• Energy Research

Abstract A transient 2D physical continuum-level model for analyzing polymer electrolyte membrane fuel cell (PEMFC) performance is developed and implemented into the new numerical framework NEOPARD-X. The model incorporates non-isothermal, compositional multiphase flow in both electrodes coupled to transport of water, protons and dissolved gaseous species in the polymer electrolyte membrane (PEM). Ionic and electrical charge transport is considered and a detailed model for the oxygen reduction reaction (ORR) combined with models for platinum oxide formation and oxygen transport in the ionomer thin-films of the catalyst layers (CLs) is applied. The model is validated by performance curves and impedance spectroscopic experiments, performed under various operating conditions, with a single set of parameters and used to study water management in co- and counter-flow operation. Based on electrochemical impedance spectra (EIS) simulations, the physical processes which govern the PEMFC performance are analyzed in detail. It is concluded that the contribution of diffusion through the porous electrodes to the overall cell impedance is minor, but concentration gradients along the channel have a strong impact. Inductive phenomena at low frequencies are identified from physics-based modeling. Induction is caused by humidity dependent ionomer properties and platinum oxide formation on the catalyst surface.

Proton Exchange Membrane Fuel Cells (PEMFC) are energy efficient and environmentally friendly alternatives to conventional energy conversion systems in many yet emerging applications. In order to enable prediction of their performance and durability, it is crucial to gain a deeper understanding of the relevant operation phenomena, e.g., electrochemistry, transport phenomena, thermodynamics as well as the mechanisms leading to the degradation of cell components. Achieving the goal of providing predictive tools to model PEMFC performance, durability and degradation is a challenging task requiring the development of detailed and realistic models reaching from the atomic/molecular scale over the meso scale of structures and materials up to components, stack and system level. In addition an appropriate way of coupling the different scales is required.

Abstract Chemical membrane degradation causes deterioration of critical membrane properties such as gas separation which finally causes failure of polymer electrolyte membrane fuel cells (PEMFCs). In order to identify the underlying physical processes, a physics-based model of chemical membrane degradation is implemented into the novel numerical framework NEOPARD-X [1]. The existing 2D PEMFC model is extended to incorporate the mechanisms of hydrogen peroxide formation and reduction, a redox cycle of iron contaminants in the ionomer phase, radical formation due to Fenton's chemistry and radical attack on the polymer structure. Unzipping of the polymer backbone and scission of the side chains are considered as degradation mechanism. The degradation model is validated against experimental data obtained in accelerated stress tests (ASTs). From theoretical considerations, the influence of chemical membrane degradation on the cell performance is revealed. The influence of pressure, relative humidity and cell voltage on the chemical degradation is rationalized. The operating conditions strongly influence the kinetics and spacial distribution of the membrane degradation. Degradation is found to be most pronounced at elevated pressure, high relative humidity and high cell voltage close to the interface of anode catalyst layer and PEM.