Origin of Superior Activity of Ru@Pt Core-Shell ...

ECS Transactions, 75 (14) 971-982 (2016) 10.1149/07514.0971ecst ©The Electrochemical Society

Origin of Superior Activity of Ru@Pt Core-Shell Nanoparticles Towards Hydrogen Oxidation in Alkaline Media J. N. Schwämmleina, H. A. El-Sayeda, B. M. Stühmeiera, K. Wagenbauerb, H. Dietzb, and H. A. Gasteigera a

Chair of Technical Electrochemistry, Technical University of Munich, Lichtenbergstraße 4, D-85748, Garching, Germany b Walter Schottky Institut, Technical University of Munich, Am Coulombwall 4, D-85748, Garching, Germany

There is an ongoing discussion about the nature of the high hydrogen oxidation reaction (HOR) activity of bimetallic Pt-Ru catalysts in alkaline media. We provide an insight on the HOR mechanism for this type of catalyst, excluding bifunctionality as the main driving force for the oxidation of hydrogen. In order to do so, we prepared and evaluated Ru@Pt core-shell nanoparticles with sub-, mono- and multilayered Pt-shells. Rotating disk electrode (RDE) studies in 0.1 M NaOH solution of all prepared catalysts, including Pt/C as a reference show that Ru particles fully encapsulated with a Pt-shell show significantly higher activity than Ru particles that are solely covered with Pt-submonolayers. Furthermore, we found a 4-fold enhancement of Ru@Pt vs. pure Pt with respect to the surface normalized HOR activity. The Ru@Pt catalysts were characterized by COads-stripping voltammetry, as well as cyclic voltammetry (CV) in order to identify the Pt-coverage and shell thickness. Introduction Platinum is the most commonly used metal in electrocatalysis for fuel cell applications nowadays. Since the kinetics of the HOR on the anode side of proton exchange membrane fuel cells (PEMFCs) are very fast (i0313K ≈ 200 mA cm-2Pt) (1), cost competitive Pt-loadings as low as 0.05 mgPt cm-2MEA can be applied (2). On the other hand, the activity of Pt decreases by roughly two orders of magnitude in alkaline environment (i0293K ≈ 0.55 mA cm-2Pt) (1, 3-5). Amongst others, this kinetic limitation strongly hinders an economically feasible application of anion exchange membrane fuel cells (AEMFCs), due to the necessity of large quantities of scarce noble-metal catalysts (4). Bimetallic Pt-Ru electrocatalysts are commonly known to exhibit catalytic HOR activities beyond those of pure Pt electrocatalysts, with reported exchange current densities up to 1.42 mA cm-2NM (NM = noble metal) in alkaline environment at room temperature (6). Despite the extraordinary activity of this class of catalysts, the cause of the activity enhancement observed upon the addition of ruthenium to platinum remains unclear. In general, two major concepts explaining the high activity in alkaline environment are discussed in the literature, a modification of platinum’s electronic structure by Ru, termed electronic or ligand effect (7, 8), as well as a bifunctional mechanism including ruthenium as an active site at the surface (6, 9). In a bifunctional mechanism, OH- is adsorbed onto Ru to react with adsorbed hydrogen on adjacent Pt to

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ECS Transactions, 75 (14) 971-982 (2016)

form water. In fact, this requires two active sites, Pt and Ru to be present in direct vicinity to each other on the surface of the catalytically active particle. H2 + 2Pt  2Pt-Hads OH- + Ru  Ru-OHads + eRu-OHads + Pt-Hads  Ru + Pt + H2O

[1] [2] [3]

While this is in general the case for Pt-Ru alloy electrocatalysts, bimetallic Ru@Pt core-shell nanoparticles only expose Ru in the case of partially Pt-covered nanoparticles, i.e., for Pt submonolayer coverages. Hence, this class of catalysts is expected to exhibit the highest HOR activity for submonolayer Pt-coverage if the bifunctional mechanism is the only driving force for the HOR. Moreover, fully Pt-covered Ru is expected to show an activity comparable to that of pure Pt, since no Ru is exposed to the surface of the nanoparticle. Bifunctional mechanism

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Figure 1: Expected HOR activity (arbitrary units) vs. Pt coverage of Ru@Pt core-shell catalysts for a purely bifunctional mechanism (purple line) or a purely electronic effect (green line). On the other hand, considering an electronic effect, ruthenium plays only a minor role in directly catalyzing the HOR. Nevertheless, it enhances the reactivity of the already active Pt-sites, without the necessity of even being exposed to the surface of the particle. Frequently, the activity of a metal towards the oxidation of hydrogen is considered to depend on the Pt-Hads binding energy (BE) (1, 10). It is well established that the BE between an adsorbate and a metal strongly depends on the d-band center of the latter, as well as its degree of filling. In 2005, Nørskov et al. provided a density functional theory (DFT) study on the reactivity of different metals with respect to the hydrogen evolution reaction (HER) which showed that lowering the Pt-Hads BE would increase the HER activity, since Pt binds hydrogen slightly too strongly compared to the optimum BE (11). As shown in a DFT study by Ruban et al, platinum’s d-band center shifts negative for a Pt-overlayer on Ru compared to a Pt surface, weakening the Pt-Hads binding (12). Based on that, Ru@Pt core-shell nanoparticles should become increasingly active (with respect to total noble metal surface area) as Pt is deposited on Ru with a maximum HOR activity

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at full Pt-coverage or even beyond. Whether the maximum is reached for mono-, bilayer or even thicker Pt-shells on Ru nanoparticles is mainly dependent on the nature and range of ruthenium’s influence on platinum’s d-band structure. Schlapka et al. studied CO adsorbed on Pt-overlayers deposited on Ru(0001). In this study, they identified the electronic influence of Ru on the Pt-COads BE to be strongest in the first layer, quickly diminishing to be barely noticeable in the third Pt-overlayer (13). Furthermore, they identified the influence of the platinum lattice strain to be minor compared to the PtCOads BE. In accordance with this, we expect the maximum HOR activity of Ru@Pt core-shell catalyst for mono- or bilayered Ru@Pt nanoparticles. Eventually, for multilayerd Pt-shells, the activity must decay towards that of pure Pt without any strong electronic influence of the ruthenium in the core of the particle. Unlike the commonly used Pt-Ru alloy catalysts where the general availability of both metals at the surface hinders a clear distinction between the two pathways, Ru@Pt core-shell nanoparticles enable a disentanglement between the two mechanisms of the oxidation of hydrogen on bimetallic Pt-Ru electrocatalysts.

Figure 2: Different types of Ru@Pt core shell nanoparticles, including submonolayer, monolayer and multilayer particles prepared by reduction of a Pt-precursor on a Ru core. Following this concept, we identified the cause of the HOR activity enhancement on Pt-Ru catalysts, utilizing Ru@Pt core-shell nanoparticles with different Pt-coverage and shell thickness. Ru@Pt particles ranging from submonolayer Pt-coverage to multilayer Pt on Ru nanoparticles, as well as Pt nanoparticles were prepared and characterized. Experimental Synthesis of Ru@Pt Core-Shell Nanoparticles Ru was prepared from 4.2 mg RuCl3 (45-55% Ru content, Sigma Aldrich) and 2.1 mg Polyvinylpyrrolidone (PVP) (average Mw ≈ 55000, Sigma Aldrich) dissolved in 40 mL of water-free ethylene glycol (99.8%, anhydrous, Sigma Aldrich), deaereated by argon (6.0-grade, Westfalen AG) and heated from room temperature to 155 °C at a constant rate of 4 °C min-1. The temperature of the heating ramp was controlled by an automatic temperature control device (Model 310, J-KEM). After keeping the temperature constant at 155 °C for 90 min, the suspension was left to cool down to room temperature. Meanwhile, the respective amount of high purity K2PtCl4 (99.99% [metals basis], Sigma Aldrich) was dissolved in 40 mL of deaereated ethylene glycol, added at once to the Ru-containing suspension at room temperature and stirred for 10 min. To deposit Pt on Ru, the mixture was heated another time to 155 °C at the same rate as before and held at

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this temperature for 90 min. After the mixture has cooled down to room temperature, the Ru@Pt nanoparticles were added to Vulcan carbon (Tanaka Kikinzoku International K. K.) dispersed in 80 mL ethylene glycol and stirred for 14 h. The amount of carbon used as support for core-shell nanoparticles was chosen to achieve a loading of 10%wt Ru@Pt. Subsequently, the suspension was mixed with acetone and centrifuged at 11500 rpm (5 °C) in an ultra-centrifuge (5810 R, Eppendorf) to separate the catalyst and the solvent. Subsequently, the catalyst was washed three times with acetone, once in 2-propanol (Chromasolv Plus, 99.9%, Sigma Aldrich) and finally dried at 70 °C in air. Electrode and Ink Preparation Glassy carbon (GC) electrodes (5 mm diameter, Pine, Durham, NC), supported by a PTFE-body (Pine Research Instrumentation, Durham, NC) were polished with 0.05 µm Al2O3 polishing suspension (Bühler AG, Düsseldorf, Germany), sonicated various times in ultrapure water (18.2 MΩ∙cm, Merck Millipore) and cleaned by subsequent dipping in 5 M KOH (99.99% purity, Semiconductor grade, Sigma Aldrich), 2 M HClO4 (60%, Cica Reagent, Kanto Chemical Co., INC., Tokyo, Japan) and ultrapure water (18.2 MΩ ∙ cm, Merck Millipore). Inks were prepared by adding ultrapure water to the dry catalyst followed by high purity 2-propanol (Chromasolv Plus, 99.9%, Sigma Aldrich). The solvent mixture consisted of 80%v 2-propanol and 20%v H2O. The catalyst content of the ink was adjusted to achieve a total layer thickness of ≈ 0.5 µm for measurements in alkaline or ≈ 1.5 µm in acid, respectively. The catalyst suspension was sonicated for 30 min in a sonication bath (Elmasonic S 30 H, Elma) to achieve a homogeneous dispersion. The temperature of the bath was maintained at less than 25 °C to avoid evaporation of the solvent. Subsequently, Nafion (5%wt in lower aliphatic alcohols, 1520% H2O, Sigma Aldrich) was added to the suspension resulting in an ionomer to carbon ratio of 0.15/1 gI gC-1. Prior to coating, the ink was sonicated in a weaker bath (USC100T, VWR) for at least 15 min. Finally, 7 µL of ink were dropped on a GC, covered with a small glass vial and left to dry at room temperature. Setup and Measurement Procedure Electrolyte solutions were prepared from high purity NaOH∙H2O (99.9995% [metals basis], TraceSELECT, Sigma Aldrich) or H2SO4 (Ultrapur, 96%, Merck Millipore) by addition of ultrapure water. Argon and Hydrogen used for purging of the electrolyte was of high purity (6.0-grade, Westfalen AG), as well as carbon monoxide (4.7-grade, Westfalen AG) used for COads-stripping voltammetry. The single-cell PTFE setup including the cleaning procedure prior to electrochemical measurements in alkaline environment was already described in an earlier work (3). A home-made Ag/AgCl reference electrode, saturated with KCl (99.999 % purity, Sigma Aldrich) was used for measurements in alkaline electrolyte. For experiments in sulfuric acid, a glass cell was used with a reversible hydrogen electrode (RHE) as reference. The reference potential was calibrated in H2-saturated electrolyte prior to every experiment using the platinum ring of the electrode. Independent of the reference electrode used during the measurement, all potentials in this publication are given with respect to RHE. Electrochemical measurements were performed using an Autolab potentiostat (PGSTAT302N, Metrohm AG) and a rotator (Pine) with a polyether ether ketone shaft. Prior to any activity determination measurements, catalysts were cleaned by cycling the potential 15 times between 0.05 and 0.8 VRHE at 50 mV s-1. Afterwards, the electrolyte solution was

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replaced by fresh 0.1 M NaOH and saturated with H2. The resistance was determined by electrochemical impedance spectroscopy (EIS) from 100 kHz to 100 Hz at open circuit potential (OCP) with an amplitude of 10 mV. After fully saturating the electrolyte with H2, polarization curves were recorded from -0.025 to 0.8 VRHE at 10 mV s-1 and 1600 rpm while the gas was set to blanketing. To calculate i0, the cathodic going scans of the polarization curves in H2-saturated electrolyte were evaluated. To determine the ECSA, COads-stripping was performed by applying a constant potential of 0.06 VRHE and purging CO for 3 min. Subsequently, CO was removed from solution by Ar-purging for 20 min while the potential was kept constant. The adsorbed CO was oxidized in a CV from 0.05 to 1 VRHE at a scan rate of 10 mV s-1, starting at 0.06 VRHE. The second CV was used as baseline to correct for the capacitance and the roughness factors (RF) was calculated from the resulting integral, using a specific charge of 420 µC cm-2NM. Results and Discussion Electrochemical Characterization As described earlier, the exposure of Ru to the surface of the Ru@Pt core-shell nanoparticles plays a key role in distinguishing between a bifunctional mechanism and a ligand effect as the cause of the HOR activity enhancement in bimetallic Pt-Ru catalysts. According to scanning electron diffraction (SED) patterns, all prepared Ru@Pt samples were free of pure (non Pt-covered) Ru particles.

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Figure 3: Cyclic voltammogram of A) Ru/C at 20 mV s-1 in Ar-saturated 0.1 M H2SO4 and 0.1 M NaOH, respectively and of B) Ru@Pt/C core-shell nanoparticles with B) submonolayer and C) multilayer Pt shell in Ar-saturated 0.1 M H2SO4 at 20 mV s-1. All measurements were done at room temperature in stagnant electrolyte. With respect to Ru@Pt core-shell particles, the cathodic peak, corresponding to the reduction of Ru(OH)x, can be used to qualitatively probe the presence of Ru on the surface of the nanoparticles (14). Nevertheless, as shown for pure Ru in Figure 3 A, this feature is not well resolved in alkaline environment, rendering a detailed analysis of the prepared catalysts in NaOH challenging. Unlike in base, the reduction of Ru(OH)x shows

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a distinct peak in dilute sulfuric acid. The utilization of this peak to distinguish between submonolayer and monolayer Ru@Pt nanoparticles was already reported by El-Sawy et al (14). Similar to their findings, Figure 3 B shows that Ru@Pt core-shell nanoparticles with low Pt-monolayer equivalents (MLE) show a clear reduction feature at ≈ 0.4 VRHE. Due to the presence of platinum on the Ru-core, the electronic structure of Ru is slightly modified and the reduction feature is shifted anodically by approximately 100 mV in Ru@Pt compared to pure Ru. Covering the Ru nanoparticles with an increasing amount of Pt causes a decrease of this peak, eventually vanishing at 1.5 Pt-MLE. Further, no reduction peak is obtained in this potential region for catalysts with thicker Pt-shells on the Ru core (Figure 3 C). We therefore conclude that ruthenium is only exposed to the surface of Ru@Pt nanoparticles with incomplete Pt-shell, whereas mono- and multilayer nanoparticles are free of ruthenium surface sites. The charge obtained from the ad- and desorption of hydrogen at potentials between 50 and 300 mVRHE increases with the amount of Pt in Ru@Pt particles. Since no pure Pt-particles were found by SED, we attribute this to the Hupd process on Pt in Ru@Pt, developing with the thickness of the Pt-shell. The maximum cathodic current at 50 mVRHE is a first indication of the catalyst’s HER activity. This current increases with the amount of Pt in Ru@Pt up to two MLE, slightly decreasing again for Ru@Pt nanoparticles with higher Pt content. COads-Stripping Voltammetry Ru@Pt core-shell nanoparticles were characterized using COads-stripping voltammetry in sulfuric acid in order to determine the thickness and composition of the Pt-shell. Various groups have reported an enhancement of CO oxidation in acid (15-17) and base (18) for bimetallic Pt-Ru catalysts compared to pure Pt. Gasteiger et al. reported the onset of CO oxidation in 0.5 M H2SO4 to shift cathodically by roughly 200 mV with a peak current at ≈ 0.72 VRHE on Pt vs. ≈ 0.49 VRHE on a Pt-Ru alloy, depending on the Pt:Ru ratio in the alloy. Similar to other groups (19, 20), Gasteiger et al. attributed the lower CO oxidation overpotential on Pt-Ru to a bifunctional mechanism where Ru sites are freed from CO at comparably low electrode potentials, hence promoting the oxidation of COads on neighboring Pt-sites (15). Later on, other groups reported CO oxidation on Ru@Pt core-shell nanoparticles to be associated with similar overpotentials as Pt-Ru alloys (21, 22), e.g. Ochal et al. reported a shift of the main COads stripping peak from ≈ 0.8 VRHE on pure Pt in 0.5 M HClO4 to ≈ 0.59 VRHE on Ru@Pt core-shell nanoparticles (23). These results indicate that instead of a bifunctional mechanism including Ru as an active site, the increased reactivity of the bimetallic Pt-Ru system towards the oxidation of CO could be due to an altered Pt d-band structure, also suggested by DFT calculations (24). Moreover, Ochal et al. found a small additional COads stripping peak at 0.76 VRHE and tentatively assigned it to pure Pt as a side-product in the synthesis of Ru@Pt (23). Recently, El-Sawy et al. prepared a series of Ru@Pt particles with varying shell thickness also utilizing COads stripping voltammetry in order to characterize Ru@Pt (14). According to El-Sawy et al., there are different types of COads on Ru@Pt core-shell nanoparticles. CO adsorbed on the first Pt overlayer (COadsA) is oxidized at the most cathodic potential of ≈ 650 mVRHE in 0.5 M H2SO4, similar to previous reports. Further, they reported a second peak for COadsB to be associated with the removal of CO from the second Pt-layer at ≈ 785 mVRHE. They attributed the larger overpotential necessary to oxidize COadsB compared to COadsA to the stronger Pt-CO binding in the second Pt-overlayer and showed that the peak current ratio (COadsA: COadsB) is in accordance with changes in the Pt-shell thickness on Ru (14). This is further

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supported by measurements and calculations by Schlapka et al. who found a significant difference in the Pt-COads BE for the first Pt-layer on Ru compared to the second (13).

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Figure 4: Baseline-corrected, surface normalized COads stripping voltammograms in Ar-saturated 0.1 M H2SO4 at 10 mV s-1 of different Ru@Pt/C, Ru/C and Pt/C catalysts. Prior to the stripping, CO was adsorbed at 0.05 VRHE for 5 min, subsequently the solution was purged with Ar for 20 min. Beyond two Pt-layers, the BE is within the range of Pt and changes from one to the next layer are minor. Following this approach, we performed COads stripping on Ru@Pt catalysts (Figure 4 B and C) in dilute sulfuric acid, as well as for the Ru core and Pt nanoparticles (Figure 4 A) prepared by the same method. For Ru@Pt core-shell nanoparticles with submonolayer Pt-shell (Figure 4 B), only a single anodic peak for the oxidation of COads is observed at ≈ 595 mVRHE. The peak current for the oxidation of COadsA is located ≈ 230 mV cathodic with respect to pure Pt. We therefore attribute it to the oxidation of COads on the first monolayer of Pt on Ru (COadsA), according to the work by El-Sawy et al. and conclude that no pure Pt exists in Ru@Pt samples prepared by this method. Also, no additional peak was observed that could correspond to the oxidation of COads on pure Ru nanoparticles, in accordance with CVs and SEDs, as reported earlier. The potential difference of the peak current for the oxidation of COadsA of roughly 60 mV (compared to El-Sawy et al.) may be due to a different particle size of the Ru core (this publication: ca. 2-3 nm) originating from slightly different synthesis routes. For Ru@Pt core-shell nanoparticles with higher MLE in the Pt-shell (Figure 4 C), a second anodic

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peak emerges. This peak corresponds to COadsB, oxidized at ≈ 760 mVRHE, significantly more anodic with respect to COadsA, but still more cathodic with respect to pure Pt. We conclude that this current originates from the oxidation of CO adsorbed on the second Ptlayer. With the increasing COadsB peak on higher Pt-MLE, the COadsA peak decreases at the same time, qualitatively describing the thickness of the Pt-shell to increase for higher Pt-MLE. The Ru@Pt core-shell nanoparticle with the highest Pt-MLE shows only very small amount of COadsA, indicating the nearly complete formation of the second Pt-overlayer on the Ru core. HOR/HER Activities in Base HOR kinetics in alkaline electrolyte were measured in a standard RDE setup in H2-saturated 0.1 M NaOH solution, as described in an earlier work (3). The kinetics of the HOR/HER were assumed to follow the Butler-Volmer equation: ikin = i0∙RF∙[exp(αaF∙η/RT) - exp(-αcF∙η/RT)]

[4]

where ikin is the kinetic current, i0 is the exchange current density (in A cm-2NM), RF is the roughness factor, F is the Faraday constant, η is the overpotential, R is the ideal gas constant, T is the temperature, αa and αc are the anodic and cathodic transfer coefficients, respectively. To extract i0HOR/HER for Ru@Pt, we evaluated the micropolarization region of the polarization curves up to ± 10 mVRHE. In this case, the linearized form of the Butler-Volmer equation can be used due to the small HOR/HER overpotential. ikin ≈ i0∙RF (αa+αc)F/RT∙η

[5]

For a single-electron transfer reaction as the limiting step, the sum of alphas is one, as reported by various groups for HOR/HER on Pt in alkaline media (3, 4). To extract i0HOR/HER of Ru@Pt core-shell catalysts from linear fitting (Figure 5 B and C), we assumed the sum of alphas to be equal to one, similar to pure platinum. To account for the resistance of the solution, all potentials were corrected for the ohmic drop. EiR-free = Emeas - I∙RHFR

[6]

where Emeas is the measured potential, I is the measured current and RHFR is the high frequency resistance (HFR) determined by EIS. Since the concentration of the reactive species depletes at the electrode surface as the overpotential is increased, a limiting current ilim is reached in RDE measurements, depending on the electrode geometry and the rotation rate. In all conducted experiments, ilim ≈ 2.5 - 2.6 mA cm-2geo at 1600 rpm was sufficiently close to the theoretical diffusion limit for H2 in 0.1 M NaOH, excluding pure Ru which shows lower HOR activity compared to the other catalysts and the current ceases completely after the formation of the oxide, as reported by other researchers (6, 16). By now, it is well established that the evaluation of reaction kinetics via the RDE technique is limited to catalysts with an intrinsic activity low enough that the kinetic overpotential (here: ηHOR/HER) is significant with respect to ηdiff of the reactive species in the liquid medium (1, 4). Due to the high activity of the catalysts prepared in this study, it was necessary to optimize the RDE measurement procedure to obtain reliable activity data. For highly active catalysts it is especially important to prepare electrodes with a comparably low RF. Therefore, we limited the loading of the catalytic

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material to 10%wt on carbon and prepared electrodes with a catalyst layer thickness < 1 µm, resulting in a total noble metal content of ≈ 2 µgNM cm-2.

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Figure 5: Cathodic going scans (points), including linear fits (lines) in the micropolarization region for different Ru@Pt/C, Ru/C and Pt/C catalysts at 10 mV s-1 in H2-saturated 0.1 M NaOH at 1600 rpm. All measurements were carried out at 298 K, corrected for the ohmic drop and normalized to the total noble metal surface obtained from COads-stripping voltammetry in NaOH. A) Reference catalysts, B) catalysts with increasing and C) decreasing HOR activity. The most active catalyst (Ru@Pt2MLE) is shown in B) and C). The catalytically active surface of electrodes prepared this way was in the order of RF ≈ 1 cm-2NM cm-2geo. We compare the Ru@Pt catalyst directly to Pt prepared by the same method, as well as the plain Ru core. The HOR activity of Pt in alkaline solution was already reported and i0298K = 0.45 ± 0.03 mA cm-2Pt obtained in our study is in reasonable agreement with other publications (i0313K = 1.0 mA cm-2Pt (1); i0293K = 0.55 mA cm-2Pt (3); i0294K = 0.57 mA cm-2Pt (4); i0313K = 1.78 mA cm-2Pt (25)). Compared to Pt, Ru intrinsically shows a rather low HOR activity, thus provides only very small current in H2-saturated electrolyte. By adding a submonolayer of Pt on Ru ([email protected]), the current obtained from HOR increases significantly with respect to pure ruthenium. The activity normalized to the total noble-metal surface area of this catalyst is already similar to that of pure Pt (Figure 6 A). This finding supports previous reports on the high HOR activity of bimetallic Pt-Ru catalysts, e.g. as Pt-adatoms on Ru (9) or in the form of an alloy (6, 8, 26). This fact is especially interesting, since only about 30% of the Ru core is covered by Pt, resulting in a three- to four-fold activity enhancement of these Pt-atoms with respect to pure Pt. Adding more Pt on the Ru core smoothly increases the surface-normalized activity further, clearly demonstrating the absence of a sudden activity decay after completion of the full Pt-shell on Ru ([email protected]) which would be expected for a catalyst that follows a purely bifunctional HOR mechanism. The maximum activity for Ru@Pt core-shell nanoparticles was obtained for two MLE of Pt on the Ru core. This catalyst showed a ≈ 4 x HOR activity enhancement with respect to pure Pt. This is consistent with the activity of [email protected] where a single Pt-atom was roughly four times as active as pure Pt. According to these findings, we conclude that the origin of the superior activity of bimetallic Pt-Ru catalysts is due to a modification of platinum’s electronic structure by Ru, rather than a bifunctional mechanism including Ru as active site on the surface of the nanoparticles. The COads-stripping voltammogram of the most active catalyst showed two distinct peaks for the first and second Pt-monolayer on Ru, rendering it challenging to determine at this

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point whether a one- or two-layer thick Pt-shell is the optimum with respect to a shift of platinum’s d-band center. However, Ru@Pt nanoparticles with higher Pt-MLE show lower surface normalized HOR activity ([email protected] and [email protected]), while especially [email protected] consists nearly exclusively of Ru covered by two Pt-overlayers. 1200

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Figure 6: HOR activity, obtained by linear fitting in the micropolarization region for Ru@Pt/C with varying Pt-shell thickness, Ru/C and Pt/C catalysts in 0.1 M NaOH with respect to A) surface area of noble metals, B) total mass of noble metals and C) mass of Pt. The red and turquoise bars show data published in the literature; the white bars are data from this study. All values are based on at least three independent measurements of different coatings of the same catalyst, while error bars indicate the standard deviation of the individual values. According to this, we tentatively assign the first Pt-layer on Ru to be the most HOR active, followed by the second one. This finding agrees well with measurements on the Pt-CO BE, where the largest downshift of platinum’s d-band center was observed for the first monolayer of Pt on Ru, followed by the second one (13). With respect to the mass-based activity (Figure 6 B) normalized to both, Pt and Ru-mass, a similar trend is found with the highest activity for Ru@Pt2MLE. Compared to the surface-normalized trend, the mass-normalized trend is rather weak due to the fact that the mass of the Ru core is always taken into account, whereas it does not provide significant activity. Normalizing only to the mass of Pt (Figure 6 C) points out that low-MLE Ru@Pt catalysts (0.3/0.5) show very high Pt-mass based activity compared to the other catalysts in this study. During synthesis of low Pt-content Ru@Pt, Pt deposits exclusively on the Ru core, as shown by COads-stripping voltammetry. Since the amount of bilayered Ru@Pt in those catalysts is negligible, every Pt atom adds directly to the overall activity of the catalyst. Catalysts with higher Pt-content show an increasing fraction of bilayered Ru@Pt. This fact arises due to the particle size distribution of the Ru core. Small Ru nanoparticles can be easily covered by two Pt-shells due to the small amount of Pt needed therefor, whereas larger particles are covered solely by one Pt-layer. Covering the first Pt-overlayer with another Pt-layer is disadvantageous with respect to the overall activity, since a fraction of the Pt atoms cannot participate in the catalysis of the HOR. As suggested earlier, the intrinsic activity of the second Pt-layer is lower than the first one, further decreasing the HOR activity of the catalyst. In accordance with this, catalyst particles that are nearly exclusively bilayered ([email protected]/[email protected]) exhibit a lower Pt-mass based activity. Comparing the surface normalized activity of the most active Ru@Pt catalyst prepared in this study with that of bimetallic Pt-Ru alloys shows that both are within a similar range (Figure 6 A) (6). This fact further supports that the reactivity of the bimetallic Pt-Ru

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system, either in the form of an alloy or core-shell nanoparticle, is mainly determined by the reactivity of Pt rather than the bifunctional influence of Ru. The slightly higher activity of Ru@Pt noparticles compared to Pt-Ru alloys prepared by St. John et al. may well be due to the sole exposure of Pt to the surface of core-shell nanoparticles, whereas Pt-Ru alloys always expose both metals to the surface. The validity of comparing our results with those of St. John et al. is supported by the fact that the (surface normalized) HOR activity of both Pt reference catalysts is very similar. The deviation in the mass-based activity of pure Pt between our results and those obtained by St. John et al. (ECSA ≈ 49 m2Pt g-1Pt) is mainly due to the large particle size of the Pt prepared in this study (ECSA ≈ 38 m2Pt g-1Pt). In comparison with the only other study on the HOR activity of Ru@Pt core-shell nanoparticles known to us, our catalyst exhibits a more than three times higher (total noble metal) mass-based activity than that of Elbert et al. (7). Conclusions By preparing Ru@Pt core-shell nanoparticles with different Pt-shell thickness and evaluating their surface-based exchange current density, we provided an insight on the reaction mechanism for the HOR on bimetallic Pt-Ru electrocatalysts in alkaline media. A bifunctional nature of the high HOR activity of this class of materials was ruled out by showing that the exchange current density of fully Pt-covered Ru particles is higher than that of partially covered ones. Furthermore, we identified the first Pt-overlayer on Ru to be the most active one, followed by the second layer that is still more active than plain Pt. Moreover, it was shown that Ru@Pt core-shell nanoparticles, dependent on the Pt-shell thickness, are at least as active as Pt-Ru alloys, in contrast to previous reports (7). Acknowledgments The authors are very grateful for the work of the micro-analytical laboratory at TUM for quick and reliable analysis, especially Ulrike Ammari. TEM imaging and SED patterns by Marianne Hanzlik are gladly appreciated. The work of Anqi Li, related to this topic is also appreciated. Financial support within the CATAPULT project (FCH JU, GA 325268) is acknowledged. References 1.

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