Functional Nanomaterials-based Research In Electrocatalytic Performance

Energy issues attract increasing concerns in recent years for the fast exhaustion of fossil fuel. It is supposed that the world will need double energy supply by 2050. Nanotechnology has opened a new way to solve this problem by creating new materials, particularly carbon-based and transition metal-based nanomaterias. As nanomaterials have special structures and a series of fantastic physical and chemical properties, they have got wide applications in various fields including biosensor, photocatalysis or electrocatalysis, nano biomedicine, energy storage and conversion and environmental protection. In recent years, carbon-based materials and transition metal-based nanomateris have attracted wide attention for energy application due to their unique properties of high surface area, good conductivity and chemical stability. A lot of effort has been made in this field. In this thesis, we have combined the research of nanomaterials science, analytical technology and electrochemistry to synthesis some organic-inorganic nanomaterials and developed their application in electrochemical catalysis, as discussed in the follow.In Chapter 1, we introduced the research and development of nanomaterials and their applications in many fields, esperciallly in electrochemical catalysis. Then we proposed a scheme of nanomaterials-based or carbon-based matrix or their hybrid for electrocatalyst study.In Chapter 2, a highly active and stable electrocatalyst for hydrogen evolution has been developed based on the in situ formation of MoS2 nanoparticles on mesoporous graphene foams (MoS2/MGF). Taking advantage of its high specific surface area and its interconnected conductive graphene skeleton, MGF provides a favorable microenvironment for the growth of highly dispersed MoS2 nanoparticles while facilitating the access of electrolytes and allowing rapid charge transfer kinetics. The MoS2/MGF nanocomposites exhibit an excellent electrocatalytic activity for the hydrogen evolution reaction with low overpotential (～100 mV) and substantial apparent current densities. Such enhanced catalytic activity stems from the abundance of catalytic edge sites, the increase of electrochemically accessible surface area and the unique synergic effects between the MGF support and active catalyst. The electrode reactions are characterized by electrochemical impedance spectroscopy. A Tafel slope of ～42 mV/decade is measured for a MoS2/MGF modified electrode, suggesting the Volmer-Heyrovsky mechanism of hydrogen evolution.In Chapter 3, a highly active and stable electrochemical catalyst of nanoporous molybdenum carbide nanowires (np-Mo2C NWs) has been developed for the hydrogen evolution reaction (HER). The np-Mo2C NWs were synthesized simply by pyrolysis of a MoOx/amine hybrid precursor with sub-nanosized periodic structure under an inert atmosphere. The enriched nanoporosity and large reactive surface of these highly dispersed nanowires with uniform Mo2C nanocrystallites provides an efficient electrocatalysis, leading to their superior HER activity with lower onset overpotential (-70 mV) and higher current densities than Mo2C microparticles. Moreover, when np-Mo2C NWs were physically mixed with Vulcan carbon (1:1 w/w), an enhanced catalytic performance was observed. This study opens a new perspective for the development of highly active non-noble electrocatalysts for hydrogen production from water splitting.In Chapter 4, we have successfully developed a safe and facile strategy to synthesize noble-metal-like Mo2C nanoparticles on reduced graphene oxide (rGO) sheets (nano Mo2C/rGO) from the MoOx-based sub-nanometer-periodic organic-inorganic hybrid precursors. The amine pre-intercalated in the hybrid precusor leads to the synchronous reduction of organic-inorganic hybrid precusor to Mo2C and GO to rGO, and thus nano Mo2C/rGO is facily prepared with high mono-dispersity as well as the proper reduced graphene in an inert atmosphere. The intimate contact between high dispersed Mo2C nanoparticles and rGO support as well as their synergy endow nano Mo2C/rGO with high active surface. The material presents high current densities (about 100 mA/cm2 at the overpotential of 200 mV), low onset overpotential and high stability in HER system. Such earth-abundant and inexpensive materials with high electrocatalytic performance hold great promise for a variety of applications in the fields of energy conversion and storage.In Chapter 5, a novel type hybrid of nitrogen-doped graphene (N-graphene) and Nb22O5 semiconductor nanosheets with sandwich-like structure has been designed and synthesized via a strategy of intercalation-carbonization. The obtained multilayer hybrid material exhibits quite positive onset potential of oxygen reduction reaction (ORR) at approximately 0.92 V vs. RHE, good stability and excellent selectivity for ORR with high methanol tolerance. The good performance towards ORR could be attributed to the synergetic coupling effect resulting from the molecular level contact of N-graphene and Nb2O5 nanosheets, which makes it a promising substitute for platinum-based materials and also propose a good way to develop their application in energy and conversion field.In Chapter 6, a non-precious metal electrocatalyst has been developed for the oxygen reduction reaction based on nanoporous molybdenum carbide (nano-Mo2C) wires through a facile calcination of sub-nanometer periodic organic-inorganic hybrid nanowires. The highly dispersed Mo2C wires were composed of 10-15 nm nanocrystals with mesopore size of 3.3 nm. The properties of nano-Mo2C wires were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and N2 adsorption/desorption porosimetry. The highly active surface area and enriched nanoporosity for nano-Mo2C wires are unique features that make them a high-performance electrocatalyst for oxygen reduction in an alkaline medium. The eletrocatalysis and reaction kinetics results show that nano-Mo2C-based materials can be developed as new catalysts with high activity at low cost for electrochemical energy conversion applications.In Chapter 7, we summarized and proposed the objects and schemes for the further research.