Operation of the tank under real conditions indicated the possibility for applications with stationary HT-PEM fuel cell systems [ 38 ]. Reprinted with permission from reference [ 38 ]. Up to 31 hydrogenation and dehydrogenation cycles were performed without degradation. The aim of the demonstration project was to produce a lightweight system that could be connected to a HT-PEM fuel cell.
The system could improve efficiency of a combined heat-power unit for household applications [ 42 ]. A lightweight tank for hydrogen storage containing 4. Additionally, the heat evolution and temperature spikes during hydrogen absorption were studied and since NaAlH 4 has a high specific heat, the material itself acted as a heat sink, aiding in the heat management of the system. The authors also performed the first-ever radiography with fast neutrons on the operational complex-hydride based test tank [ 43 ]. Yan et al. This improved the hydrogen desorption property of the tank and also increased the hydrogen desorption time at a constant flow rate Table 3.
The hydrogen permeability decreased with increasing compression pressure, while the entire desorption process only changed slightly. The results indicated that the heat transfer behavior of the hydride bed dominated the hydrogen desorption of the tank [ 44 ]. As it was mentioned in the modeling part, the reason for the combined reactor concept was that the fast desorption reaction of the metal hydride stabilizes the pressure of the reactor at the equilibrium pressure of the complex metal hydride.
The model was developed and validated by experimental data. This reactor has been developed in order to be coupled with a HT-PEM fuel cell running at technically relevant conditions, i. Brooks et al. Once these chemicals have reacted to produce hydrogen, the byproduct must be removed from the system and off-board regenerated before it can be reused. This differs from the previous system described that can be regenerated directly onboard.
Although slurries can be difficult to handle, they can be transported on and off the vehicle using a flow-through process [ 35 ]. There are several drawbacks in handling these materials. The high densities of both slurries result in need for mixing the feed and product prior to movement. Alane was found to have some advantages over AB, which generates fuel cell impurities i. A summary of the developed hydrogen storage systems based on thermolysis is shown in Table 3.
Hydrogen release can also occur apart from being subjected to heat via hydrolysis by reaction with water. A considerable amount of work has been invested in experimental testing of hydrolysis systems for complex metal hydrides [ 47 ]. Sodium borohydride, NaBH 4 , is one of the most tested materials for hydrolysis, Equation 8 [ 48 , 49 , 50 ]. NaBH 4 was first discovered in by Schlesinger and Brown, but the work was classified and not published until [ 51 ].
NaBH 4 has a hydrogen content of Ammonia borane NH 3 BH 3 , AB has received considerable interest as a hydrogen storage material owing to its high hydrogen content When in contact with water, ammonia borane releases hydrogen according to Equation 9 , which is accelerated in acidic conditions and by the use of metal catalysts [ 52 , 53 , 54 , 55 , 56 ]. Some work has been reported for hydrolysis of other metal hydrides or complex metal hydrides, e. Experimental hydrolysis systems are based on two different reactor designs, a batch or flow reactor. In a batch reactor, the complex boron hydride and the catalyst are mixed together in water and the reactor is refueled by exchanging the entire solution including remaining boron hydride, byproduct and the catalyst [ 64 , 65 , 66 , 67 ].
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In a flow reactor, the boron hydride solution is passed over or through a solid catalyst bed followed by separation of the produced hydrogen and the byproduct solution [ 48 , 68 , 69 ]. In both reactor systems, the concentration of the boron hydride solution as well as handling of the waste solution poses problems, e. The hydrolysis reaction can also occur by a reaction between steam and a solid boron hydride [ 57 , 71 , 72 ].
The use of steam can possibly reduce the amount of water otherwise used in the boron hydride solutions, thus improving the total energy storage of the system [ 57 ]. In , a first example of a flow catalyzed hydrolysis reaction was published [ 48 ]. The hydrolysis reaction was carried out over resin beads coated with Ru metal. However, the catalyst was present in excess, which may have enabled the high conversion [ 73 ]. The reactor used the heat evolved from the exothermic reaction between NaBH 4 and water to sustain the hydrolysis reaction [ 69 ].
This system was further developed by integrating a heat exchanger to heat up the fuel feed prior to contact with the catalyst bed. The integrated reactor equipped with a heat exchanger gave a more uniform heat distribution over a range of fuel flow rates, which increased the catalyst effectiveness [ 68 ]. Recently, a new concept reactor with a magnetic containment of permanent trade magnets to recollect the catalyst made out of a magnetic support coated with Ru particles was described [ 75 ].
The reactor should allow better control of the hydrogen production kinetics. The magnetic reactor was also tested in combination with a fuel cell [ 77 ]. The flow reactors discussed so far have a tubular design. A gas channel was situated above the cavity for the catalytic hydrolysis reaction.
The design of the reactor decreased the impact of the hydrogen on the surface of the catalyst, improving the conversion efficiency and allowing for a stable hydrogen generation rate. The hydrogen gas was collected in the gas channel and was quickly moved away from the catalyst. This stopped the reverse flow of the NaBH 4 solution and a continuous flow to the catalyst was achieved. Micro-reactors have also been proposed for production of hydrogen from hydrolysis of chemical hydrides for portable customer electronics [ 79 , 80 , 81 , 82 ].
The micro reactor described by Kim et al. The catalyst support made of nickel foam was positioned between the glass plates. After assembly of the micro reactor, a Co-P-B catalyst was deposited on the catalyst support by reduction of a solution of 0. The micro reactor was used in combination with a micro fuel cell. The system had a maximum power output of mW at a current of 0. There was no difference in performance when hydrogen supplied from the micro reactor was exchanged with pure hydrogen, which showed the micro reactor generated enough hydrogen for the fuel cell [ 79 , 80 ].
Zhu et al. The micro reactor employed self-circulation by a micro pumping mechanism, which was made possible by the hydrogen bubbles produced in the hydrolysis reaction [ 82 ]. This meant hydrogen was generated without parasitic power consumption. In the system, the dissolved NH 3 BH 3 was feed into the micro reactor from a rechargeable fuel reservoir.
Platinum black was electroplated into the channels to serve as catalyst for the hydrolysis of NH 3 BH 3. A small PEM fuel cell was also used in combination with the micro reactor and allowed the generation of power for micro-scale energy devices [ 82 ]. Ferreira et al. In a later publication, Ferreira et al.
The conical shaped reactor was shown to be superior and increased both the hydrogen yield and generation time while decreasing the induction time. The better performance of the conical shaped reactor may be attributed to an enhanced contact between the NaBH 4 and the catalyst [ 70 ].
Only limited reports have been published of reactor modeling for hydrolysis purposes. The physical processes inside a hydrolysis reactor are complex and multi-phase flow, heat and mass transfer need to be evaluated. Zhang et al. Thermal runaway was encountered but could be avoided by continuously cooling the system. The tests also showed that different flow rates had an impact on the temperature profile inside the reactor [ 83 ]. Schematic of the 1 kW e NaBH 4 hydrogen generation system. Reprinted with permission from reference [ 83 ].
The results from Zhang et al. The model assumed homogeneous catalysis and adiabatic operational conditions. The data used for the geometrical evaluation of the model was the temperature inside the reactor at different positions. The model produced outputs in the form of temperature, chemical conversion, relative humidity and molar flows of hydrogen and water vapor, which all matched the experimental data well at different NaBH 4 concentrations.
Additionally, the model included two sub-models, which allowed the evaluation of non-isothermal water evaporation processes and the pressure drop of the two-phase flow through the porous catalyst. The model was one-dimensional and therefore the geometrical parameters of the reactor were not evaluated in detail and only in the flow direction. The system was initially built for vehicle applications, but it could have potential for other small portable applications [ 84 ]. In , Sousa et al. Experimental data were collected from a batch reactor at different temperatures with a very dilute alkaline solution of NaBH 4 in order to avoid problems with solubility of the hydrolysis reaction product.
The data were fitted using three different kinetic models in order to evaluate the kinetics of the hydrolysis reaction. The catalyst used in the reactor was a Ru catalyst supported on Ni-foam. At low temperatures or high NaBH 4 concentrations the kinetics followed a zero-order model, while at high temperatures or low NaBH 4 concentrations, the reaction depended on the NaBH 4 concentration and the kinetics were best described by a first order model.
The experimental data were further used in a numerical 3-dimensional non-isothermal model for a pilot scale reactor. This model described the transport phenomena and also included the kinetic model for the hydrolysis reaction. Mechanical stirring in the batch reactor was shown to have an impact on the mass transfer, since NaBH 4 was more easily allowed to reach the catalytic sites.
Furthermore, the model showed that NaBH 4 in the bottom of the reactor was used less efficiently, because the produced hydrogen decreased the stirring in the bottom. The model assumed a single-phase flow during the hydrolysis reaction, which neglected the flow of hydrogen from the catalyst after production. The gas bubbles of hydrogen were responsible for the movement of the fluid and without the incorporated artificial mixing the performance predicted by the model would be extremely low and not compatible with the experimental data obtained with the reactor.
In conclusion, the 3-dimensional model was able to simulate the reactor processes and validate the kinetic model [ 85 ]. In order to evaluate a two-phase flow inside a batch reactor and better describe the impact of hydrogen formation, a new two-dimensional reactor model was recently described [ 86 ]. Data obtained from a previous described experimental setup was reused for the new model [ 85 ]. The catalyst was again a Ru catalyst supported on Ni-foam. The previous model only focused on the area, where the reaction took place and the hydrogen storage region was not considered, whereas in the new model this is taken into account [ 85 ].
The new model used a two-dimensional plane through three reaction tubes, which enabled the analysis of three regions namely the reactive solution, thermal fluid and the metal walls. Therefore, the model allowed for better description of the impact of hydrogen production bubbles from the hydrolysis reaction.
The computational methods indicated that even though a porous support Ni-foam was used for the catalyst, the hydrolysis reaction occurred mainly on the surface of the catalyst foam. Different positions of the catalyst foam as well as a plastic support for the Ni-foam inside the reactor were also studied in order to obtain information about the optimal position for reaching the quickest conversion. The optimal position was in the central region of the reactor chamber. Interestingly, the plastic support seemed to have a positive effect on the mixing during hydrogen production.
The best result was obtained for a plastic support with multiple small holes. The authors concluded that a two-phase flow model approach gave a better representation of the real system compared to a single-phase flow model [ 86 ].
Nanomaterials for Solid State Hydrogen Storage : Robert A. Varin :
As mentioned earlier the hydrolysis reaction can also be carried out using steam and solid state NaBH 4 with an added catalyst. In one report, the reactor for this type of reaction consisted of brass mesh to contain the solid NaBH 4 inside a cobber reactor. The steam was then passed into the reactor, where the hydrolysis reaction occurred [ 57 ].
A two-part dissolution-reaction model was proposed for a steam hydrolysis system [ 72 ]. The model accounted for deliquescence, solid dissolution and hydrolysis in order to release hydrogen and the model could give reasonable estimates for the kinetic constant and the mass transfer coefficients. However, the model could as such not be used for reactor design. The kinetic model described by Sousa et al. However, the actual model of the reactor was described with molar balance equations zero dimension model for the number of moles of NaBH 4 , H 2 O and NaBO 2.
This was possible because the only variable in the reactor was the hydrogen pressure. The Langmuir-Hinshelwood model was selected for describing the kinetics of the hydrogen generation rate over a range of temperatures and in time [ 85 ]. The results showed the possibility of using a NaBH 4 hydrolysis reaction as the hydrogen source for the PEM fuel cell system.
The magnetically supported reactor described by Pozio et al. This was used to evaluate the process of catalytic hydrolysis and in a later publication the complete hydrogen generator was simulated [ 75 , 76 ]. NaBH 4 has been given a no-go recommendation as a solid-state hydrogen storage material for automotive applications by the US DoE due to its high decomposition temperature.
However, since NaBH 4 releases hydrogen by hydrolysis at room temperature and the solution is both stable and non-toxic, NaBH 4 has a large potential for defense and civil applications. For application purposes, hydrogen is usually catalytically generated from alkaline NaBH 4 solutions. An advantage of using NaBH 4 is that no external heat supply is required due to the exothermic hydrolysis reaction. A disadvantage is that the NaBO 2 byproduct although environmentally friendly , needs to be collected in a separate tank in order to be recycled off board into NaBH 4.
For more information on the role of NaBH 4 in the field of hydrogen fueled applications, the reader is referred to more in-depth articles and reviews [ 7 , 88 , 89 , 90 , 91 ]. Small-unmanned aerial vehicles UAVs have gained much interest in the fields of defense and security in order to minimize human loses.
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They have been used for surveillance missions owing to the low noise and heat emissions, minimizing detection by enemies. Instead of internal combustion engines or secondary batteries, which have low thermal efficiencies and face short flight endurance 60—90 min , fuel cell systems are regarded as a more suitable alternative power source. Fuel cell systems show high thermal efficiencies, have an electrochemical reaction rather than a combustion reaction, and silent mode of operation [ 92 ].
As fuel cells operate on hydrogen, compressed hydrogen [ 93 ], liquefied hydrogen [ 94 , 95 , 96 , 97 ], and chemical hydrides [ 94 ] have all been used for UAV applications. For example, AeroVironment Inc.
However, the manufacturers claimed 24 h flight time with a full tank [ 94 ]. In the case of compressed hydrogen, Bradley et al. Since chemical hydrides have high energy densities, the combination with a proton exchange membrane PEM fuel cell stack can significantly improve the flight duration of UAVs compared to liquefied or compressed hydrogen [ 98 ]. In this section various small UAVs, which use NaBH 4 as their hydrogen source, are discussed as to their design, manufacture, and flight tests. The flight duration was increased to 9 h through further technological developments, demonstrating the improvement in endurance using chemical hydrides instead of compressed hydrogen [ 99 ].
The lithium ion battery provided the power for take-off and maneuvers whereas the fuel cell recharged the battery and provided a continuous power supply for the plane during cruise flight. Weighing only 5. Kim et al. The generated hydrogen was purified by a dehumidifier and used to power up the fuel cell stack and Li battery, whereas NaBO 2 was separated in a gas-liquid separator where it remained for collection [ 98 ]. Operating principle of fuel cell system equipped in the unmanned aerial vehicle UAV platform.
Reprinted with permission from reference [ 98 ].
Fuel Cells And Hydrogen Energy Series
To ensure a stable performance of the fuel cell system, cooling of the stacks becomes necessary. Air flowed through the intakes, cooled the stacks and then hot air left through the exits located near the propeller. Additionally, both ground and flight tests were performed, showing a flight endurance of 2. However, various problems were encountered during the flight tests, which occurred in the pump, reactor and filters [ 92 ]. The design was later improved by replacing the two tank system for a volume-exchange fuel tank where the fuel NaBH 4 solution and spent fuel NaBO 2 exchange volume within one tank hence minimizing the volumetric density Figure 6 [ ].
Schematic of a volume-exchange fuel tank. Reprinted with permission from reference [ ]. In order to obtain a high gravimetric energy density, the fuel cell stack and hydrogen generator should be ideally reduced to a minimum. Developing new materials to reduce the weight of the graphite bipolar plates incorporated in the fuel cell stacks would increase the flight endurance significantly [ 92 ]. Fuel cells as an alternative for lithium-ion batteries for laptops and mobile phone applications have attracted considerable research interest. This is mainly due to the low capacity of the current lithium-ion batteries, resulting in short operating times.
Fuel cell technologies are regarded as the most suitable for small portable applications. Although direct methanol fuel cells are typically used, issues regarding fuel crossover and low system volumetric densities remain to be resolved [ ]. The direct hydrogen fuel cell systems that use a metal hydride based hydrogen storage tank have gained much attention owing to the high power and volumetric energy densities that could be obtained [ , ].
Unfortunately, their size requirements were too large to surpass current Li-ion secondary batteries and therefore miniaturization of the fuel cells is important to achieve commercial implementation [ ]. NaBH 4 -based refuel devices for laptops and mobile phones have also been reported [ ]. For example, Prosini et al. With a 0. The refill case was made out of plastic and therefore the refill costs were based on the price of NaBH 4 1. As far as we are aware, no other publications on sodium borohydride based refuel devices for small cellular or laptop applications have been published.
Submarines are valuable assets for naval applications owing to their invisibility when submerged under water [ , ]. Unfortunately, most of the submarines are fitted with diesel-electric propulsion resulting in limited submerged time due to the limited capacity of the batteries. In addition, when charging the batteries only a few meters below the surface snorkeling the submarine is susceptible to detection.
Therefore, the development of air independent propulsion in order to prolong the underwater performance becomes crucial [ , ]. Among the various investigated systems, low temperature PEM fuel cells were shown to be the most promising. NaBH 4 allows a higher gravimetric hydrogen storage capacity compared to compressed hydrogen and does not have the safety and reliability constrains of compressed gases [ , ].
Liquid oxygen can be used as the oxidant, although compressed air, which is often used for other purposes on-board, is also considered [ ]. Apart for space applications, direct borohydride fuel cells DBFCs in combination with H 2 O 2 are also investigated as high-density power sources for autonomous underwater vehicles [ , , ]. In liquid form, H 2 O 2 is a thousand times denser than oxygen [ ]. Protonex is also currently developing an underwater fuel cell power system using the Millennium Cell Inc. A cheap and efficient route for recycling the spent fuel NaBO 2 dissolved in alkaline aqueous solution still has to be developed.
Therefore, the dissolved NaBO 2 has to be dried, which is an energy intense process. Here, the small particle size and high surface area increase the yield of the reaction [ 53 , , ]. Prevention of the generation of hydrogen and oxygen gas during regeneration is crucial as the regeneration of NaBH 4 and H 2 O 2 Equation 10 , which occurs at 2. The authors circumvented this issue by using high over-potential materials for the electrodes and exploring the formation of perborate that hydrolyze to peroxide at 1.
Furthermore, the inhibition of the NaBH 4 crossover, a critical issue often faced for DBFCs, can be reduced by improving the membrane electrode assembly or the liquid diffusion layer [ ]. In order to further develop NaBH 4 hydrolysis systems for portable applications, additional research is needed to reduce the cell volume, increase the gravimetric hydrogen storage capacity and reach efficient recycling of the spent fuel.
The development of a continuous hydrogen generator, using a continuous feeding of ammonia borane as both fuel and spent fuel, has been the focus of many researchers [ 28 , , , ]. Unfortunately, high costs and raw material scarcity only allow ammonia borane to be used only for small-scale applications. Seo et al. The system was operated by delivering spherical solid ammonia borane beads into a semi-batch type reactor filled with tetraethylene glycol dimethyl ether, a liquid promoter. Hydrogen purification equipment with a high filter capacity and efficient drainage system for spent fuel were also integrated for application purposes.
Based on the obtained results, the authors proposed an advanced reactor concept Figure 9. Schematic of the advanced fuel cell system powered by ammonia borane for prolonged operation. There are several limitations for metal hydride-based onboard storage systems that prevent their implementation: low gravimetric and volumetric capacities, insufficient kinetics within appropriate temperatures and pressure ranges, and high cost of the overall engineering system.
Modeling provides a powerful tool for the development of strategies and improvement of full-scale tanks. Several studies on tank modeling for thermolysis have been proposed, mainly based on sodium alanate with few examples of others complex hydrides. The main purpose of these simulations was the optimization of some operating parameters temperature, pressure, thermal conductivity, coolant flow rate and coolant temperature as well as the tank design length scale, geometry and fins content.
Optimizations were then proposed on the basis of the simulation results. Several modeling tools have been built on the basis of the hierarchical methodology and resistance analysis in order to estimate performances of the system and the limiting factors. The simulations showed that a good thermal management was necessary for the absorption, whereas the pressure control was important for desorption.
Still, differences exist between the simulations, which come from the choice of the kinetic model that is implemented as the governing equation for the simulations. Therefore, precautions have to be taken, since the kinetic model is built through experimental considerations. Heat transfer in the metal hydride tank can be improved using heat exchangers, multi-tubular tank geometries and heat transfer enhancers. However, this will also increase the weight of the system. Hence, optimizations are often a compromise between heat transfer and hydrogen content. Improving the ratio between the mass of the complex metal hydride bed to the mass of the tank wall, by screening lighter materials for the tank wall and developing hydrogen storage materials exhibiting both higher gravimetric and volumetric storage properties, should be a goal in order to obtain lightweight storage systems.
There are several design principles that must be taken into account while developing a complex metal hydride storage tank, such as packing arrangement of the material, hydrogen supply, heat transfer to the heat transfer medium, effective heat conductivity of the metal hydride bed and volume expansion. Additionally, efficient removal of the reaction heat from the metal hydride tank during refueling and how to provide heat during hydrogen delivery to the fuel cell remain an unsolved problem. Thermolysis of complex metal hydrides has yet to demonstrate the fulfillment of the DoE requirements Table 1.
Especially concerning the gravimetric capacity of approximately 1. In hydrolysis, recycling of the spent fuel remains the single biggest challenge. Compare all 9 new copies. Condition: New. Language: English. Brand new Book. The authors' own research results in the behavior of various hydrogen storage materials are also presented. Seller Inventory AAC More information about this seller Contact this seller.
Book Description Springer, Never used!. Seller Inventory Book Description Hardcover. Over the past decade, important advances have been made in the development of nanostructured materials for solid state hydrogen storage used to supply hydrogen to fuel cells in a clean, in. Shipping may be from multiple locations in the US or from the UK, depending on stock availability. Book Description Springer Verlag, Condition: Brand New. In Stock. Seller Inventory x Book Description Springer.