Dataset Information

Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O2 reduction in PEM fuel cells.

ABSTRACT: The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts prepared through a pyrolysis in NH3 is mostly imparted by acid-resistant FeN4-sites whose turnover frequency for the O2 reduction can be regulated by fine chemical changes of the catalyst surface. We show that surface N-groups protonate at pH 1 and subsequently bind anions. This results in decreased activity for the O2 reduction. The anions can be removed chemically or thermally, which restores the activity of acid-resistant FeN4-sites. These results are interpreted as an increased turnover frequency of FeN4-sites when specific surface N-groups protonate. These unprecedented findings provide new perspective for stabilizing the most active Fe/N/C-catalysts known to date.

SUBMITTER: Herranz J

PROVIDER: S-EPMC3810981 | biostudies-literature | 2011 Nov

REPOSITORIES: biostudies-literature

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Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O2 reduction in PEM fuel cells.

Herranz Juan J Jaouen Frédéric F Lefèvre Michel M Kramm Ulrike I UI Proietti Eric E Dodelet Jean-Pol JP Bogdanoff Peter P Fiechter Sebastian S Abs-Wurmbach Irmgard I Bertrand Patrick P Arruda Thomas M TM Mukerjee Sanjeev S

The journal of physical chemistry. C, Nanomaterials and interfaces 20111101 32

The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts prepared through a pyrolysis in NH3 is mostly imparted by acid-resistant FeN4-sites whose turnover frequency for the O2</sub ...[more]

PMID: 24179561

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Project description:Recently Debe et al. reported that Pt3Ni7 leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface ("Pt-skin effect"). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt3Ni7 while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt-Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ?300?000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt3Ni7 case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the Oad-hydration reaction (Oad + H2Oad ? OHad + OHad) but including the barriers of O2 dissociation (O2ad ? Oad + Oad) and water formation (OHad + Had ? H2Oad). We find that the reaction barrier for the Oad-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt-Ni 30?:?70 initial composition. These simulations explain the peak in surface reactivity at Pt3Ni7, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials.

Dataset Information

Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O2 reduction in PEM fuel cells.

Publications

Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O<sub>2</sub> reduction in PEM fuel cells.

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