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 continuously increasing CO2 released from human activities poses a great threat to human survival by fluctuating global climate and disturbing carbon balance among the four reservoirs of the biosphere, earth, air, and water. Converting CO2 to value-added feedstocks via electrocatalysis of the CO2 reduction reaction (CO2RR) has been regarded as one of the most attractive routes to re-balance the carbon cycle, thanks to its multiple advantages of mild operating conditions, easy handling, tunable products and the potential of synergy with the rapidly increasing renewable energy (i.e., solar, wind). Instead of focusing on a special topic of electrocatalysts for the CO2RR that have been extensively reviewed elsewhere, we herein present a rather comprehensive review of the recent research progress, in the view of associated value-added products upon selective electrocatalytic CO2 conversion. We initially provide an overview of the history and the fundamental science regarding the electrocatalytic CO2RR, with a special introduction to the design, preparation, and performance evaluation of electrocatalysts, the factors influencing the CO2RR, and the associated theoretical calculations. Emphasis will then be given to the emerging trends of selective electrocatalytic conversion of CO2 into a variety of value-added products. The structure-performance relationship and mechanism will also be discussed and investigated. The outlooks for CO2 electrocatalysis, including the challenges and opportunities in the development of new electrocatalysts, electrolyzers, the recently rising operando fundamental studies, and the feasibility of industrial applications are finally summarized.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

Recent Development of Ni/Fe-Based Micro/Nanostructures toward Photo/Electrochemical Water Oxidation

The generation of fuels and chemicals from sunlight offers a promising approach for the storage of solar energy. Natural photosynthesis is the best-known example for converting solar energy into chemical energy. In the light-dependent reactions of photosynthesis, the photoinduced electron transport chain from water (H2O) to reduced nicotinamide adenine dinucleotide phosphate (NADPH) is a kind of Z-scheme energetics diagram, which involves two photoexcitation steps and a downhill electron flow from photosystem II (PSII) to photosystem I (PSI) (Figure 1). The Z-scheme pathway is advantageous in realizing efficient charge separation and producing active species with strong reducibility for subsequent reduction of CO2 to hydrocarbons. In practice, artificial photosynthesis is an alternative approach to generate hydrogen energy using sunlight to split Photoelectrocatalytic water splitting offers a promising approach to convert sunlight into sustainable hydrogen energy. A thorough understanding of the relationships between the properties and functions of photoelectrocatalytic materials plays a crucial role in the design and fabrication of efficient photoelectrochemical systems for water splitting. This review presents the advances in the development of efficient photoelectrocatalytic materials. First, the fundamentals involved in the photoelectrocatalytic water splitting are elaborated. Then, the critical properties of photoelectrocatalytic materials are classified and discussed according to the associated photoelectrochemical processes, including light absorption, charge separation, charge transportation, and photoelectrocatalytic reactions. The importance of heterointerfaces in photoelectrodes is also mentioned in conjunction with the illustration of some functional interlayer materials. Finally, some strategies that can be employed in material screening and optimization for the construction of highly efficient photoelectrochemical devices for water splitting are also discussed. Water Splitting

Photoelectrocatalytic Materials for Solar Water Splitting

CO2 electroreduction reaction (CO2RR) to fuels and feedstocks is an attractive route to close the anthropogenic carbon cycle and store renewable energy. The generation of more reduced chemicals, especially multicarbon oxygenate and hydrocarbon products (C2+) with higher energy density is highly desirable for industrial applications. However, selective conversion of CO2 to C2+ suffers from high overpotential, low reaction rate and low selectivity, and the process is extremely sensitive to the catalyst structure and electrolyte. Here we discuss strategies to achieve high C2+ selectivity through rational design of the catalyst and electrolyte. Current state-of-the-art catalysts, including Cu and Cu-bimetallic catalysts as well as alternative materials are considered. The importance of taking into consideration the dynamic evolution of the catalyst structure and composition are highlighted, focusing on findings extracted from in situ and operando characterizations. Additional theoretical insight into the reaction mechanisms underlying the improved C2+ selectivity of specific catalyst geometries/compositions in synergy with a well-chosen electrolyte are also provided.

Rational Catalyst and Electrolyte Design for Co2 Electroreduction Towards Multicarbon Products

https://koasas.kaist.ac.kr/bitstream/10203/274765/1/Modulating-Local-CO-Concentration-as-a-General-Strategy-for-Enhancing-CC-Coupling-in-CO-Electroreduction2020Joule.pdf

Modulating Local CO2 Concentration as a General Strategy for Enhancing C−C Coupling in CO2 Electroreduction

Development of efficient and selective electrocatalysts is a key challenge to achieve an industry-relevant electrochemical CO2 reduction reaction (CO2RR) to produce commodity chemicals. Here, we re...

Over a 15.9% Solar-to-CO Conversion from Dilute CO2 Streams Catalyzed by Gold Nanoclusters Exhibiting a High CO2 Binding Affinity

Electrochemical CO2 conversion offers a promising route for value-added products such as formate, carbon monoxide, and hydrocarbons. As a result of the highly required overpotential for CO2 reduction, researchers have extensively studied the development of catalyst materials in a typical H-type cell, utilizing a dissolved CO2 reactant in the liquid phase. However, the low CO2 solubility in an aqueous solution has critically limited productivity, thereby hindering its practical application. In efforts to realize commercially available CO2 conversion, gas-phase reactor systems have recently attracted considerable attention. Although the achieved performance to date reflects a high feasibility, further development is still required in order for a well-established technology. Accordingly, this review aims to promote the further study of gas-phase systems for CO2 reduction, by generally examining some previous approaches from liquid-phase to gas-phase systems. Finally, we outline major challenges, with significant lessons for practical CO2 conversion systems.

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Towards Higher Rate Electrochemical CO2 Conversion: From Liquid-Phase to Gas-Phase Systems

Mass transfer, kinetics, and mechanism of electrochemical CO2 reduction have been explored on a model mesostructure of highly-ordered copper inverse opal (Cu-IO), which was fabricated by Cu electrodeposition in a hexagonally-closed packed polystyrene template. As the number of Cu-IO layers increases, the formation of C2 products such as C2H4 and C2H5OH was significantly enhanced at reduced overpotentials (∼200 mV) compared to a planar Cu electrode. At the thickest layer, we observe for the first time the formation of acetylene (C2H2), which can be generated through a kinetically slow reaction pathway and be a key descriptor in the unveiling of the C C coupling reaction mechanism. Based on our experimental observation, a plausible reaction pathway in Cu mesostructures is rationalized.

Effect of mass transfer and kinetics in ordered Cu-mesostructures for electrochemical CO2 reduction

Photoelectrochemical (PEC) cells have attracted much attention as a viable route for storing solar energy and producing value-added chemicals and fuels. However, the competition between light absorption and electrocatalysis at a restrained cocatalyst area on conventional planar-type photoelectrodes could limit their conversion efficiency. Here, we demonstrate a new monolithic photoelectrode architecture that eliminate the optical-electrochemical coupling by forming locally nanostructured cocatalysts on a photoelectrode. As a model study, Ni inverse opal (IO), an ordered three-dimensional porous nanostructure, was used as a surface-area-controlled electrocatalyst locally formed on Si photoanodes. The optical–electrochemical decoupling of our monolithic photoanodes significantly enhances the PEC performance for the oxygen evolution reaction (OER) by increasing light absorption and by providing more electrochemically active sites. Our Si photoanode with local Ni IOs maintains an identical photolimiting curre...

Hakhyeon Song

Papers

Modulating Local CO2 Concentration as a General Strategy for Enhancing C−C Coupling in CO2 Electroreduction

Over a 15.9% Solar-to-CO Conversion from Dilute CO2 Streams Catalyzed by Gold Nanoclusters Exhibiting a High CO2 Binding Affinity

Towards Higher Rate Electrochemical CO2 Conversion: From Liquid-Phase to Gas-Phase Systems

Effect of mass transfer and kinetics in ordered Cu-mesostructures for electrochemical CO2 reduction

An Optically and Electrochemically Decoupled Monolithic Photoelectrochemical Cell for High-Performance Solar-Driven Water Splitting