1.1. Storage of hydrogen

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1 1.1. Storage of hydrogen Why to store hydrogen? Three isotopes of hydrogen are known, hydrogen or protium (H), deuterium (D), and the unstable tritium (T). All the isotopes of hydrogen form covalent molecules like H 2, D 2, and T 2, respectively, because of the single electron in the atom. Hydrogen has an ambivalent behaviour towards other elements, occurring as an anion (H + ) or cation (H - ) in ionic compounds, forming covalent bonds, e.g., with carbon, or even behaving like a metal to form alloys or intermetallic compounds at ambient temperature. Hydrogen is not a fuel as oil and coal. Rather, like electricity, it s an energy carrier that must be generated using another source of power. Hydrogen is the most common element in the universe, but on Earth, nearly all is bound to other elements in molecules, such as hydrocarbons and water. Hydrogen atoms must be split off these molecules to generate dihydrogen gas (H 2 ), the form needed in most fuel cells [1,2]. These devices then combine hydrogen and oxygen to make water and liberate electricity. But every time a fuel is converted from one source, such as oil, to another, such as electricity or hydrogen, it costs energy. Today, by far the cheapest way to produce hydrogen is by using steam and catalysts to break down natural gas into H 2 and CO 2. But although the technology has been around for decades, current steam reformers are only 85% efficient, meaning that 15% of the energy in natural gas is lost as waste heat during the reforming process. Current techniques for liberating hydrogen from coal, oil, or water are even less efficient. Renewable energy such as solar and wind power can also supply electricity to split water without generating CO 2. But those technologies are even more expensive. Generating electricity with solar power, for example, remains 1 times more expensive (using short range economic criteria) than doing so with a coal plant. Hydrogen is the lightest element. At room temperature and atmospheric pressure, hydrogen takes up roughly 3 times more space than gasoline containing the same amount of energy. That means the increase of its storing requires either compressing it, liquefying it, or using some other form of advanced storage system. The goal is to pack hydrogen as close as possible, i.e. to reach the highest volumetric density by using as little additional material as possible. Hydrogen storage implies the reduction of an enormous volume of hydrogen gas. At ambient temperature and atmospheric pressure, 1 kg of the gas has a volume of 11 m 3. To increase hydrogen density, work must either be applied to compress the gas and decrease the temperature below the critical temperature, or to reduced the repulsion between H atoms by the interaction of hydrogen with another material. The second important criterion for a hydrogen storage system is the reversibility of uptake and release. Materials that interact with hydrogen, therefore, as well as inert materials, are important. The reversibility criterion excludes all covalent hydrogencarbon compounds because hydrogen is only released if they are heated to temperatures above 8 C, or if the carbon is oxidized.

2 Material science challenge Hydrogen storage is a material science challenge because, for all storage methods currently being investigated, materials with either a strong interaction or without any reaction with hydrogen are needed. Besides conventional storage methods, i.e. high pressure gas cylinders and liquid hydrogen, the physisorption of hydrogen on materials with a high specific surface area, hydrogen intercalation in metals and complex hydrides, and storage of hydrogen based on metals and water are considered [3,4]. Basically, six methods of reversible hydrogen storage with a high volumetric and gravimetric density are known today. They are: (i) high pressure gas cylinders, (ii) liquid hydrogen in cryogenic tanks, (iii) adsorbed hydrogen on materials with a very large specific surface area, (iv) adsorbed on interstitial sites of metallic hydrides, (v) complex compounds, and (vi) metals and complexes together with water High pressure gas storage Nearly all of today s prototype hydrogen vehicles use compressed gas. Tanks pressurized to 7 MPa take up to eight times the volume of a current gas tank to store the equivalent amount of fuel. Because fuel cells are twice as efficient as gasoline internal combustion engines, they need fuel tanks four times as large to propel a car the same distance. Pressure, bar H metal H2 solid Critical point H2 liquid Triple point H2 gas Liquid metal H gas RT Temperature, K Fig Illustrative phase diagram for hydrogen. Liquid hydrogen only exists between the solid line from the triple point at 21.2 K and the critical point at 32 K The illustrative phase diagram for hydrogen is shown in Fig At low temperatures, hydrogen is a solid with a density of 7.6 kg m -3 at 262 C, and a gas at higher temperatures with a density of kg m -3 at C and a pressure of 1 bar. Hydrogen is a liquid in a small zone between the triple and critical points with a density

3 of 7.8 kg m -3 at 253 C. At ambient temperature ( K), hydrogen gas is described by the Van der Waals equation: p ( V ) a 2 2 n RT n = (1.1) V nb V where p is the gas pressure, V the volume, T the absolute temperature, n the number of moles, R the gas constant, a is the dipole interaction or repulsion constant, and b is the volume occupied by the hydrogen molecules. The strong repulsive interaction between hydrogen is responsible for the low critical temperature (T c = 33 K) of the gas. At high pressures, deviations from the ideal gas law are large (Fig. 1.2). The hydrogen gas density at 1 psi 1 is only two-thirds of an ideal gas. Doubling the pressure to 2 psi, if that were technically, would increase the gas density by only about 5%. High pressure tanks are complex structures containing multiple layers for hydrogen confinement, rupture strength, and impact resistance. Furthermore, the tank must be cylindrical or near-cylindrical in shape, which seriously limits the options for tank placement on a vehicle. High pressure storage is most appealing for large vehicles, such as buses, which have more available space. 6 5 Gas density, g/l Ideal gas law Beattie-Bridgeman equation Hydrogen pressure, psi Fig The Beattie-Bridgeman equation of state shows that higher gas density becomes increasingly difficult to attain with higher pressure [6] Compressed gaseous storage is closest to technical feasibility and is fundamentally appealing because of its familiarity and conceptual simplicity. The major difficulty with compressed hydrogen is its volume. One kilogram of hydrogen stored in common laboratory gas cylinders at 22 psi occupies 91.2 l. For comparison, a mere 8.2 l of gasoline carries the same energy. Hydrogen tanks of 5 and 1 psi are being developed, but even at 1 psi, the volume of hydrogen is 27 l/kg. 1 1 psi = 14.7 atm

4 Liquid hydrogen storage Demonstration fuel cell vehicles have been built using liquid hydrogen storage. Here, the volumetric situation is somewhat improved compared to compressed gas because liquid hydrogen occupies about 14 l/kg (1 MJ/l, hydrogen basis). But hydrogen vapourizes at 253 C, which necessitates an exotic superinsulated cryogenic tank. Inevitably, heat leaking into the tank will produce serious boil-off, and the tank will begin to empty itself in days in undriven vehicles. Liquid hydrogen seems best suited to fleet applications, where vehicles return nightly to a central station for refueling. Advanced tank designs may extend the boil-off period to perhaps a few weeks proposals that combine high-pressure and cryogenic capabilities in a single tank design could also mitigate boil-off. There is a large energy loss for hydrogen compression (equal to 1% of the energy content of the gas compressed) or liquefaction (3% of loss). Although this affects the storage economics, it does not impact the on-board storage system because the losses occur before the hydrogen is delivered to the vehicle Storage as adsorbed hydrogen Hydrogen can be stored as adsorbed hydrogen on materials with a very large specific area. Resonant fluctuations in charge distributions, which are called dispersive or Van der Waals ineractions, are at the origin of the physisorption of gas molecules onto the surface of a solid. In this process, a gas molecule interacts with several atoms at the surface of a solid. The interaction is composed of two terms: an attractive term, which diminishes with the distance between the molecule and the surface to the power of 6, and a repulsive term, which diminishes with the distance to the power of 12. The potential energy of the molecule, therefore, shows a minimum at a distance of approximately one molecular radius of the adsorbate. The energy minimum is of the order of.1-.1 ev (1-1 kj mol -1 ). Because of the weak interaction, significant physisorption is only observed at low temperatures (< 273 K). Materials with a large specific area like activated or nanostructured carbon and carbon nanotubes (CNTs) are possible substrates for physisorption. The big advantages of physisorption for hydrogen storage are the low operating pressure, the relatively low cost of the materials involved, and the simple design of the storage system. The rather small gravimetric and volumetric hydrogen density on carbon, together with the low temperatures necessary, are significant drawbacks Hydrogen in metal hydrides Hydrogen can be chemically bound and stored as storage in materials a solid compound. Solid-hydride storage materials release hydrogen gas under suitable

5 conditions of temperature and hydrogen pressure (generally 2-3 bar) and, in some cases, in the presence of a further reactant. Solid hydrides can be loosely sorted into two groups: (i) those for which the reverse hydriding reaction can be accomplished on-board the vehicle, generally by supplying hydrogen to the vehicle at a pressure higher than its working pressure, and (ii) those for which on board rehydriding is impractical or impossible. In this case, refueling requires replacement of the storage medium itself, either by flush-and-fill or by exchanging the entire tank. The dehydrided material must then be recharged off-board; simply disposing of the spent material is unlikely to be economically acceptable for main stream transportation applications. Perhaps the most promising solid-storage media for hydrogen are the reversible metal hydrides [5]. Among the more hydrogen-rich metal hydrides, volume is not the primary issue (Fig. 1.3). In fact, many hydrides store more hydrogen per unit volume than liquid hydrogen does. Furthermore, at modest hydrogen pressures (a few bars) some hydrides, for example LaNi 5 H 6, releases hydrogen at or near room temperature. Its hydriding kinetics are also acceptable since laboratory quantities can be dehydrided and rehydrided in 5 to 1 min. The main challenge of metal hydrides is their weight. Because the hydrogen content of LaNi 5 H 6 is only 1.4% by weight (wt%), storing 5 kg of hydrogen would require 36 kg of LaNi 5 H 6 (remind that 1 kg of hydrogen carries the same energy as 8.2 l of gasoline) Practical maximum volume 22-psi gas 5-psi gas 1-psi gas 2-psi gas Liquid H 2 LaNi 5 H 6 FeTiH 2 MgH 2 Storage method Mg 2 NiH 4 NaAlH 4 Li-N-H NaB H 4 + 2H 2 O Na H + H2O Gasoline energy equivalent Fig.1.3. A kilogram of hydrogen takes a lot of space as a gas and less as a liquid or solid hydride. Practical on-board storage needs system volumes below the dashed line [6] Some reversible metal hydrides store larger specific masses of hydrogen. Magnesium hydride (MgH 2 ) contains 7.6 wt% hydrogen (1.8 MJ/kg, based on material

6 weight only, excluding the support hardware), a value that approaches feasible energy density. Regrettably, MgH 2 suffers from a thermodynamic obstacle common to highcapacity metal hydrides: the hydrogen is too strongly bound (Fig. 1.4) and the large enthalpy of hydride formation ΔH = 37 MJ/kg of dihydrogen H 2 has several important consequences. First, at the operating hydrogen pressure of the fuel cell (typically 2-5 bar), the hydrogen release temperature is commensurately high, nearly 3 C. Second, because dehydriding is endothermic, the ΔH must be supplied as heat to release the hydrogen. This represents nearly a 3% parasitic energy loss incurred on-board the vehicle. Finally, all of that heat is released again during fueling. Rapid fueling, i.e. 5 min, requires roughly 1 MW of cooling power to extract the heat energy from MgH 2. A new, as-yet-undiscovered, light-metal hydride with a hydrogen capacity greater than that of MgH 2 but with ΔH similar to that of LaNi 5 H 6, is needed. Approximate hydrogen release temperature, o C MgH 2 H 2 capacity, wt.% 6 4 TiV 1.6 Ni.4 H 5 NaAlH4 (stage 1 ) NaAlH4 (stage 2) Li-N-H Mg 2 NiH 4 2 FeTiH 2 LaNi 5 H Enthalpy of hydride formation, ΔH, MJ/kg H2 Fig Unfortunately, the known reversible metal hydrides that store large specific masses of hydrogen also have high enthalpies of formation and require high temperatures to release the strongly bound hydrogen [6] The solid hydride NaAlH 4 (sodium alanate) is intermediate between the low-and high-temperature metal hydrides. It decomposes on heating in two steps, first to sodium aluminium hydride (Na 3 AlH 6 ) plus aluminium, and subsequently to sodium hydride (NaH) plus additional aluminium. Further decomposition of the NaH requires impractical high temperatures for PEM (proton exchange membrane) fuel-cell applications. The combined theoretical hydrogen capacity is 5.6 wt%, which corresponds to 7.8 MJ per kg of the hydride. Incorporating titanium or titanium and zirconium dopants has yielded experimental dehydriding rates of 1 wt%/h at 11 and 16 C for the first and second

7 decomposition steps, respectively, but at the cost of lower hydrogen capacity (~4.5 wt%). Even relatively slow rehydriding requires high hydrogen pressure (~8 bar). A new storage material based on transformations between a series of lithiumnitrogen-hydrogen compounds has been identified recently. Although interesting, this system still suffers from a relatively high ΔH and a modest hydrogen capacity of only 6.5 wt% Chemical hydrides Nonreversible hydrides can store and release hydrogen. Sodium borohydride (NaBH 4 ) and NaH are examples of hydrolysis hydrides; adding water releases hydrogen and forms sodium metaborate (NaBO 2 ) or sodium hydroxide (NaOH). Although these materials generate considerable quantities of hydrogen, on-board system problems such as thermal control are significant, and off-board regeneration infrastructure and energyefficiency considerations remain challenging. New hydride storage research and development on non-traditional hydrides is of great importance for fabrication effective fuel cells used in vehicles, where high gravimetric-storage density is required and where hydrogen must be liberated at temperatures compatible with the waste heat of the fuel cell (< 4 K) Storage in carbon nanotubes Since 1997, numerous reports in the technical literature and news releases have claimed that carbon nanofibers, and similar carbon nanostructures can sorb anywhere from 3 to 67 wt% hydrogen at room temperature. Most claims require hydrogen pressures of around 1 bar, but in a few cases, researchers claim high hydrogen capacity at ambient pressure. Although it is widely accepted that carbon nanotubes in 12 bar hydrogen gas can sorb up to 8 wt% hydrogen at cryogenic temperatures by simple physisorption, the binding energy is far too low to account for high hydrogen storage at room temperature. The amount of hydrogen physisorbed on activated carbon, for example, drops by an order of magnitude between 77 K and room temperature, to.7 wt% or less. Hydrogen capacities approaching 8 wt% (1 hydrogen atom/carbon atom) at room temperature would require a currently unknown carbon-hydrogen bonding mechanism intermediate in strength between physisorption and chemisorption. Worldwide efforts to verify large hydrogen storage in nanostructured carbon have met with no real success. A few claims have been proven wrong (but nevertheless continue to be cited in the technical literature) Comparative analysis Volumetric and gravimetric hydrogen densities in various media are shown in Table 1.1 and Fig It is seen that in some cases the hydrogen density in metal hydrides is higher than in liquid or even solid dihydrogen. The relatively high density of

8 hydrogen in the metal hydrides combined with the relative ease of dihydrogen recovery form the basis for their potential use as hydrogen storage materials. Existing hydrogen-storage systems using liquid, gas, or solid hydrides lack the gravimetric and volumetric energy density values to meet even the minimum performance goals required for vehicular transport. Table 1.1. Volumetric and gravimetric hydrogen densities [7] Medium Volumetric density, 1 22 atoms/cm 3 Gravimetric density, wt% H 2 gas, 1 MPa.5 1 H 2 liq., 2 K H 2 sol., 4.2 K H 2 O liq. r.t LiH PdH Mg H TiH VH FeTiH LaNi 5 H Mg 2 NiH LiAlH Mg(AlH 4 ) System gravimetric energy density, MJ/kg Compressed gas 1 psi Minimum performance goal Liquid hydrogen High-temperature hydride (e.g., MgH 2) Medium-temperature hydride (e.g., NaAlH ) 4 Low-temperature hydride (e.g., LaNi5H 6 or FeTiH 2) Gasoline Ultimate technology goal System volumetric energy density, MJ/l Fig The system gravimetric energy density and system volumetric energy density for different storage medias in comparison to minimum performance goal [6] REFERENCES 1. G. Sandrock, J. Alloys and Compounds (1999)

9 2. R.F. Service, Science, 35 (5686) 13 August 24, A. Züttel, Materials Today, September (23) T.N. Veziroglu, Intern. J. Hydrogen Energy 25 (2) E.H. Majzoub, K.J. Gross, J. Alloys and Compounds (23) F.E. Pinkerton, B.G. Wicke, The Industrial Physicist, February-March (24) 2-23.

CHAPTER-9 NCERT SOLUTIONS

CHAPTER-9 NCERT SOLUTIONS Question 9.1: Justify the position of hydrogen in the periodic table on the basis of its electronic configuration. Hydrogen is the first element of the periodic table. Its electronic