High thermoelectric conversion through an optimal contribution of electronic carriers in polymeric mixed ionic-electronic conducting films

Abstract

The optimal contribution of electronic carriers in polymeric mixed ionic-electronic conducting (MIEC) films was explored to achieve high thermoelectric (TE) conversion at a low temperature gradient (ΔT) using poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (P), potassium ferricyanide (F), and well-dispersed graphene flakes (G). In this composite (PFG) film, the active carriers were identified as CN- and electrons. The sulfonate groups in P effectively dispersed the graphene flakes into nanoscale electronic channels, while P and F provided ionic channels for CN⁻ transport. By systematically varying the G content and humidity, a series of MIECs with broad electronic (σe) and ionic (σi) conductivity ranges were developed. Among these, the PFG film containing 3 wt% G (PFG3) exhibited a remarkable total Seebeck coefficient (S) of over -40 mV K-1 under a ΔT of 5.3 K at 80 % RH at room temperature. Additionally, PFG3 demonstrated stable voltage output (Vout) even after 3,000 sec. From the residual Vout, the electronic Seebeck coefficient (Se) was determined to be –990 uV K-1, the highest value reported for polymeric TE films. The simultaneous enhancement of Se and S in the same film indicated an optimized balance of electronic and ionic carrier contributions, further confirmed by the transference numbers and the conductivity ratio (σe/σi). The power density (PD) of the PFGs was also found to depend on the σe/σi, underscoring the importance of the controlling carrier contributions. Despite its MIEC nature, PFG3 efficiently transported electronic carriers through G channels, and the relationship between of Se vs σe aligned with a degenerate electronic model for PFGs. Scaling up the PFG3 film into a TE module yielded an energy density of 36.0 J m-2 and PD of 18.6 mW m-2 for a ΔT of 4.9K. The practical potential of the PFG system was demonstrated by successfully powering a diode for an extended period using TE energy harvesting and light-triggered photo-TE systems, highlighting the versatility and promise of this material for low-grade thermal energy applications.