A new framework for physically based modeling of solid oxide fuel cells

Current solid oxide fuel cell (SOFC) cell-level models are based on the subtraction of overpotentials from the Nernst equilibrium potential. This approach has two limitations: First, the Nernst equation cannot be applied to non-equilibrium gas feeds (e.g., CH4/H2O mixtures). Second, the Butler-Volmer equations used to describe current/overpotential relationships do not account for the complex electrochemical processes at the three-phase boundaries. We present here a new modeling approach that is based on the combination of (1) the elementary-kinetic description of electrochemistry, where multi-step chemical mechanisms account for coupled charge-transfer and surface chemistry, (2) the physical representation of electric potential steps due to inter-facial double layers, allowing to calculate the cell voltage without using the Nernst equation, (3) a quasi-three-dimensional multi-scale transport model describing surface transport towards the three-phase boundaries (nanoscopic scale), continuum mass and charge transport in the porous electrodes (microscopic scale), and gas-phase channel transport (macroscopic scale), and (4) a fully transient representation of all model equations for the prediction of electrochemical impedance spectra. Simulation results are presented for an internal-reforming SOFC operated on CH4/H2O mixtures. The predicted cell voltage is considerably below the value resulting from full equilibration. Distributions of current density, electric potential, and species concentrations are discussed. (c) 2007 Elsevier Ltd. All rights reserved.