Oxygen Reduction Reaction on Electrodeposited Pt100–xNix

Article pubs.acs.org/JPCC

Oxygen Reduction Reaction on Electrodeposited Pt100−xNix: Influence of Alloy Composition and Dealloying Yihua Liu, Carlos M. Hangarter, Ugo Bertocci, and Thomas P. Moffat* Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States S Supporting Information *

ABSTRACT: The electrocatalytic activity of electrodeposited Pt100−xNix thin films toward the oxygen reduction reaction (ORR) in perchloric acid was studied for x ranging between 2 and 95. The alloy composition was controlled by the potential applied during deposition. XRD and EDS were used to examine the structure and composition of the films before and after ORR measurements. Significant dealloying was evident for films with x > 45, and substantial shrinkage of the film thickness accompanied dealloying for films with x > 55. The onset of significant shrinkage occurs near the parting limit reported for bulk dealloying of fcc solid solutions. A maximum ORR specific activity of 2.8 mA/cm2 at 0.900 V RHE was observed for alloys between Pt45Ni55 and Pt55Ni45. This represents an enhancement factor of 4.7 compared to electrodeposited Pt, thereby matching the best published results reported for Pt−Ni nanoparticles and thin films. A peak ORR mass activity of 0.78 A/mgPt at 0.900 V RHE was observed for alloy film compositions between Pt38Ni62 and Pt45Ni55. In comparison to electrodeposited Pt, these films exhibit a 10-fold improvement in mass activity.

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ance.1,2,4,11−13 This has been demonstrated with extended surfaces11−13 and nanoparticle geometries.1,2,5,14−18 Experiments and modeling work indicate that Pt3Ni and Pt3Co alloys are of particular interest, with the (111) surface showing among the highest specific activities for ORR reported to date.19 More recent work suggests that iron group metal rich alloys may yield enhanced performance for both thin films13,20−22 and nanoparticles.5,7,8,22−30 A key challenge is to translate the above catalyst concepts into a scalable and economically viable membrane electrode assembly (MEA). To date, a MEA composed of nanoparticles supported on various forms of carbons has been the most commonly explored approach.1−4 The high surface-to-volume ratio of the particles inherently provides for effective utilization of Pt, but durability challenges increase as the particle diameter shrinks.3,31,32 The balance between loading, performance, and durability has focused attention on nanoparticles in the range 6−8 nm in diameter.1−4,31,33 However, the complexity of the multiple-step fabrication process along with durability issues such as the corrosion of the carbon supports challenge the viability of the alloy nanoparticle catalyst for real-world fuel cell application.3,4,31 Beyond nanoparticles, alternative catalyst configurations such as thin films are attracting renewed interest for fuel cell applications. The most well developed of these is the nanostructured sputtered thin films (NSTF), which exhibit greatly enhanced durability relative to a conventional carbon

INTRODUCTION High efficiency, low emissions, and renewable fuel sources have made polymer electrolyte membrane fuel cells (PEMFCs) a leading candidate to replace the internal combustion engine in motor-vehicles.1−3 Materials research is playing a central role in bringing this technology closer to the marketplace, with several prototype PEMFC powered cars having been assembled and tested. Nevertheless, questions concerning economical viability as well as several technical challenges remain.1−4 To make PEMFC technology more appealing, additional effort is needed to lower the production cost and improve performance and long-term reliability.1−4 Ongoing efforts have been directed toward cathode development for the oxygen reduction reaction (ORR), as the sluggish kinetics require significant quantities of Pt to meet the required power metrics. The precious metal loading presents a significant cost challenge that will only be exacerbated as commercialization is anticipated to drive the price of Pt even higher.2 Despite significant efforts to develop nonprecious metal catalysts, Pt and its alloys remain the leading candidates for the ORR, and the effort to reduce Pt loading continues to be a major research theme.1−5 The concept of Pt monolayer coatings that maximize catalyst utilization has received significant attention, with substantial progress occurring in the past few years.5−8 Beyond simply enhancing the Pt utilization, the core−shell scheme also offers the prospect of enhanced catalytic activity related to electronic effects between the Pt-enriched surface layer and the underlying core.9,10 In parallel with the core−shell scheme, alloying Pt with other metal(s) has also proven to be an effective way both to reduce Pt loading and to improve the intrinsic catalytic performThis article not subject to U.S. Copyright. Published 2012 by the American Chemical Society

Received: January 19, 2012 Revised: March 22, 2012 Published: March 22, 2012 7848

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supported nanoparticle catalyst.13,34,35 More generally, Pt and Pt alloy thin film electrocatalysts have been produced by a variety of means ranging from physical vapor deposition to sputtering,13,34−39 pulsed laser deposition,40 as well as several different electrodeposition schemes.41−43 These thin film methods also provide a robust test bed for exploring and quantifying the electrocatalytic performance of new alloys. Likewise, new architectures of interest can be created by either deposition on engineered 3-D substrates or by taking advantage of instabilities that accompany growth and dissolution to spontaneously generate 3-D topographies.34−38,44 For both thin film and nanoparticle Pt-iron group alloy electrocatalysts, dealloying of the iron group metal results in Ptenriched skins and/or skeleton structures.11,12,19,21,23,24 The enhanced activity arises from a combination of increased surface area and the structural and compositional gradients that develop during dealloying.21,45 The morphology of the dealloyed layers is a sensitive function of the alloy composition and the processing path. Deciphering the magnitude of these different contributions to ORR reactivity is critical for gaining a better understanding of the enhanced catalytic activity. Several recent reports indicate that iron group rich Pt alloys can exhibit enhanced ORR performance relative to films deposited with the Pt3M (M: iron group metal) stoichiometry.13,20,24,27 However, there remains significant dispersion with regard to the alloy composition associated with the maximum ORR activity.11,13,20,22,24−27,29,46−50 In certain cases catalytic activity increases with iron-group metal content passing through a broad maximum and then decaying toward the behavior of the base metal.11,25,50 More recently, an unusually sharp spike in ORR activity over a very narrow compositional range was reported for sputtered thin films of Pt−Ni and Pt−Co.13 The “alloy composition” determined for the peak activity was a function of the measurement method used (62 atom % Ni by energy dispersive spectroscopy (EDS), 69 atom % Ni by gravimetry, and 76 atom % Ni by X-ray fluorescence), speaking to the challenge of accurate composition measurement for 3D specimen topographies. The anomalous, discontinuous behavior is most likely related to nonuniformities within the asdeposited films associated with the sputtering conditions.13 Nevertheless, the results highlight the sensitivity of ORR catalyst performance to the details of thin film formation and its subsequent dealloying. Likewise, the effect of annealing has been shown to lead to a notable difference between the Pt skeleton and Pt skin alloys of nominally the same initial composition.24 Among the electrodeposition schemes used to form Pt alloys, underpotential codeposition is a simple, reproducible, and scalable approach for producing thin films. The method has been used to produce binary Pt−(Ni,51 Co,52 Fe,53 Cu,43 Pb54) alloys as well as ternary Pt−Ni−Pd49 and Pt−Fe−Ni55 and Pt− Fe−Co56 films. The alloy composition is a monotonic function of deposition potential, with chemically homogeneous films being obtained by potentiostatic deposition at room temperature. Given a suitable conductive support, thin alloy catalyst films can be synthesized in a matter of seconds to minutes. Furthermore, if desired, the growth potential can be easily modulated to generate compositional and related gradients within the film. Previously, the ORR on electrodeposited Pt100−xNix films had been examined and the peak activity was reported for films that contained ∼80 atom % Ni in the as-deposited state.20 However, in contrast to recent work with sputtered films,13 no singularity

in the compositional dependence was observed although the data density was relatively limited. Herein, this system is revisited to take a closer look at the influence of growth potential on the deposition and electrocatalytic performance of these alloys. The Pt100−xNix films with x from 5 to 95 are grown from an electrolyte that is rich in Ni2+ and relatively dilute in Pt2+, i.e. [Ni2+]/[Pt2+] = 33. Co-deposition of Pt-rich alloys occurs via kinetic trapping of underpotentially deposited Ni species (Niupd), while the Ni-rich alloys are codeposited in the overpotential region but at rates that are substantially faster than those for the deposition of pure Ni. In contrast to vacuum deposition methods, electrodeposition of Pt100−xNix involves ion transfer reactions and potential dependent adsorption as well as parasitic reactions, such as proton reduction, that are relevant to the deposition of Ni-rich alloys. Electrochemical quartz crystal microbalance (EQCM) in combination with EDS measurements were used to determine the alloy composition and, hence, the current efficiency as a function of the deposition potential. These measurements provide a firmer basis for evaluating the Pt loading and thereby mass activity associated with the thin film electrocatalyst. Likewise, the evolved surface roughness was examined by underpotentially deposited hydrogen (Hupd), and film shrinkage was examined by microscopic cross sections. X-ray diffraction (XRD), scanning electron microscopy (SEM), and EDS were used to follow the compositional and structural changes that accompany dealloying.

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EXPERIMENTAL METHODS Electrodeposition. Pt100−xNix thin films were prepared by potentiostatic deposition at 0.050 V increments between −0.100 V and −0.900 V versus a saturated calomel electrode (SCE). The alloy films were grown from a solution consisting of 0.003 mol/L K2PtCl4 + 0.1 mol/L NiCl2 + 0.5 mol/L NaCl at a pH of 2.5, adjusted by NaOH and/or HCl additions. All solutions reported in the study were prepared using 18 MΩ·cm water. The films were electrodeposited on an idled Au rotating disk electrode (RDE). Before each experiment the RDE was mechanically polished to a mirror-like finish, using a 0.05 μm aluminum slurry. A Pt foil counterelectrode was held in a compartment isolated from the working cell by a fine porous alumina frit. The separation was implemented to minimize the possible influence of counterelectrode oxidation byproducts, i.e. oxidation of Pt(II) to Pt(IV), on the deposition process. For films used to quantify the ORR behavior, a deposition time of 2 min was used, yielding films with a potential-dependent thickness (derived from EQCM measurements) between 15 and 80 nm. Growth was terminated by removing the RDE from solution while it was still connected to the potentiostat followed by immediately rinsing the deposit with copious amounts of 18 MΩ·cm water. Since the freshly emersed deposits were covered with a thin layer of electrolyte, performing the rinsing step promptly helped minimize galvanic displacement of the deposited Ni component by Pt. For the purpose of establishing the potential dependence of alloy structure and composition, films approximately 500 nm thick, as verified by SEM, were deposited on Au seeded Si wafer fragments for times ranging from 14 min to 4.3 h, depending on the deposition potential. The thin film substrates were produced by electron-beam evaporation of a 5 nm thick Ti adhesion layer followed by the 25 nm Au seed layer. Electroplater’s tape was used to mask the working area of the 7849

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electrode to 1.27 cm2. The films were characterized by XRD, EDS, and SEM. EQCM. To determine the current efficiency of the deposition, EQCM was utilized. The experiment was performed on a Au-coated quartz crystal that has a resonant frequency of 5 MHz. The solution was deaerated with Ar gas for at least an hour prior to the measurement. The counterelectrode, a Au wire, was placed in a compartment connecting with the working cell through a porous glass frit. A SCE served as the reference electrode. Both the current (i) and resonant frequency response were recorded while the potential was stepped from 0.500 V SCE to −0.800 V SCE in increments of 0.050 V. The rate of mass change (dm/dt) on the crystal was derived from the change in the resonant frequency using the Sauerbrey equation. The current efficiency of the deposition (ε) is given by zF ε=

0.002 V, was recorded and used to correct the data. A value of 0.309 V was obtained when SCE was measured against a Pt wire in a H2-purging 0.1 mol/L HClO4 solution at ambient conditions. A Pt foil was used as the counterelectrode. Electrochemically Active Surface Area. Immediately following ORR measurements the RDE was transferred directly to another five port 0.1 L glass vessel where the Hupd measurements were conducted in 0.1 mol/L HClO4 to determine the electrochemically active surface area of the catalyst. The cell was deaerated by purging the solution with 99.998% pure Ar gas for at least an hour before use. The RDE was idle during the measurements. The electrochemically active surface area of the catalyst was determined by integrating the anodic scan from 0.050 V RHE to 0.500 V RHE, correcting for the double layer capacitance, and dividing by the Hupd desorption charge density corresponding to a flat polycrystalline Pt electrode (i.e., 210 μC/cm2). The ORR and Hupd measurements were repeated on three distinct samples for each alloy composition (i.e., deposition potential). The average from the three sets of repeated experiments was used to determine the potential (i.e., alloy composition) dependent behavior of the systems with the standard deviation treated as the measurement error in ORR activity and Hupd derived surface area. The error of the derived quantities presented in the study was statistically evaluated by propagation of the random error. Structure and Composition. Crystallographic data was collected with a Siemens D500 X-ray diffractometer with a monochromatic Cu Kα source operated at 30 mA and 40 kV. (Identification of commercial products in this paper was done to specify the experimental procedure. In no case does this imply endorsement or recommendation by the National Institute of Standards and Technology.) Spectra were collected for 2θ ranging from 35° to 100° in 0.03° increments. Peak positions were used to calculate lattice parameters of Pt100−xNix films while the peak width was used to determine the coherence length. A Hitachi S4700 field emission scanning electron microscope (FE-SEM) was used to image Pt100−xNix films deposited on Au/Ti/Si substrates. SEM images of morphology were collected in plan view with no sample preparation. Cross sections of mechanically cleaved specimens were imaged without further preparation. Film compositions were analyzed with an EDS system (Oxford Instrument) attached to a Hitachi S4700 FE-SEM. These measurements were performed on 500 nm thick asdeposited Pt100−xNix thin films in plan view. Pure Pt and Ni bulk standards were used for analysis. The spectral overlap between Au Lα and Pt Lα lines was minimized by using a thin 25 nm Au seed with a thick 500 nm Pt100−xNix overlying film. The ZAF (Z: atomic number, A: absorption, F: fluorescence) normalization was used with the Pt Lα and Ni Kα peaks, selected to avoid peak overlap with Au Kα.57 The operating conditions for spectra collection were 15 kV, 30 μA, 5 K magnification, and a 60 s dwell time. As with the EQCM analysis, the EDS analysis assumes the sample is fully dense. The limitation of this approximation will be addressed in a subsequent publication.

( ddmt )

i(xMNi + (100 − x)MPt)

(1)

where z = 2 for both Ni and Pt reduction; F is the Faraday’s constant; x is the atomic composition of Ni in percentage as determined independently by EDS; and M is the molecular weight of Pt or Ni. The film thickness (d) for a deposition charge density of Q is d=

εQ (xMNi + (100 − x)MPt zF(xρNi + (100 − x)ρPt)

(2)

given the mass density ρ of the respective elements and assuming negligible free volume. For determining the mass activity of the alloys, the mass of codeposited Pt (mPt) is mPt =

εQ (100 − x)MPt zF

(3)

Catalyst Activation. After Pt100−xNix electrodeposition, the RDE was preconditioned before measuring the ORR kinetics. This was done by transferring the RDE to a 0.1 mol/L HClO4 solution exposed to ambient conditions and cycling the potential between −0.250 V SCE and 0.850 V SCE at the rate of 100 mV/s for 30 scans with a Pt plate being used as the counterelectrode. For the thicker 500 nm films, 150 cycles were utilized instead. The pretreatment resulted in dealloying and area change; the latter was monitored by tracking the charge associated with Hupd of the film. This dealloying strategy produced stable Hupd curves on the time scale relevant to the ORR measurements. ORR Measurements. The RDE with the activated Pt100−xNix thin film was transferred to an O2-saturated 0.1 mol/L HClO4 solution in a 0.1 L cell for ORR measurements. The solution was purged with pure oxygen gas for at least a half hour before the experiment. The RDE was set to a fixed rotation speed of 1600 rpm. The ORR was examined as the potential was cycled at 20 mV/s between 0.050 and 1.100 V versus a reversible hydrogen electrode (RHE) at room temperature. The RHE was of the trapped H2 variety based on a glass tube with Pt wire sealed through the upper end while the bottom of the electrolyte filled tube was open to the electrolyte. The RHE electrode was shown to be stable over the time span of 72 h or more. To further ensure the accuracy of the potential measurements, the value of the RHE was checked against a SCE just before each set of ORR measurements. Deviation from the standard potential value, usually within

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RESULTS Electrodeposition of Pt100−xNix Films. The potential dependence of the composition of the electrodeposited Pt100−xNix films is summarized in Figure 1. The Ni content 7850

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Figure 1. The as-deposited film appears optically black to the eye, and in some cases, this “mud-cake” layer as shown in Figure 2b spalls from the substrate during rinsing of the freshly electrodeposited material. EDS analysis of A1 indicates a high level of oxygen species, consistent with Ni(OH)2 precipitation with subsequent cracking due to dehydration. The complex microstructure calls for caution in evaluating the catalytic behavior of films grown below −0.750 V SCE. Control of the film thickness was derived from knowledge of the current efficiency of Pt100−xNix thin film deposition obtained from EQCM and independently from SEM crosssection measurements of thicker specimens. The results are summarized in Figure 3. The EQCM data (black dots) are

Figure 1. Composition by EDS for ∼500 nm thick electrodeposited Pt100−xNix films as a function of deposition potential. Note: films grown at −0.800 V SCE and −0.850 V SCE exhibit two domains in composition reflecting the bilayer nature of the specimens as detailed in the text.

obtained from the EDS measurement increases monotonically from 2 atom % Ni at −0.100 V SCE to 82 atom % Ni at −0.750 V SCE. A parallel study on the chemical uniformity of the films by transmission electron microscopic (TEM) cross sections indicates that the films are reasonably uniform with no obvious differences in the degree of homogeneity for films grown at −0.650 V SCE and −0.350 V SCE.58 Accordingly, the composition reported in Figure 1 for films having thickness of ∼500 nm is considered applicable to the thinner films grown for 2 min that are the focus of the ORR measurements reported herein. These results are congruent with a previous study showing that homogeneous Pt100−xNix alloys are obtained by potentiostatic deposition at the corresponding potential.51 Deposition is thought to occur by kinetic trapping of Niupd concurrent with ongoing Pt deposition. In contrast, at more negative potentials, −0.800 V SCE and −0.850 V SCE, a bilayer structure is apparent, consisting of a thin base layer covered by a porous and cracked overlayer with a distinctly different composition. The morphologies of films grown above and below −0.750 V SCE are compared in Figure 2. At more positive potential, the

Figure 3. Current efficiency of Pt100−xNix electrodeposition. The reported current efficiency is derived using two different methods: (black dots) differential measurementEQCM as given by dm/dt in eq 1 and (red dots) integral methodfilm thickness as determined from the SEM cross section compared to the measured charge using eq 2.

presented in differential form to give the current efficiency in accord with eq 1, where dm/dt and the current density are measured and combined with EDS derived knowledge of the alloy composition. Alternatively, the integral method (red dots), which combines the film thickness as measured by SEM cross section and the deposition charge density normalized to the EDS determined alloy composition, is used to determine the current efficiency as defined in eq 2. Qualitative agreement between the trends of two measurements is evident, with a minimum current efficiency observed at −0.500 V SCE. For the ex-situ thickness measurements, losses arise primarily from three sources: (i) oxygen reduction from exposure of the electrolyte to the laboratory ambient; (ii) proton reduction below −0.390 V SCE; and (iii) water reduction at potentials negative of ∼ −0.750 V SCE. For in situ EQCM measurements, only the latter two contributions are present, since the cell was deaerated. The red data points are derived from ex situ measurements of the deposit thickness and the assumption that the deposits are fully dense; the error bars derive largely from the uncertainty in measuring the film thickness by SEM, as indicated in column three of Table S1 (see Supporting Information). The shift of the ex situ data to relative higher efficiency than the EQCM results is amplified by the assumption of a fully dense deposit, as a parallel TEM study indicates some porosity in the as-deposited materials.58 The density error also impacts EQCM data but is expected to be somewhat obscured by incorporation of electrolyte species as well as viscoelastic coupling to the electrolyte via surface

Figure 2. SEM micrographs in plan view of Pt100−xNix thin films that were grown for 2 min at (a) −0.650 V SCE and (b) −0.850 V SCE. Inset in part b: a lower magnification image.

films are compact and metallic in nature, while a companion study shows the films are reasonably uniform in terms of composition. In contrast, the bilayer nature of the film grown at potentials below −0.750 V SCE is readily evident in Figure 2b. The thicker outer A1 layer has a Ni content in excess of 90 atom % compared to 55−65 atom % for the inner A2 layer. The different compositions of the respective layers are reflected in 7851

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roughness. The films grown at −0.800 V SCE and −0.850 V SCE are not included in this comparison as the cross-sectioning resulted in delamination of the top layer of the bilayer film. For the purposes of characterizing the ORR kinetics on Pt100−xNix alloys, attention is focused on thinner films rather than the thick films used to establish the composition− deposition potential relationship as detailed above. Thinner films not only minimize Pt loading but also facilitate more accurate measurements of the ORR mixed control kinetics by avoiding complications associated with the limited accessibility of O2 into deeply recessed, dealloyed regions of thicker films. However, a fully coalesced thin film of some thickness is required so that the geometric and electronic influences arising from the substrate material are minimized; the study of such effects is deferred to the future. Congruent with the objectives, a deposition time of 2 min was used to produce films that range from 13 to 85 nm in thickness. The charge density associated with deposition is indicated in Figure 4 for two different data

Figure 4. Thickness of Pt100−xNix thin films prepared by electrodeposition of 2 min at various potentials. Film thickness (○) derived from the deposition charge using eq 2 is compared with that (●) obtained from the EDS measurements using the thin film analysis program STRATAGem. The deposition charges associated with the respective films are presented as blue symbols.

sets. Using the current efficiency data determined from the EQCM and the composition−potential relationship shown in Figure 1 enables the film thickness to be calculated from the deposition charge using eq 2, and the results are given by the open symbols in Figure 4. Alternatively, the ThinFilmID software (Oxford Instrument) with a fully integrated STRATAGem thin film analysis program enables the film thickness to be determined from an EDS spectrum by considering the Si/Au/Pt100−xNix thin film geometry; in this case, the thicknesses obtained are indicated by the solid symbols. The resulting film thickness values derived from the two different methods are in reasonable agreement. For potentials below −0.500 V SCE, the increase in film thickness reflects codeposition of Ni that sums with the transport limited Pt deposition. At more positive potentials, a slight minimum is evident near −0.450 V SCE due to the fall off in current efficiency that accompanies the onset of proton reduction. ORR Measurements. The voltammetric ORR activities on a selection of electrodeposited Pt100−xNix, Pt, and Ni thin films, as well as a conventional Pt RDE, are summarized in Figure 5a. The Pt100−xNix films are identified according to the deposition potential used in their formation. Prior to measuring the ORR kinetics, the specimens were dealloyed using the pretreatment

Figure 5. (a) ORR measurements for activated Pt100−xNix thin films with the films identified by the respective deposition potential (SCE), electrodeposited Pt thin film−Pt ED, electrodeposited Ni film, and the as-polished Pt RDE. The nominal ORR kinetic activities evaluated using the Levich−Koutecky relationship at (b) 0.900 V RHE and (c) 0.950 V RHE, respectively. Measurements conditions: 20 mV/s in cathodic scan, O2-saturated 0.1 mol/L HClO4 solution, ambient temperature, and 1600 rpm.

specified in the Experimental Section. The evolution of the ORR response with the pretreatment will be detailed elsewhere. The voltammetric data shown correspond to the positive-going potential scan. The results were not corrected for the resistive electrolyte losses (iR) or for the background contributions associated with the metal oxide formation; the latter is evident as the small positive current observed at potentials above 1 V RHE. These two effects will be examined in more detail in another publication. The Pt100−xNix thin films show a higher catalytic activity toward ORR than the electrodeposited Pt (Pt ED) film. The ORR activity of the Pt100−xNix films exhibits a 7852

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maximum for the films grown at −0.650 V SCE with the ORR half-wave potential shifted by 70 mV relative to Pt ED. The Pt ED film shows a slightly higher ORR activity than the polished Pt RDE. By comparison, the electrodeposited Ni surface is inactive toward ORR, exhibiting a current of 0.043 mA/cm2 at 0.500 V RHE compared to 6 mA/cm2 for Pt- and Pt-based alloys. The interface kinetics of the respective films can be quantified by using the Levich−Koutecky expression to correct for the effect of O2 depletion. The diffusion limited current at 0.500 V RHE was used to determine the kinetic current density for the projected area of the respective specimens at various potentials. The accuracy and precision of the extracted kinetics decrease as the diffusion limited value is approached, so the kinetics were sampled at two different potentials, 0.900 V RHE and 0.950 V RHE, to provide for a more robust assessment. The respective dependence of the ORR activity of Pt100−xNix thin films on the deposition potential is summarized in Figure 5b and c. Each error bar represents the standard deviation of measurements performed on three distinct electrodeposited specimens grown under identical conditions. The nominal ORR kinetic activity at 0.900 V RHE reaches a maximum of 23.3 mA/cm2 for the Pt100−xNix film grown at −0.600 V SCE. For reference purposes, a Pt film grown at −0.350 V SCE exhibits a current of 2.3 mA/cm2. Thus, the maximum ORR activity of the alloy is ∼10-fold greater than Pt ED. The ORR performance of the Pt100−xNix films grown at various potentials can be clustered into three groups. For films grown between −0.300 V SCE and −0.500 V SCE, the activity enhancement scatters between 4 and 6. A steady decrease in activity is evident for films deposited between −0.600 V SCE and −0.750 V SCE followed by a sharper drop for films grown at potentials more negative than −0.750 V SCE. The same trends are present for the ORR kinetics measured at 0.950 V RHE. It is also useful to compare the ORR on Pt and Pt100−xNix thin films grown at the same potential. For Pt and Pt100−xNix films grown at −0.350 V SCE, a 4-fold difference in the ORR activity is observed. Comparison at more negative potentials is precluded by sharp attenuation of the deposition of pure Pt below −0.400 V SCE. However, comparing the ORR activity of the electrodeposited Pt film to that of a conventional polished Pt RDE reveals favorable agreement, as shown in Figure 5b and c. Consequently, these two electrodes are used as internal standards for all the ORR measurements reported herein. A more complete perspective on the ORR activity of the Pt and Pt100−xNix films requires an assessment of both the area specific kinetics and the Pt mass activity. Electrochemically Active Surface Area. Following the ORR measurements, the activated specimens were transferred to an Ar-saturated solution for Hupd surface area measurements. The well-known voltammetric signatures for Hupd and oxide formation and reduction are clearly evident in Figure 6a. At least two overlapping waves are evident in the Hupd region. The Hupd wave at more negative potential is dominant for the Pt100−xNix thin films while for the Pt film a significant contribution is also apparent at more positive potentials. These differences most likely reflect variations in the surface texture of the respective thin films. No definitive effect of Ni alloying was evident in the oxide formation or reduction region. This indicates that Ni has largely been removed from the near surface of the alloys following the 30 voltammetric cycles used in the activation treatment.

Figure 6. (a) Cyclic voltammograms of Hupd on activated Pt100−xNix thin films and as-deposited Pt thin film, obtained at a scan rate of 20 mV/s in an Ar-saturated 0.1 mol/L HClO4 solution. (b) Hupd desorption charge and relative surface area for films prepared at various deposition potentials.

The electrochemically active surface area is quantified in terms of the charge associated with the Hupd desorption waves, and the results for different specimens are summarized as a function of the growth potential in Figure 6b. The relative surface area is calculated on the assumption that the examined specimens all have the same Hupd charge density of 210 μC/ cm2 for their geometric planar projection, and the results are summarized in Figure 6b. The dependence of the surface area on deposition potential is very similar to the observed trends for the ORR activity presented in Figure 5b and c. Accordingly, the Hupd derived surface area for the films as a function of the deposition potential can be grouped into clusters. For films grown between −0.300 V SCE and −0.500 V SCE, a 6-fold increase beyond the geometric projected surface area is evident. A maximum 9-fold expansion in surface area is observed for the films deposited between −0.600 V SCE and −0.750 V SCE followed by a sharp decrease for film deposited below −0.750 V SCE. Comparing the Pt100−xNix to Pt electrodeposited at −0.350 V SCE reveals that the activated alloy film exhibits an active area that is almost twice that of the elemental film. Closer inspection of the potential dependence of the nominal ORR activity and Hupd surface area reveals some important differences. For films grown between −0.350 V SCE and −0.500 V SCE, there is a small monotonic decrease in the surface area while the opposite trend is observed for the ORR reactivity. For films grown between −0.600 V SCE and −0.750 V SCE, the expanded surface area is constant while a monotonic decrease in the ORR activity occurs. The sharp 7853

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drop in the electroactive area for films grown at more negative potentials gives an electroactive area similar to that of a polished Pt RDE. The sharp drop in area is thought to be related to massive dealloying of the Ni-rich films and perhaps the further loss of remnant Pt due to undercutting of the small ligaments by Ni dissolution or spalling of the film (see Figure 2b). These measurements demonstrate that the surface area of the activated Pt100−xNix thin film alloy is a strong function of deposition potential and the dealloying processes associated with a given alloy composition. The activation treatment, thus, must be carefully considered in evaluating the electrocatalytic behavior. Specific Activity. The Hupd derived surface area was used to normalize the ORR mass-transport-corrected kinetic data shown in Figure 5b and c. The resulting area specific activity is summarized in Figure 7 as a function of the electrodeposition

Surprisingly, for the Ni-rich alloys grown at the most negative potentials, severe dealloying leaves behind a Pt-rich remnant that has a specific ORR activity close to that of films grown at −0.550 V SCE. This is thought to be associated with the bilayer nature of the films deposited at −0.800 V SCE and −0.850 V SCE noted earlier. As indicated in the Figure 1 EDS measurements, the composition of the inner layer is close to that of films prepared at −0.550 V SCE. If the outer layer plays no role in Hupd or if it is detached or dissolved during the dealloying pretreatment, the subsequent ORR behavior would be dominated by the remaining inner layer. As a result, the activated films grown at −0.800 V SCE and −0.850 V SCE exhibit specific ORR activity very similar to that of the more Ptrich films grown between −0.600 V SCE and −0.550 V SCE despite the substantial difference of Hupd surface area development and nominal ORR activity. The Pt reference film, electrodeposited at −0.350 V SCE, has a specific activity of 0.61 mA/cm2. This is lower than the value observed on mechanical polished Pt RDE, 0.92 mA/cm2, measured under the same experimental conditions. This result is similar to that of a previous study where the area normalized ORR kinetics for electrodeposited Pt20 was shown to be a function of the electroactive area in a manner analogous to that reported for Pt nanoparticles of differing sizes.33,59 The origins of the effect have been attributed to site blocking by anions coupled with shape (size) dependent changes in the potential of zero charge. The same trends are evident for the ORR specific activity measured at 0.950 V RHE. A slight decrease in the maximum enhancement factor for ORR on the Pt100−xNix alloys from a factor of 4.7 to 4.1 is apparent. While the difference is within a 15% error, the lower value might be expected to be more accurate due to the decreased sensitivity to the Levich− Koutecky correction. Determination of Pt Loading. Quantification of mass activity is an important metric for fuel cell engineering as it relates to minimizing the Pt loading associated with nanoparticle catalysts. For thin films it is a less informative parameter, particularly for those grown beyond coalescence where the mass loading scales with film thickness, albeit with some variations possible if surface roughness evolves in nonobvious ways with film thickness, such as through dealloying. To ascertain the Pt mass loading, EQCM measurements combined with EDS analysis of the composition of the as-deposited films were used to determine the current efficiency of film growth and the Pt mass loading in accord with eq 3. The results of the analysis in Pt loading are summarized in Figure 8 for Pt100−xNix films electrodeposited for 2 min at the indicated potentials. The error bars reflect the random error propagating from the standard deviation of alloy composition (Figure 1), current efficiency (Figure 3), and charge transfer measurements (Figure 4). Compared with electrodeposited Pt, the Pt100−xNix films grown at −0.350 V SCE have a very similar Pt loading. At more negative potentials, the Pt codeposition rate is a variable function of the deposition potential. This is in contrast with previous work on Pt−Cu alloys, where Pt codeposition in a sulfate-based electrolyte occurs under diffusion limited conditions over most of the range of alloy deposition.43 Between −0.300 V SCE and −0.350 V SCE, and −0.600 V SCE and −0.750 V SCE, Pt codeposition after 2 min is in the neighborhood of 33 μg/cm2, similar to that observed for Pt ED. In contrast, a local minimum of 22 μg/cm2 is evident for films grown between −0.450 V SCE and −0.500 V SCE.

Figure 7. Specific ORR activity (i.e., Hupd corrected) of Pt100−xNix and Pt thin films evaluated at (a) 0.900 V RHE and (b) 0.950 V RHE. The enhancement factor in specific activity for Pt100−xNix films relative to Pt ED is shown on the right side ordinate.

potential. At 0.900 V RHE, the maximum ORR activity of 2.8 mA/cm2 is attained by films prepared at −0.500 V SCE and −0.550 V SCE. The peak in activity seen in the nominal kinetic data is still evident although the difference in magnitude relative to Pt ED is reduced from 10-fold to a 4.7-fold enhancement and the peak activity is slightly shifted such that films grown at −0.550 V SCE provide the most catalytic surface on a Hupd area corrected basis. The Pt-rich Pt100−xNix films grown at more positive deposition potentials, −0.350 V SCE or −0.300 V SCE, are less active but still show a 2.7 enhancement in ORR activity over Pt deposited under the same conditions. 7854

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trends and improvement factors relative to electrodeposited Pt are observed for Pt100−xNix ORR measurements performed at 0.900 V RHE and 0.950 V RHE, respectively. Pt100−xNix films prepared at −0.550 V SCE exhibit the highest ORR mass activity of 0.78 A/mgPt at 0.900 V RHE. This corresponds to an order of magnitude improvement compared to the case of electrodeposited Pt. Comparing Pt100−xNix and Pt films grown at the same potential, −0.350 V SCE, gives an improvement factor of 5. A monotonic increase in the mass activity is recorded as the deposition potential is lowered from −0.300 V SCE to −0.500 V SCE and is followed by a linear decrease with deposition potential to −0.750 V SCE. The uncertainty in the determination of Pt loading for the film grown at −0.800 V SCE and −0.850 V SCE prevents meaningful comparison with the films grown at the other potentials. The decrease in area expansion between −0.300 V SCE and −0.500 V SCE (Figure 6b) indicates that the rise in mass activity over the same potential range is due to an increase of the specific activity. On the other hand, the decrease in the specific activity at more negative potential coupled with the increased mass loading leads to the reduction in the mass activity between −0.550 V SCE and −0.750 V SCE. To assess the contribution of surface area expansion to the mass activity, the Pt loading (Figure 8) can be normalized by the Hupd surface area (Figure 6) to give the electroactive area in terms of mass loading. As shown in Figure 10, the result leads to a relatively

Figure 8. Pt mass loading per unit of geometric area for Pt100−xNix and Pt thin films grown for 2 min.

This is coincident with the onset of a parasitic proton reduction process and will be explored in more detail in another publication. A significant drop in the amount of codeposited Pt is also evident at deposition potentials more negative than −0.800 V SCE, presumably due to the Ni(OH)2, or alternatively hydride or even hydrogen, formation that obstructs Pt codeposition. Mass Activity. The Pt mass loadings for the as-deposited films were used to normalize the ORR activity data given in Figure 5b and c. The dependence on the deposition potential for Pt and Pt100−xNix is summarized in Figure 9a and b. Similar

Figure 10. Electrochemically active surface area per gram of Pt mass loading for Pt100−xNix and Pt thin films deposited for 2 min at the stated potential.

flat potential dependence of the electrochemically active surface area per unit Pt mass loading. Nevertheless, two domains are suggested by a step occurring between −0.500 V SCE and −0.550 V SCE. For films grown between −0.300 V SCE and −0.500 V SCE, the area per gram Pt scatters around 21.6 m2/ gPt, while growth between −0.550 V SCE and −0.750 V SCE yields a value of 26.9 m2/gPt. This compares to 11.5 m2/gPt for the elemental Pt film grown at −0.350 V SCE. If the enhanced mass activity of the Pt100−xNix catalysis were to be solely attributed to area expansion and catalysis of available Pt sites,60 the enhancement factor would be limited to 1.89 and 2.34 for the respective films. Thus, the additional improvement must be ascribed to enhanced specific activity associated with alloying effects, as indicated earlier. Dealloying Measurements. The specific and mass dependent ORR activities of Pt100−xNix thin films are strongly dependent on the initial alloy composition and subsequent

Figure 9. ORR (Pt) mass activity for Pt100−xNix and Pt thin films grown for 2 min. The ORR was evaluated at (a) 0.900 V RHE and (b) 0.950 V RHE. The ORR enhancement factor for Pt100−xNix films relative to the Pt ED is summarized on the right-hand ordinate. 7855

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dealloying associated with the activation process. Thus, a closer examination of the evolution of the morphology, structure, and composition of the films upon dealloying was performed using SEM, XRD, and EDS, respectively. Due to limitations in the intensity of the laboratory X-ray source and SEM resolution, attention was focused on studying the dealloying of ∼500 nm thick films similar to those used to establish the relationship between alloy composition and deposition potential. Moreover, the parallel TEM cross section study demonstrates consistency between the thin and thick films in terms of composition and structure for both the as-deposited and dealloyed states.58 Following examination of the as-deposited films, the ORR activation treatment was performed in 0.1 mol/L HClO4 solution that was open to the ambient atmosphere. This involved cycling the potential 150 times between −0.250 V SCE and 0.850 V SCE at 100 mV/s. Composition. EDS measurements of bulk compositions for as-deposited and dealloyed Pt100−xNix films are summarized in Figure 11. The monotonic dependence of the composition of

Figure 12. XRD spectra of Pt100−xNix thick films before (black) and after (red) dealloying (Pt100−xNix films as indicated by the deposition potential versus SCE). The thickness of the as-deposited films is about ∼500 nm. Scattering from the Si/Ti/Au substrate as well as that with electrodeposited Pt and Ni is evident.

(200) peaks are evident, with the former being dominant (note log scale), indicating some (111) texturing that follows from the more strongly textured Au substrate. In contrast, electrodeposited Ni is more strongly (111) textured. The peak positions of the Ni and Pt films are shifted from their bulk phase values due to a combination of coherency strain and other effects associated with deposition on Au(111). For the as-deposited Pt100−xNix films, a monotonic shift of the (111) peak position with deposition potential is evident. The data is consolidated by converting the peak position to a lattice parameter value while the peak width at half the maximum intensity represents the scattering coherence length perpendicular to the surface normal. The potential dependence of the lattice parameter is shown in Figure 13. The coherence Figure 11. Composition by EDS of ∼500 nm thick Pt100−xNix films before and after dealloying.

the as-deposited films on potential is replotted from Figure 1. Following the activation treatment, a threshold for bulk dealloying is evident for films deposited below ∼ −0.450 V SCE. At more positive potentials, the Pt-rich films are largely unaffected by the activation process, although Ni dealloying from the near surface results in a slight shift of the overall film composition. In contrast, extensive dealloying of the Ni−rich Pt100−xNix materials occurs, although some Ni content (∼20 to 30 atom %) remains within the dealloyed remnant. This suggests that bulk dealloying does not propagate significantly beyond an overall Pt3Ni stoichiometry, but the extensive porosity associated with these dealloyed films makes accurate composition measurements difficult. An anomalous result is observed for films grown at −0.750 V SCE. EDS measurements indicate significant compositional variation across the specimen with some regions containing much higher Ni concentrations. The films represent a discontinuity from the compositional trend observed for all other dealloyed samples. Structure. The structures of the respective Pt100−xNix films were examined by XRD, and the results are summarized in Figure 12. Diffraction data from electrodeposited Pt and Ni films are shown for reference; the Pt film was electrodeposited at −0.350 V SCE in a Ni-free solution while Ni was grown in a Pt-free solution at −0.850 V SCE. For the Pt film the (111) and

Figure 13. Lattice parameter of Pt100−xNix films before and after dealloying as derived from the peak position in the XRD data shown in Figure 12. The (111) peak for the films grown at −0.800 V SCE and −0.850 V SCE was deconvoluted into two components using Pearson VII distribution fitting.

length is summarized in Figure 14. For the as-deposited films the shift in the lattice parameter with potential is congruent with the EDS data shown in Figure 1. Inspection of the shape of the (111) diffraction peak indicates that the films deposited at or above −0.750 V SCE are relatively homogeneous, in contrast to the strong deviations in peak symmetry evident in the films grown at more negative potentials, such as −0.850 V 7856

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Examination of the width of the peak envelopes summarized in Figure 14 shows the as-deposited films have on average a scattering coherence length on the order of 7 ± 2 nm, as determined by the Scherrer equation. This is consistent with the grain size previously reported in a TEM study of these materials.51 For the bilayer films grown at −0.800 V SCE and −0.850 V SCE, curve fitting for the X-ray data indicates that regions with the higher Pt content have a smaller coherence length of ∼3 nm. The absence of diffraction from other planes due to the strong as-deposited texture prevents a more robust assessment of the contributions of grain size versus straininduced broadening in the as-deposited films. The variation in coherence length between the as-deposited specimens does not appear to have an obvious relationship to the deposition potential. However, following voltammetric activation, the coherence length of the Ni-rich films grown below −0.450 V SCE is sharply diminished to values as low as 3 nm while the Pt-rich films deposited at more positive potentials are not significantly altered by the activation treatment. The diminished scattering length most likely reflects the characteristic length of the Pt-rich alloy ligaments and gradients therein that comprise the low density bicontinuous structure generated by the dealloying process. Shrinkage. As reported previously, the Ni-rich Pt100−xNix alloys become highly porous, and cross sections of the film reveal substantial shrinkage. Herein the level of shrinkage was quantitatively evaluated by measuring the thickness of the films before and after the dealloying using SEM. To minimize experimental error, the following approach was taken: a Aucoated Si wafer with the as-deposited Pt100−xNix film was first cleaved in the vertical direction, and one of these two pieces was subjected to the activation or dealloying treatment. To avoid edge effects, the two original pieces were cleaved again but horizontally with respect to the direction of the first cleave and approximately at the same vertical location; SEM was used to examine the horizontally cleaved cross sections to determine the thickness of the films, and the average thickness is reported. Despite these precautions, a ∼10% uncertainty in determination of the shrinkage in film thickness remains due to surface roughness and other nonuniformities related to the film growth. The results of the analysis as a function of the deposition potential are summarized in Figure 15. For films grown above −0.550 V SCE, no shrinkage was discernible above the

Figure 14. Coherence length of Pt100−xNix thick films before and after dealloying derived from symmetric XRD data in Figure 12 using the Scherrer equation. Note: for the bilayer films grown at −0.800 V SCE and −0.850 V SCE, the spots with the higher Pt content exhibit a smaller value in the coherence length.

SCE. The asymmetry reflects a significant element of structural dispersion congruent with the bilayer morphology detailed earlier. The diffraction envelopes for the −0.850 V SCE and −0.800 V SCE are thus more properly characterized by at least two components (not shown). The dominant portion is a mixture of a highly Ni-rich alloy and amorphous Ni(OH)2, as derived from the EDS analysis; the other peak centered at a position is congruent with that for a film produced near −0.500 V SCE. In two previous studies, SEM examination of the surface revealed a small quantity of Pt- and Pd-rich particles embedded in a more Ni rich matrix for films grown at more negative potentials.20,49 XRD examination of the same films after voltammetric activation reveals several important changes. For films deposited above −0.450 V SCE a slight shift of the (111) scattering envelope to lower angles is apparent, indicating some removal of Ni. This most likely corresponds to transient formation of a Pt-rich surface skin that, when convolved with the scattering envelope of the substrate, leads to the shift in the diffraction peak to a slightly lower angle. In contrast, much more significant dealloying is evident for films deposited at potentials more negative than −0.500 V SCE. The (111) peaks shift substantially to a lower angle, reflecting the change in the bulk alloy composition toward more Pt-rich compositions. For example, the peak position for the film deposited at −0.650 V SCE shifts to a value congruent with that of a film deposited at −0.350 V SCE. This is consistent with evolution of the Ni-rich film toward an overall alloy composition near Pt 3 Ni stoichiometry as suggested by the EDS measurements. More complex behavior is seen for the films grown at more negative potentials. For the film deposited at −0.750 V SCE, two components are evident in the scattering envelope in Figure 12: a Pt-rich remnant combined with a significant portion of the original film that appears to remain largely unaltered. The inhomogeneous nature of the film is congruent with the dispersion evident in the EDS data although the inferred composition values seem to differ significantly. The anomalous behavior was verified by repeating the experiment four times. At still more negative potentials, −0.850 V SCE, the scattering envelope associated with the bilayer as-deposited film shifts to lower angles, forming a very broad scattering peak that reflects the extensive Ni dealloying that occurs during activation.

Figure 15. Shrinkage (1 − dD/dA) in film thickness upon dealloying for electrodeposited Pt100−xNix films of ∼500 nm thick as a function of deposition potentials. Data used in the evaluation of shrinkage is presented in Supporting Information Table S1. 7857

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heterogeneities evident in EDS, SEM, and XRD measurements. Film deposition positive to the reversible Ni/Ni2+ potential (i.e., −0.550 V SCE) occurs by kinetic trapping of a Niupd layer during ongoing Pt deposition. This corresponds to Pt100−xNix alloys where x < 55. For alloys with 55 < x < 80 the films are still homogeneous and the underpotential deposition process controls alloy deposition as the overpotential nucleation and growth of elemental Ni is kinetically hindered. By combining Figures 1 and 7, the dependence of the specific ORR activity on the as-deposited film composition is summarized in Figure 17 for 0.900 V RHE and 0.950 V

background defined by the measurement error. In contrast, for films deposited at −0.600 V SCE, the film thickness decreases by 32%. For more negative deposition potentials, the shrinkage was even greater, amounting to 52% for films deposited at −0.700 V SCE and −0.750 V SCE. Shrinkage of up 30 vol % has been previously noted for dealloying of Ag-rich Au100−xAgx alloys.61 According to recent work, deformation associated with surface stress that accompanies the formation of nanoscale ligaments may contribute to the shrinkage.62 For the most Nirich materials, measurements were greatly hampered by the friable nature of the as-deposited bilayer films. Furthermore, dealloying of transition metal rich alloys might also lead to materials loss by undercutting or separation of the finest Pt ligaments. The threshold behavior associated with film shrinkage is also reflected in both the EDS and XRD measurements.

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DISCUSSION To facilitate comparison with materials produced by other processes, the ORR performance of the electrodeposited films is recast in terms of the as-deposited Pt100−xNix composition. The EDS and X-ray data for the as-deposited films are summarized in Figure 16 and compared with that of Pt100−xNix

Figure 16. Comparison of the dependence of the lattice parameter on composition for Pt100−xNix alloys prepared by four different processes.

Figure 17. ORR specific activity for films evaluated at potentials of (a) 0.900 V RHE and (b) 0.950 V RHE as a function of the as-deposited alloy composition. (open circles) Pt ED; (open triangles) Pt RDE; (closed circles) electrodeposited Ni; (red squares) electrodeposited Pt100−xNix thin films; (blue squares) activity corresponding to the composition for the more Pt-rich regions of the bilayer Pt100−xNix thin films electrodeposited at −0.800 V SCE and −0.850 V SCE.

alloys produced by other means such as thermomechanically processing,63 pulsed laser deposition,40 and sputtering.39 Reasonable agreement between the composition and lattice parameters for alloys produced by four different processes is evident and speaks to the utility of the potential controlled codeposition process to produce alloys of specific composition. The dashed line corresponds to a Vegard’s law extrapolation for the fcc phase and captures the data to within the scatter. Close examination of the thermomechanically processed specimens suggests perhaps a slight convex deviation. The thin film methods are all low temperature processes whereby the limited mobility of the atoms is expected to preclude the formation of the ordered L12 and L10 phases that otherwise form in this system. The simplicity, low cost, and scalability of the underpotential codeposition process is noteworthy and lies in contrast to other time-consuming, higher thermal budget, batch processes that are used to make nanoparticle catalyst. In the present work, electrodeposits with Ni content below 80 atom % appear to be homogeneous while the higher Ni content alloys are more complicated with compositional and morphological

RHE, respectively. A broad maximum in activity is apparent for Pt100−xNix with x between 40 and 65. Compared to electrodeposited Pt, these alloys have a specific ORR activity that is up to 4.7-fold greater. The Pt75Ni25 alloy is 2.7 more active than Pt, in good agreement with the results from numerous other investigations of this well studied composition.12,14,15,17,19,25,27,64 Current thinking suggests that the enhanced intrinsic ORR activity is associated with electronic perturbation of a Pt enriched surface associated with the strain gradient that develops between the dealloyed Pt-enriched surface layer and the underlying Pt100−xNix alloy.12,21,24,65 More recently a theoretical study66 of the ORR on the segregated (111) surfaces of Pt, Pt75Ni25, Pt50Ni50, and Pt25Ni75 indicates that Pt50Ni50 should be the most active surface followed by 7858

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dealloying process in expanding access to Pt sites on a mass basis. As shown in Figure 10, all of the Pt100−xNix thin films have a higher electrochemically active surface area than Pt. For Pt76Ni24 grown at −0.350 V SCE, the surface area is almost twice that of Pt film deposited under the same conditions, yet the dealloying is thought to be limited to the near surface. As shown in a previous study, the Hupd voltammetry changes in minor ways with cycling while SEM examination of thicker films reveals substantial surface roughness is associated with the film growth process.20 In contrast, the Ni-rich films are substantially smoother in the as-deposited state and the relatively high surface area of 28 m2/gpt is the result of bulk dealloying of the thin film. For the most Ni-rich thin film, the electrode area per unit Pt mass decreases due to a combination of structural collapse (shrinkage) or loss of Pt. Close examination of these films grown by overpotential deposition (≤ −0.800 V SCE) reveals compositional gradients across the film that may also facilitate Pt loss by undercutting. The ORR dependence on the as-deposited alloy compositions shown in Figure 17 is in particularly good agreement with recent results in the literature for nanoparticles.25 A comparative summary is given in Figure 18 in terms of

Pt25Ni75, Pt75Ni25, and Pt in descending order, in good agreement with the trends shown in Figure 17, although the lattice parameters reported in ref 66 (i.e., PtNi-L10, a = 0.386 nm) are notably different from those reported herein and elsewhere67 (Pt50Ni50, a = 0.376 nm) and, thus, may undercut the relevance of such a comparison. The nominal ORR activity enhancements for Pt100−xNix (Figure 5b) are even greater, as much as a factor of 10 (e.g., for x ∼ 60) better than the case for Pt, due to the combination of the intrinsic activity of the surface coupled with the expansion of the electroactive surface area as a result of dealloying. Metallurgical analysis of the activated Pt100−xNix thick films reveals significant bulk dealloying when x > 35. For alloys that are slightly Ni-rich (x ≤ 55), the extent of dealloying is constrained such that the shrinkage and thereby strain relaxation is limited, resulting in significantly enhanced intrinsic activity. The limited extent of dealloying may be associated with Pt surface segregation known to occur on PtNi surfaces.68 In contrast, for films that are more Ni-rich (x ≥ 55) the thickness and extent of the overlying dealloyed material is greater and the stress state of the overlayer presumably more relaxed. The sharp onset in shrinkage observed for thick films with x > 55 marks the transitions between improved intrinsic ORR behavior and its subsequent relaxation seen in the thin films. The threshold for massive dealloying of x = 55 is in good agreement with the previous study of the dealloying of combinatorially sputtered nanostructured Pt100−xNix thin films.39 The sharp compositional transition is also congruent with the reported parting limit of x = 58.4 for bulk dealloying of the more reactive element in binary fcc solid solutions.69 This value is also close to the high density percolation threshold for removal of reactive metal species with 9-like nearest neighbors. Simply excluding dissolution from 10- and 11-cooridnated sites gives a parting limit near x = 55. The EDS analysis for the thick electrodeposited Pt100−xNix films presented in Figure 11 suggests that the overall composition of the remnant of the dealloyed Ni-rich alloys is close to the Pt3Ni stoichiometry. On the other hand, absent a robust assessment of the effect of strain, a simplistic Vegard’s analysis of the X-ray data (Figures 13 and 16) suggests that the remaining Ni content is lower than 20 atom % for the same films. In comparing the ORR thin film results with dealloying analysis drawn from the thick films, appropriate care must be given to the influence of film thickness. For films grown by underpotential codeposition, e.g. Ni55Pt45, previous work indicates the composition of the films to be quite homogeneous as a function of thickness.51 Nevertheless, it must be recognized that, during termination of the growth and emersion, there is a brief moment when cementation of Pt via Ni displacement might result in a Pt enriched surface layer. Resolving the magnitude of such an effect(s) may provide a means for controlled formation of a compressed Pt overlayer by modulation of the deposition potential just prior to growth termination. The increased catalytic activity due to surface area expansion associated with dealloying has been long recognized, e.g. Raney nickel. In the case of Pt alloys, the benefits of using dealloying to obtain enhanced ORR performance in this manner have been noted.60 In the present work, thick Pt100−xNix films were used to study dealloying with EDS, XRD, and dimensional analysis. The onset of significant dealloying is evident for x > 35. Since the thinner films were used to investigate the ORR kinetics, it is worthwhile to recall the effectiveness of the

Figure 18. Comparison of the composition dependence of the ORR specific activity for two types of Pt100−xNix alloyselectrodeposited thin films vs nanoparticle ensembles. ORR measurements in these two studies were conducted under very similar experimental conditions. The improvement factor in the specific activity of the Pt100−xNix alloy is relative to that of the elemental Pt counterpart.

enhancement factors whereby each data set is normalized to the behavior and the geometry of the Pt control used in the respective studies. HAADF STEM analysis of the nanoparticles with the 1:1 stoichiometry nanoparticles revealed formation of a thin Pt skin, ∼0.5 nm, and a Ni depleted core that was close to the Pt3Ni stoichiometry in composition following the ORR measurements. There was a clear correlation between Ni content and Pt skin thickness with the reported catalytic activity.25 Mapping this result onto the electrodeposited films would suggest that the thickness of the dealloyed Pt overlayer is optimized for films grown at −0.550 V SCE (∼55 atom % Ni). In terms of nominal ORR activity, a further increase is evident up to −0.650 V SCE that reflects the favorable trade-off between decreased specific ORR activity and increasing thickness of the dealloyed Pt-rich skin that is more than compensated by the surface area expansion. For more negative deposition potentials, an excessive build up in the thickness of the Pt overlayer leads to deterioration of its activity.45 7859

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In a previous study,20 the ORR kinetics on electrodeposited Pt100−xNix films grown at −0.650 V SCE were reported to be the most catalytic, in contrast to the present study where films grown at −0.550 V SCE and −0.500 V SCE were found to be more catalytic. Importantly, the specific ORR activities of the films grown at −0.650 V SCE are identical between the two studies. As to the origin of the difference and to the higher activity observed for film grown at −0.500 V SCE and −0.550 V SCE in the present study, two possible sources come to mind: insufficient sampling in the former study or, perhaps more likely, differences in the activation step used prior to ORR measurement. In the previous work, activation was limited to a small number of cycles to 1.0 V RHE, whereas the present study involved more extended cycling to an upper limit of 1.15 V RHE. It is known that the durability of the ORR activity of Pt100−xNix is highly sensitive to excursion to high potentials although it remains to be determined if this is in fact the origin of the difference between the results of these two studies. More recently, studies of both planar and nanostructured thin film (NSTF) Pt100−xNix sputtered alloys have shown enhanced ORR behavior for Ni-rich materials that included a spike in activity for film approximating the Pt3Ni7 stoichiometry.13,46,47 The composition associated with the peak activity of NSTF materials was found to be a function of the measurement method: 69 atom % Ni by gravimetry, 76 atom % Ni by X-ray fluorescence, and 62 atom % Ni by electron microprobe analysis.13 The variation speaks to the difficulty in using the latter two methods for measurements of noncompact threedimensional structures. In contrast, no evidence of discontinuous behavior in the composition dependence of the ORR kinetics was observed for the electrodeposited materials in this work. The absence of the spike in activity more likely reflects the more homogeneous nature of the planar electrodeposited film in comparison with the NSTF sputtered alloys, although it is always possible that densification of the alloy compositions examined might reveal otherwise. For example, compositional variations and gradients can arise from the sputtering process due to parallax effects associated with the relative position of Ni and Pt targets and the rotating substrate that are further accentuated when the deposition rate is high relative to the substrate rotation.13,46,47 These effects are further amplified when sputtering onto the three-dimensional NSTF substrates. The films associated with the spike in ORR activity had a lattice parameter close to 0.3708 nm for the as-deposited state, a value that reflects the convolution of the compositional gradients and stress state within the NSTF alloy catalyst. Absent detailed evaluation of these factors, XRD of bulk phase Pt100−xNix indicates that a lattice parameter of 0.3708 nm be associated with x = 60. For the as-deposited electrodeposited films, a similar lattice parameter is observed for Pt55Ni45 films grown at −0.500 V SCE, which exhibited the maximum ORR specific activity. Similarly, the phase in the bilayer materials grown at −0.800 V SCE and −0.850 V SCE that exhibited high specific ORR activity also had a similar lattice parameter, 0.3700 nm for the as-deposited state. Such correlations are probably fortuitous, with the ORR activity actually being correlated with the convolution of compositional gradients and the resulting state of stress that will also be sensitive to the detailed geometry of the interface. Further work will be required to fully characterize the composition, structure, and state of stress associated with the most active Pt100−xNix electrocatalyst.

Article

CONCLUSION Electrodeposition of Pt100−xNix thin films produced from a simple room temperature process is demonstrated along with an assessment of the ORR activity relevant to fuel cell cathodes. The composition and structure of the films are direct functions of the deposition potential, with the films containing x ≤ 80 atom % Ni being homogeneous in nature; films deposited at more negative potentials, that of −0.800 V SCE and −0.850 V SCE (i.e., x > 80 atom % Ni), display a more complex bilayer structure. Ni itself is not catalytically active toward ORR, but alloying with Pt leads to an increase in the intrinsic activity that exceeds that of elemental Pt. Relative to electrodeposited Pt, Pt45Ni55 alloy exhibits the highest specific activity with an enhancement factor of 4.7 at 0.900 V RHE and 4.1 at 0.950 V RHE. By comparison, the specific activity of Pt76Ni24 is 2.7 times greater than Pt. The increase of activity with increasing Ni content up to an equiatomic ratio of Pt/Ni is in good agreement with several recent reports for electrodeposited and sputtered thin films as well as nanoparticles. The enhanced activity has been attributed to a Pt skin that forms on top of the dealloyed substrate. Perturbation of the electronic band structure convolved with compressive strain due to the dealloying-induced misfit stress is believed to account for the enhanced ORR activity. The combination leads to the intrinsic activity increasing monotonically as the initial film composition increases from 20 atom % Ni (as-deposited at −0.300 V SCE) to 55 atom % Ni (as-deposited at −0.550 V SCE). The buildup in strain due to transient dealloying is constrained to the near surface region when negligible shrinkage in the overall film thickness was evident. In contrast, for alloys with greater than 55 atom % Ni, massive dealloying leads to rapid area expansion and shrinkage in the overall film thickness that presumably also results in a partial relaxation of the strain in the Pt skin. The nominal ORR activity is found to be greatest on Pt38Ni62 films due to the onset of substantial dealloying that leads to a sharp increase in electroactive area combined with the intrinsic activity of the dealloyed surface to give a 10-fold improvement over the comparable activity of electrodeposited Pt. A similar improvement factor is found for the mass activities of Pt45Ni55, Pt38Ni62, and Pt30Ni70.

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ASSOCIATED CONTENT

* Supporting Information S

Table listing as-deposited films’ thicknesses and that upon dealloying. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: thomas.moff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.L. and C.M.H. acknowledge the National Research Council for a postdoctoral fellowship. REFERENCES

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