The fuel cell is an outstanding example for a multi-scale system. This situation is shown schematically in Figure 1. Chemical and electrochemical reactivity takes place on a nanometer scale; it is strongly dependent on nano- and microstructural properties. Mass, charge and heat transport takes place from a micrometer over millimeter up to decimeter scale. Time scales vary from sub-nanoseconds (electrochemical and chemical reactions) over seconds (transport) up to days or even months (structural and functional degradation). All process are strongly, and often nonlinearly, coupled over the various scales. Processes on the microscale can therefore dominantly influence macroscopic behavior. For this reason, a detailed understanding of the relevant processes on all scales is required for a computer-based optimization of fuel cell design, performance and durability.
Due to the high complexity of the processes, it is not possible to treat all scales within one single group. The focus of DLR’s activities are the surface, electrode, cell and system levels.
Surface level: Elementary electrochemical and degradation kinetics. Elementary kinetics means the resolution of chemistry into single steps that represent reactivity on the molecular scale. The surface reactivity is described via a mean field approximation: Reaction rates are modeled with Arrhenius expressions, and the surface state is described based on averaged quantities (surface coverages, thermodynamic and kinetic adsorbate properties). This approach allows the simulation of macroscopic electrochemical behavior while still including atomistic details of reactivity. Semi-empirical approaches such as Butler-Volmer equations are not used. The approach allows a detailed understanding of competing reaction pathways, rate-determining steps, coupling of reaction kinetics with microstructure, and the origin of degradation processes.
Electrode level: Multi-phase porous mass and charge transport. The activities include combined continuum and microstructure-resolved approaches. Continuum models are applied to integrate porous transport processes into macroscopic cell-level simulations. Microstructure-resolved models, on the other hand, are used to determine important parameters needed for the continuum models, such as effective surface areas and phase boundaries, transport coefficients, relative permeabilities etc.
Cell level: Mass, charge and energy transport. Transport processes on the cell level are modeled using methods of computational fluid dynamics (CFD). Two different approaches are followed. One the one hand, reduced-dimensional models (e.g. 1D+1D) are used. They include only a simplified geometrical description, but are computationally very efficient and can therefore be used for multi-scale approaches such as downscaling (coupling to detailed processes on the electrode and surface levels) and upscaling (use in stack and system level models). On the other hand, full three-dimensional models are applied. Due to their high computational effort they can include only simplified models of electrochemistry; however they allow a detailed investigation of transport processes on the realistic cell geometry.
System level: Process simulation. The activities on the system level aim at the understanding, optimization, and control of the interaction between the fuel cell stack and balance-of-plant components. Important aspects are the prediction of load characteristics and transient behavior for different power ranges; assessment and optimization of system efficiency; choice of components and optimum system design; as well as control strategies and control system design.