Preparation Of Mn-based Oxides Cathodic Catalysts And Their Catalytic Performance For Oxygen Reduction Reaction

The rapid depletion of fossil fuel and the increase of environmental pollution drive us to search the sustainable and clean energy resources.Direct methanol fuel cells?DMFCs?can transform chemical energy directly into electrical energy,and have the advantages of great energy density,non-pollution and comparatively lower running temperature.It is considered to be a promising portable and auxiliary power device technology.At present,DMFCs have great potential application,such as portable power supply,electric vehicle power supply.However,the sluggish kinetics of oxygen reduction reaction?ORR?in cathode,low methanol tolerance and high cost of Nafion membrane noble-based cathodic catalysts are the major obstacles preventing the wide used in commercial development of DMFCs.In this paper,we aim at preparing non-noble cathodic catalysts with good ORR catalytic performance,high methanol tolerance and low cost,to reduce the cost and accelerate the ORR efficiency.Therefor,this subject has done the following three aspects of work.?1?3D structured Mn₂O₃ with flower-like?Mn₂O₃-F?,globe-like?Mn₂O₃-G?or rhombohedral?Mn₂O₃-R?morphologies were successfully synthesized by solvothermal method with controlling the amounts of Tween surfactant.DMFCs were equipped with PtRu/C worked as anode catalyst,polymer fiber membrane?PFM?and Mn₂O₃ as cathode catalyst.And the Mn₂O₃-G shows higher ORR catalytic activity than Mn₂O₃-F and Mn₂O₃-R.The maximum power density of Mn₂O₃-F-,Mn₂O₃-G-and Mn₂O₃-R-based DMFCs are 11.44,21.03 and 10.22mW·cm^-2 at room temperature,respectively.The influence of Tween surfactant on microstructure and ORR properties was also studied systematically.Compared with Mn₂O₃-F and Mn₂O₃-R,the test results indicated that Mn₂O₃-G has more oxygen vacancies,which can promote the electrical conductivity and act as an catalytic active center to absorb and store available O₂ for stable ORR property.?2?Based on the good ORR property of Mn₂O₃,heterojunction structural Fe₂O₃/Mn₂O₃ composite and MnFe₂O₄ solid solution were synthesized,and the differences of catalytic properties between the two kinds of Fe-Mn bimetallic oxides were compared.Fe₂O₃/Mn₂O₃ is made up of Fe₂O₃ and Mn₂O₃,and Fe₂O₃nano-particles uniformly disperse on the Mn₂O₃ substrate and a distinct heterojunction boundary between Fe₂O₃ nanoparticles and Mn₂O₃ substrate.CV test shows Fe₂O₃/Mn₂O₃?3:1?exhibits higher oxygen reduction reaction?ORR?activity than Fe₂O₃/Mn₂O₃?1:1?,Fe₂O₃/Mn₂O₃?1:3?,Fe₂O₃/Mn₂O₃?5:1?and MnFe₂O₄.The heterojunction can provide an intensive inside electric field and increases the electron transfer,while different Fe/Mn ratios represent the difference density of heterojunctions.With increasing the Fe/Mn ratio,the density of heterojunctions increases gradually,and ORR activity is gradually improved.However,when the Fe₂O₃ is excessive,lots of Fe₂O₃ nanoparticles wrap in the Mn₂O₃ matrix impeding Mn₂O₃ catalytic sites contacting with O₂ and electrolyte,and the ORR catalytic activity was reduced.DMFC tests show the maximum power density of Fe₂O₃/Mn₂O₃?3:1?is 20.29 mW·cm^-2 at room temperature,and 32.4 mW·cm^-2 at 40?.The much superior catalytic performance is due to its larger surface area,the improvement of charge transfer efficiency by heterojunction and the synergistic effect between Fe₂O₃ and Mn₂O₃,which can improve O₂ adsorption capacity and accelerate O₂ transport.?3?In order to improve the conductivity of non-noble cathodic catalysts,theurea-formaldehyde resins carbon?UFC?and Fe-containing UFC?Fe-UFC?have been prepared by direct carbonization of urea-formaldehyde resins?UF resins?.On this basis,N-doped materials of UFC/MnO₂ and Fe-UFC/MnO₂ composites were successfully synthesized by the situ redox method.UF resins are served as carbon and nitrogen source.Maximum power densities of UFC/MnO₂-and Fe-UFC/MnO₂-based DMFCs are 15.96 mW·cm^-2 and 24.30 mW·cm^-2 at room temperature,26.86 mW·cm^-2 and 40.80 mW·cm^-2 at 40?,respectively.The superior ORR catalytic activity is due to Fe-N_x existing and a large number of pyridine-N in Fe-UFC/MnO₂.The lone pair electrons of pyridine-N can coordinate with transition metal element to form Fe-N_x group on Fe-UFC/MnO₂,which can act as the electrocatalytic active sites for ORR.On the other hand,the presence of iron on the as-prapared catalyst can reduce the band gap,make it easier for electrons to change from valence band to conduction band,facilitate the transfer from Mn^III to Mn^IV,and provide free electrons for the adsorbed O₂ or the active oxygen species,so that O₂ can be reduced to OH^-.Thus iron in catalyst can accelerate the conductivity and the ORR efficiency.