Interfacial Behavior And Electrochemical Performance Of Aluminum And Magnesium Anodes In Aqueous Electrolytes

Aluminum and magnesium anodes are attractive electrode materials for metal/air batteries owing to their negative thermodynamic electrode potentials,high specific capacities,abundant reserves in the earth's crust,low cost,environmental friendliness and recyclability.However,their applications are still impeded by critical issues,including hydrogen evolution corrosion and surface passivation,which result in the discounted specific capacity and electrode potential.In the present work,we explored the anode/electrolyte interface by electrochemical and electron microscope technique and optimized the composition of anodes and electrolytes.As a result,high performance metal anode/electrolyte systems were proposed for applying in aluminum/air and magnesium/air batteries.The results are as follows:The aluminum/alkaline solution interface structure schematic was optimized based on the Gouv-Chapman-Stern(GCS)electrical double layer theory through electrochemical impedance spectrum and spectroscopic microscopy analysis,considering the inherent surface film on the aluminum anodes.Results show that the interface film is duplex structure,including the protective passivation film and the porous hydrate oxide layer.The electrical double layer capacitance based on passivation film and the space charge capacitance of porous hydrate oxide layer cause the high-frequecny and low-frequecnv capacitive arcs respectively.The passivation film separates the aluminum atoms and water molecules to avoid the drastic hydrogen evolution corrosion,meanwhile it possesses ionic conductivity acting as the ion channel between electrode and solution with the action of interface electric field.Therefore,enhancing the ionic conductivity of passivation film is key to improving the electrochemical performace of aluminum anodes.The influence of alloying and electrolyte additives on the electrochemical performance of aluminum anodes are investigated in order to alleviate the hydrogen evolution corrosion and surface passivation of aluminum anode in alkaline solution.Results show that solid-solute magnesium in aluminum anode not only inhibits the hydrogen evolution corrosion,but also enhances the discharge activity by Mg2+ doping in the passivation film.Metallic tin can be uniformly deposited on the electrode surface with the synergistic effect of solid solution magnesium and sodium stannate.which further reduces the corrosion rate and improves the discharge activity of aluminum anodes.As a result,a high-performance aluminum anode/electrolyte system Al-0.5%Mg/6 M NaOH+0.05 M Na2SnO3 was proposed to be applied in aluminum/air batteries,displaying the coulombic efficiency of 96%and electrode potential of-1.78 V vs.SCE at operating temperature of 50? and discharge current density of 100 mA·cm-2.The discharge performace exceeds that of multi-component aluminum alloys reported in literatures.The application of magnesium anodes in NaCl solution is impeded by quite low anodic efficiency.In this work,the electrochemical performance of Mg-xAl,Mg-xAl-0.3%Mn and Mg-xAl-1%Zn-0.3%Mn(AZ series)alloys with different aluminum contents are investigated in order to explore the effect of aluminum and optimize its additive amount.The aluminum content in AZ series alloys is optimizaed to be 6%for application of magnesium anodes.Results show that the effect of aluminum on the magnesium anodes including:1)The solid-solute aluminum decreases the discharge activity of magnesium anodes by stablilizing the passivation film;2)The Mg17Al12 precipitated phase aggravates the local corrosion of magnesium anodes.With the increase of precipitated phase amount,it inhibits the anodic hydrogen evolution acting as corrosion barrier;3)The formed Al8Mn5 precipitated phase in Mg-xAl-0.3%Mn and AZ series magnesium alloys can also repress the anodic hydrogen evolution.In addition,CH3COONa solution is proposed to be used in magnesium/air battery as a promsing electrolyte.The coulombic efficiency of magnesium anode in CH3COONa solution is 84%at 10 mA·cm-2 which is much higher than 59%in NaCl solution,the specific energy of magnesium/air batteries based magnesium anode are 1770 and 1370 Wh.kg-1,respectively.It is found that anodic hydrogen evolution of magnesium anode in NaCl is attributed to the exposure of active magnesium surface that caused by the attack of Cl-to the passivation film.By contrast,the proptective passivation film of magnesium anode in CH3COONa solution inhibits the anodic hydrogen evolution,leading to the improvement of the anode coulombic efficiency and specific energy of magnesium/air batteries.