The electrochemical thermoelectric (TE) coefficient (SEC≡∂V∂T; V and T are the redox potential and temperature, respectively) is a significant material parameter, because it enable us to convert heat into electricity. Here, we systematically investigated the TE properties of cobalt hexacyanoferrate (Co-HCF), LixCo[Fe(CN)6]y, against the Li concentration (x). |SEC| is higher than the Seebeck coefficient (= 0.2 mV/K at room temperature) of Bi2Te3 and distributes from 0.2 to 0.8 mV/K. We further observed a sign reversal behavior of SEC: SEC is negative at y =0.71 while it is negative (positive) at x≤0.3 (x≥0.6) at y =0.90. Based on the ionic model, we qualitatively reproduced the sign reversal behavior by including the volume expansion effect. These arguments suggest that SEC in solid is mainly governed by the electrostatic energy.
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