Transient mass transport and cell performance of a PEM fuel cell

This study aims to establish an unsteady, three-dimensional mathematical model of proton exchange membrane fuel cells (PEMFCs) with four serpentine flow field designs. The effects of the serpentine flow field designs on the transient characteristics of the PEMFCs are evaluated in terms of transient response of local current density and mass concentration under sudden change in loading. When the operating voltage is 0.7 V, the weak electrochemical reactions result in a local current density distribution that is only minimally affected by the serpentine flow field designs. However, when operating voltage instantaneously drops from 0.7 V to 0.5 V, electrochemical reactions increase. To ensure a sufficient fuel cell oxygen supply, the oxygen mass fractions are high and uniform in the cathode gas diffusion and cathode catalyst layers, which cause overshoot phenomena in the local current density distribution. Because the inlet fuel flow rate is identical in all PEMFCs, the inlet fuel flow velocity decreases as the cathode inlet number increases. When the operating voltage abruptly drops, the low inlet fuel velocity prevents the PEMFC from maintaining a steady state. Therefore, the transient response time needed to reach a steady state in the quadruple serpentine flow field design is longest when overshoot or undershoot phenomena occur. Additionally, cells with large inlet fuel flow rates perform best and reach a steady state condition fastest when cell voltage changes occur suddenly.