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Li–O2 batteries can provide greater gravimetric energy than Li-ion batteries but suffer from poor efficiency and cycle life due to the instability of aprotic electrolytes. In this study, we show that the apparent four-electron oxygen reduction to form Li2O in Li–O2 batteries with molten nitrate is facilitated by the electrochemical reduction of nitrate to nitrite, and subsequent chemical oxidation of nitrite to nitrate by molecular oxygen, instead of a four-electron oxygen reduction aided by disproportionation of Li2O2 generated from two-electron reduction of molecular oxygen. By examining a series of transition metal catalysts using experiments and computation, optimizing the surface binding of nitrate to enhance the kinetics of the electrochemical reduction of nitrate to nitrite, as well as increasing the kinetics of nitrite oxidation by O2 was shown to increase the discharge voltage and render the observed high-rate capability for NiO-based surfaces in Li–O2 batteries.

Understanding the nature of active sites is central to controlling (electro)catalytic activity. Here we employed surface X-ray scattering coupled with density functional theory and surface-enhanced infrared absorption spectroscopy to examine the oxygen evolution reaction on RuO2 surfaces as a function of voltage. At 1.5 VRHE, our results suggest that there is an –OO group on the coordinatively unsaturated ruthenium (RuCUS) site of the (100) surface (and similarly for (110)), but adsorbed oxygen on the RuCUS site of (101). Density functional theory results indicate that the removal of –OO from the RuCUS site, which is stabilized by a hydrogen bond to a neighbouring –OH (–OO–H), could be the rate-determining step for (100) (similarly for (110)), where its reduced binding on (100) increased activity. A further reduction in binding energy on the RuCUS site of (101) resulted in a different rate-determining step (–O + H2O – (H+ + e−) → –OO–H) and decreased activity. Our study provides molecular details on the active sites, and the influence of their local coordination environment on activity. [Figure not available: see fulltext.]