Insight into the effect of electrolyte ions docked in subnanopores of metal-free carbon cathode on the ORR activity

Abstract

We combined electrocatalytic measurements to reactive dynamics and meta-dynamics simulations in voltage-polarized conditions to better understand the mechanism of metal-free nanoporous-carbon-assisted-O₂ reduction reaction (ORR) in an aqueous alkaline electrolyte. We use a reactive constant voltage simulation framework to show that the surface in pores larger than 1 nm can be sufficiently polarized to induce O₂ dissociation as experimentally observed. Simulations showed that at 0V, pH = 13 reduces a free energy barrier for O₂ adsorption inside sub-nanopores (pores less than 1 nm) and at 0.7V vs. RHE potential of a cathode O₂ splitting occurs, leading to OH⁻ formation in these pores and also in larger pores. ORR in these latter environments is induced by a strong and global surface electrostatic field that is the consequence of the ion docking in sub-nanopores. The applied voltage causes cations to enter the pores only partially hydrated or bare. Their docking increases the charge on carbon atoms, and when O₂ is in their proximity it splits. Overall, combining the simulation results with experimental ones suggests that the extent of metal-free nanoporous-carbon-assisted O₂ reduction is affected by the amount of sub-nanopores. Surface chemistry/some level of carbon hydrophilicity in larger pores is also important since it affects the electrolyte and oxygen transport to these subnanopores.