Graphene-Supported Silver Nanoparticles for pH ... - IEEE Xplore

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 13, NO. 4, JULY 2014

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Graphene-Supported Silver Nanoparticles for pH-Neutral Electrocatalytic Oxygen Reduction Hongliang Sun, Kongliang Xu, Guolong Lu, Hongbin Lv, and Zhenning Liu

Abstract—Developing an electrocatalyst with desired activity and affordable cost for oxygen reduction reaction (ORR) of microbial fuel cell (MFC) remains a key challenge for practical application of MFC in wastewater treatment. In order to find an economic replacement of Pt-based catalysts while maintaining comparable catalytic efficiency, an electrocatalyst of graphene-supported silver nanoparticles (AgNPs/rGO) was prepared via a facile coreduction and its activity toward ORR in pH-neutral MFC was examined. It has been demonstrated that one-pot aqueous coreduction yielded high-quality AgNPs/rGO catalyst, as revealed by X-ray diffraction and transmission electron microscope. Interestingly, the XPS profiles also indicated the presence of oxygen-containing groups on graphene surface, which provided nuclei to form AgNPs. The resultant AgNPs/rGO catalyst displayed good ORR catalytic activity under pH-neutral condition in cyclic voltammogram and its selectivity of four-electron reduction was verified by rotating disk electrode (RDE) measurement. Moreover, AgNPs/rGO could deliver power generation and sustainability comparable to those of commercial Pt/C in a double-chamber MFC. Thus, we have demonstrated, for the first time, that graphene sheets may provide an alternative way for preparation of Ag nanocomposite catalyst and AgNPs when loaded onto graphene surface can function as a promising replacement of Pt-based catalysts under pH-neutral condition. Since Ag is less expensive and more resistant to poisons than Pt, AgNPs/rGO has better potential to be applied to MFC for recovering energy during wastewater treatment. Index Terms—Electrocatalyst, graphene, microbial fuel cell (MFC), oxygen reduction reaction (ORR), pH-neutral, silver nanoparticle.

I. INTRODUCTION ISING energy demand and intensifying environmental problems impose a prominent challenge to human beings and in recent years, a great deal of attention has been given to the development of technologies to recycle energy from waste,

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Manuscript received January 16, 2014; revised April 16, 2014; accepted April 15, 2014. Date of publication May 6, 2014; date of current version July 2, 2014. This work was supported in part by the Fundamental Research Fund of Jilin University (JLU [2011] 450060445670). The review of this paper was arranged by Associate Editor J. Li. (Correspondence author: Zhenning Liu.) H. Sun is with the Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun, Jilin, China, 130022, the School of Chemistry and Molecular Engineering, Peking University, Beijing, China, 100871, and also with the School of Mechanical and Electronic Engineering, Yunnan Open University, Kunming, Yunnan, China, 650223 (e-mail: [email protected]). K. Xu, G. Lu, and Z. Liu are with the Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun, Jilin, China, 130022 (e-mail: [email protected]; [email protected]; [email protected]). H. Lv is with the School of Mechanical and Electronic Engineering, Yunnan Open University, Kunming, Yunnan, China, 650223 (e-mail: hongbin_lv@ aliyun.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2014.2322091

as such technologies can ease energy and environment crisis simultaneously. Microbial fuel cell (MFC), a bioelectrochemical technology capable of recovering electricity directly from organic wastewater, has demonstrated remarkable potential in wastewater treatment and energy recovery [1], [2] and thus become the interest of much research. MFC utilizes anophilic microorganisms to decompose organic matters, delivering electrons and protons, both of which are transferred to cathode to react with electron acceptor. When oxygen is used as the electron acceptor, oxygen reduction reaction (ORR) takes place to reduce oxygen to water. The current bottleneck of MFC is the sluggish ORR caused by high overpotential in pH-neutral electrolyte [3]. In brief, in order to maintain normal microbial metabolism, the medium of MFC has to be an aqueous solution of pH-neutral (i.e., pH 7) rather than of strong acids or alkaline electrolyte, which limits ORR activity and MFC efficiency [3]. Thus, the development of ORR electrocatalyst that is highly active and selective under pH-neutral condition is of particular importance for MFC application. Despite tremendous efforts, developing an ORR catalyst with high activity and affordable cost remains a great challenge. Among the cathodic catalysts investigated to date, Pt-based catalyst has the best performance owing to its low overpotential [4], [5], but its high cost, limited supply and sensitivity to poisons have hindered its applications in MFC to recoup energy from wastewater. Thus, extensive efforts have been made in pursuit of economic replacements of Pt-based catalysts while maintaining comparable catalytic efficiency, especially with advanced carbon materials, such as carbon nanotubes and graphene [6]–[13] ([6] for a comprehensive review about graphene-based ORR catalysts). Graphene is a 2-D monolayer of sp2 -hybridized carbons and possesses unique electrical, chemical and mechanical properties. Recent studies have shown that graphene loaded with nonprecious metal catalysts or doped with electronegative elements exhibits high ORR catalytic activity in fuel cells [13]–[16]. However, most of the ORR performance had been demonstrated under alkaline conditions while pH-neutral requirement of MFC had been overlooked. Recently, Ag nanoparticles (AgNPs) have been found to possess high ORR activity, especially in common pH-neutral media of MFC [17]. Thus, it has been postulated that AgNPs when loaded onto graphene surface can function as a promising replacement of Pt-based catalysts under pH-neutral condition. In this paper, we loaded AgNPs onto reduced graphene oxide (rGO) by using a facile method of one-pot coreduction and examined the activity of the resultant rGO-supported AgNPs (AgNPs/rGO) toward ORR under pH-neutral condition. It has

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been found aqueous coreduction can yield high-quality AgNPs/ rGO catalyst and such a catalyst can generate voltage, power density, and sustainability comparable to those of commercial Pt/C. It has also been verified that AgNPs/rGO can selectively reduce oxygen to water via four-electron pathway. Since Ag costs less than Pt and is more resistant to poisons, it is believed that AgNPs/rGO has better potential to be applied to MFC in wastewater treatment. II. MATERIALS AND METHODS A. Preparation of Catalysts All chemical reagents used were of analytical grade. Homogenous solution of graphene oxide (GO) was prepared by Hummer’s method with KMnO4 as previously described [18]. Briefly, graphite was fully mixed with a solution of KMnO4 , H2 SO4 , H2 O2 , and K2 S2 O8 . The mixture was then treated with ultrasound and centrifuged, followed by repeated wash with deionized (DI) water. The resultant solution was dried to yield GO powder, followed by resuspension in DI water to obtain homogenous GO solution. To prepare graphene-supported silver nanoparticles (AgNPs/ rGO), 0.1 g of AgNO3 dissolved in DI water was added to 10 mL GO solution and the mixture was stirred for 12 h. Then 10 mL NaBH4 (20 mmol/L) solution was dropped into the mixture and stirred for 1 more hour to coreduce both AgNO3 and GO. The resultant AgNPs/rGO was then filtered out and washed with DI water for several times. The obtained catalyst was dried and kept at 4 °C for later use. B. Characterization of Morphology and Composition Crystal structures of graphene oxide and AgNPs/rGO were characterized by X-ray diffraction (XRD, Bruke D8 Adv, Germany) using CuK α source. The nanostructure and morphology of samples were observed with F-30ST transmission electron microscope (TEM, Tecnai, U.S.) and atomic force microscope (AFM, Ivecco, U.S.). The elemental compositions of samples were identified and analyzed by X-ray photoelectron spectroscope (XPS, PH1-5700, ESCA, U.S.). C. Characterization of Electrocatalytic Activity The ORR activity of graphene and AgNPs/rGO nanocatalyst in pH-neutral electrolyte was tested by a three-electrode system, consisting of the working electrode of glass carbon electrode, counter electrode of Pt, and reference electrode of Ag/AgCl. After dispersing the catalyst powder evenly on the surface of the glass carbon electrode, samples were fixed with 5% Nafion solution and dried for later measurements. Following the measurements, γ-Al2 O3 fine powders were used to polish the surface of the glass carbon electrode for reuse. The pHneutral electrolyte was phosphate-buffered solution (PBS), prepared with Na2 HPO4 , NaH2 PO4, and DI water. Cyclic voltammogram (CV) was measured with the voltage range of –0.8– 0.4 V and scan rate of 5.0 mV/s. The rotating disk electrode (RDE) measurement was performed on rotating-ring disk electrode (RRDE)-3A apparatus (BAS Inc., Japan) by stepwise vari-

Scheme 1.

The schematic representation of MFC setup.

ation of rotating speed (ω) from 400 to 3600 rpm and the RRDE measurement was tested at a rotating rate of 1600 rpm. The Pt-ring electrode potential was set at 0.4 V for the oxidation of hydrogen peroxide ions. D. MFC Setup and Measurements Double-chamber MFC, consisting of a biofilm-coated anode, an open-to-air cathode, and a proton exchange membrane (PEM, Nafion117, Dupont) was used in the experiments as shown in Scheme 1. The body of the anode and the cathode was made of a plexiglass tube with the length of 4.0 cm, crosssectional diameter of 3.0 cm, and total volume of 28.3 cm3 . Both the anode and cathode were covered with carbon cloth (Toray, E-Tek), especially with the carbon cloth on cathode facing air. The air diffusion electrode was made as previously described by Kim et al. [19]. In brief, 30% PTFE was coated on the carbon cloth of the cathode as a diffusion layer and then solidified at 400 °C for 20 min. When PTFE coating was dried, the powder of the catalyst was fixed to the other side of the cathode with 5% Nafion solution in ethanol to make the air diffusion cathode. The anaerobic sludge supplied by Kunming Third Wastewater Treatment Plant (Yunnan, China) was used as the inoculums for the anode of MFC at constant external resistance of 1000 Ω. Maturing of the biofilm on the anode was allowed over a period of at least two weeks, during which the culture was frequently replenished with the sludge. The culture media contained carbon (1.0 g/L glucose) and nitrogen (0.1 g/L NH4 Cl) sources. Na2 HPO4 and NaH2 PO4 were also added to maintain pH-neutral environment. The polarization and power density curves were evaluated by recording peak cell voltage yielded across various external resistances (100–10 000 Ω). The electrode potentials were measured versus SCE reference electrode. III. RESULTS AND DISCUSSION A. Morphology and Structure Characterization Three-dimensional morphology of graphene oxide (GO), the key intermediate before coreduction to form AgNPs/rGO, was characterized under AFM tapping mode. As shown in the 3-D and sectional images of GO [see Fig. 1(a) and (b)], GO

SUN et al.: GRAPHENE-SUPPORTED SILVER NANOPARTICLES FOR PH-NEUTRAL ELECTROCATALYTIC OXYGEN REDUCTION

Fig. 1. (a) Three-dimensional AFM image and (b) sectional AFM image of GO. The height difference between two red arrows in panel B is around 2 nm.

Fig. 2. (a) TEM image of AgNPs/rGO, (b) high-resolution TEM image of AgNPs, (c) size distribution of AgNPs, and (d) XRD profiles of GO and AgNPs/rGO.

has a few-layer laminar structure, with sectional height around 2 nm. Previous observation by other group has found the GO thickness of 1.376 nm corresponded to 1–2 layers of graphene oxide [20]. Thus, we speculate that our GO consists of 2–3 layers of graphene oxide. AgNPs/rGO was prepared by the coreduction of GO and AgNO3 with NaBH4 . Postcoreduction, TEM and XRD were conducted to reveal the morphology and structure of resultant AgNPs/rGO. As shown by TEM images [see Fig. 2(a)], AgNPs

Fig. 3.

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XPS spectra of AgNPs/rGO (a: C1s; b: O1s; c: Ag3d).

are relatively evenly dispersed on rGO surface, whereas the sizes of particles do vary probably due to unbalanced growth of Ag nuclei and aggregation of AgNPs. Thus, we profiled the size distribution of AgNPs using TEM and found the size of AgNPs had a relatively narrow distribution [see Fig. 2(c)] with the average particle size around 13 nm. The curve only shows half of a typical “bell-shaped” standard curve, since tiny particles below the detection limit is immeasurable. Moreover, most of AgNPs are below 20 nm in size with only a few big out-standers, which can be rationalized with previous findings that particles above 20 nm tend to form even larger aggregations [21]. Neumann et al. have recently demonstrated that AgNPs of 18 nm in diameter supported on glassy carbon possess ORR catalytic activity [22]. Thus, although it is difficult to control the particle size in our synthesis, it is still reasonable to assume most of the resultant AgNPs can function as ORR catalysts. Next we examined the structure of AgNPs/rGO by XRD. As shown in Fig. 2(d), the XRD profile of AgNPs/rGO exhibits typical peaks of Ag crystals at 2θ = 38.18◦ , 44.36°, 64.5°, and ˚ 77.46°, corresponding to d values of 2.36, 2.04, 1.44, and 1.23 A, which represent planes of (111), (200), (220), and (311), respectively and are in line with d values from JCPDS cards (No. 652871). High-resolution TEM image (see Fig. 2b) has also revealed the characteristic lattice parameter of 0.236 nm for the plane (111) of Ag. In comparison, GO has a characteristic diffraction peak at 2θ = 10◦ , which has disappeared in the profile of AgNPs/rGO, indicating that during the coreduction of Ag+ and GO, nearly all GO had been reduced to rGO. Thus, the XRD profiles have verified the formation of AgNPs/rGO. As shown in Fig. 3, the XPS spectra of AgNPs/rGO not only exhibited the characteristic peaks of each element in the catalyst, but also verified the presence of oxygen-containing functional

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Fig. 5. Polarization and power density curves of MFC with different cathode catalysts (a in black: Pt/C; b in red: AgNPs/rGO; c in green: AgNPs).

Fig. 4. CVs of three types of electrocatalysts in pH-neutral PBS at the scan rate of 5.0 mV/s (green: rGO; red: AgNPs; blue: AgNPs/rGO).

groups, such as −C = O, −COOH and −OH [see Fig. 3(a) and (b)], which can assist the formation of Ag nuclei and the growth of nanoparticles to a certain size. Consistently, two peaks are observed for Ag3d as shown in Fig. 3(c), with one peak at 368.37 eV corresponding to Ag3d3/2 and the other peak at 374.37 eV corresponding to Ag3d5/2 . It is noteworthy that both peaks have a little shift toward higher energy level compared to the typical Ag3d peaks of Ag element, suggesting the interaction between graphene and AgNPs leads to a decrease in the electron density of Ag atoms, probably due to the conjugation between the d orbit of Ag atom and π bond of graphene. The presence of oxygen-containing groups is in line with the assumption that the unreduced oxygen can function as nucleation sites of AgNPs. These results indicate that high-quality AgNPs can be loaded onto rGO by a facile one-pot coreduction. B. Electrocatalytic Activity of Catalysts The ORR catalytic activity of AgNPs/rGO in pH-neutral electrolyte was assessed by CVs with control groups at same loading conditions. As shown in Fig. 4, the CV curve of cyclic voltammetry shows good ORR activity within the scan range from −0.8 to +0.4 V (versus Ag/AgCl) setting a current window of 0.13 to −0.18 mA. In contrast, rGO alone has almost none of ORR catalytic activity, with a current window around 0.10 mA. Interestingly, AgNPs possess some ORR catalytic activity (current window from 0.05 to –0.05 mA) under the same condition, which is consistent with findings reported by other groups [23]. Previous studies have found that, the ORR catalytic activity of Ag is reversely correlated with the particle size and Ag in bulk is of low ORR catalytic activity [24], probably due to the large gap between the d orbit of Ag atom and the Fermi-level of lowdimensional crystal, which makes it difficult to break π bond of O2 . However, when the size of Ag particle is below a critical threshold of nanoscale, the quantum effect of nanoparticles and

the increased surface area will lead to a significant improvement of catalytic activity [16]. As aforementioned, the AgNPs obtained in this study have an average diameter of 13 nm and thus possess substantial ORR catalytic activity. More importantly, when AgNPs were loaded onto rGO surface, the ORR catalytic activity of resultant AgNPs/rGO had been significantly improved, probably due to the enriched polar functional groups on the rGO surface [see Fig. 3(b)], making it a good platform to form relatively small AgNPs. In addition, rGO has a large specific surface area and thus has a better loading capacity of AgNPs than common supporting materials. We assume that the combined effect of these changes is the most likely reason to account for the enhanced ORR catalytic activity of AgNPs on rGO. Then we compared the power generation of our AgNPs/rGO catalyst to that of commercial Pt/C (20% wt) under pH-neutral condition. As shown in Fig. 5, AgNPs/rGO and Pt/C yield nearly identical open-circuit voltage, both around 0.78 V. AgNPs/rGO can also produce a maximum power density of 474.5 mW/m2 , which is close to 482.6 mW/m2 for Pt/C. In contrast, although AgNPs has some level of ORR catalytic activity, it can only generate much lower open-circuit voltage (0.61 V) and maximum power density (186.7 mW/m2 ). These observations are consistent with above findings that graphene loading can significantly improve the ORR catalytic activity of AgNPs. Birry et al. have compared iron phthalocyanine (FePc) with commercial Pt/C in MFC under pH-neutral condition and found FePc delivered around 70% power generation of Pt/C in terms of open-circuit voltage and maximum power density [25]. Although an appleto-apple comparison is almost impossible due to the difference in catalyst load, active surface area and cathode configurations, it would be reasonably fair to infer that AgNPs/rGO can outperform FePc, one of the best replacements of Pt/C for pH-neutral MFCs. Yet a direct comparison with FePc and other alternatives of Pt/C still need further investigation. One important application of ORR catalyst is the cathode of MFC. Therefore, we evaluated the sustainability of our

SUN et al.: GRAPHENE-SUPPORTED SILVER NANOPARTICLES FOR PH-NEUTRAL ELECTROCATALYTIC OXYGEN REDUCTION

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Fig. 6. Current production of MFC using different cathode catalysts under glucose concentration of 1.0 g/L and external resistance of 1000 Ω (green: Pt/C; red: AgNPs/rGO; black: AgNPs).

AgNPs/rGO within the setting of pH-neutral MFC, by refreshing anode substrate (1.0 g/L glucose solution) for three cycles. As shown in Fig. 6, although AgNPs/rGO generates a little lower current than commercial Pt/C in the first cycle (0.45 versus 0.49 mA), it can sustain current generation comparable to that of Pt/C in the following two cycles, producing almost same amount of current, both around 0.48 mA. Meanwhile, the current generated by AgNPs was markedly lower at 0.33 mA. Thus within the horizon of our examination, AgNPs/rGO catalyst has demonstrated stability almost same as that of commercial Pt/C. In order to gain further insights about the ORR catalytic activity of AgNPs/rGO, RDE was utilized to verify the ORR selectivity of different catalyst at different rotating speeds, as displayed in Fig. 7. In consistency with previous findings, rGO alone yields little current, less than 20 μA, even at the rotating speed of 4000 rpm and −0.8 V. Interestingly, although AgNPs/rGO and AgNPs alone generate comparable current under 450 rpm and −0.7 V (63 and 60 μA, respectively), AgNPs/rGO can afford much higher current than AgNPs alone at 4000 rpm and −0.7 V (96 and 66 μA, respectively). Such a dramatic difference can be ascribed to the good conductivity of graphene, which can enhance electron transfer from cathode to AgNPs and thus uplift the limited rate set by electron transfer to achieve a higher current. On the contrary, without graphene, electron transfer would become rate-limiting at high rotating speed. Kinetic parameters were determined for different catalysts with widely accepted Koutecky–Levich equations [26]: 1 1 1 = + i iK iL iL = 0.62bFAD2/3 ω 1/2 ν −1/6 CO 2

(1) (2)

where i is the measured disk current, iK is the kinetic current, iL is the limiting diffusion current, b is the overall number of transferred electrons, F is the Faraday constant (96 485 C/mol), A is the area of glass carbon electrode, D is the oxygen diffusion coefficient (1.7 × 10−5 cm2 /s), ω is the electrode rotation rate, υ is the kinematic viscosity (0.01 cm2 /s), and CO2 is the oxygen

Fig. 7. RDE measurements and Koutecky–Levich plots for three electrocatalysts under pH-neutral condition (a: rGO; b: AgNPs; c: AgNPs/rGO). Insets are Koutecky–Levich plots obtained from corresponding RDE curves.

concentration in solution (2.2 × 10−7 mol/L). The insets of Fig. 7 display the corresponding Koutecky–Levich plots (i−1 versus ω −1/2 ) and the overall numbers of transferred electrons have been estimated from the slopes. Unsurprisingly rGO alone has low ORR activity and can only gain two electrons for O2 [see Fig. 7(a)], reducing O2 to H2 O2 , while the overall number of transferred electrons for AgNPs is around 3.1 [see Fig. 7(b)], between 2.0 and 4.0, implying that O2 can be reduced on AgNPs surface via both two-electron and four-electron mechanisms to yield a mixture of H2 O2 and H2 O. As expected, the overall

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number of transferred electrons for AgNPs/rGO is around 3.8 [see Fig. 7(c)], corresponding to a four-electron reduction of O2 to H2 O. These observations are in line with CV results and have demonstrated that AgNPs/rGO can efficiently catalyze fourelectron ORR under pH-neutral condition.

IV. CONCLUSION In conclusion, high-quality AgNPs/rGO catalyst can be prepared via a facile aqueous coreduction method, which has good micromorphology and relatively uniform distribution of particle size as revealed by TEM and XRD. The XPS results have not only verified the elemental composition of the catalyst, but also offered insights on the formation of AgNPs on rGO. The CV scan has demonstrated that AgNPs/rGO possesses high ORR catalytic activity in pH-neutral PBS, which has been further verified by RDE measurement, showing AgNPs/rGO can selectively reduce oxygen to water, through four-electron pathway. Moreover, in a typical double-chamber MFC, AgNPs/rGO can generate voltage, power density, and stability comparable to those of commercial Pt/C. In future studies, it is conceivable that functionalization of rGO nanosheet with specific polar groups, particularly in certain pattern, would help to control the growth of AgNPs on rGO surface and render better tunability of particle size, which would lead to the improvement of ORR catalytic activity for our AgNPs/rGO catalyst. Since Ag costs less than Pt and is more resistant to poisons, AgNPs/rGO has the potential to maintain the performance of current MFC using Pt/C, while reducing the cost, and thus to justify its application in wastewater treatment and to shift such an energy-consuming process to an energy-neutral or energygaining one in light of future advance in biology, electrochemistry, and nanotechnology areas.

ACKNOWLEDGMENT The authors would like to thank Prof. Y. Li of Peking University for her generous support and kind suggestions for this paper.

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