2D and 3D Characterization of PtNi Nanowire Electrode Composition

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2D and 3D Characterization of PtNi Nanowire Electrode Composition and Structure Sarah Shulda, Johanna Nelson Weker, Chilan Ngo, Shaun M. Alia, Scott Mauger, Kenneth C. Neyerlin, Bryan S. Pivovar, and Svitlana Pylypenko ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02097 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Applied Nano Materials

2D and 3D Characterization of PtNi Nanowire Electrode Composition and Structure

Sarah Shulda†, Johanna Nelson Weker‡, Chilan Ngo†, Shaun M. Alia₴, Scott A. Mauger₴, K.C. Neyerlin₴, Bryan S. Pivovar₴, and Svitlana Pylypenko†*

†Department

of Chemistry, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401, USA

‡Stanford

Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA

₴Chemical

and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Pkwy, Golden, CO 80401, USA

*Corresponding author: e-mail address: [email protected]

Keywords: PtNi, Extended Surface Electrocatalysts, XAFS, TXM, PEMFC

ACS Paragon Plus Environment

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Abstract Extended surface Pt or Pt-M (M=Ni or Co) catalysts are a viable alternative to supported nanoparticle catalysts for the oxygen reduction reaction (ORR) at the cathode in proton exchange membrane fuel cells (PEMFCs). The activity and durability of these catalysts in membrane electrode assemblies (MEAs) is greatly dependent on the surface and bulk properties of the catalyst material, integration with ionomer as well as the three-dimensional structure of the electrode, necessitating extensive characterization of the catalyst in the electrode at multiple scales. In this work, extended surface PtNi nanowire-based electrocatalysts derived by spontaneous galvanic displacement, and post-treated to obtain high specific and mass activities, are characterized by X-ray spectroscopies to assess catalyst composition and to determine the extent of Pt-Ni alloying. Transmission x-ray microscopy (TXM) is used to generate twodimensional and three-dimensional images of catalyst layers with varied compositions within the electrode, to show the distribution of nanowires and to study Ni dissolution and redeposition within the electrode. These techniques are complemented by electron microscopy, which is used to confirm leaching of nickel from the nanowires and its redeposition within the electrode. This process is shown to have detrimental effect on the MEA performance, which can be, at least in part, recovered by soaking electrodes in acid. The combination of these techniques provides unprecedented detail of the evolution of the nanowire-based extended surface catalyst and electrode structure guiding targeted design of high performance electrodes based on this class of materials.

Introduction


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Proton exchange membrane fuel cells (PEMFCs) are of high interest for clean, sustainable, transportation applications1-2. The performance and cost of PEMFCs are highly dependent on the platinum required for efficient oxygen reduction. Platinum based nanoparticles on high surface area carbon (HSC) are the current state-of-the-art catalyst for the oxygen reduction reaction (ORR) at the cathode, but are prone to performance losses due to dissolution, Oswald ripening, aggregation, and detachment from the carbon support due to corrosion3-4. The substantial cost of the catalyst layer, which can account for half of the total fuel cell cost, is a further limiting factor in the widespread commercialization of PEMFCs5. With current Pt loadings, Pt/HSC catalysts do not meet all of the 2020 Department of Energy targets for both performance and cost2, 6. Both Pt or Pt-M (M=Ni, Co, or Pd) catalysts with extended surfaces are promising alternative to nanoparticle-based Pt and Pt-M catalysts supported on HSC 2, 7-12 as evidenced by the growing body of work describing increased activity and durability of various structures, including 3M’s nanostructured thin film (NSTF) whiskers13, nanowires14-22, and dendrite10 structures. Generally, Pt-M alloys demonstrate higher specific activities than their Pt counterparts due to Pt d-band shifts that decrease the extent of hydroxyl species adsorbing and blocking O2 adsorption sites23-25. Catalysts with extended surfaces inherently have higher specific activities than Pt and Pt-M nanoparticles13, and despite early extended surface materials suffering from low surface areas and lower mass activities (activity gmPt-1), catalysts with higher surface areas have more recently been synthesized13, 15. Relative to Pt/HSC, extended surface Pt catalysts have also proven to be more durable in rotating disk electrode (RDE) testing and under fuel cell operating conditions17-19, largely due to significantly reduced susceptibility to dissolution and agglomeration8, 13.


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PtNi and PtCo extended surface nanowire catalysts derived from Ni nanowires via galvanic displacement were reported to have initial specific and mass activity orders of magnitude higher than Pt/HSC 14-15, 26. However, following potential cycling in RDE the catalysts suffered from a loss in activity14-15. More recent studies by our group have focused on elucidation of the effects of post-synthesis processes on PtNi to improve catalyst durability while maintaining, or improving, the kinetic gains in activity from alloy formation 26-27. The ORR activity of the optimized catalysts, as reported previously by our group, had 3 times higher activity than the as synthesized nanowires, and demonstrated substantial gains in durability during RDE testing26. While RDE testing is a great starting point to estimate catalyst kinetic activity, high RDE performance does not necessarily correlate to high performance in fuel cells where mass and charge transport is an inherent challenge. When incorporated into the membrane electrode assembly (MEA) and exposed to relevant fuel cell conditions, catalyst performance is dependent on the three-dimensional architecture of the catalyst layer, in addition to the composition and morphology of the catalyst itself. The structure of extended surface catalysts allows for electrocatalyst layer structures unobtainable with catalysts based on Pt nanoparticles. For example, MEAs prepared with Pt nanoparticles typically utilize carbon black supports to increase electrical conductivity, while MEAs consisting of NSTF catalyst have the Pt whiskers in contact with one another, eliminating the need for carbon black and resulting in much thinner MEAs with less O2 mass transfer impedance13. Other work has had similar results demonstrating improved conductivity, or reduced charge transfer resistance, and improved mass transport from the 3D structure of extended surface catalysts7, 16, 18-19. The nature of extended surface catalysts enables control over void volumes and catalyst-to-catalyst contact through the manipulation of


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in-situ growth parameters7, catalyst loadings9, 28-29, and ionomer content30. Each of these aspects provides a pathway to further optimize MEA mass transfer and conductivity. However, in order to fully capitalize on these materials, accurate characterization of the structure and chemical properties of the catalyst, and clear visualization of the catalyst structure within the catalyst layer is required. This is particularly important for the Ni and Co-containing systems, since both Ni and Co are prone to leaching during fuel cell testing and operation. This is likely to alter the extended surface catalyst morphology and surface composition and lead to contamination of the MEA due to redeposition of the non-noble metal in the electrode layer, or within the Nafion membrane, substantially degrading fuel cell performance31-35. While electron microscopy methods are readily used to visualize the as-synthesized catalyst material, including size and morphology, only limited information can be derived regarding the three-dimensional catalyst structure of an electrode. Visualization with scanning electron microscopy (SEM) is limited to the in-plane view of the top layer of the cathode or anode catalyst layer or to a cross-sectional view of the MEA. Transmission electron microscopy (TEM) of the as-prepared catalyst layer is hindered by sample size and thickness, although electron tomography has successfully been used to follow the Pt distribution in a fresh and degraded MEA36 and to study the ionomer distribution in the catalyst layer37. More recent studies using X-ray microscopy methods have shown it to be an effective, nondestructive tool for imaging electrode and membrane structures. X-ray computed tomography has been used effectively to image the microstructure, formed from the carbon support and Nafion, of a PEMFC electrode from which pore sizes were quantitatively determined38. Soft X-ray scanning transmission X-ray microscopy (STXM) has been applied to image carbon species and Pt nanoparticles in PEMFCs MEAs,39-41 as well as iron and gold


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electrode deterioration42. Hard X-ray TXM methods have been used to visualize the threedimensional morphology of solid oxide fuel cells43-45; however, despite the advantage of high contrast between metal and carbon at these energies, hard X-ray microscopes have not been used to study the catalyst structure within PEMFCs electrodes mainly due to the limited resolution of the technique that does not allow visualization of the Pt or Pt-M nanoparticles in the state-of-the art catalysts. This work focuses on the in depth investigation of PtNi extended surface catalysts. Herein, both the catalyst material and the electrode catalyst structure are thoroughly analyzed to provide insights towards optimizing the catalyst layer. For clarification purposes, the catalyst material will be referred to as the as-synthesized PtNi nanowires and the catalyst incorporated into an electrode will be referred to as the electrocatalyst layer. The dimensions of the catalysts – 200-300 nm in diameter and 100-200 µm in length – necessitate a multimodal, multi-scale characterization approach to address evolution of the catalyst composition as a function of synthesis conditions and electrode fabrication. For the as-synthesized catalyst, the material morphology is visualized using SEM and scanning transmission electron microscopy (STEM), surface speciation is studied with X-ray photoelectron spectroscopy (XPS), and the extent of PtNi alloying is determined with X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. For the investigation of the catalyst electrodes, we use synchrotron hard X-ray techniques to i) study the evolution of the catalyst composition, and track changes in the degree of alloying between the Pt and Ni phases and ii) visualize the 3D architecture of the catalyst layer with a focus on the Ni distribution. The use of monochromatic radiation at or above an element’s absorption edge allows for areas containing that element to have high contrast compared to all


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other material in the system46. Herein, hard x-rays near the Ni 1s edge are used for TXM imaging. At this energy carbon absorbs poorly allowing for Ni to be visualized with minimal interference from the carbon species present. The ability to clearly image the structure of electrodes prepared with PtNi nanowires, using 2D and 3D TXM imaging is demonstrated by analyzing the PtNi nanowire electrodes prepared with and without graphitized carbon nanofibers (GCNFs). Acid treatment of the electrodes is used to leach Ni from the catalyst layers. The complementary nature of the EXAFS, 2D TXM imaging, and TEM energy dispersive spectroscopy (EDS) imaging, are used to study the changes in nanowire structure, extent of Ni leaching and redistribution of Ni within the catalyst layer, with negative effects of this process on performance demonstrated by testing in MEA.

Experimental Details Catalyst preparation: PtNi nanowires were synthesized via spontaneous galvanic displacement of Ni nanowire templates with Pt. They were then annealed in hydrogen at 250°C, and leached in 0.1M nitric acid. Detailed description of the synthesis conditions has been previously reported by our group and is summarized briefly in Figure 115.

50 nm

PtNi nanowires synthesized via SGD of Ni with Pt

30 nm

30 nm

PtNi nanowires annealed in hydrogen at 250℃

PtNi nanowires leached in 0.1M nitric acid

50 nm

Figure 1. Scheme demonstrating steps involved in synthesis of PtNi nanowires, Pt is red and Ni is green. Electrode preparation: The PtNi was weighed into a glass vial, followed by the addition of water (0.26 mL/mg PtNi). This vial was then placed in a beaker filled with ice water, where the


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mixture was tip sonicated for 10 sec (QSonica Misonix S-400, 20 kHz, Amplitude = 1). 1propanol (0.22 mL/mg PtNi) was then added to the vial containing a 20 wt% Nafion solution (D2020, Ion Power; 1.5 µL/mg PtNi) and followed by tip sonication in the ice bath for 30 sec and bath sonication for 30 min. Tip sonication and bath sonication were repeated two more times followed by a final tip sonication. After sonication, a solution of polyacrylic acid (Mv = 4,000,000 g/mol, 0.99 wt% in water) was added followed by vortex mixing for 1 min. For the ink containing graphitized carbon (Tanaka Kikinzoku Kogyo, GCNF-2), carbon was added prior to the addition of PAA and followed by 30 sec of tip sonication and 30 min of bath sonication. The inks were sprayed onto 25 µm thick conductive Kapton using a Sono-tek spray system with an Accumist ultrasonic nozzle. The Kapton was held in place on a porous aluminum vacuum table heated to 80 ˚C. The liquid flow rate was 0.15 or 0.3 mL/min depending on the ink as some inks clogged the nozzle at 0.15 mL/min. The translational speed of the spray system was 50 mm/s. Electron microscopy: Electrodes were evaluated as-received via SEM by cutting small areas off the electrodes and mounting them with carbon tape onto an aluminum stub. For TEM analysis of catalyst, nanowires were brushed directly onto Cu TEM grids with holey C support films, while for analysis of electrodes, material on the electrodes was mechanically exfoliated before depositing it on the TEM grid using procedure described above. SEM imaging was performed on a JEOL JSM-7000F SEM operated at 15 kV. STEM analysis was conducted on an FEI Talos F200X operated at 200 kV, with chemiSTEM detector for EDS. Bruker Esprit software was used to collect and analyze EDS hypermap data, with collection times of 10 - 20 minutes and each pixel corresponding to an elemental spectrum. XPS data collection and analysis: The as-synthesized PtNi nanowires were pressed on nonconductive tape for XPS acquisition. XPS was completed on a Kratos Axis Nova X-ray


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photoelectron spectrometer with a monochromatic Al Kα source operated at 300 W, providing charge neutralization. CasaXPS software was used for data analysis with all spectra aligned to Pt at 71.1 eV, using Shirley background for Pt 4f and Ni 2p and a linear background for C 1s and O 1s spectra. XAS data collection and analysis: XANES and EXAFS spectra were collected at the Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory (SLAC) beamline 4-1 at the Pt L3 edge using a Ge array fluorescence detector. The as-synthesized PtNi nanowires were mounted between two layers of kapton tape and MEAs were mounted on one layer of kapton tape. A Pt foil reference was collected in the transmittance mode simultaneously with all samples. Three scans were averaged for each sample. Athena47 was used for all data processing including normalization, calibration, and alignment. Artemis47 was used for all EXAFS fitting with a k-range 3-12.105 Å-1. TXM sample preparation, data collection and analysis: For electrode imaging, a corner of the prepared electrode was cut and attached, perpendicular to the incident beam, to the sample holder. TXM imaging data were collected at the SSRL at SLAC National Accelerator Laboratory beamline 6-2. Details on the instrumentation, experiment set-up, and data analysis have previously been reported46, 48-49. The electrodes were imaged at 8355 eV. The pixel size, with a binning of 2, at this energy is 35 nm. The field of view is ~30 microns, but this was increased with mosaic imaging, or stitching multiple fields of view together. For 2D imaging, each electrode was imaged with a 3x3 mosaic with a 30 % overlap of each “tile” or field of view. Five images with 0.5 sec exposure time each were averaged for each tile. 20 reference images with 0.5 sec exposure time were taken and averaged for every 1250 exposures of the sample. For 3D tomography, each electrode was imaged with a 1x3 mosaic at every degree for 180˚ as the


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sample was rotated with a 30% overlap of each “tile” or field of view. Five images with 0.5 sec exposure time each were averaged for each tile and 20 reference images each with a 0.5 sec exposure time were taken and averaged at the start of the experiment and after every 36˚. Data processing was carried out using the TXM Wizard software49 including subtracting the references from each image, averaging the images, magnification correction, aligning or “stitching” together the mosaic tiles, and tomographic reconstruction using algebraic reconstruction technique50 with 20 iterations. Avizo 9.1 was used for 3D visualization and relevant calculations. MEA fabrication and testing: Catalyst-coated membrane electrodes were ultrasonic spray coated onto Nafion 212 membranes (PtNi nanowires at the cathode and Pt nanoparticles/HSC at the anode). MEAs were assembled into single test cells using Sigracet SGL 25BC gas diffusion layers (25% compression) and poly(tetrafluoroethylene) gaskets. The active area was 5 cm2 with single serpentine flow channels. The MEAs acid leached for XAS and TXM study were leached with 1M H2SO4 for 17 hours. For electrochemical testing, MEAs were leached for 17 hours in 1 L bath 0.01 M sulfuric acid at room temperature. After leaching the MEAs were rinsed with copious amounts of water and dried on a heated vacuum plate at 50 ˚C. Prior to testing, the MEAs were preconditioned via voltage cycling using previously published protocols51. Electrochemical surface area (ECA) was determined from the hydrogen underpotential deposition (HUPD) region of cyclic voltammograms recorded at 50mV/s using either a Gamry or Autolab potentiostat. ECA was determined at 30 ˚C, 150 kPa total cell pressure, 100 %RH, H2/N2 at 0.20/0.05 slpm respectively. Hydrogen-oxygen and hydrogen-air polarization curves were measured at 80 ˚C, 100 %RH, 150 kPaabs, and 0.4 slpm.


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Results and Discussion Catalyst Characterization The dimensions of the PtNi nanowires are shown in SEM and STEM images in Figure 2. Of particular interest is the EDS mapping that demonstrates a continuous 5-10 nm extended surface layer of Pt formed through this series of post-synthesis treatments (Figure 2b). XPS analysis of this material, where the Pt 4f7/2 peak is found at 71.1 eV, indicates that Pt is in the metallic state (Figure 3b)52. Some of the Ni on the surface is also found in the metallic state (peak at binding energy 852.5 eV), but both oxides and hydroxides are also detected, as indicated by the peaks at 854.1, and 855.7 eV respectively (Figure 3a)53-54. The extent of Pt-Ni alloying is estimated from XANES and EXAFS analysis. The feature in the Pt L3 XANES spectrum directly after the edge at approximately 11575 eV confirms the presence of an alloy (Figure 3c). The results of the Pt L3 EXAFS fit agree with the XANES analysis, as well as XRD results, previously reported by our group,26 providing further evidence of alloy formation. Namely, the presence of a Ni scatterer in the Pt first shell nearest neighbor is required to achieve a good fit of the model to the experimental data with a Pt-Pt coordination number of 6.4 and a Pt-Ni coordination number of 3.7 (Figure 3d) and absorber-scatterer distances that align with expected values for a Pt metal and Pt-Ni alloy at 2.72 Å and 2.58 Å, respectively26.


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Figure 2. Electron microscopy characterization of PtNi nanowires: (a) plane-view SEM , and (b) dark-field STEM and EDS mapping of Pt and Ni.


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Figure 3. Characterization of PtNi nanowires with X-ray spectroscopies: (a) Ni 2p5/2 XPS spectrum, (b) Pt 4f7/2 and 4f5/2 XPS spectrum, (c) Pt L3 XANES spectrum, (d) Fourier transform of the k3-weighted EXAFS data (solid line) and first-shell fits (dashed line) of the PtNi nanowires, and (e) parameters of the absorber-scattering atoms derived from the EXAFS fits for the PtNi nanowires, including the coordination numbers (N), interatomic distance (R), disorder term (σ2), and the R-factor. Electrode Characterization - Effect of Electrode Composition The structure of PtNi nanowires in the electrocatalyst layer determines the extent of wireto-wire contact (an important feature for charge transfer), void volume, and nanowire agglomeration (important features for maintaining accessibility of the catalyst surface). SEM of the electrode fabricated using ink containing PtNi nanowires shown in Figure 4a provides some information on the density and agglomeration of the nanowires, but due to inherent limitations of SEM, visualization of the electrocatalyst layer structure is limited to the top layer. 2D TXM image shown in Figure 4b is obtained through the use of incident radiation with an energy that is just above the Ni-K absorption edge and is not efficiently absorbed by C species, allowing for visualization of nanowires. Individual nanowires are resolved over a relatively large electrode area by tiling together multiple fields of view providing evidence that the nanowire distribution is heterogeneous with areas of higher and lower nanowire density. 3D tomographic reconstruction of the electrode structure provides additional information. A video of the rendered 3D volume of both electrodes can be found online in the supplemental information and selected views of this volume are displayed in Figure 4c. This electrode has heterogeneously dispersed nanowires where certain areas show large voids and other areas demonstrate significant agglomerations of the densely packed nanowires. Voids are shown to persist throughout the catalyst layer, which is clear from the 3D analysis. This type of information is critical in


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understanding MEA performance as agglomeration of the nanowires will reduce the active surface area of the catalyst. Graphitized carbon nanofibers (GCNFs) were added to the PtNi nanowire catalyst layer with the goal to improve catalyst dispersion. Figure 5 demonstrates the effect of addition of GCNFs on the distribution of PtNi nanowires within the catalyst layer. Plane-view SEM of the catalyst layer (Figure 5a) again provides very limited information on the catalyst structure, with polymeric constituents impeding visualization of the PtNi nanowires. The advantage of TXM is even more apparent with this electrode as it provides better visualization of the nanowire layer structure and reveals differences between the two types of electrodes. It is clear in the 2D TXM images that the addition of GCNFs to the electrode results in a significantly more homogeneous distribution of PtNi nanowires within the catalyst layer, relative to the electrode without GCNFs, while maintaining electronic contacts directly between nanowires (Figure 5b). The 3D rendering shown as selected rotations in Figure 5c (a video of the rendered 3D volume of this electrode can be found online in the supplemental information) further supports the conclusion: the addition of GCNFs to PtNi nanowires results in a catalyst layer with more homogeneously dispersed nanowires that contact each other, with minimal clumping of nanowires and a more even distribution of catalyst free space. It is important to note that in this electrode, catalyst free space also contains carbon from GCNFs and polymer (not detected with hard x-ray TXM).


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Figure 4. Characterization of the electrode made with PtNi nanowires (without GCNFs): (a) SEM, (b) 2D TXM, and (c), selected rotations of the 3D reconstruction.

Figure 5. Characterization of the electrode made with PtNi nanowires and GCNFs: (a) SEM, (b) 2D TXM, and (c) selected rotations of the 3D reconstruction.


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Electrode Characterization - Effect of Nickel Leaching The dissolution of the less noble metal from the bimetallic PtNi and PtCo catalysts during fuel cell operation is a concern even for nanoparticle-based catalysts, where the content of the non-noble metal is much lower than that of the nanowire-based materials studied here. Two different sulfuric acid treatments of the electrodes made with PtNi nanowires were employed to study Ni dissolution that is expected to occur under the low pH conditions within the electrodes. Acid leaching of the first electrode was performed at room temperature resulting in milder conditions that could remove lower amounts of nickel with increased probability of maintaining the integrity of the nanowire morphology. Acid leaching of the second MEA was carried out under elevated temperatures, 80 °C, and was intended to be sufficiently aggressive to remove a majority of the Ni from the nanowires’ Ni template core. Both acid leached electrodes were analyzed with SEM and 2D TXM and compared to the as-synthesized (unleached) electrode shown in Figure 5. Additionally, TEM-EDS was employed to track elemental distributions of nickel and platinum, along with fluorine and sulfur from ionomer and carbon from GCNFs and ionomer. As a starting point, SEM images of the electrodes (Figure 6a-b) provide little to no visual information on the distribution of nanowires as a function of acid leaching conditions. In contrast, TXM imaging of the electrode leached at room temperature (Figure 6c) showed nanowires with lower absorption that is indicative of Ni removal. 2D TXM also revealed areas with bright clumps indicating higher absorption, which we attribute to a redistribution of the leached Ni from the nanowires to the volume of the electrode. For the electrode exposed to harsher acid treatment (80 °C), TXM shows areas of higher absorption on the electrode


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indicating that the Ni content is significantly reduced and some of the remaining nickel is likely present as agglomerates/chunks (Figure 6d).

Figure 6. SEM (a and b) and 2D TXM (c and d) of electrodes leached in acid at room temperature (a and c) and 80 ºC (b and d). In previous work from our group with these catalysts, it was demonstrated that the alloying Pt with Ni increases catalyst activity26. Therefore, it is important to understand what fraction of alloyed PtNi is preserved after these acid treatments. EXAFS and XANES analysis of the acid leached electrodes at the Pt L3 edge were used to probe the effect of the acid treatments on the PtNi alloy. The decrease in the shoulder at approximately 11,575 eV in the XANES spectra (dashed line in Figure 7a) and the increase in absorption edge intensity, due to an increase in unoccupied Pt orbitals from the removal of electrons previously contributed from Ni, indicates that in addition to the removal of a significant amount of the unalloyed bulk Ni, evident


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from EDS (Figure 8), there is also a loss of Ni that was alloyed with Pt. This is further supported by the increase in Pt-Pt coordination number and the decrease of Pt-Ni coordination number (Figure 7b and c) in the first shell from the EXAFS analysis. These findings are consistent with previous XRD studies on acid leached PtNi nanowire powders26

Figure 7. X-ray spectroscopy of the electrodes with PtNi, Nafion, PAA, and GCNFs untreated (gray), acid leached in H2SO4 at room temperature (red) and acid leached in H2SO4 at 80°C (blue): (a) Pt L3 XANES spectra, (b) Fourier transform of the k3-weighted EXAFS data (solid line) and first-shell fits (dashed line) of the electrodes, and (c) parameters of the absorber-


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scattering atoms derived from the EXAFS fits, including the coordination numbers (N), interatomic distance (R), disorder term (σ2), and the R-factor. To corroborate results obtained with 2D TXM and to assess the distribution of Ni and Pt in relation to GCNFs and ionomer, STEM imaging and EDS mapping was conducted on the same set of electrodes. Quantification from EDS measurements shows very little Ni remaining in the electrode material after acid leaching at 80 °C (8 – 35 at. % Ni), versus the acid leached electrode at room temperature (~ 89 at. % Ni) and the PtNi nanowires prior to acid leaching (~ 95 at. % Ni). (Ni values here are reported after normalizing raw measurements to the Pt:Ni ratio). Distributions of the Pt and Ni within the catalyst layer are highlighted by using overlays with STEM signal, as well as overlays of Pt, Ni and C signals, meanwhile distribution of the Nafion ionomer is identified by the F signal (Figure 8). As expected, STEM-EDS analysis of material scraped off of the as-received electrode clearly shows PtNi nanowires which consist of a Pt shell around the Ni core (Figure 8a), as in the PtNi catalyst prior to electrode preparation (Figure 2b). Figure 8b-c confirms that after room temperature acid treatment nanowires lose Ni from the original cores but maintain nanowire structure, while nanowires leached at 80 °C have been broken down to particles and retain little resemblance to wires. EDS mapping shows that these particles are primarily comprised of Ni and Pt, embedded in a matrix of F and C – as expected due to presence of Nafion and GCNFs. On closer inspection of the Ni signal (Figure 8c, Ni + STEM) in comparison to that of Pt (Figure 8c, Pt + STEM), it is also apparent that Ni is disbursed not only across the remaining nanoparticles/wires, but also at low concentrations throughout the rest of the material (refer to SI 3 for higher magnification images). The presence of Ni in the absence of Pt across the areas with GCNFs and ionomer implies that the Ni was digested by acid and remained dispersed within the


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Nafion ionomer as atomic or ionic clusters. From EDS maps shown in Figure SI 4, it is evident that there is correlation between the S and Ni maps, indicating that Ni ions are likely interacting with the sulfonic acid sites of the Nafion ionomer impeding proton transfer to the catalyst surface.

Figure 8. High angle annular dark field STEM imaging and EDS mapping of electrodes with PtNi, PAA, Nafion, and GCNFs: (a) as-prepared, (b) leached in acid at room temperature, and (c) leached in acid at 80 ºC, all at lower and higher magnifications as labeled. Each pixel in every


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elemental map/overlay corresponds to an EDS spectrum; Pt is red, Ni is green, F is magenta, and C is grey. The horizontal streaking in (a) at higher magnification is due to sample drift during EDS acquisition. Electrode Characterization – Impact on Electrochemical Performance Electrodes with PtNi and GCNFs which showed better dispersion of nanowires relative to those made without carbon, were chosen for electrochemical evaluation. Two MEAs were prepared and tested to assess the effect of Ni leaching on performance. The nanowires containing a Ni core are expected to result in significant leaching of nickel, causing contamination of the membrane and poisoning of active sites. Due to substantial amount of Ni in the as synthesized catalyst, significant drop in the performance from nickel poisoning is likely to occur. Indeed, despite excellent performance in RDE which was reported by our group previously26, PtNi nanowires performed poorly when incorporated into MEAs. Potential cycling resulted in cyclic voltammograms (CVs) with no Pt hydrogen underpotential deposition features (Hupd), indicating issues with catalyst accessibility (Figure 9a). In agreement with the CV results, the oxygen polarization curves showed essentially no performance for the cell prepared with the nanowires containing large amount of Ni in the core (Figure 9b). The second MEA was acid-soaked prior to testing. Acid soaking the MEA improved performance with Hupd features clearly present in the CVs (Figure 9a) and significant performance gains in the oxygen polarization curves (Figure 9b). These improvements become even more pronounced with an additional acid-soak. Observed improvements in the performance motivate further optimization of this class of materials by pre-leaching nanowires to remove undesired bulk nickel species while preserving extended surface nature of the catalyst and PtNi alloying for optimum activity and durability in MEAs.


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Current Density (mA/cm2)

a)

Potential (V)

b)

Potential (V)

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Current Density (A/cm2)

Figure 9. Cycling voltammograms (a) and oxygen polarization curves (b) of MEAs prepared with PtNi nanowires and GCNFs.

Conclusions For extended surface catalysts to be a viable alternative to carbon-supported Pt-based nanoparticles for the ORR in PEMFCs, it is imperative that accurate information about the catalyst within the catalyst layer is available. Acquiring such knowledge requires extensive characterization of the asreceived catalyst material and the catalyst layer, preferably in its working environment. Morphology of the catalyst material is readily determined with SEM and STEM but provides minimal information on the 3-D distribution of the catalyst in the electrode electrocatalyst layer.


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TXM is an effective tool for visualizing the catalyst within the electrode structure, thus providing information complementary to data obtained with electron microscopy. This technique is particularly effective for characterization of catalyst layers prepared with nanowire-based catalysts, such as extended surface PtNi catalysts derived from Ni nanowires. It was clearly visible in 2D TXM imaging and more so in 3D tomography that the addition of GCNFs results in a more homogeneously dispersed layer of PtNi nanowires – important for maintaining the accessible surface area of the catalyst without compromising wire-to-wire contact needed for charge transport. 2D TXM also provides valuable information about the evolution of the distribution of Ni in the wires and within the electrode, with STEM imaging and elemental mapping of smaller, representative areas corroborating and complementing results obtained from TXM. Ni leaching leads to redeposition of nickel within the electrode, contaminating ionomer and poisoning active sites, as evidenced from electron microscopy and supported by MEA testing. This study makes it apparent that acid treatment of the electrode can be used to remove nickel species that result in poisoning and therefore significantly diminished performance of these electrodes, but a better option would be to pre-leach the catalyst prior to its incorporation into electrode. Complementary information obtained with XANES and EXAFS analysis of catalyst and electrode materials quantified the extent of Pt-Ni alloying and provided evidence of alloy degradation upon acid treatment, motivating further studies towards effective ways to preferentially remove unalloyed nickel while preserving alloyed PtNi, to maintain specific activity. The characterization approach discussed in this work provides an effective method to study the MEA’s catalyst layer, and demonstrates the ability to apply TXM to discern the 3D extended catalyst architecture within the catalyst layer ex-situ and in the future, under in-situ and in-operando conditions, allowing further optimization of MEAs made with nanowire-based catalysts.


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Supporting Information Videos of the electrocatalyst layer with and without graphitized carbon nanowires and higher magnification STEM images.

Acknowledgements The work was supported by the U.S. Department of Energy under Contract No. DEAC36-08GO28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Fuel Cell Technologies Office. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. Authors also acknowledge graduate student funding through Betchel fellowship. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.


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2D and 3D Characterization of PtNi Nanowire Electrode Composition

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