Synthesis, study and optimization of lithium-based materials for clean hydrogen fuel storage.

L. FERNÁNDEZ ALBANESI; F. C. GENNARI

Jujuy

Workshop; IWLiME 2016: 3rd International Workshop on Lithium, Industrial Minerals and Energy; 2016

IWLiME

The human wellness and society economic growth depend largely on the energy supply. This creates a growing worldwide energy demand which is currently met through the use of fossil fuel, with the consequent increase of CO2 emissions into the atmosphere. For this reason, there is a great interest to find sources of clean and renewable energy. In this context, hydrogen is an alternative, as it would store energy generated from an intermittent primary source (such as wind or solar) and then it would be converted to electricity by fuel cells. Thus, hydrogen is a renewable and clean energy vector.To enable the massive use of hydrogen technology, one of the challenges to overcome is the development of a safe and efficient storage medium and transportation. Metal hydrides, or combinations of metal/nonmetal, are promising candidates being able to absorb and storage hydrogen reversibly. The hydrogen storage material has to meet several technological requirements, such as high volumetric and gravimetric capacities (>80 kg.m-3 and 5wt%, respectively); fast hydrogenation/dehydrogenation kinetics (of the order of minutes) at moderate pressure and temperature; high tolerance after several hydrogen cycling; low cost and toxicity. In particular, complex hydrides of light elements such as Li-B-H (borohydrides) and Li-N-H (amides) systems are attractive because they meet the requirements of high gravimetric and volumetric efficiency of hydrogen storage, according the following reversible reactions:2 LiBH4(s) + MgH2(s) ↔ MgB2(s) + 2 LiH + 4 H2(g) (12.5 wt% H) (1)LiNH2(s) + LiH(s) ↔ Li2NH(s) + H2(g) (6.5 wt% H) (2)Unfortunately, the operating temperature for dehydrogenation of 2LiBH4-MgH2 and LiNH2-LiH mixtures is still superior to that estimated from thermodynamic data and the slow sorption kinetics remains a serious limitation to be overcome. Different strategies such as size reduction by ball milling or nanoconfinement, addition of metal catalysts and/or partial substitution of lithium by other metal [1-4], were explored to improve the hydrogen sorption performance of these hydrogen storage systems, keeping the hydrogen storage capacity.As a case study, the effect of AlCl3 addition (0.03, 0.08 and 0.13 mol) on the hydrogen storage properties of the Li-N-H system was investigated [1,2]. Different interactions were identified as a function of the amount of added AlCl3. For low AlCl3 addition (0.03 mol), the Al3+ is incorporated into the LiNH2 structure (represented by LiAlx(NH2)Cl3x) resulting in an expansion of the lattice cell and in a modification of the chemical environment of Li, as it was shown by XRPD and NMR. These structural changes remain in the material after hydrogen cycling, demonstrating the reversible Al3+ incorporation into LiNH2. The representative reactions can be expressed as follows:Milling and then heating under hydrogen pressureLiNH2(s) + x AlCl3 (s) + LiH(s) → LiAlx(NH2)Cl3x(s) + LiH(s) (3)During hydrogen desorption/absorptionLiAlx(NH2)Cl3x(s) + LiH(s) ↔ Li2Alx(NH)Cl3x(s) + H2(g)(4)When an extra amount of AlCl3 is added to LiNH2 (0.08 and 0.13 mol), milling and further thermal treatment under hydrogen lead to the formation of new amide-chloride phases, ascribed as Li-Al-N-H-Cl phases (represented by Li3Alx(NH2)3Cl3x). XRPD studies demonstrate that these phases are isostructural with trigonal/hexagonal and cubic Li4(NH2)3Cl. Supposing that complete AlCl3 incorporation occurs after milling and further thermal treatment, the reaction can be expressed:LiNH2(s) + x/3 AlCl3 (s) + LiH(s) → 1/3 Li3Alx(NH2)3Cl3x + LiH(s) (5)All AlCl3-doped LiNH2-LiH composites exhibited advantageous hydrogen storage performance (Figure 1). The dehydrogenation rate from these composites (Fig. 1A) is three-fold and six-fold faster than for un-doped LiNH2-LiH, depending on the Al content in the sample. The rehydrogenation kinetics (Fig. 1B) is also faster for doped material, conducting to good reversibility. In addition, a notable reduction/elimination of the NH3 emission was observed. The mechanism that enhances the hydrogen sorption properties depends on the structural modifications introduced in the Li-N-H system. For low AlCl3 amount, Al3+ incorporation into LiNH2 structure produces an expansion in the lattice, favoring the mobility of Li+ and H+ ions. The situation for high amount of AlCl3 added to LiNH2 looks different. New phases like amide-chloride and imide-chloride were formed in the hydrogenated and dehydrogenated states, respectively. These new phases promote not only Li+ migration during dehydrogenation kinetics, but also an IR red shift of the N-H bond with respect to LiNH2, indication a weakening of the N-H bonding.