Research and Development at the U.S. Department of Energy on Hydrogen Production and Storage
U.S. Department of Energy
Investigating the potential of hydrogen as an energy carrier is just one of many strategies in the U.S. Department of Energy (DOE) research and development (R&D) portfolio. Programs are under way on hydrogen, biomass, solar, wind, geothermal, and nuclear energy, as well as on improved use of fossil fuels, carbon sequestration, and advanced hybrid-vehicle technologies. In this presentation, I focus on the DOE Hydrogen Program and two critical areas of investigation—hydrogen production and hydrogen storage. Following a brief overview of hydrogen production strategies, I describe in detail the status and challenges of hydrogen storage.
U.S. petroleum dependence is driven by transportation, which accounts for two-thirds of the 20 million barrels of oil our nation uses each day (DOE, 2005a,b,c). Today, the United States imports 55 percent of its oil, and this is expected to increase to 68 percent by 2025 under a status quo scenario (DOE, 2005a,b,c). The public has few other options for transportation fuel because nearly all vehicles currently run on gasoline or diesel. To promote national energy security, we must develop alternative energy carriers.
Molecular hydrogen, the simplest diatomic molecule known, with the highest gravimetric energy content of known fuels, has the potential to be an attractive alternative energy carrier. Hydrogen could not only be clean and efficient, but it could also be derived from a variety of domestic resources, such as biomass, hydro, wind, solar, geothermal, and nuclear energy sources, as well as coal (with sequestration) and natural gas (for limited applications) in the near term
(see Figure 1). Hydrogen could then be used in high-efficiency power-generation systems, including internal combustion engines or fuel cells for both vehicular transportation and distributed electricity generation.
In addition to promoting energy security, pure hydrogen has the potential to be environmentally advantageous because the only by-products of hydrogen-powered fuel cells are water and heat. Emissions of carbon dioxide and criteria pollutants (e.g., nitrogen oxides [NOx], sulfur oxides [SOx], and carbon monoxide [CO], etc.) would essentially be eliminated from the point of use. These emissions would be easier to control at the point of generation, rather than from the tailpipes of 200 million vehicles.
However, although molecular hydrogen is abundant in the universe, it is not plentiful on Earth, and it is not a primary fuel source. The question is how can we efficiently produce and safely deliver, store, and use hydrogen to reap the benefits of reduced emissions, higher energy efficiency, and improved energy security.
HYDROGEN FUEL INITIATIVE
The Hydrogen Fuel Initiative, announced by President Bush in 2003, commits $1.2 billon over five years for accelerated R&D and demonstration pro-
grams that will support an industry decision in 2015 on the commercial viability of hydrogen and fuel-cell technologies. If industry decides to proceed, a full transition to a hydrogen economy would clearly take decades and strong government-industry partnerships. However, it is essential that R&D to address the feasibility of hydrogen occur now.
A brief overview of hydrogen production and the key technical challenges under investigation in the DOE Hydrogen Program will be followed by a discussion of the critical technical challenges of safe and efficient hydrogen storage. In 2003, DOE announced a “grand challenge” to the global technical community on hydrogen storage that culminated in the formation of the National Hydrogen Storage Project (see Figure 2). For the first time, centers of excellence (DOE, 2005a) dedicated to hydrogen storage were formed with multiple university, industry, and national laboratory partners, leveraging expertise and capabilities from all sectors to tackle this difficult problem. In addition, 17 new projects on the basic science of hydrogen storage were selected in 2005 and funded at $20 million over three years through the DOE Office of Science. The discussion that
follows includes a description of the key issues in hydrogen storage, the National Hydrogen Storage Project, and progress in the search for the “holy grail” of hydrogen storage.
SUMMARY OF RESEARCH AND DEVELOPMENT ON HYDROGEN PRODUCTION
To meet the needs of a hydrogen economy, ensure energy security, and realize environmental benefits, hydrogen must be produced from diverse resources with minimal life-cycle cost and environmental impact and with maximum energy efficiency. In the near term, to avoid large capital investments in infrastructure, small-scale distributed hydrogen production is likely, including distributed natural gas reforming or electrolysis at fueling stations. The key challenge is to meet the DOE goal of $2 to $3 per gallon gasoline equivalent (gge) by 2015. While hydrogen can, and will be, produced from different pathways and diverse resources, the $2 to $3 per gge goal must be met independent of production pathway or energy source. Recent results show that the cost of $5 per gge for hydrogen produced from natural gas in a distributed system (delivered, untaxed), (Devlin, 2005), is approaching $3 per gge.
To reduce cost further and improve durability and energy efficiency, research is under way on catalysts, membranes for separation and purification, water-gas shift reactors, and hydrogen-compression technology. Research on reforming of biomass and renewable liquids is addressing the same issues. The long-term strategy is to produce hydrogen from renewable sources, nuclear energy, and coal with sequestration (via gasification, not coal-based electricity) to achieve carbon-neutral or zero-carbon technologies.
Water electrolysis, another area of investigation, is focused on the key issues of capital cost and electricity cost. Research is under way on materials to improve electrolyzer durability and energy efficiency, and, at the same time, to reduce cost. Another approach under investigation involves high-temperature thermochemical reactions that can use heat from nuclear power plants or high-temperature solar energy. Key issues are efficiency, cost, and durability. A few of the other areas being studied are more robust materials for high-temperature operation, lower cost solar-concentrator technology, and optimized thermochemical reactions. Finally, exploratory research for long-term approaches is also under way on photobiological or photoelectrochemical production of hydrogen. In all of these areas, hydrogen purity is a key issue (e.g., 99.99 percent purity is necessary for polymer-electrolyte-membrane [PEM] fuel cells).
THE “GRAND CHALLENGE” OF HYDROGEN STORAGE
On the basis of weight, hydrogen has nearly three times the energy content of gasoline (120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline). However,
on the basis of volume, the situation is reversed (8 MJ/liter for liquid hydrogen versus 32 MJ/liter for gasoline). Onboard vehicular hydrogen storage is a critical challenge to meeting customer expectations for a driving range of more than 300 miles within the weight, volume, safety, and cost constraints of a marketable vehicle. Through the FreedomCAR and Fuel Partnership, between DOE and leading automotive and energy industries, technical targets were set for commercially viable vehicular hydrogen storage systems in the United States. Some of these system-level targets for 2010 are: gravimetric capacity of 6 weight percent (= 2.0 kWh/kg), volumetric capacity of 1.5 kWh/L (= 0.045 kg hydrogen/L) and a cost of $4/kWh (DOE, 2005d). Figure 3 shows various material capacities and total-system capacities for a limited number of systems, illustrating that both fundamental properties and system-engineering issues must be addressed to meet the targets (Ordaz et al., 2005).
Current hydrogen storage technologies include: high-pressure tanks, cryogenic storage, metal hydrides, chemical hydrides, and high-surface-area sorbents, such as nanostructured carbon-based materials. High-pressure and cryogenic tanks, high-surface-area sorbents, and many metal hydrides can be categorized as “reversible” onboard hydrogen storage because “refueling” with hydrogen can take place directly on board the vehicle. For chemical hydrogen storage and some high-temperature metal hydrides, hydrogen regeneration is not possible on board the vehicle; thus with these systems, hydrogen must be regenerated off board (see Figure 4).
High-pressure and cryogenic tanks meet some of the near-term DOE targets and are already in use in prototype vehicles. The state of the art in high-pressure tanks is 10,000 psi (or about 700 atm), as developed by Quantum and others (Ko and Newell, 2004). Remaining challenges include volumetric capacity and issues related to high pressure and cost. Refueling times, compression energy penalties, and heat-management during compression must also be addressed because the mass and pressure of onboard hydrogen would have to be increased to provide a driving range of more than 300 miles.
Cryogenic, or liquid, hydrogen (LH2) tanks can, in principle, store more hydrogen in a given volume than compressed tanks, because the volumetric capacity of liquid hydrogen is about 0.07 kg/L (compared to roughly 0.04 kg/L even at 700 atm). Key issues related to LH2 tanks are hydrogen boil-off, the energy required for hydrogen liquefaction (typically 35 percent of the lower heating value of hydrogen), insulation requirements, and cost.
Metal hydrides function by dissociatively adsorbing (or absorbing) hydrogen into their metal lattices, thereby allowing for higher energy densities than liquid hydrogen. Figure 5 shows that the optimum “operating P-T window” for PEM fuel cell vehicular applications is in the range of 1–10 atm and 25–120oC (Wang et al., 2004). 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 and its cost is too high for vehicular hydrogen storage applications. However, at the present time, LaNi5H6 is one of the few commercially available metal hydrides.
Metal hydrides have been studied for decades, and Sandrock and Thomas (2001) have compiled a database of metal hydride properties to help guide the development of improved materials (see also SNL, 2005). Because most metals in traditional metal hydrides are heavy, the gravimetric capacity of such systems is unacceptable. However, in 1997, a breakthrough by Bogdanovic and Schwickardi (1997) demonstrated that titanium species could act as a catalyst in reversibly storing hydrogen in “complex” metal hydrides (e.g., NaAlH4). Such systems, with light elements (in this case Na and Al), can achieve much higher gravimetric capacities without compromising volumetric capacities. Although NaAlH4 cannot meet the DOE targets, this recent discovery has spurred activity around the world on complex metal hydrides.
New systems, such as Li2NH + H2 = LiNH2 + LiH, have recently been discovered (Chen et al., 2002; Luo, 2004; Nakamori and Orimo, 2004). The substitution by light metals, such as Mg, is an active area of research to adjust operating temperatures and pressures and improve kinetics. Another promising discovery in 2005 by Vajo and coworkers is that metal hydrides such as LiBH4 can be “destabilized” to achieve high capacity at lower pressures and tempera-
tures than previously known. Vajo’s LiBH4/MgH2 material showed a hydrogen storage capacity of 10 weight percent, which is prompting attention worldwide on this approach (Vajo et al., 2005).
One of the main issues related to metal hydrides is that the heat of reaction is typically 30–40 kJ/mol. This means that, to meet refueling time targets (~3 minutes for 5 kg H2), close to 0.5 MW must be rejected during the charging of typical metal hydride systems. Thus, certain metal hydrides, such as AlH3, appear to be more suitable for off-board regeneration. However, significant engineering challenges lie ahead, such as thermal management and reactor design optimization to meet weight, volume, and cost targets. Other issues related to metal hydrides include low gravimetric capacity and slow uptake and release kinetics.
Chemical hydrogen storage refers to chemical reactions, such as the hydrolysis of sodium borohydride (NaBH4 + 2H2O → NaBO2 + 4H2), which has been demonstrated by Millennium Cell and others (Amendola et al., 2000; Wu et al., 2004), or the dehydrogenation of organic compounds, such as methylcyclohexane or decalin, which have been studied for decades, particularly in Japan. In 2004, a new type of liquid-phase hydrogen storage material was demonstrated. This liquid, based on N-ethylcarbazole, can attain more than 5.5 weight percent and 0.05 kg/L of hydrogen storage; several dehydrogenation/ hydrogenation cycles of N-ethylcarbazole have recently been shown (Figure 6)
(Cooper et al., 2004). Another exciting recent discovery is that, by combining ammonia borane (NH3BH3) in a mesoporous scaffold structure, the hydrogen-release reaction can be tailored in terms of by-product release and temperature; roughly 6 weight percent hydrogen storage has been demonstrated (Gutowska et al., 2005).
The most significant issue related to chemical hydrogen storage to be addressed is that the covalent bonds broken to release hydrogen cannot be easily replaced on board a vehicle. The “spent” fuel must be reclaimed from the car and regenerated off board at a central plant or at the fueling station. The energy requirements and total life-cycle analysis, including environmental impact, are under study.
Finally, high-surface-area sorbents, such as nanostructured carbon materials or metal organic frameworks (MOFs), and perhaps clathrates, can be used to adsorb hydrogen physically (Dillon et al., 2004; NREL, 2005). There is still some controversy as to how much hydrogen some of these materials can store. Although they have potentially high gravimetric capacity, because they have high surface areas (e.g., ~4,000 m2/g), their volumetric capacity for hydrogen storage will probably be low. However, one advantage of these materials is that the binding of hydrogen is weak so the release of hydrogen should not require high temperatures. Therefore, power consumption during hydrogen release, as well as heat rejection during refueling, would not be as challenging as for metal hydrides. Engineering issues, such as reactor design, as well as tuning fundamental material properties must also be addressed.
With improved theoretical modeling, combinatorial/high-throughput screening techniques and understanding at the nanoscale, future work worldwide will focus on “tailoring” materials to meet the targets for hydrogen storage. In addition to activities in the United States, DOE is supporting the mission of the International Partnership for the Hydrogen Economy (IPHE), formed in November 2003, to help accelerate global activities to achieve a hydrogen economy. Working with other countries, DOE has helped organize IPHE conferences on hydrogen storage, hydrogen production, and other research areas to identify, evaluate, and coordinate multinational R&D and demonstration programs to accelerate the advancement of hydrogen and fuel cell technologies (IPHE, 2005).
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