Water Splitting with Cu 4 TiX 4 (X = S and Se) Nanoparticle Electrocatalysts: Effect of the Lattice Structure and Anion Lability

Herein, two bimetallic chalcogenides, Cu4TiX4 (X= S and Se), prepared through a solid-state synthetic technique, are used as bifunctional electrode materials to study electrocatalytic water splitting. Despite the iso-stoichiometric formula, the Cu4TiSe4 lattice adopts a cubic arrangement, while the Cu4TiS4 lattice possesses a tetragonal structure. The difference in the bulk structure and atomic arrangement in the lattice arises from the structural disorder present in Cu4TiSe4, as well as the difference in the ionic radius of S2– and Se2–. The structural disorder present in Cu4TiSe4 also leads to a large variation in the Cu–Ti intermetallic distance and the presence of a dominant amount of Ti4+. Microscopic studies identified (111) and (101) planes as one of the major exposed facets for Cu4TiSe4 and Cu4TiS4, respectively. During the electrocatalytic water splitting study, Cu4TiSe4 exhibits an overpotential of 323 mV (@10 mA cm–2) for the oxygen evolution reaction (OER) and 228 mV (@20 mA cm–2) for the hydrogen evolution reaction (HER), whereas the Cu4TiS4 delivered overpotentials of 342 mV (for the OER) and 264 mV (for the HER). A steady performance (>18 h) in the chronoamperometric OER and HER conditions of Cu4TiSe4 allows fabricating a water-splitting electrolyzer (NF/CTSe(+)/(−)CTSe/NF), which delivers a cell potential of 1.74 V at 10 mA cm–2 with a Faradaic efficiency of 92 ± 3% for hydrogen production, significantly lower cell potential than that of the previously reported water-splitting electrolyzer fabricated with copper-based materials. The postelectrocatalytic characterization of the electrodes established the superior stability of the Cu4TiSe4 over Cu4TiS4 due to strong covalent-type atomic connectivity between Cu2+, Ti4+, and Se2–. The improved bifunctional activity of Cu4TiSe4 can be attributed to its lattice stability, synergism between two 3d metals, and the presence of a significant quantity of Ti3+ that controls the electrochemical activity.