Catalyst Degradation in Polymer Electrolyte Fuel Cells with Multi-Modal Techniques: Understanding Phenomena Under Varied Gas and Relative Humidity

Abstract

For widespread commercialization of polymer electrolyte fuel cells (PEFCs), cost and durability need to be addressed. Reduction in cost should not negatively affect PEFC durability and performance. At high volume production, platinum (Pt) electro catalyst contributes to 41% of the total fuel cell stack cost for light duty vehicles. This cost contribution increases to 53% for middle and heavy-duty vehicles. When membrane electrode assemblies (MEAs) are designed for durability, this cost will increase further. A complex balance exists between cost, performance and durability. With smaller nanoparticle size better Pt dispersion is achieved which gives improved performance and reduces Pt loading and cost. But this negatively affects durability, as smaller nanoparticles have higher surface energy which causes faster electrocatalyst degradation1. During an actual drive cycle, PEFCs need to operate over a wide range of operating conditions such as temperature (-40C to 120C), potential (0V to 1.5V) and relative humidity which can further accelerate Pt dissolution Under load cycling during typical drive cycle, repeated oxidation and reduction of Pt surface leads to Pt dissolution2. Dissolved Pt ions redeposit on nearby nanoparticles effectively increasing their particle size2. This electrochemical surface area (ECSA) loss via increase in particle size is well known as Ostwald ripening. Some Pt ions are reduced in the electrolyte membrane by crossover hydrogen which leads to formation of a Pt band, usually at the membrane and cathode catalyst layer interface3. At > 1V, carbon corrosion can lead to Pt detachment2. This can happen during start-up/shut down cycles when anode is exposed to hydrogen and air. Such degradation mechanisms need to be studied systematically with effect of operating conditions like gas flow rates, temperature and relative humidity under dynamic load cycling.In this study, we investigate the effect of cathode gas environment and gas flow rates on Pt particle size during aging experiment. A DOE adopted accelerated stress test (AST) protocol simulating the drive cycle operating conditions is used. It consists of a 0.6 V – 0.95 V square wave with 3s hold time at each potential. In situ electrochemical characterization at regular stages is used to identify ECSA loss trends and polarization performance. Figure 1 shows normalized ECSA loss for the four cells cycled in different environments during catalyst AST. Measurements were taken beginning of life (BOL), after 1,000, 5,000, 15,000 and 30,000 cycles. We observed that the cell that was cycled in N2 and 100 % RH degraded the most , having 25 % of ECSA left after 30,000 cycles compared to BOL. Electrochemical measurements are supplemented by detailed post-mortem ex-situ x-ray characterization by using micro x-ray fluorescence, x- ray computed tomography and micro x-ray diffraction, SEM-EDS. 1cm2 active areas of the catalyst layer at the inlet, middle and outlet of the flow field were mapped using micro x-ray diffraction to understand Pt particle size distribution under land/channel. Catalyst layer loading before and after the AST were confirmed using micro x-ray fluorescence.References(1) Cherevko, S.; Kulyk, N.; Mayrhofer, K. J. J. Durability of Platinum-Based Fuel Cell Electrocatalysts: Dissolution of Bulk and Nanoscale Platinum. Nano Energy 2016, 29, 275–298. https://doi.org/10.1016/j.nanoen.2016.03.005.(2) Meier, J. C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H. J.; Topalov, A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K. J. J. Design Criteria for Stable Pt/C Fuel Cell Catalysts. Beilstein J. Nanotechnol. 2014, 5 (1), 44–67. https://doi.org/10.3762/bjnano.5.5.(3) Zhang, J.; Litteer, B. A.; Gu, W.; Liu, H.; Gasteiger, H. A. Effect of Hydrogen and Oxygen Partial Pressure on Pt Precipitation within the Membrane of PEMFCs. J. Electrochem. Soc. 2007, 154 (10), B1006. https://doi.org/10.1149/1.2764240. Figure 1