Methylamine Treated Mn ₃ O ₄ Nanoparticles as a Highly Efficient Water Oxidation Catalyst under Neutral Condition

Electrochemical water splitting is one of the most important ways to produce hydrogen fuel for an environmentally friendly energy system. Low-cost and efficient electrocatalysts for oxygen evolution reaction (OER) have been studied to overcome the sluggish kinetics of OER, which is considered to be a bottleneck that affects the overall efficiency of water splitting. However, the atomic-level understanding and control of the catalyst surface still remains elusive. Here, we show that methylamine treatment of manganese oxide nanoparticles can effectively control their surface charge and enhance their OER catalytic activity under neutral condition. Methylamine treatment can reduce the overpotential for OER by nearly 100 mV at the current density 5 mA cm ⁻² . Through spectroscopic studies, potentiometric titration, and zeta potential analysis, we discovered that methylamine does not exist on the surface after methylamine treatment but deprotonates the surface of nanoparticles and induce additional negative charge on the surface. Surprisingly, treatment with not only methylamine but other amines can also improve the OER performance and the catalytic activity of the amine treated nanoparticles is influenced by the pK _a values of amines.