Characterization of distributed performance of polymer electrolyte fuel cells under low humidity conditions

by 1976- Dong, Qunlong

iii Low humidity operation is helpful to restrain flooding within polymer electrolyte fuel cells (PEFCs), as well as to reduce system overhead. However, low humidity operation also brings challenge on membrane hydration. In order to better understand the water transport in PEFCs under low humidity condition, as well as provide detailed experimental validation for development of advanced modeling of PEFCs, an experimental study combining simultaneous current, resistance and species distribution measurements was conducted, and a quasi-two-dimensional mass transport model was developed to analyze the effect of transport properties of fuel cell materials on water management for low humidity operation. Two segmented fuel cells with very different flow field configurations and thermal boundary conditions were designed and developed for this study. The actively control on the backing plate temperature of the improved design enables an isothermal boundary condition. Two new techniques for in-situ species measurement, a Micro Gas Chromatograph (Micro GC) and a Real Time Gas Analyzer (RTGA), have been applied with the ability to measure up to saturated levels in the flow stream. Typical PEFC steady-state and transient behaviors under low humidity operating conditions were investigated respect to various operation parameters, including inlet humidity, reactant stoichiometry, temperature, and pressure. The results show that, under reduced humidity conditions the anode water content has dominating effect on current density profile. The current distribution curves look qualitatively similar to water mole fraction in the anode curves, and the inverse of the high frequency resistance (HFR) profiles. Based on these experimental results, a characteristic profile of current density, species, and HFR distributions is proposed for low humidity operation. The change of thermal boundary does affect the distributed performance, although the essential distribution profiles are maintained, reaffirming the importance of thermal management on water management. The dynamic response of fuel cell performance to step change in voltage is heavily affected by humidification condition. For a dry cathode condition, the anode iv water content adjusts relatively slowly from high to low current, while for a dry cathode condition, the adjustment is relatively fast. For both cases, the current density adjustment time corresponds to the time of mass adjustment in the anode side, not the cathode side, indicating a non-linear water content profile in the electrolyte membrane. With low inlet humidities, the fuel cell performance is more limited by insufficient electrolyte hydration instead of depletion of oxygen. In general, the higher overall input relative humidity results in the higher bulk cell performance, despite very different local distributions. The humidifying effect from water production on the cathode catalyst layer is another critical factor for the performance PEFCs under low humidity condition. Higher cathode stoichiometry, elevated cell temperature, and lower exit pressure can all increase the amount of water required for humidification and reduce the humidifying effect of water production, resulting in lower cell performance. However, a high overall inlet relative humidity can negatively impact the cell performance under the combination of high temperature and low pressure due to the extremely low oxygen concentration. A mass transport model was developed to examine the transport properties of Gore SELECT® MEAs, as well as be a tool to look at ideal low humidity operation design. The mass transport model found that the water diffusion coefficient in Gore SELECT® reinforced membrane roughly equals the coefficient in Nafion® membrane. It was also detected that the mass transport resistance of porous gas diffusion media can create considerable water activity difference between the gas flow channel and the catalyst layer, which boosts the humidifying effect of generated water. The relatively high binary diffusion coefficient of H2O/H2 in the anode compared to the air cathode contributes to the dominating effect of local anode water profiles on local cell performance. These findings also provide direction to the development of MEA for low humidity operation.