Amorphous Transition Metal Sulfide Electrocatalysts for Green Hydrogen Generation from Solar-Driven Electrochemical Water Splitting

The synthesis and electrocatalytic properties of amorphous first- and third-row transition metal sulfides (a-TMS) for green hydrogen generation have been comprehensively reviewed. These electrocatalysts can be prepared by several solution processes, including chemical bath deposition, electrodeposition, sol–gel, hydrothermal reaction and thermolysis. The deposition method strongly influences the electrochemical properties of the synthesized a-TMS electrocatalyst. Based on overpotential at 10 mA/cm2, the electrocatalytic activity of mono-metallic a-TMS for hydrogen evolution is ranked as follows: a-NiSx > a-CuSx > a-CoSx > a-WSx > a-FeSx. The best performing a-NiSx prepared by chemical bath deposition has an overpotential at 10 mA/cm2 of 53 mV and Tafel slope of 68 mV/dec in 1 M KOH electrolyte. The integration of Ni into the a-TMS network structure is crucial to achieving high activity in multi-metallic a-TMS electrocatalyst, as demonstrated by the bifunctional (NiFe)Sx/NiFe(OH)y nanocomposite catalyst. The critical role of Ni in a-TMS catalyst design can be attributed to the lower free energy change for hydrogen adsorption on Ni. Finally, the emerging catalyst design strategy of amorphous–crystalline heterostructures with a three-dimensional morphology will be discussed together with the need to identify hydrogen adsorption sites on a-TMS electrocatalysts in future.