A chemists view: Metal oxides with adaptive structures for thermoelectric applications

Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low-cost materials with reasonable thermoelectric performances and cost-efficient preparation techniques are required. In this context metal oxides are an interesting class of materials because of their inherent high-temperature stability and relative high sustainability. Their thermoelectric performance, however, needs to be improved for wide application. Compounds with adaptive structures are a very interesting class of materials. A slight reduction of early transition metal oxides generates electrons as charge carriers and crystallographic shear planes as structure motif. The crystallographic shear planes lead to a reduction of intrinsic thermal conductivity. At the same time, the electronic transport properties can be tuned by the degree of reduction. So far only a few transition metal oxides with adaptive structures have been investigated with respect to their thermoelectric properties, leaving much room for improvement. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties. The discovery of a large thermopower in cobalt oxides in 1997 leads to a surge of interest in oxides for thermoelectric application. Whereas conversion efficiencies of p-type oxides can compete with those of non-oxide materials, n-type oxides show significantly lower thermoelectric performances. In this context Magnéli oxides have gained attention. A combination of crystallographic shear and intrinsic disorder leads to low thermal conductivities. At the same time, the electronic transport properties can be tuned by the degree of reduction. Current peak-zT values of 0.3 around 1100 K for titanium and tungsten Magnéli oxides are encouraging for future research. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties.