Exploring the Impact of Excess Cations in Acidic CO2 Reduction Reaction
Authors: Yoo Suhwan , Park Sejin , Hwang Yun Jeong 
Published: December 2024

To anticipate high performance in the electrocatalytic reactions, a comprehensive understanding of the electric double layer (EDL) is essential. Experimental, spectroscopic, and computational studies of the EDL have been reported for understanding the performance in electrocatalytic reactions such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and CO2 reduction reaction (CO2RR). Notably, in the CO2RR, previous studies have revealed the accumulation of cations near the outer Helmholtz plane (OHP) significantly has an impact on the reaction performance, product selectivity, and partial current density. The presence of these cations induces various effects, such as creating a strong electric field, altering local pH levels, stabilizing the intermediates, and specific adsorption. However, unraveling the sole cation effect remains challenging due to the system's complexity and hierarchical characteristics. Moreover, experiments predominantly conducted under favorable conditions for understanding the electric field and stabilization effects have left the specific adsorption effect relatively unexplored. Specific adsorption involves direct coordination with the electrode at the inner Helmholtz plane (IHP), with the degree determined by catalyst type and electrolyte conditions. Previous studies have highlighted the significant impact of specific adsorption on current density and intermediate stabilization, with potassium (K+) and cesium (Cs+) cation exhibiting the most pronounced effect owing to their minimal hydration degree compared to the lithium (Li+) and sodium (Na+).

Acidic CO2RR has recently garnered attention for its high carbon efficiency (CE) and energy efficiency (EE). However, the abundance of hydronium ions can lead to a favorable occurrence of HER over CO2RR due to its facile protonation. To mitigate this challenge, excess cations have been employed to suppress the hydronium ion migration and enhance the performance of the CO2RR, which means accumulated cation layer at the OHP can suppress the HER and stabilize the intermediates of the CO2RR simultaneously. Meanwhile, in the excess cation condition, adsorbed cations can be dominant on the electrode originated from the excess cations, which is not investigated intensively. For the better performance of the electrocatalytic reactions in the acidic condition, we need to elucidate the EDL formed in the presence of the excess cations.

In here, we investigated the electrode-electrolyte interface in the different cation species during the acidic CO2RR through the attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS), which can give the local information such as the adsorbed species on the metal and free species near the electrode. We confirmed the different product distribution and interface depending on the cation species, especially in the excess cations (1 M M+, M = Li, K, Cs). Specific adsorption in the presence of the excess cations can alter the product distribution, and it differs from the low cation concentration (0.1 M M+, M = Li, K, Cs). In final, to clarify the effect originated from the excess cations, we confirmed the interface on the copper and silver, which major products are C2+ and C1, respectively.