Entering new era of thermoelectric oxide ceramics with high power factor through designing grain boundaries

Thermoelectric oxide ceramics could play a significant role in waste heat recovery using thermoelectric generators, accelerating clean energy generation and achieving net-zero emissions. In 2003, the thermoelectric performance of single-crystal Ca3Co4O9+δ was evaluated with a dimensionless thermoelectric figure-of-merit (ZT) of 0.87. Polycrystalline Ca3Co4O9+δ ceramics are typically only about 30% as efficient as single crystals since they have low electrical conductivity and low Seebeck coefficient. To improve the thermoelectric performance of polycrystalline Ca3Co4O9+δ, a unique approach was developed to drive dopants segregation at the grain boundaries to dramatically increase the Seebeck coefficient and electrical conductivity and total energy conversion efficiency of the oxide. This review exploited our pertinent results from five sets of dopants to elucidate that the grain boundary can be engineered to reverse their detrimental impact on electrical properties and provide the design domain to improve the electrical transport properties significantly. The approach to engineer the grain boundary can be used for the selection of dopants with the appropriate size that will ultimately result in oxide ceramics outperforming single-crystals. The present review unveils the atomic structure origin of the dopant segregation at grain boundaries and presents a feasible and valuable approach for treating the grain boundaries as a two-dimensional intergranular secondary phase complexion that is with magnitudes higher Seebeck coefficient than that of the intragrains. Such intergranular secondary phase complexion is independently tunable to decouple the strongly correlated physical parameters and simultaneously enhance the Seebeck coefficient, electrical power factor, and ZT over a broad temperature range.