Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.
In
the past, sodium alanate, NaAlH
4
, has been widely
investigated for its capability to store hydrogen, and its potential
for improving storage properties through nanoconfinement in carbon
scaffolds has been extensively studied. NaAlH
4
has recently
been considered for Li-ion storage as a conversion-type anode in Li-ion
batteries. Here, NaAlH
4
nanoconfined in carbon scaffolds
as an anode material for Li-ion batteries is reported for the first
time. Nanoconfined NaAlH
4
was prepared by melt infiltration
into mesoporous carbon scaffolds. In the first cycle, the electrochemical
reversibility of nanoconfined NaAlH
4
was improved from
around 30 to 70% compared to that of nonconfined NaAlH
4
. Cyclic voltammetry revealed that nanoconfinement alters the conversion
pathway, and operando powder X-ray diffraction showed that the conversion
from NaAlH
4
into Na
3
AlH
6
is favored
over the formation of LiNa
2
AlH
6
. The electrochemical
reactivity of the carbon scaffolds has also been investigated to study
their contribution to the overall capacity of the electrodes.
Metal borohydrides have very high hydrogen densities but their poor thermodynamic and kinetic properties hinder their use as solid hydrogen stores. An interesting approach to improve their functionality is nano-sizing by confinement in mesoporous materials. In this respect, we used the 0.725 LiBH4–0.275 KBH4 eutectic mixture, and by exploiting its very low melting temperature (378 K) it was possible to successfully melt infiltrate the borohydrides in a mesoporous CMK-3 type carbon (pore diameter ~5 nm). The obtained carbon–borohydride composite appears to partially alleviate the irreversibility of the dehydrogenation reaction when compared with the bulk LiBH4-KBH4, and shows a constant hydrogen uptake of 2.5 wt%–3 wt% for at least five absorption–desorption cycles. Moreover, pore infiltration resulted in a drastic decrease of the decomposition temperature (more than 100 K) compared to the bulk eutectic mixture. The increased reversibility and the improved kinetics may be a combined result of several phenomena such as the catalytic action of the carbon surface, the nano-sizing of the borohydride particles or the reduction of irreversible side-reactions.
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