Palladium Content Effect on the Electrocatalytic Activity of Palladium–Polypyrrole Nanocomposite for Cathodic Reduction of Oxygen

Abstract

Application of a catalyst preparation method known as ‘water-in-oil’ microemulsion allowed to obtain a series of palladium/polypyrrole (Pd/PPY) electrocatalysts with various Pd contents (2–20 wt%) and Pd particle size within a narrow and similar range of 6–8 nm. The textural features of the samples (surface area, porosity) as well as the palladium phase (metal particle size, shape) were characterized by a number of techniques. Electrocatalytic activity of the prepared materials towards oxygen reduction reaction (ORR) was studied in 0.5 M H2SO4 using rotating ring disc electrode voltammetry (RRDE) and cyclic voltammetry techniques. All prepared Pd/PPY samples exhibited electrocatalytic activity towards ORR. At a constant palladium loading on the electrode, the polymer to Pd weight ratio was found to play a decisive role in the electrocatalytic performance of the samples. Their catalytic performance in terms of the increased reduction current and positively shifted half-wave potential (RRDE measurements) was improved when Pd content in polymer was grown from 2 to 12 wt% Pd. At the optimum Pd content, 10–12 wt%, the analysis of the RRDE data confirmed that molecular oxygen underwent mainly a desired four-electron reduction. By further increasing the Pd content and thus decreasing the PPY to Pd ratio, the selectivity to water continuously decreased while the fraction of undesired H2O2 product significantly increased. Therefore, a role of PPY to Pd weight ratio in the electrocatalytic performance of the investigated catalysts is discussed here.