The investigation of nanoscale complex hydrides for hydrogen storage application has gained the spotlight in the past decade. Herein, the thermodynamic behavior of complex borohydrides confined in mesoporous hollow carbon spheres is investigated. The pressure−composition−temperature (PCT) shows no variation of equilibrium plateau pressure upon changes in the hydrogen sorption temperatures. This is interpreted as a result of the high pressure within the carbon nanopores and provides a means to control the thermodynamics of nanosized hydrides.
With the renewed interest for hydrogen as an energy carrier, means to produce, but most importantly store, transport, and distribute, "green" hydrogen over long distances has become important. In this context, liquid organic molecules that can be hydrogenated and dehydrogenated under mild conditions of temperature and pressure continue to attract significant attention. These liquid organic molecules referred as "liquid organic hydrides" in the early 1980s include molecules such as cyclohexane, methylcyclohexane, decalin, N-heterocycles, methanol, ethanol, and formic acid. However, current liquid organic hydrides still suffer from limitations along with the emergence of more effective catalysts to meet the requirement of competing (de)hydrogenation reactions, as well as new chemistry to enable their (de)hydrogenation reactions under milder conditions and extended cycle lives. Herein, we critically review common state-ofthe-art catalyst designs, which remain one of the main barriers to the effective emergence of liquid organic hydrogen carriers enabling the widespread transport and distribution of hydrogen. Many of the most effective current catalysts are based on noble metals. Transitioning away from these rare critical elements to enable hydrogen uptake/release from organic compounds under economically viable chemical routes is a necessity.
Catalysis is at the core of previous energy transition. It has enabled the use of oil and natural gas as our primary energy sources in unprecedented ways and led to feedstocks enabling exceptionally high living standards in human history. In a decarbonized economy with hydrogen as the new energy vector, catalysis is already playing a key role in producing hydrogen. However, catalysts for the effective storage of hydrogen must be advanced. Many solid hydrogen storage materials such as magnesium‐based hydrides, alanates, and/or borohydrides display promising hydrogen densities far superior to the current state of compressed or liquid hydrogen. These solid materials have thermodynamic and kinetic barriers which severely hinder their practical hydrogen uptake and release. To date, most of these barriers for solid hydrides (especially boron or nitrogen compounds) are modified via catalysis; however, the catalytic species per se and their roles are obscure. Herein, a comprehensive overview of various catalysts for solid hydrogen storage materials, their catalytic roles, and the underpinning mechanisms is provided. The current state of knowledge is critically reviewed and gaps where further research intensification is needed to support rapid hydrogen generation and storage in solid materials for the emerging hydrogen economy are identified.
Lithium aluminum hydride (LiAlH4) is considered
as a
promising hydrogen storage material due to its high gravimetric hydrogen
storage density. However, sluggish hydrogen kinetics and poor reversibility
have prevented its use in practical applications. Improvements of
the hydrogen properties of LiAlH4 have been proposed through
a nanostructuring approach of the material. Herein, we developed a
method to encapsulate freestanding LiAlH4 nanoparticles
within a Ti shell upon reduction of TiCl3 at their surface.
The LiAlH4@Ti core–shell nanostructures obtained
through precise control in the reduction kinetics and the concentration
of the Ti precursor led to a significant improvement in the dehydrogenation
temperatures and desorption kinetics. Indeed, 5.8 mass % of hydrogen
was released within 25 min at 150 °C from LiAlH4@Ti.
More remarkably, the formation of the core–shell structure
led to the disappearance of the well known exothermic decomposition
path of LiAlH4, which evidenced the possibility of altering
the hydrogen thermodynamics of LiAlH4. Partial hydrogen
reversibility was observed at 150 °C and a 10 MPa hydrogen pressure,
mainly because of the loss of the core–shell structure upon
hydrogen cycling. However, this demonstrates that with finer control
over the nanostructuring of LiAlH4, practical hydrogen
storage conditions are within reach.
LiAlH4 and NaAlH4 are considered to be promising hydrogen storage materials due to their high hydrogen density. However, their practical use is hampered by the lack of hydrogen reversibility along with poor kinetics. Nanosizing is an effective strategy to enable hydrogen reversibility under practical conditions. However, this has remained elusive as the synthesis of alanate nanoparticles has not been explored. Herein, a simple solvent evaporation method is demonstrated to assemble alanate nanoparticles with the use of surfactants as a stabilizer. More importantly, the roles of the surfactants in enabling control over particle size and morphology was determined. Surfactants with long linear carbon chains and matching the hard character of alanates are more prone to lead to the formation of small particles of ~10 nm due to steric hindrance. This can result in significant shifts in the temperature for hydrogen release.
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