Article pubs.acs.org/cm
Degradation Mechanisms of Platinum Nanoparticle Catalysts in Proton Exchange Membrane Fuel Cells: The Role of Particle Size Kang Yu,† Daniel J. Groom,† Xiaoping Wang,∥ Zhiwei Yang,‡ Mallika Gummalla,‡ Sarah C. Ball,§ Deborah J. Myers,∥ and Paulo J. Ferreira*,† †
Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States United Technology Research Center, East Hartford, Connecticut 06118, United States § Johnson Matthey Technology Center, Blount’s Court, Sonning Common, Reading, RG4 9NH, United Kingdom ∥ Argonne National Laboratory, Argonne, Illinois 60439, United States ‡
S Supporting Information *
ABSTRACT: Five membrane-electrode assemblies (MEAs) with different average sizes of platinum (Pt) nanoparticles (2.2, 3.5, 5.0, 6.7, and 11.3 nm) in the cathode were analyzed before and after potential cycling (0.6 to 1.0 V, 50 mV/s) by transmission electron microscopy. Cathodes loaded with 2.2 and 3.5 nm catalyst nanoparticles exhibit the following changes during electrochemical cycling: (i) substantial broadening of the size distribution relative to the initial size distribution, (ii) presence of coalesced particles within the electrode, and (iii) precipitation of submicron-sized particles with complex shapes within the membrane. In contrast, cathodes loaded with 5.0, 6.7, and 11.3 nm size catalyst nanoparticles are significantly less prone to the aforementioned effects. As a result, the electrochemically active surface area (ECA) of MEA cathodes loaded with 2.2 and 3.5 nm nanoparticle catalysts degrades dramatically within 1000 cycles of operation, while the electrochemically active surface area of MEA cathodes loaded with 5.0, 6.7, and 11.3 nm nanoparticle catalysts appears to be stable even after 10 000 cycles. The loss in MEA performance for cathodes loaded with 2.2 and 3.5 nm nanoparticle catalysts appears to be due to the loss in electrochemically active surface area concomitant with the observed morphological changes in these nanoparticle catalysts.
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INTRODUCTION There is an intense drive to switch to more efficient and less polluting energy conversion devices for transportation applications. Proton exchange membrane fuel cells (PEMFCs) are a potential choice due to their high efficiency, high energy density (approximately 0.7−0.9 W/cm2), large dynamic range of operation, and low operating temperatures (approximately 80 °C).1,2 Despite these advantages, PEMFCs still face significant technical challenges before they become viable; one of the most critical is the durability of the nanoparticle catalyst in the cathode, particularly during cycling, which falls short of the 5000 operating hour target set by the United States Department of Energy.3 Several studies in the past have related morphological changes in the Pt nanoparticle catalysts, during fuel cell operation, particularly in the cathode, with performance degradation.3,4 Wilson et al.5 first reported catalyst morphology changes in PEMFCs and correlated them to lower electrochemical performance. Later, several degradation mechanisms were proposed for PEMFC durability loss,8−10 namely, (i) electrochemical Ostwald ripening, (ii) particle migration and coalescence, (iii) detachment from carbon support (mainly from carbon corrosion), and (iv) platinum dissolution and reprecipitation inside the ionomer phase. Shao-Horn et al.8 and Campbell et al.11 have claimed that © XXXX American Chemical Society
the dissolution rate and solubility should be accelerated by smaller initial particle sizes due to their higher specific surface energy.8,11 Holby et al.12 proposed that the role of the Gibbs− Thomson energy on the stability of particles is critical for particle size below 5 nm. In particular, as the particle size decreases from 5 to 2 nm, the Gibbs−Thomson energy increases significantly. Ascarelli et al.13 proposed that under pure Ostwald ripening, the PSDs of Pt should be broader and with a shift to larger sizes of the peak. On the other hand, PSDs with tails to larger sizes are correlated with migration and coalescence.18 There is extensive research work that discusses changes in Pt catalysts as a function of cycling, changes in carbon support, platinum dissolution, and membrane degradation. However, the relationship between initial nanoparticle size and the various degradation mechanisms is still not clear. The answer to this question is critical for optimal MEA performance and for maintaining performance over the lifetime of the fuel cell while also minimizing Pt loading and thus cost. In this paper, we conduct a systematic study of the influence of nanoparticle size on active degradation mechanisms and, ultimately, on the electrochemical performance of MEAs. To our Received: May 23, 2014 Revised: September 13, 2014
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dx.doi.org/10.1021/cm501867c | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
knowledge, this is the first time that a thorough and systematic transmission electron microscopy analysis has been performed on Pt catalysts to establish a fundamental correlation between particle size and the various active degradation mechanisms in PEMFCs.
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particles in used MEAs that addresses some of the shortcomings encountered during TEM analysis. 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.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. Because each image contains a different number of distinguishable platinum particles, a second protocol was needed to determine how many particles should be collected from each representative image. The analysis represented in this paper selected randomly one of the representative images and collected all the distinguishable particles from this image. Once completed, another representative image was randomly chosen and all of its distinguished particles were outlined. This process continued until 200 particles were collected. If a total of 200 particles were reached before analyzing all the distinguishable particles within an image, a random generator picked out the number of particles needed to complete the set of 200 from all the distinguishable particles in that specific image. In the case of MEA samples, the cathode region was divided into three areas, each approximately 3 μm wide and thereby covering a cathode width of approximately 10 μm. Two hundred particles were analyzed from each of the regions to give a total of six hundred particles for a cathode section in a used MEA. The cathodes were intensively investigated relative to the anode side because previous studies7 have shown that cathodes undergo substantial change relative to the anodes during fuel cell operation. Cathode Catalyst Surface Area and Performance Analysis. The electrochemically active surface area (ECA) of the cathode catalyst was measured using a cyclic voltammogram taken at 10 mV/s between 0.03 and 1.0 V (vs anode) with nitrogen flowing on the cathode, 4% hydrogen (balance nitrogen) on the anode, and a cell temperature of 80 °C. The ECA values (m2/g-Pt) were calculated by integrating the hydrogen adsorption charge in the cathodic-going sweep of the voltammogram, multiplying by the cell area, and dividing by 210 μC/ cm2 (theoretical hydrogen monolayer adsorption on Pt) and the cathode Pt loading.14 Aqueous Dissolution Rates. The dissolution rate of Pt from the five Pt/C catalysts was studied under the same potential cycling profile as was used for the MEAs. Gas diffusion electrodes (GDE) were prepared using the five catalysts powders as described in Ahluwalia et al.16 The GDE samples were immersed in 0.57 M HClO4 (GFS, double distilled,
