Reports: AC5
47684-AC5 Transmission Infrared Studies of Hydrogen Storage Materials
The goal of this project is to use transmission IR spectroscopy to determine details of the mechanism by which hydrogen is released from metal borohydrides upon heating. An understanding of these mechanisms is essential for developing borohydrides as hydrogen storage materials. Among the many formidable obstacles standing in the way of practical hydrogen fuel cell-powered vehicles is the challenge of storing enough hydrogen on board so that performance comparable to today's gasoline-powered vehicles can be achieved. Complex chemical hydrides are seen as one of the most promising ways of storing hydrogen at high enough densities to meet a target of 9.0% hydrogen by system weight. The material used in the storage system must therefore have an even higher hydrogen weight percent, which generally means that only hydrides of elements of low atomic number are likely to meet the challenge.
Boron is second only to carbon in the rich variety of compounds formed with hydrogen, and boron hydrides and related compounds are being intensely studied as potential hydrogen storage materials. Because they are stable and relatively safe to handle, NaBH4, and especially LiBH4 with a hydrogen weight percentage of 18.1, are of particular interest for hydrogen storage applications. A recent report showed that Ca(BH4)2 can be formed from the interaction of H2 gas with solid CaB6, implying that Ca(BH4)2 can serve as a material for the reversible storage of hydrogen according to the following chemical equation,
3Ca(BH4)2 ↔ CaB6 + 2CaH2 + 10H2,
which implies 9.6% of the mass of calcium borohydride is available for reversible release. Accordingly, our research is focusing on the borohydrides of Li, Na, and Ca. Recent research has indicted that stable intermediates containing the B12H122- anion are formed from the thermal decomposition of borohydrides. In the case of LiBH4, the release of hydrogen might then occur according to the following equation:
12LiBH4 ↔ Li2B12H12 + 10LiH + 13H2 ↔ 12LiH + 12B(s) + 18H2(g),
which could limit the total amount of hydrogen released. To ascertain if our IR spectra contain evidence of such an intermediate being formed from annealing of the corresponding borohydrides, we have obtained IR spectra of the K2B12H12 salt at both room temperature and as a function of annealing temperature. No previous such studies of this compound have been reported.
To conduct transmission IR studies of various borohydrides, we constructed an apparatus that allows us to obtain spectra of the solid compounds over a wide range of temperatures under both a low vacuum and under an ambient of gas. We have obtained spectra in the mid-IR range for LiBH4, NaBH4, Ca(BH4)2, and K2B12H12. Each compound displays intense peaks in the B-H stretch region, and for the BH4- compounds there is an intense peak in the BH4 deformation region. It was found that lithium borohydride is quite hygroscopic and so its spectrum displayed features due to water at 3420 and 1635 cm-1, which disappeared upon annealing to 170 °C. Water peaks were not observed for the other compounds. Once the water was removed from LiBH4, its spectrum was quite similar to that of NaBH4, which contained four prominent peaks at 1127 cm-1 due to the asymmetric BH4 deformation fundamental (ν4), the B-H asymmetric stretch fundamental (ν3) at 2291 cm-1, the overtone of ν4 (2ν4) at 2224 cm-1, and a peak at 2385 cm-1 due to a combination of the symmetric deformation mode (ν2) with the ν4 mode. The high intensity of the combination and overtone peaks in the B-H stretch region is presumably due to a Fermi resonance with the intense B-H stretch fundamental. In contrast, the B-H stretch region in the IR spectra of K2B12H12 shows a single intense peak at 2484 cm-1, and a weaker peak due to a B-H bending mode at 1079 cm-1. These are the only two peaks observed in the spectrum. The high symmetry of the icosahedral B12H122- anion accounts for the simplicity of its IR spectrum, with only three IR active fundamentals predicted. A low frequency fundamental that occurs at ~ 720 cm-1 was below the wavenumber range available at the time the spectrum was acquired. Since the B-H stretch of B12H122- occurs at a much higher value than the B-H stretch of BH4-, it should be possible to detect formation of the former if it forms from the decomposition of the latter. Spectra obtained of NaBH4 after annealing to high temperature shows the development of a peak at 2433 cm-1. While this is higher than the peaks in the B-H stretch region of BH4-, it is still below the value for the IR active B-H stretch of B12H122-. Also, there was no clear evidence for development of the 1079 cm-1 peak of B12H122-. Thus, while the results suggest formation of a distinct intermediate that contains B-H bonds, the details are not entirely consistent with formation of an Na2B12H12 intermediate. Somewhat similar results have been obtained for the other borohydrides. Because of the somewhat ambiguous results obtained so far, a variety of approaches are being taken to optimize the quality of the spectra. The spectra are obtained by pressing a mixture of the borohydrides with KBr into a tungsten grid to produce a sample that is translucent to visible light. The sharpness of the peaks depends critically on the MBH4:KBr ratio, with lower ratios giving sharper peaks but weaker signals. Experiments are being conducted to find the optimum ratio. We have also found that KBr is fairly volatile at the elevated temperatures used, which causes evaporation of the sample with deposition onto the IR cell windows. This is less of a problem when CaF2 is used as the background, but it is also harder to obtain clear samples using calcium fluoride. Thus more work is needed to optimize the experimental conditions. Once the optimum conditions are found, the experiments will be repeated and the spectra further analyzed to arrive at conclusions that are fully consistent with all available evidence. When this point is reached, the work will be submitted for publication.
