E. Klindtworth, N. Guntermann, I. Delidovich, R. Palkovits
RWTH Aachen University,
Germany
Keywords: borohydride, ionic liquids, hydrogen storage, hydrolysis catalyst
Summary:
The world's continuously increasing demand for sustainable energy sources calls for the development of alternative energy systems, such as H2, an environmentally benign energy carrier [1]. H2 can be produced via water electrolysis, using renewable energy sources. After temporary storage of H2, it is fed to a fuel cell, recovering the retained energy, e.g. to charge a phone [2]. The H2 lifecycle is illustrated in Fig. 1. Particularly for portable fuel cell applications, the H2 storage is a key issue [3]. Current H2 storage technologies include high pressure tanks, cryo storage or chemically bound H2, such as solid LiBH4 or NaBH4 with a H2 capacity of 18.4 wt% and 10.6 wt% H, respectively [4, 5]. BH4- hydrolyzes when in contact with H2O, forming H2 and the corresponding borate (Eq. (1)) [6, 7]. BH4- (aq) + 4 H2O (l) → B(OH)4- (aq) + 4 H2 (g) (1) For the introduction of BH4- in portable electronic applications, a hydrogen storage medium with a melting point below room temperature (RT) and the tailored catalytic H2 release is essential. In the present study, we generate H2 from liquid storage materials based on BH4- in ionic liquids (ILs) via hydrolysis under ambient conditions, using numerous catalysts as releasing agents. For this purpose, various BH4- ionic compounds were synthesized (Fig. 2). Our findings indicate that EMIM BH4- 2 and PMIM BH4- 3 with a hydrogen storage capacity of max. 3 wt% H are promising storage media [1]. Subsequently, the BH4- ILs were hydrolyzed with H2O, applying supported metal as well as acidic catalysts. All hydrolyses of EMIM BH4- and PMIM BH4- with metal/acid catalysts revealed saturation curves, whereas highest H2 yields were observed in a semi-batch process with continuous acid addition. Additionally, mass transport limitations as well as activation energies of the PMIM BH4- hydrolysis with Cox/support catalysts were investigated. A mechanism for the hydrolysis of PMIM BH4- is proposed based on the 11B NMR analysis of the H+ catalyzed hydrolysis of BH4- at various reaction times [1].