Pyroelectric heat harvesting, what’s next?

Harvesting all-present environmental waste heat of decentralized, disordered, and diffused forms promises energy sustainability and carbon neutrality to meet the UN’s climate target [1]. The non-static waste heat or temporal temperature change (dT/dt), which is of equal importance as the spatial temperature gradient (dT/dx) [2], though commonly existed in the surroundings (e.g., human respiration, water vapours, and exhaust pipes; Fig. 1 and Table S1) according to the second law of thermodynamics, is still far from practice due to inefficiency, intricacy, and instability in powering consumer electronics [3,4]. Providentially, the pyroelectric effect allows for scavenging temporal temperature variations via spontaneous polarization change, making it an attractive approach for direct heat-to-electricity conversion from non-static thermal sources. Pyroelectricity is typically determined by p = ∂P/∂T (P = PS + PE, PE = εE) [5,6], where p is the pyroelectric coefficient, PS and PE are spontaneous and electric polarizations with respect to the applied thermal field and electric field (E), respectively, and ε is the dielectric constant. While tremendous efforts have been made to improve the p of polar materials (up to ∼ 10 mC m−2 °C−1) [7] and the power density of heat harvesters (up to ∼ 10 mW m−2) [8–10] over the past 50 years (Table S2), their large intrinsic impedance (in the level of MΩ) and low energy conversion efficiency (0.1‰) [11] have hindered the potential implications in the sustainable power supply of ever-increasing IoT-based electronics demands. In this short review, we first discuss the fundamental of electric polarization manipulation of typical polar materials for boosting p. Then, the state-of-the-art p versus Curie temperature (TCurie) of various pyroelectric materials is benchmarked. Next, paradigm-changing progress in tailoring the material properties and device configurations, as well as external electric/thermal field modulations, is surveyed. Finally, the review concludes by proposing challenges and opportunities for the next sustainable pyroelectric heat harvesting.