The Synthesis Of Tungsten Chalcogenide/carbon Nanofiber Nanostructures And Their Hydrogen Evolution Performance

Among all of the approaches for the synthesis of hydrogen, water electrolysis has been widely used because of its environmental friendly, high efficitncy and considerable product purity, however, the costly and scarce catalytic electrode materials always astrict its development. As alternatives to Pt for the catalysis of the hydrogen evolution reaction(HER), transition metal dichalcogenides have shown great promise and attracted much interest, which is abundant on earth, low cost, easily prepared with good electrocatalytic activity. Many studies have suggested that, the exposed edges or defects of the transition metal dichalcogenides always have the lowest Gibbs absorption free anargy, and serves as catalytically active sites. Therefore, to improve the efficiency of transition metal dichalcogenides as HER catalysts, the identification of their active sites is vital. Based on the good chemical stability as well as the fast electronic transmission of carbon nanofiber(CNFs), we employed electrospinning technology to prepare polyacrylonitrile nanofibers(PAN NFs), then, the CNFs were prepared through the carbonization of PAN NFs. Using in situ reduction along with chemical vapor deposition approach, we synthesized WS2, WSe2 and W(SxS1-x)2 nanomaterials with different diamension, size, structure and morphology, to obtain a series of tungsten chalcogenide/carbon nanofiber nanostructures. Finally, we studied and explored the electric catalytic regularity and mechanism of the nanomaterials, herein, three main research aspects were carried out as shown below:The edges of the transition metal dichalcogenides are always served as the electrocatalytic active sites, thus, it is important to construct the edges as much as possible. Using(NH4)2WS4 contained electrospun polyacrylonitrile nanofibers(W-PAN NFs) as the precursor, after high-temperature calcination and S vapor inducing reaction treatments, WS2 nanosheets and WS2 nanoflowers were synthesized on the surface of CNFs. It is believed that S vapour controls the nucleation and growth of WS2 nanosheets, and the structure of WS2 nanoflowers can be tailored by CNFs, exposing abundant active sites. What’s more, increasing the amount of S vapour, the WS2 nanosheets tend to wrap the CNFs. The synthesized catalysts are directly used as the electrode for hydrogen evolution reaction(HER) and the WS2 nanoflowers exhibit good electrocatalytic activity.To further increase the active sites, combined with the influence of driving force to the crystal structure in crystal growth, we chose(NH4)6H2W12O40·nH2O contained PAN nanofiber mats(W-PAN FMs) as the precursor, WSe2 nanostructures are synthesized on CNFs through CVD method. dendritic WSe2 and 2D WSe2 nanosheets on CNFs are designed and synthesized by a diffusion-controlled CVD method. The dendtitic WSe2 exhibits remarkable enhancement in HER performance due to the effectively exposed edge sites, such as low overpotential(–150 mV), small Tafel slop(80 mV dec–1) and large current density(31.2 mA cm–2 @ –300 mV vs RHE).Doping is an another usful approach to increase the defects in transition metal dichalcogenides, which can further increase the active sites. In this study, S was chosen to substitute the Se in WSe2 to synthesize W(SexS1-x)2/CNFs nano materials. The cover density of WSe2 nanoflakes can be easily controlled by adjusting the initial W precursorr. Using the mixture vapor of Se and S, the substitution of selenium with sulfur can be easily realized. The W(SexS1-x)2 nanoflakes with more defects and dislocations are configured in the alloy form without any obvious phase separation. The electrocatalytic materials synthesized here exbited small overpotential, large exchange current density and long-term stability.