Effect of Commercial Gas Diffusion Layers on Catalyst Durability of Polymer Electrolyte Fuel Cells in Varied Cathode Gas Environment

Abstract

Improvement in durability of polymer electrolyte fuel cells (PEFCs) accelerates the commercialization of the technology in both light-duty and heavy-duty commercial vehicle applications. The Department of Energy (DOE) has adopted an accelerated stress test (AST) protocol to evaluate the durability of catalyst in PEFCs1. The protocol simulates a typical drive cycle and consists of potential cycling (30,000 square wave cycles) in nitrogen gas environment on cathode. The potential is cycled from lower potential limit (LPL) of 0.6 V to upper potential limit (UPL) of 0.95 V with a hold time of 3 seconds at each potential. Multiple studies have shown UPL, relative humidity (RH) and temperature as the primary stressors of catalyst degradation2,3. Gas diffusion layers (GDLs) play a key role of water management in the cathode catalyst layer and can therefore directly affect the local RH4. Currently, properties such as porosity, micro porous layer (MPL) thickness, PTFE content and thermal conductivity of commercially available GDLs have been optimized for peak power density at beginning of life under varying operating conditions including wet (high RH) and dry (low RH). Little is known about the effect of GDL on the durability of catalyst in ASTs.In this study we compare Pt catalyst aging in membrane electrode assemblies using three different commercially available GDLs, with distinct thermal, mechanical, and electrical properties. In the first part of the study, the DOE adopted AST protocol is used at 80°C operating temperature and 100% RH. In situ electrochemical characterization at regular intervals is utilized to identify electrochemical surface area loss (ECSA) trends and polarization performance. Figure 1 A shows before and after AST polarization curves for different GDLs and Figure 1 B shows ECSA loss as a function of AST cycles. Measurements were taken before AST, after 1,000, 5,000, 15,000 and 30,000 cycles. It was observed that for AST conducted in nitrogen cathode gas environment, catalyst durability is independent of GDL used. This is also confirmed by the ECSA loss trend. The indifference in Pt catalyst degradation can be attributed to lack of water produced due to absence of oxygen reduction reaction in nitrogen environment. All the GDLs lost approximately 80% of their ECSA from before AST. The second part of this study aims to elucidate effect of GDL properties on catalyst durability by conducting AST in air environment with open circuit potential as UPL. Figure 1 A. Before and After AST polarization curves collected at 80°C, 100% RH and 150kPa (a) backpressure in differential conditions B. ECSA loss as function of AST cycles.