Scalable free-standing polypyrrole films for wrist-band type flexible thermoelectric power generator

Abstract

Highly flexible free-standing polypyrrole films were prepared by the interfacial chemical polymerization of pyrrole (in cyclohexane) using aqueous FeCl3 as an oxidant and p-toluene sulphonic acid (PTSA) as a dopant. Morphological characterization revealed the granular morphology of the synthesized films and also suggested a reduction in their thickness with the increase in PTSA concentration. The increase of the doping content in these polypyrrole films, also resulted in the enhancement of electrical conductivity from 4.8 S/cm to 162.7 S/cm without much affecting the Seebeck coefficient ∼4–8 μV/K. The combined results of X-ray photoelectron spectroscopy and Electron spin resonance spectroscopy (ESR) confirms the enhancement of doping content (N+/N content) i.e. formation of charge carriers such as polarons/bipolarons with the increase in PTSA concentration. The synergetic combination of high electrical conductivity along with moderate Seebeck coefficient in the doped free-standing PPy films resulted in the highest average power factor of ∼0.45 μW/mK2. A wrist-band type thermoelectric power generator was also designed by integrating seven number of free-standing PPy films to manifest their potential for practical thermoelectric applications. The fabricated device exhibited maximum open-circuit voltage and current respectively as 336 μV and ∼46 nA, for a temperature difference of 80 °C. The work reveals new avenues towards the development of wearable energy harvesting devices that can be possibly designed using free-standing PPy films.