Polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC) electrode and cell level modeling
The PEFC or DMFC single cell is a complex arrangement of multiple functional layers, including the polymer membrane, catalyst layers, microporous layers, and gas diffusion layers. A central task of the modeling activities is to describe, understand and optimize the functionalities of these layers. Typically, multiple different phases and materials are involved, that is, solid membrane, solid catalyst, solid diffusion layer, liquid aqueous phase, and gas phase (Fig. 1), all of which have different transport and chemical properties that need to be included in the model.
As PEFCs typically operate around 80°C, the reaction product water can occur in both the gas phase and the liquid phase. Consequently, the models include two-phase water transport processes. Here, diffusive and pressure-driven transport is coupled to evaporation and condensation as well as water production from the electrochemical reaction. These processes furthermore strongly depend on the temperature distribution in the cell. Only detailed numerical simulations allow to unravel these complex interdependencies. Simulations are used to predict the macroscopic electrochemical behavior of the cell (Fig. 2) as well as the two-dimensional distribution of species concentrations, temperature, flow velocity, and humidity (Fig. 3). The goal is to identify cell designs and operating conditions that allow steady-state and dynamic operation under optimum water management.