Modeling and Simulation of the Degradation of Perfluorinated Ion-Exchange Membranes in PEM Fuel Cells

Abstract

A polymer electrolyte membrane ͑PEM͒ fuel cell model that incorporates chemical degradation in perfluorinated sulfonic acid membranes is developed. The model is based on conservation principles and includes a detailed description of the transport phenomena. A degradation submodel describes the formation of hydrogen peroxide via distinct mechanisms in the cathode and anode, together with the subsequent formation of radicals via Fenton reactions involving metal-ion impurities. The radicals participate in the decomposition of reactive end groups to form carboxylic acid, hydrogen fluoride, and CO 2 . Degradation proceeds through unzipping of the polymer backbone and cleavage of the side chains. Simulations are presented, and the numerical code is shown to be extremely time-efficient. Known trends with respect to operating conditions are qualitatively captured, and the exhibited behavior is shown to be robust to changes in the rate constants. The feasibility of a chemical degradation mechanism based on peroxide and radical formation is discussed. The proton exchange membrane ͑PEM͒ fuel cell has long been recognized as an important component of the future hydrogen economy. Although significant progress has been made on issues related to performance, the commercial viability of the PEM fuel cell is largely dependent upon overcoming a host of durability and degradation issues; 1 these include poisoning of the anode by carbon monoxide and hydrogen sulfide, 2-6 platinum sintering and dissolution, 7,8 degradation of carbon supports, 9,10 and membrane failure.11,12 Indeed, the lifetime of the PEM fuel cell is highly dependent on the lifetime of the ion-exchange membrane. The majority of membranes used in PEM fuel cells have a perfluorinated backbone and are modified to include sulfonic groups that facilitate the transport of protons. Typical examples of such materials are Aciplex, Flemion, Dow, and DuPont's Nafion, shown in Fig. 1.There are several known failure modes for these perfluorosulfonic acid ͑PFSA͒ membranes during fuel cell operation, involving mechanical, thermal, and electrochemical processes. Mechanical failure can occur in many forms, including tears, cracks, punctures, pinholes, and inadequate sealing, which can lead to reactant crossover and unwanted reactions. Among the causes of mechanical failure are manufacturing imperfections, 13 The most cited cause of membrane failure is chemical degradation, although in reality failure is likely to be a combination of both mechanical and chemical phenomena, the interplay between which is not fully understood. The two paramount steps in the sequence that leads to chemical degradation are ͑i͒ formation of the attacking species and ͑ii͒ attack by the species on the membrane structure. Both remain the subject of much debate. There is, however, a consensus that the chemical degradation of PFSA membranes is initiated by free radical attack on reactive end groups.11,12 Carboxylic acid ͑or other H-containing͒ end groups can form during polymerization or as a result of chemi...

Reduced-dimensional models for straight-channel proton exchange membrane fuel cells

“…The low RH operating conditions are in fact the trend of fuel cell application in order to eliminate or reduce parasitic power required for humidification and the effects of liquid water flooding. However, this comes at the cost of reduced membrane conductivity and an increased rate of chemical membrane degradation [14,19]. Table 2 Summary of properties and parameters used for the baseline calculation.…”

Non-isothermal modelling of the all-vanadium redox flow battery

“…High temperatures (caused primarily by large potential gradients) are harmful to perfluorinated membranes, such a Nafion ® [28][29][30], can enhance corrosion of the plates and can lead to water loss. The numerical simulations conducted in this work suggest that without heat dissipation from the cell, the temperature would rise to unacceptable levels in a relatively short period of time.…”

“…; y = 0 and y = h (Membrane) (29) Note that the proton flux across the membrane in the x direction is non-zero. At the interfaces between the electrodes and membrane the proton concentration is constrained to satisfy c H + = c f .…”

Development of the all-vanadium redox flow battery for energy storage: a review of technological, financial and policy aspects

“…In pilot studies, modelling can be used to systematically reduce the number of test cases and to analyse the results of tests and trials, as is already the case for fuel cells and static batteries. It can also provide valuable insight into the reaction environment at the cell level (details of the distributions of reactants, temperature, potentials and current density during operation) [96][97][98][99]. There is also , a generous battery capacity factor of 50%, zero fuel costs, the exclusion of housing infrastructure costs, grid connection and a capital recovery factor of 0.13 at a 5% interest rate for a 10-year battery life expectancy.…”