Subzero water distribution in proton exchange membrane fuel cells: Effects of preconditioning method

Automotive subzero cold-starts are challenging for proton exchange membrane fuel cells (PEMFCs) and depend on material structure and initial water content (λinitial). PEMFC cold-start capabilities are commonly investigated with isothermal water fill tests (WFTs) after partially dehydrating the cell at operational temperatures, then freezing, followed by applying low loads under 100 mA cm−2 to quantify the subzero water storage capacity. Herein, isothermal WFTs were performed using two preconditioning methods to evaluate the ionomer proton conductivity and water redistributions at −20 °C (at 10 mA cm−2): (i) Equilibrium purged to a specific relative humidity (44% or 75%, λinitial=3.2and5.3) and (ii) Load terminated with inlet relative humidities of 17% or 44% sealed immediately after polarization curves at 80 °C (calculated λinitial=4.0and4.7). The load terminated preconditioning method more closely emulates an on-board stack cold-start. For the two preconditioned tests with identical preconditioning inlet relative humidities (44%), the load terminated case showed a 2.3x reduction in protonic resistance and a 20% increase in electrochemical surface. Additionally, during the WFT the initial on load voltage of the load terminated preconditioning was 30–60 mV higher; however the time on load prior to freeze-out was 23 min compared to the 31 min for equilibrium purged.