Catalytic Hydrogen Release from Borohydride in Ionic Liquids

E. Klindtworth, N. Guntermann, I. Delidovich, R. Palkovits
RWTH Aachen University,

Keywords: borohydride, ionic liquids, hydrogen storage, hydrolysis catalyst


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].