Ionic thermoelectric materials: Innovations and challenges

As innovative energy materials, ionic thermoelectric materials (i-TEs) possess a distinctive combination of a high Seebeck coefficient and low thermal conductivity. This unique feature positions them as ideal candidates for efficiently harvesting low-grade heat. This review categorizes i-TEs into thermally diffusive type and thermogalvanic type, with a focus on the former due to its higher Seebeck coefficient. Notably, quasi-solid state materials within this category are explored in the most detailed way for their safety, flexibility, and versatile polymer network design. Strategies to enhance i-TE performance, such as improving the Seebeck coefficient, electrical conductivity, and reducing thermal conductivity, are discussed. Within this context, the proposal of hybridizing electron-type and ion-type materials emerges as an innovative approach aimed at enhancing performance. The future prospects underscore the critical necessity for challenge of establishing a systematic theoretical framework for i-TEs and emphasize the potential of electrode-electrolyte interactions in improving device performance to tackle the challenge of low energy density in diverse applications. In essence, i-TEs play a pivotal role in the effective harvesting of waste heat, making significant contributions to global sustainability and facilitating progress in green energy practices.