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Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis.

ABSTRACT: Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.

SUBMITTER: Chattot R

PROVIDER: S-EPMC6109589 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature

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Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis.

Chattot Raphaël R Le Bacq Olivier O Beermann Vera V Kühl Stefanie S Herranz Juan J Henning Sebastian S Kühn Laura L Asset Tristan T Guétaz Laure L Renou Gilles G Drnec Jakub J Bordet Pierre P Pasturel Alain A Eychmüller Alexander A Schmidt Thomas J TJ Strasser Peter P Dubau Laetitia L Maillard Frédéric F

Nature materials 20180716 9

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly oppo ...[more]

PMID: 30013055

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Project description:Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe-N-C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe-N-C catalysts or because of its capability to increase the Fe-Nx site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe-Nx site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe-N-C catalysts. The Fe-N-C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0-16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors. The carbon with the highest sulfur content was also activated through steam treatment at 800 °C for different durations, which allowed us to modulate the carbon pore volume and surface area (1296-1726 m2 g-1). The resulting catalysts were tested in O2-saturated 0.5 M H2SO4 electrolyte, and the site density (SD) was determined using the NO-stripping technique. Here, we demonstrate that sulfur doping has a porogenic effect increasing the microporosity of the carbon support, and it also facilitates the nitrogen fixation on the carbon support as well as the formation of Fe-Nx sites. It was found that the Fe-N-C catalytic activity [E1/2 ranges between 0.609 and 0.731 V vs reversible hydrogen electrode (RHE)] does not directly depend on sulfur content, but rather on the microporous surface and therefore any electronic effect appears not to be determinant as confirmed by X-ray photoemission spectroscopy (XPS). The graph reporting Fe-Nx SD versus sulfur content assumes a volcano-like shape, where the maximum value is obtained for a sulfur/iron ratio close to 18, i.e., a too high or too low sulfur doping has a detrimental effect on Fe-Nx formation. However, it was highlighted that the increase of Fe-Nx SD is a necessary but not sufficient condition for increasing the catalytic activity of the material, unless the textural properties are also optimized, i.e., there must be an optimized hierarchical porosity that facilitates the mass transport to the active sites.

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Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis.

Publications

Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis.

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