Electrocatalytic Water Oxidation by Mononuclear Cu(II) Aliphatic Tetraamine Complexes

Abstract

AbstractWe report here electrocatalytic water oxidation by a series of mononuclear Cu(II) aliphatic tetraamine complexes at various pH. In phosphate buffer solution of pH 12, the Cu(II)‐Tren (Tren is tris‐(2‐aminoethyl)amine) complex is decomposed to generate a Cu oxide/hydroxide film with phosphate incorporated on the electrode surface which can catalyze water oxidation with a small onset overpotential of 420 mV and a low Tafel slope of 48 mV dec−1. By contrast, the other two Cu(II) complexes, Cu(II)‐Me3(Tren) (Me3(Tren) is (tris‐(2‐(methylamino)ethyl)amine)) and Cu(II)‐Me6(Tren) (Me6(Tren) is tris‐(3‐methyl‐3‐azabuty)amine), having the ligands that sharing the same molecular skeleton of Tren but with methyl group substituents, are noneffective to form the surface‐bound active species. The experimental results indicate that the Cu(II)‐Tren complex decomposes by intra‐complex proton‐coupled electron transfer (PCET) from the amine ligand to the higher‐oxidation‐state Cu with significant pH/buffer base effects. At pH 9, water oxidation with Cu(II)‐Tren transforms from heterogeneous to homogeneous featuring a single‐site catalysis mechanism. Moreover, the comparison of the three Cu(II) complexes at this pH also suggests a steric effect of methyl groups on the catalysis performance. As pH decreases further to 7, Tren, with pKb ∼5.9, becomes less coordinating and precipitation of a Cu3(PO4)2 film during electrolysis leads to electrode passivation, which inhibits the homogeneous water oxidation at this pH.