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. This review provides a comprehensive overview of the state of the art in modeling of PEMFC, covering all relevant scales from atomistic up to system level as well as the coupling between these scales. Furthermore, it focuses on the modeling of PEMFC degradation mechanisms and on the coupling between performance and degradation models.

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 nonisothermal, 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

No abstract

The loss of electrochemical active surface area (ECSA) at the cathode is one of the main causes of performance degradation in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). In order to investigate the catalyst degradation and the influence of the operating conditions we develop a multiscale degradation model which includes the formation and reduction of platinum oxides, platinum dissolution, particle growth due to Ostwald ripening, platinum ion transport through the ionomer and platinum band formation in the membrane. This degradation model is coupled with a 2D PEMFC performance model and predictions regarding ion concentration, ECSA evolution and particle growth are validated with dedicated experiments and literature data. Degradation under several AST protocols and under steady state operation are compared and discussed. The importance of a spatially resolved catalyst degradation model is conveyed by the occurrence of a depletion zone in the catalyst layer close to the membrane due to the platinum migration into the membrane. By comparing the correlation between platinum mass loss in the catalyst layer and the ECSA loss we conclude that catalyst degradation under AST conditions with nitrogen is not representative for the degradation under normal operation.

High cost and performance degradation are still the main issues which hinder the commercialization of low temperature fuel cells on a grand scale [1]. Both of these issues are closely related to the catalyst which accounts for up to 40% of the PEMFC stack cost [2] while the loss of electrochemically active surface area (ECSA) poses a major contribution to the overall performance degradation. Furthermore, chemical membrane degradation is another main issue as membrane thinning and pinhole formation are limiting the lifetime of the cells. Detailed physical models provide a better understanding of the underlying mechanisms leading to these degradation mechanisms and therefore can help in developing strategies to increase the durability of the cells. Here we present a two-step approach to achieve this goal. First we develop a transient, two-dimensional single cell model, which includes all relevant mechanisms to describe the cell performance, i.e., electrochemistry, two-phase multi-component transport in the porous layers, charge and heat transport as well as water and gas permeation through the membrane. This model is implemented in our in-house code NEOPARD-X which is based on the open-source framework DuMux [3]. It provides important insights on the local conditions within the cell which are often not accessible in experiments but determine the local degradation rates. In particular we discuss the water management and how simulations of electrochemical impedance spectra (EIS) can be used for process identification. In the second step we discuss detailed physical models for the degradation mechanisms. A multi-step chemical membrane degradation model is presented which incorporates the formation and decomposition of hydrogen peroxide, iron ion redox cycle, radical formation and degradation via “unzipping” and “side chain scission” mechanism. The model provides insights on the local degradation rates depending on the operating conditions. Strongest degradation is obtained at the anode side during OCV while at higher current densities the degradation is strongly reduced and shifts to the cathode side (left figure). The model is validated with fluoride emission rate (FER) measurements under various operating conditions. Finally, a catalyst degradation model due to platinum dissolution and particle growth is discussed. Since the dissolution kinetics strongly depends on the platinum oxide coverage, a platinum oxide model has been developed and validated with dedicated CV experiments. This model is able to describe the experimentally observed logarithmic growth of the oxide coverage. This coverage affects the surface energy of the particles and thus influences the platinum dissolution. Therefore, taking into account the kinetics of the oxide formation is crucial for describing the catalyst degradation under dynamic operating conditions such as fast potential cycling which is typically used as an AST for the catalyst. By coupling the degradation model to the single cell model we investigate the catalyst degradation in AST and long-term degradation tests. The degradation model is validated with experimental data for the ECSA loss during these tests as well as with particle size distributions (PSD) obtained with TEM (right figure). The occurrence of heterogeneities in the catalyst degradation is discussed. Figure: simulated local FER (mol m-3 s-1) due to chemical membrane degradation at various cell voltages (a); comparison between simulated and measured catalyst PSD evolution (b). Figure 1