Decreasing the Stability of Borohydride with Ionic Liquids for Hydrogen Storage and Carbon Dioxide Conversion

Abstract

The world needs to move from a fossil fuel-based to a renewable energy-based society. With the increasing electricity production from wind and PV, short and long-term energy storage is becoming one of the main challenges of the 21st century. While batteries are the preferred method for short term storage, a different technology is required for seasonal storage. Hydrogen is a valuable energy carrier, owning its high energy density (122 kJ/g). However, compact and safe storage of H2 is challenging. Complex hydrides are especially attractive for solid-state H2 storage due to their high gravimetric and volumetric hydrogen density. Sodium borohydride (NaBH4) contains more than 10 mass% of H2, but high temperatures are needed to release the H2 (505°C). The work presented in this thesis focused on the combination of borohydride and ionic liquid (IL) cations, in order to modify the stability of borohydride. The materials developed in this thesis are able to release H2 at moderate temperature (< 100°C), thanks to the charge transfer between the IL cation and borohydride, making them interesting candidates for solid-state H2 storage. In addition, I will present the unique reactivity of these IL borohydrides with CO2. Due to the destabilization of the B-H bond, direct CO2 capture and reduction can be achieved under ambient conditions. Up to three CO2 molecules are fixed per borohydride anion, even at low CO2 concentrations. The obtained reaction product can be easily transform to formic acid (HCOOH), or used as formylation agent in organic synthesis. These results represent a promising path for CO2 capture and valorisation.