Self-supported cobalt/cobalt selenide heterojunction for highly efficient overall water splitting

Electrocatalytic water splitting is a vastly reliable method for hydrogen production. Nonetheless, its wide practical application urgently requires the development of highly active and stable bifunctional electrocatalysts with self-supported structures. This paper reports on a novel, simple way to fabricate an efficient cobalt/cobalt selenide (CS) catalytic electrode with a unique 3-D finger-like structure. More specifically, porous Co substrate was fabricated by a phase-inversion tape-casting and sintering method, followed by one-step selenization in Ar atmosphere. The resultant CS electrode manifested superior catalytic performance in both HER and OER (hydrogen and oxygen evolution reaction, respectively) that took place in an alkaline medium, with low overpotential and high long-term stability. Furthermore, the electrolysis test for overall water splitting was successfully performed by assembling the CS electrodes as both anode and cathode. The excellent electrocatalytic performance is ascribed to the high conductivity of the Co substrate, the distinctive finger-like structure, and the numerous in-situ developed heterojunctions, which suitably adjust the electronic structure and decrease the energy barrier of water splitting. Moreover, the density function theory (DFT) calculations revealed that the heterojunction enhances the density of the state near the Fermi level and optimize the kinetics of HER and OER processes. As a result, the hydrogen adsorption free energy (ΔGH*) is close to zero, and the energy barrier of the rate-determining step of OER is low due to the heterojunction formed.