Self-powered electronic sensors and devices are suitable for use in applications such as health monitoring and information collection under battery-free conditions. Thermoelectric (TE) materials can utilize the temperature difference between the body and environment to achieve self-power. In this work, a flexible cellulose-based TE sponge (CP:PP sponge) was prepared via the electrostatic assembly of poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) on cellulose sponges crosslinked with branched polyethylenimine (CP sponge). X-ray photoelectron spectroscopy (XPS) confirmed the adsorption of the PEDOT:PSS onto the CP sponge. The 3D structures, which were composed of thin sheets, typical of cellulose sponges, were maintained within the CP:PP sponges. These CP:PP sponges exhibited reasonable piezoresistive characteristics and excellent flexibility. Upon the application of several press-release cycles, the resistance varied without attenuation. It was demonstrated that the electrical conductivity of the sponge could be enhanced from 2 mS/cm to 6.7 mS/cm via further assembly of the PEDOT:PSS using an immersive layer-by-layer (LbL) strategy, and the thermal conductivity was maintained as 0.0449 W/mK. The maximum figure-of-merit (ZT) value was 1.88x10(-6) at 310 K. A TE generator was fabricated by sandwiching the as-prepared CP: PP sponge, with enhanced electric conductivity and inherent low thermal conductivity, between commercial cotton fabrics. At an ambient temperature of 291 K, the device was shown to generate a voltage of 0.3 mV when one side of the device was attached to forearm skin (307 K). Such CP:PP sponges could potentially be used in artificial intelligence products or remote medical monitoring devices as general, flexible thermal energy harvesting materials.