Application of graphene-based nanomaterials as ...

Journal of Power Sources 327 (2016) 548e556

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Application of graphene-based nanomaterials as novel cathode catalysts for improving power generation in single chamber microbial fuel cells Alireza Valipour, Sivasankaran Ayyaru, Youngho Ahn* Department of Civil Engineering, Yeungnam University, Gyeongsan, 38541, South Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

RGOHI-AcOH and RGO/Ni nanoparticles are studied as MFC cathode catalysts. HI-AcOH reductant positively influences the formation of RGO frameworks. Cathode catalyst loading plays a role in the MFC power generation enhancement. RGOHI-AcOH-DL MFCs (1683 ± 23 mW/ m2) showed higher performance than other non-Pt MFCs. RGOHI-AcOH could be a cost-effective alternative to Pt-based cathode catalyst.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 March 2016 Received in revised form 23 July 2016 Accepted 25 July 2016

The low catalytic activity, limited resources, complexity and costs, and non-environmentally friendly nature are key factors limiting the application of non-precious metals and their composites at the cathode in microbial fuel cells (MFCs). This study evaluated the feasibility of graphene-based nanomaterials (RGOHI-AcOH vs. RGO/Ni nanoparticle composite) as novel cathode catalysts in single chamber air-cathode MFCs. A series of MFCs with different catalyst loadings were produced. The electrochemical behavior of the MFCs were evaluated by cyclic voltammetry (CV) and impedance spectroscopy (EIS). As a result, the MFCs with the RGOHI-AcOH cathodes showed greater maximum power densities (>37%) than those with the RGO/Ni nanoparticle cathodes. In the MFCs, the highest maximum power density of 1683 ± 23 mW/m2 (CE ¼ 72 ± 3%), which covers 77% of that estimated for Pt/C (2201 ± 45 mW/m2, CE ¼ 81 ± 4%), was obtained from the double loading RGOHI-AcOH cathodes. Among the MFCs with the RGO/Ni nanoparticle composite cathodes, those loaded with a double catalyst (1015 ± 28 mW/m2, CE ¼ 70 ± 2%) showed better power performance than the others. Both CV and EIS showed good agreement with the MFC results. This study suggests that the RGOHI-AcOH cathode, particularly with a double catalyst loading, is promising for sustainable low-cost green materials, stable power generation and the long-term operation of MFCs. © 2016 Elsevier B.V. All rights reserved.

Keywords: Microbial fuel cell Cathodic catalyst Graphene Nickel nanoparticle Nanocomposite

1. Introduction * Corresponding author. E-mail address: [email protected] (Y. Ahn). http://dx.doi.org/10.1016/j.jpowsour.2016.07.099 0378-7753/© 2016 Elsevier B.V. All rights reserved.

Microbial fuel cells (MFCs) are a novel, environmentally friendly and promising alternative for generating power directly from

A. Valipour et al. / Journal of Power Sources 327 (2016) 548e556

biodegradable organic matter using bacteria [1]. Microbes in the anodic compartment produce electrons and protons from the oxidation of organic matter, with CO2 and biomass as the final products. The electrons produced are transferred to the cathode compartment through an external circuit, reacting with electron acceptors and producing an electric current [2,3]. This can open up a wide range of applications where energy is produced from wastewater whilst simultaneously achieving the treatment objectives [4]. On the other hand, the low power output of MFCs restricts their practical applications [5]. In addition to the other factors affecting the MFC performance, such as the cell design, bacterial inoculum, substrate, ion-selective membrane, and anode surface area and material [6], the cathode can be considered to bottleneck the MFCs performance as a result of the poor kinetics of oxygen reduction in the medium [1]. Therefore, to facilitate the cathode reaction in MFCs, it is important to look into the catalyst materials, which ideally have a high surface area, high conductivity, high catalytic activity, good stability, and low cost. Noble metal nanoparticles, such as platinum (Pt), gold (Au), palladium (Pd), and their alloys have shown promising catalytic activities towards the oxygen reduction reaction (ORR). Among these, Pt is the most common fuel cell catalyst at the cathode. On the other hand, their high cost and limited availability along with catalyst poisoning are obstacles to the real-world and large-scale application [7]. Therefore, many studies have attempted to improve the ORR reactivity at the cathode surface using inexpensive catalysts (e.g. Co/Fe/N, CoOx/FePc, CoTMPP, FePc, MnOx, MnPc) [4,8e11]. The point of the data generated by these studies emphasis that Pt can be replaced by inexpensive metal catalysts, while at the same time increasing power generation. The different electrodes could exhibited different behaviors and electrode modification, proved to be a good alternative for enhancing the MFCs performance. In view of that, carbon-based materials such as Vulcan XC72 (VC), Ketjenblack (KJB), activated carbon (AC), graphite (G) and carbon nanotubes (CNTs) have been used as a support composite for non-precious metal catalysts to overcome their self-aggregation and improve their catalytic activity in fuel cells. Nevertheless, these carbonaceous materials are relatively inactive toward the electroreduction of oxygen [9,12]. The recent emergence of graphene nanosheets has opened a new avenue for utilizing two-dimensional (2D) carbon materials as a catalyst or in composite catalysts because of its theoretically high surface area, high electrical conductivity, good chemical stability, and high mechanical strength [5,13]. Such intense research activities have been performed using graphene and N-functionalized graphene alone and in combination with Fe, MnO2 or Pt/Co alloy nanoparticles in MFC cathodes [5,7,14e16]. Accordingly, 3D graphene-nickel foam anode has also been studied in MFC application [17]. The chemical reduction of graphene oxide (GO), and concomitantly with metal salts, using common reducing agents, such as N2H4 and NaBH4, is a scalable method to obtain graphene (i.e. reduced graphene oxide, RGO) and its composite catalyst. On the other hand, pure RGO agglomerates as a powdery precipitate due to van der Waals forces, resulting in a loss of its electrochemical properties. Moreover, the use of reductants (as N2H4, and NaBH4) can pose an environmental and health risks as they are toxic and harmful [7,18e20]. For example, these reductants cause growth inhibition and mortality of aquatic organisms. Hepatic changes due to reductants inhalation have been demonstrated in animals. They are known to possibly cause liver damage and affect the skin and central nervous system. The reductant vapors are readily absorbed through the lung. These reductants can also cause cancer in human who are exposed to them [21,22]. Metal nanoparticles between the RGO sheets can also act as spacers that can prevent the aggregation of the sheets during drying. Ni-nanoparticles would also be more interesting because of

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their abundant and low cost. Despite this, there are no reports on the use of RGO/Ni nanoparticle composites as a cathode catalyst in MFCs. On the other hand, the fabrication of composite catalysts could be complex and expensive, and even non eco-friendly because of the toxic nature of the metal nanoparticles used, hindering their practical application. Despite this, it is still important to study RGO/ Ni nanoparticle composites for the cathode catalyst. This can be a practical rationale to gain information regarding the synergistic effect between graphene and Ni nanoparticles, that play a crucial role in the catalytic activity and selectivity of composite catalysts. In addition, the application of single highly conductive graphene materials as an alternative cathode catalyst can be a practical approach to overcoming the field-of-view restrictions on the composite catalysts and enhancing the MFC efficiency. Hydriodic acid with acetic acid (HI-AcOH) is an effective, non-toxic and versatile reducing agent for the synthesis of RGO with high electrical conductivity. Therefore, it is important to assess RGOHI-AcOH (as an efficient metal-free cathode catalyst) as a means of improving the performance of MFCs. To the best of the authors' knowledge, there are no reports describing the use of RGOHI-AcOH in the cathode for MFC applications. This study examined the feasibility and potential effectiveness of a highly conductive graphene material (RGOHI-AcOH) and a graphene nanoparticle composite (RGO/Ni) as the cathode catalyst in air-cathode MFCs for power generation. In addition, a commercial Pt/C catalyst was used as a benchmark material for comparison. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to examine the electrochemical properties of the MFCs. 2. Materials and methods 2.1. Catalysts preparation and characterization The details of preparation and characterization of RGOHI-AcOH and RGO/Ni nanoparticle composite catalysts can be found in a previous publication [23]. Fig. 1 presents a schematic flow sheet for the synthesis of catalysts. Briefly, GO was prepared from natural graphite powder using a simple room temperature method. The RGO catalyst was synthesized through a one-pot chemical reduction method using HI-AcOH. Accordingly, GO (0.4 g) was dispersed in acetic acid (150 ml). This dispersion was sonicated until it became clear with no visible particulate matter. HI (8 ml) was then added and stirred for 24 h at 40 C. The product was isolated by filtration and washed sequentially with saturated sodium bicarbonate, distilled water, and acetone. The product was then vacuum dried at 40 C. The RGO/Ni nanoparticle composite catalyst at a mass ratio of 1:1 was prepared by one-step hydrothermal process using GO and NiCl2$6H2O (20:80 mg/mg) with urea (0.1 g) and hydrazine hydrate (1 ml) at 140 C for 12 h. After cooling naturally to room temperature, the resulting black solid was filtered off, washed with distilled water, and vacuum dried at 40 C. Here, we provided just a brief overview of main characterization of resulting materials using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. SEM images were obtained using a field-emission scanning electron microscope (S-4200, Hitachi, Japan). XPS data were determined using a Thermo Scientific K-Alpha XPS system (Thermo Fisher Scientific, UK) that is equipped with a monochromatic Al Ka source with a spot size of 400 mm and a pass energy of 30 eV. Raman spectra were collected from a Raman spectroscopy (HORIBA Jobin Yvon LabRAM HR, France). Besides, the specific surface area of as-synthesized catalysts were determined using the methylene blue (MB) adsorption method.

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Fig. 1. Schematic diagram of the process synthesis of RGOHI-AcOH, and RGO/Ni nanoparticle composite catalysts.

2.2. Preparation of electrodes Initially, the electrode surfaces were cleaned by sequential sonication (15 min each) in acetone, a 1 M HCl solution, deionized water, and ethanol, followed by drying. The anodes were a graphite brush (2.5 cm diameter, 2.5 cm long, 0.22 m2 surface area, 95% porosity and average fiber diameter of 7.2 mm) (PANEX33 160 K, ZOLTEK, USA) treated with ammonia gas [24,25]. The air-cathode was prepared using a previously reported procedure [26]. 30 wt % wet-proofed carbon cloth (type B-1B, E-TEK; 7 cm2 total exposed surface area) was used as the backbone cathode electrode. On the air facing side, one carbon base layer and four polytetrafluoroethylene (PTFE, 60 wt % dispersion in H2O, Sigma Aldrich, USA) diffusion layers were applied. Catalyst ink (for 1 mg of catalyst) was prepared by sonicating (30 min) the catalyst with 10 wt % carbon black (Vulcan XC-72, Gashub) in 3.31 ml isopropyl alcohol (Duksan Chemical, Korea), 0.83 ml distilled water, and 6.67 ml Nafion solution (Sigma Aldrich, USA), and then painted on carbon cloth (liquid side). The catalyst employed was maintained at a loading of 0.5 mg/ cm2 (single loading (SL)), 1 mg/cm2 (double loading (DL)), and 1.5 mg/cm2 (triple loading (TL)). For comparison, a commercial Pt/C catalyst (10% Pt on Vulcan XC-72, BET surface area ~220 m2/g, Etek, USA) was loaded into the air cathode at 0.5 mg/cm2 using the same procedure. 2.3. MFC construction Air-cathode single chamber cylindrical MFCs (made of acrylic

glass) with a liquid chamber 4 cm long by 3 cm in diameter (28 ml liquid volume), were constructed, as previously described [2,27]. The cathodes were placed on one side of the MFCs as the catalystcoated layer facing the solution and the PTFE layers were exposed directly to air. The brush anode was positioned horizontally on the other side of the chamber at a distance of 1 cm from the cathode without any membrane used. Titanium wire was used to connect the circuit with an external load. 2.4. MFC start-up and operation The MFCs were inoculated by mixing an anaerobic sludge with the culture media (1:9 v/V), and replaced 5 times (over 120 h) to allow a biofilm to form over the anode surface. Subsequently, the MFCs were continued to feed with a pure culture medium. The culture medium was prepared containing 1 g/L sodium acetate in a 50 mM phosphate buffered solution (PBS) amended with 12.5 ml metal and 12.5 ml vitamin solutions [26]. The 50 mM PBS contained 0.13 g/L KCl, 3.32 g/L NaH2PO4$2H2O, 10.32 g/L Na2HPO4$12H2O and 0.31 g/L NH4Cl. All MFCs were operated in batch mode with an external resistance of 46 U at 30 ± 1 C. Initially, the voltage production was low and the voltage increased gradually as the anode biofilm was enriched with electrochemically-active bacteria, rendering it stable. The chamber was refilled each time the voltage decreased to less than 5e10 mV, forming a complete cycle of operation. All MFC tests were conducted in triplicate and the average value was calculated. The best MFC at the end of the experiments (under optimized condition) was fed with a culture


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medium containing 1 g/L glucose for a better comparison (see Supplementary Information for details). 2.5. Analytical methods The cell voltage produced during the experiments was recorded using a precision multimeter and a data logger (GL820, Graphtec, Yokohama, Japan). The current was calculated as I ¼ V/R, where I (mA) is the current, V (mV) is the voltage and R is the external resistance (U). The power was calculated according to Ohm's law, P ¼ IV [28,29]. The ‘‘single cycle’’ method has been used to obtain the maximum power densities of MFCs. Accordingly, the polarization, under the stable stage of MFCs (i.e. when the substrate was sufficient and the voltage output was kept stable), were plotted after stabilization of the open circuit potential (OCP) by varying the external resistance from 10000 U to 20 U in decreasing order (at a time interval of 30 min) [30]. During polarization test, each electrode potential (anode and cathode) was recorded with respect to a saturated Ag/AgCl (3 M KCl) electrode by placing it into the reactor closed to the cathode using a multimeter (Fluke 111) [31]. The coulombic efficiency was calculated as CE ¼ CP/CT 100%, where CP is the total coulombs calculated by integrating the current over time, and CT is the theoretical amount of coulombs available based on the chemical oxygen demand (COD) removed in the MFC [28,29]. The influent and effluent COD concentrations were determined using a HACH DR 2500 spectrophotometer. The electrochemical behavior, cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) of the MFCs were evaluated under open circuit condition using an Autolab PGSTAT128N electrochemical workstation (Metrohm Autolab B.V., The Netherlands). The working electrode connector was connected to the anode and both the counter electrode and reference electrode connectors were connected to the cathode. CV was performed in the potential range between 1 and -1 V at scan rates of Z V2 10 mV/s. The absolute area under the CV curves was calculated as AUC ¼ IðVÞdV, where AUC (I.V) is the absolute area, V2 e V1 is V1 the potential range, and I(V) is the response current (A). EIS was carried out over the frequency range, 0.1 Hze1 MHz, with an amplitude of 0.05 mV. 3. Results and discussion 3.1. MFC performance Fig. 2 presents the output voltages of the MFCs during four cycles (at an external resistance of 46 U). After an acclimatization period, all MFCs exhibited reproducible cycles of voltage generation. With the replacement of fresh culture media (under batch operation), the voltage increased rapidly, maintained its steady value over a period time, and decreased gradually due to substrate depletion. The voltage profiles exhibited a similar trend of an increase and decrease, but at different levels depending on the type of catalyst and its strength: Pt > RGOHI-AcOH > RGO/Ni nanoparticle catalyst cathode MFCs. Accordingly, the cycle of voltage generation lasted for approximately 25 h. Over four batch cycles of operation, the average maximum voltage reached 263 ± 0.8 mV for Pt/C MFCs. The highest voltage reached 174 ± 1.1 mV for SL, 232 ± 0.7 for DL, and 211 ± 1.1 for TL RGOHI-AcOH cathode catalyst MFCs. In the RGO/ Ni nanoparticle MFCs, the highest voltage reached 133 ± 0.5 mV for SL, 177 ± 0.4 mV for the DL, and 165 ± 0.3 mV for the TL cathode catalyst. Fig. 3a and b shows the power density and polarization curves as a function of the current density of the different MFCs. Obviously, a high power density (2201 ± 45 mW/m2, 1.02 ± 0.01 mA/cm2) is achieved when the MFCs are equipped with the Pt/C cathodes. This

Fig. 2. Voltage generation (at an external resistance of 46 U) of the MFCs produced with different cathodes.

can primarily be explained by the fact that ORR proceeds favorably via the four-electron reduction pathway on Pt/C catalyst, while the two-electron ORR pathway occurs on graphene-based materials [32]. MFCs with a RGOHI-AcOH catalyst on the cathodes highlight the remarkable performance compared to those of the MFCs using RGO/Ni nanoparticle cathodes. This can attributed to the ideal properties of RGOHI-AcOH [23], which make it more suitable for scaling up the MFC systems. According to the SEM image in Fig. 4aei, RGOHI-AcOH has crumpling features with clear layers, which could be responsible for the high surface area and surface nanostructure (343 m2/g). SEM (Fig. 4a-ii) also confirmed the spherical-like Ni nanoparticles with a relatively uniform size grown (stacked randomly) on or between the RGO layers, which might hinder aggregation (245 m2/g). Fig. 4b shows the broad scan XPS spectra (0e1350 eV) indicating carbon, oxygen, and nickel as the main species in the as-prepared catalysts. XPS analysis revealed the atomic ratio of oxygen and carbon (C/O) in RGOHI-AcOH to be 5.17. A high C/O ratio indicates the high current conductivity of the graphene materials, suggesting the efficiency of a particular reduction method. The fraction of oxygen in the RGO/Ni nanoparticle composite could be due either to surface oxidation while transferring sample from the deposition chamber to the XPS system or the adsorption of OH (/H2O) during its fabrication. The Raman spectra (Fig. 4c) of carbonaceous materials revealed the characteristic D, G, and 2D bands at ~1348, ~1590, and ~2921 cm1, respectively. The Raman intensity ratio of the G/D bands was higher in RGOHI-AcOH (ID/IG ¼ 1.2) than in the RGO/Ni nanoparticle composite (ID/ IG ¼ 0.98), verifying the enhanced degree of graphitization. The performance of the MFCs can also be influenced by the amount of catalyst loaded on the cathodes (Fig. 3a and b). The MFCs with RGOHI-AcOH-DL cathodes (1683 ± 23 mW/m2, 0.90 ± 0.006 mA/ cm2) showed highest catalytic activity and power generation (14e66%) than the MFCs using RGOHI-AcOH-SL (988 ± 21 mW/m2, 0.69 ± 0.007 mA/cm2), RGOHI-AcOH-TL (1455 ± 27 mW/m2, 0.83 ± 0.008 mA/cm2), RGO/Ni nanoparticle-SL (581 ± 19 mW/m2, 0.42 ± 0.007 mA/cm2), RGO/Ni nanoparticle-DL (1015 ± 28 mW/m2, 0.56 ± 0.008 mA/cm2), and RGO/Ni nanoparticle-TL (904 ± 21 mW/ m2, 0.53 ± 0.006 mA/cm2) cathodes (Table 1). The results suggest that the RGOHI-AcOH-DL can be a feasible cathode catalyst for MFCs by covering 77% of the estimated power density compared to Pt/C. RGOHI-AcOH-SL MFCs delivered almost similar power density

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Fig. 3. (a) power density curves, (b) polarization curves and (c) cathode and anode potentials as a function of the current density in MFCs with different cathodes.

Fig. 4. (a) SEM images, (b) XPS survey spectra, and (c) Raman spectra of (i) RGOHI-AcOH, and (ii) RGO/Ni nanoparticle composite catalysts.


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Table 1 Performance comparison of MFCs using different cathodic catalysts. Parameters

OCP (mV) Max. potential (mV) Max. power (mW) Max. current (mA) Max. power density (mW/m2) Max.volumetric power density (W/m3) Max. current density (mA/cm2) Max.volumetric current density (A/m3) Coulombic efficiency (%)

MFCs Pt/C

RGOHI-AcOH SL

DL

TL

SL

DL

TL

820 ± 12 215 ± 2.20 1.54 ± 0.03 7.17 ± 0.07 2201 ± 45 55 ± 1.13 1.02 ± 0.01 256 ± 2.62 81 ± 4

666 ± 11 144 ± 1.50 0.69 ± 0.02 4.80 ± 0.05 988 ± 21 25 ± 0.52 0.69 ± 0.007 172 ± 1.80 75 ± 3

727 ± 9 188 ± 1.30 1.18 ± 0.02 6.27 ± 0.04 1683 ± 23 42 ± 0.60 0.90 ± 0.006 224 ± 1.60 72 ± 3

726 ± 12 174.8 ± 1.60 1.02 ± 0.02 5.83 ± 0.05 1455 ± 27 36 ± 0.67 0.83 ± 0.008 208 ± 1.91 74 ± 2

602 ± 7 136.8 ± 2.20 0.41 ± 0.01 2.97 ± 0.05 581 ± 19 15 ± 0.50 0.42 ± 0.007 106 ± 1.71 73 ± 3

683 ± 11 180.8 ± 2.50 0.71 ± 0.02 3.93 ± 0.06 1015 ± 28 25 ± 0.70 0.56 ± 0.008 140 ± 2 70 ± 2

680 ± 9 170.6 ± 2 0.63 ± 0.02 3.71 ± 0.04 904 ± 21 23 ± 0.53 0.53 ± 0.006 133 ± 1.60 71 ± 3

RGO/Ni nanoparticles

SL: single load, DL: double load, TL: triple load.

(merely 9% higher) to the MFCs with the RGO/Ni nanoparticle-TL cathodes. Among the MFCs using the RGO/Ni nanoparticle cathodes, the RGO/Ni nanoparticle-DL MFCs produced a higher power density (11e43%) than the others. In this context, the deposition of Ni nanoparticles on the graphene sheets has a benefit to maximize the availability of surface area for mass transport of reactants and provide a better collection and acceptance of electrons to the graphene [33]. The good performance of MFCs with double loading catalyst cathodes can be attributed to the appropriate amount of the catalysts loaded on the cathode electrode leading to a suited thickness of catalyst layer and less aggregation of catalyst, which causes a higher catalytic mass activity. On the other hand, the thick catalyst layer (as triple loading) can interrupt electron transfer and electrochemical reaction in the cathode, possibly due to the water generated from the catalyst layer [34,35]. Fig. 3c presents the individual electrode (cathode and anode) potentials versus current densities. The potential variations for the cathode were much more distinctive than the anode potential variation for the different MFCs. The anode potentials were similar for all MFCs, whereas the cathode potentials varied over a wide range. The variation in cathode potentials can be attributed to the efficiency of the different catalysts and their loading towards oxygen reduction. Table 1 lists the other important parameters of the MFCs with different cathodes. A high OCP (820 ± 12 mV) was obtained in the MFCs with Pt/C cathodes, suggesting the intrinsic catalytic activity of the catalyst. OCPs of the MFCs using RGOHI-AcOHDL (727 ± 9 mV) and -TL (726 ± 12 mV) cathodes were similar, but higher than all the other non-precious (/metal) cathode MFCs (covering 89% of the estimated OCP in Pt/C MFCs). Among the RGO/ Ni nanoparticle cathode MFCs, the OCPs of the MFCs with RGO/Ni nanoparticl eDL (683 ± 11) and eTL (680 ± 9) cathodes were also identical. The OCPs of the MFCs with a single loading RGOHI-AcOH (666 ± 11) and the RGO/Ni nanoparticle (602 ± 7) cathodes were lower than that of the MFCs with the double and triple loading of the same cathode materials. The MFCs with Pt/C cathodes (ca. 80%) show significantly higher CEs than all others due to the both higher catalytic activity and current generation. Despite the difference in maximum power densities produced by RGOHI-AcOH and RGO/Ni nanoparticle catalysts and their loading, the CEs were all ca. 70%. This suggests that the CE was either mainly determined by the performance of the anode (i.e. by the ability of bacteria to oxidize the substrate), or that, the differences in the cathode materials for the conditions examined here was not sufficiently altered in terms of oxygen diffusion or other characteristics. In fact, almost similar reaction kinetics have been constructed at the respective anodes in this study. The current study illustrated the feasibility of using a highly conductive graphene material (RGOHI-AcOH), rather than a graphene

nanoparticle composite (RGO/Ni) as a cathode catalyst in MFCs practices. Therefore, in order to ensure the viability of the findings, the MFCs with a double loading RGOHI-AcOH cathodes have been compared particularly further with those of the MFCs used either graphene or transition metal-carbon composite materials as a cathode catalyst in the literature (Table 2). The table shows that the MFCs with Pt/C cathode in this study yielded a higher performance than cited Pt/C MFCs. There are also definite differences between the values reported for Pt/C MFCs in the literature. This can be mainly correlated to the anode type, the anodic bacterial respiration, and the process of operational strategy. The RGOHI-AcOH-DL MFCs exhibited higher power production performance than the reported MFCs equipped with cathodes using graphene either as a catalyst (crumpled RGO, flat RGO, N/RGO) or in the composite catalyst (RGO/Pt/Co, RGO/MnO2, Fe/N/RGO). The MFCs with the cathodes containing the RGOHI-AcOH-DL catalysts were also found to be more efficient than those reported for transition metal-carbon composite catalysts (Co/Fe/N/CNT, CoOx/FePc/VC, MnO/VC, FePc/ KJB, MnPc/VC, CoTMPP/KJB). A previous study reported that nonprecious metal catalysts generally have low catalytic activity and stability compared to Pt [36]. 3.2. Electrochemical analysis The CV results (Fig. 5a) revealed obvious redox peaks in all nonprecious (/metal) cathode MFCs. Each anodic peak indicated the presence of the redox species in the microorganisms. No distinct redox peaks were observed from the MFCs with the Pt/C cathodes during the CV tests, but the current response (AUC ¼ 14.82 I.V, limiting current ¼ 7.09 mA) was high, indicating the catalytic activity of Pt/C catalyst and electrogenic activity of the microbial population on the anode surface. Indeed, the lack of clear redox peaks does not necessarily mean no catalytic activity [5]. MFCs with RGOHI-AcOH cathodes showed a larger absolute area (7.52 I.V AUC for SL, 10.08 I.V AUC for DL, and 8.40 I.V AUC for TL) compared to the RGO/Ni nanoparticle cathode MFCs (4.81 I.V AUC for SL, 6.30 I.V AUC for DL, and 5.84 I.V AUC for TL). Similarly, the limiting current of RGOHI-AcOH MFCs (5.06 mA for SL, 6.28 mA for DL, 5.73 mA for TL) was higher than those with RGO/Ni nanoparticles cathodes (3.74 mA for SL, 4.97 mA for DL, 3.90 mA for TL). This could be attributed to the enhanced surface area and electrical conductivity of the RGOHI-AcOH catalyst. As the effect of the amount of catalysts loaded on the cathodes, the RGOHI-AcOH-DL MFCs showed larger current responses among the other non-precious (/metal) cathode MFCs (covering 68% of the AUC and lowering 11% limiting current estimated in Pt/C MFCs). The RGO/Ni nanoparticle-DL MFCs revealed greater current responses than the RGO/Ni nanoparticleSL and -TL MFCs. Reduced current responses were observed with

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Table 2 Performance comparison of MFCs using RGOHI-AcOH-DL cathodes with other literature values. Catalyst cathode

Cathode materials

Anode materials

Substrate

OCP (mV)

P Max (mW/m2)

P Max (W/m3)

CE (%)

Reference

RGOHI-AcOH RGOHI-AcOH Crumpled RGO Flat RGO N/RGO RGO/MnO2 RGO/Pt/Co Fe/N/RGO Co/Fe/N/CNT CoOx/FePc/VC MnO/VC FePc/KJB FePc/KJB MnPc/VC CoTMPP/KJB

Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon

Carbon brush/Ammonia-treated Carbon brush/Ammonia-treated Carbon brush/Heat-treated Carbon brush/Heat-treated Carbon brush Carbon cloth Carbon cloth/Acid/heat- treated Carbon felt Carbon paper Carbon cloth Carbon cloth Graphite felt Carbon cloth/Ammonia-treated Carbon cloth/Ammonia-treated Carbon cloth/Ammonia-treated

Sodium acetate Glucose Sodium acetate Sodium acetate Sodium acetate Sodium acetate Glucose Sodium acetate Glucose Sodium acetate Sodium acetate Sodium acetate Glucose Glucose Glucose

727 696 650 650 600 812 710 160 473 695

1683 (2201) 1381 e e 1618 (1423)

42 (55) 35 3.3 (4.8) 2.5 (4.8) e 5 (6) e e e e

72 57 e e e 12 72 e e 27 e e 21 20 20

This study

cloth cloth cloth cloth cloth cloth cloth paper cloth cloth cloth paper paper paper paper

(820) (750) (750) (555) (839) (770) (110) (330) (820)

801 (800) 760 (800) 786 (800)

1378 (1406) 1150 (561) 751 (498) 654 (850) 606 (NA) 676 (820) 634 (474) 353 (474) 483 (474)

e e e e

( ): Values obtained in MFCs with Pt/C cathode as a control. CNT: carbon nanotube, VC: vulcan XC-72, KJB: ketjenblack.

Fig. 5. (a) Cyclic voltammetry, (b) Nyquist impedance plots, and (c) related-impedance parameters of MFCs with different cathodes.

(81)

[5]

(13) (52)

(28)

(20) (20) (20)

[15] [7] [14] [16] [9] [10] [11] [4] [8]


increasing non-precious (/metal) catalyst (at triple strength) on the cathodes. The lowest current responses were obtained for the MFCs equipped with the single loading catalyst cathodes. The EIS Nyquist plots of the MFCs with different cathode electrodes are shown in Fig. 5b. All the plots were comprised of a single semicircle in the high frequency region (not completely obvious) followed by a straight line in the low frequency region due to the diffusion process (Warburg diffusion). By fitting the data of the Nyquist plots, the each impedance value was obtained (Fig. 5c). The intersection of the semicircle on the real axis represents the ohmic resistance (Rohm) of the MFCs. Since all MFCs were operated under exactly the same conditions, the difference in the ohmic resistance should be attributed to the conductivity and the type of catalyst used [37]. The charge-transfer resistance (RCT) at the electrode/ electrolyte interface corresponds to the diameter of the semicircles. The diffusion coefficients (D) of the substrate diffusing into the cathode electrode are calculated as D ¼ 0.5 (RT/AF2sC)2, where R is the gas constant (J/mol$K), T is the absolute temperature (K), A is the surface area of the cathode electrode (cm2), F is the Faraday's constant (amp.s/mol), s is the slope of the straight lines represents the values of the Warburg coefficient (ohm/s1/2), and C is the molar concentration of the PBS solution (mol/cm3) [38]. From this, the MFCs with the Pt/C cathodes have the lowest ohmic resistance (15.86 U), the smallest charge-transfer resistance (1.6 U) (except RGOHIeAcOH-DL MFCs) and the most ideal electrolyte diffusion (11.96 1010 cm2 s1), suggesting its higher catalytic activity than the other non-precious (/metal) cathodes used. A higher conductivity of Pt/C catalyst could be lead to a higher energy of the electron which transport to the electrode, resulting a lower chargetransfer resistance and a higher electrolyte diffusion. Regardless of the impedance of the Pt/C MFCs, the MFCs with RGOHI-AcOH cathodes showed a lower ohmic resistance, a smaller chargetransfer resistance, and faster electrolyte diffusion than the RGO/ Ni nanoparticle MFCs. According to the characterization results (Fig. 4), this could be attributed to the RGOHI-AcOH catalyst having a multi-layer structure, higher surface area, and higher degree of graphitization than the hydrazine-produced RGO-based materials, thus a higher conductivity and catalytic activity was obtained. As a result of the catalyst loading on the cathode, the RGOHI-AcOH-DL MFCs exhibited the lowest ohmic resistance (18.26 U), smallest (invisible) charge-transfer resistance (0.85 U) and a better electrolyte diffusion (9.26 1010 cm2 s1) than all other non-precious (/metal) cathode MFCs. Here, the apparent charge-transfer resistance was smaller than those with the Pt/C MFCs. The possible reason could be explained by a double catalyst loading which can provide a higher quantitative catalytic activity and surface area in contact with the solution. In fact, the use of larger amounts of catalyst material increases the surface area for the charge transfer reaction between active material and electrolyte [39]. Accordingly, the impedance analysis also highlights the effectiveness of the RGO/Ni nanoparticle-DL as a catalyst cathode material in enhancing the ohmic resistance (20.62 U), charge-transfer resistance (4.73 U) and electrolyte diffusion ability (6.14 1010 cm2 s1) among the MFCs with the RGO/Ni nanoparticle cathodes. A higher catalyst load (triple loading) had a slightly negative effect on the impedancerelated parameters of MFCs possibly due to the thickness of the catalyst layer and aggregation of the catalyst [34,35]. MFCs with the single load catalyst cathodes showed inferior impedance performance behaviors.

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performed under steady-state condition (with sodium acetate substrate) to confirm the durability of the RGOHI-AcOH catalyst (Fig. 6). Approximately 27 cycles were obtained at the MFCs with RGOHI-AcOH-DL cathode over a 30 day period. The power density of the MFCs with the RGOHI-AcOH-DL cathode was maintained at a similar level. These results highlight the good stability and reproducibility of the RGOHI-AcOH tested cathode material. 3.4. Pre-estimation of capital cost Financial analysis can be considered an important tool describing a sustainable technology. The costs associated with MFCs concentration depend on the type of material and market forces. A pre-estimation of economic analysis was done for the 1 m3 fed-substrate (required cathode area of 25 m2) based on the results generated by bench scale trials (Table S1). The total capital investment of the cathode electrodes with RGOHI-AcOH-DL and Pt/C catalyst for an estimated 1 m3 fed-substrate were 800 US$/m2 (by 100 US$/m2 catalyst cost) and 2800 US$/m2 (by 2100 US$/m2 catalyst cost) cathode area, respectively. This suggests that the cathode electrode using RGOHI-AcOH-DL as a catalyst could lead to 72% lower total cost than that of a Pt/C catalyst (95% lower costbased catalyst), which could make the MFC process more sustainable. The estimated cost is consistent with reported literature indicating 15 US$/m2 for activated carbon cathode and 1814 US$/ m2 for Pt/C cathode in MFC applications [40]. In this study, the cathodes were prepared by spreading activated carbon (8.8 mg/ cm2) and Pt/C (0.5 mg/cm2) catalysts on a stainless steel mesh and carbon cloth (30% wet proof), respectively. Here, in the case of stainless steel mesh a higher catalyst loading was preferable. It should be noticed that due to lower cost and higher conductivity than many carbon based materials, stainless steel mesh could be used as a base material for the MFC cathode [41]. Activated carbon is usually pressed onto a stainless steel mesh since it performs lower catalytic performance on carbon cloth [42]. Nevertheless, the corrosion of stainless steel mesh is possible when used in MFCs, which results in an increased ohmic resistance and a decreased performance. This suggests that metal based electrode supporting materials may lack the stability needed and therefore may be limited in their application in MFCs for treating certain types of wastewaters [41]. In contrast, carbon cloth is a very strong, thin, flexible, and lightweight synthetic matrix with a high porosity

3.3. Stability of the RGOHI-AcOH catalyst Based on the above findings, the RGOHI-AcOH (particularly with double loading) could be chosen as an alternative cathodic catalyst to platinum in the MFCs. Accordingly, a long term stability test was

Fig. 6. Performance of the MFCs equipped with RGOHI-AcOH-DL cathodes during longterm operation under steady-state condition at an external resistance of 46 U.

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and good electrical conductivity, which could offer a significant advantage to be used as the backbone cathode electrode in MFCs [23]. On the other hand, activated carbon is aggregated after loading on electrode based materials, and consequently a large fraction of carbon atoms cannot be accessed by the ions in the electrolyte [43]. As such, being a material made up of one single atomic layer (as RGOHI-AcOH), it is lighter, reduced require catalyst loading, reduced aggregation and ecologically friendly, unlike most other catalyst. If one catalyst material has been reduced the aggregation, it means a higher surface area in contact with the solution and better catalytic activity. Additionally, the different cost estimation performed for platinum cathode in the literature can be because of the variety of pricing supported by different companies and bulk purchases. The purpose of this pre-estimation was to verify the advantages of using the RGOHI-AcOH as cost-effective catalyst cathode in MFCs. Further research may be verified the possibility of stainless steel mesh as the backbone cathode electrode coated with RGOHI-AcOH in the MFCs to increase the power generation and reduce the total cathode cost. However, disadvantages of stainless steel mesh are unavoidable. Moreover, the result of bench scale MFCs may not translate directly to a full scale. Bench scale tests often show higher efficiencies than pilot and field experiments due to scale and edge effects. A detailed cost analysis of this technology will be needed in subsequent pilot and field experiments. 4. Conclusion

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.07.099. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

This paper highlights the potential use of highly conductive graphene materials (RGOHI-AcOH) instead of graphene nanoparticle composites (RGO/Ni) as an alternative cathode catalyst for MFCs applications. Both graphene and graphene-based catalyst cathode MFCs exhibited enhanced performance with a double catalyst load on the cathode (1 mg/cm2). As a result, the load of the catalyst on the cathode has a significant impact on the MFCs performance. The MFCs with a double loading RGOHI-AcOH cathodes deliver higher power generation (1683 ± 23 mW/m2, CE ¼ 72 ± 3%) than other non-precious (/metal) cathode MFCs, covering 77% of the power density estimated for Pt/C MFCs (2201 ± 45 mW/m2, CE ¼ 81 ± 4%). Moreover, they showed good stability for long operating periods over 30 days (27 cycles), with the additional benefit offered by the lower cathode cost (72%) than the Pt/C cathode. The catalytic activity of RGOHI-AcOH could be due mainly to the high surface area and degree of graphitization. The MFCs with the double loading RGO/Ni nanoparticle electrodes possessed power densities as high as 1015 ± 28 mW/m2 (CE ¼ 70 ± 2%). Electrochemical analyses using CV and EIS were in good agreement with the experimental result of the MFCs. Overall, by taking advantage of MFCs equipped with the double loading RGOHI-AcOH cathodes, RGOHI-AcOH was found to be a green catalyst that can be produced using a facile preparation method in MFC applications, which should be encouraged. This idea could open up a new opportunity for the scale up and commercialization of MFC technologies.

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

[30] [31] [32]

[33] [34] [35] [36] [37] [38] [39] [40]

Acknowledgments This study was supported by the 2016 Yeungnam University Research Grant.

[41] [42] [43]

X. Zhang, K. Li, P. Yan, Z. Liu, L. Pu, Bioresour. Technol. 187 (2015) 299e304. Y.H. Ahn, B.E. Logan, Bioresour. Technol. 101 (2010) 469e475. J. Choi, Y. Ahn, J. Environ. Manag. 130 (2013) 146e152. L. Birry, P. Mehta, F. Jaouen, J.P. Dodelet, S.R. Guiot, B. Tartakovsky, Electrochim. Acta 56 (2011) 1505e1511. L. Xiao, J. Damien, J. Luo, H.D. Jang, J. Huang, Z. He, J. Power Sources 208 (2012) 187e192. Y. Zhang, G. Mo, X. Li, W. Zhang, J. Zhang, J. Ye, X. Huang, C. Yu, J. Power Sources 196 (2011) 5402e5407. S. Khilari, S. Pandit, M.M. Ghangrekar, D. Das, D. Pradhan, RSC Adv. 3 (2013) 7902e7911. E.H. Yu, S. Cheng, K. Scott, B. Logan, J. Power Sources 171 (2007) 275e281. L. Deng, M. Zhou, C. Liu, L. Liu, C. Liu, S. Dong, Talanta 81 (2010) 444e448. J. Ahmed, Y. Yuan, L. Zhou, S. Kim, J. Power Sources 208 (2012) 170e175. T.M. Huggins, J.J. Pietron, H. Wang, Z.J. Ren, J.C. Biffinger, Bioresour. Technol. 195 (2015) 147e153. S. Lin, C. Shen, D. Lu, C. Wang, H.J. Gao, Carbon 53 (2013) 112e119. Y. Li, Y. Li, E. Zhu, T. McLouth, C.Y. Chiu, X. Huang, Y. Huang, J. Am. Chem. Soc. 134 (2012) 12326e12329. Z. Yan, M. Wang, B. Huang, R. Liu, J. Zhao, Int. J. Electrochem. Sci. 8 (2013) 149e158. L. Feng, L. Yang, Z. Huang, J. Luo, M. Li, D. Wang, Y. Chen, Sci. Rep. 3 (2013) 3306e3314. S. Li, Y. Hu, Q. Xu, J. Sun, B. Hou, Y. Zhang, J. Power Sources 213 (2012) 265e269. H. Wang, G. Wang, Y. Ling, F. Qian, Y. Song, X. Lu, S. Chen, Y. Tong, Y. Li, Nanoscale 5 (2013) 10283e10290. G. Wu, H. Huang, X. Chen, Z. Cai, Y. Jiang, X. Chen, Electrochim. Acta 111 (2013) 779e783. J. Xiao, Y. Xu, S. Yang, High-performance supercapacitors based on novel graphene composites, in: A.R. bin Mohd Yusoff (Ed.), Graphene-based Energy Devices, Wiley-VCH, Germany, 2015, pp. 145e171. Y. Xin, J.G. Liu, Y. Zhou, W. Liu, J. Gao, Y. Xie, Y. Yin, Z. Zou, J. Power Sources 196 (2011) 1012e1018. G. Choudhary, H. Hansen, Chemosphere 37 (1998) 801e843. Y.H. Kao, C.H. Chong, W.T. Ng, D. Lim, Occup. Med. 57 (2007) 535e537. A. Valipour, N. Hamnabard, Y.H. Ahn, RSC Adv. 5 (2015) 92970e92979. S. Cheng, B.E. Logan, Electrochem. Commun. 9 (2007) 492e496. J. Choi, Y.H. Ahn, Bioprocess Biosyst. Eng. 37 (2014) 2549e2557. S. Cheng, H. Liu, B.E. Logan, Electrochem. Commun. 8 (2006) 489e494. S. Ayyaru, P. Letchoumanane, S. Dharmalingam, A.R. Stanislaus, J. Power Sources 217 (2012) 204e208. B.E. Logan, Microbial Fuel Cells, Wiley & Sons, Inc., New Jersey, 2008. B.E. Logan, B. Hamelers, R. Rozendal, U. Schrorder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006) 5181e5192. J. Heilmann, B.E. Logan, Water Environ. Res. 78 (2006) 531e537. J. Ahmed, Y. Yuan, L. Zhou, S. Kim, J. Power Sources 208 (2012) 170e175. L. Xiao, Z. He, Application of graphene-based materials to improve electrode performance in microbial fuel cells, in: A.R. bin Mohd Yusoff (Ed.), Graphenebased Energy Devices, Wiley-VCH, Germany, 2015, pp. 355e370. X. Ji, X. Zhang, X. Zhang, J. Nanomater. 2015 (2015) 1e9. J.Y. Nam, C. Moon, E. Jeong, W.T. Lee, H.S. Shin, H.W. Kim, Environ. Eng. Res. 18 (2013) 145e150. G. Yang, Z. Liao, Z. Yang, Z. Tang, B. Guo, J. Appl. Polym. Sci. 132 (2015) 41832e41840. M. Mahmoud, T.A. Gad-Allah, K.M. El-Khatib, F. El-Gohary, Bioresour. Technol. 102 (2011) 10459e10464. Y.R. He, F. Du, Y.X. Huang, L.M. Dai, W.-W. Li, H.Q. Yu, J. Mater. Chem. A 4 (2016) 1632e1636. Z. He, Z. Wang, Z. Huang, H. Chen, X. Li, H. Guo, J. Mater. Chem. A 3 (2015) 16817e16823. Y. Barsukov, J. Qian, Battery Power Management for Portable Devices, Artech House, 2013. W. Yang, W. He, F. Zhang, M.A. Hickner, B.E. Logan, Environ. Sci. Technol. Lett. 1 (2014) 416e420. A. Janicek, Y. Fan, H. Liu, J. Power Sources 280 (2015) 159e165. X. Zhang, X. Xia, I. Ivanov, X. Huang, B.E. Logan, Environ. Sci. Technol. 48 (2014) 2075e2081. M.F. Hossain, J.Y. Park, Sci. Rep. 6 (2016) 21009e21018.