La0.7Mg0.3-xCaxNi2.5Co0.5(x=0～0.15)储氢合金的放电容量衰退机理研究

采用感应熔炼结合粉末烧结两步法制备了La_(0.7)Mg_(0.3-x)Ca_xNi_(2.5)Co_(0.5)(x=0～0.15)储氢合金,并对合金的放电容量衰退机理进行了研究.研究结果显示随着Ca含量的增加,合金的相结构没有发生明显变化,只是晶胞参数逐渐增大,即Ca主要替代了超晶格结构AB2结构单元中的Mg,但在AB5结构单元中少量的Ca恰恰对合金的放电容量产生了重要的影响.Ca在AB2和AB5两种结构单元中的存在会降低储氢过程中晶胞内部的膨胀应力,Ca的溶解能够抑制Mg的腐蚀,生成微溶于水的腐蚀产物,并提高了合金表面具有催化活性的Ni含量,改善了合金的循环寿命.较高的Ca含量会严重破坏合金的相结构,生成过量的腐蚀产物因不能完全溶于水而在合金表面形成包覆层,阻碍了电极反应,造成合金循环过程中放电容量的急剧下降.

Study on degradation mechanism of discharge capacities of La_(0.7)Mg_(0.3-x)Ca_xNi_(2.5)Co_(0.5)(x=0-0.15)hydrogen storage alloys

The La_(0.7)Mg_(0.3-x)Ca_xNi_(2.5)Co_(0.5)(x=0-0.15)hydrogen storage alloys were prepared by induction melting combined with powder sintering method,and the degradation mechanism of discharge capacity was also studied.The results showed that phase structures of the alloys were not changed significantly with the increase of Ca contents,while the lattice parameters increased gradually,indicated that Ca mainly replaced Mg in AB2 structural unit of superlattice structure.But the small amount of Ca dopants in AB5 structural unit had an important impact on discharge capacity.The existence of Ca in AB2 and AB5 structural units reduced the expansion stress during the hydrogen absorption and desorption.The dissolution of Ca inhibited corrosion of Mg and formed slightly soluble corrosion products,which increased catalytic Ni content on the surface and improved the cycle life.Nevertheless,the phase structures of the alloys with a large amount of Ca destroyed and resulted in a sharp decline in discharge capacities.This was due to the excessive corrosion products were not completely soluble in water and hindered electrochemical reaction.