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.     

Reversible hydrogen absorption in sodium intercalated fullerenes

The hydrogen absorption of sodium intercalated fullerenes (Na_xC₆₀) was determined and compared to pure fullerenes (C₆₀). Up to 3.5 mass% hydrogen can reversibly be absorbed in Na_xC₆₀ at 200 °C and a hydrogen pressure of 200 bar. The absorbed amount of hydrogen is significantly higher than for the case when only the sodium would be hydrogenated (∼1 mass% for x = 10). At 200 bar the onset of hydrogen absorption is observed at 150 °C. At a pressure of 1 bar hydrogen the major desorption starts at 250 °C and is completed at 300 °C (heating rate 1 °C min⁻¹). This absorption and desorption temperatures are significantly reduced compared to pure C₆₀, either due to a catalytic reaction of hydrogen on sodium or due to the negatively charged C₆₀. The hydrogen ab/desorption is accompanied by a partial de/reintercalation of sodium. A minor part of the hydrogen is ionically bonded in NaH and the major part is covalently bonded in C₆₀H_x. The sample can be fully dehydrogenated and no NaH is left after desorption. In contrast to C₆₀, where the fullerene cages for high hydrogen loadings are destroyed during the sorption process, the Na_xC₆₀ sample stays intact. The samples were investigated by X-ray, in-situ neutron powder diffraction and infrared spectroscopy. Na_xC₆₀ was synthesized by reacting sodium azide (NaN₃) with C₆₀ (molar ratio of Na:C₆₀ is 10:1).     

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.     

Tests on a metal hydride based thermal energy storage system

In this paper, the performance tests on Mg + 30% MmNi₄ based thermal energy storage device is presented. Experiments were carried out at different supply pressures (10–30 bar) and absorption temperatures (120–150 °C). The effects of hydrogen supply pressure and absorption temperature on the amount of hydrogen/heat stored and thermal energy storage coefficient are presented. The maximum hydrogen storage capacity of 2.5wt% is reported at the operating conditions of 20 bar supply pressure and 150 °C absorption temperature. For a given absorption temperature of 150 °C, the thermal energy storage coefficient is found to increase from 0.5 at 10 bar to 0.74 at 30 bar supply pressure. For the given operating conditions of 20 bar supply pressure and 150 °C absorption temperature, the maximum amount of heat stored is about 0.714 MJ/kg and the corresponding thermal energy storage coefficient is 0.74.     

Computational study on metal hydride based three-stage hydrogen compressor

A mathematical model for predicting the performances of a three-stage metal hydride based hydrogen compressor (MHHC) is presented. The performance of the MHHC is predicted by solving the unsteady heat and mass transfer characteristics of the coupled metal hydride beds of cylindrical configuration. The governing equations for energy, momentum and mass conservations, and reaction kinetic equations are solved simultaneously using the finite volume method. Metal hydrides chosen for a three-stage MHHC are LaNi₅, MmNi_4.6Al_0.4 and Ti_0.99Zr_0.01V_0.43Fe_0.99Cr_0.05Mn_1.5. Numerical results obtained for a single-stage MHHC using MmNi_4.6Al_0.4 are in good agreement with the experimental data reported in the literature. Using three-stage compression, a maximum pressure ratio of 28 is achieved for the supply conditions of 20 °C absorption temperature and 2.5 bar supply pressure. A maximum delivery pressure of 100 bar is obtained for the operating conditions of 20 °C absorption temperature and 120 °C desorption temperature.     

Engineering improvement of NaAlH4 system

The significant engineering challenges associated with developing lower-pressure, materials-based, hydrogen storage systems for hydrogen fuel cell light-duty vehicles are being addressed by focusing on the role that powder consolidation can play. NaAlH₄ with 4 mol % TiCl₃ was selected as the model material. We focused on the changes in the physical (density and thermal conductivity) and mechanical properties (biaxial flexure strength) and on how these impacted the volumetric capacity of the hydrogen storage system. Both the thermal conductivity and the density of the ball milled material improved with applied pressure in a uniaxial press over the range of 14 MPa–281 MPa. The thermal conductivity reached a value of (1.64 ± 0.02) W/m/K, which was a factor seven higher than that of the unconsolidated powder. The volume of the material was reduced by 42% at the highest applied pressure. A method was developed for determining the strength of NaAlH₄ pellets before and after hydrogen absorption and desorption cycles. It is based on a biaxial flexure test that was originally designed for determining the strength of green ceramic materials. The tests showed that the pellets were strong with biaxial flexure strength of 1.4 kpsi which was unaltered over three studied hydrogen absorption/desorption cycles. The increased materials density did not affect the hydrogen absorption and desorption kinetics, which is important in order to benefit from the improved volumetric capacity. The new material properties of the compacted NaAlH₄ were used in finite element modeling of a hydrogen storage system that targeted a fast refueling time. The results clearly show an improvement of the volumetric capacity of the system by powder consolidation but the gravimetric capacity remains below target, as expected. A system level study of a light-duty vehicle with such a hydrogen storage system is required in order to determine whether the amount of hydrogen stored in the pore volume of the sodium alanate will still be enough to enable one cold start from room temperature to its operating temperature (120–140 °C) or that a buffer volume needs to be installed. While it is recognized that a sodium alanate based hydrogen storage system has its limitations, it has been demonstrated that powder consolidation can address some of those limitations by improving the thermal conductivity and volumetric capacity.     

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.     

Hydrogenation properties of Hf-Ni intermetallics - Experimental and theoretical investigation

The hydrogenation properties of HfNi and Hf₂Ni₇ intermetallics were investigated at the constant pressure of 1 bar and in the temperature ranges 373-573 K for HfNi and 323-473 K for Hf₂Ni₇. The kinetic parameters, rate constants and activation energies of the absorption processes were determined. Maximal hydrogen absorption, i.e., number of hydrogen atoms absorbed per metal atom, H/M, are 1.05 and 0.04 achieved at 373 K for HfNi and Hf₂Ni₇, respectively. Multiple hydriding/dehydriding was found to influence the improvement of the kinetic parameters. XRD and SEM methods were used to investigate the structural and morphological changes of the samples due to hydrogen absorption. The thermodynamic parameters of hydriding together with the structural properties of the intermetallics and their hydrides, calculated using the full-potential linearized augmented plane waves (FP-LAPW) code based on the density functional theory (DFT), were utilized for the sake of explaining the experimental investigations.     

Hydrogen release properties of lithium alanate for application to fuel cell propulsion systems

In this paper the results of an experimental study on LiAlH₄ (lithium alanate) as hydrogen source for fuel cell propulsion systems are reported. The compound examined in this work was selected as reference material for light metal hydrides, because of its high hydrogen content (10.5 wt.%) and interesting desorption kinetic properties at moderate temperatures. Thermal dynamic and kinetic of hydrogen release from this hydride were investigated using a fixed bed reactor to evaluate the effect of heating procedure, carrier gas flow rate and sample form. The aim of this study was to characterize the lithium alanate decomposition through the reaction steps leading to the formation of Li₃AlH₆ and LiH. A hydrogen tank was designed and realized to contain pellets of lithium alanate as feeding for a fuel cell propulsion system based on a 2-kW Polymeric Electrolyte Fuel Cell (PEFC) stack. The fuel cell system was integrated into the power train comprising DC-DC converter, energy storage systems and electric drive for moped applications (3 kW). The experiments on the power train were conducted on a test bench able to simulate the vehicle behaviour and road characteristics on specific driving cycles. In particular the efficiencies of individual components and overall power train were analyzed evidencing the energy requirements of the hydrogen storage material.     

Thermal modeling of LmNi4.91Sn0.15 based solid state hydrogen storage device with embedded cooling tubes

A 2-D mathematical model is developed for predicting the minimum charging/discharging time of the metal hydride based hydrogen storage device by varying the number of cooling tubes embedded in it. This study is extended to 3-D mathematical model for predicting the hydriding and dehydriding characteristics of LmNi_4.91Sn_0.15 based hydrogen storage device with 60 embedded cooling tubes (ECT) using COMSOL Multiphysics 4.3. The performance of the hydrogen storage device during hydriding/dehydriding process is presented for different supply pressure (10–35 bar), hot fluid temperature (30–60 °C) and effective thermal conductivity of hydride bed (0.2–2.5 W/(m?K)). It is observed that the rate of heat transfer and the hydriding and dehydriding rates are enhanced when the number of ECT is increased from 24 to 70. For the reactor with 60 ECT, the rate of hydrogen absorption is rapid for the supply pressure of 35 bar and hydride bed effective thermal conductivity of 2.5 W/(m?K). The numerically predicted hydrogen storage capacity (wt%) and amount of hydrogen desorbed (wt%) are compared with experimental data and found a good accord between them.     

Modeling the kinetic behavior of the Li-RHC system for energy-hydrogen storage: (I) absorption

The Lithium–Boron Reactive Hydride Composite System (Li-RHC) (2 LiH + MgB₂/2 LiBH₄ + MgH₂) is a high-temperature hydrogen storage material suitable for energy storage applications. Herein, a comprehensive gas-solid kinetic model for hydrogenation is developed. Based on thermodynamic measurements under absorption conditions, the system's enthalpy ΔH and entropy ΔS are determined to amount to −34 ± 2 kJ∙mol H₂⁻¹ and −70 ± 3 J∙K⁻¹∙mol H₂⁻¹, respectively. Based on the thermodynamic behavior assessment, the kinetic measurements' conditions are set in the range between 325 °C and 412 °C, as well as between 15 bar and 50 bar. The kinetic analysis shows that the hydrogenation rate-limiting-step is related to a one-dimensional interface-controlled reaction with a driving-force-corrected apparent activation energy of 146 ± 3 kJ∙mol H₂⁻¹. Applying the kinetic model, the dependence of the reaction rate constant as a function of pressure and temperature is calculated, allowing the design of optimized hydrogen/energy storage vessels via finite element method (FEM) simulations.     

In-situ synchrotron X-ray diffraction study on the dehydrogenation behavior of NaAlH4 modified by multi-walled carbon nanotubes

The effect of multi-walled carbon nanotubes (MWCNTs) addition on the dehydrogenation behavior of NaAlH₄ (sodium alanate) is investigated using high-pressure thermal gravimetric analysis (HPTGA) and in-situ synchrotron X-ray diffraction (in-situ synchrotron XRD) technique. The HPTGA results show that the addition of MWCNTs facilitates dehydrogenation of NaAlH₄ by lowering the first-step dehydrogenation temperature to 80 °C. In-situ synchrotron XRD analysis demonstrates that the dehydrogenation pathway can be modified by the addition of MWCNTs which resulting in an enhanced hydrogen desorption rate and reduced desorption temperature.     

Kinetics of methane reforming over Ru/γ-Al2O3-catalyzed metallic foam at 650-900 °C for solar receiver-absorbers

The kinetics of methane reforming over Ru/γ-Al₂O₃-catalyzed high porosity Ni-Cr-Al foam were examined at temperatures of 650-900 °C in a quartz tubular reactor using an electric furnace. The kinetic data were analyzed by four different types of kinetic models based on the basic, Eley-Rideal, Langmuir-Hinshelwood, and stepwise mechanisms. Validation of the kinetic models was carried out by calculating the determination coefficient r² between the predicted and the experimental results for each model. The absolute average deviation percentage (AAD%) between the predicted and the experimental results was also estimated for each model. The kinetic model based on the reversible stepwise mechanism provided the best prediction of the experimental reforming rates with an AAD value of 6% in the range 650-850 °C.     

Improvement of hydrogen storage properties of the AB2 Laves phase alloys for automotive application

Hydrogen storage properties of the Ti_1.1CrMn AB₂-type Laves phase alloys, for both low (−30 °C) and high (80 °C) temperature applications, are improved by substituting Zr at Ti site. In agreement with the larger radius of Zr than Ti, the lattice volume of (Ti_1−xZr_x)_1.1CrMn (x=0, 0.05, 0.06 and 0.1) alloys, prepared by arc melting, increases with x. The increase in the Zr content leads to a decrease in the equilibrium hydrogen sorption pressure plateau and faster absorption kinetics, associated with an increase in the hydrogen storage capacity from 1.9 to 2.2 wt% for Ti_1.1CrMn and (Ti_0.9Zr_0.1)_1.1CrMn alloys, respectively. At −5 °C, (Ti_0.9Zr_0.1)_1.1CrMn alloy reversibly absorbs and desorbs 2.2 wt% at 160 bar within 250 s. Based on thermodynamic calculated values, the optimized Zr substituted alloy (Ti_0.9Zr_0.1)_1.1CrMn desorbs hydrogen at 3.2 bar at −30 °C and 135 bar at 80 °C. This is a significant reduction of the sorption pressure plateau as compared with the current technology for mobile applications based on Ti_1.1CrMn alloy with hydrogen desorption plateau above 400 bar at 80 °C. Finally, the mechanism of improved hydrogen storage properties is discussed based on the radius and the hydrogen affinity of the substituting element.     

The beryllium / strontium doped hydrogen storage alanate nano powders for concentrating solar thermal power applications

Nanopowders with MeH₂ and Me_m:1-7AlH_n:4-7 (Me:Be/Sr) stoichiometry are synthesized with different aluminum: alkaline earth metal (beryllium and strontium) 0.1 atomic weight ratio step. Hydrogen storage nanoparticles (beryllium hydride-strontium hydride-strontium alanate) have orthorhombic crystal structure, with one step dissociation near 150 °C with ~8 wt% hydrogen in alane to one step in BeH₂ at 250 °C with 18 wt%, to a one step in SrH₂ at 240 °C with 2.1 wt%, and two steps in strontium alanate at 147 and 240 °C with 1.1 and 5.1 wt%. Particles are near 20–70 nm with small crystallite sizes about 6–21 nm in all hydride, alanate and residual ((metallic aluminum and beryllium) and alloyed aluminum, AlSr, Al₄Sr, Al₂Sr intermetallic) phases. Results indicate a controllable process for hydride and alanates nanopowders formation with exact dissociation temperatures/hydrogen release. Nanoparticles show consistent reversibility and cyclic behavior without reducing storage capacity, suitable for concentrating solar power applications.     

Decomposition of lithium magnesium aluminum hydride

The quaternary aluminum hydride LiMg(AlH₄)₃ contains 9.7 wt% hydrogen, of which 7.2 wt% can be released in a two-step decomposition reaction via first formation of LiMgAlH₆ and then the binary hydrides MgH₂ and LiH. In-situ synchrotron radiation powder X-ray diffraction and thermal desorption spectroscopy measurements were performed to analyze the product distributions formed during the thermal decomposition of LiMg(AlD₄)₃. The first decomposition step occurs at about 120 °C and the second at about 160 °C for the as-milled sample, while for a purified sample of LiMg(AlD₄)₃, the decomposition temperatures involving release of hydrogen increase to 140 and 190 °C, respectively, suggesting that pure samples of LiMg(AlD₄)₃ are kinetically stabilized. Studies of the purified LiMg(AlD₄)₃ also showed that the second decomposition step can be divided into two reactions: 3LiMgAlD₆ → Li₃AlD₆ + 3MgD₂ + 2Al + 3D₂ and Li₃AlD₆ → 3LiD + Al + 3/2D₂. Addition of TiCl₃ to LiMg(AlD₄)₃ under a variety of ball milling conditions consistently led to decomposition of LiMg(AlD₄)₃ during milling. Correspondingly, all attempts to rehydrogenate the (completely or partially) decomposed samples at up to 200 bar hydrogen pressure failed. Decomposition of MgD₂ was observed at relatively low temperatures. This is ascribed to thermodynamic destabilization due to the formation of different Al_xMg_y phases, and to kinetic destabilization by addition of TiCl₃. A thermodynamic assessment was established for the calculation of phase stability and decomposition reaction relationships within the Li-Mg-Al−H system. The calculations confirmed the metastability of the LiMg(AlH₄)₃ phase and the irreversibility of the Li-Mg alanate phase decomposition reactions. The Li-Mg alanate decomposition pathways followed experimentally could be explained by the endothermicity of the calculated decomposition enthalpies, in that an impure or catalyzed LiMgAlH₆ intermediate phase could more directly access an endothermic decomposition reaction at lower temperatures, while a kinetically-hindered, purified LiMgAlH₆ would require higher temperatures to initiate the two-step decomposition through an exothermic reaction.     

Kinetic limitations in the Mg-Si-H system

Magnesium silicide (Mg₂Si) has attracted interest as a hydrogen storage material due to favorable thermodynamics (ΔH_desorption = 36 kJ/mol H₂) for room temperature operation. To date, direct hydriding of Mg₂Si under hydrogen gas to form MgH₂ and Si has only been attempted at low pressure and has been hindered by poor kinetics of absorption. In this paper we study the dehydrogenation reaction with in-situ neutron powder diffraction and present results of our attempts to hydrogenate Mg₂Si under both hydrogen and deuterium gas up to temperatures of 350 °C and pressures of 1850 bar. Even under these extreme absorption conditions Mg₂Si does not absorb any measureable quantity of hydrogen or deuterium.