Synthesis Of Non-precious Metal/Graphene Composites And Their Electrocatalysis Towards Oxygen Reduction Reaction

The cathodic oxygen reduction reaction(ORR) is a critical half reaction for different energy conversion and storage devices in sustainable and clean energy solution. The sluggish kinetics of ORR, however, possesses as a main obstacle for development of high-performance energy devices such as fuel cells. Precious Pt has been recognized as one of the best ORR electrocatalysts, but its scarcity, high price and limited stability confine its wide application. Therefore it is very important to seek alternative electrocatalysts and extensive research has been devoted to efficient non-precious metal catalysts. In this thesis, a variety of transition metal(hydroxide, oxide)-graphene nanocomposites have been synthesized and their ORR activities have been evaluated by using electrochemical methods combined with other physio/chemical characterizations. The details are as follows:1. Carbon-supported iron oxides and oxyhydroxides have been demonstrated promising ORR catalytic activities, but there is limited knowledge regarding the influence of iron species types and their crystalline phases on their ORR activities. In this part we synthesized four iron/graphene composites with iron oxides(α-Fe2O3, Fe3O4) and oxyhydroxides(α-FeOOH, and α/γ-FeOOH) supported on graphene oxide(GO) and systematically investigated their ORR behaviors. It is found that decorating GO with iron oxides and oxyhydroxides can dramatically improve the ORR activity of pristine GO in alkaline solution in terms of onset potential and current density, and the α-Fe2O3/GO composite exhibits highest ORR activity among all four composite materials investigated. This study unveils diverse ORR behaviors on GO-supported iron species, and explores the benchmark of their catalytic performances, and thus provides useful insights into the Fe-enabled high ORR activity for developing efficient transition metal-based ORR electrocatalysts.2. Metal cobalt nanoparticle/graphene composite(Co/rGO) was prepared and it is found that the Co/rGO composite has high specific surface area, and cobalt metal nanoparticles with diameter of 8-10 nm were distributed on the surface of graphene uniformly. As-prepared Co/rGO demonstrates essentially improved ORR activity compared to graphene itself. The onset potential is close to that of commercial Pt/C, and the electron transfer number is 3.9, and it also exhibits good stability and resistance to methanol. Investigations in this thesis not only unveil some promising ORR electrocatalysts based on non-precious metal, but also offer useful clues for the research in rational design and synthesis of non-noble metal electrocatalyst.