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An important direction in the development of energy saving policy is harvesting and conversion into electricity of low-grade waste heat. The present paper is devoted to the improvement of the efficiency of thermo-electrochemical cells based on carbon fiber electrodes and potassium ferri-/ferrocyanide redox electrolyte. The influence of the carbon fiber electrode surface modification (magnetron deposition of silver and titanium or infiltration implantation of nanoscale titanium oxide) on the output power and parameters of the impedance equivalent scheme of a thermo-electrochemical cell has been studied. Two kinds of cell designs (a conventional electrochemical cell with a salt bridge and a coin cell-type body) were investigated. It was found that the nature of the surface modification of electrodes can change the internal resistance of the cell by three orders of magnitude. The dependence of the equivalent scheme parameters and output power density of the thermoelectric cell on the type of electrode materials was presented. It was observed that the maximum power for carbon fiber modified with titanium metal and titanium oxide was 25.2 mW/m2 and the efficiency was 1.37%.

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.

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