Electrodepositing technique for improving the performance of crystalline and amorphous carbonaceous anodes for MFCs

Abstract

The properties and cost of anode materials are essential factors affecting the microbial fuel cell (MFC) performance. Therefore, in this study, an electrodeposition technique is presented as a cheap, easy, efficient, and straightforward strategy to increase the exoelectroactive bacterial adhesion and improve the surface properties of the crystalline and amorphous carbonaceous materials for use as anodes in the microbial fuel cells enriched with unconditioned industrial wastewater. Individually, the surfaces of commercial activated carbon AC (amorphous), carbon paper CP (crystalline), and carbon cloth (CC) were modified by an iron electrodeposition technique. In air-cathode microbial fuel cells, the suggested modification strategy strongly enhanced the power generation as the observed increase was 18.5%, 47.5% and 65.8% for the activated carbon, carbon cloth and carbon paper, respectively. Moreover, the coulombic efficiency (CE) is increased after iron electrodeposition modification process to reach up to 80% in case of treated activated carbon anode. Overall, the results confirmed the successful electrodeposition of iron, as an effective, simple and cheap surface treatment technique, is more efficient in the crystalline materials as compared to the amorphous materials.

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1. Introduction

Nowadays, bioenergy has attracted considerable attention as a new, valuable and cost-effective renewable energy source to overcome the energy crisis.¹ Therefore, microbial fuel cells (MFCs) have received significant attention due to their ability to overcome energy and water crises simultaneously.² MFC is an electrochemical device that converts the chemical energy stored in the organic matter into electricity via invisible vital creatures: microorganisms.^3–5 The main difference between MFC and other types of the fuel cell is that the living organism acts as catalyst and oxidizes the organic materials present at the anaerobic anode in the fuel cell.^6,7 Accordingly, particular attention should be drawn to the anode material to reduce the biocatalytic requirements. Moreover, wastewater is used as an electrolyte in the MFCs because it carries organic substrates and a mixed culture of bacteria,^8–12 which adds more importance to the MFCs for use as promising devices for wastewater treatment associated with power generation.

Natural wastewater bacteria can be divided into three different categories; fermentative bacteria, methanogens, and an exoelectrogenic active bacteria.^8,10,13 The last type is the most favorable and significant because it can generate and transfer electrons without external mediators.¹⁴ However, the presence of the other types of bacteria at the anode surface is unfavorable. The growth of the undesired microorganisms on the anode surface through the initial immunization stage tends to occupy a large space of the anode that results in reducing the available space for the exoelectrogens active bacteria as well as hindering the electron generation process. Hence, improvement in the anode surface properties to modify the selectivity should be considered to limit the attachment of unfavorable methanogenic bacteria and enhance the adhesion of the desired electrochemically active ones.

The carbonaceous materials are the optimal anode candidates due to their special properties, such as high electrical conductivity, specific surface area, and good biological compatibility for the biofilm growth.¹⁵ However, the lack of good selectivity is one of the main disadvantages of the carbonaceous materials. The introduced strategies to enhance the selectivity can be divided into three groups: (i) chemical or physical treatment, (ii) coating of the surface of the anode by highly conductive or electroactive materials, and (iii) fabrication of metal–graphite composite anodes.¹⁶ Among the familiarized modification techniques, the coating is still the simplest and most efficient strategy due to the clear enhancement in the growth of the electrochemically active bacteria on the anode and the distinct increase in the generated power.^17,18 Different materials have been reported to coat the carbonaceous anodes including conductive polymers (e.g. PANI and polypyrrole), carbon nanotubes (CNTs),¹⁹ mediators,²⁰ metal oxide and metals,²¹ and composites of these materials.²¹ Moreover, iron and its oxides are considered attractive candidates that can be used as effective coating materials.²² Fe could enhance the biocompatibility and produce higher power and current densities in addition to higher coulombic efficiency (CE) compared to the Fe-free anodes.²³ For example, Lowy et al.²¹ reported that graphite modified by a graphite paste containing Fe₃O₄ achieved 1.5–2.2 times greater kinetic activity than pure graphite. This enhancement increased the efficiency of the initial contact of the electrochemically active bacteria with the surface upon iron oxide deposition,²¹ as well as accelerated the propagation of exoelectrogenic types of microorganism.^21,24

However, the reported coating techniques are either expensive, suitable for specific anode materials, or applicable for specific MFCs (mediator-based or working with simulated anolyte). Moreover, the corresponding increase in the power is still unsatisfactory. In this study, an electrodeposition technique, using the simple electrolytic cell for the deposition of Fe on different anodes materials, is introduced as a novel, efficient and simple surface treatment strategy. Moreover, the effect of the crystallinity of the material on the electrodeposition was studied using AC (highly amorphous) and CP (highly crystalline).

The influence of the proposed treatment strategy on MFCs was investigated based on the unconditioned industrial wastewater without the addition of external microorganisms or mediators. The results indicated that the proposed strategy is very effective as a distinct increase in the power was achieved.

2. Materials and methods

2.1 Electrodes

Untreated or pristine carbon paper (CP) and carbon cloth (CC) from Electro Chem. Inc., USA were used. Commercial activated carbon sheet (AC; from Jeonju, South Korea local market) was used as anodes, where the projected area of the electrode was 2.5 cm × 2.5 cm. The cathode was carbon paper with pt. 0.5 mg cm⁻² (Electro Chem, Inc., USA). Furthermore, the single chamber air-cathode MFC model was used to investigate the performance of the electrodeposition technique on the anodes behaviour.

2.2 Electrodeposition technique

The iron electrodeposition technique was carried out in an electrolytic cell as shown in Fig. S1.† In more details, the different anode materials CC, CP and AC were cleaned in an ultrasonic bath by acetone for 1 h to remove the adsorbed contaminations. They were then rinsed with distilled water and finally the electrodes were dried in an oven at 60 °C for 12 h. After that, the cleaned electrodes were introduced in the electrolytic cell as cathode, whereas the graphite rod was used as an anode. Moreover, the two electrodes were immersed in a 100 ml of the iron acetate solution (0.2 M), and the distance between the electrodes was 1 cm. Moreover, a voltage of 10 V was applied using DC power supply between the two electrodes for 20 min at room temperature.

However, due to the voltage (10 V) difference between the cell electrodes, Fe ions (cations) moved from the solution to the cathode (the commercial anodes of MFC cell) and were precipitated as iron particles on the surface of the cathode. Finally, the electrode was removed from the cell and dried at 80 °C for 1 h before being used as an anode in the MFC.

2.3 Microorganisms media (electrolyte)

Food wastewater was collected from the local food plants in Jeonju, South Korea, and used as an electrolyte without any pretreatment. The collected wastewater was characterized in the Water Environment Research Center, Jeonju, South Korea; the characterization results are presented in Table S1.† An agar cultivation analysis was conducted to confirm the presence of mixed culture bacteria. Agar dishes were examined under a microscope (U-25ND25 (BXSL) OLYMPUST2, Japan) using 10× magnification (MPLOON10*0.25), as shown in Fig. S2A,† and the number of bacteria was counted, as shown in Fig. S2B.†

2.4 MFC structure

Single air-cathode type MFC, where the cathode is in direct contact with air and has 84 cm³ capacity, was utilized, as shown in Fig. 1A–C. The distance between the anode and cathode was 4 cm. Moreover, a Ag/AgCl reference electrode was located in the anode compartment to measure the potential of the electrodes. Cation exchange membrane (CEM, CMI-7000) purchased from Membrane International Inc., NJ, USA, was used as the proton exchange membrane. The membrane was first treated by dipping in a NaCl solution (1 M) for 12 h at room temperature, and then it was kept in distilled water until use. Two 1.0 mm thick high corrosion resistance stainless steel plates were used as the current collectors. The cell was assembled as follows: first, the cathode was sandwiched between the cation exchange membrane and cathode current collector, and then, this assembly was placed on one side of the bio container, where the anode was attached to the other current collector, as shown in Fig. 1. The current density was calculated based on the projected area of the anode (6.25 cm²).

Fig. 1 (A) Schematic for a single air-cathode type MFC. (B) Setup of a MFC system, single air-cathode type MFC connected to (LSV) and mini data logger multiple channels. (C) Individual component of a single air-cathode MFC.

2.5 Electrochemical operation and measurements

First, 80 ml of the real industrial wastewater was inserted into the anode chamber after being purged with nitrogen gas for 20 minutes to remove the dissolved oxygen. The cell was then operated in a batch mode until stabilization of the open circuit voltage (OCV), which was measured over the time by the potentiostat, was achieved. The anode and cathode potentials were measured using Ag/AgCl as the reference electrode. After OCV stabilization (the anode oxidation and cathode reduction reactions reached to an equilibrium), the cell circuit was closed and a polarization curve was obtained. The polarization curve defining the current density as a function of the voltage was obtained using a linear sweep voltammetry method (LSV, HA-151A POTENTIOSTAT/GALVANOSTAT, Japan). Briefly, the relationship between the generated current (I) and the corresponding cell voltage (V) can be determined by changing the external resistance (load) of the circuit. The relationship between the current and the voltage at different resistance was measured, starting from the maximum OCV to a zero voltage using 1 mV s⁻¹ as a scan rate. The current density was measured for 2 h at a constant cell voltage of 0.2 V to check the cell stability. All the data was obtained using a GL220 midi Logger.

The current density (CD) and the power density (PD) were calculated using eqn (1) and eqn (2) to evaluate the MFC performance,

CD = I/A_a (1)

PD = CD × V (2)

where A_a, is the projected anode surface area (6.25 cm²).

Moreover, the coulombic efficiency (CE) was calculated based on eqn (3):

where M is the molecular weight of oxygen (32), F is the Faraday's constant, b = 4 indicates the number of electrons exchanged per mole of oxygen, V_an is the analyte volume in the anode chamber, and ΔCOD is the chemical oxygen demand (COD) change over time.

2.6 Characterization

The hydrophilicity of the anode materials was measured by a DPRO image standard device, which is used to measure the water contact angle. The surface morphology of the different anodes before and after the electrodeposition was investigated using scanning electron microscopy (SEM, JEOL JSM-5900, Japan) and field-emission scanning electron microscopy equipped with an EDX analysis tool (FE-SEM, Hitachi S-7400, Japan). The crystal structures of the anode materials were examined by XRD characterization using Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu Kα (λ = 1.540 Å). The diffraction pattern was obtained over a 2θ range from 10° to 80° at a step size of 0.02°.

3. Results and discussion

3.1 Effect of the electrodeposition on the characterization of the anodes

The hydrophilicity of the treated anode was investigated by measuring the water contact angle (WCA) before and after electrodeposition, and the results are presented in Fig. 2.

Fig. 2 Water contact angles of the anodes before and after electrodeposition.

The results indicated that the hydrophilicity of the treated anodes increased due to Fe deposition on the surface of the treated anode. The WCA of the AC, CC, and CP treated anodes decreased by 23%, 33%, and 33%, respectively. However, the morphology and chemical composition of the surface exhibited a strong effect on the water contact angle (the wettability).

Herein, the enhancement in the hydrophilicity of the treated anode was attributed to the change in the chemical composition of the anode surface as well as increase in the surface roughness (Fig. 4) after the deposition of Fe. Overall, the surface of the anodes treated by iron converted the hydrophobic nature of the electrodes into highly hydrophilic, as shown in Fig. 2. Moreover, the wastewater (electrolyte of MFC) will easily wet the hydrophilic anode, which will facilitate the contact of the microorganism with the anode surface as well as enhance the attachment of the microorganism to the anode surface. Accordingly, very well attachment for the cells occurred in case of the treated electrodes, which minimized the interfacial resistance for the electron passing from the cell wall to the anode surface.¹⁸

However, the electrodeposition of Fe on the anode surface was analysed using the XRD for the treated anodes, where the (110) crystal plane of Fe (JCDPS database; #06-0696) at 44.3° appeared, as shown in Fig. 3. The appearance of the peak at 26.3° (002) crystal plane can be assigned to the original anode material (carbon) (JCDPS database; #41-1487). Another important notification that can be determined by XRD results is that the activated carbon had a more amorphous structure compared to the other two formulations. In contrast, the carbon paper showed very high intensity at the carbon fingerprint peak, which indicates very high crystallinity compared to that of the other two formulations. In addition, EDX was performed to demonstrate the effect of the material crystallinity on the electrodeposition process, as shown in Fig. S3.† Moreover, as indicted by the results, the highly crystalline materials had a positive effect on the electrodeposition process, whereas the decrease in crystallinity had a negative effect on the deposition process. Typically, the electrodeposition of Fe on the CP (high crystallinity) was 26.2% and this percentage was decreased to 6.4% for AC (highly amorphous).

Fig. 3 XRD patterns for the treated anode materials.

An investigation of the influence of the electrodeposition on the anode surface morphology is vital and was done using SEM analysis. From the results in Fig. 4D–F, the rough surface was noticed for the treated anodes and the Fe particles were successfully deposited on the treated anode surface. Furthermore, SEM images confirmed the abovementioned results, where the crystalline anode, CP (Fig. 4C and F), shows the Fe electrodeposition layer covering most of the anode surface, whereas the amorphous anode, AC, exhibited a low Fe deposition on the surface, as shown in Fig. 4A and D, respectively.

Fig. 4 Scanning electron microscope images for different anodes materials before treatment; (A) activated carbon (B) carbon cloth and (C) carbon paper. After treatment by iron electrodeposition; (D) treated activated carbon (E) treated carbon cloth and (F) treated carbon paper.

Overall, the results highlighted the availability of the electrodeposition technique for the deposition of Fe on both the amorphous and crystalline surfaces. Moreover, the crystalline surface is affected more by electrodeposition than the amorphous one.

3.2 Evaluation of the treated anodes in MFC for power harvesting

3.2.1 Electrode potential and open cell potential (OCV). Fig. 5 shows the open circuit voltage (OCV) and the anode potential (E_a) versus time for the MFCs based on the pristine and modified anodes. As shown, for all the formulations, the anode potential decreased with time, which lead to a corresponding increase in the open cell voltage (OCV). The decrease in the anode potential was attributed to the reactions occurred through the fermentation process. The microbe generated most of its energy through ATP and NADH, which must be converted back to NAD⁺ to complete the metabolism reaction. Therefore, during the open circuit mode, this stage is called the accumulation period, where the concentrations of the reduced species, such as NADH, are built up within the growth of the bacteria until the stabilization point is achieved. After this point, the potentials decreased and became unfavorable, and therefore, the respiration process stopped. Once the MFC circuit was closed, the pressure released and the electrons could transfer to the cathode to generate electricity.¹⁵ After this explanation, the decrease in the anode potential during the MFC operation can be attributed to the adhesion of the exogenous microorganisms.

Fig. 5 Relationship between the open circuit voltage (OCV) & anode potential (E_anode) versus time before (A) and after (B) anode treatment.

Based on the results displayed in Fig. 5A, it can be concluded that the decrease in the anode potential reached to its minimum value while utilizing the activated carbon as an anode followed by carbon cloth and finally carbon paper. As shown in Fig. 5A, for the pristine anodes, E_anode decreased from the initial values of 0.379, 0.222, and 0.217 V to −0.268, −0.205, and −0.112 V in the case of AC, CC, and CP, respectively. These results suggested that the corresponding decrease in the anode potential was 0.647, 0.427, and 0.329 V for the same abovementioned order. This indicated that the pristine activated carbon electrode could attract the maximum number of microorganisms, which revealed the best performance among the chosen electrodes. This high performance of the pristine activated carbon sheet can be attributed to the excellent biocompatibility, more porosity, and the low hydrophobicity of AC compared to those of CC and CP, which enhanced the bacteria adhesion.

However, the treated anodes achieved a large decrease in the anode potentials and a consequent increase in the OCV for all the anodes. Typically, the observed E_anode reached −0.286, −0.218, and −0.209 V, as well as the corresponding OCV increased from starting values of 0.344, 0.489, and 0.498 V to a maximum values of 0.835, 0.827, and 0.806 V in case of the cells working by AC, CC, and CP treated electrodes, respectively. Table S2† summarizes the electrochemical measurement results. Accordingly, the further decrease in the anode potential can be assigned to the iron deposited on the anode surface, which enhanced the efficiency of the initial contact of the electrochemically active bacteria with the modified anode surface and improves the electron transfer. Overall, these results might be ascribed to the hydrophilic properties of the treated anodes, which enhanced the physicochemical interactions between the bacteria and the anode surface. The adhesion of the microorganisms was affected by many interacting forces, such as hydrophilic interactions, van der Waals interactions, and electrostatic interactions.²⁵

Moreover, the proposed treatment technique decreased the starting up operation time by 30.8%, 21.3%, and 12.6% for AC, CC, and CP – based MFCs, respectively. As shown in Table S2,† the time required for cell stabilization decreased from 133, 100.5, and 79 h to 92.5, 79, and 69 h for the MFCs based on AC, CC, and CP, respectively. The decline in the startup time confirmed the effectiveness of the iron deposition process to increase and accelerate the attachment of the microorganisms to the anode surface and enhance the biocompatibility properties of the anode surface. Overall, these results are due to the enhancement of the hydrophilicity of the treated anodes and increase in the surface roughness. Moreover, the decreasing of initial start up time due to the accelerating adhesion of microorganisms is totally agree with those observed by Guo et al.,²⁵ who found that the decrease in the startup time for the hydrophilic surface was nine days shorter than that of the hydrophobic surface.

However, the biofilm on the treated and primary anodes was investigated by SEM, as shown in Fig. 6. The figure showed the pristine anode surfaces, AC, CC and CP (first row) (A, B and C, respectively), and the treated anode surfaces, AC, CC, and CP (second row) (D, E and F, respectively), after using in the MFCs. From the figure, the biofilm was observed clearly on the anode surface. However, the treated anodes exhibited greater attachment of the microorganism to the anode surface due to the presence of Fe on the surface. Moreover, due to the abovementioned advantages of Fe electrodeposition on the anode surface, the treated anodes possessed a larger surface area available for bacterial adhesion, which lead to an enhanced electron transfer operation between the microorganisms and electrode surface.

Fig. 6 Scanning electron microscope images of the biofilm on the pristine anodes; activated carbon (A), carbon cloth (B) and carbon paper (C). Treated anodes; activated carbon (D), carbon cloth (E) and carbon paper (F) after using the MFC.

Overall, the iron electrodeposition technique enhanced the properties of the anode materials as well as the MFC performance for three reasons: (1) increasing the propagation of electrochemically active bacteria on the electrode during the initial inoculation step; (2) acceleration of the attachment of the microorganisms to the anode surface, which lead to a more than 30% decrease in the startup time with MFC based on the activated carbon anode; and (3) increasing the efficiency of the electron transfer process from the bacteria to the modified anode surface.

3.2.2 Power generation. The amount of generated power is a strong indicator of the MFC performance. The production of power from MFCs depends on the ability of microorganisms to generate and transfer the electrons directly. In Fig. 7A and B, the generated powers from the assembled MFCs based on the treated and pristine anodes were calculated. Moreover, the influence of the treated anodes on the power generation from MFC was investigated, and the results concluded that for all the formulations, a visible increase in power generation was observed.

Fig. 7 Power generation and polarization curves versus current density. (A) For CC & CP anodes before and after iron electrodeposition. (B) For AC anodes before and after iron electrodeposition.

Numerically, the proposed treatment process led to an 18.5%, 47.5% and 65.8% increase in the maximum power produced for the treated AC, CC and CP anodes, respectively, compared to that for the pristine anodes, as shown in Fig. 7A and B and Table S3.† This significant increase in power generation is related to the promising and new biological, physical and electrochemical properties added to the anode surfaces as a result of Fe electrodeposition.

In the point of crystallinity, the CP (high crystallinity) achieved the maximum increase in generating the power; 65.8% (as show in Fig. 7A) due to the enhancement of the Fe deposition as explained above. However, AC (low crystallinity) exhibited the lowest power increase, only 18.5% higher than that of the pristine one, as shown in Fig. 7B. Overall, these results indicated a positive effect of Fe on the anode surface as well as the importance of the crystallinity in the electrodeposition technique.

Compared to the most recent reports about enhancing the anode performance using the coating procedure, the results obtained in this study were superior, as shown in Table 1.

Table 1 Power generation rates of MFCs with different anodes reported in the literature versus those obtained in this study

Types of MFC	Microorganism media	Anode types	Modification method	Improvement in the performance
				Increase in current (%)	Increase in power (%)	Ref.
Microbial electrolysis cell (MEC)	Shewanella oneidensis MR	Graphite disk	Surface coating Pd nanoparticle	50–150	—	22
Single chamber air-cathode MFC	Mixed-culture anaerobic granular sludge	Carbon paper	Surface coating carbon nanotube	—	20	26
Mediator MFC	Preacclimated bacteria from an active MFC	Graphite felt	Surface coating treatment electrochemical oxidation	39.5	—	27
Mediator MFC	Shewanella oneidensis MR	Glassy carbon	Surface coating carbon nanotube	1–82	—	28
Two chamber MFC	Escherichia coli	Carbon paper	Surface coating Pt-decorated carbon nanotube	—	6	29
Microbial electrolysis cell (MEC)	Shewanella oneidensis MR-1	Graphite disk	Surface coating Au nanoparticle	20	—	22
Two chamber MFC	Clostridiaceae	Carbon felt	Surface treating PANI deposition	—	35	30
Single chamber air-cathode MFC	Food wastewater	Carbon paper	Surface coating iron electrodeposition	194.6	65.8	This study
Single chamber air-cathode MFC	Food wastewater	Carbon cloth	Surface coating iron electrodeposition	231.3	47.5	This study
Single chamber air-cathode MFC	Food wastewater	Activated carbon sheet	Surface coating iron electrodeposition	42	18.5	This study

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3.2.3 Internal resistance of the system. The internal resistance of the MFC means the resistance of ions to flow through the cation exchange membrane (CEM) and the anodic electrolytes, in addition to the resistance of electrons to flow through the electrodes and interconnections.^31,32 The internal resistance (R_int) of the MFC can be determined from the slope of the linear polarization curve. Fig. 8 displays the internal resistance distribution in the case of the pristine and treated carbon cloth as an example.

Fig. 8 (A) Calculation of the internal resistance for the MFC system based on carbon cloth as the anode material before and after the electrodeposition treatment. (B) Comparison between the internal resistance of MFC based on the treated and untreated anode materials.

Overall, the internal resistance can be divided into three regions through the polarization curve. The first zone is called the “activation zone”, which appears at a low current and high potential part in the polarization curve. Within this region, the electrons generated from the exoelectrogenic microorganisms need to overcome the back potential provided to the cell from the external resistance to produce electricity; this results in lowering the cell voltage compared to the theoretical value. As shown in Fig. 8A, within the activation zone, there was almost no distinct influence on the proposed treatment strategy because a small decrease in the resistance between the two cells can be observed. The second zone called the ohmic resistance zone has a vital role in determining the point of the maximum achievable power. Moreover, the ohmic resistance is mainly related to the cell components as it represents the resistance of the electrodes, membrane, electrolyte, and external connections. As shown in the middle zone in Fig. 8A, the ohmic resistance of the cell based on the treated CC electrode is significantly lower than for that based on the pristine CC. The ohmic resistances were 4800 and 2500 ohm for the cells based on pristine and treated anodes, respectively. Considering that the difference between the two cells is only in terms of the anode material, the proposed treatment process led to a strong enhancement in the electrical conductivity of the anode material.

The third zone is called the mass transfer resistance; a very high cell voltage drop until zero at the maximum current density takes place within this region. In this zone, the electrical resistance reaches to its maximum value. This zone could be clarified by considering that numerous electrons need to be generated and transferred from the anode to the cathode to produce a high current. Therefore, a high digestion rate for the substrate is needed. Consequently, the concentration of the substrate molecules at the surface of the biofilm decreased sharply, which created concentration polarization.

Fig. 8B compares the overall internal resistances for MFCs based on the treated and untreated anodes. The internal resistance decreased by 35.6, 22 and 19% in case of MFC based on treated CC, AC, and CP, respectively. These results confirmed that the electrodeposition of iron on the anode surface led to a decrease in the electron charge resistance and enhanced the electron transfer, which reflects an increase in power generation and current, as observed in the previous results.

3.2.4 Current generation & Coulomb efficiency. The stability of the current produced from the used MFCs was measured over time at 0.2 volts, as shown in Fig. 9A. In the case of the MFC based on the untreated anodes, the AC anode achieved the maximum current density followed by CC and finally CP: 1690, 252 and 186.8 mA m⁻², respectively. Moreover, in the case of the MFC based on the treated anodes, the current density increased for the three different anodes to 2400, 835, and 548 mA m⁻² for the abovementioned anodesin the same order. These results suggested that the iron electrodeposition technique increased the generation of current by 3.3, 2.93 and 1.42 fold in the case of the AC, CC and CP electrodes, respectively. However, these results confirmed the effectiveness of the electrodeposition method to enhance the power and current generation in the MFCs. Increase in the current density upon iron electrodeposition can be attributed to the corresponding decrease in the charge transfer resistance, which led to an enhanced electron flow and subsequently improved current density. Moreover, the current density started at a high value and then decreased with time until a stable state was achieved. The decrease in the initial current density is normal due to the rapid depletion of the accumulated electrons on the anode surface during MFC operation. Once the current started to flow, these electrons became depleted until the MFC reached the steady state conditions. Later, the current produced matched the instantly produced electrons from the microorganisms, which led to the observed current stability in all formulations.

Fig. 9 (A) Stability of current generation versus time for the different utilized anodes and (B) Coulomb efficiency for the MFC based on the treated and untreated anodes.

Moreover, the coulombic efficiency (CE%) was estimated using eqn (3), based on the generated current and with respect to the organic substrate removal, as shown in Fig. 9B. The CE increased by 1.72, 1.63 and 1.08 times for CC, CP and AC, respectively, after treatment with Fe. This explained the increase in the current production after applying the proposed treatment strategy. In other words, more organic pollutants were digested to produce the electrons. As abovementioned, the CE was determined based on the change in the COD of the utilized wastewater upon using the MFC. Moreover, the high percentage of COD removal (more than 60%) with all the treated anodes, as shown in Fig. S4,† confirmed that the MFC is an appropriate technology for wastewater treatment and energy production simultaneously.

4. Conclusion

Electrodeposition of Fe on the surface of different carbonaceous electrodes (crystalline and amorphous) can be performed using a simple electrolytic cell using iron acetate as the precursor. The electrodeposition of iron enhanced the properties of the treated anodes, such as total surface area, roughness, wettability, and the electrical conductivity of the carbonaceous electrodes. Consequently, the proposed treatment strategy strongly enhanced the performance of the investigated materials as anodes in the microbial fuel cells based on the industrial wastewater. However, the treated anodes exhibited an improvement in the generation of power and current compared to the pristine anodes. Overall, the electrodeposition of iron from the aqueous solution is recommended as a simple, efficient and low-cost anode surface modification strategy to maximize the power generation by the microbial fuel cells.

Acknowledgements

This work was financially supported by the National Research Foundation of Korea (NRF) and Grant by the Korean Government (MOE) (No. 2014R1A1A2058967).

References

1. G. Nikhil, G. V. Subhash, D. K. Yeruva and S. V. Mohan, Energy, 2015, 88, 281–291.
2. D. R. Lovley, Curr. Opin. Biotechnol., 2006, 17, 327–332.
3. R. S. Berk and J. H. Canfield, Appl. Microbiol., 1964, 12, 10–12.
4. J. Rao, G. Richter, F. Von Sturm and E. Weidlich, Bioelectrochem. Bioenerg., 1976, 3, 139–150.
5. D. Hidalgo, T. Tommasi, S. Bocchini, A. Chiolerio, A. Chiodoni, I. Mazzarino and B. Ruggeri, Energy, 2016, 99, 193–201.
6. K. Scott, G. Rimbu, K. Katuri, K. Prasad and I. Head, Process Saf. Environ. Prot., 2007, 85, 481–488.
7. C.-H. Lay, M. E. Kokko and J. A. Puhakka, Energy, 2015, 91, 235–241.
8. L. T. Angenent, S. Sung and L. Raskin, Water Res., 2002, 36, 4648–4654.
9. A. Rinaldi, B. Mecheri, V. Garavaglia, S. Licoccia, P. Di Nardo and E. Traversa, Energy Environ. Sci., 2008, 1, 417–429.
10. S. Dollhopf, S. Hashsham, F. Dazzo, R. Hickey, C. Criddle and J. Tiedje, Appl. Microbiol. Biotechnol., 2001, 56, 531–538.
11. H. O. Mohamed, M. Obaid, K. A. Khalil and N. A. Barakat, Int. J. Hydrogen Energy, 2016, 41, 4251–4263.
12. M. A. Islam, M. R. Khan, A. Yousuf, W. C. Wai and C. K. Cheng, 2016 4th International Conference on the Development in the in Renewable Energy Technology (ICDRET), 2016.
13. A. Wheatley, Anaerobic digestion: a waste treatment technology, Chapman & Hall, 1990.
14. E. T. Kasem, T. Tsujiguchi and N. Nakagawa, Key Eng. Mater., 2013, 534, 76–81.
15. J. R. Kim, Y. Zuo, J. M. Regan and B. E. Logan, Biotechnol. Bioeng., 2008, 99, 1120–1127.
16. J. Wei, P. Liang and X. Huang, Bioresour. Technol., 2011, 102, 9335–9344.
17. B. H. Kim, H. Park, H. Kim, G. Kim, I. Chang, J. Lee and N. Phung, Appl. Microbiol. Biotechnol., 2004, 63, 672–681.
18. B. Li and B. E. Logan, Colloids Surf., B, 2004, 36, 81–90.
19. D. Jiang and B. Li, Water Sci. Technol., 2009, 59, 557.
20. A. Morozan, L. Stamatin, F. Nastase, A. Dumitru, S. Vulpe, C. Nastase, I. Stamatin and K. Scott, Phys. Status Solidi A, 2007, 204, 1797–1803.
21. D. A. Lowy, L. M. Tender, J. G. Zeikus, D. H. Park and D. R. Lovley, Biosens. Bioelectron., 2006, 21, 2058–2063.
22. Y. Fan, S. Xu, R. Schaller, J. Jiao, F. Chaplen and H. Liu, Biosens. Bioelectron., 2011, 26, 1908–1912.
23. J. R. Kim, B. Min and B. E. Logan, Appl. Microbiol. Biotechnol., 2005, 68, 23–30.
24. H. Hussein, S. Farag, K. Kandil and H. Moawad, Process Biochem., 2005, 40, 955–961.
25. K. Guo, S. Freguia, P. G. Dennis, X. Chen, B. C. Donose, J. Keller, J. J. Gooding and K. Rabaey, Energy Environ. Sci., 2013, 47, 7563–7570.
26. J.-J. Sun, H.-Z. Zhao, Q.-Z. Yang, J. Song and A. Xue, Electrochim. Acta, 2010, 55, 3041–3047.
27. X. Tang, K. Guo, H. Li, Z. Du and J. Tian, Bioresour. Technol., 2011, 102, 3558–3560.
28. L. Peng, S.-J. You and J.-Y. Wang, Biosens. Bioelectron., 2010, 25, 1248–1251.
29. T. Sharma, A. L. M. Reddy, T. Chandra and S. Ramaprabhu, Int. J. Hydrogen Energy, 2008, 33, 6749–6754.
30. C. Li, L. Zhang, L. Ding, H. Ren and H. Cui, Biosens. Bioelectron., 2011, 26, 4169–4176.
31. G. Hoogers, Fuel cell technology handbook, CRC press, 2002.
32. J. Larminie, A. Dicks and M. S. McDonald, Fuel cell systems explained, J. Wiley, Chichester, UK, 2003.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22867d

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