3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate

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3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports Brian T Sneed, David A. Cullen, Kimberly S Reeves, Ondrej E. Dyck, David A. Langlois, Rangachary Mukundan, Rodney L. Borup, and Karren L More ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09716 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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ACS Applied Materials & Interfaces

3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports Brian T. Sneed,1 David A. Cullen,2 Kimberly S. Reeves,1 Ondrej E. Dyck,1 David A. Langlois,3 Rangachary Mukundan,3 Rodney L. Borup,3 and Karren L. More1,* 1

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA

2

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA

3

Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA

KEYWORDS: PEM fuel cells; electron tomography; electrochemical active surface area; 3D; graphitized carbon ABSTRACT: Understanding the mechanisms associated with Pt/C electrocatalyst degradation in proton exchange membrane fuel cell (PEMFC) cathodes is critical for the future development of higher-performing materials; however, there is a lack of information regarding Pt coarsening under PEMFC operating conditions within the cathode catalyst layer. We report a direct and quantitative 3D study of Pt dispersions on carbon supports (high surface area carbon (HSAC), Vulcan XC-72, and graphitized carbon) with varied surface areas, graphitic character, and Pt loadings ranging from 5-40 wt.%. This is accomplished both before and after catalyst-cycling accelerated stress tests (ASTs) through observations of the cathode catalyst layer of membrane electrode assemblies. Electron tomography results show Pt nanoparticle agglomeration occurs predominantly at junctions and edges of aggregated graphitized carbon particles, leading to poor Pt dispersion in the as-prepared catalysts and increased coalescence during ASTs. By contrast, tomographic reconstructions of Pt/HSAC show much better initial Pt dispersions, less agglomeration, and less coarsening during ASTs in the cathode. However, a large loss of the electrochemically active surface area (ECSA) is still observed and is attributed to accelerated Pt dissolution and nanoparticle coalescence. Furthermore, a strong correlation between Pt particle/agglomerate size and measured ECSA is established, and is proposed as a more useful metric than average crystallite size in predicting degradation behavior across different catalyst systems.

INTRODUCTION The performance of proton exchange membrane fuel cells (PEMFCs) is strongly influenced by the electrochemically active surface area (ECSA) of the catalyst and the stability of this metric over the device lifetime.1 A typical fuel cell electrocatalyst consists of Pt or Pt-based alloy nanoparticles distributed on a nanocarbon support.2 The uniformity of the Pt distribution can vary based on catalyst loading and the carbon support morphology, with poor catalyst particle dispersion and agglomeration in the as-prepared catalyst powders facilitating rapid coarsening during the early stages of operation. Furthermore, ECSA degradation of these catalysts occurs not only through coarsening resulting from particle coalescence3, 4 and dissolution/redeposition5-7 mechanisms, but also through encapsulation due to carbon corrosion.8 Therefore, strategies to increase catalyst dispersion while mitigating catalyst degradation by optimization of Pt nucleation, carbon support structure, ionomer distribution, and other ink processing variables, are of primary interest to manufacturers moving forward,9-12 as is the assessment and decoupling of the reaction mechanisms at play.13, 14 The carbon support is crucial to the durability of the catalyst, and advances made towards enhancing durability will come from synthesis and tailoring of new nanocarbons and the catalyst/carbon interface. Castanheira et al. recently reported a detailed study on degradation of different carbon support architectures observed during carbon-corrosion accelerated stress tests (ASTs).15 Graphitized carbons improve durability by increasing their tolerance to oxidation.15-21 However, these carbons are not without their own limitations, as they often have lower surface areas and highly hydrophobic surfaces, which yield non-optimized Pt dispersions.12, 15, 22 Pt nanoparticles tend to preferentially nucleate and pack into crevasses and defect-rich graphitic edge sites rather than on the more chemi-

cally inert basal planes of the highly graphitized surfaces.23, 24 In effect, these factors negate the benefits to durability associated with corrosion-resistance by promoting coalescence from increased initial particle contact. High surface area carbon (HSAC) supports have the advantage of accessible surface pore structures and less hydrophobicity, which creates better initial dispersion of Pt, but these catalysts and supports still undergo extreme ECSA loss that is believed to be due to a combination of coarsening, dissolution, and encapsulation of particles from severe carbon corrosion.8, 12, 15, 25 The issue of catalyst durability is critical as the PEMFC industry strives to improve Pt utilization due to the high cost of the precious metal. The ability to accurately assess and monitor variations of the Pt dispersion and its effect on lifetime are equally important to improve durability and performance. Information regarding Pt size and distribution changes that occur during PEMFC operation can be obtained through quantitative electron tomography, in which objects can be characterized in three spatial dimensions (3D) at the nanoscale. Metrics such as size, aspect ratio, and interparticle spacings can be generated for representative catalyst volumes. While more and more 3D information is being obtained through electron tomography studies,26-48 to date, there exist only a handful of pioneering electron tomography studies of fuel cell catalysts,49-56 and these studies have only been conducted for a limited number of carbon supports. Importantly, electron tomography of catalysts and their supports incorporated in a membrane electrode assembly (MEA), before and after fuel cell testing, have not yet been reported. The advancement of electron microscopy and tomography in recent years has resolved the issue of scale for nanomaterials, but beam-induced damage, material instabilities under the electron beam, and contamination remain challenges that must be over-

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come to enable high fidelity 3D reconstructions. In this work, with best practices in mind, a series of Pt/C electrocatalysts

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have been successfully characterized and quantitative 3D metrics reproducibly obtained.

Scheme 1. Illustrations of the primary Pt nanoparticle dispersions on various nanocarbon supports: Pt/HSAC, Pt/Vulcan, and Pt/LSAC, left to right (simplified representations based on HRTEM/STEM imaging).

To better understand Pt/C electrocatalyst dispersion at the beginning of life (BOL) and the degradation processes of different Pt-catalyst/C-support systems in 3D, we performed electron tomography in a scanning transmission electron microscope (STEM) to establish a quantitative 3D comparison of three alternate Pt/C electrocatalyst systems representing carbons of different surface areas and graphitic character, and for a range of Pt loadings. Scheme 1 shows illustrations of the three catalyst/support architectures examined in this study: the industry standard, Ketjen Black, a porous, turbostratic, high surface area carbon (HSAC) with a surface area ~800 m2/g, a mid-range surface area carbon Vulcan XC-72 (V) with a surface area of ~250 m2/g, and an alternative highly graphitized low surface area carbon (LSAC) with a surface area of ~150 m2/g, which has been shown to have higher resistance to oxidation due to extensive graphitization, or ordered stacking, of the carbon basal planes and minimization of defects and boundaries typically associated with carbon black particles. To the best of our knowledge, Pt dispersions and degradation on this alternate LSAC support have yet to be characterized in 3D. The Pt dispersion is characterized through analysis of Pt nanoparticle size, aspect ratio, sphericity, agglomeration, and interparticle distances of the different Pt/C systems. The metrics extracted from 3D data and reconstructions of the asprepared Pt/C catalyst powders are compared with these same systems incorporated into the cathode catalyst layers of MEAs after ASTs using fuel cell test conditions specifically designed to hasten Pt coarsening (following the U.S. Department of Energy guidelines).57 RESULTS AND DISCUSSION 3D characterization. Electron tomography was performed for three Pt/C support systems (HSAC, V, and LSAC) spanning a wide range of carbon support surface areas. Three Pt loadings were chosen: 5, 20, and 40 wt.% Pt. The higher Pt loadings (20 and 40 wt.%) were selected to approximate typical loadings for cathode catalyst layers currently utilized by the fuel cell industry, while the low Pt loading (5 wt.%) was chosen as a value that could be attractive for future Pt loadings, although not practical at the current stage of PEMFC design. 3D reconstructions were generated for these nine different Pt/C catalyst powders (three Pt loadings on three carbon supports). Reconstructions were also obtained for the three 40

wt. % Pt/C catalysts from cathode catalyst layers of the MEAs subjected to the catalyst degradation AST (30,000 cycles 0.61.0 V, 15s, triangle wave, 80°C). The 3D reconstruction was accomplished using a model-based iterative reconstruction (MBIR) algorithm developed by S. Venkatakrishnan et al.58, 59 Briefly, the main differences of MBIR as compared to the commonly applied simultaneous iterative reconstruction technique (SIRT) is that MBIR uses a minimization of a cost function to ensure convergence, an image model that correlates voxel neighbors, and a probabilistic model to account for noise.58, 59 Further details of the tomography process and reconstruction methods are given in the methods section. The 3D reconstructions are shown in Figure 1 and are arranged by the carbon support surface area (highest to lowest carbon surface area, HSAC, V, LSAC, top to bottom) and Pt loading (increasing loading, 5, 20, 40, left to right). In the reconstructions, Pt is viewed as a red surface on a translucent gray carbon volume, which were segmented based on intensity thresholding. We observe some general trends in Pt dispersion; for example, reduced agglomeration is observed for lower catalyst loadings, indicating better dispersion across all three carbon supports for lower Pt loadings. A similar conclusion is drawn from representative 2D images of the same Pt/C systems obtained by high-resolution aberration-corrected STEM (ac-STEM) bright field (BF) and high angle annular dark field (HAADF) imaging of the different electrocatalysts, which are shown in Supplementary Information (SI) Figures S1 and S2. We note here that single Pt atoms and sub-nm Pt clusters (highlighted by blue arrows in Fig. S2) were observed via ac-STEM imaging to a varying extent on the different supports, especially Pt/HSAC (Fig. S2 (a-c), blue arrows). However, because of their extremely small size relative to the voxel size of the reconstructions, single atom dispersions and small clusters were not captured in the 3D analysis. It is important to note that these single atoms and small clusters do not have a significant impact on catalysis (and degradation) as they are removed through detachment and/or dissolution during the ink preparation or conditioning of the MEA, e.g., before AST or fuel cell operation. The electron tomography data sets were captured at a midrange magnification (225kx) to balance the collection of smaller particles (~1 nm) while still providing some statistical relevance with a relatively large sample volume (>107 nm3). In addition, we found that these conditions were optimal for cir


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cumventing detrimental effects of the electron beam, such as damage to the carbon support, movement of the Pt particles due to beam interactions, and carbon deposition (contamination), all of which can reduce fidelity. Sample images ac

quired at the first tilt (-70°) before and after acquisition of the full tilt series (140° range) are given in Figure S3 and show that structural integrity is retained over the course of the tomography experiment.

Figure 1. 3D reconstructions from electron tomography of as-prepared Pt/C catalysts (segmented Pt surfaces appear in red over gray/translucent C volumes). Catalysts with 5, 20, and 40 Pt wt.% loading (left to right) are shown for (a-c) Pt/HSAC, (d-f) Pt/V, and (g-i) Pt/LSAC. The 3D visuals are rendered at the same scale; note the images contain depth.

Analysis of z-slice cross-sections. To provide a more definitive location of Pt nanoparticles in relation to their carbon supports, e.g., location on the carbon surface(s) or residing within pores, 2D cross-sectional z-slices located near the center of the reconstructed volumes were obtained and are shown in Figure 2 (the Pt/C systems have the same arrangement as in Figure 1). The positions/locations of Pt nanoparticles in relation to the carbon support can also be observed from rotation of the 3D reconstructions (Movie S1). Scheme 1 depicts the fundamental structural differences between the carbon supports: 1) HSAC is highly disordered (defective) with a mesographitic outer ‘shell’ and amorphous carbon ‘core’, has less surface hydrophobicity, and Pt particles tend to primarily reside in pores below the surface; 2) mid-range surface area V carbon has a concentric domain structure with ~4-5 nm mesographitic domain sizes, hydrophobic surfaces, and with Pt depositing mainly on the surface at the domain boundaries; and 3) LSAC has highly ordered graphitic basal plane surfaces and exhibits faceted graphitic ‘shells’ with hollow cores, high hydrophobicity, and with Pt depositing predominantly where

the faceted basal plane surfaces intersect (boundaries/corners/edges), but not within the hollow void space below the surface. These specific Pt/C features are confirmed by the z-slices shown in Figure 2. Platinum nanoparticles primarily nucleate within and occupy the internal amorphous core of HSAC particles (Fig. 2a-c, yellow arrows). In contrast, Pt favors nucleation and growth on the external surface defects (domain boundaries) of the V particles (Fig. 2d-f) and the corners and particle intersections (defects/boundaries) of LSAC (Fig. 2g-i) supports, but Pt nanoparticles are not present within the interior void space of LSAC. As the Pt loading increases for HSAC, Pt begins to appear on the external surface (green arrows, Fig. 2b,c) in addition to being present in the core (~72% internal for 20 wt.% Pt/HSAC). LSAC supports show clustering of Pt nanoparticles in and around surface (particle) junctions of the nanocarbon aggregates (pink arrows), likely due to a preference for nucleation and stabilization by the graphitic edge planes or boundaries. For LSAC, we observed ~96% of the particles on the external surfaces for 20 wt. % Pt/LSAC, where agglomerations of the nanocarbon have en-



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capsulated a few. Even at higher loadings, the hollow interior of the LSAC remains largely unoccupied by catalyst nanoparticles. Agglomeration of Pt is observed on the V support at higher loadings. Comparison of nanoparticle and crystallite size. Following segmentation of Pt from the support by intensity thresholding, materials statistics were generated for each Pt/C catalyst volume to quantitatively assess the Pt dispersion. The 3D-derived size distributions by equivalent diameter (diameter of sphere of identical voxel volume) are shown for each catalyst in Figure S4. Table S1 summarizes the mean 3D minimum interparticle distances, aspect ratios, and sphericities. The plots shown in Figure 3(a,b) give the averages for particle and crystallite sizes measured from 3D electron tomography

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and 2D ac-STEM images, respectively. Equivalent diameters for the crystallite size were calculated from manual measurements of 300-500 individual particles performed on the 2D acSTEM images. Importantly, we note here that the Pt segments from the 3D datasets do not necessarily reflect the actual Pt crystallite size (obtained by high-resolution 2D ac-STEM imaging), but rather, the measurements from 3D data reflect a contribution by both individual Pt particles and multi-particle agglomerates, as Pt particles in close contact cannot effectively be separated from one another during segmentation/thresholding. We emphasize this distinction between Pt crystallite and Pt nanoparticle to avoid confusion, where different trends might be expected if simply viewed as measures of individual crystallites.

Figure 2. Representative cross-sectional z-slices of volumes from electron tomography of as-prepared Pt/C catalysts (corresponding with the arrangement of Figure 1). Catalysts with 5, 20, and 40 wt. % loading of Pt (left to right) are shown for (a-c) Pt/HSAC, (d-f) Pt/V, and (g-i) Pt/LSAC. The Pt nanoparticle size from electron tomography for the samples with low Pt loading are in general agreement with those reported in the literature, with the majority falling between ~2-4 nm.8, 12, 15, 51, 60 These values also fall near those collected for crystallite size by ac-STEM (Fig. 3b), albeit with slightly larger sizes, an important difference. The larger size suggests agglomeration exists in all the Pt/C samples, but that the majority at low Pt loadings (5 wt.% Pt) contain well-

spaced, individual particles rather than dense multi-particle agglomerates. When Pt loading is increased, the distribution shifts to larger particle sizes and shrinking of the inter-particle spacing. In contrast, the ac-STEM image data shows a much smaller increase in crystallite size as a function of Pt loading. This is an indicator that the agglomerates identified via tomography consist of multiple individual crystallites. The data indicate better Pt dispersion at lower Pt loadings, aligning with


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the qualitative comparison of tomograms (Figures 1 and 2). However, there are noteworthy trends observed between the Pt dispersions on the different carbon supports that are revealed by the 3D measurements. Pt/LSAC shows larger Pt particle sizes compared to the other Pt/C catalysts. This is an indicator of increased agglomeration (since there is much less change in the crystallite size) due to the limited number of available defect and surface sites for Pt nucleation and deposition on the basal plane surfaces of this carbon support. In contrast, Pt/V and Pt/HSAC low-loading distributions (Figure S4) appear similar, especially when compared to Pt/LSAC, yet there is more broadening of the Pt distributions at high loadings for Pt/V and Pt/LSAC that is not as apparent for Pt/HSAC, which can be attributed to confinement of Pt in the many pores of HSAC. Aspect ratios and sphericities show that Pt/HSAC has the most uniform and spherical shape, while the Pt/LSAC has more non-spherical, elongated-shaped nanoparticles, with V falling in between.

difference in the size and morphology of the Pt/C observed in the 3D renders and z-slices of AST samples (Figure 4) compared to the respective as-prepared Pt/C 40 wt.% catalysts (Figures 1 and 2), where the size increases and the particle shape becomes more spherical, which is especially evident for the Pt/LSAC catalyst. The z-slice cross-sections for the HSAC-supported Pt show a reduction in the surface-populated Pt after the stress tests. Conversely, the interior surface remains inaccessible for Pt/LSAC during ASTs because of the dense, defect-free graphitized shell, and Pt nanoparticles agglomerate and coarsen at the junctions between the graphitic nanocarbon particles.

Figure 4. 3D visuals (left) and representative cross-sectional zslices of volumes (right) from electron tomography of 40 wt. % Pt catalysts on (a) HSAC, (b) V, and (c) LSAC supports after ASTs of MEAs.

Figure 3. Average Pt (a) nanoparticle and (b) crystallite equivalent diameter for each of the Pt/C catalysts from 3D electron tomography and 2D ac-STEM imaging, respectively. Additional Pt measurements are summarized in Table S1. Changes after accelerated stress tests. Reconstructions were also acquired after catalyst-cycling ASTs were performed on MEAs with cathode catalyst layers comprised of the three different Pt/C systems with 40 wt. % Pt loading (Figure 4). It is important to distinguish the catalyst coarsening AST (30,000 cycles, 0.60 – 1.0 V) from a carbon corrosion protocol (typically a high potential hold), which will yield different results for component-specific degradation. Following the catalyst-cycling AST employed here, there is a stark

To better visualize the Pt coarsening trends associated with each of the catalyst supports, the average Pt nanoparticle equivalent diameter is plotted in Figure 3(a). The HSACsupported Pt shows the most consistent mean nanoparticle equivalent diameter (~2.8 nm) across the different loadings and a relatively constant decrease in interparticle distances (Table S1) for the as-prepared catalysts from ~9 to ~4 nm with increasing wt. % Pt, evidence of better initial dispersion of Pt in this Pt/HSAC system. The more consistent size and interparticle spacing trends of Pt/HSAC could be a result of increased available surface area for deposition as well as the physical confinement of Pt in pores. In general, the interparti



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cle distance for all the as-prepared Pt/C catalysts decreases with increased Pt loading, but there is clearly a leveling of this effect for Pt/LSAC at 40 wt. % Pt loading. V carbon achieves the smallest Pt nanoparticle size at 5 wt. % loading of ~2.3

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nm; however, this trend is not retained at higher Pt loadings (e.g., ~4 nm at 40 wt. % Pt), and can be attributed to increased catalyst agglomeration.

Figure 5. (a) ORR polarization curves and (b) ECSA of the different 40 wt. % Pt/C systems before and after ASTs in MEAs (30,000 cycles, 0.60-1.0 V, 15 s, triangle wave, 80°C). Plots of ECSA vs. 3D-derived nanoparticle size and 2D-derived crystallite size are shown in (c) and (d), respectively.

After the ASTs, all Pt/C catalyst/support systems show increases in Pt nanoparticle size and inter-particle distances, consistent with coarsening of the Pt crystallites/nanoparticles via coalescence and dissolution/redeposition mechanisms. Here we note that the extent of increase in size and interparticle distances are both larger for V and LSAC-supported Pt after AST compared to Pt/HSAC after AST. This is likely due to increased initial Pt nanoparticle contact and agglomeration of Pt particles on these catalysts leading to accelerated Pt nanoparticle coalescence and coarsening. Furthermore, the interparticle spacing for Pt/HSAC only shifts upward slightly, echoing the hypothesis of pore confinement as a physical barrier to extensive coarsening of these Pt nanoparticles. In comparison to the particle sizes from tomography, the plot of crystallite size is relatively flat, (Fig. 3b), except for the AST crystallite size increase due to coarsening. We again emphasize here that, prior to the ASTs, the crystallite size and particle size are most similar for the lowest loading where interparticle spacing is also largest. It is also interesting to note that the Pt/V particle and crystallite size have >3 nm difference after AST, which suggests clustering and coarsening, but not additional coalescence of crystallites. This was not observed with

LSAC where both values converge at ~7.8 nm. We speculate this difference in coalescence of Pt crystallites may be due to some preferential orientation or alignment of Pt nanoparticles on the highly graphitized support in contrast to Pt nanoparticles on a more turbostratic, more defective carbon support. Correlating nanoparticle size to ECSA. The trends observed in the tomography-derived data can be used to understand the origins of the ECSA loss of the different Pt/C systems after ASTs. Figure 5 shows the ORR polarization curves (Fig. 5a) and loss of ECSA (Fig. 5b) resulting from ASTs from BOL to end-of-life (EOL) for the Pt electrocatalysts in cathode layers of MEAs. The HSAC-supported Pt showed the highest initial ECSA and the smallest, but still significant, drop from 69.3 m2/g to 45.8 m2/g (~34% ECSA loss). Conversely, the LSAC-supported Pt catalyst underwent a similar ECSA loss from a much lower initial ECSA of 46.9 m2/g to 29.9 m2/g (~36% ECSA loss). The ECSA of the V-supported Pt suffered the highest ECSA loss and lowest EOL value, falling from 55.1 m2/g to 25.0 m2/g (~55% loss). The oxygen reduction reaction (ORR) polarization curves for these different catalysts during cycling ASTs show similar trends in performance (Fig. 5a). We might expect that the Pt on the HSAC-


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support having the most ideal Pt dispersion properties, would retain ECSA much better than the other systems; however, the benefit of consistent size and shape metrics was only marginal in terms of % ECSA loss compared to LSAC. Because of this we believe that for Pt/HSAC, Pt dissolution, and perhaps to a smaller extent, isolation of Pt in sub-surface pores, may play more significant roles, since the smaller sized Pt nanoparticles undergo dissolution more readily (are electrochemically less stable) than larger ones.4 Remarkably, the amount of Pt ECSA loss is similar between HSAC and LSAC supported catalysts, despite having large differences in initial agglomerate and crystallite size, and subsequently, ECSA. We can infer that Pt/LSAC ECSA loss is dominated by Pt nanoparticle coalescence and coarsening and is limited more by initial agglomeration (e.g., initial inter-particle spacing), while Pt/HSAC favors dissolution. This could also be the reason for why the Vsupported Pt catalysts saw the largest drop in ECSA, where both agglomeration and dissolution degradation mechanisms are operative simultaneously. The nanoparticle and crystallite size for each catalyst are

plotted vs. ECSA in Figure 5c and 5d, respectively. Here we observe a strong linear correlation between Pt nanoparticle size (derived from 3D tomography) and ECSA values, both before and after ASTs (adj. R2 = 0.82). In contrast, the crystallite size (measured from 2D ac-STEM images) does not yield such a fit (adj. R2 = 0.53). For example, the V-supported Pt ECSA decreases the most to the smallest EOL value, yet the crystallite size at EOL is significantly smaller than the Pt/LSAC EOL. An important conclusion that can be drawn from these data is that the initial Pt nanoparticle size is a much better metric for predicting ECSA loss than the simple crystallite size because 3D tomography more accurately accounts for the prevalence of multi-particle agglomerates, which can pose a real challenge for traditional 2D image analysis (or X-ray diffraction analysis for that matter). This further implies that when discussing the dispersion of a catalyst, one should consider the presence of crystallites already in contact with each other, or reduced interparticle spacing, in addition to their crystallite size.

Figure 6. Representative high-resolution ac-STEM BF/HAADF image pairs for Pt/HSAC (a-d), Pt/V (e-h), and Pt/LSAC (i-l) 40 wt. % catalysts before (a/b, e/f, i/j) and after (c/d, g/h, k/l) ASTs for Pt coarsening in the MEA cathode layers.

Changes to morphology. A significant change in Pt nanoparticle morphology is observed after the ASTs. This was quantified by assessing the change in the average aspect ratio and sphericity (Table S1). The aspect ratios were measured by taking the maximum over the minimum Feret diameter for

each Pt nanoparticle. All of the as-prepared Pt/C systems show increases in aspect ratio and decreases in sphericity for higher Pt loadings, which is further indicative of Pt agglomeration. All AST-Pt/C systems exhibited a large drop in the Pt nanoparticle aspect ratio and an increase in the sphericity,



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which is consistent with coarsening of Pt nanoparticles into larger, more spherical single-crystal particles through coalescence and dissolution/redeposition. Here we can conclude that the shape of particles is more irregular for V-supported and LSAC-supported Pt compared to HSAC, with LSAC showing the most non-spherical particle shapes. This can be directly attributed to the wetting behavior of Pt on LSAC due to its inherent hydrophobicity, and is additional evidence for the formation of observed initial agglomeration; multi-particle agglomerates will deviate strongly from a spherical shape, especially if Pt nanoparticles pair or link together with deposition restricted in one direction due to the hydrophobic nature of the support interface. Interestingly, the relatively low sphericity and high aspect ratio of as-prepared Pt/LSAC, especially at higher loadings, supports Pt nanoparticles ‘linking/stacking’ to form agglomerates rather than concentrically ‘balling up’, a potentially important difference between Pt on the different supports. This makes sense considering the less agglomerated 5 wt. % Pt/C systems should be more representative of the individual crystallite shape (truncated octahedra), e.g., those having higher sphericities and lower aspect ratios. We confirmed these differences in the size and shape of agglomerates before and after ASTs for Pt coarsening via high-resolution ac-STEM BF and HAADF imaging of the 40 wt. %, as shown in Figure 6 (corresponds to data shown in Figure 5). The Pt morphology becomes more spherical during coarsening for each of the Pt/C systems, in contrast to the more faceted and anisotropic morphology of the as-prepared Pt nanoparticles. This aligns with the afore-mentioned conclusion from the analysis of aspect ratios and sphericities from the 3D reconstructions. For Pt/LSAC, the linking/stacking of individual Pt crystallites, shown in Figure 6i and 6j, was commonly observed, whereas the Pt nanoparticles on HSAC are not sufficiently agglomerated to show this. Linking of Pt on V is also observed (Fig. 6e and 6f), but the resulting coarsened Pt nanoparticles (Fig. 6g and 6h) were more often found in contact with other large Pt particles, which can explain the smaller change in sphericity compared to the other Pt/C catalysts. The differences in agglomerate morphology are due to the underlying carbon support structure, surface orientation, and hydrophobicity, and how nanoparticles can agglomerate in pores compared to surface junctions/defects, and curved vs. planar surfaces, in addition to chemical nature of the support, i.e. sp2 vs. sp3 carbon bonds. Apart from the Pt agglomerate shapes, the individual Pt crystallite shape is more faceted, flattened, elongated, or disk-like. This is suggested by the observed non-ideal aspect ratios and sphericities for the 5 wt. % as-prepared samples. In particular, particle coalescence could be accelerated by preferred morphology, orientation, or alignment of particles on the support surface, which is the subject of future investigations. Several state-of-the-art high performing shape-controlled Pt-alloy catalysts have been generated in recent years and studied,61-64 but durability of these catalysts in the cathodes of MEAs of PEMFC devices remains a challenge to be addressed. Some portion of the explanation for this comes down to the carbon supports,65-67 which were not necessarily designed for the finely tailored catalysts that have been developed. We believe the crucial role of the catalyst interface with the carbon support may be frequently overlooked, and highlight this through our study of industry relevant Pt catalysts directly incorporated and studied in fabricated MEAs. Better

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control over the extent of graphitization, the controlled introduction of surface defects, and improved access to open pore space in LSAC particles may increase the number of catalyst nucleation sites. If excessive carbon support amorphization can be avoided during PEMFC operation15 and better control of surface hydrophobicity can be achieved, enhanced catalyst dispersion can be attained while retaining improved corrosion resistance. It is our hope that this information can spawn further research into achieving the same control for the carbon support that has been demonstrated for catalyst nanoparticles. CONCLUSION STEM-based electron tomography was performed for Pt/C electrocatalysts to explore how catalyst loading and carbon support type impact catalyst dispersion both before and after ASTs. These results provide specific guidelines toward carbon support optimization. LSAC supports showed poor Pt dispersion, which may be improved by functionalization of the surface-exposed basal planes. Enhancement can be made to Pt/HSAC by increasing the Pt nanoparticle size to reduce Pt dissolution. Additionally, we showed a strong correlation of the 3D-derived nanoparticle size to ECSA loss, and how this and interparticle spacing play a key role in controlling and predicting Pt coarsening behavior. METHODS Sample preparation. Tanaka Kikinzoku Kogyo K.K. (TKK) catalyst powders were obtained from Ion Power (sample IDs: Pt/HSAC: TEC10E05E, TEC10E20E, TEC10E40E; Pt/Vulcan: TEC10V05E, TEC10V20E, TEC10V40E; and Pt/LSAC: TEC10EA05E, TEC10EA20E, TEC10EA40E) and samples for STEM imaging and tomography were prepared by sonicating as-made powders in isopropyl alcohol and dropcasting 5 µL of the dispersion onto lacey carbon coated Cu grids (low mesh number grids were used to reduce eclipsing of the Cu grid at high tilt angles). The catalyst cycling ASTs were performed at Los Alamos National Laboratory for cathode catalyst layers in MEAs comprising the respective 40 wt. % catalysts. The MEAs were prepared using a decal transfer technique with the catalyst ink first being applied on a web and then pressed onto the membrane.68 The anode catalyst used was TEC10V20E and the anode loadings for all the MEAs were kept constant at 0.05 mgPt/cm2. A ~1 cm2 area of the MEA was cut out from the center of the MEA submitted to AST, the cathode catalyst layer was removed by scraping, and the collected powder was prepared for STEM characterization following the same drop-casting procedure. Electron tomography. STEM tomography was performed on a 200kV FEI Talos F200X STEM using a Gatan high tilt tomography holder for FEI instruments. Typically, the holder was plasma-cleaned for 5 minutes prior to use, after which the sample was loaded and a ~20 minute ozone cleaning treatment (10 minutes/side) was used to further reduce possible hydrocarbon deposition (contamination). A ~5 minute beam shower was performed prior to tilt series acquisition if necessary to further reduce contamination. The beam current used was ~100 pA and the beam convergence angle was ~10 mrad. Bright field (BF)/dark field (HAADF) image pairs (1024 by 1024 pixels, 0.465 nm/pixel, 10 µs dwell time) were acquired at a magnification of 225kx in 2° tilt increments over a 140°


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tilt range (±70°, 71 image pairs). The beam was blanked between images/tilts. Reconstruction and analysis. Tilt series alignment and tilt-axis corrections were performed manually for each image stack using the Gatan Microscopy Suite (GMS), as well as Fiji/ImageJ and Tomviz (tomviz.org); this was inspected and adjusted iteratively prior to reconstruction. Typically, a relatively large particle located near the central axis with few eclipsing neighbors was first used as the common feature for alignment and this was then refined by inspection of single slice reconstructions and the tilt axis prior to reconstructing the volume. The 3D reconstruction was performed for the BF tilt series using a BF model-based iterative reconstruction algorithm (MBIR) which accounts for diffraction contrast.58, 59 The BF series was selected for reconstruction for better contrast of the carbon support. Images were cropped to regions of interest that were typically ~436 by 700 pixels prior to reconstruction. The reconstruction generated binned rectangular volumes with cubic voxel edge lengths of 0.93 nm across. Visualizations were rendered with FEI’s Aviso (v. 9.1.1) software. Intensity thresholding and top-hat filter thresholding (to find local maximums from smaller particles that could not be segmented based on a simple intensity threshold) were applied to the reconstructed volumes for segmentation of the Pt from the carbon support. Materials statistics were computed for the Pt segments using FEI’s Aviso software. ac-STEM imaging. The Pt/C samples were also characterized through high-resolution BF/HAADF STEM imaging on a JEOL JEM-2200FS aberration-corrected STEM (acSTEM) system operated at 200 kV. For crystallite size measurements, equivalent diameters were calculated for individual particles from high resolution images after segmentation of Pt was performed by an intensity threshold.

AUTHOR INFORMATION Corresponding Author *K.L. More, email: [email protected]

SUPPORTING INFORMATION Animation of a 3D-reconstructed volume of Pt/LSAC 20 wt. % catalyst, animations through the tilt series and stack of zslices of the Pt/HSAC 40 wt. % AST catalyst, high resolution ac-STEM BF/HAADF images of the different Pt/C systems, STEM images before and after tilt series acquisition, histograms of equivalent diameter, a table containing average values for 3D metrics in addition to the ac-STEM-measured Pt crystallite size for each catalyst, comparison of MBIR to SIRT reconstructions, and larger scale TEM images showing the three different carbon support structural motifs.

ACKNOWLEDGMENT Research supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE). A portion of the research was performed as part of a user project through Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which is a U.S. DOE Office of Science User Facility, and by instrumentation provided by the U.S. DOE Office of Nuclear Energy, Fuel Cycle R&D Program, and the Nuclear Science User

Facilities. We would like to thank S. Venkatakrishnan, M. Lei, E. Padgett, and M. Holtz for valuable discussions.

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3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate

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