Decoding the Oxygen Evolution Reaction Mechanism in a Novel Octanuclear Nickel(II) Double Cubane Cluster: Unleashing...

The synthesis of efficient electrocatalysts for the oxygen evolution reaction (OER) in water splitting remains challenging due to slow reaction kinetics. In the present investigations, a novel octanuclear nickel(II) double cubane molecular cluster, [Ni^II ₄(LH₃)₃(µ₃-OH)(η¹-H₂O)₂]₂·2NO₃·2H₂O·CH₃OH (3), has been synthesized using a Schiff-base ligand LH₄ (2-[2-hydroxy-3-(hydroxymethyl)-5-methylbenzylideneamino]-2-methylpropane1,3-diol). Complex 3 was immobilized on activated carbon cloth (CC), referred to as CC-3, and investigated as an electrocatalyst for the OER in an alkaline medium. Single-crystal X-ray diffraction revealed that complex 3 comprised two distorted cubane-like [Ni₄(µ₃-OR)₃(µ₃-OH)]⁺ cores interconnected by flexible pendant ─CH₂OH groups from the ligand. Electrochemical analysis showed that CC-3 exhibited excellent OER activity in 1.0 m KOH, which was reflected in a low overpotential of 290 mV at 10 mA cm^{- 2} with a Tafel slope of 48 mV dec^-1. The catalyst showed remarkable stability, undergoing negligible degradation after 2000 CV cycles and 20 h of chronoamperometric measurement. Mechanistic studies suggest the formation of catalytically active high-valent Ni³⁺─OH intermediates during the OER. Fe³⁺ addition to the electrolyte enhanced OER activity, attributed to high-valent reactive species stabilization. This work demonstrates polynuclear nickel complexes as effective molecular electrocatalysts for OER, providing insights into their catalytic mechanisms.