Bridging the Gap: PEMFC Isothermal Water Storage Capacity Tests and Automotive Cold-Starts

Abstract

Automotive Proton Exchange Fuel Cells (PEFCs) are approaching technology readiness levels where personal transportation manufacturers are predicting eminent preliminary adaptation into the commercial market within the next decade. To further decrease PEFCs cost, reductions in platinum (Pt) catalyst loadings on the cathode are being sought. As the structure and the volume of the cathode catalyst layer (CL) are altered, a tangential impact will be made to sub-zero cold-start performance and durability will be realized.1 Automotive cold-starts below 0°C are challenging due to the inherent presence and production of water. The hydrophilic domains of the proton conducting ionomer in the CLs and the membrane must maintain a minimum hydration level of freezing point depressed water for operational proton conduction even at subzero temperatures. A sub-zero cold-start actively hydrates the ionomer due to production and mobility of freezing point depressed water. Saturation of the ionomer results in product water migration to the cathode open pore space which then likely exists as metastable supercooled water or ice. A successful cold-start is characterized by elevating the cell temperature above freezing for product water removal before the porous space within the cathode CL becomes blocked restricting mass transport of oxygen. Sub-zero cold-start performance in PEFCs is impacted by preconditioning hydration levels, CLs/membrane materials, and sub-zero operational conditions. The amount, state, and connectivity of water is controlled by the purge process after operational shutdown prior to freeze. Multiple research groups have established that the product water fill capacity of the CLs/membrane is extended for lower relative humidity (RH) preconditioning levels, which is at odds with the minimum parasitic losses mandated by the US Department of Energy.2 The hydration state of the CLs/membrane below zero can be examined via electrochemical impedance spectroscopy (EIS) using symmetric 4% H2 feeds.3 The impedance spectra acquired at -20°C in Figure 1 compares the preconditioning RH for two distinct shutdown methods prior to freeze: (grey) capped immediately after operational 80°C polarization curve and (black) equilibrium purged with RH inert gasses. The capped tests retained additional water that remind non-frozen at -20°C as seen by the low resistance shift in the high frequency intercept and a smaller high frequency arc related to improved charge transfer resistance. The subzero isothermal water storage capacity (WSC) is material processing dependent. Figure 2 compares the WSC at 10 mA/cm2 (-20°C) for two distinct catalyst layer (CL) fabrication methods with the same initial water content. The cathode CL prepared by decal transfer had a lower initial voltage, while the one directly sprayed onto the membrane suffered from limited WSC. Differences in initial voltage suggest significant CL ohmic contributions to overpotential. Differences in water storage capacity suggest CL structure differences. This may suggests a more ohmically connected, yet less porous electrode is produced using the direct spray. Finally, cold-start testing (Figure 3) is being evaluated using a quasi-adiabatic single cell fixture developed in-house, expanding upon the original proto-type designed at United Technologies Corporation4 , 5. Heating pads were integrated into the fixture and powered at levels equivalent to 1x and 2x adjacent cells to compensate for non-adiabatic losses. Higher water content favorable impact initial performance, however at long time scales seems to insignificantly impact cold start probability. Cold start probability is significantly impacted by the heating pad condition indicating significance of bipolar plate and coolant thermal mass impact on ability for CL to self-heat to above 0°C.