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 (Fe 2+ /Fe 3+ ) exhibits a high temperature coefficient of redox potential of +1.76 mV/K, which is complementary to the cyanide electrolyte (Fe(CN) 6 3− /Fe(CN) 6 4− ) 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/m 2 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.

Carbon-based porous electrodes have led to remarkable improvements in the performance of thermochemical cells or thermocells that electrochemically harvest low-grade waste thermal energy. However, the output current from the thermocells is hampered by the diffusion effect, which leads to depleted ion concentration as the ions permeate through the porous electrode. Here, we advance a theoretical basis for a quantitative description of the diffusion effect on current generation in such porous electrodes. One single dimensionless parameter of Thiele modulus describes the effect according to the theory adopted from the well-established results in the literature. Experimental results for carbon fiber electrodes are illustrated and quantified by the theory. The theory presented here would provide a basis for the choice and design of porous electrodes for thermocells. The results should also provide a basis for devising electrochemical devices with highly porous electrodes.

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