Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermo-Electrochemical Cell

Abstract

Low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo-electrochemical cells (or "thermocells"). We demonstrate thermocells, in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. Thermocell efficiency is further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions. The efficiency of thermocells with MWNT electrodes is shown to be as high as 1.4% of Carnot efficiency, which is 3-fold higher than for previously demonstrated thermocells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy.

Huge Seebeck coefficients in nonaqueous electrolytes

“…The possibility of converting heat into electricity has also been studied in thermogalvanic cells using two identical electrodes at different temperatures, where chemical reactions take place. [4][5][6][7] Thermogalvanic cells use electrolytes having large Seebeck coefficients (of the order of 1 mV/K in aqueous potassium ferrocyanide/ferricyanide solutions). 4,6 However the efficiency of the thermal to electrical energy conversion is not governed solely by a material's Seebeck coefficient.…”

An electrochemical system for efficiently harvesting low-grade heat energy

“…Despite recent progress, however, the figure of merit (ZT) of thermoelectrics is limited to 2 at high temperatures and 1.5 below 100°C 10,11 . Seebeck effect in electrochemical system is also investigated for thermal energy harvesting in similar architectures as a TE device, but the efficiency achieved is usually lower than 0.5% below 100°C since the thermopower is limited by poor ionic conductivity of electrolyte, which is more than three orders of magnitude smaller than the electronic conductivity in state-of-the-art TE materials [12][13][14][15] . An alternative approach of electrochemical system for thermal energy harvesting is to explore thermodynamic cycle as in thermomechanical engines.…”

Electrical power production from low-grade waste heat using a thermally regenerative ethylenediamine battery

“…Most of the reported TESs rely on using a chemical that has temperature-dependent reduction and/or oxidation potentials in aqueous solutions, but the performance of these TESs needs to be improved in terms of electrical power densities and thermalelectric conversion efficiencies [11,13]. For example, a maximum power density of 1.5 Wm -2 -electrode area with a Carnot efficiency of 1.4% was achieved in a TES operated with a ferrocyanide/ferricyanide redox solution and carbon nanotube electrodes when operated with a temperature difference of 60 °C [14]. The power density was increased to 6.6 W m -2 -electrode area (Carnot efficiency of 3.95%) using carbon nanotube aerogel sheets with a 51 °C temperature difference, but this required the use of platinum [15].…”