The accelerated degradation in the front cells of a polymer electrolyte membrane fuel cell(PEMFC)stack seriously reduces the reliability and durability of the whole stack.Most researches only focus on the size and configuration of the gas intake manifold,which may lead to the maldistribution of flow and pressure.In order to find out the mechanisms of the accelerated degradation in the front cells,an extensive program of experimental and simulation work is initiated and the results are reported.It is found that after long-term lifetime tests the accelerated degradation in the front cells occurs in all three fuel cell stacks with different flow-fields under the U-type feed configuration.Compared with the rear cells of the stack,the voltage of the front cells is much lower at the same current densities and the membrane electrode assembly(MEA)has smaller active area,more catalyst particle agglomeration and higher ohmic impedance.For further investigation,a series of three dimensional isothermal numerical models are built to investigate the degradation mechanisms based on the experimental data.The simulation results reveal that the dry working condition of the membrane and the effect of high-speed gas scouring the MEA are the main causes of the accelerated degradation in the front cells of a PEM fuel cell stack under the U-type feed configuration.Several mitigation strategies that would mitigate these phenomena are presented:removing cells that have failed and replacing them with those of the same aging condition as the average of the stack;choosing a Z-type feed pattern instead of a U-type one;putting several air flow-field plates without MEA in the front of the stack;or exchanging the gas inlet and outlet alternately at a certain interval.This paper specifies the causes of the accelerated degradation in the front cells and provides the mitigation strategies.
LI PengchengPEI PuchengHE YonglingYUAN XingCHAO PengxiangWANG Xizhong
The stoichiometric ratios and related regimes, which can promote anti-flooding of polymer electrolyte membrane fuel cell (PEMFC) with in-plate adverse-flow flow-field (IPAF), were investigated. Two flow combinations, which are the simple and complex adverse-flow between plates (ABP) that can be realized by IPAF, were employed. Constant stoichiometric ratios examination indicates that the complex ABP could improve anti-flooding of PEMFC better in the medium (greater than 200 mA/cm2 and less than 1 000 mA/cm2 ) and high (greater than 1 000 mA/cm2 ) current densities than the simple ABP. More stoichiometric ratios were introduced to find the cathode critical stoichiometry. Under the condition of cathode critical stoichiometry, the maximal local relative humidity of both electrodes of complex ABP is equal to 100% and below while the anti-flooding of the cathode of simple ABP is not satisfactory in the medium and high current densities. Further study shows that the mechanism of fuel cell, which is the interdependence between the electrodes effect, can make significant contribution to anti-flooding.
