|A liquid-phase hydrogen storage material. Credit: ACS, Luo et al. Click to enlarge.|
Researchers at the University of Oregon (UO) have developed a boron-nitrogen-based liquid-phase storage material for hydrogen that works safely at room temperature; is both air- and moisture-stable; releases H2 controllably and cleanly at temperatures below or at the proton exchange membrane fuel cell waste-heat temperature of 80 °C; utilizes catalysts that are cheap and abundant for H2 desorption; features reasonable gravimetric and volumetric storage capacity; and does not undergo a phase change upon H2 desorption.
The development of a liquid-phase hydrogen storage material has the potential to take advantage of the existing liquid-based distribution infrastructure, the team notes in their paper published in the Journal of the American Chemical Society.
The appeal of a safe, liquid-phase hydrogen storage material is clear. The US has a network of over 150,000 miles (244,000 km) of pipeline dedicated to delivering liquid petroleum products, and many nations worldwide have similar networks in place. The transition to a hydrogen-based energy economy will be greatly facilitated if it can take advantage of the existing liquid-based distribution channels such as pipelines, tankers, and retail outlets. Two potential liquid-phase hydrogen storage materials that have received recent attention in the literature are formic acid, HCO2H and hydrous hydrazine, N2H4·H2O. One disadvantage of these compounds is that they have decomposition pathways that potentially generate side products that are toxic to fuel cell catalysts (e.g., CO and NH3) in addition to safety concerns (e.g., for hydrazine).
Liquid organic hydrides (i.e., hydrocarbons) are another class of potential hydrogen carriers, but for carbon-rich systems, the hydrogen liberation step is strongly endothermic, typically requiring reaction temperatures of 350–500 °C, well above the “waste heat” temperature of 80—90 °C provided by a standard proton exchange membrane (PEM) fuel cell.
...we disclose herein the development of BN-methylcyclopentane (1), which is an air- and moisture-stable liquid at room temperature. We report that 1 is capable of releasing 2 equiv of H2 per molecule of 1 (4.7 wt %) both thermally, at temperatures above 150 °C, and catalytically using a variety of cheap and abundant metal halides, at temperatures below 80°C.—Luo et al.
The team, led by team Shih-Yuan Liu, professor of chemistry and researcher in the UO Material Sciences Institute, originally discovered six-membered cyclic amine borane materials that readily trimerize—form a larger desired molecule—with the release of hydrogen. These initial materials, however, were solids. By tweaking the structure, including reducing the ring size from a 6- to a 5-membered ring, the group succeeded in creating a liquid version that has low vapor pressures and does not change its liquid property upon hydrogen release.
Challenges to move this storage platform forward, researchers cautioned, are the needs to increase hydrogen yield and developing a more cost- and energy-efficient regeneration procedure.
Initially, the new platform could be more readily adopted for use in portable fuel cell-powered devices, said Liu, who also is a member of Oregon BEST (Built Environment & Sustainable Technologies Center).
Wei Luo, Patrick G. Campbell, Lev N. Zakharov, and Shih-Yuan Liu (2011) A Single-Component Liquid-Phase Hydrogen Storage Material. Journal of the American Chemical Society doi: 10.1021/ja208834v