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

AbstractThe 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 TBABF4 (0.1M) 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 (−1.5V vs. Ferrocene, c.f. −2.2V), 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 TBA+ is in fact a lack of deactivation given by the M+ system, rather than any enhancement offered by the former

[M(CO)4(2,2′-bipyridine)] (M = Cr, Mo, W) as efficient catalysts for electrochemical reduction of CO2 to CO at a gold electrode

Group 6 complexes of the type [M(CO)4(bpy)] (M=Cr, Mo, W) are capable of behaving as electrochemical catalysts for the reduction of CO2 at potentials less negative than those for the reduction of the radical anions [M(CO)4(bpy)].−. Cyclic voltammetric, chronoamperometric and UV/Vis/IR spectro-electrochemical data reveal that five-coordinate [M(CO)3(bpy)]2− are the active catalysts. The catalytic conversion is significantly more efficient in N-methyl-2-pyrrolidone (NMP) compared to tetrahydrofuran, which may reflect easier CO dissociation from 1e−-reduced [M(CO)4(bpy)].− in the former solvent, followed by second electron transfer. The catalytic cycle may also involve [M(CO)4(H-bpy)]− formed by protonation of [M(CO)3(bpy)]2−, especially in NMP. The strongly enhanced catalysis using an Au working electrode is remarkable, suggesting that surface interactions may play an important role, too