Top-Down Synthesis of Nanostructured Platinum-Lanthanide Alloy

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Top-Down Synthesis of Nanostructured Platinum-Lanthanide Alloy Oxygen Reduction Reaction Catalysts: PtxPr/C as an Example Johannes Fichtner, Batyr Garlyyev, Sebastian Watzele, Hany A Elsayed, Jan Schwaemmlein, Weijin Li, Frédéric Maillard, Laetitia Dubau, Jan Michalicka, Jan Macak, Alexander W. Holleitner, and Aliaksandr S. Bandarenka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20174 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Top-Down Synthesis of Nanostructured PlatinumLanthanide Alloy Oxygen Reduction Reaction Catalysts: PtxPr/C as an Example

Johannes Fichtner,a,(1) Batyr Garlyyev,a,(1) Sebastian Watzele,a,b Hany A. El-Sayed,c Jan N. Schwämmlein,c Wei-Jin Li,d Frédéric M. Maillard,e Laetitia Dubau,e Jan Michalička,f Jan M. Macak,f Alexander Holleitner,g and Aliaksandr S. Bandarenkaa,b,* a

Physics of Energy Conversion and Storage, Technical University of Munich, James-FranckStraße 1, 85748 Garching, Germany b c

Technical Electrochemistry, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching, Germany d

e

f

Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany

Chair of Inorganic and Metal-Organic Chemistry, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching, Germany

Laboratoire d’electrochimie et physicochimie de materiaux et des interfaces, University of Grenoble Alpes, 1130 Rue de la Piscine, 38402 Saint Martin d’Hères, France

Central European Institute of Technology, Brno University of Technology, Purkynova 123, 612 00 Brno, Czech Republic g

Walter Schottky Institute, Technical University of Munich, Am Coulombwall 4a, 85748 Garching, Germany (1)These

authors contributed equally to this work.

*Corresponding Author: Tel. +49 (0) 89 289 12531, E-mail: [email protected] (A.S. Bandarenka)

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KEYWORDS: Electrocatalysis, Cathodic Corrosion, Fuel Cell, Oxygen Reduction, Lanthanides, Platinum Alloys, Top-Down Synthesis

ABSTRACT The oxygen reduction reaction (ORR) is of great interest for future sustainable energy conversion and storage, especially concerning fuel cell applications. The preparation of active, affordable and scalable electrocatalysts and their application in fuel cell engines of hydrogen cars is a prominent step towards the reduction of air pollution, especially in urban areas. Alloying nanostructured Pt with lanthanides is a promising approach to enhance its catalytic ORR activity, whereby the development of a simple synthetic route turned out to be non-trivial endeavor. Herein, for the first time we present a successful single-step, scalable top-down synthetic route for Pt-lanthanide alloy nanoparticles, as witnessed by the example of Pr-alloyed Pt nanoparticles. The catalyst was characterized by

high-resolution

transmission

electron

microscopy,

energy-dispersive

X-ray

spectroscopy, X-ray diffraction and photoelectron spectroscopy and its electrocatalytic oxygen reduction activity was investigated using a rotating disk electrode technique. PtxPr/C showed ~3.5-times higher (1.96 mA/cm2Pt, 0.9 V vs reversible hydrogen electrode


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(RHE)) specific activity and ~1.6-times higher (696 mA/mgPt, 0.9 V vs RHE) mass activity compared to commercial Pt/C catalysts. Based on previous findings and characterization of the PtxPr/C catalyst, the activity improvement over commercial Pt/C originates from a lattice strain introduced by the alloying process.

Introduction Over the past few decades, the application of metal nanoparticles in catalysis has increased drastically, owing to their high surface to volume ratio.1,2,3,4 Especially as the global energy demand increases and a decoupling of the global energy supply from the carbon cycle is targeted, more research has been performed in developing nanoparticle based electrocatalysts for applications in energy conversion systems such as e.g.,



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polymer electrolyte membrane fuel cells.5 The oxygen reduction reaction (ORR) involved in hydrogen fuel cells is one of the most important energy-related reactions. Platinum and platinum-alloy nanoparticles have shown benchmark results as catalysts for the electrocatalytic reduction of oxygen.6 For instance, the development of commercial PtCo nanoparticles made the 2015th version of the fuel cell powered Toyota Mirai more efficient compared to the previous 2008th version, where pure Pt nanoparticles were used.7 Rotating disk electrode (RDE) measurements in liquid electrolyte suggested that Pt3Ni nanoframes are among the most active ORR catalysts,8 but the exceptional activity could not yet be reproduced in single-cell fuel cell tests.9 Pt-lanthanides are another group of bimetallic alloys that have shown a promising ORR activity in RDE studies.10 Measurements on Pt-lanthanide bulk alloys provided an indication that the origin of the high ORR activity for that class of catalysts is the lattice mismatch between Pt and the lanthanide atoms. The associated strain effect can be used as a descriptor for the ORR activity, as the OH binding energy shifts to more favorable values on the so-called volcano plot.11,12 However, the synthesis of the nanostructured Pt-lanthanide alloys turned out to be a challenging task. Partly due to the oxophilicity of the lanthanide metals and due to 4 ACS Paragon Plus Environment

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the large differences in reduction potentials (reduction potential of Pr3+/Pr is -2.35 V)13 compared to Pt. Only few demanding, multi-step bottom-up procedures were employed to prepare Pt-lanthanide nanoparticles.14,15 In order to implement Pt-lanthanide nanoparticles in commercial devices, efficient and scalable synthetic techniques need to be developed.

One way of single-step, scalable nanoparticle synthesis is so-called cathodic corrosion. Formation of metal “dusts” by cathodic corrosion of metallic electrodes was firstly studied by Bredig and Haber16 and then it was explored for the preparation of intermetallic compounds by Kabanov et al.17 In the last decade, Marc Koper’s18,19 and Zelin Li’s20 groups further developed the cathodic corrosion methodology for the production of metal nanoparticles. While several noble or transition metal nanoparticles and their alloys21,22,23 were synthesized using the cathodic corrosion technique, so far, the method was not utilized in synthesizing Pt-lanthanide alloyed nanoparticles. The mechanism of the nanoparticle formation through the application of alternating voltages is still not wellunderstood.18



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In this work, we are taking advantage of cathodic corrosion as a method to circumvent the complex bottom-up synthesis of Pt-lanthanide alloy nanoparticles by a simple topdown approach. For the first time, we have successfully synthesized PtxPr alloyed nanoparticles via cathodic corrosion, without any surface capping agents or chemical reducing agents. We chose to study the PtPr alloyed nanoparticles because the polycrystalline bulk Pt5Pr disk electrode has shown 4-times higher specific activity towards the ORR compared to a pure polycrystalline Pt electrode.12 Based on a combination of high-resolution transmission electron microscopy (HR-TEM) analysis, scanning

transmission

electron

microscopy

(STEM),

energy-dispersive

X-ray

spectroscopy (EDX), X-ray diffraction and photoelectron spectroscopy (XRD and XPS, respectively) of the synthesized catalyst, we show that we indeed produce PtxPr alloys and also introduce lattice strain and a high defect density into the particle structure.10 Further evaluation of the catalyst by electrochemical rotating disk electrode (RDE) measurements showed an up to 1.6-times increased mass activity and an up to 3.5-times increased specific activity compared to commercial Pt/C catalyst.


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Results and Discussion The synthesis of PtxPr nanoparticles was carried out in 8 M KOH electrolyte. The schematic description of the synthetic route and setup are shown in Figure 1. In brief, the commercial bulk Pt5Pr disk electrode was immersed in the electrolyte and an alternating sinusoidal voltage (Frequency: 200 Hz) between ±8 V vs reversible hydrogen electrode (RHE) was applied. PtxPr nanoparticles were produced immediately after applying the potential.

Figure 1. Schematic cell set-up and synthetic route of PtxPr nanoparticles supported on carbon through cathodic corrosion.



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Figure 2A-B shows a representative TEM image of unsupported nanostructured PtxPr particles, prepared by cathodic corrosion and their particle size distribution, respectively. The investigation of the particles reveals a number-averaged diameter of ~6.1 ± 1.1 nm and a surface-averaged diameter of ~6.28 nm, which was determined from the analysis of more than 100 particles. Vulcan carbon was used as a support material for the activity measurements of PtxPr nanoparticles. A typical TEM image of supported particles is shown in Figure 2C and will be further referred to as PtxPr/C in this work. To get an insight into the single particle structure, HR-TEM was performed on the sample. Figure 2D gives an overview of different particles investigated. Overall, the particles show rather non-uniform, defective shapes compared to spherically-shaped commercial Pt/C.24 Investigating the morphology of the particles by TEM, different facets can be observed, including high-index facets (Table 1). The degree of surface distortion is further increased by dislocations of certain atom planes. The partially defective character of the particle surface is increasing the number of catalytically active sites exposed to oxygen and therefore improving the ORR activity.25,26 The presence of defects, low- and high-index facets is most likely owed to the harsh conditions of the synthesis process, where the particles are forced to leave an energetically favorable environment within a very short period of time by the application of high voltages. Thereby, in comparison to the nucleation driven growth of a wet-chemical bottom-up synthesis,27,28 a lack of particle symmetry is expected due to the increased possibility of a defective particle surface.


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(A)

(B)

(C)

(D)

Figure 2. (A) TEM images of unsupported PtxPr nanoparticles. (B) Particle size distribution of unsupported PtxPr nanoparticles with a number-averaged diameter (𝑑𝑁) of ~6.1 nm and a surface-averaged diameter (𝑑𝑆) of ~6.3 nm. The full TEM image used for determination of the particle size distribution is given in Figure S1. (C) TEM image of PtxPr particles supported on



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Vulcan carbon (PtxPr/C). (D) HR-TEM images of selected single PtxPr particles indicating the presence of different crystal facets and defects on the surface. Elemental analysis of the catalyst composition was performed using EDX. Figure 3A-D depicts the respective distribution of Pt and Pr in a selected group of particles. The global analysis reveals an average Pt:Pr ratio of ~4:1 with a Pr amount comprising in a range of ~10-25%. Locally, a distinct Pr enrichment at the particle surface can be observed, leading to Pr contents up to ~50%.


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(A)

(B)

(C)

(D)

Figure 3. (A) STEM image of the PtxPr/C catalyst. Corresponding STEM-EDX elemental mapping of (B) pure Pt, (C) pure Pr and (D) combined Pt and Pr.

XPS gives further insight into the surface composition of the PtxPr nanoparticles. In Figure 4A, the Pt 4f and Pr 3d detail scans of the unsupported PtxPr nanoparticles are displayed. The fitted XPS spectrum of the Pt 4f core-level suggests the presence of metallic Pt and PtO on the surface of the nanoparticles. The deconvolution of the XPS spectrum of the Pr 3d orbital shows a 11 ACS Paragon Plus Environment


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contribution of Pr4+ and metallic Pr (Figure 4B). The elemental ratio of Pt:Pr on the surface of the investigated particles is approximately 1:1, matching well with the Pr enrichments found by EDX analysis. XRD studies further validate the presence of different types of PtxPr alloys, majority being Pt5Pr and Pt2Pr, as listed in Table S1. Moreover, the existence of pure Pt and several Pr-oxide phases has been monitored. All phases found by XRD are summarized in the supporting information and compared to literature data, showing a good agreement between the experimental and literature values (Table S1). Compared to the 2θ values of pure Pt (green vertical bars in Figure 4, 2θ values listed in Table S2), a shift to higher angles can be observed for all phases, indicating the alloying between Pt and Pr. In order to further prove the presence of alloyed PtxPr nanoparticles and the fact that they are not a simple mixture of independent Pt and Pr nanoparticles, we treated the sample with highly concentrated acid (3.6 M H2SO4 for 2 hours). Following XRD diffractogram confirmed that even in highly corrosive environment the alloyed PtxPr nanoparticles conserved their composition. The diffractogram is shown in Figure S2 and again the main phases of pure Pt are marked as green vertical bars. The fact that Pt5Pr is one of the main phases of PtxPr can be justified by the identical composition of the initial bulk electrode, indicating that the nanostructured Pt alloy composition is influenced by the choice of the initial bulk electrode. Details on the bulk Pt5Pr electrode surface after the cathodic corrosion treatment can be found in the SI (Figure S3). As mentioned in our previous analysis of the HR-TEM images, the PtxPr nanoparticles consist of several low- and high-index facets. An overview on the facets found by XRD is summarized in Table 1.


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(A)

(B)

(C)

Figure 4. XPS spectra and assignment in the (A) platinum 4f and (B) praseodymium 3d core level regions of PtxPr nanoparticles. (C) XRD diffractogram of the PtxPr nanoparticles produced by the cathodic corrosion. Green vertical bars correspond to



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the peak positions of pure Pt as adapted from PDF card number 04-0802. For all phases a positive angle shift can be observed.

Table 1. PtPr alloy compounds and corresponding indices found by XRD. Phase PtPr

Index 440 610 222 400 002 227

Pt2Pr Pt5Pr

As mentioned before, M. Escudero-Escribano et al. showed that exploiting the so-called strain effect by alloying Pt with lanthanide metals is a powerful tool to fine-tune the activity of Pt and one of the main reasons for its improved catalytic activity over pure Pt.10 Thus, the ORR activity of PtxPr/C was evaluated by RDE measurements in acidic media and compared to the commercial Pt/C catalyst. A typical cyclic voltammogram (CV) of PtxPr/C is shown in Figure 5A. Characteristic H-adsorption/desorption peaks of Pt were observed between ca. 0.1-0.4 V vs RHE and oxide-formation peaks were observed at potentials more positive than ca. 0.7 V vs RHE in the CV. The inset of Figure 5A shows the normalized and background-corrected CO-stripping peaks of as-synthesized PtxPr/C and commercial Pt/C catalyst. In comparison to commercial Pt/C, the CO-stripping peak of PtxPr/C is shifted to lower potentials by ca. 40 mV.


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A characteristic anodic scan of the polarization curve of PtxPr/C in O2-saturated 0.1 M HClO4 is depicted in Figure 5B. The inset of Figure 5B shows the kinetic current, as calculated from the polarization curve and normalized to the electrochemically active surface area (ECSA) of the PtxPr/C. The ECSA of PtxPr/C was determined through a CO-stripping experiment (inset of the Figure 5A, assumed Pt charge density: 420 μC/cm2) and it was found to be 36 m2/gPt. (A)

(B)

Figure 5. (A) Typical cyclic voltammogram of PtxPr/C in Ar-saturated 0.1 M HClO4 at 50 mV/s scan rate. The inset shows the background-corrected CO-stripping peaks of PtxPr/C and commercial Pt/C catalyst recorded in Ar-saturated 0.1 M HClO4 at a scan rate of 20 mV/s after initial Ar/CO saturation of the solution at a constant potential of 0.6 V vs RHE. (B) Typical iR-drop corrected anodic polarization curve of PtxPr/C in O2-saturated 0.1 M HClO4, rotating at 1600 rpm, normalized to the geometrical area of the glassy



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carbon electrode (0.196 cm2). The curve was corrected by subtraction of the (pseudo)capacitive currents obtained from a CV recorded at a scan rate of 10 mV/s. From thermogravimetric analysis, a PtxPr weight loading of ~4%wt was determined for the catalyst. PtxPr electrode loading: 1.5 μg/cm2. The inset shows the kinetic current normalized to the ECSA of PtxPr nanoparticles. A typical CV and polarization curve of commercial Pt/C is given in the supporting information (Figure S4, S5).

In order to compare the activity of PtxPr/C with commercial Pt catalyst, similar electrocatalytic activity measurements were done for the commercial Pt/C. The ECSA and activity measurements observed for the commercial Pt/C catalyst were in good agreement with the literature values. In detail, one of the best benchmarks was published by A. Orfanidi et al., describing an ECSA of ~74 m2/gPt, a mass activity of ~548 mA/mgPt and a specific acitivity of ~0.83 mA/cm2Pt for the commercial Pt/C compared to ~68 m2/gPt, ~424 mA/mgPt and ~0.61 mA/cm2Pt we found, respectively. It has to be noted that the literature measurements have been conducted in an optimized single-cell PEMFC setup instead of a RDE setup, using a different ionomer.29 Specific activity comparison between PtxPr/C with commercial Pt/C catalyst are shown in Figure 6A. PtxPr/C showed ~3.5-times higher specific activity (1.96 mA/cm2Pt at 0.9 V vs RHE) compared to commercial Pt/C catalyst (0.61 mA/cm2Pt at 0.9 V vs RHE). However, it should be mentioned


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that the commercial Pt/C catalyst has an approximate diameter of 3 nm and ECSA of 68 ± 2 m2/gPt compared to ~6 nm and 36 ± 2 m2/gPt for PtxPr/C.30 Considering the particle size effect on the ORR of Pt based nanostructures, for higher nanoparticle sizes an increased specific activity is the expected outcome, however the activity increase of ~3.5-times cannot fully be explained by the size effect.31,32 Thus, the increased activity we observe for PtxPr nanoparticles is mainly due to a strain/ligand effect rather than a size effect. Interestingly, the polycrystalline PtPr disk electrode showed 4-times higher activity compared to a polycrystalline Pt electrode12 and a comparable activity enhancement is observed for the PtxPr/C over commercial Pt/C catalyst. In order to test the stability of the PtxPr/C electrocatalyst, we performed an accelerated stress test, consisting of 1000 potential cycles between 0.6 and 1.0 V vs RHE at a scan rate of 100 mV/s in O2-saturated 0.1 M HClO4 (25°C). The development of the CO-stripping peak of PtxPr/C before and after stress testing is depicted in Figure S6. We conducted the same stability test for the commercial Pt/C catalysts (Figure 6A). The stability of PtxPr/C was slightly lower compared to the stability of the commercial Pt/C catalyst. However, further stability tests need to be performed in experimental conditions closer to the fuel cell operating conditions, i.e. at elevated temperatures or in a membrane electrode assembly.33 For determination of the mass activity, we measured the mass loading of PtxPr/C via thermogravimetric analysis (TGA) and determined the Pt content with average values from the EDX analysis. Mass activities were calculated by normalizing the kinetic currents at 0.9 V vs RHE to the Pt mass of PtxPr/C and of commercial Pt/C catalyst (Figure 6B). The mass activity of PtxPr/C was ~1.6-times higher (0.70 A/mgPt) compared to Pt/C catalyst (0.42 A/mgPt). Furthermore, we



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compared the mass and specific activities of PtxPr/C nanoparticles with other Pt-lanthanide/-rareearth alloy nanoparticles from the literature, given in Table S3. (A)

(B)

Figure 6. (A) Specific activity of PtxPr/C before and after 1000 voltage cycles is presented in comparison with the activity and stability results of commercial Pt/C catalyst. (B) Mass activities of PtxPr/C and commercial Pt/C catalyst before voltage cycling. Activities are compared at 0.9 V vs RHE.

Conclusions In conclusion, for the first time, we have successfully synthesized PtxPr nanoparticles by a scalable top-down approach, without using any surface capping material or chemical reducing agents.


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Characterization of PtxPr/C using HR-TEM, XRD, XPS and electrochemical techniques revealed that the procedure leads to the formation of PtxPr alloy phases, mainly being Pt5Pr and Pt2Pr, which is similar to the composition of the bulk disk electrode. The nanostructured Pt-lanthanide alloy shows a strained and highly defective structure, improving its catalytic activity towards the ORR in acidic media. At 0.9 V vs RHE, PtxPr/C demonstrated a ~3.5-times increased specific and ~1.7times increased mass activity compared to commercial Pt/C catalyst.

Experimental Section PtxPr/C was prepared by cathodic corrosion of a Pt5Pr bulk electrode. The Pt5Pr electrode, a carbon sheet and a Hg/Hg2SO4 electrode were immersed in 8 M KOH (85%, Grüssing, Germany) and contacted as working, counter and reference electrode, respectively. Briefly, an alternating sinusoidal potential of ±8 V vs RHE (frequency: 200 Hz) was applied, followed by a release of PtxPr nanoparticles into the electrolyte. Subsequently, the particles were cleaned 5 times by centrifugation with ultra-pure water (Evoqua, Germany) at 6000 rpm. Ink preparation was performed by adding 4 mg of Vulcan XC 72R (Cabot, USA) and 6 µL of Nafion dispersion (5%wt in lower aliphatic alcohols and water, DuPont, USA) to the 5 ml dispersion of nanoparticles and



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ultrasonicating the slurry until homogeneous dispersion was observed. As a reference, commercial 19.6%wt TEC10V20E Pt/C catalyst powder produced by Tanaka Kikinzoku was used as received. For the ink preparation, 10 mg of Pt/C were mixed with 1.466 ml of isopropanol, 3.6 ml of ultrapure water and 30 µL of Nafion dispersion using ultrasonic treatment for 10 minutes. The overall weight loading of PtxPr/C was determined by thermogravimetric analysis using a METTLER TOLEDO TGA/DSC 1 (STARe system). Xray photoelectron spectroscopy was performed on a setup by SPECS GmbH, including a SPECS XR50 X-ray source with Al anode (emission line at 1487 eV), a SPECS spectrometer and a SPECS hemispherical energy analyser PHOIBOS 150. X-ray diffraction was performed on a X’Pert PRO diffractometer from PANalytical B.V. The sample was investigated in a 2θ range of 5-90° with a step size of 0.01313° (300 ms per step) in continuous mode. As X-ray source, Cu Kα was used, the filter material consisted of Ni. Samples for TEM analyses of PtxPr/C and unsupported nanoparticles were prepared by drop-casting few microliters of catalyst ink or pure dispersed particles on formvar-supported carbon-coated Cu400 TEM grids (Science Services, Germany). Imaging was performed using a Philips CM100 EM operated at 100 kV with a resolution 20 ACS Paragon Plus Environment

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of 0.5 nm. HR-TEM imaging of the unsupported particles was performed using a HR-TEM FEI TITAN Themis 60-300 with X-FEG type emission gun operated at 300 kV, equipped with Cs image corrector and a STEM high-angle annular dark-field detector (HAADF). The point resolution was 0.06 nm in TEM mode. STEM-EDX analysis was conducted utilizing a JEOL 2100F microscope, operated at 200 kV and equipped with a retractable large angle Silicon Drift Detector. Quantitative analysis was performed on Pt M and L lines using the K-factors provided by the respective JEOL software. For studying the electrocatalytic activity of the nanoparticles for the ORR, a glassy carbon rotating disk electrode configuration was used. 34 Electrochemical measurements were performed on a glassy carbon rotating disk electrode with diameter of 5 mm (Pine instruments, USA) with a BioLogic VSP-300 potentiostat (BioLogic, France). The electrode was polished with 1, 0.3, and 0.05 µm alumina paste and carefully rinsed with ultrapure water previous to each experiment. Subsequently, the electrode was coated with ink homogeneously. An electrochemical glass cell was used and equipped with a Hg/Hg2SO4 (Schott, Germany) reference electrode and a Pt wire counter electrode (99.9%, Goodfellow, Germany). In order to ensure reproducible results and to avoid 21 ACS Paragon Plus Environment


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contaminations, the cell was frequently cleaned with a 3:1 ratio solution of concentrated sulfuric acid and hydrogen peroxide (30%), followed by boiling the cell with ultra-pure water. For all electrocatalytic activity experiments, 0.1 M HClO4 (Suprapur, Merck, Germany) was used as an electrolyte. Conflict of interest The authors declare no conflict of interest. Acknowledgements Financial support from the cluster of excellence Nanosystems Initiative Munich (NIM), DFG project BA 5795/4-1, TUM IGSSE cohort 11 (ActiveElectroCat, Project 11.01) are gratefully acknowledged. Ministry of Youth, Education and Sports of the Czech Republic (projects nos. LM2015041, LQ1601) is also acknowledged for financial support of this work. We are thankful to Prof. Hubert A. Gasteiger (Technical University of Munich) for providing the commercial Pt/C catalyst and to Prof. Hendrik Dietz (Technical University of Munich) for providing TEM access.

Supporting Information


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TEM images, FEG-SEM images, XRD diffractogram, Reference CV curves, Reference ORR polarization curves, CO-stripping curves

References

1. Narayanan, R.; El-Sayed, M. A. Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability. J. Phys. Chem. B 2005,

109, 12663–12676. 2. Strasser, P.; Gliech, M.; Kuehl, S.; Moeller, T. Electrochemical Processes on Solid Shaped Nanoparticles with Defined Facets. Chem. Soc. Rev. 2018, 47, 715-735. 3. Guo, S.; Zhang, S.; Sun, S. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2013, 52, 8526-8544. 4. Mahmoud, M. A.; Garlyyev, B.; El-Sayed, M. A. Determining the Mechanism of Solution Metallic Nanocatalysis with Solid and Hollow Nanoparticles: Homogeneous or Heterogeneous. J. Phys. Chem. C 2013, 117, 21886–21893. 5. Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and 23 ACS Paragon Plus Environment


Page 24 of 30

Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552–556. 6. Han, B.; Carlton, C. E.; Kongkanand, A.; Kukreja, R. S.; Theobald, B. R.; Gan, L.; O'Malley, R.; Strasser, P.; Wagner, F. T.; Shao-Horn, Y. Record Activity and Stability of Dealloyed Bimetallic Catalysts for Proton Exchange Membrane Fuel Cells. Energy Environ.

Sci. 2015, 8, 258-266. 7. Yoshida, T.; Kojima, K. Toyota MIRAI Fuel Cell Vehicle and Progress Toward a Future Hydrogen Society. Electrochem. Soc. Interface 2015, 24, 45-49. 8. Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with ThreeDimensional Electrocatalytic Surfaces. Science 2014, 343, 1339-1343. 9. Stephens, I. E. L.; Rossmeisl, J.; Chorkendorff, I. Toward Sustainable Fuel Cells.

Science 2016, 354, 1378-1379. 10. Escudero-Escribano, M.; Malacrida, P.; Hansen, M. H.; Vej-Hansen, U. G.; Velázquez-Palenzuela, A.; Tripkovic, V.; Schøitz, J.; Rossmeisl, J.; Stephens, I. E. L.; 24 ACS Paragon Plus Environment

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chorkendorff, I. Tuning the Activity of Pt Alloy Electrocatalysts by Means of the Lanthanide Contraction. Science 2016, 352, 73–76. 11. Čolić, V.; Bandarenka, A. S. Pt Alloy Electrocatalysts for the Oxygen Reduction Reaction: From Model Surfaces to Nanostructured Systems. ACS Catal. 2016, 6, 5378–5385. 12. Garlyyev, B.; Pohl, M. D.; Čolić, V.; Liang, Y.; Butt, F. K.; Holleitner, A.; Bandarenka, A. S. High Oxygen Reduction Reaction Activity of Pt5Pr Electrodes in Acidic Media. Electrochem. Commun. 2018, 88, 10-14. 13. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements.; Elsevier; 2012, pp. 1232–1235. 14. Escudero-Escribano, M.; Verdaguer-Casadevall, A.; Malacrida, P.; Grønbjerg, U.; Knudsen, B. P.; Jepsen, A. K.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Pt5Gd as a Highly Active and Stable Catalyst for Oxygen Electroreduction. J. Am. Chem. Soc. 2012, 134, 16476–16479. 15. Roy, C.; Knudsen, B. P.; Pedersen, C. M.; Velázquez-Palenzuela, A.; Christensen, L. H.; Damsgaard, C. D.; Stephens, I. E. L.; Chorkendorff, I. Scalable Synthesis of Carbon-Supported Platinum–Lanthanide and −Rare-Earth Alloys for Oxygen Reduction. ACS Catal. 2018, 8, 2071– 2080. 16. Bredig, G.; Haber, F. Ueber Zerstäubung von Metallkathoden bei der Elektrolyse mit Gleichstrom. Eur. J. Inorg. Chem. 1898, 31, 2741-2752.



Page 26 of 30

17. Kabanov, B. N.; Astakhov, I. I.; Kiseleva, I. G. Formation of Crystalline Intermetallic Compounds and Solid Solutions in Electrochemical Incorporation of Metals into Cathodes.

Electrochim. Acta 1979, 24, 167-171. 18. Yanson, A. I.; Rodriguez, P.; Garcia‐Araez, N.; Mom, R. V.; Tichelaar, F. D.; Koper, M. T. M. Cathodic Corrosion: A Quick, Clean, and Versatile Method for the Synthesis of Metallic Nanoparticles. Angew. Chem. Int. Ed. 2011, 50, 6346-6350. 19. Feng, J.; Chen, D.; Sediq, A. S.; Romeijn, S.; Tichelaar, F. D.; Jiskoot, W.; Yang, J.; Koper, M. T. M. Cathodic Corrosion of a Bulk Wire to Nonaggregated Functional Nanocrystals and Nanoalloys. ACS Appl. Mater. Interfaces 2018, 10, 9532-9540. 20. Huang, W.; Chen, S.; Zheng, J.; Li, Z. Facile Preparation of Pt Hydrosols by Dispersing Bulk Pt with Potential Perturbations. Electrochem. Commun. 2009, 11, 469472. 21. Chen, X.; Chen, S.; Huang, W.; Zheng, J.; Li, Z. Facile Preparation of Bi Nanoparticles by Novel Cathodic Dispersion of Bulk Bismuth Electrodes. Electrochim. Acta 2009, 54, 7370-7373. 22. Rodriguez, P.; Tichelaar, F. D.; Koper, M. T. M.; Yanson, A. I. Cathodic Corrosion as a Facile and Effective Method to Prepare Clean Metal Alloy Nanoparticles. J. Am. Chem. Soc. 2011, 133, 17626–17629.


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Hersbach, T. J. P.; Kortlever, R.; Lehtimäki, M.; Krtil, P.; Koper, M. T. M. Local Structure and Composition of PtRh Nanoparticles Produced through Cathodic Corrosion. Phys. Chem. Chem. Phys. 2017, 19, 10301-10308. 24. Sneed, B. T.; Cullen, D. A.; Reeves, K. S.; Dyck, O.E.; Langlois, D. A.; Mukundan, R.; Borup, R. L.; More, K. L. 3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports. ACS Appl. Mater. Interfaces 2017, 9, 29839−29848. 25. Dubau, L.; Asset, T.; Chattot, R.; Bonnaud, C.; Vanpeene, V.; Nelayah, J.; Maillard, F. Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction. ACS Catal. 2015, 5, 5333–5341. 26. Chattot, R.; Le Bacq, O.; Beermann, V.; Kühl, S.; Herranz, J.; Henning, S.; Kühn, L.; Asset, T.; Guétaz, L.; Renou, G.; Drnec, J.; Bordet, P; Pasturel, A.; Eychmüller, A.; Schmidt, T. J.; Strasser, P.; Dubau, L.; Maillard, F. Surface Distortion as a Unifying Concept and Descriptor in Oxygen Reduction Reaction Electrocatalysis. Nat. Mater. 2018, 17, 827–833.



Page 28 of 30

27. Harada, M.; Kamigaito, Y. Nucleation and Aggregative Growth Process of Platinum Nanoparticles Studied by in Situ Quick XAFS Spectroscopy. Langmuir 2012, 28, 2415−2428. 28. Kundu, P.; Nethravathi, C.; Deshpande, P. A.; Rajamathi, M.; Madras, G.; Ravishankar, N. Ultrafast Microwave-Assisted Route to Surfactant-Free Ultrafine Pt Nanoparticles on Graphene: Synergistic Co-reduction Mechanism and High Catalytic Activity. Chem. Mater. 2011, 23, 2772–2780. 29. Orfanidi, A.; Madkikar, P.; El-Sayed, H. A.; Harzer, G. S.; Kratky, T.; Gasteiger, H. A. The Key to High Performance Low Pt Loaded Electrodes. J. Electrochem. Soc. 2017,

164, F418-F426. 30. Harzer, G. S.; Schwämmlein, J. N.; Damjanovic, A. M.; Ghosh, S.; Gasteiger, H. A. Cathode Loading Impact on Voltage Cycling Induced PEMFC Degradation: A Voltage Loss Analysis. J. Electrochem. Soc. 2018, 165, F3118-F3131. 31. Nesselberger, M.; Ashton, S.; Meier, J. C.; Katsounaros, I.; Mayrhofer, K. J. J.; Arenz, M. The Particle Size Effect on the Oxygen Reduction Reaction Activity of Pt Catalysts:


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Influence of Electrolyte and Relation to Single Crystal Models. J. Am. Chem. Soc. 2011,

133, 17428–17433. 32. Shao, M.; Peles, A.; Shoemaker, K. Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity. Nano Lett. 2011, 11, 3714– 3719. 33. Dubau, L.; Maillard, F. Unveiling the Crucial Role of Temperature on the Stability of Oxygen Reduction Reaction Electrocatalysts. Electrochem. Commun. 2016, 63, 65-69. 34. Schmidt, T. J.; Gasteiger, H. A.; Stäb, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. Characterization of High‐Surface‐Area Electrocatalysts Using a Rotating Disk Electrode Configuration. J. Electrochem. Soc. 1998, 145, 2354-2358.



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TOC picture


Top-Down Synthesis of Nanostructured Platinum-Lanthanide Alloy

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