Thermal efficiency of a thermocell made of Prussian blue analogues.

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. Among battery materials, Prussian blue analogues (PBAs) are promising materials for thermocell, because α changes from approximately -0.3 mV/K in Nax Mn[Fe(CN)6 ]0.83 3.5 H2 O (NMF83) to approximately 1.3 mV/K in Nax Co[Fe(CN)6 ]0.9 2,9H2 O (NCF90). In this work, we systematically investigated the thermal efficiency (η) of the NMF83/NCF90 thermocell relative to the difference (ΔT) between low (TL  = 282 K) and high (TH  = 292-338 K) temperatures. We found that the thermal efficiency (η) increased proportionally with ΔT. The linear increase in η is ascribed to the linear increase in the cell voltage (Vcell ) and the charge (QNCF90 ) extracted from NCF90. Moreover, η reached 3.19% at ΔT = 56 K, which corresponds to 19% of the Carnot efficiency (ηcarnot  = 17.0%). We further confirmed that the magnitude of QNCF90 is quantitatively reproduced by the slopes of the discharge curves of NMF83 and NCF90.