High-efficiency thermocells driven by thermo-electrochemical processes

Abstract

Thermocells have a significantly higher Seebeck coefficient than traditional thermoelectric generators, being promising for harvesting low-grade heat. The efficiency and power density of thermocells can be improved by utilization of appropriate redox couples and electrolyte/electrode materials. Engineering strategies, such as introducing membranes or additives, further improve the performance and validity of thermocells. There are increasing direct and indirect thermocell applications, where the latter can be represented by ionic thermoelectric supercapacitors or thermally regenerative electrochemical cycles. Thermocells (also called thermo-electrochemical cells) are a promising technology for converting low-grade heat (<200°C) into electricity through temperature-dependent redox reactions and/or ion diffusion. Very recently, there have been several breakthroughs in thermocells regarding Seebeck coefficients up to 34 mVK–1 and efficiencies up to 11% by optimizing thermo-electrochemical processes. Proof-of-concept devices can obtain a power output on the order of 100 mW by harvesting ambient body heat or solar energy, which are effective power sources for various electronic devices. The rapid pace of advances in this field, however, also trigger rigorous controversies, including volatility, low power density, and the degradation of redox couples. Herein, we provide a holistic discussion on the current-state knowledge for improving thermocell performance and examine a few state-of-the-art engineering strategies for broadening the application of thermocells. Thermocells (also called thermo-electrochemical cells) are a promising technology for converting low-grade heat (<200°C) into electricity through temperature-dependent redox reactions and/or ion diffusion. Very recently, there have been several breakthroughs in thermocells regarding Seebeck coefficients up to 34 mVK–1 and efficiencies up to 11% by optimizing thermo-electrochemical processes. Proof-of-concept devices can obtain a power output on the order of 100 mW by harvesting ambient body heat or solar energy, which are effective power sources for various electronic devices. The rapid pace of advances in this field, however, also trigger rigorous controversies, including volatility, low power density, and the degradation of redox couples. Herein, we provide a holistic discussion on the current-state knowledge for improving thermocell performance and examine a few state-of-the-art engineering strategies for broadening the application of thermocells. the interaction of an ion and its solvation shell with the solution. the two parallel layers of charge surrounding the surface exposed to a fluid, where the first layer refers to the adsorbed ions by chemical interactions and the second layer refers to ions attracted to surface charge by Coulomb force, electrically screening the first layer. the goodness of thermoelectric materials for generation, defined as ZT = S2σT/κ. the use of term ‘ion’ here is to compare with traditional electronic thermoelectric materials and rationalize the definition of ionic thermoelectric tensors, such as ionic Seebeck coefficient. In fact, only a portion of thermocells (e.g., thermodiffusion cells) are based on ion transport, while the others are based on the whole system containing redox couples, electrolytes, and electrodes, which cannot be mixed with thermoelectric materials. under assumption of microscopic reversibility or detailed balance, the flux-like property (J, e.g., electrical current) is proportional to a force-like property (X, e.g., potential gradient), namely J = LX, where L is a phenomenological coefficient matrix. Provided a proper choice of flux and force, L is symmetrical (i.e., Li,k = Lk,i). partial derivative of entropy with changes in the molar composition under constant temperature and pressure. the competence of thermoelectric materials to generate thermo-voltage from temperature gradient, defined as S = δV/δT. in an isotropic fluid system with no external forces, concentration gradient can be generated by the driving force from applied temperature gradient. for the sake of better understanding, here the entropy conductivity is written as thermal conductivity.