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Spatial Mapping of Methanol Oxidation Activity on a Monolithic Variable Composition PtNi Alloy using Synchrotron Infrared Microspectroscopy Kaiyang Tu, Michael J. Lardner, Tyler A Morhart, Scott M Rosendahl, Steven Creighton, and Ian J. Burgess J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08127 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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The Journal of Physical Chemistry

Spatial Mapping of Methanol Oxidation Activity on a Monolithic Variable Composition PtNi Alloy Using Synchrotron Infrared Microspectroscopy Kaiyang Tu1, Michael J. Lardner1, Tyler A. Morhart1, Scott M. Rosendahl2, Steven Creighton3, and Ian J. Burgess1,§ 1

Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9 Canada 2 Canadian Light Source, Saskatoon, Saskatchewan, S7N 0X4 Canada 3 Saskatchewan Research Council, 15 Innovation Boulevard, Saskatoon, SK S7N 2X8 Canada §

corresponding author : email ([email protected]) / tel (+1 306-966-4722)

Abstract The use of synchrotron-sourced infrared radiation to map the electrochemical activity of a binary metal (Pt and Ni) alloy is demonstrated. The alloy is created in such a way that its metal concentration varies along one of its dimensions thus creating a continuum of electrocatalyst compositions on a single electrode. Localized methanol oxidation activity is determined spectroscopically by measuring the rate of CO2 production at variable positions along the alloy concentration gradient using an infrared microscope. Numerical simulations of the kinetically controlled reaction demonstrate that qualitative assessment of relative reaction rates is possible as long as the reaction is followed on time scales smaller than those that lead to diffusional broadening. Characterization of the alloy before and after electrochemical experiments reveal significant levels of base metal leaching. Highly dealloyed regions of the sample show the highest rates of methanol activity and have a final Ni atomic composition of approximately 5%. Surface roughening from the de-alloying process is shown to be at least partially responsible for enhanced activity.

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1. Introduction Increasing conversion efficiencies of electrocatalysts is paramount to their successful implementation. Improved fuel cell technology requires alternative materials that lower reaction overpotentials, reduce catalyst costs, and provide desired chemical conversion. Most electrocatalytic materials rely on precious metals to unlock electrons by reductively or oxidatively breaking bonds. The high cost of metals such as Pt has driven efforts to lower loading levels by alloying expensive, highly-active elements with much more abundant transition metals such as Ni.1 Incorporation of oxophilic base metals also improves catalyst specific activity by reducing susceptibility to poisoning from incomplete oxidation products such as strongly adsorbed CO during methanol oxidation.2 This bi-functional effect is augmented by an electronic effect where the alloying metal tunes the Pt d-band electron density.3 A binary alloy of the form Pt1-x M x represents a single dimension optimization problem but even greater improvements in catalyst efficiency have been reported by extending to ternary,4 quaternary5-6 and even high orders7 of metal compositions. Characterizing the performance of complex catalyst alloys or materials is time consuming and greatly benefits from methodologies that embrace parallel assessment.8 The use of combinatorial approaches is an appealing means to determine the optimal elemental composition for various electrocatalytic applications.9 Since Redding et al10 first introduced high-throughput screening to the field of fuel-cell catalyst development, efforts have been directed toward designing better catalyst libraries and improving means to characterize electrocatalytic performance. Libraries of binary and ternary metal catalysts can be produced through sputtering techniques or ink-jet printing of discrete arrays on a conductive support. Analysis of electrochemical efficiency can be made on the basis of parallel measurements of traditional current-voltage relationships11-12 or, alternatively, via assessment of reactivity through the direct or indirect measurement of electrocatalysis reaction products. Scanning

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electrochemical microscopy (SECM) has been shown to be particularly powerful for rapid screening of electrocatalytic currents from arrays covering a wide range of catalyst compositions.13-16 High throughput optical methods also offer considerable appeal due to their relative simplicity. For example, some of the first combinatorial approaches for fuel-cell catalyst screening were based on the fluorescence activation of a reporter molecule caused by electrogenerated pH changes.8, 10, 17-19 Disadvantages of fluorescence based optical techniques include the possible perturbing influence of the fluorophore itself and the restricted pH range where the signal transduction is operative. Alternative detection methodologies that provide more robust and simple screening such as bubble formation20 represent attractive approaches but have limited chemical selectivity. High-throughput infrared (IR) spectroscopy offers excellent chemical characterization of heterogeneous catalysts21 that produce or consume species other than homonuclear diatomic molecules.22 Spatially resolved IR assessment of catalysts can be achieved via IR microspectroscopy23 which has been used for screening supported organic reactions,24-27 mapping composition gradients in polymer blends,28 and evaluating methanol oxidation on individually addressable Pt microelectrodes.29 Utilization of infrared radiation sourced from a synchrotron provides greatly enhanced spatial resolution limits30 and has been used, for example, to assess the activity of zeolite single crystals.31-32 The excellent spatial resolution of synchrotron infrared radiation (SIR) microspectroscopy should allow for the mapping of local activity along a single electrode surface. The advantage of using a variable composition sample is that a single surface provides an extensive combinatorial library as recently illustrated by Loget and Schmuki33 who probed hydrogen production as a function of TiO2 nanotube dimension. In the present study we report the use of SIR microspectroscopy to map methanol oxidation activity across a monolithic Pt1-x Ni x alloy with a defined compositional gradient along one direction. Our results demonstrate a proof-of-concept and qualitatively

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support previous evaluations of the optimal loading of base transition metal in binary Pt alloys.

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Experimental

Alloy Fabrication and Characterization Full details of the fabrication of the Pt1-x Ni x alloy are provided in the Supporting Information and are shown pictorially in Figure 1. Briefly, a nickel layer was electrochemically deposited on one face of a Pt-foil. The resulting sample was annealed in a reducing atmosphere. Inter-diffusion of the two metals results in a binary alloy with the transition metal atomic concentration, x, varying along the length of the sample. The atomic composition of the alloys were determined before and after the SIR experiments using wavelength dispersive X-ray spectroscopy (WDS) analysis at the Saskatchewan Research Council and Auger electron spectroscopy (AES) at NanoFab Alberta (full details on quantification methods are provided in the Supporting Information). Leaching of the base metal is a major concern when using Pt1-x Ni x alloys in comparison to more stable binary alloy of Pt and Ru. However, it is very difficult to electrodeposit smooth Ru layers beyond several microns in thickness34 and our Ru electroplating efforts resulted in thin, highly defected films unsuitable for producing uniform concentration gradients upon annealing. After cell assembly (see below) the potential of the monolithic Pt1-x Ni x alloy was cycled for several hours in 0.1 M H2SO4 with 1.0 M CH3OH until a stable CV was observed. The alloy was then re-characterized by WDS and AES after the SIR measurements were performed. Cell Construction and SIR Measurements Modifications of previous SIR spectroelectrochemical cells35 were made to allow SIR measurements in transflectance mode. In brief, the cell was constructed from a poly(vinyl)chloride (PVC) base with an embedded 0.5 mm ring-shaped Au-wire serving as

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the counter electrode (CE), the PtNi alloy serving as the working electrode and a 0.3 mm diameter Pt wire providing a pseudo-reference electrode (RE) (Figure 1). For greater potential stability, the Pt pseudo-reference electrode was capacitively coupled to an Ag/AgCl reference electrode in the effluent stream via a 50 µF capacitor. The cell was assembled by compressing a CaF2 (25 mm diameter, 1 mm thickness, Crystan, UK) window and a nominally 25 µm thick PFTE spacer onto the polished face of the PVC to create a thin cavity for the electrolyte. An inlet and outlet allows for automated injection of electrolyte via a TTL-signal controlled syringe pump. All SIR measurements were made at the 01B1 Mid-IR beamline at the Canadian Light Source (CLS) and full details are provided in the Supporting Information.

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Results and Discussion To demonstrate the applicability of SIR mapping for screening eletrocatalyst materials an

alloy with a well-defined compositional gradient was prepared. The Pt1-x Ni x system is known to form face-centred cubic solutions over the whole range of compositions and was chosen for these studies due to its wide applicability as an electrocatalyst.36 As described in the Experimental section, a monolithic sample of variable composition was produced by alloying a Ni film that had been electrodeposited on one face of a Pt foil. The extent of intermetallic diffusion37 was manipulated by controlling the thickness of the electrodeposited Ni film as well as the duration of the annealing (see Figure S1). The bulk versus surface distribution of the metals in Pt1-x Ni x alloy catalysts has been the subject of many, often conflicting, studies3846

and can be complicated by the fact that surface segregation in binary metal nanoparticles is

likely considerably different from that in extended alloys.47 For example, surfaces of clean, annealed Pt 0.78 Ni 0.22 single-crystal and polycrystalline alloys have been reported to be pure Pt with enriched Ni in the second layer and have been termed “Pt-skin” structures.48-49 However,

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excellent correlation between the initial bulk and initial surface composition can be achieved either by very mild sputtering45 or by avoiding thermal treatment.12, 50 Furthermore, although almost all studies report that the amount of transition metal at the alloy surface is lowered after exposure to acid solutions or potential cycling, there is uncertainty about alloy stability and surface composition in operating environments.51 In order to help interpret the results of the SIR mapping of methanol oxidation on our monolithic alloy it was necessary to characterize its composition before and after the SIR measurements. Bulk and Surface Characterization of Pt1-x Nix Alloy Figure 2 shows the atomic composition gradient resulting from annealing a nominally 70 µm Ni film for 96 hours as obtained from both AES and wavelength dispersive X-ray spectroscopy. WDS measurements were made at different positions along the direction perpendicular to the gradient and provided the same elemental profiles over several millimeters thus confirming that the gradient is constrained to one direction. The twodimensional image of the alloy surface shown in Figure S2 confirms the uniformity of the gradient and provides no evidence of the formation of pure metal grains within the region of intermetallic mixing. The escape depth of Auger electrons is only several nanometers whereas X-ray emission from WDS extends to depths of several hundreds of nanometers. Although the AES signal is perturbed by the emission from several sublayers, it is much more surface sensitive compared to the WDS signal and provides a semi-quantitative measure of the surface composition. Figure 2a shows that before exposing the alloy to the electrochemical environment the surface and bulk composition are in very good agreement and there is no evidence of surface segregation. The smooth WDS profile is exactly as expected for an alloy formed by metallic interdiffusion37 and eliminates the possibility that grains of pure metals are formed during the annealing process. Any such grains would need to have volumes much smaller than the WDS beam probe (~1.5 µm diameter × ~ 0.7 µm

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depth of penetration) to mimic the profile shown in Figure 2a. The absence of surface segregation can be explained by the sample preparation employed whereby the alloy was mechanically polished prior to characterization thus removing the outer layers present after thermal annealing. After cycling the alloy in 0.1 M H2SO4 plus 1.0 M CH3OH for one hour and then applying the SIR measurement potential step sequence described below for three hours, the sample was removed from the electrolyte and re-characterized. WDS data reveal that the bulk alloy composition remains unchanged but AES shows extensive, near-surface, de-alloying. The dotted line in Figure 2b represents 50% removal of the original surface Ni concentration and it can be seen that when the original surface concentration of base metal was below ~60%, this is the extent of Ni loss. This result is in excellent agreement with work reported by Bonakdarpour et al on thin films.52 Higher original Ni concentrations see a much more precipitous loss and are extensively de-alloyed at the surface after electrochemical treatment. This is illustrated in Figure 2b by the last two data points near 200 µm length where the Ni concentration has been lowered from nearly 60% to less than 10%. The same effect was reported by Bonakdarpour et al52 and implies that Ni requires a statistically large number of Pt nearest neighbours in order to remain chemically resistant in corrosive electrolyte. We also note that because several atomic layers contribute to the AES signal, it is possible that positions along the alloy with the highest original Ni compositions actually result in a Pt-skin structure even though low levels of Ni are detected. Electrochemistry of Monolithic PtNi Alloy Electrochemical characterization of the alloy in a large volume electrochemical cell and de-aereated 0.1 M H2SO4 with 1.0 M CH3OH shows a typical voltammetric response for methanol oxidation. Although the electrochemical response represents an overall current for the monolithic alloy, cyclic voltammetry provides a diagnosis of the system stability and can be used to characterize the average performance with respect to the MOR. After cycling the

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electrode between -0.25 V < E < 1.00 V for 30-45 min, the cyclic voltammogram (CV) of the Pt-Ni alloy provides a steady response as shown in Figure 3. Reversible hydrogen adsorption at -0.25 V < E < -0.10 V and the onset of hydrogen evolution at ca. -0.25 V arise from Pt at the alloy surface and remain invariant with scanning after the initial electrochemical conditioning. The onset of methanol oxidation in the anodic scan occurs at ~0.25 V and maximum catalytic activity is seen at ~0.70 V. When the alloy was assembled in the thincavity spectroelectrochemical cell, the voltammetric features are only slightly perturbed due to mass-transfer effects and the presence of dissolved oxygen (see Figure S5). On the basis of the stable CV, a potential step sequence was developed whereby the electrode potential was originally held at an inert reference potential during the injection of fresh aliquots of electrolyte into the SIR spectroelectrochemical cell and subsequently stepped to a sample potential that supports the methanol oxidation reaction (MOR). To ensure the electrode was returned to its pre-step state the electrode was subjected to a series of potential pulses (see Supporting information for details) to remove any oxide or poisoning intermediates that may form on the electrode.53 Spatial mapping IR measurements require the electrochemical behavior of the alloy to remain constant over extended periods in order to increase S/N via the co-addition of interferograms. The current-time traces during the step from the reference potential to the sample potential were recorded throughout the SIR measurements and indicate excellent electrochemical stability and reproducibility (Figure S6). Numerical Simulations As will be described below, the interpretation of the IR results is predicated on the assumption that the dissolved CO2 detected in the electrolyte originates solely from the electrode area illuminated by the IR beam. This requires that lateral diffusion across the electrode is negligible in comparison to diffusion normal to the electrode surface. Rosendahl and co-workers54 fully characterized the diffusion problem in thin-layer SIR cells and

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demonstrated that radial diffusion effects can largely be ignored. This is somewhat intuitive as the concentration gradient is steepest in the direction normal to the electrode surface and almost negligible along the electrode surface. However, such an analysis applies for an electrode with a single valued heterogeneous rate constant such that, ignoring edge effects,55 the rate of electrolysis is uniform across the electrode surface. The electrode in the present study has been deliberately engineered to have a variable heterogeneous rate constant and the resulting concentration gradients parallel to the surface cannot be ignored. In simple terms some of the CO2 produced on a “hotter’ spot on the electrode will diffuse into the volume directly above an adjacent spot with lower MOR activity. In an effort to illustrate this effect numerical simulations of CO2 diffusion in the thincavity were performed following the general procedure described in a recent report56 and adapted as per the Supporting Information. The intrinsic level of electrochemical activity is defined by a reduced heterogeneous rate constant, K = kh D , where D is the CO2 diffusion coefficient and h is the thickness of the thin-cavity cell. The dotted line in the inset in Figure 4 shows a single step in K along a simulated electrode surface such that the rate of oxidation is twice as large on the more active region. The magnitudes of K have been chosen on the basis of reported values of D (~2 x 10-5 cm2 s-1)57 and the standard heterogeneous rate constant for MOR on platinum.58 The evolution of the CO2 concentration as a function of reduced time, T = tD h 2 , was then numerically simulated using the method of finitedifferences in time and finite elements in space to solve the pertinent diffusion equation (see Supporting Information). The resulting concentration profiles were integrated over appropriate spatial domains to provide a simulated IR response. The results for three positions (I, II and III) along the electrode are plotted as solid lines in the main body of Figure 4 whereas the dotted lines labelled K1 and K2 provide the expected CO2 concentration profiles in the absence of diffusional broadening. Position I is chosen to be adjacent to the

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electrode edge in order to illustrate that edge-effects provide a substantial and negative deviation from the true slope. This is somewhat intuitive as the absence of MOR beyond the edge of the electrode provides a steep concentration gradient that drives a rapid depopulation of CO2 molecules above the electrode edge. This effect is increasingly attenuated nearer the electrode centre. The effects of diffusion overlap near the K step can be seen by analysis of curves II and III. At longer times the CO2 transients diverge from their expected linear dependencies with the higher activity position showing a negative deviation and vice versa for the lower activity simulated data due to diffusional overlap. The extent of this error, however, is rather small and can be illustrated by superimposing the slopes of the simulated data on the true rate constant profile as shown in the inset of Figure 4. Although the measured reactivity near the step edge is “blurred” the simulated data demonstrate that the IR method can qualitatively capture regions of higher electrochemical activity. It is also important to note that the deviations only become pronounced at long times due to the fact that the diffusion of CO2 progresses almost exclusively in the direction normal to the electrode at short times. Lateral diffusion only becomes problematic once the diffusion front transcends the total thickness of the thin-cavity. The effects of diffusional broadening should be minimal as long as the rate constants are extracted from fitting experimental CO2 transients at short times. SIR Mapping of the MOR It has previously been shown that the high brilliance of SIR microscopy provides neardiffraction limited spatial resolution allowing for the detection of electrochemically produced or consumed species emanating from electrodes on the dimension of tens of microns.54, 56, 59 The high spatial resolution of SIR microspectroscopy can be used in the present application to determine the rate of MOR as a function of x in the Pt1-x Ni x alloy. This requires the timeresolved measurement of CO2 (aq) at different positions along the alloy’s concentration

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gradient. The rectangular apertures of the IR microscope were narrowed to 20 µm × 80 µm with the longer dimension perpendicular to the concentration gradient. As WDS measurements indicate that there is very little heterogeneity in the alloy composition in this direction, the measured IR response is the average signal in a 20 µm interval along the gradient. Figure 5 provides a time-resolved progression of the infrared spectra following a potential step from the rest potential (E = +0.0 V) to E = +0.60 V. An absorption peak at 2343 cm-1 is seen to increase with time and can be associated with the asymmetric stretching mode of aqueous CO2 which is shifted to lower wavenumbers compared to CO2 vapor.60-61 The line-shape of this absorption is distinctly symmetric providing further evidence that it originates from dissolved carbon dioxide rather than atmospheric CO2. The inset in Figure 5 shows a representative time-resolved progression of CO2 absorbance for one position along the PtNi gradient. At short times the absorbance varies linearly with time although deviations from linearity are often apparent at longer times due to overlapping diffusion zones (vide supra). At low overpotentials the potential-dependent heterogeneous rate constant determines the observed rate of the methanol oxidation reaction, υMOR . Kinetic control is confirmed in Figure 6 where the relative rate of the CO2 absorbance change is plotted as a function of potential for SIR focused on a representative position along the alloy. The data closely fit the expected exponential Tafel relationship between reaction rate and overpotential confirming that the oxidation is not limited by mass transport of methanol to the electrode surface. To explore the influence of the alloy composition on the rate of MOR, CO2 (aq) absorption transients were measured at different positions along the catalyst surface. Relative values of υMOR were obtained by linear fitting of the IR absorption peak intensity as a function of time and dividing the resulting slopes by the value obtained for the position corresponding to 0% Ni in the as-prepared alloy. Ideally, the electrochemical surface area (ECSA) of each illuminated spot would be determined so that the rates of MOR could be properly normalized

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by area. Area determination is often accomplished by measurement of the charge associated with hydrogen underpotential deposition but this is not feasible on the monolithic alloy. Hence no effort was made to correct the spatially resolved data for effective electrochemical area. The measured activity as a function of position along the gradient is plotted in Figure 7 along with the final atomic percentage of (near) surface Ni as determined by AES. The results indicate that the MOR reaction rate is highest for the most heavily de-alloyed part of the Pt1-x Ni x electrode which corresponds approximately to x ≈ 0.05. On the basis of combinatorial electrochemistry studies the optimal loading of ruthenium in Pt1-x Ru x alloys for MOR is about 10%12, 62 and has been rationalized as providing the highest probability of three Pt surface atoms adjacent to a single base metal atom.63 Similarly, Mathiyarasu et al64 reported that Pt 92 Ni8 provided maximum catalytic activity for MOR which is largely consistent with the data shown herein. Nevertheless aspects of Figure 7 warrant more indepth consideration. The final Ni composition profile is approximately bell-shaped whereas the MOR activity plot increases monotonically. This implies that the same atomic composition can give two different activities, for example, the reaction rate is a factor of six higher at 200 µm compared to 75 µm even though both positions have approximately 5% surface Ni. It is possible that the difference in MOR activity is due to different distribution of the atoms within these regions such as the formation of a Pt-skin from the heavily de-alloyed region. It is also notable that over an approximate 50 µm window centred at 150 µm υ MOR varies by a factor of two even though the final surface Ni composition remains essentially constant at 20 ±2 % on this section of the alloy. Qualitative roughness measurements (see below) show that a differential increase in the ECSA, caused by greater Ni leaching from higher Ni loadings in the original metal, is partly responsible for the increased MOR activity. Ex situ AFM measurements are largely non-diagnostic for determining the extent the surface roughens as they are dominated by the effects of the mechanical polishing procedure.

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Consequently, a surrogate in situ measurement was developed based on the integrated intensity of reflected infrared radiation between 4000 cm-1 and 5000 cm-1. This high frequency region was chosen as it is devoid of major adsorption from any species in the electrolyte and because scattering from relatively shorter wavelengths is most sensitive to small length scale roughness.65 The reflected intensity was measured as a function of position along the Pt1-x Ni x alloy before and after extensive potential cycling. The results (see Supporting Information) show a decrease in IR signal strength across the entire surface with a definite bias toward the edge which initially had the highest Ni content. The trend shown in Figure S9 closely follows the kinetic data for the MOR activity (Figure 7). Although the maximum change in reflected intensity is less than 10% it is important to note that light scattering is strongly dependent on the wavelength, size, shape, and distribution of surface features and hence it is difficult to use the results to provide a quantitative measurement of ECSA changes. Nevertheless, the results of this simple in situ light scattering experiment provide qualitative evidence that surface roughening is likely a contributing factor for enhanced MOR activity on de-alloyed sections of the anode.

4.

Conclusions This work has demonstrated the use of spatially-resolved IR microspectroscopy for

evaluating electrocatalytic performance. A monolithic Pt1-x Ni x alloy was engineered with a variable composition profile in one direction such that a continuum of Ni concentrations were accessible on a single anode. Characterization of the alloy shows that below an initial Ni concentration threshold, nearly half of the original base metal atoms on the surface are leached after prolonged cycling in strongly acidic electrolyte. Methanol oxidation activity was assessed by measuring spatially-resolved IR signatures of electrochemically generated CO2 produced at different rates along the surface. Numerical simulations have demonstrated that the effects of diffusional broadening can be sufficiently mitigated by restricting the

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measurement to relatively short times making a qualitative assessment of relative rates possible. The results have shown that the greatest rates of methanol oxidation activity occur when the final Ni surface composition is ~ 5%. There is some evidence that surface roughening caused by the de-alloying process may be at least partially responsible for the apparently high activity on these regions of the monolithic alloy. This proof-of-concept study lays the framework for future uses of SIR-sourced IR mapping of electrocatalytic materials. The alloying method used herein removes the problem of the inverse relationship between number of samples in the combinatorial library and compositional resolution. Furthermore, the method is amenable to the introduction of additional metals for characterizing high order alloys. The high spatial resolution afforded by synchrotron IR light makes it possible for the entire breadth of the gradient to be confined to short distances thus diminishing the overall size of the electrode and reducing the electrode response time. We are currently investigating approaches to decrease the cavity thickness in our spectroelectrochemical cell. Working in reflection mode means that IR radiation needs to pass through ~ 50 µm of water-based electrolyte which greatly diminishes sensitivity due to water absorption. This renders our current design very insensitive to the formation of formic acid which is a known product of the MOR. We are also currently modifying our method in an effort to further mitigate diffusional broadening which should allow quantitative measurements of catalytic activity. Even in its existing form, the method of IR mapping of anodes and cathodes under load could be of significant value for analyzing current distributions across commercial electrocatalyst materials where particle size and density are often heterogeneously distributed.

Acknowledgements This work was funded by a grant from the Natural Science and Engineering Research Council (NSERC) of Canada. Research described in this paper was performed at the 01B1-1 (mid-IR) beamline at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of 14


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Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research.

Supporting Information Available Details concerning the production and characterization of the binary metal alloy, spectroelectrochemical cell design, spectroelectrochemistry in the thin-cavity IR cell, instrument interfacing, numerical simulations and electrode roughening from the de-alloying process. This material is available free of charge via the Internet at http://pubs.acs.org/

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Figures and Tables Figure 1 : Alloy preparation and cell design for reflectance mode IR spectroelectrochemistry cell with a monolithic PtNi alloy of variable atomic composition serving as the working electrode.

Figure 2 : Heterogeneous PtNi alloy composition gradient measured a) before and b) after initial treatment in 0.1 M H2SO4 and 1.0 M CH3OH. WDS determined Ni (―) and Pt (―) bulk concentration as well as AES determined Ni (□) and Pt (●) surface concentrations. Dashed line in b) indicates 50% loss of Ni based on the initial WDS profile.

Figure 3 : Cyclic voltammogram of PtNi alloy in 0.1 M H2SO4 and 1.0 M CH3OH in conventional electrochemical cell. Sweep rate was 100 mV/s.

Figure 4 : Simulated IR absorption response of CO2 (IRsim) generated from a an electrode with two regions of MOR activity. Evolution of CO2 signal with time is shown for areas I (―), II (―) and III (―) on the simulated electrode as indicated in the inset. Dashed lines are the ideal (no lateral diffusion) IRsim transients for -5 > x ≥ 0 (blue) and 0 > x > 5 (red). Inset shows calculated rates Kfit (■) based on linear fitting of simulated IR responses for all positions on the electrode. Dashed line indicates actual K values. Figure 5 : Time resolved IR absorbance change of CO2 (peak maximum at 2343 cm-1) after inducing methanol oxidation reaction by stepping the potential to +0.6 V. a) spectral region of CO2 at a surface area with Ni content of ~ 33% Ni. b) Time resolved progression of CO2 on ~ 33% Ni (■) with CO2 progression (―) based on linear fitting the initial 3 seconds of data.

Figure 6 : Relative change of CO2 IR absorbance with time υ (■) measured at different potentials and corresponding exponential fit (―).

Figure 7 : Rates of MOR / . (○) measured at different position along the PtNi gradient, normalized to the rate of MOR on pure platinum. Horizontal error bars represent the SIR beam spot dimension, limited by a 20 × 80 µm blade aperture. Overlaid is the corresponding surface nickel content (■) as determined by AES.

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