Incorporation of silver graphene oxide and graphene oxide nanoparticles in sulfonated polyether ether ketone membrane for power generation in microbial fuel cell

Journal of Power Sources xxx (xxxx) xxx

Contents lists available at ScienceDirect

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

Incorporation of silver graphene oxide and graphene oxide nanoparticles in sulfonated polyether ether ketone membrane for power generation in microbial fuel cell Kien Ben Liew a, e, Jun Xing Leong b, Wan Ramli Wan Daud b, c, Azizan Ahmad d, Jenn Jiang Hwang a, Wei Wu e, * a

Department of Greenergy Technology, National University of Tainan, Tainan, 70005, Taiwan Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia c Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia d School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia e Department of Chemical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan b

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

� AgGO-GO-SPEEK composite membrane is synthesized and characterized. � AgGO-GO-SPEEK composite membrane achieves higher proton conductivity. � AgGO-GO-SPEEK composite membrane ensures lower oxygen diffusion coefficient. � AgGO nanoparticle in membrane re­ duces biofouling in microbial fuel cell separator. � Microbial fuel cell with AgGO-GOSPEEK membrane achieves higher power generation.

A R T I C L E I N F O

A B S T R A C T

Keywords: Sulfonated polyether ether ketone Graphene oxide Silver graphene oxide Microbial fuel cell Biofouling

This study aims to design a new membrane for Microbial Fuel Cell (MFC) applications, where the membrane should have good proton conductivity, low oxygen permeability and anti-biofouling properties. The selffabricated composite membranes are Graphene Oxide/Sulfonated Polyether Ether Ketone (GO-SPEEK) and Sil­ ver Graphene Oxide/Graphene Oxide/Sulfonated Polyether Ether Ketone (AgGO-GO-SPEEK). The proton con­ ductivity of AgGO-GO-SPEEK membrane is 54.2% higher and the oxygen diffusion coefficient is 76.7% lower than Nafion® 117 membrane, yielding higher selectivity as separator in MFC applications. At the earlier opti­ mum stage, MFC with GO-SPEEK generates the highest maximum power density (1134 mW m 2). Voltage achieved for first three feed cycles after enrichment are 620 mV–650 mV for all MFC system. However, the highest maximum power density (896 mW m 3) is attained by MFC with AgGO-GO-SPEEK membrane after 100 days of operation. The Electrochemical Impedance Spectroscopy (EIS) results show that AgGO-GO-SPEEK membrane has the lowest increment in terms of the membrane’s resistance. AgGO-GO-SPEEK,a better anti-

* Corresponding author. E-mail address: [email protected] (W. Wu). https://doi.org/10.1016/j.jpowsour.2019.227490 Received 8 April 2019; Received in revised form 22 October 2019; Accepted 21 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Kien Ben Liew, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227490

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

biofouling property membrane, leads to a longer and stable power production, caused by the better ion transport through membrane. The results indicated that AgGO-GO-SPEEK membrane is a promising alternative membrane to replace Nafion® 117 as a separator in the MFC.

1. Introduction

SPEEK) and Silver Graphene Oxide/Graphene Oxide/Sulfonated Poly­ ether Ether Ketone (AgGO-GO-SPEEK) composite membranes are syn­ thesized and analyzed. SPEEK polymer is a suitable material used in fabricating the membrane for fuel cell applications, due to its superior mechanical and thermal strength [28]. In addition, the original material of SPEEK, Polyether Ether Ketone (PEEK) polymer exhibits a wide range of solubility that simplifies the fabrication process making it cheaper [29]. Recently, graphene oxide is widely used as a filler in the composite membrane fabrication process, because of its promising properties. Introducing graphene oxide into the membrane’s matrix can help in improving the proton conductivity and mechanical strength of the cor­ responding membrane [30–34]. Also, graphene oxide can be modified to further improve its properties. Several studies have sulfonated the gra­ phene oxide [35–37], or modified graphene oxide with other materials such as polydopamine [38], platinum [39], and sodium dodecylbenzene sulfonate [40] to further improve its conductivity and/or mechanical strength. In this study, graphene oxide is reduced with silver particles o form silver graphene oxide. Silver graphene oxide is used to inhibit the growth of biofilm on the membrane surface, as the silver particle is proven to be one of the most effective antibacterial materials [41,42]. The present study will analyze the performance of MFC using a nanoparticle AgGO in SPEEK membrane compared to SPEEK and GO-SPEEK membrane that has been studied previously [43]. In addition, durability tests and biofouling analyses are applied in this research to examine the anti-fouling properties of the membrane after 100 days of MFC opera­ tion using EIS technique.

The energy crisis around the world has led to the growth and development of renewable energy development. Among the renewable energy technologies such as fuel cell emerge as one of the most prom­ ising technology, due to its efficiency and environmental friendly concept in producing electrical power [1]. Microbial Fuel Cell (MFC) is one of the most recent developed bio-electrochemical devices that uti­ lizes and converts the bio-resources into electrical power. MFC is well recognized to its duel functions ability which is capable of generating electricity while simultaneously treating wastewater [2,3]. At present, commercialization of MFC is still not reliable due to the low power density generation, short durability and high cost [1,4–6]. Previous studies reported that the membrane is one of the factors that can greatly affect the performance of MFC [1,7]. Membrane locates between the anode and cathode compartments can prevent the solutions from both chambers to merge together. If the substrate (anode solutions) reaches cathode side, severe biofouling can happen on cathode surface [8,9], which will affect the Oxygen Reduction Reaction (ORR) of cath­ ode catalyst hence deteriorating the MFC performance [10]. On the contrary, oxygen from the cathode chamber can reach the anaerobic anode chamber without the presence of membrane between the two compartments. The anaerobic fermentation process in anode chamber will be inhibited due to the presence of oxygen, where substrate loss occurs via the oxidation process by aerobic bacteria [9,11]. This revealed that the membrane used in MFC system should have the ability to prevent the oxygen from diffusing to the other side. In addition, as a separator in MFC, the membrane that possesses good ionic conductivity should enhance the proton diffusion to improve its performance [12]. The commercialized membrane used in MFC, Nafion® 117 [13–16], has some major drawbacks, which are expensive [17], susceptible to oxygen crossover [12,14] and susceptible to the growth of biofilm on membrane’s surface [18]. To overcome these issues, new types of membranes are introduced apart from modifying the Nafion® mem­ brane. The developement of Nafion® type membrane includes inte­ grating the membrane polymer with other fillers such as activated carbon nano-fiber [19], polyaniline [20] and polyvinylidene fluoride (PVDF) [21]. These modification has improved the properties of the membrane, which indirectly increases the MFC’s performance in terms of power density and coulombic efficiency. Similarly, different types of composite membranes that have been applied in MFC also show better fuel cell’s performance as compared to the MFC equipped with Nafion® 117 membrane, including polyethersulfone/ferric oxide (PES/Fe3O4) [22], sulfonated polyether ether ketone/sulfonated titanium oxide (SPEEK-­ TiO2-SO3H) [23], sulfonated polyether ether ketone/montmorillonite (SPEEK-MMT) [24], sulfonated polystyrene-ethylene-butylene-poly styrene/sulfonated titanium oxide (SPSEBS-S-TiO2) [25], sulfonated polyether ether ketone/charged surface modifying micromolecules (SPEEK-cSMM) [26], sulfonated polyether ether ketone/rutile titanium oxide (SPEEK-TiO2) [27] and others. However, none of the studies above concern about the anti-biofouling properties of the self-synthesized membrane. Although membraneless technology is introduced to over­ come the biofouling issues, however, some problems arise when no separator is placing between the two compartments, as described afore [8–10]. The main objective of this study is to fabricate new types of mem­ branes specifically for MFC applications, which have better properties and longer durability than Nafion® 117 membrane. In order to accomplish the objective mentioned above, two types of composite membrane, Graphene Oxide/Sulfonated Polyether Ether Ketone (GO-

2. Experimental 2.1. Sulfonation of PEEK polymer SPEEK polymer was synthesized via sulfonation of PEEK polymer using 95–98% concentrated sulphuric acid, as described in our previous study [43]. The self-synthesized SPEEK polymer has a Degree of Sulfo­ nation (DS) of 80%, as determined by the 1H nuclear magnetic reso­ nance (FT-NMR ADVANCE 111 600 MHz with Cryoprobe) spectroscopy (Bruker, Karlsruhe, Germany). 2.2. Synthesizing and characterization of graphene oxide and silver graphene oxide Modified Hummers’ method was implemented to synthesize the graphene oxide from graphite flakes [44]. The chemicals used during the graphene oxide synthesizing process include graphite flake, 95–98% concentrated sulphuric acid (H2SO4), sodium nitrate (NaNO3), potas­ sium permanganate (KMnO4), 30 wt% hydrogen peroxide (H2O2) solu­ tion and 5 wt% hydrochloric acid (HCl) solution. The details of the oxidation process were described in the previous study [43]. For silver graphene oxide preparation, two solutions was first pre­ pared separately. The first solution involves graphene oxide solution, which is the dispersion of graphite oxide (1 g) in deionized water (200 ml) via ultrasonication. The second solution involves silver nitrate so­ lution, which was prepared by dissolving 0.679 g of silver nitrate (AgNO3) in 800 ml of deionized water. Both solutions were then mixed together and mechanically stirred for 30 min to obtain a homogeneous solution. The mixture was then added with freshly prepared reducing agent solution (100 ml of 0.01 mM sodium borohydride (NaBH4) solu­ tion). The temperature was controlled at around 15 � C during the mixing process to obtain a proper reduction process, and an instant change of colour of the solution (from brown to grey) is observed. The mixture was 2

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

left to stir overnight for completing the reaction. The silver graphene oxides collected at the end of the reaction process are then washed with deionized water through centrifugation for several times, dried in a vacuum oven and kept in desiccator prior use. The self-synthesized graphene oxide and silver graphene oxide were then analyzed using Fourier Transform Infrared Spectroscopy (FT-IR, Thermo Electron, USA), Transmission Electron Microscopy (TEM-Hita­ chi HT 7700, Japan) and X-ray Photoelectron Spectroscopy (XPS, ULVAC-PHI, Inc, Japan) to identify/observe the chemical functional groups, surface morphology, and chemical states for graphene oxide and silver graphene oxide, respectively.

ko ¼

Do ¼ koL

Both composite membranes, GO-SPEEK and AgGO-GO-SPEEK were fabricated via a dry phase inversion method, and the solvent used is NMethyl-2-Pyrrolidone (NMP). For GO-SPEEK membrane fabrication, SPEEK polymer was first fully dissolved in NMP solvent through me­ chanical stirring. Then, the SPEEK solution was added with the graphene oxide solution (dispersion of graphene oxide in NMP solvent), and the stirring process was continued for another 24 h. The final mixture contained 15 wt % of SPEEK polymer with respect to the solvent, and 0.5 wt % of graphene oxide with respect to the SPEEK polymer. Before casting the membrane solution onto the glass plate, the mixture was ultrasonic for 30 min to obtain a well-dispersed homogeneous solution. The cast membrane was then dried at room temperature overnight, then at 45 � C and 60 � C for 48 h, respectively. The dried membrane was immersed in deionized water in order to peel off from the glass plate. The membrane was then immersed in 1 M H2SO4 solution for 24 h to activate the sulfonic groups of the polymer matrix. Then, the membrane was flushed with large amount of deionized water until neutral and kept in deionized water prior to use. For AgGO-GO-SPEEK membrane, the fabrication steps and pre­ treatment process is similar as described above. The difference is that there are three solutions prepared at the first stage, which include the SPEEK solution, graphene oxide solution and silver graphene oxide so­ lution. The concentration of each material in the final mixture of AgGOGO-SPEEK solution was 15 wt % of SPEEK polymer with respect to the solvent, 0.5 wt % of graphene oxide and 0.5 wt % of silver graphene oxide with respect to the SPEEK polymer, respectively.

σ¼

W2

� 100

L RA

(4)

2.5. Set-up and performance test of microbial fuel cell A dual-chamber MFC chamber was used in this study to analyze its performance and was operated in batch mode. The anolyte solution was made up of 90 vol % of synthetic wastewater and 10 vol % of mixed bacteria culture. The composition of the synthetic wastewater (modified Geobacter medium) is similar to the other study [43] while the mixed bacteria culture was obtained from a sewage treatment plant in Malaysia (Indah Water Konsortium). The catholyte solution was 50.0 mM phos­ phate buffer solution. Carbon paper was used as the electrode for both chambers, and platinum catalyst is applied on the cathode electrode surface, with a loading of 0.5 mg cm 2. A 1 kΩ external resistor is connected between the two electrodes, and the electrodes are connected to Fluke 8846A multimeter to record the voltage produced by the MFC system. At the beginning of each cycle, nitrogen gas was purged into the anode compartment, in order to create an oxygen-free environment for the anaerobic bacteria. For the cathode compartment, air was purged continuously for creating an oxygen-rich environment for the Oxygen Reduction Reaction (ORR). The MFC’s performance tests include the determination of Open Circuit Voltage (OCV), internal resistance, maximum current density, maximum power density, Chemical Oxygen Demand (COD) removal rate and Coulombic Efficiency (CE). The polarization curve of the MFC system is obtained by using the AUTOLAB potentiostat (PGSTAT128 N, Utrecht, Netherlands), and the scan rate used was 1 mV s 1. OCV and internal resistance were then determined from the polarization curve directly. Using the data from polarization curve (voltage and current density), the power density plot was plotted. The maximum current density and maximum power density were then determined by the power density plot. COD removal rate was measured by using the HACH spectrophotometer, following the Standard Method 8000 (HACH DR2800 Spectrophotometer Procedure Manual): Thermoreactor Diges­ tion Method. The HR COD reagent used can measure the COD range from 0 to 1500 COD L 1. For Coulombic Efficiency (CE) determination, the following formula was used [48]:

The water uptake level, oxygen crossover rate and proton conduc­ tivity for self-fabricated GO-SPEEK and AgGO-GO-SPEEK membranes are conducted and compared with Nafion® 117 membranes. Water uptake measurement is the percentage difference of the mass of a fully hydrated membrane (W1) and the mass of a fully dried membrane (W2), which is determined using the following equation [45]: W2

(3)

L is the membrane’s thickness (cm), R is the measured membrane’s resistance (Ω), and A is the membrane’s area (cm2).

2.4. Characterization tests of the membrane

W1

(2)

In this equation, V refers to the volume of the buffer solution in compartment A (cm3), A is the membrane’s area (cm2), t is the time when the oxygen concentration in compartment A, c is recorded (s) and L is the membrane’s thickness (cm). The Electrochemical Impedance Spectroscopy (EIS) was applied in determining the proton conductivity of the membrane. EIS was first used to measure the membrane’s resistance. The membrane is clamped be­ tween two stainless steel electrodes, and the electrodes were connected to the potentiostat (AUTOLAB potentiostat, PGSTAT128 N, Utrecht, Netherlands). A perturbation amplitude of 0.005 V was applied during the measurement, and the frequency range of measurement is from 1 Hz to 100 kHz. The Nyquist plot obtained from the EIS measurement is then fitted to obtain the membrane’s resistance, and the resistance is substituted into the following formula to obtain the membrane’s con­ ductivity, σ [47].

2.3. Fabrication of GO-SPEEK and AgGO-GO-SPEEK composite membranes

Water uptake ð%Þ ¼

V co c ln At co

(1)

For the oxygen crossover test, the set-up of a simple dual-chamber MFC chamber contained 50 mM phosphate buffer solution was used. Dissolved Oxygen (DO) probe is first inserted into one of the compart­ ments labeled as A. Nitrogen gas is then purged into the compartment A until the DO level reached its saturated minimum value, and the data is recorded. Air is then purged into the next compartment labeled as B. The measured oxygen concentration in compartment A, c (mol.cm 3) using the DO probe was recorded at a time interval of 15 min until the oxygen concentration reached saturated value. Besides, the saturated oxygen concentration in the compartment B, co (mol.cm 3) was also measured with DO probe and recorded. The mass transfer coefficient of oxygen, ko (cm.s 1) and oxygen diffusion coefficient, Do (cm2.s 1) were determined using the following equations [46]:

Ec ¼

Cp Mi � 100% Fbi ΔCODv

(5)

Cp is the collection of coulombs produced in one batch, Mi is the oxygen 3

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

2.6. Determination of MFC system impedance EIS technique was applied in measuring the impedance of the MFC system. The measurement was conducted by using Autolab potentiostat equipped with Frequency Response Analyser (FRA) function (PGSTAT302 N, Utrecht, Netherlands). Two electrodes configuration was used for the measurement, with the working electrode connected to the anode electrode, while the reference electrode was connected to the cathode electrode of the MFC. The external resistor was removed during the measurement process to create an open circuit condition. The measurement’s frequency was in the range of 0.1 Hz–100 kHz, and the perturbation amplitude was fixed as 0.005 V. The obtained Nyquist plot from the EIS measurement was then fitted, in order to obtain the membrane’s resistance in the MFC system. 3. Results and discussions 3.1. TEM, FT-IR and XPS analysis of graphite, graphite oxide and silver graphite oxide TEM images for graphene oxide and silver graphene oxide are shown in Fig. 1. A single layer of graphene oxide was successfully exfoliated from the self-synthesized graphite oxide, as shown in Fig. 1 (a). Shrinkage of the graphene oxide layer was also observed in the TEM imaging, probably due to the drying process during the sample prepa­ ration. From the TEM image of silver graphene oxide (Fig. 1 (b)), it shows that silver particles (black dots) were distributed evenly on the graphene oxide surface, and the size of the deposited silver particles is in the range of less than 10 nm. The FT-IR spectra for graphite, graphite oxide and silver graphite oxide are shown in Fig. 2. The graphite oxide and silver graphite oxide are named in FT-IR and XPS analysis instead of graphene oxide and silver graphene oxide. This is because thin graphene oxide and silver graphene oxide are in the dry state and stacked in multiple layers during analysis. Therefore, it stayed in the form of graphite rather than gra­ phene as shown in TEM images. No peak is shown in FT-IR spectra of

Fig. 1. TEM images for (a) graphene oxide (b) silver graphene oxide.

molecular mass (Moxygen ¼ 32 g mol 1), F is Faraday’s constant (96 485 C per mole of electron), bi is the number of mole of electrons produced per mole oxygen (boxygen ¼ 4), ΔCOD is the amount of COD removal (g.L 1), and v is the volume of anode solution (L).

Fig. 2. FT-IR spectra for graphite, graphite oxide and silver graphite oxide. 4

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

increasing the intensity of C–C peak. For the binding energies of O1s, it was resolved to two distinct peaks, as in graphite (532.57 eV, 531.63 eV), graphite oxide (533.22 eV, 532.09 eV) and silver graphite oxide (532.96 eV, 531.87 eV), that were attributed to the C¼O and C–O bonds, respectively [56]. It is obvious that the intensity of C–O peak for graphite oxide is higher than pristine graphite, due to the presence of oxygenated functional groups in graphite oxide. Similarly, the intensity of C–O peak is decreased in silver graphite oxide when graphite oxide is reduced to silver graphite oxide. For the binding energy of silver (Ag3d) in silver graphite oxide, there exist two distinct peaks, 368.20 eV and 374.20 eV, which represent the Ag3d5/2 and Ag3d3/2, respectively. The difference of 6.0 eV between the two cores of the 3d doublet of silver indicating that metallic silver is formed in silver graphite oxide [58]. 3.2. Water uptake, oxygen diffusion coefficient, and proton conductivity Fig. 3. Wide scan XPS spectrum for graphite, graphite oxide and silver graphite oxide.

The water uptake and oxygen diffusion coefficient for GO-SPEEK, AgGO-GO-SPEEK and Nafion® 117 membranes are listed in Table 1 showing higher water uptake value compared to Nafion 117 membrane. This attributes to the morphology of the SPEEK membrane of the high water uptake [59]. The hydrophilic groups (sulfonate groups) in SPEEK matrix are attached to the hydrophobic backbone directly, but in Nafion® matrix, the hydrophilic groups are connected to the hydro­ phobic backbone through a long side chain. The small separation be­ tween the hydrophilic and hydrophobic regions in SPEEK creates a repulsive force between the two regions, and voids are formed in the membrane matrix. For SPEEK with a high degree of sulfonation, more voids will be formed. More water molecules can occupy the voids when the SPEEK membrane is immersed in water, causing a high water uptake [59]. When compared to the pristine SPEEK membrane (water uptake ¼ 146.69%) that was reported in our previous study [43], both composite membranes have about two-fold lower water uptake value. The presence of fillers (graphene oxide and silver graphene oxide in this study) in the corresponding membrane matrix helps to reduce the water uptake. The fillers can fill up the void hence reduce the water uptake value [33,35]. The water uptake value for Nafion® 117 membrane remains the lowest among all the membranes, however, the membrane has the highest oxygen diffusion coefficient. The repulsion force between the hydrophilic and hydrophobic regions is more pronounce in Nafion® 117 than SPEEK. This is because the backbone of Nafion® 117 is more hy­ drophobic and its ionic site is more hydrophilic, as compared to SPEEK. In a hydrated environment, a narrow pathway that filled with the absorbed water molecules is created in.the Nafion® 117 matrix. The said pathway promotes the diffusion of oxygen molecules. Hence, higher oxygen transport in Nafion® 117 membrane [60]. In SPEEK, isolated water clusters are formed rather than the pathway. Thus, oxygen mol­ ecules cannot shuttle as easy as in Nafion® 117 membrane [60,61]. Incorporating the fillers such as graphene oxide and silver graphene oxide also helps in preventing the diffusion of oxygen molecules, as the diffusion pathway are probably “blocked” by the fillers [62]. For proton conductivity, the membrane’s resistance is first obtained from the fitted Nyquist plot that fitted with the proper equivalent circuit (Fig. 5), as described in the previous study [43]. The values of the resistance and calculated proton conductivity are listed in Table 2. Among all the tested membranes, GO-SPEEK membrane has the highest proton conductivity (3.80 � 10 2 S cm 1) while the AgGO-GO-SPEEK membrane (3.70 � 10 2 S cm 1) has slightly lower conductivity than the former membrane. Meanwhile, Nafion® 117 membrane (2.40 � 10 2 S cm 1) shows about 37% lower in proton conductivity than GO-SPEEK. Generally, SPEEK membrane will have lower proton con­ ductivity than Nafion® type membrane [63–65], due to the dissimilar structure of both membranes. The long side-chain of Nafion® 117 membrane allows the ionic sites to move freely (segmental motion). Thus, protons are being “pushed” while diffusing inside the Nafion® 117 matrix, hence improving the proton conductivity [31]. However, both composite membranes have better conductivity than Nafion® 117

graphite as the structure of graphite is only a carbon layer with no functional groups [49,50]. However, for graphite oxide, there are many new peaks introduced as compared to the graphite spectra. The peaks at 3401.24 cm 1, 1731.64 cm 1, 1631.72 cm 1, 1405.99 cm 1, 1224.37 cm 1, 1049.78 cm 1 are attributed to the stretching of O–H, stretching of C¼O, skeletal vibration of epoxy group, bending vibration of O–H in carboxylic group, stretching of C–OH, and stretching of C–O of the functional groups in graphite oxide [51–53]. The existence of these peaks revealed that the oxygenated functional groups were successfully attached to the carbon layer of graphite during the oxidation process, changing the structure to become graphite oxide. When the graphite oxide was further reduced to silver graphite oxide, the intensities of the peaks assigned to the oxygenated functional groups decreased as shown in the spectra. This indicates that the oxygenated groups on graphite oxide layer have been partially reduced [52,54,55]. Further, a pro­ nounced reduction of the O–H peak intensity at 3206.92 cm 1 showing the hydrogen atoms of O–H groups are mostly replaced by the silver atoms [55]. For XPS analysis, the wide scans for each material were first analyzed, as shown in Fig. 3. Initially, the pristine graphite flakes contain a weight percentage of 85 wt % and 15 wt % for C1s and O1s, respectively. After the oxidation reaction, the weight percentage of O1s for graphite oxide has increased to 41.56 wt %. The presence of oxygenated groups on the graphite oxide has attributed the above increment, indicating the graphite layer has been covered with oxygenated functional groups. When the graphite oxide was reduced to silver graphite oxide layer, a new peak is observed in the wide scan of silver graphite oxide. In addition, the weight percentage of O1s for silver graphite oxide spectra showed a decrement as compared to the graphite oxide. The weight percentage of C1s, O1s, and Ag3d for silver graphite oxide is 50.04%, 38.58%, and 11.38%. This is because some of the oxygenated groups were reduced during the deposition of silver parti­ cles on graphite oxide surface via reduction reaction. The core level for each signal was analyzed by revolving the signal to distinct peaks with curve fitting, using the mixed Gaussian and Lor­ entzian line shape as displayed in Fig. 4. The binding energies of C1s for all the tested materials were resolved to three distinct peaks, which represents the C–C (284.81 eV for graphite, 284.57 eV for graphite oxide and 284.63 eV for silver graphite oxide), C–O (285.72 eV for graphite, 286.82 eV for graphite oxide and 286.74 eV for silver graphite oxide) and C¼O bonds (286.71 eV for graphite, 288.29 eV for graphite oxide 288.30 eV for silver graphite oxide) [56,57]. The result showed that the intensity of C–C peak increases in the order of graphite oxide, silver graphite oxide, and graphite. The bonding between the carbon atoms of graphite was broken when oxidation takes place, in producing graphite oxide from graphite. When graphite oxide is reduced to form silver graphite oxide, the graphite oxide experienced reduction reaction, thus 5

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

Fig. 4. Core level XPS signals of (a) graphite C1s (b) graphite O1s (c) graphite oxide C1s (d) graphite oxide O1s (e) silver graphite oxide C1s (f) silver graphite oxide O1s (g) silver graphite oxide Ag3d.

proton conductivity is lesser compared to GO-SPEEK, indicating that silver graphene oxide does not promote/enhance the proton conduc­ tivity. This is probably due to nature hydrophobic property of silver particles [68]. In AgGO-GO-SPEEK membrane matrix, the proton is assumed to diffuse through the selected paths that contain graphene oxide, but not the silver graphene oxide. This explains why both mem­ branes have almost similar proton conductivity.

Table 1 Water uptake and oxygen diffusion coefficient for GO-SPEEK, AgGO-GO-SPEEK and Nafion® 117membrane. Membrane

Water uptake (%)

Oxygen diffusion coefficient, Do ( � 10 cm2 s 1)

Nafion® 117 GO-SPEEK AgGO-GOSPEEK

23.76 76.36 75.27

4.47 1.10 1.04

6

3.3. MFC’s performance

membrane, indicating that graphene oxide can increase the conductivity of the membrane. The presence of oxygenated functional groups on the sp3 carbon layer of graphene oxide helps to promote the proton’s diffusion, by forming the hydrogen bonding networks between the ionic sites of SPEEK polymer, oxygenated functional groups of graphene oxide and water molecules [66,67]. For AgGO-GO-SPEEK membrane, the

The polarization and power density curves for all the MFC systems at the optimum stage are shown in Fig. 6 (a), while the results of OCV, internal resistance, maximum current density, maximum power density, COD removal rate and CE are listed in Table 3. Results show that the MFC system equipped with GO-SPEEK membrane has the lowest internal resistance and produces the highest maximum current density and 6

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

Fig. 5. Nyquist plot with the fitting curve for Nafion® 117, GO-SPEEK and AgGO-GO-SPEEK Membranes. Table 2 Proton conductivity for Nafion® 117, GO-SPEEK, and AgGO-GO-SPEEK membranes. Membrane

Resistance, R (Ω)

Membrane thickness, l (cm)

Membrane area, A (cm2)

Proton conductivity, σ ( � 10 2 S cm 1)

Nafion® 117 GO-SPEEK AgGO-GOSPEEK

0.592

0.022

1.54

2.40

0.205 0.211

0.012 0.012

1.54 1.54

3.80 3.70

power density, followed by MFC system equipped with AgGO-GO-SPEEK membrane and Nafion® 117 membrane. As membrane contributes to the total internal resistance of the membrane, GO-SPEEK membrane is said to have lower resistance due to its higher conductivity [69]. This can be seen in Fig. 6 (b) the internal resistance of the three membrane with values 8.5 Ω, 9.3 Ω and 10.8 Ω for GO-SPEEK, AgGO-GO-SPEEKand Nafion® 117 respectively. This shows that membrane with higher pro­ ton conductivity has a lower resistance when applied in the MFC system. For the COD removal rate, all systems showed a high removal rate of around 83% which indicates efficient treatment of wastewater. For CE, MFC with GO-SPEEK membrane showed the highest efficiency while Nafion® 117 registered as the lowest. This is because the Nafion® 117 membrane is more susceptible to the oxygen crossover issue, which lowering its efficiency in producing the electrons [70]. Fig. 7 showed the voltage generated by each MFC connecting to a 1000Ω resistor. The enrichment process of bioanode for all MFCs equipped with different membranes took approximately 18 days. It can be observed that all MFC able to produce stable voltage ranging between 620 mV and 650 mV after three feed cycles after enrichment process the enrichment process. Each feed cycle can last for 4–5 days before the voltage drop started to drop gradually.

Fig. 6. (a) Polarization curves and power density curves (b) Nyquist plot for MFC systems with different membranes at the optimum stage. Table 3 Open circuit voltage, internal resistance, maximum current density, maximum power density, COD removal and coulombic efficiency for MFC systems with different membranes. Type of membrane in MFC system

Nafion® 117

GOSPEEK

AgGO-GOSPEEK

Open Circuit Voltage, OCV (V) Internal resistance (Ω) Maximum current density (A. m 2) Maximum power density (mW. m 2) COD removal (%) Coulombic efficiency (%)

0.793 57.66 2.18

0.828 48.59 2.62

0.825 52.91 2.60

1013

1134

1049

82.65 12.31

83.01 18.06

83.36 16.88

followed by GO-SPEEK membrane (22.13%) and AgGO-GO-SPEEK membrane (14.59%). The results indicate that MFC system with AgGO-GO-SPEEK membrane has better durability than MFC equipped with GO-SPEEK, albeit both MFC systems have a similar power perfor­ mance after enrichment. This is probably due to the AgGO-GO-SPEEK membrane has better anti-biofouling property than other candidates, which helps in enhancing the performance of MFC. In ensuring the AgGO-GO-SPEEK membrane has better anti-biofouling property, the MFC system’s impedance is measured again using EIS technique, and fitted with adequate equivalent circuit to obtain the membrane’s resis­ tance at the last cycle of operation. The results displayed in Fig. 8 (b) show that the membrane’s resistance has increased 125.93% to 24.4 Ω for Nafion® 117 membrane, 108.24% to 17.7 Ω for GO-SPEEK mem­ brane, but only 74.19% for AgGO-GO-SPEEK membrane (16.2 Ω). The silver particles added into the AgGO-GO-SPEEK membrane’s matrix help

3.4. Durability of MFC The polarization and power density curves were obtained after 100 days of MFC operations, to monitor the durability of the corresponding system, as shown in Fig. 8 (a). From the results presented in Table 4, the highest maximum current density and power density was generated by the MFC equipped with AgGO-GO-SPEEK membrane, followed by MFC with GO-SPEEK membrane and Nafion® 117 membrane. The record increased in internal resistance was 39.58%, 35.07% and 23.83% for MFC with Nafion® 117, GO-SPEEK and AgGO-GO-SPEEK membrane respectively. This causes the maximum power density has the greatest decrement in MFC system with Nafion® 117 membrane (31.79%), 7

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

Table 4 Open circuit voltage, internal resistance, maximum current density and maximum power density for MFC systems in last operation batch. Type of membrane in MFC system

Open Circuit Voltage, OCV (V)

Internal resistance (Ω)

Maximum current density (A. m 2)

Maximum power density (mW.m 2)

Nafion® 117 GO-SPEEK AgGO-GOSPEEK

0.749 0.817 0.770

80.48 65.63 65.52

1.70 1.97 2.35

691 883 896

to mitigate the adhesion of microorganism on the membrane’s surface. Less biofilm on the membrane’s surface can enhance the shuttling of generated protons through the membrane easily, yielding lower mem­ brane resistance [71]. From the membrane’s resistance data, GO-SPEEK also showed lower resistance than Nafion® 117 membrane. Graphene oxide is believed to have the ability in reducing the growth of biofilm, probably due to the hydrophilicity of graphene oxide, which can prevent the adhesion of bacteria [72,73]. 4. Conclusion In this study, two composite membranes, GO-SPEEK and AgGO-GOSPEEK were successfully synthesized for the application in MFC. Higher proton conductivity and lower oxygen diffusion coefficient of both GOSPEEK and AgGo-GO-SPEEK membrane increase their selectivity as a separator in MFC application. More specifically, SPEEK membrane with inclusion of this nanoparticle exhibited a higher proton conductivity, even though at a low dosing of 0.5 wt% as indicated in this study. During full cell test, MFC equipped with the higher conductivity composite membranes shows better performance than MFC equipped with Nafion® 117 membrane, by producing higher maximum current density and power density. MFC systems that equipped with the composite mem­ branes also produced higher coulombic efficiency, because both com­ posite membranes have higher resistance towards the oxygen diffusion. After the MFC systems have operated for approximately 100 days, MFC with AgGO-GO-SPEEK membrane displayed lowest internal resistance and produced the highest maximum power density. The AgGO-GOSPEEK membrane has the lowest resistance compared to GO-SPEEK and Nafion® 117 membranes, revealing that the inclusion of small amount of silver nanoparticle in membrane can efficiently inhibit the formation of biofilm. The AgGO-GO-SPEEK membrane demonstrated a stable performance over time and prospective properties as a replace­ ment of conventional Nafion® 117 membrane as a separator in fuel cell applications. It is suggested that proper modification of the structure of AgGO-GO-SPEEK membrane could be done in future research. Applying a layer of silver graphene oxide on the surface of the membrane rather than blending into the membrane matrix appear to be a better improvement in anti-biofouling property of membrane. Overall, the two studied membrane showed better performance compared to the commercially available Nafion® 117.

Fig. 7. Voltage as a function of time of MFC with (a)Nafion 117 (b) GO-SPEEK (c) AgGO-GOSPEEK Membrane.

Acknowledgement The authors would like to acknowledge the financial support from the research grant of DIP-2012-27 by Universiti Kebangsaan Malaysia, Malaysia, and the Ministry of Education, Taiwan for its partial financial support of this research under grant 107RSG0011. References [1] J.X. Leong, W.R.W. Daud, M. Ghasemi, K.B. Liew, M. Ismail, Ion exchange membranes as separators in microbial fuel cells for bioenergy conversion: a comprehensive review, Renew. Sustain. Energy Rev. 28 (2013) 575–587.

Fig. 8. (a) Polarization curves and power density curves (b) Nyquist plot for MFC systems with different membranes at last operation cycle. 8

K. Ben Liew et al.

Journal of Power Sources xxx (xxxx) xxx

[2] Y. Ahn, B.E. Logan, Effectiveness of domestic wastewater treatment using microbial fuel cells at ambient and mesophilic temperatures, Bioresour. Technol. 101 (2010) 469–475. [3] M. Ghasemi, M. Ismail, S.K. Kamarudin, K. Saeedfar, W.R.W. Daud, S.H. Hassan, L. Y. Heng, J. Alam, S.-E. Oh, Carbon nanotube as an alternative cathode support and catalyst for microbial fuel cells, Appl. Energy 102 (2013) 1050–1056. [4] Z. Du, H. Li, T. Gu, A state of the art review on microbial fuel cells: a promising technology for wastewater treatment and bioenergy, Biotechnol. Adv. 25 (2007) 464–482. [5] K.B. Liew, W.R.W. Daud, M. Ghasemi, J.X. Leong, S.S. Lim, M. Ismail, Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: a review, Int. J. Hydrogen Energy 39 (2014) 4870–4883. [6] J.T. Babauta, M. Kerber, L. Hsu, A. Phipps, D.B. Chadwick, Y.M. Arias-Thode, Scaling up benthic microbial fuel cells using flyback converters, J. Power Sources 395 (2018) 95–105. [7] Y. Kim, S.-H. Shin, I.S. Chang, S.-H. Moon, Characterization of uncharged and sulfonated porous poly (vinylidene fluoride) membranes and their performance in microbial fuel cells, J. Membr. Sci. 463 (2014) 205–214. [8] M. Ghangrekar, V. Shinde, Performance of membrane-less microbial fuel cell treating wastewater and effect of electrode distance and area on electricity production, Bioresour. Technol. 98 (2007) 2879–2885. [9] Y. Oon, S. Ong, L. Ho, Y. Wong, Y. Oon, H.K. Lehl, W. Thung, Hybrid system upflow construceted wetland integrated with microbial fuel cell for simultaneous wastewater treatment and electricity generation, Bioresour. Technol. 186 (2015) 270–275. [10] B. Tartakovsky, S.R. Guiot, A comparison of air and hydrogen peroxide oxygenated microbial fuel cell reactors, Biotechnol. Prog. 22 (2006) 241–246. [11] J.R. Kim, G.C. Premier, F.R. Hawkes, R.M. Dinsdale, A.J. Guwy, Development of a tubular microbial fuel cell (MFC) employing a membrane electrode assembly cathode, J. Power Sources 187 (2009) 393–399. [12] P.N. Venkatesan, S. Dharmalingam, Characterization and performance study on chitosan-functionalized multi walled carbon nano tube as separator in microbial fuel cell, J. Membr. Sci. 435 (2013) 92–98. [13] P. Pandey, V.N. Shinde, R.L. Deopurkar, S.P. Kale, S.A. Patil, D. Pant, Recent advances in the use of different substrates in microbial fuel cells towards wastewater treatment and simultaneous energy recovery, Appl. Energy 168 (2016) 706–723. [14] B. Min, S. Cheng, B.E. Logan, Electricity generation using membrane and salt bridge microbial fuel cells, Water Res. 39 (2005) 1675–1686. [15] B. Min, J. Kim, S. Oh, J.M. Regan, B.E. Logan, Electricity generation from swine wastewater using microbial fuel cells, Water Res. 39 (2005) 4961–4968. [16] D.H. Park, J.G. Zeikus, Electricity generation in microbial fuel cells using neutral red as an electronophore, Appl. Environ. Microbiol. 66 (2000) 1292–1297. [17] M. Ghasemi, W.R.W. Daud, A.F. Ismail, Y. Jafari, M. Ismail, A. Mayahi, J. Othman, Simultaneous wastewater treatment and electricity generation by microbial fuel cell: performance comparison and cost investigation of using Nafion 117 and SPEEK as separators, Desalination 325 (2013) 1–6. [18] K.J. Chae, M. Choi, F.F. Ajayi, W. Park, I.S. Chang, I.S. Kim, Mass transport through a proton exchange membrane (nafion) in microbial fuel cellsy, Energy Fuels 22 (2007) 169–176. [19] M. Ghasemi, S. Shahgaldi, M. Ismail, Z. Yaakob, W.R.W. Daud, New generation of carbon nanocomposite proton exchange membranes in microbial fuel cell systems, Chem. Eng. J. 184 (2012) 82–89. [20] N. Mokhtarian, M. Ghasemi, W.R.W. Daud, M. Ismail, G. Najafpour, J. Alam, Improvement of microbial fuel cell performance by using nafion polyaniline composite membranes as a separator, J. Fuel Cell Sci. Technol. 10 (2013), 041008. [21] S. Shahgaldi, M. Ghasemi, W.R.W. Daud, Z. Yaakob, M. Sedighi, J. Alam, A. F. Ismail, Performance enhancement of microbial fuel cell by PVDF/Nafion nanofibre composite proton exchange membrane, Fuel Process. Technol. 124 (2014) 290–295. [22] M. Rahimnejad, M. Ghasemi, G. Najafpour, M. Ismail, A.W. Mohammad, A. Ghoreyshi, S.H. Hassan, Synthesis, characterization and application studies of self-made Fe 3 O 4/PES nanocomposite membranes in microbial fuel cell, Electrochim. Acta 85 (2012) 700–706. [23] S. Ayyaru, S. Dharmalingam, Improved performance of microbial fuel cells using sulfonated polyether ether ketone (SPEEK) TiO2–SO3H nanocomposite membrane, RSC Adv. 3 (2013) 25243–25251. [24] M.M. Hasani-Sadrabadi, E. Dashtimoghadam, S.N.S. Eslami, G. Bahlakeh, M. A. Shokrgozar, K.I. Jacob, Air-breathing microbial fuel cell with enhanced performance using nanocomposite proton exchange membranes, Polym 55 (2014) 6102–6109. [25] S. Ayyaru, S. Dharmalingam, A study of influence on nanocomposite membrane of sulfonated TiO2 and sulfonated polystyrene-ethylene-butylene-polystyrene for microbial fuel cell application, Energy 88 (2015) 202–208. [26] A. Mayahi, H. Ilbeygi, A.F. Ismail, J. Jaafar, W.R.W. Daud, D. Emadzadeh, E. Shamsaei, D. Martin, M. Rahbari-Sisakht, M. Ghasemi, SPEEK/cSMM membrane for simultaneous electricity generation and wastewater treatment in microbial fuel cell, J. Chem. Technol. Biotechnol. 90 (2015) 641–647. [27] P.N. Venkatesan, S. Dharmalingam, Effect of cation transport of SPEEK–Rutile TiO 2 electrolyte on microbial fuel cell performance, J. Membr. Sci. 492 (2015) 518–527. [28] M.J. Parnian, S. Rowshanzamir, A.K. Prasad, S.G. Advani, High durability sulfonated poly(ether ether keton)-ceria nanocomposite membranes for proton exchange membrane fuel cell applications, J. Membr. Sci. 556 (2018) 12–22.

[29] P. Xing, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, K. Wang, S. Kaliaguine, Synthesis and characterization of sulfonated poly (ether ether ketone) for proton exchange membranes, J. Membr. Sci. 229 (2004) 95–106. [30] Y.-C. Cao, C. Xu, X. Wu, X. Wang, L. Xing, K. Scott, A poly (ethylene oxide)/ graphene oxide electrolyte membrane for low temperature polymer fuel cells, J. Power Sources 196 (2011) 8377–8382. [31] B.G. Choi, Y.S. Huh, Y.C. Park, D.H. Jung, W.H. Hong, H. Park, Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets, Carbon 50 (2012) 5395–5402. [32] C.Y. Tseng, Y.S. Ye, M.Y. Cheng, K.Y. Kao, W.C. Shen, J. Rick, J.C. Chen, B. J. Hwang, Sulfonated polyimide proton exchange membranes with graphene oxide show improved proton conductivity, methanol crossover impedance, and mechanical properties, Adv. Energy Mater. 1 (2011) 1220–1224. [33] L. Wang, J. Kang, J.-D. Nam, J. Suhr, A.K. Prasad, S.G. Advani, Composite membrane based on graphene oxide sheets and nafion for polymer electrolyte membrane fuel cells, ECS Electrochem. Lett. 4 (2015) F1–F4. [34] L.S. Wang, A.N. Lai, C.X. Lin, Q.G. Zhang, A.M. Zhu, Q.L. Liu, Orderly sandwichshaped graphene oxide/Nafion composite membranes for direct methanol fuel cells, J. Membr. Sci. 492 (2015) 58–66. [35] Y. Heo, H. Im, J. Kim, The effect of sulfonated graphene oxide on sulfonated poly (ether ether ketone) membrane for direct methanol fuel cells, J. Membr. Sci. 425 (2013) 11–22. [36] P.P. Sharma, V. Kulshrestha, Synthesis of highly stable and high water retentive functionalized biopolymer-graphene oxide modified cation exchange membranes, RSC Adv. 5 (2015) 56498–56506. [37] B. Zhang, Y. Gao, S. Jiang, Z. Li, Guang He, H. Wu, Enhanced proton conductivity of Nafion nanohybrid membrane incorporated with phosphonic acid functionalized graphene oxide at elevated temperature and low humidity, J. Membr. Sci. 518 (2016) 243–253. [38] Y. He, J. Wang, H. Zhang, T. Zhang, B. Zhang, S. Cao, J. Liu, Polydopaminemodified graphene oxide nanocomposite membrane for proton exchange membrane fuel cell under anhydrous conditions, J. Mater. Chem. 2 (2014) 9548–9558. [39] D. Lee, H. Yang, S. Park, W. Kim, Nafion/graphene oxide composite membranes for low humidifying polymer electrolyte membrane fuel cell, J. Membr. Sci. 452 (2014) 20–28. [40] Z. Jiang, X. Zhao, Y. Fu, A. Manthiram, Composite membranes based on sulfonated poly (ether ether ketone) and SDBS-adsorbed graphene oxide for direct methanol fuel cells, J. Mater. Chem. 22 (2012) 24862–24869. [41] Q. Bao, D. Zhang, P. Qi, Synthesis and characterization of silver nanoparticle and graphene oxide nanosheet composites as a bactericidal agent for water disinfection, J. Colloid Interface Sci. 360 (2011) 463–470. [42] A.F. Faria, C. Liu, M. Xie, F. Perrault, L.D. Nghiem, J. Ma, M. Elimelech, Thin-film compasite forward osmosis membranes functionalized with graphene oxide-silver nanocomposites for biofouling control, J. Membr. Sci. 525 (2017) 146–156. [43] J.X. Leong, W.R.W. Daud, M. Ghasemi, A. Ahmad, M. Ismail, K.B. Liew, Composite membrane containing graphene oxide in sulfonated polyether ether ketone in microbial fuel cell applications, Int. J. Hydrogen Energy 40 (2015) 11604–11614. [44] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958), 1339-1339. [45] T.H. Choi, Y.-B. Won, J.-W. Lee, D.W. Shin, Y.M. Lee, M. Kim, H.B. Park, Electrochemical performance of microbial fuel cells based on disulfonated poly (arylene ether sulfone) membranes, J. Power Sources 220 (2012) 269–279. [46] S.S. Lim, W.R.W. Daud, J.M. Jahim, M. Ghasemi, P.S. Chong, M. Ismail, Sulfonated poly (ether ether ketone)/poly (ether sulfone) composite membranes as an alternative proton exchange membrane in microbial fuel cells, Int. J. Hydrogen Energy 37 (2012) 11409–11424. [47] Z. Gu, J. Ding, N. Yuan, F. Chu, B. Lin, Polybenzimidazole/zwitterion-coated polyamidoamine dendrimer composite membranes for direct methanol fuel cell applications, Int. J. Hydrogen Energy 38 (2013) 16410–16417. [48] B.E. Logan, B. Hamelers, R. Rozendal, U. Schr€ oder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5192. [49] L.-L. Tan, W.-J. Ong, S.-P. Chai, A.R. Mohamed, Reduced graphene oxide-TiO2 nanocomposite as a promising visible-light-active photocatalyst for the conversion of carbon dioxide, Nanoscale Res. Lett. 8 (2013) 1–9. [50] Y. Wang, J. Cao, Y. Zhou, J.-H. Ouyang, D. Jia, L. Guo, Ball-milled graphite as an electrode material for high voltage supercapacitor in neutral aqueous electrolyte, J. Electrochem. Soc. 159 (2012) A579–A583. [51] M. Hilder, O. Winther-Jensen, B. Winther-Jensen, D.R. MacFarlane, Graphene/zinc nano-composites by electrochemical co-deposition, Phys. Chem. Chem. Phys. 14 (2012) 14034–14040. [52] I. Roy, D. Rana, G. Sarkar, A. Bhattacharyya, N.R. Saha, S. Mondal, S. Pattanayak, S. Chattopadhyay, D. Chattopadhyay, Physical and electrochemical characterization of reduced graphene oxide/silver nanocomposites synthesized by adopting a green approach, RSC Adv. 5 (2015) 25357–25364. [53] J. Wang, S. Liang, L. Ma, S. Ding, X. Yu, L. Zhou, Q. Wang, One-pot synthesis of CdS–reduced graphene oxide 3D composites with enhanced photocatalytic properties, CrystEngComm 16 (2014) 399–405. [54] S.W. Chook, C.H. Chia, S. Zakaria, M.K. Ayob, K.L. Chee, N.M. Huang, H.M. Neoh, H.N. Lim, R. Jamal, R.M.F.R.A. Rahman, Antibacterial performance of Ag nanoparticles and AgGO nanocomposites prepared via rapid microwave-assisted synthesis method, Nanoscale Res. Lett. 7 (2012) 1–7. [55] L. Yuan, L. Jiang, J. Liu, Z. Xia, S. Wang, G. Sun, Facile synthesis of silver nanoparticles supported on three dimensional graphene oxide/carbon black

9

K. Ben Liew et al.

[56] [57] [58] [59] [60] [61]

[62] [63] [64]

Journal of Power Sources xxx (xxxx) xxx [65] B. Yang, A. Manthiram, Sulfonated poly (ether ether ketone) membranes for direct methanol fuel cells, Electrochem, Solid State Lett 6 (2003) A229–A231. [66] M.R. Karim, K. Hatakeyama, T. Matsui, H. Takehira, T. Taniguchi, M. Koinuma, Y. Matsumoto, T. Akutagawa, T. Nakamura, S.-i. Noro, Graphene oxide nanosheet with high proton conductivity, J. Am. Chem. Soc. 135 (2013) 8097–8100. [67] R. Kumar, M. Mamlouk, K. Scott, A graphite oxide paper polymer electrolyte for direct methanol fuel cells, Int. J. Electrochem. 2011 (2011). [68] V.V. Nikonenko, A.B. Yaroslavtsev, G. Pourcelly, Ion transfer in and through charged membranes: structure, properties, and theory, Ionic Interactions in Natural and Synthetic Macromolecules (2012) 267–335. [69] E. Ji, H. Moon, J. Piao, P.T. Ha, J. An, D. Kim, J.-J. Woo, Y. Lee, S.-H. Moon, B. E. Rittmann, Interface resistances of anion exchange membranes in microbial fuel cells with low ionic strength, Biosens. Bioelectron. 26 (2011) 3266–3271. [70] S. Oh, J. Kim, J. Joo, B. Logan, Effects of applied voltages and dissolved oxygen on sustained power generation by microbial fuel cells, Water Sci. Technol. 60 (2009) 1311. [71] M. Zhang, K. Zhang, B. De Gusseme, W. Verstraete, Biogenic silver nanoparticles (bio-Ag 0) decrease biofouling of bio-Ag 0/PES nanocomposite membranes, Water Res. 46 (2012) 2077–2087. [72] S. Madaeni, S. Zinadini, V. Vatanpour, A new approach to improve antifouling property of PVDF membrane using in situ polymerization of PAA functionalized TiO 2 nanoparticles, J. Membr. Sci. 380 (2011) 155–162. [73] C. Sun, J. Miao, J. Yan, K. Yang, C. Mao, J. Ju, J. Shen, Applications of antibiofouling PEG-coating in electrochemical biosensors for determination of glucose in whole blood, Electrochim. Acta 89 (2013) 549–554.

composite and its application for oxygen reduction reaction, Electrochim. Acta 135 (2014) 168–174. J. Ma, J. Zhang, Z. Xiong, Y. Yong, X. Zhao, Preparation, characterization and antibacterial properties of silver-modified graphene oxide, J. Mater. Chem. 21 (2011) 3350–3352. X.-Z. Tang, Z. Cao, H.-B. Zhang, J. Liu, Z.-Z. Yu, Growth of silver nanocrystals on graphene by simultaneous reduction of graphene oxide and silver ions with a rapid and efficient one-step approach, Chem. Commun. 47 (2011) 3084–3086. C. Gunawan, W.Y. Teoh, C.P. Marquis, J. Lifia, R. Amal, Reversible antimicrobial photoswitching in nanosilver, Small 5 (2009) 341–344. X. Wu, X. Wang, G. He, J. Benziger, Differences in water sorption and proton conductivity between Nafion and SPEEK, J. Polym. Sci., Part B: Polym. Phys. 49 (2011) 1437–1445. V. Silva, B. Ruffmann, S. Vetter, M. Boaventura, A. Mendes, L. Madeira, S. Nunes, Mass transport of direct methanol fuel cell species in sulfonated poly (ether ether ketone) membranes, Electrochim. Acta 51 (2006) 3699–3706. Y. Yin, H. Wang, L. Cao, Z. Li, Z. Li, M. Gang, C. Wang, H. Wu, Z. Jiang, P. Zhang, Sulfonated poly(ether ether ketone)-based hybrid membranes containing graphene oxide with acid-base pairs for direct methanol fuel cells, Electrochim. Acta 203 (2016) 178–188. Y.-S. Park, Y. Yamazaki, Low water/methanol permeable Nafion/CHP organic–inorganic composite membrane with high crystallinity, Eur. Polym. J. 42 (2006) 375–387. L. Li, J. Zhang, Y. Wang, Sulfonated poly (ether ether ketone) membranes for direct methanol fuel cell, J. Membr. Sci. 226 (2003) 159–167. M. Othman, A.F. Ismail, A. Mustafa, Physico-chemical study of sulfonated poly (ether ether ketone) membranes for direct methanol fuel cell application, Malaysian Polym. J. 2 (2007) 10–28.

10