Study On Faraday Process Of Manganese Oxide(Mn O_x) Controlled By Manganese Valence State

Supercapacitor,which combines the merits of rechargeable batteries and dielectric capacitors,has relatively higher power density and higher energy density as compared with other new energy storage devices.This unique property makes it have greatly potential applications in electric automobiles and other electronic devices as backup or auxiliary power sources,and attract much investigation.The electrode material plays a decisive role in its performance.Manganese dioxide(Mn O₂)is regarded as an outstanding candidate for metal oxide supercapacitor electrode material,due to its low cost,natural abundance,environmental friendliness and high theoretical capacitance with a relatively large voltage window.However,with the further research,it is found that the electrochemical performance of Mn Ox is limited by relatively poor ionic(10–13 Scm^-1)and electronic conductivity(10^–5–10^–6Scm^-1),resulting in a certain gap between the actual measured capacity and the theoretical capacity and poor rate performance,which limits its application.In order to improve its electrochemical performance,the current research work mainly focuses on two aspects,one is to construct porous manganese oxides with a certain micro/nano morphology structure.The other one is to combine the manganese oxides with carbon materials,so as to improve the ionic and electronic conductivity.Referring to the previous work,the above methods can effectively improve the electrochemical properties of manganese oxide,but the capacity value is still far from the theoretical 1370 F g^-1.According to its energy storage mechanism,the capacity of Mn Ox comes from electric double layer capacitance and pseudocapacitance,and how to promote Faraday process and improve pseudocapacitance is the focus of current research.It has been reported that the valence state of Mn in Mn O₂can be changed by inserting a metal cation(K⁺)into Mn O₂,thus promoting its Faraday energy storage process.Therefore,the aim of this work is to promote Faraday reaction and improve the overall capacitance of the electrode by regulating the valence state of manganese.The specific work is carried out as follows:(1)Relationship between electrolyte cation and electrochemical performancePorous Mn O₂assembled by nanoflowers was successfully obtained by a simple template-free method at room temperature.Through studying the relationship between electrolyte cations(Na₂SO₄,Li₂SO₄,K₂SO₄,Mg SO₄)and electrochemical performance,it is found that electrolyte with smaller cation radius,such as Li⁺and Mg²⁺,are more likely to be embedded in Mn O₂,thus promoting pseudo-capacitance reaction.Moreover,Mg²⁺with two charges can cause multiple electron transfer in the energy storage process,which can provide nearly twice the pseudo-capacitance,thus effectively improving the overall specific capacitance of the electrode.But at the same time,the ionic with smaller radius presents the larger hydrated ion radius,resulting in the lower the rate performance.And the cycling performance is also relatively reduced due to the more ions embedded in the surface bulk phase.(2)Relationship between structure and electrochemical propertiesThe relationship between the microstructure and electrochemical properties of Mn O₂nanoflowers was explored by selecting representative monovalent and bivalent Na₂SO₄and Mg SO₄electrolyte.The results showed that the capacitance or specific capacity increases with the increase of the specific surface area(SSA)and the content of Mn³⁺.The former reflects the double layer storage mechanism,while the latter corresponds to Faraday reaction.Due to the poor rate performance of Faraday reaction,the rate performance of the system depends on the balance and optimization of the two,that is,the size and pore structure of nanoflowers.Therefore,the porous nanoflowers with good morphology,appropriate particle size(60-80 nm)and high SSA exhibited high capacity and excellent rate performance both in Na₂SO₄and Mg SO₄,the specific capacitance is 245 F g^-1at 0.5 A g^-1in Na₂SO₄,which still can maintain 39.5%when increased to 20 A g^-1.In Mg SO₄,a high capacity of 260 m Ah g^-1at 0.2 A g^-1was obtained,and 68 m Ah g^-1can be maintained even at a large current density of 10 A g^-1.However,due to the difference of energy storage mechanism in Na₂SO₄and Mg SO₄,the existence of low valence Mn in Mn O₂nanoflowers makes it perform worse cycling performance in Mg SO₄(Mg SO₄:?72%,5000 times,Na₂SO₄:99.1%,5000 times).(3)Regulation of manganese valence state and its effect on ElectrochemicalIXPerformancei)Defect oxygen:oxygen vacancies were introduced into porous Mn O₂with different crystallinity and morphology by means of vacuum and reduction,and their electrochemical properties were systematically studied.The results show that:(1)The introduction of oxygen vacancy can promote the formation of active sites and Mn³⁺,thus promoting the electric double layer and Faraday reaction,and effectively improving the specific capacitance of the electrode.For example,the capacitance of Mn O₂nanoflowers and 3D network nanoflowers increases from 230 F g^-1and 240 F g^-1to 336 F g^-1and 325 F g^-1,respectively,by introducing oxygen vacancies.(2)The introduction of oxygen vacancies can regulate the Fermi level to improve their rate performance.However,excessive oxygen vacancies will lead to defect association and even material transformation.Therefore,the introduction of oxygen vacancies must be appropriate.Such as,the rate performance of 3D network manganese dioxide with appropriate oxygen vacancies can be improved by more than 10%.(3)The oxygen vacancy can increase the strength of Mn-O band,and the local distortion around it can effectively eliminate the electrochemical strain produced during the reversible ion intercalation/de-intercalation process,thus leading to the improvement of the electrode cycle life.ii)Metal doping:the low valence manganese(Mn³⁺)are introduced by doping low valence metal Ni ion,which effectively promotes the Faraday reaction.At the same time,Ni doping regulates the band gap of Mn O₂and enhances its electrical conductivity,thus increasing the rate performance of Mn O₂nanoflowers.The results show that the sample prepared with 5%Ni/Mn molar ratio exhibits the best electrochemical performance due to its abundant pores,large specific surface area and suitable doping amount.The specific capacitance of the sample can reach 324 F g^-1,which is twice as much as that of undoped Mn O₂(161 F g^-1).The capacitance retention rate is 40.7%when the current density increases to 20 A g^-1,and it shows excellent cycle life(108%,5000 cycles).iii)Chemical bonding:the valence state of Mn in the three-dimensional porous Mn₃O₄/RGO composite was controlled by C-O-Mn chemical bond.The results ofXelectrochemical performance show that C-O-Mn not only promotes the Faraday reaction of the electrode,but also improves the conductivity and cycle stability of the material.Compared with the three-dimensional porous Mn₃O₄electrode material,the specific capacitance of Mn₃O₄modified by C-O-Mn increases from 226 F g^-1to 315F g^-1(0.5 A g^-1),the rate performance increases from 50%to 64.7%(30 A g^-1),and exhibits excellent cycle stability(105%,5 A g^-1,5000 cycles).