Free-standing flexible multiwalled carbon nanotubes paper for wearable thermoelectric power generator

Abstract

Most of the thermoelectric research, these days, is being focussed towards development of flexible thermoelectric power generators (TEG) for wearable applications. Organic thermoelectric materials being flexible and solution processable, though, seem promising cannot render themselves for designing of conventional thermoelectric devices which require both p- and n-type of legs; due to lack of air-stable and flexible n-type materials. This work shows the possibility of conversion of a p-type flexible free-standing multiwalled carbon nanotubes (MWCNTs) paper into n-type on treatment with polyethylenimine (PEI). X-ray photoelectron spectroscopy results suggest that ‘as-grown’ MWCNTs attained n-type nature due to electron donation by imine group of PEI. Power factors of ~0.2 and ~0.06 μW/mK2 observed respectively for p- and n-type MWCNTs papers, owing to extremely low thermal conductivity (~0.05W/mK), resulted in figure-of-merit (ZT) of 1.27 × 10−3 and 3.02 × 10−4 at 68 °C. A prototype TEG designed using ‘as-grown’ and ‘PEI-modified’ MWCNTs as p- and n-type thermoelements respectively exhibited output of 227 μV/7.6 μA for a temperature difference of 40 °C. In short, facile scalability of MWCNTs when collaborated with such a low cost, environment friendly method that can easily modify its conduction to n-type can certainly open opportunities for scalable production of flexible roll-to-roll type wearable thermoelectric modules.