Electrochemically Derived Crystalline CuO from Covellite CuS Nanoplates: A Multifunctional Anode Material.

In the present era, electrochemical water splitting has been showcased as a reliable solution for alternative and sustainable energy development. The development of a cheap, albeit active, catalyst to split water at a substantial overpotential with long durability is a perdurable challenge. Moreover, understanding the nature of surface-active species under electrochemical conditions remains fundamentally important. A facile hydrothermal approach is herein adapted to prepare covellite (hexagonal) phase CuS nanoplates. In the covellite CuS lattice, copper is present in a mixed-valent state, supported by two different binding energy values (932.10 eV for Cu I and 933.65 eV for Cu II ) found in X-ray photoelectron spectroscopy analysis, and adopted two different geometries, that is, trigonal planar preferably for Cu I and tetrahedral preferably for Cu II . The as-synthesized covellite CuS behaves as an efficient electro(pre)catalyst for alkaline water oxidation while deposited on a glassy carbon and nickel foam (NF) electrodes. Under cyclic voltammetry cycles, covellite CuS electrochemically and irreversibly oxidized to CuO, indicated by a redox feature at 1.2 V (vs the reversible hydrogen electrode) and an ex situ Raman study. Electrochemically activated covellite CuS to the CuO phase (termed as CuS EA ) behaves as a pure copper-based catalyst showing an overpotential (η) of only 349 (±5) mV at a current density of 20 mA cm -2 , and the TOF value obtained at η 349 (at 349 mV) is 1.1 × 10 -3 s -1 . A low R ct of 5.90 Ω and a moderate Tafel slope of 82 mV dec -1 confirm the fair activity of the CuS EA catalyst compared to the CuS precatalyst, reference CuO, and other reported copper catalysts. Notably, the CuS EA /NF anode can deliver a constant current of ca. 15 mA cm -2 over a period of 10 h and even a high current density of 100 mA cm -2 for 1 h. Post-oxygen evolution reaction (OER)-chronoamperometric characterization of the anode via several spectroscopic and microscopic tools firmly establishes the formation of crystalline CuO as the active material along with some amorphous Cu(OH) 2 via bulk reconstruction of the covellite CuS under electrochemical conditions. Given the promising OER activity, the CuS EA /NF anode can be fabricated as a water electrolyzer, Pt(-)//(+)CuS EA /NF, that delivers a j of 10 mA cm -2 at a cell potential of 1.58 V. The same electrolyzer can further be used for electrochemical transformation of organic feedstocks like ethanol, furfural, and 5-hydroxymethylfurfural to their respective acids. The present study showcases that a highly active CuO/Cu(OH) 2 heterostructure can be constructed in situ on NF from the covellite CuS nanoplate, which is not only a superior pure copper-based electrocatalyst active for OER and overall water splitting but also for the electro-oxidation of industrial feedstocks.