Storing enough hydrogen onboard a vehicle to achieve a driving range of greater than 300 miles is a significant challenge. On a weight basis, hydrogen has nearly three times the energy content of gasoline (120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline). However, on a volume basis the situation is reversed (8 MJ/liter for liquid hydrogen versus 32 MJ/liter for gasoline). On-board hydrogen storage in the range of 5-13 kg H2 is required to encompass the full platform of light-duty vehicles.
Modes of Storage
Today's state-of-the-art for hydrogen storage includes 5000- and 10,000-psi compressed gas tanks and cryogenic liquid hydrogen tanks for on-board hydrogen storage.
Compressed Hydrogen Gas Tanks
The energy density of gaseous hydrogen can be improved by storing hydrogen at higher pressures. This requires material and design improvements in order to ensure tank integrity. Advances in compression technologies are also required to improve efficiencies and reduce the cost of producing high-pressure hydrogen.
Carbon fiber-reinforced 5000-psi and 10,000-psi compressed hydrogen gas tanks are under development by Quantum Technologies and others. Such tanks are already in use in prototype hydrogen-powered vehicles. The inner liner of the tank is a high molecular weight polymer that serves as a hydrogen gas permeation barrier. A carbon fiber-epoxy resin composite shell is placed over the liner and constitutes the gas pressure load-bearing component of the tank. Finally, an outer shell is placed on the tank for impact and damage resistance. The pressure regulator for the 10,000-psi tank is located in the interior of the tank. There is also an in-tank gas temperature sensor to monitor the tank temperature during the gas-filling process when heating of the tank occurs.
The driving range of fuel cell vehicles with compressed hydrogen tanks depends, of course, on vehicle type, design and the amount and pressure of stored hydrogen. By increasing the amount and pressure of hydrogen, a greater driving range can be achieved but at the expense of cost and valuable space within the vehicle. Volumetric capacity, high pressure and cost are thus key challenges for compressed hydrogen tanks. Refueling times, compression energy penalties and heat management requirements during compression also need to be considered as the mass and pressure of on-board hydrogen are increased.
Issues with compressed hydrogen gas tanks revolve around high pressure, weight, volume, conformability and cost. The cost of high-pressure compressed gas tanks is essentially dictated by the cost of the carbon fiber that must be used for light-weight structural reinforcement. Efforts are underway to identify lower-cost carbon fiber that can meet the required high pressure and safety specifications for hydrogen gas tanks. However, lower-cost carbon fibers must still be capable of meeting tank thickness constraints in order to help meet volumetric capacity targets. Thus lowering cost without compromising weight and volume is a key challenge.
Two approaches are being pursued to increase the gravimetric and volumetric storage capacities of compressed gas tanks from their current levels. The first approach involves cryo-compressed tanks. This is based on the fact that, at fixed pressure and volume, gas tank volumetric capacity increases as the tank temperature decreases. Thus, by cooling a tank from room temperature to liquid nitrogen temperature (77°K), its volumetric capacity will increase by a factor of four, although system volumetric capacity will be less than this due to the increased volume required for the cooling system.
The second approach involves the development of conformable tanks. Present liquid gasoline tanks in vehicles are highly conformable in order to take maximum advantage of available vehicle space. Concepts for conformable tank structures are based on the location of structural supporting walls. Internal cellular-type load bearing structures may also be a possibility for greater degrees of conformability.
Compressed hydrogen tanks [5000 psi (~35 MPa) and 10,000 psi (~70 MPa)] have been certified worldwide according to ISO 11439 (Europe), NGV-2 (U.S.), and Reijikijun Betten (Iceland) standards and approved by TUV (Germany) and The High-Pressure Gas Safety Institute of Japan (KHK). Tanks have been demonstrated in several prototype fuel cell vehicles and are commercially available. Composite, 10,000-psi tanks have demonstrated a 2.35 safety factor (23,500 psi burst pressure) as required by the European Integrated Hydrogen Project specifications.
Liquid Hydrogen Tanks
The energy density of hydrogen can be improved by storing hydrogen in a liquid state. However, the issues with liquid hydrogen (LH2) tanks are hydrogen boil-off, the energy required for hydrogen liquefaction, volume, weight, and tank cost. The energy requirement for hydrogen liquefaction is high; typically 30% of the heating value of hydrogen is required for liquefaction. New approaches that can lower these energy requirements and thus the cost of liquefaction are needed. Hydrogen boil-off must be minimized or eliminated for cost, efficiency and vehicle range considerations, as well as for safety considerations when vehicles are parked in confined spaces. Insulation is required for LH2 tanks and this reduces system gravimetric and volumetric capacity.
Liquid hydrogen (LH2) tanks can store more hydrogen in a given volume than compressed gas tanks. The volumetric capacity of liquid hydrogen is 0.070 kg/L, compared to 0.030 kg/L for 10,000 psi gas tanks.
Liquid tanks are being demonstrated in hydrogen-powered vehicles and a hybrid tank concept combining both high-pressure gaseous and cryogenic storage is being studied. These hybrid (cryo-compressed tanks) insulated pressure vessels are lighter than hydrides and more compact than ambient-temperature, high pressure vessels. Because the temperatures required are not as low as for liquid hydrogen, there is less of an energy penalty for liquefaction and less evaporative losses than for liquid hydrogen tanks.
Materials-based Hydrogen Storage
There are presently three generic mechanisms known for storing hydrogen in materials: absorption, adsorption, and chemical reaction.
Absorption. In absorptive hydrogen storage, hydrogen is absorbed directly into the bulk of the material. In simple crystalline metal hydrides, this absorption occurs by the incorporation of atomic hydrogen into interstitial sites in the crystallographic lattice structure.
Adsorption. Adsorption may be subdivided into physisorption and chemisorption, based on the energetics of the adsorption mechanism. Physisorbed hydrogen is more weakly energetically bound to the material than is chemisorbed hydrogen. Sorptive processes typically require highly porous materials to maximize the surface area available for hydrogen sorption to occur, and to allow for easy uptake and release of hydrogen from the material.
Chemical reaction. The chemical reaction route for hydrogen storage involves displacive chemical reactions for both hydrogen generation and hydrogen storage. For reactions that may be reversible on-board a vehicle, hydrogen generation and hydrogen storage take place by a simple reversal of the chemical reaction as a result of modest changes in the temperature and pressure. Sodium alanate-based complex metal hydrides are an example. In many cases, the hydrogen generation reaction is not reversible under modest temperature/pressure changes. Therefore, although hydrogen can be generated on-board the vehicle, getting hydrogen back into the starting material must be done off-board. Sodium borohydride is an example.
Metal hydrides have the potential for reversible on-board hydrogen storage and release at low temperatures and pressures. The optimum "operating P-T window" for polymer electrolyte membrane (PEM) fuel cell vehicular applications is in the range of 1-10 atm and 25-120ºC. This is based on using the waste heat from the fuel cell to "release" the hydrogen from the media. In the near-term, waste heat less than 80ºC is available but as high temperature membranes are developed, there is potential for waste heat at higher temperatures. A simple metal hydride such as LaNi5H6, that incorporates hydrogen into its crystal structure, can function in this range, but its gravimetric capacity is too low (~ 1.3 wt. %) and its cost is too high for vehicular applications.
Complex metal hydrides such as alanate (AlH4) materials have the potential for higher gravimetric hydrogen capacities in the operational window than simple metal hydrides. Alanates can store and release hydrogen reversibly when catalyzed with titanium dopants, according to the following 2-step displacive reaction for sodium alanate:
First formulas for the two-step conversion of sodium alanate to hydrogen. First reaction: NaAlH4 = 1/3 Na3AlH6 + 2/3Al + H2
Second formulas for the two-step conversion of sodium alanate to hydrogen. Second reaction: Na3AlH6 = 3NaH + Al + 3/2H2
At 1 atm pressure, the first reaction becomes thermodynamically favorable at temperatures above 33ºC and can release 3.7 wt.% hydrogen, while the second reaction takes place above 110ºC and can release 1.8 wt.% hydrogen. The amount of hydrogen that a material can release, rather than only the amount the material can hold, is the key parameter used to determine system (net) gravimetric and volumetric capacities.
Issues with complex metal hydrides include low hydrogen capacity, slow uptake and release kinetics, and cost. The maximum material (not system) gravimetric capacity of 5.5 wt.% hydrogen for sodium alanate is below the 2010 U.S. Department of Energy (DOE) system target of 6 wt.%. Thus far, 4 wt.% reversible hydrogen content has been experimentally demonstrated with alanate materials. Also, hydrogen release kinetics are too slow for vehicular applications. Furthermore, the packing density of these powders is low (for example roughly 50%) and the system-level volumetric capacity is a challenge. Although sodium alanates will not meet the 2010 targets, it is envisioned that their continued study will lead to fundamental understanding that can be applied to the design and development of improved types of complex metal hydrides.
Recently, a new complex hydride system based on lithium amide has been developed. For this system, the following reversible displacive reaction takes place at 285ºC and 1 atm:
Formula for converting lithium amid to lithium hydride. Reaction: Li2NH + H2 = LiNH2 + LiH
In this reaction, 6.5 wt.% hydrogen can be reversibly stored, with potential for 10 wt.%. However, the current operating temperature is outside of the vehicular operating window. However, the temperature of this reaction can be lowered to 220ºC with magnesium substitution, although at higher pressures. Further research on this system may lead to additional improvements in operating conditions with improved capacity.
One of the major issues with complex metal hydride materials, due to the reaction enthalpies involved, is thermal management during refueling. Depending on the amount of hydrogen stored and refueling times required, megawatts to half a gigawatt must be handled during recharging on-board vehicular systems with metal hydrides. Reversibility of these and new materials also needs to be demonstrated for over a thousand cycles.
Chemical Hydrogen Storage
The term 'chemical hydrogen storage' is used to describe storage technologies in which hydrogen is generated through a chemical reaction. Common reactions involve chemical hydrides with water or alcohols. Typically, these reactions are not easily reversible on-board a vehicle. Hence, the 'spent fuel' and/or byproducts must be removed from the vehicle and regenerated off-board.
Hydrolysis reactions involve the oxidation reaction of chemical hydrides with water to produce hydrogen. The reaction of sodium borohydride has been the most studied to date. This reaction is:
In the first embodiment, a slurry of an inert stabilizing liquid protects the hydride from contact with moisture and makes the hydride pumpable. At the point of use, the slurry is mixed with water and the consequent reaction produces high purity hydrogen.
Formula for converting sodium borohydride to hydrogen. NaBH4 + 2H2O = NaBO2 + 4H2
The reaction can be controlled in an aqueous medium via pH and the use of catalyst. While the material hydrogen capacity can be high and the hydrogen release kinetics fast, the borohydride regeneration reaction must take place off-board. Regeneration energy requirements, cost and life-cycle impacts are key issues currently being investigated.
Millennium Cell has reported that their NaBH4-based Hydrogen on Demand™ system possesses a system gravimetric capacity of about 4 wt.%. Similar to other material approaches issues include system volume, weight and complexity and water availability.
Another hydrolysis reaction that is presently being investigated by Safe Hydrogen, is the reaction of MgH2 with water to form Mg(OH)2 and H2. In this case, particles of MgH2 are contained in a non-aqueous slurry to inhibit premature water reactions when hydrogen generation is not required. Material-based capacities for the MgH2 slurry reaction with water can be as high as 11 wt.%. However, similar to the sodium borohydride approach, water must also be carried on-board the vehicle in addition to the slurry and the Mg(OH)2 must be regenerated off-board.
Hydrogenation and dehydrogenation reactions have been studied for many years as a means of hydrogen storage. For example, the decalin-to-naphthalene reaction can release 7.3 wt.% hydrogen at 210ºC via the reaction:
Formula for converting decalin to naphthlene via dehydrogenation. C10H18 = C10H8 + 5H2
A platinum-based or noble metal-supported catalyst is required to enhance the kinetics of hydrogen evolution.
Recently, a new type of liquid phase material has been developed. This material, developed by Air Products and Chemicals, Inc., has shown 5-7 wt.% gravimetric hydrogen storage capacity and a greater than 0.050 kg/L hydrogen volumetric capacity. Future research is directed at lowering dehydrogenation temperatures. The advantages of such a system are that, unlike other chemical hydrogen storage concepts, the dehydrogenation does not require water. Since the reaction is endothermic the system would use waste heat from the fuel cell or internal combustion engine to produce hydrogen on-board. Furthermore, liquids lend themselves to facile transport and refueling. There are also no heat removal requirements during refueling since regeneration would take place off-board the vehicle. Thus, the replenished liquid must be transported from the hydrogenation plant to the vehicle filling station. Off-board regeneration efficiency and cost are important factors.
New Chemical Approaches
New chemical approaches are needed to help achieve the 2010 and 2015 hydrogen storage targets. The concept of reacting lightweight metal hydrides such as LiH, NaH, and MgH2 with methanol and ethanol (alcoholysis) has been put forward. Alcoholysis reactions are said to lead to controlled and convenient hydrogen production at room temperature and below. However, as is the case with hydrolysis reactions, alcoholysis reaction products must be recycled off-board the vehicle. The alcohol must also be carried on-board the vehicle and this impacts system-level weight, volume, and complexity.
Another new chemical approach may be hydrogen generation from ammonia-borane materials by the following reactions:
Example of hydrogen generation from ammonia-borane material. Two step formula: NH3PH3 equals = NH2BH2 + H2 = NHBH + H2
The first reaction, which occurs at less than 120ºC releases 6.1 wt.% hydrogen, while the second reaction, which occurs at approximately 160ºC, releases 6.5 wt.% hydrogen. Recent studies indicate that hydrogen release kinetics and selectivity are improved by incorporating ammonia-borane nanosized particles in a mesoporous scaffold.
Single-walled carbon nanotubes are being studied as hydrogen storage materials because of published hydrogen gravimetric capacities in the range of 3-10 wt.% at room temperature. However, there has been controversy due to difficulty in reproducing these results. Hence, the current research and development focus for carbon nanotubes has been on establishing reproducibility. Recent results at the National Renewable Energy Laboratory (NREL) show that while no hydrogen storage was observed in pure single-walled carbon nanotubes, roughly 3 wt.% was measured in metal-doped nanotubes at room temperature, as is shown in the Graph.
The room temperature gravimetric capacity measured in carbon nanotubes is below the 2010 system target of 6.0 wt.% and further improvements must be made. In addition, low-cost, high-volume manufacture processes must be developed for single-walled carbon nanotubes in order for them to be economically viable in vehicular applications. The U.S. Department of Energy (DOE) Hydrogen Program has a go/no-go decision point planned on carbon nanotubes at the end of fiscal year 2006 based on a reproducibly demonstrated material hydrogen storage gravimetric capacity of 6 wt.% at room temperature.
High Surface Area Sorbents and New Materials and Concepts
There is a pressing need for the discovery and development of new reversible materials. One new area that may be promising is that of high-surface area hydrogen sorbents based on microporous metal-organic frameworks (MOFs). Such materials are synthetic, crystalline, and microporous and are composed of metal/oxide groups linked together by organic struts. Hydrogen storage capacity at 78°K (-195ºC) has been reported as high as 4 wt.% via an adsorptive mechanism, with a room temperature capacity of approximately 1 wt.%. However, due to the highly porous nature of these materials, volumetric capacity may still be a significant issue.
Another class of materials for hydrogen storage may be clathrates, which are primarily hydrogen-bonded H2O frameworks. Initial studies have indicated that significant amounts of hydrogen molecules can be incorporated into the sII clathrate. Such materials may be particularly viable for off-board storage of hydrogen without the need for high-pressure or liquid hydrogen tanks.
Other examples of new materials and concepts are conducting polymers. New processes such as sonochemistry may also be applicable to help create unique, nano-structures with enhanced properties for hydrogen storage.
Hydrogen Storage Challenges
For transportation, the overarching technical challenge for hydrogen storage is how to store the amount of hydrogen required for a conventional driving range (>300 miles), within the vehicular constraints of weight, volume, efficiency, safety, and cost. Durability over the performance lifetime of these systems must also be verified and validated and acceptable refueling times must be achieved. Requirements for off-board bulk storage are generally less restrictive than on-board requirements; for example, there may be no or less restrictive weight requirements, but there may be volume or "footprint" requirements. The key challenges include:
Weight and Volume. The weight and volume of hydrogen storage systems are presently too high, resulting in inadequate vehicle range compared to conventional petroleum-fueled vehicles. Materials and components are needed that allow compact, lightweight hydrogen storage systems while enabling greater than 300-mile range in all light-duty vehicle platforms.
Efficiency. Energy efficiency is a challenge for all hydrogen storage approaches. The energy required to get hydrogen in and out is an issue for reversible solid-state materials. Life-cycle energy efficiency is a challenge for chemical hydride storage in which the by-product is regenerated off-board. In addition, the energy associated with compression and liquefaction must be considered for compressed and liquid hydrogen technologies.
Durability. Durability of hydrogen storage systems is inadequate. Materials and components are needed that allow hydrogen storage systems with a lifetime of 1500 cycles.
Refueling Time. Refueling times are too long. There is a need to develop hydrogen storage systems with refueling times of less than three minutes, over the lifetime of the system.
Cost. The cost of on-board hydrogen storage systems is too high, particularly in comparison with conventional storage systems for petroleum fuels. Low-cost materials and components for hydrogen storage systems are needed, as well as low-cost, high-volume manufacturing methods.
Codes & Standards. Applicable codes and standards for hydrogen storage systems and interface technologies, which will facilitate implementation/commercialization and assure safety and public acceptance, have not been established. Standardized hardware and operating procedures, and applicable codes and standards, are required.
Life-Cycle and Efficiency Analyses. Lack of analyses of the full life-cycle cost and efficiency for hydrogen storage systems.
Hydrogen storage, The National Renewable Energy Laboratory (NREL).
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