2.3. Ionogel thermoelectric nanogenerators
Further study of the thermoelectrochemical properties of PVDF-HFP-based ionogel was aimed at revealing their capability as thermoelectric self-powered monitoring devices. The working mechanism of thermoelectric ionogel is as illustrated in Figure 4 a. In the initial state (Figure 4a-i), there is an absence of internal temperature gradient, causing random distribution of anions and cations throughout the entire gel. This leads to a relatively low ionic concentration gradient and subsequently results in a lower ionic potential difference. Under a temperature gradient (Figure 4a-ii), the ions exhibit migration from the cold end to the hot end due to the ion thermal diffusion effect. Furthermore, the dissimilar radii of the anions [EMIM]+ and [DCA]-contribute to distinct migration rates. This disparity in migration rates establishes a concentration gradient across the temperature gradient, culminating in the generation of a voltage. Over time, electrons will naturally diffuse from the hot end to the cold end within the external circuit when an external resistor is connected in series on both sides of the ionogel. When the external circuit is disconnected and the gel is left in an open circuit, the ion concentration gradient will be gradually reestablished due to the temperature difference, resulting in the restoration of voltage after a certain period of time (Figure 4a-iii).[37,52,53] Further, By performing CV scans on the ionogel at different rate (50, 100, 200, 400, 500, and 1000 mV/s), which shows the excellent electrochemical properties.
The properties of thermoelectric ionogel are usually evaluated by their Seebeck coefficients, which is measured by a customized thermoelectric test system in this work. Figure 4b demonstrates the S of the PVDF-HFP-based ionogel as a function of the three types of ionic liquid types and concentrations (loading capacity of 8 wt%, 16 wt%, 24 wt%, and 32 wt%). Apparently, the S of PVDF-HFP-based ionogel doped with [EMIM]DCA ionic liquids are significantly better than those of the other two types of ionic liquids. For [EMIM]DCA ionic liquids, the S exhibit a significant increase with increasing loading, reaching a maximum value of 11.435 mV K-1 at the loading of 32 wt%. The ionic maximum conductivity of 8.55 mS cm-1was get when the ionic liquid loading was 32 wt% measured by the electrochemical workstation at room temperature (25 ℃) (Figure S14a-c). The ionic conductivity of the ionogel can be calculated by taking the impedance value corresponding to the real part when the imaginary part is 0 and introducing the ionic resistance value Rion. And the Nyquist plot shows a straight line without the semicircular shape of typical polymer gels, which because of the dielectric relaxation of the ions as reported by some published reports. Thermal conductivity is one of the basic parameters for evaluating the performance of thermoelectric ionogel, and therefore it was investigated (Figure 4c). The thermal conductivity of [EMIM]DCA-32 wt% gel at 36 ℃ (the normal temperature of human skin) was measured to be 0.22 w m-1k-1using a thermal conductivity tester (the inset in Figure 4c shows the test principle of the Hot Disk method). Combined with another parameter, the thermoelectric figure of merit, which is usually used to evaluate the comprehensive performance of thermoelectric materials, the final result calculated shows that the thermoelectric figure of merit for this ionogel is as high as 0.11 (Figure 4c). In summary, the [EMIM]DCA / PVDF-HDF-32 wt% ionogel exhibited a large Seebeck coefficient, excellent ionic conductivity, thermal conductivity, and excellent thermoelectric conversion efficiency, which shows great potential as materials for making thermoelectric generators.
Therefore, the output voltage, current, and power were tested by externally loading different resistance values (200, 675, 1000, 2000, 10,000, and 20,000 Ω) (Figure 4d). It is obvious that as the external resistance increases, the output voltage gradually increases and the current gradually decreases. The maximum voltage is 30 mV and the minimum current is 1.5 μA when the external resistor is 20,000 Ω. The current value reaches a maximum of 11 μA while the external resistor is changed to 200 Ω, and the minimum voltage is 3 mV. Based on the above characteristics of output voltage and current under different resistive loads, the power curve is plotted (Figure 4e), from which it can be seen that the overall trend of the curve increases with the increase of the connection resistance, and when the resistance is increased to a specific value, the power curve peaks, i.e., the maximum output power, which decreases as the load resistance continues to increase. The maximum power reaches 0.12 μW when the external resistance is 675 Ω. The inset in Figure 4e shows a schematic diagram of the test equivalent circuit for an external load of thermoelectric ionogel. The output electrical performance tests were performed in the presence of a 5 K temperature difference, while the open-circuit voltages and short-circuit currents of the ionogel outputs at a temperature difference of 8 K were studied (Figure S15).
The thermoelectric conversion capability of the ionogel was examined by further heating / cooling cycle experiments (Figure 4f), it can be seen that the ionogel exhibits excellent thermoelectric responsiveness during a single heating / cooling cycle and the overall curve waveform are well behaved. However, there is a slight decrease in the overall amplitude of the curves, which may be attributed to the decrease in the thermoelectric potential due to the thermal hysteresis effect of the thermocouples and the material when it is heated, but overall, there is a reversible thermal voltage switching during the 5 repetitive heating / cooling cycles. The results of the I-V curves obtained by connecting test electrodes at both ends of the ionogel and measuring its sensing performance under different pressures (Figure 4g). The resistance of the gel decreases linearly with increasing pressure due to an expansion of conductive channels within the ionogel and a decrease in ion spacing, which enables the ionogel to function as both pressure sensors and piezoelectric generators.