Understanding Catalytic Mechanisms and Cathode Interface Kinetics in Nonaqueous Mg-CO₂ Batteries

We leverage first-principles density functional theory (DFT) calculations to understand the electrocatalytic processes in Mg-CO₂ batteries, considering ruthenium oxide (RuO₂) as an archetypical cathode catalyst. Our goal is to establish a mechanistic framework for understanding the charging and discharging reaction pathways and their influence on overpotentials. On the RuO₂ (211) surface, we found reaction initiation through thermodynamically favorable adsorption of Mg followed by interactions with CO₂. However, we found that the formation of carbonate (CO₃^2-) and oxalate (C₂O₄^2-) intermediates via the activation of CO₂ at the catalytic site is thermodynamically unfavorable. We predict that MgC₂O₄ will form as the discharge product due to its lower overpotential compared to MgCO₃. However, MgC₂O₄ is thermodynamically unstable and is expected to decompose into MgCO₃, MgO, and C as final discharge products. Through Bader charge analysis, we investigate the covalent interactions between intermediates and catalyst sites. Moreover, we study the electrochemical free energy profiles of the most favorable reaction pathways and determine discharge and charge overpotentials of 1.30 and 1.35 V, respectively. Our results underscore the importance of catalyst design for the cathode material to overcome performance limitations in nonaqueous Mg-CO₂ batteries.