Materials for Hydrogen fuel cells
Overall Nature and Background of the Project
The desire to reduce the use of traditional fossil fuels has sparked great initiatives into alternative ways to produce, store and utilize energy that are more environmentally. While looking into all three aspects of alternative energy is important, new ways to store energy is vital. If we cannot store the energy produced by sources such as wind, solar, biomass etc., we waste the full potential from the production of cleaner sources of energy.
Hydrogen fuel cells have the best known potential for energy storage and delivery for both large and small applications. Hydrogen has the highest energy output by weight outside of nuclear reactions and nearly three times that of gasoline. Hydrogen is abundant and can be chemically extracted from a large variety of sources. The problem with hydrogen is that naturally it is a gas with a very low density, only 0.09 grams per liter which would correspond to only 0.003 kilowatt hours per liter (kW hr/l) energy density. In order for hydrogen with its natural density to match the energy production of gasoline, you would need 3000 gallons of hydrogen gas to match 1 gallon of gas. Hydrogen can be compressed to increase the density but a standard gas cylinder only reduces the ratio of gallons of hydrogen to gallons of gasoline down to 22.5 to 1. The hydrogen cars available use a compressed hydrogen gas but the tank required (the red tank in figure 1) to travel reasonable distances occupies most of the trunk space. Liquefaction of hydrogen would increase the density even more but the cryogenics required to prevent the hydrogen from boiling makes liquefaction too expensive
While compressed or liquefied hydrogen can be a short-term solution in developing hydrogen as an economically viable energy source, the real challenge is finding materials that can pack more hydrogen into a smaller volume thus increasing the density of available hydrogen. Materials with a high hydrogen density would eliminate the need for large pressurized tanks which have some safety concerns. With hydrogen dense materials, hydrogen fuel cells would not only for be used for transportation purposes but also feasible for more everyday household uses such as small electronics, household appliances, or supplementation for more intermittent energy sources such as wind or solar. The materials used for hydrogen storage should be stable and economically viable in its production, processing and disposal. Most importantly, these storage materials should reliably store and release hydrogen on demand. This project will focus on the synthesis of a set of materials that show promise for high density storage of hydrogen.
The materials studied in this project are the layered transition metal dichalcogenides (TMDC). These materials contain a metal with sulfur, selenium or tellurium (together are known as the chalcogens) in a 1:2 mole ratio. These materials are an attractive target due to a layered or sandwich like nature. The structure of these materials has the metal atoms sandwiched between the two chalcogen layers (see figure 2). The MX2 sandwich layers stack on top of each other leaving a gap between the layers. The MX2 layers have a weak van der Waals attraction to each other which allows for other chemical species such as hydrogen molecules, H2, to insert between the layers.
The layered nature of the transition metal dichalcogenides makes them also attractive candidates for hydrogen storage. The seven metals best suited for these layered materials are highlighted on the periodic table shown in figure 3. Our preliminary investigation into these materials indicates that hydrogen will not stay between the layers without high pressure but will store in the layers if there are other metal atoms (intercalates) dispersed between the layers along with the hydrogen.
Some of our preliminary data indicates that a small addition of small amount of another metal such as manganese enhances the storage of hydrogen in TMDC. For example, titanium disulfide with some added manganese, will store up to 1% hydrogen by mass. The energy density of this material with just 1% hydrogen by mass is enough for small energy applications such as electronics.
The titanium sulfide example is just one of the possible combinations we can explore for these compounds. This project will focus on the synthesis of several combinations containing one of the seven highlighted metals in figure 3 with sulfur, selenium or tellurium. Our preliminary hydrogen storage data suggests that higher levels of intercalate leads to higher storage of hydrogen in these materials. For any base material combination, the limit of intercalate that can be added to the material without changing the layered nature will need to be determined. Those materials with high intercalate levels would later be studied for their hydrogen storage capabilities.