Hydrogen from sodium borohydride and fossil source: An energetic and economical comparison

In this paper, sodium borohydride (NaBH₄) is examined as a method of hydrogen storage and transport, and compared with hydrogen obtained from fossil sources. This chemical hydride has a very high storage density capability due to its large hydrogen content. Hydrogen is released as the main product of the reaction of NaBH₄ with water, with sodium metaborate (NaBO₂) as a by-product. The main disadvantage of the process is the production cost of the borohydride.     

Fabrication of zinc‐loaded hollow glass microspheres (HGMs) for hydrogen storage

The greatest challenge for a feasible hydrogen economy lies on the production of pure hydrogen and the materials for its storage with controlled release at ambient conditions. Hydrogen with its great abundance, high energy density and clean exhaust is a promising candidate to meet the current global challenges of fossil fuel depletion and green house gases emissions. Extensive research on hollow glass microspheres (HGMs) for hydrogen storage is being carried out world‐wide, but the right material for hydrogen storage is yet underway. But many other characteristics, such as the poor thermal conductivity etc. of the HGMs, restrict the hydrogen storage capacity. In this work, we have attempted to increase the thermal conductivity of HGMs by ZnO doping. The HGMs with Zn weight percentage from 0 to 10 were prepared by flame spheroidization of amber‐colored glass powder impregnated with the required amount of zinc acetate. The prepared HGMs samples were characterized using field emission‐scanning electron microscope (FE‐SEM), environmental SEM (ESEM), high‐resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy and X‐ray diffraction (XRD) techniques. The deposition of ZnO on the microsphere walls was observed using FE‐SEM, ESEM and HRTEM which was further confirmed using the XRD and ultraviolet–visible absorption data. The hydrogen storage studies done on these samples at 200 °C and 10‐bar pressure for 5 h showed that the hydrogen storage increased when the Zn percentage in the sample increased from 0 to 2%. The percentage of zinc beyond 2, in the microspheres, showed a decline in the hydrogen storage capacity. The closure of the nanopores due to the ZnO nanocrystal deposition on the microsphere surface reduced the hydrogen storage capacity. The hydrogen storage capacity of HAZn2 was found 3.26 wt% for 10‐bar pressure at 200 °C. Copyright © 2015 John Wiley & Sons, Ltd.     

Dynamic modeling and simulation of hydrogen supply capacity from a metal hydride tank

The current study presents a modeling of a LaNi₅ metal hydride-based hydrogen storage tank to simulate and control the dynamic processes of hydrogen discharge from a metal hydride tank in various operating conditions. The metal hydride takes a partial volume in the tank and, therefore, hydrogen discharge through the exit of the tank was driven by two factors; one factor is compressibility of pressurized gaseous hydrogen in the tank, i.e. the pressure difference between the interior and the exit of the tank makes hydrogen released. The other factor is desorption of hydrogen from the metal hydride, which is subsequently released through the tank exit. The duration of a supposed full load supply is evaluated, which depends on the initial tank pressure, the circulation water temperature, and the metal hydride volume fraction in the tank. In the high pressure regime, the duration of full load supply is increased with increasing circulation water temperature while, in the low pressure regime where the initial amount of hydrogen absorbed in the metal hydride varies sensitively with the metal hydride temperature, the duration of full load supply is increased and then decreased with increasing circulation water temperature. PID control logic was implemented in the hydrogen supply system to simulate a representative scenario of hydrogen consumption demand for a fuel cell system. The demanded hydrogen consumption rate was controlled adequately by manipulating the discharge valve of the tank at a circulation water temperature not less than a certain limit, which is increased with an increase in the tank exit pressure.     

Simulation of high temperature thermal energy storage system based on coupled metal hydrides for solar driven steam power plants

Concentrating solar power plants can achieve low cost and efficient renewable electricity production if equipped with adequate thermal energy storage systems. Metal hydride based thermal energy storage systems are appealing candidates due to their demonstrated potential for very high volumetric energy densities, high exergetic efficiencies, and low costs. The feasibility and performance of a thermal energy storage system based on NaMgH₂F hydride paired with TiCr_1.6Mn_0.2 is examined, discussing its integration with a solar-driven ultra-supercritical steam power plant. The simulated storage system is based on a laboratory-scale experimental apparatus. It is analyzed using a detailed transport model accounting for the thermochemical hydrogen absorption and desorption reactions, including kinetics expressions adequate for the current metal hydride system. The results show that the proposed metal hydride pair can suitably be integrated with a high temperature steam power plant. The thermal energy storage system achieves output energy densities of 226 kWh/m³, 9 times the DOE SunShot target, with moderate temperature and pressure swings. In addition, simulations indicate that there is significant scope for performance improvement via heat-transfer enhancement strategies.     

Modeling of a thermal energy storage system based on coupled metal hydrides (magnesium iron – sodium alanate) for concentrating solar power plants

Concentrating solar power plants represent low cost and efficient solutions for renewable electricity production only if adequate thermal energy storage systems are included. Metal hydride thermal energy storage systems have demonstrated the potential to achieve very high volumetric energy densities, high exergetic efficiencies, and low costs. The current work analyzes the technical feasibility and the performance of a storage system based on the high temperature Mg₂FeH₆ hydride coupled with the low temperature Na₃AlH₆ hydride. To accomplish this, a detailed transport model has been set up and the coupled metal hydride system has been simulated based on a laboratory scale experimental configuration. Proper kinetics expressions have been developed and included in the model to replicate the absorption and desorption process in the high temperature and low temperature hydride materials. The system showed adequate hydrogen transfer between the two metal hydrides, with almost complete charging and discharging, during both thermal energy storage and thermal energy release. The system operating temperatures varied from 450 °C to 500 °C, with hydrogen pressures between 30 bar and 70 bar. This makes the thermal energy storage system a suitable candidate for pairing with a solar driven steam power plant. The model results, obtained for the selected experimental configuration, showed an actual thermal energy storage system volumetric energy density of about 132 kWh/m³, which is more than 5 times the U.S. Department of Energy SunShot target (25 kWh/m³).     

Effect of alloys modified by sodium borohydride alkaline solutions on the kinetics of hydrogen evolution reaction at Mm(Ni3.6Co0.7Mn0.4Al0.3)1.15 hydride electrodes

The effects of treating the alloy powder of an electrode with 6 M sodium hydroxide solution containing y M (y = 0.0, 0.005, 0.01, 0.02, 0.03, 0.05) sodium borohydride (NaBH₄) on the kinetics of hydrogen evolution reaction (HER) at the Mm(B₅)_1.15 hydride electrode (Mm = cerium-rich mischmetal; B₅ = Ni_3.6Co_0.7Mn_0.4Al_0.3) are described. Several kinetic parameters are considered, including Total, Volmer and Tafel overpotentials, exchange current densities and activation energies of the elementary reactions. Such parameters are investigated by measuring the overpotential decay immediately after the interruption of an applied cathodic current. The effects of NaBH₄ treatment and temperature on the mechanism of the HER are analysed on the basis of the dependence of the Volmer and Tafel overpotentials on current density. The effect of the NaBH₄ concentration on the performance of the hydrogen storage alloy as a negative electrode material in nickel-metal hydride batteries and/or alkaline fuel cells is discussed from the standpoint of relative velocity of the elementary reactions.     

Controlling the growth of NaBH4 nanoparticles for hydrogen storage

Sodium borohydride (NaBH₄) is a promising hydrogen storage material due to its potential for reversible hydrogen storage with high hydrogen capacity (10.8 mass%). However, the temperature at which hydrogen can be released from NaBH₄ is too high (> 500 °C) for practical use and the reversible release of hydrogen is only feasible under severe conditions of temperature and pressure. Nanosizing is an effective strategy to improve the hydrogen properties of NaBH₄. In this context, control over the particle size of NaBH₄ has become important. Herein, we report on a solvent evaporation method for the synthesis of isolated NaBH₄ nanoparticles with varying mean particle sizes and morphologies. In particular, the effects of surfactants’ carbon chain lengths, their steric hindrance and functional groups in facilitating the stabilisation of NaBH₄ nanoparticles of various sizes and shapes were investigated. Longer linear carbon chains bearing hard type functional groups were found to favour the growth of smaller NaBH₄ nanoparticles.     

Hydrogen release from sodium borohydrides at low temperature by the addition of zinc fluoride

The major challenge of complex hydride for hydrogen storage is to release hydrogen at a moderate temperature with rapid kinetics. This paper reports a new composite system (NaBH₄/ZnF₂) to generate hydrogen at a low temperature below 100 °C. The NaBH₄ and ZnF₂ mixture was prepared by a mechanical milling in a 2:1 M ratio. Dehydrogenation of the mixture during a heating process shows that the initial dehydrogenation temperature was below 100 °C with favorable kinetics. The structural analysis confirmed the formation of NaBF₄ and Zn in the reaction products. The study concerning the reaction mechanism reveals that ZnF₂ plays a role of reagent rather than catalyst. The produced intermediate of NaZnF₃, however, affects the decomposition of the emitted B₂H₆. This work suggests that the extension of the decomposition mechanism of NaBH₄/ZnF₂ composite system would favor the development of new catalysts for complex hydride systems.     

Novel hydrogen generator/storage based on metal hydrides

A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and higher efficiency than a classical electrolyser. The prototype comprises a six-electrode cell assembly using an AB₅ type metal hydride and Ni plates as counter electrodes, in a KOH solution. Metal hydride electrodes with chemical composition LaNi₄.₃Co_0.4Al₀.₃ have been prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study of the alloy has been performed in a Sievert-type apparatus produced by Labtech Ltd. In the present prototype during charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging the hydrogen stored in the metal hydride it is released to an external H₂ storage. Released hydrogen is delivered into the hydrogen storage up to a pressure of 15 bar. It is anticipated that the device will be integrated as a combined hydrogen generator in a stand-alone system associated to a 1 kW fuel cell.     

Energetic evaluation of hydrogen storage in metal hydrides

Metal hydrides are considered as promising candidates for hydrogen storage as they exhibit higher energy densities than compressed gas storage storages. This study represents a theoretical thermodynamic analysis of metal hydride‐based hydrogen storage systems, focusing mainly on the energy demand to operate the storage system and the resulting efficiency. The main energy demand occurs during hydrogen release. This energy demand is composed of three contributions: the heat required to heat the hydride up to desorption temperature, the heat of reaction and the work of compression to reach the targeted outlet pressure. A sensitivity analysis was performed to demonstrate the impact of several parameters, for example, heat of reaction and hydrogen uptake on the energy balance. The most influential parameter is the heat of reaction. The hydrogen uptake does not have a noticeable influence as long as it is not too low. Several possibilities to improve the efficiency of the storage system are discussed (heat integration and the application of a heat storage system). Heat integration can significantly improve the overall efficiency, whereas the application of a heat storage system does not seem realistic. Compared with other hydrogen storage technologies, metal hydrides can feature higher efficiencies than low‐temperature hydrogen storage concepts, for example, liquefied or cryo‐adsorbed hydrogen. The efficiencies of a metal hydride storage system are similar to those reached with a system based on liquid organic hydrogen carriers. Copyright © 2016 John Wiley & Sons, Ltd.     

Production of NaBH4 and hydrogen release with catalyst

The purpose of this study was to investigate the production of NaBH₄ as hydrogen storage material and testing its hydrogen storage capacity in presence of catalyst. Synthesis of NaBH₄ was investigated with NaH and trimethyl borate which was also produced in previous studies. Different reaction temperatures, times and reactant ratios constitute the three parameters of the production process. The best combination determined by FT-IR, XRF and XRD analyses, was found to be 1.413 (mol/mol) TMB over NaH at 250 °C for 90 min. In order to increase the NaBH₄ purity ethylene diamine was used as solvent at 75 °C. After 4 extraction and crystallization processes 85.17% NaBH₄ purity was obtained. Moreover, hydrogen content of the product NaBH₄ was measured in a system with different catalysts since catalyst efficiency is important in decreasing the water dependence of dehydrogenation. CoCl₂ catalyst proved best by taking out more hydrogen, both from the NaBH₄ structure and from water.     

Complex hydrides as solid storage materials: First safety tests

Hydrogen technology requires efficient and safe hydrogen storage systems. For this purpose, storage in solid materials, such as high capacity complex hydrides, is studied intensely. Independent from the actual material to be used eventually, any tank design will combine nanoscale powders of highly reactive material with pressurized hydrogen gas and so far, little is known about the behavior of these mixtures in case of incidents. For a first evaluation of a complex hydride in case of a tank failure, NaAlH₄ (doped with Ti) was investigated in a small-scale tank failure test. 80–100 ml of the material was filled into a heat exchanger tube, and sealed under argon atmosphere with a burst disk. Subsequently, the NaAlH₄ was partially desorbed by heating. When the powder temperature reached 130 °C and the burst disk ruptured at 9 bar hydrogen overpressure the behavior of the expelled powder was monitored using a high speed camera, an IR camera as well as sound level meters. Expulsion of the hydrogen storage material into (dry) ambient atmosphere yields a dust cloud of finely dispersed powder which does not ignite spontaneously. Similar experiments including an external source of ignition (spark/water reacting with NaAlH₄) yield a flame of reacting powder. The intensity will be compared to the reaction of an equivalent amount of pure hydrogen.     

Simultaneous preparation of sodium borohydride and ammonia gas by ball milling

Hydrogen and ammonia are both potential energy carriers being considered for large scale energy transport. Sodium borohydride (NaBH₄) has been widely applied as a potential solid-state hydrogen storage material. It can produce hydrogen gas and sodium metaborate (NaBO₂) after hydrolysis with water at room temperature. The regeneration of NaBH₄ from NaBO₂ could significantly reduce the cost of NaBH₄ and enable its wide-spread industrial use. In this work, we demonstrate a simple method for regenerating NaBH₄ from NaBO₂·4H₂O using Mg₂N₃, which combines NaBH₄ production with NH₃ gas production in a single step at room temperature. NaBH₄ is successfully synthesized from NaBO₂·4H₂O using the Mg₃N₂ reducing agent via ball milling under a hydrogen atmosphere. NaBH₄ is formed at a 76.6% yield by planetary ball milling at 600 rpm under 40 bar hydrogen for 12 h. NH₃ gas is also formed, which can be easily separated from the solid products. Therefore, this one-step process could produce two different types of carbon-free hydrogen carriers suitable for energy export from renewable sources.     

Hydrogen storage systems based on hydride materials with enhanced thermal conductivity

The reaction of hydrogen gas with a metal to form a metal hydride is exothermic. If the heat released is not removed from the system, the resulting temperature rise of the hydride will reduce the hydrogen absorption rate. Hence, hydrogen storage systems based on hydride materials must include a way to remove the heat generated during the absorption process. The heat removal rate can be increased by (i) increasing the effective thermal conductivity of the metal hydride by mixing it with high-conductivity materials such as aluminum foam or graphite, (ii) optimizing the shape of the tank, and (iii) introducing an active cooling environment instead of relying on natural convection. This paper presents a parametric study of hydrogen storage efficiency that explores quantitatively the influence of these parameters. An axisymmetric mathematical model was formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank, and to predict the transient temperatures and mass of hydrogen stored as a function of the thermal conductivity of the enhanced hydride material, aspect ratio of the cylindrical tank, and thermal boundary conditions. The model was validated by comparing the transient temperature at selected locations within the storage tank with concurrent experiments conducted with LaNi₅ material. The parametric study revealed that the aspect ratio of the tank has a stronger influence when the effective thermal conductivity of the metal hydride bed is low or when the heat removal rate from the tank surface is high (active cooling). It was also found that for a hydrogen filling time of 3 min, adding 30% aluminum foam to the metal hydride maximizes hydrogen absorption under natural convection, whereas the addition of only 10% aluminum foam maximizes the hydrogen content under active cooling. For filling times beyond 3 min, the amount of aluminum foam required to maximize hydrogen content can be reduced for both natural convection and active cooling. This study should prove useful in the design of practical metal hydride-based hydrogen storage systems.     

Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: A review

Hydrogen generation from the hydrolysis of sodium borohydride (NaBH₄) solution has drawn much attention since early 2000s, due to its high theoretical hydrogen storage capacity (10.8 wt%) and potentially safe operation. However, hydrolysis of NaBH₄ for hydrogen generation is a complex process, which is influenced by factors such as catalyst performance, NaBH₄ concentration, stabilizer concentration, reaction temperature, complex kinetics and excess water requirement. All of these limit the hydrogen storage capacities of NaBH₄, whose practical application, however, has not yet reached a scientific and technical maturity. Despite extensive efforts, the kinetics of NaBH₄ hydrolysis reaction is not fully understood. Therefore, better understanding of the kinetics of hydrolysis reaction and development of a reliable kinetic model is a field of great importance in the study of NaBH₄ based hydrogen generation system. This review summarizes in detail the extensive literature on kinetics of hydrolysis of aqueous NaBH₄ solution.     

Thermal Modeling of Mg2Ni-Based Solid-State Hydrogen Storage Reactor

In this paper, a numerical study of coupled heat and hydrogen transfer characteristics in an annular cylindrical hydrogen storage reactor filled with Mg₂Ni is presented. An unsteady, two-dimensional (2-D) mathematical model of a metal hydride reaction bed of cylindrical configuration is developed for predicting the hydrogen storage capacity. The effect of volumetric radiation is accounted in the thermal model. Effects of hydride bed thickness, initial absorption temperature, hydride bed thermal conductivity, and hydrogen supply pressure on the hydrogen storage capacity are studied. A thinner hydride bed is found to enhance the hydriding rate, accomplishing a rapid reaction. At an operating condition of 20 bar supply pressure and 573 K initial absorption temperature, Mg₂Ni stores about 36.7 g hydrogen per kg alloy. For a given bed thickness and an overall heat transfer coefficient, there exists an optimum value of hydride bed thermal conductivity. The present numerical results are compared with the experimental data reported in the literature, and good agreement was observed.     

Discharge dynamics of coupled fuel cell and metal hydride hydrogen storage bed for small wind hybrid systems

In small hybrid wind systems, excess wind energy is stored for later use during the deficit power generation. Excess wind energy can be stored as hydrogen in a metal hydride storage bed and reused later to generate power using a fuel cell. This paper deals with the discharge dynamics of the coupled fuel cell and metal hydride storage bed during the power extraction. Thermal coupling of the fuel cell and metal hydride bed is also discussed. The waste heat generated in the fuel cell is removed using a water coolant. The exit fuel cell coolant stream is passed through the metal hydride storage bed to supply the necessary heat required for desorption of hydrogen from the bed. This will also lead to a reduction in the load on the radiator. The discharge dynamics and the thermal management of the coupled system are demonstrated through a system simulation model developed in Matlab/Simulink platform.     

Thermo-economic analysis of a hydrogen production system by sodium borohydride (NaBH4)

In the spectrum of current energy possibilities, hydrogen represents a solution of great interest toward a future sustainable energy system. No single technology can sustain the energy needs of the whole society, but integration and hybridization are two key strategic features for viable energy production based in hydrogen economy.In the present work, a hydrogen energy model is analyzed. In this model hydrogen is produced through the electrolysis of water, taking advantage of the electrical energy produced by a renewable generator (photovoltaic panels). The produced hydrogen is chemically stored by the synthesis of sodium borohydride (NaBH₄). NaBH₄ promising features in terms of safety and high volumetric density are exploited for transportation to a remote site where hydrogen is released from NaBH₄ hydrolysis and used for energy production.This model is compared from an economic standpoint with the traditional hydrogen storage and transportation technology (compressed hydrogen in tanks).This paper presents a thermodynamic and economic analysis of the process in order to determine its economic feasibility. Data employed for the realization of the model have been gathered from recent important progresses made on the subject.The innovative plant including NaBH₄ synthesis and transportation is compared from an economic standpoint with the traditional hydrogen storage and transportation technology (compressed hydrogen in tanks). As a final point, the best technology and the components' optimal sizes are evaluated for both cases in order to minimize production costs.     

Magnesium and iron loaded hollow glass microspheres (HGMs) for hydrogen storage

The development of a safe and efficient method for hydrogen storage is essential for the use of hydrogen with fuel cells for vehicular applications. Hollow glass microspheres (HGMs) have characteristics suitable for hydrogen storage and are expected to be a potential hydrogen carrier to be used for energy release applications. The HGMs with 10–100 μm diameters, 100–1000 Å pore width and 3–8 μm wall thicknesses are expected to be useful for hydrogen storage. In our research we have prepared HGMs from amber glass powder of particle size 63–75 μm using flame spheroidisation method. The HGMs samples with magnesium and iron loading were also prepared to improve the heat transfer property and thereby increase the hydrogen storage capacity of the product. The feed glass powder was impregnated with calculated amount of magnesium nitrate hexahydrate salt solution to get 0.2–3.0 wt% Mg loading on HGMs. Required amount of ferrous chloride tetrahydrate solution was mixed thoroughly with the glass feed powder to prepare 0.2–2 wt% Fe loaded HGMs. Characterizations of all the HGMs samples were done using FEG-SEM, ESEM and FTIR techniques. Adsorption of hydrogen on all the Fe and Mg loaded HGMs at 10 bar pressure was conducted at room temperature and at 200 °C, for 5 h. The hydrogen adsorption capacity of Fe loaded sample was about 0.56 and 0.21 weight percent for Fe loading 0.5 and 2.0 weight percentage respectively. The magnesium loaded samples showed an increase of hydrogen adsorption from 1.23 to 2.0 weight percentage when the magnesium loading percentage was increased from 0 to 2.0. When the magnesium loading on HGMs was increased beyond 2%, formation of nano-crystals of MgO and Mg was seen on the HGMs leading to pore closure and thereby reduction in hydrogen storage capacity.     

Charging dynamics of metal hydride hydrogen storage bed for small wind hybrid systems

Small hybrid wind systems are capable of storing and supplying power for residential applications. In this paper, the excess wind energy is converted into hydrogen by electrolysis and is stored in a metal hydride. Metal hydride beds are known for their high volumetric capacity compared to the compressed hydrogen storage, and offers hydrogen storage at a reasonable operating temperature and pressure. A system simulation model is developed in Matlab/Simulink platform for the dynamics of the metal hydride hydrogen storage system, which is charged by the wind energy. The thermal loads of the metal hydride storage system is met by passing water at ambient temperature for cooling the bed while hydrogen is being absorbed. The effect of the transient turbulent wind velocity profile on the storage system is analyzed. The thermal management of the storage system plays an important role in the overall design, and hence it is discussed in detail.