On the Mechanism of Carbon Dioxide Reduction on Sn-Based Electrodes: Insights into the Role of Oxide Surfaces

Author(s)

Damas, Giane B. ^[1] ; Miranda, Caetano R. ^[2] ; Sgarbi, Ricardo ^[3] ; Portela, James M. ^[3] ; Camilo, Mariana R. ^[3] ; Lima, Fabio H. B. ^[3] ; Araujo, C. Moyses ^[1]

Affiliation

^[1] Uppsala Univ, Dept Phys & Astron, Mat Theory Div, S-75120 Uppsala - Sweden

^[2] Univ Sao Paulo, Inst Phys, BR-05508090 Sao Paulo - Brazil

^[3] Univ Sao Paulo, Inst Chem, BR-13560970 Sao Carlos, SP - Brazil

• Document type: Journal article
• Source: CATALYSTS; v. 9, n. 8 AUG 2019.
• Web of Science Citations: 0

Abstract

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. (AU)

• FAPESP's process: 14/26699-3 - Development of electrocatalysts for the electroreduction of CO2 and photo-electrooxidation of water: investigation of the reaction products by on-line mass spectrometry
• Grantee: Mariana Romano Camilo Casale
• Support type: Scholarships in Brazil - Post-Doctorate
• FAPESP's process: 16/13323-0 - Carbon Dioxide and Water Electrochemistry: Application in Energy Conversion and Storage
• Grantee: Fabio Henrique Barros de Lima
• Support type: Regular Research Grants
• FAPESP's process: 13/16930-7 - Electrocatalysis V: electrocatalytic processes of chemical and electrical energy interconversion
• Grantee: Edson Antonio Ticianelli
• Support type: Research Projects - Thematic Grants
• FAPESP's process: 17/02317-2 - Interfaces in materials: electronic, magnetic, structural and transport properties
• Grantee: Adalberto Fazzio
• Support type: Research Projects - Thematic Grants