processes

Article Enhanced Performance of Microbial Fuel Cells with from Ethylenediamine and Phenylenediamine Modified Graphite Felt

Egidijus Griškonis 1 , Arminas Ilginis 1, Ilona Jonuškiene˙ 2, Laurencas Raslaviˇcius 3 , Rolandas Jonynas 4 and Kristina Kantminiene˙ 1,*

1 Department of Physical and Inorganic Chemistry, Faculty of Chemical Technology, Kaunas University of Technology, Radvilenu˛pl.˙ 19, 50254 Kaunas, Lithuania; [email protected] (E.G.); [email protected] (A.I.) 2 Department of Organic Chemistry, Faculty of Chemical Technology, Kaunas University of Technology, Radvilenu˛pl.˙ 19, 50254 Kaunas, Lithuania; [email protected] 3 Department of Transport , Faculty of Mechanical Engineering and Design, Kaunas University of Technology, Studentu˛g. 56, 51424 Kaunas, Lithuania; [email protected] 4 Department of Energy, Faculty of Mechanical Engineering and Design, Kaunas University of Technology, Studentu˛g. 56, 51424 Kaunas, Lithuania; [email protected] * Correspondence: [email protected]

 Received: 15 July 2020; Accepted: 2 August 2020; Published: 5 August 2020 

Abstract: A microbial (MFC) is a promising option, which enables the effective and sustainable harvesting of electrical power due to bacterial activity and, at the same time, can also treat wastewater and utilise organic wastes or renewable . However, the practical implementation of MFCs is limited and, therefore, it is important to improve their performance before they can be scaled up. The surface modification of material is one way to improve MFC performance by enhancing bacterial cell adhesion, cell viability and extracellular transfer. The modification of graphite felt (GF), used as an anode in MFCs, by electrochemical oxidation followed by the treatment with ethylenediamine or p-phenylenediamine in one-step short duration reactions with the aim of introducing amino groups on the surface of GF led to the enhancement of the overall performance characteristics of MFCs. The MFC with the anode from GF modified with p-phenylenediamine provided approx. 32% higher voltage than the control MFC with a bare GF anode, when electric circuits of the investigated MFCs were loaded with resistors of 659 Ω. Its surface power density was higher by approx. 1.75 times than that of the control. Decreasing temperature down to 0 ◦C resulted in just an approx. 30% reduction in voltage generated by the MFC with the anode from GF modified with p-phenylenediamine.

Keywords: ; anode modification; putrefaciens; graphite felt; surface power density; diamines

1. Introduction Expectations of contemporary society for production and wastewater reclamation or effluent‘s return to the cycle with minimal environmental issues have evolved over time [1]. Thus, is seen as a sustainable process if sustainable innovations and technologies can be adopted. Among these technical innovations and progressive developments, microbial fuel cells (MFCs) as an emerging technology that may serve customers in a variety of industry sectors are bringing new opportunities. MFCs provide a new technology that can act as pollutant removal devices by using available in wastewater as catalysts to oxidize

Processes 2020, 8, 939; doi:10.3390/pr8080939 www.mdpi.com/journal/processes Processes 2020, 8, 939 2 of 15 substrates and produce much needed . MFC technologies have unique features and abilities to solve energy and environmental issues [2]. Electricity generation, effective and efficient harvesting of practically usable power for distributed power systems, still remains a critical milestone and a key challenge for further MFCs development and their successful deployment [3]. The MFCs have had—and still have—poor stability, high costs, and a low output of generated power for practical application. Nevertheless, during the most recent couple of decades, they have been a target of global-scale research. In general, the microbial fuel cell research is a highly new cross-discipline involving [4]: power electronics (application of the charge pumps, off-the-shelf capacitors, rechargeable batteries, etc.) [3], environmental engineering [3–5], biochemistry [4], circuitry [3], programing [3], bioelectrochemistry [4,6], microbiology [4,7–9], robotics [3,10], molecular biology [4], and materials science [4,11–15]. Despite the inherent simplicity of MFCs and their environmentally friendly , the anode is a crucial component of the whole system, both functionally and structurally. An ideal anodic material usually encompasses four major properties, the compatibility of which is affected by a combination of several reasons: low price and high biocompatibility, conductivity, and chemical stability. However, compatibility problems arise even when the anodic material selection and design is supposedly proper. It is a major reason underlying the low efficiency in different microbial fuel cell prototypes and is still a primary setback for its practical applications. Recently, some active measures among the researchers worldwide have been taken to improve the performance of , including their surface modifications and architecture design. Various macroporous carbons of different pore sizes, containing one-dimensionally (contain channels that do not interconnect with each other), two-dimensionally (interconnection of pores in only one direction) and three-dimensionally (interconnection of pores in two crystallographic directions) connected voids were developed for anodes in MFCs, such as reticulated vitreous carbon, graphite felt, electrospun carbon fiber materials, carbon paper, carbon fiber cloth fabrics, carbon rods, carbon nanotubes, activated carbon, carbon materials derived from natural , activated carbon fibers, carbon fiber mesh, graphite fiber brush, and layered corrugated carbon [16]. Graphite felt (GF) or carbon felt is a fiber fabric that is much thicker than materials such as carbon paper, graphite plates or sheets, and carbon cloth [17]. Most MFC studies involving the materials like bare GF only focus on maximizing power densities on a volume basis, resulting in difficulties in quantitative comparisons among different materials [17]. The loose texture of bare GF confers more space for bacterial growth than other carbon materials having plane structure, but the bacterial growth is restricted by the mass transfer of substrate and products on its surface [17]. Surface modification of graphite-based anode materials could enhance bacterial cell adhesion, cell viability and facilitate extracellular electron transfer [14]. C-, N-, O-, and S-containing functional groups have been frequently investigated in anode surface modification with the purpose to enhance bacterial attachment [18]. The carbon-based material surface containing nitrogen-functional groups is more prone to absorb negatively charged , such as Gram-negative , due to its increasing N/C ratio and positive charge on the surface leading to more favourable electron transfer and improved anode biocompatibility [19]. Anode surface modification with hydrophilic substances has been demonstrated to enhance bacterial affinity and thus promote power output [12]. It has been shown that the combined effect of the usage of phosphate buffer and the introduction of hydrophilic amino groups via the treatment of carbon cloth with ammonia increased MFC power generation by 48% [20]. Therefore, the reported research involves modification of bare GF with ethylenediamine (EDA) and p-phenylenediamine (pPDA) in order to introduce N-containing amino groups onto the surface of GF and thus improve overall performance characteristics of the microbial fuel cell. The obtained results could provide theoretical and practical support for improving the production capacity of bio-electrochemical systems. Processes 2020, 8, 939 3 of 15

2. Materials and Methods

2.1. General Conditions All chemicals were of analytical or biochemical grade and were purchased from Sigma-Aldrich, St. Louis, MO, USA; Eurochemicals, Vilnius, Lithuania; Carl Roth GmbH + Co. KG, Karlsruhe, Germany; and TCI Europe N.V., Zwijndrecht, Belgium. All microbial experiments were performed under strictly sterile conditions.

2.2. Electrodes Investigation of MFC was carried out with the three types of anodes:

1. Bare graphite felt (GF) as a control; 2. GF modified with ethylenediamine (EDA); 3. GF modified with p-phenylenediamine (pPDA).

Polyacrylonitrile-based graphite felt AvCarb G200 (Fuel Cell Store, College Station, TX, USA) with the dimensions of 7 7 0.65 cm (length width thickness) was used for all MFC electrodes × × × × ( and anodes). Before any wet processing, GF samples were wetted with 10% (v/v) aqueous propan-2-ol solution and washed well with distilled water. Thus, treated GF samples, while still wet, were placed into MFCs as cathodes and a control anode (bare GF).

2.3. Anode Material Treatment

2.3.1. Electrochemical Oxidation of GF First (before modification with diamines), GF was electrochemically oxidized for 2 h in a flow-through reactor with the network of interdigitate channels, through which 10% aqueous H2SO4 solution was circulated continuously at a 50 mL/min rate. The flow direction of the solution was reversed every 10 min. The GF electrode being electrochemically oxidized was polarized positively (as an anode), whereas another GF electrode was polarized negatively (as an auxiliary ). Electrodes were separated by a proton-exchange membrane (PEM) Nafion®NRE-212 (Fuel Cell Store, College Station, TX, USA). Potential difference between GF anode, which was being electrochemically oxidized, and the auxiliary cathode was changed in square waves from 0 to 3.5 V. The impulse period was 2 s with a duty cycle of 50%, i.e., t(on) = t(off) = 1 s. Electrochemically oxidized GF (GF-OX) was thoroughly washed with distilled water until a neutral medium of rinsing water was obtained. The GF-OX was then dried at 80 ◦C for 6 h. The hydrophilicity of dried GF-OX was tested by placing a drop of distilled water on a surface of the sample.

2.3.2. Treatment with Ethylenediamine (Ethane-1,2-Diamine or EDA) Method A. For direct treatment with EDA, a GF-OX sample was put into a mixture solution containing 70 mL of EDA and 35 mL of N,N-dimethylformamide (DMF) and kept at 50 ◦C for 3 h. The sample was washed with ethanol and water, and dried at 80 ◦C overnight. Method B. A GF-OX sample was put into a mixture solution containing 100 mL of SOCl2 and 5 mL of N,N-dimethylformamide (DMF) and was heated at 70 ◦C for 48 h. The sample was washed with tetrahydrofuran (THF) and dried at 80 ◦C overnight. Afterwards, it was put into a mixture solution containing 100 mL of EDA and 25 mL of DMF and heated at a reflux temperature of the mixture (130 ◦C) for 48 h. The sample was washed with ethanol and water, and dried at 80 ◦C overnight [21]. Electrochemically oxidized graphite felt, treated with EDA, is hereinafter referred to as GF-OX-EDA. Processes 2020, 8, 939 4 of 15

2.3.3. Treatment with p-Phenylenediamine (Benzene-1,4-Diamine or pPDA) Method A. For direct treatment with pPDA, a GF-OX sample was put into a solution of 100 mL of DMF and 20 g of pPDA and was kept at 50 ◦C for 3 h. The sample was washed with ethanol and water, and dried at 80 ◦C overnight. Method B. A GF-OX sample was put into a mixture solution containing 100 mL of SOCl2 and 5 mL of DMF and was heated at 70 ◦C for 48 h. The sample was washed with THF and dried at 80 ◦C overnight. Afterwards, it was put into a solution containing 100 mL of DMF and 20 g of pPDA and heated at reflux temperature of the mixture (130 ◦C) for 48 h. The sample was washed with ethanol and water, and dried at 80 ◦C overnight. Electrochemically oxidized graphite felt, treated with pPDA, is hereinafter referred to as GF-OX-pPDA.

2.4. Characterization of Electrode Materials Qualitative and quantitative chemical composition of the surface of bare and modified GF electrode filaments were investigated by Fourier-Transform Infrared (FT-IR) spectroscopy, Scanning Electron 1 Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis. The FT-IR spectra (ν, cm− ) of the samples were recorded on a Spectrum GX FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA) with additional equipment for the measurement in horizontal attenuated total reflection (HATR) mode. 1 The FT-IR spectra were recorded at room temperature in the wavenumber range of 4000–800 cm− with a resolution of 1 cm 1. Each spectrum was averaged from 16 scans at a scan rate of 0.2 cm s 1. Surface − · − morphology and elemental composition of the samples were investigated with a high-resolution scanning electron microscope Hitachi S-3400N (Hitachi, Ltd., Tokyo, Japan) equipped with EDX detector Bruker X Flash Quad 5040 (Bruker AXS GmbH, Karlsruhe, Germany).

2.5. Cell Cultures and Media Shewanella putrefaciens wild-type NCTC (The National Collection of Type Cultures) 10695 [22] (DSM No.: 1818, DSMZ—German Collection of Microorganisms and Cell Cultures GmbH, Braunschweig, Germany) were used as in the MFCs. Sterilized LB broth [23] and minimal medium [24] (18 mmol/L sodium DL-lactate, PIPES buffer 15.1 g L 1, NaOH 3.0 g L 1, NH Cl 1.5 g L 1, KCl 0.1 · − · − 4 · − g L 1, NaCl 5.8 g L 1, NaH PO H O 0.6 g L 1, Wolfe’s mineral solution 10 mL L 1 (ATTC, Manassas, · − · − 2 4· 2 · − · − VA, USA), Wolfe’s vitamin solution 10 mL L 1 (ATTC, Manassas, VA, USA), L-glutamic acid 2 g L 1, × − · − L-arginine g L 1, serine g L 1) were used for liquid culture preparation and LB Agar (Sigma-Aldrich, · − · − St. Louis, MO, USA) plates were used for culture maintenance. Single colonies on LB Agar plates freshly streaked from a frozen glycerol stock culture of S. putrefaciens were transferred to 15 mL of LB broth and incubated aerobically at room temperature while shaking at 100 rpm (ES-20 Compact Shaker-Incubator, Grant Instruments, Cambridge, UK) for 48 h. Afterwards, 15 mL of culture were spun down at 3000 rpm during 10 min (Hettich Universal 320R, Andreas Hettich GmbH & Co, Tuttlingen, Germany). The pellet was resuspended in 15 mL of minimal media and transferred to a sterile Erlenmeyer flask with 185 mL of minimal media for 72 h of cultivation using the same conditions. Finally, 200 mL of media were centrifuged at 3000 rpm during 10 min; the pellet was resuspended in 15 mL of minimal media and injected in the microbial fuel cell.

2.6. MFC Design and Operation Three two-chamber MFCs (approx. 32 mL volume of anode and cathode chambers with the network of interdigitate channels, separated by a 10 10 cm size PEM Nafion®NRE-212 (Fuel Cell Store, × College Station, TX, USA) were constructed and used to investigate operation performance of the MFCs by using bare GF and surface modified anodes. All MFCs were operated in batch mode using minimal medium as an anolyte with aqueous 60% (v/v) sodium DL-lactate added to maintain its concentration in the medium at 18 mmol/L. Anolyte, supplied from the anolyte reservoir, was circulated by means of Processes 2020, 8, 939 5 of 15 a peristaltic pump (BS100-1AQ, Fluid Technology Co., Ltd., Baoding, China) at 6.25 mL min 1 rate · − under anaerobic conditions provided by constant flow of nitrogen in the anode part of the system. In all MFCs, cathodes were made of bare GF and phosphate-buffered saline (PBS, pH 7.2), supplied from catholyte reservoir and aerated with air, was used as a catholyte, which was circulated by means of a peristaltic pump at 18.18 mL min 1 rate. The short-term operation of MFCs under different loads · − of the electric circuit was investigated by using the potentiostat-galvanostat SP-150 (BioLogic Sciences Instruments, Seyssinet-Pariset, France) connected directly to each MFC. The resistance of electric circuit was changed from 15 Ω to 2000 Ω programmatically by controlling the current such that the ratio voltage/current kept constant. Open circuit potential (OCP) and chronoamperometric (CA) measurements of bare GF cathode in aerated and deaerated PBS (aeration with air or passing of N2 gas, respectively, at the rate of ~0.5 dm3/min for 2 h before and throughout the experiment) were performed in a standard three-electrode electrochemical cell using potentiostat/galvanostat SP-150 (BioLogic Sciences Instruments, Seyssinet-Pariset, France) interfaced with the EC-Lab v10.39 software. In an electrochemical cell, a bare GF cylinder with the dimensions of 6 6.5 mm (diameter thickness) × × mounted in a special Plexiglas holder was used as a working electrode, which investigated surface area of 12.5 mm2 was obtained by the 4 mm diameter hole in the clamping plate of the holder. A rectangular Pt plate with a surface area of approx. 10 cm2 and an Ag/AgCl electrode filled with saturated aqueous KCl solution (E vs. SHE = 0.197 V) were used as counter and reference electrodes, respectively. The long-term performance of MFCs was investigated by connecting a passive electric load to each of them, i.e., a voltage divider consisting of 620 and 39 Ω resistors connected in series. The total electric load was 659 Ω. The voltage variation over time was measured across the 39 Ω resistor by using data logger USB TC-08 (Pico Technology Ltd., St Neots, UK) connected to the personal computer for data collection. The actual values of voltage and current generated by each MFC were calculated by applying Ohm’s law for a part of the circuit. In order to study the operation of MFCs at lower than room temperatures, all MFCs were placed in a refrigerator, in which temperature was controlled by the electronic thermostat PCR-110 (Honeywell, Charlotte, NC, USA). In the refrigerator, the temperature of all MFCs was lowered from room temperature (20 1 C) down to 0 C in approx. 5 C increments. ± ◦ ◦ ◦ 3. Results and Discussion

3.1. Anode Material Treatment Initially, as a preparation for electrochemical oxidation, bare GF samples were wetted with 10% (v/v) aqueous propan-2-ol solution, which has a lower surface tension than pure water to wet the surface of GF filaments. Propan-2-ol displaced absorbed air gas from the surface of GF filaments and interfilamental cavities, molecules of propan-2-ol and water absorbed instead and thus made GF wettable, i.e., temporarily hydrophilic. Washed well with distilled water and still wet bare GF was oxidized electrochemically in H2SO4 solution under flow-through mode with the aim of introducing -containing functional groups (mainly, carboxylic and epoxy groups), which later would participate in the reaction with organic diamines and form amide -CO-NH- and C-NH- bonds on the ≡ surface of GF. Schematic representation of electrochemical oxidation of bare GF is provided in Figure1. Processes 2020, 8, 939 6 of 15 Processes 2020, 8, x FOR PEER REVIEW 6 of 16

Figure 1. Schematic representation of electrochemical oxidation of graphite felt (GF) in 10% Figure 1. Schematic representation of electrochemical oxidation of graphite felt (GF) in 10% H2SO4 H SO solution. 2 4 solution. As seen from the EDX analysis results (Table1), long-term (2 h) electrochemical oxidation resulted, As seen from the EDX analysis results (Table 1), long-term (2 h) electrochemical oxidation as expected, in a significant increase in oxygen atomic concentration (from 1.0% to 18.8%) on the resulted, as expected, in a significant increase in oxygen atomic concentration (from 1.0% to 18.8%) filament surface of GF-OX, whereas the concentration of carbon and nitrogen decreased by 14.9% and on the filament surface of GF-OX, whereas the concentration of carbon and nitrogen decreased by 2.9%, respectively. These changes have proven the formation of the oxygen-containing functional 14.9% and 2.9%, respectively. These changes have proven the formation of the oxygen-containing groups on the GF-OX surface. functional groups on the GF-OX surface. Table 1. Data of Energy Dispersive X-ray (EDX) analysis of bare GF, GF-OX, GF-OX-EDA, Table 1. Data of Energy Dispersive X-ray (EDX) analysis of bare GF, GF-OX, GF-OX-EDA, and GF- and GF-OX-pPDA samples. OX-pPDA samples. Concentration of Element, at. % Sample Concentration of Element, at. % Sample C CONO N Bare GFGF 92.9 92.9 1.01.0 6.1 6.1 GF-OXGF-OX 78.0 78.0 18.8 18.8 3.2 3.2 GF-OX-EDA 77.8 9.0 13.2 GF-OX-pPDAGF-OX-EDA 77.8 77.9 9.09.8 13.2 12.3 GF-OX-pPDA 77.9 9.8 12.3

The formation formation of carboxylic carboxylic and epoxy epoxy groups groups during during electrochemicalelectrochemical oxidation oxidation has been been confirmedconfirmed byby the FT-IRFT-IR analysisanalysis data.data. As the comparison of FT-IRFT-IR spectraspectra of bare GFGF andand GF-OXGF-OX has 1 revealedrevealed (Figure(Figure2 2),), inin the the spectrum spectrum of of the the latter, latter, new new absorption absorption bands bands at at 1742 1742 and and 1262 1262 cm cm−−1 are 1 presentpresent andand thethe increaseincrease inin intensityintensity ofof thethe absorptionabsorption band,band, thethe maximummaximum ofof whichwhich isis atat 10921092 cmcm−−1, 1 isis observed.observed. An absorption bandband atat 17421742 cmcm−−1 has been attributed to stretching frequencies ofof CC=O=O in 1 thethe –COOH–COOH groups, and and the the peaks peaks at at 126 12622 and and 1092 1092 cm cm−1− correspondcorrespond to tothe the stretching stretching of ofphenolic phenolic C- 1 C-OO and and epoxy epoxy C- C-O-CO-C groups, groups, respectively. respectively. A A very very intensive intensive peak peak with the maximum atat 16261626cm cm−−1 isis presentpresent inin thethe FT-IRFT-IR spectraspectra ofof allall samplessamples (both(both unmodifiedunmodified GF and modifiedmodified GF-OX,GF-OX, GF-OX-EDA,GF-OX-EDA, and GF-OX-pPDAGF-OX-pPDA samples). samples) It. is It characteristic is characteristic of the of aromatic the aromatic C=C bond C=C bending bond bending in graphite in structure. graphite 1 Similarly,structure. theSimilarly, peak characteristic the peak characteristic of all sample of all spectra sample at 1384spectra cm −at 138has4 been cm−1 assignedhas been toassigned the C-H to rock the inC- methylH rock in groups methyl at groups the edges at ofthe graphite edges of structure graphite [structure25]. [25].

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FigureFigure 2. 2. FourierFourier-Transform-Transform Infrared Infrared (FT (FT-IR)-IR) spectra spectra of of bare bare GF, GF, electrochemically electrochemically oxidized oxidized (GF (GF-OX)-OX) and modified with ethylenediamine (EDA) (GF-OX-EDA) and pPDA (GF-OX-pPDA). and modified with ethylenediamine (EDA) (GF-OX-EDA) and pPDA (GF-OX-pPDA). The presence of O-containing functional groups on the surface of GF makes it hydrophilic. A drop The presence of O-containing functional groups on the surface of GF makes it hydrophilic. A of distilled water, placed on the surface of dried GF-OX, seeped into GF-OX, confirming that GF-OX drop of distilled water, placed on the surface of dried GF-OX, seeped into GF-OX, confirming that had become permanently hydrophilic and could be used in further experiments without wetting with GF-OX had become permanently hydrophilic and could be used in further experiments without low surface tension aqueous alcohol solutions. wetting with low surface tension aqueous alcohol solutions. The N-(2-aminoethyl)carbamoyl and 2-aminoethylamino or N-(4-aminophenyl)carbamoyl and The N-(2-aminoethyl)carbamoyl and 2-aminoethylamino or N-(4-aminophenyl)carbamoyl and 2-aminophenylamino moieties were introduced onto the GF-OX filament surface via one-step reaction 2-aminophenylamino moieties were introduced onto the GF-OX filament surface via one-step in DMF at 50 C in a relatively short period of time (3 h) (Method A) (Figure3), contrasting to the reaction in DMF◦ at 50 °C in a relatively short period of time (3 h) (Method A) (Figure 3), contrasting two-step procedure reported by Zhu et al. for modification of carbon felt [21]. to the two-step procedure reported by Zhu et al. for modification of carbon felt [21]. For comparison reasons, the N-(2-aminoethyl)carbamoyl and 2-aminoethylamino or N-(4-aminophenyl)carbamoyl and 2-aminophenylamino moieties were introduced onto the GF-OX surface by this two-step procedure as well (Method B). GF-OX was treated with SOCl2 at 70 ◦C for 48 h to form acyl chloride -COCl groups. Subsequently, it was treated with EDA or pPDA at 130 ◦C for another 48 h in order to enable diamine reaction with acyl chloride -COCl groups (Figure4)[ 22]. The results of EDX analysis of thus obtained GF-OX modified with EDA or pPDA were identical to the ones of GF-OX modified with these diamines via one-step reaction. As the results of the EDX analysis have shown (Table1), nitrogen atomic concentration increased up to ~13% (at.) (when treated with EDA) and ~12% (at.) (when treated with pPDA). Oxygen atomic concentration decreased to ~9% (at.) and ~10% (at.), respectively.

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FigureFigure 3. 3.Schematic Schematic view view of of one-step one-step modification modification (Method(Method A)A) of electrochemically oxidized oxidized graphite graphite Processesfeltfelt (GF-OX)20 (20GF, 8-,OX x FOR with) with PEER ethylenediamine ethylenediamine REVIEW and andp -phenylenediamine.p-phenylenediamine. 9 of 16

For comparison reasons, the N-(2-aminoethyl)carbamoyl and 2-aminoethylamino or N-(4- aminophenyl)carbamoyl and 2-aminophenylamino moieties were introduced onto the GF-OX surface by this two-step procedure as well (Method B). GF-OX was treated with SOCl2 at 70 °C for 48 h to form acyl chloride -COCl groups. Subsequently, it was treated with EDA or pPDA at 130 °C for another 48 h in order to enable diamine reaction with acyl chloride -COCl groups (Figure 4) [22]. The results of EDX analysis of thus obtained GF-OX modified with EDA or pPDA were identical to the ones of GF-OX modified with these diamines via one-step reaction. As the results of the EDX analysis have shown (Table 1), nitrogen atomic concentration increased up to ~13% (at.) (when treated with EDA) and ~12% (at.) (when treated with pPDA). Oxygen atomic concentration decreased to ~9% (at.) and ~10% (at.), respectively.

FigureFigure 4. 4.Reaction Reaction schemescheme ofof two step modification modification (Method B) B) of of electrochemically electrochemically oxidized oxidized graphite graphite feltfelt (GF-OX) (GF-OX) with with ethylenediamine ethylenediamine andand pp-phenylenediamine.-phenylenediamine.

TheThe formation formation ofof anan amideamide bondbond duringduring modificationmodification reactions with with EDA EDA and and pPDA pPDA has has been been confirmedconfirmed by by the the FT-IR FT-IR analysis analysis data data (Figure (Figure2). In2). the In FT-IRthe FT spectra-IR spectra of GF-OX-EDA of GF-OX-EDA and GF-OX-pPDA and GF-OX- 1 −1 samples,pPDA samples a new, shoulder a new shoulder of absorption of absorption bands atbands 1576 at cm 1576− is cm present is present and has and been has attributedbeen attributed to the N-Hto the bending N-H bend of theing amide of the group amide [25 group]. Amides [25]. Amides are the most are the stable most carboxylic stable carboxylic acid derivatives; acid derivatives therefore,; ittherefore has been, assumedit has been that assumed they should that notthey hydrolize should not under hydroli mildze conditions under mild of minimalconditions media of minimal used as anmedia anolyte used in as MFCs. an anolyte in MFCs.

3.2.3.2. MFC MFC PerformancePerformance TheThe e efficiencyfficiency ofof allall threethree MFCs,MFCs, thethe onlyonly didifferencefference of which was in in the the anode anode material material used, used, waswas investigatedinvestigated whil whilee maintaining maintaining all all other other parameters parameters and andconditions conditions (temperature, (temperature, electric electric load, load,aeration aeration , intensity, etc.) etc.)identical identical.. The scheme The scheme of operating of operating MFC MFC and andthe measurement the measurement of its of itselectrochemical electrochemical characteristics characteristics are are shown shown in inFigure Figure 5. 5.

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Figure 5. Schematic view of an operating microbial fuel cell (MFC) and measurement of its Figure 5. Schematic view of an operating microbial fuel cell (MFC) and measurement of its electrochemical characteristics. Here: 1—flow-through MFC, 2—PEM, 3—bare or modified GF anode, electrochemical characteristics. Here: 1—flow-through MFC, 2—PEM, 3—bare or modified GF anode, 4—bare GF cathode, 5—anolyte reservoir, 6—catholyte reservoir, 7—peristaltic pump, 8—diaphragm 4—bare GF cathode, 5—anolyte reservoir, 6—catholyte reservoir, 7—peristaltic pump, 8—diaphragm air pump, 9—airlock. air pump, 9—airlock. The overall electrochemical processes, which take place on the surface of the anode and cathode The overall electrochemical processes, which take place on the surface of the anode and cathode during MFC operation, could be described by the following equations: during MFC operation, could be described by the following equations: Anodic process: Anodic process:

− − ++ − H3CCH(OH)COOH3CCH(OH)COO−(aq)(aq)+ 6H + 6H2O(l)2O(l) →3HCO 3HCO3−3(aq)(aq)+ +14H 14H (aq) + 12e12e− → Cathodic process: lactate ion Cathodic process: + O (g) + 4H (aq) + 4e− 2H O(l) 2 → 2 + − The OCP and CA measurementsO2 in(g) aerated + 4H (aq) and + deaerated4e → 2H2O(l) PBS have revealed that PBS aeration significantlyThe OCP influenced and CA the measurements electrochemical in aerated activity and of thedeaerated bare GF PBS cathode have (Figurerevealed6). that The PBS OCP aeration value ofsignificantly the bare GF electrodeinfluenced in the the electrochemical aerated PBS, which activity stabilized of the bare after GF 1 cathode h, was higher(Figure by 6). approx.The OCP 20 value mV thanof the the bar onee GF in deaeratedelectrode in PBS. the Furthermore,aerated PBS, which the cathodic stabilized polarization after 1 h, was of thehigher bare by GF approx. electrode 20 mV in 25than mV stepsthe one from in thedeaerated OCP resulted PBS. Furthermore in an evident, the increase cathodic in cathodicpolarization current of the in aeratedbare GF PBSelectrode in contrast in 25 tomV the deaeratedsteps from one.the OCP Although resulted the in recorded an evident current increase density in cathodic values current were quite in aerated small (inPBS the in ordercontrast of tensto andthe deaerated hundreds one of .µ AlthoughA/cm2), which the recorded is typical current for electrodes density values possessing were quite no catalytic small (in properties the order inof oxygentens and electrochemical hundreds of reduction,A/cm2), which the cathodic is typical current for electrodes density possessing was approx. no 65 catalyticµA/cm 2propertieseven at the in lowestoxygen cathodic electrochemical polarization reduction, of 25 mV. the This cathodic current current density density value waswas more approx. than 65 10 A/cm times2 highereven at than the thelowest current cathodic density polarization values (maximum of 25 mV. values This werecurrent 4–5.5 densityµA/cm value2) generated was more by than the 10 investigated times higher MFCs. than Thethe current current density density generated values (maximum by the MFC values was calculatedwere 4–5.5 according A/cm2) togenerated Ohm’s lawby the by dividing investigated the measuredMFCs. The MFC current voltage density values generated by 659 Ω byelectric the MFC load was and calculated the cathode according and anode to O workinghm’s law surface by dividing area, whichthe measur was equaled MFC to 49 voltage cm2). Therefore,values by 659 it can  electric be stated load that and the the electrochemical cathode and anode oxygen workin reductiong surface on thearea, bare which GF cathode was equal was to not 49 acm limiting2). Therefore, process it can in all be MFC stated electrochemical that the electrochemical performance oxygen experiments. reduction

Processes 2020, 8, x FOR PEER REVIEW 11 of 16 Processes 2020, 8, x FOR PEER REVIEW 11 of 16 Processeson the 2020 bar,e8 , 939GF cathode was not a limiting process in all MFC electrochemical performance10 of 15 on the bare GF cathode was not a limiting process in all MFC electrochemical performance experiments. experiments.

(a) (b) (a) (b) Figure 6. Curves ofof openopen circuitcircuit potential potential change change over over time time (a )(a and) and chronoamperograms chronoamperograms (b) ( ofb) bareof bare GF Figure 6. Curves of open circuit potential change over time (a) and chronoamperograms (b) of bare cathodeGF cathode in aerated in aerated and deaerated and deaerated PBS. Each PBS. step Each in chronoamperograms step in chronoamperograms corresponds corresponds to an increase to an of GF cathode in aerated and deaerated PBS. Each step in chronoamperograms corresponds to an cathodicincrease of polarisation cathodic polarisation by 25 mV starting by 25 mV from starting open circuitfrom open potential circuit (OCP). potential (OCP). increase of cathodic polarisation by 25 mV starting from open circuit potential (OCP). After filling filling all MFCs with minimal medium as an anolyte and PBS as a catholyte, anolytes and After filling all MFCs with minimal medium as an anolyte and PBS as a catholyte, anolytes and catholytes werewere first first circulated circulated at aat constant a constant rate rate for 12 for h in12 order h in toorder allow to full allow absorption full absor of theption solutions of the catholytes were first circulated at a constant rate for 12 h in order to allow full absorption of the bysolutions the porous by the GF porous electrodes, GF hydrationelectrodes, of hydrat PEM,ion and of temperature PEM, and stabilizationtemperature of stabiliz all partsation ensuring of all parts MFC solutions by the porous GF electrodes, hydration of PEM, and temperature stabilization of all parts operation.ensuring MFC All threeoperation. MFCs A generatedll three MFCs 0 V voltage generated prior 0 introductionV voltage prior of S. introduction putrefaciens ofinto S. theputrefaciens systems. ensuring MFC operation. All three MFCs generated 0 V voltage prior introduction of S. putrefaciens Thereafter,into the systems. an equal Thereafter, volume (15 an mL) equal of bacterialvolume (15 culture mL) wasof bacterial added to culture the anolyte was added of each to MFC. the anolyte Voltage into the systems. Thereafter, an equal volume (15 mL) of bacterial culture was added to the anolyte generationof each MFC. and Voltage its steady generation growth and was its already steady observedgrowth was during already the firstobserved hours during (Figure the7). first Over hours the of each MFC. Voltage generation and its steady growth was already observed during the first hours first(Figure 3 h 7 after). Over the the introduction first 3 h after of bacteria the introduction into anolytes, of bacteria the fastest into increaseanolytes, ofthe voltage fastestand increase current of (Figure 7). Over the first 3 h after the introduction of bacteria into anolytes, the fastest increase of wasvoltage observed and current for the MFC was withobserved the anode for the from MFC GF modified with the with anodep-phenylenediamine. from GF modified After with 3p h,- voltage and current was observed for the MFC with the anode from GF modified with p- thephenylenediamine voltage generated. After by this3 h, MFCthe voltage was 76.8 generated mV (approx. by this 7.2 MFC times was higher 76.8 than mV that(approx of the. 7.2 control), times phenylenediamine. After 3 h, the voltage generated by this MFC was 76.8 mV (approx. 7.2 times thehigher MFC than with that the of anode the control) from GF, modifiedthe MFC withwith ethylenediaminethe anode from generatedGF modified 36.1 with mV (approx.ethylenediamine 3.4 times higher than that of the control), the MFC with the anode from GF modified with ethylenediamine highergenerated than 36.1 that mV of the (approx. control), 3.4 while times the higher control than MFC that with of barethe control), GF as an anodewhile generatedthe control just MFC 10.7 with mV. generated 36.1 mV (approx. 3.4 times higher than that of the control), while the control MFC with Itbare can GF be as assumed an anode that generated the inoculation just 10.7 and mV. vital It can activity be assumed of S. putrefaciens that the inoculationbacteria were and more vital favoured activity bare GF as an anode generated just 10.7 mV. It can be assumed that the inoculation and vital activity onof S. the putre surfacefaciens of bacteria modified were GF filaments.more favoured on the surface of modified GF filaments. of S. putrefaciens bacteria were more favoured on the surface of modified GF filaments.

Figure 7. Dependence of voltage generated by MFCs with different anodes on duration and ambient Figure 7. Dependence of voltage generated by MFCs with different different anodes on duration and ambient temperature at an electric load of 659 . Surface power density values for MFCs with different anodes temperature atat an electric load of 659 Ω. Surface Surface power power density density values values for for MFC MFCss with with different different anodes calculated over a 48–72 h period (presented above a corresponding curve). calculated over a 48–7248–72 h period (presented(presented aboveabove aa correspondingcorresponding curve).curve).

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The voltage voltage of of all all MFCs MFCs reached reached maximum maximum values values and stabilized and stabilized after 24 after h. This 24 can h. beThis attributed can be attributedto the complete to the inoculationcomplete inoculation of bacteria of on bacteria the anodes. on the Aanodes. particularly A particularly sudden voltagesudden risevoltage and rise the andmaximum the maximum stable average stable voltageaverage value voltage (approx. value 177(approx. mV) over 177 themV) period over th ofe 24–72 period h wereof 24 observed–72 h were in observedthe case of in the the MFC case withof the the MFC GF-OX-pPDA with the GF anode.-OX-pPDA Meanwhile, anode. MFCsMeanwhile, with bare MFCs GF with and GF-OX-EDAbare GF and GFanodes-OX-EDA generated anodes voltage generated with voltage significantly with significantly lower average lower values average over values the same over period the same of time period but veryof time close, but approx.very close, 134 approx. and 142 13 mV,4 and respectively. 142 mV, respectively. Values of voltage Values generatedof voltage generated by all MFCs by remainedall MFCs remainedstable at the stable same at level the for same 3 and level more for days 3 and (one more month). days The (one average month) voltage. The avalueverage generated voltage by value the generatedMFC with by the the anode MFC from with GF-OX-pPDA the anode from was GF 1.32-OX times-pPDA (32%) was higher1.32 times than (32%) that ofhigher the control than that MFC, of whereasthe control the MFC MFC, whereas with the the anode MFC from with GF-OX-EDA the anode from generated GF-OX an-EDA average generated voltage an 6% average higher thanvoltage the 6%MFC higher with than a bare the GF MFC anode. with a bare GF anode. TThehe values of the surface power density for each MFC (Figure 77)) werewere calculatedcalculated overover thethe periodperiod of 48 48–72–72 h according to the following equation:equation: U2·R SPDU =2 R SPD = · S Here: SPD—surface power density of MFC; U—Svoltage of MFC, V; R—electric load (resistance of electricHere: circuit) SPD—surface equal to power 659  density; S—surface of MFC; area U—voltage of face-to-face of MFC, anode V; and R—electric cathode, load m2. (resistance of electricIt can circuit) be concluded equal to 659 thatΩ introduction; S—surface area of benzene of face-to-face rings along anode with and amino cathode, groups m2. in the form of N-(4-Itaminophenyl)carbamoyl can be concluded that introductionand 2-aminophenylamino of benzene rings moieties along withonto aminothe surface groups of inGF the filaments form of favouredN-(4-aminophenyl)carbamoyl the formation and of S. 2-aminophenylamino putrefaciens bacteria moietiesand the electron onto the transfer surface processes of GF filaments to the anode.favoured the biofilm formation of S. putrefaciens bacteria and the electron transfer processes to the anode. WWhenhen the stable MFC operation operation had been been achieved achieved,, short short-term-term MFC MFC performance performance efficiency efficiency tests were performed by varying the amount of electrical load. It had been observed that the most stable surface power densitydensity waswas providedprovided by by all all MFCs MFCs at at electrical electrical circuit circuit loads loads greater greater than than 500 500Ω (Figure (Figure8). 8).

FigureFigure 8. 8.Dependence Dependence of of MFCs’ MFCs surface’ surface power power density density (over (over a 10 a min10 min periods) period ons) appliedon applied electric electric load. load. Meanwhile, at electrical circuit loads of 500 Ω and lower, a power jump (peak) was observed afterMeanwhile, each 15 min at open electrical circuit circuit period, loads after of which 500  the and power lower of, alla power MFCs jump decreased (peak) steadily, was observed but did afternot reach each stable15 min values open circuit during period, the 10 after min closedwhich the circuit power period. of all It MFCs should decreased be noted steadily, that, already but did at notthis reach stage stable of the values investigation, during the it was 10 min obvious closed that circuit the MFC period with. It should the GF-OX-pPDA be noted that anode, already generated at this stagethe highest of the power.investigation After calculating, it was obvious the surface that the energy MFC densitywith the generated GF-OX-pPDA by all anode MFCs generated over a 10 minthe periodhighest overpower. the After entire calculating range of applied the surface electrical energy loads, density it had generated been proven by all that MFCs the over MFC a with 10 min the periodGF-OX-pPDA over the anode entire generatedrange of applied the most electrical energy, loads, and values it had ofbeen generated proven energythat the by MFC MFCs with with the bareGF- OX-pPDA anode generated the most energy, and values of generated energy by MFCs with bare GF and GF-OX-EDA anodes were lower yet similar to each other (Figure 9). The amount of energy

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GFProcesses and 20 GF-OX-EDA20, 8, x FOR PEER anodes REVIEW were lower yet similar to each other (Figure9). The amount of energy13 of 16 generated by the MFC with the GF-OX-pPDA anode over a 10 min period was significantly higher than thegenerated amount by of the energy MFC generated with the GF by- otherOX-pPDA MFCs anode at lower over (250–15 a 10 minΩ) electricalperiod was circuit significantly loads. A higher sharp dropthan inthe surface amount power of energy density generated occurs when by other the circuit MFCs is at again lower loaded (250– with15  500) electricalΩ or lower circuit electric loads load. A aftersharp the drop open in circuitsurface period power is density supposedly occurs related when to the the circuit polarisation is again of bioanode.loaded with 500 Ω or lower electric load after the open circuit period is supposedly related to the polarisation of bioanode.

FigureFigure 9.9. DependenceDependence of of MFCs’ MFCs surface’ surface energy energy density density (over (over a 10 a min 10 min period) period) on applied on applied electrical electrical load. load. As another stage of the investigation of the performance of three MFCs with different anodes, the influenceAs another of temperature stage of the oninvestigation MFC performance of the performance was evaluated. of three MFCs with different anodes, the influenceAfter placing of temperature all MFCs with on MFC their anolyteperformance and catholyte was evaluated. reservoirs and peristaltic pumps ensuring theirAfter circulation placing in a all refrigerator MFCs with and their reducing anolyte the temperature and catholyte from reservoirs room (average and peristaltic 20.9 ◦C) to pumps 0 ◦C, aensuring synchronous their decreasecirculation in in the a voltagerefrigerator of all and MFCs reducing was observed the temperature (Figure 10 from). S. putrefaciensroom (averageis a 20.9 specific °C) spoilageto 0 °C, a organism synchronous of refrigerated decrease in marinethe voltage fish, of some all MFCs vacuum-packed was observed meats, (Figure and 10 chicken). S. putrefaciens [26]. It isis wella specific known spoilage that lowering organism the of temperature refrigerated lowers marine the fish, metabolic some ratevacuum of prokaryotes-packed meats, [27]. and The chillingchicken process[26]. It is reduces well known the growth that lowering rate of S. the putrefaciens temperatureas well. lowers The the critical metabolic aspect rate of ofS. prokaryotes putrefaciens is[27 its]. abilityThe chilling to grow process at 0 ◦C[ reduces28]. Cold-adapting the growth mechanism rate of S. includes putrefaciens increased as well. fluidity The critical of lipid aspect membranes of S. byputrefaciens the ability is toits finelyabilityadjust to grow lipids at 0 ° compositionC [28]. Cold- [adapting29]. Therefore, mechanismS. putrefaciens includes increasedshould be fluidity active inof anlipid MFC membranes at low temperatures by the ability as well. to finely adjust lipids composition [29]. Therefore, S. putrefaciens shouldThe be most active significant in an MFC drop at inlow voltage temperatures of all MFCs as well. was observed by cooling them from approx. 6.2 to 0.0 ◦C. At 0.0 ◦C, the highest voltage drop was observed in the MFC with the GF-OX-EDA anode (down to ~127 mV) and the lowest one was obtained in the MFC with the GF-OX-pPDA anode (down to ~42 mV). After the cooling was terminated and MFCs were allowed to naturally warm up to room temperature, the voltage values of all of them returned to the same levels that were recorded before the cooling experiment. Therefore, it can be assumed that when temperature decreased down to values close to 0 ◦C, the slowing down of bacteria resulted in a decrease in voltage generation by MFCs. It can be envisaged that more time might be needed for a low-temperature adaptation mechanism of S. putrefaciens at a temperature close to 0 ◦C.

Processes 2020, 8, 939 13 of 15 Processes 2020, 8, x FOR PEER REVIEW 14 of 16

FigureFigure 10. 10. Dependence of voltage generated by MFCs with didifferentfferent anodes anodes on on temperature temperature and and durationduration atat anan electricelectric loadload ofof 659659 Ω..

4. ConclusionsThe most significant drop in voltage of all MFCs was observed by cooling them from approx. 6.2 toIn 0.0 summary, °C. At 0.0 graphite °C, the felt highest was electrochemically voltage drop was oxidized observed and in treatedthe MFC with with ethylenediamine the GF-OX-EDA or panode-phenylenediamine (down to ~127 mV in one-step) and the shortlowest procedure one was obtained resulting in in the introduction MFC with the of aminoGF-OX groups-pPDA anode on its surface.(down to A ~42 microbial mV). After fuel the cell cooling with the was anode terminated treated and with MFCs ethylenediamine were allowed generated to naturally just warm slightly up higherto room voltage temperature, than the the MFC voltage with values the anode of all fromof them bare returned GF, whereas to the same the MFC level withs that the were anode recorded from GFbefore modified the cooling with pexperiment.-phenylenediamine Therefore, provided it can be approx. assumed 32% that higher when voltage temperature than thedecreased control down MFC, whento values electric close circuits to 0 °C, of investigatedthe slowing MFCsdown were of bacteria loaded metabolism with resistors resulted of 659 Ω in. Surfacea decrease power in densityvoltage ofgeneration the latter MFCby MFCs. was approx.It can be 1.75 envisaged times higher that than more that time of themight control. be needed Decreasing for a temperature low-temperatur downe toadaptation 0 ◦C resulted mechanism in just anof S. approx. putrefaciens 30% reductionat a temperature in voltage close generated to 0 °C. by the MFC with the anode from GF modified with p-phenylenediamine. 4. Conclusions Author Contributions: Conceptualization, E.G. and K.K.; methodology, E.G. and K.K.; formal analysis, E.G. and K.K.;In investigation, summary, graphite E.G., A.I., felt I.J. was and electrochemically K.K.; writing—original oxidized draft and preparation, treated with E.G., ethylenediamine L.R., R.J. and K.K.; or writing—reviewp-phenylenediamine and editing, in one E.G.-step and short K.K.; procedure visualization, resulting E.G. and in A.I.; introduction project administration, of amino groups K.K.; funding on its acquisition,surface. A E.G.,microbial L.R., R.J., fuel K.K. cell All with authors the haveanode read treated and agreed with toethylenediamine the published version generated of the manuscript. just slightly Funding:higher voltageThis research than the is anMFC outcome with ofthe the anode Kaunas from University bare GF, of where Technologyas the institutional MFC with multidisciplinarythe anode from projectGF modified “Innovative with microbial p-phenylenediamine fuel cells for sustainable provided production approx. of 32 % higher (MicrobElas)” voltage than that the received control funding MFC, from KTU RDI Fund (grant No. MTEPI-P-17002). when electric circuits of investigated MFCs were loaded with resistors of 659 Ω. Surface power Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study;density in theof collection, the latter analyses, MFC was or interpretation approx. 1.75 of data;times in higher the writing than of the that manuscript, of the control. or in the Decreasing decision to publishtemperature the results. down to 0 °C resulted in just an approx. 30% reduction in voltage generated by the MFC with the anode from GF modified with p-phenylenediamine. References Author Contributions: Conceptualization, E.G. and K.K.; methodology, E.G. and K.K.; formal analysis, E.G. and 1.K.K.; Li, investigation, W.W.; Yu, H.Q.; E.G., He, A.I., Z. Towards I.J. and sustainable K.K.; writing wastewater—original treatment draft preparation, by using microbial E.G., L.R., fuel R.J. cells-centered and K.K.; writingtechnologies.—review andEnergy editing, Environ. E.G. Sci.and 2014K.K.;, 7visualization,, 911–924. [CrossRef E.G. and] A.I.; project administration, K.K.; funding 2.acquisition,Deb, D.; E.G., Patel, L.R., R.; R.J., Balas, K.K. V.E. All A authors Review have of Control-Oriented read and agreed to Bioelectrochemical the published version Mathematical of the manuscript. Models of Microbial Fuel Cells. Processes 2020, 8, 583. [CrossRef] Funding: This research is an outcome of the Kaunas University of Technology institutional multidisciplinary 3. Wang, H.; Park, J.-D.; Ren, Z.J. Practical energy harvesting for microbial fuel cells: A review. project “Innovative microbial fuel cells for sustainable production of bioenergy (MicrobElas)” that received Environ. Sci. Technol. 2015, 49, 3267–3277. [CrossRef][PubMed] funding from KTU RDI Fund (grant No. MTEPI-P-17002).

Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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4. Zhen, G.; Lu, X.; Kumar, G.; Bakonyi, P.; Xu, K.; Zhao, Y. Microbial electrolysis cell platform for simultaneous waste biorefinery and clean electrofuels generation: Current situation, challenges and future perspectives. Prog. Energy Combust. Sci. 2017, 63, 119–145. [CrossRef] 5. Raslaviˇcius,L.; Striugas,¯ N.; Felneris, M. New insights into factories of the future. Renew. Sustain. Energ. Rev. 2018, 81, 643–654. [CrossRef] 6. Bajracharya, S.; Srikanth, S.; Mohanakrishna, G.; Zacharia, R.; Strik, D.; Pant, D. Biotransformation of in bioelectrochemical systems: State of the art and future prospects. J. Power Sources 2017, 356, 256–273. [CrossRef] 7. Greenman, J.; Ieropoulos, I.A. Allometric scaling of microbial fuel cells and stacks: The lifeform case for scale-up. J. Power Sources 2017, 356, 365–370. [CrossRef] 8. Hao, L.; Zhang, B.; Tian, C.; Liu, Y.; Shi, C.; Cheng, M.; Feng, C. Enhanced microbial reduction of vanadium (V) in groundwater with bioelectricity from microbial fuel cells. J. Power Sources 2015, 287, 43–49. [CrossRef] 9. Grattieri, M.; Suvira, M.; Hasan, K.; Minteer, S.D. Halotolerant extremophile bacteria from the Great Salt Lake for recycling pollutants in microbial fuel cells. J. Power Sources 2017, 356, 310–318. [CrossRef] 10. Tommasi, T.; Salvador, G.P.; Quaglio, M. New insights in Microbial Fuel Cells: Novel solid phase anolyte. Sci. Rep. 2016, 6, 29091. [CrossRef] 11. Santoro, C.; Arbizzani, C.; Erable, B.; Ieropoulos, I. Microbial fuel cells: From fundamentals to applications. A review. J. Power Sources 2017, 356, 225–244. [CrossRef][PubMed] 12. Du, Q.; An, J.; Li, J.; Zhou, L.; Li, N.; Wang, X. Polydopamine as a new modification material to accelerate startup and promote anode performance in microbial fuel cells. J. Power Sources 2017, 343, 477–482. [CrossRef] 13. Jiang, H.; Yang, L.; Deng, W.; Tan, Y.; Xie, Q. Macroporous graphitic carbon foam decorated with polydopamine as a high-performance anode for microbial fuel cell. J. Power Sources 2017, 363, 27–33. [CrossRef] 14. Hindatu, Y.; Annuara, M.S.M.; Gumel, A.M. Mini-review: Anode modification for improved performance of microbial fuel cell. Renew. Sust. Energ. Rev. 2017, 73, 236–248. [CrossRef] 15. ElMekawy, A.; Hegab, H.M.; Losic, D.; Saint, C.P.; Pant, D. Applications of graphene in microbial fuel cells: The gap between promise and reality. Renew. Sustain. Energ. Rev. 2017, 72, 1389–1403. [CrossRef] 16. Shen, Y.; Zhou, Y.; Chen, S.; Yang, F.; Zheng, S.; Hou, H. Carbon nanofibers modified graphite felt for high performance anode in high substrate concentration microbial fuel cells. Sci. World J. 2014, 130185. [CrossRef] 17. Wei, J.; Liang, P.; Huang, X. Recent progress in electrodes for microbial fuel cells. Bioresour. Technol. 2011, 102, 9335–9344. [CrossRef] 18. Li, C.; Cheng, S. Functional group surface modifications for enhancing the formation and performance of exoelectrogenic biofilms on the anode of a bioelectrochemical system. Crit. Rev. Biotechnol. 2019, 39, 1015–1030. [CrossRef] 19. Xie, Y.; Ma, Z.; Song, H.; Stoll, Z.A.; Xu, P.Melamine modified carbon felts anode with enhanced electrogenesis capacity toward microbial fuel cells. J. Energy Chem. 2017, 26, 81–86. [CrossRef] 20. Cheng, S.A.; Logan, B.E. Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells. Electrochem. Commun. 2007, 9, 492–4496. [CrossRef] 21. Zhu, N.; Chen, X.; Zhang, T.; Wu, P.; Li, P.; Wu, J. Improved performance of membrane free single-chamber air-cathode microbial fuel cells with nitric acid and ethylenediamine surface modified activated carbon fiber felt anodes. Bioresour. Technol. 2011, 102, 422–426. [CrossRef][PubMed] 22. Sharma, M.; Bajracharya, S.; Gildemyn, S.; Patil, S.A.; Alvarez-Gallego, Y.; Pant, D.; Rabaey, K.; Dominguez-Benettona, X. A critical revisit of the key parameters used to describe microbial electrochemical systems. Electrochim. Acta 2014, 140, 191–208. [CrossRef] 23. Wu, X.; Zou, L.; Huang, Y.; Qiao, Y.; Long, Z.; Liu, H.; Li, C.M. Shewanella putrefaciens CN32 outer membrane MtrC and UndA reduce electron shuttles to produce electricity in microbial fuel cells. Enzyme Microb. Technol. 2018, 115, 23–28. [CrossRef] 24. Carmona-Martínez, A.A.; Harnisch, F.; Kuhlicke, U.; Neu, T.R.; Schröder, U. Electron transfer and biofilm formation of Shewanella putrefaciens as function of anode potential. Bioelectrochemistry 2013, 93, 23–29. [CrossRef][PubMed] 25. Verma, S.; Dutta, R.K. A facile method of synthesizing ammonia modified graphene oxide for efficient removal of uranyl ions from aqueous medium. RSC Adv. 2015, 5, 77192. [CrossRef] 26. Bagge, D.; Hjelm, M.; Johansen, C.; Huber, I.; Gram, L. Shewanella putrefaciens adhesion and biofilm formation on food processing surfaces. Appl. Environ. Microbiol. 2001, 67, 2319–2325. [CrossRef] Processes 2020, 8, 939 15 of 15

27. Price, P.B.; Sowers, T. Temperature dependence of metabolic rates for microbial growth, maintenance, and survival. Proc. Natl. Acad. Sci. USA 2004, 101, 4631–4636. [CrossRef] 28. Yang, S.P.; Xie, J.; Cheng, Y.; Zhang, Z.; Zhao, Y.; Qian, Y.F. Response of Shewanella putrefaciens to low temperature regulated by membrane fluidity and fatty acid metabolism. LWT Food Sci. Technol. 2020, 117, 108638. [CrossRef] 29. Gao, X.; Liu, W.; Mei, J.; Xie, J. Quantitative Analysis of Cold Stress Inducing Lipidomic Changes in Shewanella putrefaciens Using UHPLC-ESI-MS/MS. Molecules 2019, 24, 4609. [CrossRef]

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