Spectroscopic Evidence of Size-Dependent Buffering of Interfacial pH by Cation Hydrolysis during CO₂ Electroreduction

Abstract

The nature of the electrolyte cation is known to affect the Faradaic efficiency and selectivity of CO electroreduction. Singh et al. (J. Am. Chem. Soc. 2016, 138, 13006-13012) recently attributed this effect to the buffering ability of cation hydrolysis at the electrical double layer. According to them, the pK of hydrolysis decreases close to the cathode due to the polarization of the solvation water molecules sandwiched between the cation's positive charge and the negative charge on the electrode surface. We have tested this hypothesis experimentally, by probing the pH at the gold-electrolyte interface in situ using ATR-SEIRAS. The ratio between the integrated intensity of the CO and HCO bands, which has to be inversely proportional to the concentration of H, provided a means to determining the pH change at the electrode-electrolyte interface in situ during the electroreduction of CO. Our results confirm that the magnitude of the pH increase at the interface follows the trend Li > Na > K > Cs, adding strong experimental support to Singh's et al.'s hypothesis. We show, however, that the pH buffering effect was overestimated by Singh et al., their overestimation being larger the larger the cation. Moreover, our results show that the activity trend of the alkali-metal cations can be inverted in the presence of impurities that alter the buffering effect of the electrolyte, although the electrolyte with maximum activity is always that for which the increase in the interfacial pH is smaller.

In‐Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2: Theory, Practice and Challenges

“…However, the local pH depends considerably on the purity of the salt used to prepare the electrolyte, and exactly opposing trends can be found if a set of alkaline‐metal carbonates of lower purity is used (see Figure , right panel), which might explain these discrepancies . Nonetheless, the highest pH increase always coincided with the lowest current densities, and vice versa …”

“…However, this disadvantage can be turned into an advantage, and be used to determine the local pH at the interface in operando conditions. Ayemoba and Cuesta used ATR‐SEIRAS to experimentally determine the pH at the electrode‐electrolyte interface during the electroreduction of CO 2 by determining the ratio between the integrated areas of the absorption bands corresponding to CO 2 and HCO 3 − . Using gold films chemically deposited on silicon, they demonstrated that, at a given potential, the increase in the pH near the electrode surface follows the sequence Li + >Na + >K + >Cs + (Figure , left), a sequence which is exactly the inverse of that observed for the current density and selectivity for C 2 products of the CO 2 electroreduction process .…”

Electrolyte Effects on the Electrochemical Reduction of CO2

“…This mechanism is dependent on cation radius and will alter the local pH and the local CO 2 concentration near the electrode surface, ultimately altering product selectivity. Ayemoba and Cuesta have further confirmed this effect by performing surface‐enhanced infrared spectroscopy (SEIRAS) experiments on a thin‐film gold electrode …”

Structure‐Sensitivity and Electrolyte Effects in CO2 Electroreduction: From Model Studies to Applications

“…The electrolyte composition (Figure A) plays an important role on both the activity and selectivity of the CO 2 RR, as it can: i) regulate the pH and minimise the HER against CO 2 RR, ii) affect the solubility of CO 2 , iii) stabilise the reaction intermediates …”