It is globally challenging to develop technologies that can control CO2 emissions without reducing the energy supply. The heat energy that is all around us is an attractive green energy source.1-3 Geothermal power generation4 and the Seebeck effect5 are the two main methods of generating electricity from heat. Regarding the former, electricity is generated via the rotation of a steam turbine, in which the heat used to generate the steam is supplied by a geothermal source. A major setback of this method is the availability of suitable land since the water demand is high. In the latter, electricity is generated via a temperature difference, which induces an electric potential. The device consists of at least two dissimilar conductors, and a complex design is required to ensure that it works under all possible conditions. To solve these problems, a new power generation method that could directly convert heat into electricity, therefore eliminating concerns with radioactive waste and enabling the use of renewable energy to generate electricity with a low environmental load using low-cost heat, is desirable.
We recently reported a sensitized thermal cell (STC) that is a new thermal energy conversion system6-11, which was inspired by the concept of a dye-sensitized thermal cell (DSSC).12-16 Utilising STCs, electric power can be generated “directly” from heat via the redox reactions of electrolyte ions with thermally excited carriers in semiconductors (Fig. 1). 6,7 Since this battery can function at a constant temperature, we first expected that the generation of electricity would stop at some point when equilibrium is reached. attained. However, we surprisingly observed that the reaction could be restarted by the simple flip of an on/off switch in the external circuit. The discovery of the restart phenomenon means that STC is a technology that directly converts heat into electricity, which could affect the price of oil.
In this work, we examined the cause of this restart and acquired a guideline for designing batteries that can generate power at the desired temperature. As a result, we successfully reduced the power generation temperature from 80 °C17 to room temperature (RT, 30 °C). The fabricated STCs were thinner than 0.5 mm and can be rendered flexible.
What we firstly examined
STC possesses a simple layered structure with a working electrode containing a semiconductor and a counter electrode sandwiching an electrolyte (Fig. 2a). To examine the “Restart” phenomenon, we focused on the distance between the working and counter electrodes. We fabricated 4 different cells with interelectrode distances of 85, 114, 228, and 342 μm, respectively. The obtained cells were named according to their corresponding electrode distances (Cell-85, -114, -228, and -342). The employed electrolyte volume was proportionally based on the interelectrode distance to eliminate the difference in the electrode/electrolyte contact area between each cell. A Ge-based semiconductor and a copper ion-based polyelectrolyte were selected for the STC materials.9-11,17
The fabricated STCs demonstrated stable power generation at 80 °C (Supporting Information (SI) 1). The open circuit voltage (VOC) of Cell-85, -114, -228, and -342 were approximately the same at 362, 373, 399 and 406 mV, respectively. The VOC value is caused by the difference between the redox potential of the electrolyte ions and the Fermi level of the working electrode.10 Comparing the discharge capacities of the 1st 200 nA discharge, that of Cell-342 was the smallest, thereby indicating that the sustainability of the power generation was influenced by ion convection inside the electrolyte.
After the initial discharge, all the batteries demonstrated voltage recoveries when they were switched off (SI 2). However, the time required to stabilise the voltage increased with increasing interelectrode distance. Cell-85 recovered up to 0.35 V in few tens of seconds, while Cell-228 and -342 recovered up to 0.25 V in 4 h, and this indicates that the interelectrode distance could affect the restart of STC. Notably, all the batteries were left in the thermostatic bath during discharge and restart processes.
The 200 nA discharge characteristics of each battery at 80°C were tested 4 times. In Cell-228 and -342, the VOC after the 1st discharge was 100 mV smaller than the initial VOC (Fig. 2b). Contrarily, in Cell-85 and -114, the VOC attained the initial value after each discharge.
The 2nd and 3rd discharge curves are shown in Figs. 2c and 2d, respectively. The voltage rapidly dropped to zero in Cell-228 and -342. Meanwhile, the 2nd discharge capacities in Cell-85 and -114 were 0.21 and 0.24 mAh/g, while the 3rd discharge capacities were 0.34 and 0.08 mAh/g, respectively. It is noteworthy that the discharge capacity of Cell-85 was higher than that of Cell-114.
These results strongly indicated that ion convection was crucial to the restart phenomenon (the cell can restart if sufficient ions are supplied to the electrode interface). Here we should mention that the 2nd and 3rd discharge capacities dropped more than the 1st discharge. We think this is because of the polyelectrolyte18,19. During the first discharge, the electrolyte was not completely mixed at the molecular level, thus causing the diffusion of ions via local concentration distribution. Conversely, in the 2nd and 3rd discharge processes, power was generated after the complete mixing of the electrolyte in the first discharge, resulting in the shorten discharge time. These results suggested that ion diffusion in the electrolyte is reflected by the restoration of power generation by STC.
Production of thinner cells
Since the maximum characteristics were obtained with the thinnest cell (Cell-85) among the tested ones, we applied comb-shaped electrodes to assess what would happen if the interelectrode distance was narrower. Thus, we fabricated Ge and Pt comb-shaped electrodes on quartz, glass and a Kapton tape (Fig. 3a).
Two interelectrode distances (2 and 5 mm) were examined. The cells were referred to as Cell-2 and -5. These comb-shaped electrodes were filled with the electrolyte, and the battery characteristics were measured on a hot plate while controlling the temperature.
Surprisingly, Cell-2 did not generate power at 80 °C (Figs. 3b and c). However, it generated power well as the temperature was lowered, and the best battery characteristics were obtained at 30 °C, which was near RT (SI 3a). We were so surprised that we also tried to generate power at room temperature (RT, ~25 °C under air conditioner), and confirmed that the power generation capacity (270 mV, Fig. 3d) was stable at least over 7000 s. (Fig. 3e). This power generation and restore experiments will continue for several days, and the room temperature will change during that long experiments. In order to eliminate the effect of this temperature change, the following results were measured at 30°C fixed by a hot plate.
Cell-5 exhibited the same trend; it achieved good voltage recovery even after repeated discharge cycles (Fig. 3f). Particularly, the discharge time of Cell-5 at 30 °C was longer than that of Cell-2 (Fig. 3g). To understand the phenomenon, we measured the cyclic voltammetries (CVs) of Cell-2 at different scan rates at 30 °C and 50 °C. We observed a capacitor-like behaviour at 50 °C (SI 3b and 3c). These results indicated that extremely short or long interelectrode distances are not desirable for restarting STC and that there is a ‘just right’ interelectrode distance.
These results suggested that the ion diffusion length from the electrode surface affected the generation of power. Thus, we measured the ion diffusion length using the AC impedance measurement20 (SI 4). The results of the temperature change revealed that the ion diffusion length increased at higher temperatures (Fig. 4a). Conversely, the ion diffusion lengths of Cell-85, 114, 228 and 342 increased with decreasing interelectrode distances at 80 °C (Fig. 4b). During the power generation, a potential difference, V, was generated between the working and counter electrodes. The magnitude of the electric field, which was applied to the entire electrolyte, was equal to V divided by the interelectrode distance. Resultantly, the electric field was larger with a shorter interelectrode distance, which made it easier for electrolyte ions to diffuse, thereby increasing the ion diffusion length.
From these measurements, it was confirmed that power generation occurred when the value S (the interelectrode distance divided by the ion diffusion length) was between 1 and 10. The same measurements were conducted for other STC systems, and power was generated at 1 £ S < 10 (SI 5).
Based on these results, we considered the power generation and restore mechanism of STC (Figure 5). At S < 1, the electrolyte ions behaved as a capacitor and did not move from the electrode interface. Hence, power was not generated (Fig. 5a). When S is too large, the electric field became small and the ions, which were generated at the Ge electrode (or counter electrode) interface did not diffuse to the counter electrode (or Ge electrode) interface and there was no restart (Fig. 5c). Put differently, STC generated and restored power well at 1 < S < 10, where ion diffusion constantly occurred.
Now we can design favourable STCs
The ion diffusion length is a value that is determined by the electrolyte material and temperature. That is, if you measure the ionic diffusion length of the electrolyte at the temperature where you want to install the STC, and design the interelectrode distance to be several times longer than that, you can obtain an STC that would generate and restart power well. Since STC can be ‘buried’ in heat, it has been applied to operate liquid crystal displays at 80 °C9, Bluetooth communication devices on asphalt in the summer (SI 6). In principle, VOC of STC is independent of the temperature and is the difference between the Fermi level of the working electrode and the redox level of the electrolyte ions.10 Regarding Ge/(Cu ions in polyethylene glycol (PEG)) STC in this work, VOC was ~0.35 V. Assuming that the theoretical short-circuit current value was the number of reactive ions reaching the electrode interface per second, it would be estimated as ~60 and 150 mA/cm2 at RT and 80 °C, respectively, for the Ge/(Cu ion in PEG) STC comb-shaped electrode (5 mm) based on the saturation concentration of the electrolyte, the ion mobility21 and electric field that were applied to the electrolyte (SI 7). This indicated that the output powers were 0.2 and 40 mW/cm2 at RT and 60 °C, respectively, which are comparable to that of solar cells (6 mW/cm2) and are sufficient to employ STCs in IoT devices (sensor: mW~, wireless power transmission: mW~)22. Evidently, these theoretical values were ‘instantaneous’ maxima, and the discharge and restore times would be the other major parameters to be considered for the practical application of STCs.
We strongly expect our findings to offer huge energy markets for industries and academia as well as help achieve the control of CO2 emissions without reducing the energy supply, thereby contributing towards solving global environmental issues.