SADâGLAD PtâNi@Ni Nanorods as Highly Active Oxygen Reduction

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SAD-GLAD Pt-Ni @Ni nanorods as Highly Active Oxygen Reduction Reaction Electrocatalysts Nancy N. Kariuki, Mehmet F. Cansizoglu, Mahbuba Begum, Mesut Yurukcu, Fatma M Yurtsever, Tansel Karabacak, and Deborah J. Myers ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00454 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 17, 2016

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SAD-GLAD Pt-Ni @Ni nanorods as Highly Active Oxygen Reduction Reaction Electrocatalysts Nancy N. Kariuki*1, Mehmet F. Cansizoglu2, Mahbuba Begum2, Mesut Yurukcu2, Fatma M. Yurtsever2, Tansel Karabacak2, and Deborah J. Myers1 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne,

IL 60439-4837, USA 2

Department of Physics and Astronomy, University of Arkansas at Little Rock, Little

Rock AR 72204, USA *[email protected] ABSTRACT: Vertically aligned catalysts comprised of platinum-nickel thin films on nickel nanorods (designated as Pt-Ni@Ni-NR) with varying ratios of Pt to Ni in the thin film were prepared by magnetron sputtering and evaluated for their oxygen reduction reaction (ORR) activity. A glancing angle deposition (GLAD) technique was used to fabricate the Ni nanorods (NRs) and a small angle deposition (SAD) technique for growth of a thin conformal coating of Pt-Ni on the Ni-NRs. The Pt-Ni@Ni-NR structures were deposited on glassy carbon for evaluation of their ORR activity in aqueous acidic electrolyte using the rotating disk electrode technique. The Pt-Ni@Ni-NR catalysts showed superior area-specific and mass activities for ORR compared to Pt-Ni alloy nanorod catalysts prepared using the GLAD technique and compared to conventional high-surface-area Pt and Pt-Ni alloy nanoparticle catalysts. KEYWORDS: magnetron sputtering techniques, glancing angle deposition (GLAD), small angle deposition (SAD), nickel nanorods, Pt-Ni thin film catalysts, oxygen

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reduction reaction (ORR), polymer electrolyte fuel cell polymer electrolyte membrane fuel cell (PEMFC)

INTRODUCTION Hydrogen-fueled polymer electrolyte membrane fuel cell (PEMFC) energy conversion systems have drawn significant attention as an alternative to traditional power sources, especially for automotive applications, due to their high energy conversion efficiencies, low emissions, and robustness compared to other types of fuel cells.1, 2The sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode, however, necessitate a high loading of platinum catalyst significantly increasing the system cost.3 Major cost reductions can be achieved by increasing the ORR kinetics of platinum-based catalysts1, 4-7 and by more efficient utilization of the platinum component of the catalyst. A number of Pt-transition metal alloys have been developed with enhanced ORR kinetics compared to Pt and these enhancements have been attributed to modification of the structural and/or electronic properties of surface Pt.8-14 Among the most active alloys are Pt-Fe,15,

16

Pt-Co,15,

17-19

and Pt-Ni8,

chemical29-32 de-alloying.

15, 20-26

with or without electrochemical11,

27-29

or

Despite immense improvements in activity, the limited

durability of the advanced carbon-supported ORR electrocatalysts, associated with corrosion of the catalyst particles and support,33-35 remains a major obstacle. The nanostructured thin film (NSTF) catalyst concept,36,

37

developed by 3M, has

been shown to address these issues: the ORR specific activity38,

39

and durability40 of

NSTF can be significantly higher than conventional carbon-supported nanoparticle Pt catalysts. Recently, 3M has demonstrated high ORR mass and area-specific activities in


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membrane-electrode assemblies (MEAs) for a Pt3Ni7 NSTF catalyst (0.4 A mg-1Pt and 2.5 mA cm-2Pt at 0.9 V, respectively).39, 41, 42 The NSTF catalysts are comprised of a 520 nm thick film of Pt or Pt alloy, which forms ~5 nm diameter whiskerettes, deposited on 500-2000 nm long, ~50 nm diameter organic pigment (N, N-di(3,5-xylyl)perylene3,4:9,10 bis(dicarboximide) or perylene red) whiskers. van der Vliet et al.43 have shown that thermal treatment of a PtNi NSTF catalyst at 400°C in a reducing atmosphere causes physical transformations in the catalyst. These transformations include conversion of the three-dimensional metal whiskerettes into a two-dimensional, flat, ordered, more homogeneous thin film with crystalline features and (111) surface domains. They termed these catalysts “mesostructured thin films” (meso-TF). PtNimeso-TF had approximately two times the area-specific ORR activity of PtNi-NSTF.43 However, the thermal treatment also caused evaporation of the perylene red substrate leaving the meso-TF unsupported, potentially raising issues of mechanical robustness needed for incorporation of this structure into a fuel cell electrode. In addition to the limited thermal stability of the perylene red support, perylene red is not an electron conductor.

Electronic

conductivity to the catalytic sites is provided by the thin Pt-based film, limiting the practical nanorod length and the corresponding electrochemically-active surface area (ECA) and electrode layer thickness due to electron conduction losses.42 This work describes our efforts in developing a versatile ORR electrocatalyst fabrication approach employing the advantageous design principles of 3M’s NSTF catalyst,42 which address the activity and stability issues of conventional carbonsupported Pt nanoparticle catalysts, while also addressing the thermal instability and lack of support electron conductivity issues associated with the perylene red support. The


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system demonstrated here is a thin film of a highly-active ORR catalyst deposited on an array of vertically-aligned, thermally-stable, electron-conducting, non-precious metal nanorods.

The support material demonstrated here is nickel metal, however, the

approach is applicable to, and is currently being extended to, support materials that are both thermally and chemically stable such as oxides, carbides, and silicides. Welladherent and continuous Pt-Ni thin film catalysts, with different molar ratios of Pt to Ni, were formed on Ni nanorods (NRs) by combining the magnetron sputtering techniques of glancing angle deposition (GLAD)44 for fabrication of Ni-NRs and small angle deposition (SAD)45, 46 for the growth of a thin conformal coating of Pt-Ni on the Ni-NRs (designated Pt-Ni@Ni-NR). The Pt-Ni thin film compositions (Pt to Ni atomic ratios of 3:1, 2:1, 1:1, 3:7, and 1:3) studied in this work were chosen on the basis of formation of thermodynamically-stable phases of Pt3Ni, PtNi and PtNi347 as well as the ORR activity trends observed in our previous work on Pt-Ni nanorods24 and other literature data on PtNi ORR electrocatalysts.9, 12, 14, 22, 41, 43. The most notable is the Pt3Ni7composition, which was shown by 3M to have unusually high ORR activity relative to other NSTF catalyst compositions.41 The Pt-Ni@Ni-NR structures were supported on glassy carbon (GC) for evaluation of their ORR activity in aqueous acidic electrolyte using the rotating disk electrode (RDE) technique.

EXPERIMENTAL Fabrication:Pt-Ni@Ni-NRs were fabricated by combining magnetron sputtering GLAD for the deposition of Ni-NRs and SAD for deposition of a conformal coating of Pt-Ni alloy thin films on the Ni-NRs in a vacuum system (Excel Instruments, India) with


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a base pressure of 7.5 x 10-7Torr.

The base pressure was achieved using a turbo

molecular pump backed by a mechanical pump. Ni-NR deposition: Ni-NRs were deposited on native oxide p-Si (100) silicon wafers and also on glassy carbon (GC) disk

substrates (3 mm OD x 4 mm thick, Pine

Instrument) using a 99.99% purity 2 inch diameter Ni cathode as the source (i.e., target). The substrates were mounted on a sample holder located approximately 15 cm from the cathode. Deposition was performed using 300 Watts (W) RF power with an ultra-high purity argon (Ar) working gas pressure of 2.4 × 10-3Torr. The GLAD growth was performed at θ = 85° (with respect to substrate normal) with substrate rotation at 20 rpm in order to produce vertically-aligned NRs with sufficient separation between the rods. A growth time of 30 minutes was used to produce Ni-NRs of approximately 98 nm length and 23 nm mean diameter. We note that GLAD is a physical self-assembly technique and the growth mechanism does not have a strong dependence on the substrate material used.44The geometry of the GLAD nanostructures mainly depends on the substrate morphology and deposition parameters including deposition angle and rotation speed. Therefore, in this study, GLAD produces nanorods of similar geometry and dimensions on the silicon wafer and GC substrates. Pt-Ni alloy thin film deposition: After deposition of the Ni-NRs, the silicon wafer and GC substrates were tilted at θ = 45° towards Pt and Ni sputter sources placed at a 90° angle with respect to each other to improve the conformality of Pt-Ni alloy on the NiNRs using SAD.

The substrates were rotated at 20 rpm and the deposition was

performed at a relatively high Ar gas pressure of approximately 2.8 x10-2Torr. Use of a high pressure environment during SAD decreases the mean free path of incident atoms,


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which increases the angular flux distribution and can further enhance the uniformity of the film coating on the nanorods.48 Thin film catalysts with several different molar ratios of Pt to Ni were obtained at constant 50 W DC power for Pt and different RF power values of 90, 125, 200, 415, and 490 W for Ni, designed to produce Pt to Ni atomic ratios in the thin films of 3:1, 2:1, 1:1, 3:7, and 1:3, respectively. Deposition time was varied for the different Pt-Ni film compositions, designed to obtain a similar sidewall thickness for all the films. The thicknesses of Pt-Ni films on the tops and sidewalls of the nanorodswere estimated from SEM images by measuring the diameters and lengths of the Ni-NR supports and subtracting them from the diameters and lengths of the Pt-Ni@NiNRs. Deposition time for each sample was set based on preliminary depositions performed for Pt and Ni to separately determine their individual deposition rates at 45° angle and 2.4 x10-2Torr pressure. The deposition parameters for the Pt-Ni thin films of different alloy ratios (Pt DC power fixed at 50 W) are summarized in Table 1 in the results and discussion section. Details of the Pt and Ni calibration experiments used to measure the individual Pt and Ni deposition rates at different sputter power values are presented in the Supplementary Information section (See Figs. S1 and S2). Electron Microscope Imaging: The morphology of samples, deposited on silicon wafer substrates, was characterized using a 15 kV SEM (FESEM-6330F, JEOL Ltd, Tokyo, Japan). Elemental composition and Pt weight loading: The elemental composition and weight loading of Pt in the Pt-Ni thin films were measured by quartz crystal microbalance (QCM) (Inficon Q-pod QCM monitor, crystal: 6 MHz gold coated standard quartz).


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X-ray diffraction measurements: The crystal structure was determined by X-ray diffraction (XRD) (Bruker D8 discover) using Cu Kα radiation (λ= 1.540598 Å). Electrochemical measurements: Cyclic voltammetry (CV) using an RDE was performed with a CH Instruments potentiostat (Model 760) and a standard three-electrode cell with a mercury/mercuroussulphate (Hg/Hg2SO4 in 0.5 M H2SO4) reference electrode, a Pt wire counter electrode, and 0.1 M HClO4 (GFS Chemicals, Inc.; 18 MΩ Millipore water) supporting electrolyte. The reference electrode was calibrated against a reversible hydrogen electrode (RHE) and all potentials reported here are with respect to RHE. The working electrode was the as-prepared Pt-Ni@Ni-NR arrays deposited on a 0.196 cm2 geometric area GC substrate, which was assembled into the RDE electrode tip and shaft (Pine Research Instruments). Hence, during the electrochemical measurements, the nanorods are arranged in a vertically-oriented array on the GC electrode surface.

RESULTS AND DISCUSSION Fabrication and microscopic characterization of Pt-Ni@Ni-NRs. The Pt-Ni@Ni-NRs were fabricated using magnetron sputtering in the GLAD (Ni-NRs) and SAD (Pt-Ni alloy thin films) geometries in a vacuum system (Excel Instruments, India) with a base pressure of 7.5 x 10-7Torr. A schematic of the custom-made SAD-GLAD system used in the fabrication of the Pt-Ni@Ni-NRs is shown in Figure 1. The main difference between GLAD and SAD techniques is that in SAD the incidence angle of the deposition flux (θ) is typically less than 45° (illustrated in Figure 1) and is adjusted based on the substrate surface pattern. In the SAD technique, the sidewalls and bottom portions of the support


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nanorods are effectively exposed to the deposition flux through use of a small tilt angle and rotation of the substrate. This can lead to conformal coatings of very thin layers of catalyst on the support nanorods.45 Figure 2 show top and cross-sectional SEM images of isolated and vertically-aligned Ni-NR supports and Pt-Ni@Ni-NR catalysts formed using the SAD-GLAD technique. As shown in the top and cross-sectional SEM images,the nanorods are isolated from each other leading to a channeled porosity aligned vertically with respect to the substrate surface. This geometry canimprove the transport of reactants to the catalyst sites and the removal of product water from the electrode layer.

Figure 1. Illustration of the SAD-GLAD system used for fabrication of SAD-GLAD PtNi@Ni-NR array electrodes.


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Figure 2.: Top and cross-sectional view SEM images of (a) base GLAD Ni nanorods and SAD-GLAD Pt-Ni@Ninanorods (Ni core, Pt-Ni shell) with different Pt:Ni compositions of (b) 3:1, (c) 2:1, (d) 1:1, (e) 3:7, and (f) 1:3. Elemental composition and Pt weight loading. The elemental composition and weight loading of Pt in the Pt-Ni thin films were measured using a quartz crystal microbalance (QCM) and were determined by separately depositing Ni and Pt onto 6 MHz quartz crystals under identical conditions to those used for depositing the Pt-Ni thin film catalysts on the Ni-NR supports. The oscillation frequencies of QCM crystals were recorded before and after the depositions and the amount of material was calculated using Sauerbrey’s49equation:


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∆Fm= − (2nF02∆m)/(µqρq)1/2

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(1)

where ∆Fmis the frequency change, ∆m is the mass added, n is the harmonic number, F0 is the resonant frequency of the crystal, and µq and ρq are the quartz shear modulus and density, respectively. The Pt-Ni elemental composition and weight loading of Pt on the GC substrates for the Pt-Ni thin films of different alloy ratios are summarized in Table 1. The Pt-Ni thin film elemental compositions determined by QCM closely matched the compositions expected based on Pt DC and Ni RF power. By controlling the GLAD deposition parameters, such as target sputter power and deposition time, the composition of the Pt-Ni thin film catalysts can be controlled while closely maintaining the thickness of the films (~9 nm at the sidewalls near the nanorodtips). Table 1: Experimental and measured parameters for the different compositions of Pt-Ni thin film catalysts

2.79 x 10-2

QCMdetermined composition (% Ni) 25

XRD calculated composition (% Ni) 19

QCMdetermined Pt loading (mg/cm2) 0.030

139

2.16 x 10-2

35

32

0.016

200

201

2.79 x 10-2

51

44

0.023

3:7 (70)

415

90

3.12 x 10-2

69

84

0.010

1:3 (75)

490

67

2.02 x 10-2

76

75

0.008

Desired composition Pt:Ni (%Ni)

Ni power (watts)

Deposition time (sec)

Operating pressure (Torr)

3:1 (25)

90

258

2:1 (33)

125

1:1 (50)

Structural characterization. Thicker Pt-Ni films were also deposited on silicon wafers for crystallographic determination by XRD. Figure 3 shows XRD patterns of the Pt-Ni thin films with peaks between the known two-theta values for the (111) reflections for pure Pt and Ni phases (39.9º and 44.5º, respectively), indicating that the deposited 10 ACS Paragon Plus Environment

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films are indeed alloys. The peaks are also primarily symmetrical, which indicates homogeneity in the alloy composition. The peaks are shifted to higher two-theta values with increasing Ni deposition power consistent with the increase in Ni content in the films. However, XRD peaks are weaker for the 1:1 and 3:7 Pt-Ni ratio films. This might be due to the incomplete crystal structure transition from a Pt-rich to Ni-rich alloy film, which requires further investigation. The weak and broad (111) peak of the elemental Ni film is also positioned at a slightly smaller 2θ angle. This is believed to originate from small compressive stress, which is common in sputter deposited films. However, we are not expecting any notable stress in the SAD Pt-Ni films of this study mainly due to the compliant property produced by the underlying Ni nanorods50and high working gas pressures used to grow the Pt-Ni films51. In addition, the XRD pattern of the 3:7 Pt-Ni film shows weak and diffuse side bands alongside the main (111) peak, presumably due to irregularities in the atom distribution in the alloy structure.52, 53 By fitting the (111) peak, the d spacing in the 3:1, 2:1, 1:1, 3:7, and 1:3 Pt-Ni films were calculated to be 2.22 Å, 2.19 Å, 2.16 Å, 2.07 Å, and 2.09 Å, respectively. The approximate Ni content in the alloys calculated from these d spacing and using Vegard’s law (dPtNi = X dPt + (1– X) dNi, where X is the mole fraction Pt in the Pt-Ni films and dPtNi, dPt and dNi are the d spacings of the (111) planes of PtNi, Pt, and Ni, respectively) are shown in Table 1. With the exception of the 3:7 Pt-Ni film, the XRD-determined compositions agree with those determined by QCM (difference in at% Ni: 0.64 to 6.7). The large difference (15 at% Ni) between XRD and QCM-determined composition for the 3:7 Pt-Ni films is attributed to non-uniformity in the distribution of atoms in the alloy structure, evidenced by the observed side bands, which may cause displacement of the main peak.


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Figure 3.X-ray diffraction patterns of Pt-Ni thin films deposited on silicon substrates at different Ni RF power corresponding to different Pt-Ni composition. DC power for Pt was fixed at 50 W. Pattern of Ni thin filmon silicon substrates is included for comparison. Theoretical peak positions for elemental Pt and Ni are marked with dashed lines. Electrochemical

characterization.

As

preparation

for

the

ORR

activity

measurements using RDE, the nanorod coated GC RDE disks were cycled between 0.01 V and 1.1 V at a scan rate of 100 mV/s in de-aerated 0.1 M HClO4 electrolyte to electrochemically clean (activate) the electrode surface. Figure 4Ashows the CV of the Pt-Ni@Ni-NR electrode with a Pt-Ni 3:7 film before and after electrochemical activation. During the potential cycling, the voltammetric profile evolved toward one with features characteristic of a Pt surface, with the typical underpotentially-deposited hydrogen (Hupd) and hydrogen desorption peaks (0.05–0.40 V vs. RHE), the double layer capacitive current plateau (0.4–0.6 V vs. RHE), and the Pt hydroxide/oxide peaks (0.7–1.0 V vs. RHE). The decrease in the capacitive charging appearing as a broad anodic plateau between 0.3–0.6 V is concurrent with an increase of the hydrogen adsorption/desorption


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peaks during the potential cycling. The broad anodic plateau likely indicates the presence of Ni on the nanorod surface, likely in the form of an oxide, whereas the hydrogen adsorption/desorption region indicates that Pt is also present on the nanorod surface. The dissolution of Ni is an irreversible process under the chosen potential conditions. This observation is consistent with our previous work on GLAD Pt-Ni alloy nanorods24.

Figure 4. (A) Cyclic voltammetry curves of 3:7 Pt-Ni@Ni-NRs thin film before and after electrochemical

activation

in

de-aerated

0.1

M

HClO4.(B)

Background

voltammetriccurves for all of the electrodesin fresh de-aerated 0.1 M HClO4. Scan rate, 20 mVs-1

Figure 4B shows the background voltammetric curves for all of the electrodes in fresh Ar-saturated 0.1 M HClO4 solution at a scan rate of 20 mVs-1. The Hupd region, at potentials below 0.4 V, exhibits broad features for all Pt-Ni compositions, similar to those observed by Stamenkovic et al. for a Pt3Ni(110) single crystal8 and Pt-Ni NSTF catalysts.5 This is an indication of a high degree of coordination of the Pt atoms on the surface and small crystalline facets. With the exception of the 3:1 composition, it is clear that the voltammetric signatures of the compositions are similar, exhibiting similar ECAs


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and crystal face dominance. Subtle differences are observed in the strongly-bonded Hupd peak, which has been attributed to Hupd on [100]-like facets.8 The ECAs of the Pt-Ni films were determined by integrating the charge in the Hupd region (between 0.05 V and 0.4 V), after subtracting the double layer charge and using the following equation to convert the measured charge to ECAs:54

 QHupd (C )  6 ECA(cm2 ) =  10 −2   210µCcmPt 

(2)

where, a QHupd of 210 µC/cm2 is the charge of a full coverage of H for clean polycrystalline Pt. Electrochemically-active surface areas (Table 2, column F) of 61, 43, 25, 34, 25 m2/g-Pt were measured for the catalysts with Pt:Ni ratio of 1:3, 3:7, 1:1, 2:1, and 3:1, respectively, as determined from the Hupd charges and the Pt loadings obtained using QCM (Table 1). By utilizing image processing software (ImageJ), the number density and average diameter of the nanorods were measured from the top view SEM images (Figure 2). The average length of the nanorods was measured from the cross-sectional SEM images (Figure 2). The number density of the Ni-NRs was estimated to be 53x1013nanorods per m2. For the Ni-NR supports, the measured average diameter and length were 23 ± 4 nm and 98 ± 6 nm, respectively. The measured average dimensions and calculated geometric specific surface areas of the Pt-Ni@Ni-NRs are summarized in Table 2. As can be seen in Table 2 column C, Pt-Ni@Ni-NRs with Pt:Ni of 3:1, 2:1 and 1:1 have top films that are thicker than those of the Ni-rich 3:7 and 1:3 compositions. However, all the samples have similar nanorod sidewall thicknesses with Ni-rich compositions having slightly higher values. This might be attributed to the differences in surface free energy density 14 ACS Paragon Plus Environment

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(Pt =2.48 J/m2, Ni = 1.70 J/m2) leading to different surface diffusion rates for the different alloy composition. In other words, Ni seems to have a sufficiently high surface diffusion, which favors conformal coating.We also note that thin film coating on nanorod sidewalls has been experimentally confirmed by our previous studies46 using TEM and energy dispersive X-ray spectroscopy (EDS). To estimate catalyst utilization in the electrochemical measurements, the geometric surface areas (GSAs) of the nanorodsnormalized to the Pt loading (i.e., specific areas) were calculated from the Pt-Ni@Ni-NR dimensions (Table 2) measured from the SEM images (Figure 2) and the Pt loading on the GC electrodes (Table 1), and assuming smooth nanorod surfaces. The GSAs of the nanorods(Table 2 column E) were calculated using (πr2 + 2πrh)*n /gPt, where r is the radius of the Pt-Ni@Ni-NRnanorod, h is the full length of the nanorod, n is number density, and g is the weight of Pt in the thin film determined using QCM. As expected, the GSAs calculated from the nanorod dimensions and the ECAs measured from voltammetry show increasing specific area with decreasing Pt content in the Pt-Ni films.Catalyst utilization was calculated from the ratio of the measured ECA to the GSA (Table 2 column G). Results show similar utilization (~50 %)for all the Pt-Ni@Ni-NR electrode layers, except for the 3:1 Pt-Ni film, which shows a much higher (77%) utilization. Considering the well-controlled Pt-Ni composition, and sidewall thickness, catalyst utilizations of

SADâGLAD PtâNi@Ni Nanorods as Highly Active Oxygen Reduction

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SADâGLAD PtâNi@Ni Nanorods as Highly Active Oxygen Reduction