Fluorine-Free Pt Nanocomposites for Three-Phase Interfaces in Fuel

Sep 7, 2016 - Fluorine-Free Pt Nanocomposites for Three-Phase Interfaces in ... under real working conditions in a single hydrogen/oxygen fuel cell, ...
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Fluorine-Free Pt-Nanocomposites for ThreePhase Interfaces in Fuel Cell Electrodes Delphine Dru, Stève Baranton, Janick Bigarré, Pierrick Buvat, and Christophe Coutanceau ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02145 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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ACS Catalysis

Fluorine-Free Pt-Nanocomposites for Three-Phase Interfaces in Fuel Cell Electrodes Delphine Dru1,2, Stève Baranton1, Janick Bigarré2, Pierrick Buvat2,*, Christophe Coutanceau1,*. 1

IC2MP, Université de Poitiers, UMR CNRS n°7285, Poitiers, 86022, France.

2

CEA, DAM, Le Ripault, F-37260 Monts, France.

ABSTRACT: First generation of proton exchange membrane fuel cells uses costly and unsafe perfluorinated sulfonic acid polymers (PFSAs) as membranes and as ionomers impregnating electrodes to achieve the three-phase boundaries. PFSAs imply paramount issues for large scale manufacture, use, commercialization and recycling. Alternative non-fluorinated polymers should allow not only obtaining membranes, but also adequate ionomer suspensions in convenient solvents for preparing efficient catalytic layers, which is not yet achieved. Here, we propose a universal solution consisting in the transposition of the three-phase boundary at the molecular level by grafting directly at the surface of carbon supported Pt nanoparticles a non-fluorinated proton conducting polymer combining catalytic activity of the former and transport properties of the latter. The length of polystyrenesulfonate polymer chain (as model polymer) and the number of polymer feet per platinum nanoparticles have been optimized in order to achieve the highest active surface area and activity as possible. It was shown that low grafting density and high degree of polymerization were the best configuration. The great potency of such nanocomposites as cathode catalysts for PEMFC was evidenced not only in standard three-electrode cell but also under real working conditions in a single hydrogen/oxygen fuel cell, where higher activity and stability were obtained with a nanocomposite material than with a classical Pt/C + Nafion electrode.

KEYWORDS: fuel cell, Nanocomposite catalysts, Oxygen reduction reaction, Platinum, Sulfonated hydrocarbon ionomers.

INTRODUCTION The proton exchange membrane fuel cell (PEMFC) will play a pivotal role for electric energy production in the emerging hydrogen economy; it is becoming a mature technology and several car manufacturers are starting or planning the commercialization of fuel cell vehicles. The first generation of PEMFCs is based on perfluorinated sulfonic acid polymers (PFSAs, with NafionTM being the most recognized representative)1,2 as electrolytic membranes and as ionic polymers impregnating electrodes of Pt-based nanoparticles (Pt-NPs) from 1 to 10 nm diameters3,4 dispersed on a high surface area electron conductive carbon powder5. Indeed, electrode reactions mainly occur at confined spatial sites (three-phase boundaries, TPBs), where the reactant (oxygen or hydrogen), the ionic conductor (ionomer) and the Pt catalytic site are together6. Such electrode formulation leads to significantly increase the catalyst efficiency, allowing further the decrease of the Pt loading in electrodes7,8. But Pt activity in classical 3-D catalytic layers was estimated to be only 10 to 20 % of its full potential9. Moreover, several drawbacks make perfluorinated ionomers a not ideal choice for future commercialization. Beyond their low temperature and high relative humidity working ranges10, the fluorine based chemistry for their syntheses makes these polymers expensive. Moreover, fluorine based chemistry generates

lack of safety for PFSA manufacturing, use (degradation11) and recycling12, involving exhaust of environmentally non-friendly, toxic, and corrosive compounds13. Sulfonated hydrocarbon ionomers (SHC) are interesting alternatives if they can not only be integrated in membranes but also in electrodes to create three-phase boundaries (TPBs)14-17. Attempts to replace PFSA by SHC ionomers in electrodes, always involving impregnation methods, are still not successful, and SHC display lower performances than PSFA18,19. Studies have shown that the distribution of the ionomer inside the active layer and its interactions with the Pt catalytic surface and/or the carbon support play an important role on proton, gas and water transport through the catalytic layer and further on the electrode performance17. The optimization of these properties is greatly depending on the catalytic ink formulation and on the catalytic layer deposition process, where ionomer agglomeration can occur. In the case of SHC, another limitation concerns the possibility of obtaining ionomer suspensions in protic solvents with low boiling points, which is considered as a requirement for PEMFC manufacture17,20. Here, we propose to overcome these limitations by using the high potency of nanocomposite materials, demonstrated in numerous applications (chemistry, electronics, optics, information storage, medical, biotechnology, catalysis, etc.), where active polymers decorating metal-NPs act as multifunctional and smart systems. The universal solution we emphasize is to create TPBs at the molecular level by grafting the polystyrenesulfonic acid (PSSA)21-23 as

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model SHC ionomer directly on the Pt-NP surface in order to optimize the ionomer distribution in the active layer and its interaction with Pt active sites. The catalyst activity is maximized by tuning the grafting densities and the chain lengths. This nanocomposite material design combines proton conducting pathways between the electrolytic membrane and Pt active sites through hydrophilic SO3H groups, with oxygen transport pathways from the diffusion layer to the Pt active sites through hydrophobic polymer backbone24. The activity and selectivity of the nanocomposite materials are determined using the rotating disc electrode and rotating ring disc electrode techniques, and the optimizations of the polymer grafting density and of the chain length are performed. Then, the nanocomposite catalysts are tested under fuel cell working conditions in order to determine the platinum utilization rate, the electrical performance, the stability and to compare results with classical Nafion-based Pt/C electrodes. This work will allow evidencing the great potency of such nanocomposites as cathode catalysts for PEMFC in standard three-electrode cell as well as under real working conditions in a single hydrogen/oxygen fuel cell.

EXPERIMENTAL SECTION 1

H NMR (200 MHz) spectra were recorded using an Advance DPX 200 NMR spectrometer from Bruker with a QNP probe for a 5 mm tube. The chemical displacements are expressed in parts per million on the δ scale with respect to the tetramethylsilane (TMS) singlet taken as an internal reference. Thermogravimetric analyses (TGA) were carried out using a DTA Instruments Q600 thermobalance. The measurements were made by heating the samples from 25 to 800 °C at a heating rate of 10 °C min−1 under an air flow of 100 mL min−1, followed by an isothermal step at 800 °C for 5 min. Transmission electron microscopy (TEM) measurements were carried out with a JEOL JEM 2010 (HR) with a resolution of 0.35 nm. Synthesis of the disulfanediyldiethane-2,1-diyl bis[4(chloromethyl) benzoate] initiator for atom transfer radical polymerization (ATRP): 2-hydroxyethyldisulfid (1.53 g; 9.9 mmol; 1 eq.), chloroform (30 mL) and triethylamine (4.22 g; 41.7 mmol; 4.2 eq.) were introduced in a 100 mL two-neck round-bottom flask under Argon. The two-neck round-bottom flask was sealed and placed in a cooling bath at 0 °C. 4chloromethylbenzoyl chloride was introduced dropwise and the mixture was then allowed reaching room temperature overnight. The crude product was washed 4 times (with an acidic aqueous solution, then a neutral one, a basic one and finally a neutral one). The compound was dried over MgSO4 and the solvent was evacuated using a rotavapor. The product was then dried at 60 °C overnight before characterizations (yield: 97 %). 1

H NMR (400 MHz, CDCl3, δ = 7.26 ppm): 7.9 (m, 2H, H aromatic), 7.4 (m, 2H, H aromatic), 4.5 (m, 4H, Ph-CH2-Cl and O-CH2-CH2), 3.0 (t, 2H, O-CH2-CH2-S). Elemental analysis: (C20H20Cl2O4S2): C (52.1%), H (4.4%), Cl (15.5%), O (14%) S (14%).

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ATRP of sodium styrene sulfonate. 48 mL of MilliQ water (18.2 MΩ cm) were introduced in a two-neck round-bottom flask and degassed by bubbling argon (15 min). Sodium styrene sulfonate (SSNa) was introduced under argon atmosphere and the mixture was maintained by bubbling argon. In parallel, 16 mL of methanol were put in a piriform flask and degassed by bubbling argon for 15 min. Then, the initiator was introduced under argon until complete dissolution. Once SSNa was totally dissolved, copper chloride (37 mg) and bipyridine (116 mg) were introduced in the two-neck round bottom flask under argon flow, and the mixture was kept under argon atmosphere. The initiator in solution in methanol was then dropwise introduced under argon flow using a syringe (conditioned under argon). The two-neck round-bottom flask was heated in an oil bath at 45 °C for 21 h, and the polymerization was stopped by venting the system. The mixture was filtrated on silica gel in order to remove the copper chloride trapped in the sodium polystyrene sulfonate (PSSNa). The filtrate was then concentrated under vacuum in order to facilitate the precipitation of the polymer in methanol. The polymer was dried overnight at 60 °C before characterizations. Polymer characteristics are given in table 1. 1

H NMR (200.13 MHz, D2O): δ 8-7.2 (m, Harom), 7-6 (m, Harom), 1-2 (m, CH3 and CH2) ppm.

Table 1. Nominal (Th.) and actual (Exp.) values of the degree of polymerization (DPn), conversion rate (Conv.), molecular mass (Mn and Mw), polymerization index (Ip) of the different polystyrene sulfonate polymer synthesized by ATRP. Th. Conv. Th. Mn (%)a DPn 500 103554 71 1000 206649 65 1500 309744 69 2000 412839 70 2500 515934 67 a 1 H NMR measurements, raphy in water.

Exp. Exp. Exp. Ip DPn Mnb Mwb 355 76441 85614 1.12 1.15 650 138266 159006 1035 219886 250670 1.14 1400 297266 356719 1.20 1675 355566 433791 1.22 b Steric Exclusion Chromatog-

Elemental analyses of PSSNa with DPn from 500 to 2500 showed the extreme uniformity of the polymer composition (C: 45.3 %, H: 3.3 %, O: 15 – 15.1%, Na: 10.7 - 10.8 %, Cl < 0.1%). Synthesis of platinum nanoparticles supported on carbon (Pt-NP/C). Appropriate amount of H2PtCl6•6H2O (99.9% purity, Alfa Aesar) was dissolved in 100 mL ethylene glycol (puriss. p.a., 99.5 % Fluka) in order to reach a concentration of 1 gPt L-1. Then, the pH of the solution was adjusted to 11 by adding dropwise a solution of NaOH (1 M) in ethylene glycol. Carbon Vulcan XC72 (150 mg) thermally treated for 4 h at 400 °C under nitrogen (L’Air Liquide, UQuality) was added to the solution in order to obtain a nominal metal loading of 40 wt % on carbon and the

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mixture was ultrasonically homogenized for 5 min. The reactor equipped with a cooler was placed in a MARS 5 microwave oven from CEM Corporation. The synthesis of catalysts was performed under continuous microwave irradiation at a power of 1600 W for ca. 2 min (until reaching a temperature of 100 °C), and then microwave pulses were applied for 5 min to maintain the reactor temperature. The pH at the end of the synthesis procedure was ca. 11 at 20 °C, and was reduced to ca. 2 by adding HCl. Then 50 mL of ultrapure water were added and the mixture was placed for 5 min in an ultrasonic bath for homogenization. The catalytic powder was recovered by filtration, abundantly washed with ultrapure water and dried overnight at 60 °C. The catalytic powder was thermally treated at 200 °C under air for 2h to remove remaining adsorbed organics from the synthesis. Grafting of PSSNa on platinum nanoparticles. Pt-NP/C and hexylamine were stirred in a 25 mL roundbottom flask until obtaining a stable suspension and, in order to control the grafting density, appropriate amounts of polymers in solution in water/hexylamine (50:50 v/v) were introduced dropwise under stirring. After 12 to 15 h reaction time, the solution was allowed to rest about 30 min without stirring. The modified Pt-NP(PSSNa)/C powder was recovered by filtration and washed successively with acetone (3 x 30 mL), ethanol (3 x 30 mL) and MilliQ water (3 x 30 mL) in order to remove hexylamine and polymer chains that did not graft on platinum nanoparticles. The modified catalytic powder was then dried overnight at 60 °C. Electrochemical measurements. Electrochemical measurements have been performed in a conventional three-electrode electrochemical cell at room temperature using a reversible hydrogen electrode (RHE) and a glassy carbon plate (3 cm2 geometric surface area) as reference and counter electrodes, respectively. The supporting electrolyte was a 0.1 mol L-1 HClO4 (Suprapur, Merck) solution in ultra-pure water (Millipore, MilliQ, 18.2 MΩ cm). The working electrode was prepared by deposition of a catalytic ink onto a glassy carbon disc (0.0707 cm-2 geometric surface area). For Pt-NP(Nafion)/C catalyst, the catalytic ink consisted in the dispersion of a given amount of Pt-NP/C catalytic powder in 2.646 ml of ultra-pure water and 0.354 mL of Nafion® solution (5 wt% Nafion® perfluorinated resin solution in aliphatic alcohols). For the PtNP(PSSA)/C materials (PSSA being the acid form of the polystyrenesulfonate), the catalytic inks consisted in the dispersion of given amounts of Pt-NP(PSSA)/C catalytic powders in 3 ml of ultra-pure water, without addition of Nafion solution. After homogenization in an ultrasonic bath for ca. 1 min, 3 µL of catalytic ink were deposited on the freshly polished glassy carbon disc, leading to a metal loading of 100 µgPt cm-2.

duced in a N2-saturated 0.1 mol L-1 HClO4 electrolyte and several cyclic voltammograms (CVs) were performed between 0.05 V and 1.20 V vs RHE at the scan rate of 50 mV s-1 until stable CVs were obtained. Then, 5 CVs were recorded over the same potential range at the scan rate of 5 mV s-1, the last one allowing determining the electrochemical active surface area (EASA). Studies of oxygen reduction reaction (ORR) were performed in an O2saturated (L’Air Liquide, U-Quality) 0.1 M HClO4 electrolyte from 1.05 V to 0.3 V vs RHE under quasi-steady state conditions at the scan rate of 1 mV s-1. The rotating disc electrode (RDE) technique with different rotation rates (in revolutions per minute) was used to determine the kinetics parameters (number of electron exchanged, limiting current densities, kinetics current densities, Tafel slopes). The ratio of hydrogen peroxide formed during the oxygen reduction reaction was determined using the rotating ring disc electrode (RRDE) technique (Pine instrument) at 2500 rpm. The RDE and RDDE measurements were performed using Voltalab PGZ402 potentiostats (Radiometer analytical). Fuel cell measurements. Electrodes (5 cm2 geometric surface area) were prepared by deposition of a catalytic ink containing either the desired amount of Pt-NP/C + 25 wt% Nafion or the desired amount of Pt-NP(PSSA)/C nanocomposite without Nafion on a commercial gas diffusion layer (SIGRACET 24BC). The anode consisted in Pt-NP/C + 25 wt% Nafion with a Pt loading of 0.2 mg cm-2. The cathodes were loaded with 0.4 mgPt cm-2. For Pt-NP/C + 25 wt% Nafion electrodes, the membrane electrode assembly (MEA) was manufactured by hot pressing of the electrodes on both sides of a Nafion® 211 membrane (20 µm thickness) at 115 °C and 3.5 MPa for 2.5 min, whereas for Pt-NP(PSSA)/C MEAs no hot pressing was applied. The MEAs were inserted and tightened between two monopolar plates. Fuel cell measurements were carried out at a cell temperature Tcell = 60 °C under the following experimental conditions: for the cathode, O2 humidification at room temperature, PO2 = 2 barabs; for the anode, H2 humidification at room temperature, PH2 = 2 barabs.

RESULTS AND DISCUSSION Sodium polystyrenesulfonate (PSSNa) polymers with nominal degrees of polymerization (DPn)25 from 500 to 2500 were first synthesized from styrene sulfonate monomers by atom transfer radical polymerization (ATRP)26 in the presence of disulfanediyldiethane-2,1-diylbis [4(chloromethyl) benzoate] as polymerization initiator.

Cyclic voltammograms were recorded using a Model 362 Scanning Potentiostat from Princeton Applied Research. The freshly prepared working electrode was intro-

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Table 2: Electrochemical characterization and ORR kinetic data for the Pt-Np(PSSA)/C nanocomposites. The zero grafting density sample is a Nafion containing sample.

0 500 1000 1500 2000

2500

Feet /Pt NP 0 0.103 0.103 0.103 0.103 0.036 0.051 0.103 0.206 0.309 0.411

Pt wt% 40 39 39 38 37 39 38 37 34 32 30

1

H NMR spectrum is given in Figure 1a for a polymer with a DPn of 2500. The polymolecularity indexes27 for all polymers are ca. 1.1-1.2, indicating narrow chain length distributions, and the theoretical ionic exchange capacity is ca. 4.8 meqH+ g-1 (based on calculations from elemental analysis). This high value of the ionic exchange capacity is expected to induce ionic conduction from the polymer electrolyte directly to the platinum active sites. Moreover, it has been established that the thiol function of a molecule can create a strong ionocovalent bond with a metallic surface28-30.

a

11 10

9

8

7

6

5

4

3

2

1

0

δ (ppm)

b Hexylamine 12 h at room temperature

PSSNa

Pt-NP (PSSNa)/C

EASA EC 2 -1 /m gPt 80 55 63 69 73 89 98 86 83 76 68

a

EASA FC 2 -1 /m gPt 44 18 27 30 34 52 45 44 36 -

b

B -1 /V dec. 0.067 0.072 0.066 0.065 0.068 0.069 0.074 0.067 0.077 0.083 0.095

jk @ 0.9 V -2 /mA cm -4.6 -1.4 -4.0 -3.5 -3.7 -3.9 -8.6 -4.2 -2.9 -1.3 -1.3

The symmetrical molecular structure of the disulphide polymers allows grafting the polymers on the Pt-NPs surface from both sides of the disulfide bridge, as it was shown in the case of bromide p-mercaptoaniline31. Moreover, it has been established in previous works that the post-grafting of a disulphide molecule took place only on Pt nanoparticle surface, not on the carbon support24. The schematic polymerization reaction for the synthesis of functionalized Pt nanoparticles is presented in Figure 1b. By separating the polymer synthesis and the polymer grafting reaction, it has been possible to control the degree of polymerization (DPn) and the grafting density of the polymer, i.e. the average number of polymer feet on the Pt-NPs. Figures 1c and 1d show typical TEM images of carbon supported Pt-NPs synthesised by a microwaveassisted polyol method before and after grafting of PSSNa (DPn = 2500), respectively, as well as the corresponding size distribution histograms (Figure 1e). In both cases, good dispersions of Pt particles are obtained, with mean sizes of ca. 3.0 nm, corresponding to the appropriate range for fuel cell applications, and narrow size distributions. Characterization data for the different PtNP(PSSA)/C catalysts are given in Table 2 (PSSA being the acid form of the polystyrenesulfonate). 100

▬ Pt-NP/C ▬ Pt-NP(PSSA)/C

90

7 6 5

c

■ Pt-NP/C ■ Pt-NP(PSS A)/C

70

4 3

50

2

d e

39.78 % 220.94 °C

1

Figure 1. (a) H NMR spectrum of a PSSNa polymer; (b) PtPSSNa/C nanocomposite material synthesis process flow; (c,d) TEM images of a Pt-NPs/C catalyst and a PtNP(PSSNa)/C catalyst (DPn = 2500, grafting density = 5.63 10 7 -1 molPSSNa gPt ), respectively; (e) corresponding histograms of the size distribution.

30

Deriv. Weight (%/°C)

Grafting Density -1 /mol gPt 0 5.63 5.63 5.63 5.63 1.97 2.82 5.63 11.3 16.9 22.5

DPn

Weight (%)

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

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1

38.48 %

0 50 100 150 200 250 300 350 400 450 500 550 600 650 Temperature (°C)

Figure 2. Thermogravimetric analyses of a Pt-NP/C and a PtNP(PSSNa)/C nanocomposite material (DPn = 2500, grafting -7 -1 density = 5.63 10 molPSSNa gPt ), TGA was performed in air −1 with a temperature variation rate of 10 °C min .

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Thermogravimetric analysis of Pt-NP(PSSNa)/C (Figure 2) shows mass losses related to exothermal reactions from ca. 220 °C to ca. 300 °C, which corresponds to PSSNa combustion, and for temperatures above 350 °C the combustion of carbon substrate takes place. Finally, ca. 40% of the initial mass is remaining at 800 °C, corresponding to platinum and PSSA combustion residue. Crossing these data with those from elemental analysis reveals a good reliability on PSSA and platinum loadings as determined by TGA. This analysis also evidenced the thermal stability of the organic crown up to 220 °C. 0.6 0.4

I (mA)

0.2 0.0 -0.2 -0.4 -0.6

a

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0.4

0.6

0.8

1.0

1.2

E vs RHE (V)

80 40 0

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ACS Catalysis

-40 -80

b -120 0.0

0.2

0.4

0.6

0.8

E vs DHE (V) Figure 3: (a) Cyclic voltammograms recorded at 25 °C in N2−1 saturated 0.1 M HClO4 solution at a scan rate s = 5 mV s ; (b) Cyclic voltammograms recorded at 60 °C in a fuel cell under N2 flow at the cathode and H2 flow at the anode, at a scan −1 rate s = 50 mV s . Pt-NP(Nafion)/C (Black line); Pt-7 NP(PSSA)/C: DPn = 1000, grafting density = 5.63 10 molPSSA -1 -7 gPt (blue line); DPn = 2500, grafting density = 5.63 10 -1 molPSSA gPt (purple line); green line: DPn = 2500, grafting -1 -7 density = 2.82 10 molPSSA gPt (green line).

Typical cyclic voltammograms recorded in 0.1 M HClO4 solution of Pt-NP(Nafion)/C and Pt-NP(PSSA)/C with different DPn and grafting densities are shown in Figure 3a, as examples. The electroactive surface areas (EASA) of the different composite materials, determined from the hydrogen desorption region after correction of the double layer capacitive currents32, are given in Table 2. For a given grafting density (for example 5.63 10-7 molPSSA gPt-1), the EASA increases with the DPn over the range studied. For a given DPn (for example 2500), the EASA reaches a maximum value of ca. 95 m2 gPt-1 for a grafting density of ca. 2.82 10-7 molPSSA gPt-1 against 80 m2 gPt-1 for the pure PtNP/C catalyst with Nafion, and then decreases towards values lower than that for pure Pt-NP/C with Nafion. In

other words, a low density of polymer feet on Pt nanoparticles and a high length of the polymer chain lead to an increase of the EASA. Since all catalysts have similar Pt particle sizes and size distributions, all experiments are carried under similar conditions and considering that the number of polymer feet, determined from the grafting density values, are very low (maximum average value of 0.4 polymer foot per Pt-NP), the first increase of EASA with low PSSA grafting densities compared with pure PtNP(Nafion)/C catalyst can be explained in terms of the presence of impurities in recast Nafion used as binder in the pure Pt-NP/C catalytic layer33,34 , whereas no Nafion binder is added in the case of Pt-NP(PSSA)/C electrodes. For higher grafting densities the decrease of the EASA is related to higher Pt surface coverage by the polymer. However, possible changes in the porous structure of the Pt-NP(PSSA)/C catalyst layers compared to the Nafion containing catalytic layer, leading to a higher accessibility of platinum to protons, may not be discarded to explain the increase of EASA with the increase of DPn. In addition, the decrease of charges involved in the Pt-NP surface oxidation/surface oxide reduction processes between 0.5 and 1.2 V vs RHE in presence of PSSA (Figure 3a) is explained in terms of limited water accessibility to the Pt surface. This speaks up for lower water transport from the bulk electrolyte to the Pt-NP(PSSA)/C surface than in the case of the Pt-NP(Nafion)/C material. Figure 3b shows as an example a set of cyclic voltammograms recorded on Pt-NPs (modified by PSSA or not) at the cathode of a PEMFC at 60 °C under N2 flow, the anode under H2 flow acting as counter electrode and dynamic hydrogen reference electrode (DHE). The active surface areas in fuel cell configuration have been determined from these CVs (Table 2). While all catalysts contain the same loading of platinum, it appears clearly that the highest EASA is achieved with the Pt-NP(PSSA)/C catalyst with a DPn of 2500 and a grafting density of 2.82 10-7 molPSSAgPt-1, confirming the previous observations made in aqueous acidic solution. It is worth to note that this latter nanocomposite catalyst leads to higher active surface area than the Pt-NP(Nafion)/C material, indicating a higher platinum utilization rate. For the study of the oxygen reduction reaction (ORR), quasi-stationary current vs potential curves were recorded during slow negative potential sweeps (scan rate of 1 mV s-1) between 1.05 and 0.3 V vs RHE for different rotation rates ω (from 500 to 2500 rpm) of the rotating disc electrode (RDE) in O2-saturated 0.1 M HClO4 supporting electrolyte. A typical set of current density (referred to the geometric surface area) vs potential curves are shown in Figure 4 for a Pt catalyst modified with PSSA (DPn of 2500 and grafting density of ca. 2.82 10-7 molPSSA gPt-1), as an example. All polarization curves exhibit three distinguishable potential regions. The first one from 0.3 V to ca. 0.6 V vs RHE corresponds to the diffusion limiting current density plateau, the second one from ca. 0.7 V to ca. 0.9 V vs RHE to a mixed kinetic-diffusion control region and the third one from ca. 0.9 V to 1.05 V vs RHE to the

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j (mA cm-2)

-3 -4 -5

ω = 500 rpm ω = 1000 rpm ω = 1500 rpm

-6

ω = 2000 rpm

-7

ω = 2500 rpm

-8 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

potentials by the RDE methods) was confirmed by rotating ring disc electrode (RRDE) experiments. Figures 5a,b show typical disc and ring currents recorded with a PtNP(PSSA)/C catalyst (DPn of 2500 and grafting density of ca. 2.82 10-7 molPSSA gPt-1) at 2500 rpm as an example, and the H2O2 proportion as a function of the electrode potential, respectively. The very low proportion of H2O2 (X(H2O2)) produced via a two-electron process indicates that the presence of the organic crown around the platinum particles does not affect their selectivity for the oxygen reduction reaction into water via a four-electron process. The number of exchanged electrons per reduced oxygen molecule is always between 3.9 and 4.0.

The analysis of data was carried out by separating the contribution of the diffusion of molecular oxygen in the bulk electrolyte from that of the surface processes involved in the oxygen reduction reaction35,36. For this purpose the generalized Koutecky-Levich equation (Equation 1) was used in order to take into account the mixed limitations of the oxygen diffusion in the 3-D porous catalytic film and of the oxygen adsorption on Pt nanoparticles37,38. The graphs from which were determined the number of exchanged electron (n), the values of the limiting current density (jlFilm), the kinetic current density jk and the Tafel slope b in the case of a Pt-NP(PSSA)/C catalyst with a DPn of 2500 and a grafting density of ca. 2.82 10-7 molPSSA gPt-1 are given in the Supporting Information. Table 2 summarizes the kinetics data related to ORR for all studied catalysts and calculated from mathematical treatment of the following equation (Supporting Information).

1 1 1 1 1 1 = + Diff = + Film + Diff j je jl jk jl jl

(1)

where, j is the current density, jlDiff the limiting current density of O2 in electrolyte, which is dependent on the electrode rotation rate and can be calculated, je is the current density resulting from processes occurring at the electrode. This current density is decomposed into jk, the kinetic current density and jlFilm, the limiting current density of oxygen diffusion in the catalytic film. The catalytic activity, exemplified by the value of jk, the kinetic current density at 0.9 V, follows almost the same trend as that of the active surface area: for a fixed value of grafting density (5.63 10-7 molPSSA gPt-1) jk increases with the DPn reaching a plateau value (ca. 4 mA cm-2), and for a given DPn of 2500 jk increases first to reaches a maximum value of ca. 8.6 mA cm-2 (against 4.6 mA cm-2 for pure PtNP/C) for a grafting density of 2.82 10-7 molPSSA gPt-1 and then decreases for higher values of the grafting density. These results are summarized in Table 2.

6 4 2 0.0

a

-0.2

ID (mA)

Figure 4. Hydrodynamic polarization curves recorded for different electrode rotation rates ω (in rpm) on a Pt-7 NP(PSSA)/C (DPn = 2500, grafting density = 2.82 10 molPSSA -1 gPt ) at 25 °C in O2-saturated 0.1 M HClO4 electrolyte with a −1 scan rate s = 1 mV s .

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Figure 5. (a) Hydrodynamic polarizations curves at the RRDE for the oxygen reduction on a Pt-NP(PSSA)/C disc a platinum ring electrodes and at 25 °C in O2-saturated 0.1 M -1 HClO4 electrolyte at a scan rate of s = 1 mV s , Pt ring potential = 1.2 V vs RHE; (b) Amount of hydrogen peroxide produced (X(H2O2)) in red line and water produced (X(H2O)) in black line.

The overall consequence of these observations is that such nanocomposite materials can be used as cathode catalysts for electrochemical systems: their activity and selectivity are comparable and even higher after optimization of the DPn and grafting density than those of nonmodified platinum nanoparticles for the ORR, and their electrochemical and thermal stabilities were previously demonstrated22.

The number of exchanged electrons per reduced oxygen molecule (n was found equal to 4 for all catalysts and all

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Figures 6c and 6d show the fuel cell performances as a function of the nominal DPn and of the grafting density, respectively; the cell performance, exemplified by the current density achieved at a cell voltage of 0.6 V, increases with the DPn (chain length), whereas it decreases with the grafting density. Here again, the same trends as in aqueous acidic electrolyte is observed. This clearly evidences that the presence of grafted PSSA does favor the accessibility of protons to the platinum catalytic sites as higher active surface area (Pt utilization rate) are obtained with increasing DPn. Moreover, the enhancement of fuel cell performances with DPn (chain length) also evidences that oxygen transport is not disturbed by the presence of polymer, and is even improved.

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oxygen at the anode and cathode (gas pressures of 2 bars absolute, stoichiometries of λH2 = 1.5 and λO2 = 1.5), respectively, by determining polarization and currentpower density curves (Figures 6a and 6b) from membrane-electrodes assemblies fitted with Pt-NP(PSSA)/C having different nominal DPn and different grafting densities, respectively. The platinum amount at the PtNP(PSSA)/C nanocomposite cathode is 0.4 mg cm-2, whereas that of the Pt/C anode is 0.2 mg cm-2, and the solid electrolyte is a Nafion® 211 membrane (ca. 20 µm nominal thickness).

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Figure 6. (a-b) Polarization and power density curves in fuel cell working at 60 °C with Pt-NP(PSSA)/C cathode without -7 -1 Nafion. (a) Grafting density = 5.63 10 molPSSA gPt , DPn = 1000 (black), DPn = 1500 (red), DPn = 2000 (green), DPn = -7 2500 (blue); (b) DPn = 2500, grafting density (GD) = 2.82 10 -1 -7 -1 molPSSA gPt (black), GD = 5.63 10 molPSSA gPt (blue), GD = -6 -1 -6 -1 1.13 10 molPSSA gPt (green), GD = 1.69 10 molPSSA gPt (red). (c-d) H2/O2 Fuel cell performances @ 0.6 V as a function of the DPn (c) and grafting density (d).

The potency and the capabilities of our new active layer architectures will be further demonstrated under PEMFC working conditions at 60 °C with pure hydrogen and

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Time (h) Figure 7. Fuel cell performances at 60 °C with a PtNP(PSSA)/C nanocomposite-based cathode with DPn = 2500 -7 -1 and grafting density = 2.82 10 molPSSA gPt without Nafion (black line) and a Pt-NP(25 wt% Nafion)/C cathode (blue lines): (a) Polarization and power density curves, (b) Stability -2 at 1.0 A cm .

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The membrane-electrode assembly (MEA) based on a PtNP(PSSA)/C cathode catalyst (DPn 2500, grafting density 2.82 10-7 molPSSA gPt-1) without Nafion nor hot pressing against the membrane allows reaching the same, or even higher activity, as a MEA based on Pt-NPs/C cathode catalysts with 25 wt% Nafion after hot pressing against the membrane (Figure 7a), with a maximum power density of ca. 0.95 W cm-2 at 2.25 A cm-2. At last, the stability under fuel cell working conditions was evaluated (Figure 7b). A voltage decay of ca. 60 µV h-1 at a current density of 1 A cm-2 was observed over more than 250 hours of experiment with the Pt-NP(PSSA)/C cathode catalyst (DPn 2500, grafting density 2.82 10-7 molPSSA gPt-1), whereas the classical Pt-NP(Nafion)/C cathode catalyst led to ca. 90 µV h-1 (i.e. 50 % more decay with Nafion), This confirms that the organic polymer is stable under these conditions. This represents a very important result, particularly considering the role of the grafted thiol molecules for the stabilization of the small Pt core as well as for a stable electrochemical response as it was proposed by Volatron et al.39. These authors concluded that organically grafted Pt electrocatalysts can be of interest for fuel cell application, and we are demonstrating here their high potency.

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and by making possible the use of membranes alternative to Nafion as solid electrolyte. The replacement of perfluorosulfonic acid polymer by carbon polymer is of paramount importance because fluorine based chemistry is costly and could imply environmental hazards during fabrication and use, and for facilitating both the platinum and the polymer recycling in MEAs by avoiding the pollutant and costly treatment of perfluorosufonic acid polymers. The polymer investigated here (PSSA) acts as a model polymer, and obviously future works should be devoted to design polymers exhibiting different important properties for new active layers (such as hydrophobic/hydrophilic character, ionic conductivity, electronic conductivity, acid/base character, etc.) by controlling the nature and structure of the grafted polymer, and the functional groups introduced. At last, the concept emphasized here is transposable to solid alkaline fuel cells by grafting anion conducting polymers (bearing quaternary amine groups for example).

ASSOCIATED CONTENT Supporting Information. Mathematical treatment of the generalized Koutecky-Levich (KL) equation.

AUTHOR INFORMATION

CONCLUSION

Corresponding Author

The possibility of bringing specific properties of proton and oxygen transport toward the same Pt catalytic site by grafting ionic conducting polymer on the platinum nanoparticle surface with enhancement of both catalytic activity and selectivity is a reality, i.e. the transposition of the three-phase boundary at the molecular level has been realized. By optimizing the chain length and the number of grafted polymer feet on Pt nanoparticles, very active and stable cathode catalysts, in classical three-electrode cell as well as under fuel cell working conditions, have been fabricated.

* [email protected] * [email protected]

Some of the authors of this paper, belonging to CEA, have worked on composite PVDF-HFP-based proton-exchange membranes embedding PSSA-grafted silica particles and have obtained good preliminary results under fuel cell configuration40. In their paper, they explained that “further investigations will be conducted in order to assess the durability of the composite membranes in parallel to the development of specific membrane electrode assemblies in order to further improve the performance of the cell”. It is recognized that the problem of the three-phase boundary when changing the ionomer is of great importance. The work presented in the present article concerns then the second aspect of the fuel cell performance, i.e. the optimization of the active layer structure. From now, after having optimized the membrane and the electrode architecture, the next step will be to optimize the membrane-electrodes assemblies in order to improve the fuel cell performances. This work is in progress. Such a possibility is highly attractive for simplifying the electrode material processing and lowering PEMFC system costs, by increasing the metal utilization efficiency, by avoiding the addition of an ionomer in the active layer

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We acknowledge the ADEME (French Environment and Energy Management Agency) for its financial support through the EXALAME project.

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