Physical Model of Catalyst Dissolution, Platinum Band Formation, Hydrogen Peroxide Production and Chemical Degradation of Proton Exchange Membrane in Low Temperature Fuel Cells

Abstract

Low temperature fuel cells with perfluorinated proton exchange membrane (LT-PEMFC) are recognized as one of the most promising technologies for clean electricity production in automotive sector. Despite significant progress in their efficiency and reliability, the degradation of fuel cells still remains an important issue, significantly hindering their wider market adoption. Degradation processes can to some extent be avoided by the use of new, more durable materials and components, but also by properly managing the fuel cell operation and thus avoiding the conditions in which the fuel cell degradation is most pronounced. Fast and reliable computational models of fuel cell operation and degradation are crucial for efficient development of such mitigation strategies.The catalyst layer and the membrane are the components recognized as being most susceptible to degradation phenomena. In the catalyst layer, high temperature, electric potential and relative humidity lead to electrochemical corrosion of carbon support and dissolution of the catalyst, most commonly being platinum or platinum-based binary alloy. Dissolved platinum, diffusing into the membrane, plays an important role in its chemical and consequent mechanical degradation. The platinum particles in the membrane serve as the catalyst for hydrogen peroxide formation from crossover fluxes of hydrogen and oxygen, while the presence of transition metal ions, such as copper or cobalt, originating from the binary catalyst dissolution, initiate the decay of hydrogen peroxide into various radical species via Fenton-like reactions [1]. The radical’s attack on the perfluorinated chains and sulphonic groups leads to a reduction of membrane conductivity, thinning of the membrane and consequent increase in its susceptibility to mechanical degradation, caused by the swelling and shrinking of the membrane due to the changes in its humidity, which can result in formation of cracks and pinholes in the membrane.To properly model the entire causal chain of chemical membrane degradation, several individual processes mentioned above need to be coupled into a unified model. To achieve this, we present a physically based spatially and temporally resolved model of fuel cell operation, providing the detailed picture of species concentration and electric potential distribution in the fuel cell [2], coupled with innovative model of hydrogen peroxide formation, diffusion and decay into reactive radicals, causing the chemical degradation of the membrane. The model describes the electrochemical dissolution of binary alloy catalyst, its diffusion into the membrane and the formation of so-called Pt band. The hydrogen peroxide production is modeled by including both its formation in the catalyst layers and inside the Pt band by consistently taking into account the cross-over fluxes of reactants and local electric potential across the membrane [3]. The rate of radical formation (.H, .OH, .OOH) via Fenton-like reactions is modeled by considering the concentration of transition metal ions, originating from electrochemical dissolution of binary alloy catalyst and their diffusion into the membrane. The chemical degradation due to reactive radical’s attack on the membrane is described by recently developed 0-D reactor approach [4].The output of the model is a detailed spatially and temporally resolved profile of chemical groups concentration in the membrane, which enables the calculation of changes in both membrane thickness and proton conductivity. The results, indicating most pronounced membrane degradation in vicinity of Pt band, are compatible with the Raman measurements of chemical group concentrations in the aged membrane [5]. Furthermore, the testing of the model on long-term degradation experiments is used to identify the optimal fuel cell operating conditions in which the durability of the fuel cell membrane is maximized.[1] Strlič M, Kolar J, Šelih VS, Kočar D, Pihlar B. A comparative study of several transition metals in fenton-like reaction systems at circum-neutral pH, Acta Chim Slov 2003;50:619–32.[2] Tavčar G, Katrašnik T. A Real Time Capable Quasi 3D System Level Model of PEM Fuel Cells, Fuel Cells 2020;20:17–32.[3] Burlatsky SF, Gummalla M, Atrazhev V V., Dmitriev D V., Kuzminyh NY, Erikhman NS. The Dynamics of Platinum Precipitation in an Ion Exchange Membrane, J Electrochem Soc 2011;158:B322.[4] Frühwirt P, Kregar A, Törring JT, Katrašnik T, Gescheidt G. Holistic approach to chemical degradation of Nafion membranes in fuel cells: modelling and predictions, Phys Chem Chem Phys 2020;22:5647–66.[5] Ohma A, Yamamoto S, Shinohara K. Membrane degradation mechanism during open-circuit voltage hold test. J Power Sources 2008;182:39–47. Figure 1