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Keywords:

  • cathode;
  • catalyst;
  • microbial fuel cell;
  • nanocomposite;
  • Pani/V2O5

SUMMARY

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Polyaniline (Pani), vanadium oxide (V2O5), and Pani/V2O5 nanocomposite were fabricated and applied as a cathode catalyst in Microbial Fuel Cell (MFC) as an alternative to Pt (Platinum), which is a commonly used expensive cathode catalyst. The cathode catalysts were characterized using Cyclic Voltammetry and Linear Sweep Voltammetry to determine their oxygen reduction activity; furthermore, their structures were observed by X-ray Diffraction, X-ray Photoelectron Spectroscopy, Brunauer–Emmett–Teller, and Field-Emission Scanning Electron Microscopy. The results showed that Pani/V2O5 produced a power density of 79.26 mW/m2, which is higher than V2O5 by 65.31 mW/m2 and Pani by 42.4 mW/m2. Furthermore, the Coulombic Efficiency of the system using Pani/V2O5 (16%) was higher than V2O5 and Pani by 9.2 and 5.5%, respectively. Pani–V2O5 also produced approximately 10% more power than Pt, the best and most common cathode catalyst. It declares that Pani–V2O5 can be a suitable alternative for application in a MFC system. Copyright © 2013 John Wiley & Sons, Ltd.

INTRODUCTION

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Global energy shortages and environmental pollution have led to a crisis affecting human survival and development. There has been a recent increase in the amount of research focused on the use of waste materials as an inexpensive and abundant source of renewable energy [1]. This great concern has caused the advancement of Microbial Fuel Cell (MFC). MFC is a type of fuel cell that uses bacteria as a biocatalyst, to oxidize organic and inorganic matter to produce electricity [2, 3]. MFC is able to simultaneously produce electricity and treat wastewater. A typical MFC consists of two chambers (anode and cathode) that are separated by a Proton Exchange Membrane (PEM) [4]. The performance of an MFC is dependent on several factors, including the microorganism used as a biocatalyst, the cathode catalyst, the distance between the cathode and the anode, the type of PEM used, etc. [5, 6]. However, the main problem that limits the practical application of MFC is the high cost of Pt, which is used as a cathode catalyst. Therefore, finding or developing an alternative catalyst to Pt is necessary to make MFC more practical [7]. The structure of the supporting materials can also affect the performance of the oxygen reduction reaction (ORR) as well as the catalyst. Vanadium is one of the most abundant metals on Earth (widely distributed within the Earth's crust) [8]. Both the pure and composite forms of vanadium are used in many chemical reactions as a catalyst to obtain promising yields and reduce environmental problems [9]. Vanadium has a wide range of applications, including steel additives, batteries, catalysts, etc. [10, 11]. Vanadium's active sites have an important role in its catalytic activity. However, their nature is still not fully understood [12-14]. There are different types of vanadium oxides, which are composed of single and mixed valence states, as well as different structures [15]. However, their compositional stability, phase co-existence, and transformation to different states of vanadium multivalent vanadium are still unclear [16-18]. Meanwhile, interest in the development of conducting polymers as nanocomposites has dramatically increased. This is due to their attractive properties, such as physical and chemical stability, high conductivity, biocompatibility, etc. [19]. Conducting polymer/metal nanocomposites show higher conductivity and stability, compared to polymers, which exhibit limited conductivity and lower stability in ultraviolet irradiation, heat, and other environmental conditions. Furthermore, metal nanoparticle-conducting polymer composites offer appropriate catalysis properties, with a high selectivity in chemical reactions, as the polymer effectively influences controlling the surrounding metal [16]. Polyaniline (Pani)-supported Pd nanoparticles have been applied in the oxidation coupling of the 2, 6-di-t-butyl phenol [17]. Pt/Pani and PtO2/Pani have been used for the selective hydrogenation of α, β-unsaturated aldehyde citral. PtO2/Pani produced a highly dispersed supported catalyst, which was able to hydrogenate the C=C bond of citral, whereas Pani-supported Pt exhibited more selectivity to the reduction of the carbonyl groups [18]. The catalytic activity of Pani-supported cobalt has been examined in trans-stilbene oxidation. The nanocomposite catalyst indicates a significant improvement in the reaction yield under mild conditions [20-23]. In a recent study, Qiao et al. synthesized Carbon Nano Tube/Pani nanocomposite for use as an electrode within an MFC system. They found that the performance of the composite was superior to that of neat Pani and the composite was able to produce more power than Pani [24]. In this research, we synthesized Pani and Pani/V2O5 nanocomposite catalysts using micelle technique and investigated their application in the MFC, to see whether the composite could be used as a cathode catalyst in MFC. Moreover, these results were compared to the performance of Pt, as it is the most commonly used cathode catalyst in MFCs.

MATERIALS AND METHOD

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

Synthesis of V2O5 nanocomposite using a micelle solution

V2O5 nanoparticles were synthesized using a Cetyl trimethyl ammonium bromide (CTAB) as a surfactant and micelle solution. For a typical preparation, 75 ml of 0.05 M CTAB solution was mixed with 10 ml of 0.15 M NaOH and 1.2 mmol of vanadyl sulphate hydrate (VOSO4. xH2O), then stirred at room temperature for 2 h, followed by aging at an ambient temperature for 48 h to allow precipitation. Next, the solution was washed several times with deionized water and absolute ethanol to remove the surfactant, residual reactants, and by-products. The precipitate was then dried in a furnace at 70 °C for 5 h and calcined at 400 °C for 2 h [25].

Synthesis of Pani using a micelle solution

For the preparation of pure Pani, 1.5 mmol of aniline was added to 75 ml of 0.05 M CTAB solution and mixed vigorously. Next, 3.7 × 10−5 mol of ammonium peroxydisulfate (NH4)2S2O8, was added as an initiator and mixed constantly for 2 min. This solution was then filtered and washed with deionized water and absolute ethanol to extract any oligomers or reactants. The resulting powder was then dried in oven at 70 °C for 5 h [26].

Preparation of the Pani/V2O5 nanocomposite using a micelle solution

The nanocomposite was synthesized via a micelle solution method using a hexadecyltrimethyl ammonium bromide (CTAB) cationic surfactant. First, the sample, including 75 ml of the 0.05 M CTAB solution, was prepared. Then, a 10 ml solution of 0.15M NaOH was added and mixed thoroughly; 6 mmol of aniline was added to the solution; then 1.2 mmol of vanadyl sulphate hydrate was added to the mixed solution and agitated for 90 min at room temperature; it was then allowed to stand for 40 min. Next, 1.5 × 10−4 mol of ammonium peroxydisulfate (NH4)2S2O8, was added and mixed vigorously for 2 min. The mixed micelle solution was kept at room temperature for 48 h. A precipitated fine powder was then obtained by centrifugation, which was then washed several times with absolute ethanol and distilled water to remove the residual surfactant, unreacted aniline monomer, and by-products. All the products were then oven-dried for 36 h at 65 °C [27].

MFC configuration

Two cylindrical and H-shaped chambers were constructed from Plexiglas, with an inner diameter of 6.2 cm and a length of 14 cm, separated with Nafion 117, which acted as a PEM. Oxygen was continuously fed to the cathode using an air pump (80 ml/min). Both the cathode's and the anode's surface areas were 12 cm2 and the MFC operated in an ambient temperature and at a neutral pH (6.5–7) within the anode and cathode's compartments. The pH was adjusted using a phosphate buffer solution. Plain carbon paper (Gas Hub, Singapore) was used as an ‘untreated’ anode. The cathode consisted of carbon paper coated with 0.5 mg/cm2 Pt [28].

Calculation and analysis

The current and the power produced by the system were measured using Formulas (1) and (2), respectively:

  • display math(1)
  • display math(2)

Where I represents the current (ampere), V is the voltage (volt), R is the resistance (ohm), and P is the power in watts.

The Coulombic Efficiency (CE) was calculated by integrating the current over time, relative to the time at which the maximum theoretical current was achieved. The evaluated CE over time was calculated using Formula (3).

  • display math(3)

Where M is the molecular weight of oxygen (32), F is Faraday's constant, b = 4 indicates the number of electrons exchanged per mole of oxygen, Van is the volume of the liquid in the anode compartment, and Δ COD is the change in Chemical Oxygen Demand (COD) over time (t) [29].

A Potentiostat–Galvanostat (HAK-MILIK FRIM 04699A-2007) was used to test the oxidation and reduction of organic compounds using microorganisms as a biocatalyst. The potential range was between −0.3 and 0.75 V. The working electrode, for the attachment of microorganisms, was made of Carbon Paper, and the reference electrode was Ag/AgCl. Pt was applied as a counter electrode. The scan rate was adjusted to 50 mV/s [30].

Catalyst characterization

The infrared (IR) spectra were recorded at room temperature in KBr pellets, using a Perkin Elmer Paragon 2000 FTIR spectrometer, under atmospheric conditions. The phase structures of the catalysts were determined from X-ray Diffraction (XRD) patterns, using a Bruker AXS D8 Advance X-ray Powder Diffractometer, with Cu Kα (λ = 0.15406 nm) at the angles 2θ = 10–60. The surface area of the catalyst was measured using the Brunauer–Emmett–Teller (BET) method (N2 adsorption) with a Gemini apparatus (Micromeritics 2010 Instrument Corporation). The morphology and microstructure data for the samples were obtained from Field-Emission Scanning Electron Microscopy (FESEM) using a LEO 1450VP, equipped with an Energy Dispersive X-ray detector. All samples were analysed in a high vacuum at 20 kV. X-ray Photoelectron Spectroscopy (XPS) were acquired using a Kratos (XSAM HS) spectrometer, equipped with a hemispherical electron analyser and Mg Kα (hv 12536 eV, 1eV = 1.6302 × 10−19 J) 120 W X-ray source. The samples were analysed at 3 × 10−9 mbar, using C1s line at 284.5 eV using adventitious carbon as a reference for the binding energies.

RESULT AND DISCUSSION

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

FTIR analysis

Figure 1 depicts the FTIR spectra of V2O5 nanoparticles, pure Pani, and Pani/V2O5 nanocomposite. The strong bands in the region of 750–1800 cm−1 are characteristic of Pani. Their position and intensity show that the conductive form of the polymer has been produced. These nanocomposite bands exhibit a slight shift compared to Pani, suggesting an interaction of the polymer with V2O5 in the nanocomposite. Pure Pani and nanocomposite show the same peaks, at approximately 1460–1490 cm−1 and 1590–1640 cm−1, which are assigned to benzenoid and quinoide ring in the polymer, respectively. The peak, at 1200–1300 cm−1, corresponds to the C–N stretching of the secondary aromatic amine present in Pani [21, 22]. In the case of the V2O5 nanoparticle, the strong band observed below 1000 cm−1 is related to the V=O stretching, which is overlapped by the plane bending vibration mode of C–H in the composite. The peaks at approximately 500 and 800 cm−1 revealed the V–O–V symmetric and asymmetric stretching modes, respectively, which show that both modes shift to higher wave numbers upon interaction with Pani. The data suggest a highly interacting Pani/V2O5 composite system, in which the coordination environment at the metal centre is affected by interaction with the organic conducting component [31].

image

Figure 1. FTIR spectrum for a) V2O5 nanoparticle, b) Pani, and c) Pani/V2O5.

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XRD analysis

The XRD results are shown in Figure 2. These patterns reveal that the synthesized vanadium pentoxide crystallized into an orthorhombic lattice form. The effect of adding V2O5 to the Pani and its composite were analysed using the same XRD technique. This pattern also shows partial crystallinity in the synthesized nanocomposite. As reported in most literature, most forms of Pani are essentially amorphous and show the presence of a broad high-angle asymmetric scattering peak, stretching from 2θ values between 15 and 25°. XRD results show that the diffraction peaks of V2O5 become wider and weaker after the intercalation of aniline. This indicates that the intercalation of aniline caused the V2O5 lattice structure to collapse and that the product being relatively crystalline is ascribed to the limited short-range order, in accordance with Li's result [25]. The slight shift to higher 2θ in the nanocomposite, compared to V2O5, confirms the effect of the interaction between Pani and V2O5 at the surface of the nanocomposite.

image

Figure 2. XRD pattern for a) Pani/V2O5, b) V2O5 nanoparticle, and c) Pani.

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XPS analysis

An XPS measurement was performed to determine the surface composition and chemical state of the prepared nanocomposite, the results of which are shown in Figure 3. The major feature of the core-level spectrum of C1s is a peak at approximately 284.5 eV, which is characteristic of the neutral carbon species. The V2p band showing the V2p3/2 and V2p1/2 is attributed to the oxide form of V+5. The peak of vanadium 2p3/2 in vanadium oxide occurs at 517.3 eV. This spectrum shifts to a lower binding energy (approximately 516.5 eV in the Pani/V2O5 composite), which means the vanadium converts to a lower oxidation state in the presence of Pani. In other words, the vanadium core is influenced by the Pani electrons.

image

Figure 3. XPS spectra of a) N1s in Pani and Pani/V2O5, b) V2p level in: V2O5 and Pani/V2O5.

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The XPS spectrum of the nitrogen (1s level) in the Pani reveals a peak centred at 399.6 eV, which shifts to a higher binding energy at about 1 eV in the Pani/V2O5 composite. The shifting of N1s to the higher binding energy in the composite is presumably related to the interaction of nitrogen in the Pani with the V2O5 oxygen atom. These results are in agreement with the FTIR and XRD results [32, 33].

BET surface area analysis

The nitrogen adsorption/desorption isotherms of Pani and Pani/V2O5 nanocomposite are shown in Figure 4. Clearly, the isotherm depicts type IV isotherms with H3 type hysteresis loops. Both Pani and Pani/V2O5 nanocomposites represent a mesoporous structure.

image

Figure 4. N2 adsorption/desorption isotherms of a) Pani and b) Pani/V2O5.

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The nanocomposite exhibits a highly mesoporous structure, which is of great interest for its application as electrodes, because it represents an optimization of the electrode–electrolyte interface [34]. The average pore sizes were determined at 22.9 nm for Pani and 16.2 nm for Pani/V2O5 nanocomposite. The BET surface area for both compounds was also identified at 35.2 and 9.2 m2/g for Pani and nanocomposite, respectively.

FESEM analysis

The morphologies of the prepared Pani/V2O5 nanocomposite and pure Pani are shown in Figure 5. Both pure Pani and its nanocomposite present a specific structure with a fairly uniform size distribution. The V2O5 nanoparticles are very fine and are scattered in all places of the Pani support. The nanocomposite exhibits a highly mesoporous structure, which leads to interest in its application as a catalyst [35].

image

Figure 5. FESEM micrographs of the: a) Pani/V2O5 and b) Pani.

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The FESEM results are totally compatible with the results obtained from the N2 adsorption/desorption isotherms. In terms of the nanocomposite, no bare nanoparticles were observed, which confirms that the micelle technique produced a well-dispersed nanoparticle.

Electrochemical properties

To determine whether Pani, V2O5, and Pani/V2O5 contribute to the catalytic reaction, their oxygen reduction properties in a half cell were analysed using Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV).

Figure 6 shows the CV graph of the catalysts. Apparently all three catalysts exhibited an ORR peak. The position and size of the peaks indicate whether the material is good enough to be applied as a catalyst and whether the reaction is spontaneous or not. In this case, all three peaks were in negative potential range, which confirms that the reaction was spontaneous. Furthermore, the Pani/V2O5 peak was larger than that of both V2O5 and Pani, which shows that Pani/V2O5 has a higher potential for ORR than Pani or V2O5.

image

Figure 6. CV of the fabricated electrodes.

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The results from the LSV test for different catalysts are shown in Figure 7. The figure shows that among all catalysts, Pani/V2O5 nanocomposite has the highest electrocatalytic activity for ORR. This may be due to the smaller size of the nanocomposite (as mentioned in the BET results) and also the significant electronic interaction of V2O5 and Pani in the nanocomposite, which makes Pani/V2O5 a better redox catalyst. Furthermore, the homogeneous mesoporous structure of the nanocomposite produces promising catalytic activity and, consequently, an effective interaction between Pani/V2O5 and O2 during the reduction reaction [36].

image

Figure 7. LSV of the fabricated electrodes.

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Power density

Figure 8 shows the power density graph of different cathode catalysts in MFC. The Pani/V2O5 nanocomposite cathode catalyst produced 79.26 mW/m2 power density at 194.29 mA/m2 current density. This was higher than the power produced by Pt., which was 72.1 mW/m2 at 173.3 mA/m2. This shows that the Pani/V2O5 nanocomposite catalyst can produce even more power than Pt as a traditional cathode catalyst. This could be due to the unusual properties of composites compared to neat materials. A V2O5 production of 65.31 at 208.66 demonstrates a good potential for being a cathode catalyst, followed by Pani with 42.4 mW/m2 at 132.9 mA/m2.

image

Figure 8. Generated current density from different MFCs.

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COD removal and CE

To observe the performance of MFCs in wastewater treatment and the electricity production, the COD removal and the CE of different MFCs were calculated (Figure 9). As the figure shows, the MFC using the Pani/V2O5 nanocomposite had a higher CE (16%) than the MFC using with V2O5 (9.2%) or Pani (5.5%). This result is in agreement with the power density and ORR data and means that a higher percentage of consumed substrates can be converted into electricity in the MFC using with a Pani/V2O5 cathode catalyst. This is due to a better and faster ORR in the system, which maintained the MFC pH balance, and the microorganisms tended to produce electrons and protons faster [37]. Furthermore, the COD removal data shown in Figure 9 confirms that MFC is a suitable tool for wastewater treatment and the production of electricity [38, 39].

image

Figure 9. COD removal and CE of the different MFCs.

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CONCLUSIONS

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

The main objective of this study was to find an alternative for Pt, which is a common but expensive cathode catalyst used in MFCs. Therefore, three cathode catalysts were prepared and tested for their power production ability in an MFC system. The results showed that Pani/V2O5 produced as much power as Pt. Moreover, V2O5 could produce approximately 80% of the power of Pt. This implies that Pani/V2O5 could be a good alternative to Pt in MFC. Moreover, by improving the properties of V2O5 (i.e. the attachment of some groups to make a composite, etc.), it could serve as a cathode catalyst in MFCs.

ACKNOWLEDGEMENT

  1. Top of page
  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES

The authors appreciate the financial support rendered by the National University of Malaysia by young researcher's grant (GGPM-2013-027) for this project.

REFERENCES

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  2. SUMMARY
  3. INTRODUCTION
  4. MATERIALS AND METHOD
  5. RESULT AND DISCUSSION
  6. CONCLUSIONS
  7. ACKNOWLEDGEMENT
  8. REFERENCES
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