Wearable Thermocells Based on Gel Electrolytes for the Utilization of Body Heat

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.

Advances in Thermo-Electrochemical (TEC) Cell Performances for Harvesting Low-Grade Heat Energy: A Review

Thermo-electrochemical cells (also known as thermocells, TECs) represent a promising technology for harvesting and exploiting low-grade waste heat (<100–150 °C) ubiquitous in the modern environment. Based on temperature-dependent redox reactions and ion diffusion, emerging liquid-state thermocells convert waste heat energy into electrical energy, generating power at low costs, with minimal material consumption and negligible carbon footprint. Recent developments in thermocell performances are reviewed in this article with specific focus on new redox couples, electrolyte optimisation towards enhancing power output and operating temperature regime and the use of carbon and other nanomaterials for producing electrodes with high surface area for increasing current density and device performance. The highest values of output power and cell potentials have been achieved for the redox ferri/ferrocyanide system and Co2+/3+, with great opportunities for further development in both aqueous and non-aqueous solvents. New thermoelectric applications in the field include wearable and portable electronic devices in the health and performance-monitoring sectors; using body heat as a continuous energy source, thermoelectrics are being employed for long-term, continuous powering of these devices. Energy storage in the form of micro supercapacitors and in lithium ion batteries is another emerging application. Current thermocells still face challenges of low power density, conversion efficiency and stability issues. For waste-heat conversion (WHC) to partially replace fossil fuels as an alternative energy source, power generation needs to be commercially viable and cost-effective. Achieving greater power density and operations at higher temperatures will require extensive research and significant developments in the field.

Ionic Gelatin-Based Flexible Thermoelectric Generator with Scalability for Human Body Heat Harvesting

The prosperity of intelligent wearables brings an increasingly critical problem of power supply. Regular rechargeable lithium or disposable button batteries have some problems, such as limited capacity, frequent replacement, environmental pollution, etc. Wearable energy harvester (WEH) can fundamentally solve these problems. Among WEHs, thermoelectric generator (TEG) is a promising option due to its independence of light condition or the motion of the wearer, and thermoelectric conversion (TEC) has the characteristics of quietness and continuity. Therefore, TEG has become a suitable choice for harvesting low-grade heat energy such as human body heat. Ionic thermoelectric gel (iTEG) has the advantages of a large Seebeck coefficient, freely defined shape and size, low processing cost, wide material sources, easy encapsulation, etc. In this paper, the gelatin-based iTEG is regulated and optimized by silica nanoparticles (SiO2 NPs). The optimal compound quantity of SiO2 NPs is determined, and the optimization mechanism is discussed through a series of characterization tests. Based on the iTEG, a kind of scalable flexible TEGs is proposed, and its preparation method is described in detail. A small wristband TEG (STEG) was made, and its Seebeck coefficient is 74.5 mV/K. Its bendability and stretchability were verified, and the impedance matching experiment was carried out. By charging a capacitor, the STEG successfully lights up an LED at a temperature difference (ΔT) of ~15.5 K. Subsequently, a large extended oversleeve TEG (LTEG) was prepared, and a set of heat sinks was added at the cooling end of the LTEG. Being worn on a volunteer’s forearm, the LTEG output a voltage of more than 3 V at ~20 °C. Through storing the converted energy in a capacitor, the LTEG directly drove a calculator without a DC–DC booster. The proposed iTEG and TEGs in this paper have the prospect of mass production, extending to people’s clothes, harvesting human body heat and directly powering wearable electronics.

Wearable Photo-Thermo-Electrochemical Cells (PTECs) Harvesting Solar Energy

Solar induced thermal energy is a vital heat source supplementing body heat to realize thermo-to-electric energy supply for wearable electronics. Thermo-electrochemical cells (TECs), compared to the widely investigated thermoelectric generators (TEGs), show greater potential in wearable applications due to the higher voltage output from low-grade heat and the increased option range of cheap and flexible electrode/electrolyte materials. In this work, a wearable photo-thermo-electrochemical cell (PTEC) is firstly fabricated through the introduction of a polymer-based flexible photothermal film as a solar-absorber and hot electrode, followed by a systematic investigation of wearable device design. The as-prepared PTEC single device shows outstanding output voltage and current density of 15.0 mV and 10.8 A m^-2 , 7.1 mV and 8.57 A m^-2 , for the device employing p-type and n-type gel electrolytes, respectively. Benefiting from the equivalent performance in current density, a series connection containing 18 pairs of p-n PTEC devices is effectively made, which can harvest solar energy and charge supercapacitors to above 250 mV (1 sun solar illumination). Meanwhile, a watch-strap shaped flexible PTECs (8 p-n pairs) that can be worn on a wrist is fabricated, and the realised voltage above 150 mV under light shows the potential for use in wearable applications. This article is protected by copyright. All rights reserved.

All-polymer wearable thermoelectrochemical cells harvesting body heat

Wearable thermoelectrochemical cells have attracted increasing interest due to their ability to turn human body heat into electricity. Here, we have fabricated a flexible, cost-effective, and 3D porous all-polymer electrode on an electrical conductive polymer substrate via a simple 3D printing method. Owing to the high degree of electrolyte penetration into the 3D porous electrode materials for redox reactions, the all-polymer based porous 3D electrodes deliver an increased power output of more than twice that of the film electrodes under the same mass loading using either n-type or p-type gel electrolytes. To realize the practical application of our thermocell, we fabricated 18 pairs of n-p devices through a series connection of single devices. The strap shaped thermocell arrangement was able to charge up a commercial supercapacitor to 0.27 V using the body heat of the person upon which it was being worn and in turn power a typical commercial lab timer.

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A compatible high electrical conductivity polymer film works as underlying substrate

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3D printable polymer ink with suitable rheological properties

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A serial 18 pairs of n-p devices charged supercapacitor to power a lab timer

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3D-printed all-polymer electrode thermocell device for harvesting body heat

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.

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.

Colossal thermo-hydro-electrochemical voltage generation for self-sustainable operation of electronics

Thermoelectrics are suited to converting dissipated heat into electricity for operating electronics, but the small voltage (~0.1 mV K⁻¹) from the Seebeck effect has been one of the major hurdles in practical implementation. Here an approach with thermo-hydro-electrochemical effects can generate a large thermal-to-electrical energy conversion factor (TtoE factor), −87 mV K⁻¹ with low-cost carbon steel electrodes and a solid-state polyelectrolyte made of polyaniline and polystyrene sulfonate (PANI:PSS). We discovered that the thermo-diffusion of water in PANI:PSS under a temperature gradient induced less (or more) water on the hotter (or colder) side, raising (or lowering) the corrosion overpotential in the hotter (or colder) side and thereby generating output power between the electrodes. Our findings are expected to facilitate subsequent research for further increasing the TtoE factor and utilizing dissipated thermal energy.

Thermoelectrics are suited to converting dissipated heat into electricity for operating electronics but limited by the small voltage from the Seebeck effect. Here, the authors report a thermo-hydro-electrochemical hybrid device with −87 mV K⁻¹.

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.

Ionic thermoelectric materials for waste heat harvesting

Ionic thermoelectric polymers are a new class of materials with great potential for use in low-grade waste heat harvesting and the field has seen much progress during the recent years. In this work, we briefly review the working mechanism of such materials, the main advances in the field and the main criteria for performance comparison. We examine two types of polymer-based ionic thermoelectric materials: ionic conductive polymer and ionogels. Moreover, as a comparison, we also examine the more conventional ionic liquid electrolytes. Their performance, possible directions of improvements and potential applications have been evaluated.

Thermogalvanic Hydrogel for Synchronous Evaporative Cooling and Low-Grade Heat Energy Harvesting

Efficient heat removal and recovery are two conflicting processes that are difficult to achieve simultaneously. Here, in this work, we pave a new way to achieve this through the use of a smart thermogalvanic hydrogel film, in which the ions and water undergo two separate thermodynamic cycles: thermogalvanic reaction and water-to-vapor phase transition. When the hydrogel is attached to a heat source, it can achieve efficient evaporative cooling while simultaneously converting a portion of the waste heat into electricity. Moreover, the hydrogel can absorb water from the surrounding air to regenerate its water content later on. This reversibility can be finely designed. As an applicative demonstration, the hydrogel film with a thickness of 2 mm was attached to a cell phone battery while operating. It successfully decreased the temperature of the battery by 20 °C and retrieved electricity of 5 μW at the discharging rate of 2.2 C.

A Photowelding Strategy for Conductivity Restoration in Flexible Circuits

Light-driven micropumps, which are based on electro-osmosis with the electric field generated by photocatalytic reactions, are among most attractive research topics in chemical micromotors. Until now, research in this field has mainly been focused on the directional motion or collective behavior of microparticles, which lack practical applications. In this study, we have developed a photowelding strategy for repeated photoinduced conductivity recovery of cracked flexible circuits. We immersed the circuit in a suspension of conductive healing particles and applied photoillumination to the crack; photocatalysis of a predeposited pentacene (PEN) layer triggered electro-osmotic effects to gather conductive particles at the crack, thus leading to conductivity recovery of the circuit. This photowelding strategy is a novel application of light-driven micropumps and photocatalysis for conductivity restoration.

Hydrogels Containing the Ferri/Ferrocyanide Redox Couple and Ionic Liquids for Thermocells

Thermoelectrochemical cells are a promising new technology for harvesting low-grade waste heat. The operation of these cells relies on a redox couple within an electrolyte, which is most commonly water-based, and improvement of these materials is a key aspect of the advancement of this technology. Here, we report the gelation of aqueous electrolytes containing the K3Fe(CN)6/K4Fe(CN)6 redox couple using a range of different polymers, including polyvinyl alcohol (PVA), sodium carboxymethyl cellulose (Cmc), polyacrylamide (PAAm), and two commercial polyurethane-based polymers: HydroMed D640 and HydroSlip C. These polymers produce quasi-solid-state electrolytes with sufficient mechanical properties to prevent leakage, and allow improved device flexibility and safety. Furthermore, the incorporation of various ionic liquids within the optimized hydrogel network is investigated as a route to enhance the electrochemical and mechanical properties and thermal energy harvesting performance of the hydrogels.

Quasi-solid-State Electrolytes for Low-Grade Thermal Energy Harvesting using a Cobalt Redox Couple

Thermoelectrochemical cells, also known as thermocells, are electrochemical devices for the conversion of thermal energy directly into electricity. They are a promising method for harvesting low-grade waste heat from a variety of different natural and manmade sources. The development of solid- or quasi-solid-state electrolytes for thermocells could address the possible leakage problems of liquid electrolytes and make this technology more applicable for wearable devices. Here, we report the gelation of an organic-solvent-based electrolyte system containing a redox couple for application in thermocell technologies. The effect of gelation of the liquid electrolyte, comprising a cobalt bipyridyl redox couple dissolved in 3-methoxypropionitrile (MPN), on the performance of thermocells was investigated. Polyvinylidene difluoride (PVDF) and poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) were used for gelation of the electrolyte, and the influence of the different polymers on the mechanical properties was studied. The Seebeck coefficient and diffusivity of the cobalt redox couple were measured in both liquid and gelled electrolytes, and the effect of gelation on the thermocell performance is reported. Finally, the cell performance was further improved by optimizing the concentration of the redox couple and the separation between the hot and cold electrodes, and the stability of the device over 25 h of operation is demonstrated.

Thermo-electrochemical cells for waste heat harvesting - progress and perspectives

Thermo-electrochemical cells (also called thermocells) are promising devices for harvesting waste heat for the sustainable production of energy. Research into thermocells has increased significantly in recent years, driven by advantages such as their ability to continuously convert heat into electrical energy without producing emissions or consuming materials. Until relatively recently, the commercial viability of thermocells was limited by their low power output and conversion efficiency. However, there have lately been significant advances in thermocell performance as a result of improvements to the electrode materials, electrolyte and redox chemistry and various features of the cell design. This article overviews these recent developments in thermocell research, including the development of new redox couples, the optimisation of electrolytes for improved power output and high-temperature operation, the design of high surface area electrodes for increased current density and device flexibility, and the optimisation of cell design to further enhance performance.

Self-Powered Multimodal Temperature and Force Sensor Based-On a Liquid Droplet

Herein we report a self-powered multimodal temperature and force sensor based on the reverse electrowetting effect and the thermogalvanic effect in a liquid droplet. The deformation of the droplet and the temperature difference across the droplet can induce an alternating pulse voltage and a direct voltage, respectively, which is easy to separate/analyze and can be utilized to sense the external force and temperature simultaneously. In addition, an integral display system that can derive information from external temperature/force concurrently is constructed. Combined with advantages of excellent sensing properties and a simple structure, the droplet sensor has promising applications in a wide range of intelligent electronics.