Pt Particles Functionalized on the Molecular Level as New

Article pubs.acs.org/Langmuir

Pt Particles Functionalized on the Molecular Level as New Nanocomposite Materials for Electrocatalysis Anne-Claire Ferrandez,†,‡ Stève Baranton,† Janick Bigarré,‡ Pierrick Buvat,*,‡ and Christophe Coutanceau*,† †

Université de Poitiers, IC2MP, UMR CNRS no. 7285, Poitiers 86022, France CEA, DAM, Le Ripault, F-37260 Monts, France

‡

ABSTRACT: A nanocomposite material consisting of platinum nanoparticles surrounded by an ionic conducting polymer dispersed on carbon Vulcan XC72 was synthesized. The aim of this nanocomposite material is to translate the triple-phase boundary to a molecular level in electrochemical systems involving a polymer electrolyte. The ionic conducting polymer is a poly(styrenesulfonic acid) (PSSA, or PSSNa in its sodium form) synthesized by atomtransfer radical polymerization. The polymer has a terminal thiol group to ensure bonding with platinum nanoparticles. The nanocomposite material (Pt-PSSA/C) exhibited thermal stability up to 160 °C and electrochemical stability up to 1 V versus RHE. Compared to a Pt/C catalyst, the nanocomposite catalyst has a lower active surface area but comparable catalytic activity for the oxygen reduction reaction. Furthermore, this nanocomposite material exhibits similar behavior in a fuel cell active layer without Nafion as a classical Pt/C catalyst with Nafion included in the active layer.

1. INTRODUCTION Before proton-exchange membrane fuel cell technology can gain a significant share of the electrical power market, important issues have to be addressed.1 Energy conversion is achieved with costly Pt-based catalysts,2,3 and the low oxygen reduction reaction (ORR) kinetics implies high Pt loadings at the cathode.4,5 Electrode reactions can occur only at confined spatial sites called triple-phase boundaries where the reactant, the ionic conducting polymer, and the electronic conducting substrate are present on the same platinum particle.6 Efforts to increase the amount of such active sites are therefore of paramount importance in enhancing the cell performance by increasing the platinum utilization efficiency in 3D Pt/C porous electrodes. In current systems, triple-phase boundaries are achieved by adding Nafion ionomer to the catalytic ink used for the preparation of the active layer. Most of the investigations concerning the effect of the amount of ionomer in fuel cell electrodes7,8 concluded that the optimal Nafion content depends on the fuel cell working point; high Nafion content increases the electrode active area and the cell performance at low current densities, whereas it induces mass transport limitations at high current densities because the pores fill up. To balance both of these antagonist effects, a compromise of ca. 30 wt % Nafion is often used in electrodes.9 Moreover, the Pt utilization efficiency in current commercially offered prototype fuel cell electrodes remains very low (20−30%), and reaching a higher utilization efficiency is still a crucial and therefore very active research topic.10−12 The combination of the limiting ORR kinetics at the cathode13,14 with the low Pt utilization © 2012 American Chemical Society

efficiency becomes detrimental to the cell performance and cost. To overcome these limitations, the current electrode active-layer architecture paradigm has to be abandoned. The new concept emphasized here is the transposition of the triple-phase boundaries on the molecular scale by grafting an ionic conducting polymer directly onto platinum nanoparticles. We also show the great potency of such nanocomposite materials as cathode catalysts. The triple-phase boundaries are created on the molecular level by grafting a polymer bearing ionic groups to the Pt nanoparticle surface during the catalyst synthesis. This nanomaterial design presents a high potency for creating a proton-conducting pathway between platinum active sites and the conducting membrane. For the implementation of this new concept, a two-step synthesis of the nanocomposite material was developed. A water-in-oil microemulsion method15,16 (w/o) of carbon-supported Pt nanoparticle synthesis was carried out in the presence of sodium poly(styrenesulfonate) (PSSNa) as a model cationic conducting polymer17 (the acid form of the polymer, the poly(styrenesulfonic acid), is noted as PSSA).

2. EXPERIMENTAL SECTION 1

H NMR (200 MHz) spectra were recorded using an Avance 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 Received: September 6, 2012 Revised: November 22, 2012 Published: November 26, 2012 17832

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Figure 1. (a) Pt-PSSNa/C nanocomposite material synthesis process flow. (b) 1H NMR characterization of the PSSNa polymer. 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. 2.1. Synthesis of 2-Bromo-N-{4-[4-(2-bromo-2-methyl-propionylamino)-phenyldisulfanyl]-phenyl}-2-methyl initiator (C20H22Br2N2O2S2). The synthesis of the C20H22Br2N2O2S2 initiator was carried out in a flask by dissolving 343.6 mg of bipyridine (purity 98.8%) and 248 mg of 4-aminophenyl disulfide (purity 98%) in 10 mL of chloroform (anhydrous, purity 99%) under vigorous stirring. The flask was cooled to 0 °C before the dropwise addition of 272 μL of bromoisobutyrate (purity 98%), and the reaction mixture was allowed to reach room temperature for 10 h under stirring. Then, 10 mL of water was poured into the flask, and the reaction mixture was transferred to a separatory funnel. The aqueous phase was washed with

dichloromethane (anhydrous, purity 99.8%). The organic layer was dried over MgSO4 and filtered. The solvents were removed using a vacuum rotary evaporator. The product was purified by chromatography on silica gel with CH2Cl2/CH3OH solvent (100/0 → 90/10, rf = 0.8). The yield was 95%. All reagents were purchased from SigmaAldrich. 1H NMR (200.13 MHz, CDCl3): δ 8.47 (s, 1H, NH), 7.56− 7.42 (m, 4H, Harom), 2.04 (s, 6H, CH3). 13C NMR (200.13 MHz, CDCl3): δ 170.1 (CO), 137.2 (NH-Carom), 132.9 (S−Carom), 130.1 (HN−C arom −CH arom −CH arom −S), 12.6 (H 2 N−C arom −CH arom − CHarom−S), 63.1 (Br−C−(CH3)2, 32.6 (CH3). Elemental analysis (atom %): C (42.5), H (4.2), Br (30.0), N (4.3), O (7.9), S (11.1). 2.2. Synthesis of Sodium Polystyrenesulfonate (PSSNa). In a two-necked flask, 10 mg of styrene sulfonate (purity 90%) was dissolved under stirring in a 3/1 water/methanol solution under a stream of argon (U Quality from L’Air Liquide) before 115 mg of bipyridine (purity 98.8%) and 36.4 mg of copper chloride (anhydrous, purity 99.995%) were added. Then 50 mg of initiator was added, under argon, to the reaction mixture. The polymerization reaction was carried out for 20 h at 45 °C under a stream of argon. The solution was filtered through silica gel to remove the copper chloride. Then the 17833

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Figure 2. Pt-PSSA/C nanocomposite characterization: (a) TEM images at different magnifications of carbon-dispersed platinum nanoparticles encapsulated by a PSSNa crown, with their size distribution. (b) Thermogravimetric analysis of the nanocomposite material. TGA was performed in air at 10 °C min−1. 2.4. Electrochemical Measurements. The working electrode is prepared by the deposition of catalytic ink on a glassy carbon disk according to a method proposed by Gloagen et al.18 The catalytic powder (10 and 12 mg of Pt/C and Pt-PSSA/C catalysts, respectively) is added to a mixture of 0.2 mL of Nafion solution (5 wt % from Aldrich) in 1 mL of water (Milli-Q, Millipore, 18.2 MΩ cm). After ultrasonic homogenization of the catalyst/C-Nafion ink, 3 μL was deposited from a microsyringe onto a freshly polished glassy carbon substrate, yielding a catalytic powder (metal catalyst + carbon) loading of 293 μg cm−2 (117 μgPt cm−2 for the Pt/C catalyst) and 350 μg cm−2 (120 μgPt cm−2 for the Pt-PSSA catalyst), leading to a catalytic layer thickness close to 1−1.5 μm.19 Water was then evaporated in a stream of pure nitrogen at room temperature. The electrochemical experiments were carried out with a Voltalab PGZ 401 (Radiometer) in a standard three-electrode electrochemical cell thermostated at 20 °C in an N2-purged supporting electrolyte. The working electrode was a glassy carbon disk (0.071 cm2 geometric surface area), the counter electrode was a glassy carbon plate (8 cm2 geometric surface area), and the reference electrode was a reversible hydrogen electrode (RHE). All potentials are referenced to the RHE. The supporting electrolyte was a 0.5 M H2SO4 (Suprapur, SigmaAldrich) solution in ultrapure water. Electrochemical quartz crystal nanobalance (EQCN) measurements were conducted with an Elchema EQCN-701 electrochemical quartzcrystal nanobalance fitted with a 10 MHz AT-cut quartz crystal (Elchema). The quartz crystal coated with Ti as an intermediate layer and Au as the outer layer (0.196 cm2) was placed in a Teflon holder using a horizontal configuration with the crystal being at the bottom. Experimental work was performed using a homemade, threeelectrode electrochemical glass cell and a carbon counter electrode. The Pt-PSSA/C catalyst was deposited directly onto the Au surface

reaction product was obtained by removing the water under vacuum in a rotary evaporator. The polymer was precipitated in an acetone/ methanol solution, filtered, and dried in an oven at 75 °C overnight. The yield was 92%. All reagents were purchased from Sigma-Aldrich. 1 H NMR (200.13 MHz, D2O): δ 8−7.2 (m, Harom), 7−6 (m, Harom), 1−2 (m, CH3 and CH2). Elemental analysis (atom %): C (39.4), H (4.3), Br (21.0), N (3.8), O (10.3), S (15.1), Na (6.1) %. Polymerization index: 1.8. 2.3. PSSNa Grafting during the Water-in-Oil Microemulsion Method (w/o) for Pt Nanoparticle Synthesis. A microemulsion is prepared by dispersing 1 mL of an aqueous solution containing 0.2 mol L−1 hexachloroplatinic acid hexahydrate (purity 99.9%) in 18.71 g of n-hexane (purity 95%) in the presence of 5.3 g of Brij 30. After allowing the microemulsion to equilibrate for 30 min, platinum salt reduction is performed by the direct addition of 116 mg of sodium borohydride (purity 96%) powder to the microemulsion. PSSNa (10 mg) is added to the stable, black Pt colloid solution under vigorous stirring. After 1 h of stirring, 60 mg of Vulcan XC72 carbon powder is added and the suspension is sonicated for 30 min. Composite material cleaning is performed by filtration after the first addition of 50 mL of acetone. Three subsequent acetone addition and filtration steps are performed followed by an acetone/water (50/50 v/v) solution addition and filtration step and three pure water addition and filtration steps. This cleaning procedure is repeated for the removal of Brij 30 from the nanocomposite material. Finally, the nanocomposite material is dried at 135 °C to ensure the thermal decomposition of the remaining adsorbed Brij 30 molecules. Hexachloroplatinic acid hexahydrate was purchased from Alfa Aesar, and Vulcan XC72 carbon powder was purchased from Cabot corp. All other reagents were purchased from Sigma-Aldrich. 17834

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Figure 3. Potentiostatic stability in an acidic medium for Pt PSSA nanocomposite material by electrochemical quartz crystal nanobalance. The mass variation (red line) is presented with the corresponding cyclic voltammograms (black line) for four different upper limit potentials: 0.85 V vs RHE, 1.0 V vs RHE, 1.2 V vs RHE, and 1.4 V vs RHE. Cyclic voltammograms are recorded at 25 °C in dearated 0.5 M H2SO4 solution at a scan rate of s = 5 mV s−1. ification temperature Tair = 60 °C, air pressure Pair = 3 barabs; (anode) H2 humidification temperature TH2 = 80 °C; H2 pressure PH2 = 3 barabs.

using an ink method and allowed to dry in a N2(g) atmosphere at room temperature. Then, the Au electrode covered with the catalyst was placed in the Teflon holder and rinsed several times with freshly prepared electrolyte. Prior to each experiment, the working electrode potential was set to 0.10 V and the electrolyte was degassed by purging with high-purity N2(g) for 20 min. Electrochemical measurements in a 0.5 M aqueous H2SO4 solution were performed at a scan rate of s = 5 mV s−1 using an Elchema PS-205 potentiostat and Voltscan acquisition software. Frequency changes (Δf) during electrochemical measurements were converted to mass variations (Δm). After the measurements, EQCN was calibrated using Cu deposition and stripping in a 0.1 M aqueous HClO4 solution according to the procedure described in the instrument manual; the deviation between the measured and calculated calibration constant values was 4%. An Elchema EQCN-602 Faraday cage was used to isolate the experimental setup from any external electromagnetic interference. 2.5. Fuel Cell Measurements. Electrodes (5 cm2 geometric surface area) are prepared by deposition on a gas diffusion layer of a catalytic ink containing the desired amount of 40 wt % Pt/C + 25 wt % Nafion or Pt polymer/C nanocomposite to obtain Pt loadings of 0.4 and 0.1 mg cm−2 for the anode and cathode, respectively. Gas diffusion layers are prepared from a commercial carbon cloth (Electrochem. Inc. CC-060) on which a Vulcan XC 72 (Cabot Corp.) + 20 wt % Teflon (Electrochem. Inc. PTFE emulsion EC-TFE) microporous layer was brushed. Once the catalytic ink is dried, membrane electrode assembly (MEA) is performed by hot pressing the electrodes on both sides of a Nafion 117 membrane at 115 °C and 35 kg cm−2 for 5 min. The MEA is inserted and tightened between two monopolar plates. Fuel cell measurements are carried out at a cell temperature of Tcell = 80 °C under the following experimental conditions: (cathode) air humid-

3. RESULTS AND DISCUSSION Figure 1a presents the schematic reaction protocol for the synthesis of functionalized Pt nanoparticles. The PSSNa was previously synthesized in a yield of 92% by atom-transfer radical polymerization (ATRP)20 in the presence of 2-bromoN-{4-[4-(2-bromo-2-methyl-propionylamino)-phenyldisulfanyl]-phenyl}-2-methyl-propionamide as the polymerization initiator. The polymer 1H NMR is shown in Figure 1b. The expected degree of polymerization was MN = 100 000. The polymerization index of PSSNa is ca. 1, and its ion-exchange capacity is ca. 4.8 meqH+ g−1. This high value of the ionexchange capacity is expected to involve ionic conduction from the polymer electrolyte directly to the platinum active sites. After the polymer is added to the Pt colloidal solution obtained at the end of the w/o synthesis reduction step, the disulfide bridge, introduced in the polymer by the initiator, allows the grafting of PSSNa on the platinum nanoparticle surface, leading to the formation of the nanocomposite material (Figure 1a) through a Pt−S covalent bond.21,22 Figure 2a gives TEM images of the catalytic powder recorded at different magnifications. The catalyst synthesis method leads to a repartition of platinum nanoparticles on the carbon substrate comparable to that observed on classical Pt/C catalysts with similar metal loading.16 A narrow size distribution 17835

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Figure 4. (a−c) Electrocatalytic activity and selectivity of Pt-PSSA nanocomposite material and (d−f) Pt-Nafion catalyst towards an oxygen reduction reaction. (a, d) Polarization curves recorded at 25 °C in O2-saturated 0.5 M H2SO4 electrolyte at a scan rate of s = 3 mV s−1. The electrode rotational rate (Ω) was set at 400, 900, 1600, and 2500 rpm. (b, e) Current densities related to phenomena occurring at the electrode (je) for the ORR as a function of the electrode potential with the resulting Tafel plot (inset). (c, f) Total exchanged number of electrons per reduced dioxygen molecule (nt) with the associated amount of hydrogen peroxide produced (X(H2O2)).

respectively, in total agreement with the mass losses recorded by TGA, indicating that the PSSNa residue assumed after combustion is actually very low. This analysis also provides evidence of the thermal stability of the PSSNa crown up to 160 °C. Before discussing the beneficial effect of the platinum functionalization on fuel cell performance, we examine the electrochemical and electrocatalytic behaviors (active surface area, stability under potential control, activity, and selectivity toward ORR) of the nanocomposite material. The electrochemical active surface area (EASA) of the nanocomposite material was determined from the cyclic voltammograms (CV) recorded in deaerated sulfuric acid electrolyte, according to eq 1, and compared to that measured on the Pt/C catalyst

and a mean particle size of ca. 3.5 nm are obtained (Figure 2a, bottom), which lies in the optimal particle size range for the ORR.23 Nanoparticles observed at higher magnifications clearly exhibit the presence of a surrounding PSSNa organic crown. The thermogravimetric analysis of the nanocomposite material is presented in Figure 2b. Several mass losses are observed. Water desorption (endothermic mass loss) occurs in the range from 25 to 150 °C, and then an exothermal reaction from ca. 160 to ca. 350 °C corresponds to the PSSNa combustion. At temperatures above 350 °C, the combustion of the carbon substrate takes place. Finally, 34% of the initial mass remains at 800 °C, corresponding to platinum. In addition to the thermal behavior characterization, crossing the data with those from elemental analysis reveals good reliability of the PSSNa and platinum loadings determined by TGA. The elemental analysis indeed gives 38 and 9 wt % loadings for Pt and PSSNa, 17836

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Article 1 s

the generally accepted detailed reaction mechanism described by Tarasevich et al.,28 involving the formation of hydrogen peroxide (H 2 O2 ) as an intermediate or product, was considered. The Koutecky−Levich equation considered is

∫ i(E) dE

H Q monolayer

(1)

with s being the electrode potential scan rate, i(E) being the current measured over the hydrogen desorption potential range, and QHmonolayer being the charge density involved for hydrogen monolayer desorption from a smooth polyoriented platinum surface (210 μC cm−2).24 A drastic decrease in the electrochemical active surface area (EASA) from ca. 40 m2 gPt−1 for the Pt/C catalyst down to ca. 25 m2 gPt−1 for the supported nanocomposite material evidenced the presence of the poly(styrenesulfonic acid) (PSSA) crown. It is likely that sulfonate anions of the ionomer do not adsorb as strongly as sulfate anions of the electrolyte; hence, the active surface area value of the Pt/C catalyst is certainly underestimated in comparison to that of the Pt-PSSA/C catalyst. However, Feliu et al.25 showed that the adsorption of the anion, which could occur as soon as 0.2 V, led to an approximately 10% difference in the active surface area determination in sulfuric or perchloric acid (perchlorate does not adsorb on platinum). Moreover, the active surface area of Pt-PSSA is found to be about 35% smaller than that of Pt/C, and not only sulfonate anions but also sulfate anions can adsorb on Pt-PSSA. The influence of sulfate anion adsorption on the comparison between both catalysts certainly lies in the standard deviation of the measurements. The only conclusion that we can draw from these measurements is that the grafting of PSSA on Pt particles leads to a decrease in the active surface area of ca. 35−40%. The stability under potentiostatic conditions of the organic crown was followed by measuring the electrode mass loss during several voltammetric cycles as a function of the upper-limit potential. The mass variations measured by the electrochemical quartz crystal nanobalance for five voltammetric cycles and the corresponding CVs recorded are given in Figure 3. For better readability, the CVs and the corresponding mass variations are displayed on the same current and mass scales, respectively. An important degradation of the organic crown is observed only for upper potential limits higher than 1.2 V versus RHE, as evidenced by the significant electrode mass decrease, leading to higher EASA. With a potential upper limit fixed at 1.2 V versus RHE, although the mass loss is not significant, a very limited degradation of the organic crown is pointed out by the more defined hydrogen adsorption/desorption peaks between 0.05 and 0.3 V versus RHE compared to those observed in the CVs recorded with a potential upper limit set at 1.0 V versus RHE. The nanocomposite material displayed excellent stability up to 1.0 V versus RHE, which is the PEMFC open circuit cathode voltage. Characteristic sets of E(j) polarization curves for the ORR recorded during a positive-going potential scan at different rotational rates (Ω) of the electrode in 0.5 M H2SO4 at 25 °C on the Pt-Nafion/C catalysts and Pt-polymer/C nanocomposite material are given in Figure 4a,d. All polarization curves exhibit two distinguishable potential regions: from 0.4 V versus RHE to ca. 0.6 V versus RHE, a diffusion limiting current density plateau and from ca. 0.7 V versus RHE to ca. 0.8 V versus RHE, a mixed kinetic−diffusion control region. The current densities are roughly proportional to Ω1/2. A general analysis of data was carried out by separating the contribution of the diffusion of molecular dioxygen from that of the surface processes involved in the oxygen reduction reaction using the j−1 versus Ω−1/2 Koutecky−Levich plots.26,27 For that purpose,

1 1 1 1 1 = + ads + film + diff j jl jl jl j0 (θ /θe)eη / b

(2)

where j0 is the exchange current density, η is the overpotential, b is the Tafel slope, θ and θe are the coverage of a platinum surface by species coming from oxygen adsorption at potential E and at the equilibrium potential Eeq (1.185 V vs RHE), respectively, jlads is the limiting current density related to the adsorption, jlfilm is the limiting current density related to the diffusion in the catalytic film, and jldiff is the limiting current density related to the diffusion in the electrolyte. The oxygen reduction polarization curves were recorded under quasi-stationary conditions at a potential scan rate of 3 mV s−1. If we assume that in the considered potential range the adsorption process of oxygen is more rapid than the chargetransfer steps, then θ ≈ θe for the whole electrode potential range.27 The kinetic current density can then be defined as jk = j0 (θ /θe)eη / b = j0 eη / b

(3)

Both jlads and jlfilm are limiting current densities that are independent of the rate of rotation of the electrode, and it is then impossible to separate their contributions. They are included in a single limiting current jl, defined as

1 1 1 = ads + film jl jl jl

(4)

Finally, for the implementation of Koutecky−Levich analysis, the phenomena occurring at the electrode (i.e., independent of the rotation rate) are separated from the diffusion in the electrolyte by introducing je: 1 1 1 1 = + ads + film je jk jl jl

(5)

The slopes of the Koutecky−Levich straight lines (with data taken from Figure 4a,d) at different electrode potentials lead to the determination of the total number of exchanged electrons per reduced dioxygen molecule (nt) as a function of the electrode potential, and the intercept at the origin gives the current density related to the phenomena occurring at the electrode (je). Then plotting je−1 as a function of the electrode potential (Figure 4b,e) allows a determination of the limiting current density (jl). In the case of a catalytic film, the limiting current density (jl) contribution was previously defined as a combination of the oxygen diffusion limiting current density in the catalytic film (jlfilm) and the oxygen adsorption limiting current density on the catalytic sites (jlads).27 The values of jl were found to be ca. −55 and −40 mA cm−2 for Pt-Nafion/C and Pt-PSSA/C catalysts, respectively. This significant difference in jl values clearly indicates that the presence of the organic crown affects the oxygen diffusion in the catalytic film and/or the oxygen adsorption on the catalytic sites. The plot of η versus log(je/(j1 − je)) gives straight lines (Figure 4b,e), from which the Tafel slopes (b) and the exchange current densities (j0) are obtained. Two distinguishable overpotential regions are characterized by different values of the Tafel slopes; in the overpotential range above −0.30 V, the Tafel slope is ca. −70 mV decade−1 whereas in the overpotential range below −0.30 17837

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V, the Tafel slope is ca. −110 mV decade−1, as expected for ORR on platinum materials.29 The number of exchanged electrons per reduced dioxygen molecule and the corresponding proportion of H2O2 (XH2O2) produced via a two-electron process indicates that the presence of the organic crown around the platinum particles does not affect their ability to reduce oxygen to water via a four-electron process (Figure 4c,f). The kinetic constants calculated from the Koutecky−Levich analysis are reported in Table 1.

hydration management of the membrane easier over the whole polarization curve. Hence, reproducible polarization curves could be obtained. Both systems lead to very comparable cell performances in terms of achieved power and current densities (i.e., Pmax ≈ 0.3 W cm−2 at j ≈ 0.6 A cm−2). However, different behaviors can be observed depending on the considered current density range. In the kinetically controlled current density region (j < 0.2 A cm−2),31 the Pt-polymer/C nanocomposite material presents a higher catalytic activity for ORR than does the classical electrode material, whereas in the electrolyte ionicresistance-controlled region (0.2 A cm−2 < j < 0.6 A cm−2), the Pt-polymer/C nanocomposite material leads to a higher absolute value of the slope of the U(j) polarization curve (higher proton-transfer resistance) than with the classical electrode material. This latter behavior could be due to a percolation of proton conduction pathways that is not fully optimized, whereas our new active-layer architecture leads not only to a higher platinum utilization efficiency but also to the achievement of a better interface, on the molecular scale, between the ionic conducting phase and the platinum active sites, explaining the higher cell voltage in the kinetically controlled region,32 which clearly validates the concept emphasized here (i.e., the transposition of triple-phase boundaries on the molecular scale). As we discussed earlier, the kinetic current density jk is higher for Pt-PSSA/C than for Pt/C, whereas the limiting current density jl is lower. The higher kinetic current density (i.e., the higher exchange current density j0, which is directly related to the electrocatalytic activity), in addition to better proton transport through the catalytic film, may explain the higher voltage in the activation region (from j = 0 to 0.2 A cm−2) with the PSSA-Pt/C catalyst than with the Pt/C catalyst. However, for current densities higher than 0.4 A cm−2, the Pt/C catalyst leads to a higher cell voltage (i.e., higher power densities than for Pt-PSSA/C, which displays a mass transport (oxygen) limitation). The lower value of jl may be due to an oxygen diffusion limitation in the catalytic film. Both of these effects are more or less counterbalanced according to the working potential of the cell. The classical preparation of active layers, consisting of physically mixing the Pt/C catalyst and Nafion ionomer, leads to very low platinum utilization, as explained in the Introduction. The modification of the platinum particles with a proton-conductive polymer leads to an increase in the utilization of platinum that can counterbalance the decrease in the platinum active surface area. Although a loss of active surface area occurs with the grafting of PSSA on Pt particles, the activity toward ORR is certainly enhanced by the increase of the number of triple-phase boundaries (active sites for the reaction), hence increasing the number of platinum nanoparticles available for electrode reactions. Moreover, we believe that Pt utilization is not only increased by the greater number of triple-phase boundaries but also by the increased proton conductivity implied by the presence of PSSA, which can lead to a higher turnover frequency. The improvement in the Pt utilization is not only quantitative (in terms of the number of available active sites for the reaction) but also qualitative (in terms of the rate of reaction per active site available). The percolation of proton-conduction pathways in the catalytic layer can be improved by adding 10 wt % Nafion to the Pt-PSSA/C nanocomposite catalyst. The addition of a small amount of Nafion leads to an increase in the fuel cell voltage, especially in the resistance-controlled region (0.2 A cm−2 < j