Relationship between H2 sorption, electrochemical cycling and aqueous corrosion properties in A5Ni19 hydride-forming alloys (A = Gd, Sm)

A5B19 compounds (A = rare earth, B = transition metal) are promising materials as negative electrodes in Ni-MH batteries. They are built from stacking along the c axis of one [A2B4] and three [AB5] subunits. The former one exhibits a good ability to absorb hydrogen, and the later one a good cycling stability, thus A5B19 compounds are expected to benefit the large capacity and the cycling stability of both units. In this paper, Gd5Ni19 and Sm5Ni19 were synthesized and their hydrogen sorption properties were investigated using the Sieverts' method. Calendar corrosion was also studied and X-ray diffraction, magnetic measurements and thermal gravimetric analysis were made to determine the amount of corroded alloy. Electrochemical cycling was performed for both compounds. After 100 cycles, the electrochemical capacity of Gd5Ni19 remains constant whereas the Sm5Ni19 one decreases sharply after five cycles. This good capacity retention is linked to the low corrosion rate of Gd5Ni19 which corrodes 3 times less than Sm5Ni19 after 18 weeks in KOH. For Sm5Ni19, we conclude that the capacity decrease was due for 19% to crystallinity loss and 53% to corrosion. Finally, a comparison is made between A5Ni19 and A2Ni7-type compounds and the structure influence on the hydrogenation and corrosion properties is discussed.