Iron (II/III) perchlorate electrolytes for electrochemically harvesting low-grade thermal energy

Self-assembled monolayers for electrostatic electrocatalysis and enhanced electrode stability in thermogalvanic cells

Waste heat is ubiquitous; as such, sustainable and long-lasting devices are required to convert it into more useful forms of energy that can make use of this abundant potential resource. Thermogalvanic cells (or thermocells) can use the thermoelectrochemical properties of redox couples to achieve this; entropy-driven redox reactions allow them to act as liquid thermoelectrics. However, excellent electrocatalysis at the electrode surface is required for optimum conversion efficiency. Serendipitous observation of Nafion-based electrocatalysis prompted the exploration of electrostatically charged self-assembled monolayers (SAMs) inside a thermocell. Both electrostatic electrocatalysis and improved electrode stability were observed; in an aqueous K3[Fe(CN)6]/K4[Fe(CN)6]-based cell, modification with (3-trimethylammonium bromide)thiopropane resulted in higher electrical power, and protection against [Fe(CN)6]3-/4--induced gold passivation, relative to bare gold. Molecular-based electrostatic electrocatalysis could be an alternative to precious metal-based nanomaterial electrocatalysis, and could be integrated with (nano)carbon-based electrodes to further enhance the ability of thermogalvanic and other electrochemical energy conversion devices, e.g. redox flow batteries.

Concentration dependence of resistance components in solutions containing dissolved Fe2+/Fe3

Electrolyte solutions containing Fe2+/Fe3+ are suitable for liquid thermoelectric conversion devices (LTEs), because they are inexpensive materials and exhibit a high electrochemical Seebeck coefficient α. Here, we investigated the concentration (c) dependence of resistance components, i.e., solvent (Rs), charge-transfer (Rct), and diffusion (Rdif) resistances, of dissolved-Fe2+/Fe3+-containing aqueous, methanol (MeOH), acetone, and propylene carbonate (PC) solutions. We found that the c dependence of Rs and Rdif are well reproduced by empirical formulas, and , where η(c) is viscosity at c. We further found that the magnitudes of Cs and Cdif are nearly independent of solvent, suggesting that η is one of the significant solution parameters that determine Rs and Rdif.

Stretchable and Durable Bacterial Cellulose-Based Thermocell with Improved Thermopower Density for Low-Grade Heat Harvesting

Low-grade heat exists ubiquitously in the environment, and gel-state thermogalvanic cells (GTCs) can directly convert thermal energy into electricity by a redox reaction. However, their low ionic conductivity and poor mechanical properties are still insufficient for their potential applications. Here, we designed a bacterial cellulose (BC) nanofiber-macromolecular entanglement network to balance the GTC's thermopower and mechanical properties. Therefore, the BC-GTC shows a Seebeck coefficient of 3.84 mV K-1, an ionic conductivity of 108.5 mS cm-1, and a high specific output power density of 1760 μW m-2 K-2, which are much higher than most current literature. Further connecting 15 units of BC-GTCs, the output voltage of 3.35 V can be obtained at a temperature gradient of 65 K, which can directly power electronic devices such as electronic calculators, thermohydrometers, fans, and light-emitting diodes (LEDs). This work offers a promising method for developing high-performance and durable GTC in sustainable green energy.

Design and Optimization Strategies for Flexible Quasi-Solid-State Thermo-Electrochemical Cells

Currently, efficient utilization of low-grade thermal energy is a great challenge. Thermoelectricity is an extremely promising method of generating electrical energy from temperature differences. As a green energy conversion technology, thermo-electrochemical cells (TECs) have attracted much attention in recent years for their ability to convert thermal energy directly into electricity with high thermal power. Within TECs, anions and cations gain and lose electrons, respectively, at the electrodes, using the potential difference between the hot and cold terminals of the electrodes by redox couples. Additionally, the anions and cations therein are constantly circulating and mobile via concentration diffusion and thermal diffusion, providing an uninterrupted supply of power to the exterior. This review article focuses mainly on the operation of TECs and recent advances in redox couples, electrolytes, and electrodes. The outlook for optimization strategies regarding TECs is also outlined in this paper.

Gigantic and Continuous Output Power in Ionic Thermo-Electrochemical Cells by Using Electrodes with Redox Couples

The main obstacle of ionic thermo-electrochemical cells (TECs) in continuous power supply lies in a low heat-to-electricity energy conversion efficiency because most TECs work in thermodiffusion mode in which the ions are confined in a liquid/electrolyte media. The introduction of the redox couple onto the electrode surface may overcome the obstacle by resolving the low mass transport rate of ions caused by the redox process occurring near but not on the electrode surface. Herein, the authors demonstrate enhancement of TECs by integrating the redox couple directly onto the electrode surface to maximize the mass transport efficiency. A discontinuous interfacial modification strategy is developed by using a carbon cloth/iron (II/III) phytate as the symmetric electrodes. The gelled electrolyte consisting of a polyacrylamide matrix and phytic acid is shown to promote selective ion diffusion. A synergistic combination consisting of the thermodiffusion effect and redox reactions on the electrode is established in a pre-treated layout. Such TEC affords a high output voltage of 0.4 V, an excellent instantaneous output power density (20.26 mW m-2 K-2 ) and a record-high 2 h output energy density (2451 J m-2 ) under TH = 30 °C with TC = 15 °C, with an ultrahigh Carnot-relative efficiency of 1.12%.

Biomass-Derived Sustainable Electrode Material for Low-Grade Heat Harvesting

The ever-increasing energy demand and global warming caused by fossil fuels push for the exploration of sustainable and eco-friendly energy sources. Waste thermal energy has been considered as one of the promising candidates for sustainable power generation as it is abundantly available everywhere in our daily lives. Recently, thermo-electrochemical cells based on the temperature-dependent redox potential have been intensely studied for efficiently harnessing low-grade waste heat. Despite considerable progress in improving thermocell performance, no attempt was made to develop electrode materials from renewable precursors. In this work, we report the synthesis of a porous carbon electrode from mandarin peel waste through carbonization and activation processes. The influence of carbonization temperature and activating agent/carbon precursor ratio on the performance of thermocell was studied to optimize the microstructure and elemental composition of electrode materials. Due to its well-developed pore structure and nitrogen doping, the mandarin peel-derived electrodes carbonized at 800 °C delivered the maximum power density. The areal power density (P) of 193.4 mW m^-2 and P/(ΔT)² of 0.236 mW m^-2 K^-2 were achieved at ΔT of 28.6 K. However, KOH-activated electrodes showed no performance enhancement regardless of activating agent/carbon precursor ratio. The electrode material developed here worked well under different temperature differences, proving its feasibility in harvesting electrical energy from various types of waste heat sources.

High Seebeck coefficient thermogalvanic cells via the solvent-sensitive charge additivity of cobalt 1,8-diaminosarcophagine

Thermogalvanic devices can chemically convert low grade (<200 °C) waste thermal energy into electrical energy. A temperature gradient across the device drives an entropically favourable electrochemical redox reaction, resulting in continuous current production. The voltage correlates with the entropy change during the redox reaction, which favours high valence metal complexes with high charge densities. Here we investigate cobalt (II/III) sarcophagine ([Co(SAR)]^2+/3+) for application in thermogalvanic cells, as a function of solvent; the two uncoordinated amine groups 1,8-diaminosarcophagine are typically protonated to form tetracationic/pentacationic [Co(SARH₂)]^4+/5+. In water, [Co(SARH₂)]^4+/5+ gave a thermogalvanic Seebeck coefficient (S_e) of +0.43 mV K^-1, which is entropically consistent with just the Co^2+/3+ core valence, whereas DMSO and ionic liquid solvents gave S_e values of +1.84 and +2.04 mV K^-1, respectively, in line with the 'Co^4+/5+' overall complex. This work proves how the ionic charge on pendant moieties can undergo charge-additivity with the metal core to significantly boost entropically-driven processes, but only in suitably low dielectric and bulky solvents.

Temperature switch of electrochemical Seebeck coefficient of Fe2+/Fe3+ via formation of [FeCl4]2-/[FeCl4]. RSC Adv 2023;13:3971-3975. [PMID: 36756564 PMCID: PMC9890644 DOI: 10.1039/d2ra07886d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The electrochemical Seebeck coefficient (α = dV/dT; V and T are the redox potential and temperature, respectively) is important parameter for thermoelectric conversion. Here, we found that α of Fe²⁺/Fe³⁺ in dimethyl sulfoxide (DMSO) with small amount of 19 M LiCl aqueous solution exhibits a crossover behavior from 1.4 mV K^-1 (20 °C ≤ T ≤ 40 °C) to ≈0 mV K^-1 (50 °C ≤ T ≤ 80 °C). The molar absorption (ε) spectra revealed that the crossover is ascribed to the transformation of the dominant Fe³⁺ complex from [FeL₆]³⁺ (L is solvent molecule) to [FeCl₄]^-.

Advancement of Electrochemical Thermoelectric Conversion with Molecular Technology

Thermocells are a thermoelectric conversion technology that utilizes the shift in an electrochemical equilibrium arising from a temperature difference. This technology has a long history; however, its low conversion efficiency impedes its practical usage. Recently, an increasing number of reports have shown drastic improvements in thermoelectric conversion efficiency, and thermocells could arguably represent an alternative to solid thermoelectric devices. In this Minireview, we regard thermocells as molecular systems consisting of successive molecular processes responding to a temperature change to achieve energy generation. Various molecular technologies have been applied to thermocells in recent years, and could stimulate diverse research fields, including supramolecular chemistry, physical chemistry, electrochemistry, and solid-state ionics. These research approaches will also provide novel methods for achieving a sustainable society in the future.

Control of Fe3+ coordination by excess Cl- in alcohol solutions

We spectroscopically investigated coordination state of Fe³⁺ in methanol (MeOH) and ethanol (EtOH) solutions against Cl^- concentration ([Cl^-]). In both the system, we observed characteristic absorption bands due to the FeCl₄ complex at high-[Cl^-] region. In the MeOH system, the proportion (r) of [FeCl₄]^- exhibits a stationary value of 0.2-0.3 in the intermediate region of 10 mM < [Cl^-] < 50 mM, which is interpretted in terms of [FeCl _n L_6-n ]^3-n (n = 1 and 2). In the EtOH system, r steeply increases from 0.1 at [Cl^-] = 1.5 mM to 0.7 at [Cl^-] = 3.5 mM, indicating direct transformation from [FeL₆]³⁺ to [FeCl₄]^-. We further found that the coordination change significantly decreases the redox potential of Fe²⁺/Fe³⁺.

Paper-Based Ionic Thermocouples for Inexpensive and High-Precision Measurement of Temperature

Accurate and yet cost-effective temperature measurements are required in various sectors of academia and industry. Thermocouples (TCs) are most widely used for temperature measurements; however, their low temperature sensitivity and high thermal conductivity should be improved to ensure the reliable measurement of output voltage for small temperature differences. To address this, a paper-based ionic thermocouple (P-iTC) presented here utilizes a pair of paper strips soaked with the electrolytes of potassium ferri-/ferrocyanide and iron (II/III) chloride redox couples, which are used as p- and n-type elements, respectively. The fabricated P-iTC provides 70× higher temperature sensitivity (α, 2.8 mV/K) and 30× lower thermal conductivity (k, 0.8 W/m K) than those of commercial K-type TCs, thereby yielding a remarkably high α/k ratio of 3.5 mV m/W. Reliable sensing performance is measured during three weeks of operation, which indicates that the P-iTC should be stable in long-term operation. To demonstrate the practicality of the P-iTC, a 3 × 3 planar array of P-iTCs is fabricated to monitor the temperature profile of a surface in contact with heat sources. Using pencil-drawn graphite electrodes on paper, a highly cost-effective P-iTC with the material cost of ∼0.5 cents per device is also fabricated, which is successfully used to monitor cold chain temperatures while retaining its excellent temperature-sensing performance.

Operando magnetic resonance imaging for mapping of temperature and redox species in thermo-electrochemical cells

Low-grade waste heat is an abundant and underutilised energy source. In this context, thermo-electrochemical cells (i.e., systems able to harvest heat to generate electricity) are being intensively studied to deliver the promises of efficient and cost-effective energy harvesting and electricity generation. However, despite the advances in performance disclosed in recent years, understanding the internal processes occurring within these devices is challenging. In order to shed light on these mechanisms, here we report an operando magnetic resonance imaging approach that can provide quantitative spatial maps of the electrolyte temperature and redox ion concentrations in functioning thermo-electrochemical cells. Time-resolved images are obtained from liquid and gel electrolytes, allowing the observation of the effects of redox reactions and competing mass transfer processes such as thermophoresis and diffusion. We also correlate the physicochemical properties of the system with the device performance via simultaneous electrochemical measurements.

High seebeck coefficient in middle-temperature thermocell with deep eutectic solvent

Deep eutectic solvent (DES) was applied to the solvent of thermocell and high Seebeck coefficient (S_e) of the thermocell was achieved at high-temperatures operation. The S_e of a redox couple of ferricyanide and ferrocyanide ([Fe(CN)₆]^3−/4−) reaches − 1.67 mV/K in a DES consisting of ethylene glycol and choline chloride. Spectroscopic analysis reveals that this is due to the strong interactions between the redox couple and the DES. Furthermore, the cell can operate over a wide temperature range of 135–165 °C. This result is a desired feature for waste-heat recovery applications.

An Electricity-Generating Window Made of a Transparent Energy Harvester of Thermocells

Windows are primarily for admitting light or air and allowing people to see out. Presented here are windows that can generate electricity while retaining the primary functions. These windows are made of transparent thermocells that convert a temperature difference across the window to electricity. Interconnected p-type and n-type or p-n thermocells are introduced and utilized to scale up the output power of a thermocell window (T-window). The T-window consisting of 2 p-n thermocells provides an output voltage of 60 mV and a power density of 0.5 μW/cm² for a small temperature difference of 10 °C with an optical transparency of ∼50% in the visible range. The T-window introduced here could pave the way to enhancing energy efficiency in residential environments by capturing naturally available low-grade heat, a new renewable energy source that is otherwise discarded to the surrounding environment.

A Novel Thermocell System Using Proton Solvation Entropy

The entropy change associated with proton-coupled electron transfer (PCET) reactions significantly enhance the Seebeck coefficient (S_e ) of thermocells. A redox pair of [Ru(H_x im)₆ ]^2+/3+ (Him=imidazole, x=0≈1) releases three protons in their one-electron redox reactions in thermocells, which gave a remarkably high S_e of -3.7 mV K^-1 as confirmed by temperature-dependent square wave voltammetry. The value of S_e is proportional to the redox reaction entropy (ΔS_rc ), which increased with the number of dissociating protons. This result demonstrates the utility of PCET reaction toward efficient thermoelectric conversion.

Thermosensitive crystallization-boosted liquid thermocells for low-grade heat harvesting

Low-grade heat (below 373 kelvin) is abundant and ubiquitous but is mostly wasted because present recovery technologies are not cost-effective. The liquid-state thermocell (LTC), an inexpensive and scalable thermoelectric device, may be commercially viable for harvesting low-grade heat energy if its Carnot-relative efficiency (η_r) reaches ~5%, which is a challenging metric to achieve experimentally. We used a thermosensitive crystallization and dissolution process to induce a persistent concentration gradient of redox ions, a highly enhanced Seebeck coefficient (~3.73 millivolts per kelvin), and suppressed thermal conductivity in LTCs. As a result, we achieved a high η_r of 11.1% for LTCs near room temperature. Our device demonstration offers promise for cost-effective low-grade heat harvesting.

Enhanced Seebeck coefficients of thermocells by heat-induced deposition of I3-/hydrophobized α-cyclodextrin complexes on electrodes

Ethylated α-cyclodextrin (Et18-α-CD) is used as a host matrix for I-/I3- thermocells. Although Et18-α-CD is not soluble in water at ambient temperature, it becomes soluble by complexation of the I3- anion. Meanwhile, the complex is precipitated upon elevating the temperature. The change in thermo-responsive solubility of the I3-/Et18-α-CD complex increases the Seebeck coefficient (Se) of the thermocell up to 2.6 mV K-1. The underlying mechanism of the increased Se is elucidated by UV-vis spectroscopy, Raman spectroscopy, and electrochemical measurements. This result shows the temperature-dependent solubility changes of redox-active species as a potential means to improve the performance of electrochemical thermocells.

Thermocells for Hybrid Photovoltaic/Thermal Systems

The photovoltaic conversion efficiency of solar cells is highly temperature dependent and decreases with increasing temperature. Therefore, the thermal management of solar cells is crucial for the efficient utilization of solar energy. We fabricate a hybrid photovoltaic/thermocell (PV/T) module by integrating a thermocell directly into the back of a solar panel and explore the feasibility of the module for its practical implementation. The proposed PV/T hybrid not only performs the cooling of the solar cells but also produces an additional power output by converting the heat stored in the solar cell into useful electric energy through the thermocell. Under illumination with an air mass of 1.5 G, the conversion efficiency of the solar cell can improve from 13.2% to 15% by cooling the solar cell from 61 °C to 34 °C and simultaneously obtaining an additional power of 3.53 μW/cm² from the thermocell. The advantages of the PV/T module presented in this work, such as the additional power generation from the thermocell as well as the simultaneous cooling of the solar cells and its convenient installation, can lead to the module’s importance in practical and large-scale deployment.

Diffusion and Current Generation in Porous Electrodes for Thermo-electrochemical Cells

Carbon-based porous electrodes have led to remarkable improvements in the performance of thermochemical cells or thermocells that electrochemically harvest low-grade waste thermal energy. However, the output current from the thermocells is hampered by the diffusion effect, which leads to depleted ion concentration as the ions permeate through the porous electrode. Here, we advance a theoretical basis for a quantitative description of the diffusion effect on current generation in such porous electrodes. One single dimensionless parameter of Thiele modulus describes the effect according to the theory adopted from the well-established results in the literature. Experimental results for carbon fiber electrodes are illustrated and quantified by the theory. The theory presented here would provide a basis for the choice and design of porous electrodes for thermocells. The results should also provide a basis for devising electrochemical devices with highly porous electrodes.