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V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater
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V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater Md.T. Noori a, M.M. Ghangrekar b,*, C.K. Mukherjee a a b
Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur, 721 302, India Department of Civil Engineering, Indian Institute of Technology, Kharagpur, 721 302, India
article info
abstract
Article history:
Performance of air-cathode microbial fuel cell (MFC) was evaluated using vanadium
Received 13 September 2015
pentoxide microflowers (V2O5-MFs) catalyst on cathode and the results were compared
Received in revised form
with a-MnO2 nanotubes (a-MnO2-NTs). The oxygen reduction reaction (ORR) kinetics was
18 December 2015
determined using linear sweep voltammetry (LSV) and electrochemical impedance spec-
Accepted 24 December 2015
troscopy (EIS). MFC using V2O_5MF produced 31% higher current density and 32% lower
Available online xxx
charge transfer resistance (Rct) than a-MnO2-NT. An improved power density of 6.06 W m3 was observed in MFC using V2O5-MF catalyst, followed by the MFC using a-MnO2-NT
Keywords:
(5.5 W m3) and that without catalyst (0.81 W m3). Furthermore, the coulombic efficiency
a-MnO2-NT
of MFC using V2O5-MF (15.2%) was 17% and 52.6% higher than a-MnO2-NT and control,
Catalyst
respectively. MFCs demonstrated significant fish market wastewater treatment with 80%
Electrochemical analysis
removal of chemical oxygen demand (COD) and 60% protein removal efficiency. The overall
Fish market wastewater
performance of MFC using V2O5-MF catalyst was found to be superior, asserting it as a good
V2O5-MF
catalyst, among low-cost metal based catalysts.
Wastewater treatment
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A microbial fuel cell is a recently developed novel technology that can generate bioelectricity using bacteria as biocatalyst and wastewater as fuel source [1]. Performance of MFC is affected by many parameters such as inoculum pretreatment, type of proton exchange membrane (PEM) used, oxidant used in cathodic chamber and cathode catalyst used [2,3]; among which cathodic reduction kinetics play the most important role. Performances of MFCs using various oxidants in catholyte have been reported in literature [4,5]; however,
the oxygen from air is the most promising oxidant for MFC application because of free and abundant availability, and high redox potential. More practical design of MFC is an air-cathode type MFC in which one side of carbon electrode is exposed to air and the other to water. However, the poor oxygen reduction reaction (ORR) and high overpotential losses, which typically results in 300e400 mV loss in the MFC voltage, hampers the overall performance [6]. Therefore, it has been of great interest to develop sustainable high-efficiency cathode catalyst to enhance ORR for improving power density in MFCs. The most widely used catalyst to improve power production in MFC is
* Corresponding author. Tel.: þ91 3222 283440. E-mail address: [email protected] (M.M. Ghangrekar). http://dx.doi.org/10.1016/j.ijhydene.2015.12.163 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163
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Platinum (Pt) or Platinum-based alloys [7]; although, high cost of Pt makes it impractical for commercial use in MFC. The need for affordable catalyst has prompted various research efforts in recent years to aim for developing low-cost transition-metal based electro-catalyst, metal oxides or organic polymer based transition metal oxides to work as a catalyst on cathode [8,9]. Manganese (Mn) and Vanadium (V) metal oxides, MnOx, and VOx are low cost, available in abundance and have good electrochemical properties. Considerable efforts have been made to improve ORR in MFC by incorporating MnOx as a catalyst with different support of carbon [10]. In a similar way, Vanadium is freest metal distributed in the earth's crust [9]. Vanadium based compounds such as vanadium carbide and nitride have shown great ORR catalytic activity and has gained enormous attention in the field of fuel cell research [11,12]. Many researchers have reported potential application of vanadium pentoxide as catalyst to enhance electrochemical kinetics in supercapacitors and lithium ion batteries [13]; however, very few works are available on its use as cathode catalyst in MFC. Ghoreishi et al. [9] used vanadium pentoxide (V2O5)/Polyaniline (PANI) nanocomposite as catalyst for cathode and compared performance with Platinum treated cathode in MFC. The MFC with V2O5/PANI composite cathode produced a maximum power density of 79.26 mW m2, which was higher than the value obtained with Platinum treated cathode (72.1 mW m2). The study had put forward potential application of V2O5/PANI composite to replace the widely used costly Platinum as catalyst. Moreover, vanadium was reported to be efficient to be used as sole electron acceptor for enhancing power in dual-chamber MFC [14]. Furthermore, vanadium oxide (VOx) of nano size such as nano-rods, nano-wires and nano-tubes have been successfully synthesized and used in supercapacitors and lithium ion polymer battery to enhance electrochemical kinetics. Although the nanostructure of VOx showed high electrochemical kinetics, but agglomeration of the catalyst due to large surface area of such particles is still a challenge [15]. As compared to nano-VOx, micro-sized VOx has advantage over agglomeration and could be used for longer time in supercapacitors [15]. The promising results using 3D micro-size vanadium oxide as catalyst in energy storage devices have received enormous attention among the researchers [16]. In this research, V2O5 microflowers (V2O5-MFs) were synthesized using hydrothermal process and its applicability as catalyst in air-cathode MFC was investigated. The electrochemical activities such as oxygen reduction reaction (ORR) and charge transfer resistance of cathode were studied by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS), respectively. Performance of MFC having cathode coated with V2O5-MFs was compared with the performance of MFC having low-cost a-MnO2 nanotubes (aMnO2-NTs) catalyst on cathode.
Experimental Synthesis of catalysts Vanadium pentoxide microflowers (V2O5-MFs) were synthesized using hydrothermal method reported elsewhere [15]. In
a typical synthesis, 1.2 g of V2O5 and 2.49 g of oxalic acid hydrate (H2C2O4$2H2O) were dissolved in 40 mL of deionized water under vigorously stirring condition at a temperature of 80 C. An aqueous solution of VOC2O4 (0.33 M, 5 mL) obtained from the above reaction mixture and 1 mL of H2O2 (35% by weight) were mixed in a 50 mL Teflon container under stirring conditions. After 20 min, 15 mL ethanol was added to the mixture and stirred for another 30 min. The container was then sealed in a stainless steel autoclave and kept in an electric oven at a temperature of 170 C for 2 h. After completion of heating time, the autoclave was allowed to cool down naturally to obtain a blue colored precipitate of vanadium oxide (VO). This VO was collected by centrifugation, washed with absolute ethanol and dried overnight at 80 C. Yellow colored V2O5 microflowers were then obtained by calcination of VO in a muffle furnace at a temperature of 350 C. Nanotubes of a-MnO2 (a-MnO2-NTs) were synthesized using hydrothermal process as well. Briefly, 30 mL aqueous solution of KMnO4 (0.06 M) and 10 mL of 1 M HCl was stirred for 15 min in a Teflon container. The container was sealed in a stainless steel autoclave and kept in a preheated muffle furnace at 150 C for 12 h. The resulting deep brown precipitates were collected by centrifugation and washed three times with ethanol [10]. All the chemicals used in these synthesis process were procured from SigmaeAldrich (Germany).
Cathode preparation Three layered (the catalyst layer, the base layer, and the diffusion layer) cathodes were prepared in laboratory as reported earlier [17]. Carbon cloth was used as current collector, and polydimethyl siloxane (PDMS) was used as catalyst binder [18]. For a comparative study, three different electrodes were prepared using V2O5-MFs, a-MnO2-NTs catalyst layer and one without catalyst (only carbon layer in water facing side). The base layer of the air-facing side of carbon cloth was coated with a solution containing PDMS (6.25 mg cm2) and Vulcan XC (1.56 mg cm2). A diffusion layer with same solution was coated on the base layer to prevent water leakage. The catalyst layer containing V2O5-MFs or a-MnO2-NTs (5 mg cm2 Vulcan XC, 0.5 mg cm2 catalyst, 5% w/v PDMS solution) was applied to the water-facing side of carbon cloth and the same solution without any catalyst was applied to the cathode of control MFC. The electrodes were air dried for 2 h and then dried in an electric oven at 80 C for 30 min after applying each layer.
MFC fabrication and operation Three cubic-shape single-chambered MFCs were fabricated using acrylic sheet with an anode chamber of 2.5 cm width and diameter of 7 cm. Stainless steel mesh with an actual surface area of 15 cm2 was used as anode in all three MFCs. The anode and cathode were separated by a 3 cm 3 cm proton exchange membrane (PEM) (Nafion-117, DuPont, USA). The PEM and the cathode in respective MFCs were held together with nut-bolt joint. Hereafter, the MFC having V2O5MFs, MnO2-NTs and without catalyst on cathode are referred as MFC-V, MFC-M, and MFC-C, respectively (Fig. 1). MFCs were inoculated with ultrasonic pretreated anaerobic septic tank
Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163
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sludge [19] and wastewater collected from IIT Kharagpur fish market was used as substrate. The anode and the cathode were connected using concealed copper wire across 1000 U external resistance. All experiments were performed in batch mode of 4 days fed-batch cycle.
Characterization of catalysts The crystallographic structure of synthesized catalysts were investigated by X-ray diffraction (XRD) (Diffraktometer D8, Bruker Axs, Germany) with Cu Ka radiation (l ¼ 0.1541 nm). Diffractograms were obtained with 2q ranging from 10 to 90 at a scanning rate of 0.12 /s. Fourier transform infrared spectroscopy (FTIR) spectra were collected on Spectrum Rx, (PerkinElmer, Inc., USA) spectrophotometer. Surface morphology was observed using Scanning electron microscopy (SEM, ZEISS EVO 60, Carl ZEISS SMT, Germany) equipped with tungsten filament and Oxford EDS detector. Images were taken by applying electron beam having an acceleration voltage of 20.00 kV.
Electrochemical test of cathodes Electrochemical analysis of fabricated electrodes was performed using potentiostat (AUTO LAB AUT58696, Metrohm), and the electrodes were characterized by electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) in an abiotic electrochemical cell. All measurements were carried out using three electrode electrochemical cell consisting of a working electrode (V2O5-MF, MnO2-NT or without catalyzed), an Ag/AgCl reference electrode (CH Instruments, Inc., RE-5B; þ0.197 V vs. a standard hydrogen electrode, SHE) and a Platinum (Pt) counter electrode. LSV was performed in voltage ranging from þ0.3 V to 0.2 V (vs. Ag/AgCl) at a scan rate of 10 mV s1. The EIS tests were conducted with a frequency ranging from 100 kHz to 100 mHz with sinusoidal perturbation of 10 mV. The solution resistance (Rs) and the charge transfer resistance (Rct) were calculated by fitting the EIS spectra in an equivalent circuit [20].
Fig. 1 e Digital image of fabricated MFCs; the electrodes used for LSV and EIS are shown in front of respective MFC.
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Analytical measurements and calculations The output voltage (E) across an external resistance of 1000 U produced by MFC was measured using digital multimeter (CIE 122, Taiwan). Polarization studies were performed by varying external resistance from 10,000 U to 5 U in decreasing order of resistance, and the corresponding voltages were recorded using data logger (Agilent, Malaysia) connected to a personal computer. Power density (W m2 or W m3) in MFCs was calculated by dividing power (E2/R) to the total projected anode surface area (m2) or total volume (m3) of anodic chamber. Coulombic efficiency (CE) was calculated according to CE (%) ¼ Cp/Cth 100, where Cp (C) is total harvested Coulombs and Cth (C) is theoretical Coulombs available per gram of COD removal. COD was determined using standard closed reflux protocol [21]. A Bradford assay kit was used to quantify total protein [22].
Results and discussions Characterization of catalyst The phase structure of synthesized V2O5-MF and MnO2-NT/ Vulcan XC composites were identified using X-Ray Diffraction (XRD) (diffractograms are shown in Fig. S1A). The XRD peaks of V2O5/Vulcan XC composite appeared at 2q ¼ 15.4 , 20.2 , 26.1 , 31 , 32.3 , 34 , 41 , 45 and 52.7 corresponding to the reflections of 200, 001, 110, 301, 011, 310, 002, 411 and 402, respectively. The reflections confirmed orthorhombic phase plane of V2O5 (JCPDS file no. 41-1426; space group Pmmn ¼ 59) having lattice parameter of a ¼ 11.5 A , b ¼ 3.5 A and c ¼ 4.3 A [15]. The major peaks at 2q ¼ 20.2 (001), 26.1 (110) and 31 (301) were due to the insertion of oxygen in VO crystalline plane during annealing of VO at 350 C [23]. However, the peaks were found to be weak probably due to the interaction between V2O5 and Vulcan XC. The XRD peak can be perfectly indexed to tetragonal crystal system of a-MnO2 (JCPDS file no. 44-0141; space group l4/m (87), a ¼ 9.78 A , b ¼ 9.78 A and c ¼ 2.86 A (Fig. S1A)). The sharp peak of a-MnO2-NT in the diffractogram is indicative of its highly crystalline nature [10]. The strong peak around wave number 1023 cm1 (Fig. S1B, FTIR spectra of V2O5) is associated with the stretching vibration of V]O. The weak peak around 723 cm1 and 523 cm1 revealed the symmetric and asymmetric stretching mode of single chemical bond associated with vanadium and oxygen (VeOeV) [24]. It has been documented that the vanadyl oxygen present in V]O bond plays an important role in catalytic and electrochemical activity [25]. However, a recent study has exposed that the oxygen present in the lattice structure of VeOeV is most relevant in oxidation process [24]. Therefore, it is expected that the V2O5 has superior potential to catalyze oxygen when used in air cathode MFCs. The strong and broad peak around 570 cm1 and 1100 cm1 indicated the vibration of MneO and MneOH group (Fig. S1B). Broad peaks in both V2O5 and a-MnO2 around wave number of 1700 cm1 and 3400 cm1 were due to absorbed water molecule and carbon dioxide, respectively. The typical characteristics obtained speculated that the synthesized catalyst had
Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163
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high surface-to-volume ratio [26]. The morphological structure of V2O5 and a-MnO2 was further investigated using field emission surface microscope (FESEM). The V2O5 comprised of flower-like porous structure in micro scale (Fig. 2A); whereas, a-MnO2 has nano-tube like structure (Fig. 2B) [10].
Electrochemical property of cathodes Electrochemical analysis of cathodes was performed with LSV and EIS in absence of bacteria. A typical LSV of V2O5-MF, aMnO2-NT catalyzed and that without catalyzed cathode is shown in Fig. 3A. V2O5-MF catalyzed cathode showed higher current response over an applied potential range between þ0.3 V and 0.2 V (vs. Ag/AgCl reference electrode) than aMnO2-NT catalyzed cathode. A maximum current density of 5.11 A m2 was recorded in the V2O5-MF catalyzed cathode, resulting in a 1.3-fold improvement as compared to the aMnO2-NT catalyzed cathode (3.95 A m2). The onset potential was found to be 0.05 V and 0.03 V in V2O5-MF and a-MnO2-NT catalyzed cathode, respectively, indicating both the catalysts have decent catalytic activity. Moreover, at onset potential, the current density was found to be 31% higher in V2O5-MF
Fig. 3 e (A) LSV of cathode fabricated with V2O5-MF, aMnO2-NT and Vulcan XC carbon powder without catalyst represented by a, b and c, respectively, (B) EIS spectra of cathode consisting of V2O5-MF, a-MnO2-NT and only Vulcan XC without catalyst.
Fig. 2 e SEM micrograph showing surface morphology of as-synthesized catalysts (A) Flower-like structure of V2O5 after calcination of VO at 350 C in muffle furnace; image was taken at 20 kX magnification (B) a-MnO2; image was taken at 15 kX magnification.
catalyzed cathode as compared to a-MnO2-NT catalyzed cathode. Vanadium pentoxide is an N-type semiconductor having an optical energy gap of 2.3 eV [24]. This typical characteristic of V2O5 is used in different redox coupling reactions as catalyst. There are two steps of V2O5 catalyst based electrochemical reaction: (1) the lattice oxygen (V]O, or VeOeV) participates in oxidation reaction and leaves an oxygen vacancy, and (2) the filling up of oxygen in the lattice structure; thereby regenerating V2O5 [24]. The mechanism of ORR achieved by V2O5 catalyst is still not fully understood. However, Liu and Anson [27] promoted a mechanism of four electron electrochemical reduction of O2 in presence of V2O5 (salen) catalyst. In this mechanism, reduction of VVOþ to VIIIþ (halfcell potential Eo ¼ 0.7 V) creates an oxygen vacancy in the lattice of V2O5 as per the following equations:
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VV Oþ þ 2Hþ þ 2e/VIIIþ þ H2 Oðoxygen vacancyÞ
(1)
during cathode fabrication process. Thus, this V2O5-MF could be used as cathode catalyst for prolonged time.
VIIIþ þ O2 /O2 Vþ ðinsertion of oxygenÞ
(2)
Wastewater treatment
O2 Vþ þ VIIIþ /½VOOV2þ
(3)
½VOOV2þ /2VV Oþ ðfast regenerationÞ
(4)
Bi-functional property of MFC as efficient wastewater treatment and electricity generation device has been documented extensively [30,31]. Wastewater collected from fish market, which was used as anode substrate in the MFCs, was characterized and found to have a COD of 1150 ± 5 mg L1 and a protein content of 265 ± 12 mg L1. The pH was found in slightly acidic range of 5.5e6.5. Hence, pH of feed was adjusted to 7.5 ± 0.1 using phosphate buffer (50 mM) before adding it to the MFCs. Wastewater treatment efficiency concerning COD and protein removal was monitored for 12 fed-batch cycles with each cycle having a retention time of 4 days. COD removal efficiencies of 83 ± 0.34%, 82.8 ± 0.24% and 84 ± 0.8% were found in MFC-V, MFC-M and MFC-C, respectively (Fig. 4). The results of COD removal are similar to the COD removal efficiency of 85% obtained earlier in MFC treating fish processing wastewater [32]. These MFCs also demonstrated successful protein removal from wastewater. The average protein removal efficiencies were found to be 64.5 ± 1.7%, 62.6 ± 4% and 63 ± 1.5%, respectively in MFC-V, MFC-M and MFC-C (Fig. 4). The protein removal observed in these MFCs was lower than the protein removal efficiency of ~94% reported in earlier studies while treating wastewater containing bovine serum albumin (BSA) and peptone in MFC, respectively [33]. The lower protein removal efficiency achieved in this study could be possibly because of the complexity of substrate used wherein the wastewater carries higher chain peptides, protein, lipid, and fats. The overall wastewater treatment efficiency of fish market wastewater was found to be satisfactory based on COD and protein removal from fish market wastewater. However, no significant difference in the treatment efficiency was observed in three MFCs.
The combination of Equations (1)e(4) gives the following four electron ORR in presence of V2O5 catalyst. O2 þ 4Hþ þ 4e /2H2 O
(5)
Furthermore, a-MnO2 catalyst also favors four electron electrochemical reduction pathway of ORR in alkaline medium [10,28]. The electrochemical equations involved in a typical redox coupling reaction catalyzed by a-MnO2 have been explained by previous researchers [10]. The cathode prepared without catalyst (only Vulcan XC) had very little current over an applied voltage range (þ0.3 to 0.2 V vs. Ag/ AgCl). It suggests that only carbon matrix without catalyst is not capable of catalyzing ORR [17]. EIS was performed to evaluate interfacial charge transport behavior of the cathodes. Nyquist plots (imaginary component vs. real component of impedance) of all three cathode viz. with V2O5-MF and a-MnO2-NT as catalyst and that without catalyst are presented in Fig. 3B. As shown in figure, diameter of the semi-circle produced in the Nyquist plot was found to be highest for the cathode without catalyst followed by aMnO2-NT and V2O5-MF. It indicates a higher kinetic driving force for ORR by creating lower hindrance in charge transfer in V2O5-MF catalyzed cathode. Cathodic resistances such as charge transfer resistance (Rct) and solution resistance (Rs), were estimated by fitting the EIS spectra in an equivalent circuit. Solution resistances (Rs) of the three different cathodes were found to be same because of the same catholyte (1 M KCL solution) and cell configuration used in the electrochemical study. At high overpotential, Rs becomes dominant. However, solution resistance can be reduced by reducing electrode distance and change in reactor design [18]. Rct is responsible for regulating ORR overpotential. Lower Rct value of 3.7 U was found in the cathode using V2O5-MF, thus offering excellent electrochemical property; whereas the cathode containing a-MnO2NT catalyst had slightly higher Rct (5.5 U) and the cathode without any catalyst had a Rct of 13 U. This 3.5 and 1.5-fold lower Rct of V2O5-MF catalyzed cathode when compared to control cathode and a-MnO2 catalyzed cathode, respectively, exhibited excellent charge transport and lower ORR overpotential, thereby increasing the reductive current as confirmed by LSV data. The improved electrochemical kinetics of V2O5-MF decorated cathode is attributed to its porous structure and less agglomeration of catalyst. The porous structure facilitated higher oxygen penetration and increased contact area between electrode material and oxygen [15,29]. The specific property assisted faster regeneration of catalyst during oxidation process as per Eq. (4). Moreover, the flower like microstructure of V2O5-MF provided less agglomeration
Fig. 4 e COD and protein removal efficiency of the MFCs.
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Power production Effect of different low-cost cathode catalysts on power production and coulombic efficiency was evaluated. MFCs were operated in a batch mode of 4 days fed-batch cycle and monitored for 12 batch cycles. All three MFCs started giving voltage from the day of start-up, endorsing presence of active exoelectrogenic bacteria in septic tank mix consortia pretreated with ultrasonication. With days of operation, the anode potential (Ea) decreased, and the cathode potential (Ec) was found to increase in all three MFCs. Stable performance in all MFCs were found after 4 sequential fed-batch cycles. After 4th batch cycle, no significant difference was observed in Ea, and it was about 450 mV (vs. Ag/AgCl) in all three MFCs. The minimum Ec of 90 mV (vs. Ag/AgCl) was recorded in control MFC (MFC-C); and for MFC using a-MnO2-NT (MFC-M), Ec was 290 mV and it was 350 mV in MFC using V2O5-MF catalyzed cathode (MFC-V). Hence, it is clear that only the cathode performance had a major influence on the performance of MFCs. MFCs were operated under 1000 U external resistance, and current was measured daily. Power obtained from each MFC was normalized to total liquid volume of anode. In daily observations of power production from MFCs, MFC-V could generate a 32% and 74% higher power density with an average value of 2.30 ± 0.02 W m3 as compared to that of MFC-M (1.56 ± 0.02 W m3) and MFC-C (0.58 ± 0.01 W m3), respectively (Fig. 5A). Likewise, the coulombs recovery in MFC-V was found to be substantially enhanced, with an average CE of 15.2 ± 0.11%, which was 17% and 52.6% higher than that of MFC-M (12.2 ± 0.12%) and MFC-C (7.2 ± 0.11%), respectively (Fig. 5A). It is to be noted that the CE obtained from this study using fish market wastewater (COD ~ 1200 mg L1) was higher than those reported with air-cathode MFCs (3e12%) using platinum as catalyst on cathode treating domestic wastewater [34]. In the present study, significant improvement in CE was demonstrated using low-cost catalyst (V2O5-MF) as compared with costly platinum and a-MnO2 catalyst used in MFC treating real wastewater. Enhanced power density and coulombic efficiency in MFC using V2O5-MF as catalyst on cathode (MFCV) demonstrated higher electrochemical kinetics (high reduction current and low charge transport resistance) due to porous structure and N-type semiconductor property of V2O5MF catalyst. However, MFC-M also offered reasonable and comparable performance as reported earlier [35,36]. Polarization studies were performed to get optimum power density and corresponding current density normalized to total anodic volume and actual surface area of anode. The maximum power density was found to be enhanced from 0.8 W m3 (36.7 mW m2) in MFC-C to 5.5 W m3 (249 mW m2) in MFC-M and to 6.06 W m3 (273 mW m2) in MFC-V (Fig. 5B). This 7.7 and 1.1 times higher power (normalized to anodic volume) than MFC-C and MFC-M, respectively, in MFC-V shows excellent electrochemical property of V2O5-MF as catalyst in MFC. Ghoreishi et al. [9] reported power density of 79.26 mW m2 and 72.1 mW m2 using V2O5/PANI composite and platinum as catalyst, respectively, which is much lower than the value obtained in this study. It can be clearly observed from Fig. 5B (voltage vs. current density), a rapid voltage drop occurred upto a current density
Fig. 5 e (A) Coulombic efficiency (left Y-axes) and power generation (right Y-axes) of MFC-V, MFC-M and MFC-C under different batch cycles; (B) Polarization curves of MFCV, MFC-M, and MFC-C, Y-axes to the left represents cell potential, and Y-axes to the right represents power density. of 14 Am3 and beyond 30 Am3 in MFC-M as compared to MFC-V, indicating higher activation and concentration loss, respectively in MFC-M [1]. The voltage drop between current density of 14 A m3 e 30 A m3 was observed almost similar in MFC-M and MFC-V, suggesting these MFCs had same polarization overpotential (ohmic loss). As compared to these two MFCs, MFC-C supported less current density of 16.4 Am3. In comparison with a-MnO2 catalyst in this study and platinum catalyst [9,34], the MFC with V2O5 catalyst cathode had superior performance, ascertaining it as a highly attractive lowcost catalyst. Due to lower fabrication cost and highly appreciable performance, V2O5 could be used to fabricate larger MFCs (scalable MFC) to handle high volume of wastewater and simultaneous generation of bio-electricity. A low total internal resistance of 55 U was obtained from MFC-V as compared to the MFC-M (63 U) and MFC-C (133 U). This internal resistance trend observed is in good agreement with the EIS results. However, the internal resistance
Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163
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observed was higher as compared to earlier studies that might be attributed to the difference in separator-electrode assembly (SEA) and reactor configuration used with more distance between the electrodes [37]. Based on the results obtained from LSV, EIS and performance of MFCs, it may be concluded that the V2O5-MF catalyzed cathode had lower ORR overpotential due to which MFC-V produced better power as compared to the other two MFCs (MFC-M and MFC-C) by recovering additional coulombs.
Conclusion Effective application of V2O5 microflower porous catalyst in cathode of MFC was demonstrated. The LSV and EIS spectra showed enhanced electrochemical kinetics and ORR using V2O5-MF catalyst on cathode due to better contact of oxygen within the porous structure of catalyst. The power density of MFC using V2O5-MF catalyst on cathode was found to be enhanced by 9.2% and 79.6% with a significant increase in CE by 17% and 52.6% when compared with MFC using a-MnO2-NT as a cathode catalyst and using only carbon powder as cathode material, respectively. However, organic matter and protein removal efficiency were similar in all MFCs. Thus, V2O5MF can be used as a cathode catalyst to improve power produced from MFC for commercial application.
Acknowledgment The grant received from Department of Science and Technology, Govt. of India (File No. DST/TSG/NTS/2010/61) to undertake this research work is duly acknowledged.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.12.163.
references
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V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater Md.T. Noori a, M.M. Ghangrekar b,*, C.K. Mukherjee a a b
Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur, 721 302, India Department of Civil Engineering, Indian Institute of Technology, Kharagpur, 721 302, India
article info
abstract
Article history:
Performance of air-cathode microbial fuel cell (MFC) was evaluated using vanadium
Received 13 September 2015
pentoxide microflowers (V2O5-MFs) catalyst on cathode and the results were compared
Received in revised form
with a-MnO2 nanotubes (a-MnO2-NTs). The oxygen reduction reaction (ORR) kinetics was
18 December 2015
determined using linear sweep voltammetry (LSV) and electrochemical impedance spec-
Accepted 24 December 2015
troscopy (EIS). MFC using V2O_5MF produced 31% higher current density and 32% lower
Available online xxx
charge transfer resistance (Rct) than a-MnO2-NT. An improved power density of 6.06 W m3 was observed in MFC using V2O5-MF catalyst, followed by the MFC using a-MnO2-NT
Keywords:
(5.5 W m3) and that without catalyst (0.81 W m3). Furthermore, the coulombic efficiency
a-MnO2-NT
of MFC using V2O5-MF (15.2%) was 17% and 52.6% higher than a-MnO2-NT and control,
Catalyst
respectively. MFCs demonstrated significant fish market wastewater treatment with 80%
Electrochemical analysis
removal of chemical oxygen demand (COD) and 60% protein removal efficiency. The overall
Fish market wastewater
performance of MFC using V2O5-MF catalyst was found to be superior, asserting it as a good
V2O5-MF
catalyst, among low-cost metal based catalysts.
Wastewater treatment
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A microbial fuel cell is a recently developed novel technology that can generate bioelectricity using bacteria as biocatalyst and wastewater as fuel source [1]. Performance of MFC is affected by many parameters such as inoculum pretreatment, type of proton exchange membrane (PEM) used, oxidant used in cathodic chamber and cathode catalyst used [2,3]; among which cathodic reduction kinetics play the most important role. Performances of MFCs using various oxidants in catholyte have been reported in literature [4,5]; however,
the oxygen from air is the most promising oxidant for MFC application because of free and abundant availability, and high redox potential. More practical design of MFC is an air-cathode type MFC in which one side of carbon electrode is exposed to air and the other to water. However, the poor oxygen reduction reaction (ORR) and high overpotential losses, which typically results in 300e400 mV loss in the MFC voltage, hampers the overall performance [6]. Therefore, it has been of great interest to develop sustainable high-efficiency cathode catalyst to enhance ORR for improving power density in MFCs. The most widely used catalyst to improve power production in MFC is
* Corresponding author. Tel.: þ91 3222 283440. E-mail address: [email protected] (M.M. Ghangrekar). http://dx.doi.org/10.1016/j.ijhydene.2015.12.163 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163
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Platinum (Pt) or Platinum-based alloys [7]; although, high cost of Pt makes it impractical for commercial use in MFC. The need for affordable catalyst has prompted various research efforts in recent years to aim for developing low-cost transition-metal based electro-catalyst, metal oxides or organic polymer based transition metal oxides to work as a catalyst on cathode [8,9]. Manganese (Mn) and Vanadium (V) metal oxides, MnOx, and VOx are low cost, available in abundance and have good electrochemical properties. Considerable efforts have been made to improve ORR in MFC by incorporating MnOx as a catalyst with different support of carbon [10]. In a similar way, Vanadium is freest metal distributed in the earth's crust [9]. Vanadium based compounds such as vanadium carbide and nitride have shown great ORR catalytic activity and has gained enormous attention in the field of fuel cell research [11,12]. Many researchers have reported potential application of vanadium pentoxide as catalyst to enhance electrochemical kinetics in supercapacitors and lithium ion batteries [13]; however, very few works are available on its use as cathode catalyst in MFC. Ghoreishi et al. [9] used vanadium pentoxide (V2O5)/Polyaniline (PANI) nanocomposite as catalyst for cathode and compared performance with Platinum treated cathode in MFC. The MFC with V2O5/PANI composite cathode produced a maximum power density of 79.26 mW m2, which was higher than the value obtained with Platinum treated cathode (72.1 mW m2). The study had put forward potential application of V2O5/PANI composite to replace the widely used costly Platinum as catalyst. Moreover, vanadium was reported to be efficient to be used as sole electron acceptor for enhancing power in dual-chamber MFC [14]. Furthermore, vanadium oxide (VOx) of nano size such as nano-rods, nano-wires and nano-tubes have been successfully synthesized and used in supercapacitors and lithium ion polymer battery to enhance electrochemical kinetics. Although the nanostructure of VOx showed high electrochemical kinetics, but agglomeration of the catalyst due to large surface area of such particles is still a challenge [15]. As compared to nano-VOx, micro-sized VOx has advantage over agglomeration and could be used for longer time in supercapacitors [15]. The promising results using 3D micro-size vanadium oxide as catalyst in energy storage devices have received enormous attention among the researchers [16]. In this research, V2O5 microflowers (V2O5-MFs) were synthesized using hydrothermal process and its applicability as catalyst in air-cathode MFC was investigated. The electrochemical activities such as oxygen reduction reaction (ORR) and charge transfer resistance of cathode were studied by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS), respectively. Performance of MFC having cathode coated with V2O5-MFs was compared with the performance of MFC having low-cost a-MnO2 nanotubes (aMnO2-NTs) catalyst on cathode.
Experimental Synthesis of catalysts Vanadium pentoxide microflowers (V2O5-MFs) were synthesized using hydrothermal method reported elsewhere [15]. In
a typical synthesis, 1.2 g of V2O5 and 2.49 g of oxalic acid hydrate (H2C2O4$2H2O) were dissolved in 40 mL of deionized water under vigorously stirring condition at a temperature of 80 C. An aqueous solution of VOC2O4 (0.33 M, 5 mL) obtained from the above reaction mixture and 1 mL of H2O2 (35% by weight) were mixed in a 50 mL Teflon container under stirring conditions. After 20 min, 15 mL ethanol was added to the mixture and stirred for another 30 min. The container was then sealed in a stainless steel autoclave and kept in an electric oven at a temperature of 170 C for 2 h. After completion of heating time, the autoclave was allowed to cool down naturally to obtain a blue colored precipitate of vanadium oxide (VO). This VO was collected by centrifugation, washed with absolute ethanol and dried overnight at 80 C. Yellow colored V2O5 microflowers were then obtained by calcination of VO in a muffle furnace at a temperature of 350 C. Nanotubes of a-MnO2 (a-MnO2-NTs) were synthesized using hydrothermal process as well. Briefly, 30 mL aqueous solution of KMnO4 (0.06 M) and 10 mL of 1 M HCl was stirred for 15 min in a Teflon container. The container was sealed in a stainless steel autoclave and kept in a preheated muffle furnace at 150 C for 12 h. The resulting deep brown precipitates were collected by centrifugation and washed three times with ethanol [10]. All the chemicals used in these synthesis process were procured from SigmaeAldrich (Germany).
Cathode preparation Three layered (the catalyst layer, the base layer, and the diffusion layer) cathodes were prepared in laboratory as reported earlier [17]. Carbon cloth was used as current collector, and polydimethyl siloxane (PDMS) was used as catalyst binder [18]. For a comparative study, three different electrodes were prepared using V2O5-MFs, a-MnO2-NTs catalyst layer and one without catalyst (only carbon layer in water facing side). The base layer of the air-facing side of carbon cloth was coated with a solution containing PDMS (6.25 mg cm2) and Vulcan XC (1.56 mg cm2). A diffusion layer with same solution was coated on the base layer to prevent water leakage. The catalyst layer containing V2O5-MFs or a-MnO2-NTs (5 mg cm2 Vulcan XC, 0.5 mg cm2 catalyst, 5% w/v PDMS solution) was applied to the water-facing side of carbon cloth and the same solution without any catalyst was applied to the cathode of control MFC. The electrodes were air dried for 2 h and then dried in an electric oven at 80 C for 30 min after applying each layer.
MFC fabrication and operation Three cubic-shape single-chambered MFCs were fabricated using acrylic sheet with an anode chamber of 2.5 cm width and diameter of 7 cm. Stainless steel mesh with an actual surface area of 15 cm2 was used as anode in all three MFCs. The anode and cathode were separated by a 3 cm 3 cm proton exchange membrane (PEM) (Nafion-117, DuPont, USA). The PEM and the cathode in respective MFCs were held together with nut-bolt joint. Hereafter, the MFC having V2O5MFs, MnO2-NTs and without catalyst on cathode are referred as MFC-V, MFC-M, and MFC-C, respectively (Fig. 1). MFCs were inoculated with ultrasonic pretreated anaerobic septic tank
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sludge [19] and wastewater collected from IIT Kharagpur fish market was used as substrate. The anode and the cathode were connected using concealed copper wire across 1000 U external resistance. All experiments were performed in batch mode of 4 days fed-batch cycle.
Characterization of catalysts The crystallographic structure of synthesized catalysts were investigated by X-ray diffraction (XRD) (Diffraktometer D8, Bruker Axs, Germany) with Cu Ka radiation (l ¼ 0.1541 nm). Diffractograms were obtained with 2q ranging from 10 to 90 at a scanning rate of 0.12 /s. Fourier transform infrared spectroscopy (FTIR) spectra were collected on Spectrum Rx, (PerkinElmer, Inc., USA) spectrophotometer. Surface morphology was observed using Scanning electron microscopy (SEM, ZEISS EVO 60, Carl ZEISS SMT, Germany) equipped with tungsten filament and Oxford EDS detector. Images were taken by applying electron beam having an acceleration voltage of 20.00 kV.
Electrochemical test of cathodes Electrochemical analysis of fabricated electrodes was performed using potentiostat (AUTO LAB AUT58696, Metrohm), and the electrodes were characterized by electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) in an abiotic electrochemical cell. All measurements were carried out using three electrode electrochemical cell consisting of a working electrode (V2O5-MF, MnO2-NT or without catalyzed), an Ag/AgCl reference electrode (CH Instruments, Inc., RE-5B; þ0.197 V vs. a standard hydrogen electrode, SHE) and a Platinum (Pt) counter electrode. LSV was performed in voltage ranging from þ0.3 V to 0.2 V (vs. Ag/AgCl) at a scan rate of 10 mV s1. The EIS tests were conducted with a frequency ranging from 100 kHz to 100 mHz with sinusoidal perturbation of 10 mV. The solution resistance (Rs) and the charge transfer resistance (Rct) were calculated by fitting the EIS spectra in an equivalent circuit [20].
Fig. 1 e Digital image of fabricated MFCs; the electrodes used for LSV and EIS are shown in front of respective MFC.
3
Analytical measurements and calculations The output voltage (E) across an external resistance of 1000 U produced by MFC was measured using digital multimeter (CIE 122, Taiwan). Polarization studies were performed by varying external resistance from 10,000 U to 5 U in decreasing order of resistance, and the corresponding voltages were recorded using data logger (Agilent, Malaysia) connected to a personal computer. Power density (W m2 or W m3) in MFCs was calculated by dividing power (E2/R) to the total projected anode surface area (m2) or total volume (m3) of anodic chamber. Coulombic efficiency (CE) was calculated according to CE (%) ¼ Cp/Cth 100, where Cp (C) is total harvested Coulombs and Cth (C) is theoretical Coulombs available per gram of COD removal. COD was determined using standard closed reflux protocol [21]. A Bradford assay kit was used to quantify total protein [22].
Results and discussions Characterization of catalyst The phase structure of synthesized V2O5-MF and MnO2-NT/ Vulcan XC composites were identified using X-Ray Diffraction (XRD) (diffractograms are shown in Fig. S1A). The XRD peaks of V2O5/Vulcan XC composite appeared at 2q ¼ 15.4 , 20.2 , 26.1 , 31 , 32.3 , 34 , 41 , 45 and 52.7 corresponding to the reflections of 200, 001, 110, 301, 011, 310, 002, 411 and 402, respectively. The reflections confirmed orthorhombic phase plane of V2O5 (JCPDS file no. 41-1426; space group Pmmn ¼ 59) having lattice parameter of a ¼ 11.5 A , b ¼ 3.5 A and c ¼ 4.3 A [15]. The major peaks at 2q ¼ 20.2 (001), 26.1 (110) and 31 (301) were due to the insertion of oxygen in VO crystalline plane during annealing of VO at 350 C [23]. However, the peaks were found to be weak probably due to the interaction between V2O5 and Vulcan XC. The XRD peak can be perfectly indexed to tetragonal crystal system of a-MnO2 (JCPDS file no. 44-0141; space group l4/m (87), a ¼ 9.78 A , b ¼ 9.78 A and c ¼ 2.86 A (Fig. S1A)). The sharp peak of a-MnO2-NT in the diffractogram is indicative of its highly crystalline nature [10]. The strong peak around wave number 1023 cm1 (Fig. S1B, FTIR spectra of V2O5) is associated with the stretching vibration of V]O. The weak peak around 723 cm1 and 523 cm1 revealed the symmetric and asymmetric stretching mode of single chemical bond associated with vanadium and oxygen (VeOeV) [24]. It has been documented that the vanadyl oxygen present in V]O bond plays an important role in catalytic and electrochemical activity [25]. However, a recent study has exposed that the oxygen present in the lattice structure of VeOeV is most relevant in oxidation process [24]. Therefore, it is expected that the V2O5 has superior potential to catalyze oxygen when used in air cathode MFCs. The strong and broad peak around 570 cm1 and 1100 cm1 indicated the vibration of MneO and MneOH group (Fig. S1B). Broad peaks in both V2O5 and a-MnO2 around wave number of 1700 cm1 and 3400 cm1 were due to absorbed water molecule and carbon dioxide, respectively. The typical characteristics obtained speculated that the synthesized catalyst had
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high surface-to-volume ratio [26]. The morphological structure of V2O5 and a-MnO2 was further investigated using field emission surface microscope (FESEM). The V2O5 comprised of flower-like porous structure in micro scale (Fig. 2A); whereas, a-MnO2 has nano-tube like structure (Fig. 2B) [10].
Electrochemical property of cathodes Electrochemical analysis of cathodes was performed with LSV and EIS in absence of bacteria. A typical LSV of V2O5-MF, aMnO2-NT catalyzed and that without catalyzed cathode is shown in Fig. 3A. V2O5-MF catalyzed cathode showed higher current response over an applied potential range between þ0.3 V and 0.2 V (vs. Ag/AgCl reference electrode) than aMnO2-NT catalyzed cathode. A maximum current density of 5.11 A m2 was recorded in the V2O5-MF catalyzed cathode, resulting in a 1.3-fold improvement as compared to the aMnO2-NT catalyzed cathode (3.95 A m2). The onset potential was found to be 0.05 V and 0.03 V in V2O5-MF and a-MnO2-NT catalyzed cathode, respectively, indicating both the catalysts have decent catalytic activity. Moreover, at onset potential, the current density was found to be 31% higher in V2O5-MF
Fig. 3 e (A) LSV of cathode fabricated with V2O5-MF, aMnO2-NT and Vulcan XC carbon powder without catalyst represented by a, b and c, respectively, (B) EIS spectra of cathode consisting of V2O5-MF, a-MnO2-NT and only Vulcan XC without catalyst.
Fig. 2 e SEM micrograph showing surface morphology of as-synthesized catalysts (A) Flower-like structure of V2O5 after calcination of VO at 350 C in muffle furnace; image was taken at 20 kX magnification (B) a-MnO2; image was taken at 15 kX magnification.
catalyzed cathode as compared to a-MnO2-NT catalyzed cathode. Vanadium pentoxide is an N-type semiconductor having an optical energy gap of 2.3 eV [24]. This typical characteristic of V2O5 is used in different redox coupling reactions as catalyst. There are two steps of V2O5 catalyst based electrochemical reaction: (1) the lattice oxygen (V]O, or VeOeV) participates in oxidation reaction and leaves an oxygen vacancy, and (2) the filling up of oxygen in the lattice structure; thereby regenerating V2O5 [24]. The mechanism of ORR achieved by V2O5 catalyst is still not fully understood. However, Liu and Anson [27] promoted a mechanism of four electron electrochemical reduction of O2 in presence of V2O5 (salen) catalyst. In this mechanism, reduction of VVOþ to VIIIþ (halfcell potential Eo ¼ 0.7 V) creates an oxygen vacancy in the lattice of V2O5 as per the following equations:
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5
VV Oþ þ 2Hþ þ 2e/VIIIþ þ H2 Oðoxygen vacancyÞ
(1)
during cathode fabrication process. Thus, this V2O5-MF could be used as cathode catalyst for prolonged time.
VIIIþ þ O2 /O2 Vþ ðinsertion of oxygenÞ
(2)
Wastewater treatment
O2 Vþ þ VIIIþ /½VOOV2þ
(3)
½VOOV2þ /2VV Oþ ðfast regenerationÞ
(4)
Bi-functional property of MFC as efficient wastewater treatment and electricity generation device has been documented extensively [30,31]. Wastewater collected from fish market, which was used as anode substrate in the MFCs, was characterized and found to have a COD of 1150 ± 5 mg L1 and a protein content of 265 ± 12 mg L1. The pH was found in slightly acidic range of 5.5e6.5. Hence, pH of feed was adjusted to 7.5 ± 0.1 using phosphate buffer (50 mM) before adding it to the MFCs. Wastewater treatment efficiency concerning COD and protein removal was monitored for 12 fed-batch cycles with each cycle having a retention time of 4 days. COD removal efficiencies of 83 ± 0.34%, 82.8 ± 0.24% and 84 ± 0.8% were found in MFC-V, MFC-M and MFC-C, respectively (Fig. 4). The results of COD removal are similar to the COD removal efficiency of 85% obtained earlier in MFC treating fish processing wastewater [32]. These MFCs also demonstrated successful protein removal from wastewater. The average protein removal efficiencies were found to be 64.5 ± 1.7%, 62.6 ± 4% and 63 ± 1.5%, respectively in MFC-V, MFC-M and MFC-C (Fig. 4). The protein removal observed in these MFCs was lower than the protein removal efficiency of ~94% reported in earlier studies while treating wastewater containing bovine serum albumin (BSA) and peptone in MFC, respectively [33]. The lower protein removal efficiency achieved in this study could be possibly because of the complexity of substrate used wherein the wastewater carries higher chain peptides, protein, lipid, and fats. The overall wastewater treatment efficiency of fish market wastewater was found to be satisfactory based on COD and protein removal from fish market wastewater. However, no significant difference in the treatment efficiency was observed in three MFCs.
The combination of Equations (1)e(4) gives the following four electron ORR in presence of V2O5 catalyst. O2 þ 4Hþ þ 4e /2H2 O
(5)
Furthermore, a-MnO2 catalyst also favors four electron electrochemical reduction pathway of ORR in alkaline medium [10,28]. The electrochemical equations involved in a typical redox coupling reaction catalyzed by a-MnO2 have been explained by previous researchers [10]. The cathode prepared without catalyst (only Vulcan XC) had very little current over an applied voltage range (þ0.3 to 0.2 V vs. Ag/ AgCl). It suggests that only carbon matrix without catalyst is not capable of catalyzing ORR [17]. EIS was performed to evaluate interfacial charge transport behavior of the cathodes. Nyquist plots (imaginary component vs. real component of impedance) of all three cathode viz. with V2O5-MF and a-MnO2-NT as catalyst and that without catalyst are presented in Fig. 3B. As shown in figure, diameter of the semi-circle produced in the Nyquist plot was found to be highest for the cathode without catalyst followed by aMnO2-NT and V2O5-MF. It indicates a higher kinetic driving force for ORR by creating lower hindrance in charge transfer in V2O5-MF catalyzed cathode. Cathodic resistances such as charge transfer resistance (Rct) and solution resistance (Rs), were estimated by fitting the EIS spectra in an equivalent circuit. Solution resistances (Rs) of the three different cathodes were found to be same because of the same catholyte (1 M KCL solution) and cell configuration used in the electrochemical study. At high overpotential, Rs becomes dominant. However, solution resistance can be reduced by reducing electrode distance and change in reactor design [18]. Rct is responsible for regulating ORR overpotential. Lower Rct value of 3.7 U was found in the cathode using V2O5-MF, thus offering excellent electrochemical property; whereas the cathode containing a-MnO2NT catalyst had slightly higher Rct (5.5 U) and the cathode without any catalyst had a Rct of 13 U. This 3.5 and 1.5-fold lower Rct of V2O5-MF catalyzed cathode when compared to control cathode and a-MnO2 catalyzed cathode, respectively, exhibited excellent charge transport and lower ORR overpotential, thereby increasing the reductive current as confirmed by LSV data. The improved electrochemical kinetics of V2O5-MF decorated cathode is attributed to its porous structure and less agglomeration of catalyst. The porous structure facilitated higher oxygen penetration and increased contact area between electrode material and oxygen [15,29]. The specific property assisted faster regeneration of catalyst during oxidation process as per Eq. (4). Moreover, the flower like microstructure of V2O5-MF provided less agglomeration
Fig. 4 e COD and protein removal efficiency of the MFCs.
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Power production Effect of different low-cost cathode catalysts on power production and coulombic efficiency was evaluated. MFCs were operated in a batch mode of 4 days fed-batch cycle and monitored for 12 batch cycles. All three MFCs started giving voltage from the day of start-up, endorsing presence of active exoelectrogenic bacteria in septic tank mix consortia pretreated with ultrasonication. With days of operation, the anode potential (Ea) decreased, and the cathode potential (Ec) was found to increase in all three MFCs. Stable performance in all MFCs were found after 4 sequential fed-batch cycles. After 4th batch cycle, no significant difference was observed in Ea, and it was about 450 mV (vs. Ag/AgCl) in all three MFCs. The minimum Ec of 90 mV (vs. Ag/AgCl) was recorded in control MFC (MFC-C); and for MFC using a-MnO2-NT (MFC-M), Ec was 290 mV and it was 350 mV in MFC using V2O5-MF catalyzed cathode (MFC-V). Hence, it is clear that only the cathode performance had a major influence on the performance of MFCs. MFCs were operated under 1000 U external resistance, and current was measured daily. Power obtained from each MFC was normalized to total liquid volume of anode. In daily observations of power production from MFCs, MFC-V could generate a 32% and 74% higher power density with an average value of 2.30 ± 0.02 W m3 as compared to that of MFC-M (1.56 ± 0.02 W m3) and MFC-C (0.58 ± 0.01 W m3), respectively (Fig. 5A). Likewise, the coulombs recovery in MFC-V was found to be substantially enhanced, with an average CE of 15.2 ± 0.11%, which was 17% and 52.6% higher than that of MFC-M (12.2 ± 0.12%) and MFC-C (7.2 ± 0.11%), respectively (Fig. 5A). It is to be noted that the CE obtained from this study using fish market wastewater (COD ~ 1200 mg L1) was higher than those reported with air-cathode MFCs (3e12%) using platinum as catalyst on cathode treating domestic wastewater [34]. In the present study, significant improvement in CE was demonstrated using low-cost catalyst (V2O5-MF) as compared with costly platinum and a-MnO2 catalyst used in MFC treating real wastewater. Enhanced power density and coulombic efficiency in MFC using V2O5-MF as catalyst on cathode (MFCV) demonstrated higher electrochemical kinetics (high reduction current and low charge transport resistance) due to porous structure and N-type semiconductor property of V2O5MF catalyst. However, MFC-M also offered reasonable and comparable performance as reported earlier [35,36]. Polarization studies were performed to get optimum power density and corresponding current density normalized to total anodic volume and actual surface area of anode. The maximum power density was found to be enhanced from 0.8 W m3 (36.7 mW m2) in MFC-C to 5.5 W m3 (249 mW m2) in MFC-M and to 6.06 W m3 (273 mW m2) in MFC-V (Fig. 5B). This 7.7 and 1.1 times higher power (normalized to anodic volume) than MFC-C and MFC-M, respectively, in MFC-V shows excellent electrochemical property of V2O5-MF as catalyst in MFC. Ghoreishi et al. [9] reported power density of 79.26 mW m2 and 72.1 mW m2 using V2O5/PANI composite and platinum as catalyst, respectively, which is much lower than the value obtained in this study. It can be clearly observed from Fig. 5B (voltage vs. current density), a rapid voltage drop occurred upto a current density
Fig. 5 e (A) Coulombic efficiency (left Y-axes) and power generation (right Y-axes) of MFC-V, MFC-M and MFC-C under different batch cycles; (B) Polarization curves of MFCV, MFC-M, and MFC-C, Y-axes to the left represents cell potential, and Y-axes to the right represents power density. of 14 Am3 and beyond 30 Am3 in MFC-M as compared to MFC-V, indicating higher activation and concentration loss, respectively in MFC-M [1]. The voltage drop between current density of 14 A m3 e 30 A m3 was observed almost similar in MFC-M and MFC-V, suggesting these MFCs had same polarization overpotential (ohmic loss). As compared to these two MFCs, MFC-C supported less current density of 16.4 Am3. In comparison with a-MnO2 catalyst in this study and platinum catalyst [9,34], the MFC with V2O5 catalyst cathode had superior performance, ascertaining it as a highly attractive lowcost catalyst. Due to lower fabrication cost and highly appreciable performance, V2O5 could be used to fabricate larger MFCs (scalable MFC) to handle high volume of wastewater and simultaneous generation of bio-electricity. A low total internal resistance of 55 U was obtained from MFC-V as compared to the MFC-M (63 U) and MFC-C (133 U). This internal resistance trend observed is in good agreement with the EIS results. However, the internal resistance
Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163
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observed was higher as compared to earlier studies that might be attributed to the difference in separator-electrode assembly (SEA) and reactor configuration used with more distance between the electrodes [37]. Based on the results obtained from LSV, EIS and performance of MFCs, it may be concluded that the V2O5-MF catalyzed cathode had lower ORR overpotential due to which MFC-V produced better power as compared to the other two MFCs (MFC-M and MFC-C) by recovering additional coulombs.
Conclusion Effective application of V2O5 microflower porous catalyst in cathode of MFC was demonstrated. The LSV and EIS spectra showed enhanced electrochemical kinetics and ORR using V2O5-MF catalyst on cathode due to better contact of oxygen within the porous structure of catalyst. The power density of MFC using V2O5-MF catalyst on cathode was found to be enhanced by 9.2% and 79.6% with a significant increase in CE by 17% and 52.6% when compared with MFC using a-MnO2-NT as a cathode catalyst and using only carbon powder as cathode material, respectively. However, organic matter and protein removal efficiency were similar in all MFCs. Thus, V2O5MF can be used as a cathode catalyst to improve power produced from MFC for commercial application.
Acknowledgment The grant received from Department of Science and Technology, Govt. of India (File No. DST/TSG/NTS/2010/61) to undertake this research work is duly acknowledged.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.12.163.
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Please cite this article in press as: Noori MdT, et al., V2O5 microflower decorated cathode for enhancing power generation in air-cathode microbial fuel cell treating fish market wastewater, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2015.12.163