A pH‐Sensitive Stretchable Zwitterionic Hydrogel with Bipolar Thermoelectricity

Abstract Amid growing interest in using body heat for electricity in wearables, creating stretchable devices poses a major challenge. Herein, a hydrogel composed of two core constituents, namely the negatively‐charged 2‐acrylamido‐2‐methylpropanesulfonic acid and the zwitterionic (ZI) sulfobetaine acrylamide, is engineered into a double‐network hydrogel. This results in a significant enhancement in mechanical properties, with tensile stress and strain of up to 470.3 kPa and 106.6%, respectively. Moreover, the ZI nature of the polymer enables the fabrication of a device with polar thermoelectric properties by modulating the pH. Thus, the ionic Seebeck coefficient ( S i ) of the ZI hydrogel ranges from −32.6 to 31.7 mV K −1 as the pH is varied from 1 to 14, giving substantial figure of merit ( ZT i ) values of 3.8 and 3.6, respectively. Moreover, a prototype stretchable ionic thermoelectric supercapacitor incorporating the ZI hydrogel exhibits notable power densities of 1.8 and 0.9 mW m −2 at pH 1 and 14, respectively. Thus, the present work paves the way for the utilization of pH‐sensitive, stretchable ZI hydrogels for thermoelectric applications, with a specific focus on harvesting low‐grade waste heat within the temperature range of 25–40 °C.