Achilles' heel of Iron-Based Catalysts During Oxygen Reduction in Acidic Medium

Abstract

Fuel cells efficiently convert chemical into electric energy, with promising application for clean transportation. In proton-exchange membrane fuel cells (PEMFCs), rare platinum metal catalyzes today the oxygen reduction reaction (ORR) while iron(cobalt)-nitrogen-carbon materials (Fe(Co)-N-C) are a promising alternative. Their active sites can be classified as atomically dispersed metal-ions coordinated to nitrogen atoms (MeNxCy moieties) or nitrogen functionalities (possibly influenced by sub-surface metallic particles). While their durability is a recognized challenge, its rational improvement is impeded by insufficient understanding of operando degradation mechanisms. Here, we show that FeNxCy moieties in a representative Fe-N-C catalyst are structurally stable but electrochemically unstable when exposed in acidic medium to H2O2, the main ORR byproduct. We reveal that exposure to H2O2 leaves iron-based catalytic sites untouched but decreases their turnover frequency (TOF) via oxidation of the carbon surface, leading to weakened O2 binding on iron-based sites. Their TOF is recovered upon electrochemical reduction of the carbon surface, demonstrating the proposed deactivation mechanism. Our results reveal a hitherto unsuspected deactivation mechanism during ORR in acidic medium. This study identifies the N-doped carbon surface as Achilles' heel during ORR catalysis in PEMFCs. Observed in acidic but not in alkaline electrolyte, these insights suggest that durable iron-nitrogen-carbon catalysts are within reach for PEMFCs if rational strategies minimizing the amount of H2O2 or reactive oxygen species (ROS) produced during ORR are developed.

A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

“…A similar deactivation is also observed for pyrrolic complexes and is well-documented in the literature [148][149][150]. These observations highlight the important role of the carbon framework in increasing the relative stability of Fe-N4 sites in Fe-N-C materials against oxidative and protolytic decomposition induced by the acidic conditions and the presence of parasitic amounts of H2O2 151. Indeed, we posit that extending the aromatic periphery around the (phen2N2)FeCl active site could enhance stability and provide a path towards the bottom-up synthesis of robust Fe-based ORR catalysts.CONCLUSIONSThe results reported herein provide a molecular perspective on the structure and oxygen reduction reactivity of Fe-N4 active sites in Fe-N-C materials.…”

Facile Synthesis of the Amorphous Carbon Coated Fe-N-C Nanocatalyst with Efficient Activity for Oxygen Reduction Reaction in Acidic and Alkaline Media

“…Previous researchers have found that the Fe-N-C catalyst has a stable structure but unstable electrochemical performance when exposed to H 2 O 2 (ORR by-product) in acidic electrolyte [ 49 ]. The catalytic active site of the Fe-N-C catalyst was not be affected, but its conversion frequency was reduced through the oxidation of the carbon surface, resulting in the weakening of the binding ability of the Fe-N-C catalyst to O 2 .…”

Identification of Durable and Non-Durable FeNx Sites in Fe-N-C Materials for Proton Exchange Membrane Fuel Cells

“…This hypothesis is in line with our recent work that demonstrated decreased TOF of Fe-based sites by chemical reaction of Fe0.5 with H2O2. 25 The activity drop from 0 to 5 h of operation is thus assigned to both decreased TOF via mild surface oxidation of carbon (vertical arrows in Figure 5e), and decreased number of S1 sites (arrows along the dashed lines in Figure 5e). After 5 h operation, the TOF of the Fe sites seems stabilized, leading to linear correlations between the overall activity and either D1 or D1+D2.…”