Thermocells Driven by Phase Transition of Hydrogel Nanoparticles

Exploring Pyrazine-Based Organic Redox Couples for Enhanced Thermoelectric Performance in Wearable Energy Harvesters

Thermoelectric generators (TEGs) based on thermogalvanic cells can convert low-temperature waste heat into electricity. Organic redox couples are well-suited for wearable devices due to their nontoxicity and the potential to enhance the ionic Seebeck coefficient through functional-group modifications.  Pyrazine-based organic redox couples with different functional groups is comparatively analyzed through cyclic voltammetry under varying temperatures. The results reveal substantial differences in entropy changes with temperature and highlight 2,5-pyrazinedicarboxylic acid dihydrate (PDCA) as the optimal candidate. How the functional groups of the pyrazine compounds impact the ionic Seebeck coefficient is examined, by calculating the electrostatic potential based on density functional theory. To evaluate the thermoelectric properties, PDCA is integrated in different concentrations into a double-network hydrogel comprising poly(vinyl alcohol) and polyacrylamide. The resulting champion device exhibits an impressive ionic Seebeck coefficient (Si) of 2.99 mV K-1, with ionic and thermal conductivities of ≈67.6 µS cm-1 and ≈0.49 W m-1 K-1, respectively. Finally, a TEG is constructed by connecting 36 pieces of 20 × 10-3 m PDCA-soaked hydrogel in series. It achieves a maximum power output of ≈0.28 µW under a temperature gradient of 28.3 °C and can power a small light-emitting diode. These findings highlight the significant potential of TEGs for wearable devices.

Phase interface engineering enables state-of-the-art half-Heusler thermoelectrics

In thermoelectric, phase interface engineering proves effective in reducing the lattice thermal conductivity via interface scattering and amplifying the density-of-states effective mass by energy filtering. However, the indiscriminate introduction of phase interfaces inevitably leads to diminished carrier mobility. Moreover, relying on a singular energy barrier is insufficient for comprehensive filtration of low-energy carriers throughout the entire temperature range. Addressing these challenges, we advocate the establishment of a composite phase interface using atomic layer deposition (ALD) technology. This design aims to effectively decouple the interrelated thermoelectric parameters in ZrNiSn. The engineered coherent dual-interface energy barriers substantially enhance the density-of-states effective mass across the entire temperature spectrum while preser carrier mobility. Simultaneously, the strong interface scattering on phonons is crucial for curtailing lattice thermal conductivity. Consequently, a 40-cycles TiO2 coating on ZrNi1.03Sn0.99Sb0.01 achieves an unprecedented zT value of 1.3 at 873 K. These findings deepen the understanding of coherent composite-phase interface engineering.

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%.

Direct Conversion of Phase-Transition Entropy into Electrochemical Thermopower and the Peltier Effect

A thermocell generates thermopower from a temperature difference (ΔT) between two electrodes. The converse process of thermocells is an electrochemical Peltier effect, which creates a ΔT on the electrodes by applying an external current. The Seebeck coefficient (Se ) of the electrochemical system is proportional to the entropy change of the redox reaction; therefore, a redox system having a significant entropy change is expected to increase the Se . In this study, a thermoresponsive polymer having a redox-active moiety, poly(N-isopropyl acrylamide-co-N-(2-acrylamide ethyl)-N'-n-propylviologen) (PNV), is used as the redox species of a thermocell. PNV2+ dication undergoes the coil-globule phase transition upon the reduction to PNV+ cation radical, and a large entropy change is introduced because water molecules are freed from the polymer chains. The Se of PNV thermocell drastically increased to +2.1 mV K-1 at the lower critical solution temperature (LCST) of PNV. The entropy change calculated from the increment of Se agrees with the value evaluated by differential scanning calorimetry. Moreover, the electrochemical Peltier effect is observed when the device temperature is increased above the LCST. This study shows that the large entropy change associated with the coil-globule phase transition can be used in electrochemical thermal management and refrigeration technologies.

Proton-Coupled Electron Transfer Aided Thermoelectric Energy Conversion and Storage

Low-grade heat is ubiquitous in the environment and its thermoelectric conversion by the ionic conductors remains a challenge because of the low efficiency and poor sustainability. Here we demonstrate that the thermoelectric performances can be boosted by combining the Soret effect of protons and proton-coupled electron transfer (PCET) reaction of benzoquinone and hydroquinone in hydrogels. An overall enhancement of thermopower (25.9 mV K-1 ), power factor (5 mW m-1  K-2 ), figure of merit (>2.4) and continuity of power output is achieved. Moreover, an energy-storage function can be achieved by the redox couple, and a retained power output of 27.7 %, or 14 mW m-2 for more than 3 hours is obtained by the re-balance of PCET reactants in the hydrogel after the removal of the temperature gradient.

High-Performance Isotropic Thermo-Electrochemical Cells Using Agar-Gelled Ferricyanide/Ferrocyanide/Guanidinium

An isotropic thermo-electrochemical cell is introduced with a high Seebeck coefficient (S _e) of 3.3 mV K^-1 that uses a ferricyanide/ferrocyanide/guanidinium-based agar-gelated electrolyte. A power density of about 20 µW cm^-2 is achieved at a temperature difference of about 10 K, regardless of whether the heat source is on the top or bottom section of the cell. This behavior is very different from that of cells with liquid electrolytes, which exhibit high anisotropy, and for which high S _e values are achieved only by heating the bottom electrode. The guanidinium-containing gelatinized cell does not exhibit steady-state operation, but its performance recovers when disconnected from the external load, suggesting that the observed power drop under load conditions is not due to device degeneration. The large S _e value and isotropic properties can mean that the novel system represents a major advancement from the standpoint of harvesting of low-temperature heat, such as body heat and solar thermal heat.

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.

Current state of knowledge on intelligent-response biological and other macromolecular hydrogels in biomedical engineering: A review

Because intelligent hydrogels have good biocompatibility, a rapid response, and good degradability as well as a stimulus response mode that is rich, hydrophilic, and similar to the softness and elasticity of living tissue, they have received widespread attention and are widely used in biomedical engineering. In this article, we conduct a systematic review of the use of smart hydrogels in biomedical engineering. First, we introduce the properties and applications of hydrogels and compare the similarities and differences between traditional hydrogels and smart hydrogels. Secondly, we summarize the intelligent hydrogel types, the mechanisms of action used by different hydrogels, and the materials for preparing different types of hydrogels, such as the materials for the preparation of temperature-responsive hydrogels, which mainly include gelatin, carrageenan, agarose, amylose, etc.; summarize the morphologies of different hydrogels, such as films, fibers and microspheres; and summarize the application of smart hydrogels in biomedical engineering, such as for the delivery of proteins, antibiotics, deoxyribonucleic acid, etc. Finally, we summarize the shortcomings of current research and present future prospects for smart hydrogels. The purpose of this paper is to provide researchers engaged in related fields with a systematic review of the application of intelligent hydrogels in biomedical engineering. We hope that they will get some inspiration from this work to provide new directions for the development of related fields.

A Novel Gel Thermoelectric Chemical Cell for Harvesting Low-Grade Heat Energy

The use of gel thermoelectric chemical cells to capture low-grade heat for conversion to electricity is an attractive approach. However, there are few studies on whether the distribution of redox species in the electrolyte has an effect on the performance of cells. Herein, this concern was discussed by constructing a novel gel thermoelectric chemical cell (Cu-C-cg). Using cellulose-like rice paper as a separator, a concentration gradient of electrolyte was carefully constructed, so that the concentration of potassium ferrocyanide gradually decreased from the hot electrode to the cold electrode while the concentration of potassium ferricyanide gradually increased. Through electrochemical measurement and analysis, it was found that the thermoelectric performance of this cell outperformed the cell without electrolyte concentration gradients. Meanwhile, this performance could be enhanced by the use of asymmetric electrodes composed of copper foil and carbon electrodes. After optimizing the conditions, the open-circuit voltage, output power, and Seebeck coefficient of the Cu-C-cg cell at 12 K temperature difference were 0.450 V, 183 μW, and 7.82 mV K^-1 , respectively. This work not only provides a novel idea in gel-based cell design but also an excellent thermoelectric chemical cell.

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.

Thermoelectrochemical Seebeck coefficient and viscosity of Co-complex electrolytes rationalized by the Einstein relation, Jones-Dole B coefficient, and quantum-chemical calculations

The Seebeck coefficient (Se) and the viscosity of a redox electrolyte are the key characteristics of thermoelectrochemical cells that generate electric power from waste thermal energy. However, the recent upsurge of research in this field is seriously disconnected from the knowledge of solution chemistry explored in the previous century. Herein, we systematically investigate five redox couples of cobalt complexes containing different aromatic ligands and anions in γ-butyrolactone solvent to demonstrate how the Einstein relation of hydrodynamic theory and the Jones-Dole B coefficient obtained from viscosity measurements can be used to account for such electrolyte properties. In essence, we reveal that the outer-shell (solvent reorganization) and inner-shell (metal-ligand reorganization) contributions to the redox reaction entropy ΔS_rc (∝Se) can be quantified by the analyses using the B-coefficients and quantum-chemical simulations, respectively, while the distinct regimes found in the viscosity and conductivity are well accounted for by the Einstein relation, despite its classical hydrodynamic origin.

Efficient Low-Grade Heat Conversion and Storage with an Activity-Regulated Redox Flow Cell via a Thermally Regenerative Electrochemical Cycle

Efficient and cost-effective technologies are highly desired to convert the tremendous amount of low-grade waste heat to electricity. Although the thermally regenerative electrochemical cycle (TREC) has attracted increasing attention recently, the unsatisfactory thermal-to-electrical conversion efficiency and low power density limit its practical applications. In this work, a thermosensitive Nernstian-potential-driven strategy in the TREC system is demonstrated to boost its temperature coefficient, power density, and thermoelectric conversion efficiency by rationally regulating the activities of redox couples at different temperatures. With a Zn anode and [Fe(CN)₆ ]^4-/3- -guanidinium as the catholyte, the TREC flow cell presents an unprecedented average temperature coefficient of -3.28 mV K^-1 , and achieves an absolute thermoelectric efficiency of 25.1% and apparent thermoelectric efficiency of 14.9% relative to the Carnot efficiency in the temperature range of 25-50 °C at 1 mA cm^-2 . In addition, a thermoelectric power density of 1.98 mW m^-2 K^-2 is demonstrated, which is more than 7 times the highest power density of reported TREC systems. This activity regulation strategy can inspire research into high-efficiency and high-power TREC devices for practical low-grade heat harnessing.

Regulating Thermogalvanic Effect and Mechanical Robustness via Redox Ions for Flexible Quasi-Solid-State Thermocells

The design of power supply systems for wearable applications requires both flexibility and durability. Thermoelectrochemical cells (TECs) with large Seebeck coefficient can efficiently convert low-grade heat into electricity, thus having attracted considerable attention in recent years. Utilizing hydrogel electrolyte essentially addresses the electrolyte leakage and complicated packaging issues existing in conventional liquid-based TECs, which well satisfies the need for flexibility. Whereas, the concern of mechanical robustness to ensure stable energy output remains yet to be addressed. Herein, a flexible quasi-solid-state TEC is proposed based on the rational design of a hydrogel electrolyte, of which the thermogalvanic effect and mechanical robustness are simultaneously regulated via the multivalent ions of a redox couple. The introduced redox ions not only endow the hydrogel with excellent heat-to-electricity conversion capability, but also act as ionic crosslinks to afford a dual-crosslinked structure, resulting in reversible bonds for effective energy dissipation. The optimized TEC exhibits a high Seebeck coefficient of 1.43 mV K^-1 and a significantly improved fracture toughness of 3555 J m^-2, thereby can maintain a stable thermoelectrochemical performance against various harsh mechanical stimuli. This study reveals the high potential of the quasi-solid-state TEC as a flexible and durable energy supply system for wearable applications.

Thermoelectrochemical Cells Based on Ferricyanide/Ferrocyanide/Guanidinium: Application and Challenges

Ferricyanide/ferrocyanide/guanidinium-based thermoelectrochemical cells have been investigated under different loading conditions in this work. Compared with ferricyanide/ferrocyanide-based devices, the device with guanidinium-added electrolytes shows higher power and energy densities. We observed that the enhanced performance is not due to the ionic Seebeck effect of guanidinium but because of the configuration entropy change resulting from the selective binding of Gdm+ to Fe(CN)64-. However, the device with guanidinium-added electrolyte does not show steady-state operation. The two possible reasons include (1) the difficult diffusion of Fe(CN)63- into the crystal layer of (Gdm+)n[Fe(CN)64-] at the hot electrode and (2) the difficult precipitation of (Gdm+)n[Fe(CN)64-] formed at the cold side upon the binding of the reduced Fe(CN)64- with Gdm+. Nevertheless, the performance recovers once the device is disconnected from the external loading. Due to the high thermopower after adding guanidinium, we successfully fabricate self-powered sensors by connecting four flexible cells in series. The sensors can transfer humidity, temperature, and air pressure data wirelessly using body heat. Therefore, ferricyanide/ferrocyanide/guanidinium is a promising electrolyte material for applications of low-grade energy harvesting.

A flexible quasi-solid-state thermoelectrochemical cell with high stretchability as an energy-autonomous strain sensor

The design of effective energy systems is crucial for the development of flexible and wearable electronics. Regarding the direct conversion of heat into electricity, thermoelectrochemical cells (TECs) are particularly suitable for low-grade heat harvesting to enable flexible and wearable applications, despite the fact that the electrolyte leakage and complex packaging issues of conventional liquid-based TECs await to be further addressed. Herein, a quasi-solid-state TEC is assembled using the polyacrylamide/acidified-single-walled carbon nanotube (PAAm/a-SWCNT) composite hydrogel, developed via a facile in situ free-radical polymerization route with tin(IV) chloride/tin(II) chloride (Sn⁴⁺/Sn²⁺) as the redox couple. The as-fabricated TEC with a 0.6 wt% a-SWCNT content presents a large thermoelectrochemical Seebeck coefficient of 1.59 ± 0.07 mV K^-1 and exhibits excellent stability in thermoelectrochemical performance against large mechanical stretching and deformation. Owing to this superior stretchability, the as-fabricated TEC is further assembled into an energy-autonomous strain sensor, which shows high sensitivity. The strategy of utilizing a quasi-solid-state TEC for energy-autonomous strain sensing unveils the great potential of heat-to-electricity conversion in flexible and wearable electronics.

Rational Design of Thermocells Driven by the Volume Phase Transition of Hydrogel Nanoparticles

Thermocells are thermoelectrochemical conversion systems for harvesting low-temperature thermal energy. Liquid-state thermocells are particularly desirable because of low cost and their high conversion efficiency at temperatures around physiological temperature, and they have, thus, been extensively studied. However, the performance of the thermocells has to be improved to utilize them as energy chargers and/or batteries. Recently, we reported that a liquid-state thermocell driven by the volume phase transition of hydrogel nanoparticles showed highly efficient thermoelectric conversion with Seebeck coefficient (S_e) of -6.7 mV K^-1. Here, we report the design rationale of the thermocells driven by the phase transition. A high S_e of -9.5 mV K^-1 was achieved at temperature between 36 and 40 °C by optimizing choice and amount of redox chemical species. The figure of merit (ZT) of the thermocell was improved by selecting appropriate electrolyte salt to increase the ionic conductivity and prevent the precipitation of nanoparticles. Furthermore, screening of nanoparticles revealed the high correlation between S_e and the pH shift generated as a result of phase transition of the nanoparticles. After optimization, the maximum ZT of 8.0 × 10^-2 was achieved at a temperature between 20 and 70 °C.