Research On The Mechanism And Performance Of Aqueous Vanadium Diboride-Air Battery

Water-based vanadium diboride?VB₂?air battery is currently known as the primary battery with the highest theoretical specific capacity(4060 m Ah g^-1),and its theoretical discharge potential is 1.55 V.In addition,based on the high stability of VB₂ to water and air,the battery also shows good environmental adaptability and high safety.Therefore,VB₂-air batteries have excellent commercial application potential.However,there are few reports about this battery system so far.There is not enough understanding of the electrochemical reaction mechanism and main problems of the anode,and there is no corresponding research on the discharge performance at high rate(?250 m A g^-1).In view of the limited research work of VB₂-air batteries,we have carried out the following parts to promote the development of the battery.In order to better design an efficient optimization strategy for discharge performance,we first carried out research on anodic electrochemical mechanism and performance dominant factors.The three-electrode configuration was used to test the open circuit potential-time curve and anodic polarization curve with VB₂ membrane electrode as the working electrode.Based on the color characteristics of the electrolytic cell during the two curve tests,combined with the Raman spectroscopy analysis of the solution at the corresponding stage and the XRD and XPS analyses results of the working electrode film after discharge,we summarized a reasonable new anode charge transfer reaction mechanism and the chemical changes of product ions in solution.Subsequently,under the guidance of the new mechanism,the research on the anodic passivation mechanism continued.Through the analyses of the in-situ impedance on cells during galvanostatic discharge and anodic polarization curves with different initial conditions,it was determined that the anodic active substance VB₂ does not undergo surface passivation during the standing and discharging processes of the batteries.Therefore,the possibility of cell polarization and low Coulomb efficiency caused by passivation is ruled out.In order to clarify the real factors that affect the performance of the anode,we studied the factors of the reactants such as OH^-ion concentration,VB₂ membrane area and areal density.The results show that as the OH^-concentration and the contact area of VB₂ with the electrolyte increase,the oxidation dissolution rate of VB₂increases significantly,and the anode potential also shifts significantly.This work to some extent reveals the working principle of VB₂-air batteries and provides a strategic basis for improving discharge performance.The new anode discharge mechanism shows that the oxidation discharge of VB₂ must involve OH^-ions.Based on this,we designed a strategy for VB₂ coated with zwitterionic polydopamine?PDA?.The PDA membrane contains abundant amine groups and phenolic hydroxyl groups exhibiting fully reversible,p H-switchable permselectivity for both cationic and anionic molecules in specific p H range.At high p H,the membrane has a net negative charge that excludes anions but passes cations,while at low p H it is positively charged and excludes cations but passes anions.Based on this characteristic,the PDA coating layer was used to avoid the occurrence of corrosion reactions during battery storage by inhibiting the hydroxide ions from contacting VB₂.The results of VB₂ powder corrosion test showed that the remaining rate of VB₂ with the PDA coating is up to 90 wt%but only 80 wt%for the uncoated one.During discharge,the hydroxyl ions near the anode would be rapidly consumed by the electrode reaction,thereby reducing the p H of the electrolyte near the membrane.Although hydroxide ions can continuously diffuse toward the anode,the continuous anode discharge reaction causes continuous consumption of the hydroxide ions,leading to a dynamic equilibrium.As a result,the PDA membrane exhibits positive charge and attracts hydroxide ions into the surface of VB₂during discharge.For the pouch cells with 325 m Ah capacity discharged at 250 m A g^-1,the VB₂@PDA based cell attains a Columbic efficiency of 87%,19%higher than that of the VB₂based cell.Moreover,the discharge voltage platform of VB₂@PDA based cell displays an upgrade about 0.05 V.Therefore,coating VB₂ with PDA is an effective strategy to suppress hydrogen evolution corrosion.Based on the experimental consideration of anionic conductors,we further designed the strategy for suppressing corrosion by modifying VB₂ particles with Fe Ni-LDH,a typical layered double hydroxide.Studies have shown that LDH materials can conduct OH^-ions through the"Grotthuss mechanism",that is,hydroxyl ions are transported along adsorbed water molecules and hydroxide host layers through rapid hydrogen bond formation and cleavage.The LDH layer was synthesized in situ on the surface of VB₂ as the transmission channel of OH^-ion to ensure the normal charge transfer reaction of the anode,while inhibiting the entry of the depolarizer H⁺ions to avoid self-discharge,thereby improving the discharge efficiency of the battery.The results show that the discharge specific capacity of the battery reaches 2750 m Ah g^-1 at 500 m A g^-1,which is 700 m Ah g^-1 higher than that of the uncoated battery.Therefore,the use of layered double hydroxides with the ability to conduct OH^-ions can effectively improve the anode efficiency of VB₂-air batteries.Although the coating strategy has greatly improved the anode efficiency of VB₂-air batteries under high-rate discharge conditions,the actual discharge potential of the battery differs greatly from the theoretical value.Although the"nanoized VB₂"strategy reported from the literature has reduced the anodic polarization to some extent,it is not enough for full cells.Therefore,we carried out the research on cathode ORR catalysts to improve cathode reaction kinetics and reduce cathode polarization.Based on the characteristics of carbon-based catalysts,we designed the strategy for etching carbon nanotubes with strong oxidizing potassium permanganate.This etching process can not only generate defects and highly active edge carbon electrons on the carbon nanotubes,but also introduce Mn O₂ doping.Using the etched carbon nanotubes directly as the conductive agent and catalyst of the cathode,the discharge potential of the VB₂-air battery is basically close to the discharge potential of the noble metal Pt,which better improves the discharge polarization problem of the battery and reduces the cost.