The Effect of Material Properties on the Subzero Water Storage Capacity of Cathode Catalyst Layers for Proton Exchange Membrane Fuel Cells

Abstract

Proton Exchange Membrane Fuel Cell cold start performance and durability from subzero conditions are dependent upon the cathode catalyst layer (CL) structure and interfacial binding to the membrane. The CL constituents and fabrication methodology impact the amount and location of freezing point depressed water available for proton conduction at subzero temperatures, water storage capacity and mechanical stability. Two common types of CL fabrication methods were studied to compare and contrast the nominal operational performance (80°C polarization curves) and water storage capacity at −10°C, −20°C, and −30°C; Decal Transfer versus Direct Spray. The cathode platinum on carbon CL constituents were also varied; ionomer-to-carbon support loadings (20wt% and 30wt%) and ionomer equivalent weight (830EW, 1000EW, and 1100EW). The different cathode CLs were assessed for resistance to proton conduction () and electrochemical surface area (ECSA). The Sprayed cathode CL had the least and highest ECSA, while decreasing the EW negatively impacted the electronic continuity of the CL. The baseline configuration (1100EW 30wt%) nominal operational iR-free voltages at 500 mA cm−2 were 50 mV higher for the Spray method. Subzero isothermal water fill tests (WFTs) at −10 mA cm−2 measured water storage capacity at freeze-out; Decal transfer was 2-2.5x higher than Spray CLs.