The pending shortage of fossil fuels and the deleterious emissions resulting from their use have increased interest in the development of hydrogen-fueled vehicles. In technologically advanced countries, and increasingly in ‘emerging economies’, energy use for transportation is a large fraction of the total energy consumed. Use of a clean fuel such as hydrogen would decrease reliance on fossil fuels and positively impact atmospheric pollution. Western Europe, Japan, and the US have begun efforts to develop hydrogen-fueled vehicles based on modified internal combustion engines or fuel cells. However, it is not clear whether hydrogen can be produced and transported to local distribution points in an acceptable manner, if vehicles can be refueled safely and rapidly, and whether sufficient onboard storage capacity can be developed.
The US Department of Energy is basing its considerations upon a small automobile that uses a hydrogen fuel cell – the ‘Freedom Car’. Three types of onboard hydrogen storage are being considered – liquid hydrogen, compressed hydrogen gas, and storage in the form of hydrides. All have major disadvantages, but the most promising appears to be hydrides. Liquid hydrogen storage meets volumetric and gravimetric requirements, but ‘boil off’ and refueling present safety hazards. In addition, liquefaction requires significant energy. Storage as high-pressure gas will not meet volumetric density requirements even at a pressure of 650 bar; significant energy is required for compression; and the high flammability of hydrogen presents hazards in collisions. Storage as metal or complex hydrides appears to hold promise, as a subset of these hydrides can nominally meet volumetric and gravimetric density (9 wt.%) requirements.
Major obstacles must be overcome if hydride storage is to become a usable onboard storage system. Chief among these is the fact that hydrides having a sufficiently high gravimetric density have ionic or covalent bonding of hydrogen to the metal sublattice. Consequently, desorption of hydrogen requires temperatures that are well above the desired range (−20°C to 100°C). Thus the ‘accessible’ gravimetric densities and desorption rates are too small. None of the metallically bonded hydrides that do desorb hydrogen in the desired range (generally the ‘heavy’ metal hydrides) have sufficient gravimetric densities. Development of a metal hydride storage system will require modification of the ionic bonding of hydrogen in ‘light’ metal hydrides, e.g. MgH2 or AlH3, toward metallic bonding, perhaps by alloying with other light metals. To date, there has been no success in this direction. A large number of ‘complex hydrides’ incorporate hydrogen with ionic and covalent bonding. While these have high total gravimetric densities of hydrogen, almost all require high temperatures for desorption and absorption of hydrogen. Further, desorption often occurs in multiple stages at increasing temperatures and pressures. Desorption of hydrogen often requires dissociation of the metal sublattice into metallic clusters and hence subsequent absorption of hydrogen (refueling) is very slow.
Studies have utilized catalysts to try and decrease the temperature range at which hydrogen is desorbed, but with limited success. Incorporation of transition metal oxides into Mg has enhanced the rates of absorption and desorption but has not significantly decreased the temperatures at which these occur. Incorporation of TiCl3 into complex hydrides, e.g. NaAlH3 and Na3AlH6, has a similar effect with the desorption activation enthalpy largely unaffected. These observations suggest that the nature of the bonding of hydrogen to the metal sublattice is not affected, but that the catalyst enhances the transport of hydrogen across the inevitable surface oxides. Thus, while catalysts are part of the solution, they are not likely to be sufficient to increase the amount of hydrogen that can be desorbed in the desired temperature and pressure range.
In order to effectively use research resources, it is necessary to focus on critical issues. Many issues proposed for study, while scientifically interesting, are not on the critical path toward a usable hydride storage material. Since absorption and desorption cycles convert the hydrides into nanophase particles and grains, there is little need for nanophase materials from which hydrides can be formed. Similarly, diffusion of hydrogen is not rate limiting during absorption and desorption. There is, however, a need to understand the role of surface oxides and the effects of catalysts on hydrogen transport across surfaces. Additionally, the exothermic heats of formation of the metal and complex hydrides are large. Since it is desirable to ‘refuel’ in a relatively short time, the heat generated by hydrogen absorption requires careful thermal management. The major issue is controlling the bonding of hydrogen in the hydrides. Development of a suitable hydride system will require adequate and ‘patient’ funding for basic science. While it is always dangerous to predict the course of science, I am not particularly optimistic about our ability to develop a hydride having the desirable properties. Without this there will be no ‘Freedom Car’.
[1] Howard K. Birnbaum is a professor emeritus at the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign.