Co 9 S 8 /CoTe 2 n‐n Type Heterojunction: A Heterogenic Interfacial Integration of Co 9 S 8 and CoTe 2 Phases for Energy‐Efficient Hydrazine Oxidation Assisted Hydrogen Production

Abstract Controlled and optimized heterogenic interfacial coupling is the key to enhance the electrochemical performance. Herein, for the first time, telluride‐based Co 9 S 8 /CoTe 2 heterostructure is reported as a bifunctional catalyst for energy‐efficient H 2 generation. Detailed investigations suggest that the heterogenic interfacial coupling leads to superior bifunctional electrochemical performance of the Co 9 S 8 /CoTe 2 heterostructure. Furthermore, it displays excellent hydrazine oxidation reaction (HzOR) performance at industrial‐level current density (500 mA cm −2 @ 338 mV) using an electrochemically neutral carbon paper substrate. The overall hydrazine electrooxidation‐assisted water splitting (OHzWS) cell performance of the Co 9 S 8 /CoTe 2 heterostructure couple is 10 mA cm −2 @ 0.22 V, which is 1.60 V less than the conventional overall water splitting. Band energy profiles of Co 9 S 8 /CoTe 2 heterostructure reveal an n‐n type heterojunction formation between Co 9 S 8 and CoTe 2 phases. The work function difference between the phases results in an impulsive electron flow from the CoTe 2 to the Co 9 S 8 phase and the creation of a built‐in electric field. This prompts the electrochemical kinetics of HzOR and hydrogen evolution reaction (HER) by enabling faster charge transport at the interface. Furthermore, DFT supports the experimental findings, and Gibbs's free energy profile for HzOR studies indicate that the Co 9 S 8 /CoTe 2 heterostructure interface is favorable for HzOR compared to Co 9 S 8 and CoTe 2 .