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On the Mechanism of Carbon Dioxide Reduction on Sn-Based Electrodes: Insights into the Role of Oxide Surfaces
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0002-5853-0819
Institute of Physics, University of São Paulo, São Paulo 05508-090, Brazil.
Institute of Chemistry, University of São Paulo, São Carlos 13560-970, Brazil.
Institute of Chemistry, University of São Paulo, São Carlos 13560-970, Brazil.
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2019 (English)In: Catalysts, E-ISSN 2073-4344, Vol. 9, no 8, article id 636Article in journal (Refereed) Published
Abstract [en]

The electrochemical reduction of carbon dioxide into carbon monoxide, hydrocarbons and formic acid has offered an interesting alternative for a sustainable energy scenario. In this context, Sn-based electrodes have attracted a great deal of attention because they present low price and toxicity, as well as high faradaic efficiency (FE) for formic acid (or formate) production at relatively low overpotentials. In this work, we investigate the role of tin oxide surfaces on Sn-based electrodes for carbon dioxide reduction into formate by means of experimental and theoretical methods. Cyclic voltammetry measurements of Sn-based electrodes, with different initial degree of oxidation, result in similar onset potentials for the CO2 reduction to formate, ca. −0.8 to −0.9 V vs. reversible hydrogen electrode (RHE), with faradaic efficiencies of about 90–92% at −1.25 V (vs. RHE). These results indicate that under in-situ conditions, the electrode surfaces might converge to very similar structures, with partially reduced or metastable Sn oxides, which serve as active sites for the CO2 reduction. The high faradaic efficiencies of the Sn electrodes brought by the etching/air exposition procedure is ascribed to the formation of a Sn oxide layer with optimized thickness, which is persistent under in situ conditions. Such oxide layer enables the CO2 “activation”, also favoring the electron transfer during the CO2 reduction reaction due to its better electric conductivity. In order to elucidate the reaction mechanism, we have performed density functional theory calculations on different slab models starting from the bulk SnO and Sn6O4(OH)4 compounds with focus on the formation of -OH groups at the water-oxide interface. We have found that the insertion of CO2 into the Sn-OH bond is thermodynamically favorable, leading to the stabilization of the tin-carbonate species, which is subsequently reduced to produce formic acid through a proton-coupled electron transfer process. The calculated potential for CO2 reduction (E = −1.09 V vs. RHE) displays good agreement with the experimental findings and, therefore, support the CO2 insertion onto Sn-oxide as a plausible mechanism for the CO2 reduction in the potential domain where metastable oxides are still present on the Sn surface. These results not only rationalize a number of literature divergent reports but also provide a guideline for the design of efficient CO2 reduction electrocatalysts.

Place, publisher, year, edition, pages
2019. Vol. 9, no 8, article id 636
Keywords [en]
electrocatalysis, carbon dioxide conversion, formic acid, tin-based electrodes, tin oxide, tin-carbonate, reaction mechanism
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-390068DOI: 10.3390/catal9080636ISI: 000482799100047OAI: oai:DiVA.org:uu-390068DiVA, id: diva2:1340307
Funder
StandUpSwedish Research CouncilSwedish National Infrastructure for Computing (SNIC)Available from: 2019-08-05 Created: 2019-08-05 Last updated: 2019-10-02Bibliographically approved

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