Thermostatics of Concentration Polarization and the Possible Effects of Surface Ad-Ions in SOFC Composite Cathodes

Abstract

In situations common to high-temperature SOFC (Solid-Oxide Fuel Cell) operation at steady-state, the dependence of the activity of oxide ions on the oxygen partial pressure (p_O2) becomes relatively unimportant for p_O2>0.15 bar. Nevertheless, one might question what would be the indirect effect (if any) of surface ad-ions on the evaluation of concentration polarization due to changes on the oxygen partial pressure near to the grain boundary between electronic, ionic, and gas phases. From thermostatics considerations and macroscale-continuum phenomenological equations it is shown that the definition of concentration polarization, as commonly found in the literature, may contain a series of limitations and, among these limitations, it is discussed the relative importance of oxygen surface ad-ions on the formation of concentration cells,when it is compared to the concentration gradients of “bulk” gas species in the porous channels of SOFC cathodes. This theoretical discussion about the influence of contact chemistry near the TPB (three-phase boundary) region indicates that concentration polarization could in fact either increase or decrease, depending on whether the chemical activity of oxide ions is affected by the local chemical activity of O2 near the gas/solid interface. In order to verify this, one needs to know how the differences of interfacial potential step and chemical activity relate to changes in the contact chemistry and space charge zone near this interface. Particularly, the evaluation of the potential step between the electronic conductor and the ion adsorbed layer requires the prescription of certain phenomenological parameters. In the present study, one of these parameters is the density of “active” surface sites near the TPB region, which represents just a fraction of the total surface site density available over the entire electrode phase. It is a fraction because only those sites sufficiently close to the TPB “hot spot” are considered “active” for the adsorption of charged species, which are then subsequently able to undergo electrochemical reduction at the TPB. So to calculate this density of “active” surface sites and thus the corresponding electrostatic potential step, a phenomenological governing equation for oxygen ad-ions on the electrode surface is proposed (i.e., in terms of surface coverage of ad-ions) to account for electrocatalytic and interfacial transport processes (i.e., competing surface diffusion and adsorption processes near the TPB “hot spot”) within a composite porous cathode.