How Stable Are Spherical Platinum Nanoparticles Applicable to Fuel

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

How Stable Are Spherical Platinum Nanoparticles Applicable to Fuel Cells? Sadaf Tahmasebi, Gregory Jerkiewicz, Stève Baranton, Christophe Coutanceau, Yoshihisa Furuya, and Atsushi Ohma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10617 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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The Journal of Physical Chemistry

How Stable Are Spherical Platinum Nanoparticles Applicable to Fuel Cells?

Sadaf Tahmasebi1, Gregory Jerkiewicz1*, Stève Baranton2*, Christophe Coutanceau2, Yoshihisa Furuya3, Atsushi Ohma3 1

Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada

2

IC2MP, UMR CNRS 7285, Équipe Catalyse et Milieux Non Conventionnel, Université de Poitiers 4 rue Michel Brunet, TSA 51106, 86073, Poitiers Cedex 9, France

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Research Division, Nissan Motor Co., Ltd. 1-Natsushima Cho, Yokosuka, Kanagawa 2378523, Japan

* Corresponding authors: [email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT: We report on the synthesis, characterization and degradation behavior of spherical platinum nanoparticles (Pt-NPs).

The Pt-NPs were synthesized with and without

carbon-support using the “water-in-oil” microemulsion method. X-ray diffraction (XRD) was used to examine their average crystallite size, which was ca. 4.0 nm. The shape, size, and size distribution of the Pt-NPs were evaluated using transmission electron microscopy (TEM); the average size was ca. 4.0 nm, thus in agreement with the XRD data. The agreement between the XRD and TEM data indicates that the Pt-NPs were single crystallites in nature. Thermogravimetric analysis (TGA) measurements were carried out to evaluate the metal loading, which was close to the target value of 40 wt%. Cyclic voltammetry (CV) experiments were performed in 0.50 M aqueous H2SO4 in the s = 1.00−50.0 mV s−1 potential scan rate to determine the specific surface area (As) of the catalysts and to assess the cleanliness of the system. The Pt surface oxide growth and reduction were successfully examined using in-situ confocal Raman spectroscopy. The results allow monitoring the appearance and disappearance of crystallinity in the surface oxide layer. The stability of the catalyst was evaluated by recording 500 CV profiles in 0.50 M aqueous H2SO4 solution in the 0.05 V ≤ E ≤ 1.55 V range at s = 50.0 mV s−1. The corrosion behavior of Pt-NPs was studied using potentiodynamic polarization (PDP) measurements at s = 0.10 mV s−1 in the presence of different gaseous environments (N2(g), O2(g), or H2(g)). The nature of the dissolved gas has a profound impact on the stability/corrosion behavior of the Pt-NPs. The Pt nanocatalysts are stable in the electrolyte saturated with H2(g), undergo slight corrosion in the electrolyte saturated with N2(g), and undergo significant corrosion in the electrolyte saturated with O2(g). The carbon support also undergoes corrosion and porosity changes.

The corrosive degradation of the Pt-NPs and carbon support is

pronounced the most in the case of the anodic PDP. Cyclic voltammetry measurements were employed to determine the loss of the electrochemically active surface area (Aecsa) of the Pt-NPs prior to and after PDP measurements; the results correlate with the corrosion rates. The new and original results on the characterization and corrosive degradation of the Pt-NPs represent an important contribution that will benefit the fuel cell science and technology.


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INTRODUCTION As electrochemical systems, polymer electrolyte membrane fuel cells (PEMFCs) can generate electrical energy and thermal energy (heat) through electrochemical oxidation (“combustion”) of hydrogen. They operate without generating greenhouse gases and involve two concurrently occurring electrochemical reactions, namely the hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode. Both reactions take place at the surface of electrocatalysts, which are Pt nanoparticles (Pt-NPs) dispersed on nano-structured carbon support (typically ca. 50 nm in diameter).

Carbon supported Pt-NPs possess an excellent

electrocatalytic activity towards the HOR and ORR; hence, the synthesis and characterization of Pt-NPs received a tremendous amount of interest.1-8 Numerous methods of preparing Pt-NPs were developed, but wet chemistry approaches attract the most attention because they are simple and versatile, and facilitate direct deposition of dispersed Pt-NPs on carbon support. Among the wet chemistry approaches, the so called “water-in-oil” microemulsion method offers an easy and convenient procedure for synthesizing NPs of transition metals (e.g. Pt, Au, Pd, Ru, Ni, Fe) with controlled shape, size, and size distribution.9-12 In the case of the synthesis of Pt-NPs using this method, H2PtCl6 is dissolved in an aqueous phase that contains meso-droplets of water; the latter are stabilized by a surfactant. These meso-droplets act as tiny reactors in which individual PtNPs are formed. It is important to add that a single nanoparticle can contain several crystallites and be polycrystalline in nature. The formation of Pt-NPs occurs through the reduction of a soluble Pt4+-containing compound, with hydrazine (N2H4) or sodium borohydride (NaBH4) being used as a reducing agent. Due to the simplicity of using NaBH4 as compared to N2H4, the former is often the preferred reducing agent. The suggested overall reaction between H2PtCl6 and NaBH4 is presented in eq 1.9,10


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PtCl 62 − (aq) + BH −4 (aq) + 3 H 2 O(l) → Pt 0 (s) + H 2 BO 3− (aq) + 4 H + (aq) + 6 Cl − (aq) + 2 H 2 (g)

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(1)

Although Pt is a noble metal and is considered to be inert (hence the term noble metal), it still undergoes very slow but unavoidable chemical or electrochemical degradation. The relationship between the electrocatalytic activity of Pt-NPs and their degradation through corrosive phenomena is complex, because there are many possible degradation reactions. Existing data indicate that small Pt-NPs (2–4 nm in size) possess the highest electrocatalytic activity towards the HOR and ORR, but at the same time the rate of their corrosive degradation is also high due to their small size.13-21,22,23 In the case of Pt-NPs that are part of PEMFCs operating for several years (a fuel cell stack is expected to operate for at least 10 years prior to being reconditioned)24, their slow degradation translates into irreversible materials losses and, therefore, into significant deterioration of the PEMFCs’ performance. Successful improvement of long-term operation of PEMFCs will result in their implementation as a practicable mean of electrical energy production for personal transportation as well as for other applications (e.g., stationary electrical energy generation in remote areas).

Consequently, research on the stability and degradation of

nanostructured Pt materials is of high importance to the emerging hydrogen economy, which envisages water electrolyzers as hydrogen-generating devices and fuel cells as electrical and thermal energy-generating devices.14,25-30 Potentiodynamic polarization (PDP) at low potential scan rates (s) generates polarization transients that can be used to determine: (i) the corrosion potential (Ecorr) and corrosion current density (jcorr); (ii) the activity towards anodic and cathodic faradaic reactions (such as the hydrogen and oxygen evolution and oxidation reactions, abbreviated as HER and OER, HOR,


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and ORR); (iii) the exchange current densities (jo) and Tafel slopes (b) of these reactions; and (iv) the potential ranges of the active and passive regions as well as the active-passive transition (if applicable). Experimentally determined corrosion current density values can be converted into corrosion rates expresses as mass loss per unit surface area and per unit of time or as thickness loss per unit of time. Potentiodynamic polarization is a very useful technique due to its experimental simplicity, short response time, and straightforward analysis of results. Interestingly, while polarization transients are available for a broad range of non-noble transition metals and metallic alloys used in the construction of metallic structures, they are unavailable for Pt materials; existing transients are limited to the potential ranges of faradaic reactions (i.e., ORR and HOR).31-36 In this contribution, we report results on the synthesis at room temperature of small and spherical Pt nanoparticles (Pt-NPs, ca. 4.0 nm in diamater) using the “water-in-oil” microemulsion method. The shape, size, and size distribution of the spherical Pt-NPs were examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). In the case of carbon-supported Pt-NPs, thermo-gravimetric analysis (TGA) was used to evaluate the metal loading. Cyclic voltammetry was employed to determine their electrochemically active surface area and to characterize their behavior in aqueous H2SO4 solution. In-situ confocal Raman spectroscopy was also employed to further complete the electrochemical characterization of the Pt-NPs by monitoring the formation and reduction of surface oxides. Potentiodynamic polarization measurements were conducted in both anodic (positive-going) and cathodic (negative-going) scan directions in order to advance the understanding of Pt-NPs degradation in relation to their chemical state, as well as any simultaneously occurring electrochemical degradation of the carbon support through electrooxidation (C(s) → CO2(g)) that could result in


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physical detachment of Pt-NPs from the support. 37

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We also report new results on the

polarization behavior of carbon-supported Pt-NPs in aqueous H2SO4 solution saturated with a reactive gas, such as H2(g) or O2(g). These gases are selected in order to mimic some conditions encountered by nanostructured Pt materials in operating PEMFCs. The polarization transients acquired in aqueous H2SO4 solution in the presence of H2(g) or O2(g) are compared to those obtained in the same electrolyte outgassed using N2(g).

EXPERIMENTAL SECTION Preparation of Carbon-Supported Pt-NPs. Platinum nanoparticles were synthesized using the “water-in-oil” microemulsion method.9,12 The oil phase was prepared by mixing 37.0 g of nheptane (Sigma Aldrich, HPLC grade) and 16.1 g of polyethylene glycol dodecyl ether (Brij® L4, Sigma Aldrich) under continuous stirring conditions until a homogeneous solution was obtained. The aqueous phase was prepared by dissolving hexachloroplatinic acid hexahydrate (99.95 %, Alfa Aesar) in ultra-high purity water (Milli Q®, Millipore 18.2 MΩ cm in resistivity) to obtain an aqueous H2PtCl6 solution having a concentration of 0.10 mol L−1. A 1.6 mL aliquot of the aqueous phase was added to the oil phase containing the Brij L4 surfactant under continuous stirring conditions until a translucent and stable microemulsion was obtained. Solid sodium borohydride (NaBH4, 99 % purity, Sigma Aldrich) was added in large excess (ca. 100 mg) to the mixture to reduce Pt4+ to Pt0 (see eq 1). In the case of the unsupported Pt-NPs, their cleaning was carried out by repetitive soaking with acetone (technical grade, 5 times), ethanol (HPLC grade, 5 times), and water (ultra-high purity, UHP, water (5 times). Then, the Pt-NPs were separated from the solution by allowing them to settle at the bottom of glass bottle and by removing the solvent. An ink was prepared by dispersion of the Pt-NPs in UHP water using an


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ultrasonic bath. In the case of the carbon-supported Pt-NPs, carbon (Vulcan XC72, CABOT) pre-treated under N2(g) at 400 °C for 4 hr was directly added to the colloidal solution in order to reach ca. 40 wt% metal loading; the dispersion of Pt-NPs was facilitated by ultrasonication for 15 min. Finally, the mixture containing the carbon-supported Pt-NPs was filtered using a hydrophilic polyvinylidene difluoride (PVDF) membrane with a 0.22 µm pore diameter (Millipore). Then, the carbon-supported Pt-NPs were washed several times (at least five times) with acetone (technical grade) and UHP water, respectively. Afterwards, the Pt-NPs were dried overnight in air at 60 °C using an oven. Such prepared catalytic powder was then thermally treated for 2 hr at 200 °C under air condition to remove any remaining surfactant through its oxidation (combustion). X-Ray Diffraction (XRD) Analysis. XRD patterns were acquired using PANalytical Empyrean X-ray diffractometer. Measurements were performed from 2θ = 20 ° to 2θ = 140 ° in a step mode, with a step interval of 0.06 ° and a fixed acquisition time of 10 s at each value of 2θ. Transmission Electron Microscopy (TEM) Characterization. TEM analysis was performed using a JEOL JEM 2100 (UHR) microscope with a resolution of 0.19 nm. The mean size and size distribution of the Pt-NPs were determined using the Feret’s diameter and by counting 500 isolated NPs using the ImageJ free software.38 Thermo-Gravimetric Analysis (TGA). TGA measurements were carried out to determine the exact value of the Pt loading using a TA Instrument model SDT Q 600 apparatus.

The

measurements involved gradually heating the sample from 25 °C to 900 °C with a heating rate of 10 °C min−1 under an air flow of 100 mL min−1. In-situ Confocal Raman Spectroscopy Characterization.

In-situ Raman spectra were

acquired using a HORIBA iHR320 confocal Raman spectroscopic microscope. They were


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obtained during anodic-going (0.050 V → 1.45 V) and cathodic-going (1.45 V → 0.050 V) potential transients, respectively; the potential scan rate was s = 0.20 mV s–1. The Raman spectra were acquired at a 50.0 mV interval with each measurement lasting 200 s.

Each Raman

spectrum is attributed to an average value of the lowest and highest potentials of the range (i.e., the spectrum recorded from 0.800 V to 0.840 V is assigned to a potential of 0.820 V). Electrochemical Characterization.

Electrochemical measurements were conducted in a

conventional, three-electrode, two-compartment electrochemical cell at room temperature. A reversible hydrogen electrode (RHE) was used as a reference electrode (RE) and all potential values are reported on the RHE scale. A glassy carbon plate having a geometric surface area of ca. 3 cm2 was used as a counter electrode (CE); its surface area was significantly greater (at least ten times) than that of the working electrode (WE). The working electrode was prepared by dispensing 3.00 µL of a catalytic ink onto a glassy carbon disk polished to a mirror-like finish and having a geometric surface area of 0.0707 cm2. The catalytic ink was prepared as follows: first, 17.7 mg of Pt/C catalytic powder were added to 2.646 mL of UHP water and the ink was homogenized using an ultrasonic bath for ca. 15 min; then, 354 µL of commercial solution of Nafion dissolved in aliphatic alcohols (5 wt%, Sigma Aldrich) were added to the abovedescribed suspension; such prepared Nafion-containing suspension was homogenized in an ultrasonic bath for ca. 30 s; the final volume of the ink was 3.00 mL. Dispensing of a volume of 3.00 µL of the ink on 0.0707 cm2 of the glassy carbon substrate resulted in a loading of 100 µgPt cm−2. Electrochemical experiments were conducted in 0.50 M aqueous H2SO4 (96 %, Suprapur Merck) solution at a temperature of 298 K. All aqueous solutions were prepared using UHP water. Prior to conducting experiments, the electrolyte solution was outgassed by purging through it N2(g) (99.999% in purity, Praxair) for at least 30 min. Cyclic voltammetry (CV)


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experiments were conducted using an EG&G Princeton Applied Research model 362 potentiostat. Cyclic voltammetry profiles were acquired at different pre-determined potential scan rate values, namely s = 1.00, 2.00, 5.00, 10.0, 20.0, and 50.0 mV s–1. Degradation and corrosion behavior of the carbon-supported Pt-NPs were examined by recording polarization transients using a Princeton Applied Research VersaSTAT 4 potentiostat-galvanostat.

All

measurements were conducted according to the following procedure: first, N2(g) was purged through the electrolyte for at least 30 min to outgas it; then, ten CV profiles were recorded at s = 50.0 mV s−1 and, subsequently, an anodic/cathodic polarization transient was acquired in the −0.100 V ≤ E ≤ 2.00 V range at s = 0.10 mV s–1 with an electrode rotation rate of ω = 2500 rpm. The following procedure was employed to acquire anodic/cathodic polarization transients in 0.50 M aqueous H2SO4 saturated with H2(g) or O2(g). First, we recorded ten CV profiles in 0.50 M aqueous H2SO4 outgassed with N2(g) to examine the system’s cleanliness. Then, a reactive gas (H2(g) or O2(g)) was purged through the WE compartment for ca. 30 min and ten CV profiles were acquired (0.05 V ≤ E ≤ 1.45 V, s = 50.0 mV s−1). Afterwards, an anodic/cathodic potentiodynamic polarization measurement was performed followed by ten CV profiles (also in the presence of H2(g) or O2(g); 0.05 V ≤ E ≤ 1.45 V, s = 50.0 mV s−1). Each experiment reported in this contribution was repeated at least three times to confirm reproducibility of data. The stability of the Pt-containing nanocatalysts was examined by acquiring 500 CV profiles in the 0.05 V ≤ E ≤ 1.45 V range at s = 50.0 mV s−1 and by analyzing changes in their electrochemically active surface area. The electrochemically active surface area was determined using a well-establish procedure that involves evaluation of the charge associated with the adsorption and desorption of the under-potential deposited H (HUPD).

39

Prior to any

electrochemical measurement, repetitive potential cycling (at least 10 transients) in the 0.05 –


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1.45 V range was performed to verify the cleanliness of the system and the stability of the carbon-supported Pt-NPs (see Supporting Information). The ten CV transients are practically superimposable (the first and second slightly differ in the Pt oxide formation region).

RESULTS AND DISCUSSION Thermo-Gravimetric Analysis (TGA). The mass variation profile for the carbon-supported PtNPs, the green solid transient in Figure 1, presents the mass variation with respect to the initial mass in percentage (m/mi × 100%, where mi is the initial sample mass and m is the sample mass at a specific temperature) as a function of temperature (T). The gray solid transient in Figure 1 shows the first derivative of the mass variation profile (dm/dT) as a function of temperature. The two plots reveal that the first mass transition occurs in the 293–400 K range, which corresponds to the desorption of physisorbed water. The second mass transition occurs in the 600–800 K range and is attributed to the combustion of the carbon support. The remaining material is pure Pt and its mass of 2.691 mg is used to determine the Pt loading (wPt in wt%) by applying eq 2:

wPt =

mPt × 100% mPt + mC

(2)

where mPt is the mass of the Pt-NPs and mC is the mass of the carbon support (both determined on the basis of TGA measurements). The value of wPt is found to be 36.3 wt% and is very close to the target value of ca. 40 wt%. X-Ray Diffraction (XRD) Characterization. Figure 2 presents an XRD pattern for the carbonsupported Pt-NPs powder (the black points) in the 20 ° ≤ 2θ ≤ 140 ° range. It was analyzed


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using the Levenberg-Marquardt method and deconvoluted using a pseudo-Voigt fitting by means of a computer refinement program (Fityk).40 All the diffraction peaks were indexed to the face centered cubic (fcc) structure of Pt and their position was found to be consistent with the JCPDS 04-0802 card except for the small peak at 2θ = 23.1 °, which is attributed to the {002} set of planes of turbostratic graphite.41,42 The peak assignments according to the JCPDS 04-0802 card are presented as the solid, vertical blue lines. The fitted curve in Figure 2 (the green profile that consists of several individual peaks shown as gray, solid peaks) closely agrees with the experimental data thus validating the deconvolution procedure. The average crystallite size (the Scherrer length, Lv) of the Pt-NPs was determined to be 4.1 ± 0.3 nm. Transmission Electron Microscopy (TEM) Characterization.

Transmission electron

microscopy images were acquired in order to evaluate the particles’ shape, size, and size distribution. Figures 3a and 3b present TEM images for the carbon-supported Pt-NPs and the unsupported Pt-NPs, respectively. These images reveal that the nanoparticles are distinguishable from the carbon support and have a well-defined spherical shape (a spherical shape).43 Figure 3c presents a histogram of the particle size distribution prepared on the basis of the analysis of the TEM images acquired for the carbon-supported Pt-NPs using several TEM images. The average size of these spherical Pt-NPs was determined to be 4.0 ± 0.5 nm. According to these results, the “water-in-oil” microemulsion method is very convenient for the synthesis of small Pt nanospheres with a narrow size distribution. It is interesting to observe that the average particle size of the Pt-NPs determined by TEM is consistent with the average crystallite size (4.1 ± 0.3 nm) determined by XRD. This is an important observation indicating that each nanoparticle is a single crystallite.


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Electrochemical and In-situ Raman Spectroscopy Characterization. Figure 4a presents a series of CV profiles for the carbon-supported Pt-NPs in the 0.05 V ≤ E ≤ 1.45 V range acquired in 0.50 M aqueous H2SO4 solution outgassed with N2(g) at different potential scan rate values, namely s = 1.00, 2.00, 5.00, 10.0, 20.0, and 50.0 mV s−1. The cathodic and anodic CV features in the 0.05 V ≤ E ≤ 0.40 V range correspond to the adsorption and desorption of the underpotential deposited H (HUPD), respectively. The anodic feature observed in the 0.80 V ≤ E ≤ 1.45 V range corresponds to the formation of Pt surface oxide and the cathodic one in the 0.60 V ≤ E ≤ 1.10 V range to the reduction of Pt surface oxide. The very small anodic feature observed in the 0.40 V ≤ E ≤ 0.70 V range corresponds to the oxidation of hydroquinone groups on the surface of the carbon support.44 The expected feature due to the reduction of quinone groups overlaps the main cathodic feature in the 0.50 V ≤ E ≤ 1.20 V range, which is due to the Pt surface oxide reduction and is indistinguishable due to its tiny size.45,46 The other featureless characteristics of the CV profiles with small current (I) values are due to the double-layer charging. Figure 4b presents capacitance (C, where C = I / s) versus E profiles prepared using the CV profiles presented in Figure 4a. An analysis of the capacitance profiles leads to the following observations: (i) the transients are almost perfectly superimposable in the HUPD adsorption and desorption regions indicating that the same number of Pt surface atoms is involved in surface reactions independently of the potential scan rate values; (ii) the size of the anodic feature due to the Pt surface oxide formation and the cathodic one due to the Pt surface oxide reduction decreases with an increase in the potential scan rate value; this behavior is expected because the oxide formation is a slow process and the oxidation time experienced by the Pt-NPs increases as the potential scan rate decreases and, consequently, the oxide layer


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increases its thickness; and (iii) the oxide reduction peak shifts towards higher potential values with a decrease in the potential scan rate value.47 Figures 5a and 5b present Raman spectra for the unsupported Pt-NPs deposited on the glassy carbon substrate. These spectra are recorded during anodic (Figure 5a) and cathodic (Figure 5b) potential transients at s = 0.20 mV s–1. They were acquired using an in-situ confocal Raman microscope/spectrometer coupled to a potentiostat; the experimental setup facilitates simultaneous spectroscopic and electrochemical measurements.

The electrochemical and

confocal Raman spectroscopy measurements were conducted in the 0.05 V ≤ E ≤ 1.45 V range; a Raman spectrum acquired at E = 0.40 V (in the double layer region) served as a reference spectrum. The Raman spectra obtained during the anodic transient (Figure 5a) reveal features associated with the formation of Pt surface oxide at potential values greater than 0.90 V; its intensity increases with a rise in the electrode potential. At E = 0.90 V, the Raman shift at 455 cm–1 is attributed to the formation of Pt surface oxide, which possesses a degree of crystallinity. When the potential reaches a value of E = 1.05 V the spectral feature undergoes a blue shift towards 525 cm–1 and remains at this Raman shift value even when the applied potential is further increased. The blue shift is attributed to an increase in the crystallinity of the Pt oxide layer brought about by an increase in its thickness (an increase in the applied potential).46,48 Upon reversal of the scan direction, the peak attributed to Pt surface oxide is still observed when the applied potential is in the 0.80 V ≤ E ≤ 1.45 V range. Its intensity commences to decrease when the potential of E = 0.90 V is reached and is practically invisible in the case of E = 0.80 V. This decrease in peak intensity is accompanied by a red-shift down to 510 cm–1. At this stage of the discussion it is important to relate the electrochemical results presented in Figure 4 to the spectroscopic results shown in Figure 5. In the case of the anodic


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scan, the Raman feature attributed to the surface oxide on Pt-NPs appears only when the potential of 0.90–0.95 V has been reached, while in the CV measurements at low potential scan rates the onset of surface oxidation is observed already at ca. 0.80 V (Figure 4b). In the case of the cathodic scan, the Raman feature attributed to the surface oxide is clearly visible in the 0.90 V ≤ E ≤ 1.45 V range and disappears when the potential of 0.85–0.80 V has been reached, while in the CV measurements complete reduction of the surface oxide is achieved only when the applied potential reaches 0.50 V.

Elsewhere, it was explained that in the case of bulk

polycrystalline Pt, the formation of surface oxide commences at ca. 0.85 V and as the applied potential and/or oxidation time increases its thickness also increases. It was also explained that the surface oxide reduction occurs in the 0.50 V ≤ E ≤ 1.10 V range.45-48 In the case of Pt-NPs, the surface oxide formation commences at a lower potential value giving rise to a CV feature but without generating a distinguishable peak in Raman spectra.

We propose the following

hypothesis to explain the relation between the CV and Raman spectroscopy results: (i) in the 0.80 V ≤ E ≤ 0.95 V range of the anodic scan, PtO in the form of chemisorbed O (Ochem) forms in a sub-monolayer quantity without giving rise to any Raman feature; (ii) at E > 0.95 V a quasithree dimensional PtO comprising Pt2+ and O2– develops giving rise to a unique Raman feature; the process involves interfacial structural transformation (the so-called interfacial place exchange); and (iii) because Pt-NPs contain facets having different crystallographic orientations, it is conceivable that some facets are covered with an oxide layer, while others are not; this would give rise to the development of surface oxide “patches” with sufficient thickness and crystallinity to generate a Raman signal. The in-situ Raman spectroscopy results and the above hypothesis allow us to conclude that an oxide layer of a specific (minimum) thickness and a certain degree of crystallinity needs to develop, even in patches (with nanoparticle facets playing


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the role of patches), to be measurable using confocal Raman spectroscopy. The confocal Raman spectroscopy measurements reveal an original and intriguing behavior. Specifically, in the anodic transient the spectral feature due to the Pt surface oxide appears when the potential of 0.85–0.90 V has been reached, but according to the CV measurements only ca. 18% of the surface oxide is formed (ca. 82% of the surface is oxide-free). Here, it is proposed that the coverage of O on certain nanoparticles facets is sufficiently high to initiate the interfacial structural transformation between top-most Pt surface atoms and Ochem surface species, thus to create crystallinity. In the cathodic transient, the Raman feature disappears as soon as 0.80 V or a lower potential value has been reached, although a complete oxide reduction is accomplished only at 0.50 V. In fact, in the case of 0.80 V only 37.5% of the surface oxide is reduced. Although the individual steps involved in the Pt oxide reduction remain unknown, we propose that an interfacial process occurring in the 1.10–0.80 V range leads to a loss of crystallinity. This crystallinity loss can be localized in the sense that it can take place at facets having some specific surface orientations. Our confocal Raman spectroscopy results, which at the present time are limited to spherical Pt-NPs of one size, call for additional research employing preferentially oriented Pt-NPs of various shapes and sizes. The results of in-situ confocal Raman spectroscopy measurements presented in this contribution demonstrate that it is a very powerful technique capable of monitoring crystallinity appearance and disappearance in very thin surface oxide layers. The stability of the carbon-supported Pt-NPs placed on a glassy carbon substrate was examined by recording 500 CV profiles in 0.50 M aqueous H2SO4 solution in the 0.05 V ≤ E ≤ 1.45 V range at s = 50.0 mV s−1 and T = 298 K. Degradation of carbon-supported Pt-NPs is pronounced the most within the initial several hundred cycles; thus, we limited our


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measurements to 500 transients.12

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The CV transients were used to determine the

electrochemically active surface area (Aecsa) that was subsequently converted to the specific surface area (As in m2 gPt–1). Figure 6 presents CV profiles for a gradually increasing number of transients and the inset in Figure 6 shows the evolution of the value of As versus the cycle number. We observe that the value of As decreases linearly with the number of potential transients and after 500 cycles its value equals ca. 60 % of the initial one. It is also worthwhile mentioning that the value of the current in the double layer region does not change as the repetitive potential cycling progresses. Because the carbon support has a significantly greater specific surface area (As = 250 m2 g–1) than the Pt nanocatalysts, the majority of the current in this region is attributed to the carbon-electrolyte interactions. Since its value remains practically the same, it may be concluded that the corrosion of the carbon support is very slow as compared to the degradation of the Pt-NPs. In order to advance our understanding of the degradation behavior of the carbon-supported Pt-NPs, we performed potentiodynamic polarization transient measurements at a very low potential scan rate and in the presence and absence of reactive gases. Corrosion

Behavior

of

Carbon-Supported

Potentiodynamic Polarization.

Pt

Nanoparticles

Examined

Using

The corrosion behavior of carbon-supported Pt-NPs was

investigated by recording potentiodynamic polarization (PDP) transient in 0.50 M aqueous H2SO4 between a lower potential limit of EL = –0.10 V and an upper one of EU = 2.00 V at s = 0.10 mV s–1 and T = 298 K. We selected a very low potential scan rate, because steady-state conditions can only be achieved when s is very low (here 0.10 mV s–1). An anodic direction refers to a scan from EL to EU, and a cathodic direction to a scan from EU to EL. These experiments were performed using 0.50 M aqueous H2SO4 solution saturated with different gases (i.e., N2(g), O2(g), or H2(g)) to establish whether the nature of the dissolved gas had any impact


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on the corrosion of Pt-NPs and to assess the magnitude of any such effect. Figure 7 shows three sets of anodic (Figure 7a) and cathodic (Figure 7b) PDP transients; they are color-coded with the gray, blue, and red transients referring to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. In these graphs, the x-axis is the applied potential (E) and the y-axis is a logarithm of the absolute value of the current density (logj). Each graph includes a diagram showing potential domains in which specific surface and faradaic reactions can take place. It is important to emphasize that the nature of the dissolved gas influences the nature of the faradaic reactions that can take place and the chemical state of the Pt-NPs is different in the case of the anodic and cathodic transients. Thus, the six diagrams are different. Electrolyte Saturated with N2(g). In the case of the anodic (positive-going) polarization transient acquired in the electrolyte saturated with N2(g) the significant value of j at negative potentials is due to the HER; as expected, the value of j decreases as the applied potential approaches the potential of the H+/H2 redox couple (E = 0.00 V). The well-defined minimum at 0.50 V is the corrosion potential (Ecorr = 0.50 V) and the corrosion current density equals jcorr = 100 nA cm–2. The minor feature at 0.55–0.60 V is attributed to the oxidation of hydroquinone groups on the surface of the carbon support to quinone ones. 49 As the applied potential increases, the Pt surface becomes passivated through the formation of PtO. The OER commences at E = 1.23 V but a noticeable current density value is observed only at higher potentials due to very low kinetics (a high activation overpotential) of the process; the value of j gradually rises in the case of E > 1.5 V. It is important to mention that the latter process is accompanied by the formation of PtO2. 50 The PDP transient reveals a local maximum at 1.80 V followed by a decrease in the value of j with a local minimum at 1.90 V and a subsequent increase in j up to the upper potential limit (EU = 2.00 V) of the measurement. The existence of the local


17


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maximum at E = 1.80 V is attributed to carbon corrosion, which gives rise to a loss of physical contact (and electrical contact) between the substrate and some Pt-NPs. In the case of the cathodic (negative-going) polarization transient, the application of a high positive potential gives rise to the formation of Pt surface oxide (comprising both PtO and PtO2) and the OER. The value of j decreases steeply and linearly in the 1.97 V – 2.00 V range; it decreases even further and also linearly (but the slope is different) as the applied potential is reduced down to 1.60 V. The PDP transient shows a well-defined corrosion minimum with Ecorr = 1.55 V and jcorr = 25 nA cm–2. In the case of the cathodic PDP transient the dramatically higher value of Ecorr (by 1.05 V) and the significantly lower value of jcorr (by a factor of four) than in the case of the anodic PDP transient is attributed to the passivated state of the Pt-NPs. The cathodic PDP transient shows a local maximum at E = 0.50 V that is attributed to the reduction of Pt surface oxide (with a small contribution due to the reduction of surface quinone groups). At the potential of E = 0.00 V the HER commences and the value of j increases as the magnitude of the applied negative overpotential rises. Electrolyte Saturated with O2(g). In the case of the anodic (positive-going) polarization transient acquired in the electrolyte saturated with O2(g) the significant value of j at negative potentials is due to the concurrently occurring HER and ORR. The value of j decreases slightly as the applied potential approaches E = 0.00 V but the value of j is higher than in the case of the electrolyte saturated with N2(g) due to the ORR. The value of j remains almost constant in the 0.00 V – 0.70 V range and is attributed to the ORR. As the potential increases, the current density of the ORR on metallic Pt decreases because the overpotential for this reaction is progressively smaller.

Then, as the surface oxide formation commences, the value of j

continues to decrease because the Pt surface oxide is less active towards the ORR than metallic


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Pt. The current density of the ORR also decreases because its overpotential is progressively smaller. The PDP transient reveals a well-defined minimum that is attributed to Pt corrosion with Ecorr = 1.02 V and jcorr = 250 nA cm–2. As the applied potential increases, the formation of PtO begins giving rise to a small current density. As expected, the OER starts at E = 1.23 V and as in the case of the electrolyte saturated with N2(g) a noticeable current density is observed only at high potentials (E > 1.5 V); at high anodic potentials the OER is accompanied by the formation of PtO2. Again, the PDP transient reveals a local maximum at E = 1.70 V followed by a decrease in the value of j with a local minimum at 1.85 V and a subsequent increase in j up to the upper potential limit (EU = 2.00 V) of the measurement. Similarly to the behavior of the nanocatalysts in the electrolyte saturated with N2(g), this feature is assigned to carbon corrosion. In the case of the cathodic (negative-going) polarization transient, the behavior between EU and Ecorr is very similar to that observed in the case of the electrolyte saturated with N2(g), but the value of Ecorr is 1.05 V (lower by ca. 0.50 V), while jcorr = 50 nA cm–2 (twice higher). The lower value of Ecorr and the higher value of jcorr for the electrolyte saturated with O2(g), as compared to that saturated with N2(g), is assigned to a more active surface state of the Pt-NPs. Because the Pt-NPs are covered with a layer of surface oxide comprising both PtO and PtO2, and because the surface oxide enhances the surface area, the passive layer does not offer protection but rather facilitates the material corrosion. This corrosion behavior clearly stems from an interplay of several factors. As the applied potential decreases from Ecorr down to 0.00 V, the observed current density is mainly due to the ORR. However, the value of j is small and increases concurrently with the reduction of the Pt surface oxide (its complete reduction is achieved at 0.40 V). At E = 0.00 V the HER commences and the value of j increases as the magnitude of the applied negative overpotential increases. It is important to mention that the


19


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HER and ORR occur simultaneously, but the rates of individual processes cannot be determined on the basis of only electrochemical measurements. In the case of the electrolyte saturated with O2(g), we do not observe any feature in the PDP transients that could be attributed to the oxidation of hydroquinone groups or the reduction of quinone groups because the current density associated with these processes is much smaller than that due to the ORR. Electrolyte Saturated with H2(g). In the case of the anodic (positive-going) polarization transient acquired in the electrolyte saturated with H2(g) the significant value of j at negative potentials is due to the HER; the magnitude of j decreases as the applied potential approaches E = 0.00 V. At positive potentials, the current density is due to the HOR; it remains practically unchanged up to E = 0.90 V at which the surface oxide formation commences. Because the Pt surface oxide is less active towards the HOR than metallic Pt, the value of j decreases as the thickness of the Pt surface oxide increases.51 The PDP transient does not reveal any minimum characteristic of Pt corrosion indicating that in the case of the electrolyte saturated with H2(g) there is no corrosion of the nanocatalysts. The increase in j at E > 1.6 V is due to the OER, which is accompanied by the HOR the rate of which is unknown but is expected to be small. In the case of the cathodic (negative-going) polarization transient, the application of a high positive potential gives rise to the OER again accompanied by the HOR. It is important to observe that in the case of carbonsupported Pt-NPs there is no formation of Pt surface oxide when the electrolyte is saturated with H2(g). If a surface oxide were formed, then we would observe a feature at lower potential values due to its reduction. The value of j remains practically constant over a very broad potential range and is due to the HOR. As the applied potential approaches 0.00 V, the magnitude of j decreases; it increases once the applied potential is lower than 0.00 V and the HER commences.


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It is of vital importance to identify the nature of the anodic and cathodic processes associated with the Ecorr and Icorr using the PDP transients and the diagrams depicting possible surface and faradaic reactions. These reactions refer to the anodic and cathodic processes occurring within the vicinity of Ecorr. In the case of the anodic PDP transient in the electrolyte outgassed using N2(g), the anodic process refers to Pt degradation, with several processes being feasible (e.g., direct Pt dissolution, anodic Pt oxide formation followed by chemical dissolution). The cathodic process refers to the hydrogen evolution reaction (HER) occurring at potentials as high as 0.5 V.

This might seem counterintuitive because the standard potential of the

H+(aq)/H2(g) redox couple is E°(H+(aq)/H2(g)) = 0.000 V. However, it refers to an electrolyte saturated with H2 and the pressure of H2(g) above the electrolyte surface being 1.00 bar (the amount of dissolved H2 is determined by the Henry’s law). It is important to add that every tenfold decrease of the H2(g) pressure shifts the potential of the H+(aq)/H2(g) redox couple by 0.0296 V towards higher potentials. The amount of dissolved H2 is extremely tiny and it originates from the reference electrode compartment. In the case of the cathodic PDP transient in the electrolyte outgassed using N2(g), the anodic process refers to Pt oxidation, again, with several processes being possible. However, the Pt-NPs are covered with an oxide layer, thus the anodic process involves not metallic Pt but Pt oxide (e.g., PtO oxidation to PtO2 that could be accompanied by its subsequent chemical dissolution). The HER is very unlikely to be the cathodic process, because Ecorr = 1.55 V is very high. Thus, the cathodic process is attributed to a cathodic reduction of Pt oxide. However, this proposal does not explain the steady-state current, which is expected to remain the same, because it should vanish once the entire Pt oxide has been cathodically reduced. Therefore, at this stage of the discussion we are unable to offer an indisputable explanation and suggest some tentative explanation that might require additional


21


experimental verification.

Page 22 of 46

In the case of the anodic and cathodic PDP transients in the

electrolyte saturated with O2(g), the anodic process refers to Pt oxidation and the cathodic process refers to the oxygen reduction reaction (ORR). There is a small difference in the value of Ecorr, which is attributed to the chemical state of the electrode. Namely, it is covered with a thin layer of PtO in the case of the anodic transient and with a thick layer comprising both PtO2 and PtO in the case of the cathodic transient. In order to facilitate a comparison of the results obtained in the presence of different gases, we combine the anodic and cathodic PDP transients and present them in Figures 8a and 8b; we employ the same set of colors as those in Figures 7a and 7b. The results clearly show that the nature of the dissolved gas has a profound impact on the anodic and cathodic polarization behavior of the carbon-supported Pt-NPs and on their corrosion. The nanocatalysts do not undergo corrosion in the electrolyte saturated with H2(g) but undergo corrosion in the same electrolyte saturated with N2(g) or O2(g). In the case of the electrolyte saturated with O2(g), the values of Ecorr for the anodic and cathodic PDP transients are 1.05 V and 1.02 V, respectively. However, the respective values of jcorr are 250 and 50 nA cm–2 and this difference is attributed to the chemical state of the electrode, the passivated state of the Pt-NPs that decreases the corrosion rate. In the case of the electrolyte saturated with N2(g), the values of Ecorr for the anodic and cathodic PDP transients are 0.50 V and 1.55 V, respectively. This large difference is due to the passivated nature of the Pt-NPs in the case of the cathodic PDP transient; the values of jcorr are 100 nA cm–2 and 25 nA cm–2, respectively. It is also interesting to observe that the two types of PDP transients are very similar in the very narrow potential regions within the vicinity of the lower and upper limits (EU and EL).


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In order to examine the influence of PDP measurements on any changes to Aecsa of the carbon-supported Pt-NPs, we recorded CV profiles prior to (the solid lines) and after (the dashed lines) the anodic (Figure 9a) and cathodic (Figure 9b) measurements. For consistency of the presentation, the profiles are color-coded with the grey, blue, and red transients referring to the electrolyte saturated with N2(g), O2(g), or H2(g), respectively. In the case of the electrolyte saturated with N2(g) or H2(g), the current associated with the double-layer charging is greater after the polarization measurements than prior to acquiring them. This change can be explained by an increase in the surface area of the carbon support due to its moderate degradation and development of porosity.52 In all CV profiles, the pseudo-capacitive current associated with the adsorption and desorption of HUPD and the surface oxide formation and reduction is smaller after performing PDP measurements than prior to acquiring them (the difference is the greatest in the case of the electrolyte saturated with O2(g)). This behavior is assigned to a loss of Aecsa of the PtNPs because of their degradation.53 Above, we explain that the CV profiles for the Pt nanocatalysts acquired after PDP measurements differ from those recorded prior to performing them. It is important to add that this difference is the most pronounced in the case of the electrolyte saturated with O2(g), as compared to the electrolyte saturated with N2(g) or H2(g). In fact, in the case of the anodic polarization (in the presence of O2(g)) the usual features due to HUPD adsorption and desorption as well as the surface oxide formation and reduction are hardly discernable. These results point to significant dissolution of the Pt-NPs and degradation of the carbon support. In the case of the cathodic polarization (in the presence of O2(g)), we concluded that there is little degradation of the carbon support because the double-layer charging current remains almost the same. The experimentally observed more significant degradation of the Pt-NPs during the anodic


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polarization than during the cathodic one is supported by the jcorr values reported in Table 1. Concerning the degradation of the carbon support, it is more severe in the case of the anodic PDP measurements than in the case of the cathodic ones. The CV profiles presented in Figure 9 create a basis for the determination of Aecsa of the Pt-NPs, which is then converted to a specific surface area (As). A comparison of the values of As prior to and after PDP measurements yields the loss of As (expressed as percentage) with respect to its original value (Table 1). An analysis of the data shown in Table 1 clearly demonstrates that the degradation of Pt-NPs is the most significant in the electrolyte saturated with O2(g) and in the case of the anodic (positive-going) PDP measurements. The jcorr values reported in Table 1 are converted to a mass loss (in mg) per cm2 per year with the assumption that a majority of the degraded Pt is in the form of Pt2+; thus, the process is accompanied by the transfer of two electrons. Finally, it is important to add that the loss of As data correlates with the jcorr values.


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Table 1: Values of the corrosion potential (Ecorr), the corrosion current density (jcorr), the mass loss and the loss of the specific surface area (As) after anodic and cathodic potentiodynamic polarization.

Gaseous Environment N2

Anodic Potentiodynamic Polarization Mass loss per cm² per Ecorr (V) jcorr (nA cm−2) year (mg cm–2 year–1) 0.50 100 3.19

As loss after anodic PDP (%) 37

O2

1.05

250

7.98

90

H2

N.A.

N.A.

N.A.

≈2

Gaseous Environment N2

Cathodic Potentiodynamic Polarization Mass loss per cm² per As loss after Ecorr (V) jcorr (nA cm−2) –2 –1 year (mg cm year ) cathodic PDP (%) 1.55 25 0.798 25

O2

1.02

50

1.60

61

H2

N.A.

N.A.

N.A.

≈2

CONCLUSIONS Spherical platinum nanoparticles (Pt-NPs) were successfully synthesized using the water-in-oil microemulsion method and characterized using physical and electrochemical techniques. The TGA measurements determined the Pt loading in the carbon-supported catalyst; it was found to be 36.3 wt%, thus very close to the target value of 40 wt%. The XRD pattern was typical of the fcc structure of Pt, and the XRD and TEM results revealed that the Pt-NPs have an average particle size of ca. 4 nm and are individual crystallites in nature. Cyclic voltammetry profiles were acquired in 0.50 M aqueous H2SO4 solution using different potential scan rates in the 1.00 – 50.0 mV s–1 range; they showed a dependence of the oxide growth and reduction features on the potential scan rate. The Pt oxide formation and reduction were successfully investigated using in-situ confocal Raman spectroscopy. A clearly distinguishable band assigned to Pt oxide was observed in the anodic CV transient for potentials E > 0.90 V; it was also observed that it


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underwent a progressive blue shift with increasing potential, which is attributed to an increase in the crystallinity of the oxide layer. The Pt oxide band is also clearly visible in the cathodic CV transient but it is practically invisible when E = 0.80 V is reached, although the entire oxide layer is not completely reduced yet, as revealed by the CV profile. This behavior is attributed to a loss of crystallinity in the oxide layer (a fraction of the oxide is reduced but the remaining oxide loses its crystallinity).

These observations indicate that the confocal Raman spectroscopy is a

powerful technique capable of monitoring surface oxide formation and reduction in the monolayer range as well as the development and disappearance of crystallinity. An evaluation of the behavior of the Pt-NPs upon repetitive potential cycling (500 CV transients) revealed they are very stable. A degradation study was carried out in the presence of neutral (N2) and reactive (H2 or O2) gases using anodic and cathodic polarization measurements at a very low potential scan rats. The results demonstrate that the nature of the dissolved gas has a profound influence on corrosion behavior of Pt-NPs and the main observations are as follows: (i) in the case of measurements carried out in the electrolyte saturated with H2(g), no corrosion of Pt-NPs was observed but the carbon support became more porous; (ii) in the case of measurements carried out in the electrolyte saturated with N2(g), the Pt-NPs underwent a slight degradation and the carbon support slight degradation; and (iii) in the case of measurements carried out in the electrolyte saturated with O2(g), the Pt-NPs underwent significant corrosion, especially in the case of the anodic PDP transient; the carbon support also underwent degradation that was also more pronounced in the case of the anodic PDP transient. The chemical state of the Pt-NPs and their corrosion behavior are important because of their application as catalysts in fuel cells and water electrolyzers.

The new and original results presented in this contribution make an


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important contribution to our understanding of their corrosion in aqueous acidic media in the presence of different gaseous environments.

ACKNOWLEDGEMENTS S.T., S.B., C.C. gratefully acknowledge financial support from the European Communities (FEDER) and the Région Nouvelle Aquitaine. G.J. and S.T. gratefully acknowledges financial support from the Natural Sciences and Engineering Research Council of Canada (the Discovery Grant) and Automotive Partnership Canada (Catalysis Research for Polymer Electrolyte Fuel Cells Network, CaRPE-FC 31-619293 RGPNM/477963-2015).

SUPPORTING INFORMATION Supporting Information Available: The initial ten CV profiles recorded during the electrochemical cleaning step of the carbon-supported Pt-NPs.


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39- Biegler, T.; Rand, D. A. J.; Woods, R. Limiting oxygen coverage on platinized platinum; Relevance to determination of real platinum area by hydrogen adsorption. J. Electroanal. Chem. 1971, 29, 269−277. 40- Wojdyr, M. Fityk: a general-purpose peak fitting program. J. Appl. Crystal. 2010, 43, 1126– 1128. 41- Zhu, H.; Li, X.; Han, F.; Dong, Z.; Yuan, G.; Ma, G.; Westwood, A.; He, K. The effect of pitch-based carbon fiber microstructure and composition on the formation and growth of SiC whiskers via reaction of such fibers with silicon sources. Carbon 2016, 99, 174–185. 42- Yi, H.; Wang, H.; Jing, Y.; Peng, T.; Wang, Y.; Guo, J.; He, Q.; Guo, Z.; Wang, X. Advanced asymmetric supercapacitors based on CNT@Ni(OH)2 core-shell composites and 3D graphene networks. J. Mater. Chem. A 2015, 3, 19545–19555. 43- Coutanceau, C.; Urchaga, P.; Brimaud, S.; Baranton, S. Colloidal syntheses of shape- and size-controlled Pt nanoparticles for electrocatalysis, Electrocatalysis 2012 3, 75–87. 44- Álvarez, G.; Alcaide, F.; Miguel, O.; Cabot, P. L.; Martínez-Huertac, M.V.; Fierro, J. L. G. Electrochemical stability of carbon nanofibers in proton exchange membrane fuel cells. Electrochim. Acta 2011, 56, 9370– 9377. 45- Xing, L.; Hossain, M. A.; Tian, M.; Beauchemin, D.; Adjemian, K. T.; Jerkiewicz G. Platinum electro-dissolution in acidic media upon potential cycling. Electrocatalysis 2014, 5, 96–112. 46- Tahmasebi, S.; McMath, A. A.; Van Drunen, J.; Jerkiewicz, G. Catalytic duality of platinum surface oxides in the oxygen reduction and hydrogen oxidation reactions. Electrocatalysis 2017, 8, 301–310. 47- Furuya, Y.; Mashio, T.; Ohma, A.; Tian, M.; Kaveh, F.; Beauchemin, D.; Jerkiewicz G.; Influence of electrolyte composition and pH on platinum electrochemical and/or chemical dissolution in aqueous acidic media. ACS Catal. 2015, 5, 2605−2614. 48- Baroody, H. A.; Jerkiewicz, G.; Eikerling. M. H. Modelling oxide formation and growth on platinum. J. Chem. Phys. 2017, 146, 144102. 49- Kocha, S. Principles of MEA preparation. In: Vielstich, W., Lamm, A., Gasteiger, H. A. (Eds), Handbook of Fuel Cells, John Wiley & Sons, New York, 2003, Vol. 3 Chapter 43, pp. 538–565.


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50- Tremiliosi-Filho, G.; Jerkiewicz, G.; Conway, B. E. Characterization and significance of the sequence of stages of oxide film formation at platinum generated by strong anodic polarization. Langmuir 1992, 8, 658–667. 51- Iden, H.; Takaichi, S.; Furuya, Y.; Mashio, T.; Ono, Y.; Ohma, A. Relationship between gas transport resistance in the catalyst layer and effective surface area of the catalyst, J. Electroanal. Chem. 2013, 694, 37–44. 52- Dubau, L.; Castanheira, L.; Berthomé, G.; Maillard, F. An identical-location transmission electron microscopy study on the degradation of Pt/C nanoparticles under oxidizing, reducing and neutral atmosphere. Electrochim. Acta 2013, 110, 273– 281. 53- Sellin, R.; Grolleau, C.; Arrii-Clacens, S.; Pronier, S.; Clacens, J. M.; Coutanceau, C.; Léger, J. M. Effects of temperature and atmosphere on carbon-supported platinum fuel cell catalysts. J. Phys. Chem. C 2009, 113, 21735–21744.


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Captions to Figures Figure 1. Mass variation with respect to the initial mass and derivative of the mass loss as function temperature from TGA measurements on carbon-supported Pt-NPs. Figure 2. XRD pattern (the black dots), deconvolution peaks (the grey lines) and a fitting curve (the green line). Figure 3. (a) TEM image for spherical carbon-supported Pt-NPs, (b) TEM image for spherical unsupported Pt-NPs, and (c) histogram of the size distribution of carbon-supported Pt-NPs. Figure 4a. CV profiles for carbon-supported spherical Pt-NPs; Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 1.00, 2.00, 5.00, 10.0, 20.0, and 50.0 mV s–1. Figure 4b. Capacitance graphs for carbon-supported spherical Pt-NPs; electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 1.00, 2.00, 5.00, 10.0, 20.0, and 50.0 mV s–1. Figure 5. Raman spectra acquired during (a) an anodic potential transient showing the oxide formation and (b) a cathodic potential transient showing the oxide reduction; electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 0.20 mV s–1. Figure 6. CV profiles for a gradually increasing number of transients; the inset shows the evolution of the value of As versus the cycle number; s = 50.0 mV s–1; electrolyte: 0.50 M aqueous H2SO4; T = 298 K. Figure 7. Anodic (a) and cathodic (b) PDP transients; the gray, blue, and red transients refer to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 0.10 mV s−1. The corresponding potential domains of surface and faradaic reactions occurring at Pt-NPs are presented above each PDP transient.


33


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Figure 8. Anodic (a) and cathodic (b) PDP transients; the gray, blue, and red transients refer to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 0.10 mV s−1. Figure 9. CV profiles prior to (solid lines) and after (dashed lines) anodic (a) and cathodic (b) PDP measurements. The gray, blue, and red transients refer to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 50.0 mV s−1.


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Page 35 of 46 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


TOC Graphic


35


0.10 100 0.08 80

0.222 mg H2O

0.06 4.724 mg C

60

0.04

40

0.02

20

0.00

dm / dT (mg K-1)

( m / mi) x100 (%)

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

Page 36 of 46

36.3 % Pt

0 200

400

600

800

1000

-0.02 1200

T (K) Figure 1. Mass variation with respect to the initial mass and derivative of the mass loss as function temperature from TGA measurements on carbon-supported Pt-NPs. ACS Paragon Plus Environment

Page 37 of 46

(111)

(220)

(222)

(331)

Pt JCPDS Card 04-0802 (200)

(311)

(400)

(420)

a.u.

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


C

20

40

60

80

100

120

140

2(°) Figure 2. XRD pattern (the black dots), deconvolution peaks (the grey lines) ACS Paragon Plus Environment and a fitting curve (the green line).


a

b 70

Frequency (%)

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

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c

60 50 40 30 20 10 0 2.5

3.0

3.5

4.0

4.5

Particle size (nm)

Figure 3. (a) TEM image for spherical carbon-supported Pt-NPs, (b) TEM image for spherical unsupported Pt-NPs, and (c) histogram of the size distribution of carbon-supported Pt-NPs. ACS Paragon Plus Environment

5.0

Page 39 of 46

300 200 100

I (µA)

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


0 -100 Pt/C- s = 1.00 mV s-1 Pt/C- s = 2.00 mV s-1 Pt/C- s = 5.00 mV s-1 Pt/C- s = 10.0 mV s-1 Pt/C- s = 20.0 mV s-1 Pt/C- s = 50.0 mV s-1

-200 -300 -400 -500 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E (V vs. RHE) Figure 4a. CV profiles for carbon-supported spherical Pt-NPs; electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 1.00, 2.00, 5.00, 10.0, 20.0, and 50.0 mV s–1. ACS Paragon Plus Environment


14 12 10 8

C (mF)

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

6

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Pt/C - s = 1.00 mV s-1 Pt/C - s = 2.00 mV s-1 Pt/C - s = 5.00 mV s-1 Pt/C - s = 10.0 mV s-1 Pt/C - s = 20.0 mV s-1 Pt/C - s = 50.0 mV s-1

b

4 2 0 -2 -4 -6 -8 -10 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E (V vs. RHE) Figure 4b. Capacitance graphs for carbon-supported spherical Pt-NPs; electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 1.00, 2.00, 5.00, 10.0, 20.0, and 50.0 mV st1. ACS Paragon Plus Environment

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E (V vs RHE)

E (V vs RHE)

1.42 V

Pt oxide

50 CPS

a

Pt oxide

50 CPS

b

0.07 V

1.43 V

0.08 V 500

1000

1500

2000

Raman shift (cm -1)

500

1000

1500

2000

Raman shift (cm -1)

Figure 5. Raman spectra acquired during (a) an anodic poten4al transient showing the oxide forma4on and (b) a cathodic poten4al transient showing the oxide reduc4on; electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 0.20 mV s–1. ACS Paragon Plus Environment


400 200 0

I (µA)

-200 70 -1

As (m g )

-400 -600 -800 0.0

60

2

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

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Pt / C After 50 Cycle Pt / C After 100 Cycles Pt / C After 200 Cycles Pt / C After 300 Cycles Pt / C After 400 Cycles Pt / C After 500 Cycles

0.2

0.4

0.6

50 40 30

0

100

200

300

400

500

Cycle number

0.8

1.0

1.2

1.4

1.6

E (V vs. RHE) Figure 6. CV profiles for a gradually increasing number of transients; the inset shows the evolution of the value of As versus the cycle number; s = 50.0 mV s–1; electrolyte: 0.5 M aqueous H2SO4; T = 298 K. ACS Paragon Plus Environment

Page 43 of 46

E°(H+/H2) = 0.000 V

0.0

E range of O2(g) evolution

0.4

E range of H2(g) evolution

1.2

1.6

E ≥ 1.60 V PtO2 formation

-2

E°(Pt2+/Pt) = 1.188 V

-4

-4

-6

-6

-8

-8

b

E°(O2/H2O) = 1.229 V 0.85 - 1.60 V PtO formation

E (V vs RHE)

0.8

-2

E°(H+/H2) = 0.000 V


A.N. ads / des

UPD H

A.N. ads / des

UPD H E range of H2(g) evolution

a

E°(O2/H2O) = 1.229 V 0.85 - 1.60 V PtO formation

E (V vs RHE)

E ≥ 1.60 V 0.0 0.4 0.8 1.2 1.6 0.10 - 0.40 V 0.5 - 1.10 V PtO2 formation PtO2 reduction PtO reduction E°(Pt2+/Pt) = 1.188 V

-10

-10 E°(H+/H2) = 0.000 V

0.85 - 1.60 V PtO formation

0.0

0.4

-2

E range of O2(g) reduction


1.2

E range of H2(g) evolution

E ≥ 1.60 V 1.6 PtO2 formation

E°(Pt2+/Pt) = 1.188 V

-4

-2

-6

-8

-8

-10

-10 E°(H+/H2) = 0.000 V


0.0



A.N. ads / des 0.4


E (V vs RHE)

0.8

1.2

1.6

E ≥ 1.60 V PtO2 formation

E°(Pt2+/Pt) = 1.188 V

-3



A.N. ads / des

E (V vs RHE)

E ≥ 1.60 V 0.0 0.4 0.8 1.2 1.6 0.10 - 0.40 V 0.5 - 1.10 V PtO2 formation PtO2 reduction PtO reduction E°(Pt2+/Pt) = 1.188 V

-3

-4

-4

-5

-5

-6 -0.5

E°(O2/H2O) = 1.229 V

E range of H2(g) oxidation

E°(O2/H2O) = 1.229 V

E range of H2(g) oxidation

E (V vs RHE)

E ≥ 1.60 V 0.8 1.2 1.6 0.0 0.4 0.10 - 0.40 V 0.5 - 1.10 V PtO2 formation PtO2 reduction PtO reduction E°(Pt2+/Pt) = 1.188 V E range of O2(g) reduction

-4

-6

E°(H+/H2) = 0.000 V


A.N. ads / des

UPD H

E (V vs RHE)

0.8

E°(O2/H2O) = 1.229 V


A.N. ads / des


E°(H+/H2) = 0.000 V

E°(O2/H2O) = 1.229 V

log (|j|)

log (|j|)

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


-6 0.0

0.5

1.0

1.5

E (V vs. RHE)

2.0

2.5

-0.5

0.0

0.5

1.0

1.5

2.0

E (V vs. RHE)

Figure 7. Anodic (a) and cathodic (b) PDP transients; the gray, blue, and red transients refer to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 0.10 mV s−1. The corresponding potential domains of surface and faradaic reactions occurring at ACS Paragon Plus Environment Pt-NPs are presented above each PDP transient.

2.5


-2

a

-2 -3 -4

-4

-5

-5

-6 -7

-6 -7

-8

-8

-9

-9

-10 -0.5

0.0

0.5

1.0

1.5

2.0

b

-3

log (|j|)

log (|j|)

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

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2.5

-10 -0.5

0.0

0.5

1.0

1.5

2.0

E (V vs. RHE)

E (V vs. RHE)

Figure 8. Anodic (a) and cathodic (b) PDP transients; the gray, blue, and red transients refer to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 0.10 mV s−1.


2.5

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300

a

CV Profile - Prior to Anodic Polarization CV Profile - After Anodic Polarization

200 100

300

100

0

0

-100

-100

-200

-200

-300 300

b

CV Profile - Prior to Cathodic Polarization CV Profile - After Cathodic Polarization

200

-300 300

CV Profile- Prior to Anodic Polarization CV Profile-After Anodic Polarization

0

CV Profile- Prior to Cathodic Polarization CV Profile-After Cathodic Polarization

0

I (µA)

I (µA)

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


-300 -600 -900 800

-600 -900 800

CV Profile- Prior to Anodic Polarization CV Profile-After Anodic Polarization

600

-300

400

400

200

200

0

0

-200

-200

-400

-400 0.0

0.2

0.4

0.6

0.8

CV Profile- Prior to Cathodic Polarization CV Profile-After Cathodic Polarization

600

1.0

1.2

1.4

1.6

0.0

0.2

E (V vs. RHE)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

E (V vs. RHE)

Figure 9. CV profiles prior to (solid lines) and after (dashed lines) anodic (a) and cathodic (b) PDP measurements. The gray, blue, and red transients refer to the electrolyte solution saturated with N2(g), O2(g), and H2(g), respectively. Electrolyte: 0.50 M aqueous H2SO4; T = 298 K; s = 50.0 mV s−1.



-2

N2 O2 H2

70

-3

60 50

-4

40 30

-5

20 10 0 2.5

3.0

3.5

4.0

4.5

Particle size (nm)

5.0

log (|j|)

Frequency (%)

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

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-6 -7 -8

log│jcorr │

-9 -10 -11 -0.5

log│jcorr │ Ecorr

0.0

0.5

1.0

Ecorr 1.5

E (V vs. RHE)


2.0

2.5

How Stable Are Spherical Platinum Nanoparticles Applicable to Fuel

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