Amorphous-Crystalline Interfaces Coupling of CrS/CoS<sub>2</sub> Few-Layer Heterojunction with Optimized Crystallinity Boosted for Water-Splitting and Methanol-Assisted Energy-Saving Hydrogen Production

Abstract: Large-scale hydrogen production through the electrochemical water splitting technique is an important way for addressing the impending energy and environmental crisis. This approach requires highly efficient and robust bifunctional cost-effective electrocatalysts. Engineering amorphous and crystalline phases within electrocatalysts is a key method for enhancing the catalytic kinetics of water electrolysis, due to their unique physicochemical properties. The interface and amorphous regions constructed within heterostructures serve as highly active sites that play a crucial role in electrochemical reactions. On the other hand, highly crystalline regions within the heterostructure demonstrated high tolerance in harsh environments, which helps to improve the stability of the overall catalyst. However, effectively tailoring the crystalline state of catalysts within a microenvironment presents a significant challenge. Herein, construction of a novel CrS/CoS2 heterojunction with precise control over crystallinity were presented. The optimized amorphous CrS/highly crystalline CoS2 heterojunction (A-CrS/HC-CoS2) exhibits a low overpotential of 90.6 mV (at 10 mA∙cm−2) and 370.5 mV (at 50 mA∙cm−2) for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations reveal that charge redistribution induces variations in the d-band center value at the A-CrS/HC-CoS2 heterostructure interface, enhancing the catalytic activity for both HER and OER. The displacement of the d-band due to charge redistribution in the Cr―S―Co bond within A-CrS/HC-CoS2 contributes to the modulation of the adsorption capacity of H* and OOH* intermediates on the catalyst surface, thereby optimizing the rate-determining step for HER and OER. The amorphous/highly crystalline structure also facilitates the structural and compositional evolution of A-CrS/HC-CoS2 during water electrolysis, ensuring excellent stability. As a bifunctional catalyst in a methanol-assisted energy-saving hydrogen production device, A-CrS/HC-CoS2 operates at a low cell voltage of 1.51 V to deliver a current density of 10 mA∙cm−2, making it a promising candidate among metal-based catalysts. The well-preserved amorphous/crystalline heterointerfaces in A-CrS/HC-CoS2, along with favorable changes in surface composition, contribute to robust HER and OER stability. This work provides valuable insights into the manipulation of catalytic activity through crystalline control within amorphous/crystalline heterojunctions for bifunctional transition metal compound electrocatalysts.