Hydrogen grain-boundary diffusion in nano-crystalline Mg-films - experimental and theoretical study
Final Report Abstract
Grain boundary (GB) diffusion differs from bulk diffusion in crystals. For substitutional diffusion of solvents in metals it is several orders of magnitude faster, for interstitial diffusion of solvents it can be either slower or faster, depending on the solvent concentration. Kirchheim provided a Gaussian siteenergy model that explains both types of diffusivity upon site-filling in the low-concentration α-phase of the Pd-H system. Based on the Kirchheim model, we suggested in this project that grain boundaries can act as pathways for fast hydrogen uptake. This is of special interest for extra-stable hydrides as those of Magnesium (Mg), because of an extremely low bulk diffusivity. Mg is regarded as a promising energy storage material, because of its high gravimetric hydrogen density in the MgH2 phase. But, practical application is hindered by its generally slow kinetic behaviour and the formation of the socalled blocking layer. MgH2 can be regarded as a model system to investigate the hydrogen GB diffusivity in the hydride phase because of its extreme low bulk diffusivity. This constraint was expected to lead to dominant influence of the GBs on the systems’ kinetics. The targets of this project was firstly to clarify if GBs and interfaces can be considered as fast hydrogen diffusion paths in MgH2 and secondly, to unravel the interplay between grain boundaries and hydride morphologies including GB diffusion on the overall hydrogen diffusion. Physically deposited Mg films were used as nano-crystalline model materials containing a large number of grain boundaries. Hydrogen was loaded from the gas phase or from the electrolyte. It turned out that the “somewhat mysterious” formation of the blocking layer had to be addressed, before. We deduced the impact of the driving force on the hydride formation leading to thick blocking layers by low chemical potentials. A model was suggested here, based on the performed mechanical stress and strain measurements. The hydrogen overall diffusivity was measured by permeation methods and in-situ XRD peak evolution. FEM simulations helped to understand the contribution of GBs on the overall diffusion coefficient. It turned out that the obtained hydrogen overall diffusion coefficients in the hydride phase reflect the GB diffusion. This allows to optimize the film conditions for fast hydrogen uptake. We have achieved ultrafast lateral hydrogen diffusion during hydrogenation of adhered 30 nm thin Mg films, as measured by hydrogenography. According to these measurements, the hydrogenation kinetics was affected by the arising mechanical stress and the front propagation kinetics. It was further found, that Fe-additives did not strongly promote the hydrogenation kinetics. A successful visualization of the hydride formation in a Mg film was depicted by environmental TEM (ETEM). Two modes of hydride formation were detected: one confirmed the mentioned model of half-spherical hydrides. The other mode propagates along the film surface. It was found that hydride formation goes along with nano-crystallization with grains that are separated by low angle grain boundaries. Fast diffusion seems to be bound to the initial high angle grain boundaries, it seems to be slower in low angle grain boundaries. Further fast diffusion appears at the Pd/Mg interface and the Mg/MgH2 interface. All diffusion kinetics are orders of magnitude faster that hydrogen diffusion in the bulk hydride. A model was suggested describing the observed findings. The results provide routes to optimize the micro-structure for fast hydrogen uptake in Mg materials and allow to achieve thick hydride layers, without the hindrance by the blocking layer. The latter is beneficial for high hydrogen storage capacity in Mg energy storage materials.
Publications
- (2011). In-situ XRD measurement of nanocrystalline magnesium films during hydrogen loading. Hamburger, Beamline B2: Synchrotron Labor Annual Report
Uchida, H., Wagner, S., Bell, A. M., & Pundt, A.
- (2014). Hydrogen in Metals. In D. E. Laughlin, & K. Hono, Physical Metallurgy (S. 2597-2705). Oxford: Elsevier
Kirchheim, R., & Pundt, A.
(See online at https://doi.org/10.1016/B978-0-444-53770-6.00025-3) - (2015). Absorption kinetics and hydride formation in magnesium films: Effect of driving force revisited. Acta Materialia, 85, 279-289
Uchida, H., Wagner, S., Hamm, M., Kürschner, J., Kirchheim, R., Hjörvarsson, B., et al., Pundt, A.
(See online at https://doi.org/10.1016/j.actamat.2014.11.031) - “Hydrogen absorption property of nanocrystalline-magnesium films”, Dissertation thesis, Göttingen 2015
Uchida, H.
- (2017) FEM simulation supported evaluation of a hydrogen grain boundary diffusion coefficient in MgH2, International journal of hydrogen energy, 42 (35), 22530–22537
Hamm, M.; Pundt, A.
(See online at https://doi.org/10.1016/j.ijhydene.2017.05.050) - (2018) Fast lateral hydrogen diffusion in magnesium-hydride films on sapphire substrates studied by electrochemical hydrogenography. International journal of hydrogen energy, 43 (3), 1634–1642
Teichmann, N.; Hamm, M.; Pundt, A.
(See online at https://doi.org/10.1016/j.ijhydene.2017.11.101) - “Hydrogen diffusion and hydride formation in grain boundary rich magnesium”, Dissertation thesis, Göttingen 2018
Hamm, M.
- In situ hydrogen loading of magnesium thin-films observed with environmental transmission electron microscopy, International journal of hydrogen energy, 2019
Hamm, M., Bongers, M.D., Roddatis, V., Dietrich S., Lang K.-H., Pundt, A.
(See online at https://doi.org/10.1016/j.ijhydene.2019.10.057)
