Reversible molten carbonate cells: Testing and preliminary formulation of a kinetic model

Abstract

Reversible molten carbonate cell technology allows for dual operation depending on the applied polarization, alternating between Gas to Power mode as a fuel cell and Power to Gas mode as an electrolysis cell. While the first has already been used in industrial applications, the second is emerging as a promising solution for energy storage and direct syngas production from a CO2-rich process waste stream despite several issues in effective operation. This work aims at the characterization of the reversible system operation, coupling experimental observations with a theoretical phenomenological analysis. A cell composed of state-of-the-art materials is tested, recording the electrochemical performance under variable working conditions. A rigorous discussion on the oxygen and fuel electrode reaction paths results in a preliminary kinetic formulation derived from the Butler-Volmer equation. The model considers innovatively specific equations and kinetic parameters for the anodic and the cathodic polarization of each electrode, which permits predicting the asymmetric behaviour of the cell when switching between fuel cell and electrolysis mode. Since alternating the operation mainly involves a change in the fuel electrode feed, a sensitivity analysis is performed at variable hydrogen and CO2 compositions. Increasing hydrogen content favours the reaction evolution by avoiding starvation in fuel cell mode, while it shows an indirect influence in electrolysis by improving the diffusion of reactants. The CO2 effect is more emphasised during electrolysis mode, resulting in the limiting reactant under the considered conditions, while it has an almost negligible dependence in a fuel cell.