High Seebeck Coefficient Electrochemical Thermocells for Efficient Waste Heat Recovery

An electrochemical thermocell realizes thermal to electric energy conversion when two electrodes operate the same reversible reaction but at different temperatures. Its Seebeck coefficient is determined by the entropy change of the redox reaction. Here we report a thermocell containing acetone and iso-propanol as the redox couple, which can achieve the highest reported Seebeck coefficient of −9.9 mV K^–1 when the hot side is above the boiling point of acetone. Vaporization entropy of acetone increases the total entropy change in the conversion of iso-propanol to acetone. In addition, a concentration gradient of acetone caused by evaporation and condensation increases the cell voltage significantly. Stable performance of the thermocell is enabled by a Pt–Sn catalyst operating in a neutral pH electrolyte solution. The possibility of utilizing a liquid–gas phase change to increase the Seebeck coefficient of thermocells opens a new venue for exploration

Supramolecular Thermo-Electrochemical Cells: Enhanced Thermoelectric Performance by Host–Guest Complexation and Salt-Induced Crystallization

Thermo-electrochemical cells have potential to generate thermoelectric voltage 1 order higher than that given by semiconductor materials. To overcome the current issues in thermoelectric energy conversion, it is of paramount importance to grow and fulfill the full potential of thermo-electrochemical cells. Here we report a rational supramolecular methodology that yielded the highest Seebeck coefficient of ca. 2.0 mV K–1 around ambient temperatures. This is based on the encapsulation of triiodide ions in α-cyclodextrin, whose equilibrium is shifted to the complexation at lower temperatures, whereas it is inverted at elevated temperatures. This temperature-dependent host–guest interaction provides a concentration gradient of redox ion pairs between two electrodes, leading to the eminent performance of the thermo-electrochemical cells. The figure of merit for this system, zT reached a high value of 5 × 10–3. The introduction of host–guest chemistry to thermoelectric cells thus provides a new perspective in thermoelectric energy conversion

Supramolecular Thermo-Electrochemical Cells: Enhanced Thermoelectric Performance by Host–Guest Complexation and Salt-Induced Crystallization

Thermo-electrochemical cells have potential to generate thermoelectric voltage 1 order higher than that given by semiconductor materials. To overcome the current issues in thermoelectric energy conversion, it is of paramount importance to grow and fulfill the full potential of thermo-electrochemical cells. Here we report a rational supramolecular methodology that yielded the highest Seebeck coefficient of ca. 2.0 mV K^–1 around ambient temperatures. This is based on the encapsulation of triiodide ions in α-cyclodextrin, whose equilibrium is shifted to the complexation at lower temperatures, whereas it is inverted at elevated temperatures. This temperature-dependent host–guest interaction provides a concentration gradient of redox ion pairs between two electrodes, leading to the eminent performance of the thermo-electrochemical cells. The figure of merit for this system, <i>zT</i> reached a high value of 5 × 10^–3. The introduction of host–guest chemistry to thermoelectric cells thus provides a new perspective in thermoelectric energy conversion

Molecular Design of Organic Ionic Plastic Crystals Consisting of Tetracyanoborate with Ultralow Phase Transition Temperature

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]−) and various organic cations. In particular, the OIPC composed of [TCB]− and spiro-(1,1′)-bipyrrolidinium ([spiropyr]+) cations can transform into its plastic phase at ultralow temperature (Tp = −55 °C) while maintaining a high melting point (Tm = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high Tm was attributed to the relatively small interionic distance between [spiropyr]+ and [TCB]−. At the same time, the low Tp was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input

Molecular Design of Organic Ionic Plastic Crystals Consisting of Tetracyanoborate with Ultralow Phase Transition Temperature

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]−) and various organic cations. In particular, the OIPC composed of [TCB]− and spiro-(1,1′)-bipyrrolidinium ([spiropyr]+) cations can transform into its plastic phase at ultralow temperature (Tp = −55 °C) while maintaining a high melting point (Tm = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high Tm was attributed to the relatively small interionic distance between [spiropyr]+ and [TCB]−. At the same time, the low Tp was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input

Thin Solid Electrolyte Layers Enabled by Nanoscopic Polymer Binding

To achieve high-energy all-solid-state batteries (ASSBs), solid-state electrolytes (SE) must be thin, mechanically robust, and possess the ability to form low resistance interfaces with electrode materials. Embedding an inorganic SE into an organic polymer combines the merits of high conductivity and flexibility. However, the performance of such an SE-in-polymer matrix (SEPM) is highly dependent on the microstructure and interactions between the organic and inorganic components. We report on the synthesis of a free-standing, ultrathin (60 μm) SEPM from a solution of lithium polysulfide, phosphorus sulfide, and ethylene sulfide (ES), where the polysulfide triggers the in situ polymerization of ES and the formation of Li3PS4. Reactant ratios were optimized to achieve a room-temperature conductivity of 2 × 10–5 S cm–1. Cryogenic electron microscopy confirmed a uniform nanoscopic distribution of β-Li3PS4 and PES (polyethylene sulfide). This work presents a facile route to the scalable fabrication of ASSBs with promising cycling performance and low electrolyte loading

Structure and Solution Dynamics of Lithium Methyl Carbonate as a Protective Layer For Lithium Metal

Lithium methyl carbonate (LMC) is synthesized in a single step and investigated as a coating material for Li protection. The LMC layer can be formed <i>in situ</i> during Li plating on Cu foil. The chemical and crystal structure of the ∼1 μm thick coating is confirmed, which remains intact during lithium cycling. SEM also confirms the dissolution of the LMC layer after being transferred into the commercial LiPF₆-carbonate electrolyte. Presaturating the electrolyte with LMC powder prevents the dissolution of the LMC coating layer, which improves the Li plating/stripping Coulombic efficiency from 90.5% (bare Cu in commercial electrolyte) to 96.1% at 0.5 mA cm^–2 and 1 mAh cm^–2

Thin Solid Electrolyte Layers Enabled by Nanoscopic Polymer Binding

To achieve high-energy all-solid-state batteries (ASSBs), solid-state electrolytes (SE) must be thin, mechanically robust, and possess the ability to form low resistance interfaces with electrode materials. Embedding an inorganic SE into an organic polymer combines the merits of high conductivity and flexibility. However, the performance of such an SE-in-polymer matrix (SEPM) is highly dependent on the microstructure and interactions between the organic and inorganic components. We report on the synthesis of a free-standing, ultrathin (60 μm) SEPM from a solution of lithium polysulfide, phosphorus sulfide, and ethylene sulfide (ES), where the polysulfide triggers the in situ polymerization of ES and the formation of Li3PS4. Reactant ratios were optimized to achieve a room-temperature conductivity of 2 × 10–5 S cm–1. Cryogenic electron microscopy confirmed a uniform nanoscopic distribution of β-Li3PS4 and PES (polyethylene sulfide). This work presents a facile route to the scalable fabrication of ASSBs with promising cycling performance and low electrolyte loading