Energy harvesting thermocell with use of phase transition

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 V_cell 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 (T_c) for the phase transition in cobalt Prussian blue analogue (Co-PBA; Na_xCo[Fe(CN)₆]_y) to just above room temperature, by finely adjusting the Fe concentration (y = 0.82). With increase in the cell temperature (T_cell), V_cell of the Na_xCo[Fe(CN)₆]_0.82 (NCF82)/Na_xCo[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.