Durability study of platinum nanoparticles supported on gas-phase synthesized graphene in oxygen reduction reaction conditions

Accepted Manuscript Full Length Article Durability Study of Platinum Nanoparticles Supported on Gas-Phase Synthesized Graphene in Oxygen Reduction Reaction Conditions Erwan Bertin, Adrian Münzer, Sven Reichenberger, Rene Streubel, Thomas Vinnay, Hartmut Wiggers, Christof Schulz, Stephan Barcikowski, Galina Marzun PII: DOI: Reference:

S0169-4332(18)32757-0 https://doi.org/10.1016/j.apsusc.2018.10.061 APSUSC 40629

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

1 July 2018 30 September 2018 8 October 2018

Please cite this article as: E. Bertin, A. Münzer, S. Reichenberger, R. Streubel, T. Vinnay, H. Wiggers, C. Schulz, S. Barcikowski, G. Marzun, Durability Study of Platinum Nanoparticles Supported on Gas-Phase Synthesized Graphene in Oxygen Reduction Reaction Conditions, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.10.061

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Durability Study of Platinum Nanoparticles Supported on Gas-Phase Synthesized Graphene in Oxygen Reduction Reaction Conditions

Erwan Bertin1, Adrian Münzer2, Sven Reichenberger1, Rene Streubel1, Thomas Vinnay4, Hartmut Wiggers2,3, Christof Schulz2,3, Stephan Barcikowski1,3 and Galina Marzun1,3

1. Technical Chemistry I, University of Duisburg-Essen, 45141 Essen, Germany 2. Institute for Combustion and Gas Dynamics – Reactive Fluids (IVG), University of DuisburgEssen, 47057 Duisburg, Germany 3. Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 47057 Duisburg, Germany 3. Carl Padberg Zentrifugenbau GmbH, 77933 Lahr, Germany

Keywords: pulsed laser ablation in liquids, graphene, gas-phase synthesis, microwave plasma reactor, platinum nanoparticles, oxygen reduction reaction

* Corresponding author ([email protected])

ABSTRACT Ligand-free platinum nanoparticles were prepared by pulsed laser ablation in liquids (PLAL) and employed as a benchmarking catalyst to evaluate the durability of a new gas-phase synthesized graphene support in oxygen reduction conditions. Raman measurements showed that the graphene, as compared to Vulcan, was almost defect free. Transmission electron microscopy and initial electrochemically active surface area measurements confirmed good Pt nanoparticle dispersion of the catalysts on both supports. During durability tests, graphene supported Pt nanoparticles showed much better ECSA retention (75% on graphene as compared to 38% on Vulcan), ultimately retaining a higher ECSA than a commercial sample subjected to the same procedure.

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1. INTRODUCTION Proton Exchange Membrane Fuel Cells (PEMFCs) are promising candidates to replace internal combustion engines, as they operate at low (~80°C) temperature with high efficiency.[1] Unfortunately, they still face significant challenges inhibiting their commercialization. The activity of current catalysts for the oxygen reduction reaction (ORR), mainly platinum nanoparticles (Pt NPs) on carbon, remains low due to kinetic limitations, leading to high loading requirements and costs.[1, 2] Stability is another key issue. Platinum nanoparticles are prone to Ostwald ripening and agglomeration, but also to dissolution and detachment from the carbon support.[3, 4] The carbon support itself suffers from corrosion, which can start thermodynamically above 0.207 V vs the standard hydrogen electrode (SHE).[5] Hence, as summarized in recent studies, the challenge is to identify a support that allows good dispersion of the Pt NPs and provides excellent resistance to carbon corrosion.[3, 6] Graphene has been recognized as an alternative, and has been used as nanosheets to support Pt catalyst [7, 8] or as aerogel to support Pt NPs for formic acid oxidation. [9] The use of graphene in MEA has been thoroughly investigated[10], as well as its durability, albeit on reduced graphene oxide with fluorine doped SnO2 (FTO) rather than as prepared graphene.[11] Further applications of graphene as a support have recently been reviewed by Daud et al.[12] Recently, Münzer et al. succeeded with the lab-scale synthesis with a production rate of several hundreds of mg/h of an almost defect-free gas-phase synthesized graphene (GPG) [13], similar to [14, 15]. The GPG also features high electrical conductivity and thus has promising characteristics for a catalyst support material in fuel cells. Therefore, this investigation aims at evaluating the potential of this specific GPG as a catalyst support for PEMFCs. Surfactant-free Pt NPs were prepared by Pulsed Laser Ablation in Liquids (PLAL) [16, 17] and subsequently deposited on GPG and on Vulcan carbon as reference. 3

The electrocatalysts were subjected to accelerated durability tests (ADT) ascertaining the robustness of the graphene in ORR conditions.

2. EXPERIMENTAL Platinum nanoparticles were prepared by PLAL using the Amphos 500 flex picosecond laser (10.1 MHz, 46,3 µJ, 3 ps pulse duration, 467 W).[18] Milli-Q water (resistivity of 18.2 M∙cm at 25°C) was pumped through the ablation chamber at a flow rate of 480 ml/min.[18] The colloid was subsequently centrifuged using a continuously operating tube centrifuge (CEPA, type Z11, 40000 RCF, 3 L/h). The concentration of Pt nanoparticles in the colloid was evaluated from the UV-Vis spectra (ThermoFisher Evolution 301) recorded before and after centrifugation.[16] The hydrodynamic particle size distribution was determined by Analytical Disk Centrifuge (CPS Instruments, Inc. Model DC24000 UHR) with a detector wavelength of 405 nm and a disk rotation speed of 24.000 rpm. GPG was generated from ethanol as described previously. [19], using a commercial Cyrannus microwave slot antenna (iplas) connected to a 2 kW microwave generator (Muegge) operating at 2.45 GHz. The experimental setup is schematically represented in Figure 1. A precursor mixture consisting of an ethanol/argon/hydrogen spray was injected via a twosubstance nozzle in the center of the tube. The nozzle is surrounded by a coaxial eddy flow of argon and hydrogen that stabilizes the central gas flow and provides a suitable gas mixture for the formation of a stable plasma. The ethanol flow rate was adjusted by a syringe pump. Typical operation parameters for the central gas flow are 5 standard liter per minute (slm) argon, 0.1 slm hydrogen, and 1 ml/min of ethanol. The coaxial swirl was fed with 0.5 slm hydrogen and 5 slm argon and the reactor pressure was 1200 mbar abs. Highest production rates were found at 1500 mbar. 4

The surface area of the graphene material was measured via nitrogen adsorption (BET, Brunauer-Emmet-Teller) [20] using a Quantachrome Nova 2000. X-ray photoelectron spectra (XPS) were measured with a VersaProbe II (Ulvac-Phi) with Mg K radiation at 1253.6 eV and an emission angle of 45° to determine the composition and the chemical nature of the materials. Raman spectra were obtained using a Renishaw inVia Reflex MicroRaman spectroscopy system with a 514 nm laser to study the carbon species of the graphene samples. To evaluate the D, G and 2D band intensity in the measured Raman spectra, Lorentzian fit has been applied as it is a common method in literature to do structural analysis of graphene-related systems by Raman spectroscopy. [21, 22] The specific electrical conductivity of the graphene samples was measured with a Keithley 4200-SCS using a four-point measurement setup. Colloidal deposition on the support, either GPG or Vulcan (Cabot VXC72R), was performed by mixing the appropriate amount of colloids to achieve a 20 wt.% loading with the support material, dispersed in 0.1 mM phosphate buffer at pH 7 after 1 h sonication, according to the procedure developed by Marzun et al.[16] After sedimentation, the supernatant was removed and supported Pt nanoparticles were dried overnight in an oven at 60°C in air. Transmission Electron Microscopy (TEM) measurements of the supported catalysts were performed using a Zeiss EM 910 at an acceleration voltage of 100 kV on a carbon-coated copper grid. High resolution TEM (HR-TEM) pictures of the same samples were taken with a Csaberation-corrected JOEL 2200 FS at acceleration voltage of 200 kV. The catalyst inks were prepared by combining 4.9 mg of catalyst with 5 mL of a 80:20 deionized water and isopropanol (VWR, Normapur) mixture, followed by addition of 40 L of a 5 wt.% Nafion ionomer solution (Sigma Aldrich, 15–20% water). The inks were sonicated for 30 min prior to use. The ink was then drop-casted on a glassy carbon rotating disk electrode

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(RDE, Ametek, 0.196 cm2) at 100 RPM and dried for 15 min at 700 RPM.[23] Electrochemical experiments were performed using a VersaStat3F with a Pt wire as counter electrode (CE) and a saturated mercury sulfate reference electrode (MSE) in a Luggin capillary. However, all potentials are quoted with respect to the RHE (Reversible Hydrogen Electrode) scale. The electrochemically active surface area (ECSA) was determined from integration of the hydrogen desorption region after double layer correction in an Ar-saturated (Air Liquide, 5.0) 0.1 M HClO4 solution (Alfa Aesar, ACS, 60-62%), considering a conversion factor of 210 C∙cm-2. The Ohmic drop was evaluated by impedance spectroscopy and compensated at 85% for the ORR measurements in O2 saturated solution (Air Liquide, 99.998%).

3. RESULTS AND DISCUSSION The GPG, Vulcan and the Pt NPs were first characterized. The Raman spectra of the GPG and the Vulcan are presented in Figure 2 and were used to investigate the quality of the synthesized graphene. The G band occurs at 1584 cm–1 which means that GPG consist of graphene sheets [14]. The intensity of the G band in the GPG is relatively high which is attributed to a larger number of graphene layers as it is known that the G band intensity increases almost linearly with the graphene layer thickness [24]. Furthermore, the G band of the GPG is relatively narrow and its maximum below 1590 cm-1 which can be attributed to the fact that graphene and not graphene oxide sheets are formed [14, 25, 26]. From the almost symmetric shape and the position of the 2D band signal of GPG at 2694 cm–1 it can be deduced that the gas-phase graphene consists of less than five layers [27]. A shoulder of the G band has been attributed to the presence of structural disorder in graphene sheets [28]. Ferrari demonstrated that in Raman spectroscopy, edges of graphene sheets are interpreted as defects and therefore signals indicating defects will also appear in nominally defect-free graphene [28]. Taking this into account it is 6

obvious from the Raman measurements that GPG structure in the sheets seems to be almost free of defects. Indeed, in comparison to the GPG (ID/IG = 0.431) the ID/IG ratio of 1.015 of the Vulcan indicates much higher structural disorder, a result of the topological defects in graphite-like areas, impurities from synthesis and the presence of amorphous carbon. The relatively broad and weak line around 2950 cm–1 (D + G band) of Vulcan is also originating from structural disorder.[21, 29] In contrast, the ID/IG = 0.431 of GPG indicates a smaller quantity of defects and a much higher degree of ordering.[21, 28, 30-32] To further characterize the graphene support, it’s electrical conductivity and BET surface area were also measured. In respect with the electrical conductivity measurement, the pristine GPG powder was pressed into pellets and electrically characterized via four-point-measurement. For details see [13]. The material showed an electrical conductivity of at least 4000 S/m. The results are in good agreement with literature. Rani et al. measured bulk conductivities of pressed graphene powder assays (PGPA,  = 0.99 g/cm3) in the range of 1000 S/m [33], however, our GPG show superior conductivity. The BET surface area was evaluated at 294 m2/g with the latest value being comparable to commercial Vulcan XC72, and promising for an application as catalysts support in PEMFCs.[34] Finally, the XPS spectrum of GPG were recorded to investigate and quantify their surface oxidation more in detail. In case of gas-phase graphene only traces of oxygen could be identified (Figure 3) and fitting of the spectra resulted in an overall oxygen content of less than 1%. The C1s signals can be attributed to C–C and C–H bonds and also the C1s spectrum gives no indication for an important oxidation. According to Lascovic et al. [35], the XPS spectra were also used to evaluate the content of sp2 hybridized carbon taking into account the D parameter. Based on the value of 20.6, the percentage of sp 2 carbon was calculated to be about 86% for

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GPG, which means that the samples consist mostly of graphitic carbon. The mass-weighted size distribution of the Pt nanoparticles is depicted in Figure 4. As typically observed for nanoparticles generated using a picosecond laser, a fraction of the NPs is above 20 nm. Thus, the nanoparticles were centrifuged to remove the larger nanoparticles, resulting in a mass-weighted mode centered at 5.1 nm. Accordingly, the ECSA also increases from 5±1 m2/gPt to 31±3 m2 g-1Pt (see insert). After centrifugation, the nanoparticles were adsorbed on the support, and analyzed by TEM (Figure 5 A and C). Vulcan displays a smooth, roughly spherical morphology and a size of ca 40–50 nm. On the contrary, graphene shows far less contrast, and almost linear edges. The HRTEM (Figure 5 B and D) clearly display the long, layered structure of graphene nanosheets, as opposed to the shorter, curved features of the Vulcan carbon. Qualitatively, the Pt nanoparticles appear well dispersed in both samples, and display similar size distributions (Figure 5 C and F). XRD diffractograms were also recorded and are displayed in Figure 6 for the Pt NPs on Vulcan and on graphene. Both diffractograms displays two clearly distinct series of peaks, respectively associated with the Pt NPs (blue) and the carbon support (black). The corresponding Pt peaks are nearly identical between each diffractogram, as expected. The peaks associated with the carbon support are however radically different. Vulcan only give a broad defined peak at 24.6°. The graphene support gives much sharper peaks, allowing higher angle ones to be discernable. The most intense diffraction peak, at 26.2°, is sharper and much better defined than its counterpart in Vulcan carbon, confirming the high degree of order and homogeneity of our graphene support. In agreement with the TEM micrographs, the initial ECSA of the Pt nanoparticles on Vulcan and on graphene are almost identical, being respectively of 32±5 m2 g-1Pt and 31±3 m2 g1

Pt.

The ORR performances are also very comparable: the specific activities are 8

–0.54±0.07 mA cm-2Pt (Vulcan) and –0.59±0.08 mA cm-2 Pt. (GPG), in agreement with previously published values for NPs of this size (Figure 7).[3] The mass activities follow the same trend, being respectively -171±6 A g-1Pt and -197±16 A g-1Pt, on Vulcan and graphene. To accurately simulate the aging of a catalyst layer in fuel cells is challenging. The US Department of Energy (DOE) durability working group proposed an ADT separated in two phases.[36] First, cyclic voltammograms (CV; 50 or 100 mV/s) between 0.6–1.0 V are performed to simulate the load cycles, then cycling between 1.0–1.5 V to simulate the start and stop cycle.[36] The first part was sought to focus on platinum dissolution, whereas the second would be critical for carbon corrosion. However, Pizzutilo et al. recently demonstrated that both occurred simultaneously using in-situ techniques, and that voltammograms between 0.5-1.5 V were the harshest conditions. The DOE protocol is also commonly performed under inert atmosphere, whereas Dubau et al. demonstrated that under oxygen atmosphere, expected in the FC, the degradation was accelerated.[37] Nevertheless, in all cases, the degradation is monitored through the decrease in the ECSA of the catalysts. In this study, we have implemented a mixed ADT in O2-saturated solution. At first, 18000 CVs at 100 mV s-1 are applied between 0.6–1.0 V. Then, from the results of Pizzutilo et al., 800 CVs between 0.6–1.5 V, also in O2 saturated HClO4 0.1 M, are performed. During these tests, the electrode is not rotated, with the exception of a last set of 3 CVs performed at the end of the ADT tests, which results are displayed in Figure 7. Note that these scans are recorded without any attempt to reactivate the catalyst. Clearly, at the end of the ADT, all catalysts undergo pronounced deactivation. The changes in the limiting current suggest that part of the ink may have detached, that incomplete reduction of the Pt oxides occurred or that under prolonged cycling the blanket configuration might not be sufficient to maintain oxygen saturation. After each ADT, the ECSA is measured under Ar. The results are displayed in Figure 8, both in terms of actual ECSA and 9

remaining ECSA (ECSA at t / Initial ECSA) to better highlight the changes. The blank CV under Ar atmosphere are also displayed in Figure 8A, displaying the classical features of Pt catalyst, as well as a slightly higher capacitance for the commercial sample, which may reflect the different BET surface area of this carbon. All samples showed a decrease of variable amplitude during the ADT. Pt nanoparticles supported on Vulcan retain 60% of their ECSA in the first part of the ADT as compared to 95% when supported on graphene. As both Pt nanoparticles are identical, dissolution and Ostwald ripening should occur at the same low extent in this potential window.[4] Thus, the difference in stability is likely to rather arise from either Pt NPs aggregation, carbon corrosion, or both. However, carbon corrosion is not expected to occur significantly in the 0.6– 1.0 V range. Hence, Pt NPs appear to be more strongly adsorbed on graphene, thus reducing migration and agglomeration. On the other hand, corrosion of carbon can also lead to increased migration.[3] Carbon corrosion is also known to occur to a larger extent on defect-rich carbon.[6] Considering the differences between the supports displayed in the Raman spectra, it is not possible to exclude a contribution for carbon corrosion on Vulcan even in this range. In the second phase of the ADT (Figure 8), between 0.6–1.5 V, the degradation of the catalyst layer is accelerated.[4] After additional 800 CVs, only 38% of the initial ECSA of the Pt nanoparticles dispersed on Vulcan remains, as compared to 75% when the same particles are supported on graphene. In this range, both Pt dissolution and carbon corrosion occur. It is clear however that these degradation mechanisms are inhibited on the graphene supported nanoparticles. Assuming the dissolution is similar for NPs of the same size, the benefits of using carbon structures which are resistant to corrosion is clearly apparent. Direct comparison of these results with the literature is not trivial as many variations of the ADT protocols are employed, alongside with NPs of different size. Nevertheless, the same ADT was also applied to a commercial 20 wt.% Pt/C from Alfa Aeasar (HiSPEC 3000). Initially, 10

the commercial sample displays much higher ECSA: 81±12 m2 g-1Pt, with a specific activity of 0.40±0.02 mA cm-2Pt and a mass activity of -299±33 A g-1Pt. As compared to our results, the NPs of the commercial samples are smaller, leading to a higher ECSA and mass activity, but lower specific activity.[38]. After the ADT in the load cycles (0.6-1.0 V), the commercial NPs retain 87% of their ECSA, in line with the results of Pizzutilo et al., 84±2% (500 mV/s, Ar, 10 000 CVs, 46 wt.% TKK) but higher than reported by Castanheira et al., 60%, (100 mV/s, O2, 10 000 CVs, 40 wt.% Pt/Vulcan) and likely reflect not only the difference in the test procedure, but also in the Pt preparation. As previously stated, Pt NPs prepared by PLAL on graphene retain 95% of their ECSA. On the other hand, after an additional 800 cycles (0.6–1.5 V), the commercial sample only retains 35% of its ECSA, as the Vulcan supported NPs. This is less than reported in this range by Pizzutilo et al., ~40% after 1000 CVs, likely due to the use of an O2-saturated atmosphere in the present study. As the graphene-supported Pt NPs retained 75% of their ECSA under these conditions, they now display a higher durability than the commercial sample. We suspect the lack of defects in the GPG to lead to better corrosion resistance, hence better durability in ORR conditions.

4. CONCLUSIONS Platinum nanoparticles were prepared by PLAL and centrifuged to reduce polydispersity, reaching a size of ~5 nm, as determined by ADC and TEM. The particles were adsorbed on an almost defect free (as determined from Raman & XPS spectroscopy) high surface area GPG (BET 294 m2·g-1) and on Vulcan XC72. TEM and initial ECSA measurements confirmed good dispersion on both supports. Through long term stability tests, graphene supported nanoparticles displayed much better ECSA retention (75%) than Pt on carbon black (38%) and a commercial Pt/C catalyst (35%), likely due to better resistance to corrosion. 11

ACKNOWLEDGEMENTS EB gratefully acknowledges the support of the Alexander von Humboldt Foundation and of the Fonds de Recherche du Québec – Nature et Technologies. AM, CS, and HW gratefully acknowledge funding by the German Research Foundation within the Research Unit FOR2284 (WI 1958/3-1). SR. thanks the Mercator Research Center Ruhr (MERCUR) for financial support (Pr-2014-0044). In addition, the authors thanks Prof. R. Schmechel, L. Kühnel, and P. Fortugno for conductivity measurements, the Interdisciplinary Center for Analytics on the Nanoscale (ICAN) for the Raman and HR-TEM (M. Heidelmann) measurements and J. Jakobi for the TEM ones. SB, GM and TV gratefully acknowledge the financial support of the German Federal Ministry of Education and Research within the KMU-innovative program (Kontikat, KFZ: 02P16K591).

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CAPTIONS Fig.1 Schematic illustration of the reactor used for the gas-phase synthesis of graphene nanosheets. Fig. 2 Raman spectra of GPG and Vulcan, as well as their corresponding ID/IG ratio. Fig. 3 High resolution XPS spectra of GPG in the O1s and C1s region, with their respective curve fitting. Fig. 4 Mass weighted size distribution of the Pt NPs before and after centrifugation. The insert represents the ECSA of the Pt NPs before and after centrifugation, and the number weighted size distribution. Fig. 5 TEM and HRTEM micrographs as well as number-weighted size distribution of PtNP, respectively of laser-generated Pt NPs on Vulcan (A, B, C) and on synthesized graphene in (D, E, F). Fig. 6 XRD diffractograms of the Pt NPs on Vulcan and on Graphene Fig. 7 Initial and post ADT ORR curves for Pt NPs on Vulcan and on GPG, as well as on the commercial sample, recorded at 20 mV/s in 0.1M HClO4. Fig. 8 In (A), Pt CVs before and after ADT. In (B) and (C), Pt ECSA after the load ADT and after the start/stop ADT protocol for nanoparticles supported on Vulcan, on GPG and for commercial Pt/C, both normalized (B) and raw (C).

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Figure 1

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Intensity / arb. u.

D

G

ID/IG = 1.015 I2D/IG = 0.116

GPG Vulcan 2D

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500

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Figure 2

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C-C / C 1s C-H hn = 1486.6 eV sample-analyzer 45°

Intensity / a. u.

O 1s hn = 1486.6 eV sample-analyzer 45°

C-C, sp2 O: <1at%, C: >99 at% D-Parameter: 20.6  sp2: ~86%

p-p*

536

534

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Binding energy / eV

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Figure 3

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Pt NPs as prepared Pt NPs after centrifugation 0.30

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Figure 4

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PDI=0.083

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Figure 5

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Normalized intensity / -

ª

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¨ Graphite, 2h, P 63/m m c, ICSD: 76767 ª Platin, F m -3 m (225), ICSD: 76951 ª ª

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Figure 7

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p o s t 1 8

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Figure 8

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Highlights -

Laser-generated ligand-free Pt nanoparticles as benchmarking catalyst

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Homogeneous particle distribution on almost defect-free gas phase synthesized graphene

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Graphene supported Pt nanoparticles show a long term stability in oxygen reduction reaction due to good resistance to corrosion

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Graphical abstract

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