SnO2

Research Article www.acsami.org

Microwave-Assisted Synthesis of Reduced Graphene Oxide/SnO2 Nanocomposite for Oxygen Reduction Reaction in Microbial Fuel Cells Nadia Garino,*,† Adriano Sacco,† Micaela Castellino,† José Alejandro Muñoz-Tabares,† Angelica Chiodoni,† Valeria Agostino,†,‡ Valentina Margaria,† Matteo Gerosa,†,‡ Giulia Massaglia,†,‡ and Marzia Quaglio† †

Center for Space Human Robotics @Polito, Istituto Italiano di Tecnologia, Corso Trento 21, 10129 Torino, Italy Applied Science and Technology Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

‡

S Supporting Information *

ABSTRACT: We report on an easy, fast, eco-friendly, and reliable method for the synthesis of reduced graphene oxide/SnO2 nanocomposite as cathode material for application in microbial fuel cells (MFCs). The material was prepared starting from graphene oxide that has been reduced to graphene during the hydrothermal synthesis of the nanocomposite, carried out in a microwave system. Structural and morphological characterizations evidenced the formation of nanocomposite sheets, with SnO2 crystals of few nanometers integrated in the graphene matrix. Physico-chemical analysis revealed the formation of SnO2 nanoparticles, as well as the functionalization of the graphene by the presence of nitrogen atoms. Electrochemical characterizations put in evidence the ability of such composite to exploit a cocatalysis mechanism for the oxygen reduction reaction, provided by the presence of both SnO2 and nitrogen. In addition, the novel composite catalyst was successfully employed as cathode in seawater-based MFCs, giving electrical performances comparable to those of reference devices employing Pt as catalyst. KEYWORDS: reduced graphene oxide, tin oxide, oxygen reduction reaction, microbial fuel cells, microwave

1. INTRODUCTION

acceptor, receiving electrons from the anode and combining them with protons.7 Nowadays, the standard catalyst for speeding-up the ORR is platinum, supported on different typologies of carbon substrates with high surface areas.8 However, great research effort is currently focused on the study of other high performing catalysts alternative to platinum, characterized by a lower cost. In this field, nonprecious metal oxides, such as manganese oxides,9 attract particular attention because they are inexpensive, earth-abundant, environmentally friendly, and have been shown to give electrocatalytic activities for the fourelectrons reduction of oxygen. In addition, carbon-based electrodes, used as common supports for the ORR catalysts, have been also identified as materials with an intrinsic activity for the electro-reduction of oxygen to peroxide.10,11 Since its discovery in 2004,12 graphene gained a key role among carbon-based materials in the field of energy production, conversion and storage.13−15 Because of its excellent electric, mechanic, and electrochemical properties, graphene is a

Microbial fuel cells (MFCs) are bioelectrochemical systems that convert chemical energy directly into electrical energy from the respiratory metabolic profit of electrochemically active bacteria.1,2 MFCs are a versatile emerging technology, offering a broad range of biotechnological applications.3 In particular, the greatest potential of MFCs lies on the energy production from biomass and on the use of wastewater as fuel,4 which allows combining wastewater treatment and energy recovery.5 In recent years, MFCs gained enormous interest because they can produce renewable energy in a sustainable way.6 However, there are still some efficiency limits that do not allow a MFC widespread. Rapid advances in MFC technology focused the attention on the need to find alternative electrodic materials and catalysts in order to obtain better performances, long-term resistance, lower costs, and increased eco-friendliness. In the detail of an open-air cathode MFC, electrogenically active bacteria operate in the anodic compartment, oxidizing organic compounds with the production of electrons and protons. The anode electrode acts as solid electrons acceptor and transfers them to the external circuit, whereas protons are transferred to the cathode through the electrolyte. At the cathode, the oxygen reduction reaction (ORR) occurs with oxygen as the electron © 2016 American Chemical Society

Received: November 19, 2015 Accepted: January 26, 2016 Published: January 26, 2016 4633

DOI: 10.1021/acsami.5b11198 ACS Appl. Mater. Interfaces 2016, 8, 4633−4643

Research Article

ACS Applied Materials & Interfaces

Then, a stainless-steel disk, used as support, was coated with the obtained solution to form a uniform film. The resulting deposition was dried at room temperature for 24 h. XRD measurements were performed with Bragg−Brentano symmetric geometry in a PANalytical X’Pert Pro instrument using Cu Kα (40 kV and 40 mA) radiation. Continuous scan mode was used to collect XRD data in a range of 10° to 74° and 0.02° step size. Rietveld analysis was performed in order to analyze each spectrum by using Topas Academic software (4.1). Convolution-based method was used where the source emission profiles with full axial instrument contributions was modeled,26 whereas background was fitted with a Chebyshev polynomial function with eight parameters. Average crystallite size was assumed to be isotropic in all cases and modeled by applying the integral breadthbased method, whereas lattice strain, which was assumed to be anisotropic, was modeled by considering symmetrized spherical harmonics expansion. Field emission scanning electron microscopy (FESEM, ZEISS, Merlin) and energy dispersive X-ray spectroscopy (EDS, OXFORD Xact) were used to evaluate the general quality, the composition, and the morphology of the different composite materials. The samples were prepared by dispersing the composite on top of a carbon tape. Samples for transmission electron microscopy (TEM) were prepared by suspending the obtained nanocomposite powder in ethanol and by immersing the vessel containing the dispersion in ultrasonic bath. A suspension droplet was then drawn and applied to a standard holey carbon TEM Cu grid, analyzing the specimen after the complete evaporation of the solvent. TEM observations were performed with a FEI Tecnai F20ST equipped with a field emission gun (FEG) operating at 200 kV. Specific surface areas (SSAs) were determined using the Brunauer− Emmet−Teller (BET) method27 on a Quadrasorb evo (Quantachrome Instruments). Prior to adsorption, 50 mg of powder was placed in the cell and evacuated overnight at about 70 °C. Raman measurements were performed by a Renishaw InVia Reflex micro-Raman spectrometer (Renishaw plc, Wotton-under-Edge, UK) equipped with a cooled CCD camera, exploiting a 514.5 nm laser excitation, through a microscope objective (50×), in backscattering light collection, providing a photon flux of 1 MW/cm2. The integration time was 10 s. A PHI 5000 Versaprobe scanning X-ray photoelectron spectrometer (monochromatic Al Kα X-ray source with 1486.6 eV energy), was used to investigate the material chemical composition. A spot size of 100 μm was used in order to collect the photoelectron signal for both the high resolution (HR) and the survey spectra. Different pass energy values were exploited: 187.85 eV for survey spectra and 23.5 eV for HR peaks. All samples were analyzed with a combined electron and argon ion gun neutralizer system, in order to reduce the charging effect during the measurements. The semiquantitative atomic compositions and deconvolution procedures were obtained using Multipak 9.6 dedicated software. All core-level peak energies were referenced to C 1s peak at 284.5 eV (C−C/C−H sp2 bonds) and the background contribution in HR scans was subtracted by means of a Shirley function. 2.3. Electrochemical Characterization. All the electrochemical measurements were carried out through a CH Instrument 760D electrochemical workstation and an ALS RRDE-3A rotating ring disk electrode apparatus. For all the measurement, the catalyst was deposited on the working electrode (a BioLogic glassy carbon disk/ Pt ring, area 0.13 cm2) through the following method. Before the catalyst deposition, the working electrode was properly polished with ethanol. The catalyst (2 mg) was dispersed in a solution containing 25 μL of water, 175 μL of 5% Nafion solution, and 100 μL of isopropyl alcohol. The mixture was ultrasonicated for 2 min to form a uniform black dispersion. 10 μL of this formulation was cast-coated onto the disk surface to form a uniform film. The resulting deposition was dried at room temperature for 1 day. For comparison purpose, commercial Pt/C (Sigma-Aldrich) was used as a reference catalyst. The deposition procedure is the same as described above. The final catalyst loading was 0.5 mg/cm2 for both rGO/SnO2- and Pt-based materials.28

versatile functional and structural material to design well performing devices. In addition, its uncommon properties can be exploited to prepare graphene-based composites with high surface/volume ratio.16,17 In this way, by combining nontoxic metal oxide in a graphene structure, new nanocomposite materials for new generation MFC cathode electrodes can be designed to have good efficiency for the ORR. Generally, to synthesize and obtain this kind of composites, complicated and time-consuming processes are necessary, like a traditional hydrothermal route.18,19 However, many recent research works report on the exploitation of different techniques that allow shorter reaction time and lower energy consumption, like the microwave-assisted route.20−22 The main characteristic of this technique is to guarantee a uniform heating of the precursors and to present outstanding reaction rates, if compared to other preparation routes. Indeed, with microwave irradiation it is possible not only to decrease drastically the reaction times but also to reduce the graphene oxide in a single step, having, at the same time, a proper and homogeneous functionalization of the graphene sheets. In fact, during the microwave irradiation, metal oxide nanoparticles (NPs) directly crystallize on the surface of graphene, and neither additional treatments nor toxic chemicals are needed for the synthesis. In this work, we present a fast, one-step, green, and microwave-assisted hydrothermal synthesis of reduced graphene oxide (rGO)/SnO2 nanocomposites. Similar composite materials have been investigated for various applications such as solar cells, batteries, electrochemical sensors or biosensors23,24 and as anode material for MFC,25 due to their good biocompatibility. Here we propose the application of rGO/ SnO2 as a catalyst for the ORR, showing a good electrochemical behavior, thus indicating to be a promising candidate as electrode material for cathode application in MFCs. To the best of our knowledge, this is the first paper reporting the employment of such composite material as ORR catalyst. Different composites with various Sn concentrations were prepared, and their physical, chemical, and electrochemical properties were evaluated through different techniques. Finally, the performances of MFCs based on an rGO/SnO2 cathode were compared with those of reference devices based on a standard Pt cathode.

2. EXPERIMENTAL SECTION 2.1. Nanocomposite Synthesis. The general synthesis procedure was obtained by adapting and modifying the process proposed by ElDeen et al.20 All the chemicals were used as purchased without further purification. In a typical reaction, 50 mg of GO (Cheap Tubes Inc., USA) was added to 30 mL of double distilled water, with 20 mg of urea (Sigma-Aldrich). Then, a specific amount (12%, 25% or 40%, corresponding to sample B, C, or D, respectively) of SnCl2·2H2O (Sigma-Aldrich) was added and dissolved in the as-prepared mixture. A reference rGO sample (i.e., without SnO2 NPs, denominated as sample A) was synthesized by the same procedure in a SnCl2·2H2Ofree solution. For all the samples, the precursor mixtures were sonicated for 40 min and the resultant slurries were then transferred in a 100 mL Teflon reactor, equipped with pressure and temperature probes, connected with the microwave furnace (Milestone STARTSynth, Milestone Inc., Shelton, Connecticut). The mixtures were irradiated for 15 min at 180 °C (800 W) and then the reactor was cooled to ambient temperature. The resultant product was collected in a small vessel, frozen with liquid nitrogen and freeze-dried until all the water was removed. 2.2. Physical and Chemical Characterization. Samples for Xray diffraction (XRD) were prepared by suspending the nanocomposite in ethanol and ultrasonicated to form a uniform dispersion. 4634


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L; Na−acetate 2.50 g/L; Wolfe’s vitamin solution 10.00 mL/L (ATCC) and Wolfe’s trace mineral solution 10.00 mL/L (ATCC). The medium was bubbled with 100% N2 to maintain anoxic condition. The culture was subjected to 6 sequential enrichments for 18 days of total growing and each subculture was obtained by transferring 10 mL of inoculum into 90 mL of fresh medium and was incubated at room temperature and 160 rpm orbital shaking. 2.6. MFC Operation and Characterization. The MFC reactors were inoculated with the sixth subculture in a concentration of 10% v/ v with respect to the total chamber volume. MFC operation medium consisted in 50 mM phosphate buffer solution (PBS) added with 1 g/ L of Na−acetate (as organic electron donor) and 1.25 g/L of bacteriological peptone (as nitrogen source).33 Dissolved oxygen was removed from the anolyte by bubbling the medium with nitrogen. Tests were conducted in duplicate in feed batch mode for 3 cycles of 5 days each. The performances of the MFCs were evaluated through the linear sweep voltammetry (LSV) technique using a Bio-Logic VSP potentiostat. The potential was ranged between open circuit and short circuit, and the sweep rate was 1 mV/s. All the tests were conducted at ambient temperature (23 ± 2 °C) and pressure (1 atm).

Pt was used as counter electrode and Ag/AgCl as reference electrode. The electrolyte was 0.1 M KOH O2-saturated aqueous solution (unless otherwise specified), and the experiments were conducted at room temperature. All the potentials are always reported with respect to Ag/AgCl. Cyclic voltammetry (CV) measurements were carried out in O2saturated and N2-saturated solutions from +0.2 to −0.8 V, with a scanrate of 10 mV/s. For the rotating disk electrode (RDE) measurements, the working electrode was scanned from +0.2 to −0.8 V, with a rate of 5 mV/s and different rotation speeds in the 625−2500 rpm range. Koutecky− Levich plots were analyzed to calculate the electron transfer number n through the following equation:29

1 1 1 = + J JK 0.62nFCO2DO2 2/3υ−1/6ω1/2

(1)

where J is the measured current density, JK is the kinetic current density, F is the Faraday constant, CO2 is the oxygen bulk concentration, DO2 is the oxygen diffusion coefficient, υ is the kinematic viscosity of the electrolyte, and ω is the electrode rotation speed. For the rotating ring disk electrode (RRDE) measurements, the disk electrode was scanned from +0.2 to −0.8 V, with a rate of 5 mV/s and a fixed rotation speed of 2500 rpm, while the ring electrode was maintained at a fixed potential of 0.2 V. The percentage of HO2− and the electron transfer number were calculated from the following equations:30 HO2−% = 200 ×

n=4×

IR /N ID + IR /N

ID ID + IR /N

3. RESULTS AND DISCUSSION 3.1. Physical and Chemical Characterization. 3.1.1. XRD Analysis. Figure 1 shows the XRD spectra obtained

(2)

(3)

where IR and ID are the ring and disk currents, respectively, and N is the current collection efficiency of the Pt ring. Electrochemical impedance spectroscopy (EIS) measurements were carried out at 0, −0.3, and −0.6 V potentials and 2500 rpm rotation speed, with a small signal of 10 mV and frequency range 10−2−104 Hz. Chrono-amperometry (CA) tests were conducted at fixed −0.3 V potential and 2500 rpm rotation speed. 2.4. MFC Fabrication and Assembly. In this work squared single-chamber MFC devices with an open-air cathode configuration were used. Devices were fabricated using a Benchman 4000 computer numerical control (Light Machines Corp) and were composed of three main sections: the anodic compartment, the reaction chamber itself, and the cathodic compartment. The area of both anodic and cathodic electrodes was 9 cm2, whereas that of the intermediate region was 6.25 cm2, allowing a total volume of the cell of around 15 mL. The cathodes were built starting from Commercial Carbon Felt (Soft felt SIGRATHERM GFA 5, SGL Carbon, Germany). To guarantee the diffusion of the oxygen through the electrode into the cathodic chamber, a layer of polytetrafluoroethylene (PTFE)31 was applied on one side of the carbon felt. The catalytic materials were deposited onto stainless-steel grids using a Nafion/isopropyl alcohol mixture to ensure the adhesion, thereafter the grid was put in contact with the side of electrode not covered by the PTFE layers, and they were pressed together during the assembly of the device. To ensure the electric contact, stainless-steel wires were inserted into the cathode. The anodic electrode was made of commercial carbon felt, and the electric contact was based on a stainless-steel screw. Details on the cell architecture are reported in the Supporting Information (SI), together with a sketch of the device (Figure S1). 2.5. Inoculum Enrichment. A microorganisms’ biofilm was grown on the anodic electrode starting from a seawater inoculum. The sample was taken from the Ligurian Sea and enriched with a chemical method,32 in order to improve the percentage of anode-respiring bacteria. Enrichment medium consisted in Fe(III) citrate 13.70 g/L; NaHCO3 2.50 g/L; NH4Cl 1.50 g/L; NaH2PO4 0.60 g/L; KCl 0.10 g/

Figure 1. X-ray diffraction patterns of rGO/SnO2 nanocomposites: sample A is bare rGO, whereas samples B, C, and D are rGO/SnO2 prepared with 12%, 25%, and 40% of Sn precursor, respectively.

from reference sample (A) and from samples B, C, and D. An additional spectrum from the stainless-steel support without sample (not shown here) was used to model the spectra background and the instrument contribution. Such models were then applied in the subsequent refinements. For sample A, strong reflections from metallic stainless-steel support (cubic, space group 225) are found at 43.5 and 50.6° (2θ) in addition to a broad peak at 24.7° (2θ) corresponding to an interplanar distance of 0.36 nm. The above-mentioned broad peaks can be attributed unambiguously to rGO, because the interplanar distance is shorter than the basal spacing ((002) plane) resulting from the graphite oxidation (0.75 nm, due to the 4635


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Figure 2. (a−d) FESEM micrographs of samples A, B, C, and D, respectively (sample A is bare rGO, whereas samples B, C, and D are rGO/SnO2 prepared with 12%, 25%, and 40% of Sn precursor, respectively). In the insets, a magnification is reported for sake of comparison.

Figure 3. High resolution TEM and FFT of a single crystallite of sample C (left) and selected area electron diffraction pattern of sample C (right).

was found regarding phase quantification, where SnO2 wt % contents were found to be 11.0 ± 0.2%, 22.3 ± 0.4%, and 38.0 ± 0.7% that correspond to a final Sn at. % concentration of 1.07%, 2.43%, and 4.94%, respectively. Finally, the spectra obtained from all samples with SnO2 showed a sharp peak at 32.7° (2θ), as well as other negligible peaks attributed to residual compounds from the Sn precursor that could not be identified. The full list of structural parameters obtained from refinement is reported in Table S1 in the SI. 3.1.2. Morphological Analysis. In Figure 2, the field emission scanning electron microscopy characterizations of the reference sample as well as of the samples B, C, and D are reported. Figure 2a shows the morphology of the reference

introduction of oxygen-containing groups on the basal plane), and larger than the spacing in densely packed graphene layers of natural graphite (0.33 nm).34 Concerning the XRD spectra from samples B, C, and D, the strong reflections from the substrate are still present as well as the broad peak of rGO. However, in these spectra it is possible to observe the appearance of broad peaks at 26.6°, 33.7°, and 50.8° (2θ) that correspond to the (110), (011), and (211) planes for the SnO2 (Tetragonal, space group 136). The grain size calculated from Rietveld refinement were 3.18 ± 0.01, 2.97 ± 0.07, and 3.24 ± 0.61 nm for samples B, C, and D, respectively. This indicates that the initial Sn precursor concentration does not have influence in the final crystal size. However, the opposite 4636


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one at 1350 cm−1 (peak D), a second one at 1520 cm−1 that refers to the amorphous contribution (peak A), and the third one at 1585 cm−1 for the graphitic part (peak G).38 The background was subtracted using a linear function. I(D)/I(G) ratios were calculated for all the samples, obtaining: 0.84 for the GO, 0.91 for the rGO and 1.03 for the rGO/SnO2 composite. As the I(D)/I(G) ratio is a measure of the ordering of the sp2 phase, the I(D)/I(G) ratio increase and the shift toward higher frequencies of the G peak position (from 1587 up to 1590 cm−1) demonstrate an increase of the sp2 phase. However, the graphitization is not completed or crystal relaxation is partial, as evidenced from the broadening of D and G bands. Further information on the chemical composition of the produced material and proof of the reduction of GO into rGO were provided by XPS analysis. Figure S4a of the SI shows the survey spectra collected for each sample: apart from C, O, and N peaks, the presence of Sn is clearly evident in samples B, C, and D with different intensities, due to the different Sn precursor loading. The following relative atomic concentrations: 1.2 at. % for B, 2.2 at. % for C, and 3.9 at. % for D were obtained, which are in agreement with those found by XRD. Figure 4a reports the high resolution spectra of the C 1s region for all the rGO samples. By comparing the four deconvoluted spectra, it can be clearly seen that contributions to the C 1s peak by carbon−oxygen bonds are dramatically reduced after the hydrothermal process. In fact the C(C,H) sp 2 contribution, which is attributed to peak I, represents the larger component of the HR spectrum for each sample. Curves II and III are assigned to −C(O,N) and −CO bonds, respectively.39 Moreover, there is another component, peak IV (also shown at larger magnification in the inset of Figure 4a), called π−π* (HOMO−LUMO) transition, that is a characteristic shakeup line (satellite peak located at ∼6 eV from main C 1s peak)40 for carbon in aromatic compounds (e.g., aromatic rings), coming from the ring excited by the exiting photoelectrons. This feature is the fingerprint of extended delocalized electrons in the material and it is present in the rGO C 1s peak, whereas it is usually absent in the GO one.41,42 Figure 4b shows the N 1s HR peak, whose presence is due to the urea used in the reduction process. It is made up by the superposition of a higher peak at lower binding energy (399.1−399.6 eV) and a lower peak at higher chemical shift (400.9−401.7 eV). The first component can be assigned to N atoms implanted in the graphene lattice as pyrrolic-like nitrogen, sp3 coordinated, whereas the second one is due to a quaternary or graphitic configuration, sp2 coordinated.43,44 Figure S4b shows the HR spectra of the Sn 3d doublet, which can be assigned to the SnO2 species according to Suzer et al.45 3.2. Electrochemical Characterization. The performances of the rGO/SnO2 nanocomposite as a catalyst for the ORR were evaluated through different electrochemical techniques. The results of the CV measurements are reported in Figure 5a. By observing the curves related to the bare rGO sample, the presence of a cathodic peak (centered at −0.39 V) in O2saturated solution and the simultaneous absence of any peaks in N2-saturated solution, confirms that the ORR is catalyzed even if SnO2 is not present. This feature has to be attributed to the above-described functionalization of the graphene sheets by means of the graphitic-like nitrogen atoms included in the graphene lattice, hybridized in a sp2 network, whose effectiveness in the catalysis of ORR is well-known in the literature.46,47 Concerning the behavior of the rGO/SnO2

sample, which is composed by the characteristic rGO flakes, without evident structural damaging. In Figure 2b−d, details of the surface of the composite as a function of the Sn initial loading are reported. All the images show, as the reference sample, flakes without evident structural damage, confirming the goodness of the synthesis procedure, with a minimal modification of the initial rGO morphology. In addition, it is possible to notice the presence of small particles, attributed to tin oxide crystals, uniformly distributed and perfectly integrated in the flake matrix. By comparing the insets of Figure 2b−d, it is possible to notice an increase of nanocrystals concentration while increasing the initial Sn precursor loading, in agreement with XRD spectra analysis. In addition, the EDS spectrum of sample C (reported in the SI as an example, Figure S2) evidences not only the presence of C, O, and Sn, as expected, but also of N. This finding, better discussed by the XPS analyses below, opens new possibilities for the understanding of the composite as catalyst. Sample C was the only one selected and analyzed by high resolution TEM, as reported in Figure 3, since SnO2 size is not affected by the initial Sn loading, as already put in evidence by XRD and FESEM, whereas the three samples differ for the number of nanocrystals dispersed in the rGO matrix. In this image it is possible to observe an important number of nanocrystals homogeneously dispersed in the rGO matrix. In the same figure, a high magnification image of one of these nanocrystals and the relative fast Fourier transform (FFT), indexed as SnO2 in [111] axis zone, are reported. The size of the nanocrystals, as observed by HRTEM, ranged between ∼3 and 5 nm, confirming the structural analyses by XRD. In addition, in Figure 3 the selected area electron diffraction pattern of the same sample, collected by considering an area of ∼20 μm3 on a rGO flake, is reported. This pattern presents two superimposed and well-defined type of diffractions. The first is a rings diffraction pattern, which corresponds to the randomly oriented SnO2 nanocrystals, according to the interplanar distances calculated from them and which were in agreement with XRD analysis (see labels on Figure 3). The second is a diffraction pattern composed by bright spots with an hexagonal arrangement, that was identified as rGO in [001] axis zone (with the observation axis perpendicular to the basal plane of the hexagonal rGO cell). In this pattern, it is significant to note that the spots do not have a perfect round shape but present an arc shape, which could be interpreted as the stacking of few rGO layers (no more that 3−4 as it is extracted from XRD parameters, Rietveld analysis in Table S1) slightly misoriented one with respect to the others. Moreover, the contrast observed by both SEM and TEM shows clearly that the restacking of the rGO sheets is not present. These structural and morphological characterizations provide important information about the composite and confirm, as for similar microwave-assisted routes,35 that the synthesis does not affect the structure of the material, providing rGO with a uniform distribution of SnO2 nanocrystals. In addition, BET analysis (data not shown) confirmed elevated SSA values, typical of exfoliated GO-based materials.36,37 In particular, values equal to 400.2 and 353.5 m2/g were obtained for rGO (sample A) and rGO/SnO2 composite (sample B), respectively. 3.1.3. Raman and XPS Analysis. Raman spectra reported in Figure S3 of the SI show the G and D peaks region for the pristine GO, rGO (sample A), and rGO/SnO2 (sample B). The fitting procedure was done considering three peaks: the first 4637


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nanocomposite, the voltammograms of sample B are reported in Figure 5a (similar curves were obtained for the other samples). By comparing the results with those obtained for bare rGO sample, a higher current was measured for the composite, and it is worthy to note that the cathodic peak is characterized by a more positive potential (−0.24 V). This means that the synergistic effect of the N-functionalized graphene and the SnO2 NPs brings to a lower overpotential for ORR and to an enhanced reduction capability, evidencing the properties of cocatalysis of the composite material.16 ORR measurements were carried out by performing an LSV for different electrode rotation speeds. In Figure S5a of the SI, the results of this characterization for the rGO/SnO2 (sample B) catalyst are reported as an example. The typical curves observable in this kind of graph can be appreciated in this figure, with diffusion-limiting current values which become larger while increasing the rotation speed ω, due to the reduction of the diffusion distance at higher speeds.29 By fixing a potential value, and reporting the inverse of the current density J as a function of ω−1/2, the so-called Koutecky−Levich plots can be obtained.48 In Figure S5b of the SI, the data related to sample B are reported for three different potential values. The linearity of the curves and their parallelism evidence a first order reaction kinetics as a function of the dissolved oxygen.16 By exploiting eq 1, it is possible to calculate the electron transfer number n from the slope of the Koutecky−Levich plots. As reported in Figure S5b, n values for sample B are around 3.80, with a slight dependency on the potential. Such high values demonstrate a dominant four-electrons ORR, similar to the case of the reference Pt/C sample (around 3.94, data not shown) and to the theoretical one (dashed line in Figure S5b). The LSVs of all the samples measured at 2500 rpm are compared in Figure 5b. As it is clearly evident, a dependency of the current density on the amount of SnO2 NPs in the composite is exhibited by the analyzed catalysts. In particular, a nonmonotonic trend was found, with diffusionlimiting current values that increase for low SnO2 amount, and then reduce for larger NPs content. In accordance with this result, the electron transfer numbers at −0.65 V were calculated from the Koutecky−Levich plots related to each catalyst, and these values are presented in the inset of Figure 5b. Interestingly, the bare nitrogen-functionalized rGO exhibits an electron transfer number of 3.38, which indicates a

Figure 4. (a) XPS C 1s high resolution spectra of rGO/SnO2 composites with deconvolution peaks showing four components for each spectrum: curve I for C(C,H) bond, II for C(O,N), III for −CO, and IV for π−π* satellite transitions. In the inset, the high magnification of peaks IV is shown. (b) N 1s high resolution spectra of rGO/SnO2 composites with deconvolution peaks showing two components for each spectrum: curve I for pyrrolic-like nitrogen and II for graphitic-like nitrogen. In all the graphs, sample A is bare rGO, whereas samples B, C, and D are rGO/SnO2 prepared with 12%, 25%, and 40% of Sn precursor, respectively.

Figure 5. (a) Cyclic voltammograms of rGO (sample A) and rGO/SnO2 (sample B) catalysts in O2-saturated and N2-saturated solutions. (b) ORR polarization curves of the different samples at 2500 rpm rotation speed (sample A is bare rGO, whereas samples B, C, and D are rGO/SnO2 prepared with 12%, 25%, and 40% of Sn precursor, respectively); in the inset the electron transfer numbers at −0.65 V evaluated from Koutecky− Levich plots of the different samples is shown. 4638


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the samples. The produced HO2− species were found to be as low as about 10%, whereas n values resulted to be in the range 3.7 and 3.8, in good agreement with the RDE analysis reported above; in both cases, the curves exhibit a very small dependency on the potential. In contrast, the bare rGO catalyst and the composite prepared with high SnO2 content, are characterized by larger peroxide percentages (as high as 50% for samples A and D at −0.3 V), and correspondently lower electron transfer number. These data confirm that a mixed two/four electrons ORR pathway is exhibited by the proposed nanocomposite material when a nonoptimal amount of SnO2 NPs is present on the rGO surface. To investigate the dependence of catalytic properties of rGO/SnO2 nanocomposite on the amount of SnO2 NPs, EIS measurements were carried out. The Bode plots (modulus and phase vs frequency) of the impedance of all the samples measured at 0 V are reported in Figure S7 of the SI. In the phase plots (and correspondently in the modulus ones), three main features (each associated with a different process) can be recognized: a high frequency (HF) peak (and a corresponding change in the modulus slope) in the kHz range, related to the charge diffusion limitation inside the material;49 a middle frequency (MF) peak (in the Hz range) accounting for the interface between the catalyst material and the liquid electrolyte;50 and a low-frequency (LF) peak (below 1 Hz) related to the diffusion of species in the solution.50 By looking at Figure S7, the curves related to the bare rGO sample exhibit a HF process that is slower (the phase peak is left-shifted with respect to those of the other samples) and less effective (the modulus values are larger); on the contrary, no difference can be appreciated for the SnO2-containing samples. Regarding the LF process, instead, the phase peaks of the samples with null or low SnO2 amount are characterized by a faster diffusion, compared to those of the other two samples. Interestingly, the total impedance (i.e., the value of the modulus at the lowest frequency) related to the bare rGO sample is quite similar to those of high SnO2-laden samples, further confirming that upon reaching the optimal NPs concentration (sample B, whose total impedance is 4-fold lower), higher SnO2 amounts have a disadvantageous effect on the catalysis properties of the nanocomposite materials. With the aim of numerically evaluating the resistances associated with each process, the experimental data were fitted by using the equivalent electrical circuit reported in Figure S8, in which each process is modeled through a parallel between a resistance R and a constant phase element (CPE) Q (the CPE is a generalization of the common capacitance, used to account frequency dispersion on the measured spectra);51 a resistance Rs is added accounting for all the ohmic resistances. The fitting curves are reported in Figure S7 superimposed to the experimental data. By looking at this graph, a good match between measured and computed curves can be appreciable, thus evidencing the proper choice of the equivalent circuit (for all the calculated curves, the fitting error was lower than 5%). In Figure 7, the resistance values extracted from the fitting procedure related to each sample are compared. In accordance with the above-reported qualitative discussion, the resistance R1 (related to the HF process) is larger than the corresponding values of other samples, which do not exhibit any dependence on the amount of SnO2 NPs. On the contrary, the resistance R2, related to the charge transfer at the solid/ liquid interface, experiences a monotonic decrease while increasing the amount of SnO2: the presence of such crystallites on the graphene surface significantly speeds up the charge

predominant four-electrons ORR pathway, and further confirms that the presence of the graphitic-like nitrogen atoms in the graphene lattice is able to catalyze the reduction reaction. As already discussed above, the presence of these surface groups, combined with the coverage by SnO2 NPs, is responsible for a cocatalysis effect, which can enhance the effectiveness of the proposed material. For this reason, samples B and C are characterized by n values that are larger than that of the bare rGO catalyst. However, sample C exhibits an electron transfer number of 3.50, i.e., lower than the sample containing a minor amount of SnO2 (3.80). This means that an optimal concentration of NPs allows obtaining a synergistic effect17 (of SnO2 together with nitrogen) for the catalysis of the ORR, whereas a larger amount of SnO2 can, to a certain extent, limit the performances of the N atoms in the graphitic-like groups. This hypothesis is further confirmed by the n value related to the highest NP-containing composite (sample D, 3.28), which resulted to be even lower than that of the bare rGO catalyst. To validate the catalytic pathways of the proposed materials discussed above, 4 electrodes RRDE measurements were performed. For these measurements, while scanning the disk potential at a fixed rotation rate, the currents at disk and ring electrode are measured. The disk current is related to the fourelectrons ORR current of the analyzed material, whereas the current of the ring electrode (whose potential is fixed at a large value) is associated with the two-electrons ORR (intermediate) peroxide species.17 In Figure S6a,b of the SI, the ring and disk currents measured for sample B are reported. As expected, the measured ring current appears very low (2 orders of magnitude) if compared to the corresponding disk current. In fact, as discussed above for this sample, the preferred reduction pathway relies in the four electrons reaction, and, as consequence, the current related to the two electrons reduction is low. By applying eqs 2 and 3, the electron transfer number and the percentage of peroxide species formation HO2−% can be calculated. The curves obtained from this calculation are reported in Figure 6, and compared to the results related to all

Figure 6. Comparison of electron transfer number (left axis) and peroxide percentage (right axis) evaluated from RRDE measurements of the different samples at 2500 rpm rotation speed and different potentials (sample A is bare rGO, whereas samples B, C, and D are rGO/SnO2 prepared with 12%, 25%, and 40% of Sn precursor, respectively). 4639


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nanocomposite exhibits a loss of activity lower than 10%, in line or even better with respect to other nonprecious-metal-based catalysts reported in the literature.29,52 This excellent durability result has to be expected, because the rGO/SnO2 sample is characterized by a four-electrons ORR pathway, and usually the stability of catalysts based on nonprecious metal is correlated with the amount of peroxide species produced during the reaction,53 which in this case resulted to be very low (as shown in Figure 6). For comparison, the CA test was carried out also on the reference Pt/C sample. As shown in the inset of Figure 8, this sample is characterized by a reduction of the current equal to 12% with respect to the initial value after just 3000 s, while in the same period the decrease related to the rGO/SnO2 is as low as 3%. This result evidences the improved stability of the proposed composite material with respect to a commercial Pt/C catalyst. 3.3. MFC Performances. The electrochemical characterizations reported in Section 3.2 showed that the proposed nanocomposite material exhibits noticeable features for its application as an ORR catalyst. In addition, the accurate selection of the optimal Sn concentration during the synthesis procedure allowed obtaining catalysis performances comparable to those of more expensive precious-metal-based materials, widely exploited to speed up the cathodic reaction. To verify further the effectiveness of such composite material as cathode in a real working device, rGO/SnO2 samples were employed into air cathode MFCs and the performances of these cells were compared with those of devices based on a Pt/C cathode. In Figure 9, the typical LSV curves related to both types of MFCs are presented. As can be clearly evident, the perform-

Figure 7. Comparison of the resistances obtained from the fitting procedure of the EIS spectra for the different samples at 2500 rpm rotation speed and 0 V (sample A is bare rGO, whereas sample B, C, and D are rGO/SnO2 prepared with 12%, 25%, and 40% of Sn precursor, respectively); in the inset the resistances obtained from the fitting procedure for the sample B at different potentials are shown.

transfer process. In addition, as expected and differently from what happens with the HF transport process, the MF one exhibits a slight dependence also on the potential, i.e., with the reaction kinetic, as can be observable in the inset of Figure 7. Finally, the resistance R3 is characterized by a marked dependence on the potential and by a nonmonotonic dependence on the amount of SnO2: in accordance with the results of all the other electrochemical characterization, sample B shows the lower resistance, whereas the catalysts with high SnO2 amount are characterized by larger values. By comparing this result with the RRDE analysis, it can be concluded that the LF process is the rate-limiting step of the ORR for this kind of nanocomposite materials. The durability of the proposed catalyst material during prolonged ORR was investigated through CA tests. The result of this characterization is reported in Figure 8. As it can be clearly visible, after more than 3 h of continuous operation, the

Figure 9. Typical potential/current density (left axis, full symbols) and power density/current density (right axis, empty symbols) curves of MFCs based on rGO/SnO2 (sample B, prepared with 12% of Sn precursor) and reference Pt/C as cathodic catalysts.

ances of the rGO/SnO2-based MFCs are comparable with the Pt/C-based devices, used as reference, thus validating the above-reported electrochemical characterization. Both LSV curves show an open circuit voltage of about 0.5 V, and short circuit current density values in the range 500−600 mA/m2. In the same figure, the power density curves demonstrate a maximum value equal to 80 mW/m2 for the nanocompositebased devices and a slight lower value for the reference Pt-based ones (about 50 mW/m2).

Figure 8. Chronoamperometric curves of rGO/SnO2 catalyst (sample B, prepared with 12% of Sn precursor) measured at −0.3 V with 2500 rpm rotation speed and normalized with respect to the initial current value. In the inset, a comparison between the curves of sample B and of reference Pt/C catalyst is shown. 4640


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ACS Applied Materials & Interfaces Bringing into comparison these values with those proposed in the literature is difficult because MFC performances are affected by several factors such as microbial inoculum, chemical substrate, operational conditions, and architecture of the device.54 There are several works employing Pt/C as the cathode in an open-air cathode MFC in which the obtained electrical power density varies from 26 to 620 mW/m2 as a function of the biofilm source and of the organic electron donor.31,55−57 In addition, the power density values obtained in this work with the reference Pt-based cell are in line with a previous study that uses a similar seawater inoculum and the Na−acetate as an organic electron donor in a two-chambered MFC.33 With these premises, the direct correlation between the nanocomposite-based cells with the Pt/C-based devices, together with the low discrepancy of the duplicates, once again confirms the intriguing properties of the rGO/SnO2 composite material as an effective catalyst for the ORR also in a real working MFC.

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AUTHOR INFORMATION

Corresponding Author

*N. Garino. E-mail: [email protected]. Tel.: +39 011 5091922. Fax: +39 011 5091901. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Marco Armandi for the help with BET measurements and Dr. Paola Rivolo and Dr. Alessandro Virga for the help with the Raman analysis.

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4. CONCLUSIONS A novel reduced graphene oxide/SnO2 nanocomposite was successfully prepared through a microwave-assisted synthesis and employed as a catalyst for ORR in MFCs cathodes. Different samples were prepared as a function of the initial Sn precursor. The morphological and structural characterizations revealed the formation of nanocomposite sheets, with crystals of a few nanometers integrated in the graphene matrix. XRD and XPS confirmed the presence of tin oxide within the rGO matrix, providing also an indication on the relative concentration in the nanocomposite. FESEM and TEM evidenced the formation of SnO2 nanoparticles, whereas XPS confirmed the functionalization of the graphene matrix by the presence of nitrogen derived by one of synthesis precursors. Electrochemical characterizations put in evidence the ability of such composite to be an effective catalyst for the ORR, in particular exploiting the cocatalysis mechanism provided by the presence of both SnO2 and nitrogen. Furthermore, it was found a dependence of the catalyst performance on the initial Sn concentration, giving an indication on the best conditions to obtain the optimal electrode for the ORR. When used as cathodic material in seawater-based MFCs, the novel composite catalyst allowed obtaining electrical performances comparable to those of reference devices employing Pt as a catalyst. The easiness, fastness, eco-friendliness, and reliability of the method adopted for the synthesis of the rGO/SnO2 nanocomposite, together with its low cost (especially if compared to materials based on precious metals) and with its promising electrochemical properties, make the proposed catalyst the ideal candidate for a new generation of efficient cathodic material for application in single-chambered MFCs.

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sample B; ring current and disk current of sample B; EIS measurements on the different samples; equivalent circuit exploited for the fitting of the impedance spectra (PDF).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11198. Structural parameters obtained from XRD analysis; sketch of the MFC prototype; EDS spectrum of sample C; Raman spectra for the pristine GO, rGO (sample A) and rGO/SnO2 (sample B); XPS survey spectra and Sn 3d high resolution spectra of rGO/SnO2 composites; ORR polarization curves and Koutecky−Levich plots of 4641


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