(415d) Hydrogen Storage in Small PtPd Alloy Nanoparticles: A DFT Study

Metal hydrides are promising hydrogen storage materials as they can store large amounts of hydrogen in a small space (volumetric densities >100 kg H₂ m^-3). Transition metal hydrides are one of the few systems that exhibit the ability to reversibly absorb hydrogen near ambient conditions.^1,2 However, they are heavy and expensive. One possible way to reduce the amount of metal needed and/or increase the amount of hydrogen stored is to use smaller nanoparticles. Surprisingly however, Pd exhibits very different behavior compared to Pt when nanosized. While the hydrogen storage capacity of Pd decreases with decreasing size,³ that of Pt in fact increases.⁴ Insights on the mechanisms by which hydrogen is stored in metals are needed to help us understand the effects of nanosizing on their capacity.

To gain insights into the mechanisms by which hydrogen is stored in small nanoparticles, we performed density functional theory (DFT) calculations to study: i) the effect of the metal, ii) the effect of core-shell morphology, and iii) the effects of alloyed surfaces. At realistic hydrogen pressures, high coverages of hydrogen are expected on the nanoparticles. We thus studied the adsorption of multiple hydrogens on surface as well as subsurface sites. The most stable configurations of the different hydrogen coverages on the nanoparticle were obtained by an iterative process involving fitting of a Cluster Expansion (CE) Hamiltonian,^5,6 which parameterizes the DFT energies we calculated in terms of point and multibody interactions, and employing the CE Hamiltonian in a simulated annealing procedure to locate stable minima.

Our results show that surface sites can contribute significantly to the hydrogen storage capacity in small nanoparticles. This is as opposed to the commonly-held belief that hydrogen is only stored in the bulk.^3,4 The insights we have gained will serve as a guide for designing higher capacity, less costly, hydrogen storage materials in the nanosized regime.

References

1 L. Schlapbach and A. Züttel, Nature, 2001, 414, 353–358.

2 A. Züttel, Mater. Today, 2003, 6, 24–33.

3 M. Yamauchi, R. Ikeda, H. Kitagawa and M. Takata, J. Phys. Chem. C, 2008, 112, 3294–3299.

4 M. Yamauchi, H. Kobayashi and H. Kitagawa, ChemPhysChem, 2009, 10, 2566–2576.

5 J. M. Sanchez and D. De Fontaine, in Structure and Bonding in Crystals, eds. A. Nsvrotsky and M. O’Keeffe, Academic Press, Inc, New York, 1981, vol. 2, p. 117.

6 D. J. Schmidt, W. Chen, C. Wolverton and W. F. Schneider, J. Chem. Theory Comput., 2012, 8, 264–273.