Fabrication Of Nanoscale MoS₂-based Electrocatalysts For Hydrogen Evolution Reaction

Clean, efficient and reanewable hydrogen energy is expected to play a major role in the development of sustainable energy and environment in order to eliminate the effects of energy crisis and carbon emission. Up to now, it is well-known that water electrolysis is the most environmental and sustainable way of hydrogen production. However, a large overpotential of hydrogen evolution causes high energy consumption. Therefore, an effective method to reduce overpotential is developing efficient catalysts for hydrogen evolution reaction (HER). It has been demonstrated that Pt-group noble metals possess the highest catalytic activity for HER, while its scarcity and high cost hinders its large-scale application. To date, earth-abuntant transition metal compounds are a type of new-emerging, promising electrocalasts for HER, MoS2, as the representative of them, whose catalystic activity is close to Pt. Nevertheless, MoS2 nanomaterials still bare a lot of problems of their shortage of catalystic active sites and low conductivity. Thus, It is necessary to increase the catalytic activity of MoS2 by raising active sites and improving conductivity. Herein, we take four measures to increase the catalytic activity of MoS2, which are nanocrystallization, transition metal Co-doping, ZnS composite and forming composite with sufur doped graphene (SGO), and four kinds of MoS2-based electrocatalysts are synthesized by solvothermal method, named nano-MoS2, Co-MoS2, ZnS/MoS2 and MoS2/SGO, repectively. Then, a variety of measures was conducted to characterize the structure, morphology, chemical state and composition of the products, as well as the property of catalyzing hydrogen production was totally explored.Firstly, we synthesize MoS2 nanomaterials by using solvothermal method in polar, medium-polar and non-polar solvents, separately. It is found that the type of solvents shows an great effect of the structure and morphology of MoS2, which further influences the performance of MoS2 for HER. The MoS2 synthesized in three types of solvents is composed of nanoparticles, which exhibit low crystallinity and amorphous structure with short range order. Among all the products, products synthesized in non-polar solvents are made up of particles with a size of several hundred nanometers, and exhibite preferable catalytic activity; ones synthesized in medium-polar solvents have a global shape with the diameter of 70nm, whose catalytic activity take the second place; MoS2 synthesized in polar solvents is composed of nanoparticles with the size of about 200-300nm, and its activity is the worst. It is also found that 180℃×8h is the best synthesis conditions, and MoS2 which is synthesized with sublimed sulfur as sulfur source displays smaller size and much higher catalystic activity than the ones synthesized with thioacetamide. The heat treatment procedure was optimized and 350℃ was verified as the proper temperature for raising activity of MoS2, and the MoS2 synthesized in acetonitrile showed the highest catalytic activity after 350℃×2h treatment. Comparisons of catalystic activity were made between the optimal electrocatalyst MoS2-350-acetonitrile and bulk MoS2, the results reveal that bulk MoS2 is almost electrocatalystic inert, and the activity parameters of MoS2-350-acetonitrile are:η= 284mV (10mA·cm-2), Tafe slope= 88.3mV/dec, ECSA= 20.4mF/cm2, Rct≈15kΩ.Secondly, we fabricate Co-MoS2 and ZnS/MoS2 electrocatalysts by Co and Zn incorporation. It is proved that dopant amount of Co and Zn has a great influence on the acitivity, and the optimal amount of Co and Zn is 1/30 and 1/3, respectively. The two transition metal elements show different mechanisms toward improving activity of MoS2:Co prevents the oxidation of terminal Mo and activates the S edge sites, which, thus increasing the reactive activity sites. On the contrary, a kind of coated composite structure forms after Zn doping, in which ZnS lies in inner space, MoS2 at the surface. In addition, the catalytic activity of the two doped products was further improved by heat treatment. Finally, two optimal doped products are obtained, they are Co-MoS2-350 and ZnS/MoS2-350, the activity parameters of them are:Co-MoS2-350, η= 251mV (10mA·cm-2), Tafel slope= 84.8mV/dec, ECSA= 36.6mF/cm2, Rct≈7kΩ; ZnS/MoS2-350, η= 247mV (10mA·cm2), Tafel slope= 86.4mV/dec, ECSA=24.5mF/cm2, Rct≈8kΩ, respectively.In order to further increase the activity sites and improve the electrical conductivity of MoS2, MoS2/SGO composite catalyst was prepared by combination of MoS2 and SGO. The experimental results indicate that the catalytic activity raised by 4 times and the transfer resistance reduced by more than half of MoS2 after its recombination with SGO, and the optimal loading of SGO is 2mg/mL. However, the activity of MoS2/SGO reduced after heat treatment of 350℃, which is caused by the serious aggregation of MoS2. The prepared composite electrocatalyst of MoS2/SGO exhibites higher catalytic activity than Co-MoS2 and ZnS/MoS2, and the activity parameters of which are:η= 220mV (10mA·cm-2), Tafel slope= 52.5mV/dec, ECSA= 29.1mF/cm2, Rct≈5kΩ. Compared with the state-of-the-art Pt/C electrocatalyst, MoS2/SGO possesses the comparable kinetics of 52.5mV/dec, which is very close to 48.6mV/dec of Pt/C for HER.