Thermodynamic Analysis of Carbon Deposition on the Fuel Electrode of a Reversible Solid Oxide Cell

Abstract

Fouling via carbon deposition occurs in reversable solid oxide cells (RSOCs) when it operates with carbon-based fuels or performs CO2 electrolysis. Carbon deposition is known to degrade performance and, in some cases, lead to delamination and irreversible cell damage. In this study, research was conducted using thermodynamic analysis to investigate the potential of solid carbon formation during electrolysis mode by analyzing the chemical potential of electro-neutral oxygen (O2) at the interface between the electrolyte and the fuel electrode. The chemical potential of electro-neutral oxygen is calculated based on a local chemical equilibrium assumption and addresses the isothermal transport of ions and electrons across the electrolyte. The O2 chemical potential at the fuel electrode/electrolyte interface is used to evaluate the affinity of carbon deposition reactions by the corresponding Gibbs free energy changes. Our analysis shows that regardless of the chemical makeup and physical properties of a cell an oxygen chemical potential threshold is ultimately reached as applied cell voltage during electrolysis is increased which beyond this threshold carbon deposition is inevitable. Our study is expanded to include the role of fuel electrode ionic resistance on the chemical potential of O2 at the fuel electrode/electrolyte interface and shows that an increase in ionic resistance results in more favored carbon deposition. Additional findings indicate that at 800 deg. C the electrochemical reduction of carbon monoxide is thermodynamically favored at lower local CO partial pressures over the Boudouard reaction during electrolysis mode.