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

Abstract

Abstract:The initial performance and decay trends of polymer electrolyte membrane fuel cells (PEMFC) cathodes with Pt3Co catalysts of three mean particle sizes (4.9 nm, 8.1 nm, and 14.8 nm) with identical Pt loadings are compared. Even though the cathode based on 4.9 nm catalyst exhibited the highest initial electrochemical surface area (ECA) and mass activity, the cathode based on 8.1 nm catalyst showed better initial performance at high currents. Owing to the low mass activity of the large particles, the initial performance of the 14.8 nm Pt3Co-based electrode was the lowest. The performance decay rate of the electrodes with the smallest Pt3Co particle size was the highest and that of the largest Pt3Co particle size was lowest. Interestingly, with increasing number of decay cycles (0.6 to 1.0 V, 50 mV/s), the relative improvement in performance of the cathode based on 8.1 nm Pt3Co over the 4.9 nm Pt3Co increased, owing to better stability of the 8.1 nm catalyst. The electron OPEN ACCESSCatalysts 2015, 5 927 microprobe analysis (EMPA) of the decayed membrane-electrode assembly (MEA) showed that the amount of Co in the membrane was lower for the larger particles, and the platinum loss into the membrane also decreased with increasing particle size. This suggests that the higher initial performance at high currents with 8.1 nm Pt3Co could be due to lower contamination of the ionomer in the electrode. Furthermore, lower loss of Co from the catalyst with increased particle size could be one of the factors contributing to the stability of ECA and mass activity of electrodes with larger cathode catalyst particles. To delineate the impact of particle size and alloy effects, these results are compared with prior work from our research group on size effects of pure platinum catalysts. The impact of PEMFC operating conditions, including upper potential, relative humidity, and temperature on the alloy catalyst decay trends, along with the EMPA analysis of the decayed MEAs, are reported.

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

“…Gummalla et al reported this catalyst to have a metal surface area of 35 m 2 /g-Pt, as determined by gas phase CO adsorption on the catalyst powder, an XRD-determined lattice spacing of 3.848 Å, and Pt and Co contents of 37.9 and 3.88 wt% (74.7 mol% Pt and 25.3 mol% Co), respectively, as determined by inductively-coupled plasma optical emission spectroscopy (ICP-OES). 17 A representative TEM image of the catalyst powder and the PSDs derived from TEM images are shown in Figure 1. Speder et al have shown limitations of PSD analyses based on TEM alone, based on the fact that particle sizes might be over/underestimated due to the limited contrast between the nanoparticles and the support.…”

“…Therefore any comparisons between larger annealed Pt-alloys and typical commercial Pt catalysts with smaller mean particle sizes are not straightforward. 6,17 In this work, efforts were made to investigate the durability and degradation mechanisms of similar sized Pt and Pt 3 Co electrocatalysts in the aqueous and membrane-electrode assembly (MEA) environments.Recent work has shown that anomalous small-angle X-ray scattering (ASAXS) is an effective technique for observing in-situ and inoperando evolutions of Pt catalyst PSDs leading to insights into their degradation mechanism(s).14,18-22 A comprehensive discussion of the * Electrochemical Society Student Member. z E-mail: dmyers@anl.gov ASAXS method and its use in characterizing carbon-supported metal catalysts can be found in several publications and textbooks.…”

“…Therefore any comparisons between larger annealed Pt-alloys and typical commercial Pt catalysts with smaller mean particle sizes are not straightforward. 6,17 In this work, efforts were made to investigate the durability and degradation mechanisms of similar sized Pt and Pt 3 Co electrocatalysts in the aqueous and membrane-electrode assembly (MEA) environments.…”

Potential Dependence of Pt and Co Dissolution from Platinum-Cobalt Alloy PEFC Catalysts Using Time-Resolved Measurements

“…These alloy catalysts have much higher mass activity for ORR than Pt/C catalysts, and, due to their larger particle size (4-5 nm), can be more stable than the 2-3 nm Pt/C traditionally used to obtain high ORR mass activities. 4 However, Co, with a standard dissolution potential of −0.28V vs. NHE 24 is unstable at typical PEFC cathode potentials (∼0.6 to ∼0.9 V). One objective of this work is to provide initial quantitative data on the dissolution behavior of Co in commercial PtCo/C catalysts as a function of potential and potential profile.…”

“…Whereas only in-cell studies can properly characterize the performance and stability of advanced polymer electrolyte fuel cell (PEFC) oxygen reduction reaction (ORR) catalysts, [1][2][3][4] the fuel cell environment presents challenges to the real-time measurement of the catalyst degradation processes. Therefore, the bulk of our understanding of Pt and transition metal dissolution from Pt and Pt alloy catalysts, including real-time resolution of the dissolution processes during potential cycling, has been obtained through aqueous studies.…”