In situ X-ray Investigation of Hydrogen Charging in Thin Film Bimetallic Electrodes
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In situ x-ray investigation of hydrogen charging in thin film bimetallic electrodes Najeh M. Jisrawi, Harold Wiesmann, M. W. Ruckman, T. R. Thurston, G. Reisfeld, B. M. Ocko, and Myron Strongin Department of Physics, Brookhaven National Laboratory, Upton, New York 11973 (Received 1 July 1996; accepted 1 May 1997)
Hydrogen uptake and discharge by thin metallic films under potentiostatic control was studied using x-ray diffraction at the National Synchroton Light Source (NSLS). The formation of metal-hydrogen phases in Pd, Pd-capped Nb, and PdyNb multilayer electrode structures was deduced from x-ray diffraction data and correlated with the cyclic voltammetry (CV) peaks. The x-ray data were also used to construct a plot of the hydrogen concentration as a function of cell potential for a multilayered thin film.
I. INTRODUCTION
Metal hydrides are becoming important materials in batteries and are used in hydrogen storage devices.1–4 In the past, hydrogen-containing electrodes have been examined by electron microscopy,5 x-ray scattering,6–11 or neutron scattering.12 In situ voltammetry has been relied upon for dynamical studies of hydrogen charging due to the lack of suitable probes. Much of the difficulty in probing electrochemical systems in situ arises because the probing radiation is not intense enough to penetrate the cell walls, electrolytes, or electrode material. The radiation must penetrate the bulk of the electrode material, in order to determine whether certain phenomena are controlled by processes occurring at surfaces or in the body of the electrode.13 The construction of powerful synchroton x-ray sources14 now provides beams of sufficient intensity to penetrate thin electrochemical cells and makes it feasible to conduct slow scan cyclic voltammetry15 during the x-ray characterization of working electrodes. In this paper, the results of an in situ investigation of model metal-hydride electrodes using x-ray diffraction and cyclic voltammetry (CV) are reported. Changes in the diffraction patterns from the electrodes were employed to identify metal-hydrogen phases connected with features in the cyclic voltammograms. Specifically, Pd, Pd-capped Nb, and multilayer PdyNb films were investigated and straightforward identification of metalhydrogen phases, even in the more complicated metal multilayers, was found to be possible. The x-ray data showed that the same metal-hydrogen phases formed during charging and discharging as observed in gas phase experiments.16 An interesting aspect of the x-ray data is the observation that buried Nb layers charge with H before the Pd layers. This shows that the hydrogen freely moves between the layers, and the hydrogen chemical potentials of the Pd and Nb layers determine the hyJ. Mater. Res., Vol. 12, No. 8, Aug 1997
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drogen distribution during charging and discharging, as well as in the steady state.
II. EXPERIMENTAL PROCEDURES
The metallic films for this experiment were pre
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