Transition metal/polymer catalysts forO₂reduction

Abstract

Transition metal/polymer catalysts for O 2 reduction have been investigated extensively as a less expensive, more abundant alternative to platinum catalysts for polymer electrolyte fuel cell (PEFC) cathodes. Embedding metals into polymers offers many potential advantages analogous to the function of metal‐containing enzymes, which include bringing the metal centers into proximity for cooperative action and moderating the local pH near the metal to improve catalyst stability. In this article, we critically summarize the developments in this category of catalysts. While many of the anticipated benefits of the transition metal/polymer catalysts are still to be demonstrated, the most successful aspect of using polymer matrices thus far has been the improved durability. Some newer systems have shown stability for hundreds of hours in the real (acidic) PEFC environment, which was not previously observed for nonprecious catalysts of any type. Although the activity must be increased before any nonprecious metal catalysts are commercially viable, the rich chemistry of polymers allows for many pathways of improvement. We discuss current efforts to understand the structure and action of the active sites and, based on that discussion and research reported to date, suggest future work in the field of oxygen reduction electrocatalysis using transition metal/polymer catalysts.

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

“…Co-based catalysts of the type Co/N/C and catalysts based on Co and Fe (Co–Fe/N/C) have also shown interesting properties, even if, according to Figure , Co is a much less common metal than Fe. Several book chapters and review articles have already been written about Fe- and/or Co-based catalysts made to perform the oxygen reduction reaction (ORR) in fuel cells. ,− In this literature, it is also reported that, besides the non-noble metal-based catalysts, some metal-free catalysts made of carbon doped with several heteroatoms (the most common one being nitrogen) may also reduce O 2 electrochemically in acid medium. − …”

Multitechnique Characterization of a Polyaniline–Iron–Carbon Oxygen Reduction Catalyst

“…Several recent literature reviews have demonstrated the importance of elucidating the ORR active sites in this class of catalyst. ,− This study is a follow-up on a previous effort in the development of PANI–Fe–C ORR catalyst at Los Alamos National Laboratory. , The purpose of the study was to elucidate the identity of the active site in the PANI–Fe–C class of catalysts by correlating ORR activity with the catalyst morphology and chemical speciation as a function of variation in the catalyst preparation procedure, specifically heat treatment temperature and presence of sulfur-containing precursor. The main technique utilized in this study was X-ray absorption fine structure (XAFS) spectroscopy, which comprises X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) and enables the measurement of atomic structure, bonding characteristics, and oxidation state with elemental specificity.…”

Cobalt and Iron Complexes with N-heterocyclic Ligands as Pyrolysis Precursors for Oxygen Reduction Catalysts

“…Later, it was found that the pyrolysis products of these compounds, as well as other complexes obtained from Co and Fe salts, N-donor ligands and/or polymers, supported on different carbons displayed a higher activity and stability than their precursor materials, comparable in some cases to that of Platinum [16][17][18]. Recently, metal-free catalysts, consisting mainly in Ndoped carbons (graphenes, nanotubes, carbon nitride, etc) have shown ORR activity which was even higher than that of Platinum in alkaline medium [19][20][21].…”

“…Although catalysts for ORR have been synthetized for decades [13][14][15][16][17][18], there is still a big debate about the ORR active site for Ptfree catalyst [22,23]. Three main models of the structure of the active site have been proposed: i) Fe-N 4 or Co-N 4 moieties bound to the graphite structure during thermal treatment [24].…”

“…Independently from the mechanism or the nature of the active site, experimental evidences have shown that: the nature and amount of metal employed, pyrolysis temperature, carbon support, N-rich polymer-and/or ligand-structure are key parameters that determine the ORR activity of these catalysts [18][19][20][21][22][23][24][25][26][27][28].…”