IN-SITU X-RAY-ABSORPTION SPECTROSCOPY STUDIES OF METAL HYDRIDE ELECTRODES

In situ x-ray absorption spectroscopy (XAS) studies were done on three metal hydride electrodes, LaNi5, LaNi4.8Sn0.2, La0.8Ce0.2Ni4.8Sn0.2, in 6M KOH. Ex situ measurements were also made on dry uncycled electrodes and on material from an La0.8Ce0.2Ni4.8Sn0.2, electrode that had been cycled 25 times. Comparison of the in situ XAS at the Ni K and at the La L(3) edge of charged and discharged electrodes indicates large changes in the electronic and structural characteristics on introduction of hydrogen. Results at the Ce L(3) edge in La0.8Ce0.2Ni4.8Sn0.2 show a transition from a mixed valent or to a gamma-like Ce state as the lattice expands during charge. Ex situ x-ray absorption near-edge structures (XANES) at the Ni K edge indicate that the additions of either Ce or Sn fill empty Ni 3d states. The Ni K edge extended x-ray absorption fine structures (EXAFS) for all three alloys in the dry uncharged state were similar, indicating that minor substitutions for either the A or B component do not substantially change the structure. The Sn substitution causes an increase both in a and c axis as evidenced from increase in the Ni-Ni and the Ni-La distances. Partial substitution of La by Ce causes a slight contraction in the Ni-La distance. The Ni XANES and EXAFS indicate that about 6% of the Ni in the La0.8Ce0.2N4.8Sn0.2 corroded after 25 cycles. Ce XANES on the cycled electrode indicates some corrosion of Ce and the formation of Ce (III) state. The results indicate that XAS is a very useful technique for the study of alloy hydrides, particularly the role of electronic structure, the environment around minor constituents, and the corrosion of individual components.