Research On The Catalytic Performance Of Non-noble Metal Oxides In Electrocatalysts

The existence and development of human society are strongly dependent on energy sources.Fossil fuels,as a major part of the world's energy sources,will be exhausted because of over exploitation and utilization.At the same time,excessive exploitation and use of large amounts of fossil fuels would cause serious environmental pollution.The above negative situations could restrict the sustainable development of today's society.Therefore,the search for alternative clean energy sources has become a major task for scientific researchers in the field of energy sources.Fuel cells can directly convert chemical energies to electric energies and thus have high power generation efficiency because they are not restricted by the Carnot Cycle.Among all kinds of fuel cells,direct methanol fuel cells?DMFCs?have many advantages,such as little environmental pollution and easily available fuel,so they are regarded as the most promising mobile energy device in the next few years.Pt-based materials have been the most efficient catalysts for DMFCs,however,the cost of them is high and they are easily poisoned by the intermediate species during the electrocatalysis.Therefore,it is necessary to develop a non-precious metal catalyst with high activity and stability for DMFCs.Based on the above considerations,in the present work,we choose Ag to replace Pt and introduce non-noble metal oxides into the electrocatalysts due to their prominent promoting effect.We design and synthesize a series of metal oxides including Co₃O₄,Mn₃O₄,composite Co₃O₄-Mn₃O₄ oxides,Mn_yO_x with different morphologies,Polyaniline?PANI?-decorated MoO₃ nanorods.The catalytic proformances of these metal oxides-promoted catalysts for ORR and MOR are investigated.Specific results are as follows:1.Co₃O₄,Mn₃O₄,and Co₃O₄-Mn₃O₄ composite metal oxides are synthesized and used as the promoter to Ag catalysts.The catalytic performances of the Ag/Co₃O₄/C,Ag/Mn₃O₄/C and Ag/Co₃O₄-Mn₃O₄/C catalysts toward ORR in 0.5 M KOH electrolyte solution are investigated.For a better understand of the promoting effect of metal oxides,Ag/C sample was also synethsized in this work.From the LSV curves,it can be seen that both onset potentials of Ag/Co₃O₄/C and Ag/Mn₃O₄/C are located at0.87 V,which is about 20 mV more positive than that on Ag/C catalyst?0.85 V?.For the composite metal oxide-supported catalyst Ag/Co₃O₄-Mn₃O₄/C,the onset potential is located at 0.89 V,which shows ca.20 mV more positive than Ag/Co₃O₄/C and Ag/Mn₃O₄/C catalysts,and ca.40 mV more positive than Ag/C catalyst.Results show that the metal oxides such as Co₃O₄ and Mn₃O₄ could significantly improve the catalytic performances of Ag to ORR,and the composite Co₃O₄-Mn₃O₄ shows the highest promoting effects among the catalysts probably due to the co-synergistic effects of Co₃O₄-Mn₃O₄.2.In this work,Mn_yO_x nanocrystals with different morphologies including nanorods,nanoplates and corallines are synthesized through the simple chemical precipitation method.They are used as the promoter and support for Ag catalysts.The morphologies and structures of the catalysts are comprehensively characterized by SEM,TEM,XPS and XRD.The results of the electrochemical techniques show that the onset potentials of ORR on the Ag/Mn_yO_x/C catalysts with different morphologies are almost the same?0.87 V?,which is positively shifted by 20 mV as compared with Ag/C sample.Among the catalysts,Ag/Mn_yO_x/C with Mn_yO_x nanorods as the promoter shows the highest catalytic properties.The kinetic current of ORR on Ag/Mn_yO_x-R is 14.09 mA cm^-2,which is 1.5 times that of Ag/C sample.Results show that the introduction of Mn_yO_x into Ag catalysts can increase their catalytic activity and the morphologies of Mn_yO_x promoters have an important effect on the catalytic performances of Ag.The electronic interaction of Ag between the Mn_yO_x promoters is responsible for this phenomenon.3.In this work,we investigated the promoting effect of MoO₃ to Pt catalyst by rationally designing and tuning the nanostructure of the catalysts.MoO₃ nanorods are firstly synthesized through hydrothermal method and used as the substrate for the deposition of polyaniline?PANI?layer.The PANI-MoO₃ composite nanostructures are then used as the support for Pt catalyst.Depending on the preparation method of Pt nanoparticles,the nanostructure can be PANI nanotube supported Pt?Pt/PANI?through etching MoO₃ nanorods with NaBH₄ and PANI-MoO₃ composite nanorods supported Pt?Pt/PANI-MoO₃?.The catalytic properties of the two catalysts toward MOR are investigated.Results show that the current of methanol oxidation on Pt/PANI-MoO₃ catalyst is comparable to that on Pt/PANI(0.150 A mg_Pt^-1),while the onset and peak potential of MOR?0.40 and 0.64 V?on the former is lowered by 150and 180 mV,respectively,as compared with the latter(?0.55 and 0.82 V?.This phenomenon suggests a much higher catalytic activity of Pt/PANI-MoO₃.Under 0.6V,the methanol oxidation current on Pt/PANI-MoO₃ is 0.032 A mg^-1_Ptt at 3600 s,which is much higher than that on Pt/PANI sample(0.003 A mg_Pt^-1).Our findings indicate that the presence of MoO₃ can promote the catalytic properties of Pt,and the co-synergistic effects of PANI and MoO₃ is responsible for the improvement.