Hydrogen Storage: Methods and Challenges, Essays (university) of Energy and Environment

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ESSAY ON HYDROGEN STORAGE

1. INTRODUCTION

Hydrogen is the most abundant element in the universe and has great potential as an energy source. It can be easily generated from a renewable energy source and is non-polluting. This lighter-than-air gas makes the perfect fuel as it contains three times the energy of liquid hydrocarbons and when it reacts with oxygen to produce energy, forms water as a harmless by-product. But, one of the biggest challenges to using hydrogen as a fuel is finding a way to store it. Furthermore, hydrogen has similarities to some of our current fuels; particularly natural gas yet for the acceptance of hydrogen as an energy provider there must be an understanding, in terms of safety, of both the physical and chemical properties.

However, Hydrogen has several beneficial properties that increase its potential as a fuel. This includes hydrogen's low density of 0.08Kg /m2 (under normal conditions) and a high ignition temperature of 585°C. Due to its low-density any leakage of hydrogen would disperse quickly, minimising the risk of an explosion. A hydrogen flame also emits minimal radiation due to its small heating value per volume.

Presently hydrogen can be stored in three forms; gaseous, liquid or as a solid combined with a metal hybrid. The most suitable storage method is dependent upon safety aspects, environmental issues, economic criteria and the end-use of hydrogen.

2. GASEOUS HYDROGEN

This can also be seen as compressed gas which is the most mature storage technology, but compression adds inefficiencies to the hydrogen life-cycle and requires stronger, costlier materials for tank construction. Also, it is the most commonly used and simplest method to store hydrogen in its natural form as a gas. Storage of gaseous hydrogen is primarily limited by volume considerations as a result of hydrogen's low density, as even at high-pressure very large volumes are required resulting in high material costs.

Significant amounts of hydrogen can also be stored within high-pressure storage tanks, that can be situated above ground or underground which is similar to the storage of natural gas. The construction material properties in above ground storage impose limitations on the quantity of gaseous hydrogen that can be stored and the hydrogen can be stored at an increased pressure in an underground pressure tank or underwater tank. Extensive materials research is being conducted to improve compressed gas storage method; advancements have already been made in carbon-fibre wrapped tanks, which are lighter and safer than traditional steel tanks. Below are two of the most promising methods to store hydrogen gas under high pressure: composite tanks and glass micro spheres.

2.1 COMPOSITE TANKS

A schematic of a typical high-pressure H2-storage C-fibre-wrapped composite tank is shown in Figure

3. The main advantages with such composite tanks are their low weight (meets targets), and that they are commercially available, well-engineered and safety tested (extensive prototype experience exists), and have codes that are accepted in several countries for pressures in the range 350- bars.

Figure 3 Schematic of a typical compressed H2 gas composite tank (Source: Quantum Technologies) [1].

2.2. Glass Micro Spheres The basic concept for how glass micro spheres can be used to store hydrogen gas onboard a vehicle can be described by a three step process (charging, filling, and discharging): First, the hollow glass spheres are filled with H2 at high pressure (350-700 bars) and high temperature (ca. 300_C) by permeation in a high-pressure vessel. Next, the micro spheres are cooled down to room temperature and transferred to the low-pressure vehicle tank. Finally, the micro spheres are heated to ca. 200- 300_C for controlled release of H2 to run the vehicle. Table 3 compares the main merit factors for two promising gaseous H2 storage options for the future, namely composite tanks and glass micro spheres. In general it can be concluded that it is possible to build safe systems, but costs need to be reduced.

Table 3 Merit factors for gaseous H2 storage – comparison of composite tanks and glass micro spheres. Parameter Composite Tanks Glass Micro Spheres Value Comment Value Comment Temperature, T + No heat exchanger needed – High T needed

Pressure, p – High p compressors needed + Low onboard p possible

Energy density – Only partially-conformable + Up to 5 wt.% H2, conformable Robustness + Extensively tested – Breakable spheres Safety + Existing codes & standards + Inherently safe

Cost – 500-600 USD/kg H2? Needs to be determined

3. LIQUID HYDROGEN

Hydrogen stored in a liquid form is substantially more compressed than in gaseous form and superficially it appears an appealing means of energy storage but there are various contributory negative factors. Primarily that the liquidification requires a large expenditure of energy and secondly, through the use of insulation, liquid hydrogen must be continually kept at a low temperature (<20K). There are risks associated with this constant low temperature, due to the high expansion ratio of liquid hydrogen to gaseous hydrogen. If there was a warming of liquid hydrogen extremely high pressures could accumulate and result in damage or an explosion. This section discusses the three most promising methods: cryogenic H2, NaBH4 solutions, and rechargeable organic liquids.

3.1. Cryogenic Liquid Hydrogen (LH2) Cryogenic hydrogen, usually simply referred to as liquid hydrogen (LH2), has a density of 70.8 kg/m at normal boiling point (-253_C). (Critical pressure is 13 bar and critical temperature is – 240°C). The theoretical gravimetric density of LH2 is 100%, but only 20 wt. % H2 of this can be achieved in practical hydrogen systems today. The main advantage with LH2 is the high storage density that can be reached at relatively low pressures. Liquid hydrogen has been demonstrated in commercial vehicles (particularly by BMW), and in the future it could also be co-utilized as aircraft fuel, as it provides the best weight advantage of any H2-storage.

3.2. NaBH4 Solutions Borohydride (NaBH4) solutions can be used as a liquid storage medium for hydrogen. The catalytic hydrolysis reaction is:

NaBH4 (l) + 2H2O (l) ® 4H2 (g) + NaBO2 (s) (ideal reaction) Eqn. 1

The main advantage with using NaBH4 solutions is that it allows for safe and controllable onboard generation of H2. The main disadvantage is that the reaction product NaBO2 must be regenerated

4.1. Carbon and other high surface area materials 4.1.1. Carbon-based Materials (Nanotubes and Graphite Nanofibers) Carbon-based materials, such as nanotubes (Figure 5a-c) and graphite nanofibers (Figure 5d) have over the last decade received a lot of attention in the research community and in the public press. The general consensus today is that the high H2-storage capacities (30-60 wt%) reported a few years ago are impossible and were the result of measurement errors.

4.1.2. Other High Surface Area Materials The most predominant examples of other high surface area materials are zeolites, metal oxide frameworks (MOFs), and clathrate hydrates. The definitions and main features for these materials are as follows: · Zeolites : Complex aluminosilicates with engineered pore sizes and high surface areas. Well known as “molecular seives”. The science for capturing non-H2 gases is well known. · Metal Oxide Frameworks (MOFs) : Typically ZnO structures bridged with benzene rings. These materials have an extremely high surface area, are highly versatile, and allow for many structural modifications. · Clathrate hydrates : H2O (ice) cage structures, often containing “guest” molecules such as CH4 and CO2. The cage size and structure can often be controlled by organic molecules (e.g., THF). The materials are all characterized by extremely high surface areas that can physisorb molecular H2. They have been shown to store a few wt.% H2 at cryogenic temperatures. However, the R&D question is if they can be engineered to reversibly store high levels of H2 near room temperature. These materials, particularly metal oxide frameworks and clathrate hydrates, represent new storage ideas and should be studied to determine the potential for the near future.

4.2. Rechargeable Hydrides The two main reversible hydriding reactions in rechargeable metal hydride batteries in addition to Research & Development on rechargeable hydrides has been going on for decades, and a large database with information about their properties exists today within the IEA HIA Annex 17 (http://hydpark.ca.sandia.gov). In summary of this database (the metal hydride “family tree”), it could be seen clearly that it is the complex hydrides that provide the hope for the future, particularly the non-transition metal types such as borohydrides, alanates, and amides. These are discussed in some more detail below.

4.2.1. Alanates The key properties of the most known alanates are provided in Table 7, were particularly NaAlH4 is being intensely studied on many fronts. The low-temperature kinetics and reversibility of these alanates are improved by adding a catalyst (e.g., Ti).

Table 7 Alanates – Key properties of most common types. Type Storage density, wt.% H2 Desorption temperature, _C* LiAlH4 10.6 190 NaAlH4 7.5 100 Mg(AlH4) 9.3 140 Ca(AlH4) 7.8 > 230 *Theoretical l maximum

4.2.2. Borohydrides The key properties of the most known borohydrides are provided in Table 8, which shows that the borohydrides have much higher capacity potential than the alanates. However, they are also much less studied than alanates. In general borohydrides are too stable, and are not as reversible as alanates, but are at the same time considered more safe than the alanates.

Table 8 Borohydrides – Key properties of most common types. Type Storage density, wt.% H2 Desorption temperature, _C* LiBH 4 18.5 300 NaBH4 10.6 350 KBH4 7.4 125 Be(BH4)2 20.8 125 Mg(BH4)2 14.9 320 Ca(BH4)2 11.6 260 *Theoretical maximum

4.3. Chemical Hydrides (H2O-reactive) Chemical hydrides can be handled in a semi-liquid form, such as mineral oil slurry. In this form hydrides can be pumped and safely handled. Controlled injection of H2O during vehicle operation is used to generate H2 via hydrolysis reactions. The liberation of H2 is exothermic and does not require waste heat from the vehicle power source. The overview of the hydrolysis reactions for the most common chemical hydrides provided in Table 9 shows that the theoretical potential storage density is around 5-8 wt.% H2. In practical systems, one approach for NaH is to encapsulate small spheres in polymeric shells. In general, MgH2 probably offers the best combination of H2-yield and cost. In all cases, this is an energy intensive process, and it is doubtful cost can be reduced to vehicle targets.

Table 9 Hydrolysis reaction for selected H2O-reactive chemical hydrides Hydrolysis Reaction Storage density, wt.% H* LiH + H2O Þ H2 + LiOH 7. NaH + H2O Þ H2 + NaOH 4. MgH2 + 2H2O Þ 2H2 + Mg(OH)2 6. CaH2 + 2H2O Þ 2H2 + Ca(OH)2 5. *Theoretical maximum

4.4. Chemical Hydrides (thermal) Ammonia borane is another group of chemical hydrides that potentially could be used to store hydrogen in a solid state. An overview of the decomposition reactions (and corresponding temperatures) and the potential storage density is provided in Table 10. They are toxic and would likely contaminate the fuel cell catalysts.

Table 10 Decomposition reactions for a thermal chemical hydrides Decomposition reaction Storage density, wt.% H2 Temperature†, °C* NH4BH4 Þ NH3BH3 + H2 6.1 < 25 NH3BH3 Þ NH2BH2 + H2 6.5 < 120 NH2BH2 Þ NHBH + H2 6.9 > 120 NHBH Þ BN + H2 7.3 > 500 *Theoretical maximum † Decomposition temperature

5. CONCLUSIONS & RECOMMENDATIONS

This paper has discussed the main issues (possibilities and gaps) for storage of H2 in gaseous, liquid, and solid form. Below is a summary of the main findings with respect to technology status, best option(s), and main Recommendations and Developments issues:

Gaseous H2 Storage: Status: Commercially available, but costly. Best option: C-fiber composite vessels (6-10 wt% H2 at 350-700 bar). R&D issues: Fracture mechanics, safety, compression energy, and reduction of volume.