*3.3. Effect of Vacuum Pressure on Thermoelectric Performance of the Novel TEP*

Table 1 displayed that the thermal conductivities of the novel TEP tended to rise as the vacuum pressure decreased, resulting from the increase in phase change phenomena of the polymeric nanofluid. The overall increment of the thermal conductivity coefficient was about 10% under the vacuum pressure in the TEP tube decreasing from 760 to 400 torr. The relationships between electric performances and temperatures of the novel TEP at different vacuum pressures are shown in Table 2. The power density increased while reducing the vacuum pressure and increasing the thermal conductivity during the heating processes, and the overall temperature of the internal fluid increased, resulting in increasing the power density. These electric performances were current density and power density based on the electrode reaction surface area of the novel TEP, of 24.76 cm2. Table 2 exhibited that the current and power densities were not greatly affected by vacuum pressure at 25 ◦C. However, with the increase in temperature and the decrease in vacuum pressure of the novel TEP, the current and power densities had an increasing trend. As the vacuum pressure decreased, the boiling point of the solution inside the tube decreased and the power density tended to increase. Since the reduced vacuum pressure was conducive to the formation of positive electrochemical reactions and the reduction of the thermal resistance inside the tube, the temperature of the polymeric nanofluid inside the tube increased, so that the overall current and power densities rose. In addition, more ions were transported to the electrodes through the auxiliary driving forces of the chemical gradient and potential activity. Reducing the vacuum pressure of the tube increased the rate of the redox reaction, thereby improving the electrical performance. Finally, the overall growth rate was respectively about 22% and 28%.


**Table 1.** Thermal conductivities W/(m·K) of the novel TEP under various pressures.

**Table 2.** Electric performances (μA/cm<sup>2</sup> and μW/cm2) of the novel TEP under various pressures.

