Vibrational entropy as an indicator of temperature coefficient of redox potential in conjugated polymers

Abstract

Recently, it was demonstrated that environmental heat can put a battery on the charged state by using the difference in the thermal coefficient α (= dV /dT) of the redox potential (V) between the cathode and anode materials. In this battery, α is the key parameter that determines the device performance. Here, we systematically investigated α in prototypical conjugated polymers, i.e., polythiophene and four fluorene-based polymers. We found that α steeply increases with the number ratio (r) of thiophene in the main chain from 0.19 mV/K at r = 0.0 to 1.08 mV/K at r = 1.0. This trend is well understood in terms of variation of the vibrational entropy S vib between the neutral and oxidized states of the constituent molecule.

Energy harvesting thermocell with use of phase transition

“…For example, the NMF83/NCF90 cell (α cathode −α anode = 1.7 mV/K) 6 shows a thermal voltage of V cell ~ 40 mV between T L (=286 K) and T H (=313 K). So far, α of wide range of battery materials were reported; α = −0.3-1.4 mV/K in Prussian blue analogues 8 , α = 0.2-1.1 mV/K in several conjugated polymers 9 , α = 0.0−0.9 mV/K in Na x CoO 2 10 and α = 0.9 mV/K in Li x FePO 4…”

Origin of the material dependence of temperature coefficient of redox potential in conjugated polymers

“…Therefore, we need to search the origin for the material dependence in another physical parameter. In our previous work, 18) the material dependence was explained by the fact that α vib of thiophane (C 4 H 4 S) is larger than that of benzene (C 6 H 5 ). In this model, the main chains of the polymers were approximated to be composed of the C 6 H 5 and C 4 H 4 S units.…”

“…polyacetylene, 14) poly(p-phenylene) (PPP) 15,16) and polythiophene, 17) show the reversible redox process among the reduced, neutral and oxidized states. The characteristic of conjugated polymers is that α strongly depends on the main chain structure; 18) α = 1.08 mVK −1 for poly(3-hexylthiophene-2,5-diyl) (P3HT), 0.38 mVK −1 for poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) and 0.19 mVK −1 for poly [9,9-dioctylfluorene-co-N-(4-butylphenyl) diphenylamine] (TFB). In our previous work, 18) the material dependence is interpreted in terms of the vibrational entropy of the benzene (C 6 H 5 ) and thiophene (C 4 H 4 S) units.…”

“…The characteristic of conjugated polymers is that α strongly depends on the main chain structure; 18) α = 1.08 mVK −1 for poly(3-hexylthiophene-2,5-diyl) (P3HT), 0.38 mVK −1 for poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) and 0.19 mVK −1 for poly [9,9-dioctylfluorene-co-N-(4-butylphenyl) diphenylamine] (TFB). In our previous work, 18) the material dependence is interpreted in terms of the vibrational entropy of the benzene (C 6 H 5 ) and thiophene (C 4 H 4 S) units. In this model, the main chains of the polymers were approximated to be composed of the C 6 H 5 and C 4 H 4 S units.…”