We demonstrate that a sodium-ion secondary battery (SIB)-type thermocell consisting of two types of Prussian blue analogue (PBA) with different electrochemical thermoelectric coefficients (S-EC equivalent to partial derivative V/partial derivative T; V and T are the redox potential and temperature, respectively) produces electrical energy during heat cycles. The device produces an electrical energy of 2.3 meV/PBA per heat cycle between 295 K (=T-L) and 323K (=T-H). The ideal thermal efficiency (eta = 1.0%), which is evaluated using the heat capacity (C = 4.16 meV/K) of ideal Na2Co[Fe(CN)(6)], reaches 11% of the Carnot efficiency (eta(th) = 8.7%). Our SIB-type thermocell is a promising thermoelectric device that harvests waste heat near room temperature. (C) 2018 The Japan Society of Applied Physics