Comparative study of single-phase and two-phase physics-based models for proton-exchange membrane fuel cells

A physics-based non-isothermal proton-exchange membrane fuel cell model is developed to analyze the common hypothesis of neglecting liquid water in the system. Simulations at different temperatures and relative humidities are performed to evaluate the limitations of this assumption. The tests indicate that the increase of relative humidity causes significant deviations in the polarization curve at high current densities, which have been mainly related to a reduction in oxygen's content at the catalyst layer indirectly caused by the inaccurate water vapor's molar fraction predicted by the single-phase submodel, resulting in higher cathodic activation overvoltages. Moreover, this assumption considerably affects the profiles for other key properties – such as temperature and water content in the ionomer – even when the deviations in the polarization curve are minor. These effects are more pronounced at lower temperatures, where a relative humidity of 50 % is already enough to affect the results, and deviations due to ohmic overvoltage become noticeable, which was not the case at higher temperatures. Therefore, the hypothesis of single-phase operation should be limited to fuel cells at higher temperatures, low humidities, and with small ohmic overvoltage contributions. However, even under those conditions, important properties for water management may be affected.