Manganese Oxide-based Electrocatalytic Design And Its Application In Zinc-air Batteries

Increasing energy demand due to rapid population growth and intensive industrialization has prompted society to shift from fossil fuels to more sustainable sources of energy.With the rapid development of modern industry and society,energy and environmental problems have come along.More and more attention has been paid to the development of green and sustainable energy.As an important energy conversion/storage system,zinc-air batteries have attracted much attention due to their high theoretical energy density and specific capacity,low cost and high safety.In addition,rechargeable ZABs contains two basic reactions,including oxygen reduction(ORR)and oxygen evolution(OER).However,due to the complex electron transfer process,both reactions always exhibit slow dynamics,which makes the widespread use of rechargeable ZABs more challenging.Therefore,it is very important to design active and efficient dual-function electrocatalysts to quickly realize rechargeable ZABs charging(OER)/discharging(ORR)function and obtain practical battery performance.At present,platinum(Pt)-based catalysts are still considered to be the most active and stable ORR catalysts,which are difficult to be completely replaced by other elements due to their excellent performance and stability.Iridium(Ir)or ruthenium(Ru)-based oxides are considered to be the most efficient catalysts for OER.However,the shortcomings of precious metal catalysts such as low content,high cost and single function hinder the application of precious metal catalysts.Transition metal oxide(TMO),such as cobalt,nickel and manganese oxides,have been extensively studied as corrosion resistant bifocals.However,the relatively low electronic conductivity greatly limits the use of TMO as an electrocatalyst.Among TMO,manganese oxide(MnOx)has been explored as ORR and OER catalysts because of its rich oxidation state,multiple crystal structures,low cost and environmental friendliness.Based on this,this paper mainly designed a nd studied the application of manganese oxide catalyst in zinc-air battery,including the following work:(1)α-MnO2 with rod-like structure was prepared by reducing potassium permanganate with silicon nanowires and hydrofluoric acid.The α-MnO2 prepared at 120℃ exhibits excellent ORR activity.It showed a large half-wave potential of 0.85 V vs.RHE and a high initial potential of 1.01 V vs.RHE in 0.1 M KOH electrolyte.The diffusion-limited current density(7 mA cm-2)of the α-MnO2-120 catalyst is also quite steady.The exceptional ORR activity of α-MnO2-120 is attributed to the high Mn III content and sufficient oxygen vacancies.In addition,α-MnO2-120 catalyst is used to assemble Zn-air battery showed an open circuit potential of 1.27 V,a maximum power density of 240 mW cm-2 at a current density of 0.33 A cm-2,and excellent stability.(2)The carbon dots bridging nickel oxide and manganese trioxide through the oxygen-containing group of carbon dots(NiO-Mn2O3-CDs)for ORR and OER is proposed.The optimal NiO-Mn2O3-CDs yields remarkable electrocatalytic activity with low overpotential(298 mV)to derive the 10 mA·cm-2 for OER and a high half wave potential(0.84 V vs.RHE)for ORR.The outstanding performances are attributed to the CDs covalently bridging Ni and Mn atoms,which reprogram the electronic structure of active sites and enhance charge transfer.Consequently,the rechargeable zinc-air battery test with NiO-Mn2O3-CDs as the air cathode deliver the excellent performances,including a high open voltage(1.51 V),a maximum power density(287 mW cm-2@0.35 A cm-2)and robust cycling life,outperforming the commercial Pt/C+IrO2.(3)UV reduction method was used to attach Ag to MnO2 for oxygen reduction reaction.In the experiment,it was found that the Ag-MnO2 sample had better catalytic activity,which was due to the conductivity,surface area and surface area of Ag-MnO2.The active sites are significantly increased,thereby realizing the modification of the catalyst.