The Crystal Plane Effect Of Manganese Oxide For Oxygen Reduction And Oxygen Evolution

With the increasing global warming and depletion of oil resources,we face enormous environment and energy challenges.In order to meet the global energy demand,the metal air batteries with clean green energy are considered as one of the potential energy storage devices.However,the kinetics of oxygen reduction（ORR）and oxygen evolution（OER）reaction at the air electrode are particularly slow,requiring electrocatalysts to increase their efficiency,which is a key factor limiting the further development of metal-air batteries.Although precious metals such as platinum and rhodium are the first choice for catalyzing ORR/OER,their high price limits the application of the catalyst.However,manganese oxide catalysts have attracted much attention due to their low cost,abundant resources and the higher electrocatalytic activity,but their catalytic properties need to be further improved.Based on this,the focus of this paper is to design a manganese oxide with a unique structure,and to improve the electrochemical performance of the catalyst by increasing the activity site by the modification of Ag and the loading of NiO.The following are the main research contents:（1）Mn₂O₃ with unique cubic and octahedral structure was synthesized by simple hydrothermal method.The relationship between different crystal faces and electrochemical performance of the catalyst was studied.The results of electrochemical studies show that the cubic structure of Mn₂O₃ has better ORR and OER activity than the octahedral structure of Mn₂O₃.This is because the cubic Mn₂O₃ exposed（001）plane has larger oxygen vacancies concentration and the stronger.electron transporting capability than the octahedral Mn₂O₃exposed（111）plane,at the same time,the cubic structure shows better stability than the octahedron.（2）Since the relatively inexpensive noble metal Ag can provide more active sites for the oxygen reduction reaction,in order to improve the catalytic performance of Mn₂O₃,Ag is modified on the surface of Mn₂O₃ with different structures by dipping method to study the influence of different crystals on the growth of Ag nanoparticles and the interaction between Ag and different crystal faces.The study found that different crystal faces affect the growth of Ag nanoparticles,and Ag modified cubic structure of Mn₂O₃ has better ORR catalytic performance,which is close to the commercial Pt/C with the most excellent performance.This is because the modification of Ag produces a higher energy interface structure and provides more active sites,which is beneficial to charge transfer.The catalyst has good stability and methanol resistance.（3）Mn₃O₄ with octahedral structure and nanosphere particles was synthesized by hydrothermal method.The electrochemical properties of different crystal planes of Mn₃O₄ and the effect of NiO loading on the relationship between crystal plane and electrochemical performance were studied.It was observed from the SEM that NiO is adsorbed on the surface of octahedral and spherical particles.From the TEM analysis,it is found that the Mn₃O₄octahedron is exposed to the（011）low energy surface,while the Mn₃O₄ nanosphere particles are exposed to the（103）high energy crystal plane.The electrochemical results show that the ORR and OER activities of Mn₃O₄ nanospheres with high energy crystal plane are higher than those of Mn₃O₄ with octahedral structure.Compared with Mn₃O₄ octahedral and nanosphere particles,NiO/Mn₃O₄ octahedron and NiO/Mn₃O₄ nanosphere particles exhibit excellent ORR and OER properties due to the synergistic effect between NiO and Mn₃O₄ increasing catalyst activity Site,which improves its electrochemical performance.The kinetic results show that the ORR process of NiO/Mn₃O₄ nanospheres is a 4e reaction process,and it has excellent electrocatalysis ORR/OER performance.ItsΔE value is small and can be used as a bifunctional catalyst for oxygen electrodes.This is because the high-energy crystal face structure of the nanosphere particles has a large number of active sites,which facilitates the rapid transfer of charge and enhances the catalytic activity of the electrocatalyst.This study provides a new idea for the design of highly efficient metal-air battery dual-function catalysts.