Thirty-second Joule-heating synthesis of Cu-intercalated Mo₆S₈ Chevrel phases with pH-symmetric hydrogen evolution...

Chevrel-phase compounds are structurally robust transition-metal chalcogenides, and their tunable ion-intercalation chemistry makes them of considerable interest for electrochemical applications. However, the conventional synthesis of high-purity Chevrel phases generally relies on prolonged high-temperature sintering, which is unfavorable for rapid materials screening and compositional regulation. Herein, we propose a Joule-heating strategy that enables the rapid synthesis of Cu-intercalated Mo₆S₈ Chevrel phases with a holding time of 30 s at 1500 °C. Among the prepared Cu _x Mo₆S₈ samples, Cu_2.76Mo₆S₈ exhibits a high phase purity of 99.6% and a relatively uniform Cu distribution within the examined particles. Hydrogen evolution reaction (HER) measurements show that Cu_2.76Mo₆S₈ displays a nearly pH-symmetric electrocatalytic response, requiring overpotentials of 403 and 400 mV to reach a current density of 10 mA cm^-2 in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. The corresponding Tafel slopes are also comparable in acidic and alkaline electrolytes, indicating relatively balanced HER kinetics under different pH conditions. Further electrochemical cycling tests reveal electrolyte-dependent structural reconstruction: enhanced HER activity in acidic media is accompanied by structural changes consistent with partial Cu deintercalation, whereas moderate catalytic degradation in alkaline media is accompanied by more complex evolution of Cu species. XRD and XPS characterizations indicate that the Mo₆S₈ cluster framework is largely retained after cycling, while the Cu species undergo electrolyte-dependent evolution. This work provides an effective route for the rapid preparation of high-purity Chevrel phases and demonstrates that Cu-intercalated Mo₆S₈ can serve as a model system for investigating pH-dependent HER behavior and electrochemical reconstruction mechanisms.