Proton Transport and Durability of Polymer Electrolyte Membrane Fuel Cells’ Catalyst Layer

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are promising power sources for transportation applications due to their low carbon dioxide emission, and high energyefficiency features. However, there are still some problems that hinder the wide commercialization of fuel cell electric vehicles (FCEVs), including poor durability, and highcost. For the state-of-the-art PEMFC system, the cost of catalyst still takes about 30% of the total cost. The cost of fuel cell system is still above the Department of Energy (DOE) 2020 cost target of 40 $=KW. As platinum (Pt) is still the most commonly used catalyst for PEMFC, it is hard to reduce this portion of raw material cost by mass production.Within the fuel cell, the oxygen reduction reaction (ORR) that happens at the cathode has very sluggish kinetics compared with the hydrogen oxidation reaction (HOR) that happens at the anode. Because of that, the fuel cell cathode requires much more loading of catalyst than the anode. Though there has been a lot of progresses in the development of alternative non-platinum group metal (non-PGM) catalysts, the specific activity and durability are stillnot comparable with conventional Pt catalyst. In the near term, increasing the utilization of Pt is a more viable solution to reduce the catalyst loading and cost while maintainingthe required power density. Scientists and engineers have put significant efforts into developing new platinum group metal (PGM) catalyst using alloying or nano-structuringtechniques. Another approach that is of equivalent importance is to make the catalyst surface area electrochemically accessible, without contact with the polymer electrolyte that poisons the catalyst. Part of this dissertation, we studied the proton transport properties of anew ionomer-free catalyst configuration, and the ionomer-free regions in the conventional carbon supported Pt catalyst (Pt/C) catalyst. The water and electrode potential dependence of the proton transport process were studied in detail. The information will be instructive to new catalyst structure design and operating condition optimization.Another issue challenging automotive PEMFC systems is their durability. Unlike stationary power system, automotive applications requires the PEMFC system to experiencequick variation of power demand, a wide variety of weather conditions, including subfreezing conditions, and thousands of cycles of start-up/shutdown processes. The powerdemand variation and subfreezing condition operation can occasionally cause hydrogen starvation in the fuel cell catalyst layer, leading to cell reversal and potentially carboncorrosion damage. Start-up/shutdown cycles can also lead to electrolyte potential perturbations and slow degradation to the cathode. In this work, we examined the hydrogenstarvation at regular temperatures by looking at the oxygen evolution reaction (OER) catalyst failure mechanism at the anode, and tested a new anode configuration with enhancedreversal tolerance. We also studied the degradation mechanism related to subfreezing condition operations. As for the start-up/shutdown cycle durability, we tested a series of selective anode catalysts and their effectiveness in preventing cathode degradation.