Synthesis And Performance Of Molybdenum Dioxide Based Materials As Electro Hydrogen Evolution Reaction Catalyst

Since 1900’s, the human society has entered a period of rapid development. Growth of population and development of science and technology have turned the world a lot. Human’s requirement of energy is increasing in order to support the daily life of modern society. Pollution caused by uncontrolled spend of fossil fuels has become a serious problem worldwide. Therefore, the exploration of durable and clean energy is of great significance to the sustainable development of earth. Hydrogen is an environmental friendly energy carrier, as its only combustion product is water. In that case, hydrogen has attracted much attention of countries all over the world as important future energy. However, as a kind of secondary energy source, the production of hydrogen must go through the step of energy conversion. And the way of large-scale hydrogen producing due to the methane reforming reaction will generate a large amount of greenhouse gases during the hydrogen evolution process. Therefore, it is necessary to looking for new hydrogen production method without pollution.Water electrolysis is an efficient and non-polluting hydrogen evolution route, which can convert electric energy into chemical energy directly. At present, the electrodes used in the water electrolysis are mainly platinum carbon materials, which exhibit small overpotential as well as large current density. However, the large scale applications of platinum carbon catalysts are limited due to the rare content in earth and the expensive price. In this case, it is significant to find an alternative electrocatalytic hydrogen production catalyst. Molybdenum dioxide, as a metallic semiconductor, displays good electrical conductivity. At the same time, benefitting from its special crystalline structure, it has been widely used in the field of organic catalysis.This thesis is mainly aimed at the preparation of molybdenum dioxide and its composites as well as their applications in the field of electrocatalytic hydrogen evolution. First of all, we prepared the molybdenum dioxide nanoparticles decorating on graphene nanosheets by a simple redox reaction between molybdenum trioxide and molybdenum powder and tested its electrocatalytic activities of hydrogen production; then, we used molybdenum foil as current collector instead of molybdenum powder to obtain a phosphorus doped molybdenum dioxide nanoparticles on Mo foil, which achieved good HER activity. Besides, we found there is a strong metal-support interaction between Pt and MoO2, in that case we prepared a uniform ultralow platinum load molybdenum oxide/carbon nanotube composite for HER, and it is found that the catalysts showed excellent activity of hydrogen production which is comparable to commercial Pt/C catalysts. From the above, we found that MoO2, due to its special characteristics, is a promising electrocatalytic hydrogen production catalyst.The content of this paper is divided into the following four chapters:The first chapter is the introduction. First, the concept and basic principle of hydrogen production technology as well as hydrogen electrode reaction are summarized. Then the selection and the design principle of hydrogen evolution reaction catalysts are briefly introduced, followed by the recent development of hydrogen evolution reaction catalysts, such as sulfides, phosphides and composite catalyst. A brief description of the content and significance of the dissertation is given to end the chapter.In Chapter 2, a simple redox hydrothermal method has been developed to fabricate noble-metal-free MoO2/rGO composite for highly efficient HER. GO nanosheets provide oxygen-containing functional groups for precursor attachment, and restrict growth of MoO2 nanoparticles (NPs) with a small size due to the space confinement effect among GO layers as well. Benefitting from a synergistic effect between metallic MoO2 NPs and graphene, the obtained MoO2/rGO composite exhibits excellent HER activity with small onset overpotential of 190 mV, a large cathodic current density and small Tafel slope of 49 mV per decade, while MoO2 NPs or rGO itself is not that efficient HER catalyst. Additionally, MoO2/rGO composite displays good stability after 1000 potential circles in both acidic and alkaline condition. Dramatically improved HER activity and excellent stability are attributed to small size, more active sites, high conductivity as the synergistic effect of MoO2 NPs and graphene.In Chapter 3, we report the fabrication of noble-metal-free P doped MoO2 nanoparticles (NPs) on Mo foil as electrode for highly efficient HER. Benefiting from a strong interaction between P doped MoO2 NPs and Mo foil as current collector, the obtained electrode exhibits excellent HER activity with a small onset overpotential of 80 mV, a large cathodic current density of 10 mA cm-2 at 135 mV and a small tafel slope of 62 mV per decade, much better than MoO2-based catalysts. Additionally, P doped MoO2 film/Mo foil electrode displays good stability even after 2000 potential cycles in acidic media. The development of a novel route to prepare P doped MoO2 on Mo foil as an active HER catalyst broadens the insight of designing noble-metal-free HER efficient catalysts with cost-effective and environmentally friendly.In Chapter 4, we take advantage of Pt, MoO2 and carbon nanotubes (CNTs) to develop a composite material with ultralow Pt loading as an active and durable catalyst for the HER in acid media. This Pt-MoO2/MWCNTs catalyst is fabricated via a simple redox reaction, exhibiting highly efficient electrocatalytic activity and excellent stability with a negligible onset potential, a Tafel slope of 43 mV per decade and reaches 10 and 20 mA cm-2 at 60 and 84 mV, respectively. Due to the SMSI and good electro-conductivity of MoO2, the HER activity and stability of the Pt-MoO2/MWCNTs composite are comparable to that of the commercial Pt/C catalyst. Remarkably, high HER mass activity and enhanced long term stability are achieved for the composite catalyst with a 0.5 wt% Pt loading mass. Our work opens up a new track to explore highly efficient catalysts with reduced consumption of Pt, meanwhile maintaining the optimal catalytic activity and durability.