Tunable Electrochemical Entropy through Solvent Ordering by a Supramolecular Host

Abstract

An aqueous electrochemically controlled host− guest encapsulation system demonstrates a large and synthetically tunable redox entropy change. Electrochemical entropy is the basis for thermally regenerative electrochemical cycles (TRECs), which utilize reversible electrochemical processes with large molar entropy changes for thermogalvanic waste-heat harvesting and electrochemical cooling, among other potential applications. A supramolecular host−guest system demonstrates a molar entropy change of 4 times that of the state-of-the-art aqueous TREC electrolyte potassium ferricyanide. Upon encapsulation of a guest, water molecules that structurally resemble amorphous ice are displaced from the host cavity, leveraging a change in the degrees of freedom and ordering of the solvent rather than the solvation of the redox-active species to increase entropy. The synthetic tunability of the host allows rational optimization of the system's ΔS, showing a range of −51 to −101 cal mol −1 K −1 (−2.2 to −4.4 mV K −1 ) depending on ligand and metal vertex modifications, demonstrating the potential for rational design of high-entropy electrolytes and a new strategy to overcome theoretical limits on ion solvation reorganization entropy.

Solvation of Nanoscale Materials

“…Although research in this area remains a frontier, recent examples have demonstrated that synthetic manipulation of the solvation environment serves as a pathway for enhancing DSr and the energy density of the cell. For example, solvent mixtures, 157 the use of dissolved polymers that undergo phase changes upon redox, 158 metallocages that intercalate charge balancing ions through a desolvation process, 159 and polyoxometalate anions 160 all involve large supramolecular changes to solvation structure (Figure 4A). We predict that the ability of solvent to intercalate into pores and the frustration solvent to assemble into ordered shells at porous surfaces will strongly influence the thermoelectrochemical behavior of porous colloids.…”

Converting Heat to Electrical Energy Using Highly Charged Polyoxometalate Electrolytes

“…As an alternative, solution-state electrochemical cells may reach Seebeck coefficients in the mV K -1 range either as thermally regenerative electrochemical cycles (TRECs) or as thermogalvanic cells (TGCs). 5,6 Both technologies involve redox electrolytes that equilibrate to different mixtures of oxidized and reduced forms when exposed to different temperatures. This temperature dependence of the equilibrium constant produces a free energy difference (DG) between the electrodes at dissimilar conditions and hence a usable voltage.…”

“…Recent studies include the use of gel-based electrolytes, 12 deep eutectic solvents, 10 and ionic liquids, 13 and materials such as carbon nanotubes for creating thermogalvanic cells that display a high ionic conductivity while retaining a low thermal conductivity, in order to avoid thermal equilibration across the cell. 14 Additionally, the use of solvent mixtures, 8,15-18 polymers with redox-induced phase changes, 19 and supramolecular host-guest interactions 5,6,8 have shown promise for greatly altering the ΔSrc for redox couples due to the concomitant ordering or release of solvent molecules following redox reactions. Although these recent studies employ varying methods for controlling solvation entropy, few have explored increasing charge density of the electrolyte.…”

Converting Heat to Electrical Energy Using Highly Charged Polyoxometalate Electrolytes

“…Despite decades of research, S e for most solid-state thermoelectrics remains in the μV K –1 range, , prohibiting their use in harnessing low-grade heat. As an alternative, solution-state electrochemical cells may reach Seebeck coefficients in the mV K –1 range either as thermally regenerative electrochemical cycles (TRECs) or as thermogalvanic cells (TGCs). , Both technologies involve redox electrolytes that equilibrate to different mixtures of oxidized and reduced forms upon exposure to different temperatures. This temperature dependence of the equilibrium constant produces a free energy difference (Δ G ) between the electrodes at dissimilar conditions and hence a usable voltage.…”

“…Recent studies include the use of gel-based electrolytes, deep eutectic solvents, and ionic liquids, as well as materials such as carbon nanotubes for creating thermogalvanic cells that display a high ionic conductivity while retaining a low thermal conductivity, in order to avoid thermal equilibration across the cell . Additionally, the use of solvent mixtures, ,− polymers with redox-induced phase changes, and supramolecular host–guest interactions ,, have shown promise for greatly altering the Δ S rc for redox couples due to the concomitant ordering or release of solvent molecules following redox reactions, reaching Seebeck coefficients in the range of up to −4 mV K –1 or higher. ,, Although these recent studies employ varying methods for controlling solvation entropy, few have explored increasing the charge density of the electrolyte.…”