Thermochemistry of Hydrogen Storage by Mg - a Density Functional Theory Study
Researchers from Accelrys have carried out a study of the thermochemistry of hydrogen storage by Mg, a promising material for mobile applications. They probed the hydrogen storage behavior in the nanometer and subnanometer thickness range using DMol3 , a density functional theory (DFT) module in MS Modeling.
The study revealed that, as the film thickness decreased, so did the enthalpy. It was also found that it is thermochemically advantageous if there is product cohesion during hydrogen desorption.
These findings are important because of the pressing needs in lowering the operating pressure-temperature (P-T) conditions of such promising materials like Mg, that will eventually aid fuel cell-powered mobile application design.
Magnesium is one of the most promising mobile hydrogen storage candidates. Its formation, however, requires high temperatures (300 °C, 1 bar pressure), and is very slow.
To improve this situation, one proposal involved magnesium alloying before hydrogen exposure but, even though reaction times were reduced, reaction thermodynamics were not appreciably improved.
Another approach involved reducing the magnesium particle size (to the the micro or nano size range), thus increasing the particle surface area available for reaction.
It has been reported 1 that reducing the particle size to the nanometer range does not change the thermochemistry. In this paper 2 the researchers question this statement - surely with the finer particles resulting in a greater surface area, the greater the structural modification due to the increasing importance of surface relaxation that happens on the particle surfaces.
Using MS Modeling's DMol3 , the DFT study was aimed at answering the following questions:
Is there a critical particle size at which the thermochemistry will change?
What about the crystal chemistry during the hydrogen absorption/desorption process?
The known value of enthalpy of formation for the bulk materials was reproduced to within error limit. Thin films of Mg and MgH2 of thickness ranging from 1 to 9 unit cell depths (sub-nanometer to low nanometer thickness) were studied. As the film thickness decreased, so did the enthalpy change, to a smallest value of 29 KJ/molH at 2 unit-cell depth (0.6 nm MgH2 ), close to the upper bound of the desired enthalpy change for systems that are suitable for mobile storage. On the other hand, the enthalpy change approaches that of the bulk value (or to within the error limit of experimental observations) as the thin film thickness increased beyond the low nanometer regime.
The results also suggest that it is thermochemically advantageous if there is cohesion of the products during the desorption process.
The results are significant because of the insight into tailoring the thermochemistry as a function of feature size in the nanometer and subnanometer regime and will greatly aid the design of H storage for mobile applications.
1. A. Zaluska, L. Zaluska, and J. O. Stroem-Olsen, J. Appl. Phys. A , 2001, 72, 157; E. Akiba and H. Iba, Intermetallics 6 , 1998, 461; T. Kuriiwa et al. , J. Alloys Compounds, 1999, 295, 433: M. Tsukahara et al. , J. Electrochem. Soc. , 2000, 147 , 2941.
2. Jian-jie Liang, Theoretical insight on tailoring energetics of Mg hydrogen absorption/desorption through nano-engineering, Applied Physics A: Materials Science & Processin, 2005, 80 ( 1 ), 173.
Figure 1 Structure models of MgH2. a) DMol3 optimized 2D repeating unit of 1 unit-cell depth cleaved along the 001 surface; (b) DMol3 optimized 2D repeating unit of 2 unit-cell depth cleaved along the 001 surface; (c) bulk unit cell
Figure 2 Variation of the enthalpy of formation of hydriding Mg thin films. The thin films hypothetically assumes arbitrary thickness (in terms of the number of unit-cell depth) before and after hydration. Solid dots represent data points of comparable film thickness before and after hydration (e.g. 1 unit-cell depth before and after hydration).