Isothermal cold start of polymer electrolyte fuel cells

Experimental procedures, aimed at elucidating fundamentals of polymer electrolyte fuel cell startup from subzero temperatures, are reported. A method of equilibrium purge using partially humidified gas with well-controlled relative humidity is introduced to effectively control initial water distribution inside a cell prior to cold start. Isothermal cold start, in which single cells with sufficiently large thermal mass are used to fix the cell temperature constant at the startup ambient temperature, is proposed to study intrinsic cold start capability of the membrane-electrode assembly. The proton conductivity of membranes with low water content and at subzero temperatures is measured in situ. The cumulative product water in mg/cm(2) has been used to quantify performance of isothermal cold start and is shown to exhibit a nonlinear relationship with the membrane water uptake potential, defined as the difference between the membrane hydration water content and initial value prior to cold start. It is found that the membrane is a key component to enhance the intrinsic capability of isothermal cold start from - 30 degrees C. Finally, when the current density is high, the pore volume of the cathode catalyst layer is not fully utilized for ice storage, thereby decreasing cold start performance. (c) 2006 The Electrochemical Society.