In Situ Anomalous Small-Angle X-ray Scattering Studies of Platinum Nanoparticle Fuel Cell Electrocatalyst Degradation

Abstract

Polymer electrolyte fuel cells (PEFCs) are a promising high-efficiency energy conversion technology, but their cost-effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically active surface area (ECA) of the Pt nanoparticle electrocatalysts. While numerous studies using ex situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood. Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS patterns were analyzed to obtain particle size distributions (PSDs) of the Pt nanoparticle electrocatalysts at periodic intervals during the potential cycling. Oxide coverages reached under the applied potential cycling protocols were both calculated and determined experimentally. Changes in the PSD, mean diameter, and geometric surface area identify the mechanism behind Pt nanoparticle coarsening in an aqueous environment. Over the first 80 potential cycles, the dominant Pt surface area loss mechanism when cycling to 1.0-1.1 V was found to be preferential dissolution or loss of the smallest particles with varying extents of reprecipitation of the dissolved species onto existing particles, resulting in particle growth, depending on potential profile. Correlation of ASAXS-determined particle growth with both calculated and voltammetrically determined oxide coverages demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA. This understanding potentially reduces the complex changes in PSD and ECA resulting from various voltage profiles to a response dependent on oxide coverage.

In-Operando Anomalous Small-Angle X-Ray Scattering Investigation of Pt3Co Catalyst Degradation in Aqueous and Fuel Cell Environments

“…That is Pt dissolution and its dependence on particle size being the predominant factor controlling surface area loss and a minor contribution from Pt re-deposition and/or coalescence through re-deposition. 16,19,51,52 However, the mean diameter increase of the Pt 3 Co particles (∼0.3 to 0.5 nm) observed here is similar to the mean diameter increase observed for a Pt catalyst (∼0.2 to 0.4 nm) with an much smaller initial mean diameter (∼3 nm). 20 The similar growth for nanoparticles of significantly different size is not consistent with what would be expected if all changes were due to the effect of particle size (Gibbs-Thomson) on Pt dissolution rates.…”

“…This type of mechanism is consistent with previous findings on Pt catalysts with different initial mean diameters. 19,20 EXAFS and ASAXS data are consistent with an initial intraparticle structure comprised of a slightly Pt-rich shell which evolves with potential cycling into a structure with a slight predominance of heteroatomic bonds over homoatomic bonds such as one with a Pt-rich shell, a Co-rich underlayer, and a Pt-Co alloy core.…”

“…53 They reported a mean diameter increase of ∼0.06 nm after 1800 triangle cycles between 0.56 V and 1.16 V at 50 mV/s; a cycling regime more degrading than the one used here due to the higher upper potential limit (1.16 vs. 1.0 V). 19 Thus, we find that for the same mean particle size, the Pt and Pt 3 Co show significantly different rates and extent of particle size increase and GSA loss, with the Pt 3 Co degrading significantly faster. This same observation was made by Gummalla et al for these same two catalysts subjected to triangle wave cycling in an MEA environment.…”

“…19 The higher dissolved Pt concentrations in the Pt 3 Co system lead to higher rates of Pt re-depositon resulting in greater extent of coarsening which is evident in the Pt PSD evolutions (Figure 2) and as an increase in the number of particles with diameters larger than 6 nm (Figure 7a). Coarsening was not observed for the 5 nm Pt catalyst (Figure 7b).…”

“…14,[18][19][20][21][22] A comprehensive discussion of the ASAXS method and its use in characterizing carbon-supported metal catalysts can be found in several publications and textbooks. [23][24][25][26][27][28][29] One advantage this technique has is its ability to obtain element specific PSDs.…”