Modelling of gas diffusion limitations in Ni/YSZ electrode material in CO₂ and co-electrolysis

Carbon formation during CO₂ and co-electrolysis (combined electrolysis of H₂O and CO₂)has been observed in recent studies, under operating conditions where carbon formation,based on the bulk gas composition, should be thermodynamically unfavorable. The carboncan principally be formed by the Boudouard reaction (2CO à CO₂ + C(s)) or the COreduction reaction (CO+H₂ à H₂O + C(s)), and will disintegrate the cell structure as itgrows. It is therefore of great importance to be able to predict when the carbon is formed,and subsequently take actions to prevent formation.The literature offers suggestions that the carbon formation is caused by diffusionlimitations within the Ni/YSZ electrode, but this has not been verified. To do so, thediffusion has been modelled with the dusty gas model and the effect of the electrodetortuosity , porosity , temperature T, electrode thickness dc, and current density i, hasbeen investigated. It is shown that diffusion limitations on reactant transport may lead tovery significant increases in equilibrium temperatures for the two carbon forming reactions.For given electrode properties (e, t and dc) increasing current density leads to increasingequilibrium temperatures. The model can be used to calculate limitations on operatingconditions (T, i) that ensure no carbon formation.