Advances in Thermo-Electrochemical (TEC) Cell Performances for Harvesting Low-Grade Heat Energy: A Review

Abstract

Thermo-electrochemical cells (also known as thermocells, TECs) represent a promising technology for harvesting and exploiting low-grade waste heat (<100–150 °C) ubiquitous in the modern environment. Based on temperature-dependent redox reactions and ion diffusion, emerging liquid-state thermocells convert waste heat energy into electrical energy, generating power at low costs, with minimal material consumption and negligible carbon footprint. Recent developments in thermocell performances are reviewed in this article with specific focus on new redox couples, electrolyte optimisation towards enhancing power output and operating temperature regime and the use of carbon and other nanomaterials for producing electrodes with high surface area for increasing current density and device performance. The highest values of output power and cell potentials have been achieved for the redox ferri/ferrocyanide system and Co2+/3+, with great opportunities for further development in both aqueous and non-aqueous solvents. New thermoelectric applications in the field include wearable and portable electronic devices in the health and performance-monitoring sectors; using body heat as a continuous energy source, thermoelectrics are being employed for long-term, continuous powering of these devices. Energy storage in the form of micro supercapacitors and in lithium ion batteries is another emerging application. Current thermocells still face challenges of low power density, conversion efficiency and stability issues. For waste-heat conversion (WHC) to partially replace fossil fuels as an alternative energy source, power generation needs to be commercially viable and cost-effective. Achieving greater power density and operations at higher temperatures will require extensive research and significant developments in the field.

Low-Grade Thermal Energy Harvesting and Self-Powered Sensing Based on Thermogalvanic Hydrogels

“…However, some recent studies have reported record conversion efficiencies of 11% by establishing high concentration ratios at the cold and hot ends [ 74 ]. The Seebeck coefficient, conductivity, power density, and number of charge/discharge cycles of some thermogalvanic hydrogels is shown in Table 1 [ 21 , 23 , 45 , 56 , 75 ]. Despite these low efficiencies, since the primary use of thermogalvanic hydrogels is to harvest wasted energy, the efficiency requirements for commercial viability are relatively low.…”

“…Among them, applications of inorganic semiconductive thermoelectric generators are expensive, have mechanical brittleness, and are challenging to synthesize. Their thermoelectric efficiency is limited not only by the small Seebeck coefficient (thermodynamic quantity, determined by the change in the electron chemical potential caused by temperature changes) of about 100–200 μV K −1 but also by the electrical and thermal conductivity [ 22 , 23 , 24 ].…”

“…Inexpensive and environmentally friendly electrode and electrolyte materials form simple device structures, as well as excellent thermoelectric power performance [ 30 , 31 , 32 ]. As shown in Figure 1 , the number of publications and citations using hydrogels to harvest and sense thermal energy has increased from 2000 to 2022 [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , ...…”

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

“…48 To date, numerous review articles have assessed the research progress of ionic liquids-based thermo-electrochemical cells from various standpoints. 49,316,318,321 For instance, Wang et al 318 summarized some common ionic thermoelectric materials and their related thermoelectric conversion mechanism. Li et al 316 fully discussed recent research progress and thermoelectric performance of p-type and n-type thermoelectric cells.…”

Experimental Investigation of Coupled Transport Mechanisms in a PEM Based Thermoelectric Energy Converter

“…For steady operation of the latter, the reactants must either be recirculated or the electrodes sequentially heated and cooled [10]. In addition to these different concepts, TGCs also vary in redox couples and electrolytes, which is well summarized in [11]. Due to its increasing availability and without any climate impact, the use of green hydrogen as a reactant in TGC shows great potential, which has already been proven in other thermoelectric energy converters [12][13][14].…”