Electrochemical water oxidation using single-site Cu(II) molecular complexes: A mechanism elucidated by computational studies

Abstract

In this work, the penta-coordinated mononuclear Cu(II) complexes [Cu(QCl-Tpy)Cl2], [Cu1] (where QCl-Tpy = 3-([2,2':6',2''-terpyridin]-4'-yl)-2-chloroquinoline) and [Cu(8HQ-Tpy)Cl2], [Cu2] (where 8HQ-Tpy = 2-([2,2':6',2''-terpyridin]-4'-yl)quinolin-8-ol) had been utilized as an electrocatalyst for water oxidation in 0.1 (M) H_2 〖PO〗_4^-/H〖PO〗_4^(2-) buffered solution at pH = 12.0. Detailed electrochemical measurements implies that both complexes follow first order kinetics and liberates oxygen via Water Nucleophilic Attack (WNA) pathway. The active participation of buffer under electrochemical conditions implies its strong Atom Proton Transfer (APT) ability and thereby facilitating the O-O bond formation. The TOF and TOFmax of the catalysts had been elucidated using Foot of the Wave Analysis (FOWA) and catalytic Tafel plot. The complexes [Cu1] and [Cu2] exhibits 〖TOF〗_max of 139005 s-1 and 183019 s-1 respectively as elucidated by FOWA assuming first order rate constant. The catalytic Tafel plot supports the superior electrocatalytic activity of the catalyst [Cu2] in contrast to catalyst [Cu1]. Theoretical studies also affirms that both catalysts undergo WNA and thereby following first order kinetics. Moreover, theoretical studies also attribute the Oxygen Evolution Reaction (OER) is thermodynamically more favourable at higher alkaline pH owing to the better nucleophilicity of the hydroxide ions.