The Interparticle Distance Effect on Transient Platinum Dissolution: Degradation at High and Low Loadings
Authors: Sandbeck Daniel John Seale , Inaba Masanori , Bucher Jan , Zana Alessandro , Arenz Matthias , Cherevko Serhiy 
Published: February 2020

In hopes of reducing the necessary quantities of Pt in PEMFCs, the vast majority of research efforts in the past decades have been aimed at increasing the activity of catalysts towards the oxygen reduction reaction (ORR).1 These strategies include: Pt metal alloy nanoparticles, shape controlled Pt nanoparticles and non-precious metal catalysts, among others. With the discovery and development of advanced catalysts the required loadings of Pt in PEMFCs could be reduced, but there are still many questions to be answered regarding Pt catalyst stability at lower loadings.
It has been observed in full cell membrane electrode assembly (MEA) experiments that at low cathode loadings and high current density performance drastically decreases,2 and irreversible degradation drastically increases.3 Such experiments refer to decreasing catalyst layer (CL) loadings by depositing less Pt/C material, and therefore decreasing the CL thickness. In efforts to elucidate the effect of CL thickness on transient platinum dissolution, one of the major degradation mechanisms of the CL,4 a scanning flow cell coupled to an inductively coupled mass spectrometer (SFC-ICP-MS) was employed, where it was shown that specific dissolution decreases with increasing layer thickness.5 As both CL thickness and Pt amount are changing simultaneously, a solid conclusion on the contribution of each of these parameters to the overall PEMFC’s performance deterioration could not be drawn. To address this issue, a different approach was taken in the current work.
In the study presented here the transient dissolution of Pt from Pt/Vulcan catalysts with varying wt % loading and constant CL thickness was monitored via the SFC-ICP-MS technique.5 The catalysts were synthesized using a two-step surfactant free “toolbox” approach which allows for precise control over the particle size, metal content and hence interparticle distance.6

Seven samples with 8-60 wt % Pt and particle size of ca. 2 nm were tested under different electrochemical conditions, including cyclic voltammetry scans from 0.05-1.5 VRHE at 10 mV·s-1 which deconvolutes anodic and cathodic dissolution processes and provides a basis for comparison to previous work.5 Different scan rates were employed from 2-500 mV·s-1 revealing a similar dependence for all samples. Additionally, the samples were subjected to an aggressive ADT protocol in the potential range of 0.6-1.5 VRHE causing severe Pt dissolution between 20-60 % total loss. The data clearly demonstrate that specific dissolution decreases with increasing loading and decreasing interparticle distance. Therefore, high wt % loading or high Pt surface area utilization should be desirable in Pt/C materials to minimized degradation via Pt dissolution.

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