An electrocatalytically active nanoflake-like Co₉S₈-CoSe₂ heterostructure for overall water splitting

Abstract

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/cm ²for 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/cm ². 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/cm ² 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.