Comparison of electrocatalytic reduction of CO2 to HCOOH with different tin oxides on carbon nanotubes

•Two SnOx/MWCNT and two SnO2/MWCNT cathodes were prepared for the electrochemical reduction of CO2 to formate.•A 60% faradaic efficiency with 25% energy efficiency was obtained with the two SnOx/MWCNT cathodes.•The efficiencies were not strongly influenced by the morphology of the particle agglomeration.

A comparative study is reported on the electrocatalytic reduction of CO2 to HCOOH in aqueous alkaline solution with differently prepared tin-oxide particles on multi-walled carbon nanotubes from SnCl2 or SnCl4 precursors. The highest faradaic and energy efficiencies of 64% and 27% were obtained at − 1.40 V vs. SCE with particles that were obtained by KBH4 reduction from a SnCl2 precursor. At lower potentials, competitive reduction reactions occur. A SnCl2 versus SnCl4 precursor favors retention of a Sn(II) valence state in a surface tin oxyhydroxide surface layer. Different morphologies of the particle agglomerates made little difference in the electrocatalytic selectivity and activity. The two SnOx/CNT electrodes both showed a current density retention of ~ 70% after a 20-h electrolysis.