Development of high performance mediator - less Microbial Fuel Cell

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Development of high performance mediator - less Microbial Fuel Cell comprising a catalytic steel anode Preetha Chandraserkharan Meenu, Bhuvanendran Revamma Sreelekshmy, Rubina Basheer, Suma Malini Sadasivan, Rajee Mole Vijayakumari Ramakrishnan, and Sheik Mohammadu Aboobakar Shibli ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00337 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Applied Bio Materials

Development of high performance mediator - less Microbial Fuel Cell comprising a catalytic steel anode Preetha Chandraserkharan Meenu 1, Bhuvanendran Revamma Sreelekshmy2, Rubina Basheer2, Suma Malini Sadasivan.1,Rajee Mole Vijayakumari Ramakrishnan1, Sheik Mohammadu Aboobakar Shibli*1 1 Department of Chemistry, University of Kerala, Thiruvananthapuram, Kerala 695 581, India 2 Department of Biotechnology, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala 695 581, India Abstract The present paper reports for the first time, construction of a sugarcane bagasse mediated double chambered MFC, consisting of a novel bioanode of Iron/Titanium Ni-P composite. This anode could facilitate uninterrupted extracelluar electron transfer (EET) from bacteria (mixed culture). The Ni-P composite anode had significant corrosion resistance and enhanced electrocatalytic activity. The corrosion rate was reduced to 0.187 mmpy which was 3 times lesser when compared with the non composite anode. A steady decrease in internal resistance from 3.84 × 103 ohm to 2.94 × 102 ohm was achieved with the incorporation of the Iron/Titanium based composite on the anode surface. The presence of Fe (III) ion centers in the composite surface favored electro active biofilm formation and enhanced the capacitive nature of the anode, thereby accelerating EET. The constructed MFC showed an internal resistance as low as 1.12 x 10–2 Ω in comparison with control MFC. This lead to achievement of very high power density of ~ 2.1 W/m2, which was 20% higher than that of control MFC. While Stacked MFC obtained an maximum open circuit potential of 3.2 V with power density and current density out put of 6.3 W/m2 and 2.7 mA/m2 respectively. Even though extensive papers are available in this field, this report is first of its kind that deals about such a simple reproducible system which can be extended to other similar systems. Key words: Microbial Fuel Cell; Catalytic anode; Ni–P coating; Biocompatible *S.M.A.Shibli, Department of Chemistry, University of Kerala, Thiruvananthapuram, India Ph:+91 8547067230. Email:[email protected],[email protected] 1

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1. INTRODUCTION Microbial fuel cells (MFCs) are the innovative and challenging technology in which microorganism convert chemical energy to electrical energy by the oxidation of organic/inorganic matter1. MFCs have wide advantage over other conventional energy sources due to their high conversion efficiency, different operating conditions as well as eco–friendly nature. Various types of MFCs have been reported over the past few decades for their applications in power supplies for biosensors2,3, wastewater treatment4 and bioelectricity generation.

5-7

Despite the huge efforts,

most of the works were restricted to lab scale due to their low power output and lack of long term stability. In MFCs, microorganisms can mediate electrons to the exogenous acceptor by three extracellular electron transfer (EET) mechanisms: direct electron transfer; indirect electron transfer via shuttling of excreted mediators; and through electrically conductive pili.

8,9

Electron transfer

from microbial cells to the electrode can also be carried out using external chemical mediators such as thionine, neutral red and so on. Since most of the mediators used are expensive and toxic, MFC’s employing any particular mediator has not been commercialized. The systems without such exogenous mediator are classified as mediator-less.10 Thus improvement of EET is mostly carried out either by isolation and genetic engineering of efficient electrochemically active bacteria (EAB) or by modifying the anode. The second approach is considered to be easier than the first one.11-13 Therefore, development and modification of anode characteristics, which can directly affect the bacterial attachment, electron transfer released by substrate oxidation simultaneously, and lowering the material requirement and operation cost for the enhanced MFC performance are the major challenges.14 Therefore it is of great importance to seek appropriate anode materials to accelerate the extracellular electron transfer (EET) for improved power production in MFCs. Generally different materials such as carbon (paper/mesh)15, graphite (rod/fiber brush/felt) etc. are utilized as anodes in MFC, but these materials lack electrochemical activity and some are expensive too.16-18 Mild steel is a common industrial material has wide applications, due to their high tensile strength and low cost, but limited to non-corrosive environment. Therefore electroless Nickel-Phosphorous (Ni-P) coatings is widely used to protect the substrate because of its high corrosion resistance and wear resistance characteristics. Various literature studies revealed that the incorporation of transition metal oxides such as TiO2, Fe2O3, Al2O3 into the Ni-P matrix could greatly improve its characteristics and coating performance leading to enhancement in 2


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electrocatalytic activity.19-23 Titanium dioxide (TiO2) is of great research interest because of its structural

stability,

low

cost,

good

biocompatibility

and

tailorable

morphology.24-26

Electrocapacitive materials like Fe3O4, Fe2O3 and conducting polymer/RuO2 composite have been developed, which can accumulate the anodic charge like an internal capacitor and then discharge over a short period to power the device. A synergistic catalytic effect of Fe2O3 along with TiO2 could be achieved if both the oxides are produced in a composite form, where Fe2TiO5 composite has the unique charge transport property and high active specific surface area with high catalytic efficiencies.27-29 In this context, the present study highlights the possibility of using highly electroactive iron/titanium composite Ni-P coating on mild steel as a promising anode for enhanced extracellular electron transfer (EET) from bacteria thereby enhancing the power generation in MFC. An optimum iron/titanium based composite – Ni– P coating with high corrosion resistance and enhanced biocompatibility and electrocatalytic properties is critical. The electrode can be used as an anode for operation of sugarcane bagasse based mediator - less MFC. The present study focuses on fabrication of such a MFC with the catalytic anode with low internal resistance and high electrocapacitive Fe (III) centres to achieve very high power density. This is for the first time, such an iron/titanium based composite – Ni– P coated anode is applied for MFCs. This approach is novel and versatile due to its ease of reproducibility, wide availability of rawmaterials, exhibition of high electrocatalytic activity and biocompatibility compared to other available literature in this field. 2. MATERIALS AND METHODS 2.1 Microorganism, substrate and inoculum preparation The sugarcane bagasse effluent collected from a Sugarcane Seed Farm, Panthalam, Kerala, India, was used as the bacterial substrate and for isolating the microorganism capable of degrading cellulose. The bacterial consortium isolated form this effluent was used as inoculum. The isolated microorganisms were characterized from the morphological and biochemical test results (Table S1). All the organisms were maintained on agar slant at 4 °C. The cells from the agar slant were scraped off and resuspended in 10 mL of sterile nutrient broth. This suspension culture was grown at 37 °C in an incubator at 250 rpm until optical density (OD) at λ600 reached 0.7 (5.6 × 108 CFU/mL). This was used as the stock culture for the entire experiments with MFCs. The entire 3



studies were carried out with the milled sugarcane bagasse samples after pretreatment with dilute H2SO4 (1% v/v in water) to remove hemicellulose followed by alkaline treatment with 1% NaOH to remove lignin from the bagasse sample and to reduce its recalcitrance during cellulose degradation by bacteria. The initial pH of the medium containing the pretreated sugarcane bagasse (PSB) was adjusted to 7.0 prior to autoclaving (120 °C for 20 min). The presence of microorganisms after 30 days of the MFC operation was identified from the morphological and biochemical test results. 2.2 Preparation of electrode and its characterization All the chemicals were purchased from sigma Aldrich. Fe2TiO5 composite was prepared by thermal decomposition method by mixing required amount of TiCl4 and anhydrous FeCl3 in isopropanol and which was then evaporated to dryness followed by heating at 120 °C for 1 h. The resulting dry powder was then subjected to annealing at 700 °C for 2 h in a muffle furnace.19 For the preparation of anode, the mild steel substrate (AISI 304 grade) was mechanically polished using emery paper (80 to 1800 grade) followed by treatment with 5% NaOH solution and 3% HCl (ASTM B 656). The cleaned substrate was activated with SnCl2 (10 g/L) and PdCl2 (1 g). Then it was immersed into the Ni-P bath containing nickel sulphate (30 g/L), succinic acid (25 g/L) sodium hypophosphite (25 g/L) at 4.5 pH and 80-85 ˚C with constant stirring for 2 h.19 Different amounts (1 g/L, 2 g/L, 5 g/L, 10 g/L) of Fe2TiO5 composite (FTO) were added into the bath during the coating process. The composite and electrodes were characterized by X-ray diffraction (XRD) recorded on a Philips Xpert MPD X-ray powder diffractometer using Cu-Kα radiation. X-ray photoelectron spectroscopy (AXIS Ultra DLD (XPS), Kratos Analytical Ltd. UK), Field Emission Scanning Electron Microscopy (FESEM, Nova NanoSEM 450), Confocal Laser Scanning Microscopy (CLSM, Leica Germany) and High-resolution Transmission Electron Microscopy (HRTEM, JOEL JEM-2100). All bio-electrochemical measurements were performed in Electrochemical Workstation (BioLogic Science Instruments, SP200 France). The measurements were carried out using 3 electrode system - where composite coated mild steel acts as working electrode, Ag/AgCl/Cl- as reference electrode and platinum mesh of negligible impedance as counter electrode. In every case, the exposed area of the electrode was set at 1 cm2. The analysis of data thus obtained were processed using EC-Lab software, version 10.38. All tests were conducted in 4


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50 mL pretreated sugarcane bagasse (PSB) medium. The pH was adjusted to 7.0 and the medium was sterilized by autoclaving for 20 min at 121 °C and 15 psi. The bacterial consortium as detailed in section 2.1 was used as inoculum. EIS was carried out at 10 mV applied voltage with a frequency range of 100 mHz to 200 kHz. 2.3 MFC configurations and operation A MFC reactor consist of 2 rectangular plexiglass chambers of approximately 50 mL capacity each with dimension of 4.5 cm × 4.5 cm × 1 cm. Both the chambers were separated using proton exchange membrane (PEM, Nafion 117) sealed between 2 mm thick silicon rubber gaskets. Stainless steel of grade SS304 was used as the cathode and mild steel modified with FTO composite and without composite as anodes. The anode compartment (anaerobic) loaded with freshly prepared bacterial suspension in PSB medium (pH 7.0) and the cathode compartment (aerobic) loaded with 100 mM potassium phosphate buffer solution (PBS, pH 7.5) containing 50 mM potassium ferricyanide.30-31 The catholyte was replaced with fresh PBS at regular intervals. The external resistance of 100 kΩ was connected to the reactor and all the tests were conducted in batch mode at room temperature. Later, to enhance the power output of MFC, 4 individual MFCs of same dimensions were stacked and connected in series. The working volume of each electrode chamber was 50 mL amounting to a total volume of 200 mL in an MFC stack. Each MFC reactor in the stack was connected in series mode in which, the cathode of the first MFC was connected to the anode of the second MFC and so on. When the fuel cell was initially set up, the anode and cathode potentials were recorded using a digital multimeter (Mastech, Model M3900, India). The electrochemical characterizations, chronoamperometry of the electrode biofilm were performed at potential of 0.2V vs Ag/AgCl/KCl. The internal resistance of the MFC was calculated using Electrochemical impedance spectroscopy (EIS). EIS was conducted over a frequency range of 10000-0.05 Hz with sinusoidal excitation signal of 10 mV. All the electrochemical characterizations were carried out using three electrode systems in which, the developed electrodes acted as anode (working electrode) along with stainless steel cathodes (counter electrodes) and Ag/AgCl/Cl- reference electrodes. The working electrode and the reference electrode were immersed in the anode chamber and the counter electrode in the cathode chamber. Cyclic voltammetry (CV) was performed for the highly sensitive detection of EET at high precision and also to measure the catalytic activity of EABs. CV was performed at the 5



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scan rate of 10 mV/s. All the Fitting were done using EC-lab V 10.40. The polarization and power density of MFC were drawn out from the sampling of voltage or electrode potential and current (I) across the resistance. The current densities (i) were normalized to projected area A using i = V/A, whereas power density was calculated as, P = VI/A, where V is the voltage drop across the resistance , I is the current, A is the exposed surface area of the anode and normalized by liquid volume of anodic chamber (vol) using P =VI/ vol. In series circuit connection, the current flowed through 4 MFCs and the total current generation were measured using an ammeter ( Ipgi 100). The corresponding voltage across the resistor terminals was recorded with time and the electrical outputs were estimated. 3. Results and discussion 3.1. Characterization of Fe2TiO5 composite Fig. 1(A) displays the XRD results of as prepared Fe2TiO5 composite (FTO). The XRD patterns exhibit intense peaks at 2θ = 25.28° (101), 33.12° (230), 36.02° (411). These peaks correspond to the presence of orthorhombic pseudobrookite Fe2TiO5 (JCPDS 41-1432).

In

addition, peaks at 24.06° and 54.02° were attributed to rhombohedral Fe2O3 phase.32 As the crystallization of orthorhombic iron-titanate phase starts at 700 °C, the composite annealed at this temperature shows orthorhombic TiO2 along with traces of anatase and rutile TiO2, indicating the co-existence of three phases. However certain percentage of Fe3+ present at the surface of TiO2 diffuses into the bulk producing a substitutional solid solution. Since the radius of Fe3+ is equal to Ti4+, the substitution of iron in the lattice of TiO2 is a favorable process resulting in formation of Fe2TiO5 phase.33,34 Addition of Fe limits the crystal growth of TiO2 with an average crystallite size of ~84 nm as calculated using Scherrer formula. The HRTEM images (Fig. 1(B)) shows that the prepared composite have agglomerated graining structure with pseudo cube shape and truncated corners. The particle size is estimated to be in the range of 20-30 nm. The SAED patterns showed distinct concentric rings with bright spots. The absence of diffuse rings proved that the composite is purely crystalline in nature with the d-spacing of approximately 0.35 nm.

6


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Fig. 1. (A) XRD, (B) HRTEM and (C) SAED pattern of as prepared FTO composite The chemical state of the Ti and Fe ions on the surface layer of FTO was clearly depicted by XPS spectra (Fig. 2). Fe 2p spectra (A) exhibited a sharp peak for Fe 2p3/2 at the binding energy of 709.1 eV.

35

The shift in peak from ~706.7 eV of metallic Fe indicates that Fe3+ cation penetrate

into the TiO2 lattice and substitute the Ti4+ cations.35,36 Also, the broad structure of higher binding energy peak of Fe 2p1/2 at ~724 eV was due to the Fe3+ states owing to oxidation. The Ti 2p3/2 and Ti 2p1/2 spin – orbital splitting photoelectrons were located at binding energies of ~456.1 eV and 462.2 eV respectively, of which Ti 2p3/2 peak was relatively narrower with lower FWHM values.35 These values correspond to Ti4+ oxidation state, the shift in binding energy to higher values when compared to those of metallic Ti, which may be due to the formation of Ti-O-Fe bond in the crystal lattice. The constituting elements are Fe, Ti and O as revealed from EDS (Fig. S1)

7



Fig. 2. XPS spectra of (A) Fe 2p and (B) Ti 2p of FTO composite showing its oxidation state

3.2. Characterization of FTO-Ni-P electrodes XRD pattern reveals the extent of incorporation of FTO when loaded with different amount of composite (0 g/L, 1 g/L, 2 g/L, 5 g/L and 10 g/L) in the electroless Ni-P bath (FTO-Ni-P) (Fig. S2). The incorporation of FTO into the Ni–P had significant influence on the formation of different phases as well as determination of grain size of Ni–P alloys. Sharp peak at 38.77° can be indexed to monoclinic P (310) (JCPDS – 75 – 0577) and the broad peak at 44.96° can be indexed to Ni (III) texture (JCPDS – 01 – 1266). This was in agreement with previous reports that super saturation of P in Ni resulted in the formation of amorphous structure.38 In addition the peaks at 2θ = 25.286° and 36.028° for the plane (101) and (411) corresponding to pseudobrookite FTO (JCPDS – 41–1432). Decrease in incorporation of FTO in Ni-P coating with increase in loading concentration was due to the composite aggregation which prevents their entry into the microcrystalline phase of Ni–P coating.

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Fig. 3. (A) Tafel polarization curve and (B) Impedance spectra of (a) pure Ni-P and different amount of FTO (b) 1 g L-1 (c)2 g L-1 (d) 5 g L-1 (e)10 g L-1 incorporated Ni-P coatings on mild steel. The coating of Ni-P and also incorporation of FTO can influence the electronic characters. The catalytic activity of the composite Ni-P coatings were studied by using tafel polarization techniques (Fig. 3A). The electrochemical kinetic parameters such as corrosion potential (Ecorr), corrosion current (Icorr) and corrosion rate (CR) are summarized in Table 1. The Ecorr of FTO-Ni-P coatings shifted to more anodic side compared to that of the bare Ni-P which indicates the passivation behavior of composite coating. It was due to its ability to act as a physical barrier between the corrosive environment and the metal substrate. This shift also decreases the oxidation rate of the coating resulting in relative lowering in corrosion rate. The FTO-Ni-P coatings with relatively lower Icorr revealed the formation of passive film, which can provide protection and slow down the corrosion rate. The anode with 2 g/L FTO incorporated Ni-P coating was selected as the optimal anode based on the better corrosion potential and lower corrosion current (minimum corrosion rate of 0.187 mmpy and maximum inhibition efficiency of 71.03 %). The anode performed best at this combination due to the uniform incorporation of FTO into the Ni-P matrix, eventually forming dense surface with low porosity. However, the presence of higher concentration of composite (> 2 g/L in the bath) coatings exhibited a cathodic shift in corrosion potential from (-585 mV (vs Ag/AgCl)) for pure Ni-P coating to (-647 mV (vs Ag/AgCl)) for composite (10 g/L) coatings due to agglomeration of composite because of magnetic property of Fe which impairs the surface morphology suggesting the poor corrosion inhibition of the coatings. A big fluctuation in curve b (Fig. 5A) is associated with charging current disturbance. It is clear 9



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from the figure that the coating exhibits relatively passive nature in the potential range from -0.40 V to -0.25 V with low current density. This is due to the presence of a thin passive film formed on the surface. After the breakdown potential (Eb) at the range of -0.2 to -0.1 V, a rapid increase in current density is noticed. This indicates the breakdown of the film at that moment. The coating developed from 1 g/L had least stability compared to all other coatings. When large amount of the composite is dispersed in the bath, the composites get attached loosely on the activated surface by weak physical adhesion.37 Table 1. Fitted parameters obtained from the Tafel plots Samples

Ecorr (mV vs ref.)

icorr (µA)

βa (mV)

βc (mV)

Corrosion rate (mmpy)

Rp (Ώ)

Pure Ni-P

-585.477

198.054

247.0

229.3

0.648

0.260

1 g L-1 FTO-Ni-P

-481.632

60.634

516.5

378.4

0.198

1.564

2 g L FTO-Ni-P

-556.461

57.372

221.3

151.8

0.187

0.681

5 g L-1 FTO-Ni-P

-514.323

58.702

433.5

361.1

0.192

1.457

10 gL-1 FTO-Ni-P

-647.044

143.127

314.9

184.7

0.468

0.353

-1

Fig. 3B shows the Electrochemical Impedance Spectroscopy (EIS) plots of various composite modified electrode to examine biofilm growth and biofilm conductivity. From the figure, all the curves appeared as an single incomplete semicircle but differ considerably in radius. This indicates that a same fundamental process was occurring on all the electrodes but differ in their electroactive surface area. The microbial respiration is basically a single big redox reaction that takes place at the electrode/solution interface. Thus, impedance signal (circular arc) can be influenced by breakdown of the nutrients within the electrolyte, the deposition of biofilm materials on the electrode surface, the charge transfer through the attachment of cells and microbial nanowires and the presence of microbial cell in close proximity to the electrode surface. The relative contribution of these mechanisms to the overall impedance may vary depending on the growth period of the microorganism and the adherent surface.

38-45

The fitted equivalent circuit (Fig. S4) composed of

Rs - solution resistance, Rct - charge transfer resistance, Rpore - passive biofilm/biofilm pore resistance and QdL and Qp - CPE parameters for double layer and biofilm respectively (Table S2). 10


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The obtained Rct value for the composite coating with optimum loading of FTO (2 g/L) (2.43 × 102 Ωcm2) was lower than that of pure Ni-P and all other studied coatings. The low Rct value indicated the higher catalytic performance of the FTO-Ni-P electrodes due to high electrical conductivity and the superior electrocatalytic activity generated by the synergistic effect of iron/titanium in the Ni-P coatings. Further, the presence of Fe (III) centers on the coating with optimum loading of FTO (2 g/L) enhanced the biofilm formation by EAB. Thus EABs generated more number of electrons by oxidation of fuel in the medium, which in turn accelerated EET. The high αL value (1) for FTO-Ni-P (2 g/L) coating indicated that surface was almost homogeneous due to the fact that FTO particles are chemically stable and thus effectively incorporated and decreased the metallic area prone to corrosion. Incorporation of FTO into the electroless Ni-P matrix leads to enhancement in the surface morphology and topography of the coated electrode. The FESEM images (Fig. 4A) shows the pure Ni-P coating has refined, smooth surfaces and uniform grains with nodular structure which indicated the typical amorphous nature of Ni-P coating.47 Further incorporation of the composite as shown in Figure 4B had resulted in the formation of Ni-P coatings with small granules like structure non-uniformly distributed on the surface leading to higher surface area enhancing the catalytic activity. The metallurgical improvement was achieved with the incorporation of mixed oxide into the coating. The incorporated FTO composite was of regular shape and size and showed clear boundaries, indicating that the coating had a perfectly crystalline structure.48 The presence of Ni, P, Fe, Ti and O were confirmed by the EDS spectrum (Fig. S5).

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Fig. 4. Scanning electron micrographs of (A1, A2, A3) Pure Ni-P and (B1, B2, B3) FTO - Ni-P coating on mild steel substrate at 1000X, 2000X, 5000X magnification respectively.

3.3. Biocompatibility of the composite coated Ni-P electrode The biocompatibility of the composite coated electrode was revealed by FESEM (Fig. 5A) and CLSM (Fig. 5B) images. From the figures, the bacterial cells seem to spread throughout the entire surface eventhough the most of the bacterial cells were embedded in the EPS layer. The cells spread in a non specific manner and finally localizing throughout the entire surface. The biofilm was highly heterogeneous with respect to height and localization. The thickening of biofilm occurs through adhesion of planktonic cells on the biofilm surface as well as division of cells embedded within the biofilm matrix.49 Adherence of more and more planktonic cells on the biofilm results in formation of cell clusters. From CLSM image, green fluorescence indicates about the presence of live bacteria on the surface of the electrode.50 The biofilm comprised of group of microorganisms in which cells stick to each other and often also to the surface. These adherent cells became embedded within a matrix of extracellular substances produced by the bacteria. During CLSM imaging, only live cells could be stained and it generated fluorescence green colour whereas polymeric conglomeration of extracellular polysaccharides could not be stained and hence, that appeared as dark. The images thus obtained were stacked on Z-axis for 12


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determining the thickness of the biofilm formed (Fig. S6). The height profile of the images is represented by a colour coding system, where blue → red indicates increase in height of the biofilm. The thickness of the biofilm is estimated to be more than 25 µm.

Fig. 5. (A) FESEM and (B) CLSM images showing biocompatibility of the composite coated Ni-P electrodes containing thick biofilm layer. Since the surface of the material is the point of interaction with the bacteria, the factors such as adhesiveness, roughness and wettability play very important role in determining the rate of biofilm formation on the electrode surface. Higher the cell hydrophobicity, higher is the cell-tocell adhesion and aggregation.51 Hydrophobicity has been proven to influence microbial interactions with any surfaces.52 The hydrophobic interactions of the Ni-P coated electrodes with and without composite were studied by contact angle measurement studies (Fig. S7). The contact angle with water (θw) measured for composite coated electrode was 127.2º and that for un-added one was 118.7º. Surface showing θw of less than 50º are considered as hydrophilic.53 When the contact angle of a surface is > 90º, it is named non-wettable. Thus, from the results, it can be concluded that both the composite coated surface as well as the uncoated were hydrophobic in nature. It is noteworthy that hydrophobicity of the surface increased with the incorporation of iron/ titanium based composite in Ni-P coatings. 3.4. Power generation by the developed MFC Fig. 6A shows the typical i-V behaviour of both anode and cathode. The change in open circuit voltage (OCV) values are linked to potential of anode (Ea) values, since stainless steel 13



cathode potential (Ec) values are rather constant between ~285 and ~225 mV. The potential of FTO-Ni-P anode ranges from -828.2 to -756.3 mV which was lower than that of Ni-P (-685 to 498.1 mV) revealed that the least thermodynamics driving force were utilized for the oxidation of medium for the FTO-Ni-P anode. The stability of the anode in MFC was checked upto 3 months (Fig. S9). With in 4 days of operation the anode potential and reached a plateau of -828.2 V, then it gradually decreased to -756.3 V. The highest cell voltage of ~ 1012 mV was attained in the MFC equipped with FTO incorporated Ni-P anode within 4 days of operation. This indicated the time for complete biofilm formation. The obtained cell voltage remained stable for 3 months. This may be the time period required for complete microbial degradation of the biomass. The MFC equipped with the pure Ni-P generated a cell voltage of only ~720 mV. Further, the discharge of voltage was steeper when compared to the FTO- Ni-P coated anode. This could be due to the fact that, the pure Ni-P can support only lesser biofilm growth when compared to that at FTO-Ni-P. The presence of Fe (III) in the FTO-Ni-P anode favors electroactive biofilm formation and persuades the bacteria to donate electrons. The fuel cell performance and power density curve were illustrated in the Fig. 6B. From the figure, FTO-Ni-P decorated anode exhibited maximum power density of ~2.1 W/m2 (3.6 mW/m2/mL) and current density. A significant part of increased power in the beginning was due to the electrochemical charge of FTO in the anode. A sudden increase in maximum value was obtained thereafter due to the increased hydrophobicity thus favoring the electrostatic interaction with bacterial membrane which results in complete biofilm development on the surface of the anode in the system. The extracellular electron transfer was accelerated by increase in bacterial growth and colonization on the anode surface. Once the entire surface was covered with biofilm, the bacterial metabolism as well as rate of electron transfer at the anode-biofilm interface reached a steady state. The steady state got maintained for more than 3 months of the present study time. The performance of FTO-Ni-P modified anode was highly stable. A drop in power was observed due to increase in ohmic losses and over potentials. The complex electrochemical reaction in MFC was affected by the interaction of microorganism with electrodes and the effect of medium and product diffusion.

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Fig. 6. (A) i-V curve and (B) polarization curve of MFC with FTO –Ni-P (-○-) and Ni-P (- ∆ -) as anode. EIS was adopted as the method of choice for the characterization of aqueous biointerfaces and internal resistance (Fig. 7B). From the figure, it was clear that the nyquist plot of the FTO-NiP coated electrode exhibited smaller radius than that of the Ni-P electrode, which revealed higher charge transfer efficiency and lower internal resistance. A significant decrease in Rct was obtained by FTO-Ni-P modified anode (-248.6 and 3.7 x 102 Ω) compared to that of the pure Ni-P. This revealed high charge transfer ability of the bioanode with FTO-Ni-P electrodes which resulted in an enhanced power output. Biofilm that developed on the electrode provided a large conductive surface that facilitated the electron transfer and reduced the charge transfer resistance. The Cdl was increased from 4.1 x 10-7 (pure Ni-P) to 4.3 x 10-6 F in FTO-Ni-P coated electrode in MFCs. These results demonstrated that incorporation of the FTO-Ni-P coatings on the mild steel anode decreased the internal resistance and increased the capacitance, thus enhancing the performance of anode and increasing the power output. The charge storage efficiency of different electrodes was studied from chronoamperogram recorded at OCP, which is illustrated in Fig. 7A. From the figure it was clear that FTO-Ni-P modified anode exhibited a high initial current of approximately 1 mA which was 40% higher than that of MFC with pure Ni-P anode that rapidly decreased to a steady value. Thus the magnitude of the current at the beginning of the discharge was dependent on the specific capacitance of the electrode material. The amount of charge released was evaluated using the equation Q=0∫t I dt, where Q is the total charge accumulated, I is the discharge current and t is 15



the discharge time. Introducing FTO-Ni-P coated electrode exhibited 8.7 C/cm2 of charge which was 47 times higher than that of pure Ni-P anode. This confirmed that the FTO-Ni-P modified anode could act as a good biocapacitor by accumulating more electrons and then discharging them efficiently. Thus incorporation of FTO increased the charge storage capacity of the bioanode. Under open circuit conditions, electron generated by oxidation of the substrate was temporarily stored in the cytochromes of EAB, and then the electrons were transferred into the anode surface.54-55 In this way, a capacitive anode can make a bridge between EAB and the anode which can control the flow of electron generated by EAB to the anode. Thus higher the capacitance of the anode modifier, the better is the performance.

Fig. 7. (A) Chronoamperometric response (i-t) and (B) Nyquist plot of MFCs with (a) FTO- Ni-P coated anode (b) pure Ni-P as working electrodes The cyclic voltammetry (CV) was used to study the catalytic activity and capacitive behavior of the electrode when the electroactive biofilm was formed on its surface in the MFC. The measurement of CV was carried out using Electrochemical Workstation using a threeelectrode system at a slow scan rate of 10 mV/s (Fig. S8). The MFC with the FTO incorporated electroless Ni-P coated mild steel as anode showed a well-defined anodic/oxidizing and cathodic/reduction peak at ~0.2 V and ~ -0.452 V respectively. The absence of peak for Ni-P coated electrode revealed that the bare Ni-P anode had no significant chemical catalytic activity in the bacterial medium. The oxidative current generated by biological oxidation of the substrate rapidly rose at the potential 0.2 V (vs. Ag/AgCl) for the composite incorporated Ni-P anode. This 16


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significant increase in the current can be explained by the capacitance value and the electrochemically accessible surface area (ECSA). The increased ECSA as well as the biocompatibility of the composite incorporated Ni-P anode resulted in increased bacterial growth and attachment of biofilm on the surface which in turn increased the amount of electrons involved in charge transfer, improving the anode performance. In general, the increase in redox peak current with the FTO modification was attributed to the inherent electron transfer ability of bacteria to insoluble Fe (III) metal centers of the composite. In the case of surface bound biofilm on the composite coated anode, the supply of electron accepting Fe (III) within the surface matrix is limited. This is the reason for the characteristic peak obtained. The current that increased with oxidation of Fe (III) suddenly began to drop to zero with complete stoichiometric oxidation of the Fe (III). Such an effect or characteristic peak was not seen in the case of the bare Ni-P coated anode because the capacitance of the Ni-P was very low. Thus, the CV analysis proved about the ability of the developed anode to efficiently transfer the electrons collected from the bacterial catalytic centers to the circuit. This contributed to increase in the power output. The proposed mechanism for the efficient electron transfer in highly active FTO-Ni-P coated anode was attributed to the increased surface area and reversible redox reaction between the oxidation states of Fe in the FTO. The presence of Fe (III) centers in the FTO and also increased surface area favors bacterial attachment and electroactive biofilm formation and also persuades the bacteria to donate electrons and was more efficient in capturing electrons from the biofilm. Then these electrons liberated from microbial oxidation of glucose were stored in the FTO-Ni-P. Moreover, the high capacitance of the iron moiety amplified the rate of electron efflux. Once the electron acceptor is available, the stored electron can be immediately released and transferred to the electron acceptor. Morphological studies revealed that the pure Ni-P anode exhibited some crack on its surface after polarization whereas FTO-Ni-P modified anode retained their microstructure in bacterial cultivation medium with a thin layer of biofilm on the surface (Fig. S10). These results confirmed the stability of FTO-Ni-P coatings to be used as anode in MFC revealing the feasibility of the electrode. 3.5 MFC stacking performance Voltage can be further increased by stacking MFC reactors i.e, connecting the individual reactor in series to provide higher voltage output (Fig. 8A). According to the design, individual 17



four individual MFC reactors can be placed in close series so that they can be connected to each other through the external circuit. During the initial hours, no significant increase in cell voltage could be detected. Then, the cell voltage increased linearly with time up to a highest of 3.0 V (external load 100 kΩ) and had an OCV of 3.2 V after 4 days of continuous operation. This is the period of rapid biofilm development on the surface of the anode in the system during which extracellular electron transfer accelerated with increase in bacterial growth and colonization on the anode surface. Once the entire surface was covered with biofilm, the bacterial metabolism as well as rate of electron transfer at the anode-biofilm interface reached a steady state.

Fig. 8. (A) Schematic Diagram and (B) polarization curve of stacked MFC The power density /polarization curve corresponding to overall power output of stacked MFC is given in Figure 8B. As expected there was a sharp shoot up in the power when the MFCs were stacked. Highest power density exhibited by the system was 6.3 W/m2 (44.59 mW/m2/mL) at a current density of ~2.7 mA/m2 beyond which, there was a sharp decrease in the power due to inability of the bacteria to metabolize. Also, the drop in cell voltage followed a slow path showing an increase in capacitance of the whole system. The current produced could be used to light an LED light of 3.2 V after 4 days of uninterrupted operation (Fig. S11). The LED light worked continuously when connected to the MFC system without any break for more than 3 weeks which proved that the current generation by bacteria in the developed system was a continual process. This OCV achieved in this study was comparable to that obtained in the work reported by Zhao et al., 2017. The OCV obtained was 1.15 V when 2 MFCs were connected in series.56 18


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4. CONCLUSION We demonstrated about fabrication of a sustainable MFC comprising of an electroactive Iron/Titanium based Ni-P composite steel anode. The Fe2TiO5 composite with Fe3+ oxidation state could shoot the band gap down from 3.2 eV to 2.2 eV making it a suitable for incorporation into a biocompatible anode for MFC. The catalytic anode was prepared by incorporating the catalytic composite into Ni-P electroless coating. The surface of the anode had homogenous morphology and uniform distribution of nucleating sites. Mild steel anodes with high corrosion resistance and stability were achieved with the incorporation of an optimum amount of Fe2TiO5. The existence of Fe (III) centres in the coatings enhanced the active surface area which favored bacterial attachment and subsequent electroactive biofilm formation. This phenomina persuaded the bacteria to donate electrons and the capacitive nature of the compoaite became more efficient in capturing elecrons from the biofilm resulting in achievement of power density as high as 2.1 W/m2. This power was 20% higher than that of the control. The internal resistance was suppressed to 1.12 x 10-2 Ω while the capacitance was increased to 47 times when compared to the control system. During long time operation, the MFC sustained an OCV of ~ 1000 mV. The microstructure of the anode surface was retained during the operation of the MFC. Stacked MFC obtained an maximum open circuit potential of 3.2 V with power density and current density out put of 6.3 W/m2 and 2.7 mA/m2 respectively. This report is first of its kind that deals about such a simple reproducible system which can be extended to other similar systems for practical applications. ASSOCIATED CONTENT Supporting Information The following files are available free of charge EDS analysis showing the presence of Fe, Ti and O in the FTO composite; XRD patterns of FTO composite incorporated electroless Ni-P coatings; 2D, 3D optical profilometer images of 1000 µm x 1000 mm surfaces and the height profile along the marked line of a portion with and without coatings; Equivalent circuits used for fitting the impedance spectra based on a double layer model of surface film - Rs – Solution resistance, Rct – charge transfer resistance, Rpore – pore resistance, Qp – CPE parameters, Qdl – CPE parameters for double layer, αL - Dispersion parameters; EDS 19



pattern of FTO incorporated electroless Ni-P coating showing the peaks corresponding to Ni, P, Fe, Ti and O; the hydrophobicity of Ni-P and composite incorporated Ni-P electrode using contact angle measurements; Cyclic voltammogram showing electrochemical activity of the bare Ni-P coated mild steel and FTO incorporated electroless Ni-P coated mild steel as anode; Scanning electron micrograph showing the stable microstructure of the coating after polarization; the stacked MFC with LED light glowing; List of identified bacterial isolates and their GenBank accession numbers; Fitted parameters obtained from the Nyquist plot of the coatings. (PDF) AUTHOR INFORMATION Corresponding Author [email protected]; [email protected] Acknowledgements: The authors are grateful for the authorities of University of Kerala, for providing facilities for the present work. The authors gratefully acknowledge the funding received from, KSCSTE, (Grant Number: 022/SRSPS/2013/CSTE), State Government of Kerala, India, to complete the work. References (1) Samsudeen, N.; Radhakrishnan, T. K.; Matheswaran, M. Bioelectricity Production from Microbial Fuel Cell Using Mixed Bacterial Culture Isolated from Distillery Wastewater. Bioresour. Technol. 2015, 195, 242-247. (2) Ren, H.; Pyo, S.; Lee, J-I.; Park, T-J.; Gittleson, F. S.; Leung, F. C. C.; Kim, J.; Taylor A. D.; Lee, H-S.; Chea, J. A High Power Density Miniaturized Microbial Fuel Cell having Carbon Nanotubes Anodes. J. Power Sources. 2015, 273, 823-830. (3) ElMekawy, A.; Hegab, H. M.; Pant, D.; Saint, C. P. Bio‐analytical Applications of Microbial Fuel Cell–based Biosensors for Onsite Water Quality Monitoring. J. Appl. Microbiol. 2018, 124(1), 302-313. (4) Gregory, K. B.; Lovley, D. R. Remediation and Recovery of Uranium from Contaminated Subsurface Environments with Electrodes. Environ. Sci. Technol. 2005, 39, 8943-8947.

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TOC GRAPHIC

Fe2TiO5 FTO-Ni-P-Mild steel electrode

eee-

Biomass/ Organic substances

ee-

e-

e- e

Microbial Fuel Cell stacking

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CO2 + H2O

Development of high performance mediator - less Microbial Fuel Cell

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