Efficient electrochemical CO₂ reduction (CO2RR) into liquid fuels can potentially meet three goals at once – large-scale energy storage, CO₂ capture and a carbon neutral energy cycle through utilization of renewable energy. Metal catalysts have high overpotential and low Faradic Efficiencies for direct conversion of CO₂ to methanol and other liquid fuels. Using computational electrocatalysis simulations based on density functional theory, we show that RuO₂-based electrocatalysts, which have been observed experimentally to evolve methanol at low overpotential, can catalyze the reaction through an alternative reaction mechanism. These pathways have oxygen-coordinated intermediates and the oxide catalyst can thus potentially bypass the limitations of the CO*-CHO* scaling relations on metal catalysts. Here, we establish adsorbate scaling relations for such reaction mechanisms and propose an activity volcano with OH* binding as descriptor. We also predict the ideal binding free energy for H* and OH* to facilitate selective CO2RR over H₂/CO evolution. ¹

Our study shows that adsorbate−adsorbate interaction effects are strong on the RuO₂(110) surface and can alter the reaction thermodynamics substantially. Steric and electronic effects on reaction intermediates from varying coverage of CO* spectators can lead to different observed product compositions from experiments. 50% CO* coverage is necessary for obtaining methanol as a product from CO2RR.²

IrO₂ and RuO₂ binds OH* and other intermediates from CO2RR strongly and falls in the strong binding leg of the activity volcano. We observe that the miscible system Ir_xRu_(1‑x)O₂ exhibits anomalous weaker OH* binding energy in the presence of CO* spectators. We attribute this to a Ru-Ir ligand effect, based on electronic structure analysis. Ir atoms at the bridge site with Ru neighbors, have electron depletion and a t_2g orbital shift. Synergistic effects from adsorbate interaction and ligand effects in compositions with intermediate Ru/Ir character for active sites places them close to the top of the activity volcano. The predicted onset potential for formic acid / methanol evolution is −0.2 V RHE. ³

References:

1. A. Bhowmik, T. Vegge, and H. A. Hansen, ChemSusChem, 9, 3230–3243 (2016)
2. A. Bhowmik, H. A. Hansen, and T. Vegge, J. Phys. Chem. C, 121, 18333–18343 (2017)
3. A. Bhowmik, H. A. Hansen, and T. Vegge, ACS Catal., 7, 8502–8513 (2017)

Figure: Theoretical activity volcano for methanol evolution from CO2RR on oxide surfaces. Combined effects from adsorbate interaction and ligand interaction provide path to efficient oxide catalyst design.