Reduced internal resistance in thermorechargeable battery with heavy active materials

Thermorechargeable battery (TB), which can be charged by temperature change $\Delta T$ via difference in the electrochemical Seebeck coefficient $\alpha$ between the cathode and anode, is a promising energy harvester. In order to put TB into practical use in society, it is necessary to increase the maximum output power ($W_{\max }$ = $\frac{V_{\mathrm{TB}}^2}{4R}$, where $V_{\mathrm{TB}}$ and $R$ are the thermal voltage and internal resistance) per unit area of electrode. Here, we investigated $R$ and its components in laminate film type Na_1.48Co[Fe(CN)${}_6$]_0.87 (Co-PBA)/Na_1.76Ni[Fe(CN)${}_6$]_0.94 (Ni-PBA) TB against active material weight $m$. We found that the charge-transfer resistance $R_{\mathrm{ct}}$ at 20 °C steeply decreases from 60 $\Omega$/cm${}^2$ in TB with light $m$ to 7 $\Omega$/cm${}^2$ in TB with heavy $m$, causing significant reduction of $R$. Reflecting the reduced $R$, $W_{\max }$ at $\Delta T$ = 30 K significantly increases from 2.5 $\mu {\mathrm{W/cm}}^2$ to 18.8 $\mu {\mathrm{W/cm}}^2$ with increases in $m$.