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Project Title:

Characterisation of metal hydrides

Ref.No.: 141

Project Type and Category:

Basic research

Project Duration:

01/1989 – 12/1998

Project Participants:

M. Groll, G. Friedlmeier, M. Wanner, Institut für Kernenergetik und Energiesysteme, Universität Stuttgart,
DLR Stuttgart; Max-Planck-Institute für Metallforschung (Institut für Werkstoffwissenschaften, Institut für Physik), Stuttgart

Sponsor:

100% DFG

Project Budget and
Funding:

1.8 Mio DM, see also Project ref. No.: 73

Project Description and Objectives:

A more efficient utilization of primary energy resources is becoming increasingly important due to the environmental problems caused by the intensive use of fossil fuels and due to their limited availability [1]. An important contribution to efficient energy usage is provided by thermal energy upgrading systems such as heat transformers or heat pumps and thermal energy storage systems. Metal hydrides are promising materials for realizing such devices. In order to design these machines it is necessary to know various characteristics of the involved metal-hydrogen reactions [2]. Especially important characteristics are dynamic pressure-composition isotherms (PCI) and the cyclic stability of application-relevant metal hydrides. PCI represent the equilibrium conditions, viz. H2 pressure as a function of the H-content in the solid phase with the temperature as a parameter; they deliver essential information for the designer such as temperature range of applicability, storage capacity and thermodynamic properties [3]. The cyclic stability concerns possible long-term materials degradation phenomena. The metal hydrides used in the considered applications must undergo several tens of thousands of absorption/desorption cycles during the lifetime of the machine. At these conditions intrinsic degradation mechanisms such as disproportionation (decomposition of the useful metal hydrides into very stable phases which remain inactive at operational conditions) can lead to power and efficiency losses in these applications even if they work as closed systems, where contamination with gas impurities can be excluded [4].
Available materials data with respect to the above-mentioned applications are far from being sufficient. Therefore an extensive materials characterisation program is necessary.

Technical Goals:

The goal of the Sub-Project B5 is to provide, at least to a certain degree, the required materials data. The program tasks include the characterisation of available commercial alloys and metals of different types (AB2, AB5,Mg-based high-temperature materials, etc.) and pursues the development of new materials as well. The measured data are conveniently compiled in a data bank created especially for this purpose together with available literature data. The development of the required experimental set-ups is also part of the present work. Volumetric PCI measurements are carried out dynamically (i. e. at a constant hydrogen mass flow rate) since this method represents the practical conditions better than the static one (measurement at a stepwise supply of hydrogen to the sample), which is usually employed in the literature [2].
A first device was developed to investigate the cyclic stability of metal hydrides by thermally cycling the samples in isochoric systems and a second one is under construction. Hydrogen absorption is induced by cooling the samples to a proper temperature, desorption by heating [4]. Cycling conditions which are representative for the considered applications have been selected. Changes of the system pressure difference between desorption and absorption during cycling indicate eventual capacity losses and are used to define a parameter (Dxrev) to Adequately quantify the cyclic stability. Dxrev represents the measured composition change During each cycle (i. e. the reversible capacity of the sample for the given cycling conditions) and is expressed as the atomic ratio of the absorbed or desorbed H to the metal in the sample (H/M). The possibility of recovering the lost capacity and the involved mechanisms are also of interest for this work. Additionally, intrinsic kinetic properties of metal-hydrogen reactions are investigated in collaboration with Sub-Project B2.

Project Status

Finalized

Preliminary or Final Results:

The AB2 alloy Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5 , which was employed in the Mercedes-Benz hydrogen car fleet in the early 80´s shows the highest cyclic stability (Dxrev) together with the maximum and minimum cycling temperatures and pressures (vmax , vmin , pmax and pmin , respectively) as a function of the number of cycles N of all materials investigated in our laboratories so far. The apparent slow and small degradation observed can mostly be attributed to the reduction of the system Pressure due to hydrogen losses to the atmosphere. This explanation is supported by the recovery of Dxrev observed after the H2 pressure was increased at N » 73000 by re-filling. The reversible capacity Dxrev is always lower than the maximum possible value corresponding to stoichiometry (1 H/M). This is caused by the large plateau slope and hysteresis of this material, which does not allow for complete hydriding and dehydriding of the sample at the given cycling conditions.
Cyclic stability (Dxrev), maximum and minimum cycling temperatures and pressures (vmax , vmin , pmax and pmin , respectively) as a function of the number of cycles N for the AB5 alloy LmNi4.49Co0.1Mn0.205AL0.205 were investigated.
Also cyclic stability (Dxrev), maximum and minimum cycling temperatures and pressures (vmax , vmin , pmax and pmin , respectively) as a function of the number of cycles N for the AB5 alloy LaNi4.7Al0.3 were investigated.
The AB5 alloy, mNi4.49Co0.1Mn0.205AL0.205 (Lm = mischmetal) shows a faster and much stronger degradation rate. The original reversible capacity was Recovered by heating the sample to temperatures between 400 and 500 °C under vacuum. Disproportionation was found to be the degradation mec0hanism. Even faster and stronger degradation but similar mechanisms were observed for the AB5 alloy LaNi4.7Al0.3 , which could succsessfully be regenerated several times at the above-mentioned conditions.
The obtained results indicate that in general high temperatures and pressures accelerate the degradation mechanisms. In order to guarantee an efficient and economically acceptable performance of metal hydride machines, special attention must be paid to the selection of sufficiently stable materials. The possibility of and need for a periodic regeneration of the used.

Related Reference Papers and Other Publications:

[1]C.-J.Winter, J. Nitsch(Hrsg.), Wasserstoff als Energieträger, 2.Auflage, Springer, Berlin,1989
[2]G. Sandrock, S. Suda and L. Schlapbach, Hydrogen in Intermetallic Compounds II, Topics in Applied Physics, Vol. 67, Springer, Berlin, 1992, Chap. 5.
[3]G. Friedlmeier, M.Schaaf And M.Groll, Z. Phys. Chem. N. F., 183(1994)185
[4]G. Friedlmeier, A. Manthey, M. Wanner and M.Groll: Cyclic Stability of Varius Application-Relevant Metal Hydrids, J. Alloys Comp., 231(1995) 880.

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Project Title: Characterisation of metal hydrides		Ref.No.: 141
Project Type and Category:	Basic research
Project Duration:	01/1989 – 12/1998
Project Participants:	M. Groll, G. Friedlmeier, M. Wanner, Institut für Kernenergetik und Energiesysteme, Universität Stuttgart, DLR Stuttgart; Max-Planck-Institute für Metallforschung (Institut für Werkstoffwissenschaften, Institut für Physik), Stuttgart
Sponsor:	100% DFG
Project Budget and Funding:	1.8 Mio DM, see also Project ref. No.: 73
Project Description and Objectives:	A more efficient utilization of primary energy resources is becoming increasingly important due to the environmental problems caused by the intensive use of fossil fuels and due to their limited availability [1]. An important contribution to efficient energy usage is provided by thermal energy upgrading systems such as heat transformers or heat pumps and thermal energy storage systems. Metal hydrides are promising materials for realizing such devices. In order to design these machines it is necessary to know various characteristics of the involved metal-hydrogen reactions [2]. Especially important characteristics are dynamic pressure-composition isotherms (PCI) and the cyclic stability of application-relevant metal hydrides. PCI represent the equilibrium conditions, viz. H2 pressure as a function of the H-content in the solid phase with the temperature as a parameter; they deliver essential information for the designer such as temperature range of applicability, storage capacity and thermodynamic properties [3]. The cyclic stability concerns possible long-term materials degradation phenomena. The metal hydrides used in the considered applications must undergo several tens of thousands of absorption/desorption cycles during the lifetime of the machine. At these conditions intrinsic degradation mechanisms such as disproportionation (decomposition of the useful metal hydrides into very stable phases which remain inactive at operational conditions) can lead to power and efficiency losses in these applications even if they work as closed systems, where contamination with gas impurities can be excluded [4]. Available materials data with respect to the above-mentioned applications are far from being sufficient. Therefore an extensive materials characterisation program is necessary.
Technical Goals:	The goal of the Sub-Project B5 is to provide, at least to a certain degree, the required materials data. The program tasks include the characterisation of available commercial alloys and metals of different types (AB2, AB5,Mg-based high-temperature materials, etc.) and pursues the development of new materials as well. The measured data are conveniently compiled in a data bank created especially for this purpose together with available literature data. The development of the required experimental set-ups is also part of the present work. Volumetric PCI measurements are carried out dynamically (i. e. at a constant hydrogen mass flow rate) since this method represents the practical conditions better than the static one (measurement at a stepwise supply of hydrogen to the sample), which is usually employed in the literature [2]. A first device was developed to investigate the cyclic stability of metal hydrides by thermally cycling the samples in isochoric systems and a second one is under construction. Hydrogen absorption is induced by cooling the samples to a proper temperature, desorption by heating [4]. Cycling conditions which are representative for the considered applications have been selected. Changes of the system pressure difference between desorption and absorption during cycling indicate eventual capacity losses and are used to define a parameter (Dxrev) to Adequately quantify the cyclic stability. Dxrev represents the measured composition change During each cycle (i. e. the reversible capacity of the sample for the given cycling conditions) and is expressed as the atomic ratio of the absorbed or desorbed H to the metal in the sample (H/M). The possibility of recovering the lost capacity and the involved mechanisms are also of interest for this work. Additionally, intrinsic kinetic properties of metal-hydrogen reactions are investigated in collaboration with Sub-Project B2.
Project Status	Finalized
Preliminary or Final Results:	The AB2 alloy Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5 , which was employed in the Mercedes-Benz hydrogen car fleet in the early 80´s shows the highest cyclic stability (Dxrev) together with the maximum and minimum cycling temperatures and pressures (vmax , vmin , pmax and pmin , respectively) as a function of the number of cycles N of all materials investigated in our laboratories so far. The apparent slow and small degradation observed can mostly be attributed to the reduction of the system Pressure due to hydrogen losses to the atmosphere. This explanation is supported by the recovery of Dxrev observed after the H2 pressure was increased at N » 73000 by re-filling. The reversible capacity Dxrev is always lower than the maximum possible value corresponding to stoichiometry (1 H/M). This is caused by the large plateau slope and hysteresis of this material, which does not allow for complete hydriding and dehydriding of the sample at the given cycling conditions. Cyclic stability (Dxrev), maximum and minimum cycling temperatures and pressures (vmax , vmin , pmax and pmin , respectively) as a function of the number of cycles N for the AB5 alloy LmNi4.49Co0.1Mn0.205AL0.205 were investigated. Also cyclic stability (Dxrev), maximum and minimum cycling temperatures and pressures (vmax , vmin , pmax and pmin , respectively) as a function of the number of cycles N for the AB5 alloy LaNi4.7Al0.3 were investigated. The AB5 alloy, mNi4.49Co0.1Mn0.205AL0.205 (Lm = mischmetal) shows a faster and much stronger degradation rate. The original reversible capacity was Recovered by heating the sample to temperatures between 400 and 500 °C under vacuum. Disproportionation was found to be the degradation mec0hanism. Even faster and stronger degradation but similar mechanisms were observed for the AB5 alloy LaNi4.7Al0.3 , which could succsessfully be regenerated several times at the above-mentioned conditions. The obtained results indicate that in general high temperatures and pressures accelerate the degradation mechanisms. In order to guarantee an efficient and economically acceptable performance of metal hydride machines, special attention must be paid to the selection of sufficiently stable materials. The possibility of and need for a periodic regeneration of the used.
Related Reference Papers and Other Publications:	[1]C.-J.Winter, J. Nitsch(Hrsg.), Wasserstoff als Energieträger, 2.Auflage, Springer, Berlin,1989 [2]G. Sandrock, S. Suda and L. Schlapbach, Hydrogen in Intermetallic Compounds II, Topics in Applied Physics, Vol. 67, Springer, Berlin, 1992, Chap. 5. [3]G. Friedlmeier, M.Schaaf And M.Groll, Z. Phys. Chem. N. F., 183(1994)185 [4]G. Friedlmeier, A. Manthey, M. Wanner and M.Groll: Cyclic Stability of Varius Application-Relevant Metal Hydrids, J. Alloys Comp., 231(1995) 880.