Ion–Electron Coupling Enables Ionic Thermoelectric Material with New Operation Mode and High Energy Density

Ionic thermoelectrics(i-TE) possesses great potential in powering distributed electronics because it can generate thermopower up to tens of millivolts per Kelvin. However,as ions cannot enter external circuit, the utilization of i-TE is currently based on capacitive charge/discharge, which results in discontinuous working mode and low energy density. Here,we introduce an ion–electron thermoelectric synergistic(IETS)effect by utilizing an ion–electron conductor. Electrons/holes can drift under the electric field generated by thermodiffusion of ions, thus converting the ionic current into electrical current that can pass through the external circuit. Due to the IETS effect, i-TE is able to operate continuously for over 3000 min.Moreover, our i-TE exhibits a thermopower of 32.7 mV K^(-1) and an energy density of 553.9 J m^(-2), which is more than 6.9 times of the highest reported value. Consequently, direct powering of electronics is achieved with i-TE. This work provides a novel strategy for the design of high-performance i-TE materials.

High-Performance Ionic Thermoelectric Supercapacitor for Integrated Energy Conversion-Storage

Converting low-grade waste heat into usable electricity and storing it simultaneously requires a new technology that realize the directional migration of electrons or ions under temperature difference and enrichment on the electrodes.Although the urgent demand of energy conversion-storage(ECS)has emerged in the field of wearable electronic,achieving the integrated bi-functional device remains challenge due to the different mechanisms of electrical transportation and storage.Here,we report an ionic thermoelectric supercapacitor that relies on the synergistic functions of thermoelectricity and supercapacitor in the thermoelectric ionogel electrolyte and high-performance hydrogel electrodes to enhance the ECS performance under a thermal gradient.The thermoelectric electrolyte is composed of polyacrylamide hydrogel and sodium carboxymethyl cellulose(PMSC),possessing cross-linked network with excellent cation selectivity,while the ionic thermoelectric properties are further improved in the presence of NaCl.The corresponding Seebeck coefficient and ionic conductivity of the NaCl–PMSC electrolyte reach 17.1 mV K^(-1)and 26.8 mS cm^(-1),respectively.Owing to good stretchability of both gel-based electrolyte and electrode,the fullstretchable integrated ECS device,termed ionic thermoelectric supercapacitor,presents promising thermal-charge storage capability(~1.3 mC,ΔT≈10 K),thus holds promise for wearable energy harvesting.

Solvation Engineering via Fluorosurfactant Additive Toward Boosted Lithium-Ion Thermoelectrochemical Cells

Lithium-ion thermoelectrochemical cell(LTEC), featuring simultaneous energy conversion and storage, has emerged as promising candidate for low-grade heat harvesting. However, relatively poor thermosensitivity and heat-to-current behavior limit the application of LTECs using LiPF_6 electrolyte. Introducing additives into bulk electrolyte is a reasonable strategy to solve such problem by modifying the solvation structure of electrolyte ions. In this work, we develop a dual-salt electrolyte with fluorosurfactant(FS) additive to achieve high thermopower and durability of LTECs during the conversion of low-grade heat into electricity. The addition of FS induces a unique Li~+ solvation with the aggregated double anions through a crowded electrolyte environment,resulting in an enhanced mobility kinetics of Li~+ as well as boosted thermoelectrochemical performances. By coupling optimized electrolyte with graphite electrode, a high thermopower of 13.8 mV K^(-1) and a normalized output power density of 3.99 mW m^(–2) K^(–2) as well as an outstanding output energy density of 607.96 J m^(-2) can be obtained.These results demonstrate that the optimization of electrolyte by regulating solvation structure will inject new vitality into the construction of thermoelectrochemical devices with attractive properties.

Quasi-Solid Thermoelectrics for Wearable Low-Grade Energy Harvesting

Ionic thermoelectricity(i-TE),as a new energy conversion and storage technology,has been widely discussed by the academic community.As one of the representatives of low-grade thermal energy recovery,i-TE has made remarkable progress and become an influential research direction in the energy field.Among them,thermoelectric ionogels have a wide range of applications in the field of energy recovery and utilization due to their excellent flexibility,stability,and thermoelectric conversion ability,providing many application possibilities for such materials.The development of highly efficient and stable ionic thermoelectric devices is largely dependent on the development of new materials and structural designs.This paper focuses on the recent strategies for improving the efficiency of thermoelectric conversion in the field of ionic thermoelectric gels,including new methods for material design,structural optimization,and innovative developments in the application of thermoelectric materials.The evaluation indicators of thermoelectric conversion efficiency are discussed,including ionic thermal voltage,ionic conductivity and power output,ductility,and self-healing properties.Additionally,various application devices based on thermoelectric materials with excellent thermoelectric conversion properties are highlighted.Further,different challenges and strategies that need to be addressed are presented in the hope of providing inspiration and guidance for the commercialization of i-TE.

Durable and stretchable nanocomposite ionogels with high thermoelectric property for low-grade heat harvesting

Stretchable ionic thermoelectric(i-TE) materials have attracted growing interest in converting low-grade thermal energy into electricity. However, substantial improvement on i-TE performance of quasi-solid ionogels remains a significant challenge.Here, a nanocomposite ionogel with skin-like stretchability, high i-TE performance, thermostability and durability is prepared by hybridizing ionic liquid(IL) and Laponite nanosheets into waterborne polyurethane(WPU). With multiple H-bond, WPU can accommodate a higher content of IL, thereby improving its ionic conductivity. After cation exchange between IL and Laponite,the negatively charged Laponite sheets and released Na+can enhance the ionic Seebeck coefficient by enlarging thermophoretic mobility difference between the cations and anions in ionogel. Besides, incorporation of Laponite causes the decrease of thermal conductivity. Thus, the WPU-IL-Laponite ionogel exhibits a high ionic thermopower of 44.1 m V K-1, high ionic conductivity of 14.1 m S cm-1and low thermal conductivity of 0.43 W m-1K-1at a relative humidity of 90%. The corresponding ionic figure of merit of the ionogel is 1.90±0.27. Moreover, the ionogel demonstrates excellent durability during repeated stretching process.The stretchable ionogel can be fabricated into ionic thermoelectric capacitor to convert thermal energy from solar radiation into electricity.