Hierarchically structured lead-free barium strontium titanate for low-grade thermal energy harvesting

Low-grade heat conversion is one of the most promising strategies to realise carbon-neutral electricity production. Here we present a study on a hierarchically structured porous pyroelectric barium strontium titanate (BST) ceramic with a low Curie temperature and improved thermal energy harvesting performance. The aligned porous structure is beneficial to achieve a greatly reduced permittivity and heat capacity, combined with a high degree of polarisation to maintain a high pyroelectric coefficient. These characteristics contributed to significantly improved sensing and harvesting pyroelectric figures-of-merit, where the thermal energy harvesting figure-of-merit (F _E ^′) increased by 510% when the porosity reached to 54 vol%, compared to that of the dense BST ceramic. After rectification of the AC pyroelectric current to a DC current, the electric output was stored on a 1 μF capacitor whose voltage after 20 s increased with increasing volume fraction of porosity; namely 5.8 V (dense, 3 vol% porosity), 6.9 V (30 vol%), 7.5 V (43 vol%) and 9.2 V (54 vol%). A maximum energy density of 4.26 μJ/mm ³ was obtained from the BST with 54 vol% porosity, which was 1.6 times higher than that of the dense counterpart (2.71 μJ/mm ³). This aligned porous structure therefore represents a new and promising architecture for facile harvesting and conversion of low-grade waste heat into electrical power.