Pound for pound, hydrogen carries three times the energy of gasoline, and fuel cells can use this energy much more efficiently than can an automobile engine. However, if we carry hydrogen as a gas in a 20-gallon gasoline tank, there is only enough energy to drive the car 500 feet. There are two ways to carry more hydrogen: liq­uefy it or compress it. Hydrogen turns into a liquid at -253°C or just 20° above absolute zero. Needless to say, it takes a lot of energy to run the cryogenic equip­ment to cool to this temperature. Even if the tank in the car is very well insulated, the hydrogen will boil off slowly overnight. In use, it has to be heated rapidly to feed the fuel cell at a rate depending on the speed of the car. On top of this, each liter or gallon of liquid hydrogen has only 30% the energy of an equal volume of gasoline. It makes more sense to compress the hydrogen.

Scuba tanks and laboratory gas cylinders are heavy. For cars, light-weight tanks made of carbon fiber composites have been developed to hold pressures as high as 10,000 psi (pounds per square inch) or 69,000 kilopascals, which is 700 times atmospheric pressure. Normal would be about 5,000 psi, which is higher than in scuba tanks. The cost of such a tank would be at least ten times higher than for a gasoline tank of equal volume. Regardless of this, can the tank be large enough to power a car for 300 miles (480 km)? A back-of-the-envelope calculation of this is given in Box. 3.6. Squeezing the hydrogen takes energy, most of which shows up as heat of compression. The compressing has to be done beforehand, since the hydrogen has to cool. Otherwise, not enough can fit into the tank. When the hydro­gen is released for use, it will be too cold for the fuel cell and has to be heated up.

Under development are ways to store hydrogen in solids. Metal hydrides can absorb hydrogen like a sponge under pressure and then release it under heat when the pressure is relieved. As shown in Fig. 3.50a, the hydrogen molecules go between the atoms of the solid, so it can hold 150% more hydrogen than an equal volume of liquid hydrogen [29]. Unfortunately, the chemicals found so far are either too heavy, react too slowly, or require too high a temperature. Some can absorb only 2% of their own weight in hydrogen, even without the pressurized container. The fuel stored this way for a 300-mile trip would weigh half a ton [29]. Magnesium hydride can store 7.6% of its weight, but needs an inconvenient tem­perature of 300°C. The most promising ones are complex hydrides combined with a “destabilizer.” For instance, lithium and magnesium borohydrides (LiBH4 + MgBH4) will combine into two other hydrides and release hydrogen at a low temperature [30]. This can hold 8.4% of hydrogen by weight (Box 3.6). The reaction is reversed when hydrogen is added under pressure at a filling station. Unfortunately,

Box 3.6 Carrying Compressed Hydrogen in a “Gas” Tank

The energy content of a gallon of gasoline is about the same as that of 1 kg of hydrogen, so 1 gallon » 1 kg H2. (In metric units, it is not as convenient: 1 L » 0.12 kg H2.) Say it takes 20 gallons of gasoline to go 300 miles. Since fuel cells are more efficient, it would take not 20 kg but only, say, 8 kg of H2 to drive a car that far. From high-school chemistry, we remember that a mole of gas occupies 22.4 L, so there are 2 g of H2 in 22.4 L. The density at stan­dard conditions is then 2/22.4 = 0.089 g/L. At 10,000 psi (700 atm) com­pressed hydrogen at room temperature would have a density 700 times higher or 63 g/L. Eight kilograms would then occupy 8,000/63 = 128 L or about 34 gallons. So to go as far as a normal car, a hydrogen car would need a 70% bigger tank, not including the mechanisms for handling the compressed gas.

There is also the question of weight. The US Department of Energy has set a goal that the weight of a tank should weigh no more than 17 times the weight of the fuel. (The fuel weight is more than 6% of the tank weight.) Hydrogen tanks so far are 25-50 times as heavy as the fuel.

the reaction rate is too slow to be useful even though the material is in the form of a fine powder to expose large surface area and reduce the path for heat conduction.

A promising new material called metal-organic frameworks (MOFs) has been invented by Yaghi [31]. These are extremely light-weight chemical structures that act as nets to trap larger molecules, as illustrated in Fig. 3.50b. Just like a net, a MOF has struts linked together with strong bonds, forming a scaffold to enclose a large space.

This atomic net has the largest area per unit weight ever obtained: 4,500 m2/g. That means that a paper clip’s weight of material can cover a football field. With consider­able chemical derring-do, hundreds of different kinds of MOFs have been created for different purposes. For storing hydrogen in cars, one liter of the compound MOF-177 can store 62 g of H2, exceeding the 6%-by-weight rule in Box 3.6. However, this has been done so far only at 77 K (kelvin: degrees centigrade above absolute zero). This is liquid-nitrogen temperature, easy to get in the laboratory but hard to maintain in a car, though much easier than the 20 K of liquid H2. A MOF that works for hydrogen at room temperature could get to 5% H2 by weight, but it is not easy to manufacture on a large scale. Another type of compound called COFs is suitable for that, and research is proceeding to make those work at room temperature. COFs can also help with carbon capture in coal plants. A tank filled with MOFs can hold nine times as much CO2 as one without MOFs [31]. Other compounds called ZIFs can actually selectively capture the CO2 going up a smoke stack.

Chemical storage of hydrogen is an active research area in laboratories, but noth­ing works well enough so far to proceed to the next step of engineering large-scale production.