Carbon Matrix Confined Sodium Alanate for Reversible Hydrogen Storage

Abstract

H2 is considered as an attractive fuel for both stationary and mobile applications in future energy scenarios. One of challenges we are facing is how to store H2 in a compact and safe way. Complex light metal hydrides are promising solid-state hydrogen storage candidates, as they have the potential to offer relatively high gravimetric and volumetric energy density, and allow reversible hydrogen storage under moderate pressure and temperature. However, the slow H2 desorption rates and H2 reabsorption rates with dehydrogenation products, and in most case unfavorable thermodynamics make complex metal hydrides unsuitable for practical applications. Sodium alanate (NaAlH4), though has an equilibrium pressure of 1 bar H2 at 30 oC, only starts to release H2 when temperature reaches its melting point (~183 oC). Furthermore, only limited reversibility of desorption can be achieved even with severe conditions ( 10% with 150 bar H2 pressure and 170 oC). One approach to combat the sluggish kinetics and improve reversibility is decreasing the particle size and confining hydrides into pores of a matrix. In this project, we investigated the impact of nanoconfinement of a porous carbon matrix on the H2 release, reversibility of desorption and thermodynamic stability of NaAlH4. Melt infiltration was applied to synthesize nanoconfined NaAlH4 in carbon with NaAlH4 loadings varied from 5 wt% to 40 wt%. It was shown that almost full pore volume of the carbon was filled with 40 wt% NaAlH4. For the loadings up to 20 wt%, NaAlH4 was mainly located in <10 nm pores of carbon and lacked long range. Studies on 20 wt% NaAlH4 with porous and non-porous carbon showed that the H2 desorption profile of NaAlH4 was modified even with the presence of non-porous graphite, making the desorption to NaH take place in a single step instead of two. The onset of H2 release temperature was shifted from ~175 oC for NaAlH4 with non-porous graphite to ~120 oC for NaAlH4 confining in porous carbon. In addition, partial reversibility of desorption from NaAlH4/porous carbon was achieved under mild rehydrogenation conditions (38% reversibility with 24 bar H2 pressure, 150 oC and 3 h), whereas almost no reversibility was found for desorption of NaAlH4 with non-porous graphite. Moreover, it was found that the equilibrium of H2 desorption for NaAlH4 confining in small porous of carbon was altered, leading to a single dehydrogenation step from NaAlH4 to NaH. Further investigation on reversibility showed that the availability of active Na species after first desorption fundamentally hinders the rehydrogenation. The unavailability of Na was mainly caused by the side reactions of Na species and impurity (especially O- containing groups) in carbon support. With addition of extra Na, or alternatively controlling the purity of carbon lead to almost full reversibility of desorption for nanoconfined NaAlH4. This approach is expected to be relevant for other metal hydrides.