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, Cu 4 TiX 4 (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 Cu 4 TiSe 4 lattice adopts a cubic arrangement, while the Cu 4 TiS 4 lattice possesses a tetragonal structure. The difference in the bulk structure and atomic arrangement in the lattice arises from the structural disorder present in Cu 4 TiSe 4, as well as the difference in the ionic radius of S 2– and Se 2– . The structural disorder present in Cu 4 TiSe 4 also leads to a large variation in the Cu–Ti intermetallic distance and the presence of a dominant amount of Ti 4+ . Microscopic studies identified (111) and (101) planes as one of the major exposed facets for Cu 4 TiSe 4 and Cu 4 TiS 4, respectively. During the electrocatalytic water splitting study, Cu 4 TiSe 4 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 Cu 4 TiS 4 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 Cu 4 TiSe 4 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 Cu 4 TiSe 4 over Cu 4 TiS 4 due to strong covalent-type atomic connectivity between Cu 2+, Ti 4+, and Se 2– . The improved bifunctional activity of Cu 4 TiSe 4 can be attributed to its lattice stability, synergism between two 3d metals, and the presence of a significant quantity of Ti 3+ that controls the electrochemical activity.