Various Carbon Chain Containing Linkages Grafted Graphene with Silver Nanoparticles Electrocatalysts for Oxygen Reduction Reaction

In this study, we have synthesized an efficient catalyst by various carbon chain containing linkers grafted graphene with silver nanoparticles (AgNPs)-decorated GO-S-(CH2)x-SH (where x = 2, 3, 4 and denoted as GO-Cx-Ag) for oxygen reduction reaction (ORR). The structural and morphological properties have investigated via several instrumental methods. Among those catalysts, the GO-C2-Ag has showed an excellent electrocatalytic performance by cyclic voltammetry (CV) and hydrodynamic techniques for ORR in alkaline media. Hydrodynamic voltammetry reveals that the GO-C2-Ag modified electrode has catalyzed effectively at higher potential. The overall electrocatalytic results showed that the GO-C2-Ag has better activity toward ORR and demonstrated nearly four electron transfer pathway into H2O due to much grafting of linker molecule and smaller size of AgNPs. The value of transferred electron number (n) and other kinetic parameters have demonstrated that the GO-C2-Ag is highly facilitated than that of GO-Ag and other GO-Cx-Ag to electrocatalytic oxygen reduction. © The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0931414jes] All rights reserved.

Graphene, which is discovered by Geim's group in UK in 2004, is composed of a single sheet of conjugated sp 2 carbon atoms packed into a honeycomb crystal structure. 1 It is an attractive material due to its unique physical and chemical properties such as its outstanding thickness, tensile strength (130 Gpa), thermal conductivity (5,300 W/mK), allowable current density (108 A/cm 2 ), and mobility (200,000 cm 2 /Vs). [2][3][4][5] However, the oxidized graphene (GO) surface is highly functionalized with oxygenated groups, such as ketones, epoxides, hydroxyls and carboxylates, 6 which make them targets for covalent modification. Graphene has been recognized and used as an important material in various technologies in chemistry such as, displays, rechargeable batteries, solar cells, automobiles, fuel cells (FCs). 7 In recent years, due to global warming, energy issues are in the spotlight. Thus, many studies have been conducted on carbon nanomaterials supported metal nanoparticles (NPs) for both anodic and cathodic reactions in FCs including oxygen reduction reaction (ORR).
ORR is important for energy conversion systems in FCs 7,8 and metal air batteries. 9 It is well-known that a platinum (Pt)-based catalyst in an acidic solution has the highest chemical stability among electrochemically active catalysts. However, Pt-based catalysts are too expensive to make FCs commercially viable. Under acidic conditions, due to the high dissociation energy (494 kJ/mol) of oxygen molecules, electrochemical oxygen reduction requires a high overpotential, and the reaction rate is slow. 10 Moreover, Pt has suffered several problems such as, poor drivability and toxic effects of methanol crossover/carbon monoxide in direct methanol fuel cells (DMFCs).
Among these options, Ag, being relatively inexpensive and abundant, is a promising candidate as a cathode electrode catalyst for FCs. It has been reported that the 20% Ag/C catalyst shifts toward negative potentials by about 50 mV in comparison with the onset potential of the 20% Pt/C catalyst and better tolerance toward methanol. 25 Various effects of the Ag/C electro-catalyst on its ORR with the particle size, amount of loading, and bonding with a non-metal were recently studied. [25][26][27] To the best of our knowledge, however, there is no reported relationship yet between the linker length and the catalytic activities of Ag-decorated graphene.
Based on the above discussions, the various length linkages in between graphene and AgNPs have synthesized and were investigated for ORR in 0.1M NaOH solution. The GO-C x -Ag catalysts were examined using cyclic voltammetry (CV) and hydrodynamic techniques including rotating ring disk electrode (RRDE) and rotating disk electrode (RDE) techniques. The catalysts showed better ORR activity in the shorter-chain GO-C 2 -Ag -modified glassy carbon electrodes (GCEs) than that of GO-Ag and other GO-C x -Ag. The electrocatalyst and the stability toward the ORR were also analyzed through Koutecky-Levich plots, which confirmed that the O 2 reduction was proceeded on via a four-electron transfer pathway.
Different length linkages on graphene (GO-C x -SH).-GO was obtained by oxidizing graphite using the improved Hummers method. 3,7,28,29 The GO-C x -SH was prepared by a modified method. 29 Briefly, the GO and the linker materials, HS-(CH 2 ) x -SH (x = 2, 3, and 4), were separately dispersed with tetrahydrofuran (THF) into four round-bottom flasks and stirred at 55 • C for 20 hours before 30-min ultrasonic agitation. The resulting black materials were separated from the mixture by filtration, washed several times with THF, methanol, ethanol, and distilled water (DW), and dried in a vacuum oven at 50 • C for 18 hours.
Synthesis of GO-C x -Ag.-To verify the support effect, the GO-C x -Ag mixture was prepared by adding 50 mg of GO-C x -SH to 15 mL of DW for 30-minute ultrasonic agitation. Then 0.1M AgNO 3 (5 mL) and 0.1M NaOH (2 mL) were added to the mixture and stirred for 20 hours. The GO-C x -Ag products were obtained via centrifugation, washed with DW, and vacuum-dried for 24 hours at 50 • C.
The GO has large surface area and provided with epoxy and hydroxyl functional groups, 6 providing a number of chemically active sites for addition of SH-C x -SH via a condensation reaction and subsequently addition of AgNPs onto the linker's molecules (Scheme 1). As a result, the GO-C x -Ag has been synthesized with dendrite like structure and consisted of a sheet decorated with AgNPs are well separated.
Physical characterization.-X-ray photoelectron spectroscopy (XPS) was performed with a VG Multilab 2000 spectrometer (Thermo VG Scientific, South-end-on-Sea, Essex, UK) in an ultra-high vacuum. The XPS data analysis program of Avantage 4.54 version (Thermo Electron Corp., England) was used. This system uses an unmonochromatized Mg K (1253.6 eV) source and a spherical section analyzer. Survey scan data were collected using a 50eV pass energy. A field emission scanning electron microscope (FE-SEM) image of the modified electrode was obtained with a JSM-7500F field emission scanning electron microanalyzer (JEOL). The high-resolution transmission electron microscopy (HRTEM) images and energy dispersive X-ray spectroscopy (EDS) were carried out with a TECNAI 20 microscope at 200 kV.
Electrochemical measurement.-For the electrode preparation, a GO-C x -Ag suspension in water (1 mg/ml) was prepared by introducing a predetermined amount of the corresponding sample under sonication. Then 16 μl of the prepared catalyst ink was dropped onto the surface of a glassy carbon electrode (GCE, 0.5 cm in diameter) prepolished with a 0.05μm alumina suspension on a polishing cloth (BAS, USA). After the coating, electroreduction was performed between the sweeping potentials of 0 V to −1.5 V at a scan rete of 50 mV/s for 60 cycles in phosphate buffer solution (PBS) at PH 5. Then the GO-C x -Ag coated GCE was employed for ORR in an O 2and/or Ar-saturated 0.1 M NaOH solution. All the voltammetric measurements were taken using a three-electrode potentiostat [CHI 700C electrochemical workstation (USA)] in a grounded Faraday cage at room temperature. Pt wire was used as an auxiliary electrode. A cali-brated Ag/AgCl electrode from Bioanalytical System Inc. (BAS) in a 3M NaCl solution was used as a reference electrode. Figure 1 shows the base CVs of the GO-C x -Ag recorded in an Ar-saturated 0.1 M NaOH solution at a scan rate of 50 mV s −1 . In the potential range of 0.1 −0.4 V vs. (Ag/AgCl), three peaks were observed: a, b, and c, which were located at about 0.15, 0.21, and 0.26 V, respectively. Moreover, the d peak was located at approximately 0.03 V. The weak peak (a) was due to the dissolution of Ag and formation of a surface monolayer of Ag 2 O film onto AgNPs. The strongly appeared peaks (b) and (c) were assigned to the formation of the bulk phases of AgOH and Ag 2 O, and the cathodic peaks were assigned to the reduction of Ag 2 O back to metallic silver. 30 It is known that the electrochemical surface area (ECSA) could reveal the degree of metal utilization in the electrode material. Therefore, these results imply a better utilization of the Ag on the surface of the GO-C 2 -Ag with the assistance of smaller size of AgNPs. The ECSA for all catalysts were calculated using Coulombic charge for the reduction of AgO, which is located at around 0 V potential regions in Figure 1 and enlisted in the Table I. We expected the Ag/C catalyst to be stable for ORR, as reported by Lee et al. 31 However, the AgNPs dissolution in 0.1M NaOH in the first measurement yielded the peak (d) current density of about 3.73 mA cm −2 , and in the second and third measurements, 3.03 mA cm −2 and 2.11 mA cm −2 , respectively.
Ag has a problem of strong dissolution in an open circuit, as seen in the following equation: 32 To solve this problem, a study was conducted on the alkaline electrolyte, 0.1 M NaOH electrolyte for the Ag based catalyst and it showed that the stability of the AgNPs significantly improved. 32,33 Therefore, we have used 0.1 M NaOH electrolyte system for avoiding AgNPs dissolution during synthesis and ORR.

Results and Discussion
Surface morphology.-The surface morphology of GO-C x -Ag was characterized with SEM and TEM investigations. The Figure 2 shows the TEM images of GO-C x -Ag and SEM images of GO-C x -Ag at Figure 2 insets, respectively. The layer-by-layer assembled 2D sheet-like morphology was observed at GO-C x -Ag ( Figure 2). 34 According to the TEM images, at the shorter chain, GO-C 2 -Ag, high content of AgNPs were appeared onto the surface and the AgNPs size was smaller than that of GO-C 3 -Ag and GO-C 4 -Ag (Figure 2a).  Moreover, the AgNPs were uniformly dispersed onto the graphene sheet. The Figure 2b and 2c had also spherical shaped AgNPs and were not uniformly dispersed. Slight agglomeration of NPs was observed which yielded a bigger size distribution. The measurement of spacing lattice planes are 0.22 nm, 0.231 mn, and 0.28 nm, for GO-C 2 -Ag, GO-C 3 -Ag and GO-C 4 -Ag, respectively, which corresponds to the (200) lattice plane of face-centered-cubic structure. 35,36 According to the Figure's insets, the SEM image of GO-C 2 -Ag is much porous like than that of GO-C 3 -Ag and GO-C 4 -Ag that probably due to much amount of AgNPs attachment onto the linker's molecules. The size distribution are displayed in a , b and c , respectively, and the size of AgNPs is increasing. The corresponding EDS spectra (Figure 2e) are suggestion a decreasing S content with higher chain values and AgNPs are almost in the same content. The Ag contents are as 24.12, 23.93 and 24.08 wt% for GO-C 2 -Ag, GO-C 3 -Ag and GO-C 4 -Ag, respectively. Figure 3a shows the XPS survey spectra for GO-C x -Ag. All spectra showed the C1s (284 eV), O1s (532 eV), Ag3d (367 eV), and S2p (162 eV) signals. The C/Ag wt% ratio was decreased as 2.55, 2.52 and 2.50 for GO-C 2 -Ag, GO-C 3 -Ag, and GO-C 4 -Ag, respectively. Indicating carbon content is decreasing due to the higher degree of chain grafting in the order of C 2 > C 3 > C 4 onto the graphene. Figure 3b shows the high-resolution S2p spectra. As can be observed in the S2p spectra, at 162.8 eV is corresponded to the double carbon liked S (-C-S-C-) and between 164 eV to 168 eV has no ant identical peaks for SO x . 37 This result actually suggests that the linker molecules are grafted onto the GO sheets to afford C-S bonded groups. The sulfur content (in wt%) is also suggested that the higher linkage was done in GO-C 2 -Ag (1.90) than GO-C 3 -Ag (1.78) and GO-C 4 -Ag (1.73). Figure 3c shows the highresolution C1s spectrum. The binding energies of 285 eV, 287 eV, and 289 eV corresponded to the C-C/C=C in unoxidized graphite carbon; C-O/epoxy and O-C=O in carboxyl groups, respectively, for GO. 2,3 Comparing C-O/epoxy peak for GO-C x -Ag C1s, the C-O/epoxy was significantly reduces upon reduction. It is, however, the C-O/epoxy peak intensity is not actually been reduced, probably due to the influence of C-S bond. 38 Moreover, little negative shift can also be observed in the C-O/C-S than C-O peak. Indicating that the linkage molecule, HS-C x -SH, was bonded with C in graphene followed by the condensation reaction. According to intensity of C-O/C-S peak, the higher degree of linkage may be in the order of C 2 > C 3 > C 4 . Figure 3d shows the core level spectrum of the Ag peaks at Ag3d 5/2 and Ag3d 3/2 , which correspond to 368.93 eV and 374.92 eV, 368.87 eV and 374.69 eV, and 368.59 eV and 374.59 eV for GO-C 2 -Ag GO-C 3 -Ag and GO-C 4 -Ag respectively, and which are consistent with the expected spin energy difference of approximately 6 eV. The binding energy is very typical for Ag in metallic form (Ag 0 ). No Ag3d peaks were detected in the Ag 2 O and AgO, which suggests that AgNPs are stable in the atmosphere. 39 Therefore, the XPS result suggesting that the constant Ag content is due to higher dispersion with smaller size and vice versa, as observed in TEM analysis.

XPS characterization.-The
ORR onto GO-C x -Ag modified electrodes.-The GO-C x -Ag could impart electrocatalytic activities toward ORR and can be compared with GO-Ag. However, the superior ORR was observed at the shorter chain containing, GO-C 2 -Ag catalyst than that of other GO-C x -Ag. As such, we exploited the possibility of GO-C x -Ag as Pt-free catalysts for electrochemical reduction of O 2 . The CVs for oxygen reduction on the GO-C x -Ag electrodes in an aqueous O 2 -saturated 0.1 M NaOH solution are shown in Figure 4a. As can be seen, the onset potential of ORR for the GO-Ag electrode is at −0.27 V with the cathodic reduction peak around −0.5 V. Upon linkage functionalization of graphene, both the onset potential and the ORR reduction peak potential shifted positively to around −0.18 and −0.4 V, respectively at GO-C 4 -Ag. However, the more positively shifted onset potential and peak potential can be observed at −0.13 V and −0.28 V, respectively, for GO-C 2 -Ag (for GO-C 3 -Ag, onset and peak potential were −0.16 V and −0.45 V, respectively), accompanied by a concomitant increase in the peak current density (Figure 4a). These results clearly demonstrated a significant enhancement in the ORR electrocatalytic activity for the GO-C 2 -Ag in respect to the other modified electrode. To further investigate the ORR performance, we carried out the linear sweep voltammetric (LSV) measurements on a rotating ring disk electrode (RRDE) with all GO-C x -Ag in same solution. As shown in Figure 4b, the ORR at the GO-C 4 -Ag and GO-C 3 -Ag electrodes commenced around −0.14 V and −0.13 V, respectively (onset potential), whereas the ORR onset potential at the GO-C 2 -Ag electrode signifi-Scheme 2. Schematic for ORR pathway.
cantly shifted positively to −0.11 V with the limiting diffusion current at −1.2 V being stronger than that of these electrodes. Therefore, the enhanced ORR on GO-C 2 -Ag due to well dispersion of AgNPs with smaller size and the higher ECSA while the Ag mass loading (16 μl) was same for all GO-C x -Ag.
Kinetic studies.-To analyze the experimental results in this study, we used the simplified model, Scheme 2. 22,40 Path 1 shows how O 2 is reduced directly to H 2 O through a four-electron transfer. Path 2 is the sequential reaction path wherein O 2 is first reduced to H 2 O 2 through a two-electron transfer, followed by a two-electron reduction to H 2 O (Path 3), or by the release of the formed H 2 O 2 into the bulk solution (Path 4).
The Figure 5 shows the ORR on GO-C x -Ag performed by an RDE in the O 2 -saturated 0.1 M NaOH aqueous solution at a scan rate of 10 mV s −1 with various rotating speeds of 100-3600 rpm. For the all GO-C x -Ag, the limiting diffusion current density increased with an increasing rotation rate, and the ORR onset potentials are −0.1 V, −0.13 V, and −0.14 V, for GO-C 2 -Ag (Figure 5a) GO-C 3 -Ag (Figure 5b) GO-C 4 -Ag ( Figure 5c) and GO-Ag (Figure 5d) respectively. The potentiometric measurement showed that a shorter chain, GO-C 2 -Ag, catalyze at most positive onset potential with higher current density toward ORR. Very significantly, in the limiting current portion (−0.5 V to −1.2 V) of LSV for all GO-C x -Ag toward ORR has only one step which is favorable for ORR at the electrodes while the two step can be seen in the at GO-Ag electrode only. 2,38 The Figure 5 shows the corresponding Kotecky-Levich plots for GO-C x -Ag and GO-Ag. In order to determine the kinetic proficiencies of all GO-C x -Ag, the Koutecky-Levich plots ( Figure 5 a , b , c and d , respectively) were constructed from the rotating disk voltammogram data. The Koutecky-Levich plots at different electrode potentials displayed good linearity, and the slopes remained approximately constant at potentials that ranged from −0.3 V to −1.2 V, which suggest that the electron transfer numbers are similar for oxygen reduction at different electrode potentials and represents first order kinetics with respect to O 2 . 41,42 Moreover, the each Koutecky-Levich plot has a distinct slope which may be indicating the oxygen reduction does not follow the diffusion limited reaction. 41 The number of transferred electron (n) per O 2 molecule was calculated from the slope of Koutecky-Levich plot which derived from Koutecky-Levich equation: 42,43 Where in j is the measured current density, j k is the kinetic current density, j L is the limiting diffusion current density, ω is the rotation rate of the electrode, n is the transferred electron number per oxygen molecule in ORR, F is the Faraday constant (F = 96,485 C mol −1 ), is the diffusion coefficient of O 2 in the 0.1M NaOH solution (D o = 1.9 × 10 −5 cm 2 s −1 ), and ν is the kinetic viscosity of the electrolyte (v = 0.01 cm 2 s −1 ). Figure 6a, in which the n values were found to be dependent on the potential for electrodes. In particular, the n value increased with a decrease in the negative potential. The n value for ORR at the GO-C 2 -Ag electrode is always higher than that of GO-C 3 -Ag and GO-C 4 -Ag electrodes over the potential range from −0.6 to −1.2 V. Within the range the n value from 3.8 to 4 which suggests the ORR proceeds is definitely via a four-electron pathway on GO-C 2 -Ag. Although, the n values of GO-C 3 -Ag and GO-C 4 -Ag are more than 3.5 and that also suggest nearly four-electron pathway. 40,42 Therefore, the number of electron per O 2 molecule was inversely proportional to the slope as observed. 44 According to Fig. 6b, the H 2 O 2 proportion at GO-C x -Ag is much lower than that of GO-Ag. This is also consistent with the relatively high calculated transferred electron number per O 2 . Among all GO-C x -Ag, the ORR at the GO-C x -Ag electrode has produced significantly low H 2 O 2 .
To further analyze the kinetic parameter in the ORR, the Tafel investigation was done for GO-C x -Ag electrodes modified (Figure 6c). Generally, there are two Tafel slopes have been observed in previous studies i.e. 120 mV/dec. and 60 mV/dec. 45,46 The 60 mV/dec. indicates the rate-limiting step for the ORR and the 120 mV/dec. indicates the rate-determining processing to the transport of oxygen to the electrocatalyst. 47 The Tafel slopes obtained at kinetic regions (between −0.3 and −0.2) were 120.2, 126.5, and 131.6 mV dec −1 , for GO-C 2 -Ag, GO-C 3 -Ag and GO-C 4 -Ag, respectively. The small Tafel slope is generally indicating faster electron transfer kinetics and less poisoning to the metal surface. 5,48 However, the results indicate that the similar behaviors of the Tafel slopes and the reaction mechanism and the rate-determining step are the same in GO-C x -Ag and much better at GO-C 2 -Ag among all catalysts. 2,41

Conclusions
In this study, the effect of the length of the linker molecules was investigated by AgNPs-supported graphene on electrocatalytic activity toward ORR. The microscopic image of GO-C 2 -Ag consisted of sheets decorated with many AgNPs with smaller particle size than GO-C 3 -Ag and GO-C 4 -Ag. Moreover, XPS suggests a much amount of smaller linker molecules can be grafted via a condensation reaction onto graphene sheet. To determine the catalytic activity, the CV and RDE techniques were used for the ORR in 0.1 M NaOH solution. All the electrochemical and the kinetic results were indicating that a better catalysis was done upon shorter chain linked catalyst, GO-C 2 -Ag, than that of GO-Ag and other higher chain linked catalysts, GO-C 3 -Ag and GO-C 4 -Ag. Therefore, the GO-C 2 -Ag catalyst had superior catalytic activity toward ORR through four-electron transfer pathway.