Preparation Of Ag/Pd Bimetallic ORR Electrocatalysts And Their Applications In Fuel Cells And Zinc-Air Batteries

The continuous consumption of fossil fuels has created maj or problems,including resource scarcity,climatic variation,environmental pollution and so on,which have attracted the attention of governments and organizations all over the world.Developing sustainable energy technologies is an effective pathway to resolve the above problems.Electrochemical energy storage and conversion devices,such as proton exchange membrane fuel cells(PEMFC)and zinc-air batteries(ZAB),have been widely concerned and studied by researchers due to their high energy density,high energy conversion efficiency,low pollution emission and so on.Both the PEMFC and the ZAB are involved in oxygen reduction reaction(ORR)during the discharge process,which is the critical step determining the energy conversion efficiency and cost for the PEMFC and the ZAB.However,the reduction kinetics of ORR is so sluggish that highly efficient ORR catalysts are required to improve the reaction rate.Consequently,it is urgent to develop ORR catalysts with high catalytic activity,long-term life and high cost-effectiveness to promote the large-scale commercial application of these electrochemical energy storage and conversion devices.The high price of the Pt based oxygen reduction catalysts which are widely used and researched at present is one of the reasons for the high cost of PEMFC and ZAB.In recent years,Pd based ORR catalysts have been widely concerned and studied since the crystal structure and physicochemical properties of Pd metal are similar to those of Pt metal,and the Pd metal possesses advantages in reserves and price.The lattice parameters of Ag metal are similar to those of Pd metal,and the price of Ag metal is much lower than that of Pd metal.The bimetallic ORR catalysts formed by Ag metal and Pd metal can adjust the electronic structure of Pd based catalysts,improve the ORR catalytic activity and reduce costs.Therefore,the research and development of Ag/Pd bimetallic oxygen reduction catalysts with high activity and long-term life have important significance for improving output power and cycle life of PEMFC and ZAB.In this thesis,preparation methods and corresponding electrocatalytic properties of the Ag/Pd bimetallic oxygen reduction catalysts with different structures were carefully studied with the aspect of improving the output power and cycle life of PEMFC and ZAB.The research contents include the following two parts.(1)In the third chapter,an Ag/Pd alloy catalytic layer was fabricated on the near surface of the Nafion membrane(Nafion-Ag/Pd membrane)by the impregnationreduction method combined with galvanic replacement.Although the metal loading amount was low,the water absorption ratio of the Nafion-Ag/Pd membrane was improved significantly,which was beneficial to increase the through-plane proton conductivity of the Nafion membrane.By constructing an Ag/Pd alloy conductive layer near the surface of the Nafion membrane,it would improve the in-plane electronic conductivity of the Nafion membrane,reduce the interface contact resistance between the cathode catalytic layer of PEMFC and the Nafion membrane,and promote the oxygen reduction reaction at the same time,which possessed a certain significance in improving the output power of PEMFC under dry conditions.(2)In the fourth chapter,ethylene glycol was used as solvent and reductant to synthesize the carbon-supported Ag@Pd core-shell oxygen reduction catalysts by onepot synthesis.The formation of the Ag@Pd core-shell structure catalysts with Ag as core and Pd as shell was confirmed by XRD,TEM,EDS,ICP and XPS characterization.The electrochemical characterization results showed that the catalytic activity(0.9 V)of Ag@Pd/C catalysts was 1.7 times higher than that of the commercial Pd/C catalysts.And the Ag@Pd/C catalysts also showed better stability after ADT test.Benefiting from the higher ORR catalytic activity and stability of Ag@Pd/C catalysts under alkaline conditions,the output power and long-term discharge stability of the home-made Znair battery with Ag@Pd/C catalysts as cathode catalytic layer were effectively improved.