Materials for Hydrogen Storage: Past, Present, and Future - The

PERSPECTIVE pubs.acs.org/JPCL

Materials for Hydrogen Storage: Past, Present, and Future Puru Jena* Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284-2000, United States

ABSTRACT With a growing world population, an increasing standard of living in many developing countries, a limited supply of fossil fuels, and its adverse effect on the environment, the need for clean and sustainable energy has never been greater. Hydrogen, the simplest and most abundant element in the universe, has the potential to meet this energy need if numerous hurdles in its efficient and safe production, storage, and use in fuel cell vehicles can be overcome. This Perspective briefly discusses the status of hydrogen storage ; past, present, and future.

enjamin Franklin once wrote “...in this world nothing can be said to be certain but death and taxes.” Well, there is at least one more thing that can be said to be certain in this world, the limited supply of fossil fuels such as coal, oil, and natural gas on which the world depends for much of its energy needs. With a growing world population and an increasing standard of living, it is not a question of if but when the world will run out of fossil fuels. In addition, fossil fuels have an adverse effect on the environment as they are responsible for the increasing CO2 content in the Earth's atmosphere. Thus, there has never been a time when the demand for clean and sustainable energy was greater. Ideally, this has to be abundant, environmentally benign, renewable, safe, and cost-effective. Because two-thirds of the oil used in the United States goes to meet the demands of the transportation industry, these alternate energy sources should also be able to meet this important industry's requirements. Currently, such alternate energy sources do not exist.

B

where hydrogen is produced by splitting water through electrolysis with solar energy, storing it reversibly in a solid, and using it on demand in a fuel cell to produce energy. However, considerable difficulties associated with its efficient production, storage, and use in fuel cells lie ahead in achieving this dream. Among these, storage of hydrogen for mobile applications is the most difficult hurdle. Currently, hydrogen is stored either in high pressure tanks or in liquid form in cryogenic tanks. These forms of storage are not suitable for widespread commercial application as the energy density of hydrogen even at 10 000 psi or in liquid form is only 4.4 and 8.4 MJ/L, respectively, compared to the energy density of gasoline, namely, 31.6 MJ/L. In addition, there are significant energy costs in storing hydrogen in these forms, not to mention associated safety issues. The alternative is to use solid materials for hydrogen storage. It has been known for more than a century that hydrogen can be stored reversibly in metals such as Pd. Unfortunately, these metals are heavy and are not suitable for commercial automotive applications because the Department of Energy's storage system target is rather stringent. The ultimate full fleet targets2 require gravimetric and volumetric densities of 7.5 wt % and 70 g/L, respectively, an operating temperature between -40 and 85 °C, a minimum delivery pressure of 12 bar, and a fueling time less than 3 min. In addition, the storage system should be safe, durable (1500 operational cycle life), and cost-effective. No existing systems meet these conditions. Realizing that much basic research needs to be done to understand the interaction of hydrogen with matter, in 2003, the Department of Energy convened a task force to examine the basic materials research needs for a successful transition to a hydrogen economy. While considerable work needs to be done on efficient production and use of hydrogen in fuel cells, the report3 concluded that storage of hydrogen poses the most difficult challenge. The current status of hydrogen

With a growing world population and an increasing standard of living, it is not a question of if but when the world will run out of fossil fuels. Hydrogen is considered as a possible solution1 for the mobile industry because it is the most abundant element in the universe, contains the highest energy density per unit mass, and burns clean, producing only water. Unfortunately, hydrogen is not an energy source but rather an energy carrier. It is not freely available in nature and needs to be produced from water or other organic compounds. Although hydrogen can be considered as renewable if it is produced from water, note that it costs more energy to produce it than one recovers while burning it. Figure 1 shows an ideal hydrogen cycle

r 2011 American Chemical Society

Received Date: November 11, 2010 Accepted Date: January 6, 2011 Published on Web Date: January 13, 2011

206

DOI: 10.1021/jz1015372 |J. Phys. Chem. Lett. 2011, 2, 206–211


storage is summarized in Figure 2. While hydrogen densities increase from high-pressure tanks to storage in liquid form and in various solids, its accessibility continually decreases. Although considerable progress has been made in our understanding of how hydrogen interacts with matter, we have a long way to go. In this Perspective, I provide a summary of the developments made and outline the important challenges that still lie ahead in our quest for that “ideal” hydrogen storage material. I must apologize that the limited space available does not allow me to discuss important works of many authors. Instead, I cite only a few selected papers4-7 with the hope that they will form the basis for further search. There are mainly three different ways that hydrogen can be adsorbed on a material (see Figure 3). In the case of physisorption (Figure 3a), hydrogen remains molecular and binds weakly on the surface with a binding energy in the meV range. Hence, it desorbs at very low temperatures. In the case of chemisorption (Figure 3b), the H2 molecule dissociates into individual atoms, migrates into the material, and binds chemically with a binding energy lying in the 2-4 eV range. The bonding is strong, and desorption takes place at higher temperatures. The third form of binding is where the bond between H atoms in a H2 molecule is weakened but not broken (Figure 3c). The strength of binding is intermediate between physisorption and chemisorption (binding energy in the 0.1-0.8 eV range)

and is ideal for hydrogen storage under ambient pressures and temperatures. This form of quasi-molecular binding has two different origins. Kubas8 has shown that charge donation from the H2 molecule to the unfilled d orbitals of transition-metal atoms and back-donation from the transition-metal atom to the antibonding orbital of the H2 molecule is responsible for this quasi-molecular bonding. Niu et al.,9 on the other hand, have shown that the electric field produced by a positively charged metal ion can polarize the H2 molecule, which can then bind to the metal cation in quasi-molecular form. In both cases, multiple hydrogen atoms can bind to a single metal atom. To achieve hydrogen gravimetric and volumetric densities required for mobile applications, host elements should be lighter than Al metal. Examples of some of these materials6 are shown in Figure 4. Light metal and chemical hydrides as well as sorbent materials that possess high hydrogen gravimetric densities are given in Table 1. Unfortunately, hydrogen atoms in the above materials are bound either too strongly or too weakly. In the former case, it is difficult for hydrogen to be desorbed under ambient conditions, while in the later case, hydrogen can only be held at very low temperatures. For the discussion to follow, I divide the hydrogen storage materials into three categories in terms of the strength of hydrogen bonding, (1) sorbent materials6,7,10-13 where hydrogen is physisorbed and weakly bound to the substrate; (2) complex hydrides5,14,15 where hydrogen is held in strong covalent bonds; these consist of light metal hydrides and chemical hydrides; and (3) nanostructured materials16-19 where hydrogen is held by an interaction that is intermediate between physisorption and chemisorption. These are composed of functionalized sorbent materials as well as nanoparticles of complex hydrides. The thermodynamics of these materials can be tuned by either suitable doping or tailoring particle size. The progress made in our understanding of these materials is briefly discussed, leaving the reader to consult original publications for details. Sorbent Materials. Carbon-based materials such as nanotubes, fullerenes, graphene, mesoporous silica, metal-organic frameworks (MOFs), isoreticular metal-organic frameworks (IRMOFs), covalent-organic frameworks (COFs), and clathrates belong to this category.6,7,10-13 A first report by

Figure 1. Hydrogen cycle (Reproduced with permission of Hydrogen as a Future Energy Carrier; Zu.ttel, A., Borgschulte, A., Schlapbach, L., Eds.; Wiley-VCH: Weinheim, Germany, 2008).

Figure 2. Hydrogen density and accessibility of different storage systems (Reproduced with permission of M. Gutowski).


207



Figure 4. Hydrogen storage density in sorbent materials and various chemical compounds and hydrides (Courtesy of W. L. Mao, ref 6; reproduced with permission of the American Institute of Physics). Table 1. Hydrogen Gravimetric Density in Selected Materials materials

materials

hydrogen (wt%)

LiAlH4

10.54

NaAlH4

7.41

MgH2

7.69

Mg(AlH4)2

9.27

LiNH2 Mg(NH2)2

8.78 7.15

NaNH2 LiBH4

5.15 18.36

NaBH4

10.57

Mg(BH4)2

14.82

Al(BH4)3

16.78

CH4

25.00

NH3BH3

19.35

NH4BH4

24.24

(H2)4CH4

50.00

sites, pore volume versus surface area, and the role of contact between the source and receptor. Work on clathrate hydrates6 also shows promise, provided that these materials can be stabilized closed to ambient conditions. In this connection, (H2)4CH4 (referred to as H4M), with 33.4% molecular hydrogen by mass or 50% total hydrogen if one includes the four hydrogen atoms within each methane molecule, is a very attractive material. Cryogenic experiments have shown that H4M is stable over a wide range of pressures and temperatures (see ref 6). Recently, a new class of porous aromatic frameworks (PAFs) with diamond-like structure has been proposed that possess higher gravimetric hydrogen density than COF-102, namely, it can store 6.53 wt % hydrogen at 298 K and 100 bar.20 Complex Hydrides. These consist of light metal hydrides and chemical hydrides. MgH2 and salts of [AlH4]- (alanates), [NH2]- (amides), and [BH4]- (borohydrides) such as NaAlH4, LiNH2, Li2NH, and Mg(BH4)2 belong to the former category,5 while NaBH4, C10H18, and NH3BH3 are classified as chemical hydrides.14,15 In most cases, chemical hydrides are generated through chemical reaction and are not reversible on board of a vehicle. In alanates, amides, and borohydrides, hydrogen is covalently bonded to central atoms in “complex” anions. Although these materials contain a high density of hydrogen, they were initially not considered as suitable hydrogen storage materials because of their poor thermodynamics, kinetics, and reversibility. Consider, for example, NaAlH4. It under-

Figure 3. Adsorption of hydrogen on substrates, (a) physisorption, (b) chemisorption, and (c) quasi-molecular bonding.

Dillon et al.12 on the hydrogen storage ability of carbon nanotubes attracted considerable attention and hope, but later studies (see ref 7) showed that these results were flawed due to difficulties in accurate measurements and impurities in samples. Although they exhibit promising hydrogen storage capacities at 77 K, only less than 1 wt % of hydrogen can be stored at 298 K and 100 atm of pressure. Hydrogen uptake at ambient temperatures, however, can be improved through a “spillover” mechanism,7 which has been known in the catalysis community for a long time. Here, hydrogen dissociates on a supported catalyst and diffuses into the support. Hydrogen spillover by different techniques (e.g., physical mixing, chemical doping, ultrasonication, and plasma-assisted doping) and spillover-assisted storage on carbon nanostructures, MOFs, IRMOFs, COFs, zeolites, and mesoporous silica have been recently reviewed by Wang and Yang.7 Much work still remains to be done for improving hydrogen uptake by sorbent materials using the hydrogen spillover mechanism under ambient thermodynamic conditions. This includes a fundamental understanding of the heats of adsorption of hydrogen on adsorbent


hydrogen (wt%)

208



goes dehydrogenation at 265 °C, giving rise to Al metal and NaH, namely ð1Þ NaAlH4 f NaH þ Al þ ð3=2ÞH2

NH3BH315 have brought new hope that chemical hydrides may be candidates for on-board hydrogen storage. Efforts are also made to modify the thermodynamics of hydrogenation/dehydrogentation reactions by using additives to form compounds or alloys in the dehydrogenated state that are energetically favorable with respect to the products of the reaction without additives. First-principles calculations are enabling identification of new destabilized metal hydride reactions for reversible hydrogen storage.30 Theoretical procedures have also been developed for crystal structure generation and prediction of ionic compounds consisting of a collection of cations and rigid complex anions.31 The synergy between theory and experiment is making it easier to search for new hydrogen storage materials with desired properties.

Further dehydrogenation of the binary metal hydride occurs at temperatures above 400 °C. The above reaction, however, happens in two different stages. ð2Þ 3NaAlH4 f Na3 AlH6 þ 2Al þ 3H2 Na3 AlH6 f 3NaH þ Al þ ð3=2ÞH2

ð3Þ

Reaction 2 occurs at around 210 °C, evolving 3.7 wt % of hydrogen, while reaction 3 occurs at around 250 °C, releasing 1.9 wt % of hydrogen. A systematic study by Bogdanovic and Schwickardi21 showed that the addition of Ti-based catalysts could bring down the first dehydrogenation temperature in the solid state to 150 °C while simultaneously improving the conditions for rehydrogenation. These findings fueled the hope that light metal hydrides such as MgH222 could function as a rechargeable hydride under moderate conditions and might be developed for application as an on-board hydrogen storage material. In this connection, Mg(BH4)2 shows promise as a hydrogen storage material due to its high gravimetric density (see Table 1). However, the lack of reversibility for hydrogen storage in LiBH4 and Mg(BH4)2 was recently attributed to the existence of very stable phases containing [B12H12]2- polyhedral complexes.23 The reader is referred to a recent review by Orimo et al.5 for an in-depth analysis of light metal hydrides. A fundamental understanding of how the catalysts work and how the hydride can be regenerated has not been easy. Catalysts are usually mixed with the hydride by mechanical ball milling, which introduces defects and impurities that are hard to characterize. A recent work24 where carbon nanotubes and fullerenes were introduced as catalysts without ball milling has shown that charge transfer from the Na atom to the electronegative substrate of the carbon nanostructures weakens the Al-H bond and reduces the energy for hydrogen desorption. Studies have also shown that nanostructuring can improve the hydrogen storage properties of these materials.25 Much of the recent work, therefore, has concentrated on how the catalysts work and how nanostructuring can improve both thermodynamics and kinetics. Defects have also been shown to play an important role in hydrogen removal and mass transport. For example, Na vacancies were shown to promote hydrogen desorption,26 while AlH3 vacancies are responsible for large-scale metal atom transport needed for rehydriding of NaAlH4.27,28 Theoretical studies on NaH, MgH2, and NaMgH3 have also demonstrated the role of defects associated with metal sites in atomic transport.29 Chemical hydrides such as sodium borohydride (NaBH4) are also a rich source of hydrogen which can be released through chemical reaction with water NaBH4 þ 2H2 O f 4H2 þ NaBO2

The synergy between theory and experiment is making it easier to search for new hydrogen storage materials with desired properties. Functionalized Nanostructures. Research on nanostructured materials has clearly demonstrated that reduced size, low dimensionality, and low coordination can lead to properties that are very different from the corresponding bulk materials. Because the physics and chemistry of matter at the nanoscale can be fundamentally altered, considerable research has focused on the role that nanostructures can play in the search for “ideal” hydrogen storage materials. It has already been shown that simply reducing the size of complex hydrides to the nanoscale regime can improve the thermodynamics and kinetics of hydrogen storage.24 Considerable theoretical work also has been carried out on pure and doped carbon and BNbased nanostructures (nanotubes, fullernes, and graphenelike), which show their potential as reversible hydrogen storage materials under ambient conditions. One of the early works by Zhang, Yildrim, and their collaborators16,17 showed that transition-metal-doped C60 fullerene and carbon nanotubes can store hydrogen up to 9 wt % reversibly with favorable kinetics and thermodynamics. However, later work18 showed that these materials may not be very stable during repeated hydrogenation/rehydrogenation cycles as transitionmetal atoms prefer to cluster. It was later shown19 that this problem can be avoided either by choosing metal dopants that bind stronger to the substrate than to each other or by having them replace a ligand site.32 For example, theoretical studies showed that Li19 and Ca33 atoms supported on C60 and nanotubes do not cluster, and up to 13 wt % hydrogen can be stored on Li12C60.19 The binding energy of hydrogen can be further enhanced by functionalization of C60. Recent theoretical work34 has also suggested that an external electric field can be used to polarize H2 molecules just as metal cations can. The thermodynamics of hydrogen storage can then be tailored by varying both the polarizability of the substrate as well as the electric field strength. Experiments are needed to verify the above predictions of theory.

The fuel can be processed on board of a vehicle by means of a catalyzed reaction generating hydrogen needed to power a fuel cell. The spent fuel containing NaBO2 can be later reprocessed back to NaBH4. Recent discoveries of Li3N14 and


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AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected]. Telephone: (804) 828 8991.

Biographies Puru Jena is Distinguished Professor of Physics at Virginia Commonwealth University and Jefferson Science Fellow alumnus at the U.S. Department of State. He is a Fellow of the American Physical Society and recipient of Virginia Commonwealth University's Award of Excellence and Distinguished Scholar Award and the Outstanding Faculty Award of the Commonwealth of Virginia. His research interest includes nanoclusters and cluster-assembled materials, surfaces, interfaces, spintronics, and hydrogen storage materials. Figure 5. Summary of various hydrogen storage materials and their limitations (Figure adapted from a talk given by Professor W. I. F. David of Oxford University).

ACKNOWLEDGMENT I am thankful to Profs. W. I. F. David, M. Gutowski, J. K. Johnson, A. Kandalam, S. Li, Q. Sun, and Q. Wang for a critical reading of the manuscript. I also thank Profs. A. Zuttel, W. Mao, M. Gutowski, and W. I. F. David for permission to reproduce the figures presented here. The work is partly supported by grants from the Department of Energy.

Figure 5 summarizes the different types of materials being studied for hydrogen storage along with the challenges that they face [Professor W. I. F. David, private communication]. Metal hydrides are reversible under ambient conditions but are too heavy. Simple chemical hydrides are reversible but at very high pressure and temperature. Complex chemical hydrides have high hydrogen density but suffer from poor reversibility. Sorbent materials offer good reversibility but require very low temperatures. Theoretical calculations have predicted many promising cluster-based materials, but they are hard to synthesize. Nanostructured materials provide opportunities but are yet to emerge as practical materials. Much research still needs to be done to identify appropriate catalysts, study the existence of stable intermediate phases and their crystal structure, and understand the kinetics. Considering the impact that a successful hydrogen economy can have on our energy needs and the progress that basic research has made in the past decade in addressing issues and challenges in hydrogen storage, it is imperative that the momentum of current research into a basic understanding of hydrogen interaction in materials be maintained. I end this Perspective with an optimistic quote from “The Mysterious Island” by Jules Verne: “...water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable...When the deposits of coal are exhausted we shall heat and warm ourselves with water. Water will be the coal of the future.”

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Much research still needs to be done to identify appropriate catalysts, study the existence of stable intermediate phases and their crystal structure, and understand the kinetics.


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Materials for Hydrogen Storage: Past, Present, and Future - The

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