Researchers at TU Delft have found that the addition of extremely small crystals to solid electrolyte material has the potential to considerably raise the efficiency of fuel cells. They have published two papers on their findings in fairly rapid sequence in the journal Advanced Functional Materials: the first in the 9 December 2010 issue, the second in the 24 March issue.
The December paper, with PhD student Lucas Haverkate as lead author, describes the theory behind the results. Fellow PhD student Wing Kee Chan is the main author of the second paper, which focuses on the experimental side of the research.
The researchers at the Faculty of Applied Sciences at TU Delft were concentrating their efforts on improving electrolyte materials for fuel cells or batteries. The electrolyte is usually a liquid, but this has a number of drawbacks. The liquid has to be very well enclosed, for example, and it takes up a relatively large amount of space. While a solid electrolyte could address those issues, however, the conductivity in solid matter is not as good as it is in a liquid.< ?p>
One of the ways of achieving this, and therefore of increasing conductivity in solid electrolytes, is to add nanocrystals (of seven nanometers to around fifty nanometers), of titanium dioxide (TiO2) in this case. TiO2 crystals attract protons, and this creates more space in the network. The nanocrystals are mixed in the electrolyte with a solid acid (CsHSO4). This latter material delivers the protons to the crystals.
Decreasing the dimensions of heterogeneous mixtures of ionic conductors towards the nanoscale results in ionic conduction enhancements, caused by the increased influence of the interfacial space-charge regions. For a composite of TiO2 anatase and solid acid CsHSO4, the strong enhancement of the ionic conductivity at the nanoscale also can be assigned to this space-charge effect. Surprisingly high hydrogen concentrations in the order of 1021 cm-3 in TiO2 are measured, which means that about 10% of the available sites for H+ ions are filled on average.
Such high concentrations require a specific elaboration of the space-charge model that is explicitly performed here, by taking account of the large occupation numbers on the exhaustible sites. It is shown that ionic defects with negative formation enthalpy reach extremely high concentrations near the interfaces and throughout the material. By performing first-principles density functional theory calculations, it is found that proton insertion from CsHSO4 into the TiO2 particles is preferred compared to neutral hydrogen atom insertion and indeed that the formation enthalpy is negative. Moreover, the average proton fractions in TiO2, estimated by the theoretical ionic density profiles, are in good agreement with the experimental observations.
—Haverkate et al.
The addition of the crystals appears to cause an enormous leap in the conductive capacity, up to a factor of 100.
—Lucas Haverkate
Chan carried out measurements on the electrolyte material using the neutron diffraction method. The way in which the neutrons are dispersed makes it possible to deduce certain characteristics of the material, such as the density of protons in the crystals.
The effect on the mobility of the protons is observed directly using quasielastic neutron scattering and nuclear magnetic resonance spectroscopy. Surprisingly large fractions of up to 25% of the hydrogen ions show orders-of-magnitude enhanced mobility in the nanostructured composites of TiO2 or SiO2, both in crystalline CsHSO4 and an amorphous fraction.
—Chan et al.
Haverkate noted that this was the first time that measurements have been taken of solid-material electrolytes in this way, and on such a small scale.
Other material combinations besides the combination of TiO2 and CsHSO4 will be tested that may achieve better scores in the area of stability, the researchers note.
At this stage, we are more concerned about acquiring a fundamental understanding and a useful model, than the concrete issue of finding out what the most suitable material is. It is important that we identify the effect of nanocrystals, and give it a theoretical basis. I think there is great potential for these electrolytes. They also have the extra benefit of continuing to function well over a wide range of temperatures, which is of particular relevance for applying them in fuel cells.
—Professor Fokko Mulder, Haverkate’s and Chan’s PhD supervisor
Resources
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Lucas A. Haverkate, Wing K. Chan, Fokko M. Mulder (2010) Ionic Nanosystems: Large Space-Charge Effects in a Nanostructured Proton Conductor. Advanced Functional Materials doi: 10.1002/adfm.201090105
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Wing K. Chan, Lucas A. Haverkate, Wouter J.H. Borghols, Marnix Wagemaker, Stephen J. Picken, Ernst R.H. van Eck, Arno P.M. Kentgens, Mark R. Johnson, Gordon J. Kearley, Fokko M. Mulder (2011) Direct View on Nanoionic Proton Mobility. Advanced Functional Materials doi: 10.1002/adfm.201001933