材料期刊网

通过NaBH_4还原CoSO_4溶液制备了Co_(0.68)B_(0.32),Co_(0.55)B_(0.45)和Co_(0.50)B_(0.50)一系列超细非晶态Co-B合金粉体. 电化学测试表明,在300 mA/g的高电流密度下,3种合金电极的首次放电容量分别高达510.6,666.4和667.2 mA·h/g,经60次循环后,放电容量仍分别有331.6,379.5和390.5 mA·h/g.3种合金电极还表现出良好的高率放电性能,在1200 mA/g的放电电流密度下,放电容量分别为336.2,373.4和390.1 mA·h/g.较高的B含量有助于提高合金的电化学性能,这是因为B的氧化溶解能提高合金电极的实际电化学反应面积,

Alkaline rechargeable batteries, such as Ni/Cd batteries and Ni/MH batteries have been widely used as power sources, however, their further applications are limited due to contamination of Cd in Ni/Cd batteries and lower discharge capacities of Ni/MH batteries. Some metal borides,such as Co-B, Ni-B, Fe-B, V-B and Ti-B, have been known to own very high discharge capacity in alkaline aqueous solution, in which Co-B alloy exhibits the highest reversible discharge capacity and the best cyclic stability. However, the reversible capacity of Co-B alloy prepared by arc melting process is usually less than 250 mA·h/g, which is only one fourth of the theoretical capacity of the alloy electrode (908 mA·h/g). In the present study, chemical reduction method was used to prepare Co-B alloy to further enhance the electrochemical capacity of the alloy. A series of ultrafine powders of amorphous Co-B alloys, Co_(0.68)B_(0.32), Co_(0.55)B_(0.45) and Co_(0.50)B_(0.50), were prepared by reducing CoSO_4 with solution of NaBH_4. Electrochemical measurements indicate that the prepared alloys exhibit excellent electrochemical properties. At a high current density of 300 mA/g, the initial discharge capacities of these alloys are 510.6, 666.4 and 667.2 mA·h/g, respectively, their discharge capacities still keep 331.6, 379.5 and 390.5 mA·h/g after 60 cyc, respectively. Even at a discharge current density as high as 1200 mA/g, the three alloys still deliver reversible capacities of 336.2, 373.4 and 390.1 mA·h/g, respectively. In the Co-B alloy electrodes, the boron atoms have two functions. First, boron can be oxidized to BO_3~(-3), thereby partly contributes to the discharge capacity. Second, most importantly, the gradual dissolution of boron into the electrolyte (in the form of BO_3~(-3)) during the charging/discharging creates a new surface in the electrodes, which can effectively increase the surface area of the active material in contact with the electrolyte. The alloy with higher boron content thereby can produce a larger reaction surface area by boron dissolution than the alloy prepared with lower B content. So the higher B content in the Co-B alloys can be helpful for improving their electrochemical properties.