Surface Structure to Tailor the Electrochemical Behavior of Mixed-Valence Copper Sulfides during Water Electrolysis

The semiconducting behavior of mixed-valence copper sulfides arises from the pronounced covalency of Cu-S bonds and the exchange coupling between Cu^I and Cu^II centers. Although electrocatalytic study with digenite Cu₉S₅ and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured Cu₉S₅ and CuS are prepared and used as electrode materials to study their electrochemistry. Powder X-ray diffraction (PXRD) and microscopic studies have found the exposed surface of Cu₉S₅ to be d(0015) and d(002) for CuS. Tetrahedral (T_d) Cu^II, distorted octahedral (O_h) Cu^II, and trigonal planar (T_p) Cu^I sites form the d(0015) surface of Cu₉S₅, while the (002) surface of CuS consists of only T_d Cu^II. The distribution of Cu^I and Cu^II sites in the lattice, predicted by PXRD, can further be validated through core-level Cu 2p X-ray photoelectron spectroscopy (XPS). The difference in the electrochemical response of Cu₉S₅ and CuS arises predominantly from the different copper sites present in the exposed surfaces and their redox states. In situ Raman spectra recorded during cyclic voltammetric study indicates that Cu₉S₅ is more electrochemically labile compared to CuS and transforms rapidly to CuO/Cu₂O. Contact-angle and BET analyses imply that a high-surface-energy and macroporous Cu₉S₅ surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of Cu₉S₅ and CuS to CuO/Cu₂O/Cu(OH)₂ electroactive species. The performance of the in situ formed copper-oxides towards electrocatalytic water-splitting is superior compared to the pristine copper sulfides. In this study, the redox chemistry of the Cu₉S₅/CuS has been correlated to the atomic arrangements and coordination geometry of the surface exposed sites. The structure-activity correlation provides in-depth knowledge of how to interpret the electrochemistry of metal sulfides and their in situ potential-driven surface/bulk transformation pathway to evolve the active phase.