Electrocatalytic Reduction of Carbon Dioxide on Tin Oxide-Based Nanoparticles: Indirect Detection of Formate Ions Via on-Line Dems
Authors: Camilo Mariana R. , Lima Fabio H. B. 
Published: February 2020

The electrochemical reduction of carbon dioxide is an important route for producing valuable chemical compounds and industrial feedstock. Additionally, this molecule can be used in regenerative cycles of electrochemical energy conversion and storage. In the last case, carbon dioxide can be selectively reduced to formate ions, which can be used as fuel in direct formate or formic acid fuel cells. As presented before [1,2], metals with high hydrogen evolution reaction (HER) overpotential, such as Pd, Sn, Hg, In, and Sn, tend to catalytically convert carbon dioxide to formate ions. In this study, we have synthesized carbon-supported Sn-based nanoparticles and tested them as electrocatalysts for the CO2 electro-reduction in aqueous KHCO3 electrolyte. The generated formate ions were indirectly and quantitatively determined via cyclic voltammetry on a porous sputtered platinum electrode, for the electro-oxidation of the formate ions, with on-line and instantaneous detection of CO2 (electro-oxidation product of formate) by Differential Electrochemical Mass Spectrometry (DEMS). For this, after potentiostatic polarizations for the CO2 electro-reduction, the CO2-saturated KHCO3 electrolyte was acidified with sulfuric acid (to reach pH close to zero) in order to decompose the bicarbonate ions, and it was purged with argon for 30 min. The acidification is necessary because the protons generated from the formate ions electro-oxidation in the near-neutral pH KHCO3 solution attack the bicarbonate ions, producing CO2. So, this procedure guarantees that all detected CO2 comes exclusively from the electro-oxidation of the generated formic acid molecules and not from the decomposition of the bicarbonate ions (the acid addition also induces the protonation of the formate species producing formic acid). This procedure offers, therefore, an alternative method for quantitative determination of formate ions during the CO2 reduction. The results showed that the production of the formate ions linearly increases with the decrease of the electrochemical potential in the potential domain from -0.9 to -1.8 V vs. Ag/AgCl, for SnO2/C nanoparticles. The maximum faradaic current efficiency for formate ions production was 23 %, achieved at -1.2 V. Interestingly, the modification of tin with other transition metals, such as cobalt, completely inhibited the production of formate. Contrarily, by synthetizing non-alloyed bimetallic Sn-Co nanoparticles, the formate ions generation was maintained. The discussion concerning the parameters that govern the selectivity to formate ions will be addressed in this presentation. The faradaic current efficiency-electrocatalyst composition/structure correlation, determined by DEMS, followed the tendencies of surface hydroxide formation, and this will also be presented and discussed.

[1] Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta 39 (1994) 1833 – 1839.

[2] Y. Chen, M. W. Kanan, J. Am. Chem.Soc. 2012, 134, 1986−1989

Acknowledgements

The authors gratefully acknowledge financial support from FAPESP (2016/13323-0 and 2013/16930-7) and CNPq (306469/20162).