X-ray and electrochemical studies of bimetallic Pt-based oxygen reduction electrocatalysts

Abstract

Dealloying carbon supported Pt alloy nanoparticles has been shown to particles with a Pt rich outer shell surrounded by an alloy rich core that are highly active electrocatalysts for the oxygen reduction reaction, which is of interest for use in fuel cell cathodes. The structure of these materials as well as how the size, elemental distribution and composition changes during fuel cell operation is important.

The catalysts were subjected to an accelerated stability test under similar conditions to those experienced in fuel cell cathodes. At various points throughout the test, the ECSA was recorded and samples taken for ex situ analysis. A variety of x-ray based spectroscopic techniques including XPS, XRD and XAS were used to investigate how the catalyst structure has been affected by the test. TEM will also be used. The 5 nm Pt/C and equivalent alloy catalysts were shown to be stable under these conditions with no significant change in structure or surface area. This shows that the protocol used here does not fully represent the conditions experienced in the fuel cell where degradation is observed. In comparison, the ECSA of 2 nm Pt/C sample was greatly decreased. Further testing for either a longer duration or using higher acid concentration is required to differentiate between the stability of the 5 nm nanoparticle samples.

Additionally, as measurements of the electrocatalytic activity made using the RDE technique differ significantly to the performance obtained in an MEA an alternative method was proposed. The GDE combines the advantages of the RDE system in terms of speed of testing and the quantity of catalyst required, with a more accurate representation of the conditions experienced in a fuel cell i.e ability to access the high current density regime. This method was shown to compare favourably with other electrode configurations from the literature such as floating electrodes in terms of ease of use and similarity to results from testing in PEMFC MEAs. Several issues remain with the system, notably quantifying the amount of the catalyst actually utilised, although this does also allow the study of fuel cell related phenomena such as flooding of catalyst layers.

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Identifiers

ORCID for Andrea E. Russell: orcid.org/0000-0002-8382-6443

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