Structural Design, Vast Synthesis And Application Of Several Low-/Non-Platinum Electrocatalysts

Fuel cells, as an electrochemical reaction device that directly converts the energy of chemical reaction to electric energy, can not only replace micro-battery to provide energy for a watch, but also can replace major power station to provide power for a city. However, the commercialization of fuel cells is still exposed to several serious problems needed to be solved. The most urgent one is to design and develop low-/non-platinum (Pt) oxygen reduction reaction (ORR) electrocatalysts with high activity and stability to partly or fully replace the commonly used noble metal Pt-based electrocatalysts, reducing the production cost of fuel cells significantly. In this dissertation, we focus our research interest on this topic. Firstly, we summarized the research progress on designing and developing low-/non-Pt ORR electrocatalysts over the last40years, particularly highlighting the achievements of designing and developing new Pt-free electrocatalysts using low cost metals or nonmetals as raw materials. Of note, the electrocatalytic activity and stability of these new electrocatalysts are often low and do not reach practical applications required. Based on this, we rationally selected transition metal chalcogenide (TMC) materials as the main research object because they possess the advantages of low cost, high yield and large room to improve the activity and stability. By several effective routes, such as understanding the active sites of TMC materials deeply, fabricating TMC materials with exposed high-index facets, or hybriding TMC materials with other functional materials to modify their electronic structures, we can enhance the catalytic performance of TMC materials to meet the requirements of practical applications. The main achievements can be summarized as follows:1. Using very simple binary/ternary mixed solvothermal methods, we selectively synthesized several TMC materials with unique shape, size, composition, and structure at large scale, such as unique ultrathin lamellar mesostructured CoSe2-Amine nanobelts, Fe7Se8polyhedra with exposed high-index facets and Fe7Se8nanorods, and urchin-like NiSe nanocrystals and NiSe nanospheres with various surface topography structures. We systematically studied the influence of the solvent, temperature and time on the phase, morphology and size of the final products. Base on the experimental results, we proposed the inner growth mechanism of these materials rationally. We carefully investigated the microstructures of such novel TMC materials and studied their inner optical, magnetic, and electrocatalytic properties deeply. After a comprehensive understanding of these materials, we came to design new ORR electrocatalysts with high activities and stabilities rationally.2. We selected lamellar mesostructured CoSe2-DETA (DETA=diethylenetriamine) nanobelts as a "model material" and combined them with other functional nanoparticles, such as metals and metal oxides, rationally, achieving new hybrid structures such as Pt/CoSe2, Fe3O4/CoSe2and Mn3O4/CoSe2nanocomposite nanobelts. The changes of electronic structure after hybriding CoSe2with foreign nanoparticles were discussed deeply. We found that the new CoSe2-based hybrid materials exhibited much higher electrocatalytic activity and stability as compared with pure CoSe2/DETA nanobelts. For examples, Pt/CoSe2nanobelts not only made CoSe2/DETA nanobelts have a largely enhanced ORR activity, and CoSe2also made Pt possess nice methanol-tolerant properties. After hybriding CoSe2/DETA nanobelts with Mn3O4nanoparticles, the achieved Mn3O4/CoSe2composite materials have clearly enhanced oxygen evolution reaction (OER) electrocatalytic activity. More importantly, such a nanocomposite shows good stability in an alkaline solution. Based on the understanding of "synergistic coupling effect" among different materials, we have already gained certain experiences of designing Pt-free electrocatalysts and have obtained many new materials with practical prospect.3. We also designed and synthesized several Pt-based nanocrystals with clean surface and high specific surface area for electrocatalytic application. We first synthesized untrasmall PtO2nanocrystals (Adams catalyst) at a large scale in pure water without surfactant. It was found that the PtO2nanocystals showed much high ORR activity and stability as compared to commercial Pt/C electrocatalysts (Johnson-Matthey,20-wt%). Additionally, we also fabricated hierarchical Fe3O4nanochain assemblies with complex building units in a polyol solution. Due to the presence of many nanopores in this nanostructures, the obtained Fe3O4has a very high specific surface area (～43.5m2g-1). It is believed that such Fe3O4nanostructures may be used as a support material to load Pt nanoparticles, which may improve the ORR performance of Pt significantly.