Abstract
Remarkable advances have recently been made in the thermocell array with series or parallel interconnection, however, the output power from the thermocell array is mainly limited by the electrolyte performance of an n-type element. In this work, we investigate iron (II/III) perchlorate electrolytes as a new n-type electrolyte and compared with the ferric/ferrous cyanide electrolyte at its introduction with platinum as the electrodes, which has been the benchmark for thermocells. In comparison, the perchlorate electrolyte (Fe2+/Fe3+) exhibits a high temperature coefficient of redox potential of +1.76 mV/K, which is complementary to the cyanide electrolyte (Fe(CN)63−/Fe(CN)64−) with the temperature coefficient of −1.42 mV/K. The power factor and figure of merit for the electrolyte are higher by 28% and 40%, respectively, than those for the cyanide electrolyte. In terms of device performance, the thermocell using the perchlorate electrolyte provides a power density of 687 mW/m2 that is 45% higher compared to the same device but with the cyanide electrolyte for a small temperature difference of 20 °C. The advent of this high performance n-type electrolyte could open up new ways to achieve substantial advances in p-n thermocells as in p-n thermoelectrics, which has steered the way to the possibility of practical use of thermoelectrics.
Highlights
Low-grade heats generated from geothermal reservoirs, power plants and various industrial processes are recycled at low efficiency or just released to the surrounding environment in the form of waste heat[1,2,3]
A temperature coefficient of redox potential (α) is related to the voltage that can be obtained from the cell at a given temperature difference and at constant pressure, which can be expressed as eq (1)
A new electrolyte that compliments and/or outperforms the cyanide electrolyte could open up new ways to achieve substantial advances in p-n thermoelectrochemical cells (TECs) as in p-n thermoelectrics, which has steered the way to the possibility of practical use of thermoelectrics
Summary
One of the favorable factors for choosing the iron perchlorate electrolyte was that the high solubility of iron perchlorate salts in water[28] such that its use can increase the output current of the TEC and at the same time decrease the thermal conductivity of electrolyte. The temperature coefficient of perchlorate electrolyte is the highest among the reported Fe2+/Fe3+ salt systems with different counter ions (e.g., α of +0.13 mV/K for ammonium iron sulfate, +0.29 mV/K for iron sulfate, +1.35 mV/K for iron triflate, and +1.34 mV/K for iron nitrate)[30], as well as higher compared to the cyanide electrolyte It is clear from the eq (1) that the sign and magnitude of α are determined by the reaction entropy change (Δs°rx) for a given redox reaction[31,32,33]. In view of the tremendous progress made in the thermocell performance with the introduction of the cyanide electrolyte, the proposed electrolyte bodes well for the significant advances that could be made with its introduction
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