A Carbon-Cloth Anode Electroplated with Iron Nanostructure for Microbial Fuel Cell Operated with Real Wastewater

: Microbial fuel cell (MFC) is an emerging method for extracting energy from wastewater. The power generated from such systems is low due to the sluggish electron transfer from the inside of the biocatalyst to the anode surface. One strategy for enhancing the electron transfer rate is anode modiﬁcation. In this study, iron nanostructure was synthesized on a carbon cloth (CC) via a simple electroplating technique, and later investigated as a bio-anode in an MFC operated with real wastewater. The performance of an MFC with a nano-layer of iron was compared to that using bare CC. The results demonstrated that the open-circuit voltage increased from 600 mV in the case of bare CC to 800 mV in the case of the iron modiﬁed CC, showing a 33% increase in OCV. This increase in OCV can be credited to the decrease in the anode potential from 0.16 V vs. Ag / AgCl in the case of bare CC, to − 0.01 V vs. Ag / AgCl in the case of the modiﬁed CC. The power output in the case of the modiﬁed electrode was 80 mW / m 2 —two times that of the MFC using the bare CC. Furthermore, the steady-state current in the case of the iron modiﬁed carbon cloth was two times that of the bare CC electrode. The improved performance was correlated to the enhanced electron transfer between the microorganisms and the iron-plated surface, along with the increase of the anode surface- as conﬁrmed from the electrochemical impedance spectroscopy and the surface morphology, respectively.


Introduction
Given the current energy and water challenges facing the world today, new technology should be developed with the aim of facing these problems. Current energy relies on fossil fuels, which not only are limited in resources, but also result in high quantities of pollution and health risks [1][2][3]. The environmental impact of fossil fuels can be minimized through increasing the efficiency of current technologies [4,5] and/or relying on renewable energy sources [6], such as biomass energy [7][8][9], that using plain CC. The results were discussed based on the effect of the plated metal on both the surface area and the electron transfer between the anode and the microorganisms.

Anode Synthesis
The carbon cloth CC (ElectroChem, Inc.) was dipped in acetone for 10 min, then in deionized water for another 10 min. It was then dried and used as the cathode in the electroplating cell, where the graphite rod was used as the anode. The two electrodes were immersed in 0.16 M of iron chloride (Sigma-Aldrich) and were connected via power supply. The distance between the two electrodes was kept at 1 cm. The electrodes were electroplated at 5 V for 2 min. The iron modified carbon cloth (Fe-CC) was washed several times with distilled water and dried at 50 • C overnight. It was later used as an anode in the MFC.

Construction of the Microbial Fuel Cell
An air-cathode MFC was constructed with an 84 mL volume for the anode chamber as shown in Figure 1. Nafion 117 (ElectroChem, Inc.) was used as an electrolyte membrane. A ready-made Pt/C on CC of loading of 0.5 mg (fuelcellstore.com) used as the cathode (6 cm 2 ). The Fe-CC or bare CC of 6 cm 2 was used as the anode. Wastewater from the water treatment plant at Sharjah, U.A.E was used as both a substrate and a microorganism source. The initial COD was in average of 700 ppm. Two MFCs with Fe-CC and bare CC anodes were constructed and operated in parallel under the same operating conditions. Where the anode chamber of the two cells was filled with wastewater, and the two cells were operated at 25 • C in a batch mode. to that using plain CC. The results were discussed based on the effect of the plated metal on both the surface area and the electron transfer between the anode and the microorganisms.

Anode Synthesis
The carbon cloth CC (ElectroChem, Inc.) was dipped in acetone for 10 min, then in deionized water for another 10 min. It was then dried and used as the cathode in the electroplating cell, where the graphite rod was used as the anode. The two electrodes were immersed in 0.16 M of iron chloride (Sigma-Aldrich) and were connected via power supply. The distance between the two electrodes was kept at 1 cm. The electrodes were electroplated at 5 V for 2 min. The iron modified carbon cloth (Fe-CC) was washed several times with distilled water and dried at 50 o C overnight. It was later used as an anode in the MFC.

Construction of the Microbial Fuel Cell
An air-cathode MFC was constructed with an 84 mL volume for the anode chamber as shown in Figure 1. Nafion 117 (ElectroChem, Inc.) was used as an electrolyte membrane. A ready-made Pt/C on CC of loading of 0.5 mg (fuelcellstore.com) used as the cathode (6 cm 2 ). The Fe-CC or bare CC of 6 cm 2 was used as the anode. Wastewater from the water treatment plant at Sharjah, U.A.E was used as both a substrate and a microorganism source. The initial COD was in average of 700 ppm. Two MFCs with Fe-CC and bare CC anodes were constructed and operated in parallel under the same operating conditions. Where the anode chamber of the two cells was filled with wastewater, and the two cells were operated at 25 o C in a batch mode.

Electrochemical Measurements
The MFCs were connected with a GL240 multi-channel midi-logger to record the anode potential vs. Ag

Electrochemical Measurements
The MFCs were connected with a GL240 multi-channel midi-logger to record the anode potential vs. Ag/AgCl and the open-circuit voltage (OCV). The cell was operated under open circuit conditions until it reached steady state conditions, meaning a constant cell voltage and anode potential. After steady state was achieved, the MFCs were operated under closed-circuit conditions. Linear sweep voltammetry was carried out at 1 mV/s from the open-circuit voltage to zero voltage to obtain the current voltage and current power curves. Then current discharge at 0.2 V was performed for 120 min to investigate the steady-state current generation. The electrochemical impedance spectroscopy (EIS) was conducted for the whole cell at 0.2 V from 100 kHz to 100 mHz.
All the cell measurements under closed-circuit conditions, i.e., linear sweep voltammetry (from open-circuit voltage to zero voltage), current discharge at 0.2 V, and EIS measurements for the whole cell at 0.2 V, were carried out using Potentiostat (Biologic VSP-200).

Anode Characterization
The surface morphology and the elemental composition of the carbon cloth before and after modification with iron were examined using a scanning electron microscope, SEM (Tescan VEGA XMU) integrated with Energy Dispersive X-Ray (EDX) for elemental analysis. Figure 2 shows the surface morphology and elemental analysis of the carbon cloth before and after the deposition of the iron. It is clear from the figure, that the bare carbon cloth is composed of smooth fibers of around 7µm diameter (Figure 2a), that are 100% carbon (Figure 2b). After iron deposition, the surface becomes rough with a Nano sheet structure, as can be seen in (Figure 2c), where the inset shows the high magnification. The surface is composed of iron oxide, as seen in the EDX analysis shown in (Figure 2d).

Anode Characterization
The surface morphology and the elemental composition of the carbon cloth before and after modification with iron were examined using a scanning electron microscope, SEM (Tescan VEGA XMU) integrated with Energy Dispersive X-Ray (EDX) for elemental analysis. Figure 2 shows the surface morphology and elemental analysis of the carbon cloth before and after the deposition of the iron. It is clear from the figure, that the bare carbon cloth is composed of smooth fibers of around 7µm diameter (Figure 2a), that are 100% carbon (Figure 2b). After iron deposition, the surface becomes rough with a Nano sheet structure, as can be seen in (Figure 2c), where the inset shows the high magnification. The surface is composed of iron oxide, as seen in the EDX analysis shown in (Figure 2d).

Microbial Fuel Cells (MFC) Under Open-Circuit Conditions
After the injection of the anolyte solution in the anode chamber, the anode potential vs. Ag/AgCl and open-circuit voltage (OCV) were recorded and shown in Figure 3. As seen in Figure 3a, the anode potential in the case of the cell using the bare CC decreased from 0.22 V vs. Ag/AgCl to reach a minimum value of 0.16 V vs. Ag/AgCl within 30 h. Following that, it was nearly constant. In the case of using Fe-CC on the other hand, the anode potential decreased from an initial value of 0.18 V to reach a minimum value of −0.01 V vs. Ag/AgCl within 40 h. Following that, it was also nearly constant. The decrease of the anode potential in the two cells resulted in the increase in the open-circuit voltage of the two cells, as can be seen in Figure 3b [48]. The steady values of the OCV were 0.79 and 0.62 V, for MFC using Fe-CC and CC, respectively. The higher activity of the Fe-CC compared to the bare CC anode can be credited to the role of the Fe nano-layer in improving the electron transfer between microorganisms. The anode surface can also be credited, due to the role of the iron had improving the electrical conductivity of the CC, as being confirmed later using EIS. Furthermore, the high roughness and the Nano sheet structure of the Fe-CC (Figure 2c) resulted in a higher surface area available for the growth and electron transfer between the microorganisms and the anode surface.  Figure 4 shows the performance of two MFCs under closed-circuit conditions. Figure 4a shows the i-V and i-P curves for the two MFCs using the CC and Fe-CC. Modifying the CC with the Fe nanostructure enhanced both the maximum power and current densities of the MFC, as they increased from 40 mW/m 2 and 275 mA/m 2 in the case of CC to 80 mW/m 2 and 890 mA/m 2 in the case of Fe-CC, respectively. This essentially reflects a two-time increase in the power output, and more than a three-time increase in current density. However, the linear sweep voltammetry measurements,  Figure 4 shows the performance of two MFCs under closed-circuit conditions. Figure 4a shows the i-V and i-P curves for the two MFCs using the CC and Fe-CC. Modifying the CC with the Fe nanostructure enhanced both the maximum power and current densities of the MFC, as they increased from 40 mW/m 2 and 275 mA/m 2 in the case of CC to 80 mW/m 2 and 890 mA/m 2 in the case of Fe-CC, respectively. This essentially reflects a two-time increase in the power output, and more than a three-time increase in current density. However, the linear sweep voltammetry measurements, i.e., i-V measurements, describe only instantaneous electrode behavior, that could significantly change under long-term operation. Due to this, the cell was operated for around 120 min under a fixed cell voltage of 0.2 V, as shown in Figure 4b. It is clear from the figure, that the cell using the modified electrode, i.e., Fe-CC, demonstrated a significantly higher steady current density, i.e., current after two hours of cell operation, of 134 mA/m 2 , is almost two times that obtained in the case of the bare CC of 71 mA/m 2 . In both cases, the initial current density rapidly decreases, which could be related to the depletion of the excess electrons, accumulated during the OCV and before the current discharge. However, the slight decrease in the current after this with cell operation (after 10 min of current discharge) especially in the case of the Fe-CC can be attributed to the depletion of the organic materials in the wastewater [23].

Cell Operation Under the Closed-Circuit Condition
Sustainability 2020, 12, x FOR PEER REVIEW 7 of 12 min of current discharge) especially in the case of the Fe-CC can be attributed to the depletion of the organic materials in the wastewater [23]. The enhanced cell performance in terms of the higher power output and higher current density can be attributed to the role of the iron in improving the electrical conductivity of the CC, that increases the electron transfer rate between the microorganisms and the anode, as confirmed from the EIS measurements shown in Figure 5. As can be seen in the figure, the ohmic resistance of the cell with the Fe-CC electrode is around 2.5 Ohms. This equates to one-third of the ohmic resistance in the case of the bare CC electrode. Such low resistivity would result in a drastic improvement in the electron transfer from the microorganisms to the anode surface, meaning a higher power output is obtained. Additionally, looking at the linear relationship between the current and voltage in i-V curves for the two cells (Figure 4a), it is clear that the ohmic resistance in the case of the modified electrode is much lower than that in case of the bare CC electrode. Moreover, the high roughness of the modified anode with a nanosheet structure would increase the available surface area of the modified anode, as shown in Figure 2c. The enhanced cell performance in terms of the higher power output and higher current density can be attributed to the role of the iron in improving the electrical conductivity of the CC, that increases the electron transfer rate between the microorganisms and the anode, as confirmed from the EIS measurements shown in Figure 5. As can be seen in the figure, the ohmic resistance of the cell with the Fe-CC electrode is around 2.5 Ohms. This equates to one-third of the ohmic resistance in the case of the bare CC electrode. The equivalent circuits in the case of the plain CC and the modified one (Fe-CC) were represented in Figure 5b. As can be seen in the Figure 5b, the circuit is composed of ohmic resistance (Rs) as can be seen at the intersection with the X-axis at high frequency, and the slight semi circuit that appears at medium frequency is modeled by the double layer capacitance (CPE1) in parallel to charge transfer resistance (Rkin), while at low frequencies, it is modeled by another double layer capacitance (CPE2) that is in parallel to mass transfer resistance (Rmass) and both are in series with other double layer capacitance (CPE3) [62]. The values of the different resistances are shown in Table 1: It is clear from the table that modifying the carbon cloth with the Fe resulted in significantly decreasing the ohmic resistance from 7.516 to 1.846 ohm, while the charge transfer resistance (Rkin) is decreased from 60.62 ohm to 9.27 ohm. Such results confirm the role of the Fe in improving the charge transfer between the microorganisms and the anode surface.

Conclusions
The electroplating technique was used for modifying the carbon cloth with a thin layer of an iron nanostructure. The synthesized iron coated carbon cloth Fe-CC was investigated as an anode for Such low resistivity would result in a drastic improvement in the electron transfer from the microorganisms to the anode surface, meaning a higher power output is obtained. Additionally, looking at the linear relationship between the current and voltage in i-V curves for the two cells (Figure 4a), it is clear that the ohmic resistance in the case of the modified electrode is much lower than that in case of the bare CC electrode. Moreover, the high roughness of the modified anode with a nanosheet structure would increase the available surface area of the modified anode, as shown in Figure 2c.
The equivalent circuits in the case of the plain CC and the modified one (Fe-CC) were represented in Figure 5b. As can be seen in the Figure 5b, the circuit is composed of ohmic resistance (Rs) as can be seen at the intersection with the X-axis at high frequency, and the slight semi circuit that appears at medium frequency is modeled by the double layer capacitance (CPE1) in parallel to charge transfer resistance (R kin ), while at low frequencies, it is modeled by another double layer capacitance (CPE2) that is in parallel to mass transfer resistance (R mass ) and both are in series with other double layer capacitance (CPE3) [62].
The values of the different resistances are shown in Table 1: It is clear from the table that modifying the carbon cloth with the Fe resulted in significantly decreasing the ohmic resistance from 7.516 to 1.846 ohm, while the charge transfer resistance (R kin ) is decreased from 60.62 ohm to 9.27 ohm. Such results confirm the role of the Fe in improving the charge transfer between the microorganisms and the anode surface.

Conclusions
The electroplating technique was used for modifying the carbon cloth with a thin layer of an iron nanostructure. The synthesized iron coated carbon cloth Fe-CC was investigated as an anode for an MFC operated with real wastewater. Modifying the carbon cloth with a thin layer of iron significantly enhanced the performance of the MFC. This is because the power and current densities increased twice and three times, respectively, compared to those of using bare carbon cloth CC. The improved performance of the Fe-CC can be credited to the role of iron in improving the electron transfer between the microorganisms and the anode surface, as well as the increased surface area of the anode. This is evident from both the roughness and nanosheet structure of the Fe-CC electrode.