Flexible Thermoelectric Paper and Its Thermoelectric Generator from Bacterial Cellulose/Ag₂Se Nanocomposites

In this research, a flexible thermoelectric (TE) paper was fabricated from bacterial cellulose/silver selenide (BC/Ag2Se) nanocomposites. Ag2Se particles were in situ synthesized in the network of BC nanofibers. Several synthesis parameters that crucially affect the formation of Ag2Se particles in the BC structure were investigated to understand the phase formation mechanism. Under the optimized conditions, the BC/Ag2Se paper with a large proportion of Ag2Se up to 75 wt % was successfully obtained. The in situ synthesis limits the Ag2Se formation within the nanopores of the BC structure. As a result, the submicrosize Ag2Se particles with a narrow size distribution were homogeneously dispersed in the BC nanofiber network. The microstructure was further improved by hot-pressing, which increases the density of the BC/Ag2Se paper and makes the BC layered structure more compact. These contributed to a significant enhancement of the TE properties, with the electrical conductivity of 23,000 S/m and the Seebeck coefficient of −167 μV/K at 400 K. The power factor was 642 μW/mK2 at 400 K, a very high value compared to other flexible TE research. The measurement of thermal conductivity yielded the κ value of 0.36 W/mK at 400 K, which led to the maximum ZT of 0.70 at 400 K. To demonstrate the TE conversion, five BC/Ag2Se paper pieces were connected in series to construct a TE module. The module is very flexible and can be curved to attach to any arbitrary shape of the hot/cold surfaces. In addition, the process for fabricating the BC/Ag2Se paper is scalable without any use of advanced or expensive instruments. This makes it a very attractive choice as a flexible TE generator.