The Fabrication Of Metal Sulfides-Based Nanoarray Materials And Their Applications In Electrocatalytic Hydrogen Evolution Reaction

In the twenty-first century, human beings have to face two serious problems, energy crisis and environmental pollution. Development of new energy resources, like Hydrogen Energy, is one of the effective ways to solve these problems. Generating hydrogen via electrolysis or photo-electrolysis of water not only makes the process more environment-friendly, but also efficiently achieves the conversion and utilization of different energy sources. Although Pt-group noble metals are the best catalysts of hydrogen evolution reaction (HER), the exorbitant price and scarcity make them difficult to implement large-scale application. Exploring the alternative electrocatalysts is of great significance for the whole water splitting, and Sulfides-based electrodes are a kind of new HER electrocatalytic materials with promising application.In this work, we prepared the CoS2 nanopyramid array and CoS2@MoS2 hierarchical nanoarray electrodes by one-step hydrothermal method as well as Co-Mo-S nanosheets electrode via hydrothermal synthesis followed by annealing sulfurization, and studied their HER electrocatalytic performance. Specific details as follows:1. The Fabrication of CoS2 Nanopyramid array Materials and their Applications in Electrocatalytic Hydrogen Evolution ReactionWith the one-step hydrothermal synthesis, CoS2 nanopyramid array were fabricated on the different substrate (Ti foil and 3D carbon fiber paper). On the Ti substrate, by controlling the reaction time, we successfully characterized the growth process of CoS2 nanopyramid array with the SEM measurement. In 0.5 M H2SO4 solution, we studied the effect of reaction time on the HER performance of CoS2 nanopyramids, and the 15h-sample was the best HER catalyst. Compared with the samples prepared at other reaction times,15h-CoS2 nanopyramids possessed the biggest electrochemical surface area and the smallest reaction resistance. Finally, due to the chemical stability of CoS2 materials, the optimized sample also showed good HER electrocatalytic activity and stability, demonstrating that CoS2 nanopyramid array was an all-pH-value-working HER catalyst.On the three-dimensional carbon fiber paper, by controlling the initial reactant ratio and reaction solvent, we successfully prepared CoS2 catalyst with different morphologies, including nanopyramid array, micropyramid array and thin film, and studied the influence of morphology on the acidic HER electrocatalytic activity. The result showed the performance of CoS2 nanopyramids was better than that of the micropyramids and thin film, which could be attributed to its large electrochemical active surface area and good electrical conductivity. Compared with the catalysts reported previously, the 3D CoS2 nanopyramid array material showed better HER electrocatalytic activity, the small onset potential of 61 mV and low overpotential of 140 mV required to reach the cathodic current density of-100 mA/cm2. Finally, we found that the 3D CoS2 nanopyramids electrode could work stable in acidic, neutral and alkaline media, further proving that CoS2 nanopyramid array as a HER catalyst could be applied in all-pH-value condition.2. The Fabrication of CoS2@MoS2 Hierarchical Nanoarray Material and its Applications in Electrocatalytic Hydrogen Evolution ReactionWith the one-step hydrothermal synthesis, CoS2 nanocubes coated by MoS2 nanosheets were fabricated on the substrate of Ti foil, and by controlling the reaction time, we successfully characterized the growth process of CoS2@MoS2 hierarchical nanoarray with the SEM measurement. There are two main steps in the formation:metallic CoS2 nanocubes growth and amorphous MoS2 nanosheets coating. The further characterizations of XRD, Raman, HRTEM and EDX spectra suggested the good crystallinity of CoS2 nanocubes and amorphous nature of Co-doped MoS2 nanosheets. The electrochemical measurements demonstrated that compared with CoS2 and MoS2 nanoarray materials, CoS2@MoS2 hierarchical nanoarray, as the cathodic electrode of water splitting in the acidic media, showed better HER electrocatalytic activity. The excellent performance could be ascribed to the novel hierarchical structure, tight binding between catalysts and the substrate, metallic properties of pyrite-phase CoS2 and the intrinsic electrocatalytic activity of amorphous Co-doped MoS2. Finally, we found that the CoS2@MoS2 electrode could work stable with the 2000 CV cycles and 15h testing, which could be attributed to the tight binding between the active materials and substrate and low adhesion to H2 bubbles.3. The Fabrication of Co-Mo-S Nanosheets Array Material and its Applications in Electrocatalytic Hydrogen Evolution ReactionWith the two-step method, hydrothermal synthesis followed by the annealing sulfurization treatment, Co-Mo-S ultrathin film coated on CoMoO4 nanosheets array were fabricated on the substrate of Ti foil. And by adjusting the initial reactant ratio in the hydrothermal method, we successfully fabricated the Co-Mo-O precursors with two morphologies: hierarchical nanosheets array and ultrathin nanosheets array. After the annealing sulfurization treatment, each Co-Mo-S nanosheet consists of numerous inconnected nanoparticles forming the mesoporous structure. The further characterizations of XRD, Raman, HRTEM and EDX spectra suggested the good crystallinity of CoMoO4 core and amorphous nature of ultrathin Co-Mo-S shell with the co-doping of Co, Mo and S elements. The electrochemical measurements demonstrated that compared with CoS2 and MoS2 nanoarray materials, Co-Mo-S hierarchical nanosheets array and ultrathin nanosheets array materials, as the cathodic electrode of water splitting in the acidic media, showed much better HER electrocatalytic activity. The excellent performance could be attributed to the mesoporous structure with large electrochemical active surface area producing more active sites, the highly open and porous structure with the electrolyte and products diffuse effectively benefitting the utilization of electrocatalytic active sites and the intrinsic electrocatalytic activity of amorphous Co-Mo-S layer resulting from the co-doping of three elements. Finally, we found that the Co-Mo-S hierarchical nanosheets array electrode could work more stable than the ultrathin nanosheets array electrode because of the robustness of hierarchical nanosheets, indicating the advantage of hierarchical-structuring.