Preparation and characterization of amorphous and crystalline LaNi₅ thin film electrodes

Amorphous and polycrystalline LaNi₅ thin films have been fabricated by electron beam evaporation in ultrahigh vacuum. Their structure and composition were examined by X-ray diffraction, transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy. The reversible hydrogen storage capacity of these films has been studied electrochemically in 6 M KOH. The charge/discharge rate was ∓ 0.42 mA cm^-2 (10 A g^-1) for 600 Å-thick amorphous films and ∓ 0.25 mA cm^-2 (2 A g^-1) for 2500 Å-thick crystalline films. The maximum discharge capacity of the amorphous films is about 160 mA h g^-1 (LaNi₅H_2.6). They can be subjected to at least 500 cycles before the discharge capacity decreases to half of the maximum value. Polycrystalline films can also be cycled 500 times without decreasing the discharge capacity (160 mA h g^-1, LaNi₅H_2.6) if they are charged to LaNi₅H₃. When they are fully charged, their maximum discharge capacity becomes 250 mA h g^-1 (LaNi₅H₄), which is greater than that of the amorphous films. However, polycrystalline films peel off the substrate when the film is thicker than 2500 Å. A plateau has been observed in the potential vs. discharge time curve for the crystalline LaNi₅ films as thin as 600 Å. There is no plateau for the amorphous films. The plateau, observed during the first several charge/discharge cycles of the 600 Å-thick crystalline film, disappeared after further cycling. This observation indicates a possible phase transformation from crystalline to amorphous. This study is aimed at correlating microstructure with hydrogen storage capability of metal hydride thin films.