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Towards more efficient and eco-friendly thermoelectric
oxides with hydrogen substitution
April 18, 2023
by Tokyo
Institute of Technology
Credit: Tokyo Tech
Today, over half of the total energy produced
from fossil fuels is discarded as waste heat, which accelerates global
warming. If we could convert the waste heat into a more useful form of
energy like electricity, we could minimize fuel consumption and reduce
our carbon footprint. In this regard, thermoelectric energy conversion
has gained momentum as a technology for generating electricity from
waste heat.
For efficient conversion, a thermoelectric material must have a high
conversion efficiency (ZT). So far, realizing a high ZT has been
possible only with the use of heavy elements like lead, bismuth, and
tellurium. However, the use of rare, expensive, and environmentally
toxic elements such as these has limited the large-scale application
of thermoelectric energy conversion.
To tackle these issues, transition metal oxides based on platforms
such as SrTiO3 have emerged as a more inexpensive and benign
alternative. However, their ZT is typically limited by a high thermal
conductivity (κ), since for a high κ, the temperature across the
material becomes uniform more quickly, and the lowered temperature
difference—the driving force behind the thermoelectric
conversion—causes electric power generation to decrease as well.
Against this backdrop, a research team including Associate Professor
Takayoshi Katase from Tokyo Institute of Technology (Tokyo Tech),
Japan recently discovered a new approach to reducing κ and boosting
the performance of SrTiO3 by hydrogen substitution.
Conventionally, the use of light elements is expected to increase the
κ originating from lattice vibration (κlat), leading to the adoption
of heavy elements to reduce the κlat. In contrast, in their study
published in Advanced Functional Materials, the team discovered that
the κlat of SrTiO3 could be reduced to less than half its original
value by substituting a light element, namely hydrogen.
They clarified the mechanism underlying their observation using
first-principles calculations, which showed that substituting a
portion of the oxygen anions (O−) with hydrogen anions (H−), yielding
compounds of the form SrTiO3−xHx, results in a mixture comprising a
strong Ti-O bond and a weak Ti-H bond. These randomly distributed Ti-(O,H)
bonds, in turn, largely decrease κlat.
The team also found that SrTiO3−xHx polycrystals exhibit high electron
mobility comparable to that of single-crystal materials without any
deterioration in electron conduction across grain boundaries. Based on
these two effects, a low thermal conductivity along with a high
electrical output power are realized at the same time, resulting in an
improved thermoelectric conversion efficiency in the SrTiO3−xHx
polycrystal.
Overall, these findings can open doors to innovative strategies for
developing next-generation thermoelectric materials. "In future, the
hydrogen substitution approach would realize excellent environmentally
benign thermoelectric materials that do not require the use of heavy
elements," concludes Dr. Katase.
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