(253f) DFT-Derived Insights of CO2 Reduction in Solid Oxide Electrolysis Cells and Inhibition By S-Containing Molecules

Solid Oxide Electrolysis Cells (SOECs) have become a system of interest due to their efficiency of splitting H₂O and reducing CO₂ into useful, high-energy products. The most common SOEC cathode is Ni/YSZ with Yttria stabilized zirconia (YSZ) as the solid electrolyte for O^2- conduction. The active site for SOEC, at sufficiently high applied potential, is often postulated to be a triple phase boundary (TPB) between metallic Ni nanoparticle and the YSZ surface. Point source impurities in CO₂ feeds can have sulfur-containing compounds such as SO₂, which are known to inhibit CO₂ reduction.

In this work, we use density functional theory and microkinetic modeling to study the mechanism of CO₂ dissociation and sulfur inhibition on Ni and Ni/YSZ interface. CO2 can adsorb at the interface, undergo electrochemical C-O scission leading to CO and O^2- ions. However, catalytic activity can be hampered by the presence of point source impurities such as S due to their strong adsorption, minimal barriers to bond activation and surface persistence, even at high temperatures (~800 ˚C). Modeling the thermodynamics and kinetics in the local environment of the electrode TPB can elucidate the mechanism of desirable and undesirable cathode half reactions.

Comparative analysis of the enthalpy of CO₂ and SO₂ adsorption on Ni(111), YSZ(111), and Ni/YSZ shows CO₂ binds weakly on Ni(111) and YSZ(111) while strongly at the interface, SO₂ binds significantly more strongly on all sites indicating competitive inhibition. SO₂ dissociation has lower thermodynamic barrier than CO₂ dissociation, indicating that persistent S-containing species can occupy active sites on Ni nanoparticle as well as the TPB. Both adsorption and dissociation of CO₂ at the interface results in charge transfer from Ni to CO₂; indicating a potential-dependent step. Subsequently, a subsurface O diffuses to the bulk leaving behind an interfacial vacancy and CO desorbs, thereby completing the cycle.