New Mecury-Replacing Techniques And Mechanisms In Alkaline Zinc Batteries

Owing to its unique merits, zinc has been widely used as anode material in many alkalinezinc batteries. But there are some common problems like self-discharge and poorrechargeability existing in alkaline zinc batteries. Using mecury as additive is considered aneffective way to solve these questions. Mecury had been replaced gradually by the additivescontaining indium from the point of view of enviromental protection. But the electrochemicalbehaviors of indium in concentrated KOH solutions are less understood. The single way of theusage of indium in alkaline zinc batteries and the rising prices of indium because of itswidespread use in many fields and rare souce become an new issues in restricting thedevelopment of alkaline zinc batteries. Therefore exploiting effecitve, eco-friendly and lowcost mecury-replacing and indium-replacing additives becomes the new challenge of alkalinezinc batteries.Several methods for replacing mecury or indium were developed and their mechanismswere investigated by electrochemical and nonelectrochemical methods,including linearpotential sweep, cyclic voltammogram, chronocoulometry, chronopotiometry,AC impedance,hydrogen evolution measurement,XRD, SEM, EDS, FTIR and ICP.The research results showed:(1) The cathodic behavior of indium in KOH solution is mainly determined by theelectrochemical step. Whereas the anodic oxidation of indium is dissolution-depositionmechanism. At lower potential, the corrosion of indium is a one electron reaction. At higherpotential, the reaction involves three electrons transfer. Corrosion product [In(OH)₃]_adisunstable and can dissolve into the solution.The second anodic peak involves the formation of the mixture of [In(OH)₃]_adand In₂O₃.The stability of the indium oxide depends on the formation potential.(2) Both electroless plating and electroplating of indium on the surface of zinc are easyto implement. Indium electroless plated on the surface of zinc can inhibit effectively thecouple reactions of zinc corrosion: hydrogen evolution and the oxidation of zinc. Besides, electroless plating indium can broaden the active-area of zinc, postpone its passivation andrestrain its corrosion in the passive-region. The indium layer with fitting thickness canenhance active dissolution of zinc at initial stage. The charge-discharge performance of zinccan also be improved to a great extent by indium plating, which is important for the usage ofindium in rechargeable zinc batteries.The content of indium in zinc-indium alloy increases higher with increasing inelectroplating time. Indium in alloy coating can enhance the overpotential of hydrogenevolution and the anodic dissolution resistance of zinc. This means that indium can inhibiteffectively the conjugated reaction of zinc corrosion in alkaline electrolyte. Indium withappropriate contents can broaden the potential region for the active dissolution of zinc andpostpone the passivation of zinc. The depth of discharge of zinc in alkaline electrolyte can beimproved in the presence of indium in zinc-indium alloy coating with appropriate content.Accordingly, the discharge capacity of zinc in alkaline electrolyte can be raised to a certaindegree. The indium with appropriate contents in Zn-In alloy coating can favor the reduction ofzinc oxide products. The charge-discharge performance of zinc can also be improved to agreat extent.(3) New free-mecury techniques with less negative influence on battery performance,low cost and environmental friendliness were developed. The surface of cathodiccurrent-collector in alkaline Zn/MnO2battery is treated by chemical plating: Zn-In alloy withhigh hydrogen evolution overpotential is prepared. Zn-In alloy raises the hydrogen evolutionoverpotential of current–collector obviously and is capable of restricting batterygas-expansion effectively and thus improve the performance of battery. Therefore, it wasanticipated that it can find promising application in mercury-free battery industry, as thereplacement of conventional In coated copper current–collector. The Zn/MnO2batteryproducted using the technique is in accordance with the national standard.(4) Both imidazole(IMZ) and Polyethylene glycol600(PEG600) can inhibit zinccorrosion in KOH solution to a certain extent. IMZ inhibits zinc corrosion by mainlydepressing the anodic reaction, whereas PEG by depressing the cathodic reaction, especiallythe evolution of hydrogen. There is a synergistic effect between IMZ and PEG when they arecombined to be used as a composite inhibitor for zinc corrosion in KOH solution, each of both playing a role in suppressing the anodic or cathodic reaction. The synergistic effectresults mainly from the different structure of cyclic IMZ and linear PEG600. IMZ bonds Znthrough nitrogen atom, whereas PEG600through oxygen atom. Thus, when both of theadditives are used in a KOH solution to protect zinc from corrosion, IMZ and PEG exhibitdistinct advantages that help remedy the limitations of using either additive individually. Thecomposite additive used as mecury substitute can not only enhance the effiency on inhibitingzinc corrosion, but also improve the discharge performance of alkaline Zn/MnO2batteries.(5) PEG600and polysorbate20(Tween20) can inhibit zinc corrosion in KOH solutionto a certain extent by depressing hydrogen evolution. There is an obivious synergetic effectbetween PEG600and Tween20on the corrosion inhibition of zinc. The polarity of Tween20is much strong than that of PEG600. Tween20adsorbs on the surface of zinc prior to PEG600. But the surface of zinc cannot be covered totally by Tween20because of its ramousstructure. The remaining active points can be covered by linear PEG600. The battery(Zn/MnO2) discharge performance tests show that the composite inhibitor can reduce theself-discharge of zinc anode effectively. This study provides alkaline zinc battery with apromising substitute inhibitor for mercury.