The Effect of Particle Size on Pt/C Electrocatalyst Degradation in Proton Exchange Membrane Fuel Cells

Abstract

Pt nanoparticle electrocatalysts in Proton Exchange Membrane Fuel Cells (PEMFCs) have been shown to undergo irreversible degradation through coalescence and Pt dissolution, the degree of which vary with initial particle size. Such degradation leads to losses in the performance and operating hours of the PEMFC [1]. In this work, we study the relationship between Pt nanocatalyst size and degradation mechanisms for PEMFC cathodes cycled between 0.6 and 1.0 V in triangular wave with 32s time period. Furthermore, to address the nanoparticle morphology, we use for the first time roundness and circularity parameters as novel metrics.Two cycled membrane electrode assemblies (MEAs), S1 and S2, whose cathodes were initially loaded with Pt nanoparticles of sizes 2 nm and 12 nm, respectively were microtomed for crosssection transmission electron microscopy (TEM) analysis. Subsequently, TEM images from three separate regions of the cathode, 3 µm in length, thereby covering the approximate 10 µm electrode thickness ( Fig. 1) were acquired in a JEOL 2010F operated at 200 kV and analyzed.In addition to individual spherical particles, the TEM observations identified nanoparticles with two distinct morphologies: (i) coalesced (Figs. 2a and 3a) and (ii) dendritic (Figs. 2b and 3b). As expected, coalesced and dendritic morphologies were more prevalent in sample S1 because smaller nanocatalysts have higher dissolution and mobility rates [1]. For example in region A, the frequency of coalesced nanoparticles was found to be 9.0% for sample S1 and 4.5% for sample S2, while the dendritic particles were 3.5% for sample S1 and 2.5% for sample S2. However, sample S1 exhibits a more pronounced dissolution than what is suggested by the dendritic frequency, as observed by the dense agglomeration of Pt nanoparticles along the cathode membrane interface, which was completely absent in the sample S2.The two morphologies were subsequently defined by a ratio R of roundness to circularity, for which R > 1 for dendritic particles and R < 1 for coalesced particles. The ratio R provides a means to quantify the severity and the mechanism of degradation for a particular nanoparticle. Roundness is more sensitive to elongation caused by coalesced particles along a central axis. Circularity is more sensitive to sharp discontinuities caused by variations in concavity and convexity, such as branching.The presence of these morphologies provides insight into specific degradation mechanisms. In particular, the dendritic particles arise from the dissolution and redeposition of Pt, whereas the coalesced particles are the result of nanoparticles migrating and coalescing on the carbon support [2].

Degradation Mechanisms of Platinum Nanoparticle Catalysts in Proton Exchange Membrane Fuel Cells: The Role of Particle Size

“…In the case of the as-received powder samples, 200 particles were analyzed from each sample, using the software Image J. To choose the 200 particles within the TEM images, a protocol outlining specific requirements for particle analysis was created . Herein, specific magnifications to obtain the TEM images were selected to set the equivalent error under 5%.…”

“…In contrast, particle size analysis by X-ray diffraction and small angle Xray scattering provides statistically significant results, but extracting information about the shape of particles is challenging. In this regard, we have used a novel protocol developed during this project 15 for counting particles in used MEAs that addresses some of the shortcomings encountered during TEM analysis.…”

“…To choose the 200 particles within the TEM images, a protocol outlining specific requirements for particle analysis was created. 15 Herein, specific magnifications to obtain the TEM images were selected to set the equivalent error under 5%. Image J's outlining hand tool was used to highlight each particle that matched the above criteria.…”

Effect of Particle Size and Operating Conditions on Pt3Co PEMFC Cathode Catalyst Durability

“…Energy Dispersive X-ray (EDX) analysis was used to confirm the Pt and Co composition. Particle size distributions were determined from at least 200 particles using a procedure described elsewhere [50]. Figure 11 shows TEM bright-field images of 40 wt.% Pt3Co/Ketjen EC 300J versions annealed at increasing temperatures (T1, T2 and T3) to generate larger average particle sizes.…”