(Invited) Exceptional Electrocatalytic Oxidation of Small Molecules Via Tunable Charge Transfer Interactions in Perovskites and Perovskite Derivatives

Abstract

The electrooxidation of urea continues to attract considerable interest as an alternative to the oxygen evolution reaction (OER) as the anodic reaction in the electrochemical generation of hydrogen due to the lower potential required to drive the reaction and the abundance of urea available in waste streams. Herein we investigate the effect of Sr substitution in a series of La2-xSrxNiO4+δ Ruddlesden-Popper catalysts on the electrooxidations of urea, methanol, and ethanol and conclude that activities toward the urea oxidation reaction increase with increasing Ni oxidation state. The 75% Sr-substituted La0.5Sr1.5NiO4+δ catalyst exhibits an exceptionally high mass activity of 588 mA mg-1and 7.85 A mg-1 cm-2 for the electrooxidation of urea in 1 M KOH containing 0.33 M urea, demonstrating the potential applications of Ni-based Ruddlesden-Popper materials for direct urea fuel cells and low-cost hydrogen production. Additionally, we find the same correlations between Ni oxidation state and activities for the electrooxidations of methanol and ethanol, as well as identify processes that result in catalyst deactivation for all three oxidations. Most importantly, this is the first demonstration of how systematically increasing Ni – O bond covalency by raising the formal oxidation state of Ni above +3 serves to increase catalyst activity for these reactions and will act as a governing principle for the rational design of catalysts for the electrooxidation of urea and other small molecules going forward.