Electrochemical Catalytic Performance Of Transition Metal Chalcogenides And Their Hybrids

As a clean and carbon-free energy, hydrogen is regarded as one of the optimal options to slow global warming and resolve energy crisis in the twenty-first century. High purity hydrogen can be produced from water by electrocatalysis, photocatalysis or photoelectrochemistry on a large scale. This process requires high-efficiency and low-cost electrocatalysts. Recently, transition metal chalcogenides (TMC) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have gained much attention as active cathode materials for hydrogen evolution reaction (HER). Various methods like grain thinning, defect engineering and layer-by-layer assembly have been utilized to improve the performance. TMC is earth-abundant and has a lot of novel optical, electrical and magnetic properties. However, the HER performance of TMC electrocatalyst is confronted with several drawbacks such as high hydrogen overpotential, poor stability and difficulty in mass production, which restricts its practical application. The goal of this doctoral dissertation is to modulate the crystal and electronic structure of WS2 by metal doping, annealing process and anodizing treatment. Amorphous nickel tungsten sulfide film (NiWS), amorphous cobalt tungsten sulfide film (CoWS) and tungsten trioxide dihydrate/tungsten disulfide hybrids (WO3·2H2O/WS2) are fabricated as stable and efficient HER catalysts. This may have potential application foreground in the fields of electrochemistry and photocatalysis. The main results are described as follows:1.2H-WS2 is a typical two-dimensional (2D) layered material and its edge sites, which are uncoordinated centers, have been identified to be catalytically active for hydrogen evolution. Based on this point, the amorphization of WS2 is proposed to expose and utilize dense uncoordinated sites. Therefore, employing a thermolytic process, we design and prepare amorphous NiWS and CoWS films as tungsten sulfide derived materials for HER catalysis for the first time. The Ni2+ or Co2+ cations can interact with tetrathiotungstate anion (WS42-) to form complexes like [M’(WS4)2]n-or M’WS4 (M’= Ni, Co; n= 2,3) in the precursor. Hence, Ni/Co incorporation and thermal treatment can be implemented together to optimize the amorphous structure and material property. The electrochemical tests indicate that amorphous NiWS annealed at 210℃ with a Ni to W ratio of 1:3 delivers the best HER efficiency. It has an onset potential of～100 mV and a Tafel slope of～55 mV dec-1, which are significantly smaller than those of the amorphous tungsten sulfide (WSx). The excellent durability of amorphous NiWS is confirmed by constant-voltage electrolysis for 24 h. Capacitance measurements imply that incorporation of Ni or Co dramatically increases the active sites of WSx, leading to the improvement of HER activity. The electrochemically active surface area of amorphous NiWS is 342 times larger than WSx, especially. The results extend the study of tungsten sulfide derived materials in the amorphous field.2. Heterogeneous composite catalysts have been widely applied in hydrogen production. But the synergistic TMC-based catalysts have mainly been used to increase the quantity of active sites, improve the electron transport, or optimize the Gibbs free energy of adsorbed hydrogen (△GH) of electrode. In order to utilize the synergy of electrochromism and hydrogen spillover effect, we propose WO3·2H2O/WS2 hybrid as a novel and active HER catalyst. Under a negative voltage, hydrogen ions (H+) could be intercalated into the lattice of WO3·2H2O, forming hydrogen tungsten bronze. The inserted hydrogen atoms are protonic in nature and could migrate to the planar defect or edge sites of WS2, and then participate in HER. Besides, this process is accelerated by the outstanding proton diffusion coefficient in layered WO3·2H2O. This concept introduces the electrochromism and hydrogen spillover effect into the design of HER catalyst and promotes further exploiture of synergy in hydrogen evolution.3. In order to verify the synergetic mechanism of WO3·2H2O/WS2 hybrid, we prepare 2H-WS2 film on a carbon fiber paper (CFP) by employing chemical vapor deposition equipment. Then, this film is treated by in-situ anodic oxidation in acid medium and the composite of WO3·2H2O nanoplates/WS2 film is formed. The hybrid displays impressive HER activity with an onset potential of only～60 mV and a Tafel slope of～54 mV dec-1. A current density of 100 mA cm-2 at 152 mV overpotential is observed for the hybrid catalyst, which is 10 times larger than that of the 2H-WS2 sample under the same voltage. Through experimental research and contrast, we find that the introduce of WO3·2H2O has little influence on the amount of active sites of catalysts. Moreover, when WO3·2H2O is turned into WO3 by thermal treatment, the HER performance of the hybrid deteriorates significantly. So, we think the enhancement of HER property derives from the synergy between WO3·2H2O and WS2, which increases the surface coverage of hydrogen (OH) on WS2. The results provide reliable experimental evidence for the synthesis of composite catalysts and the application of synergistic mechanism in hydrogen production.