Experimental Insights into the Reaction Intermediates-Selectivity Relationship for Electrochemical CO2 Reduction Reaction
Authors: Hayase Shunsuke , Morinaga Asuka , Su Qing , Yoshimura Hiroki , Sai Ryansu , Kondo Yasuyuki , Ishikiriyama Mamoru , Ibe Masaya , Katayama Yu , Yamada Yuki 
Published: December 2024

Since the industrial revolution, the use of fossil fuels has increased CO2 concentration in the atmosphere, which has caused global warming to become a serious problem [1]. Thus, to realize a "carbon neutral" society, Electrochemical CO2 Reduction Reaction (CO2RR) has attracted attention. However, there are several challenges to the practical application of CO2RR, one of which is the improvement of product selectivity: the standard electrode potential of each reaction to various products is overlapped [2], and its large overpotential [3], making it difficult to selectively produce a specific product. In the pioneering study by Hori and co-workers, various metal electrodes were investigated and classified into four groups based on their primary reaction product: HCOO–-forming metals, CO-forming metals, hydrocarbon-forming metals, and CO2RR inactive metals (hydrogen-producing metals) [4]. Since the adsorption strength of the reaction intermediates has been considered the primary descriptor for the differences in reaction products, most studies have focused mainly on the adsorption energy. In recent years, more studies have focused on aspects other than the adsorption energies of O-bound and C-bound adsorbates to further improve product selectivity. Rossmeisl et al. suggest that the binding energy of H-bound adsorbates, in addition to O-bound and C-bound adsorbates, is an essential factor in explaining the final product [5]. Furthermore, the focus now expands to electrolyte properties, such as pH [
6], cations [
7], and anions [
8] to control the stability of the reaction intermediates. As such, it is critical to experimentally probe the state of the reaction intermediates in situ to accurately understand the effect of the strategies mentioned above on the reaction pathway.

In this study, we experimentally probe the reaction intermediates at various metal electrodes with different product selectivity using operando surface-enhanced infrared spectroscopy (SEIRAS). For the operando SEIRA measurements, we used a metal working electrode composed of a thin metal film electrodeposited on a Si prism coated with a Pt surface enhancement layer. Carbon rods were used as the counter electrode, Ag/AgCl as the reference electrode, and 1 M KHCO3 as the electrolyte. To compare the reactivity of reaction intermediates on different metal electrodes, we conducted linear sweep voltammetry (LSV) from 0.50 to –0.90 V vs RHE and probed the reaction intermediates during the potential sweep using SEIRAS. The result confirms the potential-dependent change in the reaction intermediates as well as the metal-dependent change in the composition of the reaction intermediates, suggesting successful observation of the CO2RR intermediates in different metal catalysts. Through this study, we attempted to interpret the correlation between the reactivity of each electrode and the properties of the reaction intermediates.

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