Flexible polymer-based thermoelectric materials : from electronic to ionic carriers

Abstract

Thermoelectric materials based on flexible polymers have received considerable attention for their ability to directly harvest waste heat to power wearable electronics and Internet-of-Things sensors because of advantages such as facile solution-processable manufacturing, flexibility, and biocompatibility. Polymer-based thermoelectric materials include electronic conductive polymer films (e-TE) and quasi-solid-state ionic thermoelectric (i-TE) gels. However, plenty of research has concentrated on improving the TE performances of these materials, while paying little consideration to their mechanical qualities. In this dissertation, I report a systematic investigation of flexible polymer based thermoelectric materials in which electrons and ions serve as energy carriers, respectively. The effects of the ordering structure and oxidation levels on the thermoelectric and mechanical properties of e-TE poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrene sulfonic acid) (PSS) films were investigated. First, a counterion exchange reaction between the PEDOT: PSS and the ionic liquid Li:nFSI was found to strongly affect the ordering structure of the conductive polymer PEDOT. The degree of counterion exchange reaction increased with the size of nFSI−. The synergistic optimization of the thermoelectric and mechanical properties of PEDOT: PSS-x Li: nFSI resulted in an electrical conductivity of 1515 S cm−1, a high Seebeck coefficient of 24 μV K−1 and a tensile strain of 30%. The tensile strain was approximately 10 times higher than that of pristine PEDOT: PSS (3%), indicating the application potential of PEDOT: PSS-x Li: nFSI for the flexible thermoelectric devices. Second, the oxidation level of the PEDOT: PSS film was also investigated by adding the L-ascorbic acid and EMIm:TCM. The addition of L-ascorbic acid confirmed that the PEDOT polymer chain was effectively reduced, thereby changing the oxidation level and boosting the thermoelectric power factor without a considerable sacrifice of mechanical flexibility. The electrical conductivity of the corresponding film was stable (ΔR/R0 < 0.12%) over 1000 bending cycles, indicating excellent bendability. Quasi-solid-state i-TE gels are alternative flexible and stretchable polymer based thermoelectric materials. In this dissertation, a study was conducted on ionogels, including polyacrylamide (PAM)/alginate (Alg)-EMIm: BF4, agarose (AG)-Na: DBS, and Gelatin-KCl-FeCN3−/4−. The addition of PEG considerably increased the thermopower of Pam-Alg/EMIm: BF4 from 5.6 to 19.3 mV K−1, indicating the positive effect of tuning the interaction between the mobile ions EMIM+ and the polymeric gel. The micellization effect of DBS− anions could be responsible for the unprecedented high thermopower of 41.0 mV K−1. The synergy between the thermodiffusion and the thermogalvanic effects was exploited to combine the advantages such as a high thermopower and high output current. A gelatin-based i-TE gel (Gelatin-0.8M KCl-0.42/0.25M FeCN4−/3−) exhibited a high thermopower of 17 mV K−1 and high instant output power density of 0.66 mW m−2 K−2. Using this material in a quasi-continuous thermal charge/electrical discharge mode for long-time usage delivered a maximum energy density of 12.8 J m−2 in 1 hour. The tensile strain in the i-TE gels (Pam-Alg-PEG)/EMIm: BF4 has a high upper limit of > 2000%, which was considerably higher than that in the e-TE polymer film.