(Invited Plenary) Durability of Polymer Electrolyte Membrane Fuel Cells and the Development of Accelerated Stress Tests

Abstract

The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is one of the key barriers to their commercialization in automotive applications.1 All the components of the fuel cell membrane electrode assembly (MEA) including the catalyst, catalyst support, membrane and Gas diffusion layers have shown degradation during fuel cell operation. The DOE – Fuel Cell Technologies program has set lifetime durability targets of 5000 hours for automotive fuel cells and the U.S. Drive Fuel Cell Technical Team (FCTT) has developed AST protocols for the various components.2 This talk will outline the degradation mechanisms that have been observed in MEA components, and detail the Accelerated Stress Tests that have been designed to evaluate these degradation modes. The major degradation mechanism observed with Pt-alloy catalysts is the leaching of the transitional metal from the Pt –alloy and the associated mass activity loss. This is in contrast to pure Pt-catalyst where the main degradation mechanism is the increase of average catalyst particle size and associated loss in surface area. While most alloy catalysts also exhibit increase in particle size, the voltage loss in fuel cells is primarily controlled by the de-alloying of the catalyst. XRD and TEM studies reveal that the transition metal is seen to migrate out of the catalyst nano-particle into the ionomer within the catalyst layer and membrane. This mechanism has been observed during both simulated drive cycle measurements and AST measurements. Data will be presented from 2 different ASTs. The older FCTT AST that consists of potential cycling from 0.6 V to 1.0 V at 50 mV/sec3 and a newer electro-catalyst AST which is a square wave with upper and lower potentials of 0.95 V and 0.6 V with 3 seconds duration, and was based on literature reports.4 The degradation in these ASTs will be compared to that observed while performing the durability drive cycle protocol recommended by the FCTT.2 The Pt catalyst is usually supported on carbon that provides porosity and electronic conductivity to the catalyst layer. However, this carbon has been observed to corrode at high potentials. This corrosion either results in the evolution of CO2 and depletion of carbon in the catalyst layer or to an increase in oxide surface groups on the carbon and associate increase in hydrophilicity. Results from carbons with different surface areas will be presented and fuel cell performance loss will be correlated to both the collapse of the pore structure in the catalyst layer and the increasing platinum particle size resulting from support corrosion. CO2 evolution data will be presented to quantify the amount of carbon corrosion during drive cycle and AST measurements. Carbon corrosion during start/stop measurements and its effect on fuel cell performance will also be presented. Global and local membrane thinning due to chemical and mechanical degradation respectively have been observed during fuel cell operation. Data from field tests (Busses operated by Ballard) and drive cycle testing have shown that local failure primarily happens due to mechanical degradation which is further accelerated by any chemical degradation. The performance of un-stabilized and stabilized (both mechanical and chemical) membranes will be presented. The fluorine emission from the membranes is a good estimate of degradation rates and is accelerated by hot, dry conditions at open circuit and by RH cycling. A combined mechanical/chemical AST (RH cycling at OCV) was developed to mimic membrane durability under drive cycle conditions. Finally gas diffusion layer degradation and its effect on fuel cell performance will be presented. Drive cycle tests performed with Sigracet 24BC GDLs exhibited mass transport losses consistent with GDL degradation. This degradation was completely mitigated with the use of more advanced GDLs including Sigracet 29BC.