An Electrocatalytically Active Nanoflake-Like Co₉S₈-CoSe₂ Heterostructure for Overall Water Splitting

Synthesis of nonprecious metal-based efficient electrocatalysts for the development of water electrolyzers is of significant interest. Herein, we describe the synthesis of a defect-rich electrocatalytically active Co9S8-CoSe2 heterostructure and its electrochemical water splitting performance for the development of a water electrolyzer. The Co9S8-CoSe2 heterostructure is synthesized in a single step by a solvothermal approach, and it has a defect-rich nanoflake-like morphology. The in situ grown Co9S8 and CoSe2 are chemically coupled. The heterostructure is electrocatalytically active toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). It delivers the benchmark current density of 10 mA/cm2 for HER at an overpotential of 61 and 150 mV in acidic and alkaline pH, respectively. It is also highly active toward OER and requires 340 mV to attain 10 mA/cm2. The superior electrocatalytic performance of the Co9S8-CoSe2 heterostructure is ascribed to geometrical and electronic effects. Defect-rich Co9S8-CoSe2 nanoflakes have abundant catalytically active sites that promote electrolyte contact and favor facile electron transfer kinetics. Electronic interaction and chemical coupling between Co9S8 and CoSe2 modulate chemisorption energies of hydrogen- and oxygen-containing intermediates for better electrocatalytic performance. Post-OER analysis reveals that the Co9S8-CoSe2 heterostructure serves as a precatalyst and transforms to electrocatalytically active CoOOH during OER. As a proof-of-concept demonstration, a lab-made water electrolyzer is fabricated using the Co9S8-CoSe2 heterostructure-based anode and cathode. The device delivers 10 mA/cm2 current density at a cell voltage of 1.66 V and retains 85% of its initial current density even after 20 h of continuous electrolysis.