Microstructural origin of selective water oxidation to hydrogen peroxide at low overpotentials: a study on Mn-alloyed TiO2

Abstract

One key objective in electrocatalysis is to design selective catalysts, particularly in cases where the desired products require thermodynamically unfavorable pathways. Electrochemical synthesis of hydrogen peroxide (H₂O₂) via the two-electron water oxidation reaction (2e⁻ WOR) requires a +0.54 V higher potential than four-electron O₂ evolution. So far, best-performing electrocatalysts require considerable overpotentials before reaching peak faradaic efficiency. We present Mn-alloyed TiO₂ coatings prepared by atomic layer deposition (ALD) and annealing as a stable and selective electrocatalyst for 2e⁻ WOR. Faradaic efficiency of >90% at < 150 mV overpotentials was achieved for H₂O₂ production, accumulating 2.97 mM H₂O₂ after 8 hours. Nanoscale mixing of Mn₂O₃ and TiO₂ resulted in a partially filled, highly conductive Mn³⁺ intermediate band (IB) within the TiO₂ mid-gap to transport charge across the (Ti,Mn)O_x coating. This IB energetically matched that of H₂O₂-producing surface intermediates, turning a wide bandgap oxide into a selective electrocatalyst capable of operating in the dark. However, the high selectivity is limited to the low overpotential regime, which limits the system to low current densities and requires further research into increasing turn-over frequency per active site.