Expandable model and size effect of proton exchange membrane fuel cells with elevating temperature and reducing humidity

Abstract

Proton Exchange Membrane Fuel Cell (PEMFC) has become one of the key power sources for electrical transportation. As the power demand increases, the active area of PEMFC becomes larger. However, under the same operating conditions and with the same materials, performance of commercial large-area fuel cells (LAFCs) differ from those of laboratory-level small-area fuel cells (SAFCs) due to the uneven distribution of internal states, such as gas and water content. This phenomenon is referred to as size effect. Investigating the mechanism of size effect is essential for designing and optimization of LAFCs. In this study, an expandable along-the-channel dynamic model is established, which integrates the mass transport processes along proton transport and channel directions. By adjusting the number of partitions along the channel, in-plane heterogeneity can be simulated considering both accuracy and efficiency. The polarization curves and current density distribution of 44 cm² SAFC and 291 cm² LAFC under different temperatures and cathode inlet humidities are tested. Two models with different number of partitions are calibrated and used to reconstruct internal states of SAFC and LAFC. It is found that water accumulation along the channel and water transfer between anode and cathode can significantly enhance the water content in the membrane and catalyst layers in LAFC. This results in lower membrane resistance and activation polarization at low temperatures compared to SAFC. However, as temperature increases and humidity decreases, the water loss issue becomes more pronounced in LAFC, occurring in the following order: air inlet, air outlet, and central region. Although this contributes to lowering the liquid water content and enhancing oxygen diffusion, its suppression of the oxygen reduction reaction and the resultant increase in membrane resistance are expected to have a dominant effect in LAFC, leading to a significantly lower performance of LAFC than that of SAFC under high temperature and low humidity conditions. This study reveals the intrinsic mechanisms behind performance changes in fuel cells upon scaling up through modeling and experiments, laying the foundation for the optimization of LAFCs.