Review-Subzero Automotive Fuel Cells: Water Fill Tests Vs Cold-Starts

Subzero cold-starts are one of the last remaining challenges to the commercialization of automotive fuel cells. The ultimate cold-start target is to exothermically self-start a fuel cell stack from -20 degrees C to attain 50% rated power within 30 s, expending only 5 MJ of start-up/shutdown power-currently unattainable with today's materials, thus exacerbating the performance gap of future advanced materials. There exists a disconnect between published subzero isothermal water fill test results and industrially relevant cold-starts. For an isothermal water fill test in a single cell at -20 degrees C, the highest water fill capacity (7.8 mg(H2O)cm(-2) in 2100 s) was observed for the lowest applied galvanostatic load of 10 mA cm(-2) with the lowest initial water content (lambda(initial) = 2.2). Conversely, the fastest time to 0 degrees C for a 36-cell stack from -20 degrees C was 54 s under an adaptive applied galvanostatic/potentiostatic load, due to adjacent cell heating accelerating the overall stack's thermal rise. A higher lambda(initial) of 6.2 or greater enhances the cold-start performance and thermal response of a stack. For unassisted cold-starts, both convective mass/heat flux are necessary to avoid accumulation of ice and to overcome the thermal mass of the endplates of a stack with 20-cells or more.