PVDF-Modified Nafion Membrane for Improved Performance Of

membranes

Article PVDF-Modiﬁed Naﬁon Membrane for Improved Performance of MFC

Liping Fan 1,2,*, Junyi Shi 2 and Yaobin Xi 2

1 College of Information Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China 2 College of Environment and Safety Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China; [email protected] (J.S.); [email protected] (Y.X.) * Correspondence: [email protected]

Received: 9 July 2020; Accepted: 12 August 2020; Published: 13 August 2020

Abstract: Low power production and unstable power supply are important bottlenecks restricting the application of microbial fuel cells (MFCs). It is necessary to explore effective methods to improve MFC performance. By using molasses wastewater as fuel, carbon felt as an electrode, and the mixture of K3[Fe(CN)6] and NaCl as a catholyte, an MFC experimental system was set up to study the performance of MFCs with three different proton exchange membranes. A Nafion membrane was used as the basic material, and polyvinylidene fluoride (PVDF) and acetone-modified PVDF were used to modify it, respectively. The experimental results show that a PVDF-modified membrane can improve the water absorption effectively and, thus, make the MFC have greater power generation and better wastewater treatment effect. The acetone-modified PVDF can further improve the stability of output power of the MFC. When the acetone-modified PVDF was used to modify the Nafion membrane, the steady output voltage of the MFC was above 0.21 V, and the Chemical Oxygen Demand (COD) removal rate for molasses wastewater was about 66.7%, which were 96.3% and 75.1% higher than that of the MFC with the ordinary Nafion membrane. Membrane modification with acetone-modified PVDF can not only increase the output voltage of the MFC but also improve the stability of its output electrical energy.

Keywords: microbial fuel cells; membrane modiﬁcation; PVDF; acetone

1. Introduction Energy shortage and environmental pollution have become two major problems restricting the sustainable development of human beings. Due to overexploitation, traditional fossil fuels have been nearly exhausted, and the worldwide competition for resources is on the rise. In addition, the exploitation of fossil fuels damages land and villages and pollutes water resources, and the various gases and solid wastes produced in the process of using fossil fuels also pollute the environment. Developing clean energy to replace fossil fuel is an effective way to solve the environmental and energy problems, so it has gradually become the focus of people’s attention and the key development field of various countries [1–3]. Microbial fuel cells (MFCs) are a kind of biological reaction device that uses microbial metabolism to convert chemical energy in organic matter into electrical energy directly. An MFC uses organic waste as fuel to generate electric energy in the process of waste treatment, and almost no harmful substances are produced in the process of electricity generation. It can be used not only as a power generation device to solve energy problems but also as a waste disposal device to solve environmental pollution problems. Therefore, it is regarded as an efficient, low-consumption, clean, and environmentally sustainable energy [4–6].

Membranes 2020, 10, 185; doi:10.3390/membranes10080185 www.mdpi.com/journal/membranes Membranes 2020, 10, 185 2 of 14

Due to the influence of low power generation, high cost, and unstable work, the popularization and application of MFCs is still restricted [7]. The main factors affecting the performance of MFCs include microbial population, exchange membrane, electrode, internal and external resistance, substrate concentration, electrode spacing, etc. [8–11]. Researchers have tried various methods to improve the performance of MFCs. The common methods include the fabrication of new electrode materials, the development of a more efficient exchange membrane, the modification of carbon-based electrode materials and Nafion membrane, the cultivation of more efficient electrogenic bacteria, and so on. When a potassium dichromate (K2Cr2O7)-modified carbon cloth anode was used in an MFC, the power generation performance and wastewater treatment effects were significantly improved [12]. Au-g-C3N4 nanostructures have high electron transfer efficiency and can be used as MFC electrodes to improve electrochemical performance [13]. Ag@g-C3N4 NSs, which was fabricated by an innovative and simple biogenic approach, can produce highly reactive oxygen species (ROS) [14], if it is used as the cathode or catalyst of MFCs, the reaction rate of the cathode chamber can be accelerated. The MFCs using a Mo2C/CCT (molybdenum carbide nanoparticles-modified carbonized cotton textile) anode can produce higher power density [15]. Some research found that the use of mutualistic interaction of yeast and bacteria could increase the performance of the MFCs [16]. Among all these factors that affect MFC performance, the proton exchange membrane is considered to be the most important factor affecting the operation effect of MFC. A proton exchange membrane is an important part to separate the anode chamber and cathode chamber and transfer protons from anode to cathode; moreover, the cost of the membrane accounts for more than 38% of the total cost of MFCs [17,18], so it is also one of the important factor affecting the promotion and application of MFCs. Proton exchange membranes have been widely used in MFCs because of their advantages of high conductivity and low internal resistance [19–21]. The most commonly used proton exchange membrane in MFCs is perfluorosulfonic acid membrane, such as a Nafion membrane produced by the DuPont Company. The main advantages of Nafion membranes are their high proton conductivity, good chemical stability, and mechanical properties, but it still has some drawbacks such as high oxygen permeability, poor thermal stability, biological deposition, and serious water loss at high temperature, which limits the performance of MFCs [22]. In order to solve these problems, it is an effective and recognized way to improve the performance of MFCs by membrane modification. Polyvinylidene fluoride (PVDF) is a pure thermoplastic fluoropolymer. PVDF not only has the advantages of good toughness, low friction coefficient, corrosion-resistant, and anti-aging but also has the characteristics of low density, low impedance, and good chemical stability [23–27]. On the other hand, compared with a Nafion membrane itself, PVDF is a relatively cheap material. Modifying a Nafion membrane by PVDF can improve the water retention and its adaptability to temperature and humidity at a very low cost [28] and then increase the electricity-generating capacity and the wastewater treatment efficiency of MFCs, so the overall cost performance of an MFC system is improved. In this paper, PVDF particles and a kind of modified PVDF solution were used to modify the proton exchange membrane, and the effects of PVDF and modified PVDF on the power generation performance and water purification effect of MFCs were studied. In addition, a group of experiments was added to study the effect of carbon cloth and carbon felt electrodes on the operation of MFCs under the same other conditions.

2. Materials and Methods

2.1. Composition of the Experimental System The experimental system consists of three parts, namely, a dual chamber MFC, an external load and a data acquisition system (Figure1). A dual chamber MFC is mainly composed of a cathode chamber, anode chamber, proton exchange membrane, cathode, and anode. The reaction chambers were made of plexiglass. The cathode chamber and anode chamber are separated by a proton exchange membrane (Naﬁon 117; DuPont Co., Wilmington, DE, USA). The volume of both the cathode and Membranes 2020, 10, 185 3 of 14 Membranes 2020, 10, x FOR PEER REVIEW 3 of 13 anodefelt with reaction surface chambers areas of 4 was cm 500× 4.5 mL. cm. BothThe anode the anode and the and cathode cathode were materials connected were with carbon external felt with load surfacethrough areas wires of 4to cm form4.5 a cm.complete The anode closed and circuit. the cathode The voltage were connectedgeneratedwith by the external MFC loadwas throughcollected × wiresthrough to form the adata complete acquisition closed circuit.card (MPS-010602; The voltage generatedMorpheus by Electronics the MFCwas Technology collected throughCo., Beijing, the dataChina), acquisition recorded card every (MPS-010602; 60 s, and Morpheustransmitted Electronics to a computer Technology through Co., USB Beijing, interface China), for recorded storage, everyprocessing, 60 s, and and transmitted display. to a computer through USB interface for storage, processing, and display.

FigureFigure 1. 1.Schematic Schematic diagram diagram of of the the MFC MFC experiment experiment system. system. 2.2. Experimental Materials and Pretreatment 2.2. Experimental Materials and Pretreatment The anolyte of the MFC for experiment was molasses wastewater, which was prepared by mixing The anolyte of the MFC for experiment was molasses wastewater, which was prepared by the following chemicals: 3.13 g/L NaHCO3, 0.31 g/L NH4Cl, 6.338 g/L NaH2PO4 H2O, 0.13 g/L KCl, mixing the following chemicals: 3.13 g/L NaHCO3, 0.31 g/L NH4Cl, 6.338 g/L NaH· 2PO4·H2O, 0.13 g/L 0.2 g/L MgSO4 7H2O, 0.015 g/L CaCl2, 3 g/L brown sugar, 0.01 g/L MnSO4 H2O, and 6.8556 g/L KCl, 0.2 g/L MgSO· 4·7H2O, 0.015 g/L CaCl2, 3 g/L brown sugar, 0.01 g/L MnSO· 4·H2O, and 6.8556 g/L Na2HPO4 12H2O. Na2HPO4··12H2O. The catholyte used in the experiment was a mixture of potassium ferricyanide (K3[Fe(CN)6]) and The catholyte used in the experiment was a mixture of potassium ferricyanide (K3[Fe(CN)6]) and sodium chloride (NaCl). 0.2 mol/LK3[Fe(CN)6] (32.9 g) was dissolved in a phosphoric acid buffer sodium chloride (NaCl). 0.2 mol/L K3[Fe(CN)6] (32.9 g) was dissolved in a phosphoric acid buffer solution (PBS) in a 500 mL volumetric flask; we put 0.4 mol/L NaCl (11.7 g) into another 500 mL solution (PBS) in a 500 mL volumetric flask; we put 0.4 mol/L NaCl (11.7 g) into another 500 mL volumetric flask; then the K3[Fe(CN)6] solution described above were mixed with NaCl solution in a volumetric flask; then the K3[Fe(CN)6] solution described above were mixed with NaCl solution in a ratio of 1:1 to obtain the catholyte of the MFC. All the reagents used in the experiment were purchased ratio of 1:1 to obtain the catholyte of the MFC. All the reagents used in the experiment were purchased from Damao Co, Tianjin, China. from Damao Co, Tianjin, China. Many research results show that soil is rich in microorganisms [29,30]. So the electrogenic bacteria Many research results show that soil is rich in microorganisms [29,30]. So the electrogenic used in the anode chamber were obtained by culturing some soil. The soil was collected around some bacteria used in the anode chamber were obtained by culturing some soil. The soil was collected trees in the campus. Under anaerobic conditions, the prepared simulated molasses wastewater and around some trees in the campus. Under anaerobic conditions, the prepared simulated molasses the collected soil were put into the culture flask together, then some trace elements (such as carbon, wastewater and the collected soil were put into the culture flask together, then some trace elements nitrogen, phosphorus, etc.) needed for microbial growth were added, and then the culture flask was (such as carbon, nitrogen, phosphorus, etc.) needed for microbial growth were added, and then the put into the biochemical incubator and domesticated at 30 C for about 3 days. When the sludge culture flask was put into the biochemical incubator and domesticated◦ at 30 °C for about 3 days. in the culture flask was suspended as flocculate, the culture of microorganisms was considered to When the sludge in the culture flask was suspended as flocculate, the culture of microorganisms was be successful. considered to be successful. 2.3. Preparation of Membranes 2.3. Preparation of Membranes We boiled the proton exchange membrane (Nafion 117) in 5% H2O2 solution for 1 h, then took it We boiled the proton exchange membrane (Nafion 117) in 5% H2O2 solution for 1 h, then took it out and washed it repeatedly with deionized water to remove organic impurities; then boiled it in 1 out and washed it repeatedly with deionized water to remove organic impurities; then boiled it in 1 mol/LH2SO4 for 1 h to remove metal ion impurities; we then took it out and washed it repeatedly mol/L H2SO4 for 1 h to remove metal ion impurities; we then took it out and washed it repeatedly with deionized water again and boiled it for 1 h with deionized water to remove the residual H2SO4. with deionized water again and boiled it for 1 h with deionized water to remove the residual H2SO4. The pretreated proton exchange membrane was put into deionized water for standby. The pretreated proton exchange membrane was put into deionized water for standby. (1) Preparation of PVDF and modified PVDF solution

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(1) Preparation of PVDF and modified PVDF solution DMF (N,N-dimethylformamide) is a widely used excellent organic solvent with good solubility and chemical stability for a variety of organic and inorganic compounds [31]. Therefore, we used DMF as solvent to make the PVDF solution. 12.47 g PVDF was dissolved in 250 mL DMF solution and stirred for 2 h at room temperature to obtain white PVDF solution. 11.21 g PVDF was dissolved in 250 mL mixture of DMF and acetone with a volume ratio of 4:6 and stirred continuously for 2 h at room temperature, and then the modified PVDF solution with milky white color was obtained. (2) Preparation of composite membrane The Nafion membrane with a size of 10 cm 10 cm was immersed in a methanol solution with a × volume ratio of 3:1 for 1 h to make the membrane surface swell, and thus the PVDF particles can better adhere to it; then the Nafion membrane was placed in the PVDF solution and soaked at a constant temperature of 60 ◦C for 5 min; then it was taken out and dried and placed in a vacuum drying oven to dry at a constant temperature of 60 ◦C for 15 min. Repeating the above operations for four or five times, PVDF–Nafion composite membrane was obtained. We immersed another Nafion membrane in the modified PVDF solution, treated the Nafion membrane in the same way as above, and finally, the modified PVDF–Nafion composite membrane was prepared [32].

2.4. Analysis Method The performance analysis of an MFC mainly includes power generation performance and wastewater treatment effect. The current density value is generally used to evaluate the power generation performance. The output current I of the MFC can be calculated by Ohm’s law according to the measured voltage value U, that is: I = U/R (1) in which U is the output voltage of the MFC measured by data acquisition card and R is the external load resistance. In the experiment, R is always set at 500 Ω. Then the generating current density IA of the MFC can be obtained by calculating the following formula: I U I = = (2) A A AR where A is the anode surface area of the fuel cell. Wastewater treatment effect is also an important index of MFC performance. In MFCs, microorganisms degrade organics to generate electricity; meanwhile, the wastewater that is used as an anolyte (or fuel) of the MFC can also be purified well. In order to analyze the wastewater treatment effect of the MFC, the influent COD and effluent COD of the anode chamber were measured by a LH-NP2 type COD rapid detector (Lohand biological company, Dalian, China), and then the COD removal rate of the MFC was calculated, according to which the wastewater treatment effect of the MFC was analyzed subsequently. Through the physical structure and morphology of the material, the effect of the membrane modification can be analyzed more deeply in essence. Scanning electron microscope (SEM) (JSM-6360LV; JEOL Co., Tokyo, Japan) analysis of the exchange membrane before and after modification was carried out and electrochemical impedance spectroscopy (EIS) (CHI660E, CH Instrument Co., Shanghai, China) analysis of the MFC was also provided in this experiment. Membranes 2020, 10, 185 5 of 14 Membranes 2020, 10, x FOR PEER REVIEW 5 of 13

3. Results and Discussion

3.1. Comparison of Power Generation Performance Four MFC experimental systems with the same basicbasic configuration configuration were run simultaneously to guarantee that the other experimental conditions are the same except for the comparison items, so as to obtain a good comparison eeffect.ffect. Firstly, the influenceinfluence of didifferentfferent electrodeselectrodes onon thethe powerpower generationgeneration performance of the MFC with the same NafionNafion membrane was analyzed. Figu Figurere 22 showsshows thethe outputoutput voltagevoltage curvescurves ofof MFCsMFCs with didifferentfferent electrodes and didifferentfferent exchangeexchange membranes.membranes. It can be seen from the figure figure that the power generation capacity of MFC with carbon cloth electrode was ob obviouslyviously different different from that of the MFC with a carbon felt electrode. When When both the exchange membranes of the MFCs are common NafionNafion membranes, the steady-state output voltag voltagee of the MFC with a carbon cloth electrode was about 0.0250.025 V,V, while while the the steady-state steady-state output output voltage voltage of theof the MFC MFC with with a carbon a carbon felt electrode felt electrode was about was 0.107about V.0.107 That V. is That to say, is to the say, output the output voltage voltage of the MFCof the with MFC a carbonwith a carbon felt electrode felt electrode is about is328% about higher 328% thanhigher that than of that MFC of with MFC a with carbon a carbon cloth electrode.cloth electrod Thise. isThis mainly is mainly due todue the to factthe thatfact that the carbonthe carbon felt hasfelt has a larger a larger specific specific surface surface area area and and richer richer and and finer finer pores pores than than the the carbon carbon cloth. cloth. Therefore,Therefore, it is more beneficialbeneficial for microorganisms to be adhered to its surface and, thus, increases the growth space of the anode biofilmbiofilm and improves the electron transfertransfer of thethe wholewhole system.system. In view of the above experimental results, carbon felt was used asas thethe electrodeelectrode ofof thethe MFCMFC inin thethe laterlater experiments.experiments. It cancan alsoalso be be seen seen from from Figure Figure2 that 2 that the outputthe output voltage voltage of the of MFC the withMFC a PVDF-modifiedwith a PVDF-modified Nafion membraneNafion membrane was significantly was significantly higher thanhigher that than of thethat MFC of the with MFC a commonwith a common Nafion Nafion membrane. membrane. This is mainlyThis is becausemainly because the exchange the exchange membrane membrane modified bymodified PVDF improves by PVDF the improves water retention the water performance, retention soperformance, the water absorption so the water capacity absorption and proton capacity conductivity and proton are improved. conductivity The introduction are improved. of organic The materialsintroduction is also of eorganicffective tomaterials improve is the also conductivity effective to of theimprove exchange the membrane.conductivity However, of the exchange when the exchangemembrane. membrane However, was when modified the exchange by general membrane PVDF, althoughwas modified the output by general voltage PVDF, was although significantly the higheroutput thanvoltage that was of the significantly MFC with higher a normal than Nafion that of membrane, the MFC thewith output a normal voltage Nafion fluctuated membrane, so much the thatoutput it was voltage diffi cultfluctuated to stabilize. so much Although that it was the maximumdifficult to voltage stabilize. can Although reach 0.3 the V, themaximum output voltage cannotcan reach be kept0.3 V, at athe high output value voltage stably andcannot drops be sharply kept at after a high reaching value thestably peak and value; drops in the sharply early stageafter ofreaching its operation, the peak the value; start-up in the voltage early ofstage the of MFC its oper is relativelyation, the low. start-up Low startingvoltage of voltage the MFC and is fluctuating relatively outputlow. Low are starting major problems voltage and affecting fluctuating practical output application. are major Therefore, problems the affecting improved practical PVDF application. was used to modifyTherefore, the the membrane improved to furtherPVDF was improve used theto powermodify generation the membrane stability to further of the MFC. improve The stabilitythe power of thegeneration output stability voltage ofof the MFC. MFC withThe stability a modified of the PVDF–Nafion output voltage composite of the MFC membrane with a modified was improved PVDF– obviously;Nafion composite in this case, membrane the steady was state improved output obviously; voltage was in about this case, 0.21 V,the which steady was state 96.3% output higher voltage than thatwas ofabout the MFC0.21 V, with which a common was 96.3% Nafion higher membrane. than that of the MFC with a common Nafion membrane.

0.50 Nafion/carbon felt 0.45 PVDF-Nafion/carbon felt 0.40 modified PVDF-Nafion/carbon felt Nafion/carbon cloth 0.35

V 0.30 U/ 0.25

0.20 Voltage

0.15

0.10

0.05

0.00 0 25 50 75 100 125 150 t (h) Figure 2. OutputOutput voltage curves of MFCs under different diﬀerent conditions.

The stability of the output voltage of the MFC with a modified PVDF–Nafion membrane has been improved significantly, mainly because the acetone solvent accelerated the rate of acceptance of

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MembranesThe 2020 stability, 10, x FOR of the PEER output REVIEW voltage of the MFC with a modified PVDF–Nafion membrane6 of has 13 been improved significantly, mainly because the acetone solvent accelerated the rate of acceptance of PVDF by the Nafion membrane; in addition, the porous and disordered structure on the surface of PVDF by the Nafion membrane; in addition, the porous and disordered structure on the surface of the the Nafion membrane makes PVDF easier to adhere to, making the proton transmission performance Nafion membrane makes PVDF easier to adhere to, making the proton transmission performance of of the modified Nafion membrane more stable, thus improving the stability of the output electric the modified Nafion membrane more stable, thus improving the stability of the output electric energy energy of the MFC. of the MFC. In order to compare the power generation capacity of MFCs in the several different conditions In order to compare the power generation capacity of MFCs in the several different conditions more effectively, the output electrical energy values of the MFC with four different conditions were more effectively, the output electrical energy values of the MFC with four different conditions were calculated and compared. The formula that links energy W and power P is: calculated and compared. The formula that links energy W and power P is: W=Pt (3) W = Pt (3) The power can be calculated by using the Joule’s law equation, so the energy can be described by: The power can be calculated by using the Joule’s law equation, so the energy can be described by: U 2 WUItt= = U2 (4) W = UIt = R t (4) R Since the output voltage of the MFC is time-varying, the output electrical energy within 0 h to Since the output voltage of the MFC is time-varying, the output electrical energy within 0 h to 150 150 h can be expressed as: h can be expressed as: Z 2 150 uu2 WWt== dd t (5)(5) 0 0 RR Figure3 3shows shows the the electrical electrical energy energy values values generated generated by the fourby the MFCs four with MFCs di fferent with conditions. different conditions.It can be seen It thatcan thebe seen MFC that with the a carbon MFC with cloth a electrode carbon cloth produced electrode much produced less electrical much energy less electrical than the energyMFC with than a carbonthe MFC felt with electrode. a carbon The felt output electrode. electrical The output energy electrical of the MFC energy with of the the common MFC with Nafion the commonmembrane Nafion and carbon membrane cloth electrodeand carbon was cloth 3.83 electrod J. Whene awas carbon 3.83 feltJ. When electrode a carbon was used,felt electrode the electrical was used,energy the generated electrical by energy the MFCs generated using by the the common MFCs using Nafion the membrane, common Nafion the PVDF–Nafion membrane, the composite PVDF– Nafionmembrane, composite and the membrane, modified and PVDF–Nafion the modified membrane PVDF–Nafion were membrane 29.7 J, 45.3 were J, and 29.7 47.4 J, 45.3 J, respectively. J, and 47.4 TheJ, respectively. electrical energy The electrical produced energy by the produced MFC with by the the common MFC with Nafion the membrane common Nafion was significantly membrane lower was significantlythan that of the lower MFC than with that the of two the composite MFC with membranes. the two composite This shows membranes. that the PVDF-modified This shows that proton the PVDF-modifiedexchange membrane proton significantly exchange me enhancesmbrane thesignificantly power generation enhances capacitythe power of generation MFCs. The capacity electrical of MFCs.energy The of the electrical MFC with energy modified of the PVDF MFC membranewith modified was PVDF slightly membrane higher than was that slightly of the MFChigher with than a thatgeneral of the PVDF-modified MFC with a general membrane. PVDF-modified This shows thatmemb therane. use ofThis the shows modified that PVDF the use membrane of the modified can not PVDFonly significantly membrane can enhance not only the powersignificantly generation enhanc stabilitye the power of the generation MFC but alsostability improves of the the MFC power but alsogeneration improves performance the power ofgeneration MFC. Therefore, performance modification of MFC. ofTherefore, the exchange modification membrane of the by exchange modified membranePVDF has a by good modified effect onPVDF improving has a good the overalleffect on performance improving the of the overall MFC. performance of the MFC.

60 Nafion/carbon cloth Nafion/carbon felt 50 PVDF/Nafion/carbon felt modified PVDF/Nafion/carbon felt

J 40 W/

Electric Energy Electric 20

0 Figure 3. ElectricalElectrical energy energy of MFCs with different diﬀerent conditions.

In order to further analyze the effect eﬀect of the modiﬁedmodified PVDF on the stability of the MFC, the standard deviation of the output current densitydensity ofof thethe MFCMFC waswas calculated.calculated. The steady-state value of the current density that characterizes the power supply capacity of the MFC can be obtained according to the calculation formula of the average value:

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The steady-state value of the current density that characterizes the power supply capacity of the MembranesMFC can 2020 be obtained, 10, x FOR PEER according REVIEW to the calculation formula of the average value: 7 of 13

X1 + X2 + ... + Xn µ = X +XX++ ... (6) μ= 12n n (6) n 2 where µ is the average value of current density (A/m ); Xi is the value of current density of the ith where μ is the average value of current density (A/m2); Xi is the value of current density of the ith sample (A/m2); n is the number of samples. sample (A/m2); n is the number of samples. The power supply stability of the MFC can be characterized by standard deviation, i.e., The power supply stability of the MFC can be characterized by standard deviation, i.e.,: r Xn 1 n 2 σ =σ (X−i μ µ2 ) (7) =()= X i (7) n  ii=1 1 − where σ isis thethe standardstandard deviationdeviation ofof thethe outputoutput currentcurrent densitydensity (A(A/m/m22).). Standard deviationdeviation isis a a measure measure of of the the degree degree of dispersionof dispersion of the of averagethe average of a setof ofa set data. of Adata. larger A largerstandard standard deviation deviation means means that there that is there a larger is a di largerfference difference between between the sample the datasample and data itsaverage and its value,average while value, a smallerwhile a smaller standard standard deviation deviation means thatmeans the that sample the sample data is data closer is tocloser the averageto the average value. Thevalue. smaller The smaller the standard the standard deviation, deviation, the smaller the smaller the volatility the volatility and the and better the better the stability the stability [33,34 [33,34].]. Figure4 4 shows shows thethe currentcurrent densitydensity curvescurves ofof thethe MFCMFC inin didifferentfferent conditions. It can be seen that the steady-statesteady-state values values of theof the current current density density of the of MFC the with MFC the with PVDF–Nafion the PVDF–Nafion composite membranecomposite membraneand the modified and the PVDF–Nafion modified PVDF–Nafion membrane membrane were 0.219 were A/m2 0.219and 0.233A/m2 Aand/m 20.233, respectively. A/m2, respectively. Although Althoughthere was onlythere little was di onlyfference little between difference the two,between it can th bee seentwo, from it can Table be 1seen that thefrom standard Table 1 deviation that the valuesstandard of thedeviation current values density of ofthe the current MFC with densit they PVDFof the compositeMFC with membranethe PVDF andcomposite the MFC membrane with the andmodified the MFC PVDF with composite the modified membrane PVDF were composite 0.062 A/m 2membraneand 0.013 Awere/m2, respectively,0.062 A/m2 and the0.013 standard A/m2, respectively,deviation of and the currentthe standard density deviation of the MFCof the with current the density modified of the PVDF MFC composite with the modified membrane PVDF was compositesignificantly membrane lower than was that significantly of the MFC with lower the than common that PVDFof the composite MFC with membrane. the common This meansPVDF compositethat the modified membrane. PVDF This membrane means significantly that the modifi improvesed PVDF the stabilitymembrane of the sign outputificantly power improves of the MFC. the Thestability modified of the PVDF output membrane power of canthe MFC. effectively The modified overcome PVDF the serious membrane fluctuation can effectively of the output overcome electrical the seriousenergy fluctuation of the MFC of caused the output by the electrical conventional energy PVDF-modifiedof the MFC caused membrane, by the conventional which not PVDF-modified only increases membrane,the output voltagewhich not of theonly MFC, increases but also the output improves voltage the stability of the MFC, of its but output also electricalimproves energy.the stability This of is its of outputgreat significance electrical energy. for the This practical is of great application significance of MFCs. for the practical application of MFCs.

0.50 Nafion/carbon felt 0.45 PVDF Nafion/carbon felt

0.40 modified PVDF Nafion/carbon felt Nafion/carbon cloth 0.35

) 0.30 2 0.25 (A/m A

I 0.20 0.15 0.10 0.05 0.00 0 25 50 75 100 125 150 t (h) Figure 4. CurrentCurrent density curves of MF MFCsCs in different diﬀerent conditions.

Table 1. Steady-state values and their standard deviations of current density.

Nafion/ Nafion/ PVDF–Nafion/Carbon Modified PVDF– Carbon Carbon Felt Nafion/Carbon Felt Cloth Felt Steady-state value 0.062 0.181 0.219 0.233 (A/m2)

Standard deviation 0.023 0.037 0.062 0.013 (A/m2)

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Table 1. Steady-state values and their standard deviations of current density.

Nafion/Carbon Nafion/Carbon PVDF–Nafion/ Modified PVDF– Cloth Felt Carbon Felt Nafion/Carbon Felt Steady-state value 0.062 0.181 0.219 0.233 (A/m2) Standard deviation 2 0.023 0.037 0.062 0.013 Membranes 2020(A/m, 10), x FOR PEER REVIEW 8 of 13

3.2. SEM Analysis The SEM SEM images images of of the the three three different different proton proton exch exchangeange membranes membranes are shown are shown in Figure in Figure 5. It can5. Itbe can seen be that seen the that surface the surface of the of theconventional conventional Nafion Nafion membrane membrane had had no no obvious obvious morphological characteristics except for some sporadic impurities impurities,, and and it it seems to be a compact plane shape as a whole. The surface morphology of the PVDF-modifiedPVDF-modified proton exchange membrane was rough and mossy, and there were manymany granular and agglomerated substances protruding from the surface, which shouldshould bebe duedue toto the the uneven uneven distribution distribution of of the the modifier. modifier. Moreover, Moreover, the the white white PVDF PVDF particles particles on theon the surface surface of the of membranethe membrane were sparsewere sparse and uneven, and uneven, which maywhich be themay main be the reason main for reason the instability for the ofinstability the power of generationthe power performance.generation performance. However, the However, surface of the the surface acetone-modified of the acetone-modified PVDF–Nafion compositePVDF–Nafion membrane composite was membrane covered by manywas covered regular by granular many polymers,regular granular and more polymers, pores were and formed, more whichpores were is more formed, conducive which tois protonmore conducive conduction. to proton From theconduction. overall pointFrom ofthe view overall of thepoint composite of view membrane,of the composite the distribution membrane, of the PVDF distribution white particles of PVDF was densewhite andparticles uniform, was indicatingdense and that uniform, PVDF particlesindicating were that well PVDF attached particles to Nafionwere well membrane, attached thusto Nafion improving membrane, the power thus production improving and the outputpower stabilityproduction of MFCs.and output This indicatesstability thatof MFCs. the modified This indicates PVDF canthat bringthe modified a better modificationPVDF can bring effect a onbetter the protonmodification exchange effect membrane on the proton than ordinaryexchange PVDF.membrane than ordinary PVDF.

(a) (b) (c)

Figure 5. SEMSEM images. images. (a () ageneral) general Nafion Nafion membrane. membrane. (b) (PVDF–Nafionb) PVDF–Nafion composite composite membrane. membrane. (c) (modifiedc) modified PVDF–Nafion PVDF–Nafion composite composite membrane. membrane.

3.3. Hydroscopicity Analysis of Membrane The water water absorption absorption performance performance of of the the proton proton exchange exchange membrane membrane is directly is directly related related to the to thepower power generation generation performance performance of MFC. of MFC. When When the the water water content content of of the the membrane membrane is is high, high, the migration coefficient coefficient of the proton is higher than that of of water, and the conductivity is is also also high. high. Therefore, the higher the water absorption of the membrane, the better its conductivity. To compare the hydroscopicity of the three didifferentfferent membranes used in the experiment, the water absorption of the three membranes was calculated, respectively, byby measuringmeasuring the weight of the membrane before and after hydration [[35,36].35,36]. Firstly, the membrane was was immersed immersed in in deionize deionizedd water water at atroom room temperature temperature for for 24 24h. Then h. Then the themembrane membrane surface surface was was dried dried with with the the filter filter paper paper immediately immediately after after taking taking it it out, out, and and then then was weighed and marked as wet weight of the membrane W . The membrane was then placed in an oven weighed and marked as wet weight of the membrane W11. The membrane was then placed in an oven at 80 °C and baked for 4 h and, then, was taken out and cooled to room temperature, weighed, and recorded as dry weight W0. Then the water absorption of the membrane can be calculated as: − WW10 Δ=W (%) × 100% (8) W0

The water absorption rates of the three different membranes are shown in Table 2.

Membranes 2020, 10, x FOR PEER REVIEW 9 of 13

Wet weight W1(g) 3.65 3.92 3.87

Dry weight W0(g) 3.43 3.50 3.46 Membranes 2020, 10, 185 9 of 14 Water absorption 6.41 12.00 11.80 rate ΔW(％) at 80 ◦C and baked for 4 h and, then, was taken out and cooled to room temperature, weighed, and recorded as dry weight W0. Then the water absorption of the membrane can be calculated as: It can be seen from Table 2 that the water absorption rate of the PVDF membrane was 12%, while W W0 that of the modified PVDF membrane was∆ 11.8%,W(%) which = 1 wa− s 87.2%100% and 84.1% higher than that of the (8) W0 × conventional Nafion membrane, respectively. The water absorption rate of the modified PVDF composite Themembrane water absorption was basically rates the of thesame three as that diﬀerent of the membranes PVDF membrane. are shown This in Tableshows2. that the conductivity of the two kinds of PVDF-modified membranes was approximately the same, which further proves the above experimentalTable 2. results.Water absorption rate of diﬀerent membranes.

PVDF–Nafion/ Modified PVDF– Nafion/Carbon Felt 3.4. Analysis of EIS Carbon Felt Nafion/Carbon Felt

EIS canWet reflect weight theW 1dynamic(g) process, mechanism 3.65, and impedance 3.92 information of the whole 3.87 MFC system. TheDry internal weight W resistance0 (g) and its own3.43 reaction kinetics process 3.50 are also important 3.46 factors Water absorption rate affecting the operation effect of the MFC.6.41 The Nyquist plots of 12.00 the four MFCs under 11.80 different ∆W (%) operating conditions are shown in Figure 6. Each curve is composed of two parts, namely, the semicircle in the high frequency region and the straight line in the low frequency region. The value of the left intersectionIt can be seen of from the semicircle Table2 that and the the water horizontal absorption axis rate is the of thesolution PVDF internal membrane resistance was 12%, RS; while the diameterthat of theof the modified semicircle PVDF is the membrane charge transfer was 11.8%, impedance which R wasCT caused 87.2% andby redox 84.1% reaction higher on than the that of electrodethe conventionalsurface. The straight Nafion membrane,line in the low respectively. frequency Theregion water represents absorption the Warburg rate of the impedance, modified PVDF whichcomposite reflects the membrane diffusion degree was basically of ions in the the same electrode as that [3 of7– the39]. PVDF membrane. This shows that the Itconductivity can be seen from of the Figure two kinds6 that the ofPVDF-modified charge transfer impedance membranes RCT was of the approximately MFC with conventional the same, which Nafionfurther membrane proves and the carbon above experimentalfelt anode is results.about 10 Ω, which is significantly greater than that of PVDF–Nafion membrane and modified PVDF–Nafion membrane. The test results show that the MFC 3.4. Analysis of EIS with the conventional Nafion membrane has a large polarization internal resistance, while the MFC constructedEIS by can the reflect PVDF the-modified dynamic proton process, exchange mechanism, membrane and impedance reduces theinformation polarization of the internal whole MFC resistance.system. This The may internal be due resistanceto the improvement and its own of reactionproton transfer kinetics efficiency process areand also electrochemical important factors reactionaff ecting efficiency the operation of the whole effect of system the MFC. by The PVDF Nyquist-modified plots of proton the four exchange MFCs under membrane, different thus operating reducingconditions the polarization are shown internal in Figure resistance.6. Each At curve the issame composed time, it ofalso two can parts, be seen namely, that the the solution semicircle in internalthe resistance high frequency RS of MFC region with and the the ordinary straight linePVDF in– theNafion low membrane frequency region. and the TheMFC value with ofthe the left modifiedintersection PVDF–Nafion of the semicircle membrane and were the horizontalabout 6.5 Ω axis and is the2.6 solutionΩ, respectively. internal resistanceComparedR swith; the the diameter generalof PVDF the semicircle-modified is themembrane, charge transfer the modified impedance PVDFR ctmembranecaused by can redox make reaction the MFC on thes have electrode a lower surface. solutionThe resistance. straight line Therefore, in the low th frequencye modified region PVDF represents is a more the suitable Warburg modifier impedance, for improving which reflects the the performancediffusion of degreeproton ofexchange ions in the membranes. electrode [37–39].

16 Nafion/carbon felt PVDF Nafion/carbon felt 14 modified PVDF Nafion/carbon felt Nafion/carbon cloth 12

)

Ω ( 8

-Z'' 6

0 0 5 10 15 20 25 30 35 40 Z' (Ω)

FigureFigure 6. Electrochemical 6. Electrochemical impedance impedance spectra spectra of MFCs of MFCsunder underdifferent diﬀ conditions.erent conditions.

It can be seen from Figure6 that the charge transfer impedance Rct of the MFC with conventional Nafion membrane and carbon felt anode is about 10 Ω, which is significantly greater than that of PVDF–Nafion membrane and modified PVDF–Nafion membrane. The test results show that the MFC Membranes 2020, 10, 185 10 of 14 with the conventional Nafion membrane has a large polarization internal resistance, while the MFC constructed by the PVDF-modified proton exchange membrane reduces the polarization internal resistance. This may be due to the improvement of proton transfer efficiency and electrochemical reaction efficiency of the whole system by PVDF-modified proton exchange membrane, thus reducing the polarization internal resistance. At the same time, it also can be seen that the solution internal resistance Rs of MFC with the ordinary PVDF–Nafion membrane and the MFC with the modified PVDF–Nafion membrane were about 6.5 Ω and 2.6 Ω, respectively. Compared with the general PVDF-modifiedMembranes 2020, 10,membrane, x FOR PEER REVIEW the modified PVDF membrane can make the MFCs have a lower solution10 of 13 resistance. Therefore, the modified PVDF is a more suitable modifier for improving the performance of protonWhat exchange is more, membranes. it can also be seen from the comparative experimental results that the solution internalWhat impedance is more,it of can the also MFC be with seen the from carbon the comparative cloth electrode experimental was almost results the same that as the that solution of the internalMFC with impedance the carbon of felt the electrode, MFC with but the the carbon charge cloth transfer electrode impedance was almostwas quite the different. same as thatThe charge of the MFCtransfer with impedance the carbon of felt the electrode, MFC with but th thee carbon charge transfercloth electrode impedance was was about quite 30 diΩff, erent.whichThe was charge about transferthree times impedance that of ofthe the MFC MFC with with the the carbon carbon felt cloth elec electrodetrode. This was further about 30 indicatesΩ, which that was carbon about felt three is timesmore thatsuitable of the for MFC use as with an electrode the carbon of feltMFCs electrode. than carbon This cloth. further indicates that carbon felt is more suitableTo forobtain use more as an detailed electrode impedance of MFCs information, than carbon the cloth. impedance data were fitted in Zview (Scribner AssociatesTo obtain Inc., more Southern detailed Pines, impedance NC, USA). information, The equivalent the impedance circuit is shown data were in Figure fitted in 7, Zview where (Scribner Zw is the Warburg impedance reflecting the resistance encountered by the reactants in the process of diffusion in Associates Inc., Southern Pines, NC, USA). The equivalent circuit is shown in Figure7, where Zw is the Warburgsolution; impedanceCPE is a constant reflecting phase the element resistance in wh encounteredich the double by the layer reactants impedance in the is processrepresented. of di ffusion in solution; CPE is a constant phase element in which the double layer impedance is represented.

CPE

Rct Zw

Figure 7. The equivalent circuit of MFCs under different conditions. Figure 7. The equivalent circuit of MFCs under different conditions. The impedance data obtained by equivalent circuit fitting are listed in Table3. It can be seen that the Warburg impedance changed little in several different cases, but the solution impedance of The impedance data obtained by equivalent circuit fitting are listed in Table 3. It can be seen that acetone-modified PVDF composite membrane was the smallest. the Warburg impedance changed little in several different cases, but the solution impedance of acetone-modified PVDF composite membrane was the smallest. Table 3. Impedance data obtained by equivalent circuit fitting.

NafionTable/Carbon 3. ImpedancePVDF–Nafion data obtained/ byModified equivalent PVDF–Nafion circuit fitting./ Nafion/Carbon Felt Carbon Felt Carbon Felt Cloth Nafion/ Nafion/ Rs/Ω 6.191PVDF–Nafion/Carbon 6.518 Modified PVDF–Nafion/Carbon 3.162 4.977 Rct/Ω Carbon 11.36 2.897 4.138 31.38Carbon Felt Felt Zw/(Ω)Felt 0.7962 0.6684 0.8775 0.7758Cloth

3.5.Rs Water/Ω Quality6.191 Analysis 6.518 3.162 4.977

Rct/WhenΩ molasses11.36 wastewater was2.897 used as an anolyte of the MFC,4.138 the COD values of influent31.38 and effluent of MFCs with different proton exchange membranes are shown in Table4. It can be seen thatZw/( theΩ) COD0.7962 removal rate of the0.6684 MFC with conventional Nafion0.8775 membrane was 38.1%,0.7758 while the COD removal rates of the MFC with the PVDF–Nafion composite membrane and the MFC with the modified PVDF–Nafion composite membrane were 63% and 66.7%, respectively, which were 65.4% 3.5. Water Quality Analysis and 75.1% higher than that of MFC with ordinary Nafion membrane. Therefore, when the molasses wastewaterWhen molasses was used wastewater as the fuel ofwas MFC, used the as molassesan anolyte wastewater of the MFC, had the alsoCOD been values well of purified influent andand treatedeffluent while of MFCs the organicwith different matter proton was degraded exchange by me thembranes microorganisms, are shown in and Table the use4. It ofcan the be modified seen that the COD removal rate of the MFC with conventional Nafion membrane was 38.1%, while the COD removal rates of the MFC with the PVDF–Nafion composite membrane and the MFC with the modified PVDF–Nafion composite membrane were 63% and 66.7%, respectively, which were 65.4% and 75.1% higher than that of MFC with ordinary Nafion membrane. Therefore, when the molasses wastewater was used as the fuel of MFC, the molasses wastewater had also been well purified and treated while the organic matter was degraded by the microorganisms, and the use of the modified PVDF membrane further improved the water purification effect of the MFC. This indicates that MFCs can not only degrade organic matter to produce electric energy but also purify the wastewater as feed. In particular, when the proton exchange membrane was loaded by the modified PVDF, the purification effect of the MFC was significantly improved.

Membranes 2020, 10, 185 11 of 14

PVDF membrane further improved the water purification effect of the MFC. This indicates that MFCs can not only degrade organic matter to produce electric energy but also purify the wastewater as feed. In particular, when the proton exchange membrane was loaded by the modified PVDF, the purification effect of the MFC was significantly improved. Molasses wastewater is a kind of wastewater with high pollutant concentration, which is rich in sugars, proteins, amino acids, vitamins, and other organic matter. Molasses wastewater, which has high concentrations of COD and complex components, could cause serious environmental pollution in the ecosystem [40]. At present, the COD removal rate of the existing molasses wastewater treatment methods is generally not very high. For example, the COD removal rate of molasses wastewater by UV photolysis method was only 2%; the COD removal rate of a complex hybrid system was only about 15% [41,42], and the COD removal rate of molasses wastewater by anaerobic tank with better treatment effect was about 46.0% [43]. Therefore, it is an alternative method to use MFCs to treat molasses wastewater. Most importantly, MFCs can generate electric energy while treating wastewater, which provides an effective way to solve the dual pressure of environmental pollution and energy crisis at the same time. So, MFCs are a promising technology for power generation and wastewater treatment.

Table 4. Influent and effluent COD of MFCs with different membranes (with carbon felt electrode).

Nafion PVDF–Nafion Modified PVDF–Nafion Influent (mg/L) 3663 3663 3663 Effluent (mg/L) 2266 1354 1220 COD removal rate (%) 38.1 63.0 66.7

4. Conclusions Using PVDF particles to modify the Nafion membrane can improve the power generation performance and water purification effect of the MFC effectively. The membrane modified by the modified PVDF solution can further improve the adhesion of PVDF on the membrane and, thus, improve the power generation stability of the MFC. When the modified PVDF–Nafion composite membrane was used, the performance of MFC in power generation capacity, power supply stability, sewage treatment effect, and other aspects were significantly improved. In this case, the output voltage of MFC can be basically stable at 0.21 V in most of the running time, which was 96.3% higher than that of the MFC with the conventional Nafion membrane; the COD removal rate of molasses wastewater can reach 66.7%, which is 75.1% higher than that of the MFC with the ordinary Nafion membrane. Therefore, PVDF is a valuable membrane modification material, which can improve the performance of MFCs. In addition, although the water purification effect of the MFC with the carbon felt electrode is almost the same as the MFC with the carbon cloth electrode, the electricity production capacity of the MFC with the carbon felt electrode is much higher than that of the MFC with the carbon cloth electrode. Therefore, carbon felt is a better electrode for the MFC than the carbon cloth.

Author Contributions: Conceptualization, L.F.; data curation, J.S. and Y.X.; formal analysis, J.S. and Y.X.; investigation, L.F. and J.S.; methodology, L.F. and J.S.; project administration, L.F.; resources, L.F.; validation, J.S.; visualization, L.F.; writing—original draft, J.S.; writing—review & editing, L.F. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National Natural Science Foundation of China, grant 61143007; the Chinese-Macedonian Scientific and Technological Cooperation Project of Ministry of Science and Technology of the People’s Republic of China, grant [2017] 25: 5-5; and the Plan for Distinguished Professors in Liaoning province, China, grant [2014] 187. Conflicts of Interest: The authors declare no conflict of interest. Membranes 2020, 10, 185 12 of 14

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