Borohydride in ionic liquids for tailored hydrogen release

Abstract

H2 as an environmentally benign energy carrier could answer the world's continuously increasing demand for sustainable energy sources, especially in portable fuel cell applications. Thereby, H2 storage is the key issue. In the present work, we developed a liquid H2 storage material based on BH4− and ionic liquids for tailored H2 release under ambient conditions using various catalysts as releasing agents. Thereto, we synthesized four BH4− ionic liquids via ionic exchange. The most promising BH4− based ionic liquids are 1-Ethyl-3-methylimidazolium BH4− and 1-Propyl-3-methylimidazolium BH4−, containing up to 3 wt% releasable hydrogen and the latter being liquid at standard temperature. The hydrolysis of 1-Ethyl-3-methylimidazolium BH4− with various supported metal catalysts (Pt/C, Pt/CNT, Ru/CNT, Pd/CNT, 5 wt% metal) and H2O (8, 16, 24 eq.) was assayed. The highest yield was obtained with 24 eq. H2O and Pt/CNT. The catalysts were characterized via X-ray photoelectron spectroscopy and transmission electron microscopy/energy-dispersive X-ray spectroscopy. To calculate the catalyst activities, the dispersion and size distribution of the metal particles were determined. Recycling of Pt/CNT showed a reasonable stability of the catalyst activity over four recycling runs. In order to avoid spontaneous crystallization of 1-Ethyl-3-methylimidazolium BH4− as a storage material, 1-Propyl-3-methylimidazolium BH4− was introduced. The hydrolysis yield was significantly increased by the use of acidic catalysts, such as HCl and Amberlyst 36. Further enhancement of the H2 yield was achieved by adopting a semi batch process, continuously adding 1 M HCl to 1-Propyl-3-methylimidazolium BH4−. The H2 release correlated approximately linearly with the acid addition rate, excluding liquid-liquid mass transfer limitations during the hydrolysis. Based on 11B NMR analysis, a reaction mechanism of the hydrolysis of BH4− ionic liquid with HCl as catalyst is proposed.