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\n1 : 1 stoichiometric mixture, followed by heat treatment at 600 C under inert atmosphere, gave a series\nof novel nanostructured electrocatalysts AgPc/C(600), FePc/C(600) and FeAgPc/C(600) (ca. 3 wt% metal\nloadings) for the oxygen reduction reaction (ORR) in alkaline media. The catalysts were structurally\ncharacterized by XRPD, XPS, HR-TEM/STEM and chemisorption measurements. During the synthetic heat\ntreatment of AgPc/C(600) at temperatures above 250 C, the AgPc decomposed to form small finely\ndispersed carbon supported Ag nanoparticles (mean diameter 8.5 nm). This process was delayed for\nFeAgPc/C(600) to above 300 C and the resulting Ag nanoparticles were much smaller (mean diameter\n2.3 nm). As expected, at 600 C the FePc/C(600) forms highly dispersed arrays of single Fe ions\ncoordinated by four nitrogen atoms (Fe–N4 units). Electrodes coated with AgPc/C(600), FePc/C(600) and\nFeAgPc/C(600) were investigated for ORR in alkaline media by linear sweep voltammetry and the RRDE\nsystem was used to probe the production of HO2\n. The electrochemical activity of all materials was\nanalyzed by Tafel and Koutecky–Levich plots and the stability of each catalyst was followed using\nchronopotentiometry. Both Fe-containing electrocatalysts, FeAgPc/C(600) and FePc/C(600), were highly\nactive for the ORR promoting exclusively the four electron pathway also during chronopotentiometry,\nwhile AgPc/C(600) was found to produce up to 35 mol% HO2\n. Compared to FePc/C(600), the binary\nFeAgPc/C(600) catalyst displayed remarkably higher activity and stability. This experimental evidence\ncould be explained in terms of a synergistic Ag–Fe interaction which results from the unique\nnanostructure that forms during heat treatment which consists of very finely dispersed Ag nanoparticles\nand Fe–N4 moieties.