Electrochemical Effects in Thermoelectric Polymers

Abstract

Conductive polymers such as PEDOT:PSS hold great promise as flexible thermoelectric devices. The thermoelectric power factor of PEDOT:PSS is small relative to inorganic materials because the Seebeck coefficient is small. Ion conducting materials have previously been demonstrated to have very large Seebeck coefficients, and a major advantage of polymers over inorganics is the high room temperature ionic conductivity. Notably, PEDOT:PSS demonstrates a significant but short-term increase in Seebeck coefficient which is attributed to a large ionic Seebeck contribution. By controlling whether electrochemistry occurs at the PEDOT:PSS/electrode interface, the duration of the ionic Seebeck enhancement can be controlled, and a material can be designed with long-lived ionic Seebeck enhancements.

Diffuse charge dynamics in ionic thermoelectrochemical systems

“…Assuming the distance between the electrodes is much smaller than the other dimensions, which is reasonable considering devices often have separations on the order of millimeters [15][16][17], the dynamics of this system are governed by the one dimensional ion conservation equation,…”

“…Assuming a thermo-electric device is intended to operate at least at room temperature, T 0 ≈ 300 K, then O(1 K) temperature differences would readily suffice to meet this condition. Several devices operate in this regime [12,[15][16][17]. Thus, defining a small parameter δ = G f 2L/T 0 = ∆T /T 0 ≪ 1, we can express all unknowns as perturbations to their initial state by a small temperature gradient.…”

“…Due to these drawbacks, there has been recent interest in "soft" thermo-electric materials containing ionic charge carriers such as ionic liquids [9][10][11], non-aqueous organic electrolytes [12], and mixed ionic-electronic conducting polymers [13][14][15][16][17]. Although these come with a reduction in electrical conductivity compared to semiconductor based thermo-electrics, they can be competitive due to having much larger Seebeck coefficients, on the order of mV/K [12,[15][16][17].…”

“…Although these come with a reduction in electrical conductivity compared to semiconductor based thermo-electrics, they can be competitive due to having much larger Seebeck coefficients, on the order of mV/K [12,[15][16][17]. This enables such materials to store more charge [17] and provides a pathway for designing devices that have figures of merit comparable to semiconductor based devices.…”

“…Chang et al [15] used polymeric ethylenedioxythiophene (PEDOT) and polystyrenesulphonate (PSS) doped with silver ions and achieved Se = 0.1 mV/K with ZT = 0.13, stable over O(10 3 s), compared to the undoped material which exhibited a decay in Se over O(100 s). Kim et al [16] use a PSS-based thermo-electric generator to achieve an ionic Seebeck coefficient of 8 mV/K and ZT = 0.4.…”