Electrochemical Model of a Triode Solid Oxide Fuel Cell

A Triode Solid Oxide Fuel Cell is a novel cell architecture that aims to control the electrocatalytic activity under low steam reforming conditions, or poisoning conditions. This fuel cell design introduces a third electrode (auxiliary electrode) of the same nature of the cathode while the anode is common between the two circuits (Figure 1). The auxiliary circuit, which electrically connects the anode with the auxiliary electrode, is operated in electrolysis mode while the fuel cell circuit runs in its conventional way. During the triode operation, performance enhancement is recorded especially when a significant electrode overpotential is presented; this can be the case for SOFC anode feeds with natural gas and gasoline fuels, or when it is being poisoned from impurities. A two-dimensional stationary isothermal model is developed and implemented in commercial software, COMSOL Multiphysics (version 4.3b). The model is based on composite electrodes, where the equations for mass, momentum and charge transport, along with electrochemical global kinetics (Butler-Volmer equation) and charge conservation are solved simultaneously. Exchange current densities are fitted from experimental data to reproduce the observed behavior of the cell. The model, able to predict conventional and triode operation, gives guidelines for the electrodes geometry design that needs to be adopted in order to favor the expected improvements. In particular, it is noted that the aspect ratio between the electrodes distance cathode-auxiliary and the electrolyte thickness is a key parameter. The interaction between the main and auxiliary circuits is then discussed in terms of equipotential and current lines in a cross section of the cell for two different geometries: anode and electrolyte supported cells. The improvements achievable in triode operation are also calculated in terms of electric current surplus that is possible to obtain in the fuel cell circuit while imposing an electrolysis voltage of 1.7 V in the auxiliary circuit.