Mathematical approaches to modelling the mass transfer process in solid oxide fuel cell anode

Abstract

In the recent literature, the influence of lean fuel, type of fuel, velocity profile and the flow direction on the performance operation of solid oxide cells (SOCs) is commonly investigated. SOCs generate electricity when they operate in fuel cell mode (SOFC – Solid Oxide Fuel Cell) or produce hydrogen in electrolysis mode (SOEC – Solid Oxide Electrolyzer Cell). They consist of several layers, including two porous electrodes separated by a gas-tight electrolyte. Typical anode porosity (fuel electrode in SOFC mode) varies from 20% to 50% (after sintering). The porosity value may limit the mass transport and the overall performance of SOC. This work investigates computationally and experimentally, the existing challenge such as diffusion mechanism in porous medium using the example of SOC electrode. A set of mass transport models (Fick's laws, modified Fick's law, Maxwell-Stefan Model, Dusty Gas Model) are described and discussed, highlighting their similarities, differences, advantages, and disadvantages as well as restrictions in usage. Finally, a detailed computational fluid dynamics (CFD) model of the diffusive transport in porous electrode was elaborated and validated. An experimental campaign was conducted using anode supported SOC, fabricated at the Institute of Power Engineering in Poland. Simulations were performed of the mass transport of pure hydrogen and pure water vapor or carbon dioxide (substitute of real product of electrochemical reaction in SOC for validation purposes) through porous anode. Tests were conducted in the wide flow range 10 [ml/min] up to 200 [ml/min] for both gases. The results demonstrate that hydrogen mole fractions reached similar values (0.5–0.6) at both outlets only for the maximum residence time or high porosity of anode. This implies that residence time and gas diffusion can be controlled by a combination of operating and microstructural parameters and can be estimated by numerical simulation using computational fluid dynamics. The main motivation and outcome of this work were to reduce the cost and time required for the optimization of the mass transport process through SOC's porous electrode.