Kinetic Isotopic Effect Studies of Iron–Nitrogen–Carbon Electrocatalysts for Oxygen Reduction Reaction

Abstract

The kinetic isotopic effect (KIE) of oxygen reduction reaction (ORR) was studied via the investigation of both Koutecky−Levich and Tafel methods on atomically dispersed iron-containing, a.k.a. iron−nitrogen−carbon (Fe−N−C) electrocatalyst. This type of catalyst has been under intensive development for use as a platinum-group-metal-free cathode catalyst in polymer electrolyte membrane fuel cells. The KIE value derived from the Tafel method (the slopes of the semilogarithmic representation of the polarization data) is effectively 1, indicating that for this Fe–N–C electrocatalyst, the rate-determining step (RDS), i.e., first electron charge transfer, is independent of the proton/deuteron ratio (H+/D+). This finding suggests that the RDS of the Fe–N–C catalysts is not the main factor limiting its performance. Thus, through careful optimization of structure and morphology resulting in overcoming other limitations, Fe–N–C catalysts could, in principle, successfully compete with Pt-based ORR electrocatalysts. In contrast, the KIE value derived from the Koutecky–Levich method, based on the analysis of linear sweep voltammetry in the diffusion-limited region of polarization response at varied convective conditions (electrode rotating speeds), is approximately 2, thus implying that in mass-transport-controlled region of ORR, the mechanism and, hence, the RDS are H+-dependent. This behavior, combined with the understanding that Fe–N–C display multiple active sites, suggests a more complex and more limited mechanism of ORR for the sites involved in hydrogen peroxide production and further reduction, a.k.a. “parallel” or peroxide pathway. The catalysts exhibiting this hydrogen peroxide production (bifunctional or 2 × 2 e–) pathway will be intermittently inferior in ORR due to the concerted proton/electron charge-transfer process as RDS.