Electrochemical Behavior of Platinum Nanoparticles on a Carbon

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Electrochemical Behavior of Platinum Nanoparticles on a Carbon Xerogel Support Modified with a [(Trifluoromethyl)-benzenesulfonyl]Imide Electrolyte Bing Liu, Hua Mei, Darryl D Desmarteau, and Stephen Creager J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp505417e • Publication Date (Web): 10 Oct 2014 Downloaded from http://pubs.acs.org on October 16, 2014

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The Journal of Physical Chemistry

Electrochemical Behavior of Platinum Nanoparticles on a Carbon Xerogel Support Modified with a [(Trifluoromethyl)-benzenesulfonyl]imide Electrolyte Bing Liu#, Hua Mei##, Darryl DesMarteau and Stephen E. Creager* Hunter Laboratory, Department of Chemistry, Clemson University, Clemson, SC 29634 USA # Present address; Jiangnan Graphene Research Institute, 6 Xiangyun Road, Wujin Economic Development Zone, Changzhou, Jiangsu, CHINA ## Present address; Department of Chemistry, East Tennessee State University, Johnson City TN 37614 USA

ABSTRACT

A monoprotic [(trifluoromethyl)benzenesulfonyl]imide (SI) superacid electrolyte was used to covalently modify a mesoporous carbon xerogel (CX) support via reaction of the corresponding trifluoromethyl aryl sulfonimide diazonium zwitterion with the carbon surface. Electrolyte attachment was demonstrated by elemental analysis, acid-base titration and thermogravimetric analysis. The ion-exchange capacity of the fluoroalkyl-aryl-sulfonimide-grafted carbon xerogel (SI-CX) was approximately 0.18 meq g-1 as indicated by acid-base titration. Platinum nanoparticles were deposited onto the SI-grafted carbon xerogel samples by the impregnation and reduction method and these materials were employed to fabricate polyelectrolyte membrane

ACS Paragon Plus Environment

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fuel-cell (PEMFC) electrodes by the decal transfer method. The SI-grafted carbon-xerogelsupported platinum (Pt/SI-CX) was characterized by X-ray diffraction and transmission electron microscopy to determine platinum nanoparticle size and distribution and the findings are compared with carbon-xerogel-supported platinum catalyst without the grafted SI electrolyte (Pt/CX). Platinum nanoparticle sizes are consistently larger on Pt/SI-CX than on Pt/CX. The electrochemically active surface area (ESA) of platinum catalyst on the Pt/SI-CX and Pt/CX samples was measured with ex-situ cyclic voltammetry (CV) using both hydrogen adsorption/desorption and carbon monoxide stripping methods, and by in-situ CV within membrane electrode assemblies (MEAs). The ESA values for Pt/SI-CX are consistently lower than for Pt/CX. Some possible reasons for the behavior of samples with and without grafted SI layers, and implications for the possible use of SI-grafted carbon layers in PEMFC devices, are discussed.

KEYWORDS: Carbon xerogel; Aryl diazonium salt; Electrolyte grafting; Carbon-supported platinum catalyst; Proton exchange membrane fuel cell


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Introduction Polyelectrolyte membrane (PEM) fuel-cell electrode optimization may be thought of as a process that maximizes the three-phase contact regions where reactant gas, electrons and protons are in intimate contact with the supported catalyst particles. Optimization is achieved by adjusting the catalyst, catalyst support, and electrolyte structures to maximize transport rates for reactants and products to and from the catalyst particles, thereby leading to a high overall rate for the electrode reaction. Much research has been and continues to be directed at developing new materials that maximize electrode activity and thereby improve fuel-cell performance. One approach to optimizing electrode properties is to adjust the structure of the catalyst support. Most early work on fuel-cell electrodes utilized platinum supported on carbon black, however in recent years there has been much research on other nanostructured carbon supports including carbon nanofibers and nanotubes, xerogels and aerogels, and other templated mesoporous carbon materials. 1-12 Another critical part of a PEM fuel-cell electrode is the polymer electrolyte that provides a pathway for proton transport to and from the supported catalyst particles. A commonly used method for making PEMFC electrodes involves simple mixing of a carbon-supported platinum electrocatalyst with a polymeric protonic conductor such as Nafion® to form an ink from which thin-film electrodes are formed by solvent casting 13-14. The binding between electrode and electrolyte in such electrodes is relatively weak and catalyst utilization can be low15. Improvements are needed to ensure a more robust and long-lived contact of electrolyte with catalyst to achieve and retain a high catalyst utilization. One way to achieve intimate integration of electrolyte with electrode is to attach the electrolyte to the electrode via a robust covalent surface bond. Several groups have prepared carbon supports or carbon-supported catalysts onto which are attached various molecular organic acid


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groups. For example, Qi and co-workers reported on the use of 2-aminoethanesulfonic acid 16, 2aminoethylphosphonic acid 17, a sulfonated silane 18 and a sulfate salt 19 for modifying carbon or catalyst-supported carbon. Modest improvements in fuel-cell performance with a diminished addition of fluoropolymer electrolyte (e.g. Nafion) in the electrode were demonstrated, but further improvements are desired. Other teams have used sulfonated carbon black and carbon nanotubes in fuel-cell electrodes, 20-22 and as esterification catalysts.23-27 We previously reported on electrochemically grafting of a [(trifluoromethyl)benzenesulfonyl]imide (SI) superacid electrolyte onto glassy carbon electrodes via the aryl diazonium reduction technique 28. A robust layer of sulfonimide acid was formed as indicated by XPS and electrochemical probes. Extension of this work to high-surface-area carbon supports would be desirable, however electrochemical grafting of electrolyte onto dispersed powder supports is not practical, and even for a monolithic high-surface-area carbon support such as a carbon aerogel or xerogel, the scale-up of electrochemical grafting is problematic. Herein we present our continued work on covalent attachment of monoprotic [(trifluoromethyl)benzenesulfonyl]imide (SI) superacid electrolytes onto high-surface-area carbon via chemical grafting of a parent diazonium zwitterion without the help of an electrochemical reduction. Figure 1 schematically illustrates our approach which will utilize a carbon xerogel (CX) as catalyst support and as a substrate onto which electrolyte will be grafted. There has been much recent interest in chemical grafting of non-ionic organic functional groups (such as nitrophenyl) onto carbon black 29, ordered mesoporous carbon 27, 30, graphite powder 31, carbon nanotube 32, and diamond 33, even metallic substrates 34 via diazonium chemistry by either spontaneous reduction 29, 32-34 or using a reducing agent to promote diazonium reduction29, 31, 35

. The mechanism of covalent surface grafting via diazonium chemistry has also been


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investigated29. We show that SI acid groups may be attached to carbon xerogels and that the resulting materials can serve as supports for platinum nanoparticles. Platinum particles grown on the surface of SI-modified carbon are shown to be larger and to have lower electrochemically active surface area than particles grown on unmodified carbon xerogel supports. This fact has consequences for the possibly utility of Pt on SI-modified carbon in PEM fuel cell applications.

SO2NSO2CF3 SO2NSO2CF 3

SO2NSO2CF3

- N2 N2

Figure 1. Covalent attachment of a [(trifluoromethyl)-benzenesulfonyl]imide superacid electrolyte onto a carbon surface


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Experimental Materials The [(trifluoromethyl)benzenesulfonyl]imide diazonium zwitterion (N2+C6H4SO2N-SO2CF3) was prepared as described previously28, 36. Resorcinol (98%, Aldrich), formaldehyde (37 wt%, ACS Reagent, Aldrich), acetonitrile (extra dry, Acros Organics), and DMF (AR, Mallinckrodt) were used as received from the suppliers. De-ionized (DI) water was purified using a Milli-Q system to a resistivity no less than 18.2 MΩ cm prior to use. Synthesis of carbon xerogel A previously described resorcinol(R)-formaldehyde (F) sol-gel method using sodium carbonate as catalyst (C) with ambient pressure drying 37-38 was modified to synthesize the carbon xerogel samples used in the present work. In brief, resorcinol, formaldehyde, water, and catalyst sodium carbonate were mixed then gelled/cured to produce a hydrogel which was dried directly in air to produce a RF gel which was subsequently carbonized in N2 atmosphere to obtain a carbon xerogel. Further details on the CX synthesis are given in reference 39. Chemical grafting of aryl fluorosulfonimide onto carbon xerogel The CX sample prepared as described above was ground to a fine powder by mortar and pestle prior to use in grafting experiments. The CX powder sample (about 0.5 g) was suspended in 1015 mL of a N2+C6H4SO2N-SO2CF3 diazonium compound solution in acetonitrile (approx. 15-20 mg/mL) and the mixture was kept stirring overnight at room temperature. Following this treatment the grafted CX powder was collected by filtration on a 0.2 µm Nylon filter, washed


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with acetonitrile and DI water several times, then Soxhlet extracted with acetonitrile/THF overnight and dried in a vacuum oven overnight under dynamic vacuum at 120 °C. Deposition of Pt catalyst onto sulfonimide-grafted carbon xerogel SI-grafted CX powder samples (about 0.1 g) were sonicated in 30 mL DI water for 15 min. Then, a diluted H2PtCl6 (Acros Organics, 40% Pt) solution in water was added to the SI-grafted CX powder suspension and the resulting suspension was sonicated for another 30 min. The mass of platinum in the H2PtCl6 salt added to the CX suspension was sufficient to correspond to 20 wt% of platinum metal in the final material, should all the Pt be deposited onto the carbon support. After sonication, the pH of the mixture was adjusted to 11 using a 4 mol L-1 NaOH solution and an excess amount (10 x) of formaldehyde was diluted in DI water (2 mL) and added drop by drop into the grafted carbon suspension under stirring. The mixture was kept stirring for another 15 min at room temperature, then the reaction temperature was raised to 90 ºC and kept at 90 ºC for 2 hr under stirring. Then the reaction mixture was cooled down to room temperature and diluted HCl (2 mol L-1) was added to further promote Pt catalyst precipitation onto the carbon supports. The Pt-deposited SI-grafted CX powder (Pt/SI-CX) was then collected by filtration, thoroughly washed with DI water, and dried at 100 °C under vacuum. As a comparison, Pt catalyst was also deposited onto a separately prepared CX sample without any grafted electrolyte (SBET, 462 m2g-1, peak pore diameter around 14 nm, detailed characterization is given in reference 39) following the same procedures as described above (denoted as Pt/CX). For all Pt/CX and Pt/SI-CX samples subjected to further analysis, the final Pt content was approximately 20% weight as measured by thermogravimetric analysis (TGA) under O2 atmosphere using a Mettler Toledo TGA/SDA 851e analyzer at 850 °C with a heating rate of 15 ºC min-1.


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Materials characterization CX samples were characterized by the N2 sorption method at 77K using a Micrometrics model ASAP 2010 apparatus. Samples were degassed at 200 ºC for one day before measurement. The specific surface area (SBET) of the CX samples was extracted from the Brunauer-Emmett-Teller (BET) model and the mesopore and micropore properties (mesoporous volume Vmeso, mesoporous surface area Smeso, microporous volume Vmicro, microporous surface area Smicro) were extracted from the t-plot method and the Barrett-Joyner-Halenda (BJH) model to evaluate the pore size and distribution. The total pore volume (Vtotal) was recorded at P/P0 near to 1. TGA analysis for sulfonimide-grafted CX samples was performed using a Mettler Toledo TGA 851 thermal analyzer under N2 atmosphere. The acid content (also called herein the ion exchange capacity, IEC) of the sulfonimide-grafted CX samples was measured using an acid-base back-titration method. Before titration, the grafted samples were converted to acid form using diluted HCl and dried under vacuum and heat. For SI-grafted CX samples, a known amount of sample (about 0.2 g) was placed in 5.00 mL of a 1.115x10-2 mol L-1 standardized NaOH solution, and the resulting solution was sealed and kept stirring overnight in a glass bottle. The suspension was then centrifuged, and after centrifugation, 3.00 mL of the top clear solution was taken out and titrated with a 7.956x10-3 mol L-1 standard HCl solution. A pH meter (Acumet AB 15, Fisher) was used to record the pH change during the titration to identify the end point. The sulfonimide-grafted CX sample was also characterized by EDX (Energy dispersive X-ray spectroscopy) analysis via an EDX attachment on a scanning electronic microscope (SEM). The sample was mounted inside the SEM on a conductive adhesive tape for EDX analysis. The Pt/SI-CX samples were characterized in comparison with uncoated CX-supported Pt (Pt/CX) by XRD with a Scintag XDS2000 powder X-ray


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diffractometer using Cu Kα radiation of wavelength 0.1540 nm, and by TEM with a Hitachi model H7600T transmission electron microscope. The electrochemically active surface area (ESA) for platinum in the Pt/SI-CX and Pt/CX samples was measured with ex-situ cyclic voltammetry (CV) and CO stripping voltammetry using a CH Instruments model 660A electrochemical workstation with picoamp booster and Faraday cage with a standard threeelectrode cell. A locally-constructed Hg/Hg2SO4 (0.1 mol L-1 H2SO4) electrode and a Pt wire served as reference and counter electrodes respectively. The working electrode was a thin-filmcoated glassy carbon (GC) plate electrode made by attaching a graphite rod with graphite adhesive to the back of a square GC plate (5 mm each side, geometric surface area 0.025 cm2). The thin film on the GC was made from an ink mixture of the Pt/SI-CX or Pt/CX catalyst, solubilized Nafion and isopropanol. Membrane Electrode Assembly (MEA) Fabrication and Testing Membrane electrode assemblies (MEAs) were fabricated from the ink of a Pt/SI-CX or Pt/CX catalyst by the thin-film decal transfer method using Nafion 117 membranes. A detailed description of the MEA fabrication method is provided in reference 39. The active electrode area was 5 cm2, and the Pt loading on both anode and cathode was about 0.25 mg Pt/cm2. The singlecell in-situ CV measurements on MEAs were performed on a model 850C test station from Scribner Associates Company with the fuel-cell test fixture 40 operating in a two-electrode configuration in which the anode serves as both pseudo-reference and counter electrode, and the cathode serves as the working electrode.


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Results and Discussion Carbon support characterization The N2 adsorption/desorption isotherm and pore size distribution of the CX sample used in this work are presented in Figure 2. The isotherm is a type IV isotherm with H2 hysteresis loop which indicates the presence of mesopores 41. From the inset in Figure 2, the carbon xerogel has a pore size distribution from 3 to 20 nm with peak size of 8 nm in the mesopore range. Other textural properties for CX samples from the N2 gas adsorption analysis are listed in Table 1. 400

CX

0.6

3

Pore volume (cm /g)

BJH dV/dlog(D) pore volume

300

3

N2 adsorbed ( cm /g)

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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0.3

0.0 1

10

100

Pore diameter (nm)

200

100 0.0

0.5

1.0

Relative pressure (P/P0)

Figure 2 N2 adsorption/desorption isotherm and pore size distribution (inset) of carbon xerogel

Table 1 Textural properties of carbon xerogel (CX) Carbon CX

SBET

Smicro

2 -1

2 -1

Smicro/

Vtotal 3 -1

Vmicro 3 -1

Vmeso 3 -1

Vmeso/

dBET

dBJHads

dBJHdes

mg

mg

SBET

cm g

cm g

cm g

Vtotal

nm

nm

nm

532

277

0.52

0.49

0.12

0.34

0.72

3.7

6.3

5.0

SBET: BET surface area, Smicro:micropore surface area by t-plot, Vtotal:total pore volume at near saturation pressure, Vmeso:cumulative volume of pores between 1.7 and 300 nm by BJH adsorption branch, dBET, dBJHads, dBJHdes:average pore width by 4V/A


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Covalent grafting of aryl fluorosulfonimide electrolytes onto carbon supports The CX samples were subjected to a series of steps that sought to immobilize [(trifluoromethyl)benzenesulfonyl]imide electrolytes onto the supports via covalent chemical bonding onto carbon. This approach is similar to the well-studied electrochemical approach for modifying carbon electrode surfaces with substituted aryl groups via electrochemical reduction of aryl diazonium salts (or zwitterions) at carbon. Electroreduction of the aryl diazonium group is followed by rapid C-N bond cleavage to produce a phenyl radical which can covalently bind to carbon. Several recent papers have described work which indicates that under certain conditions, similar chemistry can occur on high-surface-area carbon supports that are simply suspended in a solution of the relevant aryl diazonium salt, without electrochemical potential control of the carbon. The results described below are for carbon samples that were modified using this approach, details of which are given in the experimental section. Table 2 presents elemental analysis results obtained by SEM/EDX analysis on two different CX samples. For the uncoated CX samples S, F and N were not detected, while S, F and N were detected in the grafted CX sample. This finding strongly suggests that the sulfonimide grafting onto CX samples was successful. From the F element content in the SI-grafted sample (F element is specific to the grafted acid group), the sulfonimide acid group content on the sample is estimated to be 0.35 mmol g-1.


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Table 2 Elemental analysis (EDX) of sulfonimide-grafted CX samples Sample

C%

O%

N%

S%

F%

Uncoated CX

88.16

11.84

0

0

0

SI-Grafted CX

84.93

7.55

2.86

2.66

1.99

a.

Elemental analysis was also performed using XPS, for which data were acquired as described in ref 28. The elemental composition in the table was confirmed however there was much scatter in the data such that reliable quantification could not be performed.

Acid-base back-titration curves for the SI-grafted CX powder samples using a standard HCl solution to titrate the remaining base following addition of acidified SI/CX samples to a known volume of standard NaOH solution are shown Figure 3. The difference between the titration curves without any carbon added to the standard base and those with uncoated carbons added prior to titration reflects the intrinsic acid content of the carbon support prior to sulfonimide grafting, which in this case is 0.05 mmol/g. The acid content of the SI-grafted CX powder was determined by back titration to be 0.23 mmol/g, which means that the contribution from the grafted SI electrolyte was 0.18 mmol/g. This value for the amount of SI electrolyte bonded to the CX sample is approximately half of the value estimated from EDX analysis. One possible cause for this discrepancy is that the value obtained from EDX analysis may disproportionally reflect modification in the near-surface region of macroscopic CX particles. SI modification may occur unevenly in of the CX particles such that surface regions are more thoroughly covered than the deep interior regions. EDX would be most responsive to sample regions in the topmost few micrometers, whereas titrimetric analysis would reflect the entire modified sample surface


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including regions in the deep interior of large CX particles, which may not have been modified with as much SI electrolyte. By combining the value of 0.18 mmol g-1 listed above for SI-derived acid content with the specific surface area for the CX obtained from nitrogen physisorption analysis prior to sulfonimide grafting, we estimate the average equivalent [(trifluoromethyl)benzenesulfonyl]imide acid surface coverage on the CX carbon samples to be 3.4x10-11 mol cm-2. This value is approximately 20 times lower than the value of 7.4x10-10 mol cm-2 reported in our previous paper describing [(trifluoromethyl)benzenesulfonyl]imide electrolyte coating on flat glassy carbon (GC) electrodes that were modified using the electrochemical aryl diazonium reduction grafting technique. There are several possible reasons for the coverage difference between the two sample types. Two principal reasons are as follows: (1) The electrochemical grafting was performed by cyclic voltammetry over many cycles each of which forced sulfonimide diazonium salt reduction to occur very near to the GC substrate possibly leading to high surface coverage; and (2) the CX substrates used for chemical grafting have mesoporous and microporous structures which have some very small pore sizes, smaller than the molecular size of the fluorosulfonimide agent. In such a case, attachment of the fluorosulfonimide group inside the very small micropores might be sterically restricted. On the flat, smooth surface of the GC substrate, no such steric restrictions would be present. It is instructive to compare our present findings with those of others who have prepared acidgrafted carbon supports using other acid groups and grafting techniques. Qi and coworkers reported a surface coverage of 2.0x10-10 mol cm-2 of grafted sulfonated silane groups 18 (equivalent to 0.39 mmol g-1 Pt/C after conversion using data provided in the paper) on Ptsupported carbon black (XC-72). Elemental analysis was performed in this work via electron


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microprobe analysis. These same workers reported 0.23 mmol g-1 grafted ethanesulfonic acid 16 on Pt-supported carbon black (XC-72) using a TGA method to measure acid content. This estimate of acid content corresponds to a surface coverage of 1.2x10-10 mol cm-2 assuming a carbon surface area of 195 m2 g-1 18. Wang et.al. 27 reported a much higher surface loading of 1.93 mmol g-1 grafted sulfonic acid group per gram of an ordered mesoporous carbon CMK-5 (equivalent to surface coverage of 1.3x10-10 mol cm-2 sulfonic acid on CMK-5 after conversion from the data provided by the authors). Our sulfonimide acid group surface coverage on the SIgrafted CX samples are lower than the results reported by the Qi or Wang groups. For the grafting work in Qi’s group, the substrates for grafting were mostly carbon-supported Pt catalyst, for which the Pt catalyst on the carbon might promote acid group grafting leading to higher surface coverage. For the work reported by Wang, an additional reducing agent hypophosphorous acid was used for reduction of the diazonium salt, which might help achieve a higher surface coverage. 14

Without CX Uncoated CX SI-grafted CX

12

10

pH

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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8

6

4

2 0

2

4

6

8

Titrant HCl (mL)

Figure 3 Back titration curves of sulfonimide-grafted carbon CX (●), uncoated CX (●) and without CX (○)


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It is also instructive to compare the acid contents given above for various electrolyte-grafted carbon supports with that of a typical fuel cell electrode made by the thin-film fabrication method 14, 42. A commonly used electrode formulation includes 70% Pt/carbon and 30% ionomer, typically 1100 EW Nafion, usually dispersed in a solvent to form an ink prior to electrode preparation. The sulfonic acid content (from the ionomer) in an electrode prepared in this way is approximately 0.45 mmol g-1 carbon. The acid content in our SI-grafted CX samples is lower than this so it does not provide the amount of acid needed for fuel cell electrodes made by thinfilm method. Therefore, if these grafted CX samples were to be used to fabricate electrodes, it is likely that additional Nafion ionomer would need to be added to the electrode formulation to make electrodes useful for fuel cell applications, as in fact has been reported in the literature for some other acid-grafted carbon supports 16, 18-19. Yet another reason for the need of additional ionomer in the electrodes is that is can serve as a binder necessary for holding together electrodes made by the thin-film method. The sulfonimide-grafted CX powder samples were also subjected to a thermogravimetric analysis (TGA). Representative TGA curves are shown in Fig. 4. The mass loss from sulfonimide-grafted CX at temperatures from 250 to 500 ºC is about 3.8%, and is thought to reflect cleavage of the carbon-carbon bond binding the acid group onto carbon. In contrast, for an uncoated CX sample the mass loss over the same temperature range is almost negligible. From this mass loss a sulfonimide acid content of 0.14 mmol g-1 carbon is estimated by assuming that the mass loss from 250 to 500 ºC is attributable solely to the grafted functional group having a molar mass of 288.25 g mol-1. This finding indirectly confirms the successful grafting of sulfonimide electrolyte onto CX samples and is in fairly good agreement with the value obtained from titrimetric analysis.


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102 100 98

Weight percentage (%)

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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96

Uncoated CX Sulfonimide-grafted CX

94 92 90 88 86 84 82 80 0

100

200

300

400

500

600

700

800

900

1000

Temperature (°C)

Figure 4 TGA curves of uncoated CX and sulfonimide-grafted CX samples

Pt catalyst deposition and characterization The SI-grafted carbon samples described herein are of interest in part for their possible utility as catalyst supports in PEM fuel cell applications. Therefore, it is instructive to prepare and characterize Pt catalysts deposited onto SI-grafted CX samples (Pt/SI-CX). Platinum deposition was accomplished from hexachloroplatinic acid solution using formaldehyde as reducing agent, and platinum content for both Pt/SI-CX and Pt/CX samples was determined by TGA to be approximately 20 weight percent. The resulting materials were characterized with XRD TEM, and ex-situ CV. We recently reported a similar study on Pt deposition onto CX supports without grafted electrolytes39 and that earlier work provides background to the present work on SIgrafted CX supports. The XRD diffractograms for the samples of Pt/SI-CX and Pt/CX are shown in Figure 5. The diffractogram shows features expected for Pt as labeled on the graph. From the line broadening,


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Pt nanoparticle crystallite sizes may be quantified using the Scherer equation (Equation 1) 43 using the Pt (220) line for calculation. d (nm) =

0.9λ B cos(θ )

(1)

In this equation d is the Pt crystallite size (diameter), λ is the X-ray wavelength (0.1540 nm), B is the full width at half height for the diffraction peak in radians and θ is half of the diffraction angle. Platinum crystallite size values obtained in this way for Pt/SI-CX and Pt/CX are given in Table 3. Pt crystallite sizes are slightly higher for the Pt/SI-CX sample (5.6 nm) than for the Pt/CX (4.5 nm). 1000

110

Pt/SI-CX Pt/CX

800

600

CPS

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


200 400

220

311

200

222

0 20

40

60

80

2θ (°)

Figure 5 XRD diffractograms of Pt/SI-CX and Pt/CX


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Table 3 Pt nanoparticle size and specific surface area by different methods (XRD, TEM, ex-situ CV (H), ex-situ CV (CO) and in-situ CV (H) Sample

dXRD

Pt/SI-CX

nm 5.6

Pt/CX

4.5

dTEM,

SH

SCO

dCO

dTEM,surf

nm 6.6

m2g-1 23

m2g-1 19

nm 14.8

m2g-1 6.3

mg 15

4.9

67

63

4.4

4.5

55

vol

Sinsitu 2 -1

dXRD, Pt crystallite size by XRD; dTEM,vol, volume-average particle diameter by TEM; SH, specific electrochemical surface area from ex-situ CV (H desorption charge); SCO, specific electrochemical surface area from ex-situ CO stripping CV; dCO, equivalent electrochemical Pt particle diameter; dTEM,surf, surface-average particle diameter by TEM; Sin-situ, specific surface area from in-situ CV in MEA (H-stripping)

Transmission electron microscopy (TEM) was also used to estimate Pt particle size for the Pt/SI-CX and Pt/CX samples. TEM micrographs and histogram graphs of Pt particle size for the Pt/SI-CX and Pt/CX samples are shown in Figures 6 and 7 respectively. Both TEM micrographs clearly show well-distributed Pt nanoparticles supported on carbon, and the particles appear to be generally larger on Pt/SI-CX than on Pt/CX. Histograms of the number of particles in a particular size range were obtained by counting more than 200 particles from each TEM image using ImageJ software 44. The mean Pt particle size may be estimated from the histograms as the volume-averaged particle diameter as follows; dvolume-average = Σ(nidi4) / Σ(nidi3). Volumeaveraged Pt particle diameters obtained in this way are listed in Table 3, and are in generally good agreement with values obtained by XRD.


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Figure 6 TEM micrographs of Pt/SI-CX (left) and Pt/CX (right)

Figure 7 Histograms of Pt particle size of Pt/CX (left) and Pt/SI-CX (right). Volume-averaged and surface-averaged particle diameters calculated from these distributions are listed in Table 3.

Figure 8 (top) presents ex-situ CVs for Pt/SI-CX and Pt/CX samples for the H adsorption / desorption region from which the electrochemical surface area (ESA) may be obtained. The CV shapes are as expected for Pt on carbon insofar as they exhibit well-defined regions for H adsorption/desorption and Pt oxidation/oxide reduction atop a relatively large capacitive background current for carbon. Estimates of the hydrogen adsorption or desorption charge


19


density were made as described in references 45-46, and ESA values calculated using these values combined with the known Pt loading on the electrodes (as described in reference 39) are given in Table 3. Figure 8 (bottom) shows CO stripping voltammograms for Pt/SI-CX and Pt/CX samples, and ESA values obtained as described in reference 39 are reported in Table 3. ESA values from CO stripping are in good agreement with those from hydrogen adsorption or desorption for both samples. We note that the peak potential for CO stripping is significantly more negative for the Pt/SI-CX sample than for the Pt/CX sample. This fact is indicative of larger Pt particle sizes for the Pt/SI sample.47 We note also that the specific capacitance in the double-layer region (between approximately 0 and -0.4 V vs. Hg/HgSO4 reference electrode) is higher for the Pt/CX sample than for the Pt/SI-CX sample. The reason for this finding is unclear. It may have to do with the SI layer blocking access of electrolyte to some of the carbon, possibly by blocking or filling pores, or it may reflect fundamental changes in double-layer structure at the carbon / SI interface.

Current (A/mgPt)

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3.0x10

-2

2.0x10

-2

1.0x10

-2

Pt/SI-CX Pt/CX

0.0

-1.0x10

-2

-2.0x10

-2

-3.0x10

-2

-4.0x10

-2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Potential (V)


20

8.0x10

-2

6.0x10

-2

4.0x10

-2

2.0x10

-2

Pt/SI-CX Pt/CX

1st cycle

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


Current (A/mgPt)

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2nd cycle

0.0

-2.0x10

-2

-4.0x10

-2

-0.8

2nd cy

-0.6

-0.4

-0.2

0.0

cle

ycle 1st c

0.2

0.4

0.6

Potential (V)

Figure 8 Ex-situ CVs (top) and CO stripping CVs (bottom) of Pt/SI-CX and Pt/CX

The equivalent electrochemical diameter of Pt particles from CO stripping may be estimated using equation 2 below;

dCO =

6000 ρ SCO

(2)

In this equation dCO is the equivalent electrochemical diameter of Pt particles from CO stripping in units of nm, ρ is the Pt density (21.4 g cm-3), and SCO is the specific platinum surface area obtained from CO stripping, in units of m2g-1. Values for the equivalent electrochemical diameter of Pt particles obtained in this way for the Pt/SI-CX and Pt/CX samples are listed in Table 3. These values may be compared with surface-averaged diameters for the platinum particles obtained from histograms of particle sizes from TEM as follows; dsurface-average = Σ(nidi3) / Σ(nidi2). Surface-averaged Pt particle diameters obtained in this way are listed in Table 3. The two values are in good agreement for the Pt/CX sample, which suggests that the TEM micrograph provides a representative rendering of Pt particle sizes and the Pt particles are in


21


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Page 22 of 28

good electrical contact with the carbon support for this sample. The agreement is not so good for the Pt/SI-CX sample. We believe that the most likely reason for the poor agreement is that some Pt particles on the Pt/SI-CX sample were formed so as to not be in good electrical contact with the electronically conductive carbon. This situation could also have been caused by the presence of negatively charged groups on the grafted CX surface. Another possibility is that a few especially large Pt particles might exist in the Pt/SI-CX sample that are not accounted for in the imaging experiments because they are out of the field of view. Further TEM imaging would be required to assess this possibility. In-situ CV may also be used to estimate the Pt ESA in electrodes in MEAs. Comparison of ESAs for such samples with those obtained by ex-situ CV provides information on catalyst utilization in the MEA. ESA values measured in-situ for both Pt/SI-CX and Pt/CX samples (cathodes only; see Figure 9 and Table 3) are very close to those measured ex-situ using both H adsorption/desorption and CO stripping. The good agreement between in-situ and ex-situ ESA measurements indicates that the catalyst utilization in MEAs fabricated in our lab is similar to that in the thin-film electrodes that were studied on glassy carbon supports in aqueous acid electrolyte. (By utilization, we mean, catalyst that is in contact with electrode and electrolyte in experiments with liquid electrolyte is also in contact with electrode and electrolyte in experiments with Nafion electrolyte.) The in-situ ESA of Pt/SI-CX is again lower than that of Pt/CX.


22

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Pt/SI-CX Pt/CX

0.005 2

Current density (A/cm )

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


0.000

-0.005

-0.010

0.2

0.4

0.6

0.8

Cell potential (V)

Figure 9 In-situ CVs of Pt/SI-CX and Pt/CX

The low ESA for samples of platinum on Pt/SI-CX renders those samples unsuitable for use in practical fuel-cell devices. Improvements would be needed that would enable smaller platinum particles to be obtained, with higher surface area, an improved contact with the carbon and with electrolyte. These improvements could be realized in a variety of ways, including changing the way that catalyst is deposited, and changing the way that the support is prepared prior to, and possibly subsequent to, catalyst deposition. One possibly approach that we are pursuing in separate work involves the use of electrolyte grafting via covalent attachment of electrolyte to zirconia particles embedded in the carbon.48-50 An advantage of this approach is that the electrolyte may be attached after platinum deposition, which could mitigate problems associated with having electrolyte present as platinum is deposited. These issues and many others have been much discussed in the literature and this is only one of many possible paths forward that could lead to improved fuel-cell devices.51-53

Conclusions


23


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A monoprotic [(trifluoromethyl)benzenesulfonyl]imide acid electrolyte was successfully grafted onto high-surface-area CX supports without aid of electrochemical induction. The amounts of sulfonimide electrolyte grafted onto CX support was determined by back acid-base titration, EDX and TGA. The sulfonimide acid content on the grafted CX was estimated to be about 0.18 mmol g-1 carbon from acid-base titration measurements. These findings are consistent with other analytical results from TGA and EDX. Pt catalyst particles were deposited onto the sulfonimide-grafted CX samples and the resulting materials were characterized by XRD and TEM for Pt particle size and dispersion in comparison with Pt catalyst deposited onto uncoated CX samples. Pt particle size was consistently larger on the Pt/SI-CX samples. The ESA of the Pt/SI-CX was measured via ex-situ and in-situ CV methods and compared with that of Pt/CX. The ESA for Pt/SI-CX was much smaller than that of Pt/CX, a finding which suggests that some Pt particles in the Pt/SI-CX sample may not be in electrical contact with the carbon support.

ACKNOWLEDGMENT The authors gratefully acknowledge the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-05ER15718 for financial support of the work. Also, financial support of earlier work that enabled the present work by the US National Science Foundation, from grant DMI-0303645, is gratefully acknowledged.


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REFERENCES 1. Zhang, S.; Shao, Y. Y.; Yin, G. P.; Lin, Y. H., Recent progress in nanostructured electrocatalysts for PEM fuel cells. J Mater Chem A 2013, 1, 4631-4641. 2. Xu, J. B.; Zhao, T. S., Mesoporous carbon with uniquely combined electrochemical and mass transport characteristics for polymer electrolyte membrane fuel cells. Rsc Adv 2013, 3, 16-24. 3. Muthuswamy, N.; de la Fuente, J. L. G.; Ochal, P.; Giri, R.; Raaen, S.; Sunde, S.; Ronning, M.; Chen, D., Towards a highly-efficient fuel-cell catalyst: optimization of Pt particle size, supports and surface-oxygen group concentration. Phys Chem Chem Phys 2013, 15, 38033813. 4. Ruvinskiy, P. S.; Bonnefont, A.; Savinova, E. R., 3D-ordered layers of vertically aligned carbon nanofilaments as a model approach to study electrocatalysis on nanomaterials. Electrochim Acta 2012, 84, 174-186. 5. Du, H.; Li, B.; Kang, F.; Fu, R.; Zeng, Y., Carbon aerogel supported Pt-Ru catalysts for using as the anode of direct methanol fuel cells. Carbon 2007, 45, 429-435. 6. Glora, M.; Wiener, M.; Petricevic, R.; Probstle, H.; Fricke, J., Integration of carbon aerogels in PEM fuel cells. J Non-Cryst Solids 2001, 285, 283-287. 7. Petricevic, R.; Glora, M.; Fricke, J., Planar fibre reinforced carbon aerogels for application in PEM fuel cells. Carbon 2001, 39, 857-867. 8. Smirnova, A.; Dong, X.; Hara, H.; Vasiliev, A.; Sammes, N., Novel carbon aerogelsupported catalysts for PEM fuel cell application. Int J Hydrogen Energ 2005, 30, 149-158. 9. Marie, J.; Berthon-Fabry, S.; Achard, P.; Chatenet, M.; Pradourat, A.; Chainet, E., Highly dispersed platinum on carbon aerogels as supported catalysts for PEM fuel cell-electrodes: comparison of two different synthesis paths. J Non-Cryst Solids 2004, 350, 88-96. 10. Marie, J.; Berthon-Fabry, S.; Chatenet, M.; Chainet, E.; Pirard, R.; Cornet, N.; Achard, P., Platinum supported on resorcinol-formaldehyde based carbon aerogels for PEMFC electrodes: Influence of the carbon support on electrocatalytic properties. Journal of Applied Electrochemistry 2007, 37, 147-153. 11. Marie, J.; Chenitz, R.; Chatenet, M.; Berthon-Fabry, S.; Cornet, N.; Achard, P., Highly porous PEM fuel cell cathodes based on low density carbon aerogels as Pt-support: Experimental study of the mass-transport losses. J Power Sources 2009, 190, 423-434. 12. Marie, J.; Berthon-Fabry, S.; Achard, P.; Chatenet, M.; Chainet, E.; Pirard, R.; Cornet, N., Synthesis of Highly Porous Catalytic Layers for Polymer Electrolyte Fuel Cell Based on Carbon Aerogels ECS Transactions 2006, 1, 509-519. 13. Costamagna, P.; Srinivasan, S., Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000: Part I. Fundamental scientific aspects. J. Power Sources 2001, 102, 242-252. 14. Wilson, M. S.; Gottesfeld, S., Thin-film catalyst layers for polymer electrolyte fuel cell electrodes. J. Appl. Electrochem. 1992, 22, 1-7. 15. Litster, S.; McLean, G., PEM fuel cell electrodes. J. Power Sources 2004, 130, 61-76. 16. Xu, Z.; Qi, Z.; Kaufman, A., Advanced Fuel Cell Catalysts. Electrochem. Solid-State Lett. 2003, 6, A171-A173. 17. Xu, Z. Q.; Qi, Z. G.; Kaufman, A., High performance carbon-supported catalysts for fuel cells via phosphonation. Chem Commun 2003, 878-879. 18. Easton, E. B.; Qi, Z.; Kaufman, A.; Pickup, P. G., Chemical Modification of Proton Exchange Membrane Fuel Cell Catalysts with a Sulfonated Silane. Electrochem. Solid-State Lett. 2001, 4, A59-A61.


25


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

Page 26 of 28

19. Xu, Z.; Qi, Z.; Kaufman, A., Superior Catalysts for Proton Exchange Membrane Fuel Cells. Electrochem. Solid-State Lett. 2005, 8, A313-A315. 20. Guo, L.; Chen, S.; Wei, Z., Enhanced utilization and durability of Pt nanoparticles supported on sulfonated carbon nanotubes. J Power Sources 2014, 255, 387-393. 21. Tripathi, B. P.; Schieda, M.; Shahi, V. K.; Nunes, S. P., Nanostructured membranes and electrodes with sulfonic acid functionalized carbon nanotubes. J Power Sources 2011, 196, 911-919. 22. Xu, F.; Wang, M.-x.; Sun, L.; Liu, Q.; Sun, H.-f.; Stach, E. A.; Xie, J., Enhanced Pt/C catalyst stability using p-benzensulfonic acid functionalized carbon blacks as catalyst supports. Electrochim Acta 2013, 94, 172-181. 23. Barroso-Bujans, F.; Fierro, J. L. G.; Rojas, S.; Sanchez-Cortes, S.; Arroyo, M.; LopezManchado, M. A., Degree of functionalization of carbon nanofibers with benzenesulfonic groups in an acid medium. Carbon 2007, 45, 1669-1678. 24. Geng, L.; Yu, G.; Wang, Y.; Zhu, Y., Ph-SO3H-modified mesoporous carbon as an efficient catalyst for the esterification of oleic acid. Applied Catalysis A-General 2012, 427, 137-144. 25. Liu, R.; Wang, X.; Zhao, X.; Feng, P., Sulfonated ordered mesoporous carbon for catalytic preparation of biodiesel. Carbon 2008, 46, 1664-1669. 26. Stellwagen, D. R.; van der Klis, F.; van Es, D. S.; de Jong, K. P.; Bitter, J. H., Functionalized Carbon Nanofibers as Solid-Acid Catalysts for Transesterification. Chemsuschem 2013, 6, 1668-1672. 27. Wang, X.; Liu, R.; Waje, M. M.; Chen, Z.; Yan, Y.; Bozhilov, K. N.; Feng, P., Sulfonated ordered mesoporous carbon as a stable and highly active protonic acid catalyst. Chem Mater 2007, 19, 2395-2397. 28. Creager, S. E.; Liu, B.; Mei, H.; DesMarteau, D., Electrochemical grafting of an aryl fluorosulfonimide electrolyte onto glassy carbon. Langmuir 2006, 22, 10747-10753. 29. Toupin, M.; Belanger, D., Spontaneous Functionalization of Carbon Black by Reaction with 4-Nitrophenyldiazonium Cations. Langmuir 2008, 24, 1910-1917. 30. Li, Z.; Dai, S., Surface Functionalization and Pore Size Manipulation for Carbons of Ordered Structure. Chem. Mater. 2005, 17, 1717-1721. 31. Pandurangappa, M.; Ramakrishnappa, T.; Compton, R. G., Nitroazobenzene Functionalized Carbon Powder: Spectroscopic Evidence for Molecular Cleavage. Int J Electrochem Sc 2008, 3, 1218-1235. 32. Bahr, J. L.; Tour, J. M., Highly Functionalized Carbon Nanotubes Using in Situ Generated Diazonium Compounds. Chem. Mater. 2001, 13, 3823-3824. 33. Mangeney, C.; Qin, Z.; Dahoumane, S. A.; Adenier, A.; Herbst, F.; Boudou, J.-P.; Pinson, J.; Chehimi, M. M., Electroless ultrasonic functionalization of diamond nanoparticles using aryl diazonium salts. Diamond Relat. Mater. 2008, 17, 1881-1887. 34. Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C., Grafting of Nitrophenyl Groups on Carbon and Metallic Surfaces without Electrochemical Induction. Chem. Mater. 2005, 17, 491-501. 35. Pandurangappa, M.; Lawrence, N. S.; Compton, R. G., Homogeneous chemical derivatisation of carbon particles: a novel method for funtionalising carbon surfaces. Analyst 2002, 127, 1568-1571. 36. Mei, H.; VanDerveer, D.; DesMarteau, D. D., Synthesis of diazonium (perfluoroalkyl) benzenesulfonylimide zwitterions. Journal of Fluorine Chemistry 2013, 145, 35-40.


26

Page 27 of 28

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


37. Petricevic, R.; Reichenauer, G.; Bock, V.; Emmerling, A.; Fricke, J., Structure of carbon aerogels near the gelation limit of the resorcinol-formaldehyde precursor. J. Non-Cryst. Solids 1998, 225, 41-45. 38. Saliger, R.; Bock, V.; Petricevic, R.; Tillotson, T.; Geis, S.; Fricke, J., Carbon aerogels from dilute catalysis of resorcinol with formaldehyde. J. Non-Cryst. Solids 1997, 221, 144-150. 39. Liu, B.; Creager, S., Carbon xerogels as Pt catalyst supports for polymer electrolyte membrane fuel-cell applications. J Power Sources 2010, 195, 1812-1820. 40. Cooper, K. R.; Ramani, V.; Fenton, J. M.; Kunz, H. R., Experimental methods and data analyses for polymer electrolyte fuel cells. 1.5 ed.; Scribner Associates, Inc.: 2007. 41. Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T., Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (recommendations 1984). . Pure Appl. Chem. 1985, 57, 603-619. 42. Wilson, M. S.; Valerio, J. A.; Gottesfeld, S., Low platinum loading electrodes for polymer electrolyte fuel cells fabricated using thermoplastic ionomers. Electrochim. Acta 1995, 40, 355-363. 43. Langford, J. I.; Wilson, A. J. C., Scherrer after sixty years: A survey and some new results in the determination of crystallite size. J. Appl. Crystallogr. 1978, 11, 102-113. 44. http://rsb.info.nih.gov/ij/. http://rsb.info.nih.gov/ij/ (accessed Accessed on August 4, 2009). 45. Pozio, A.; De Francesco, M.; Cemmi, A.; Cardellini, F.; Giorgi, L., Comparison of high surface Pt/C catalysts by cyclic voltammetry. J Power Sources 2002, 105, 13-19. 46. Vidakovic, T.; Christov, M.; Sundmacher, K., The use of CO stripping for in situ fuel cell catalyst characterization. Electrochim. Acta 2007, 52, 5606-5613. 47. Maillard, F.; Eikerling, M.; Cherstiouk, O. V.; Schreier, S.; Savinova, E.; Stimming, U., Size effects on reactivity of Pt nanoparticles in CO monolayer oxidation: The role of surface mobility. Faraday Discussions 2004, 125, 357-377. 48. Oh, J.-M.; Kumbhar, A. S.; Geiculescu, O.; Creager, S. E., Mesoporous Carbon/Zirconia Composites: A Potential Route to Chemically Functionalized Electrically-Conductive Mesoporous Materials. Langmuir 2012, 28, 3259-3270. 49. Park, J.; Oh, J.-M.; Creager, S. E.; Smith Jr, D. W., Grafting of chain-end-functionalized perfluorocyclobutyl (PFCB) aryl ether ionomers onto mesoporous carbon supports. Chem. Commun. 2012, 48, 8225-8227. 50. Oh, J.-M.; Park, J.; Kumbhar, A.; Jr., D. S.; Creager, S., Electrochemical Oxygen Reduction at Platinum/MesoporousCarbon/Zirconia/Ionomer Thin-Film Composite Electrodes. Electrochim Acta 2014, 138, 278-287. 51. Cao, M. N.; Wu, D. S.; Cao, R., Recent Advances in the Stabilization of Platinum Electrocatalysts for Fuel-Cell Reactions. Chemcatchem 2014, 6, 26-45. 52. Su, L.; Jia, W. Z.; Li, C. M.; Lei, Y., Mechanisms for Enhanced Performance of PlatinumBased Electrocatalysts in Proton Exchange Membrane Fuel Cells. Chemsuschem 2014, 7, 361-378. 53. Xu, Y.; Zhang, B., Recent advances in porous Pt-based nanostructures: synthesis and electrochemical applications. Chemical Society Reviews 2014, 43, 2439-2450.


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TOC Graphic


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Electrochemical Behavior of Platinum Nanoparticles on a Carbon

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