2.3 Comprehensive Evaluation of Photo-thermo-electric Conversion of the PTEH-Interlocking
As shown in Figure 4a , the oxidation reaction ([Fe(CN)6]3− − e → [Fe(CN)6]4−) occurs at the hot side and the reduction reaction ([Fe(CN)6]4− + e → [Fe(CN)6]3−) occurs at the cold side in the thermoelectric hydrogel. 42, 43 We designed a devices with hot side and cold side to estimate the voltage output as a function of temperature difference (ΔT) in the thermoelectric cell (Figure S6 ). The voltage of the PTEH-Interlocking cells increases from 7.14 mV to 34.79 mV as ΔT increased from 5°C to 25°C (Figure 4b ). While the voltages of TEH is 6.47, 12.51, 18.80, 25.42 and 31.66 mV with ΔT of 5, 10, 15, 20 and 25°C (Figure 4c ). That Seebeck coefficient of PTEH-Interlocking is -1.40 mV K-1, which is higher than that of TEH (-1.26 mV K-1) (Figure 4d ). The maximum power density of PTEH-Interlocking cells increases from 0.799 to 4.341 mW m−2 as the ΔT rises from 10°C to 30°C (Figure 4e ). We investigated the influence of different levels of [Fe(CN)6]4− oxidation on the voltage in the PTEH-Interlocking cell under constant simulated sunlight illumination (100 mW cm−2). As shown inFigure 4f , the cell voltage reaches to a maximum of 6.92 mV as the dosage of PA increases to 0.1 mol L-1. Although the color of the PA-PEI-Fe photothermal film gradually deepens as dosage of PA increases with an higher efficient photothermal conversion (Figure S5 ), more dosage of PA results in a slight decrease of cell voltage because of more consumption of [Fe(CN)6]3−. After stretching to a strain of 100% for ten times and continuous stretching to a strain of 400% for eight times, the voltage of PTEH-Interlocking cells maintains at ~34.88 mV with a ΔT of 25°C (Figure 4g ), indicating the good mechanical stability.
The combination between PA-PEI-Fe film and TEH is of importance for the heat transfer in the photo-thermo-electric cell. The thermal conductivity of PA-PEI-Fe film and TEH is 0.408 W·m-1K-1 (Figure S7 ) and 0.895 W·m-1K-1 (Figure 4h ) at room temperature, respectively. Whereas that of TEH cells with direct physical adhesion of PA-PEI-Fe thermal film decreased to 0.64 W m−1 K−1 because loose adhesion allows air to enter the interface between the TEH and PA-PEI-Fe film (where that thermal conductivity of air is only 0.0267 W m−1 K−1). Notably, that thermal conductivity of PTEH-Interlocking is the highest at 0.99 W m−1K−1, indicating the the PA-PEI-Fe photothermal film assembled in situ facilitates rapid heat transfer from the photothermal film to the thermo-electric hydrogels (Figure 4h ).1, 10 The electronic conductivities of PTEH-Interlocking (22.84 mS m−1) is similar to that of TEH (22.94 mS m−1) (Figure 4i ), indicating that the in situ PA-PEI-Fe film has no affect on the the overall electron transfer in PTEH-Interlocking cells.
The higher Seebeck cofficient of the PTEH-Interlocking cells is probably devided into two mechnasim: (1) the increasing redox entropy via oxidation of PA and PEI by [Fe(CN)6]3−in the system; 42, 43 (2) high thermal conductivity at the interface due to the interlocking structure.