Local microenvironment tuning induces switching between electrochemical CO2 reduction pathways

Gas diffusion layers (GDL) have become a critical component in electrochemical CO2 reduction (CO2R) systems because they can enable high current densities needed for industrially relevant productivity. Besides this function, it is often assumed that the choice of catalyst and electrolyte play much more important roles than the GDL in influencing the observed product selectivity. Here, we show that tuning of the GDL pore size can be used to control the local microenvironment of the catalyst and hence, effect significant changes in catalytic outcomes. This concept is demonstrated using sputtered Ag films on hydrophobic PTFE substrates with 6 different pore sizes. Although Ag is known to be a predominantly CO generating catalyst, we find that smaller pore sizes favor the generation of formate up to a faradaic efficiency of 43%. Combined experimental and simulation results show that this is due to the influence of the pore size on CO2 mass transport, which alters the local pH at the electrode, resulting in reaction pathway switching between CO and formate. Our results highlight the importance of the local microenvironment as an experimental knob that can be rationally tuned for controlling product selectivity: a key consideration in the design of CO2R systems.

Automated summary

Gas diffusion layers (GDL) are crucial in electrochemical CO2 reduction (CO2R) systems to achieve high current densities. It is commonly believed that the choice of catalyst and electrolyte is more important than GDL in determining product selectivity. However, our study shows that adjusting GDL pore size can significantly impact catalytic outcomes by controlling the local microenvironment of the catalyst. Experimental and simulation results demonstrate that smaller pore sizes promote the generation of formate instead of CO due to the influence of pore size on CO2 mass transport and local pH at the electrode. This highlights the importance of the local microenvironment as a tunable parameter for controlling product selectivity in the design of CO2R systems.