Electrochemical Study On Ultrafine Co-B Alloy

Secondary alkaline batteries have been playing an important role in the field of electric energy storing devices for several decades. However, they are increasing limited in meeting the growing energy and power demands of consumer devices. Therefore, there is continuous need to search for new energy storage materials to develop higher density batteries. Co-B alloy has attracted increasing attention due to its large discharge capacity in alkaline solutions and may serve as a new type of negative electrode with the high energy density for alkaline secondary batteries. To overcome the shortcomings of CoB alloy electrode, some original research work has been done in this paper.Co57.5B42.5 was prepared by reducing CoSO4 with alkaline sodium borohydride (NaBH4) in 1.0 ml/L ethylenediamine +25 g/L CoSO4?7H2O slightly alkaline solution at 600C. The discharge capacity of the alloy reaches 795.9mAh/g at a high current density of 300mA/g and 462.7mAh/g after 100 cycles, which is about 1.5 times that of conventional AB5-type alloy.It has been confirmed by XPS, XRD, CV and EIS that Co-B electrode reaction is not an electrochemical hydrogenation– dehydrogenation process, but a Co/Co (OH) 2 reaction. The Co/Co (OH) 2 reaction of Co-B electrode may occur in two steps represented as: Co + OH- = Co (OH) ads + e- Co (OH)ads + OH- = Co (OH)2, ads + e-The precipitation process of Co can be control by both charge transfer and diffusion of OH- in micro pores of the electrode; cobalt dissolution is an activation-passivation process, which is completely controlled by charge transfer.It was found from charge-discharge, CV, ICP and SEM results that capacity loss of Co-B electrode during cycling at room temperature is due to dissolution of a small amount of Co (OH) 2 and passivation, while the serious capacity loss at elevated temperature (550C) results from dissolution of a large amount of Co (OH) 2 into electrolyte.Study shows that the addition of K2S or 8-hydroxyquinoline to electrolyte can significantly suppress capacity loss of Co-B electrode during cycling at elevated temperature, which may contribute to the formation of cobalt sulfide or [Co(II)(8-HQ)2.2H2O] complex compound protective film, reducing the dissolution of Co (OH) 2.