Iron‐Free Cathode Catalysts for Proton‐Exchange‐Membrane Fuel Cells: Cobalt Catalysts and the Peroxide Mitigation Approach

Abstract

High‐performance and inexpensive platinum‐group‐metal (PGM)‐free catalysts for the oxygen reduction reaction (ORR) in challenging acidic media are crucial for proton‐exchange‐membrane fuel cells (PEMFCs). Catalysts based on Fe and N codoped carbon (Fe–N–C) have demonstrated promising activity and stability. However, a serious concern is the Fenton reactions between Fe2+ and H2O2 generating active free radicals, which likely cause degradation of the catalysts, organic ionomers within electrodes, and polymer membranes used in PEMFCs. Alternatively, Co–N–C catalysts with mitigated Fenton reactions have been explored as a promising replacement for Fe and PGM catalysts. Therefore, herein, the focus is on Co–N–C catalysts for the ORR relevant to PEMFC applications. Catalyst synthesis, structure/morphology, activity and stability improvement, and reaction mechanisms are discussed in detail. Combining experimental and theoretical understanding, the aim is to elucidate the structure–property correlations and provide guidance for rational design of advanced Co catalysts with a special emphasis on atomically dispersed single‐metal‐site catalysts. In the meantime, to reduce H2O2 generation during the ORR on the Co catalysts, potential strategies are outlined to minimize the detrimental effect on fuel cell durability.

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 …”

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High‐Power PGM‐Free Cathodes in Fuel Cells

“…Although Fe-N-C catalysts possess the highest activity of the transition metal catalysts, Fe ions are known to catalyze the formation of hydroperoxyl radical oxygen species (ROS) via the Fenton reactions when they meet H 2 O 2 , an unwanted byproduct of the ORR process. [49,50] The as-generated ROS further destroys the local carbon structure and MN 4 sites in the catalysts. Even worse, it attacks the ionomer and membrane in PEMFCs, thereby causing severe failure of MEAs.…”

Mapping Localized Peroxyl Radical Generation on a PEM Fuel Cell Catalyst Using Integrated Scanning Electrochemical Cell Microspectroscopy

“…In cases when the catalyst distribution is heterogeneous and dynamic, in situ spatially resolved techniques may be required. Characterization of the ORR performance is further complicated by the generation of reactive oxygen species (ROSs), namely, hydroxyl and hydroperoxyl radicals formed by decomposition of the hydrogen peroxide (H 2 O 2 ) intermediate on the carbon support of the Pt catalysts (Trogadas et al, 2011;Wang et al, 2019;Trogadas and Coppens, 2020). ROSs attack the polymer binding holding the catalysts to the electrode, resulting in substantial particle aggregation and variation in edge-toedge distances between particles.…”

“…Perhaps the most active PGM-free ORR catalysts are ligated iron species formed by sintering of organometallic precursors with argon or nitrogen (Lefèvre et al, 2009;Sun et al, 2017;Zhang et al, 2017;Li et al, 2019). Unfortunately, leached iron from ligated iron catalysts promotes decomposition of intermediate hydrogen peroxide, formed during the ORR, into ROS (Wang et al, 2019;Du et al, 2020). In addition to the deleterious effects of ROS generation, other components of PEFCs are especially sensitive to radical formation.…”

“…Specifically, the polymer membrane will degrade in the presence of ROS, eventually forming pinholes that lead to complete cell failure (Mittal et al, 2006;Coms, 2008;Gubler et al, 2011). Substantial ROS generation persists even in iron-free and PGM-free ORR catalysts, emphasizing the importance of this reaction (Wang et al, 2019;Du et al, 2020).…”