Abstract and subjects
In this paper, the electrochemical kinetics, oxygen transport and solid water formation within the cathode electrode of polymer electrolyte fuel cells (PEFCs) during cold start is investigated. We specifically evaluate the key parameters that govern the self-startup of PEFCs by considering a wide range of the relevant factors. These parameters include characteristic time scales of cell warm-up, ionomer hydration in the catalyst layer, ice build-up and melting, as well as the ratios of the time constants. Supporting experimental observation using neutron imaging and isothermal cold start experiment is discussed. Gas purge is found to facilitate the PEFC cold start but the improvement may be relatively small compared with other methods such as selecting suitable materials and modifying the cell design. We define a three-step electrode process for cold start and conduct a one-dimensional analysis, which enables the evaluation of the impact of ice volume fraction and temperature variations on the cell cold start performance. The ionic conductivity data of Nafion
® 117 membrane at subfreezing temperature, evaluated from experiment, is utilized to analyze the temperature dependence of the ohmic polarization during cold start.