Nanostructured Fe–Ag electrocatalysts for the oxygen reduction reaction in alkaline media

Abstract

The impregnation of Ketjen Black (C) with iron(II) and silver(II) phthalocyanines (MPc) individually or as a 1 : 1 stoichiometric mixture, followed by heat treatment at 600 °C under inert atmosphere, gave a series of novel nanostructured electrocatalysts AgPc/C(600), FePc/C(600) and FeAgPc/C(600) (ca. 3 wt% metal loadings) for the oxygen reduction reaction (ORR) in alkaline media. The catalysts were structurally characterized by XRPD, XPS, HR-TEM/STEM and chemisorption measurements. During the synthetic heat treatment of AgPc/C(600) at temperatures above 250 °C, the AgPc decomposed to form small finely dispersed carbon supported Ag nanoparticles (mean diameter 8.5 nm). This process was delayed for FeAgPc/C(600) to above 300 °C and the resulting Ag nanoparticles were much smaller (mean diameter 2.3 nm). As expected, at 600 °C the FePc/C(600) forms highly dispersed arrays of single Fe ions coordinated by four nitrogen atoms (Fe–N₄ units). Electrodes coated with AgPc/C(600), FePc/C(600) and FeAgPc/C(600) were investigated for ORR in alkaline media by linear sweep voltammetry and the RRDE system was used to probe the production of HO₂⁻. The electrochemical activity of all materials was analyzed by Tafel and Koutecky–Levich plots and the stability of each catalyst was followed using chronopotentiometry. Both Fe-containing electrocatalysts, FeAgPc/C(600) and FePc/C(600), were highly active for the ORR promoting exclusively the four electron pathway also during chronopotentiometry, while AgPc/C(600) was found to produce up to 35 mol% HO₂⁻. Compared to FePc/C(600), the binary FeAgPc/C(600) catalyst displayed remarkably higher activity and stability. This experimental evidence could be explained in terms of a synergistic Ag–Fe interaction which results from the unique nanostructure that forms during heat treatment which consists of very finely dispersed Ag nanoparticles and Fe–N₄ moieties.