Impact of liquid-water accumulation and drainage cycles on fuel-cell performance and stability

A transient, two-phase, 2D model of a proton-exchange-membrane fuel cell (PEMFC) is developed and used to study liquid-water dynamics. Porous structure of the cell components is taken into account via a mixed-wettability pore-size-distribution sub-model. A novel dynamic boundary condition is proposed for describing the experimentally observed cycles of liquid-water accumulation and drainage in gas-diffusion layers (GDLs). Numerical results are compared to current-density and resistance responses to varying-scan-rate polarization sweeps and voltage steps measured with an in-house single-channel cell at multiple operating conditions. This work demonstrates how high breakthrough and minimum exit capillary pressures in GDLs, coupled with fast liquid-water eruption, result in water accumulation/drainage cycles that induce current-density oscillations. Numerical simulations reveal the existence of a "resonant'' scan rate that maximizes polarization-curve hysteresis. At this scan rate, the time scales of the voltage sweep and liquid-water accumulation/drainage cycle are similar. Flooding of hydrophilic catalyst layers is shown to result in a sudden loss of performance within single seconds, which can be captured with fast-scan polarization curves. Overall, this work demonstrates the critical importance of transient analysis of PEMFC operation under wet conditions in both experimental and modeling research.