A thermocell that consists of cathode and anode materials with different temperature coefficients (α = dV/dT) of the redox potential (V) can convert environmental thermal energy to electric energy via the so-called thermal charging effect. The output voltage Vcell of the current thermocell, however, is still low (several tens mV) and depends on temperature, which are serious drawbacks for practical use of the device as an independent power supply. Here, we report that usage of phase transition material as electrode qualitatively improve the device performance. We set the critical temperature (Tc) for the phase transition in cobalt Prussian blue analogue (Co-PBA; NaxCo[Fe(CN)6]y) to just above room temperature, by finely adjusting the Fe concentration (y = 0.82). With increase in the cell temperature (Tcell), Vcell of the NaxCo[Fe(CN)6]0.82 (NCF82)/NaxCo[Fe(CN)6]0.9 (NCF90) cell steeply increases from 0 mV to ~120 mV around 320 K. Our observation indicates that the thermocell with use of phase transition is a promising energy harvesting device.

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Recently, it was reported that a thermocell can convert temperature into electric energy by using the difference in the thermal coefficient (α ≡ dV /dT ) of the redox potential (V ) between the cathode and anode materials. Here, we systematically investigated α of Na x Co 1−z Mn z [Fe(CN) 6 ] y (Co 1−z Mn z -PBA) against Mn concentration (z). The zdependence of α is interpreted in terms of the 3d-electron configuration entropy (∆S 3d ) of the redox site. Utilization of S 3d is a strategy effective for the design of high-|α| material for energy harvesting thermocell.

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Recently, it was demonstrated that environmental heat can put a battery on the charged state by using the difference in the thermal coefficient α (= dV /dT) of the redox potential (V) between the cathode and anode materials. In this battery, α is the key parameter that determines the device performance. Here, we systematically investigated α in prototypical conjugated polymers, i.e., polythiophene and four fluorene-based polymers. We found that α steeply increases with the number ratio (r) of thiophene in the main chain from 0.19 mV/K at r = 0.0 to 1.08 mV/K at r = 1.0. This trend is well understood in terms of variation of the vibrational entropy S vib between the neutral and oxidized states of the constituent molecule.