The direct CO₂ electrochemical reduction reaction (CO2RR) is a potential technology to convert waste CO₂ streams into valuable chemicals, using renewable electricity as a driving force. During this process a variety of carbon-based products such as CO, HCOOH and hydrocarbons can be formed. The selectivity of the CO2RR is determined by the nature of the catalyst interphase, which is dependent on the catalyst material, its morphology and the working electrolyte, among other factors. Therefore, it is necessary to get a holistic approach to fully understand the CO2RR and to get optimal catalytic results.

For this contribution we look at the influence of proton concentration on the selectivity of carbon based catalysts. Unlike hydrogen or methane generation, CO production has been shown to be independent of pH concentration on the NHE scale. Therefore, the proton concentration can be used to tune the CO/H₂ ratio. In aqueous electrolytes, for instance, the selectivity towards CO on Fe nitrogen-doped carbon is clearly enhanced at neutral pH whereas acidic conditions favored methane and hydrogen production. [1]

For this contribution we studied the CO2RR in aprotic media aiming to suppress the competing hydrogen evolution reaction (HER). We first studied a pure carbon electrode which in aqueous electrolyte selectively produced H₂, yet traces of CO were also obtained. By contrast, in an aprotic electrolyte (0.1M NBu4PF6 in acetonitrile) CO was obtained with a faradaic efficiency higher than 90%. Density functional theory (DFT) simulations confirmed this is attributed to the absence of protons, showing that certain carbon defects can reduce CO₂ into CO in both media. Nevertheless, carbon sites are predicted to be more active towards the HER and thus in aqueous media the formation of H₂ is the predominant process.

While it is remarkable that the pure carbon can selectively reduce CO₂, the process takes place at  high over potentials, this can be improved by intruding dopants. Metal nitrogen doped carbons, in particular, have been shown to be highly active towards the CO2RR in aqueous media. [2] Working in a non-protic electrolyte, resulted in suppression of both the HER and the CO2RR, as both reactions require protons. Nevertheless, the presence of small amounts of water resulted in a clear enhancement of the CO2RR while keeping a low HER activity. These results highlight the importance of proton concentration on determining the reaction selectivity. On one hand H^₊ concentration has to be limited to suppress the competing process of the HER. However, protons are also necessary to carry out the reduction of CO₂ into CO and methane.