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The electropolymerization of o-Phenylenediamine (oPDA), both in the absence and presence of Fe3+ in solution, was investigated in detail using cyclic voltammetry concurrently with the quartz crystal microbalance (QCMB) technique. It is shown that a set of redox peaks (A1/C1), seen between 0.05 and 0.4 V is characteristic of the phenazine-like PoPDA polymer, produced by the irreversible radical oxidation of the oPDA monomer at a Au surface and forming an oPDA radical cation that initiates the polymerization step at >0.8 V vs. RHE. The QCMB results showed that ca. 20% of the oxidized product deposits on the Au surface as a redox-active polymer film, with charge compensation by H+ injection/expulsion along with some inhalation/exhalation of water. The presence of Fe3+ in solution during electropolymerization led to very similar electrochemistry, with ca. 50% of the oxidized product forming the redox-active polymer film. The PoPDA films are very smooth and uniform, while films formed in the presence of Fe3+ are coated with Fe-containing nodules ca. 0.8 μm in diameter, which hinder the further growth of the PoPDA film.

The primary goal of this work has been to better understand the oxygen reduction (ORR) activity of a series of Fe-N-C catalysts, formed by mixing Fe chloride with o-phenylenediamine (PDA) followed by heat treatment (HT). It is shown that, for HT at less than 500oC, significant redox chemistry, typical of a o-PDA redox polymer, is revealed in fully deaerated acidic solutions. This redox chemistry disappears at higher HT temperatures, arguing that the ORR active site arises from the thermal decomposition product of the PDA surface polymer. By electrodepositing PDA, either from a solution of o-PDA or from one also containing Fe3+, followed by ORR studies, it is demonstrated that the presence of Fe in the PDA film serves to enhance the ORR activity.

This paper compares the use of o-phenylenediamine (oPDA) and its polymeric counterpart, poly-o-phenylenediamine (PoPDA), as the ligand precursors for the formation of non-precious metal (NPM, Co vs. Fe) cathode catalysts. Rotating ring disk electrode (RRDE) work revealed that high onset potentials (0.92 V vs. RHE) and low %H2O2 yields (0.5 % over a wide potential range of 0.05-0.7 V) can be obtained. The use of PoPDA proved to be a better C-N source than the o-PDA monomer, likely due to the formation of a higher number of active sites or to the improved stability of these active sites on the carbon support during high temperature pyrolysis. In all cases, the Fe-based catalyst is superior to the Co-based one. Also, this study confirms the additional benefits of using acid-leached vs. as-received carbon as the support material.

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