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

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.