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) were utilized as electrocatalysts for water oxidation in 0.1 M H₂PO₄⁻/HPO₄²⁻ buffered solution at pH = 12.0. Detailed electrochemical measurements suggest that both complexes follow first-order kinetics and liberate oxygen via the water nucleophilic attack (WNA) pathway. The active participation of the buffer under electrochemical conditions indicates its strong atom proton transfer (APT) ability, thereby facilitating the O–O bond formation. The TOF and TOF_max of the catalysts were elucidated using foot-of-the-wave analysis (FOWA) and catalytic Tafel plot. The complexes [Cu1] and [Cu2] exhibit TOF_max values of 10 × 10³ s⁻¹ and 15 × 10³ s⁻¹, respectively, as elucidated by FOWA, assuming a 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 affirm that both catalysts undergo WNA and thereby follow first-order kinetics. Moreover, theoretical studies demonstrate that the oxygen evolution reaction (OER) is thermodynamically more favourable at higher alkaline pH owing to the better nucleophilicity of the hydroxide ions.