Modular electrochemical production of hydrogen using Mott–Schottky Co9S8/Ni3S2 heterojunction as a redox mediator

Modular electrochemical production (MEP) system could decouple the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and pairing with a redox mediator (RM), respectively. Herein, Mott–Schottky Co9S8/Ni3S2 heterojunction was constructed, which was employed as a RM to separate the hydrogen and oxygen production in space and time in MEP system for H2. The MEP system for H2 involved a two−step electrochemical−electrochemical (EC−EC) looping process. The reversible redox reaction of Co9S8/Ni3S2 was paired with HER in step 1 and subsequently paired with OER in step 2. The Mott–Schottky hetero-structures enabled the redistribution of Ni central charge and accelerated the electron transfer from semiconductor Ni3S2 to metallic Co9S8 on the interface. This made the formation of Lewis acid at the Ni3S2 in the heterojunction, which bonded with the OH− Lewis base, facilitating the electrochemical redox kinetics of Co9S8/Ni3S2. Thus, the Co9S8/Ni3S2 RM presented a high area capacitance (29.60 F/cm2 at 5 mA/cm2), and an excellent stability upon operation over 5000 cycles (15 days). The MEP system can continuously produce H2 for 1502 s at 10 mA/cm2 with a Faradaic efficiency of 100%. The MEP system possessed a high energy efficiency (83%), requiring a lower cell voltage than that of a conventional water electrolysis system. The MEP system for H2 enabled flexible utilization of renewable solar energy by photovoltaic (PV) panels, thereby facilitating solar-to-hydrogen conversion.