Advancement in the storage of hydrogen energy

INTRODUCTION

The development of a sustainable energy economy, based on renewable,

carbon neutral energy is a necessary and urgent task. Hydrogen (H2) is a leading

candidate for the storage and transportation of energy provided it can be

efficiently produced from renewable energy sources and effectively stored in a

safe and concentrated manner. Reaching these goals for hydrogen requires

breakthroughs in both aspects of production and storage, and for both small to

medium personalized energy and large scale distributed applications. Thus,

any realistic and practical solution to develop a sustainable energy economy

must be diversemanaging energy conversion, storage and transport on all

scales.

The pace of fossil fuel consumption, particularly oil and gas, has increased in

recent decades, which poses several challenges to our energy systems, including

dwindling finite resources and the production of environmentally harmful by-

products, especially carbon dioxide emissions . These challenges give rise to

strong demands for more efficient energy sources. To address the problems of

global warming, pollution, and fossil fuel resource depletion, a next-generation

energy source must overcome the stringent criteria of renewability,

environment-friendly by-products, and abundance. The use of hydrogen as an

energy carrier has been widely discussed because it stores the highest energy

density by weight, it can be produced from a variety of sources (e.g., water,

biomass, and organic materials) , and its by-product is pure water. The

realization of a hydrogen economy in the near future is considered to be a

possible solution to the looming energy crisis. Hydrogen has a very high

gravimetric energy density of 120 kJ g1, about three 1mes the energy density

of gasoline and almost seven times the energy density of conventional fossil

fuels . The main concern associated with realizing a hydrogen economy is the

very low volumetric energy density of hydrogen: moving a vehicle requires a far

larger fuel tank than typical gasoline-fueled vehicles. Hydrogen storage and

transport is a major bottleneck in the development of hydrogen-based

technologies, e.g., fuel cells . The development of viable technologies and

materials for the effective, safe, and stable storage of hydrogen constitutes a

crucial step toward realizing a hydrogen economy.

Four Approaches to Storage of Hydrogen Energy

(i) high-pressure tanks:

High-pressure tanks require pressures of 350700 bar for hydrogen compression, however, even at such high pressures, the energy density is below that of conventional energy sources. The storage tanks must be prepared from strong lightweight materials that can withstand pressurized conditions, thereby increasing the application costs.

(ii) cryogenic liquefaction of molecular hydrogen,

The cryogenic liquefaction of hydrogen has been considered as a viable on-board storage option for the automotive industry; however, hydrogen liquefaction occurs only at very low temperatures (20 K). Ambient heat flow into the storage tank readily evaporates hydrogen, thereby increasing the internal pressure of the tank and causing fuel loss (boil-off) and safety challenges.

(iii) chemical solid storage materials, and

The chemical storage of solid materials is safer than compression or liquefaction and is based on dissociative hydrogen adsorption to the solid metals (usually lithium, magnesium, or aluminum). This method provides a high gravimetric hydrogen storage capacity at ambient temperatures and pressures. Unfortunately, desorption of hydrogen from these materials suffers from a large endothermic energy barrier (MgH2: 75.3 kJ mol1), thus requiring unacceptably high temperatures to release the hydrogen, which reduces the energy efficiency (poor release kinetics and storage material recycling).

(iv) physically adsorbing porous materials

Physically absorbing porous materials provide another option for safe hydrogen storage without thermodynamic energy inefficiencies. One challenge is that the adsorption processes rely on van der Waals interactions, which are intrinsically low-energy. Nonetheless, most current efforts toward the development of hydrogen storage materials have focused on the design of nanomaterials that reversibly store hydrogen molecules at temperatures approaching ambient conditions .

Established Technologies On Hydrogen Storage

1. Compressed hydrogen:

Compressed hydrogen is the gaseous state of the element hydrogen which is kept under

pressure. Compressed hydrogen in hydrogen tanks at 350 bar (5,000 psi) and 700 bar (10,000 psi) is

used for hydrogen in vehicles. Car manufacturers have been developing this solution, such as

. Honda[7] or Nissan

2. Liquid hydrogen:

BMW has been working on liquid tank for cars, producing for example the BMW Hydrogen 7.

Proposals and Research & Advancement on Hydrogen Storage 1. Chemical 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.

1. Metal hydrides: Metal hydrides, such as MgH2, NaAlH4, LiAlH4, LiH, LaNi5H6, TiFeH2 and palladium hydride, with varying degrees of efficiency, can be used as a storage medium for hydrogen, often reversibly.[9] Some are easy-to-fuel liquids at ambient temperature and pressure, others are solids which could be turned into pellets.

2. Carbohydrates: Carbohydrates (polymeric C6H10O5) releases H2 in a bioreformer mediated by the enzyme cocktailcell-free synthetic pathway biotransformation. Carbohydrate provides high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints: It can also be stored as a solid powder. Carbohydrate is the most abundant renewable bioresource in the world.

3. Synthesized hydrocarbons: An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. However, these reformers are slow to react to changes in demand and add a large incremental cost to the vehicle powertrain.

4. Liquid organic hydrogen carriers (LOHC): Unsaturated organic compounds can store huge amounts of hydrogen. These Liquid Organic Hydrogen Carriers (LOHC) are hydrogenated for storage and dehydrogenated again when the energy/hydrogen is needed. Heterocyclic aromatic compounds are most appropriate for this task. A compound that stands in the focus of the current LOHC research is N-ethylcarbazole[21] but many others do exist,[22] e.g. dibenzyltoluene[23] which is already industrially used as a heat transfer fluid.

5. Phosphonium borate: In 2006 researchers of University of Windsor reported on reversible hydrogen storage in a non-metal phosphonium borate frustrated Lewis pair

Development in collaboration with the Energy Center

Goals for hydrogen requires breakthroughs in both aspects of production and storage, and

for both small to medium personalized energy and large scale distributed applications.

Thus, any realistic and practical solution to develop a sustainable energy economy must be

diversemanaging energy conversion, storage and transport on all scales.

1. Hytech:

The HyTech project is focused on the realization of breakthroughs and advancing innovative technologies in the field of solar hydrogen production and hydrogen storage

2. PECHouse2:

The project aims at developing both materials and technologies which will lead to the production of hydrogen using solar energy, with ambitious goals in terms of process efficiency, physico-chemical stability of materials, as well as of H2 production costs.

3. NanoPEC:

This project investigates solar-driven hydrogen production via photoelectrochemical water splitting, using new concepts and methods, afforded by nanotechnology, to design innovative composite nanostructures in which each component performs specialized functions.

This DOE Hydrogen and Fuel Cells Program:

This DOE Hydrogen and Fuel Cells Program activity is focused on advanced storage of

hydrogen (or its precursors) on vehicles or within the distribution system.

Field of work:

1. High-Pressure and Cryogenic Tanks: The Office of Energy Efficiency and Renewable Energy is developing and evaluating advanced concepts to store

hydrogen at high pressures and cryogenic temperatures that improve volumetric capacity, conformability, and cost of storage. 2. Advanced Solid State and Liquid Materials: The Offices of Energy Efficiency and Renewable Energy and Fossil Energy are working to develop innovative materials for reversible hydrogen storage including high surface area adsorbents, metal organic frameworks, and metal hydrides, as well as approaches that are regenerable off-board such as chemical hydrides and liquid carriers. 3. Basic Research: In the Office of Science's basic research program, the main focus will be on basic research needs in developing novel storage materials and methods. The broad class of storage materials to be studied includes various forms of complex hydrides and nanostructured materials.

Carbon Materials For Hydrogen Storage:

The scope of the discussion in the present review is limited to recent advances in hydrogen storage using

porous carbonaceous adsorbents that have received significant attention from the hydrogen storage

research community. The prominent carbonaceous adsorbents are categorized as follows: (1) carbon

nanomaterials, carbon nanotubes and fullerene, (2) zeolite-templated carbons (ZTCs), and (3) carbide-

derived carbons (CDCs), and the preparation methods, pore characteristics, and hydrogen storage

behaviors are described. In the following section we introduce the recently developed MOF-derived

carbons (MDCs) that exhibit promising H2 storage capacities better than those of previously reported

porous adsorbents. We concentrate on the preparation and pore characteristics of MDCs to provide

guidelines for the design of suitable porous carbon materials for hydrogen storage.

Methods:

1. Carbon nanomaterials:

Numerous reports have described the uses of various carbon nanomaterials, including single- and multi-walled CNTs, nanohorns, graphite nanofibers, graphene, and fullerenes, as efficient hydrogen storage media. Hydrogen storage in carbon nanomaterials, particularly CNTs, has since been further optimized using theoretical and experimental approaches by various research groups. Since the beginning of 2000, however, reports critical of the hydrogen storage capacities of carbon nanomaterials began to emerge.

2. Activated carbons (ACs):

Activated carbons (ACs) are highly porous, amenable to large-scale preparation, and very stable to chemicals and heat. For these reasons, ACs are considered to be the most commercializable targets for hydrogen storage applications. ACs can be synthesized from a variety of organic precursors, including agricultural wastes, such as coconut shells and fibers, jute fibers, nut shells, soybeans, and oil seeds

3. Carbide-derived carbons (CDCs):

The selective etching of metal atoms from metal carbides leads to the formation of porous carbon with a density lower than that of graphite, called carbide-derived carbon (CDC) [33]. CDCs were first thought to be an undesirable by-product; however, Gogotsi et al. identified a useful application of CDCs as molecular sieves, gas storage adsorbents, catalysts, and supercapacitors [33], [34] and [35]. This group developed mass production methods for CDCs. CDCs were typically produced by the chlorination of metal carbide according to the reaction: MeC+x/2Cl2 (g)=MeClx+C, where Me indicates an extracted metal atom.

4. MOF-derived carbons (MDCs):

A new type of carbon, MDCs, was first proposed by Xu et al., who used MOF-5 as a template and furfuryl alcohol as the carbon precursor. The impregnated furfuryl alcohol was polymerized and carbonized inside the micropores of an MOF. After carbonization, the template was eliminated using HCl treatment to produce a unique nanoporous carbon material. The procedure was similar to the preparation of ZTCs, with the exception that a MOF was used as the template.

Conclusion:

Recent progress in the synthetic methods, pore characteristics, and hydrogen storage

capacities of various porous carbon materials was briefly reviewed. None of the porous

carbon materials considered above (or any other porous adsorbent for that matter) has yet

satisfied the criteria set by the DOE for hydrogen storage applications; however, the slow

pace of technological advancement in the design and manufacture of highly pressurized

cryogenic tanks may render porous carbon material adsorbents highly practical.

By selecting an appropriate pore system for storing hydrogen molecules under given

conditions, a variety of carbon materials may potentially be useful for meeting the DOE

requirements. The new DOE targets released in 2009 recommended ultimate operating

pressures of 0.41.2 MPa and a H2 delivery temperature of between 40 C and 100 C.

Very high ultramicroporous and supermicropore volumes will be required to accomplish

these target storage capacities. From this perspective, the pore characteristics of MDCs are

highly desirable. MDCs are still in the early stages of development. By analogy to the

development of MOFs, wherein several thousands of MOFs have been reported to date

thus, MDC performance optimization is expected to yield fruitful results.

Great deal of progress has been achieved over the past decade in the field of solid porous

adsorbents for hydrogen storage applications. Although it is possible that adsorptive

storage on high-surface area adsorbents has already reached the limits determined by

physical constraints, and, therefore, additional progress in this field is unlikely, recent

experimental and theoretical studies suggest that the pore size distributions may be

optimized. Small pore volumes, rather than high surface areas, decisively affect the

hydrogen storage capacity near room temperature. Current research directed toward the

development of ultrahigh-surface area materials must, therefore, be reconsidered.

Concurrent considerations associated with chemical approaches, such as spillover and alkali

metal doping, provide exciting opportunities for realizing viable H2 storage carbon

materials.