Deciphering the Impact of Cations on the Electrochemical Interface in CO Coupling Reaction

Recently, an article in the National Science Review highlighted the research conducted by Prof. Jun Cheng from Xiamen University and Dr. Jia-Bo Le from Ningbo Institute of Materials Technology and Engineering of Chinese Academy of Sciences. Their study used ab initio molecular dynamics simulation methods to investigate the microscopic properties of the electrochemical interface. The findings shed light on the cation effect and its impact on the selectivity of C₂ products in the electrocatalytic reduction of carbon monoxide, providing valuable insights for future carbon monoxide/carbon dioxide electro-reduction catalytic systems.

In numerous electrocatalytic reactions, researchers have observed that changing the electrolyte cation can significantly alter catalytic activity and selectivity. However, the microscopic mechanism underlying this phenomenon remains debated.

To address this issue, the researchers focused on the electrocatalytic reduction of carbon monoxide and systematically studied the effects of different cations, including various alkali metal ions and alkyl-ammonium ions, on the microscopic structure of the Pt(111)-CO_ad/aqueous solution interface and its correlation with the CO coupling reaction’s activity.

The study revealed the following key findings:

1) The solvation layer structure of the cation at the interface is influenced by the ion’s hydration energy. Cations with low hydration energy, such as Cs⁺, tend to undergo dehydration at the interface, leading to coordination with surface-adsorbed CO molecules. This coordination affects the adsorption stability of CO and its coupling intermediate.

2) The properties of the cation impact the double layer capacitance at the interface. Generally, cations with smaller hydration radii exhibit larger interface capacitance, explaining why alkyl-ammonium ions usually have smaller capacitance than alkali metal ions. Additionally, the researchers discovered two unique phenomena: the over-screening effect of low hydration energy ions, such as Cs⁺, which increases capacitance, and the reduction of water content and dielectric screening ability in the interface by super-sized ions like propyl-ammonium ions, leading to decreased capacitance.

3) Surface reaction calculations demonstrated a significant correlation between the CO coupling reaction’s energy on the electrode surface and surface charge, ion coordination, and hydrogen bonding with water molecules. When considering the effect of cations on the interface’s microscopic properties, the researchers were able to understand the experimental trend of different cations’ impact on the selectivity of CO electro-reduction C₂ products. Specifically, Cs⁺〜Rb⁺ > K⁺ > Li⁺ and Me₄N⁺ > Et₄N⁺ > Pr₄N⁺. These findings provide important guidance for future designs of electro-reduction catalytic systems targeting carbon monoxide/carbon dioxide.

More information:
Jia-Bo Le et al, Molecular understanding of cation effects on double layer and its significance to CO-CO dimerization, National Science Review (2023). DOI: 10.1093/nsr/nwad105

 

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