In Situ Stability Studies of Platinum Nanoparticles Supported on Mixed

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In Situ Stability Studies of Platinum Nanoparticles Supported on Mixed Ruthenium and Titanium Oxides (RTO) for Fuel Cell Cathodes Elisabeth Hornberger, Arno Bergmann, Henrike Schmies, Stefanie Kuehl, Guanxiong Wang, Jakub Drnec, Daniel Sandbeck, Vijay K. Ramani, Serhiy Cherevko, Karl J. J. Mayrhofer, and Peter Strasser ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02498 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Catalysis

In Situ Stability Studies of Platinum Nanoparticles Supported on Mixed Ruthenium and Titanium Oxides (RTO) for Fuel Cell Cathodes

Elisabeth Hornbergera, Arno Bergmannb, Henrike Schmiesa, Stefanie Kühla, Guanxiong Wangc, Jakub Drnecd, Daniel J. S. Sandbecke,f,g, Vijay Ramanic, Serhiy Cherevkoe,f, Karl J. J. Mayrhofere,f,g, Peter Strassera* a

Department of Chemistry, Chemical Engineering, Division Technical University of Berlin, 10623

Berlin, Germany b

Fritz-Haber-Institute of the Max-Planck-Society, Department of Interface Science, Faradayweg 4-6,

14195 Berlin, Germany c

School of Engineering & Applied Science, Washington University in St. Louis, 63130 St. Louis, MO,

USA d

European Synchrotron Radiation Facility (ESRF), 38000 Grenoble, France

e

Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich,

Egerlandstr. 3, 91058 Erlangen, Germany f

Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, Düsseldorf, Germany

g

Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-

Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany *Corresponding author email address: [email protected]

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Abstract Using a variety of in situ techniques, we tracked the structural stability and concomitantly the electrocatalytic oxygen reduction reaction (ORR) of platinum nanoparticles on ruthenium-titanium mixed oxide (RTO) supports during electrochemical accelerated stress tests, mimicking fuel cell operating conditions. High-energy X-ray diffraction (HE-XRD) offered insights in the evolution of the morphology and structure of RTO-supported Pt nanoparticles during potential cycling. The changes of the atomic composition was tracked in situ using scanning flow cell measurements coupled to inductively coupled plasma mass spectroscopy (SFC-ICP-MS). We excluded Pt agglomeration, particle growth, dissolution or detachment as cause for the observed losses in catalytic ORR activity. Instead, we argue that Pt surface poisoning to be the most likely cause of the observed catalytic rate degradation. Data suggest that the gradual growth of a thin oxide layer on the Pt nanoparticles due to strong metal-support interaction is the most plausible reason for the suppressed catalytic activity. We discuss the implication of the identified catalyst degradation pathway, which appear to be specific for oxide supports. Our conclusions offer previously unaddressed aspects related to oxide-supported metal particle electrocatalyst frequently deployed in fuel cells, electrolyzers or metal air batteries. Keywords: oxide supported platinum nanoparticles · in situ X-ray studies · electrocatalysis · oxygen reduction reaction · polymer-electrolyte membrane electrolyte fuel cell


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1

Introduction

Polymer-electrolyte membrane electrolyte fuel cells (PEMFC) are flexible power generation devices for automotive, stationary or portable applications. However, nanoscale Pt-based particle catalysts, typically supported on high surface area carbons, inside PEMFC electrodes still suffer from insufficient morphological stability and resulting declining catalytic activity. Some key degradation mechanisms have been identified: Pt dissolution, particle detachment or crystal growth via Ostwald ripening and migration[1]. Carbon corrosion is the key degradation process of the carbon support material causing electrical isolation and detachment of the individual Pt particles followed by rapid agglomeration and loss of surface area[1]. To mitigate catalyst degradation and boost catalytic activity, more active metal catalysts combined with more stable support materials are needed. To achieve this, two major strategies have been pursued[2]. The first strategy is aimed in tailoring of the electronic structure of the Pt particles by various procedures: a) Pt alloy/dealloy[3-11], b) core-shell[12-14], c) shapecontrolled nanocrystals[15-16] and nanoframes[17-19]. The second strategy towards more stable catalysts aims at new supports. In particular, moving away from carbon supports to oxidic supports is a promising route to lowering corrosive degradation, thanks to the oxidative stability of oxides. Beyond their oxidation stability, oxide supports also exhibit so-called strong metal-support interaction (SMSI) between metal nanoparticles and oxidic support substrates, which structurally stabilizes metal nanoparticles. Tauster et al. associated the term SMSI with the formation of an intermetallic bond between the catalytic noble metal and the oxide support[20-21]. Electron donation from occupied d states of the oxide support to the vacant d orbitals of the metal atoms results in an altered electronic structure creating special contact zones, but also masking of significant portions of it. The SMSI effect of Pt nanoparticles supported on metal oxides is claimed to enhance catalytic activity and stability towards the oxygen reduction reaction (ORR) as this reaction is one of the major obstacle of the total fuel cell system[22-24]. A. Kumar et al. studied Pt nanoparticles supported on rutile-phase tantalum-modified titanium oxide (Ta0.3Ti0.7O2) and determined reasonable ORR activity and high durability in membrane electrode assemblies (MEAs) tests. They related this improvement to an increased electron density in Pt as discovered via X-ray absorption near edge structure (XANES) analysis[25]. Until now various metal oxides have been investigated to find suitable substitutes for carbon, including SnO2[26-31], ITO[32-33], WOx[34-36], SiO2[37], TiO2[38-39] and RuO2[37, 39-40]. Wide band-gap semiconducting oxides such as TiO2 are promising metal oxide support materials[22]. Their corrosion resistance and SMSI result in enhancement of chemical stability and enhanced ORR activity[41]. However, poor conductivity of the pure oxide limits their application as cathode catalyst support. This is why doped TiO2, typically doped with more conducting transition metal oxides in order to significantly increase the electrical conductivity have become the focus of recent studies[42]. The mixed metal oxides of TiO2 and RuO2 (denoted herein as RTO) combines two favorable properties as it shows a high electrical conductivity and chemical stability both in acidic and oxidative media[43-44]. Ho et al. reported improvements of the electrocatalytically activity and durability for Pt/RTO[45]. Using a specific RTO with a molar ratio of 3 ACS Paragon Plus Environment

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3:7, the authors claimed stronger electronic interactions between Pt particles and support, compared to commercial Pt/C and PtRu/C catalysts. This study also established for RTO (3:7, hydrated) a link between the catalytic activity, durability, the tolerance of CO poisoning, the lower number of unfilled d-states and the high proton conductivity. C.-P. Lo et al. reported, that Pt/RTO (1:1)[39] if compared to Vulcan shows a ten times higher electrochemical stability during potential cycling in an aggressive AST simulating 10,000 start-up/shut-down cycles. The MEA performance equipped with Pt/RTO (1:1) exhibited promising properties, although the catalyst was not matching up the Pt/C benchmark. This might be due to large platinum particle sizes on the oxide support influencing the mass activity and non-optimized electrode preparation that increased ohmic and transport losses. However, to date, only little effort has been dedicated to a more fundamental investigation of the electronic and catalytic behavior of the metal particle catalyst/oxide support systems, in particular with emphasis on a combined concomitant analysis of Pt particle and oxide support degradation. In a previous contribution, our group reported on the stability of Pt nanoparticles supported on indium tin oxide (ITO) under electrochemical protocols to mimic operating conditions in two potential regimes[33]. During start-up/shut-down cycles, Pt/ITO shows excellent structural stability and catalytic activity compared to a commercial Pt/C, but under catalytic ORR operation conditions, the Pt/ITO ensemble suffers from ITO corrosion. In situ high-energy X-ray diffraction (HE-XRD) and in situ scanning flow cell measurements connected to inductively coupled plasma mass spectroscopy (SFC-ICP-MS) were carried out to track the catalyst degradation. The HE-XRD patterns show decreasing intensities in the ITO reflexes indicating a loss of the support material likely due to dissolution. SCF-ICP-MS supports this hypothesis by detection of a steady dissolution of In and Sn. The observed declined ORR activity of Pt/ITO could be explained by partial redeposition of In and Sn atoms and decoration effects on the Pt particle surface resulting in an oxidic Pt surface adlayer. In expanding our previous efforts to understand the mechanisms of catalyst/oxide support degradation, we here report on the in situ stability and activity of three Pt/RTO electrocatalysts for cathode material with different Pt nanoparticle loadings. We applied an electrochemical accelerated stress test mimicking fuel cell operating conditions in a rotating disk electrode (RDE) setup and HE-XRD to follow the morphological and structural evolution of Pt and RTO nanoparticles during potential cycling. With in situ SFC-ICP-MS, the evolution of the atomic composition of the catalyst/support couple was tracked. Together, we were able to track the evolution of stability and activity and discuss the beneficial role of SMSIs in the observed catalyst stability. We also identify the most plausible degradation mechanism of Pt/RTO couples and discuss its origin and implications.


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2 2.1

Experimental Methods

Synthesis of RTO

RTO powders with molar metal ratios of 1:1 and 1:9 were prepared in a wet chemical approach[39]. To achieve an equimolar ratio of both Ti and Ru, 12.5 mmol titanium(IV) oxide (TiO2, Aeroxide P25, Acros Organics) was dispersed in 250 mL deionized water and homogenized by sonification for 30 min. 12.5 mmol ruthenium(III) chloride hydrate (RuCl3·xH2O, 35–40% Ru, Acros Organics) was poured to form a mixture while stirring for 30 min. 0.05 M KOH(aq) was then dropwise added into the stirred solution until the pH reached 7. The black powder was filtered out and thoroughly washed with deionized water. The RTO powders were dried at 120 °C for 8 h and further calcined at 450 °C for 3 h in air. In order to achieve the molar ratio of 1:9, the ratio of RuCl3·xH2O vs. TiO2 was adjusted.

2.2

Synthesis of Pt Nanoparticles

An overview table of all synthesis parameter is given in the supporting information (see Table S1)

2.3

Synthesis of Pt Nanoparticles on RTO – High Loading (HL)

Pt nanoparticles were synthesized directly on RuO2·TiO2 by reducng the Pt precursor with formic acid[40]. A suspension of 5 mmol RTO (1:9) in a reaction solution of 1.9 mmol hexachloroplatinic acid hexahydrate (H2PtCl6·6H2O, ACS reagent, ≥37.50% Pt basis, Sigma–Aldrich) and 30 mL formic acid (∼98%, Fluka) in 600 mL of water was homogenized by sonification for 30 min. Under stirring, the suspension was heated at 80 °C for 2 h. The product was collected by vacuum filtration, thoroughly washed with deionized water and dried at 60 °C.

2.4

Synthesis of Pt Nanoparticles on RTO – Intermediate Loading (IL)

0.60 mmol Platinum(II) acetylacetonate (Pt(acac)2, 48% Pt min, Alfa Aesar) and 1.2 mmol 1,2tetradecanediol (90%, Sigma-Aldrich) were dissolved in 50 mL dibenzyl ether (≥98.0%, SigmaAldrich). 300 µL of each oleic acid (70%, AlfaAesar) and oleylamine (70%, Alfa Aesar) were added to the reaction solution. The reaction solution was heated up to 175 °C for 3 h. The formed Pt particles were washed with ethanol (absolute, VWR chemicals) and dispersed in 20 mL toluene (≥99.8%, Sigma-Aldrich). 100.47 mg RTO (1:1) were dispersed in 20 mL ethanol, mixed with the dispersed Pt particles, and further stirred and sonicated by a horn sonifier for 1 h. The reaction solution was stirred overnight, before being centrifuged and washed with ethanol. The supported Pt particles were freeze dried overnight.

2.5

Synthesis of Pt Nanoparticles on RTO – Low Loading (LL)

0.60 mmol Pt(acac)2 and 1.2 mmol 1,2-tetradecanediol were dissolved in 50 mL dibenzyl ether. 300 µL of each oleic acid and oleylamine were added to the reaction solution. The reaction solution was heated up to 175 °C for 1 h. The Pt particles were washed with ethanol and dispersed in 20 mL toluene. 73.13 mg RTO (1:1) were dispersed in ethanol, mixed with the dispersed Pt particles, and further stirred and sonicated by a horn sonifier for 1 h. The reaction solution was stirred overnight, 5 ACS Paragon Plus Environment

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before being centrifuged and washed with ethanol. The supported Pt particles were freeze dried overnight.

2.6

Ex situ X-ray diffraction (XRD)

Ex situ lab-based XRD experiments were performed at a Bruker D8 Advance Diffractometer in BraggBrentano geometry using Cu Kα radiation at a wavelength of 0.154 nm operated at 40 kV and 40 mA. Diffraction patterns were collected in a 2θ-range of 20–90° with a step size of 0.02° and a counting time of 7 s per step.

2.7

Inductively coupled plasma optical emission spectroscopy (ICP-OES)

A Varian 715-ES-inductively coupled plasma (ICP) analysis system with optical emission spectroscopy (OES) detection was used to quantify the Pt loading of the catalysts. 6-8 mg of the catalyst were dissolved in a 3:1:1 mixture of concentrated hydrochloric acid, sulfuric acid and nitric acid (AnalaR, NormaPur, VWR) overnight. For further digestion the solution was treated in a microwave for 10 min at 180 °C with a ramping time of 10 min. The upper limited pressure was set to 18 bar. The dissolved sample was cooled down, filtered, and diluted with ultrapure water. 10 mL of the digested sample were transferred to a sample tube and measured by ICP-OES. The standard concentrations of Pt were 1, 5 and 10 ppm. The chosen wavelengths for detecting the Pt emission lines were 203.646, 204.646, 212.863, 214.424, 217,468, 224.552 and 265.945 nm.

2.8

Transmission electron microscopy (TEM)

TEM images were acquired using a FEI TECNAI G2 20 S-TWIN instrument using a LaB6-cathode operated at 200 kV. To prepare TEM samples, a small amount of the catalyst powder was dispersed in ethanol. 10 µL of the suspension was pipetted onto a copper mesh (4300 Mesh) with lacey carbon film and dried for 3 min at 60 °C in air.

2.9

Electrode preparation

Inks for electrochemical measurements were prepared by dispersing an exactly determined amount of 3-8 mg powder in a mixture of 79.6 Vol-% ultrapure water, 20.0 Vol-% 2-propanol (≥99.95%, SigmaAldrich) and 0.4 Vol-% Nafion solution (5 wt-% in 2-propanol, Sigma-Aldrich). The mixture was horn sonified for 30 min. The glassy carbon (GC) working electrode was polished using first a micro polish solution (1 µm, Buehler Alpha) on a Nylon sheet and second a micro polish solution (0.05 µm, Buehler Alpha) on a Microcloth® sheet. Polishing solution residues on the electrode were removed by sonicating the electrodes for 5 min each in water, acetone, and ethanol. The clean electrode was dropcasted with 10 µl of the ink solution and dried in air for 8 min at 60°C resulting in a Pt loading on the GC disc of 25 µgPtcm-2 for HL- and IL-Pt/RTO and 8 µgPtcm-2 for LL-Pt/RTO.

2.10 Electrochemical measurements Electrochemical measurements were performed using a BioLogic Instruments SP-150/200 potentiostat in a glass cell suitable for a three-electrode arrangement. A GC rotating disc electrode (RDE) with a 6 ACS Paragon Plus Environment

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ACS Catalysis

total surface area of 0.196 cm2 acted as working electrode. A mercury/mercury sulfate reference electrode (Hg/HgSO4, Ametek, potential -0.72 V vs. reversible hydrogen electrode (RHE)) was integrated into the experimental setup via a Luggin-capillary. A wrapped 5x5 cm2 Pt mesh mounted on a Pt wire was used as counter electrode with a high surface. 0.1 M HClO4 (diluted with ultrapure water from 70% HClO4, 99.999% trace metal bases, Sigma-Aldrich) was used as electrolyte. Cyclic voltammetry (CV) was performed in a potential range of 0.05-1 V in N2 purged electrolyte at a scanrate of 100 mVs-1. Linear sweep voltammetry (LSV) in anodic direction for ORR activity tests was performed in a potential range of 0.05-1 V in O2 saturated electrolyte with a scanrate of 5 mVs1

and at a rotation speed of 1600 rpm. The accelerated stress test (AST) consisting of 5000 cycles was

adapted from the 2016 fuel cell targets put forward by the US Department of Energy (DoE) and performed in a potential range of 0.6-0.95 V in N2 purged electrolyte at a scanrate of 100 mVs-1. A detailed scheme of the conducted electrochemical protocol is given in the supporting information (see Figure S1). CO stripping was performed in a potential range of 0.05 - 1 V in N2 purged electrolyte at a scanrate of 50 mVs-1, after creating a CO-saturated covered catalyst surface in CO saturated electrolyte. Additionally, a reduction step at a constant potential of 1.2 V and -0.35 V was conducted after AST for 3 h. All electrode potentials are referenced against the RHE and corrected for the iRdrop in a high frequency resistance RHF range measured by potentiostatic electrochemical impedance spectroscopy at 0.5 V. To ensure reproducibility, all measurements were performed three times.

2.11 In situ high energy X-ray diffraction (HE-XRD) In situ HE-XRD experiments were performed at the beamline ID31 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. For calibration of the working distance CeO2 (NIST SRM 674b) was used as reference material. The experiments were performed at an energy of 78 keV. The monochromized X-ray beam was focused to a size of 20x6 µm (horizontal x vertical). The detector was a Pilatus3 X CdTe 2M. For each working electrode, 10x10 µl of an ink solution were drop-casted on a 1x4 cm carbon sheet (graphite diffusion layer (GDL) 39BC, SGL Group GmbH, Meitingen Germany) to maximize the amount of sample in the X-ray spot. This carbon sheet was wrapped with copper tape and inserted into the in situ transmission cell setup in a three electrode arrangement. A silver/silver chloride reference electrode (Ag/AgCl, 3 M KCl, World Precision Instruments, Berlin, Germany) was implemented into the experimental setup. A Pt wire was used as counter electrode. Electrochemical protocols were conducted with a SP-240 Potenstiotat (BioLogic Instruments).

2.12 Scanning flow cell inductively coupled plasma mass spectrometry (SCF-ICP-MS) Inks of the Pt/C and Pt/RTO catalysts were prepared using ultrapure water (18.2 MΩ·cm, PURELAB® Plus System, ELGA), a solution of 5% Nafion (Sigma-Aldrich) and an adjusted amount of catalyst powder yielding a Pt loading of 8.33 µgPt·cm-2 for the HL- and IL-Pt/RTO samples and 2.67 µgPt·cm-2 for the LL-Pt/RTO sample. The Nafion : catalyst ratio was equal to the RDE 7 ACS Paragon Plus Environment

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experiments. A GC sheet as working electrode (SIGRADUR® G, HTW) was prepared by polishing with Al2O3 suspension (1 µm) and thoroughly rinsing with ultrapure water. The ink was homogenized by sonification and pipetted in drops of 0.3 µL onto the glassy carbon sheet resulting in a catalyst spot size of ca. 0.008 cm2. Measurements of the dissolution behavior of Pt, Ru and Ti during the electrochemical cycling were pursued on a scanning flow cell connected to an inductively coupled mass spectrometer (NexION® 300X, Perkin Elmer) as described elsewhere[46-47]. Cell contact area was 0.035 cm2. A graphite rod acted as counter electrode and a saturated Ag/AgCl electrode as reference electrode. Measurements were carried out in an Ar-purged electrolyte flow of 0.1 M HClO4 (ultrapure water, Suprapur® 70% perchloric acid, Merck). The flow rate was ca. 170 µL·min-1. 10 µg·L-1 Re (187Re measured) for Pt, 10 µg·L-1 Rh (103Rh measured) for Ru and 20 µg·L-1 Sc (45Sc measured) for Ti were used as internal standards. The dissolution of the metals were obtained via integration of the ICP-MS signal data by the flow rate (normalized to total drop casted metal mass). The integration boundaries were taken where the baseline signal was approximately constant. The limit of detection and limit of quantification was taken as three and ten times the standard deviation of the blank signal divided by the slope of the calibration line, respectively.

3

Results

Figure 1 a displays the XRD patterns of the three synthesized catalysts addressed in this study, that is high loading (HL-), intermediate loading (IL)- and low loading (LL-)Pt/RTO. All three materials showed broad reflections of face-centered cubic (fcc) Pt (PDF #00-04-0802). The RTO support reflexes could be assigned to three separated crystalline phases: RuO2 (PDF #01-088-0322), rutile TiO2 (PDF #01-076-1939) and anatase TiO2 (PDF #01-076-1286). All reflexes referred to the corresponding reference pattern and no additional phases were present. Depending on the ratio of TiO2 to RuO2 in the RTO support composition, the reflexes of either TiO2 or RuO2 were more prominent. This is clearly visible at the reflex (101) of RuO2 and rutile TiO2. If the Ru content was low, as in case of HL-Pt/RTO, for which the support material has a Ru:Ti ratio of 1:9, the lower Ru content caused less intense reflexes.


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ACS Catalysis

Figure 1: Structure and morphology of Pt nanoparticles on RTO (HL- (green), IL- (blue) and LL- (orange) Pt/RTO). Panel a) shows the X-ray diffraction pattern and the corresponding reference reflexes of Pt (line: PDF #00-04-0802), RuO2 (*: PDF #01-088-0322), rutile TiO2 (+: PDF #01-076-1939) and anatase TiO2 (x: PDF #01-076-1286). Transmission electron microscopy images of the as prepared catalysts are shown in panel b), c) and d). Panel e) shows the Pt crystallite sizes as determined by Rietveld refinement from X-ray diffraction patterns taken ex situ as prepared and in situ after activation and after AST.

The XRD patterns were further analyzed by Rietveld refinement, confirming the RTO support compositions of RuO2 : TiO2 ratios of 1:9 (HL) and 1:1 (IL, LL). The weight fractions of crystalline Pt of 39 wt-% (HL-), 19 wt-% (IL-) and 7 wt-% Pt (LL-Pt/RTO) are in good agreement with those determined by compositional analysis with ICP-OES, i.e. 43 wt-% (HL-), 18 wt-% (IL-) and 6 wt-% Pt (LL-Pt/RTO). To determine the Pt particle size and distribution, transmission electron microscopy (TEM) was performed (Figure 1 b,c,d, and additional images in Figure S2a,b,c). HL-Pt/RTO initially showed a quite homogeneous distribution of Pt particles on the RTO support with an average particle size of 3.9 ± 0.8 nm (see Figure S2d). Similarly, IL- and LL-Pt/RTO showed a rather homogeneous Pt particle distribution. Since the ratio of RuO2 was higher, the experimental Z-contrast was much lower and this is why particle histograms could not be obtained. Additionally, LL-Pt/RTO showed small amounts of agglomerates. Thus, from our refinement, crystallite sizes of the Pt particles of 4.3 ± 0.1 nm (HL-), 6.1 ± 0.1 nm (IL-) and 4.3 ± 0.2 nm (LL-Pt/RTO) were obtained (see Figure 1 e). For HL-Pt/RTO, the Pt crystallite size determined by Rietveld refinement and the mean Pt particle size are in good accordance within the given error. The Pt particles of HL- and LL-Pt/RTO have a similar size while for IL-Pt/RTO the particles are slightly bigger. To investigate the long term stability of Pt/RTO during simulated fuel cell conditions, accelerated stress tests in a RDE setup were conducted. Figure 2 shows the cyclic voltammetry (CV) of HL-, IL-


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and LL-Pt/RTO in the initial state after conditioning of the catalysts with 100 cycles of activation in the potential range from 0.05-1 V and after 5,000 cycles AST in the potential range from 0.6-0.95 V.

Figure 2: Cyclic voltammetry of HL- (green), IL- (blue) and LL- (orange) Pt/RTO after activation and after the AST in N2 saturated electrolyte from 0.05-1 V with a scan rate of 100 mVs-1. Bar plots in the insets represent the ORR mass activity (jm) as determined at 0.9 V from linear sweep voltammetry recorded in O2 saturated electrolyte from 0.05-1 V with a scan rate of 5 mVs-1 and at a rotation speed of 1600 rpm. All shown graphs have been corrected for iR-drop.

The CVs show the characteristic electrochemical Pt responses in the Hupd region (0.05-0.4 V) and PtO/OH region (0.6-1 V). HL-Pt/RTO shows small capacitive currents (0.4-0.6 V), while IL-and LLPt/RTO show much larger capacitive current as the higher fraction of RuO2 contributes to the adsorption processes of O/OH[39]. After the AST, the currents in the Hupd region stay at a stable level. This can be quantified in terms of the electrochemical surface area (ECSA). For all three catalysts, the initial Hupd-ECSA of 19 ± 1 (HL-), 15 ± 1 (IL-) and 52 ± 11 m2gPt-1 (LL-Pt/RTO) decreases only by 36% after the AST (see also Table S2). The TEM based particle size distribution of HL-Pt/RTO after the AST showed a slightly lower average particle size of 3.6 ± 0.8 nm (see Figure S2e). Depending on the Pt-loading, the values of the Hupd-ECSA differ. For LL-Pt/RTO, the Pt-loading is the lowest and the particles are more likely to be locally separated from each other resulting in a higher Pt-mass based Hupd ECSA. A commercially available carbon-based Pt reference (20 wt% Pt/Vulcan, BASF) was used as reference and comparison for the Pt/RTO-supported electrocatalysts. The reference was measured under equal AST conditions[33]. The initial Pt/C Hupd-ECSA of 61 m2gPt-1 is higher than for Pt/RTO, but less stable as it decreases by 11%. In contrast to the stable Hupd-ECSA, the current in the Pt-O/OH region decreases stronger suggesting less oxidizable Pt sites. To further quantify the oxidizable Pt sites, the electrochemical CO oxidation behavior was studied with CO stripping experiments conducted after the activation CVs and after AST (see Figure S3). The CO stripping revealed initial CO-ECSAs of 17 ± 2 (HL-), 21 ± 2 (IL-) and 71 ± 20 m2gPt-1 (LL-Pt/RTO) that are diminished by 1519% after the AST. The ORR mass activity jm (see also Figure S4 + Table S2) of initially 88 ± 14 (HL-), 57 ± 6 (IL-) and 167 ± 73 mAg-1Pt (LL-Pt/RTO) drops after 5,000 CV about 26-49%. The 10 ACS Paragon Plus Environment

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ACS Catalysis

commercial Pt/C shows a higher initial ORR mass activity of 160 mAg-1Pt, that only declines about 14%[33]. Attempts to fully recover the original Pt ORR activity after a chronoamperometric reduction remained unsuccessful (see Figure S5).

In situ XRD investigations. In situ high-energy XRD was performed to investigate the structural changes of both Pt particles and RTO in terms of changes of the crystallites while potential cycling. Figure S6 shows the X-ray diffraction patterns vs. AST cycle numbers. It can be clearly seen, that the reflexes of Pt and the support phases show no large shifts or changes in intensity suggesting high structural integrity of the nanoparticle-support ensemble. To gain deeper insights, Rietveld refinement was performed of the in situ patterns.

Figure 3: a) Scheme of the in situ HE-XRD setup. The incident X-ray beam hits the sample, which is attached to a carbon sheet, and creates a diffraction pattern at the detector. RE: reference electrode, WE: working electrode, CE: counter electrode. b) Rietveld refinement results of in situ high energy X-ray diffraction. The upper panel shows the Pt weight fraction, the lower panel the Pt particle coherence length during the AST (for non-defective particles equal to the particle size). Both exhibit a high Pt particle stability of HL- (green), IL- (blue) and LL- (orange) Pt/RTO while potential cycling.

Figure 3 shows that for all three catalysts the crystalline fractions of the Pt remained constant during the AST. For HL-Pt/RTO a slight decrease of 3 wt-% was found. The crystalline Pt weight fractions were also consistent to those determined by compositional analysis, i.e. 37 wt-%, 19 wt-% and 7 wt-% Pt for HL-, IL- and LL-Pt/RTO, respectively. Furthermore, no changes within the error of the fits were observed in the Pt coherence lengths (for non-defective particles equal to the particle size) with initial values of 4.7 ± 0.1 (HL-), 5.2 ± 0.1 (IL-) and 4.6 ± 0.2 nm (LL-Pt/RTO). Simultaneously, the crystalline fractions of the RTO support remained constant during the AST as well (see Figure S7). The Ru : Ti ratios of the mixed metal oxides are depicted in the determined weight fractions of RuO2 vs. TiO2. For HL-Pt/RTO the ratio is 10 ± 1 % RuO2 vs. 52 ± 1 % TiO2, in case of IL- and LL-Pt/RTO the ratios are 42 ± 1 % RuO2 vs. 36 ± 2 % TiO2 and 51 ± 1 % RuO2 vs. 40 ± 1 % TiO2, respectively. Minor changes were observed as for IL-Pt/RTO the TiO2 rutile fraction showed a slight decrease of 11 ACS Paragon Plus Environment

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4 wt-%, while for LL-Pt/RTO a slight decrease of the RuO2 fraction of 3 wt-% was found. No significant changes of the initial coherence lengths of RuO2, TiO2 rutile and anatase were observed, for instance for RuO2 these time invariant values amounted to 10 ± 1 (HL-), 6.9 ± 0.1 (IL-) and 9.9 ± 0.1 nm (LL-Pt/RTO) (see also Figure S7). Considering the minor experimental time variations in the analyzed structural parameters over the stress test, we conclude, that RTO supports show excellent compositional and structural stability.

Figure 4: In situ scanning flow cell inductively coupled plasma mass spectrometry of HL- (green), IL- (blue) and LLPt/RTO (orange). While a loss of Pt is detected due to the start of the electrochemical protocol, during the AST no dissolution of Pt, Ti or Ru is detected.

In situ SFC-ICP-MS investigation. To learn more about the metal dissolution behavior of the Pt/RTO catalysts during cycling conditions, in situ Scanning Flow Cell (SFC) studies, coupled to an on-line ICP-MS were performed. Figure 4 shows the Pt, Ru and Ti dissolution profiles in a comparable shortened AST measurement (40 CV, 0.6 – 0.95 V, 100 mVs-1). A small Pt dissolution peak was evident for all three catalysts when the electrochemical protocol was started (denoted as start EC). 12 ACS Paragon Plus Environment

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After this, during the first 100 activation CVs and the subsequent AST no further Pt dissolution was detected. Therefore, the total Pt loss was as low as 0.02 (HL-), 0.03 (IL-) and 0.09 % (LL-Pt/RTO). As to the metals of the support material, Ru and Ti, no dissolution was detected at all. Only for ILPt/RTO a minor Ru dissolution peak was detected at starting the electrochemical protocol, which was above the limit of detection (LOD) but below the limit of quantification (LOQ). For HL-Pt/RTO, the very low Ru loading was resulting in a noisier signal and spikes of Ti dissolution were detected within the noise signal.

4

Discussion

We conducted a thorough in situ stability study in order to evaluate and better understand the degradation behavior of Pt nanoparticle supported on a recently reported new Ru, Ti mixed metals oxide. Three different Pt weight loadings (HL-, IL- and LL-Pt/RTO) were synthesized and investigated to evaluate the effect of particle density on stability. The catalysts were tested in an AST to simulate fuel cell operating conditions. Using in situ techniques, we were targeting a deeper understanding of the dynamic processes governing catalyst and support stability and degradation. We discovered, that the Pt/RTO ensembles show excellent structural stability against particle growth and agglomeration and a high stability against dissolution. However, the electrochemical responses before and after the AST reveal a reproducible and important disparity in terms of the Hupd-ECSA staying essentially constant, while both catalytic ORR activity and the CO-ECSA values declined drastically after the AST. The CO-ECSA is closely linked to the ORR activity in so far as it can be regarded as a measure of the accessibility of the Pt nanoparticle surface for CO adsorbates. Nakada et al. reported for the Pt/SnO2 couple a quite similar behavior. In the hydrogen adsorption/desorption region no shape change was observed while cycling from 0.05-1.5 V, both the anodic current due to Pt oxide formation (> ca. 0.8 V) and the cathodic current due to the Pt oxide reduction (> 0.6 V) decreased[48]. The potential window of this AST (0.6-0.95 V) was selected to simulate fuel cell operation conditions and does not include the potential range for Pt oxide formation/reduction. Certainly, the surface properties of the Pt nanoparticles appeared to have changed during and after the AST resulting in a way as to lower the ORR activity and CO-ECSA. However, neither Pt nanoparticle nor the RTO support showed any indication of structural or morphological instability based on our in situ HE-XRD and SCF ICP-MS experiments. Hence, the reason for the activity declines must lie elsewhere. To account for the declining catalytic activity in the face of declining CO-ECSA and stable HupdECSA values, we turn our attention to SMSI effects. While SMSI effects are known to result in beneficial electronic effects due to the formation of oxidic overlayers on top of the catalytically active noble metal phase (particles), generating oxide patches in atomic proximity to the noble metal surface, these “oxide decoration” processes may also constitute an unfavorable blocking. The metal surface becomes partially or completely encapsulated by an atomically thin oxide layer[49]. Labich et al.[50] 13 ACS Paragon Plus Environment

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correlated the SMSI with the surface energy (γ) of the oxide supports, whereas oxides with low surface energies (e.g. TiO2, V2O5) contribute more to the SMSI than oxides with relatively high surface energies (e.g. SiO2, Al2O3). In turn, metals having a relatively high surface energy (eg. Pt, Rh, Pd) favor encapsulation[51]. SMSI effects may create new synergistic reaction sites between the metal and the support, which is sometimes referred to as a bifunctional effect[49]. At this metal-support interface, synergetic spillover phenomena of surface adsorbates may occur. Reactive species are believed to migrate from the metal at the perimeter sites on top of the covering oxide layer, which provides the second reaction site. We believe that these well-known processes in heterogeneous gas phase catalysis, more specifically in the oxidation of CO by molecular oxygen, are also plausible mechanisms in electrocatalysis. They could influence the electrochemical CO oxidation and furthermore have a beneficial effect on the ORR mechanism, where O/OH adsorbates take a crucial part in. Oxophilic Ru centers could readily provide oxygenated surface species by dissociating water at lower potentials than platinum, which react with adsorbed CO at Pt to CO2[52]:

Pt CO Ru OH → Pt Ru CO H e

1

According to this plausible hypothesis, both metal and support would participate in the reaction leading to a decreased CO poisoning of the Pt. Figure S3 shows the performed CO stripping experiments. The influence of the Ru content to the CO reduction is clearly visible. The CO oxidation onset potential for IL- and LL-Pt/RTO is lower than for HL-Pt/RTO.

Figure 5: Illustration of the hypothetical degradation process: Formation of a thin oxide overlayer on top of the RTOsupported Pt nanoparticle. The overlayer can be penetrated by a proton, but is impermeable for oxygen and carbon monoxide.

Taking all these effects associated with the SMSI into account would actually reconcile the experimentally observed, seemingly contradicting trends in Hupd- and CO-ECSA values during stability tests. CO poisoning and stripping on SMSI-modified Pt/RTO nanoparticles is lower due to the limited accessibility of CO and O/OH adsorbates to the Pt site due to the emerging oxide overlayer during potential cycling. However, the covering oxide remains permeable for H adsorbates. 14 ACS Paragon Plus Environment

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Subnanoscale encapsulation (decoration) layers are well known in literature[53-55]. Zhang et al. showed the existence of an encapsulation layer atop the Pt nanoparticles surface supported on TiO2 by X-ray photoelectron spectroscopy (XPS) combined with Ar+ ion sputtering[56]. Figure 5 illustrates the hypothetical degradation process. Initially, the Pt nanoparticle is freely accessible for protons, oxygen and carbon monoxide, but by time of the stress test a degradation process is initiated leading to the formation of an oxide coverage on top of the Pt nanoparticle. The subnanoscale oxidic overlayer consists of TixOy and likely of RuxOy. The oxidic overlayer is than still accessible for proton, but not anymore for oxygen and carbon monoxide. Similar behavior was observed in case supported Pt nanoparticles supported on ITO[33]. The phenomena of a selectively permeable oxidic layer is well known in photo-electrochemistry and intentionally introduced to catalysts, i.e. for core/shell noblemetal/Cr2O3 catalyst, in order to inhibit the ORR and facilitate the hydrogen evolution reaction (HER)[57-58]. Indeed, the stable Hupd-ECSA is an indication for a very thin overlayer of oxide. The oxide overlayer formation appeared to be partially irreversible, as attempts to fully recover the original Pt ORR activity after a chronoamperometric reduction remained unsuccessful (see Figure S5). What is further supporting our SMSI-based oxide overlayer hypothesis, is the fact that a TEM analysis of the evaluable Pt particle size distribution of HL-Pt/RTO after the AST shows no growth of the nanoparticles (see Figure S2d,e). The TEM images taken after the AST indicate that all the three Pt/RTO catalysts show no pronounced tendency to form agglomerates. Additionally, in situ HE X-ray investigations and Rietveld refinement show no increase of the Pt crystallite size or changes in the crystallite. An increased size of a particle caused by a very thin potentially amorphous overlayer is likely to be invisible by TEM and HE-XRD investigation. Analyzing the other contributing crystallite phases of the support material, RuO2 and TiO2 anatase and rutile, no significant changes of size or fraction are observed in the AST for all the three catalysts. No formation of an additional oxide (layer) phase is observed and the existing crystallite phases show no decreased signal towards amorphization processes. Nevertheless, nanoscale amorphization processes of RuO2 or TiO2 onto Pt could not be excluded. The results of the in situ SCF-ICP-MS measurements support the idea of a thin oxide layer, as no dissolution of Pt, Ru or Ti during the AST could be detected.

5

Conclusions

We have explored and analyzed the evolution of structural, morphological and compositional stability of Pt nanoparticle supported on the mixed metal oxide of ruthenium and titanium oxides (RTO) under electrochemical potential cycling. The Pt nanoparticles were deposited on RTO resulting in a welldefined distribution. The electrochemical stability was investigated in an accelerated stress test mimicking fuel cell operating conditions. We tracked the evolution of crystal structure, particle size, and metal ion dissolution by in situ measurements. We are able to show that the Pt nanoparticles on RTO show an excellent structural stability. Despite also the high stability against dissolution, however, the electrochemical activity data decreases with time, indicating another origin for the losses in ORR 15 ACS Paragon Plus Environment

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activity. It is proposed, that the Pt particles might be covered with a thin (partial) oxide layer due to strong metal-support interaction. This strong metal-support interaction results in decreased ORR activity and CO oxidation. Hence, we discuss a degradation pathway of oxide supported Pt nanoparticles leading to a better understanding of non-carbon based cathode electrocatalysts for PEMFC.


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6

Supporting Information

Synthesis parameter, electrochemical characterization data, in situ high-energy X-ray diffraction data, TEM images after AST

7 Acknowledgments The German Research Foundation (DFG) with the grant STR 596/4-1 (“Pt-Stability”) and the German Federal Ministry of Education and Research (BMBF) with the grant 03SF0531B (“HT-linked”) supported this work. The authors acknowledge the ESRF for the synchrotron beamtime and F. Dionigi and T. Merzdorf for their kind assistance with the HE XRD measurements. We thank the ZELMI of the Technical University of Berlin for their kind assistance with the TEM measurements.

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Domen, K., Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall Water Splitting Studied by Model Electrodes. J. Phy. Chem. C 2009, 113, 10151-10157. 58.

Dionigi, F.; Vesborg, P.C.K.; Pedersen, T.; Hansen, O.; Dahl, S.; Xiong, A.; Madea, K.;

Domen, K.; Chorkendorff, I., Suppression of the water splitting back reaction on GaN:ZnO photocatalysts loaded with core/shell cocatalysts. J. Catal. 2012, 292, 26-31.


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In Situ Stability Studies of Platinum Nanoparticles Supported on Mixed

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