Mechanistic insights into acidic water oxidation by Mn(2,2'-bipyridine-6,6'-dicarboxylate)-based hydrogen-bonded organic frameworks

Acidic-stable oxygen evolution reaction (OER) catalysts based on earth-abundant materials are important but rare for the proton exchange membrane-based water electrolysis. In this study, a metal-containing hydrogen-bonded organic framework (HOF) of manganese coordinated with 2,2'-bipyridine-6,6'-dicarboxylate ligands, Mn(bda), interconnected through hydrogen bonding and π-π stacking is used as a heterogeneous OER catalyst (Mn(bda)-HOF) for acidic water oxidation and exhibits a considerable OER performance. Electrochemical results show that Mn(bda)-HOF displays a turn of frequency of 1 s⁻¹ at an overpotential of 870 mV. Meanwhile, this Mn(bda)-HOF shows an unusual pH dependence on performance, where the reaction rate increases with the decrease of pH. A comprehensive mechanistic study reveals that the charge transfer triggered coupling of two metal-oxo species Mn⁵⁺(O) is the rate-determining step, which leads to this unusual pH dependence on the OER performance.