Episode 11: Introduction of hydrogen produces eco-friendly thermoelectric oxides

Many industrial processes require heat or create it as a by-product. Now, Takayoshi Katase from the Tokyo Institute of Technology has found a way to harness this heat in an eco-friendly way, as he explains in an interview with MRS Bulletin podcaster Laura Leay. One way to harness this heat is to use thermoelectric devices to produce electricity via the Seebeck effect. Conventional thermoelectric materials, however, are composed of heavy metals such as lead and tellurium, which are toxic. To incorporate hydrogen into the structure, and so replace the toxic elements, Katase’s research team used a rapid thermal sintering process where the starting material—which already includes the hydrogen—is sealed inside a tube. Some of the oxygen sites in strontium titanate are then substituted by the hydrogen. “More than expected, the hydrogen substitution reduces thermal conductivity less than half, and also increases electronic conductivity, resulting in the large enhancement of energy conversion efficiency,” Katase says. This work was published in a recent issue of Advanced Functional Materials. 

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. Sustainability and energy efficiency are big topics that attract a lot of attention. Many industrial processes require heat or create it as a by-product. One novel way of harnessing this heat is to use thermoelectric devices to produce electricity via the Seebeck effect. Here, a temperature difference across the device causes a current to flow. Conventional thermoelectric materials are composed of heavy metals such as lead and tellurium, which are toxic and in some cases, not very abundant.

TAKAYOSHI KATASE: We try to develop ecofriendly thermoelectric oxides with the use of hydrogen. 

LAURA LEAY: That was Takayoshi Katase from the Tokyo Institute of Technology in Japan. To incorporate hydrogen into the structure, and so replace the toxic elements, his team used a rapid thermal sintering process where the starting material—which already includes the hydrogen—is sealed inside a tube. Some of the oxygen sites in strontium titanate are then substituted by the hydrogen. Conventional wisdom indicates that incorporating light elements into the structure should increase the thermal conductivity of the material, which would make it less efficient in generating an electric current. Takayoshi’s team expected the opposite to be true and this is exactly what they saw.

TAKAYOSHI KATASE: More than expected, the hydrogen substitution reduces thermal conductivity less than half, and also increases electronic conductivity, resulting in the large enhancement of energy conversion efficiency.

LAURA LEAY: Although Takayoshi’s team expected the thermal conductivity to reduce, the change was surprisingly large. To explain this result, they reached out to Dr. Terumasa Tadano at Japan’s National Institute for Materials Science to carry out state-of-the-art phonon calculations. The hydrogen was randomly distributed throughout the material and formed weak bonds with the titanium. In contrast, the titanium-oxygen bonds are much stronger. The randomly distributed titanium-hydrogen bonds enhance phonon scattering.   

TAKAYOSHI KATASE: So, even if we use light element, thermal conductivity can be reduced by introducing the inhomogeneity of chemical bonds in the material.

LAURA LEAY: So, hydrogen substitution into strontium titanate can lead to thermoelectric materials that have a lower environmental impact than conventional ones. Not only that, it’s a step in the right direction for extracting useful energy from industrial processes, turning waste heat into something useful and so further reducing humankind’s adverse effects on the natural world. This work was published in a recent issue of Advanced Functional Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.