Oxygen-induced doping on reduced PEDOT

Abstract

The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) has shown promise as air electrode in renewable energy technologies like metal-air batteries and fuel cells. PEDOT is based on atomic elements of high abundance and is synthesized at low temperature from solution. The mechanism of oxygen reduction reaction (ORR) over chemically polymerized PEDOT:Cl still remains controversial with eventual role of transition metal impurities. However, regardless of the mechanistic route, we here demonstrate yet another key active role of PEDOT in the ORR mechanism. Our study demonstrates the decoupling of conductivity (intrinsic property) from electrocatalysis (as an extrinsic phenomenon) yielding the evidence of doping of the polymer by oxygen during ORR. Hence, the PEDOT electrode is electrochemically reduced (undoped) in the voltage range of ORR regime, but O₂ keeps it conducting; ensuring PEDOT to act as an electrode for the ORR. The interaction of oxygen with the polymer electrode is investigated with a battery of spectroscopic techniques.

Fig. 1. (a) Comparing oxygen reduction reaction (ORR) in aqueous 0.1 M KCl in N₂ and in O₂ with Pt in O₂. (b) In situ resistometry of a PEDOT:Cl film vs. applied potential in N₂ (blue line) and in excess of O₂ (light blue line). The curves presented refer to the average resistance of PEDOT:Cl, as this was calculated from the data of the three studied devices. The dashed lines aside the average curves show the fluctuation of the resistance in both N₂ and O₂ in the low negative potential (<–0.6 V). Nevertheless, the resistance is stable in the potential range –0.6 V to 0.6 V.

Fig. 2. (a) Evolution of conductivity before and after exposure in air (40% RH) and (b) evolution of normalized resistance when O₂ is let in the chamber for the pristine PEDOT:Cl film and the films reduced at 0 V, –0.5 V and –0.9 V.

Fig. 3. (a) Absorbance spectra for the pristine PEDOT:Cl sample and the samples reduced at 0 V, –0.5 V and –0.9 V, both in N₂ environment and after 60 min exposure to air. (b) Evolution of absorbance at 582 nm versus exposure time in O₂ for the pristine sample and for samples reduced at 0 V, –0.5 V and –0.9 V. (c) Normalized absorbance at 582 nm versus normalized resistance change for various exposure times to O₂.

Fig. 4. (a) Infrared spectra of pristine and reduced PEDOT:Cl samples under nitrogen atmosphere. The arrows indicate peak position changes from the quinoid (pristine) to the benzoid (reduced at –0.9 V) structures. Further details of the labeled bands can be found in Table 1. (b) Normalized ratio of quinoid and benzoid ring structures, based on normalized intensities of asymmetric CC vibration of quinoid (1528–1514 cm^–1) and benzoid (1480–1469 cm^–1) structures.

Fig. 5. (a) Infrared spectra of pristine and reduced PEDOT:Cl samples, during the first 2 hours of exposure to oxygen. Green (red) boxes indicate the regions of increasing (decreasing) peak intensity by time, upon exposure to oxygen. (b) Infrared spectra of pristine and reduced PEDOT:Cl samples, followed over a week under exposure to normal atmosphere.

Fig. 6. XPS C(1s), O(1s) and S(2p) spectra for the film reduced at –0.9 V in N₂ environment ((a), (d) and (g)) and after 40 min of exposure in air ((b), (e) and (h)), and after 2 weeks ((c), (f), and (i)) respectively.

Fig. 7. Chemical structures of (a) EDOT, (b) pristine PEDOT (as prepared – oxidized form), (c) reduced PEDOT.