A review of transition metal‐based bifunctional oxygen electrocatalysts

Electrochemical energy storage and conversion devices play a key role in the development of clean, sustainable, and efficient energy systems to meet the sustainable growth of our society. However, challenging issues including the sluggish kinetics of oxygen electrode reactions involving the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are present, limiting the implementation of devices such as metal‐air batteries, water electrolyzers, and regenerative fuel cells. In this review, various monometallic and bimetallic transition metal oxides (TMOs) and hydroxides are summarized in terms of their application for ORR/OER, in which the merits and demerits of various precious metal and carbon‐based metal oxide materials are discussed, with requirements for better electrocatalysts and catalyst support being introduced as well. Following this, different approaches to improve catalytic activity such as the introduction of doping and defects, the manipulation of crystal facets, and the engineering of supports, compositions, and morphologies are summarized in which TMOs with improved ORR/OER catalytic activities can be synthesized, further improving the speed, stability, and polarization of electrochemical energy storage and conversion devices. Finally, perspectives into the improvement of performance and the better understanding of ORR/OER mechanisms for bifunctional electrocatalysts using in situ spectroscopic techniques and density functional theory calculations are also discussed.     

Combinatorial electrochemical synthesis and characterization of tungsten-based mixed-metal oxides

Automated systems for electrochemical synthesis and high-throughput screening of photoelectrochemical materials were developed and used to prepare tungsten-based mixed-metal oxides, W(n)O(m)M(x) [M = Ni, Co, Cu, Zn, Pt, Ru, Rh, Pd, and Ag], specifically for hydrogen production by photoelectrolysis of water. Two-dimensional arrays (libraries) of diverse metal oxides were synthesized by automated cathodic electrodeposition of the oxides on Ti foil substrates. Electrolytes for the mixed oxides were prepared from various metal salts added to a solution containing tungsten stabilized as a peroxo complex. Electrodeposition of the peroxo-stabilized cations gave rise to three distinguishable oxide groups: (1) mixed-metal oxides [Ni], (2) metal-doped tungsten oxides [Pt, Ru, Rh, Pd, Ag], and (3) metal-metal oxide composites [Co, Cu, Zn]. The oxides typically showed n-type semiconducting behavior. Automated measurement of photocurrent using a scanning photoelectrochemical cell showed the W-Ni mixed oxide had the largest relative zero bias photocurrent, particularly at a low Ni concentration (5-10 atomic percent Ni). Pt and Ru were also found to increase the photoactivity of bulk tungsten oxide at relatively low concentrations; however, at concentrations above 5 atomic percent, crystallization of WO(3) was inhibited and photoactivity was diminished. Addition of Co, Cu, and Zn to WO(3) was not found to improve the photoelectrochemical activity.     

Enhancement of activity of PtRh nanoparticles towards oxidation of ethanol through modification with molybdenum oxide or tungsten oxide

Electrocatalytic systems utilizing carbon (Vulcan)-supported PtRh nanoparticles (PtRh/Vulcan) admixed with either molybdenum oxide or tungsten oxide were tested and compared during electrooxidation of ethanol. The systems' performance was diagnosed using electrochemical techniques such as voltammetry and chronoamperometry. The proposed electrocatalytic materials were also characterized with X-ray diffraction (XRD), transmission and scanning electron microscopies (TEM and SEM), as well as SEM-coupled energy dispersive X-ray spectroscopy (SEM-EDX). For both systems containing molybdenum and tungsten oxides, enhancements in catalytic activities (relative to the behavior observed at bare PtRh/Vulcan nanoparticles) were found during ethanol electrooxidation at room temperature (22?°C). Further, it was from chronoamperometric current (density)–time responses that anodic electrocatalytic currents measured at 0.3?V (vs. RHE) were more than 20% higher in the case of the MoO₃-containing PtRh/Vulcan system relative to that utilizing WO₃. The diagnostic “CO-stripping” experiments were consistent with the view that addition of molybdenum oxide or tungsten oxide to PtRh/Vulcan tended to shift potential for the oxidation of inhibiting CO-adsorbate ca. 80 or 40?mV towards less negative values in comparison to the analogous but oxide-free system. The fact that carbon (Vulcan)-supported PtRu nanoparticles exhibited higher electrocatalytic reactivity observed phenomena may be attributed to specific interactions between noble metal centers and the oxides in addition to chemical reactivity of metal oxo groups in the vicinity of PtRh/Vulcan at the electrocatalytic interface.     

缺陷位工程在二维材料电催化析氢反应中的研究进展

面对不可再生资源的快速消耗和环境污染的日益加重,寻找清洁可再生能源势在必行.氢能是一种清洁可再生的能源,是目前最有希望替代化石燃料的一种能源.电化学水分解可用来产生高纯氢气,其中析氢催化剂起着至关重要的作用.尽管贵金属铂基催化剂表现出优异的析氢性能,然而稀缺性和高成本限制了其大规模应用.因此,开发高效和地球存量丰富的电...     

Mixed Transition‐Metal Oxides: Design,Synthesis, and Energy‐Related Applications

A promising family of mixed transition‐metal oxides (MTMOs) (designated as A_xB_3‐xO₄; A, B=Co, Ni, Zn, Mn, Fe, etc.) with stoichiometric or even non‐stoichiometric compositions, typically in a spinel structure, has recently attracted increasing research interest worldwide. Benefiting from their remarkable electrochemical properties, these MTMOs will play significant roles for low‐cost and environmentally friendly energy storage/conversion technologies. In this Review, we summarize recent research advances in the rational design and efficient synthesis of MTMOs with controlled shapes, sizes, compositions, and micro‐/nanostructures, along with their applications as electrode materials for lithium‐ion batteries and electrochemical capacitors, and efficient electrocatalysts for the oxygen reduction reaction in metal–air batteries and fuel cells. Some future trends and prospects to further develop advanced MTMOs for next‐generation electrochemical energy storage/conversion systems are also presented.     

核壳结构金属-有机骨架及其衍生物在锂离子电池负极材料中的应用

金属–有机框架(MOFs)材料具有比表面积较大、孔径可调、制备容易、结构与功能多样性等优势,被广泛应用于电化学能源转化与储存领域。其中独特的核壳结构材料由于表面修饰的作用往往更能表现出核内与壳层的协同作用。本文介绍了具有核壳结构MOFs作为锂离子电池负极材料的发展现状,并重点综述其衍生物(多孔碳材料、金属氧化物、金属硫/硒化物以及金属/金属氧化物)的制备方法以及在锂离子电池负极中的应用。MOFs通过高温煅烧或改变化学反应条件的方法,可制备出结构可调的传统无机电极材料并表现出更优异的电化学性能。最后总结了核壳结构MOFs材料作为锂电负极材料存在的问题和挑战,并提出可能的解决途径和未来的应用前景。     

Nanoparticles as structural and functional units in surface-confined architectures

The nanoscale engineering of functional chemical assemblies has attracted recent research effort to provide dense information storage, miniaturized sensors, efficient energy conversion, light-harvesting, and mechanical motion. Functional nanoparticles exhibiting unique photonic, electronic and catalytic properties provide invaluable building blocks for such nanoengineered architectures. Metal nanoparticle arrays crosslinked by molecular receptor units on electrodes act as selective sensing interfaces with controlled porosity and tunable sensitivity. Photosensitizer/electron-acceptor bridged arrays of Au-nanoparticles on conductive supports act as photoelectrochemically active electrodes. Semiconductor nanoparticle composites on surfaces act as efficient light collecting systems, and nanoengineered semiconductor 'core-shell' nanocrystal assemblies reveal enhanced photoelectrochemical performance due to effective charge separation. Layered metal and semiconductor nanoparticle arrays crosslinked by nucleic acids find applications in the optical, electronic and photoelectrochemical detection of DNA. Metal and semiconductor nanoparticles assembled on DNA templates may be used to generate complex electronic circuitry. Nanoparticles incorporated in hydrogel matrices yield new composite materials with novel magnetic, optical and electronic properties.     

Recent Advances in Two-dimensional Materials for Electrochemical Energy Storage and Conversion

With the increased energy demand,developing renewable and clean energy technologies becomes more and more significant to mitigate climate warming and alleviate the environmental pollution.The key point is design and synthesis of low cost and efficient materials for a wide variety of electrochemical reactions.Over the past ten years,two-dimensional(2D)nanomaterials that graphene represents have been paid much attention as a class of the most promising candidates for heterogeneous electrocatalysts in electrochemical storage and conversion.Their unique properties,such as good chemical stability,good flexibility,and good electronic properties,along with their nanosized thickness and large specific area,make them exhibit comprehensively good performances for energy storage and conversion.Here,we present an overview on the recent advances in electrochemical applications of graphene,graphdiyne,transition metal dichalcogenides(TMDs),and MXenes for supercapacitors(SCs),oxygen reduction reaction(ORR),and hydrogen evolution reaction(HER).     

Flame spray pyrolysis for the one-step fabrication of transition metal oxide films: Recent progress in electrochemical and photoelectrochemical water splitting

Developing large scale deposition techniques to fabricate thin porous films with suitable opto-electro nic properties for water catalysis is a necessity to mitigate climate change and have a sustainable environment.In this review,flame spray pyrolysis(FSP)technique,a rapid and scalable methodology to synthesize nanostructured transitional metal oxide films with designed functionalities,is firstly introduced.Furthermore,applications in electrochemical(EC)and photoelectrochemical(PEC)water splitting for the production of hydrogen fuel is also presented.The high combustion temperature and the aggregation of flame aerosol ensure that the FSP-made films possess high crystallinity,tunable porosity and high surface areas,making this method suitable either as catalysts for EC water splitting or as efficient semiconductor materials for PEC water splitting.Finally,a perspective on the next generation FSP engineered films with potential applications in energy storage and conversion is described.     

First principles scheme to evaluate band edge positions in potential transition metal oxide photocatalysts and photoelectrodes

The positions of electronic band edges are one important metric for determining a material's capability to function in a solar energy conversion device that produces fuels from sunlight. In particular, the position of the valence band maximum (conduction band minimum) must lie lower (higher) in energy than the oxidation (reduction) reaction free energy in order for these reactions to be thermodynamically favorable. We present first principles quantum mechanics calculations of the band edge positions in five transition metal oxides and discuss the feasibility of using these materials in photoelectrochemical cells that produce fuels, including hydrogen, methane, methanol, and formic acid. The band gap center is determined within the framework of DFT+U theory. The valence band maximum (conduction band minimum) is found by subtracting (adding) half of the quasiparticle gap obtained from a non-self-consistent GW calculation. The calculations are validated against experimental data where possible; results for several materials including manganese(ii) oxide, iron(ii) oxide, iron(iii) oxide, copper(i) oxide and nickel(ii) oxide are presented.     

用于超级电容器的锰钴氧化物/碳纤维材料

The development of high-performance supercapacitor electrode materials is imperative to alleviate the ongoing energy crisis. Numerous transition metals (oxides) have been studied as electrode materials for supercapacitors owing to their low cost, environmental-friendliness, and excellent electrochemical performance. Among the developed binary transition metal oxides, manganese cobalt oxides typically show high theoretical capacitance and stable electrochemical performance, and are widely used in the electrode materials of supercapacitors. However, the poor conductivity and active material utilization of manganese cobalt oxide-based electrode materials limit their potential capacitance application. Cotton is mainly composed of organic carbon-containing materials, which can be transformed to carbon fibers after calcination. The resultant carbonaceous material exhibits a large specific surface area and good conductivity. Such advantages could potentially suppress the negative effects caused by the poor conductivity and small specific surface area of manganese cobalt oxides, thereby improving the electrochemical performance. Herein, we firstly deposited manganese cobalt oxides on cotton by a simple hydrothermal method, yielding a composite of manganese cobalt oxides and carbon fibers via subsequent calcination, to improve the electrochemical performance of the electrode material. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), thermogravimetric analysis (TGA), and electrochemical characterizations were used to investigate the physical, chemical, and electrochemical properties of the prepared samples. The fabricated manganese cobalt oxides in the composite were uniformly dispersed on the carbon fiber surface, which increased the contact between the interface of the electrode material and electrolyte, and enhanced electrode material utilization. The electrode material was confirmed to have well contacted with the electrolyte during a contact angle test. Hence, a pseudo-capacitance reaction completely occurred on the manganese cobalt oxide material. Moreover, the addition of carbon fibers reduced the resistance of the material, resulting in excellent capacitive performance. The capacitance of the prepared composite was 854 F∙g^-1 at a current density of 2 A∙g^-1. The capacitance was maintained at 72.3% after 2000 cycles at a current density of 2 A∙g^-1. These results indicate that the manganese cobalt oxide and carbon fiber composite is a promising electrode material for high-performance supercapacitors. The findings presented herein provide a strategy for coupling with carbon materials to enhance the performance of supercapacitor electrode materials based on manganese cobalt oxides. Thus, novel insights into the design of high-performance supercapacitors for energy management are provided.     

A general electrochemical approach to deposition of metal hydroxide/oxide nanostructures onto carbon nanotubes

This communication describes a new and relatively general electrochemical approach to the deposition of transition metal hydroxide/oxide nanostructures onto multi-walled carbon nanotubes (MWNTs) based on the precipitation of metal hydroxide/oxide nanostructures onto MWNTs by increasing the local pH values at the electrode/electrolyte interface induced by the proton-consuming electrochemical reduction of hydrogen peroxide (H₂O₂). The results obtained with cyclic voltammetry, scanning electron microscopy, and X-ray photoelectron spectroscopy of the synthetic nanocomposites substantially suggest the deposition of the metal hydroxides/oxides onto MWNTs induced by the electrochemical reduction of H₂O₂. This study essentially offers a facile but effective and relatively general electrochemical approach to the synthesis of the nanocomposites consisting of metal hydroxides/oxides and MWNTs.     

石墨烯基催化剂的设计合成与电催化应用

为了解决能源匮乏和环境污染的问题,研究人员正致力于寻找清洁可持续的新能源。 其中,氧气还原、氧气析出、析氢反应等是紧密联系新型清洁能源获取和存贮的重要电化学反应。 为了提高其能量转化效率,电催化剂(如碳载铂Pt/C)被广泛地用于降低其反应活化能、提高能量转化效率。 近年来,石墨烯作为一种具有高比表面积和优异导电性的二维碳材料受到了广泛关注。 通过表面杂原子掺杂、缺陷调控和引入催化活性组分等方式,获得了催化性能与贵金属催化剂相媲美,且低价格和高稳定性的非贵金属石墨烯基催化材料。 针对氧气还原、氧气析出和析氢反应在燃料电池、金属-空气电池和电催化水分解中的应用,本文概括综述了通过表/界面结构性质调控提高石墨烯电催化性能和稳定性,获得具有双功能或复合催化性能的石墨烯基催化剂的最新研究进展。 最后总结和展望了亟待解决的问题及未来的发展趋势。     

甲醇选择氧化金属氧化物催化剂的结构与其催化性能的关系

甲醇是一个重要的平台分子, 实现其高效转化为能源和化学品的关键是揭示相关反应过程中催化剂的结构与催化性能之间的关系和反应机理. 围绕这个关键问题, 以甲醇选择氧化为探针反应, 本文总结了负载氧化钼、负载氧化钒和杂多酸等典型催化剂体系以及近年来发展的氧化铼、氧化钌等新催化剂体系在认识催化活性中心结构和反应机理, 进而调控它们的氧化中心和酸中心等方面所取得的进展. 这些认识将有助于设计制备性能优异的新催化剂和实现甲醇到目标氧化产物的定向转化.     

纳米 Ce1MgxZr1-xO2 固体多相催化剂的制备、表征及催化合成四氢苯并[b]吡喃衍生物的性能

 A series of Ce1MgxZr1-xO2 mixed metal oxides with different molar ratios were prepared by simple co-precipitation and were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, temperature-pro- grammed desorption of CO2, and N2 adsorption techniques. The prepared materials were tested for catalytic activity by the synthesis of tet-rahydrobenzo[b]pyran derivatives using a three component reaction (aromatic aldehydes, malononitrile, and dimedone) in an ethanol me-dium. The best catalytic activity was obtained with Ce1Mg0.6Zr0.4O2. The particle size or crystallite size was estimated using the De-bye-Scherrer equation. The addition of magnesium oxide into the ceria-zirconia lattice resulted in the formation of nanosized particles rang-ing from 5.41 to 9.78 nm. This work describes the catalytic behavior of magnesium oxide in mixed metal oxide systems.     

Controlled Assembly of Cu/Co-Oxide Beaded Nanoclusters on Thiolated Graphene Oxide Nanosheets for High-Performance Oxygen Evolution Catalysts

The use of water splitting modules is highly desired for the sustainable production of H₂ as a future energy carrier. However, the sluggish kinetics and demand of high anodic potential are the bottlenecks for half-the cell oxygen evolution reaction (OER), which severely hamper the overall conversion efficiency. Although transition metal oxides based electrocatalysts have been envisioned as cost-effective and potential contenders for this quest, nevertheless, their low conductivity, instability, and limited number of active sites are among the common impediments that need to be addressed to eventually enhance their inherent catalytic potential for enhanced OER activity. Herein, the controlled assembly of transition metal oxides, that is, Cu@CuO_x nanoclusters (NCs, ≈2 nm) and Co@CoO_x beaded nanoclusters (BNCs, ≈2 nm), on thiol-functionalized graphene oxide (G-SH) nanosheets is reported to form novel and highly efficient electrocatalysts for OER. The thiol (-SH) functionality was incorporated by selective epoxidation on the surface of graphene oxide (GO) to achieve chemically exfoliated nanosheets to enhance its conductivity and trapping ability for metal oxides in nanoscale dimensions (≈2 nm). During the electrocatalytic reaction, overpotentials of 290 mV and 310 mV are required to achieve a current density of 10 mA cm⁻² for BNCs and NCs, respectively, and the catalysts exhibit tremendous long-term stability (≈50 h) in purified alkaline medium (1 m KOH) with no dissolution in the electrolyte. Moreover, the smaller Tafel slopes (54 mV/dec for BNCs and 66 mV/dec for NCs), and a Faradic efficiency of approximately 96 % indicate not only the selectivity but also the tailored heterogeneous electrons transfer (HET) rate, which is required for fast electrode kinetics. It is anticipated that such ultrasmall metal oxide nanoclusters and their controlled assembly on a conducting surface (G-SH) may offer high electrochemical accessibility and a plethora of active sites owing to the drastic decrease in dimensions and thus can synergistically ameliorate the challenging OER process.     

Phosphate Ion-Functionalized CoS with Hexagonal Bipyramid Structures from a Metal–Organic Framework: Bifunctionality towards Supercapacitors and Oxygen Evolution Reaction

To solve energy-related environmental problems and the energy crisis, efficient electrochemical materials have been developed as alternative energy storage and conversion systems. Abundant transition metals and their sulfides are attractive electrochemical materials. Herein, we report an efficient phosphorization strategy, which improves the overall electrochemical performance of metal sulfides. In detail, CoS hexagonal bipyramids were synthesized through simple calcination combined with in situ sulfurization of a cobalt-based metal–organic framework template, and then phosphate ion-functionalized CoS (P-CoS) was prepared through a phosphorization reaction. P-CoS exhibited outstanding electrochemical activity as both supercapacitor electrode and oxygen evolution reaction (OER) catalyst. Supercapacitors based on CoS and P-CoS as the electrodes had high specific capacitances of 304 and 442 F g⁻¹, respectively, and remained stable for over 10 000 cycles at 5 A g⁻¹. For OER, P-CoS showed a current density of 10 mA cm⁻² at an overpotential of 340 mV, with a small Tafel slope. In conclusion, functionalizing CoS with phosphate ions is a promising method for enhancing chemical reactivity and accelerating ion and electron transfer.     

Polymeric Heterogeneous Catalysts of Transition‐Metal Oxides: Surface Characterization,Physicomechanical Properties,and Catalytic Activity

We investigate the physicomechanical properties of polymeric heterogeneous catalysts of transition‐metal oxides, specifically, the specific surface area, elongation at break, breaking strength, specific electrical resistance, and volume resistivity. Digital microscopy, Fourier‐transform infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, and energy‐dispersive analysis are used to study the surfaces of the catalysts. The experimental results show that polymeric heterogeneous catalysts of transition‐metal oxides exhibit high stability and can maintain their catalytic activity under extreme reaction conditions for long‐term use. The oxidation mechanism of sulfur‐containing compounds in the presence of polymeric heterogeneous catalysts of transition‐metal oxides is confirmed. Microstructural characterization of the catalysts is performed by using X‐ray computed tomography. The activity of various catalysts in the oxidation of sulfur‐containing compounds is determined. We demonstrate the potential application of polymeric heterogeneous catalysts of transition‐metal oxides in industrial wastewater treatment.     

A general route for RuO2 deposition on metal oxides from RuO4

A novel method for the deposition of RuO(2) from RuO(4)(g) on diverse metal oxides has been developed by grafting dopamine onto the otherwise un-reactive metal oxide surface. Oxygen evolution reaction on TiO(2) and the photoelectrochemical improvement of WO(3) by deposition of RuO(2) are just a few examples where this novel deposition method can be used.     

Solubility phenomena in industrial and natural system

The importance of solubility phenomena in industrial and natural systems is illustrated mainly by examples related to nuclear energy development. Illustrative examples are given of the consequences of changes in solubility due to gradients of temperature (hydrothermal mass transport), chemical composition (hydrothermal alteration of minerals) electrochemical potential (corrosion) and mechanical stress (fracture mechanics). The chemical systems of interest are primarily insoluble oxides in high temperature water, for example: transition metal oxides on surfaces of structural metal alloys of power plant steam generators; actinide and other heavy metal oxides of nuclear fuels; silicate glasses for immobilizing fission product wastes, and minerals in the rocks of an underground waste vault in contact with heated ground water. Requirements for improved solubility measurement equipment and techniques, experimental data and theoretical methods are noted. Some recent solubility data and their shortcomings are outlined for specific examples, including UO₂, basic lead carbonates, potassium aluminosilicate (feldspars), three to five component borosilicate glasses and zirconium hydride (in Zr metal).Presentation to First International Symposium on Solubility Phenomena, University of Western Ontario, London, Ontario, August 21–23, 1984.