Surface And Interface Control Of Nanoelectrocatalyst Toward Methanol Oixdation Reaction And Hydrogen Evolution Reaction

With the increasingly serious crisis in energy and environment, electrocatalysis is playing more and more vital roles in the development of next-generation energy sources and the technology of energy conversion. Among the electrolytical reaction, methanol oxidation reaction and hydrogen evolution reaction have been at the focus of scientists. Methanol oxidation reaction?MOR?, where the chemical energy of CH3 OH can be transformed into electrical power in high efficiency, is the anodic reaction of direct methanol fuel cells. However, the poisonous intermediate hinders the commercialization of this fuel cell. In contrast, hydrogen evolution reaction?HER? is a process to store renewable electricity into the bond of hydrogen molecule, which is an important part in the development of clean "hydrogen economy". The studies of HER are focusing on improving electrocatalytic activity of non-noble metal catalyst to lower the cost of water electrolyzers. Overall, it is of great research significance to employ a reaction-oriented methodology to develop highly active and durable electrocatalysts in a cost-effective and scalable method.This thesis has focused on methanol oxidation reaction as well as hydrogen evolution reaction, and five nanostructure-optimized and high-performance nanoelectrocatalyst has been designed and synthesized via surface and interface engineering. The nanoelectrocatalysts are ultradispersed and ultrafine Pt nanoclusters on carbon nanotubes?Pt_n/PDA-CNT?, strongly coupled platinum-tin dioxide-graphene nanohybrid?Pt-Sn O₂-pr GO? and hierarchically nanostructured MoS₂ nanoassembly with rich in-plane edges, threedimensionally hierarchical MoS₂-CNT nanohybrid, and ultradispersed and single-layered MoS₂-r GO nanohybrid?UDSL-MoS₂-r GO?. X-ray powder diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy are applied to characterize the structure and composition of the surface and inner part of the synthesized electrocatalysts. Transmission electron microscopy, high-resolution transmission electron microscopy and elemental mapping are employed to study the morphology and microstructure of the electrocatalyst. Based on these results, we proposed possible mechanisms for the formation of these unique nanostructures. Finally, the catalytic properties of electrocatalysts toward methanol oxidation reaction or hydrogen evolution reaction are measured. With the results of physical characterization, we have proposed the possible relationship between nanostructure and electrocatalytic performance, which might shed a light on the design of next-generation electrocatalyst. The basic information of 5 kinds of electrocatalysts are as follow:?1? Pt nanoclusters?¹.81 nm?, which are much smaller than Pt nanoparticles?³ nm? in commercial Pt/C, have been uniformly dispersed on polydopaminefunctionalized carbon nanotubes via a simple polyol method. These ultradispersed Pt nanoclusters feature large accessible active surface as well as high fraction of surface atoms with low coordination numbers, and therefore not only exhibit excellent mass activity and stability but also show improved specify activity and CO tolerance relative to Pt/C and Pt/CNT catalyst.?2? A rational, simple and integrated strategy is reported to construct strongly coupled metal-metal oxide oxide-graphene nanostructure as an electrocatalyst with high performance. We first simply synthesize the interacted Sn O₂-pr GO?protected and reduced graphene oxide? hybrid with Sn O₂ nanoparticles?⁴ nm? selectively anchored on the oxygenated defects of r GO using an in-situ redox and hydrolysis reaction. After the deposition of Pt, uniform Pt NPs are found to contact intimately and exclusively with Sn O₂ phase in Sn O₂-pr GO hybrid. This constructed nanostructure?Pt-Sn O₂-pr GO? exhibits significantly improved electrocatalytic activity?2.19 fold? and durability?2.08 fold? over that of the state-of-the-art Pt/C catalyst.?3? A hierarchically nanostructured MoS₂ electrocatalyst where separated edges emerge on the basal plane has been developed for the first time via facile and scalable solvothermal synthesis with cost-effective precursors. Rich defect sites and low crystalline have been proposed to be the cause of in-plane edges. MoS₂ catalyst with such an optimized nanostructure is measured to show remarkable HER activity with fast kinetics?onset potential:-87 m V; Tafel slope: 41 m V per decade? and exceed almost all documented MoS₂-based electrocatalyst. The surprising enhancement has been discussed and ascribed to the inplane edge morphology which can boost the number of catalytic sites and promote the intrinsic activity of each sites simultaneously, facilitating the catalytic process for H2 evolution.?4? A 3D hierarchically nanostructured MoS₂-CNT nanohybrid, where few-layered MoS₂ nanoflakes?1-3 layers? are alligned on the backbone of carbon nanotubes, is synthesized via a facile and green solvothermal method using widely accessible precusors. The size of MoS₂ nanoflakes can be easily tuned by adjusting the porpotion of water in mix solvent. The detailed characterization shows that the MoS₂-CNT nanoelectrocatalyst exhibit an optimized nanostructure which can not only stabilizes the catalytically active site on the edge of MoS₂, but also boosts the electron transfer during the operation. In addition, the 3D nanostructure on MoS₂-CNT and the mesostructure forming by the stack of MoS₂-CNT can be also attributed to its outstanding HER performance, providing the current density of ⁹00 m A cm-2 on the overpotential of only ²90 m V.?5? Ultradispersed and single-layered MoS₂ nanoflakes with tiny size were dispersed on the surface of graphene using an aqueous solution-based method, synthesizing the UDSLMoS₂/r GO. This strategy can tackle the two main problems of MoS₂ as a non-noble metal HER electrocatalyst including the number of active sites and the electrical conductivity. The HER performance of UDSL-MoS₂/r GO shows superior activity and the Tafel slope is only ³5 m V dec^-1 which is the lowest value in non-Pt electrocatalyst and very closed to that of Pt(³3 m V dec^-1). This surprisingly high activity may originate from the ultradispersed single-layered MoS₂ nanoflake with tiny size which optimize the exposition of active sites on edges and lower the resistance of electron transfer during the process of HER.