The electrochemistry and performance of cobalt-based redox couples for thermoelectrochemical cells

Large Temperature Dependence of Redox Potential Driven by Semiclathrate Hydrate Formation for Thermo-Electrochemical Conversion

The temperature dependence of the redox potential (temperature coefficient) is a critical parameter for redox couples employed in thermoelectrochemical conversion devices, such as thermogalvanic cells and thermally regenerative electrochemical cycles (TRECs). We developed a novel strategy for boosting the temperature coefficient of ferro/ferricyanide through the formation/dissociation of a semiclathrate hydrate (SCH). The aqueous solution with ferro/ferricyanide and tetrabutylammonium fluoride (TBAF) showed SCH formation/dissociation by small temperature variations, which contributed to a huge temperature coefficient (-13.8 mV K-1) near ambient temperature. The large-temperature coefficient was attributed to a significant change in the TBAF concentration in the liquid phase caused by SCH formation/dissociation, resulting in the rearrangement of the ion pair of ferricyanide and cations. We introduced the electrolyte to a charging-free TREC device driven by a small temperature swing (9 K) and achieved the highest normalized power density (4.8 mW m-2 K-2). This electrolyte design strategy will pave the way for electrochemical energy harvesting from small temperature changes such as diurnal temperature variations. In addition, this study creates a new research field for semiclathrate hydrate chemistry.

Effect of Coordination Environment and Electronic Coupling on Redox Entropy in a Family of Dinuclear Complexes

The elucidation of factors that govern the temperature sensitivity of the electrochemical potential is essential to the development of electrochemical systems with target properties. Toward this end, we report a series of isostructural homo- and heterometallic M2 (M = FeII, FeIII, ZnII) complexes supported by a phenoxo-centered tetrapyridyl ligand and ancillary carboxylate ligands that enables independent change in (i) charge, (ii) coordination environment of the redox-active center(s), and (iii) electronic coupling strength between redox centers. Variable-temperature electrochemical analysis of the series reveals the temperature coefficient for Fe-based redox couples to be highly dependent on the coordination environment of the redox-active center(s), with Fe centers in a pseudo-octahedral [FeN3O3] coordination environment affording a 2-fold greater temperature coefficient for the FeIII/FeII redox couple than those in ancillary ferrocenyl groups. In contrast, identical temperature coefficients for the FeIII/FeII redox event in Fe2 and FeZn complexes establish electronic coupling strength to have a minimal impact on the temperature dependence of the Fe-based redox couple. Taken together, these results provide important insights for the design of molecular compounds with target redox properties, and they provide the first examination of how electronic coupling influences the temperature dependence of the redox potential and the associated redox entropy in molecular compounds.

Highly efficient activation of peroxymonosulfate by cobalt ferrite anchored in P-doped activated carbon for degradation of 2,4-D: adsorption and electron transfer mechanism

The dispersing effect of carbon materials on nanoparticles can enhance the full exposure of their active sites. Herein, phosphorus (P)-doped activated carbon-supported trace cobalt ferrite composites (P-CoFe@BC_X) were achieved by two-step pyrolysis for efficient peroxymonosulfate (PMS) activation and water pollution remediation. The removal efficiency of 2,4-dichlorophenoxyacetic acid (2,4-D) was optimized by adjusting the coupling ratio of carbon substrate and cobalt ferrite. P-CoFe@BC₅/PMS oxidation system (0.10 g L^-1, 0.50 mM) eliminated 98.3% of 2,4-D (20.0 mg L^-1) within 60 min at unadjusted pH. The constructed adsorption enrichment and oxidative degradation pathways are highly efficient in utilizing reactive oxygen species (ROS), and the dual tracks of free and non-free radicals achieve the rapid degradation of 2,4-D. P-doped activated carbon acts as an electron shuttle to accelerate electron transfer between active sites and enhances the adsorption efficiency of 2,4-D and PMS onto the composites. In addition, the P-CoFe@BC₅/PMS oxidation system still exhibited strong 2,4-D removal performance at a wide pH range of 2.0-10.0. The inhibitory effect of environmental components was related to their concentration, such as chloride, bicarbonate, sulfate and humic acid. Density functional theory calculations show that ROS tends to attack the CO bond on the 2,4-D branch chain, and the degradation products show lower biological toxicity. Hence, the constructed cobalt ferrite anchored P-doped activated carbon activated PMS system has great potential in treating organic wastewater.

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 Photo-Thermo-Electric Conversion Using Polyoxovanadate in Ionic Liquid for Low-Grade Heat Utilization

A large fraction of energy, including solar energy, is dissipated into ambient atmosphere as low-grade waste heat. Efficient utilization of such energy is critical to address the current energy crisis and global warming issue. Herein, the efficient near-IR (NIR)-photothermal, thermoelectric, and thus photo-thermo-electric conversion of polyoxovanadate compound {[Ni(1,10-phenanthroline)₃ ][V₁₄ O₃₄ Cl]Cl, NiV₁₄ } in ionic liquid was achieved. The solution displayed a NIR-photothermal efficiency of 16.04 and 23.43 % at 808 and 1064 nm, respectively. Taking advantage of the synergetic thermodiffusive and thermogalvanic effects of various ion species in NiV₁₄ solution, an open circuit voltage of approximately 0.45 V was obtained at ΔT=70 K generated by physical heating or NIR irradiation, indicating a large Seebeck coefficient of 6.38 mV K^-1 and an optimized thermal power at 1.2 W m^-2 . The polyoxovanadate-ionic liquid system offers a new platform for efficiently utilizing not only low-grade thermal energy but also solar energy for electricity generation.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Thermocells Driven by Phase Transition of Hydrogel Nanoparticles

Thermoelectric conversion of low temperature, delocalized, and abundant thermal sources is crucial for the development of the Internet of Things (IoT) and/or a carbon-free society. Thermocells are of great interest in thermoelectric conversion of low-temperature heat due to the low cost and flexibility of components. However, significant improvement of the conversion efficiency is required for the practical use of the cells. Here, we report thermo-electrochemical cells driven by volume phase transition (VPT) of hydrogel nanoparticles (NPs). Entropically driven VPT of poly(N-isopropylacrylamide) NPs containing carboxylic acids and amines generates a pH gradient of up to 0.049 and -0.053 pH K^-1, respectively, around physiological temperature. The pH gradient triggers the proton-coupled electron transfer (PCET) reactions of quinhydrone on the electrodes, resulting in the highly efficient thermoelectric conversion with a Seebeck coefficient (S_e) of -6.7 and +6.1 mV K^-1. Thermocells driven by phase transition of hydrogels provide a nontoxic, flexible, and inexpensive charger that harvests carbon-free energy from abundant energy sources such as solar, body and waste heat.

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.

Fabrication of redox-active polyoxometalate-based ionic crystals onto single-walled carbon nanotubes as high-performance anode materials for lithium-ion batteries

Polyoxometalate-based ionic crystals were fabricated onto single-walled carbon nanotubes as anode materials for lithium-ion batteries with high specific capacity and excellent cycling stability.

Electrochemical Thermoelectric Conversion with Polysulfide as Redox Species

Thermocells convert waste heat to electricity without any pollution; however, the high cost and corrosivity of redox species hinder their commercialization. In this work, a thermocell that utilizes abundant polysulfide as redox species was demonstrated for the first time. 1-Butyl-1-methylpyrrolidinium polysulfide [(P₁₄ )₂ S₃ ] was synthesized, and the redox species were prepared by the addition of sulfur to the (P₁₄ )₂ S₃ solution in DMSO. In thermoelectric measurements, the Seebeck coefficient changed from -0.68 to +0.5 mV K^-1 through addition of sulfur to the cell. Operando UV/Vis spectroscopy and open-circuit voltage analysis revealed that this effect was attributed to the change in the dominating redox reactions by the addition of sulfur. This result also provides a thermodynamic view on polysulfides electrochemistry, which is of high importance for lithium-sulfur batteries.

Inversion of thermodiffusive properties of ionic colloidal dispersions in water-DMSO mixtures probed by forced Rayleigh scattering

Thermodiffusion properties at room temperature of colloidal dispersions of hydroxyl-coated nanoparticles (NPs) are probed in water, in dimethyl sulfoxide (DMSO) and in mixtures of water and DMSO at various proportions of water, [Formula: see text]. In these polar solvents, the positive NPs superficial charge imparts the systems with a strong electrostatic interparticle repulsion, slightly decreasing from water to DMSO, which is here probed by Small Angle Neutron Scattering and Dynamic Light Scattering. However if submitted to a gradient of temperature, the NPs dispersed in water with ClO₄^- counterions present a thermophilic behavior, the same NPs dispersed in DMSO with the same counterions present a thermophobic behavior. Mass diffusion coefficient [Formula: see text] and Ludwig-Soret coefficient [Formula: see text] are measured as a function of NP volume fraction [Formula: see text] at various [Formula: see text]. The [Formula: see text]-dependence of [Formula: see text] is analyzed in terms of thermoelectric and thermophoretic contributions as a function of [Formula: see text]. Using two different models for evaluating the Eastman entropy of transfer of the co- and counterions in the mixtures, the single-particle thermophoretic contribution (the NP's Eastman entropy of transfer) is deduced. It is found to evolve from negative in water to positive in DMSO. It is close to zero on a large range of [Formula: see text] values, meaning that in this [Formula: see text]-range [Formula: see text] largely depends on the thermoelectric effect of free co- and counterions.

Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions - anisotropy of the Soret coefficient under a magnetic field

Under a temperature gradient, the direction of thermodiffusion of charged γ-Fe2O3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either a positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient ST is measured in stationary conditions together with the mass-diffusion coefficient Dm using a forced Rayleigh scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of Dm with NP volume fraction Φ. The Φ-dependence of ST is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transfer ŝNP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively, with ST > 0 and with ST < 0 are then probed under an applied field H[combining right harpoon above], and an anisotropy of Dm and of ST is induced while the in-field system remains monophasic. Whatever the H[combining right harpoon above]-direction (parallel or perpendicular to the gradients and ), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.

Thermoelectricity and Thermodiffusion in Magnetic Nanofluids: Entropic Analysis

An analytical model describing the thermoelectric potential production in magnetic nanofluids (dispersions of magnetic and charged colloidal particles in liquid media) is presented. The two major entropy sources, the thermogalvanic and thermodiffusion processes are considered. The thermodiffusion term is described in terms of three physical parameters; the diffusion coefficient, the Eastman entropy of transfer and the electrophoretic charge number of colloidal particles, which all depend on the particle concentration and the applied magnetic field strength and direction. The results are combined with well-known formulation of thermoelectric potential in thermogalvanic cells and compared to the recent observation of Seebeck coefficient enhancement/diminution in magnetic nanofluids in polar media.