Basanta
Ghimire‡
ab,
Mihir
Parekh‡
*ab,
Herbert
Behlow
ab,
Morteza
Sabet
bc,
Sriparna
Bhattacharya
ab,
Nawraj
Sapkota
ab,
Pankaj Singh
Chauhan
d,
Abha
Misra
d and
Apparao M.
Rao
*ab
aDepartment of Physics and Astronomy, Clemson University, 118 Kinard Laboratory, Clemson, South Carolina, USA. E-mail: mihirp@clemson.edu; arao@clemson.edu; basantg@clemson.edu; hbehlow@clemson.edu; nsapkot@clemson.edu
bClemson Nanomaterials Institute, Clemson University, 81 Technology Drive, Anderson, South Carolina, USA. E-mail: bbhatta@g.clemson.edu
cDepartment of Automotive Engineering, Clemson University, 4 Research Dr, Greenville, South Carolina, USA. E-mail: ssabet@clemson.edu
dDepartment of Instrumentation and Applied Physics, Indian Institute of Science, Bengaluru, Karnataka, India. E-mail: pschauhan.89@gov.in; abha@iisc.ac.in
First published on 15th January 2025
We present Soret effect-driven electrochemical devices that generate >1 V with a mere 10 K temperature difference with the cold end at room temperature, i.e., a thermopower α > 100 mV K−1 – almost four to five times the record to date [Adv. Energy Mater., 2019, 9, 1901085]. We show that α depends not only on the electrolyte composition but also on the electrode porosity and microstructure, which has remained an understudied area of research. Interestingly, our devices show novel voltage oscillations (unlike electrochemical oscillations observed previously, which were a result of either (a) stochastic single-molecule electrochemistry or (b) redox reactions) arising from an interplay between ionic diffusion and ionic migration within the electric double-layer, highlighting the potential for novel applications. Notably, the real-world use of TRECO is demonstrated by (a) facile continuous operation, (b) harvesting body heat (∼825 mV obtained for a temperature difference of 6 K), and (c) powering a pocket calculator using a single large format TRECO cell to harvest waste heat from warm continuously operating lab equipment.
Broader contextWhile shifting towards renewable energy sources is one way of tackling global warming, harvesting waste heat (nearly two-thirds of primary energy is wasted as heat) is another way of reducing greenhouse gas emissions. Harvesting high-grade waste heat using thermoelectric generators based on the Seebeck effect has been explored for a while, but harvesting the ubiquitous near-room temperature waste heat (e.g., waste heat from hot mobile phones, laptops, body heat, etc.) has remained a challenge. In this work, we (a) develop Soret effect-driven thermally rechargeable electrochemical oscillators (TRECOs) to harvest near room temperature ultra-low grade waste heat, (b) demonstrate a record high (beating the previous best by a factor >4) thermopower (i.e., the voltage generated per unit temperature difference) of ∼0.1 V K−1, (c) elucidate the fundamental physics underlying the previously unobserved voltage oscillations during discharge (implying that TRECO could be used for novel applications such as generating voltage pulses, or to generate oscillating waveforms, etc.) and (d) demonstrate TRECO's potential for real-time applications by powering a pocket calculator and developing a TRECO based wearable arm-band. All of the above is attributed to electrode microstructure and porosity – previously unexplored areas of study for Soret effect-driven devices. |
In addition to the high thermopower discussed above, our ionic thermoelectric cells exhibited novel and previously unreported oscillations in the output voltage during their galvanostatic discharge. In particular, our thermally charged cells exhibited voltage oscillations in the presence or absence of a temperature gradient across them during discharge, implying an electrochemical origin for the oscillations. Hence, we refer to our ionic thermoelectric cell as a thermally rechargeable electrochemical oscillator or TRECO. In other words, TRECO is an electrochemical oscillator that can be charged by maintaining a low-temperature difference across its two electrodes. While electrochemical oscillations have been observed previously, they were found to arise from either (i) an interplay of multiple processes governing redox reactions,15–19e.g., lithium titanate anodes in lithium-ion batteries, or (ii) stochastic single-molecule electrochemical processes,20–26e.g., electrochemistry on zeptoliter volumes of hexamineruthenium(III) chloride on a Pt ultra-microelectrode. However, electrodes and electrolytes in TRECO do not undergo redox reactions (the cyclic voltammetry data shown in Fig. S1† do not exhibit any peaks corresponding to redox reactions), pointing towards a novel origin of electrochemical oscillations in TRECO.
Our study shows that TRECO generates voltage due to charge separation, leading to oppositely charged double-layers at the two electrode–electrolyte interfaces. While the nature of the oscillations depends on discharge current and electrode porosity, the oscillations are also present in TRECOs with nonporous electrodes, although to a lesser extent. Thus, the possibility of oscillations arising purely from stochastic single-molecule electrochemistry within pores just large enough to trap single solvation shell structures can also be ruled out. The nonporous electrodes ensure that single-molecule electrochemical processes do not occur, as the entire electrode surface is essentially exposed to the electrolyte. We find that (i) the interplay between Fick's diffusion (diffusion due to concentration gradient) and ion migration due to the electrostatic forces within the electric double-layer (EDL) at the electrode–electrolyte interfaces and (ii) electrode microstructure and porosity together are responsible for the voltage oscillations. As a result, choosing electrodes with appropriate microstructures helped us observe (a) extraordinarily high thermopower at room temperatures (temperature difference between cold and hot ends being ∼10 K) and (b) voltage oscillations (which were observed both with and without the temperature gradient). A detailed description and discussion elucidating the link between electrode microstructure and these two novel effects form the backbone of this work. Notably, we also demonstrate TRECO's ability to power a calculator in real-time with (i) a TRECO module made up of four coin cells connected electrically in series, (ii) a single large format TRECO cell, which harvests waste heat from a continuously operating laboratory vacuum pump, (iii) a wearable arm-band based on a single TRECO coin cell.
This study advances the field of low-grade-waste heat recovery and thermal energy harvesting in multiple ways: (a) we uncovered the effect of electrode microstructure and porosity on the thermopower, which was previously an understudied areas of research, (b) our focus on electrode microstructure and porosity helped us improve over the previous thermopower record by a factor of 4, (c) we observed novel voltage oscillations (and elucidated the physics behind the oscillations) during discharge, which were not reported previously.
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Fig. 2 Open circuit voltage as a function of time for TRECO coin cells (temperature difference was set to 10 K before inserting the cell in the setup shown in Fig. 1a) with cellulose obtained from delignified wood as the separator, an aqueous electrolyte containing sodium chlorite and polyethylene oxide, and (a) CD1, (b) CD2, and (c) BP electrodes. (d) Open circuit behavior as a function of time for a cell with Pt electrodes with a 5.5 K temperature difference (adapted from ref. 9). |
The dependence of OCV on electrode material and porosity is not unexpected. The Soret effect generates a potential difference via charge separation, which depends not only on the ion (Na+, ClO2−) diffusivity and thermophoretic mobility but also on the EDL structure at the electrode–electrolyte interfaces. The EDL structure depends largely on the electrode microstructure; hence, the electrode material and its porosity play an important role in governing the generated OCV in any ionic thermoelectric cell, such as those used previously (e.g., ref. 27) or the TRECO. The maximum (by magnitude) observed OCV for the three different electrode types and their corresponding thermopowers are listed in Table 1. It should be noted that most previous studies utilizing the Soret effect to harvest low-grade waste heat have not reported the temporal behavior of OCV at a fixed temperature difference.1,29–31 Moreover, the previous literatures focused primarily on electrolyte optimization for increasing the OCV and thermopower. However, as pointed out above, OCV depends not only on time but also on the electrode material and porosity.
The reproducibility of thermopower and its dependence on the applied temperature difference are shown in Tables S1† and 2, respectively. The large OCV and thermopower are (i) generally attributed to large differences in ionic diffusivities and thermophoretic mobilities and (ii) interestingly enough, to electrode porosity also. The sodium (Na+) and chlorite ions (ClO2−) are the cations and anions present in our aqueous electrolyte. Polar hydroxyl groups within the cellulose separator and ether groups within polyethylene oxide (PEO) favor Na+ movement over ClO2− ions.9 Moreover, the ionic size difference is expected to affect ionic diffusivity. Thermophoretic mobility difference and electrode porosity are the other factors contributing to charge separation. Moreover, within the EDL present in the electrode pores, the existing large electric field (arising from charge separation) drives ion migration in a direction opposite to Fick's diffusion (Fig. 1b). The electric field leads to an electrostatic attraction between the electrodes and the ions within the double-layer, thus holding the ions close to the electrodes. Nonetheless, Fick's diffusion tends to drive the ions away from the electrodes. As shown in Fig. 1b, this interplay between Fick's diffusion and Soret effect in the electrolyte and Fick's diffusion and ion migration in the EDL dynamically changes an ion's position, leading to the fluctuating OCV. The mechanisms leading to voltage fluctuations in TRECOs under open circuit conditions are similar to those responsible for the self-discharge of electric double-layer capacitors. In electric double-layer capacitors, voltage drops during the rest period due to charge redistribution32 within the pores of the porous electrodes and within the electrolyte. The charges redistribute because, during the charging process, the slow ionic movement does not allow ions to diffuse into the deepest pores present within the electrode, leading to ion concentration gradients within the electrode (porous part of the electrode) and within the electrolyte. Fick's law drives ions from regions of high concentration to low concentration, resulting in charge redistribution. While the voltage drops smoothly in electric double-layer capacitors owing to charge redistribution, the TRECO's voltage exhibits a stochastic behavior due to the multiple phenomena, namely, the Soret effect (which pushes ions towards the electrodes and its pores) and Fick's diffusion (which pushes the ions in the opposite direction). This complex interplay of ion flux motion and the pore size distribution in the electrode and the separator leads to non-linearities in TRECO's voltages as a function of temperature gradient (Table 2). A movie (SM1†) highlighting the similarities and differences between the electrical charging of an electric double-layer capacitor and the thermal charging of TRECO can be found in the ESI† section.
Temperature difference (ΔT) | Open circuit voltage (V) |
---|---|
0 K | ∼0 V |
7.5 K | −0.490 V |
10 K | −0.84 V |
It is well known that in an electric double-layer capacitor (EDLC), the capacitance is highly dependent upon the pore size distribution within the electrode material. Pores with sizes of the order of the solvated ions essentially ensure a minimum distance between the solid electrode material and the ion within the EDL, thus increasing the capacitance.33 However, pore sizes smaller than ionic sizes can inhibit the ions from entering the pores, thus lowering the utilized electrode surface area and, hence, the capacitance. Therefore, akin to EDLCs, our TRECO cells based on CD1 and CD2 electrodes (cf.Fig. 1c and S3†) display an increased thermopower than BP electrodes with relatively larger pores (Fig. S4†).
![]() | (1) |
To investigate the origin of oscillations further, we leverage the equivalent circuit model of the TRECO shown in Fig. 4.
In Fig. 4, Rdiff refers to the ionic diffusion resistance within the electrolyte, RSoret is the resistance to thermodiffusion within the electrolyte, Rdl1, Rdl2, Cdl1, and Cdl2 represent double-layer resistances and capacitances at the two electrode–electrolyte interfaces, respectively. While Rext is the resistance of the external connections, and I is the discharge current applied during the galvanostatic discharge of the TRECO, I1, I2, I3, I4, I5, V1, V2, V3, and V4 are assumed to vary temporally. Although the double-layer resistances, double-layer capacitances, and bulk electrolyte resistances (ionic diffusion and thermodiffusion resistance) typically vary with voltage, we assume they are constant or weakly varying. This is because the amplitude of a single voltage oscillation, for most cases, is on the order of 1 mV, which is much smaller than the TRECO voltage of ∼1 V. V1 − V4 gives the measured TRECO voltage, and for voltage to oscillate, d(V1 − V4)/dt (i.e., the slope of voltage versus time curve) should exhibit a change of sign, i.e., vary from negative to positive or vice versa. Solving the equivalent circuit model suggests that
![]() | (2) |
f1(t)Cdl1Rdl1 + f2(t)Cdl2Rdl2 + ∫[f1(t) + f2(t)]dt = I(Rdl1 + Rdl2), | (3) |
The temporal nature of I1, I4 (and I2, I5) drives the oscillations. A typical RC parallel circuit shows an exponential voltage profile (with respect to time), both during charging and discharging. Since RC circuits have represented the double-layers, the voltage oscillations imply charging–discharging of the double-layers (charging and discharging seem to happen in unequal amounts as the device as a whole is being discharged and the oscillations are riding on a typical discharge profile). Charging of the double-layer implies ions moving towards the electrode–electrolyte interface (ionic migration dominates over Fick's diffusion (cf.Fig. 1b)), and ions move away from the electrode–electrolyte interface during discharge of the double-layer (Fick's diffusion dominates over ionic migration within the double-layer (cf.Fig. 1b)). This leads to a change of sign for I1, I4, which further leads to voltage oscillations as can be seen from eqn (2) and (3). This shows that the origin of voltage oscillations is electrochemical in nature. Thus, the order of magnitude analysis and the equivalent circuit model predict that the voltage oscillations (a) do not require the temperature gradient during discharge, (b) should exist irrespective of the chosen electrode, electrolyte, and separator material's porosity, and microstructure, and (c) may be observed even during electrical charging of a typical supercapacitor. This is because the electric double-layer is present at all electrode–electrolyte interfaces. We performed additional experiments described in the next section to confirm this prediction.
Fig. 5a exhibits the presence of voltage oscillations in the positive voltage regime close to its maximum voltage. However, oscillations are absent during the initial monotonic voltage rise, which is unlike the oscillations observed in the negative voltage regimes in Fig. 3a and b but is similar to the phenomenon shown in Fig. S8a.† A comparison of Fig. 3a, b and S8a† indicate that a high current of 1000 nA (the magnitude is different and depends upon the material's microstructure, as can be seen from Fig. S8a–d†) discharges the cell too quickly, not allowing sufficient time for ions to migrate back into the EDL. The ions diffuse out of the EDL (step I in Fig. 6) via the electrolyte (step II in Fig. 6) to the opposite electrode (step III in Fig. 6), thus switching polarity. With reversed polarity, the discharging current acts like a charging current for the cell, thus increasing the electrostatic attraction for ions within the EDL, which overcomes the concentration gradient effects and prevents the ions from diffusing out of the EDL once again. While Fig. 3a and b show the presence of oscillations even before the cell switches polarity due to discharge; their absence before a polarity switch in Fig. 5a at a low discharge current of 500 nA shows that the EDL dynamics is different in the presence or absence of temperature gradient (this is expected as the Soret effect helps in creating charge separation and hence we have more ions in the EDL with temperature gradient than in the case of no temperature gradient). Therefore, we hypothesize that a lower discharge current might exhibit oscillations before switching polarities in the absence of thermal gradients during discharge. This hypothesis is confirmed in Fig. 5c.
The above models and experiments confirm the role of the EDL in voltage oscillations. EDL characteristics depend upon the electrode material, microstructure, and porosity. Furthermore, the different oscillation patterns exhibited by TRECO cells with CD1, CD2, and BP electrodes confirm the role of choice of electrode material, microstructure, and porosity. Since the oscillations are seen with multiple electrode/electrolyte materials and varying microstructures and porosities, it seems that voltage oscillations might occur in any porous electrode, irrespective of the microstructure. However, the question still remains as to whether porous electrodes are necessary for observing voltage oscillations. This question becomes even more relevant if we extend the proposed models16–18 to EDL. The authors of ref. 16–18, through their models, showed that voltage oscillations occur because lithium ions shift from one particle to another during the lithiation processes within the two-phase regime.16–18 A simple extrapolation of this concept to TRECO suggests that voltage oscillations may be due to ions shifting from one pore to another within the electrode. However, as shown in Fig. 5d, a simple experiment with nonporous stainless-steel electrodes, cellulose obtained from delignified wood and soaked in an aqueous solution of sodium chlorite and PEO suggests that electrochemical oscillations can be observed even in cells with nonporous electrodes. We suggest that the ions find it hard to escape the EDL due to an interplay between Fick's diffusion and ionic migration due to electrostatic forces.
While this confirms the role of EDL, the role of porosity cannot be ruled out because, in porous electrodes, the double-layer structure itself depends on the porosity (if we have pore sizes of the order of Debye length, which is the case for CD1, CD2, and BP electrodes). Additionally, pores with sizes of the order of ionic sizes might also act like a physical trap for ions, thus making it harder for ions to leave the pore during the discharge and contributing to oscillations. In summary, the oscillations in porous electrodes result from porosity and the dynamic EDL structure, but the dynamic EDL structure is the sole cause for oscillations in nonporous electrodes. A detailed theoretical formulation35,36 explaining the complex dependence of voltage fluctuations under open circuit conditions (thermal charging) and voltage oscillations under galvanostatic discharge conditions on ionic diffusivities/mobilities is presented in the ESI† section under the title ‘Effect of ionic mobility on charging/discharging timescales, voltage fluctuations during open circuit conditions, and oscillations during discharge’. Additionally, a detailed explanation regarding the role of electrode microstructure and porosity in governing the electric double-layer structure, and hence the thermopower and voltage oscillations, has been presented in the ESI.†
One way to think of change in polarity is that it may be achieved via very large amplitude voltage oscillations where the peak-to-peak voltage difference is greater than the initial voltage before the polarity flip. However, such oscillations would imply that
(1) ions within the electric double-layer managed to not only escape the electric double-layer,
(2) but also reach the electric double-layer at the other electrode–electrolyte interface,
(3) then escape the electric double-layer at the other electrode–electrolyte interface and
(4) eventually reach the original electric double-layer in which they were present before the initial polarity flip.
This would only be possible when the ionic migration due to electrostatic forces is negligible compared to the diffusion flux. However, such a case would have never given rise to voltage oscillations during discharge at all because the oscillations arise due to a competition between the ionic migration and Fick's diffusion within the electric double-layer. Thus, the key to achieving continuous operation is flipping the polarity twice (once from initial to opposite and then back to initial). The polarity flips may be achieved easily by splitting the task into two steps, each step accounting for one polarity flip, as elaborated in the next to next paragraph below.
As shown in Fig. 5a and S8a–c,† the change in device polarity for TRECO can be easily achieved by discharging the cell at high currents. As explained in Fig. 6, discharging the cell at a high current ensures that the electronic movement through the external circuit is extremely fast (compared to the ionic movement through the electrolyte). This rapidly changes the electric field magnitude within the electric double-layer at the electrode–electrolyte interface, thus reducing the electrostatic attraction between the electrodes and oppositely charged ions. This alters the balance between Fick's diffusion and ionic migration within the electric double-layer, and hence, instead of observing voltage oscillations within the same polarity, the voltage first flips sign, making ions switch their location (from one electric double-layer to another) and then we observe voltage oscillations with flipped polarity. This is because, with flipped polarity, the current initially acting as a discharge current now starts to act as a charging current, thus increasing the electrostatic attraction between the electrodes and oppositely charged ions and holding them tightly enough to prevent a further flip of polarity.
To ensure continuous operation, the device polarity must be returned to the original polarity (i.e., polarity before the high current discharge was started). That may further be achieved (cf.Fig. 7) by flipping the sign of the current. This allows us to control the final voltage, as different voltages may be easily achieved after the initial and second polarity flip, as is evident from Fig. S8b and c.† Notably, our scheme does not require any change within the device itself, unlike the scheme proposed in ref. 12. Furthermore, achieving back the same voltage before the initial change of electrodes was performed in ref. 12 would require them to either have both sets of electrode materials with same thermopower in magnitude and opposite in sign or adjusting the temperature difference. Adjusting the temperature difference is not always possible in real-world settings where the application governs the hot end temperature and the cold end temperature is often room temperature. Thus, our proposed continuous operation scheme in this paper is more facile regarding its real-world applicability.
As shown in Fig. 8a and b, a TRECO module was assembled by connecting four TRECO coin cells (electrically in series and thermally in parallel), and its top and bottom faces were held at 313 K and room temperature (300 K), respectively. Thus, a temperature difference of 13 K was applied across the two terminals of the module. The module's output was sufficient to power a calculator (whose battery was removed) in real-time, as shown in movies SM3 and SM4† (uploaded). We also performed experiments by connecting resistors as loads across TRECO. The corresponding experimental procedure is described under the section ‘Discharging TRECO coin cells using a constant resistance load’ in the ESI,† and the average power and energy dissipated in the resistors are tabulated in Table S4.†
Fig. 9 illustrates the details of a wearable arm-band powered by a single TRECO coin cell. Fig. 9a shows an arm badge pouch, typically used for holding ID cards during sports, repurposed to contain the TRECO coin cell. Two concentric circular holes were made in the pouch to expose the top and bottom surfaces of the coin cell to human skin (hot end) and air (cold end), respectively. Before attaching the arm-band to the human arm (i.e., ΔT = 0 K), the TRECO coin cell exhibited a negligible initial voltage, as seen in Fig. 9b. The room temperature and skin temperature were measured using an infrared thermometer (Fig. 9c and d), highlighting a temperature difference of approximately 6 K between the hot and cold ends of the TRECO coin cell. As shown in Fig. 9e, the TRECO coin cell generated about 825 mV from a small temperature difference of 6 K. Finally, Fig. 9f shows that the voltage returned to 9 mV after the TRECO coin cell was disconnected from the human arm. Informed consent was obtained from the human participant before the experiment, and a video capturing the experiment is included in the ESI section (ESI, movie SM6†).
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Fig. 10 Energy efficiency versus thermopower for TRECO and other state-of-the-art Soret effect-based ionic thermoelectric generators such as the ones based on PSSNa,37 PSSH,38 ionic liquids,30 PEO:NaOH27 electrolytes, ferricyanide/ferrocyanide redox couple3 based ionic thermoelectric and electronic thermoelectrics based on PEDOT:PSS39 and Bi2Te3 (ref. 37). Amongst Soret effect-based ionic thermoelectrics, TRECO has one of the highest efficiencies and the highest thermopower. |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta08559k |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2025 |