Direct Evidence of Local pH Change and the Role of Alkali Cation during CO2 Electroreduction in Aqueous Media.

We report, for the first time, utilizing a rotating ring-disc electrode (RRDE) assembly for detecting changes in the local pH during aqueous CO2 reduction reaction (CO2 RR). Using Au as a model catalyst where CO is the only product, we show that the CO oxidation peak shifts by -86±2 mV/pH during CO2 RR, which can be used to directly quantify the change in the local pH near the catalyst surface during electrolysis. We then applied this methodology to investigate the role of cations in affecting the local pH during CO2 RR and find that during CO2 RR to CO on Au in an MHCO3 buffer (where M is an alkali metal), the experimentally measured local basicity decreased in the order Li+ > Na+ > K+ > Cs+ , which agreed with an earlier theoretical prediction by Singh et al. Our results also reveal that the formation of CO is independent of the cation. In summary, RRDE is a versatile tool for detecting local pH change over a diverse range of CO2 RR catalysts. Additionally, using the product itself (i.e. CO) as the local pH probe allows us to investigate CO2 RR without the interference of additional probe molecules introduced to the system. Most importantly, considering that most CO2 RR products have pH-dependent oxidation, RRDE can be a powerful tool for determining the local pH and correlating the local pH to reaction selectivity.