The use of complex metal hydrides such as NaAlH4 is a promising strategy for hydrogen storage, but their application is limited by slow de/rehydrogenation kinetics. Quasicrystals (QCs), in particular Al65Cu20Fe15, exhibit remarkable catalytic effects in accelerating these processes. Understanding the mechanism of this effect is critical to optimizing catalytic properties and developing more efficient hydrogen storage materials.
A key aspect of the catalytic action of CCs is their unique atomic structure, characterized by long-range order without translational periodicity. This structure creates multiple active sites on the surface that facilitate hydrogen adsorption and dissociation. Studies show that transition metal atoms such as Fe play an important role in hydrogen adsorption, while Al atoms provide a platform for the dissociation of H2 molecules.
The hydrogen storage system presented under the number 65 is still in the prototype stage. In this paper, the effect of Al4-complex hydride NaAlHCu20Fe15 quasicrystal (QC) as a catalyst on the dehydrogenation and rehydrogenation processes of NaAlH4 is investigated. The NaAlH4 sample in which the catalyst is Al65Cu20Fe15 was ball milled and leached. NaAlH4-LBMACF showed rapid hydrogen uptake (about 3.20 wt%) within a minute, reaching a maximum level (about 4.68 wt%) in 15 minutes. At the same time, NaAlH4-LACF, NaAlH4-BMACF, NaAlH4-ACF and pure NaAlH4 absorbed significantly less: 0.50 wt.%, 1.38 wt.%, 1.10 wt.% and 0.70 wt.%, respectively, over the same period at a temperature of 130 °C and a hydrogen pressure of 100 atm. NaAlH4-LBMACF releases about 4.22 wt.% hydrogen in 15 minutes, while NaAlH4-LACF requires 45 minutes to desorb a similar volume at 130 °C and a pressure of 1 atm. NaAlH4-LBMACF maintains reversibility for up to 25 cycles with an insignificant decrease in hydrogen storage capacity (about 0.06 wt.%) during dehydrogenation and rehydrogenation. The catalytic mechanism and effect of Al–Cu–Fe on NaAlH4 were studied using structural and microstructural analysis including in situ NMR spectroscopy, in situ Raman spectroscopy and X-ray photoelectron spectroscopy.
With growing concerns about global issues including fossil fuel shortages, climate change and environmental pollution, sustainable development has become a key principle for achieving human progress. To ensure sustainability, it is necessary to use clean and renewable energy sources such as solar, wind, geothermal, hydrogen and others. Hydrogen, as the most abundant element in the universe, is a promising energy source due to its purity, renewable nature, availability and high energy density. However, to create a full-fledged hydrogen economy, it is necessary to solve technical problems in the production, storage and use of hydrogen. Hydrogen storage remains a serious challenge due to its low density. To increase the density, it is necessary to compress the gas, cool it to extremely low temperatures or reduce repulsion by interacting with other materials. These processes are expensive and risky, so the development of safe and efficient solid-state materials for hydrogen storage is an important task. The US Department of Energy has identified targets for the development of such materials, including a system gravimetric density of 5.5 wt%, a bulk density of 1.3 kWh/L, a system fill time (5 kg) of 3.3 minutes, a service life greater than 1500 cycles, and an operating temperature below 60 °C. NaAlH4 is one material that can meet these requirements.
The microstructure of the CC surface, including the presence of steps and defects, also affects the catalytic activity. These defects can serve as centers for nucleation of the hydride phase during rehydrogenation, thereby accelerating the process. An important role is played by the electronic structure of the CC, which can be tuned by changing the alloy composition. The correct choice of composition allows optimizing the energy of hydrogen binding on the CC surface, which is necessary for effective catalysis.
Modeling by the methods of density functional theory (DFT) confirms the experimental data and allows to study in more detail the mechanism of de/rehydrogenation on the surface of CC. These calculations show that the dissociation of H2 on the surface of Al65Cu20Fe15 occurs with a relatively low energy barrier, which explains the high catalytic activity of this material.
In conclusion, the excellent catalytic effect of Al65Cu20Fe15 quasicrystal in de/rehydrogenation of NaAlH4 is due to the combination of its unique atomic structure, surface microstructure and electronic structure. Further research aimed at optimizing these parameters will allow us to create even more efficient hydrogen storage catalysts.
Author: Satish Kumar Verma, Ashish Bhatnagar, Mohammad Abu Shaz, Thakur Prasad Yadav
Institute:
Center for Hydrogen Energy, Department of Physics, Banaras University, Varanasi 221005, India
Department of Physics, Materials Science and Engineering, Jaypee Institute of Information Technology, Noida, 201307, India
Department of Physics, Faculty of Science, University of Allahabad, Prayagraj, 211002, India