Fuel cells usually use compressed hydrogen as the fuel, but there are many other types of fuels that can be used. The type of fuel used depends upon the fuel cell application. Fuels are often in their final form before entering the fuel cell; however, certain fuel cell types can be processed on the inside of the fuel cell. Alternative fuel types are used because they are readily available, easier to transport, perceived to be safer or processed in-situ. Depending upon the fuel type, one of the disadvantages of certain liquid fuels is that they poison the catalyst layer over time. Metal hydrides are a good choice for portable military applications and reversible aboard hydrogen storage due to their inherent safety, good hydrogen gravimetric density, and excellent volumetric density. Metal hydrides are used for reversible onboard hydrogen storage because they have a lower weight and less volume than compressed and liquid hydrogen options and can release hydrogen at relatively low temperatures and pressures. The fuel cell system can be designed to use the waste heat generated by the fuel cell to “release” the hydrogen from the media. Fresh packs can be swapped for old ones and old packs can be refueled. Refueling a small metal hydride container only takes about 5 to 15 minutes. Figure 1 shows an illustration of a metal hydride tank.
Figure 1. Example Portable Metal Hydride Tank.
Metal hydrides are formed when metal atoms bond with hydrogen to form stable compounds. A large amount of hydrogen per unit volume can be extracted, so the storage density is good despite the fact that they can be heavy. They are often used as powders to maximize the surface area–to-mass ratio. Some of the problems with metal hydrides are high alloy cost, sensitivity to gaseous impurities, and low gravimetric hydrogen density.
Metal hydrides are safer than other alternatives because they are endothermic when releasing hydrogen and are kept under a relatively low pressure of 1 to 10 atm within the metal hydride container. The metal hydride adsorption reaction is:
M + (x/2) H2 → MHx (exothermic, ⌂ H < 0)
where the number of hydrogen atoms, x, per metal atom, M, is a function of the chemistry of the metal. The exothermicity of the reaction means that more heat causes equilibrium to shift towards free hydrogen gas, and higher partial pressure of H2 causes a shift towards adsorption and metal hydride formation. The law of mass action shows that the equilibrium constant is Keq = [H2](–x/2). If we substitute this into the free energy equation:
⌂G = –RT ln Keq = (x/2) RT ln (PH2)
A hydride with a high heat of reaction (⌂G) has a lower equilibrium pressure of hydrogen over the metal / metal hydride system at a given temperature, and a stronger metal–hydrogen bond. A metal is a useful storage medium for hydrogen if it strongly bonds to hydrogen so it can be charged up. However, if the bond is too strong, the metal will not give up its hydrogen under heating or depressurization. Hydrides are also sensitive to contaminants (some are poisoned by oxygen or water vapor) so care must be taken to only introduce pure hydrogen to the hydride.
The use of metal hydrides as a storage medium for hydrogen is dependent not only on thermodynamics, but on kinetics. The intrinsic kinetics of hydrogen dissociation are fast. The rate-determining step is heat transport into the powder. Powders have a thermal conductivity in the range of 1 to 3 W/mK, which means that they do not conduct heat well. For comparison, copper (one of the best thermal conductors) has a conductivity of 401 W/mK, window glass is at 1.0 W/mK, and fiberglass (a thermal insulator) has a thermal conductivity of 0.05 W/mK.
Heat typically needs to be transferred between the walls of the pressure vessel and the powder. While high surface area means fast hydrogen adsorption and desorption, it also means smaller powder particles. Conduction can be improved by embedding the metal hydride in aluminum foam or running channels with hot liquid through the powder. These processes can increase the net thermal conductivity to as much as 7 to 9 W/mK.
Hydrogen tends to embrittle the particles, causing them to crack into smaller pieces. This increases the total surface area of the powder, increasing the hydrogen desorption/adsorption rate, but the smaller hydride particles can be entrained in the gas flow – which requires filtering to keep the particles out of hydrogen output. This has led to concerns about certain metal hydrides’ long-term usage.
Having a hydrogen reservoir between the metal hydride and the fuel cell is useful to stabilize the hydrogen consumption rate. Complex metal hydrides have the potential for higher gravimetric hydrogen capacities than simple metal hydrides. One example is alanate (AlH4), which can store and release hydrogen reversibly when catalyzed with titanium dopants according to the following reactions:
NaAlH4 = 1/3 Na3AlH6 +2/3Al+H2
Na3AlH6 = 3 NaH + Al + 3/2H2
The first reaction can release 3.7 wt.% hydrogen at 1 atm pressure, and temperatures above 33 °C. The second reaction can release 1.8 wt.% hydrogen above 110 °C. The amount of hydrogen that can be released is more important than the amount that the material can hold; therefore, it is the key parameter used to determine system (net) gravimetric and volumetric capacities. Some current issues with metal hydrides include low hydrogen capacity, slow uptake, release kinetics, and cost. The packing density of these powders is also low (about 50 percent). Continuous research of metal hydrides will hopefully result in the design and development of complex metal hydrides in the future.
One of the newer complex hydride systems is based upon lithium amide has been developed and the reaction occurs at 285 °C and 1 atm:
Li2NH + H2 = LiNH2 + LiH
This reaction allows 6.5 wt.% hydrogen to be reversibly stored, with the potential for 10 wt.%. One issue with using this system is that if it is utilized for portable applications with a PEMFC, the reaction temperature is quite high and needs to be lowered. Further R&D on this system may lead to additional improvements in capacity and operating conditions. Table 1 summarizes the storage system characteristics of some types of metal hydrides. As you can see from the table, the system weights are relatively low (3.4 to 6.0 kg) with high hydrogen capacity (478 – 848 kg H2) and good energy density (3.683 – 5.321 MJ/kg).
|System Volume (L)||568||365||450||433||563|
|System Weight (kg)||6.0||5.0||3.4||5.0||5.44|
|Capacity (kg H2)||848||705.7||478.9||705.7||766.8|
|Capacity (MJ H2)||1.492||1.933||1.064||1.629||1.362|
|Energy Density (MJ/kg)||4.988||5.227||3.683||3.713||5.321|
Table 1. Metal Hydride Storage System Characteristics.
Many types of fuels can be used for different fuel cell types and systems. Metal hydrides maybe a good option for certain applications due to their safety and excellent volumetric density. The use of metal hydrides may be beneficial for use with fuel cells for certain applications, but the overall goal of fuel cell technology is to use pure hydrogen from renewable sources of energy other than fossil fuels.