Energy conversion technologies relying on platinum group metal (PGM) catalysts suffer from the disadvantage of high cost. Additionally, they are susceptible to be poisoned by the reaction byproducts. Thus, to realize cost-effective, clean and renewable electrochemical devices, such as metal-air batteries, there is an urgent need to develop non-PGM catalysts. In the past ten years, a wide range of materials–as alternatives to PGM-based catalysts–have been developed. Examples of these materials are transition metal oxides, covalent- and metal-organic frameworks, and metal-free carbon-based electrocatalysts [1]. Among these materials, metal-free carbon-based catalysts–such as graphene and carbon nanotubes (CNTs) doped with nitrogen or sulfur–are promising candidates due to their high activity, durability, and low cost. Nonetheless, the synthetic approaches utilized to prepare and process these catalysts are multi-steps. These complex schemes lead to a lack of control over materials structure and chemical functionality. As a consequence, the electroactivity of the synthesized materials is being compromised [2].

Here, we investigate the properties of conjugated polymers thin films as electrocatalysts. Although there are a few reports on the electroactivity of conjugated polymers toward oxygen reduction reaction (ORR), the reported values are not sufficient for the device fabrication. Additionally, the origin of the electrocatalytic activities of these materials are not well understood and often attributed to the residual metal atoms remained in the polymeric domains. Here, we show that by tailoring the chemistry of monomers, we can tune the electrochemical properties of the polymers. For instance, in the case of thiophene-based conjugated polymers, i.e. poly(thiophene) and poly(3,4-ethylenedioxythiophen) (PEDOT), the onset of the ORR overpotential are measured to be 0.61 V and 0.76 V vs reversible hydrogen electrode (RHE) [3]. Populating the monomer structure with heteroatoms, we recorded an enhanced electroactivity–comparable to Pt/C–for polymeric domains of poly(3,4-ethylenedithiathiophene) (PEDTT). More interestingly, PEDTT shows high electroactivity toward oxygen evolution reaction (OER) with overpotential onset of 1.72 V vs RHE. This observation makes PEDTT, the first conjugated polymer with bifunctional electroactivity for oxygen reactions. To demonstrate the bifunctionality of PEDTT, we assembled a rechargeable zinc-air battery using polymer in the cathode. We evaluated the performance of the assembled battery and measured an open circuit voltage of 1.4 V and charge and discharge voltages of 2.1 V and 1.1 V, respectively, when the cell was operated at 2 mA/cm². The device demonstrated a stable cycling performance for over 100 cycles of charge-discharge.

1. Fu, G., Y. Tang, and J.M. Lee, Recent Advances in Carbon‐Based Bifunctional Oxygen Electrocatalysts for Zn− Air Batteries. ChemElectroChem, 2018. 5(11): p. 1424-1434.
2. Guo, D., et al., Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science, 2016. 351(6271): p. 361-365.
3. Kaviani, S., et al., Electroactive and Conformal Coatings of oCVD Polymers for Oxygen Electroreduction. ACS Applied Polymer Materials, 2019.