  This paper investigates the thermoelectric properties of solid polymer
electrolytes (SPE) containing lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) salts, along
with carbon-based additives of various dimensionalities. Increasing salt
concentration leads to higher Seebeck coefficients as a result of the
increasing number of free charge carriers and additional, superimposed effects
by ion-ion and ion-polymer interactions. NaTFSI-based electrolytes exhibit
negative Seebeck coefficients (up to $S = -1.5\,\mathrm{mV\,K^{-1}}$),
indicating dominant mobility of $\mathrm{TFSI^-}$ ions. Quasi-one-dimensional
carbon nanotubes (CNTs) increase the Seebeck coefficient by a factor of 3.
Planar, two-dimensional graphite flakes (GF) moderately enhance it, affecting
$\mathrm{Na^+}$ and $\mathrm{TFSI^-}$ ion mobilities and electronic
conductivity. Bulky, three-dimensional carbon black (CB) additives induce a
unique behavior where the sign of the Seebeck coefficient changes with
temperature, presumably due to interaction with $\mathrm{TFSI^-}$ ions within
the CB structure. Changes in activation energy and Vogel temperature with salt
concentration suggest structural and mechanical modifications in the polymer
matrix. The choice of carbon-based additives and salt concentration
significantly influences the thermoelectric properties of SPEs thermoelectric
properties, providing insights into their potential for thermoelectric
applications. Sodium-based electrolytes emerge as promising, sustainable
alternatives to lithium-based systems, aligning with sustainable energy
research demands.
