A thermocell generates thermopower from a temperature difference (ΔT) between two electrodes. The converse process of thermocells is an electrochemical Peltier effect, which creates a ΔT on the electrodes by applying an external current. The Seebeck coefficient (Se) of the electrochemical system is proportional to the entropy change of the redox reaction; therefore, a redox system having a significant entropy change is expected to increase the Se. In this study, a thermoresponsive polymer having a redox‐active moiety, poly(N‐isopropyl acrylamide‐co‐N‐(2‐acrylamide ethyl)‐N′‐n‐propylviologen) (PNV), is used as the redox species of a thermocell. PNV2+ dication undergoes the coil–globule phase transition upon the reduction to PNV+ cation radical, and a large entropy change is introduced because water molecules are freed from the polymer chains. The Se of PNV thermocell drastically increased to +2.1 mV K−1 at the lower critical solution temperature (LCST) of PNV. The entropy change calculated from the increment of Se agrees with the value evaluated by differential scanning calorimetry. Moreover, the electrochemical Peltier effect is observed when the device temperature is increased above the LCST. This study shows that the large entropy change associated with the coil–globule phase transition can be used in electrochemical thermal management and refrigeration technologies.

Stimuli-responsive aggregation of polymer chains in water has found a variety of applications in polymer science, biology, and chemical engineering. To date, the majority of the phase transitions between the aggregated and dissolved forms has been observed by changing the solution temperature, and an active and precise control on the phase transition with a high time resolution has been challenging. Herein, a reversible phase transition of poly(allylamine-co-allylurea) (PAU) in an aqueous electrolyte is achieved by electrochemical redox cycling of hexacyanoferrate(II/III) ([Fe(CN) 6 ] 4−/3− ) ion pair. The aggregation and dissolution cycle can be completed in a high-resolution time frame of as short as 5 s. The strong electrostatic interaction between the protonated primary amino group of PAU and the tetravalent [Fe(CN) 6 ] 4− anion induces the aggregation, while the oxidation to the trivalent [Fe(CN) 6 ] 3− anion reduces the attractive force, and the polymer chain redissolves in solution. The ureido group of PAU helps the chain-folding process through the formation of inter/intrachain hydrogen-bonding networks, resulting in the sharp phase transition. By using [Fe(CN) 6 ] 4−/3− as the electron mediator, the electrochemical control on the large transparency change of polymer aqueous solution is realized for the first time.

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