Efficient Homogeneous Electrocatalytic Water Oxidation by a Manganese Cluster with an Overpotential of Only 74 mV

Abstract

Water electrolysis is among the simplest method for generating hydrogen as an alternative renewable fuel. A major challenge associated with this process is the development of cheap, simple, and environmentally benign catalysts that lead to a minimum overpotential for water oxidation. Inspired by the Mn₄ CaO_x cluster that catalyzes water oxidation in photosystem II, described here is the synthesis and characterization of the manganese cluster [Mn₁₂ O₁₂ (O₂ CC₆ H₂ (OH)₃ )₁₆ (H₂ O)₄ ] (Mn₁₂ TH) along with its electrocatalytic activity at pH 6. Electrochemical, spectroscopic, and electron microscopy studies show that Mn₁₂ TH is a homogeneous electrocatalyst for water oxidation and enables oxygen evolution with a reaction rate of 22 s^-1 , high Faradic efficiency (93 %), and an overpotential of only 74 mV, the lowest reported to date. Based on the electrochemical data, the organic ligands, which can be described as the second coordination sphere of the catalytic manganese core, play a key role in facilitating the oxidation process and accelerating the reaction.

Keywords: electrochemistry; homogeneous catalysis; manganese; reaction mechanisms; water oxidation.