Preparation And Electroactivity Of Ag-based Complex Nano-catalysts Doped With Manganese Dioxide And Ferroferric Oxide

Fuel cell is a kind of power-generation device which is able to convert chemical energy, contained in fuel and oxidant, into electric energy. As an efficient, clean and environmental energy-technology, the fuel cell is endowed with important research significance in the new century with high-speed economy and a lack of resource. Oxygen reduction reaction（ORR） is an important reaction occurring on the cathode, and the catalyst for ORR is mainly Pt and Pt-based electrocatalysts. However, it is difficult to promote fuel cell into commercial use because Pt-based electrocatalysts are expensive and easy to be got poisoning. Ag-based catalysts are easy to prepare and has excellent electrocatalytic activity for ORR in alkaline solution. Therefore, developing new Ag-based catalysts has become a hot research spot. In the thesis, acidified carbon nanotubes are applied as catalyst carrier and two different transition metal oxides, manganese dioxide（MnO₂） and ferroferric oxide（Fe₃O₄）, are used to modify Ag-based catalysts to improve the electrocatalytic activity of Ag-based catalysts for ORR. Through a series of physical characterization, the crystal structure, surface morphology and particle size of the catalyst are investigated, and the electrocatalytic activity and stability of the catalyst for oxygen reduction are also investigated through a variety of electrochemical methods. The main contents of this paper include the following aspects:（1） The chemical-reduction method is adopted to prepapre Ag-based catalyst, in which acidified carbon nanotubes are applied as catalyst carrier and sodium borohydride is used to reduce silver nitrate. Ag/MWCNT, Ag/5%MnO₂/MWCNT, Ag/10%MnO₂/MWCNT and Ag/15%MnO₂/MWCNT, with different MnO₂ loading ratios were prepared. SEM and XRD techniques were used to characterize the above-mentioned catalysts, and the results show that these catalysts particles present good dispersion. The electrocatalytic activity and stability of the catalyst for ORR was also investigated in alkaline solution by cyclic voltammetry and linear sweep electrochemical methods. Results show that when the MnO₂ doping ratio is 5%, the catalyst has the best elctrocatalytic activity for ORR. The onset potential of Ag/5%MnO₂/MWCNT for ORR in alkaline solution is 0.003 V（vs Hg/Hg O） and the ORR current density at 2000 r·min-1 is 3.38 m A·cm-2.（2） The liquid-phase redox method was adopted to prepare binary Ag-based nano-catalysts. Different proportions of MnO₂ was loaded on acidified carbon nanotubes, and then a certain amount of silver and cobalt nanoparticles（Ag:Co mole ratio is 8:2） was deposited on the surface of MnO₂/MWCNT to synthesize the Ag₈Co₂/MnO₂/MWCNT catalysts with different MnO₂ loadings, including Ag₈Co₂/MWCNT, Ag₈Co₂/5%MnO₂/MWCNT, Ag₈Co₂/10%MnO₂/MWCNT, and Ag₈Co₂/15%MnO₂/MWCNT. SEM and XRD were employed to charaterizate the structure and surface of the prepared catalysts. Also, cyclic voltammetry and linear sweep methods were adopted to study the electrocatalytic activity and stability of the catalysts for ORR in alkaline solution. And the results show that catalyst particles with different sizes spread out on the surface of the carbon nanotubes, and there are plenty of spaces between the particles, thus, the contact area betweenO₂ and the catalyst increases. The electrocatalytic activity and stability of the catalysts for ORR were also investigated in alkaline solution by cyclic voltammetry and linear sweep electrochemical methods. Results show that the ORR current density of Ag₈Co₂/10%MnO₂/MWCNT at 2000 r·min-1 is 3.38 m A·cm-2, which is higher than that of Ag₈Co₂/MWCNT.（3） Ag/MWCNT, Ag/2%Fe₃O₄/MWCNT, Ag/5%Fe₃O₄/MWCNT, and Ag/10%Fe₃O₄/MWCNT catalysts with different Fe₃O₄ loadings were prepared by modification of Ag/MWCNT with Fe₃O₄. And the results show that catalyst particles with different sizes uniformly spread out on the surface of the carbon nanotubes, and there are plenty of spaces between the particles. The electrocatalytic activity and stability of the catalyst for ORR were also investigated in alkaline solution by cyclic voltammetry and linear sweep electrochemical methods. Results show that the ORR onset potential of Ag/2%Fe₃O₄/MWCNT at 2000 r·min-1 is 3.27 m A·cm-2, higher than the current density of Ag/MWCNT.