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.

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Windows are primarily for admitting light or air and allowing people to see out. Presented here are windows that can generate electricity while retaining the primary functions. These windows are made of transparent thermocells that convert a temperature difference across the window to electricity. Interconnected p-type and n-type or p-n thermocells are introduced and utilized to scale up the output power of a thermocell window (T-window). The T-window consisting of 2 p-n thermocells provides an output voltage of 60 mV and a power density of 0.5 μW/cm2 for a small temperature difference of 10 °C with an optical transparency of ∼50% in the visible range. The T-window introduced here could pave the way to enhancing energy efficiency in residential environments by capturing naturally available low-grade heat, a new renewable energy source that is otherwise discarded to the surrounding environment.

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A thermocouple is the most widely used electrical component for cost‐effective measurement of temperature in both academia and industry. However, its low sensitivity with typically tens of microvolts per Kelvin needs to be improved to obtain precise measurements. Introduced here is an ionic thermocouple to provide the level of accuracy required of temperature measurements in medicine, precision agriculture, smart buildings, and industrial processes. Ionic conductors are utilized in place of the electrical conductors typically used in the conventional thermocouples (TCs). The ionic thermocouples (i‐TCs) are demonstrated with redox reactions of 10 × 10−3 m potassium ferri/ferrocyanide and 0.7 m iron(II/III) perchlorate, which are electrolytes used as p‐type and an n‐type elements, respectively. The voltage output of the i‐TC that is generated by a change in temperature is approximately two orders of magnitude larger than that of the conventional TC, providing almost two more significant figures in measured temperature. The i‐TC can easily be miniaturized as demonstrated for the in situ temperature measurement of the fluid flowing in the channel of a microfluidic device. A flexible and stretchable i‐TC device is also demonstrated to stably operate up to a tensile strain of 23% with no noticeable degradation in performance.