Origin Of The Material Dependence Of The Temperature Coefficient Of The Redox Potential In Coordination Polymers

The thermal coefficient alpha (= dV/dT) of the redox potential (V) is an important physical parameter for thermal energy harvesting. We systematically investigated alpha of six Prussian blue analogues (M-PBAs; M = Mn, Fe, Co, Ni, Cu, and Cd) and Na2Zn3[Fe(CN)(6)](2) (Zn-HCF) in aqueous solution of 17 mol/kg NaClO4. We found that alpha shows significant material dependence and ranges from -1.18 mV/K in Cd-PBA to 0.57 mV/K in Co-PBA. We further found that (I) alpha >= 0 (alpha <= 0) if the redox site is M ([Fe(CN)(6)]), and (II) vertical bar alpha vertical bar increases with the Na site number (N) of the Na site around the central position of the nanopore. We interpreted the observations in terms of the variation (Delta S-Na) of the Na configuration entropy (S-Na) in the vicinity of the redox site.