Researchers Show Carbon Nanostructures Can Function as Catalysts for Solid-State Hydrogen Storage

15 March 2009

Screening study results of NaAlH4/carbon mixtures. Sample key: (a) 8 nm CNT, (b) 10-20 nm CNT, (c) 10-20 nm CNT with 4 mol % Ti, (d) 50 nm CNT, (e) graphite, (f) C60[1] (g) C60[2] (h) C60[3], (i) control no carbon, ball mill 4 mol % TiCl3, and (j) control no carbon or Ti. Two pressures used for the rehydriding step (which affects the amount of hydrogen desorbed in the second cycle) are highlighted by color: high pressure experiments are blue; lower pressure experiments are red. Credit: ACS. Click to enlarge.

Researchers from the US and Sweden have shown that carbon nanostructures (fullerenes, nanotubes, and graphene) can be used as catalysts for hydrogen uptake and release in complex metal hydrides such as sodium alanate (NaAlH₄) and also developed what they characterize as an “unambiguous understanding” of how such catalysts work.

The researchers from Savannah River National Laboratory and Virginia Commonwealth University in the US and Uppsala University and the Royal Institute of Technology in Sweden set out to understand the mechanism behind the catalytic effects of carbon nanomaterials, specifically on the example of sodium alanate, which is a popular material for hydrogen storage studies. The results of their work, which combined experimental and theoretical efforts, were published online 3 March in the ACS journal Nano Letters.

Solid-state hydrogen storage materials, such as such as alanates and borohydrides, are under investigation as promising media for use in on-board hydrogen storage for vehicles. Among these, the researchers noted, sodium aluminum hydride (NaAlH₄) is the most widely studied material. However, the functional properties of these materials have to be improved by catalysts. The effect of earlier catalysts, such as Ti, has been difficult to explain.

Several other research groups have reported that carbon materials can function as catalysts for the dehydrogenation and rehydrogenation of NaAlH₄. However, most of those samples were ball milled, and ball milling is known to degrade fragile carbon nanostructures and to introduce Fe contamination from the ball mill vial and/or balls. (Fe is an excellent catalyst for dehydrogenation of NaAlH₄.) The researchers in this new effort used a solvent preparation technique to mix the NaAlH₄ and carbon without introducing metal contaminants.

We have shown that carbon nanostructures, traditionally thought of as hydrogen storage materials, can in fact be used as catalysts for hydrogenation/dehydrogenation of sodium alanate, NaAlH₄. C₆₀ materials were found to be the best carbon additive for NaAlH₄, rehydriding NaAlH₄ by 4.3 wt% over 8 h. We suspect that a contributing factor to the performance of the C₆₀ is its dispersibility. The C₆₀ molecules likely have their entire surface available for interaction with NaAlH₄. Graphite particles will only have the particle faces exposed to NaAlH₄, and CNTs are known to agglomerate and likely are bundled together, lowering the surface available to NaAlH₄. Experiments exploring the catalytic effect of carbon nanomaterials on other complex metal hydrides, for example, borohydrides, are planned.

Using density functional theory and generalized gradient approximation for exchange and correlation, we have calculated the electron affinity of several carbon substrates and have shown how this is intimately connected with the hydrogen sorption mechanism. In addition, the calculated electron affinities appear to reflect the curvature of the nanostructures, by increasing with increasing curvature. The larger the electron affinity of the substrate is, the greater the probability of Na donating its electron to the substrate. The substrate was found to have a dramatic effect on the hydrogen removal energy. Experimentally we see that the curvature of the nanostructures plays a significant role in this process and C₆₀ fullerene is a better catalyst than the nanotubes. Theory further shows that due to its very high curvature (5,0) CNT may even be a better catalyst.

—Berseth et al. (2009)

The extensive simulations were performed at Uppsala University’s Multidisciplinary Center for Advanced Computational Science (UPPMAX).

Now that the catalytic capabilities of carbon nanomaterials have been demonstrated so clearly and the mechanism that makes this behaviour possible has been understood, we expect a strong impulse on putting this effect to use in practical applications. Certainly, our findings have the strongest impact in the field of hydrogen storage, but beyond that, the same mechanism that we revealed can make carbon nanomaterials a very important catalyst in many other systems as well.

—"Professor Rajeev Ahuja, Uppsala University

Resources

• Polly A. Berseth et al. (2009) Carbon Nanomaterials as Catalysts for Hydrogen Uptake and Release in NaAlH₄. Nano Lett., Article ASAP doi: 10.1021/nl803498e