Enhanced Electrocatalytic Activities by Substitutional Tuning of Nickel-Based Ruddlesden–Popper Catalysts for the Oxidation of Urea and Small Alcohols

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 La_2–xSr_xNiO_4+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 La_0.5Sr_1.5NiO_4+δ catalyst exhibits a mass activity of 588 mA mg_ox^–1 and 7.85 A mg^–1 cm_ox^–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. This 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 will act as a governing principle for the rational design of catalysts for the electrooxidation of urea and other small molecules going forward.