Alkali metal cations are well-known to affect the rates and product selectivity of a wide range of electrocatalytic processes at the aqueous electrolyte/electrode interface. The mechanisms by which cations modulate electrocatalytic processes are still under debate. Proposed mechanisms include electrostatic and/or chemical interactions between cations and surface-adsorbed intermediates and buffering of the interfacial pH by hydrated cations. To elucidate the relative importance of these different mechanisms, it is essential to understand how alkali metal cations are distributed in the electrochemical double layer. A key challenge is the scarcity of techniques capable of probing the distribution of alkali metal cations at the interface under electrocatalytic conditions. In this work, we address this challenge. We establish tetramethylammonium, N(CH₃)₄⁺, as a vibrational probe of the alkali metal cation distribution in the electrochemical double layer. Using surface-enhanced infrared absorption spectroscopy (SEIRAS), we monitored the asymmetric CH₃ deformation mode of N(CH₃)₄⁺ at the aqueous electrolyte/polycrystalline Au interface. We found that alkali metal cations displace specifically adsorbed N(CH₃)₄⁺ from the electrode with decreasing applied potential. With DFT calculations, we identified the different degrees of electron transfer upon specific adsorption of the two types of cations as the likely origin of the potential-induced displacement. At a fixed potential, the degree to which alkali metal cations displace N(CH₃)₄⁺ increases in the order Li⁺<K⁺<Cs⁺. The absolute value of the free energy of hydration of the cations decreases in the order Li⁺>K⁺>Cs⁺. This observation suggests that alkali metal cations with softer hydration shells have a higher tendency to reside close to the electrode surface. Further, we determined that the near-surface population of alkali metal cations correlates with the rate of carbon dioxide-to-carbon monoxide conversion at the Au electrode. We will discuss the implications of this finding for the mechanistic understanding of carbon dioxide activation at aqueous electrolyte/metal interfaces.

Matthias Waegele

Boston College

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