The selective electrocatalytic reduction of CO₂ to formate ions or to formic acid is attracting the attention of several researchers. This is mainly because these products can be utilized as hydrogen carriers and as fuels for direct or for indirect fuel cells. In aqueous media, electrocatalysts formed by metals with high hydrogen evolution overpotential, such as Pd, Hg, In, and Sn, are selective for the production of formate (or formic acid, depending on the electrolyte pH). Among these metals, Sn has gained notice because it is abundant and relatively nontoxic. The reasons for the selectivity observed on Sn electrodes are still in debate and has been the subject of recent papers [1,2]. It seems that tin allows the carbon dioxide reduction intermediate to be adsorbed via the oxygen atoms after a proton-couple electron transfer step, so conducting the reaction to the production of formate. In addition, the studies have been discussed the effect of the surface hydroxides, which may enhance the CO₂ reduction mainly via two effects: (i) suppressing or decelerating the water electro-reduction and; (ii) allowing a first chemical reaction step with CO₂, so producing an adsorbed bicarbonate intermediate (via oxygen atoms), so facilitating the protonation of the carbon atom. In this paper, we will present some results obtained for the CO₂ electrochemical reduction on nanostructured tin-alloys, such as Sn-Pd and Sn-Co. The faradaic efficiencies for formate (or formic acid) production were determined via quantitative determination by cyclic voltammetry and experiments of on-line Differential Electrochemical Mass Spectrometry (DEMS) for the formate electro-oxidation after experiments of CO₂ electrolysis. It will be discussed the correlation between the nanostructure morphology and Sn oxidation state with the stability and faradaic efficiency for the production of formate (or formic acid) on different electrolytes and pHs. We expect that the obtained results will serve to further understand the main electrocatalyst features that control the selectivity and stability for the CO₂ reduction to formate.

[1] J.T. Feaster, C. Shi, E.R. Cave, T. Hatsukade, D.N. Abram, K.P. Kuhl, C. Hahn, J.K. Norskov, T.F. Jaramillo, ACS Catal. 7 (2017) 4822 – 4827.

[2] J.E. Pander, M.F. Baruch, A.B. Bocarsly, ACS Catal. 2016, 6, 7824−7833.

Acknowledgements

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