Progress in the Development of Fe‐Based PGM‐Free Electrocatalysts for the Oxygen Reduction Reaction

Abstract

Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen-fuel, a promising way to store energy produced from renewable power sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel-cell technology is the high content of platinum-group-metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low-cost PGM-free electrocatalysts synthesized from inexpensive, earth-abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe-N-This article is protected by copyright. All rights reserved. C). PGM-free Fe-N-C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel-cell technology costs. Herein, recent developments in PGM-free electrocatalysis, demonstrating increased fuel-cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM-free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active-site densities, while at the same time establishing methods to mitigate catalyst degradation.

On the Influence of Oxygen on the Degradation of Fe‐N‐C Catalysts

“…and ultimately possibly resulting in the formation of volatile carbon corrosion products, CO and CO 2 , leading in this case to metal leaching via destruction of MN x sites (irreversible degradation). More recently, using a kinetic model describing the degradation rate of PGM‐free ORR catalysts, Yin and Zelenay and Martinez et al . confirmed that peroxide and/or peroxide derived radicals are likely responsible for the performance loss in PEMFC devices.…”

Thermally Driven Structure and Performance Evolution of Atomically Dispersed FeN4 Sites for Oxygen Reduction

“…This traditional synthesis concept has primarily limited the improvement of catalyst performance due to poor control of the catalyst morphology and the local structure of the active sites. Because of the complex synthesis procedures, the nature of such MN 4 active sites and their formation mechanisms have remained elusive for several decades …”

Zinc‐Mediated Template Synthesis of Fe‐N‐C Electrocatalysts with Densely Accessible Fe‐Nx Active Sites for Efficient Oxygen Reduction

“…The single atomic (SA) iron coordinated nitrogen (Fe‐N x ) moieties in the carbon matrix are commonly recognized as active centers for catalyzing sluggish ORR kinetics . Currently reported strategies for synthesizing Fe‐N‐C electrocatalysts unavoidably involve high‐temperature pyrolysis of Fe‐ and N‐containing precursors . However, during the thermal pyrolysis, Fe species tend to agglomerate and eventually form low‐active Fe‐based clusters and nanoparticles encapsulated in carbon shells (e.g., Fe 3 C@C NPs), which are difficult to be removed and thus seriously block the formation active Fe‐N x moieties (generally, SA Fe content in Fe‐N‐C is <1.0 wt%) .…”

“…[15][16][17] Currently reported strategies for synthesizing Fe-N-C electrocatalysts unavoidably involve high-temperature pyrolysis of Fe-and N-containing precursors. [18,19] However, during the thermal pyrolysis, Fe species tend to agglomerate and eventually form low-active Fe-based clusters and nanoparticles encapsulated in carbon shells (e.g., Fe 3 C@C NPs), which are difficult to be removed and thus seriously block the formation active Fe-N x moieties (generally, SA Fe content in Fe-N-C is <1.0 wt%). [20,21] Additionally, a large amount of Fe-N x moieties encapsulated in the carbon matrix cannot participate in the ORR process as a result of their inaccessibility.…”