The influence of electrolyte identity upon the electro-reduction of CO2

The influence of supporting electrolyte cations on the voltammetric behaviour and product distribution in N-methylpyrrolidone-based carbon dioxide electroreduction systems is investigated. The reduction potentials associated with TBABF(4) (0.1 M) and corresponding alkali metal (M) electrolytes; LiBF4, NaBF4 and RbBF4 (focussing mainly on the reduction of the widely employed Li+ species) were established in both the presence and absence of CO2 at polycrystalline noble metal working electrodes. In situ and ex situ Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and qualitative element identification via flame testing were used to aid the assignment of reduction processes. It was established that CO2 reduction products in the metal cationic systems were formed at a much less negative potential than those found with the non-metal cation (-1.5 V vs. Ferrocene, c.f. -2.2 V), however the resultant alteration of the surface environment was found to deactivate the electrode to further CO2 reduction. The presence of CO2 in solution was found to affect the potential required for the bulk deposition of metal from the electrolyte through the same process. Where TBA(+) and M+ were employed simultaneously in the system, the resultant voltammetry shared the majority of features with the pure M+ system with CO2 reduction suppressed at more negative potentials therefore supporting the conclusion that any 'catalytic effect' associated with TEA is in fact a lack of deactivation given by the M+ system, rather than any enhancement offered by the former. (C) 2014 Published by Elsevier B.V.