Borohydride in ionic liquids for tailored hydrogen release

H-2 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, H-2 storage is the key issue. In the present work, we developed a liquid H-2 storage material based on BH4- and ionic liquids for tailored H-2 release under ambient conditions using various catalysts as releasing agents. Thereby, 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-Ethy1-3-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 H-2 yield was achieved by adopting a semi batch process, continuously adding 1 M HCl to 1-Propyl-3-methylimidazolium BH4-. The H-2 release correlated approximately linearly with the acid addition rate, excluding liquid-liquid mass transfer limitations during the hydrolysis. Based on B-11 NMR analysis, a reaction mechanism of the hydrolysis of BH4- ionic liquid with HCl as catalyst is proposed. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.