Enhanced volumetric hydrogen density in sodium alanate by compaction

Powder compaction is a potential process for the enhancement of the volumetric and gravimetric capacities of hydrogen storage systems based on metal hydrides. This paper presents the hydrogen absorption and desorption behaviour of compacts of sodium alanate material prepared under different levels of compaction pressure. It is shown that even at high compaction levels and low initial porosities, hydrogen absorption and desorption kinetics can proceed comparatively fast in compacted material. Furthermore, experimental hydrogen weight capacities of compacted material are higher than the experimental values obtained in case of loose powder. It is demonstrated that the kinetic behaviour of the compacted material during cycling is directly associated to the volumetric expansion of the compact, which is quantitatively measured and analyzed during both hydrogen absorption and desorption processes. The cycling behaviour and dimensional changes of compacted sodium alanate material are a key consideration point if it is used as hydrogen storage materials in practical tank systems.     

Parallel FTIR-ATR and gravimetrical in-situ measurements on sodium alanate powder samples during hydrogen desorption

An experimental set-up has been developed to carry out in-situ measurements on hydrogen storage materials. It has been used to perform FTIR–ATR measurements on sodium alanate samples during hydrogen desorption and, in parallel, to perform gravimetrical measurements on the same sample. The ATR spectra showed the typical broad Al–H vibrational features, changing according to the phase change the material undergoes during the load cycle. It is shown that the absorbance of the material at these bands and the hydrogen content correspond to each other in a reproducible way. This behavior may be used to measure the hydrogen mass released by an alanate hydrogen reservoir by measuring the optical absorbance at defined wavelengths. In this paper the results of these experiments that may give rise to the realization of a level sensor in future hydrogen storage applications are presented.     

Optimization of hydrogen storage tubular tanks based on light weight hydrides

Design of hydrogen storage systems aims at minimal weight and volume while fulfilling performance criteria. In this paper, the tubular tank configuration for hydrogen storage based on light weight hydrides is optimized towards its total weight using the predictions of a newly developed simulation model. Sodium alanate is taken as model material. A clear definition of the optimization is presented, stating a new optimization criterion: a defined total mass of hydrogen has to be charged in a given time, instead of prescribing percentages of the total hydrogen storage capacity. This yields a wider space of possible solutions. The effects of material compaction, addition of expanded graphite and different tubular tank diameters were evaluated. It was found that compaction of the material is the most influential factor to optimize the storage system. In order to obtain lighter storage systems one should concentrate on improving the ratio mass of hydride bed to mass of tank wall by screening lighter materials for the tank wall and developing hydrogen storage materials exhibiting both higher gravimetric and volumetric storage capacities.     

Experimental results of an air-cooled lab-scale H2 storage tank based on sodium alanate

One possibility to store hydrogen in fuel-cell driven automobiles is the storage in solid state hydrides. Sodium alanate (NaAlH₄) is a well-known hydride desorbing up to 5 wt.% H₂ with reasonable rates at temperatures above 120 °C. Therefore a high temperature PEM fuel cell (HT-PEM FC) system with exhaust temperatures of about 180 °C can be used to provide the required enthalpy of reaction. In this study, the absorption and desorption behaviour of a lab-scale tank containing 304 g cerium-doped NaAlH₄ is studied using (exhaust) air as heat transfer medium. For absorption reactions an optimal temperature for maximal reaction rates is identified. Additionally, the importance of an adapted heat management is shown for the present tank. For desorption experiments different operation procedures are used and the constraints in temperature and air-flow given by the HT-PEM are considered. For all 25 experiments a good cycling stability has been measured with a stabilised material capacity of more than 3.7 wt.% H₂.     

System simulation modeling and heat transfer in sodium alanate based hydrogen storage systems

In this paper, we examine the feasibility of an on-board hydrogen storage system using sodium alanate as the hydrogen storage material. A two-dimensional model is used for evaluating refueling dynamics as well as heat transfer coefficients for the system level model. A parametric study is conducted to understand the influence of different operating parameters on the refueling time. System level performance of this storage system during driving conditions is evaluated using a simulation model developed in Matlab/Simulink platform.     

Development of polymer nanocomposites with sodium alanate for hydrogen storage

The development of materials based on polymer nanocomposites for hydrogen storage with lower temperature of desorption might contribute to the consolidation of the use of hydrogen as a sustainable energy. The purpose of this work was to develop hybrid porous materials consisting of polyaniline or sulfonated polyetherimide as polymer matrices and a potential hydride for hydrogen storage – sodium alanate. Multiwall carbon nanotubes and titanium dioxide were also added in order to improve the hydrogen absorption capacity of the sodium alanate. The nanocomposites were prepared via solution mixing and analyzed by differential scanning calorimetry, thermogravimetric analysis, transmission and scanning electron microscopy and kinetic of hydrogen sorption. The nanoparticles had some influence on the polymers structures, modifying its thermal properties, such as glass transition temperature and the onset temperature of degradation. Microscope analyses showed that not all the particles were always well dispersed and distributed through the matrices. However, kinetics of hydrogen sorption tests indicated a significant amount of hydrogen (up to 1.2 wt%) in the nanocomposites after 12 h at relatively low temperature (120 °C) and 32 bar.     

Cycling and engineering properties of highly compacted sodium alanate pellets

Sodium alanate powder comprised of NaH and Al was doped with 3 mol% titanium chloride (TiCl₃) and pelletized into highly compacted cylindrical pellets. The pelletization process was performed to improve thermal conductivity and volumetric hydrogen capacity of the metal hydride, compared to loose or tapped powder, which are vital requirements for on-board hydrogen storage applications. The pelletization process was performed over a range of 69–345 MPa (10–50 kPSI) with a 95% increase in density and improvement in thermal conductivity 18 times greater compared to powder at the maximum pelletization pressure (1.60 g/cm³ and 0.82 g/cm³; 9.09 W/m K and 0.50 W/m K, respectively). Hydrogen cycling capacities and kinetics were not adversely affected by the pelletization process although 10 cycles are required to obtain full hydrogen capacity. Pellet cycling capacity maintained a stable 4 wt% H₂ over 50 cycles. Ti-doped NaH + Al pellets exhibited similar thermal cycling expansion as with the loose powder; within 30 cycles there was a 50% loss in pellet density and by 50 cycles the loss in pellet structural integrity made handling problematic.     

Sodium alanate system for efficient hydrogen storage

Hydrogen is a promising energy carrier in future energy systems. However, hydrogen storage is facing increasing challenges within the development of more environmentally friendly energy systems with high capacity, fast kinetics, favorable thermodynamics, controllable reversibility, especially for applications in vehicles with fuel cells that use proton–exchange membranes (PEMs). In this report, we present a critical review on catalyst modified and nanoconfined NaAlH₄, focusing on their thermodynamics and kinetics behaviors. Catalyst is of increasing interest and may lead to significantly enhanced kinetics, higher degree of stability and/or more favorable thermodynamic properties. Thus, catalyst–doped NaAlH₄ is expected to strongly contribute by the development of novel catalysts and synthesis methods. Additionally, nanoconfined NaAlH₄ may also have a wide range of applications in the PEM fuel cells. Selected catalyst materials, porous scaffold materials, methods for preparation of NaAlH₄ systems and their hydrogen storage properties are reviewed. This is the first review report on catalyst modified and nanoconfined NaAlH₄.     

Experimental study of powder bed behavior of sodium alanate in a lab-scale H2 storage tank with flow-through mode

Chemical hydrogen storage in complex hydrides offers the potential of high gravimetric storage densities compared to intermetallic hydrides, and is therefore a promising technology for mobile applications. The main challenge for mobile application is still the required high refuelling rate of the hydrogen storage tanks. Since hydrogen is bonded by an exothermal chemical reaction in complex hydrides, appropriate storage tanks require high heat transfer rates for the cooling of the tank. Hydride tanks that are state of the art rely on an indirect cooling and are additionally equipped with e.g. finns, foams, etc. to improve the heat transfer rate. For the present study, an improved laboratory tank, which allows for indirect as well as direct cooling by excess H₂ gas (flow-through mode), has been designed and built. This laboratory tank is filled with 87 g of NaAlH₄ (doped with 2 mol% CeCl₃) and equipped with 8 thermocouples as well as two pressure sensors. Experimental results presented in this paper show a significant influence of the cooling by gaseous excess H₂ on the flow-directional temperature profiles at the part of the reaction bed close to the inlet. Considering the overall conversion, this influence is rather small due to the low heat capacity flux (ρc_p)_H2. Furthermore, it is shown that changes in material properties, attributed to the effects of heat and mass transport as well as intrinsic reaction kinetics, can be measured and assessed by the temperature and pressure sensors. After about 10 complete charging and discharging cycles, the initial permeability K of the bed has decreased by 50% to 1.6·10⁻¹² m². In the same time, the initial thermal conductivity has increased by a factor of 1.3 to values reported in literature (0.67 Wm⁻¹ K⁻¹) and remains constant during further cycles. Additionally, it is observed that the reaction rate of the second absorption step improves, even after a total of 36 cycles.     

System simulation models for on-board hydrogen storage systems

System simulation models for automotive on-board hydrogen storage systems provide a measure of the ability of an engineered system and storage media to meet system performance targets. Thoughtful engineering design for a particular storage media can help the system achieve desired performance goals. This paper presents system simulation models for two different advanced hydrogen storage technologies – a cryo-adsorption system and a metal hydride system. AX-21 superactivated carbon and sodium alanate are employed as representative storage media for the cryo-adsorbent system and the metal hydride system respectively. Lumped parameter models incorporating guidance from detailed transport models are employed in building the system simulation models.     

Enhancement of hydrogen storage properties in 4MgH2 Na3AlH6 composite catalyzed by TiF3

In this work, we have investigated the hydrogen release and uptake pathways storage properties of the MgH₂Na₃AlH₆ with a molar ratio of 4:1 and doped with 10 wt% of TiF₃ using a mechanical alloying method. The doped composite was found to have a significant reduction on the hydrogen release temperature compared to the un-doped composite based on the temperature-programme-desorption result. The first stage of the onset desorption temperature of MgH₂Na₃AlH₆ was reduced from 170 °C to 140 °C with the addition of the TiF₃ additive. Three dehydrogenation steps with a total of 5.3 wt% of released hydrogen were observed for the 4MgH₂Na₃AlH₆-10 wt% TiF₃ composite. The re/dehydrogenation kinetics of 4MgH₂Na₃AlH₆ system were significantly improved with the addition of TiF₃. Kissinger analyses showed that the apparent activation energy, E_A, of the 4MgH₂Na₃AlH₆ doped composite was 124 kJ/mol, 16 kJ/mol and 34 kJ/mol lower for un-doped composite and the as-milled MgH₂, respectively. It was believed that the enhancements of the MgH₂Na₃AlH₆ hydrogen storage properties with the addition of TiF₃ were due to formation of the NaF, the AlF₃ and the Al₃Ti species. These species may played a synergetic catalytic role in improving the hydrogenation properties of the MgH₂Na₃AlH₆ system.     

Controlled degradation of highly compacted sodium alanate pellets

Compaction of sodium alanate doped with 3 mol% titanium chloride (TiCl₃) into rigid cylindrical pellets improves thermal conductivity, density and volumetric hydrogen capacity of a traditionally poorly conductive material. However, hydrogen cycling of alanate pellets results in significant expansion which counteracts the advantages of compaction. Restricting the area in which pellets can expand into minimizes these losses with no adverse effect to cycling capacity. Confined pellets had a 50% less decrease in density over 30 cycles, 5 times greater thermal conductivity within 10 cycles and maintain structural integrity through 50 cycles compared to free pellets. In addition, pellets within mechanical confinement fused into a rigid stack within the first few hydrogen cycles thereby reducing surface contact resistance between pellets by 3.5 times. Improved thermal conductivity and heat transfer through a pellet bed of materials such as complex metal hydrides, is a key aspect for on-board storage applications.     

Honeycomb metallic structure for improving heat exchange in hydrogen storage system

A two-dimensional model for predicting heat and mass transfer in an alanate hydride reactor with metallic honeycomb structure (MHCS) heat exchanger has been developed. Using this model, a numerical study was performed to examine the influence of the MHCS’s cell size on the profiles of temperature, concentrations of the formed species, and hydrogen charging rate. The obtained results showed that the reduction of the MHCS’s cell size combined with an external cooling design configuration permits better use of the storage system. Based on this model, a comparison of the operating performance of various reactor designs was carried out. It was found that equipping the reactor with hexagonal cooling tubes clearly improved the performance of the charging process without further loss in the gravimetric and volumetric capacities of the hydrogen storage system.     

Dynamics of hydrogen powered CAES based gas turbine plant using sodium alanate storage system

Typical compressed air energy storage (CAES) based gas turbine plant operates on natural gas or fuel oils as fuel for its operation. However, the use of hydro-carbon fuels will contribute to carbon emissions leading to pollution of the environment. On the other hand, the use of hydrogen as fuel for the gas turbine will eliminate the carbon emissions leading to a cleaner environment. Hydrogen can be produced using renewable energy sources like wind, solar etc. Storage of hydrogen is a bottleneck for such a system. A high capacity sodium alanate metal hydride bed is used in this study to store the hydrogen. The dynamics of the CAES based gas turbine plant operating with hydrogen fuel is presented along with discharge dynamics of the metal hydride bed. The heat required for desorbing the hydrogen from the metal hydride bed is provided partly by the hot flue gas exiting from the low pressure turbine and partly by external heating. Thus some of the heat from the flue gas is extracted. A novel multiple bed strategy is employed for efficient desorption. Each bed consists of a shell and tube, with alanate in the shell and heating fluid flowing through the helical coiled tube. Hydrogen combustor is modeled using a simplified Continuous Stirred Tank Reactor (CSTR) assumption in CANTERA. The NO_x emissions in the low pressure turbine exhaust stream are presented.     

CFD simulations of filling and emptying of hydrogen tanks

During the filling of hydrogen tanks high temperatures can be generated inside the vessel because of the gas compression while during the emptying low temperatures can be reached because of the gas expansion. The design temperature range goes from ?40 °C to 85 °C. Temperatures outside that range could affect the mechanical properties of the tank materials. CFD analyses of the filling and emptying processes have been performed in the HyTransfer project. To assess the accuracy of the CFD model the simulation results have been compared with new experimental data for different filling and emptying strategies. The comparison between experiments and simulations is shown for the temperatures of the gas inside the tank, for the temperatures at the interface between the liner and the composite material, and for the temperatures on the external surface of the vessel.     

Experimental investigation of a liquid cooled high temperature proton exchange membrane (HT-PEM) fuel cell coupled to a sodium alanate tank

In high temperature proton exchange membrane (HT-PEM) fuel cells, waste heat at approximately 160 °C is produced, which can be used for thermal integration of solid state hydrogen storage systems. In the present study, an HT-PEM fuel cell stack (400 W) with direct liquid cooling is characterized and coupled to a separately characterized sodium alanate storage tank (300 g material). The coupled system is studied in steady state for 20 min operation and all relevant heat flows are determined. Even though heat losses at that specific power and temperature level cannot be completely avoided, it is demonstrated that the amount of heat transferred from the fuel cell stack to the cooling liquid circuit is sufficient to desorb the necessary amount of hydrogen from the storage tank. Furthermore, it is shown that the reaction rate of the sodium alanate at 160 °C and 1.7 bar is adequate to provide the hydrogen to the fuel cell stack. Based on these experimental investigations, a set of recommendations is given for the future design and layout of similar coupled systems.     

Cerium hydride generated during ball milling and enhanced by graphene for tailoring hydrogen sorption properties of sodium alanate

In this study, NaAlH₄?based hydrogen storage materials with dopants were prepared by a two-steps in-situ ball milling method. The dopants adopted included Ce, few layer graphene (FLG), Ce + FLG, and CeH_2.51. The hydrogen storage materials were studied by non-isothermal and isothermal hydrogen desorption measurements, X-ray diffractions analysis, cycling sorption tests, and morphology analysis. The hydrogen storage performance of the as-prepared NaAlH₄ with Ce addition is much better than that with CeH_2.51 addition. This is due to that the impact of Ce occurs from the body to the surface of the materials. The addition of FLG further enhances the impact of Ce on the hydrogen storage performance of the materials. The hydrogen storage capacity, hydrogen sorption kinetics, and cycle performance of NaAlH₄ with Ce + FLG additions are all better than NaAlH₄ materials with the addition of either Ce or FLG alone. The NaAlH₄ with Ce and FLG addition starts to release hydrogen at 85 °C and achieves a capacity of 5.06 wt% after heated to 200 °C. The capacity maintains at 4.91 wt% (94.7% of the theoretical value) for up to 8 cycles. At 110 °C, the material can release isothermally a hydrogen capacity of 2.8 wt% within 2 h. The activation energies for the two hydrogen desorption steps of NaAlH₄ with Ce and FLG addition are estimated to be 106.99 and 125.91 kJ mol^?1 H₂, respectively. The related mechanisms were studied with first-principle and experimental methods.     

Thermochemical production of sodium borohydride from sodium metaborate in a scaled-up reactor

Sodium borohydride (NaBH₄) is a safe and practical hydrogen storage material for on-board hydrogen production. However, a significant obstacle in its practical use on-board hydrogen production system is its high cost. Hence, the reproduction of NaBH₄ from byproducts that precipitate after hydrolysis is an important strategy to make its use more cost effective. In this work, we focused on the optimization of thermochemical NaBH₄ reproduction reaction in a large-scaled reactor (∼100 ml), and we investigated the effects of the reaction temperature (400–600 °C) and H₂ pressure (30–60 bar) on the NaBH₄ conversion yield using Mg as a reducing agent. The conversion yield of NaBO₂ to NaBH₄ increased with an increase in H₂ pressure to 55 bar and then decreased slightly at 60 bar. The yield increased with an increase in the reactor temperature from 400 to 600 °C. The maximum yield was 69% at 55 bar and 600 °C using homogenized reactants by ball-milling for 1 h under an Ar atmosphere. Though Ca as a reducing agent makes the thermochemical reproduction reaction more favorable, the NaBH₄ yield was low after 1 h of production at 55 bar and 600 °C. This result may be due to the fact that Ca is not as effective as Mg in catalyzing the conversion of hydrogen gas to protide (H⁻), which can substitute oxygen actively in NaBO₂.