Voltammetric and Scanning Electrochemical Microscopy

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Voltammetric and Scanning Electrochemical Microscopy Investigations of the Hydrogen Evolution Reaction in Acid at Nanostructured Ensembles of Ultramicroelectrode Dimensions: Theory and Experiment Jose L Fernandez, and Cynthia G. Zoski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08976 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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The Journal of Physical Chemistry

Voltammetric and Scanning Electrochemical Microscopy Investigations of the Hydrogen Evolution Reaction in Acid at Nanostructured Ensembles of Ultramicroelectrode Dimensions: Theory and Experiment

José L. Fernándeza,† and Cynthia G. Zoskia,‡,* a

Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003

Present Address and Corresponding Author ‡,* C.G. Zoski; Telephone: (512) 475-9698; Email: [email protected] ‡

Center for Electrochemistry, Department of Chemistry, The University of Texas at Austin, Texas 78712, United States

Present Address † Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, (3000) Santa Fe (Santa Fe) Argentina. E-mail: [email protected].

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ABSTRACT Mechanistic studies of the hydrogen evolution reaction (HER) on nanoelectrode ensembles of ultramicroelectrode dimensions (UME-NEEs) are demonstrated using addressable nanoelectrode membrane arrays (ANEMAs). A kinetic model considering simultaneous occurrence of all elementary steps and the effects of mass transport to/from the UME-NEE surface was developed and used to interpret HER steady-state voltammograms (SSVs). The analysis accounts for the effect of fraction of covered area, f, and roughness factor, fR, of the nanoelectrodes on HER SSVs and illustrates the roles of these parameters in mechanistic studies of electrocatalyzed multi-step reactions on UMENEEs. The behavior of bare Au UME-NEEs was studied in acid solution and characterization of the HER on these electrodes was performed by SSV and scanning electrochemical microscopy (SECM) in the substrate generation-tip collection mode (SGTC). The activity of Au NEEs for the HER was found to be similar to that of macrosized polycrystalline Au. Selective electrodeposition of Pt NPs was performed on Au UME-NEEs, and SSV was used to study the kinetic mechanism of the HER in acid on the Pt-modified UME-NEEs with different f values. UME-NEEs with excess Pt showed HER activity equivalent to polycrystalline Pt while those with small, almost undetectable, amounts of Pt showed HER behavior that could not be explained as the addition of pure Au and pure Pt contributions. SECM activity screening on Pt-modified ANEMAs detected changes in the apparent electrocatalytic activity for the HER, in agreement with corresponding SSVs.

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INTRODUCTION Random arrays or ensembles of nanoelectrodes (NEEs) with different geometries such as disk nanoelectrodes (NEs) and nanoparticles (NPs) are of fundamental and technological importance in several physicochemical areas including sensing technologies,1,2 vibrational spectroscopies,3 electroanalysis,4 fundamental electrochemistry,5 and electrocatalysis6. Specifically, NEEs are being investigated as electrocatalysts in renewable energy devices 7 due to the need to limit the quantity of noble metals while still supporting facile mass and charge transport. 8 The NEE electroactive area represents a small fraction of the total exposed geometric electrode area which gives rise to current behavior that depends critically on the timescale of the electrochemical experiment, and on the NE radius and spacing. Thus for example, macro-NEEs largely operate either in the transient regime by diffusion only or in the steady-state regime under convective or microelectrode radial diffusion conditions.9-13 We recently reported the development of NEEs of ultramicroelectrode (UME) dimensions (UME-NEEs) which are fabricated by securing a Au-filled polycarbonate membrane (PCM) to a microfabricated addressable array patterned with regularly spaced individually addressable Au UME electrodes each of 10 µm diameter.14, 15,16 Microregions of the Au-filled polycarbonate membrane are electrically contacted through these underlying UMEs resulting in a platform of UME-NEEs, which are spaced far enough apart to prevent cross talk between neighboring UME-NEEs. The size and random placement of the disk-shaped NEs are determined by the polycarbonate membrane template and range in sizes of radius 5 nm ≤ RNE ≤ 100 nm and fractions of inactive (or blocked) area (θb = inactive area/geometric area) in the range 0.99 veH/veT > 10-4, the transition between routes shows up as a shoulder in the SSV, as in SSVs (d) and (e). If both equilibrium rates are comparable (i.e., veH/veT ≥ 0.1), then both routes operate at similar rates and there is no a clear transition from one to the other, as in SSVs (f) and (g).

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To explore the expected HER behavior of Au, a metal much less active than Pt, SSVs were calculated using veV = 5×10-10 mol cm-2s-1, veH = 7×10-15 mol cm-2s-1, veT = 6×10-9 mol cm-2s-1 and θe = 0.05, which are equilibrium rates and H(ad) coverage values on the order of those previously reported for this metal.40 However, the kinetic parameters of the HER on Au reported in the literature vary over a range spanning more than three orders of magnitude. This is caused by the strong sensitivity of these parameters to the experimental history of the Au electrode surface and adsorption of impurities and cations.50,51,52 These studies show that low activity is caused by the poor ability of Au to chemisorb H(ad), which results in very low H(ad) coverage.53 SSVs calculated with these previously listed kinetic parameters are shown in Figure 2(B) as solid lines. The SSVs simulated with these parameters are shifted toward more negative potentials with respect to those previously shown for Pt, and the current plateau that is caused by the Tafel route in this case is clearly detected on UME-NEEs with f values in the range 0.7 ≥ f > 0.01. The SSVs that result from eq. (12) are included as dashed lines in order to show that under certain conditions of f (< 0.01) and η (< 0.8 V) values, this equation is a valid approximation for the HER.

Voltammetric study of the HER on Au UME-NEEs. The electrochemical activity for the HER on Au UME-NEEs was evaluated in deareated 10 mM H2SO4/0.1 M Na2SO4 solution. The cyclic voltammograms (CVs) of the 25 Au UME-NEEs were first recorded individually using a multipotentiostat in the potential range -0.3 V ≤ E vs. Ag/AgCl ≤ 1.7 V and are shown in Figure 3a. These CVs show behavior typical of Au electrodes in acidic solution. Formation of an oxide layer on the Au NEs occurs at potentials more positive than 0.9 V. Evolution of O2 is observed at potentials more positive than 1.4 V.

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The sharp peak at 0.8 V corresponds to the complete electro-reduction of the Au oxide formed during the anodic scan. Electroreduction of O2 to hydrogen peroxide from dissolved air that was not completely evacuated by Ar purging, is detected at potentials more negative than 0.1 V producing currents no larger than 0.5 nA. These CVs show capacitive currents that are disproportionally larger than the currents usually observed for the electrochemical double layer charging on regular Au electrodes. The capacitive current that is added to a UME-NEE electrochemical response is generated by the electrical capacitance that is intrinsic to the lithographic platform components such as the thin Pt tracks.15 The AEA values of each UME-NEE were estimated from the voltammetric charge for the reduction of a monolayer of electroformed gold oxide.54 Different tested ANEMAs showed NEE AEA values that varied over different ranges, between the smallest (i.e., 4 ± 1 ×10 -7 cm2) and largest (i.e., 17 ±1 ×10-7cm2) values. For the ANEMA analyzed here, these values (tabulated in Table S1) varied in the range 1.4×10-6 – 2.8×10-6 cm2. These are surprisingly large values, even larger than the hypothetical geometric area of the underlying UMEs (AGEO = πa2 = 7.9×10-7 cm2) and suggest high porosity of the electroless-grown gold deposits that fill the membrane pores. SSVs for the HER in acid obtained by CV on individual UME-NEEs are shown in Figure 3b. Well-defined SSVs were obtained by scanning the UME-NEE potentials between -0.2 V and -1.4 V vs. Ag/AgCl. At more negative potentials (i.e., E < -1.5 V) the discharge of water to H2 through reaction (5) becomes significant. The limiting currents for H+ reduction on the UME-NEEs varied between 130 nA and 230 nA. Variations in iL,UME-NEE values observed among the UME-NEEs are related to changes in the effective UME-NEE radii due to variations in f caused by a non-uniform distribution

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of nanopores in the membrane.16 Assuming that these UME-NEEs have disk shapes, iL,UME-NEE measured values (i.e., 130 nA ≤ iL,UME-NEE ≤ 230 nA) correspond to UMENEEs with effective diameters ranging between 5 and 9 µm. Using the value for the membrane pore radius of 15 nm as a mean NE radius, RNE = 15 nm, the f value of each UME-NEE can be estimated from eq. (6). These values and the previously calculated electroactive areas lead to the calculation of fR for each UME-NEE. Both f and fR are tabulated in Table S1, which shows that while f varies in the range 0.0027 ≤ f ≤ 0.015, fR fluctuates in the range 139 < fR < 804 (except for one UME-NEE where the Au oxide reduction peak was ill-defined, thus leading to an erroneous AEA value and fR = 1420). Such large fR values suggest that the NE surface is porous, and that the solution fills the pores to a depth of several nanometers (which is different on each NE) until surface tension stops the infiltration. Thus, while such small f values cause a significant increase of mass transport rates on these UME-NEEs, the large fR values compromise this beneficial effect and brings the apparent response of these UME-NEEs close to that of a continuous disk UME. Such behavior has also been reported in so-called nanoconfinement experiments related to electrode porosity.55,56 The SSVs shown in Figure 3b were correlated using eqs.(7)-(9) with a single set of kinetic parameters. In these calculations, α = 0.5 was used, the reference equilibrium potential was Ee = -0.297 V vs. Ag/AgCl (calculated with the Nernst equation with CH+ = 20 mM) and a disk geometry was assumed for the UME-NEEs. The SSVs that resulted from these correlations using the same kinetic parameters are shown as open symbols in each graph of Figure 3b. The calculated and experimental SSVs agreed very well, using the following parameters (vei in mol cm-2s-1): veV = (1.0 ± 0.1)×10-10, veH < 1×10-14, veT =

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(5.0±0.2)×10-9, θe = 0.010± 0.005, which are close to those previously reported for the HER on polycrystalline Au UMEs measured under similar conditions,40 and lead to an exchange current density j0 = 1 × 10-5 A cm-2 calculated using eq. (14)37and in comparison to j0 = 1 × 10-5 A cm-2 for polycrystalline Au UMEs.

 vVe vHe + vVe vTe + vHe vTe  j = 2F   e e e  vV + vH + 2vT  0

(14)

Due to the large fR values, the SSV responses were almost insensitive to the value of veH. Additionally, even though the UME-NEEs have different f and fR values, as long as they can be characterized, their SSVs can be correlated with the model using exactly the same kinetic parameters.

SECM imaging of the HER on Au ANEMAs. Substrate generation-tip collection SECM activity images for the HER were measured on Au ANEMAs polarized at potentials more negative than -0.8 V vs. Ag/AgCl. A 10-µm diameter Pt tip at ET = 0.2 V vs. Ag/AgCl was used to amperometrically detect dissolved H2 evolved at and diffusing from the UME-NEE surfaces (Schematic 1).21 At this tip potential, H2 is oxidized to H+ at a mass-transport controlled reaction rate, so that the tip current is a direct measure of the local H2 concentration. Because the tip-substrate distances during these images are relatively large (i.e., d = 20 – 30 µm), H+ feedback between the tip and substrate is insignificant. A typical HER activity image of an ANEMA at -0.9 V is shown in Figure S3. This image corresponds to the ANEMA of Figure 3 that was previously evaluated by voltammetry using the multipotentiostat. The H2 concentration profiles around the single UME-NEEs are clearly defined during the tip scan, indicating that the NEEs are

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uniformly distributed over each UME.16 However, along the entire ANEMA there are small changes in the maximum tip currents that are established over the center of each UME-NEE. These changes may be caused by a slight tilt of the sample on the SECM stage, which leads to tip-substrate distances over the top rows that are larger than those over the bottom rows. Under these tilt effects, no clear correlation could be established between the SECM activity image in Figure S3 and the SSVs, especially the limiting currents, in Figure 3b.

Modification of Au ANEMAs with electrodeposited Pt. ANEMAs are platforms that can function as nanostructured catalyst supports due to the array of UME-NEEs that can act as base substrates for metal NP electrodeposition. By restricting the nucleation site down to a few nanometers it is possible to electrochemically grow single NPs of selected metals that will be distributed following the random pattern of the base Au-filled membrane.16,57 With this modification, an array of 25 UMEs with NP ensembles (UMENPEs) can be obtained that can establish well-defined mass-transport conditions by hemispherical diffusion and enable the study of electrochemical reactions under wellcontrolled mass transport conditions on supported electrodeposited metal NPs. It should also be possible to control the size and crystalline structure of the deposited metal NPs, for example by judicious selection of electrodeposition time, potential, and waveform conditions. Modification of Au-filled ANEMAs was performed by potentiostatic electrodeposition of Pt from diluted chloroplatinic acid solutions on individual UME-NEEs. Pt deposition occurred on the Au NEs by application of a potential of -0.6 V vs. Ag/AgCl for 60 s. Current vs. time curves recorded on each UME-NEE during the deposition

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process are shown in Figure S4a. A continuous current decay is observed during this 60 s period on most of the UME-NEEs, which is typical of potentiostatic Pt electrodeposition at high overpotentials. On some of the UME-NEEs, there are current spikes that could be related to either electrical noise or H2-bubble generation. Simultaneous evolution of H2 is an undesirable process that occurs during the Pt deposition process. Hydrogen bubbles lead to porous deposits and may also act as a reducing agent in the vicinity of the UMENEEs causing difficulty in controlling the deposition conditions. This phenomenon was better observed when Pt deposition was attempted using potentials even more negative than -0.6 V. In these cases, precipitation of Pt black around the UME-NEEs was evident by optical microscopy. We concluded that better control for selective modification of ANEMAs with electrodeposited Pt NPs can only be reached when using less negative potentials where essentially no H2 evolution takes place and longer deposition times.

Voltammetry and SECM characterization of Pt-modified ANEMAs. The electrochemical behavior of Pt-modified UME-NEEs prepared as described was evaluated by CV in sulfuric acid solution. Figure S4b shows the CVs of the UME-NPEs in a potential range between -0.35 V and 1.4 V vs. Ag/AgCl. These CVs show some of the typical features that are observed upon scanning the potential of polycrystalline Pt electrodes in acid, including large oxygen and hydrogen evolution currents at potentials more positive than 1.2 V and more negative than -0.3 V, respectively, and a Pt oxidation plateau and oxide reduction peak at potentials more positive than 0.4 V and at 0.3 V, respectively. The under-potential deposited (UPD) atomic hydrogen electro-adsorption peaks that are commonly observed in polycrystalline Pt electrodes are not clearly defined in these nanostructured electrodes due to the small Pt areas which produce adsorption

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peak currents that are too small to be distinguished from the large capacitive currents on these platforms.58 These CVs indicate that Pt deposition occurs on the Au NEs using the designated potentiostatic electrodeposition conditions. However, some Au regions remain exposed to the electrolyte solution as shown by the presence of voltammetric peaks related to gold surface electrochemical processes, such as the reduction of Au oxide between 0.75 and 0.8 V. This behavior indicates that the electrodeposition of Pt on the porous surface of the Au NEs at high rates and short times does not cover the entire inner area of the electrodes. Once the dissolved Pt salt is exhausted inside the pores under quiescent conditions like those used in this work, it is not replenished by mass transport of dissolved Pt salt in the bulk solution unless the electrodeposition rate is very slow, and the electrodeposition most likely proceeds onto the outer NE area. The electroactive areas of electrodeposited Pt and of uncovered Au were calculated from the voltammetric charge of the oxide reduction peaks, and are indicated in Table S2 for each UME-NEE.54 For this Pt modified ANEMA, the uncovered Au areas varied over range 1.4×10-7 – 5.1×10-7 cm2 (i.e., compared to ∼8×10-7 cm2 expected for a uniform UME-NEE), and the electrodeposited Pt areas varied over the interval 6×10-7 – 9×10-7 cm2. This could be an indication that Pt mostly deposits on the NE surfaces and grows over them as NPs. The HER was analyzed on these Pt UME-NPEs by the SSVs shown in Figure 4a. The activity of the electrodes for this reaction is clearly superior to that of the bare Au UME-NEEs shown in Figure 3b. This is demonstrated by a positive shift of the HER voltammetric waves on Pt UME-NPEs by about 0.5 V with respect to those acquired on bare Au UME-NEEs. Over this potential range, the HER current comes exclusively from the Pt area. Oxygen reduction from dissolved air, which may proceed to hydrogen

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peroxide over this potential range,59 leads to a negligible (almost undetectable) reduction current, as compared to the HER current. With the exception of one NPE, the HER limiting currents measured on the Pt UME-NEEs were larger than the corresponding unmodified Au UME-NEEs. Assuming that the Pt NPs are spherical and that they keep the radius value of the NEEs (15 nm), the f values indicated in Table S2 were calculated from the measured iL,UME-NEE values of the SSVs shown in Figure 4a and from eq. (6). These values and the previously calculated electroactive areas lead to the calculation of fR values for each UME-NPE that are also indicated in Table S2. Figure 4a shows that f varies in the range 0.002 ≤ f ≤ 0.035 (except for one UMENPE where the iL,UME-NEE value was very large leading to an abnormally large f value), although more than 60% of these values lay in the interval between 0.005 and 0.011. Moreover, while fR fluctuates in the range 10 ≤ fR ≤ 140, more than 60% of the fR values are found in the interval between 15 and 40. The correlation of these SSVs with eqs. (7) (9) lead to the following kinetic parameters (vei in mol cm-2s-1): veV = (8.0 ± 0.2)×10-6, veH < 1×10-9 (poorly sensitive), veT = (1.0 ± 0.1)×10-6, θe = 0.15± 0.03. These parameters are quite similar (i.e., only slightly smaller) and follow the same trend as those previously reported for the HER on polycrystalline Pt UMEs.40 They lead to j0 = 0.15 A cm-2, close to values reported for this metal under similar conditions.37 The exchange current densities for the parallel Volmer-Tafel (j0VT) and Volmer-Heyrovsky (j0VH) routes (calculated from eq. (14) with veH = 0 and veT = 0, respectively) are j0VT = 0.15 A cm-2 and j0VH = 4 × 10-5 A cm-2. According to these parameters, the HER on these Pt UME-NPEs follows the Volmer-Tafel route (eqs. (2) and (3)) at less cathodic potentials, and quickly reaches proton mass-transport control.

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SECM activity images were obtained on the analyzed Pt NP-modified ANEMA for the HER in the SG-TC mode. One of these images is shown in Figure 4b. Imaging conditions were the same as those used in testing bare Au ANEMAs, although in this case the electrode was polarized at -0.5 V vs. Ag/AgCl. This potential is negative enough for reaction (1) to operate under mass-transport control on Pt. The observed differences in the H2 concentration profiles among spots are likely caused by changes in the effective sizes (or the f values) of the UME-NPEs, in correlation with the HER SSVs in Figure 4a.

HER kinetic analysis on Pt-modified ANEMAs with increasing Pt. The previous results were obtained on a Pt-modified ANEMA where Pt was electrodeposited using the same conditions on all spots. Here a group of five UME-NEEs in an ANEMA were modified by potentiostatic deposition of Pt using different potentials. In contrast to the Pt modified ANEMA analyzed in Figure 4, a long electrodeposition time of 400 s and less negative potential values, E > -0.5 V vs. Ag/AgCl, were chosen to avoid poor control due to simultaneous hydrogen evolution, as discussed previously. Figure S5 shows the current vs. time curves recorded on each Au UME-NEE during Pt electrodeposition from a diluted chloroplatinic acid solution. These curves show the long-time responses of the electrode current during electrodeposition of Pt, where current values increase as applied potentials become more cathodic.57 The CVs obtained on these Pt-modified UME-NPEs in non-deareated 0.5 M H2SO4 (Figure 5I) show a transformation from the CV corresponding to a Au-NEE (curve a) to that of a Pt UME-NPE (curve e). As the amount of Pt increases and blocks the exposed Au surface, the peaks due to oxidation of gold and reduction of gold oxide that are observed at 1.3 V and 1.0 V respectively, tend to decrease, with some variation due to the intrinsic variation of the NEE porosity among

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UME-NEEs. The resulting AEA values of Au calculated from the charges of these oxide reduction peaks are indicated in Figure 5I. Modification of these ANEMAs with Pt was performed so that the area of Pt on the fabricated UME-NPEs increased from almost undetectable values at less negative electrodeposition potentials to almost complete coverage of the NEE area and overgrowth of Pt NPs at more negative electrodeposition potentials. In contrast to the UME-NPEs shown in Figure S4 that were obtained using short deposition time (60 s) and very cathodic potential (i.e., -0.6 V), in this case the Au response could be totally blocked, as in electrode (e). Increasing amounts of Pt are demonstrated by the increase in the charge of Pt oxide formation/reduction peaks at 0.6 V and 0.4 V, respectively, and a clear change in the oxygen reduction wave. With the exception of electrode (e), the voltammetric peaks of Pt oxide reduction are not clear enough to use in estimating the Pt electroactive areas, although values not larger than 10-7 cm2 could be estimated after an arbitrary background subtraction. However, it is clear that there is a direct relationship between electrodeposition potential and amount of deposited Pt to a potential of -0.4 V. At potentials more negative than this value, -0.45 V, the amount of deposited Pt decreased with respect to the electrode prepared at -0.4 V. This was not an isolated case and the same situation was observed in other independent preparations. This result could be related to weak attachment of Pt particles deposited at high currents and long times onto the gold NEEs, which favors loss of Pt NPs during subsequent manipulation steps. The mechanistic calculations on Pt shown in Figure S1 indicate that it should be possible to observe kinetic features in the SSVs for the HER over a narrow range of f

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values. Experimental SSVs for the HER are shown in Figure 5II (solid black lines). All of these curves reached limiting currents, from which f was calculated using eq. (6). Disk-shaped NEs (χ = ξ = 1, RNE = 15 nm) were assumed with the exception of the electrode shown in curve (e), on which spherical NPs were assumed (χ = πln(2), ξ = 4, RNE = 15 nm). Additionally, fR of Au (and fR of Pt in (e)) were estimated from the measured electroactive areas. All measured magnitudes are indicated in Figure 5II. The HER SSVs measured on unmodified Au UME-NEEs of an ANEMA exposed to a Pt solution, such as that shown in Figure 5II curve (a), are quite different compared to those measured on untreated Au UME-NEEs as in Figure 3b. The apparent HER half wave potential of curve (a) is shifted approximately 0.1 V to more positive values. This observation suggests that there is some degree of spontaneous Pt deposition at the open circuit potential of the Au NEEs in the chloroplatinic acid solution. Thus, even though Pt was not deliberately electrodeposited on these electrodes, they contain a small amount of this metal undetectable on the CVs and respond to the HER following the expected behavior of a Au UME-NEE contaminated with traces of Pt.60,61 An apparent shift of about 0.3 V to more anodic values of the potential interval where the SSVs for the HER operate under mixed control is observed upon adding increasing amounts of Pt to the Au UME-NEEs. This shift is concomitant with the manifestation of a shoulder, which becomes more evident at intermediate amounts of Pt as in curves (c) and (d). The highest apparent activity was detected on the Pt UME-NPE that was prepared at -0.4 V (curve ( e)), which showed a HER SSV close to the reversible case with a peak appearing before the limiting current was reached.

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To compare the experimental SSVs of Figure 5II with the model, it is necessary to account for the simultaneous response of Au and Pt. Rigorous modeling of this system is complex and outside the scope of this work. However, we developed a simplified model to treat the data as described in SI-S8. The calculated SSVs resulting from this simplified model are included in Figure 5II as red, open symbols. In performing these calculations, the kinetic parameters on Au and Pt were kept constant in all SSVs (i.e., close to their values measured on the pure metals), and only the parameters φ, fR,Pt and veV,Au were allowed to vary freely. From fitting experimental data, φ , an adjustable parameter that is related to the fraction of geometric area covered by Pt, was found to have a very low value in the Pt-contaminated Au UMENEE and to increase as electrodeposition of Pt was deliberately carried out. The calculated fR,Pt values also increased over the range 2 ≤ fR,Pt ≤ 7, implying Pt electroactive areas in the range 10-8 cm2 < APt < 10-7 cm2, in agreement with the CVs shown in Figure 5I. Curves (a) and (b) represent situations where the SSVs are mostly influenced by the activity of Au, and the Pt fraction only barely affects the region of more anodic potentials. Curves (c), (d) and (f) are affected by the activity of Au and Pt in similar proportions, and curve (e) is exclusively affected by the behavior of deposited Pt. All of these SSVs were calculated with the same kinetic parameters, which are indicated in the Figure 5 legend and in Table S3, except for veV,Au which varied over nearly one order of magnitude. In that case, veV,Au on the Au UME-NEE with traces of Pt (i.e., (2.0 ± 0.1) × 10-9 mol s-1cm-2) was found to be more than one order of magnitude larger than that measured on the unmodified Au UME-NEEs (i.e., (1.0 ± 0.1) × 10-10 mol s-1 cm-2), which caused an increase of j0 from 1 × 10-5 A cm-2 to 2 × 10-4 A cm-2. Since the Pt

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contributions are accounted for in the model, this apparent increase of veV on Au is probably an artifact caused by the presence of regions with different activities of both Pt and Au, that are not treated in the model. Upon adding more Pt, the resulting value of veV,Au increased up to (2.0 ± 0.1) × 10-8 mol s-1cm-2 (i.e., j0 = 7 × 10-4 A cm-2) which could indicate an extension of this additional region. These results suggest that interpretation of the HER on this type of mixed system requires a model taking into account the contributions not only of the pure metals but also of an intermetallic region with differential behavior. Although the transient peak observed in the SSV of electrode (e) with 3D development of Pt NPs could not be fit with the present steady-state model, the values determined for vei and θePt were found to be similar to those in Figure 4a, leading to j0 = 0.17 A cm-2 in comparison to j0 = 0.15 A cm-2 found for the HER on the Pt UMENPEs of Figure 4a.

SECM activity screening of the HER on Pt-modified ANEMAs. The Pt-modified ANEMA used in the previous section was screened by SECM in the SG-TC mode in order to demonstrate the capabilities of SECM activity screening on these platforms in detecting the same differences in the apparent HER performance observed by voltammetry. In this case, the differences in the apparent activity are caused by the addition of Pt, which affects the SSVs for the HER as demonstrated in the previous analysis. An image of the entire array at -1.0 V vs. Ag/AgCl is shown in Figure S6. This array contained the six UME-NEEs of Figure 5 and the remaining electrodes were unmodified Au UME-NEEs, although as in electrode (a) they were contaminated with small amounts of Pt spontaneously deposited at open circuit potential. At -1.0 V, all 25 UME-NPEs showed appreciable activity for the HER and no significant differences were

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observed on the Pt-modified electrodes since all operate under mass-transport limiting conditions at this potential. The small observed differences in the intensity, as discussed previously, originate in small changes in f at each UME-NEE. As the ANEMA potential is made more positive (i.e. -0.5 V), UME-NPEs with spontaneous Pt show a low SECM response for the HER and the only detected evolved H2 comes from electrodes (c) to (f), where according to the previous treatment, the coverage and roughness factor of Pt is larger, φ > 0.25 and fR,Pt > 8. At more anodic potential values, E ≥ -0.4 V, only the activity of the UME-NPE with 3D deposition of Pt NPs, electrode (e), is detected, in agreement with the SSVs in Figure 5II. This analysis demonstrates that SECM screening of HER activity by the SG-TC mode on selectively modified ANEMAs rapidly provides good visualization of the extent of Pt modification of the UME-NEEs.

CONCLUSIONS We have demonstrated the use of UME-NEEs on ANEMAs for voltammetric and SECM studies of the HER in acid. Modification of Au ANEMAs with Pt NPs was performed selectively on individual UME-NEEs by electrodeposition. The porosity of the NEs resulted in a mixed nanostructured UME-NEE containing exposed Au and Pt under most conditions. The kinetics of the HER on nanostructured surfaces (Au and electrodeposited Pt) were studied by SSV. Hemispherical diffusion to the UME-NEEs (or to the UME-NPEs) resulted in a true steady-state under well-defined high mass-transport rates controlled by the low fraction of covered area of these UME-NEEs and the porosity of individual NEs. The SSVs were processed using a model for the HER operating on nanostructured microensembles of uniform electrocatalysts through the Volmer-Heyrovsky-Tafel mechanism.

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The kinetic elementary parameters of the HER on Au NEEs and on the overgrown-Pt NPEs with excess Pt were similar to those measured on macro-sized polycrystalline metals and led to j0 values of 1 × 10-5 (Au) and 0.15 (Pt) A cm-2, close to the values reported for these metals. On both metals, the Volmer-Tafel route predominates over the Volmer-Heyrovsky route (j0VT ≅ j0 >> j0VH). On mixed Au-Pt NEEs with small amounts of Pt, an approximate model that considered the simultaneous and independent occurrence of the HER on both metals was used to correlate the SSVs. The resulting kinetic parameters were similar to those obtained on the pure metals, except for the equilibrium rate of the Volmer step on Au, which increased as Pt content increased, leading to an increase of j0 of the HER on Au from 1 × 10-5 to 7 × 10-4 A cm-2. Although such an effect could be real, it could also be caused by the simplicity of our approximate model. For example, our coarse modeling of mass transport to the bi-component porous surface could result in this apparent increase of the calculated kinetic parameter. Another possibility, not included in the model, could be the existence of intermetallic regions where the kinetics are different. Differences in the apparent activity for the HER on selectively modified UME-NPEs were detected through SECM activity images. The apparent activity was affected by the intrinsic activity of the analyzed material and by the fraction of active area.

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SUPPORTING INFORMATION Experimental details including chemicals and materials, preparation of the Au-filled nanoporous membrane and of the ANEMAs, derivation of the limiting current for a 3D disk UME ensemble of spherical NEs, derivation of the steady-state current vs. potential dependence for the HER on UME-NEEs, derivation of the reversible steady-state i(η) dependence for the HER, calculation of additional HER SSVs on UME-NEEs, tabulation of results for Au UME-NEEs and Pt UNE-NPEs and kinetic parameters, additional results on UME-NEEs, and description of the simplified model used in analyzing UNENEEs containing both Pt and Au current contributions.

ACKNOWLEDGEMENT This work was funded by the National Science Foundation (CHE-1757127, CHE1408608, CHE-0809966, C.G.Z.) and a travel grant to J.L.F. by CONICET (Argentina).

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Schematic 1. Substrate generation-tip collection (SG-TC) SECM imaging of hydrogen evolution activity (HER).

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Figure 1. (A) SSVs for the HER in acid solution (C*H+ = 0.02 M) calculated using eqs. (7) to (9) on UME-NEEs (a = 5 µm). Kinetic parameters (vei in mol cm-2s-1): veV = 2×10-5, veH = 6×10-10, veT = 1×10-6, θe = 0.2, α = 0.5. (B) Same SSVs normalized with respect to iL,UME-NEE calculated with eq. (6) for χ = 1. (I) RNE = 15 nm, fR = 1 and f = 1.0 (a), 0.1 (b), 0.04 (c), 0.02 (d), 0.01 (e), 0.003 (f), 0.001 (g), 0.0005 (h). Dashed lines: eq. (11) (reversible). Dotted line: eq. (12), Volmer-Heyrovsky route at largeη. (II) f = 0.1 (solid lines) and 0.01 (dashed lines), and disk NEs with fR = 1 and RNE (nm) = 15 (a), 30 (b), 60 (c), 150 (d).

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Figure 2. SSVs for the HER on UME-NEEs (a = 5 µm) with disk NEs (RNE = 15 nm) in acid solution (C*H+ = 0.02 M) calculated using eqs. (7) to (9) (solid lines). (A) f = 0.001 and fR = 1.0 (a), 10 (b), 100 (c). Kinetic parameters (vei in mol cm-2s-1): veV = 2×10-5, veH = 6×10-10, veT = 1×10-6, θe = 0.2. Dashed line: eq. (11), reversible. (B) fR = 1 and f = 1.0 (a), 0.7 (b), 0.1 (c), 0.01 (d), 0.005 (e), 0.002 (f). Kinetic parameters (vei in mol cm-2s-1): veV = 5×10-10, veH = 7×10-15, veT = 6×10-9, θe = 0.05. Dashed lines: eq. (12), VolmerHeyrovsky route at high η.

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Figure 3. Electrochemical analysis of Au UME-NEEs in 10 mM H2SO4/0.1 M Na2SO4. (a) CVs over -0.3 V ≤ E ≤ 1.7 V at 100 mV/s from Ei = -0.3 V, scanning positive. (b) Experimental SSVs (solid lines) for the HER, -0.2 V ≥ E ≥ -1.4 V, at 100 mV/s from Ei = -0.2 V, scanning negative. Open symbols: theoretical curves from eqs.(7)-(9) and parameters (vei in mol cm-2s-1): veV = (1.0 ± 0.1) × 10-10, veH < 1 × 10-14, veT = (5.0 ± 0.2) × 10-9, θe = 0.010 ± 0.005. 34



Figure 4. Evaluation of the HER in 10 mM H2SO4/0.1 M Na2SO4on the Pt NP-modified ANEMA of Figure 3. (a) Experimental SSVs (solid black lines) for the HER on Pt UMENPEs in an ANEMA. Scan rate: 100 mV/s. Potential range: -0.2 V ≤ E ≤ -1.0 V. Open blue circles: theoretical curves calculated using eqs. (7) - (9) and the parameters (vei in mol cm-2s-1): veV = (8.0 ± 0.2) ×10-6, veH < 1×10-9, veT = (1.0 ± 0.1)×10-6, θe = 0.15 ± 0.03. (b) SG-TC image of HER activity on an ANEMA of Pt UME-NPEs, ET = 0.2V, ES = -0.5 V vs. Ag/AgCl, scan rate: 300 µm/s. Tip: Pt, a = 5 µm (RG ≅ 5), d = 25 µm.

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Figure 5. (I) CVs of Pt-modified UME-NEEs prepared as in Fig. S5, Edep = (a) open circuit; (b) -0.2 V; (c) -0.3 V; (d) -0.35 V; (e ) -0.4 V; (f) -0.45 V. Solution: 0.1 M H2SO4. Scan rate: 100 mV/s, scan in cathodic direction from Ei = 1.5 V. (II) Experimental SSVs (solid black lines) for the HER on Pt-modified UME-NEEs in (I). Solution: 10 mM H2SO4/0.1 M Na2SO4. Scan rate: 100 mV/s, scan in cathodic direction from Ei = 0.0 V. Open red circles: SSVs calculated on a surface with Pt and Au as described in the text with kinetic parameters (vei in mol cm-2s-1): veV,Au = see graph and Table S3 in SI, veH,Au < 10-14, veT,Au = (6.0 ± 0.5) × 10-9, θeAu = 0.010 ± 0.005, veV,Pt = (4.0 ± 0.2) × 10-6, veH,Pt = (6 ± 2) × 10-10, veT,Pt = (1.5 ± 0.1) × 10-6,θePt = 0.18 ± 0.02. Values indicated for φ an fR have ∼10% uncertainties.

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