Abstract
A significant surface wettability transition from hydrophobicity to hydrophilicity after long-term operation is observed in graphite bipolar plates of a proton exchange membrane fuel cell (PEMFC). Extensive in-situ electrochemical characterization was performed to investigate the impact of bipolar plate wettability on the performance and durability of two PEMFCs (named “New cell” and “Old cell”), which use the same membrane electrode assemblies but have new and aged bipolar plates, respectively. Two-phase flow simulations are conducted to analyze water transport within bipolar plate channels with different wettabilities, utilizing the volume of fluid method. The Old cell is found to have reduced performance and accelerated degradation compared to the New cell. The degradation is attributed to increased liquid water at the cathode, which increases mass transport resistance by obstructing active catalyst sites. Simulations further demonstrated that as the hydrophobicity of the channel surface decreases, liquid water flow transitions from discrete droplet flow to semi-slug/film flow, and ultimately to slug flow. This transition results in greater water accumulation in hydrophilic channels, leading to higher pressure drops, intensified pressure oscillations, and more frequent sharp pressure drop peaks. These effects amplify the risk of cathode flooding, relating to experimentally observed performance losses and accelerated degradation in the Old cell.