Washed Away: Oxygen Reduction Drives Ionic Liquid Loss in Fuel Cell Catalyst Layers

High Resolution Image Download MS PowerPoint Slide The surface modification of electrocatalysts by ionic liquids (ILs) has emerged as an effective strategy for enhancing their electrocatalytic activity in various reactions, including the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). Moreover, the addition of ILs into the catalyst layers (CLs) of PEMFCs can improve reactant mass transport and water management. However, to be considered in real devices, the beneficial role of ILs should be sustained over the timescale of PEMFC operation. Our previous works showed that ILs may suppress the dissolution of electrocatalysts, but little is known about the stability of the ILs themselves. To fill this knowledge gap, this work introduces a method to quantify IL stability by tracking the dissolution of sulfur, an element present in most ILs utilized to improve the ORR activity of Pt-based electrocatalysts. We find that the conditioning time of IL-modified PtCo/C CLs-based membrane electrode assemblies (MEAs) in full-cell tests depends on the initial amount of IL. These new results suggest that IL removal from CLs of PEMFC is possible. Using a gas-diffusion-electrode (GDE) half-cell setup connected to an inductively coupled plasma mass spectrometer (ICP–MS), we show that IL dissolution correlates with ORR activity. Based on these findings, we propose a mechanism by which IL is washed out by the water produced during the ORR. This urges a need to stabilize the IL immobilization in the CL, such as electrocatalyst support pore-size engineering, which can be considered a vital approach to suppress IL removal. Although our work focuses on the stability of ILs in the CLs of PEMFCs, we believe that the insights gained are broadly applicable and can be transferred to other processes, such as electrocatalytic CO 2 reduction.