Electrocatalytic Oxygen Reduction Performance of Silver

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ELECTROCATALYTIC OXYGEN REDUCTION PERFORMANCE OF SILVER NANOPARTICLE DECORATED ELECTROCHEMICALLY EXFOLIATED GRAPHENE João Henrique Lopes, Siyu Ye, Jeff T Gostick, Jake Barralet, and Geraldine Merle Langmuir, Just Accepted Manuscript • Publication Date (Web): 03 Jun 2015 Downloaded from http://pubs.acs.org on June 6, 2015

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ELECTROCATALYTIC OXYGEN REDUCTION PERFORMANCE OF SILVER NANOPARTICLE DECORATED ELECTROCHEMICALLY EXFOLIATED GRAPHENE Joao Henrique Lopes, Siyu Ye, Jeff T Gostick, Jake E Barralet‡* and Geraldine Merle‡ J. H. Lopes Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) E-mail: ([email protected]) Dr. S. Ye Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, V5J 5J8 (Canada) E-mail: ([email protected]) Prof. J. T. Gostick Department of Chemical Engineering, McGill University, Montreal, H3A 0C5, (Canada) E-mail : ([email protected])

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Prof. J. E. Barralet Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) Department of Surgery, Faculty of Medicine, McGill University, Montreal, (Canada) E-mail: ([email protected]) Dr. G. Merle Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) E-mail: ([email protected])

Figure 1 – Schematic illustration of mechanism for electrochemical exfoliation of graphite employed in this paper.


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Figure 2- Bright-field TEM images of (a) bi-layer and (b, c) monolayer graphene flakes, showing typical size, shape, and morphology of the graphene. (d) Electron diffraction pattern of the graphene sheet in b, with the peaks labelled by Miller–Bravais indices. (e) TGA curves of raw graphite (black), graphene (red), and graphene oxide (blue) at a heating rate of 10 ºC/min in air. (f) Raman spectra (excitation wavelength 𝝀 = 𝟓𝟑𝟐 𝒏𝒎) for graphite (black) and graphene flakes (red).


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Figure 3 - (a) Cyclic voltammograms for bare, GRp, and GRn electrodes in 1.0 M KCl solution between 0 V and 0.5 V versus SCE. Scan rate, 100 mV s-1. (b) Nyquist diagrams for EIS measurements of bare, GRp, and GRn electrodes. Conditions: N2-saturated 1 M KCl and 50 mM PBS as background electrolytes with 10 mM 𝐊 𝟑 [𝐅𝐞(𝐂𝐍)𝟔 ]/𝐊 𝟒 [𝐅𝐞(𝐂𝐍)𝟔 ] (1:1 molar ratio). (c) Inset: Randles equivalent circuit used for data fitting.

Figure 4 – (a) Cyclic voltammograms of GC covered with graphene in the absence of silver nitrate solution 0.1 M KNO3 (dotted line) and 0.1 M KNO3 + 0.1 M AgNO3 solution at scan rates of 50 mV s−1 (-600 to 500 mV) and (b) zoom view of silver deposition.


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Figure 5 – Low-magnification TEM images (a-c), EDX composition analyses (d-f), and particle size distribution (g-i) of GRn/Ag 50 ms, GRn/Ag 100 ms, and GRn/Ag 200 ms. The insets show the corresponding HRTEM images. The parameters denoted µ and σ are, respectively, the mean and standard deviation of the variable’s natural logarithm in the lognormal model. The number of particles measured was approximately 500.


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Figure 6- Linear sweep voltammetry (LSV) obtained in 0.1 M KOH saturated with N2 or O2 for a bare and modified GC electrode. Scans start at 0.1V and scan rate: 50 mV s-1.


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Table 1- Electrochemically active surface area, capacitance, resistance values, voltammetric data, ΔEp and E1/2, and apparent electron-transfer rate constants, K0app, for the bare, GRp, and GRn electrodes

ESCA

Bare

GRp

0.36

0.56

GRn 0.75

CCV (µF.cm-2)

185 ± 40

692 ± 83

4329 ± 133

Rct (Ω.cm2)

566.1 ± 5.7

279.2 ± 3.7

70.4 ± 1.2

ΔEp [mV]

103

85

68

E1/2 [mV]

212

216

217

1.38

2.77

8.31

K0app (cm.s-1 -3

x10 )


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ELECTROCATALYTIC OXYGEN REDUCTION PERFORMANCE OF SILVER NANOPARTICLE DECORATED ELECTROCHEMICALLY EXFOLIATED GRAPHENE Joao Henrique Lopes, Siyu Ye, Jeff T Gostick, Jake E Barralet‡* and Geraldine Merle‡ J. H. Lopes Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) E-mail: ([email protected]) Dr. S. Ye Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, V5J 5J8 (Canada) E-mail: ([email protected]) Prof. J. T. Gostick Department of Chemical Engineering, McGill University, Montreal, H3A 0C5, (Canada) E-mail : ([email protected])


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Prof. J. E. Barralet Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) Department of Surgery, Faculty of Medicine, McGill University, Montreal, (Canada) E-mail: ([email protected]) Dr. G. Merle Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) E-mail: ([email protected]) KEYWORDS Electrochemical exfoliation, Graphene, Silver, ORR, Nanoparticles

ABSTRACT We have developed a potentiostatic double-pulse technique for silver nanoparticle (Ag NP) deposition on graphene (GRn) with superior electronic and ionic conductivity. This approach yielded a 2D electrocatalyst with a homogenous Ag NP spatial distribution having remarkable performance in the oxygen reduction reaction (ORR). GRn sheets were reproducibly prepared by the electrochemical exfoliation of graphite (GRp) at high yield and purity with a low degree of oxidation. Polystyrene sulfonate added during exfoliation enhanced the stability of the GRn solution by preventing the re-stacking of the graphene sheets and increased its ionic conductivity. The potentiostatic double-pulse technique is generally used to electro-deposit Pt nanoparticles and remains challenging for silver metal that exhibits nucleation and growth


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potentials relatively close to each other. We judiciously exploited this narrow margin of potential and for the first time we report Ag NP electro-deposited onto graphene with the subsequent ability to control both the density and the size of metallic nanoparticles. Considering the high activity along with the lower cost of Ag compared to Pt, these findings are highly relevant to the successful commercialization of fuel cells and other electrochemical energy devices.

Introduction Graphene exhibits a high corrosion resistance compared with graphite and has outstanding electronic, thermal, and mechanical properties,1 and it is anticipated that it can replace conventional carbon supports for energy storage and conversion applications.2 One such application is to improve the oxygen reduction reaction (ORR) in the fuel cells and other electrochemical energy devices. Considerable efforts have been made to synthesize graphene nanocomposites with high ORR catalytic activity and so improving both the graphene support quality and the catalyst itself are priority areas. In order to prepare graphene materials, methods including mechanical, CVD and liquid-phase exfoliation and chemically reduced graphene oxide (GO) have been attempted.3 However, these processes tend to involve many complex steps, making them unsuitable for scale-up, or require toxic reducing agents and in some cases actually result in low quality materials with poor electrical and thermal conductivities.1, 4 Electrochemical exfoliation while theoretically attractive suffers from low yield and the processability of exfoliated graphene remains challenging.5, 6 In recent years, some researchers have attempted to improve electrochemical methods in order to produce graphene with high quality by minimizing the oxidation of the aromatic carbon network through creation of hydroxyl, carboxyl and epoxy moieties.7, 8, 9, 10, 11, 12, 13 Here a novel and facile surfactant-assisted electrochemical procedure has


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been demonstrated to exfoliate graphene while controlling the oxidation degree of the graphene sheets. The biggest obstacle to developing energy based electrochemical apparatus is minimising the cost while retaining adequate catalytic performance. Pt and Pt alloys are the most active catalysts for ORR and their prohibitive cost prevents their widespread utilization. Silver-based catalysts have reasonable catalytic activity but are much less widely investigated, despite their nearly 2 orders of magnitude lower cost than platinum and resistance to poisoning14, 15. Previous studies have established the electrocatalytic behaviour of metallic nanoparticles to be size dependent, where smaller nanoparticles of Pt16, Pd17 and Ag18,

19, 20

displayed better oxygen reduction

reaction activity. Despite moderate success in synthesizing silver nanoparticles with different size ranges, certain limitations in terms of size, monodispersity, density and stability are encountered19,

21

. The borohydride-mediated reduction method22 requires a large excess of

reducing agent for the formation of monodispersed and uniform Ag nanoparticles15, 23. Recently, Ag nanoclusters were reported by using electrochemical reduction of AgNO3 on nitrogen doped graphene, and despite some promising results for ORR, the synthetic route required the use of expensive oligonucleotide or sulphonated aromatic templates.24,

25

Unlike gold, it is very

challenging to achieve silver nanoparticles below 10 nm from solution with high monodispersity and stability26, 27 and so peak catalytic performance is hard to attain for this metal15, 23. Among the different substrate deposition methods of metals, electrochemical approaches generally lead to a higher purity of the particles, higher control over the dimension and lower particle size distribution28 and creates strong interactions between the metal and substrate. The potentiostatic double-pulse technique (PDP) is the only method enabling the manipulation of the particle size and spatial distribution29, 30 through the control of the nucleation and the growth of


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the particles by varying the polarization and the duration of the applied pulse.29 This technique has been already applied to the preparation of silver nanoparticles however in these studies the minimum size achieved was only 100nm

31, 32

. In this paper, using a different solvent and

different potential and time, we report an electrochemical route to produce defect free graphene sheets decorated with ~3nm silver nanoparticles (GRn/Ag) in two steps. First graphite was electrochemically exfoliated in the presence of poly(sodium 4-styrenesulfonate) (PSS), as an intercalant, to reduce the strong van der Waals interactions between the graphene sheets and to prevent their tendency to aggregate in solution. As a first step, substantial amounts of graphene as individual sheets have been obtained and maintained in the reduced form, thereby improving efficiency. For the first time, we successfully achieved homogenous deposition of highly active Ag NP (2-3nm) on graphene sheets via a potentiostatic double-pulse technique. For the oxygen reduction reaction, the current density normalized to the electrode surface increased up to 5 mA·cm-2 from 4 mA·cm-2 as particle size decreased from 4-5nm to 2-3nm.

Experimental details Preparation of graphene and graphene oxide High purity graphite flakes (99.99%, Sigma-Aldrich) were used as received. The conductive carbon tape method was used to prepare the electrode with graphite flakes. A 0.15 M Na2SO4 (Fisher Scientific) and 1mM PSS (Mw=70,000, Sigma-Aldrich) aqueous solution was used to increase the exfoliation rate and the pH was maintained at 2 with sulfuric acid (98%, Fisher Scientific). The electrochemical exfoliation of graphite was performed in a three-electrode system using natural graphite flakes as the working electrode, platinum wire as the counter electrode, and a saturated calomel electrode (SCE) reference electrode. The electrochemical


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exfoliation was carried out in two steps following the method developed by Su.33 First step at +2.5 V for 10 minutes was followed by a second step at +6 V for 30 minutes under an N2-flux. The exfoliated graphitic material was collected under vacuum filtration and washed three times with water and isopropanol. Finally, the obtained powder was dispersed in isopropanol and centrifuged at low speed (1000 rpm) for 30 minutes to remove the larger particles and (4000 rpm) for 3 hours by using a centrifuge (Z300, Hermle Labortechnik GmbH, Wehingen, Germany). The precipitate was dried in a vacuum oven at 50 °C for 12 hours. Graphene oxide (GO) was produced by a method described by Marcano and co-workers34. The improved method for GO production was carried adding a 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) in a mixture of graphite flakes (3.0 g, 1 equivalent by weight) and KMnO4 (18.0 g, 6 equivalent by weight). Then, the reaction was heated to 50 °C and stirred for 12 h. The reaction was cooled to room temperature and poured onto ice (400 mL) with 30% H2O2 (3 mL). In the next step, the mixture was sifted through a metal sieve (300 µm) and then filtered through Buchner funnel with fritted disc. The filtrate was centrifuged (4000 rpm for 4 h), and the supernatant was decanted away. The remaining solid material was then washed in succession with 200 mL of water, 200 mL of 30% HCl, and 200 mL of ethanol (2x) and sieved. The obtained filtrate was centrifuged (4000 rpm for 4 h) and the supernatant decanted away. The material remaining after this extended, multiple-wash process was coagulated with 200 mL of ether, and the resulting suspension was filtered. The solid obtained on the filter was vacuum-dried overnight at room temperature.

Graphene characterisation


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Thermo gravimetric analysis (TGA) of the samples was performed on an SDT-Q600 simultaneous TGA/DSC thermo gravimetric analyzer (SDT Q600, TA Instruments, New Castle, DE, USA). Samples were heated under air (100 mL min-1) from room temperature to 800 °C at 10 °C min-1. Raman spectra were collected with a Thermo Scientific Raman spectrophotometer (DXR Raman Microscope, Thermo Electron Scientific Instruments LLC, Madison, WI, USA) in a backscattering geometry and equipped with a CCD detector. The excitation laser beam at a wavelength of 532 nm and a power at the sample surface of 1.5 mW, had an area of around 1 µm2 and was focused using an Olympus microscope coupled to a 100x objective. Based on Raman spectra, the average crystallite size ( ) of the sp2 lattice was calculated as described by Cançado et al.35 using Equation (1). = 2.4 10 ⁄

Equation 1

where , and are the laser wavelength in nm, the integrated intensities (areas) of the Raman G and D bands, respectively. Sample morphology was investigated with a SEM FEG Quanta Scanning Electron Microscope (Inspect F50, FEI Company, Hillsboro, OR, USA) and by Transmission Electron Microscopy (TEM) (Tecnai T12, FEI Company, Hillsboro, OR, USA) with an electron accelerating voltage of 120 kV. Lattice fringe images of the graphene sheets were taken using FEI Tecnai G2 F20 cryo-STEM (FEI Company, Hillsboro, OR, USA) equipped with a Gatan Ultrascan 4000 4 k × 4 k CCD camera Model 895 at an accelerating voltage of 200 kV.

Modification of glassy-carbon (GC) electrode and Ag electrodeposition Prior to Ag electrodeposition on graphene electrode, a glassy carbon electrode (4.92 mm ø) was polished with 0.05 mm alumina slurry, rinsed with distilled water and sonicated in acetone


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for 5 minutes. Then 20 µL of the as-synthesized graphene (0.5 mg) in DMF (1.0 mL) solution (ink) was drop-cast onto a freshly polished GC surface. Catalyst films were typically dried overnight at ambient temperature. The modified GC electrode, namely the graphene electrode (GC-GRn) was dried at room temperature under ambient air (12 hours). Similar procedure was applied to graphite (GC-GRp) and graphene oxide (GC-GO) electrodes. The electrochemical deposition was performed employing the GC modified electrodes in N2sparged plating solution of aqueous 1.0 mM silver nitrate (99.9%, Fisher Scientific) and 0.10 M potassium nitrate (99.9%, Fisher Scientific) as support electrolyte. A rectangular silver plate (3x5 cm) was used as reference electrode to avoid any undesired reactions with the silver salt during the potentiostatic deposition. The amount of electrodeposited silver was calculated with the charge passed at the onset of the potential step.36 After deposition, the CG modified electrode surface was rinsed with water and dried in air. Electrochemical characterisation Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were performed using potentiostat / galvanostat (VersaSTAT 4, Princeton Applied Research, Oak Ridge, TN, USA) system connected to a three electrodes cell interfaced to a computer system with corresponding electrochemical software (VersaStudio Version 2.40.4). The electrochemical cell was assembled with a conventional three-electrode system: a glassy carbon working electrode (4.92 mm diameter), Saturated Calomel Electrode (SCE) reference electrode and Pt wire counter electrode. The CV experiments were performed in 1 M KOH solution or in 10 mM [Fe(CN)6]3−/[Fe(CN)6]4− (1 : 1 molar ratio) redox couple in 1 M KCl. The capacitance at the surface of the graphene electrode (CCV, F·cm−2) was estimated from the cyclic voltammograms37


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by integrating the area under the current–potential curve divided by the sweep rate (υ, V·s-1) and the surface area of the active electrode (ECSA, cm2), according to the Equation 2: "#$ = % ⁄(&'("))

Equation 2

The EIS experiments were performed in the presence of 0.1 M phosphate buffer solution (PBS) (pH = 7), 0.1 M KCl, containing of 10 mM [Fe(CN)6]3−/[Fe(CN)6]4− (1 : 1 molar ratio) with potential perturbation of 10 mV (rms) within a frequency range of 105 Hz to 10-2 Hz, spaced logarithmically (120 per 10 decades). EIS results were analyzed using ZsimpWin and the criteria employed to estimate the fitting quality were evaluated first with the low chi-square value and then with the low estimative errors (%) for all components. To assess the ORR catalytic activity, the GRn and GRn/Ag were first loaded onto the surface of glassy carbon electrodes for linear sweep voltammetry (LSV) in N2 and O2-saturated 0.1 M KOH. For comparison, bare glass-carbon electrode was also evaluated under identical conditions.

Results and Discussion Graphene

has

been

successfully

electrochemically

exfoliated

in

poly(sodium

4-

styrenesulfonate) (PSS) (Figure 1). The applied voltage led to water oxidation accompanied by the production of hydroxyl and oxygen radicals. These radicals oxidised the edge and grain boundaries of the graphite facilitating the intercalation of sulfate ions (SO42-). In this process, PSS incorporated in the solution helped the expansion and exfoliation of the graphite flakes with the reductive decomposition of anionic sulfate ions *(+ , →↑ (+,(/) 0. Adding PSS as an intercalant to the process not only minimized the oxidation of graphene during exfoliation, but as


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well stabilised graphene as a colloid in the solution and prevented the nanosheets from aggregating.

Figure 1 – Schematic illustration of mechanism for electrochemical exfoliation of graphite employed in this paper. Characterisation of graphene To determine the number of graphene layers, several techniques such as TEM38 and Raman spectroscopy39 were employed. Both bilayer (Figure 2a) and monolayer (Figure 2b-c) graphene of approximately 10 µm in longest dimension was directly measured with DigitalMicrograph. The graphene sheets were lightly wrinkled and their edges tended to scroll slightly. No large particles with a thickness of more than a 3 layers were found suggesting successful exfoliation. The absence of both secondary diffraction spots in a hexagonal arrangement40 and intense 111002 spots compared with 121002 spots41 (graphene monolayers 1

2 ⁄1, 2 3 1)42 in the electron diffraction pattern (Figure 2d) confirmed the largely monolayered nature of the graphene, Figures 2 b-c. Figure 2f shows typical Raman spectra of graphene with 3 major bands; D band around 1350 cm-1, the G band around 1580 cm-1, and the 2D band (the overtone of the D band) around 2700 cm-1 43, related to the defects in the sp2 lattice, breathing modes of C6 rings of sp2 lattice and numbers of graphene stacked layers, respectively.44,

45 7, 46

The defect

density of the graphite structure, particularly the defects located at the edges of graphene, was


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determined by measuring the ratio between the intensity of D and G bands (ID/IG).43 We found a (ID/IG) value of 0.21 indicating a lower defect in intensity. This finding proves that our electrochemical exfoliation method produced graphene with lower defects density than the graphene obtained with chemical or thermal reduction of GO.47, 48

Figure 2- Bright-field TEM images of (a) bi-layer and (b, c) monolayer graphene flakes, showing typical size, shape, and morphology of the graphene. (d) Electron diffraction pattern of the graphene sheet in b, with the peaks labelled by Miller–Bravais indices. (e) TGA curves of raw graphite (black), graphene (red), and graphene oxide (blue) at a heating rate of 10 ºC/min in


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air. (f) Raman spectra (excitation wavelength 4 = 567 89) for graphite (black) and graphene flakes (red). As the number of the graphene layers decreases, the intensity of 2D band increases. The calculated I2D/IG ratio was higher (~0.3) than graphite (0.2), confirming the decrease in the number of layers. Furthermore, the 2D band is observed at ~2660 cm-1 and can be well fitted with a single Lorentzian peak, which is typical for a monolayer graphene[25], further supported by the absence of a peak at 2,700 cm-1 which generally increases when the number of layer increases.49 Another spectral feature of graphene is related to absorption frequency of 2D band, which typically shifts for lower energy (red-shift) in the Raman spectra for GRn compared to GRp flakes. Thermal gravimetric analysis (TGA) (Supplementary information, SI) showed the presence of PSS and of the labile oxygen-containing groups (Figure 2e). Finally, although GO exhibited the same thermal decomposition profile, the recorded weight losses were considerably higher than GRn (39 and 34%) indicating that our exfoliation process prevented GRn from being strongly oxidized.50 The low oxidation degree of graphene was confirmed by attenuated total reflection Fourier transform infrared (ATR-FTIR) (Supplementary information, SII). The electrochemical quality and properties of the graphene has been investigated via cyclic voltammetry (CV) and impedance spectroscopy. The quasi-rectangular shape without any obvious redox or faradic peaks of the CV (Figure 3a) indicates a purely capacitive behaviour51.


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Figure 3 - (a) Cyclic voltammograms for bare, GRp, and GRn electrodes in 1.0 M KCl solution between 0 V and 0.5 V versus SCE. Scan rate, 100 mV s-1. (b) Nyquist diagrams for EIS measurements of bare, GRp, and GRn electrodes. Conditions: N2-saturated 1 M KCl and 50 mM PBS as background electrolytes with 10 mM : 6 ;?)@ A/: C ;?)@ A (1:1 molar ratio). (c) Inset: Randles equivalent circuit used for data fitting.

The calculated electrochemically active surface area (ECSA) increased from 0.36 cm2 for bare GC to 0.56, and 0.75 cm2 for GRp, and GRn, respectively, suggesting a higher number of catalytic sites for graphene, i.e., higher electrocatalytic activity (Table 1). The specific capacitance of the surface-modified graphene electrode was significantly higher than graphite, indicating a higher capacity to store energy with a highly reversible separation of electrical charge37, 52 by forming an electric double-layer capacitor. The capacitance values appear to be attributed to the presence of one-atom-thick sheets of hybridized sp2-bonded carbon atom in the graphene that enables faster charge propagation

37

. Although a similar structure is present in

graphite, the electrochemical exfoliation lead to a greater exposure of sp2 hybridized structure at the electrode/solution interface.


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Table 1. Electrochemically active surface area, capacitance, voltammetric data (data extracted from Fig. SII, Supplementary information, SIII), ∆Ep and E1/2, and apparent electron-transfer rate constants, K0app, for the bare, GRp, and GRn electrodes.

ESCA CCV (µF.cm-2) ∆Ep [mV] E1/2 [mV] 0

K

app

-1

Bare

GRp

GRn

0.36 185 ± 40 103

0.56 692 ± 83 85

0.75 4329 ± 133 68

212

216

217

1.38

2.77

8.31

-3

(cm.s x10 )

The electrochemical benefit from the graphene structure with respect to the flow of electrons was investigated with electrochemical impedance spectroscopy (EIS) and by studying the heterogeneous electron-transfer (HET) rate with cyclic voltammetry in presence of K3[Fe(CN)6]/K4[Fe(CN)6] (1:1 molar ratio) (Supplementary information, SI.III). The heterogeneous charge transfer (Rct) has been determined as the semi-circle diameter of the Nyquist plots53 (Figure 3b) and calculated from the data fitted with the Randles equivalent circuit (inset -Figure 3c). The Nyquist diagrams however show some discrepancies at high frequencies associated to the charge transfer resistance-limiting process at the Grn, Grp and GO/ solution interface (RCT). The presence of a smaller semicircle for Grn indicates a lower charge transfer resistance (Rct) at the electrode/electrolyte interface which favors faster ion transfer kinetics.

Figure 3b shows that Rct follows the order: Rct(GO, )> Rct(Bare)> Rct(GRp)>

Rct(GRn). Therefore Grn with its small charge transfer resistance enables the electrolyte ions to easily diffuse into its pores access its surface. This low Rct value for GO can be the result of the


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repulsion between high oxygen containing groups at the surface of the electrode and the negatively charged ferrocyanide/ferricyanide redox couple whereas the high value for GRn can be attributed to the presence of the graphitic network of sp2 bonds, the low density of defects and/or the low oxygen containing groups which have been previously determined with the Raman and TGA analysis. At higher frequency, the slope of 45º portion of the curve, called the Warburg resistance, results of the frequency dependence of the ion diffusion in the electrolyte. From Figure 3b, Grp, Grn and GO exhibited a short Warburg curve implying a short ion diffusion path, which will allow easy access of electrolyte ions to the electrode surface. The knee frequency values of Grn were much higher than Grp and GO indicating a better ion diffusion ability thereby a higher rate performance54.

These results confirm the excellent electrical

conductivity of the graphene on the contrary to the GO, which acts as an isolator. The peak potential separation approaches 103, 85, and 68 mV for bare GC, GRp, and GRn electrodes, respectively (Table 1 and Fig. SII). The lower separation between the two peak potentials for GRn

electrode

confirms

a

higher

heterogenous

electron

transfer

rate

for

the

K3[Fe(CN)6]/K4[Fe(CN)6] redox reaction (Supplementary information I.III). Comparing previous study found in the literature with our data,52 our electrochemical exfoliation process lead to a graphene with superior conductivity likely due to the higher specific surface area and the presence of sp2 hybrized system.

Electrodeposition of Ag NP onto graphene We further demonstrate that the size and density of silver nanoparticles could be engineered on graphene electrochemically. We exploited the potentiostatic double-pulse (PDP) technique31 traditionally employed for the deposition of Pt nanoparticles with controlled size and density. To


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calculate the electrochemical parameters used in PDP technique, a cyclic voltammograms of an electrode covered with graphene in silver nitrate solution was performed (Figure 4(a-b).

Figure 4 – (a) Cyclic voltammograms of GC covered with graphene in the absence of silver nitrate solution 0.1 M KNO3 (dotted line) and 0.1 M KNO3 + 0.1 M AgNO3 solution at scan rates of 50 mV s−1 (-600 to 500 mV) and (b) zoom view of silver deposition.

From Figure 4b and taking into account the density of silver nanoparticles, particle size, and homogeneity of particle distribution, the potential chosen for the nucleation pulse (E1) was set at -500 mV, which is higher than Ecritical and the growth pulse (E2) was set at -100 mV, which is high enough to control particle growth but low enough to suppress the generation of new nuclei. The duration times of the nucleation pulse (t1) studied were 50, 100 and 200 ms, whereas for growth pulse (t2) was set in 1s (Figure 5a). The mass of deposited Ag was obtained by graphical integration of the electrical charge consumed during the deposition process QAg (C cm-2), according to following equation (3):55 JK/ =

LK/ ∙ N ('PQRS%TU 3) 4O


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where M and F are the atomic weight of Ag (107.87 g mol-1) and the Faraday constant (96485.309 C mol-1) respectively. We assumed that the current efficiency was 100% and neglected the double layer charging. The amounts of silver deposited for a pulse nucleation of 50, 100, and 200 ms were 0.196, 0.344, and 0.570 µg cm-2, respectively. As expected the amount of Ag electrodeposited increases as a function of increasing step time. The Ag NP were spherical in shape, well dispersed and randomly distributed, exhibiting a narrow size distribution Figures 5 (a-c).

Figure 5 – Low-magnification TEM images (a-c), EDX composition analyses (d-f), and particle size distribution (g-i) of GRn/Ag 50 ms, GRn/Ag 100 ms, and GRn/Ag 200 ms. The insets show the corresponding HRTEM images. The parameters denoted µ and σ are,


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respectively, the mean and standard deviation of the variable’s natural logarithm in the lognormal model. The number of particles measured was approximately 500.

The nanoparticle density increased from 7.39 x1010 to 3.65 x1011 nanoparticles per cm2 with the nucleation time. Previously a potentiostatic pulse method (single pulse)56 was employed to obtain silver nanocrystallites on graphite with a coverage of near 1010 particles cm-2. However, the authors were only able to produce particles, approximately ten times bigger than those reported in our study. The PDP method allowed a homogenous dispersion of few nanometers size particle predicting a superior catalytic activity. The crystalline structure of the Ag nanocrystallites with lattice spacing of 0.24 nm, which corresponds to the distance along the [111] projection separating (111) planes of atoms for face-centered cubic silver has been confirmed with HRTEM images (inset – Figure 5d-f).57 Furthermore, the lattice fringes related to the crystalline structure of the graphene in the (200) crystallographic planes with lattice spacing obtained from the image equals 0.32 nm were observed. EDX composition analyses of the GRn/Ag (Figures 5d-f) revealed the presence of Ag and Cu from the TEM holder, confirming that the process was free from contamination. The log-normal distribution median diameter of the Ag NP was 3.22 ±0.2, 5.28 ±0.2, and 4.76 ±0.2 nm for 50, 100 and 200 ms nucleation times, respectively (Figure 5g-i). The size of silver nanoparticles and the range of distributions are lower than the values reported from chemical deposition,15 thermal deposition,58 electrochemical deposition,31, 59 chemical vapor deposition60 or photochemical process.61 Particles nucleation was not observed preferably at wrinkles, corrugations or step sites, in agreement with other findings on other carbon surfaces.62, 63 The GRn/Ag 50 ms electrode showed the narrowest particle distribution (range of 0.5–7.5 nm)


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whereas GRn/Ag 100 ms and GRn/Ag 200 ms electrodes the wide nanoparticle size distributions with ranges from 1 to 14 and from 1 to 12 nm, respectively. These results confirm that the nucleation of particles occurs only during the nucleation pulse, while the formed nuclei grow continuously during the growth and that with a prolonged time of nucleation the growth can occur simultaneously.

Catalytic activity The electrocatalytic activity towards oxygen reduction (ORR) of these graphene supported silver nanoparticles in dependence of the particle sizes was investigated using linear sweep voltammetry (LSV) experiments (Figure 6).

Figure 6- Linear sweep voltammetry (LSV) obtained in 0.1 M KOH saturated with N2 or O2 for a bare and modified GC electrode. Scans start at 0.1V and scan rate: 50 mV s-1. As expected, under N2 atmosphere, no anodic or cathodic peaks were observed, only a slight increased of the cathodic current for GRn and GRn/Ag modified electrodes at higher potential due to the reduction of traces of oxygen present on the edges of the graphene. The ORR


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polarization curves obtained in an oxygen-saturated solution show two cathodic peaks related to )V+ → )V reaction on two different locations at GRn/Ag surface

64, 65

characteristic of the

ORR reaction. It was notable that these peaks did not appear for GRn/Ag 50 ms electrode probably due to the lower amount and smaller size of silver nanoparticle deposited. It was reported that this peak is a key parameter to predict ORR activities on Ag/C catalysts and that the catalytic activities for the ORRs greatly depend on its intensity which is in return, directly related to the metal loading and that the ORR onset potentials shift positively.66 GRn and GRn/Ag voltammograms clearly showed an ORR activity as indicated by the distinct onset potentials. The cyclic voltamogramm for GRn and GRn/Ag in O2 saturated solution compared to the N2 saturated solution unequivocally exhibits a reduction peak at around -0.45, supporting a two electron path for the oxygen reduction reaction in alkaline water based electrolyte cathodic peaks: +, + X, + + 2Y → X+, + +X .67,

68

Although a four electron to water for fuel cell

application is preferred route as a two electron69 because of the higher current available and the absence of undesirable by product such as hydrogen peroxide70,

71

, these catalysts are highly

advantageous in industry for H2O2 production and there are few studies concerning them20. With an onset potentials for the ORR shifted favourably to the more positive potential (+0.05V for GRn/Ag electrodes) compared to reported values for silver-graphene nanoribbon57 and commercially available Pt-loaded carbon supported,24, 72, 73 GRn/Ag appears to be an excellent catalyst for the ORR. Higher current densities are obtained for GRn and GRn/Ag than bare GC and GRp electrodes. This positive shift and higher current density indicated that the silver nanoparticles on the graphene reduced the ORR overpotential thereby enhancing its catalytic activity. The electrocatalytic performance in terms of current density, of the GRn/Ag electrodes follows the sequence: 50 ms>200 ms> 100ms. Although GRn/Ag 100 and GRn/Ag 200 ms


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exhibited the biggest particle size and the greatest amount of Ag, the GRn/Ag 50 ms electrode performed the best. Beside its large surface area and good electrical conductivity, the GRn/Ag 50 ms electrode with its narrow size distribution and small particle size offered not only the maximal availability of electrocatalyst surface area for electron transfer but also better mass transport of reactants to the electrocatalyst. In the literature, many attempts have been made to develop alternative catalyst, to replace or reduce the content of Pt for ORR. As far as we know, the performances of these alternative catalysts for ORR were still unsatisfactory and rarely exceed the performance with Pt/C. Although the GRn/Ag 50ms has been proved to have excellent capacity for ORR, our results are comparable to Ag nanoparticle on graphene nanoribbons with a quantity of Ag over 500 time more assuming a comparable particle density57 but remarkably higher than previously reported commercially Pt/C (1mA cm-2 without rotating disc electrode24 and 2.5mA cm-2 at 400rpm74), Ag nanoclusters24, Ag nanoparticles on graphene that was prepared with chemical approaches15, 23

and Ag/C.57,

66

It has to be pointed out that unlike these studies, our measurements were

performed without a rotating disk electrode that generally negates the problems of mass transfer by increasing the O2 flux to the catalyst surface with the rotation and therefore our results in terms of ORR performance are likely greatly underestimated. This remarkable increase of current density for the ORR compared to others similar architectures prepared with the largely described Hummer’s exfoliation method and silver nitrate reduction coincides with a greater surface contact between the high purity graphene with superior conductivity and the small silver particle size homogenously distributed. Our novel GRn/Ag 50ms catalyst could be therefore an ideal candidate for the alternative to Pt catalysts, offering a new prospective for a less expensive cathode at the same performance.


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Conclusion In summary, metal nanoparticle composites of high quality graphene with a low degree of oxidation have been prepared by surfactant-assisted electrochemical exfoliation. A double pulse technique enabled Ag-decoration of graphene with controlled nanometer size particles. We have successfully prepared Ag-graphene with good ORR performance. Ag, though not as efficient as Pt could, through this high efficiency processing route, produce a material with comparable efficiency as standard commercial Pt/C catalyst by virtue of minimising particle size and maximising graphene sheet loading offering a cost reduction and lowering hurdles to widespread adaptation of alkaline medium for fuel cells.

AUTHOR INFORMATION Corresponding Author Prof. J. E. Barralet Faculty of Dentistry, McGill University, Montreal, H3A 2B2, (Canada) Department of Surgery, Faculty of Medicine, McGill University, Montreal, (Canada) E-mail: ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.


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Funding Sources This work was carried out with the support of the São Paulo Research Foundation – FAPESP (Grant 2013/12376-5). Natural Sciences and Engineering Research Council of Canada (NSERC) and Ballard Power Systems are acknowledged for the Engage Grants. Notes Marvin was used for drawing the graphene structures, MarvinSketch 6.3.0 (ChemAxon Ltd. http://www.chemaxon.com).

ABBREVIATIONS ORR, Oxygen Reduction reaction; Grp, GRp, Graphite; GRn, Graphene, GO, graphene oxide; PSS, polystyrene sulfonate; HET, Heterogeneous electron-transfer; TGA, thermal gravimetric analysis; CV, Cyclic voltammograms. REFERENCES 1. Cooper, D. R.; D’Anjou, B.; Ghattamaneni, N.; Harack, B.; Hilke, M.; Horth, A.; Majlis, N.; Massicotte, M.; Vandsburger, L.; Whiteway, E.; Yu, V. Experimental Review of Graphene. ISRN Condensed Matter Physics 2012, 2012, 56. 2. Choi, H. J.; Jung, S. M.; Seo, J. M.; Chang, D. W.; Dai, L. M.; Baek, J. B. Graphene for energy conversion and storage in fuel cells and supercapacitors. Nano Energy 2012, 1 (4), 534551. 3. Bonaccorso, F.; Lombardo, A.; Hasan, T.; Sun, Z.; Colombo, L.; Ferrari, A. C. Production and processing of graphene and 2d crystals. Materials Today 2012, 15 (12), 564-589. 4. Coleman, J. N. Liquid Exfoliation of Defect-Free Graphene. Accounts Chem Res 2013, 46 (1), 14-22. 5. Parvez, K.; Li, R. J.; Puniredd, S. R.; Hernandez, Y.; Hinkel, F.; Wang, S. H.; Feng, X. L.; Mullen, K. Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics. ACS nano 2013, 7 (4), 3598-3606. 6. Liu, J. L.; Poh, C. K.; Zhan, D.; Lai, L. F.; Lim, S. H.; Wang, L.; Liu, X. X.; Sahoo, N. G.; Li, C. M.; Shen, Z. X.; Lin, J. Y. Improved synthesis of graphene flakes from the multiple electrochemical exfoliation of graphite rod. Nano Energy 2013, 2 (3), 377-386.


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The table of contents A controllable fabrication of Ag nanoparticle (NP)-decorated graphene and their application for oxygen reduction reaction (ORR) have been demonstrated. Ag NPs deposition was performed with a potentiostatic double-pulse technique to obtain narrow particle size, high surface area and homogenous spatial distribution. We found that the size and loading density of NPs greatly affects the performance of Ag/Gn for ORR.

Keyword: Electrochemical exfoliation, Graphene, Silver, ORR, Nanoparticles

Joao Henrique Lopes, Siyu Ye, Jeff T Gostick, Jake E Barralet§ and Geraldine Merle§* Silver nanoparticles decoration of defect free surfactant-assisted electrochemically exfoliated graphene


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Electrocatalytic Oxygen Reduction Performance of Silver

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