Preparation And Catalytic Behavious Of High-Performance Pd Electrocatalyst For Direct Formic Acid Oxidation Fuel Cells

Direct formic acid fuel cell (DFAFC) has advantages of high volume ratio energy, high energy conversion efficiency, safety and reliability. Within comparison to hydrogen oxygen fuel cells, DFAFC uses a liquid fuel, formic acid, thus easily adopting the currently conventional facility for transportation, storage and fueling of gaoline, and holds great promise for automobiles and portable electrical sources. However, due to the extensive use of noble Pt catalyst, high expense becomes one of the main obstacles to restrict the commercialization of DFAFC. Consequently, research and development of inexpensive and high performance electrocatalysts towards formic acid oxidation and oxygen reduction reaction (ORR) are highly demanded. Pd with a crystal structure and electronic properties similar to Pt has great potential as an alternative to replace Pt electrocatalyst due to its relatively lower cost and higher resistance to CO poisoning. The advances of nanoscience have provided various approaches in nanoscales to design and synthesize high performance electrocatalysts. This PhD thesis has investigated rational design and synthesis of high performance electrocatalysts in nanoscales for DFAFC with new approaches and nanostructured supports. Furthermore, the catalytic behavious are discussed and the enhancement mechanism of electrode kinetics as well as their scientific insights is explored. In more details:1. Literature review. Principle, classification, characteristics and applications of fuel cells are briefly summarized. In particular, the catalysts of DFAFC and their reaction mechanism are highlighted, and the common used catalyst supporting materials are analyzed. The challenges of DFAFC catalysts in science and practice are also discussed. The research purposes, main content and innovation point of this study are also introduced.2. Experimental design and characterization methods. The experimental apparatus and reagents used in this project are introduced, and main methods for physical/chemical analysis and electrochemical characterization of DFAFC catalysts are also described.3. DNA-Directed Pd nanocrystals/carbon nanotubes catalyst and its catalytic behavious for oxygen reduction reaction. CNTs possess high specific surface area, good chemical stability and electrical conductivity, thus have attracted enormous interest in the uses as catalyst supports. However, they are inherently inert and rely on activation of their graphitic surface, such as refluxing in concentrated acids to generate oxygen functional groups, to deposit uniformly distributed catalyst nanoparticles. The harsh oxidative process reduces electrical conductivity and worsens corrosion resistance for CNTs. In this work, DNA is used to immobilize CNTs through π-π stacking between the aromatic nucleobases of DNA and CNTs graphite surface, in which the intrinsic structural and electronic properties of CNTs can be preserved, and the regularly arranged PO43-groups on the sugar-phosphate backbone of DNA can direct the growth of ultrasmall Pd nanocrystals with an average size of3.4nm uniformly distributed on CNTs for high-performance Pd/DNA-CNTs catalyst. Comparing with Pd/CNTs and commercial Pd/C, the prepared Pd/DNA-CNTs catalyst towards ORR exhibits higher electrocatalytic activity and stability. This approach can be extended for broad applications in energy conversion and storage systems.4. DNA functionalized Graphene to direct growth of highly electroactive Pd nanocrystals and the catalytic behavious towards formic acid oxidation. Pt is one of the most widely used electrocatalysts for DFAFC, but its catalytic activity is not good enough towards formic acid oxidation and can be easily poised by CO. Graphene has large specific surface area and superior conductivity, and is functionalized by DNA as a support to direct growth of high-activity Pd electrocatalyst towards formic acid oxidation (Pd-DNA@Graphene). TEM images indicate that Pd nanocrystals are uniformly distributed on Graphene surface with an average size as small as5nm. The electrocatalytic behavious of prepared Pd-DNA@Graphene towards formic acid oxidation is investigated by CV, LS V, and i-t showing higher peak current density, io and lower Rct than that of Pd-Graphene and commercial Pd/C for better catalytic performance. Pd-DNA@Graphene is also more suitable for long-term operation than that of Pd-Graphene and commercial Pd/C.5. One-step preparation of ultrasmall Pd nanocrystals/Graphene and its catalytic behavious towards formic acid oxidation. The traditional preparation methods to prepare Pd nano-catalysts need a complex process, and the nanoparticles have serious aggregation, resulting in smaller ECSA and weaked catalytic activity. In this work, formic acid is used as a reductant to one-step prepare ultrasmall Pd nanocrystals with an average size of3.4nm while uniformly distributing on graphene under hydrothermal conditions for a high-performance Pd@Graphene catalyst. Comparing with commercial Pd/C, the prepared Pd@Graphene catalyst towards formic acid oxidation presents higher peak current density, io and lower Rct and better stability. The catalytic mechanism for high-performance Pd@Graphene catalyst is investigated. It is expected that this high performance catalysts can have broad applications due to its simpleness and available.6. Various nanostructured carbon materials as supports to prepare Pd catalysts and their catalytic behavious towards formic acid oxidation. The effect of various nanostructured carbon materials as supports to prepare Pd catalysts and their electrocatlytic behaviors towards formic acid oxidation are investigated. Zero dimensional XC-72, one dimensional CNTs, two dimensional graphene and three dimensional3D-RGO are selected as catalyst supports to prepare catalyst Pd@XC-72, Pd@CNTs, Pd@G and Pd@3D-RGO, respectively. Comparing with Pd@XC-72, Pd@CNTs and Pd@G, Pd@3D-RGO shows the highest specific surface area, largest ECSA, highest peak current density and io, the lowest Rct and best stability for formic acid oxidation. Thus3D-RGO is revealed as one kind of unique supporting materials for catalysts. This work also offers valuable fundamental for researchers to study or/and select more superior support materials. 7. Conclusions and outlook. Conclusions are delivered and future works are also discussed.