By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter
Big Picture – Hydrogen storage
Hydrogen is a useful industrial process gas and holds promise in the alternative energy sector for its use in fuel cells. However, effective utilization of industrially produced hydrogen, such as from steam reforming of natural gas, requires efficient, affordable hydrogen storage of a variety that does not currently exist.
More in-depth – MOF coating of Pd nanocrystals
In the most recent issue of Nature Materials, Guangqin Li and coauthors, predominantly from Hiroshi Kitagawa’s group at Kyoto University, demonstrate greatly enhanced hydrogen storage capacity and speed in palladium by applying a metal-organic framework (MOF) coating to cubic Pd nanoparticles. The copper-containing MOF, HKUST-1, which itself is not a hydrogen-storing material, was shown to increase hydrogen adsorption in the Pd nanoparticles by 74%, while simultaneously increasing the rate at which the hydrogen was adsorbed and desorbed. Additionally the adsorption process was almost fully reversible in the MOF-coated Pd particles, whereas some hydrogen remained irreversibly adsorbed in the bare Pd nanocubes.
The authors offer that MOF coatings are known to increase surface area, porosity, and hydrogen condensation which can all enhance the rate of hydrogen adsorption, and additionally demonstrate in the paper that there was partial electron transfer from the palladium to the copper in HKUST-1. This electron transfer is hypothesized to increase the number of electron holes in the Pd 4d band, thereby accounting for the additional accommodation of hydrogen in the nanocubes.
While the authors acknowledge that the precious metal palladium remains prohibitively expensive to use for hydrogen storage, they hope that the approach of using MOF coatings to enhance hydrogen storage will prove viable with more affordable metal storage systems.
[The figure shows a schematic representation (a) of the MOF-coating/hydrogen adsorption as well as TEM (b,c), HAADF-STEM (d), and STEM-EDX micrographs (e-g) of the MOF-coated Pd nanocubes. Reprinted by permission from Macmillan Publishers Ltd: [Nature Materials] G. Li, et al., Nat. Mater. 13 (8), 802-806, copyright 2014]