Factors affecting limiting current in solid oxide fuel cells or debunking the myth of anode diffusion polarization

Limiting current densities for solid oxide fuel cells were measured using both button cells and a flow-through cell. The cell anodes were supplied with mixtures of humidified hydrogen and various inert gasses. It was demonstrated that the true limiting current in flow-through cells is reached when either: the hydrogen is nearly or completely depleted at the anode-electrolyte interface near the outlet; or when the concentration of steam at that interface becomes high enough to interfere with adsorption or transport of the remaining hydrogen near the triple-phase boundaries. Choice of inert gas had no effect on limiting currents in the flow-through tests, indicating that diffusion within the porous anode had no significant effect on cell performance at high currents. In the button cells, the apparent limiting currents were significantly changed by the choice of inert gas, indicating that they were determined by diffusion through the bulk gas within the support tube. It was concluded that the apparent limiting currents measured in button cells are influenced more by parameters of the experimental setup, such as the proximity of the fuel tube outlet, than by the physical properties of the anode.