hydrophilic properties of Vulcan XC72 carbon powder by grafting of trifluoromethylphenyl and phenylsulfonic acid groups

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Modification of hydrophobic/hydrophilic properties of Vulcan XC72 carbon powder by grafting of trifluoromethylphenyl and phenylsulfonic acid groups Martin Weissmann, Ste`ve Baranton *, Jean-Marc Clacens, Christophe Coutanceau Laboratoire de Catalyse en Chimie Organique, UMR 6503 CNRS, 40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France

A R T I C L E I N F O

A B S T R A C T

Article history:

Carbon supports (glassy carbon and Vulcan XC72 powder) were modified by electrochemi-

Received 5 February 2010

cal and spontaneous grafting of phenylsulfonic acid (PSA) or trifluoromethylphenyl (TFMP)

Accepted 2 April 2010

groups via diazonium ion reduction. The effectiveness of the grafting was confirmed elec-

Available online 9 April 2010

trochemically, by XPS measurements and elemental analyses. The hydrophobic or hydrophilic character of carbon surfaces was evidenced by measuring the contact angles of drops of different liquids (water, ethylene glycol and glycerol) in heptane. The surface energy was calculated and it was found, for example, that spontaneous grafting of a glassy carbon surface by PSA groups led to an increase by a factor 20 of the surface energy compared with an unmodified glassy carbon surface. The study of the grafting of such groups on XC72 carbon powder indicated that a very low grafting ratio (in wt%) led to a significant change in the macroscopic properties of the powder. Thermogravimetric analysis coupled with mass spectroscopy measurements (TGA-MS) showed that these grafted layers were thermally stable even in the presence of dispersed platinum nanoparticles. It was shown by cyclic voltammetry that the carbon substrate modification did not affect the electrochemical behavior of platinum catalyst, since the same active surface area was determined on Pt-XC72, Pt-PSA-XC72 and Pt-TFMP-XC72 catalysts.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

The control of the hydrophobic/hydrophilic character of materials has a great interest in a lot of applications, such as biomaterials [1], film formation of oil in water emulsion (cosmetic formulations, pharmaceutical lotions and lubricant additives) [2], detection of molecular species in aqueous solutions [3], separation and purification of wastewater [4], and heterogeneous catalysis in liquid or gas phase [5–7]. Amongst the materials, carbon blacks are themselves used in numerous applications [8] where their surface properties have to be improved. For example, proton exchange membrane fuel cell electrodes are generally constituted of a nanostructured metallic phase dispersed on a carbon black powder [9–11],

which is a convenient substrate because of its high electrical conductivity, chemical and electrochemical stabilities and high specific surface area [12–14]. The balance of hydrophobic and hydrophilic properties of materials is of very great importance in PEMFCs [15]: the electron transfer between catalysts and electrodes may be affected by water flooding [16] notably at the cathode where water is produced, and the ionic conductivity of Nafion membrane may be affected by the dehydration of the anode [17], resulting in ohmic polarization. The gas diffusion layer on the cathode side of PEMFC requires generally hydrophobic treatments before use [18]. Polytetrafluoroethylene (PTFE) with different concentrations is generally used to adjust the hydrophobic character of the electrodes [19]. The water

* Corresponding author: Fax: +33 5 49 45 35 80. E-mail address: [email protected] (S. Baranton). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.04.003

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management is practically always performed in the gas diffusion layer of the electrodes [20–23]. However, reactions occur in the active layer of the electrodes sandwiched between the membrane and the gas diffusion layer. Therefore, it appears very important to manage water directly in the active layer. In a PEMFC, the hydration of the electrolytic membrane has to be optimized in order to favor proton conduction from the anode to the cathode. Furthermore, ionomer is added to the active layer of the electrodes in order to improve the triple phase boundary where electrochemical reactions occur [24]. This Nafion based ionomer has also to be hydrated to avoid ohmic polarization. Hence the water transport in such system is very complex and involves the electro-osmotic drag from the anode to the cathode [25] and the back-diffusion of water from the cathode to the anode [26] due to the concentration gradient (water is produced at the cathode and comes also from the electro-osmotic drag). To achieve correct hydration of the electrodes and of the membrane, hydrogen and oxygen are often humidified by bubbling in heated water, but this method is not adapted for systems having size and mass constraints. Watanabe et al. [27] first developed a self-humidified membrane by incorporating platinum particles, leading to increase the platinum loading in the system, which is a costly and rare material. Han et al. [17] have developed a new design of self-humidification method for fuel cells by protonizing a conventional hydrophobic fuel cell anode by Nafion-silica suspension. Cell performances under non-hydrated gases were higher than those obtained with non-modified Nafion, but lower than those achieved with humidified gases. In a previous article treating on the stabilization of platinum nanoparticles by formation of a molecular bridge, the limitation of carbon surface modification by oxidative methods was exposed with the reasons for preferring modification of carbon surfaces via a diazonium ion reduction method [28]. In the present work, the modification of carbon surfaces via a diazonium reduction route is studied in order to confer hydrophobic or hydrophilic characteristics to carbon substrates. Different carbon substrates (glassy carbon and Vulcan XC72 powder) were modified by 4-trifluoromethylphenyl and 4-phenylsulfonic acid groups and the grafted layers were characterized using electrochemical methods, elemental analyses, XPS measurements, TG analyses coupled with mass spectroscopy in order to verify the effectiveness of the grafting and the stability of the grafted layers. The hydrophobic/ hydrophilic character of the carbon surfaces was evaluated by measuring the contact angle of drops of different solvents, and by observation of the behavior of the carbon powders in water. At last, influence of the grafted layers on the platinum particle size and size distribution was determined by transmission electron microscopy (TEM), and the electrochemical behavior of Pt/C catalysts was compared using cyclic voltammetry.

2.

Experimental

2.1.

Modification of a glassy carbon electrode

A glassy carbon electrode was polished with A1 (grain size: 1 lm), A3 (grain size: 0.3 lm) and A5 (grain size: 0.05 lm)

alumina powders. After each polishing step the electrode was washed with ultrapure water (milliQ, Millipore, 18.2 mX cm) and sonicated for 5 min. The electrochemical modification of the glassy carbon electrode was performed in a standard three-electrode cell. The working electrode was a 3 mm diameter glassy carbon disk, the counter electrode was a glassy carbon plate and a SCE was used as reference electrode (Radiometer Analytical). The system was controlled by a Voltalab potentiostat (Radiometer Analytical, PGZ 100). Electrochemical modifications were carried out in 0.5 mol L1 HCl deaerated aqueous solution using an in situ method of diazonium ion synthesis according to the protocol described by Baranton and Be´langer [29]. Layers of 4-trifluoromethylphenyl and 4-phenylsulfonic acid molecules were grafted. The presence of grafted layers on the electrode surface was confirmed by electrochemical measurements in 0.1 mol L1 KCl aqueous solution (Acros organics, 99%) containing ferricyanide and ferrocyanide probe molecules 3 1 (½FeðCNÞ4 6  ¼ ½FeðCNÞ6  ¼ 5 mmol L , Merck), the electrochemical reactivity of probe molecules being influenced by the presence of grafted groups on the electrode surface.

2.2.

Modification of carbon powders

The Vulcan XC72 carbon powder was modified with 4-trifluoromethylphenyl (TFMP-XC72) or 4-phenylsulfonic acid (PSAXC72) layer grafted in aqueous medium by spontaneous reduction of the corresponding in situ generated diazonium cations. Carbon Vulcan XC72 (300 mg) were placed in 50 mL of a 0.5 mol L1 HCl solution containing 4 mmol L1 of the corresponding amine compound (97%, Sigma Aldrich). The solution was vigorously stirred for 30 min before controlled nitrite sodium addition (NaNO2 ACS reagent, Sigma Aldrich) up to a concentration twice that of 4-aminotrifluoromethylphenyl or 4-aminophenylsulfonic acid in order to ensure a total transformation of the amine into diazonium in spite of the nitroþ gen dioxide and nitric oxide gas release (2NO 2 þ 2H ! NO2 þ NO þ H2 O). The solution was stirred and left to react at room temperature for 24 h. The carbon was then filtered and abundantly rinsed with water, methanol, dimethylformamide, acetonitrile, methanol and water, successively. The resulting modified carbon powder was dried under air at 75 C overnight. The amounts of fluorine or sulfur in the different samples were determined by elemental analysis (CNRS Central Analysis Department). The amount of grafted groups is given in weight percent (wt%).

2.3. Thermogravimetric spectroscopy

analysis coupled with mass

TGA-MS experiments were performed on a TGA apparatus SDT Q600 from TA Instruments coupled with a mass spectrometer Prisma QMS 200 (Balzers). Samples of 10 mg were disposed in a platinum crucible and heated from 25 to 900 C (with a 10 C min1 temperature slope) under air atmosphere (50 mL min1 flow rate). The mass spectroscopy measurements, recorded during thermogravimetric analyses, were performed at m/z 64 corresponding to the SO2 signal

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and m/z 69 corresponding to the CF3 signal, for the characterization of PSA and TFMP grafted layers, respectively.

2.4. X-ray photoelectron spectroscopy (XPS) characterization XPS measurements were performed with an Escalab MKII (VG scientific) set-up using the magnesium monochromatic beam (1253.6 eV). The data were collected at room temperature and the operating pressure in the analysis chamber was set below 8 · 109 Torr. The survey spectra were recorded between 0 and 1200 eV with a resolution of 1 eV. The core level spectra were recorded with a resolution of 50 meV. The binding energies were lined up with respect to the C 1s peak at 284.6 eV.

2.5.

Electrochemical measurements on catalytic powders

Catalytic powders of platinum nanoparticles supported on carbon were prepared via the ‘‘water in oil’’ microemulsion method described elsewhere [30], but slightly modified. Molar ratio between the surfactant (Brij 30 from Aldrich) and water, n(Brij)/n(water), was set to 3.8. For all experiments, the nominal platinum weight ratio was 40 wt%. The following catalytic powders were prepared: Pt-TFMP-XC72 (carbon Vulcan XC72 modified with 8 wt% of 4-(trifluoromethyl)phenyl), Pt-PSA-XC72 (carbon Vulcan XC72 modified with 6.5 wt% of 4-phenylsulfonic acid) and Pt-XC72 (unmodified carbon). Catalytic inks containing Nafion are composed of 25 mg catalytic powders, 2.5 mL ultrapure water and 0.5 mL Nafion solution (Nafion 5 wt% in aliphatic alcohol from Aldrich). Catalytic inks without Nafion are composed of 25 mg catalytic powders, 1.5 mL ultrapure water and 1.5 mL isopropanol. Isopropanol acts as a dispersant for the catalytic powder (which is normally devoted to Nafion) to obtain a good homogeneity of the ink. Electrodes were prepared by deposition of 3 lL of the catalytic ink on a glassy carbon electrode or a gold plate, for Nafion containing inks or Nafion-free inks, respectively. The solvent was evaporated at room temperature either under nitrogen flow or under air for 20 min. Electrochemical measurements were carried out using a classical three-electrode cell where the reference was a

10

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reversible hydrogen electrode (RHE) and the counter electrode was a glassy carbon plate. The electrolyte is a 0.1 M HClO4 (suprapur from Merck) aqueous solution deaerated 30 min by nitrogen (U quality from l’Air Liquide) bubbling. Same set-up as for the electrochemical modification of the glassy carbon surface was used. Before recording the voltammograms used for the determination of the active surface area, the platinum surface was cleaned by repeating 20 voltammetric cycles between 0.05 and 1.45 V vs. RHE [31] at 50 mV s1. Then the voltammograms were recorded between 0.05 and 1.2 V vs. RHE at 20 mV s1. For each samples, three measurements were performed to verify the reproducibility.

2.6.

Transmission electronic microscopy

TEM measurements were carried out using a JEOL (200 keV) transmission electronic microscope with a resolution of 0.35 nm. ImageJ free software was used to determine nanoparticle size on a total of 500 counted particles. The size of nanoparticles is evaluated from isolated nanoparticles, i.e. nanoparticles which cannot be considered as an agglomerate of several particles on TEM pictures.

3.

Results and discussion

3.1.

Functionalization of glassy carbon

3.1.1.

Grafting of diazonium ions by electrochemical method

The grafting of 4-phenylsulfonic acid and 4-trifluoromethylphenyl was electrochemically carried out on a glassy carbon electrode in order to determine the reduction potential of the related diazonium ion precursors. The terminal functional groups were chosen because of their chemical natures close to that of the PTFE-based skeleton and of the ionic conductive groups of Nafion polymer, for TFMP (–CF3) and PSA (–SO3H), respectively. The in situ synthesis of diazonium ions in acidic aqueous medium ([HCl] = 0.5 mol L1) was carried out with 5 mmol L1 of either sulfanilic acid or 4-trifluoromethylaniline. Then sodium nitrite (10 mmol L1) was added in the acidic solution deaerated by bubbling pure nitrogen. The grafting of 4-phenylsulfonic acid and 4-trifluoromethylphenyl groups was

a

5

b

0

0

-20

I (µA)

I (µA)

-10

-30 -40

-5

-10

-50

-15

-60 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 E (V vs SCE)

E (V vs SCE) 1

Fig. 1 – Voltammograms recorded in aqueous medium ([HCl] = 0.5 mol L ) during the reduction reaction of 2 mmol L1 diazonium ions for the grafting of (a) PSA groups and (b) TFMP groups (T = 20 C; v = 50 mV s1).

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then carried out in course of a linear potential at 50 mV s1 between 0.6 and 0.7 V vs. SCE for five voltammetric cycles. Voltammograms given in Fig. 1a and b for the grafting of PSA and TFMP, respectively, report the occurrence of the reduction reaction of diazonium ions on the glassy carbon surface. The main reduction peak potentials, where the grafting reactions occur [29], are located at ca. 0.5 V vs. SCE and ca. 0.3 V vs. SCE, for PSA and TFMP, respectively. The voltammograms recorded for the grafting of PSA and TFMP present other important differences which have to be discussed. The first observation is that the reduction current of the main reduction peak achieved in the first cycle is much higher in the case of the grafting of PSA than in the case of the grafting of TFMP. It has been shown by other authors that the reduction peak intensity is correlated to the reduction potential of the diazonium salt [29,32]: the lower is the reduction potential, the higher is the charge involved in the reduction reaction. Although the reduction reaction of 4-phenylsulfonic diazonium ions is quantitatively more important than that of 4-trifluoromethylphenyl diazonium ions, the grafting reaction involving these former radicals is disfavored at so low potentials, likely to the benefit of parasite reactions between formed radicals or between formed radicals and species present in the electrolyte. The second observation concerns the persistence of the reduction reaction in the case of the PSA grafting: a reduction current peak is recorded at lower potentials over several voltammetric cycles conversely to that is observed in the case of TFMP where the surface seems completely blocked after the first voltammetric cycle. This last behavior is a more classical one [28–33]: the grafting layer in formation leads to increase the charge transfer resistance of electrons after the first voltammetric cycle, which further limits the reduction current as soon as the second cycle; the grafting reaction is auto-limiting. In the case of the PSA grafting, the reaction yield is low; a single cycle is not enough to reach the limiting surface concentration of PSA groups to achieve a complete blocking of the carbon surface and of the charge transfer. The reaction continues then over several voltammetric cycles until the charge transfer resistance becomes too important. It is also possible that the hydrophilic property of the sulfonic acid end functions of grafted PSA leads to increase the interaction between the carbon surface and the aqueous electrolyte [34], allowing the diazonium ions to reach the carbon surface. This phenomenon could partially counterbalance the charge transfer resistance, allowing the reaction to continue and a higher surface coverage to be reached. The occurrence of the grafting layers was verified by recording cyclic voltammograms on the glassy carbon electrode before and after the grafting reactions in a 5/5 mmol L1 3 1 ½FeCN4 KCl aqueous solution (Fig. 2). 6 =½FeCN6  + 0.1 mol L In both cases (surface modifications by PSA and TFMP), the typical redox reversible peak of the ferricyanide/ferrocyanide probe molecules observed on the unmodified glassy carbon surface have disappeared. However, the PSA layer seems to induce a lower charge transfer resistance than the TFMP layer, as higher currents are recorded notably above 0.4 V vs. SCE indicating that an oxidation reaction occurs on the electrode surface. It has already been shown that the intensity of the surface blocking

100

50 I (µA)

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0

-50

-100 -0.2

0.0

0.2 0.4 E (V /SCE)

0.6

Fig. 2 – Cyclic voltammograms recorded in the presence of 3 1 ) in 0.1 mol L1 KCl FeðCNÞ4 6 and FeðCNÞ6 (5/5 mmol L aqueous electrolyte on a bare glassy carbon electrode (solid line), on a glassy carbon electrode modified by PSA molecules (dash-doted line) and on a glassy carbon electrode modified by TFMP molecules (doted line) from diazonium ion reduction (T = 20 C; v = 50 mV s1). towards probe molecules was depending on the end function of the grafted groups (for equivalent surface concentrations) [29]. Hydrophilic and electric properties of the grafted layer could explain these differences. It has notably been shown that TFMP grafted layers on glassy carbon led to a charge transfer resistance ten times higher than that with a same surface concentration of nitrophenyl layer [29]. This difference was explained by the hydrophobic character of the –CF3 end functions which limits the accessibility of the surface to the electro-reactive species in solution. This surface repulsion leads to avoid electron transfer needed for the redox reaction of ferricyanide/ferrocyanide complexes. In the case of a PSA layer, phenomena occurring on the electrode surface are more complex. The pH of a ferricyanide/ferrocyanide solution is ca. 7, which is higher than the pKa of phenylsulfonic acid functions, estimated at ca. 0.5 by electrochemical impedance spectroscopy (EIS) [29]. Therefore, all acid functions are dissociated and the terminal functions are –SO 3 . Charge repulsion between ferricyanide/ ferrocyanide complexes and the electrode surface should then occur and the charge transfer resistance should increase. However, it is not the case. It has also been shown by EIS that the charge transfer resistance of a nitrophenyl grafted layer was ten times higher than that obtained with a non-dissociated phenylsulfonic acid grafted layer (with comparable surface concentrations) [29]. This effect is probably related to the hydrophilic property of the 4-phenylsulfonic acid groups. It is then likely that both phenomena (charge repulsion and hydrophilic property) are involved in the PSA 4 grafted layer for FeðCNÞ3 6 =FeðCNÞ6 redox behavior.

3.1.2. Spontaneous grafting of diazonium ions on glassy carbon Cyclic voltammetry experiments have clearly shown the possibility of the glassy carbon surface modification by PSA and TFMP groups; the electrode surface properties were greatly affected by the presence of the grafted layers. The hydrophobic or hydrophilic character of the grafted layer can

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be easily evaluated by angle contact analysis, which allows determining the surface energy. The use of glassy carbon as substrate is motivated by the fact that this material can be mirror polished in order to avoid a too important surface roughness which could alter the measurements. For the grafting reaction, a glassy carbon plate is half immerged in an aqueous solution containing 10 mmol L1 diazonium ions (PSA or TFMP) and 0.5 mol L1 HCL for 24 h (spontaneous grafting reaction [35]). The plate is then rinsed with ultrapure water and sonicated in ultrapure water 1 min. At first, a water drop (5 lL) was deposited on the different modified and unmodified carbon plates. Photographs presented in Fig. 3 clearly shows that functionalization by PSA groups leads to a better wetting of the surface (Fig. 3a) than on unmodified glassy carbon (Fig. 3b), and that modification of the surface by TFMP induces the increase of the contact angle of the water drop revealing the increase of the hydrophobic character of the surface (Fig. 3c). The surface energy changes as a function of the grafting layer was determined by measuring the contact angle of three different liquid compounds (water, ethylene glycol and glycerol) in a heptane phase. The surface energy was determined using the Good–van Oss–Chaudhury equation [36]: qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi þ  cdSV cdLV þ cþ c ð1Þ cLV ð1 þ cos hÞ ¼ 2 SV cLV SV cLV þ where cdLV is the dispersive component (dipole–dipole interac tion) and cþ LV and cLV are the polar components of the liquid surface tension; same formalism is used for the solid surface  tension (cdSV , cþ SV and cSV ). Table 1 gives the values of the different cLV, the drop angle contacts for water, ethylene glycol and glycerol on modified and unmodified glassy carbon, and the related surface energy. The sample modified with a PSA layer displays a surface energy twenty times higher than that of an unmodified surface indicating its higher hydrophilic property. In contrary, the sample modified with a TFMP layer displays a surface energy more than 20% lower than that of an unmodified glassy carbon surface. The difference in this latter case is not so important that in the case of the modification by PSA, but the glassy carbon substrate has initially a hydrophobic character which can hinder the real effect of the modification by TFMP. However, although it is known that spontaneous reduction of diazonium ions leads to grafted layers less compact than those obtained by electrochemical reduction [35], the low coverage is sufficient to significantly modify the surface properties.

3.2. Spontaneous grafting of diazonium ions on Vulcan XC72 carbon powder Previous experiments have shown that the formation of grafted layers of PSA and TFMP could be obtained via a spon-

Table 1 – Dispersive component and polar component of the liquid and solid surface tension surface tensions, drop angle contacts for water, ethylene glycol and glycerol drops of 5 lL in heptane, and related surface energy on modified and unmodified glassy carbon. Water Glycerol Ethylene glycol 2

72.8 21.8 25.5 25.5

63.4 37 3.92 57.4

48.3 29.3 1.92 47

hTFMP-C () Surface energy (mJ m2)

82 1400

69

59

hC () Surface energy (mJ m2)

75

61 1800

52

hPSA-C () Surface energy (mJ m2)

37

63 52,000

37

cLV cdLV cþ LV c LV

(mJ m ) (mJ m2) (mJ m2) (mJ m2)

Table 2 – Nominal (expected) and maximum (obtained) weight ratios determined from elementary analysis, and atomic ratios of fluor or sulfur for modified carbon powders.

Nominal weight ratio (%) Fluor or sulfur (at%) Estimated weight ratio (%)

TFMP-XC72

PSA-XC72

10 3.2 8

8 1.29 6.5

taneous reduction reaction of the corresponding diazonium ions on glassy carbon surface. From these results, the modification of a carbon powder typically used for the preparation of PEMFC Pt/C active layers was performed via the same optimized procedure as described in a previous work for the formation of a thiophenol grafted layer [28]. Table 2 compares the maximum weight ratios obtained as estimated from elementary analyses with the nominal weight ratios (expected considering that the grafting reactions give 100% yield); analyses performed on the unmodified Vulcan XC72 powder did not allow detecting fluor or sulfur. Following weight ratios of grafted molecules are obtained: 8 and 6.5 wt%, for the PSA-XC72 and the TFMP-XC72 samples, respectively. The relatively low grafting yield can be explained by the low reduction potential of the 4-phenylsulfonic acid diazonium groups, and by the high hydrophobic character of the 4-trifluoromethylphenyl molecules which could rapidly limit the accessibility of the surface, as evidenced by study of the electrochemically controlled grafting. XPS characterizations of the grafted powders were performed to confirm the grafting of PSA and TFMP groups.

Fig. 3 – Photographs of a water drop (5 lL) deposited at 20 C on (a) a PSA modified glassy carbon surface, (b) an unmodified glassy carbon surface and (c) a modified TFMP glassy carbon surface. Contact angles are also presented on the photographs.

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Table 3 – Atomic ratios of carbon, oxygen and fluor or sulfur as determined from XPS characterizations of the grafted carbon powders. Element

Atomic ratio (at%)

TFMP-XC72 C O F

87 4.2 8.8

PSA-XC72 C O S

92.7 6.6 0.7

200 CPS

a

Fig. 5 – Photographs of (a) a 6.5 wt% PSA modified XC72 carbon powder, (b) an unmodified XC72 carbon surface and (c) a 8 wt% modified TFMP XC72 carbon powder. Carbon powders were ultrasonically homogenized in water for 10 min and kept 48 h decantation before recording photographs.

b

6000 CPS

174 172 170 168 166 164 162 160 Binding energy (eV)

692

690 688 686 Binding energy (eV)

684

Fig. 4 – XPS spectra recorded for (a) S2p core level spectrum for the PSA modified XC72 carbon powder and (b) F1s core level spectrum for TFMP modified XC72 carbon powder.

The survey spectra of both samples indicated clearly the presence of sulfur and fluor on PSA-XC72 and TFMP-XC72, respectively. The relative atomic ratios in carbon, fluor, sulfur and oxygen are given in Table 3. Fig. 4 gives the S2p core level spectrum for the PSA modified carbon (Fig. 4a) and the F1s core level spectrum for TFMP modified carbon powder (Fig. 4b). In the case of the carbon powder modified by TFMP (Fig. 4b), a single signal centered at 688.6 eV is detected, indicating that all fluor atoms are in the same electronic configuration in trifluoromethyl groups. In the case of the carbon powder modified by PSA (Fig. 4a), two peaks are observed at 163.8 and 168.6 eV, both fitted with their S2p 1/2 and S2p 3/2 components, corresponding to sulfur atoms in thiophenol groups and sulfur atoms in highly oxidized groups (4-phenylsulfonic acid), respectively. The presence of thiol groups is not yet understood, notably in so high amount. The chemical reduction of sulfonic acid groups into thiol functions is very difficult to perform [37,38] and it is unlikely that it could take place during the diazonium ion synthesis or during the grafting reaction. However, sulfur is

mainly present under oxidized form corresponding to PSA groups. Moreover, acid–base titrations were performed using the following procedure: 100 mg of carbon sample (XC72 or PSA-XC72) are put in 20 mL of 102 mol L1 sodium hydroxide solution in order to neutralize the protons of the surface acid sites. The mixture is then filtered and 15 mL of the filtrate is titrated with a 5 · 103 mol L1 HCl solution. Table 4 gives the titration results. The modification of the carbon surface with PSA groups leads to a significant increase (by a factor 2) of the acid site concentration per unit surface area. The weight ratio of grafted PSA measured by acid–base titration is very close to that determined by elemental analysis (6.2 and 6.5 wt%, respectively). Considering the agreement between the determination of phenylsulfonic acid groups by acid–base titration and the measurement of the total sulfur content by elemental analysis, it could be proposed that during XPS measurements the X-ray beam reduces some sulfonic acid groups into thiol groups. Other important information from the titration experiment is that all sulfonic acid functions represent effective Bro¨nsted acid sites after grafting on the carbon powder. This may confer some ionic conductivity to the carbon support and help to improve the triple phase boundaries in the fuel cell active layer. Samples of TFMP-XC72 (30 mg), PSA-XC72 (30 mg) and unmodified Vulcan XC72 (30 mg) were dipped into 2 mL of ultrapure water, ultrasonicated for 10 min and left to stand for 48 h at room temperature. Photographs after 48 h decantation are presented in Fig. 5. The unmodified carbon powder was first well dispersed in water but after 48 h, sedimentation was observable. The PSA modified carbon powder remained very well dispersed in water even after times longer than 48 h. The TFMP could never be well mixed with water and rap-

Table 4 – Equivalent volume, acid site concentrations and grafting ratios determined by titration measurements on XC72 and modified PSA-XC72 carbon powders.

NaOH solution XC72 PSA-XC72

Veq.HCl (mL)

Acid site mass concentration (mol g1)

Acid site surface concentration (mol cm2)

Grafting ratio (wt%)

30 23.3 17.5

– 4.4 · 107 8.3 · 107

– 1.76 · 1010 3.32 · 1010

– 0 6.2

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a 0.25

Frequency

0.20 0.15 0.10 0.05 0.00

1

2

3

4

5

6

7

8

9 10 11 12 13 14

Particle size (nm)

b

0.40 0.35

Frequency

0.30 0.25 0.20 0.15 0.10 0.05 0.00

1

2

3

4

5

6

7

8

9 10 11 12 13 14

Particle size (nm)

c

0.35 0.30

Frequency

0.25 0.20 0.15 0.10 0.05 0.00

1

2

3

4

5

6

7

8

9 10 11 12 13 14

Particle size (nm)

Fig. 6 – TEM pictures and related particle size distributions of the Pt particles (40 wt% metal loading on carbon) dispersed on (a) unmodified XC72 carbon powder, (b) modified PSA-XC72 carbon powder and (c) modified TFMP-XC72 carbon powder.

idly formed aggregates above the water indicating a very high hydrophobic character. Although the low weight ratios of grafted groups (ca. 8 wt% and ca. 6.5 wt%, for PSA and TFMP grafted groups, respectively), great changes in carbon surface properties are exhibited compared to unmodified carbon powder. This is an important result which demonstrates that it is not necessary to obtain a very high grafting reaction yield to affect significantly the surface properties of materials. Platinum particles were then deposited on the different carbon powders using a ‘‘water in oil’’ microemulsion method. TEM images were recorded on the three samples

(Fig. 6a–c). The presence of grafted layers seems not to affect the dispersion of platinum particles on the substrate, despite the very different hydrophobic/hydrophilic properties of the carbon surface. Moreover, all samples display a mean particle size of ca. 6 nm, independently of the substrate. Thermogravimetric analyses coupled with mass spectroscopy measurements were carried out under air flow to verify the metal loading and the stability of the catalytic powders (Fig. 7a and b). Metal loadings were found to be close to the nominal value (40 wt%) expected for all samples, with ca. 36 wt% for the Pt-XC72 (not shown) and 38 wt% for both Pt-

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a

90 Weight (%)

80 70 60 50

4 8 ( 2 0 1 0 ) 2 7 5 5 –2 7 6 4

MS signal (a.u.)

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Table 5 – Reproducibility in the active surface area measurements of platinum particles dispersed on modified and unmodified carbon supports (determined under experimental conditions of Fig. 8). The nominal platinum loading corresponds to Pt(40 wt%)/C support. Active surface area (m2 g1)

Measurements

40 30 0

200

400 600 Temperature (°C)

800

b

100

MS signal (a.u.)

Weight (%)

90 80 70 60 50 40 30 0

200

400 600 Temperature (°C)

800

Fig. 7 – TGA curves (dashed line) coupled with MS measurements (plain line) of Pt supported on modified carbon powders recorded from 25 to 900 C (with a 10 C min1 temperature slope) under air atmosphere (50 mL min1 flow rate). (a) Pt-TFMP-XC72 sample and (b) PtPSA-XC72 sample.

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-50

-100

-150

0.0

0.2

0.4 0.6 0.8 E (V vs RHE)

1.0

1.2

Pt-XC72

Pt-TFMP-XC72

Pt-PSA-XC72

1 2 3

29 32 28

27 30 30

31 30 27

Average

30

29

29

tures higher than 120 C (not shown); it is then likely that in presence of platinum, the beginning of the degradation of the TFMP grafted layer occurs at the same temperature (ca. 120 C), forming compounds which have not been detected. However, in both cases, the presence of platinum seems not to activate the decomposition of the grafted layers and then does not lead to decrease their stability; the catalytic powders display degradation temperatures higher than 100 C, which validates their use in electrodes of PEMFC. At last, electrochemical measurements were carried out to compare the active surface area of platinum particles deposited on each carbon substrate. Because Nafion is generally used as electrolytic membrane and is also added in the active layer of PEMFC electrodes [39,40], experiments were first carried out with Nafion added in the active layer. Typical voltammograms given in Fig. 8 present the general shape of that recorded with carbon supported platinum nanoparticles [41]. Results in Table 5 indicates that the grafted groups have no influence on the active surface area of the platinum particles as determined from the hydrogen desorption region of the voltammograms [42,43]. However, an oxidation peak is detected at ca. 0.65 V vs. RHE for all catalysts. The intensity of this peak is higher when grafted layers are formed on the carbon surface (notably in the case of the PSA layer). Cyclic voltammograms recorded with Nafion-free electrodes are presented in Fig. 9. Again, no significant difference in the charge involved under desorption peaks of hydrogen is re-

Fig. 8 – Cyclic voltammograms recorded in 0.1 M HClO4 on a Pt-XC72 sample (plain line), a Pt-PSA-XC72 sample (dashed line) and on a Pt-TFMP-XC72 sample (dotted line) prepared from a Nafion containing ink (T = 20 C, v = 20 mV s1).

100 50 I (µA)

TFMP-XC72 and Pt-PSA-XC72 samples. For the TG-MS measurements, m/z 69 characteristic of CF3 fragments and m/z 64 characteristic of SO2 fragments, were considered to follow the behavior of the Pt-TFMP-XC72 sample and of the Pt-PSAXC72 sample, respectively. In the case of the Pt-TFMP-XC72 sample, the formation of CF3 fragments is detected for temperatures above 150 C (which is the same temperature as for the degradation of the TFMP-XC72 powder alone), whereas in the case of the Pt-PSA-XC72 sample, SO2 fragments are detected from only 400 C. However, TG-MS analysis of the platinum-free PSA-XC72 sample (m/z 64) has evidenced that a little degradation of the grafted layer occurred for tempera-

150

0 -50 -100 -150 -200 0.0

0.2

0.4 0.6 0.8 E (V vs RHE)

1.0

1.2

Fig. 9 – Cyclic voltammograms recorded in 0.1 M HClO4 on a Pt-XC72 sample (plain line), a Pt-PSA-XC72 sample (dotted line) and on a Pt-TFMP-XC72 sample (dashed line) prepared from a Nafion-free ink (T = 20 C, v = 20 mV s1).

CARBON

4 8 ( 20 1 0 ) 2 7 5 5–27 6 4

corded, indicating that the active surface area of platinum is not affected by the grafted layers, notably in the case of the hydrophobic active layer. In both cases, with and without Nafion, no significant difference in the double capacitive layer appears between 0.4 and 0.5 V vs. RHE. More interesting is the disappearance of the oxidation peak at 0.65 V vs. RHE in the absence of Nafion in the active layer. This observation indicates that the peak at 0.65 V vs. RHE is not due to a degradation process of the grafted layers under potential control, but rather to a complex mechanism involving the ionomer and the modified carbon supports or to an impurity revealed by the presence of Nafion in the electrode.

4.

Conclusion

Works presented in this paper indicate clearly that it is possible to modify significantly the surface characteristics of glassy carbon and of carbon powders. The grafting of adapted functionalized groups allows increasing or diminishing the water wetting of the substrate, according to the hydrophobic/hydrophilic nature of the groups. It is very important to note that very low surface concentrations (in the range of 1010 mol cm2) lead to very important macroscopic behaviors. This opens the possibility to realize multifunctional modification of the carbon substrate (for example, increase of the hydrophobic character with TFMP groups and of the stability of the platinum particles with thiol groups [28]). It is also important to note that some modifications can lead to the increase of the ionic exchange capacity of the substrate, a property which can be used to increase the quality and the quantity of the triple phase boundaries in the active layers of fuel cells in order to improve the platinum utilization efficiency. Such modified carbon materials may of course be used in the active layers of fuel cell electrodes; however it may also have a lot of other fields of application such as printable ink, reinforcing agent, surface coating, sealing compounds and formulation of plastics [8].

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