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Recent Advances in CO Reduction Electrocatalysis on Copper David Raciti, and Chao Wang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00553 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018
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ACS Energy Letters
Recent Advances in CO2 Reduction Electrocatalysis on Copper David Raciti, Chao Wang*
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218 *
Email:
[email protected] Abstract Electroreduction of CO2 represents a promising approach toward artificial carbon recycling for addressing the global challenges in energy and sustainability. The foreground of this approach dependent on the development of efficient electrocatalysts capable of selectively reducing CO2 to valuable (oxygenated) hydrocarbon products at low overpotentials. Here we present an overview of recent developments of Cu electrocatalysts for CO2 reduction. Our focus is placed on elucidation of the structure-property relationships of monometallic Cu electrocatalysts, which is believed to be the foundation for understanding alloy and other more complex catalytic systems. Reported mechanisms are discussed in terms of grain boundaries, open facets, residual oxides, subsurface oxygen, local pH effect, etc. After this discussion, remaining questions are raised for further studies and development of advanced electrocatalysts for energy- and chemically efficient CO2 reduction.
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ACS Energy Letters
The rise of CO2 levels in the atmosphere has raised substantial concerns about the environmental impacts of burning fossil fuels. It has inspired the exploration of new technologies to mitigate the anthropological emission of CO2 by capturing, sequestration and/or conversion of this greenhouse gas. Among the various approaches being explored, electrochemical reduction of CO2, an artificial way of carbon recycling, represents one promising solution for the energy and environmental sustainability challenges.1 Powered by renewable electricity (e.g., solar and wind) and using water as the proton source, the electroreduction of CO2 can produce a wide range of reduced carbon compounds, ranging from carbon monoxide (CO) and formate to methanol, methane, ethanol and ethylene.2 These molecules can either be directly used as fuels, such as ethanol being used to partially substitute gasoline for internal combustion engines, 3 or be converted into liquid fuels and other valuable chemicals through further processing, e.g., CO as a feedstock for the Fischer-Tropsch process.4 Albeit having great potential, the energy conversion and chemical transformation efficiencies of current CO2 electrolyzers are limited by the lack of efficient electrocatalysts. Between the 1980-1990s, Hori et al. conducted a comprehensive survey of various metal electrodes for CO2 reduction in aqueous electrolytes (primarily 0.1 M KHCO3).5-7 The various metals are categorized into four groups based on their product distributions (Table 1): (i) Cu, the only metal capable of reducing CO2 to hydrocarbons (or oxygenated hydrocarbons) at significant rates; (ii) Au, Ag, Zn, Pd and Ga, from which CO is the major product; (iii) Pb, Hg, In, Sn, Cd, Tl and Bi, primarily producing formate; (iv) Ni, Fe, Pt and Ti, where no CO2 reduction is observed at steady state but only hydrogen evolution. CO2 reduction capability of the metals in groups (i) and (ii) is attributed to the stabilization of *CO2∙− and/or *COOH (* denotes a surface adsorption site) on their surfaces, whereas the formation of formate on group (iii) metals is believed to occur via hydration of non-adsorbing CO2∙−. Group (iv) metals are believed to bind to the intermediate *CO too strongly, which inhibits the continual reduction of CO2, leaving only the evolution of hydrogen from the interstitial sites among adsorbed CO. Later on, computational studies of the CO2 reduction mechanisms by Norskov et al. elucidated the unique property of Cu for catalyzing the CO 2 reduction toward hydrocarbons (Figure 1; One may notice the limitation of reaction pathways considered in these studies, as seen from other reports.8-10 Our attention here is the binding energies of *CO.), where
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the binding energy of *CO is used as the descriptor for the correlations between the electrocatalytic performance and the surface property of different metals11, 12. It is suggested that *CO binds to Au and Ag weakly, leading to its desorption as CO (g) once formed from the reduction of CO 2 on these metals. Cu is distinct from the other metals by having an intermediate binding strength to *CO, achieving a balance of the barriers for activation of CO2 and hydrogenation of *CO.12, 13 Further investigation by Bagger et al. suggested adsorption energies of *H, *COOH and *CH3O can also be used as descriptors in addition to *CO.14 They classified groups (i) and (ii) metals as having *H, whereas group (iii) with little or no *H, at the CO2 reduction potential, and further distinguished Cu from the other metals by not having underpotential deposition of hydrogen but binding *CO. Thereby Cu has received the greatest attention as electrocatalytic materials for CO2 reduction. The investigations of CO2 reduction on bulk Cu electrodes with extended surfaces have revealed the dependence of product yield and distribution on the electrode potential,15, 16 surface structure,17-21 pressure of CO222-24 and electrolyte (salt concentration,22 pH,25-27 cations,28-31 etc.15).7 Despite being active for production of hydrocarbons, it still requires large overpotentials for extended Cu surfaces to reduce CO2. On polycrystalline Cu (Cu-poly), a potential more negative than −0.6 V (versus the reversible hydrogen electrode, RHE; the same reference electrode is used in the following discussion unless otherwise specified) is needed to reach a total geometric current density of 1 mA/cm2geo (Figure 2). Yet, hydrogen evolution dominates at this potential, and only CO and formate (two-electron processes) are produced from CO2 reduction with a total Faradaic efficiency (FECO2) of
