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A soy protein film doped with evenly distributed CaCl2 is prepared as positive tribolayer and the impact of CaCl2 and water on contact-separation triboelectrification is discussed. The results show water retention by CaCl2 is beneficial for improving the stretchability and electrical output of the biomechanical energy harvester.
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•Water retention in soy protein results in 32.5-fold stretchability and 2-fold power enhancement.•SP-CaCl2-0.30 functions as a stretchable positive tribolayer as well as an elastic electrode.•The power density of the protein-based biomechanical energy harvester reaches 1125 mW/m2.
To meet the rising demand for wearable electronics with high power demand, biomechanical energy harvester based on stretchable materials with high triboelectric charge densities are required. Soy protein (SP) represents a prospective tribopositive material; however, its application is limited due to its brittle nature. Herein, we report the doping of SP with hygroscopic CaCl2 to afford a tribolayer film with improved tribopositivity and stretchability, where Ca2+ disperses evenly in SP via electrostatic interactions. The water molecules adsorbed by CaCl2 form hydrogen bonds with SP chains and improve the charge-donating ability. The optimal SP–CaCl2-0.30 film with CaCl2-doping concentration of 0.30 mmol exhibits high resilience (elongation at break: 130 %) compared with the pristine SP film (elongation at break: 4 %). The device based on SP–CaCl2-0.30 as the positive tribolayer and Ecoflex as counterpart yields open circuit voltage, short circuit current, and short-circuit transferred charge of 130 V, 4.4 µA, and 44 nC, respectively. When the load resistance matches the device’s internal resistance, the peak transient power reaches 1125 mW/m2. Moreover, the device output is maintained even after 5000 of stretch and release cycles, and the harvested charge from the bending and release motions of the finger and elbow reach 3.5 nC and 40 nC, respectively. This study demonstrates a prospective approach toward protein-based energy harvesting with high stretchability and electrical output, showing significant potential for application in wearable electronics for biomechanical energy harvesting and relative humidity monitoring.