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Fluorine substituted High Capacity Hydrides for Hydrogen Storage at low working temperatures

Final Report Summary - FLYHY (Fluorine substituted high-capacity hydrides for hydrogen storage at low working temperatures)

Executive summary:

FLYHY focused on tailoring of materials thermodynamics and kinetics by anion substitution in high-capacity hydrogen storage materials (alane, borohydrides and Reactive hydride composites or RHCs), on employing novel paths of materials synthesis, on obtaining in-depth scientific understanding by extended structural and thermodynamic characterisation and modelling, on determining tank relevant materials properties like compaction behaviour and thermal conductivity, as well as on scaling up materials synthesis and testing a prototype solid state hydrogen storage tank.

H substitution by more electronegative halogens F, Cl, Br, and I in a functional group or a complex changes the bond strength of the remaining elements and thereby may facilitate release and uptake of hydrogen. FLYHY investigated whether too stable or too unstable materials could be modified in such a way that stability reached the desired state (reaction enthalpy -30 to -40 kJ / (mol H2)), while retaining high storage capacity. Modified alane and RHCs were synthesised by high energy ball milling routes. For pure and modified stable and unstable borohydrides novel wet chemical routes for synthesis and combinations with ball milling were developed.

For alane, a wide range of compositions and milling parameters were studied, but no indications of fluorine substitution were observed, agreeing with our theoretical calculations. Due to a positive mixing enthalpy of AlH3 and AlF3, fluorine substitution is rather unlikely. Work on alane was terminated in the first year of the project. For pure, stable borohydrides like LiBH4, Mg(BH4)2 and Ca(BH4)2 new routes of wet chemical synthesis with high yield were developed. They show a high cost advantage compared to the same, commercially available materials.

In pure LiBH4, Cl, Br, and I, substitution of BH4- groups was found. Ca(BH4)2 showed only substitution by Cl and I. Substitution with the heavier halogens leads to the stabilisation of the hexagonal high-temperature LiBH4 polymorph down to room temperature. Substitution was observed as a solid solution with the compound containing the larger anion as the host-structure. By addition of fluorine the stability of borohydrides could be lowered significantly and release of hydrogen, but also other compounds like boranes, be observed already below 150 degrees of Celsius. Indications of fluorine substituting for hydrogen in BH4 groups were observed, e.g. in in situ PXD and Near-edge X-ray absorption fine structure (NEXAFS) data. The hydrogenation was not completely reversible, but enhanced compared to the pure compounds.

For Ca, Li and Na based RHCs, effects of fluorine addition were found in all three systems, e.g. a lowered dehydrogenation temperature and significantly increased reaction rates. Double peaks in Differential scanning calorimetry (DSC) and NEXAFS hint on structural changes due to fluorine, especially significant in fluorinated Ca-based RHC. Cycling of that compound led to lowering of the onset of hydrogen desorption from ca. 300 degrees of Celsius down to less than 250 degrees of Celsius, but the storage capacity of ca. 7 % by weight decreased to approximately 4 % by weight in the first cycle, remaining constant in the following cycles. Modelling of reaction kinetics gave a reduction of the activation energy from 160 to 115 kJ / (mol H2) upon cycling. Increasing F content in this RHC seems to lead to lower reaction enthalpy and activation energy. A value of 49 kJ / (mol H2) was estimated from DSC measurements. This behaviour could be completely explained by the reaction pathway under the chosen conditions. Optimisation of reaction temperatures and pressures and proper additives are expected to lead to reaction pathways, which maintain the high storage capacity.

Comparison of life cycle and fuelling cost of present hydrogen storage methods showed, that solid state hydrogen storage already today is competitive with compressed and liquid storage, provided that raw materials are purchased from large-scale industrial suppliers in tonnage quantity and not from fine chemical suppliers in gram amounts. Effects of less purity of raw materials have to be studied in future projects. The developed materials synthesis routes and employed storage tank technology are especially suitable for manufacturing by Small and medium-sized enterprises (SMEs).

Project context and objectives:

The world's primary energy needs are expected to grow by 55 % from 2005 to 2030. The present energy supply is based on the limited resource fossil fuels, the use of which is causing the CO2 content of the atmosphere to increase and possibly also the global mean temperature. Renewable energy sources like sun, wind or biomass are obvious alternatives to the present use of fossil fuels, but suffering from one major problem: They are unevenly distributed both geographically and over time. Most countries need to integrate several contributions of renewable energies. Therefore, a safe, cheap and efficient energy carrier is required, which can be converted e.g. to electrical energy or heat without harm for the environment. Hydrogen would be the ideal choice, but it suffers from one fundamental drawback as hydrogen is a light gas at ambient conditions, it is very difficult to store in the density and compactness required for industrial and consumer applications.

Novel materials form the backbone of any emerging energy technology, such as the hydrogen based future economy. The scientific core of FLYHY was to carry out international cutting-edge design, preparation, characterisation and application of novel materials for hydrogen storage in the solid state.

At present there is still no hydrogen storage system available fulfilling all requirements for use especially in mobile applications simultaneously:
- high storage density;
- temperatures and heats of operation compatible with PEM fuel cells;
- high hydrogen loading and unloading rates in the range of a few minutes; and
- low-system costs.

FLYHY focused especially on the first three points by:
(i) exploiting findings on tailoring of materials thermodynamics by anion substitution in alane, borohydrides and RHCs, in order to achieve a breakthrough in the thermodynamic hydrogen sorption properties of these materials exhibiting the highest hydrogen capacities known at present;
(ii) obtaining an in depth scientific understanding of the sorption properties of the substituted compounds by extended structural and thermodynamical characterisation and modelling, to optimise the investigated materials;
(iii) determining tank relevant materials properties like e.g. cycling behaviour;
(iv) scaling-up materials production and doing first tests in a prototype tank.

Tailoring of physical properties, such as thermodynamics, i.e. working temperatures, and reaction kinetics, was achieved by adding halogens to the storage materials and employing novel paths of materials synthesis.

Substitution of an element, i.e. hydrogen, with a more electronegative element in a functional group or a complex, changes the bond strength of the remaining elements and thereby may facilitate release and possibly uptake of hydrogen. FLYHY chose the most electronegative element, fluorine, as the focus of its research and also its group members in the periodic table, chlorine, bromine and iodine. By partially substituting halogens for hydrogen or functional groups like (BH4) , the enthalpy of reaction of too stable and too unstable high capacity hydrogen storage materials should be changed to the desired range of -30 to -40 kJ / (mol H2), while retaining as much hydrogen storage capacity as possible. For achieving the targets of FLYHY, three material systems had been selected:
- alane (AlH3, theoretical storage capacity 10.1 % by weight), which up to now cannot be rehydrogenated at conditions suitable for onboard loading. Unmodified alane is also thermodynamically much too unstable for practical use;
- borohydrides show some of the highest theoretical gravimetric hydrogen contents (e.g. LiBH4 18.5 % by weight), but for practical use are much too stable or too unstable, respectively;
- RHCs (reversible capacity up to 11 % by weight;
(LiBH4+MgH2)) have the unique advantage - compared to all other methods for modifying hydrogen storage materials - that upon reaction of two or several hydrides in the composite, an average hydrogen storage capacity together with a substantially reduced reaction enthalpy is achieved.

The objectives of the FLYHY project were to obtain also fundamental knowledge on:
(i) novel routes for reproducible and safe materials synthesis;
(ii) the influence of the modified materials structures on hydrogen sorption properties;
(iii) assessment of the different storage materials with regard to raw materials and production cost and necessary expense for storage tank construction with regard to tank capacity, heat management and tank safety;
(iv) storage tank relevant materials parameters like practical storage densities and thermal conductivities of the materials, hydrogen sorption properties in larger amounts of powders, sensitivity to hydrogen purity and long term stability;
(v) (if promising materials are available at milestone in month 30) upscaled production processes as well as materials behaviour in a laboratory prototype test tank, giving input for future improvements in the time following this project.