There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.

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Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

The applications of copper (Cu) and Cu-based nanoparticles, which are based on the earth-abundant and inexpensive copper metal, have generated a great deal of interest in recent years, especially in the field of catalysis. The possible modification of the chemical and physical properties of these nanoparticles using different synthetic strategies and conditions and/or via postsynthetic chemical treatments has been largely responsible for the rapid growth of interest in these nanomaterials and their applications in catalysis. In addition, the design and development of novel support and/or multimetallic systems (e.g., alloys, etc.) has also made significant contributions to the field. In this comprehensive review, we report different synthetic approaches to Cu and Cu-based nanoparticles (metallic copper, copper oxides, and hybrid copper nanostructures) and copper nanoparticles immobilized into or supported on various support materials (SiO2, magnetic support materials, etc.), along with their applications i...

http://pubs.acs.org/doi/pdfplus/10.1021/acs.chemrev.5b00482

Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis.

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

The selective electroreduction of carbon dioxide to C2 compounds (ethylene and ethanol) on copper(I) oxide films has been investigated at various electrochemical potentials. Aqueous 0.1 M KHCO3 was used as electrolyte. A remarkable finding is that the faradic yields of ethylene and ethanol can be systematically tuned by changing the thickness of the deposited overlayers. Films 1.7–3.6 μm thick exhibited the best selectivity for these C2 compounds at −0.99 V vs RHE, with faradic efficiencies (FE) of 34–39% for ethylene and 9–16% for ethanol. Less than 1% methane was formed. A high C2H4/CH4 products’ ratio of up to ∼100 could be achieved. Scanning electron microscopy, X-ray diffraction, and in situ Raman spectroscopy revealed that the Cu2O films reduced rapidly and remained as metallic Cu0 particles during the CO2 reduction. The selectivity trends exhibited by the catalysts during CO2 reduction in phosphate buffer, and KHCO3 electrolytes suggest that an increase in local pH at the surface of the electrode i...

Selective Electrochemical Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper(I) Oxide Catalysts

Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, in situ Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu2O, Cu(OH)2, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu(OH)2 during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a CuIII oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCuIIIO2 standard, we suggest that the chemical composition of the CuIII oxide is CuO2–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurement...

In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of CuIII Oxides as Catalytically Active Species

The catalytic activities of sulfur sites in amorphous MoSx for the electrochemical hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H2SO4 electrolyte. Using X-ray photoelectron spectroscopy and linear sweep voltammetry, we found the turnover frequency for H2 production to increase linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S2– and/or bridging S22–. To distinguish the catalytic performances of these two types of atoms, we turn to quantum chemical simulations using density functional theory. The apical S2– atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (ΔGH) in excess of +1 eV, and were thus ruled out as reaction sites for HER. In situ Raman spectroscopy of the model [Mo3S13]2– cluster further demonstrate the higher catalytic reactivity of the bridging S22– over terminal S22– (which have lower electron binding energy) for proton reduction.

Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction

Amorphous molybdenum sulfide (MoSx) is currently being developed as an economically viable and efficient catalyst for the electrochemical hydrogen evolution reaction (HER). An important yet unsolved problem in this ongoing effort is the identification of its catalytically active sites for proton reduction. In this work, cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to investigate the catalytically active sites and structural evolution of MoSx films during HER in 1 M HClO4 electrolyte. Transformation of anodically deposited MoSx (x ≈ 3) to a structure with MoS2 composition during the cathodic sweep of a CV was demonstrated using XPS and operando Raman spectroscopy. Interestingly, a Raman peak at 2530 cm–1 was recorded at potentials relevant to H2 evolution, which we ascribed to the S–H stretching vibration of MoSx–H moieties. This assignment was corroborated by H/D isotope exchange experiments. Mo–H (or Mo–D) stretching vibrations were not observed, which...

Operando Raman Spectroscopy of Amorphous Molybdenum Sulfide (MoSx) during the Electrochemical Hydrogen Evolution Reaction: Identification of Sulfur Atoms as Catalytically Active Sites for H+ Reduction

The electrocatalytic reduction of CO2 and water splitting have received significant attention because recycling CO2 into fuels and chemical feedstock is a crucial step to close the anthropogenic carbon circle, whereas splitting water produces H2 gas, which is a valuable carbon-free energy carrier. The clear identification of the catalytic-active sites and elucidation of the reaction mechanisms in these systems remain a grand challenge. It requires simultaneous characterizations of the catalysts under actual reaction conditions. Raman spectroscopy is among the handful of techniques that are suitable for the in situ/operando investigations of heterogeneous catalytic systems. This Perspective will highlight primarily recent works on the application of Raman spectroscopy in unraveling the structural changes of catalysts, their possible active sites, and the intermediates formed during water electrolysis and CO2 electroreduction. Results from complementary techniques such as X-ray absorption spectroscopy, amon...

Yilin Deng

Papers

Selective Electrochemical Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper(I) Oxide Catalysts

In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of CuIII Oxides as Catalytically Active Species

Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction

Operando Raman Spectroscopy of Amorphous Molybdenum Sulfide (MoSx) during the Electrochemical Hydrogen Evolution Reaction: Identification of Sulfur Atoms as Catalytically Active Sites for H+ Reduction

Characterization of Electrocatalytic Water Splitting and CO2 Reduction Reactions Using In Situ/Operando Raman Spectroscopy