The maximum values of ZT from Figs. 3a, 5, 6d, 10, and 11d were used to calculate the maximum efficiency from Eq. (3) for a cold temperature of 300 K and are summarized in Fig. 13. Although Na0.85CoO2 has the highest potential efficiency, the poor stability limits its potential application. Among the remaining materials, for a temperature difference of around 500 K, the maximum efficiencies are in the range of 4–6%.