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 (σ_e) and ionic (σ_i) 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 conditions of a ΔT of 5.3 K, 80% RH and room temperature. Additionally, PFG3 demonstrated a stable voltage output (V_out) even after 3000 s. From the residual V_out, the electronic Seebeck coefficient (S_e) was determined to be −990 μV K⁻¹, the highest value reported for polymeric TE films. The simultaneous enhancement of S_e 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 (σ_e/σ_i). The power density (PD) of the PFGs was also found to depend on σ_e/σ_i, underscoring the importance of controlling carrier contributions. Despite its MIEC nature, PFG3 efficiently transported electronic carriers through G channels, and the relationship of S_evs. σ_e 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 a PD of 18.6 mW m⁻² for a ΔT of 4.9 K. 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.