Research On Modification Of MnO₂ Catalyst For Zinc Air Battery

Metal-air batteries,especially zinc-air batteries have received continuous attention in recent years due to its advantages of low cost,high theoretical energy density,safety and reliability.However,due to the slow redox kinetics of air cathode in rechargeable zinc-air battery,its development and application are seriously restricted.At present,precious metal materials with excellent catalytic activity are rare and expensive.Therefore,many studies have focused on finding a stable and affordable non precious metal materials with good catalytic activity to reduce the use of precious metal catalysts.Among the existing catalytic materials,MnO₂is one of the potential candidates for bifunctional catalysts due to its abundant reserves,low cost and good catalytic performance.However,as a semiconductor material,MnO₂has poor conductivity,so it is not ideal to use it as air cathode catalyst directly.Therefore,some special methods should be used to modify MnO₂to enhance its catalytic activity,so as to achieve the expected goal of commercialization of rechargeable zinc-air battery.In this paper,a kind of nano sea urchin like MnO₂material was synthesized and annealed in a low concentration reducing atmosphere.It was found that the reason for the improvement of the catalytic activity of the annealed sample was that more abundant Mn³⁺and more oxygen vacancies were produced.The specific contents include the following three parts:（1）A sea urchin like α-MnO₂ was synthesized by hydrothermal method,and the effect of hydrothermal time on the catalytic performance of the product was studied.The products were characterized by various means.The results show that there is no significant difference in all aspects of the data between the samples in the short time of 3 hour and the medium time of 9 hour.when it is extended to 15 hour,the specific surface area and catalytic performance of the samples decrease greatly.At the same time,compared with commercial MnO₂,the more excellent catalytic activity of MnO₂prepared under hydrothermal conditions is due to more Mn³⁺and large specific surface area.Finally,the charge and discharge performance of the rechargeable Zn-air battery assembled by hydrothermal 9 hour is better than that of Pt-C.（2）The optimized MnO₂ samples were annealed at different annealing temperatures for two hours in 2%NH₃atmosphere.The products were characterized by various characterization methods.The results show that the crystal form of the sample annealed at 200℃will not change,but a small part of it will turn into Mn₃O₄at 250℃and completely turn into Mn₃O₄at 300℃,which confirms the low thermal sensitivity of MnO₂.In addition,the samples annealed at 200℃also have excellent oxygen evolution（OER）performance.X-ray photoelectron spectroscopy（XPS）test shows that the reason for the improvement of OER performance of the samples annealed at 200℃is the more abundant Mn³⁺and more oxygen vacancies.Finally,the rechargeable Zn-air battery assembled by annealed samples at 200℃has better charge and discharge performance than the untreated MnO₂samples.（3）When 2% NH₃ is replaced by 5% H₂ under the same preparation conditions,it is found that the MnO₂samples prepared in H₂atmosphere have the same catalytic performance as those prepared in NH₃atmosphere.Further more,their Mn³⁺content and oxygen vacancy ratio are similar.This shows that the doping of N element in the NH₃treatment process will not affect the catalytic performance of the samples,on the contrary,Mn³⁺and oxygen vacancies are generated to improve the catalytic performance of the samples.Finally,the charge and discharge performance of rechargeable Zn-air battery assembled by MnO₂annealed at 200℃in H₂atmosphere becomes worse,which may be related to the instability of material structure.