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 Co 9 S 8 /Ni 3 S 2 heterojunction was constructed, which was employed as a RM to separate the hydrogen and oxygen production in space and time in MEP system for H 2 . The MEP system for H 2 involved a two−step electrochemical−electrochemical (EC−EC) looping process. The reversible redox reaction of Co 9 S 8 /Ni 3 S 2 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 Ni 3 S 2 to metallic Co 9 S 8 on the interface. This made the formation of Lewis acid at the Ni 3 S 2 in the heterojunction , which bonded with the OH − Lewis base, facilitating the electrochemical redox kinetics of Co 9 S 8 /Ni 3 S 2 . Thus, the Co 9 S 8 /Ni 3 S 2 RM presented a high area capacitance (29.60 F/cm 2 at 5 mA/cm 2 ), and an excellent stability upon operation over 5000 cycles (15 days). The MEP system can continuously produce H 2 for 1502 s at 10 mA/cm 2 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 H 2 enabled flexible utilization of renewable solar energy by photovoltaic (PV) panels, thereby facilitating solar-to-hydrogen conversion.