Tuning Electronic Structure Of Fractal Co-based Catalysts For Hydrogen Evolution Reaction

Hydrogen is considered as the most ideal clean and green renewable energy carrier because of its high combustion calorific value,recyclable combustion products and rich raw materials.Hydrogen produced by electrochemical water splitting has attracted much attention due to its advantages of simple process,low energy consumption,high efficiency,low pollution and high purity.The overpotential of hydrogen evolution can be greatly reduced by optimizing electrocatalysts.Cobalt is one of the more important choices for hydrogen evolving catalysts in the last decade.With the features of high activity from nanosized structure and high stability from microsized structure,fractal Co-based electrocatalysts are therefore chosen as electrocatalysts for hydrogen evolution reaction（HER）.In this paper,the main contents are as follows:（1）Co metal electrocatalysts with fractal structure were prepared by a simple template-free hydrothermal method.Fractal Co metal electrocatalyst with hexagonal phase and spherical Co electrocatalyst were successfully prepared by adjusting hydrothermal time,temperature,and so on.The fractal Co metal electrocatalyst has lower hydrogen evolution overpotential than that of the spherical Co metal electrocatalyst.In the alkaline electrolytes,the fractal Co only requires 307 m V overpotential to derive the hydrogen evolution at 10 m A/cm² of current density.Fractal Co metal electrocatalyst with preferential orientation of（002）plane exhibits large electrochemical active surface area,more active sites,which improves the mass transfer efficiency,the effective contact with water and the activity of electrocatalytic hydrogen evolution.（2）Co/TiO₂ catalyst with fractal structure was constructed by one-step template-free hydrothermal method with introduction of commercial titanium dioxide.The optimized Co/TiO₂ catalyst has lower overpotential of electrocatalytic hydrogen evolution.In the alkaline electrolytes,the Co/TiO₂ only requires 229 m V of overpotential to derive the hydrogen evolution at current density of 10 m A/cm².Moreover,the Co/TiO₂ Schottky catalyst exhibits a superior HER activity with a turnover frequency（TOF）of 0.052 s^-1 and an exchange current density of 79μA/cm²,which is about 4.3 and 4.0 times,and 4.9 and 17.3 times greater than that of pristine Co,and commercial TiO₂,respectively.Experiments and theoretical calculations suggest that the Schottky junction plays an important role to optimize hydrogen adsorption free energy(ΔG_H^*)by tuning the electronic structure,which enhances performance for HER of the Co/TiO₂ Schottky catalyst.Schottky junction tunes the electronic structure of Co metal surface,enriches electrons on Co metal surface,improves the adsorb ability of water and the utilization efficiency of electrons,thus improving the performance of electrocatalytic hydrogen evolution.（3）Fractal structure Co-based electrocatalysts（CoMn,CoFe,CoNi,CoMo,CoMnFe,CoMnCu,CoFeNi,CoNiCu and CoMnFeNiCu）with different element compositions were synthesized by hydrothermal method.It was found that fractal CoMo electrocatalyst has the best hydrogen evolution performance by electrocatalytic hydrogen evolution test.In the alkaline electrolytes,the fractal CoMo electrocatalyst only requires 293 m V overpotential to derive the hydrogen evolution at current density of 10 m A/cm².The radius of Co and Mo elements are quite different,and the alloy material will produces lattice strain,which promotes the redistribution of charge on the catalyst surface,tunes the electronic structure and effectively enhances the HER.