-based nanomaterials are representative cost-effective materials that have been widely used as highly active and stable electrocatalysts for electrochemical energy applications, such as the water oxidation reaction, the methanol/ethanol reaction and many other small molecule oxidation reactions.

Density Functional Theory Study of NiFeCo Trinary Oxy-Hydroxides for an Efficient and Stable Oxygen Evolution Reaction Catalyst

NiOOH and its doped species are widely used as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. In this work, we carried out comprehensive density functional theory (DFT) simulations of Ni-based electrocatalysts for the OER by applying suitable dopants in β-NiOOH. A range of Fe and Co atoms (%) are employed as doping agents to increase the overall catalytic ability, stability, and feasibility of NiOOH. Our simulations indicate that Ni_88%Fe_6%Co_6%OOH is efficient, stable, and provides more catalytic sites at the surface of resulting catalysts for water adsorption and dissociation, which facilitate the OER. The lower overpotential for the OER is estimated from the higher adsorption energy of water molecule over the surface of Ni_88%Fe_6%Co_6%OOH, followed by other electronic properties such as band structure, electrostatic potential, the density of states, and surface formation energy.

Dynamic active-site generation of atomic iridium stabilized on nanoporous metal phosphides for water oxidation

Designing efficient single-atom catalysts (SACs) for oxygen evolution reaction (OER) is critical for water-splitting. However, the self-reconstruction of isolated active sites during OER not only influences the catalytic activity, but also limits the understanding of structure-property relationships. Here, we utilize a self-reconstruction strategy to prepare a SAC with isolated iridium anchored on oxyhydroxides, which exhibits high catalytic OER performance with low overpotential and small Tafel slope, superior to the IrO₂. Operando X-ray absorption spectroscopy studies in combination with theory calculations indicate that the isolated iridium sites undergo a deprotonation process to form the multiple active sites during OER, promoting the O-O coupling. The isolated iridium sites are revealed to remain dispersed due to the support effect during OER. This work not only affords the rational design strategy of OER SACs at the atomic scale, but also provides the fundamental insights of the operando OER mechanism for highly active OER SACs.

Shaping well-defined noble-metal-based nanostructures for fabricating high-performance electrocatalysts: advances and perspectives

This review highlights recent advances in shaping protocols and structure-activity relationships of noble-metal-based catalysts with well-defined nanostructures in electrochemical reactions.

One-step synthesis of bifunctional iron-doped manganese oxide nanorods for rechargeable zinc–air batteries

Iron doped MnO₂ nanorods are successfully synthesized via one step hydrothermal method. The nanorods shows remarkable high bifunctional electrocatalytic activity for oxygen reduction as well as oxygen evolution reaction. For practical applications, a solid-state zinc–air battery was made for powering a light emitting diode.

MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

The oxygen evolution reaction (OER) and chlorine evolution reaction (CER) are electrochemical processes with high relevance to water splitting for (solar) energy conversion and industrial production of commodity chemicals, respectively. Carrying out the two reactions separately is challenging, since the catalytic intermediates are linked by scaling relations. Optimizing the efficiency of OER over CER in acidic media has proven especially difficult. In this regard, we have investigated the OER versus CER selectivity of manganese oxide (MnO_x), a known OER catalyst. Thin films (∼5–20 nm) of MnO_x were electrodeposited on glassy carbon-supported hydrous iridium oxide (IrO_x/GC) in aqueous chloride solutions of pH ∼0.9. Using rotating ring–disk electrode voltammetry and online electrochemical mass spectrometry, it was found that deposition of MnO_x onto IrO_x decreases the CER selectivity of the system in the presence of 30 mM Cl^– from 86% to less than 7%, making it a highly OER-selective catalyst. Detailed studies of the CER mechanism and ex-situ structure studies using SEM, TEM, and XPS suggest that the MnO_x film is in fact not a catalytically active phase, but functions as a permeable overlayer that disfavors the transport of chloride ions.

Recent Progresses in Electrocatalysts for Water Electrolysis

Abstract

The study of hydrogen evolution reaction and oxygen evolution reaction electrocatalysts for water electrolysis is a developing field in which noble metal-based materials are commonly used. However, the associated high cost and low abundance of noble metals limit their practical application. Non-noble metal catalysts, aside from being inexpensive, highly abundant and environmental friendly, can possess high electrical conductivity, good structural tunability and comparable electrocatalytic performances to state-of-the-art noble metals, particularly in alkaline media, making them desirable candidates to reduce or replace noble metals as promising electrocatalysts for water electrolysis. This article will review and provide an overview of the fundamental knowledge related to water electrolysis with a focus on the development and progress of non-noble metal-based electrocatalysts in alkaline, polymer exchange membrane and solid oxide electrolysis. A critical analysis of the various catalysts currently available is also provided with discussions on current challenges and future perspectives. In addition, to facilitate future research and development, several possible research directions to overcome these challenges are provided in this article.

Graphical Abstract

Core-Shell Au@Metal-Oxide Nanoparticle Electrocatalysts for Enhanced Oxygen Evolution

Enhanced catalysis for electrochemical oxygen evolution is essential for the efficacy of many renewable energy technologies, including water electrolyzers and metal-air batteries. Recently, Au supports have been shown to enhance the activity of many 3d transition metal-oxide thin films for the oxygen evolution reaction (OER) in alkaline media. Herein, we translate the beneficial impact of Au supports to high surface area, device-ready core-shell nanoparticles consisting of a Au-core and a metal-oxide shell (Au@M_xO_y where M = Ni, Co, Fe, and CoFe). Through a systematic evaluation, we establish trends in performance and illustrate the universal activity enhancement when employing the Au-core in the 3d transition metal-oxide nanoparticles. The highest activity particles, Au@CoFeO_x, demonstrate an overpotential of 328 ± 3 mV over a 2 h stability test at 10 mA cm^-2, illustrating that strategically coupling Au support and mixed metal-oxide effects in a core-shell nanoparticle morphology is a promising avenue to achieve device-ready, high-performance OER catalysts.

Enhanced Oxygen Evolution Reaction Electrocatalysis via Electrodeposited Amorphous α-Phase Nickel-Cobalt Hydroxide Nanodendrite Forests

We demonstrate an electrodeposition method to rapidly grow novel three-dimensional nanodendrite forests of amorphous α-phase mixed nickel-cobalt hydroxides on stainless steel foil toward high performance electrocatalysis of the oxygen evolution reaction (OER). The proposed hydrogen bubble-templated, diffusion-limited deposition process leads to the unprecedented dendritic growth of vertically aligned amorphous metal hydroxides, induced by the controlled electrolysis of the tuned water content in the primarily alcohol-based deposition solution. The hierarchical nature of these binder-free, amorphous metal hydroxide deposits leads to their superhydrophilic nature and underwater superaerophobic behavior. The combination of all of these qualities leads to exemplary catalytic performance. When directly grown on planar stainless steel substrates, these nanoforests show high OER activity with overpotentials as low as ∼255 mV to produce a current density of 10 mA cm^-2 over 10 000 accelerated stability test cycles. This work demonstrates a novel fabrication technique that can simultaneously achieve a dendritic hierarchical structure, vertical alignment, superaerophobicity, amorphous crystal structure, and intimate contact with the substrate that leads to high catalytic activity with excellent durability.

Systematic Identification of Promoters for Methane Oxidation Catalysts Using Size- and Composition-Controlled Pd-Based Bimetallic Nanocrystals

Promoters enhance the performance of catalytic active phases by increasing rates, stability, and/or selectivity. The process of identifying promoters is in most cases empirical and relies on testing a broad range of catalysts prepared with the random deposition of active and promoter phases, typically with no fine control over their localization. This issue is particularly relevant in supported bimetallic systems, where two metals are codeposited onto high-surface area materials. We here report the use of colloidal bimetallic nanocrystals to produce catalysts where the active and promoter phases are colocalized to a fine extent. This strategy enables a systematic approach to study the promotional effects of several transition metals on palladium catalysts for methane oxidation. In order to achieve these goals, we demonstrate a single synthetic protocol to obtain uniform palladium-based bimetallic nanocrystals (PdM, M = V, Mn, Fe, Co, Ni, Zn, Sn, and potentially extendable to other metal combinations) with a wide variety of compositions and sizes based on high-temperature thermal decomposition of readily available precursors. Once the nanocrystals are supported onto oxide materials, thermal treatments in air cause segregation of the base metal oxide phase in close proximity to the Pd phase. We demonstrate that some metals (Fe, Co, and Sn) inhibit the sintering of the active Pd metal phase, while others (Ni and Zn) increase its intrinsic activity compared to a monometallic Pd catalyst. This procedure can be generalized to systematically investigate the promotional effects of metal and metal oxide phases for a variety of active metal-promoter combinations and catalytic reactions.

Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering

Recent advances in power generation from renewable resources necessitate conversion of electricity to chemicals and fuels in an efficient manner. Electrocatalytic water splitting is one of the most powerful and widespread technologies. The development of highly efficient, inexpensive, flexible, and versatile water electrolysis devices is desired. This review discusses the significance and impact of the electrolyte on electrocatalytic performance. Depending on the circumstances under which the water splitting reaction is conducted, the required solution conditions, such as the identity and molarity of ions, may significantly differ. Quantitative understanding of such electrolyte properties on electrolysis performance is effective to facilitate the development of efficient electrocatalytic systems. The electrolyte can directly participate in reaction schemes (kinetics), affect electrode stability, and/or indirectly impact the performance by influencing the concentration overpotential (mass transport). This review aims to guide fine-tuning of the electrolyte properties, or electrolyte engineering, for (photo)electrochemical water splitting reactions.

Three-dimensional ordered mesoporous Co3O4 enhanced by Pd for oxygen evolution reaction

Considerable efforts have been devoted recently to design and fabrication of high performance and low cost electrocatalysts for oxygen evolution reaction (OER). However, catalytic activity of current electrocatalysts is usually restricted by high onset potential and limited active sites. Herein, we fabricated three-dimensional (3D) highly ordered mesoporous Pd-Co₃O₄ composite materials as excellent electrocatalysts for OER in alkaline solution with high activity and stability. Three-dimensional highly ordered mesoporous Co₃O₄ material was firstly synthesized using mesoporous silica KIT-6 as hard template. Then, Pd-Co₃O₄ nanomaterials were prepared by a simple reduction method. The as-prepared 3D mesoporous Pd-Co₃O₄ catalysts have ordered mesoporous structure with a high surface area of 81.0 m² g^-1. Three-dimensional highly ordered mesoporous structure can facilitate diffusion and penetration of electrolyte and oxygen. Moreover, the catalysts can also keep catalyst particles in a well dispersed condition with more catalytic active sites. Electrochemical measurements reveal that the 3D mesoporous Pd-Co₃O₄ catalysts exhibit superior performance in alkaline solution with low onset potential (0.415 V vs. SCE) and excellent long-duration cycling stability.

Edge reactivity and water-assisted dissociation on cobalt oxide nanoislands

Transition metal oxides show great promise as Earth-abundant catalysts for the oxygen evolution reaction in electrochemical water splitting. However, progress in the development of highly active oxide nanostructures is hampered by a lack of knowledge of the location and nature of the active sites. Here we show, through atom-resolved scanning tunnelling microscopy, X-ray spectroscopy and computational modelling, how hydroxyls form from water dissociation at under coordinated cobalt edge sites of cobalt oxide nanoislands. Surprisingly, we find that an additional water molecule acts to promote all the elementary steps of the dissociation process and subsequent hydrogen migration, revealing the important assisting role of a water molecule in its own dissociation process on a metal oxide. Inspired by the experimental findings, we theoretically model the oxygen evolution reaction activity of cobalt oxide nanoislands and show that the nanoparticle metal edges also display favourable adsorption energetics for water oxidation under electrochemical conditions.

Direct Growth of MoS2 Microspheres on Ni Foam as a Hybrid Nanocomposite Efficient for Oxygen Evolution Reaction

MoS2 microspheres are directly grown on the conductive Ni foam and the as-obtained MoS2 -Ni electrodes exhibit highly efficient electrocatalytic performances for oxygen evolution in 1.0 m NaOH, displaying a rather low overpotential of 0.310 V at 20.0 mA cm(-2) , a high current density, good cyclic stability, and excellent flexibility.

Electrochemical synthesis of a nanohybrid film consisting of stacked graphene sheets and manganese oxide as oxygen evolution reaction catalyst

A new electrochemical process yielded a thin film consisting of stacked graphene sheets and manganese oxide effective for oxygen evolution reaction.