Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium

Abstract

For a successful replacement of Pt, tremendous efforts have hitherto been made to develop high-performing Fe-N-C catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). In comparison to the remarkable progress in activity, the stability of Fe-N-C catalysts still remains critical, however. Fe demetallation in acidic medium is hypothesized to be one critical factor for the overall lifetime. In contrast to the general belief, we herein demonstrate using an operando spectroscopic analysis that catalytically inactive Fe particles exposed to acid electrolytes cannot be fully removed by acid washing due to a relatively high open circuit potential (ca. 0.9 VRHE) leading to the formation of insoluble ferric species, whereas these particles dissolve under PEMFC operating conditions (E cathode < 0.7 VRHE) due to operando reduction to soluble ferrous cations. To overcome this issue, we demonstrate two approaches: (i) synthesis of Fe-N-C catalysts free of Fe particles and (ii) postsynthesis removal of exposed Fe particles through the control of potential using an external potentiostat or an internal reducing agent (i.e., SnCl2). Operando spectroscopic analyses verified that Fe demetallation during a given voltammetric protocol was dramatically decreased for both synthetically and postsynthetically modified Fe-N-C catalysts, while the initial ORR activity did not significantly change. However, all of these catalysts showed similar performance decay over short-term PEMFC durability tests, demonstrating the lack of a role played by ferrous cations leached from inactive Fe particles on catalyst deactivation. This supports the view that the activity is mainly imparted by FeN x C y moieties. Nevertheless, the presented guidelines are generally applicable to the whole class of Fe-N-C catalysts in order to minimize Fe demetallation in PEMFCs, which provides important advances for the future design of stable electrocatalytic systems for long-term operation.

Investigation of Earth-Abundant Oxygen Reduction Electrocatalysts for the Cathode of Passive Air-Breathing Direct Formate Fuel Cells

“…However, on both electrolytes, these two peaks vanish after 2 voltammetric cycles, and the curves present as stable profile afterward. Probably, this metal dissolution may occur due to the carbon corrosion of some reactive carbon sites, which leads to the destruction of the neighboring metal-nitrogen species, releasing the metal ions, as evidenced in a previously published study [26].…”

Rational Design and Synthesis of Low-Temperature Fuel Cell Electrocatalysts

“…A phenomenon is usually observed in stability tests in which Fe leaching-out occurs at low potentials (< 0.7 V) without direct adverse effects on ORR activity, and carbon oxidation occurs at high potentials (> 0.9 V) with observable activity decay [149][150][151]. Based on this phenomenon, four possible reasons are suggested for the decay (schematic illuminated in Fig.…”

“…Based on this phenomenon, four possible reasons are suggested for the decay (schematic illuminated in Fig. 15a) [150,152] [155], decreasing surface basicity and is responsible for initial activity losses; (3) active sites are flooded as oxidized carbon change hydrophobic surfaces into hydrophilic surfaces [149,156]; (4) free radical species attack carbon matrixes [150,157], causing Fenton reagents to form during ORR with Fe existence, leading to corrosion of catalyst and membrane.…”

“…However, for Fe leaching, Choi et al [150] prepared four catalysts with a wide range of Fe leaching rates in a liquid electrolyte (Fig. 15c).…”

“…[149,158]. c Current density versus cumulative charge at 0.5 V for four Fe/N/C catalysts with different Fe leaching rates [150]. The numbers appearing at the end of each sample are the demetallation amounts of Fe during a given voltammetric protocol Generally, the resistance of mass/charge transport is in the order of oxygen transport > proton transfer > electron transfer.…”