Dramatically Improved Thermoelectric Properties by Defect Engineering in Cement-Based Composites

In recent years, the problem of overheating in summer has been of great concern. Pavements are continuously exposed to solar radiation, and because of high temperatures, pavement temperatures reach 60 to 70 °C. This potential low-grade heat has been unused. Cement-based composites with thermoelectric properties can convert this low-grade heat to useful electrical energy. The importance of this green technology for generating renewable energy and sustainable development has been widely accepted and noticed. However, the power factor of current cement-based composites is too low, and harvesting low-grade heat on a large scale and at low cost requires improving the thermoelectric properties of cement-based composites. In this paper, we present a method to increase the electrical conductivity of ZnO and thus improve the thermoelectric properties of cement-based composites by defect engineering, obtaining a high power factor of 224 μWm^-1 K^-2 at 70 °C, a record value recently reported for thermoelectric cement-based composites. Zinc oxide powder was treated with a reducing atmosphere to increase the content of oxygen defects and thus improve the electrical conductivity. Pretreated ZnO powder of 5.0 and 10.0 wt % expanded graphite were added to the cement matrix. The ZnO/expanded graphite cement-based composites were made and tested for their thermoelectric properties using a dry pressing process, which exhibited excellent thermoelectric properties. The result showed high conductivity (12.78 S·cm^-1), a high Seebeck coefficient (-419 μV/°C), a high power factor (224 μWm^-1 K^-2), and a high figure of merit value (8.7 × 10^-3), which facilitate future large-scale applications. Using the cement-based composites to lay a road of 1 km in length and 10 m in width, 35.2 kW·h of electricity can be collected in 8 h. This study will inspire how to improve thermoelectric performance of cement-based composites.