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Article

Performance Comparison of Different Cathode Strategies on Air-Cathode Microbial Fuel Cells: Coal Fly Ash as a Cathode Catalyst

by
Asimina Tremouli
1,*,
Pavlos K. Pandis
1,
Theofilos Kamperidis
1,
Christos Argirusis
1,
Vassilis N. Stathopoulos
2 and
Gerasimos Lyberatos
1,3
1
School of Chemical Engineering, National Technical University of Athens, 15780 Athens, Greece
2
Laboratory of Chemistry and Materials Technology, Department of Agricultural Development, Agrofood and Management of Natural Resources, National and Kapodistrian University of Athens, Psachna Campus, 34400 Psachna, Greece
3
Institute of Chemical Engineering Sciences (ICE-HT), Stadiou Str., Platani, 26504 Patras, Greece
*
Author to whom correspondence should be addressed.
Water 2023, 15(5), 862; https://doi.org/10.3390/w15050862
Submission received: 21 December 2022 / Revised: 1 February 2023 / Accepted: 21 February 2023 / Published: 23 February 2023
(This article belongs to the Special Issue Biological Technology for Wastewater Treatment)
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Abstract

:
The effect of different cathode strategies (mullite/MnO2, Plexiglas/Gore-Tex/MnO2, mullite/coal fly ash, mullite/biochar, mullite/activated carbon) on the performance of air-cathode microbial fuel cells (MFCs) was investigated. The highest maximum power output was observed using MnO2 catalyst pasted on Gore-Tex cloth (7.7 mW/m3), yet the highest coulombic efficiencies (CEs) were achieved using MnO2 (CE 23.5 ± 2.7%) and coal fly ash (CE 20 ± 3.3%) pasted on ceramic. The results showed that the utilization of coal fly ash and biochar as catalysts in MFC technology can be a sustainable and cost-effective solution.

1. Introduction

Over recent decades, microbial fuel cells (MFCs) have attracted research interest because of their unique feature of treating wastewater while producing electricity. Microbial fuel cells (MFCs) are systems in which microorganisms function as catalysts and convert the chemical energy contained in the chemical bonds of biomass and waste directly to electrical energy [1,2]. The direct power production during wastewater treatment using the MFC process could be a solution to current issues that conventional wastewater treatment practices face.
In particular, aerobically activated sludge requires large amounts of energy for aeration, recirculation and wastewater pumping. An MFC could be used in a treatment system as an alternative to an energy-demanding activated sludge system, resulting in net energy production rather than consumption. Moreover, the activated sludge process produces large amounts of sewage sludge, since it is an aerobic process, compared to use of anaerobic MFC technology, which generates only a small amount of microbial mass. Sludge handling drastically increases the operational and energy costs of typical wastewater treatment plants [3,4]. Currently, anaerobic digestion (AD) is widely applied as an alternative method for wastewater treatment since it saves energy sources and is highly effective in converting organic chemicals into methane (CH4) gas. However, AD technology, unlike the MFC process, is not, in general, feasible when treating low-strength wastewater, such as municipal wastewater. Additionally, the use of AD as an electricity producing process is a two-step process (methane generation followed by burning in an internal combustion engine), in contrast to the MFC system, which produces electricity directly [5].
Although much effort has been applied towards the practical implementation of MFC technology in the field of wastewater treatment, there are still practical barriers to overcome before utilization of these systems is possible. The main obstacles are the low power output obtained, due to high internal resistance and current instability, as well as the high costs of the materials [6]. In this context, the practical implementation of the technology can be achieved through an MFC system that has the appropriate design for scaling up and can be conveniently combined with existing wastewater treatment facilities, while ensuring high performance using cost-effective construction materials.
In this direction, several air-cathode, membrane-less, single-chamber configurations have been examined since these designs increase power production and reduce the capital cost of MFCs [7,8]. In such systems, various inexpensive and sustainable materials have been tested as separators and as cathode catalysts [9,10]. In this regard, ceramics are very promising materials to be used as separators in MFCs due to their wide availability, low cost, structural stability, durability and environmental friendliness when compared to other materials [11]. Several studies can be found in the literature using different types of ceramics as separators while treating wastewater [12,13,14,15,16]. In addition, in order to overcome the low performance of air-cathode systems because of the poor cathode oxygen reduction reaction (ORR) [17], the use of platinum has been widely examined [18]. However, since the use of Pt hinders the practical implementation of MFC technology due to its high cost and low availability, platinum cathode catalysts have been replaced by carbon-based, metal-free, transition-metal-oxide-based catalysts, as well as metal-nitrogen-carbon catalysts [19].
Beyond the cost-effectiveness of carbon metal-free catalysts, these materials have recently attracted significant attention because they do not suffer from crossover effects and have long-term operational stability [20]. In particular, activated carbon has a high specific surface area, rich porous structure, high mechanical strength and stable properties, as well as excellent acid/alkali resistance [21]. In the same context, biomass-derived black carbon (biochar) has recently attracted attention as an electrode material in MFC technology since it is a cost-effective and environmentally friendly material [22,23]. In addition, manganese dioxide is a transition metal oxide that has been extensively used as a cathode catalyst in MFC systems due to its environmentally friendly characteristics, good electrocatalytic activity and chemical stability [8,24,25].
Coal fly ash (CFA) is a solid waste that is produced by the combustion of coal in power plants. Nowadays, CFA is stored in landfills and ponds or is disposed of by simple stack. These methods are not environmentally friendly, since CFA contains contaminants, such as mercury, cadmium and arsenic, and, without proper management, these contaminants can pollute waterways, groundwater, drinking water, and the air [26]. However, CFA mainly consists of oxides, such as SiO2, Al2O3, Fe2O3, TiO2 and CaO [27], and can be considered as a valuable raw material and source of transition metals. In this context, currently fly ash is reused in sectors such as cement, concrete, structural fill, soil stabilization and agriculture [27]. Considering that less than 30% of the total amount of fly ash generated is reused, other exploitation options for commercial applications need to be explored [27]. Only, recently, Jia et al., 2018 [28], suggested the exploitation of FA mixed with sewage sludge as the anode electrode in an MFC system, whereas Chen et al., 2022 used municipal solid waste incinerator bottom ash as an electrode plate in a dual chamber MFC since FA contains several different metals and metal oxides and can potentially be used as a cathode catalyst in an MFC unit [29].
In this study, we developed five different strategies to improve the cathode performance of a single-chamber, four-air cathode MFC (4ACMFC) seeking to use cost-effective and environmentally friendly materials [30]. Specifically, we manufactured five identical 4ACMFC systems using different cathode electrode assemblies. In particular, MnO2 was used as a catalyst, while both Gore-Tex cloth and mullite were used as separators and cathode catalyst supports. In addition, activated carbon, biochar and CFA were placed on mullite tubes and their effectiveness as cathode catalysts was examined and compared. The performance of the 4ACMFCs was assessed in terms of organic matter (COD) removal and electricity production. Electrochemical characterization of the systems using impedance spectroscopy was also performed. The key novel contribution of the present work lies in the fact that, for the first time, a four-air cathode, single-chamber microbial fuel cell, which was constructed using specific selection and combination of materials, was examined using the above cathode strategies, whereby, to the best of the authors’ knowledge, CFA was applied for the first time on a ceramic and used as a cathode electrode in an MFC system.

2. Materials and Methods

2.1. MFC Construction

Five identical MFCs, but with different cathode assemblies, were constructed based on a 4ACMFC design [24,30,31,32]. The new units (V = 659 cm3) consisted of a single Ertalon® chamber, as shown in Figure 1c. The total height of each cell was 11.5 cm, whereas the “active height” (anode chamber) was 8 cm (Figure 1b). The internal diameter of each unit was 11 cm. One circular Ertalon® lid sealed the top of the anodic chamber. The lid was constructed to fit up to four tubes with an external diameter of 2 cm (Figure 1a). The tubes ran through the chamber and served as separators and as support for the cathode electrocatalyst. The new units were manufactured taking advantage of the positive aspects and avoiding the negative aspects of the 4ACMFC design [24,30] based on experience of its operation. Specifically, the new units have more flexibility and stability in their construction characteristics. In particular, the original 4ACMFC unit was made of Plexiglas [30,32], whereas, in the case of the new units used in this work, this material was replaced by Ertalon. The original 4ACMFC unit was constructed by joining a Plexiglas conical base and a circular lid to a cylinder, while the new units consisted of one cylinder with one top lid (Figure 1b). By making this modification, the design was made more compact and less sensitive to leakages. In addition, the cathode tubes which run through the cylinder had “plug and play” characteristics, which means they could be easily removed and replaced (Figure 1b,c). On the other hand, the cathode tubes of the 4ACMFC were silicon-glued and were removed with difficulty.
Graphite granules (type 00514, Le Carbone, Belgium), with diameters ranging between 1.5 and 5 mm, were used as the anodic biofilm support and conducting material, conveying electrons to a graphite rod (13 cm long by 7 mm diameter) inserted into the packed bed of granules with an active volume of 150 cm3. The cathode tubes were open to the atmosphere and no special aeration was employed.

2.2. Electrode Assembly Preparation

Four different ceramic cathode assemblies were prepared using MnO2 (EMD Tosoh Hellas) (M-MnO2), activated carbon (AC) (Sigma-Aldrich, CAS 7440-44-0), wood biochar (BC) (15–150 μm) and coal fly ash (CFA) (acquired from Megalopoli Power Plant in Arcadia, Greece) as cathode catalysts. Mullite, (Bonis S.A., Athens, Greece) (external diameter 25 mm, internal diameter 20 mm, porosity 18–20%), was used as the separator and catalyst support. In addition, Plexiglas tubes (2 cm diameter and 2 mm thickness) were used for the construction of one MFC unit (P-MnO2). The tubes were uniformly perforated with circular holes (2 mm diameter) [9].
A prior mix containing 5 g of each of the above material/catalysts with 10 mL of graphite paint (HSF54–YSHIELD, Germany) was prepared and diluted in an EtOH/2-propanol 1:1 vol% of 20 mL. The slurry was sonicated for 30 min in order to be homogenized, and then a paste was formed. The paste was brushed onto the inner surface of the ceramic tubes as reported elsewhere [10]. In the case of the Plexiglas tubes, the cloth was tightly bound on the outside wall of each perforated tube and the side covered with the MnO2 coating was placed at the air-facing side. The final form of the ceramic cathodes is depicted in Figure 2.

2.3. Operation

The five units were operated under identical conditions in order to compare the performance of the different cathode assemblies. The enrichment and adaptation of the electrochemically active bacteria were performed in batch mode, under a fixed external load of 100 Ω for each cell. During inoculation, 10% v/v of anaerobic sludge collected from the Lykovrysi Wastewater Treatment Plant of Athens, Greece, was used as the inoculum, whereas glucose-based synthetic wastewater (1 gCOD/L) was used as the feed [9]. Following the enrichment of cells, only glucose-based synthetic medium (1 gCOD/L) was used.

2.4. Analytical Methods

The performance of the units was evaluated in terms of COD removal efficiency, and CE and volumetric power density, normalized for the anodic liquid volume (150 cm3). The COD removal efficiency and the CE were calculated as described by Logan et al. [33]. The voltage of the units was recorded at 2 min intervals by a Keysight LXI Data Acquisition system. The measurements of pH and conductivity were conducted using digital instruments WTW INOLAB PH720 and WTW INOLAB, respectively. The soluble COD was measured according to standard methods [34].
All electrochemical experiments were carried out using a potentiostat–galvanostat (PGSTAT128N—AUTOLAB) with a two-electrode set-up. Linear sweep voltammetry (LSV) was conducted from open current voltage (OCV) to short circuit, with a step of 0.005 mV/s, in order to estimate the polarization curves of the cell. In addition, electrochemical impedance spectroscopy (EIS) was performed in the whole cell in order to calculate the internal resistances and further justify the results. A two-electrode setup was applied for the above measurements. The working electrode of the potentiostat was connected to the anode side, whilst the reference and counter electrode were connected to the four cathodic electrodes. The data were collected over the frequency range of 100 kHz–1 mHz, applying a stimulus of a 10 mV amplitude regarding the sinusoidal signal. The electrical equivalent circuit used to simulate the EIS curves was the same as reported elsewhere [10] and is depicted as an inset in the Nyquist Diagram in Section 3.3.

3. Results and Discussion

3.1. MFC Operation

In order to assess the different cathode strategies, following the acclimation of the units, the MFCs operated in batch mode for approximately 480 h. Figure 3 demonstrates the current output profiles produced from the five units, while the CE and COD removal efficiency from MFCs equipped with the different cathode materials (P-MnO2, M-MnO2, CFA, BC, AC) are summarized in Figure 4 and Figure 5.
The results obtained showed that the P-MnO2 assembly achieved the highest Imax of 2.7 ± 0.2 mA, followed by M-MnO2 with 2.1 ± 0.1 mA, BC with 1.5 ± 0.1 mA, CFA with 1.5 ± 0.2 mA and AC with 0.3 ± 0.1 mA. Although the maximum current output was obtained from the P-MnO2 assembly, the M-MnO2 unit outperformed in terms of coulombic efficiency (CE), with the second highest CE value achieved from the CFA unit. In particular, the CE values were 23.5 ± 2.7%, 20 ± 3.3%, 17.5 ± 3.6%, 7.6 ± 1.5%, 3 ± 1%, for the M-MnO2, CFA, P-MnO2, BC and AC units, respectively. Regardless of the performance of the MFCs in terms of current output and CE, all the systems successfully removed the organic substrate of the synthetic wastewater. Specifically, the following COD removal efficiencies were obtained: 94 ± 5% (AC), 93 ± 4% (BC), 91 ± 3% (M-MnO2), 89 ± 3% (P-MnO2), 85 ± 5% (CFA). Despite the different cathode assemblies used, the high COD removal efficiency values confirmed the presence of non-electrogenic bacteria in the anode chamber, which also consume organic matter of the wastewater along with the electrogenic bacteria [35].

3.2. Effect of the Different Cathode Strategies on the Polarization Performance of the MFC Units

Figure 6a,b show the dependence of the MFC voltage, V, and the produced power volumetric density, P, on the current density passing through the units with the different cathode assemblies. As shown in Figure 6a, the open circuit voltage (OCV) was ~0.44 V for the assemblies M-MnO2, CFA, BC and AC, whereas, the OCV value for the P-MnO2 configuration was 0.52 V. The P-MnO2 configuration achieved the highest maximum power volumetric density at 7.7 mW/m3, followed by M-MnO2 with 5.5 mW/m3, CFA with 2.9 mW/m3, BC with 1.9 mW/m3 and AC with 1.1 mW/m3 (Figure 6b).
In addition, the internal resistances (Rin) of the CFA, BC and AC assemblies, as determined by the power density peak method, were 133 Ω, 177 Ω and 298 Ω, respectively. Moreover, the almost constant slope of the polarization curves of these units (Figure 6a) indicated the very significant contribution of ohmic losses (ohmic overpotential) for the CFA, BC and AC assemblies. The internal resistance for the MnO2 catalyst was lower when compared to the other catalysts, regardless of the structural support used (Rin was 64 Ω and 70 Ω for P-MnO2 and M-MnO2, respectively). On the other hand, when an MnO2 catalyst was used, in addition to the ohmic losses that occurred, a rapid voltage drop was also observed at high current densities, indicating mass transport limitations (Figure 6a).

3.3. Electrochemical Impedance Spectroscopy Characterization

Considering the results on the EIS of the cells, Figure 7 depicts the Nyquist Diagrams of the cells.
In Figure 7, Nyquist plots for all cell strategies are depicted. All contain one arc and Warburg diffusion. This implies that all resistances in the cells were caused by the resistance of biofilm and charge transfer resistances from the electrodes.
The numerical results were calculated through Z-fit analysis and are shown in Table 1. Prior to any comment on these values, the internal resistances were verified through EIS and had the same results as the values of internal resistances from Jacobi’s law. There were obvious differences between the values of RS, RBF and RCT between all the proposed cell modifications. The values CBF and CCT refer to the capacitance of the biofilm and the capacitance of charge transfer of the electrodes, respectively. The CBF value corresponds to the accumulation rate of the biofilm in the anode side, whilst CCT refers to the accumulation of the responsible moieties for the charge transfer [35,36,37,38,39,40]. Between M-MnO2 and P-MnO2, there were certain differences in the RCT value. The higher value in the case of P-MnO2 indicated that the electrodes increased the charge transfer resistance three times in relation to the M-MnO2. In addition, the CCT value was significantly higher in the case of P-MnO2, and this is attributed to mass transport limitations, as reported also in Figure 6a. The different cathode assemblies also contributed to different values in RCT for the rest cell strategies (CFA, BC and AC). In the case of AC, the RCT value was significantly higher and, having a low value of CCT, explained the poor electrical output of the cell. BC seemed to have operated and produced lower power output in relation to the P-MnO2, M-MnO2 and CFA strategies, and this was supported by the same trend in all the resistance values. The coulombic efficiency presented mostly the same trend as CCT for the different cell modes. The high values of CCT observed imply a larger accumulation of the species responsible for the enhancement of RCT values. In addition, the high RCT values imply reduction in the power output values and coulombic efficiency of the cells.

4. Conclusions

The operation of five different MFC cathode assemblies was assessed and compared in terms of organic matter removal and electricity generation. Although the wastewater treatment was satisfactory for all cases (COD removal efficiency ≥ 85%), in terms of power generation, the MnO2 catalyst outperformed coal fly ash (2.9 mW/m3), biochar (1.9 mW/m3) and activated carbon (1.1 mW/m3). In particular, the P-MnO2 configuration achieved the highest maximum power volumetric density (7.7 mW/m3) in comparison to M-MnO2 (5.5 mW/m3). Impedance experiments undertaken supported the results from the LSV curves and provided an explanation of the mass transport limitation of the cells. Although the MnO2 configurations obtained the best performance, the results indicated that the exploitation of coal fly ash and biochar as cathode catalysts in MFC technology is promising, but more research is needed in order to further enhance their performance.

Author Contributions

Conceptualization, A.T. and G.L.; data curation, A.T., T.K. and P.K.P.; formal analysis, A.T., P.K.P. and T.K.; funding acquisition, A.T.; investigation, A.T., T.K. and P.K.P.; methodology, A.T., T.K. and P.K.P.; project administration, A.T.; resources, A.T., C.A., V.N.S. and G.L.; supervision, C.A., V.N.S. and G.L.; validation, A.T., P.K.P. and T.K.; visualization, A.T., T.K. and P.K.P.; writing—original draft, A.T., P.K.P. and T.K.; writing—review and editing, A.T., P.K.P. and T.K. All authors have read and agreed to the published version of the manuscript.

Funding

This project has received funding from the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under grant agreement No [862].

Data Availability Statement

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photos of (a) the top lid, (b) the internal view and (c) the MFC units.
Figure 1. Photos of (a) the top lid, (b) the internal view and (c) the MFC units.
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Figure 2. Ceramic electrodes produced.
Figure 2. Ceramic electrodes produced.
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Figure 3. Current output during consecutive batch experiments of the five units with the different cathode assemblies (P-MnO2, M-MnO2, CFA, BC, AC) versus time.
Figure 3. Current output during consecutive batch experiments of the five units with the different cathode assemblies (P-MnO2, M-MnO2, CFA, BC, AC) versus time.
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Figure 4. Average COD removal efficiency values with deviations of the five units with the different cathode assemblies (P-MnO2, M-MnO2, CFA, BC, AC) versus time.
Figure 4. Average COD removal efficiency values with deviations of the five units with the different cathode assemblies (P-MnO2, M-MnO2, CFA, BC, AC) versus time.
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Figure 5. Average CE efficiency values with deviations of the five units with the different cathode assemblies (P-MnO2, M-MnO2, CFA, BC, AC) versus time.
Figure 5. Average CE efficiency values with deviations of the five units with the different cathode assemblies (P-MnO2, M-MnO2, CFA, BC, AC) versus time.
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Figure 6. MFC voltage Ucell (a) and power density (b) versus current density for the different cathode strategies (P-MnO2, M-MnO2, CFA, BC, AC).
Figure 6. MFC voltage Ucell (a) and power density (b) versus current density for the different cathode strategies (P-MnO2, M-MnO2, CFA, BC, AC).
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Figure 7. Nyquist (Bode-Bode) plots from EIS measurements.
Figure 7. Nyquist (Bode-Bode) plots from EIS measurements.
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Table
Table 1. Fitting parameters from EIS measurements.
Table 1. Fitting parameters from EIS measurements.
Fitted ParametersM-MnO2CFAP-MnO2BCAC
RS (Ω)3281317590
RBF (Ω)36353539114
RCT (Ω)21786075
CBF (F)3 × 10−32 × 10−75 × 10−32 × 10−32 × 10−13
CCT (F)9 × 10−79 × 10−30.7 × 10−45 × 10−31 × 10−3
RINT (Ω) *7013368174279
Note: * RINT is the sum of all internal resistances.
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MDPI and ACS Style

Tremouli, A.; Pandis, P.K.; Kamperidis, T.; Argirusis, C.; Stathopoulos, V.N.; Lyberatos, G. Performance Comparison of Different Cathode Strategies on Air-Cathode Microbial Fuel Cells: Coal Fly Ash as a Cathode Catalyst. Water 2023, 15, 862. https://doi.org/10.3390/w15050862

AMA Style

Tremouli A, Pandis PK, Kamperidis T, Argirusis C, Stathopoulos VN, Lyberatos G. Performance Comparison of Different Cathode Strategies on Air-Cathode Microbial Fuel Cells: Coal Fly Ash as a Cathode Catalyst. Water. 2023; 15(5):862. https://doi.org/10.3390/w15050862

Chicago/Turabian Style

Tremouli, Asimina, Pavlos K. Pandis, Theofilos Kamperidis, Christos Argirusis, Vassilis N. Stathopoulos, and Gerasimos Lyberatos. 2023. "Performance Comparison of Different Cathode Strategies on Air-Cathode Microbial Fuel Cells: Coal Fly Ash as a Cathode Catalyst" Water 15, no. 5: 862. https://doi.org/10.3390/w15050862

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