Enhancing the efficiency of medium-scale dual-chamber microbial fuel cell systems through the utilization of novel electrodes material and proper selection of catholyte and external resistance

Abstract

Dual-chamber microbial fuel cells (DC-MFC) are devices that can be used to generate electricity through the degradation of substrates. In this study, the performance of DC-MFC with novel electrode materials is evaluated under different external resistance using a hydrochloric acid solution as catholyte. Hydrophilic-treated graphene was used as the electrode material, DuPont^TM Nafion 117 was used as the proton exchange membrane and domestic wastewater served as the substrate. The maximum power density achieved was 32.05 $mW\cdot m^{-2}$, obtained by degrading 69.8% of organic matter when an external resistance of 100 Ω was used as electrical load. This power density was 32 times higher than the power density obtained in the control (1.01 $mW\cdot m^{-2}$). This result obtained was similar to those reported in the literature for small-scale DC-MFC systems. And, contrary to the reported trend that as the scale of MFCs increase, the efficiency decreases. It can be stipulated with these results that is possible to scale up DC-MFC to medium-scale systems without loss performance quality by selecting the appropriate external resistance, catholyte and electrode materials.

Graphical abstract

Highlights

• •Medium-scale DC-MFCs have demonstrated efficient performance and scalability.
• •An external resistance of 100Ω ensures better performance in medium-scale DC-MFCs.
• •HCl as a catholyte enhances the degradation of substrate in medium-scale DC-MFCs.

1. Introduction

Microbial fuel cells (MFCs) are electrochemical devices that use the abilities of electrogenic microorganisms to promote the generation of electricity through the degradation of substrates such as wastewater [1]. Utilizing wastewater as a substrate in MFCs offers distinct advantages over pure organic compounds (glucose, acetate, butyrate, etc.), such as cost-effective substrate, contributing to wastewater treatment and simultaneously converting organic matter in wastewater into electrical energy [2]. This highlights the potential of MFCs as an alternative solution for properly managing wastewater and producing clean energy sources.

However, the application of MFCs in domestic and industrial scenarios still presents challenges, such as low energy efficiencies and high investment costs. The main factors that directly impact these MFCs challenges include the design used at scaling up, the materials used as electrodes, the external resistance load, and the catholyte. In the literature, extensive research on the performance of laboratory-scale MFCs is readily available, whereas studies on their efficiency at a large-scale are comparatively scarce. While extensive research on small-scale MFCs (less than 300 mL) reports power densities close to 100 $mW\cdot m^{-2}$, or even higher power densities, around 1 to 3 $W\cdot m^{-2}$ but using pure organic compounds as substrates ([3], [4], [5], [6]. Fewer studies focused on pilot-scale MFCs (100 L - 1,000 L), report power densities around 1.4 $mW\cdot m^{-2}$, that are well bellow those of small-scale systems [7]. The main difficulties hindering the practical application of MFCs in society and industry include the challenges of transferring efficiencies obtained in small-scale to large-scale MFCs. A trend has been observed in which as the scale-up of MFCs increases, efficiencies decrease [8], [9], in addition to the high cost of some system components, such as platinum as electrode material, some materials used as catalyst, and proton exchange membrane [10], [11], [12]. These components operate adequately and contribute to high efficiencies when used in small-scale MFCs. However, they become economically unsustainable if used in large scale MFCs, then exploring the performance of medium-scale and pilot-scale MFCs configurations can contribute to addressing these challenges.

The correct selection of the electrode material has a direct effect on MFCs performance. Carbon-based and metal-based materials are the most commonly used option for MFCs. Among them, carbon-based materials have stood out for their unique characteristics, such as its high conductivity, biocompatibility, and chemical stability, compared to metal-based materials [13], [14]. While high efficiencies have been reported with carbon based electrodes in MFCs, those came mostly from small-scale applications and using pure substrates [7]. It is worth to point out that the efficiency measurement in MFCs involves power density analysis, which takes into account the current, voltage, and the external resistance load [15].

External resistance is a component that is chosen by the user and its load affects the flow of electrons between the chambers [16]. It has been studied that the selection of a load resistance close to the MFC internal resistance maximizes the power production [17]. However, this behavior requires more research as MFCs scale up. in order to understand the effects of external resistance on the MFCs internal dynamics and identify the values that favors the MFCs overall performance. For example, it has been reported that the emission of $C{H}_4$ (methane) from wetland-MFC can be reduced when external resistance is connected [18]. Recent studies Another parameter that influences MFCs performance is the catholyte. Although water is commonly used in the cathodic chamber and has well-documented the positive effects on MFC performance, exploring the impacts of different catholytes in more detail will allow this parameter to be optimized to enhance the overall performance of MFCs

To address these challenges of low energy efficiencies and high investment costs, recent published work focused on optimizing the performance of MFCs, through the research of electrode and membrane materials, substrate, catholyte, pH, microbial activity, MFC configuration and scale, internal resistance, and external resistance [19], [2], [20]. However, further studies are needed to achieve optimal performance of MFCs at scales that impact the society and contribute to green energy generation and wastewater treatment. Therefore, the aim of this research is to enhance the ability of the MFCs to degraded organic matter in wastewater and to produced bioenergy. This was achieved through the evaluation of medium-scale dual-chamber microbial fuel cell (DC-MFC) systems using domestic wastewater as a substrate. A novel hydrophilic-treated graphene (HTG) material was used as electrodes, because, according to the results obtained in the previous work, HTG electrodes showed better MFCs efficiency compared to other carbon-based materials [21]. Hydrochloric acid (HCl) solution was used as catholyte. Four different external resistances (10 Ω, 100 Ω, 1.0 kΩ, 2.2 kΩ) were testing to identify the closest optimal. Additionally, control experiments were conducted for comprehensive comparison and analysis.

2. Methodology and materials

2.1. Experimental device

The dual-chamber microbial fuel cell was made of acrylic tube of 15.0 cm diameter as is shown in Fig. 1a. The total volume of the anode and cathode chambers was 3.85 L and 2.23 L, respectively. A perfluorinated Nafion® 117 (Sigma-Aldrich Quimica S de RL de CV, Toluca, México) was used as a proton exchange membrane (PEM). The membrane was previously activated following the procedures described by Lavorante et al. and Hasani-Sadrabadi et al. [22], [23]. Hydrophilic-treated graphene (HTG) plates (Hitachi Chemical co., Itd, Tokyo, Japan) were used as anode and cathode with a thickness of 2 mm and dimensions of 21.0 × 13.4 cm and 14.0 × 13.4 cm, respectively (Fig. 1b). The main characteristics of the HTG material such as surface composition, surface area and pore size, were performed in the previous work by F. Borja-Maldonado and M. Lopez Zavala [21]. The surface of the anode was sanded evenly to favor the attachment of the microorganisms. Electrical connection from chambers to external load resistor were made with AWG-16-gauge wire and a resistor connects both chambers. Resistors with values of 10 Ω, 100 Ω, 1.0 kΩ, and 2.2kΩ were used. The anode chamber was filled with 2.4 L domestic wastewater and 1.0 L of inoculum, and then magnetically mixed with a digital stirring hotplate (Cimarec™ Thermo Scientific, Monterrey city, México). The cathode chamber was filled with 2.15 L of 0.1 M HCl solution as catholyte. Oxygen-free conditions in the medium-scale DC-MFC system were promoted by first bubbling nitrogen gas into both chambers, followed by applying a vacuum (27.5 kPa) to expel air and dissolved oxygen, and the system was then perfectly sealed. A ProSense digital pressure sensor with a range of −100 kPa to 1 MPa was used in both chambers to monitor the pressure.

Figure 1.

Experimental device. (a) DC-MFC prototype, (b) HTG electrodes.

2.2. Inoculum preparation

The inoculum used in the experiments was prepared by acclimatizing activated sludge under anaerobic conditions, using the following procedure. First, samples of activated sludge from the aerobic bioreactor and raw wastewater were collected from the wastewater treatment plant “Dulces Nombres” located in Monterrey city, México. The mixtures were then placed in sealed containers, container 1 was filleted at a 1:2 proportion of activated sludge and wastewater. Container 2 was filleted in the same 1:2 proportion of activated sludge, but here the pH of the wastewater was adjusted to approximately pH 5 to promote the growth of acidogenic microorganisms, [24]. Every day, the supernatant was carefully removed by decantation to prevent any loss of inoculum. The containers were then refilled with fresh wastewater and tightly closed to promote anaerobic conditions. This procedure was carried out for a period of 10 days at a temperature of 24 ± 1 ^∘C [25].

2.3. Performance evaluation of medium-scale DC-MFC systems

The effect of 10 Ω, 100 Ω, 1.0 kΩ, and 2.2 kΩ on the capacity to treat domestic wastewater and produce energy was carried out in different scenarios, as shown in Table 1. HTG electrodes were used in all scenarios; this selection was based on the results obtained in the previous study by Borja-Maldonado and Lopez Zavala [21]. In that study, the use of HTG electrodes was demonstrated to improve the performance of DC-MFCs for power production and wastewater treatment.

Table 1.

Experimental conditions of DC-MFC evaluation.

Sc	External Resistance Ω	Anolyte	Catholyte	Operational Conditions	Electrodes	Inoculum
1	10	WW	HCl	AC and CC O2-free	HTG	Mixed consortium
2	100	WW	HCl	AC and CC O2-free	HTG	Mixed consortium
3	1,000	WW	HCl	AC and CC O2-free	HTG	Mixed consortium
4	2,200	WW	HCl	AC and CC O2-free	HTG	Mixed consortium
	Controls
5	10	WW	DW	AC O2-free CC Op-Atm	HTG	Mixed consortium
6	100	WW	DW	AC O2-free CC Op-Atm	HTG	Mixed consortium
7	1,000	WW	DW	AC O2-free CC Op-Atm	HTG	Mixed consortium
8	2,200	WW	DW	AC O2-free CC Op-Atm	HTG	Mixed consortium

Sc= Scenario; WW= Wastewater; HCl= hydrochloric acid; DW= deionized water; AC= Anodic Chamber; CC= Cathodic Chamber; Op-Atm= Open to the atmosphere.

Scenarios 1 to 4 (Table 1) were operated with HTG electrodes, using domestic wastewater as substrate, HCl as catholyte, and maintaining oxygen-free conditions in both the cathodic and anodic chambers. Scenarios 5 to 8 were operated under the same conditions with HTG electrodes, domestic wastewater as substrate, and oxygen-free conditions in the anodic chambers. The difference was in the cathodic chambers, which were filled with deionized water and left open to the atmosphere as a control. All batch tests were carried out at Temperature of 24 ± 1 ^∘C.

The evaluation of external resistance of 10 Ω, 100 Ω, 1.0 kΩ, and 2.2 kΩ and the effect on the capacity to treat domestic wastewater and produce energy was carried out in different scenarios, as shown in Table 1.

Anode supernatant samples were collected every 24 hours for analysis. The chemical oxygen demand (COD) 5520-C method, described by Standard Methods for the Examination of Water and Wastewater [26], and a Hach DRB200 Digital Reactor were used to determine the organic load of the samples. The Solids-2540 Protocol of the Standard Methods for the Examination of Water and Wastewater [26] was used to estimate volatile suspended solids of biomass. Measures of pH and conductivity were carried out using a Thermo Scientific™ Orion Star™ A211 Benchtop pH Meter, and a Thermo Scientific™ Orion 3 Star™ Conductivity Benchtop Meter, (CTR Scientific, México), respectively. The gases generated in the DC-MFC system were analyzed with a MicroGC Agilent 490 gas chromatographer, (Agilent Technologies, México city, México). The gas chromatographer was calibrated with a standard mixture of 5% of Hydrogen (H), 5% of Oxygen (O), 5% of Nitrogen (N), 5% Methane of (CH₄), 5% of Carbon Dioxide (CO₂) and 75% of Helium (He) gases.

2.4. Analyses and calculations

The potential (output voltage) generated by the DC-MFC system was monitored using a Campbells Scientific A CR850 series datalogger. Then the equations (1), (2), and (3) [27] were used to calculate the current, the power and the electrical energy, respectively.

$$I=\frac{V}{R_{ext}}$$ (1)

$$P=I\times V$$ (2)

$$E=P\times t$$ (3)

Where, V is the potential in volts (V) that is a function of the external resistance $R_{ext}$ in ohms (Ω); I the current intensity in amperes (A); P is the power in watts (W), E is the electrical energy in joules (J), t is the time in seconds (s). In addition, the power density per volume ($mW\cdot m^{-3}$) and per area ($mW\cdot m^{-2}$), and the Coulomb Efficiency ($CE\text{\%}$) (equation (4) [15]) were determined.

$$CE=\frac{{\int }_0^{t}Idt}{F{V}_I\frac{(b_0)\mathrm{\Delta }COD}{M_O}}⁎100$$ (4)

Where, I is the current intensity in amperes (A), t is the time in seconds (s), $M_O$ is the molecular weight of oxygen (32 $g\cdot mo{l}^{-1}$), $V_I$ is the volume of the mixed liquor in the anode chamber in liters (L), $b_0$ is the stoichiometric number of moles of electrons ($e^{-}$) exchanged per mole of oxygen ($4mol{e}^{-}\cdot mo{l}^{-1}$), $\mathrm{\Delta }COD$ is the change in COD ($g\cdot L^{-1}$) at time (t), F is the Faradayś constant that represents the magnitude of electric charge per mole of $e^{-}$ ($96,485Coulomb\cdot mo{l}^{-1}{e}^{-}$).

3. Result and discussion

3.1. Performance of the medium-scale DC-MFC system

The medium-scale DC-MFC operated with HCl as catholyte and 100 Ω showed maximum COD removal efficiencies in the range of 70-80%. Simultaneously, a maximum power density of 32.05 $mW\cdot m^{-2}$ was achieved.

These results are similar to those reported in the literature for scaled MFCs (Table 2), for example Goto et al. [28], reported a COD removal efficiency of 85% when evaluating MFCs with a volume of 1.5 L. Furthermore, they also reported a 47% COD removal efficiency when the MFC capacity was increased to 12 L. This demonstrated a decrease in COD removal efficiency as the scale of the MFC increased. Comparing these results with those obtained in this study, it is possible to observe that the 3.4 L MFCs scale used in this study achieved good performance in COD removal, despite the greater scale.

Table 2.

Comparison of performance and scale of MFCs.

MFC	Volume	Anode	Cathode	External	Power	COD	Reference
Configurations	(L)	material	material	resistance	Density	removal
				(Ω)	(mW ⋅ m−2)	%
SC-MFC	1,000.00	FB	AC-SSM	0.5-50	113.00	34	[59]
DC-MFC	0.45	CC	Pt-coated CC	NR	14.63	NR	[30]
SC-MFC	1.50	rGO	Pt-coated CC	3–10	2.70 Wh ⋅ m3	85	[28]
SC-MFC	12.00	rGO	Pt-coated CC	3–10	1.30 Wh ⋅ m3	47	[28]
SC-MFC	100.00	rGO	Pt-coated CC	3–10	2.10 Wh ⋅ m3	52	[28]
DC-MFC	25.00	CF	CF-CuZn	1	0.32	68	[29]
SC-MFC	11.20	GFB	Pt-coated CC	75	80.00	NR	[32]
SC-MFC	10.00	CFi-C	CFi-C	3-22	1.60	99	[31]
DC-MFC	3.85	HTG	HTG	100	32.05	70	This study

SC: single chamber; DC: dual chamber; AC: activated carbon; SSM= stainless steel mesh; rGO: reduced graphene oxide; CC: carbon cloth; GFB: graphite fiber brushes; CF: carbon felt; HTG: hydrophilic treated graphene: NR: no reported; CFi-C: carbon fiber cloth.

On the other hand, it was evident that the energy yield of this study was approximately 23 times higher than that reported in other studies of similar scale and characteristics. For example (Table 2, Das et al. [29], Nourbakhsh et al. [30], and Lu et al. [31], reported power densities of 0.32 $mW\cdot m^{-2}$, 1.39 $mW\cdot m^{-2}$ and 1.6 $mW\cdot m^{-2}$ in studies with DC-MFCs of 25 L, 0.45 L and 20 L capacity, respectively. Hiegeman et al. [32] evaluated a 45 L MFCs made up of 4 MFC of 11.2 L each one operated in a municipal wastewater treatment plant. They obtained a power density of 80 $mW\cdot m^{-2}$ but also observed a complete inhibition of the systems due to ammonia concentration >40 mg/L in the wastewater. The findings of these studies indicate that there is still considerable variation in both energy and substrate degradation efficiencies. This study demonstrate the importance of applying efforts to research focused on medium-scale and pilot-scales MFCs.

There is extensive research in the literature focused on small-scale MFC (less than 300 mL) systems. While very few have focused on the study of medium-scale MFCs (less than 100 L) and pilot-scale MFCs (100 L to 1,000 L approximately). The design features of small-scale MFCs are not always as effective when transferred to medium-scale or pilot-scale MFCs [33]. Therefore, the results obtained in this work are valuable because they contribute to the generation of information for MFCs at these scales.

3.2. Effect of external resistance on DC-MFC wastewater degradation

The influence of the external resistance on the ability of a DC-MFC to clean domestic wastewater was measured in terms of COD removal. The COD removal in a DC-MFC system using HCl as catholyte (as shown in Fig. 2a) was measured at different external resistances. The COD removal efficiencies achieved were 79.8%, 69.8%, 67.8% and 52.1% for external resistances of 2.2 kΩ, 100 Ω, 10 Ω, 1.0 kΩ, respectively.

Figure 2.

Performance of DC-MFC using different external resistances and catholyte. (a) COD removal in DC-MFC using HCl catholyte, (b) pH in DC-MFC using HCl catholyte, (c) COD removal in DC-MFC control, and (d) pH in DC-MFC control. an= anodic chamber; ca= cathodic chamber.

The rate of substrate oxidation was observed to be faster at the 100 Ω resistance compared to the 2.2 kΩ resistance during the initial hours of operation, indicating that lower external resistance promotes faster substrate oxidation [34]. Furthermore, external resistances have been reported to have an impact on the anode potential, which, in turn, influences the development of different microbial communities. As a result, various metabolic pathways can be employed for the degradation of organic matter ([35]). This behavior has also been described in previous studies. Godian et al. and Jung et al. [34], [36] reported that the population of exaelectrogenic microorganisms increased as external resistance decreased, and that lower external resistance influences the regulation of the anode potential. This, in turn, affects the competition between electrogenic and non-electrogenic active microorganisms, thus promoting high electrogenic activity [37]. It is also reported a link between external resistance and microorganisms, which suggests that a lower anode potential associated with higher external resistance could impact the behavior of the microbial community and influence the development and adaptation of diverse microorganisms [38], [39].

In addition, when HCl is used as a catholyte, it dissociates into hydronium cations ($H_{3}O$)⁺ and chloride anions ($C{l}^{-}$). The hydrogen is diffused into the anode chamber through the proton exchange membrane, causing the pH in the anodic chamber to decrease until equilibrium is reached between the two chambers, as shown in the Fig. 2b. It should be noted that the cathodic reactions occurring in this study differ from those in the conventional MFC configuration (open to atmospheric oxygen), where oxygen reduction occurs. In this study the scenario using free-oxygen conditions in both chambers of the MFC, promotes the hydrogen reduction in the cathodic chamber. However, this is still to be confirmed.

The pH values decreased from initial levels of 7-8 to approximately pH 3, this pH decrease is attributed to the diffusion of HCl from the cathode to the anode chamber through the semipermeable membrane Nafion® 117. This behavior has also been reported in the literature and indicates that when there are different pH values between the chambers of the MFCs (as observed in the scenarios operated with HCl), the pH of the chambers will change to converge [40]. The acidification process occurring in the anode chamber promotes the hydrolysis of organic matter and the growth of acidophilic microorganisms, such as acidogenic and acetogenic species, which thrive in environments with a pH below 5.5 [41], [42], [43], [24]. One advantage of promoting the acidophilic consortium is their ability to adapt to environmental fluctuations [44]. A similar behavior was observed in the assessment of electrode materials obtained in a previous work [21], which is also associated with the effect of pH on the DC-MFC performance.

Therefore, even though a maximum final COD removal (79%) was obtained using an external resistance of 2.2 kΩ, the resulting power density was very low compered to that obtained using a external resistance of 100 Ω. This could be a attributed to the participation of non-exaelectrogeneic microorganisms in the degradation of organic matter, and further studies may help to confirm this.

On the other hand, COD removal efficiencies and pH evolution were analyzed when deionized water (control) was used in the cathode chamber of the DC-MFC (as shown in Fig. 2c and 2d). The results showed a COD removal of 86.2%, 81.7%, 81.5% and 75.5% for external resistance of 2.2 kΩ, 10 Ω, 1.0 kΩ, 100 Ω, respectively, which indicate a slight improvement in COD removal efficiency when the DC-MFC was operated with deionized water in the cathode chamber. It is important to note that promoting acidophilic consortia can impact the growth of other microorganisms, including methanogens, potentially inhibiting their development. However, in the scenarios operated with deionized water, the pH (Fig. 2d) showed minimal change over time, maintaining conditions close to neutral pH. Therefore, it can be inferred that microorganisms, such as methanogens, that prefer pH close to neutral were favored in their growth. Thus, it can be attributed that they mostly participated in the oxidation of the substrate. A similar trend was observed in substrate oxidation between the scenarios using deionized water and those using HCl. The maximum COD removal of 86.2% was achieved with an external resistance of 2.2 kΩ. Furthermore, the highest COD removal (close to 70%) in the first hours of operation was observed with an external resistance of 100 Ω. Therefore, a low external resistance of 100 Ω promoted faster oxidation of organic matter in both groups of scenarios operated under HCl and deionized water [34].

Gas monitoring also provided valuable information to elucidate the effect of external resistance on the development of active microorganisms in the anode chamber. Fig. 3a displays the detection of methane ($C{H}_4$) gas in DC-MFC using HCl as catholyte, revealing relatively low concentrations of $C{H}_4$ (below 14.5%) compared to those observed in scenarios using deionized water (close to 30%), as shown in Fig. 3b. Despite these differences in concentrations, it was observed that in both groups of scenarios (with HCl as catholyte and the control), the highest methane detection was achieved with the highest external resistance of 2.2 kΩ. This behavior has also been reported in other studies, such as the work of Pinto et al. [45], they found higher $C{H}_4$ values in their evaluations of MFCs operated with resistance above estimated internal resistance values.

Figure 3.

Evolution of gases production in DC-MFC using different external resistances, (a) Methane gas in DC-MFC using HCl as catholyte, (b) Methane gas in DC-MFC control, (c) CO₂ gas in DC-MFC using HCl as catholyte, and (d) CO₂ gas in DC-MFC control. an= anodic chamber; ca= cathodic chamber.

The low detection of $C{H}_4$ gas in the anode chamber from scenarios using HCl as catholyte compared to the control scenarios, can be attributed to the low pH in the anode chamber, caused by the diffusion of HCl from the cathode chamber, which is unfavorable for the growth of methanogenic microorganisms. Conversely, the higher detection of $C{H}_4$ gas, in control scenarios (Fig. 3b), suggests that the oxidation of the organic matter was primarily carried out by methanogenic microorganisms that prefer pH values close to neutral, corresponding to these scenarios.

Additionally, a slight amount of $C{H}_4$ (less than 3%) was observed in the cathode chamber from scenarios using HCl as catholyte corresponding to the diffusion of the $C{H}_4$ gas from the anode to the cathode chamber through the Nafion® 117 membrane. Regarding the control scenarios, they were operated under open atmospheric conditions as described in section of Methodology and materials 2, consequently, no gas was detected in these cathode chambers.

Fig. 3 shows the detection of carbon dioxide ($C{O}_2$) in the DC-MFC over time. The anode chamber operated under acidic conditions exhibited a $C{O}_2$ detection of around 16% (Fig. 3c), and diffusion of $C{O}_2$ gas to the cathode chamber through the Nafion® 117 membrane was also observed. The slightly higher detection of $C{O}_2$ gas in the anode chamber compared to $C{H}_4$ gas suggests that a small proportion of methanogens may be active in these acidic conditions, however, most of the degradation was being carried out by acidophilic microorganisms. In contrast, the use of deionized water in the cathode chamber resulted in lower $C{O}_2$ gas detection (Fig. 3d), less than 5%, indicating the carbon is transformed into methane.

3.3. Effect of external resistance on DC-MFC energy production

To determine the optimal selection of the external resistance that maximizes the performance of DC-MFCs, it was necessary to include an analysis of the energy production in the MFCs systems. In each experiment, the potential, current intensity, power density, energy, and coulombic efficiency were measured according to equations (1), (2), (3) and (4). Power density produced under different external resistances is shown in Fig. 4. When HCl was used as a catholyte, the maximum power densities per area obtained were 32.05 $mW\cdot m^{-2}$, 18.12 $mW\cdot m^{-2}$, 5.41 $mW\cdot m^{-2}$ and 1.78 $mW\cdot m^{-2}$ for 100 Ω, 10 Ω, 1.0 kΩ and 2.2 kΩ (Fig. 4a), respectively. And the maximum power densities per volume obtained were 543.82 $mW\cdot m^{-3}$, 307.26 $mW\cdot m^{-3}$, 91,72 $mW\cdot m^{-3}$ and 30.23 $mW\cdot m^{-3}$ for 100 Ω, 10 Ω, 1.0 kΩ and 2.2 kΩ (Fig. 4b), respectively.

Figure 4.

Power density in DC-MFC for different external resistances (a) power density (mW ⋅ m⁻²) and (b) power density (mW ⋅ m⁻³) when HCl is used as a catholyte; (c) power density (mW ⋅ m⁻²) and (d) power density (mW ⋅ m⁻³) in the control.

On the other hand, the maximum power densities produced in the control (Fig. 4c and Fig. 4d) were 2.09 $mW\cdot m^{-2}$, 1.40 $mW\cdot m^{-2}$, 1.01 $mW\cdot m^{-2}$ and 0.11 $mW\cdot m^{-2}$ for 1.0 kΩ, 2.2 kΩ, 100 Ω and 10 Ω, respectively. These results show a significant difference between the maximum power density obtained in the scenarios operating with HCl (32.05 $mW\cdot m^{-2}$, 100 Ω) compared to the maximum power density (2.09 $mW\cdot m^{-2}$, 1.0 kΩ) obtained in the control. These results confirm those obtained in the previous work [21], which shows that the use of HCl favors the energy production in DC-MFCs.

The analysis of COD removal and energy production of all scenarios reveals that although the DC-MFC controls achieved slightly higher efficiencies in organic matter removal compared to those operated with HCl, they did not correspond with the higher energy efficiencies observed in the scenarios operated with HCl. This suggests that the COD removal in the control was mainly carried out by non-electrogenic microorganisms. This is also due to the fact that these non-electrogenic colonies growth faster because they can utilize different soluble electron acceptors and are possibly more energetically favorable [34].

Analysis of these findings revealed that the use of HCl as a catholyte and an external resistance of 100 Ω resulted in the highest power density of 32.05 $mW\cdot m^{-2}$. The value of the internal resistance (43.9 Ω) calculated from the slope of polarization curves (Fig. 5, showed a congruent linked by the proximity between the value of the external resistance of 100 Ω. This alignment promotes greater stability in DC-MFCs and consequently increases their efficiencies [35]. It was also observed that as the external resistance of the evaluation decreased from 2.2 kΩ to 100 Ω, the power density increased.

Figure 5.

Polarization curve of DC-MFC using different external resistances and HCl as catholyte.

Similar behavior has also been described in previous studies, for instance, Aelterman et al. [46] reported that when the resistance decreased from 50 to 10.5, the power density increased from 115 $mW\cdot m^{-3}$ to 203 $mW\cdot m^{-3}$. Rismani et al. [47] indicated that lower external resistance favored the electrochemical performance, they reported an increase of power density from 57.5 $mW\cdot m^{-2}$ to 66 $mW\cdot m^{-2}$ using 249 Ω and 20 Ω respectively. Gonzalez et al. [2] was also reported a decreased in the power density when the external resistance was increased. Jung et al. [36] reported a higher current flow of 248.5 $mA\cdot m^{-2}$ and anode potential -4 mV when using a low external resistance of 150 Ω, compared to 40.5 $mA\cdot m^{-2}$, and -188 mV using 9,800 Ω. They indicated that lower external resistance influences the regulation of the anode potential.

In this study the potential, current intensity, power density, current density, CE%, and COD removal are shown in Table 3. It was observed that the potentials (Fig. 6) obtained from scenarios operated with HCl and external resistance of 2.2 kΩ, 1.0 kΩ and 100 Ω (Fig. 6a) were very close to each other. It was also observed that current density increase as external resistance decrease, as was reported in previous studies.

Table 3.

Performance of DC-MFC using HTG electrodes and different external resistances.

Sc	Catholyte	External	Potential	Current	Power density	Current density	COD	CE
		Resistance Ω	mV	(mA)	mW ⋅ m−2	mA ⋅ m−3	removal %	%
1	HCl	10	102.21	10.22	18.12	177.28	67.8	10.2
2	HCl	100	430.00	4.30	32.05	74.58	69.8	9.5
3	HCl	1,000	558.42	0.56	5.41	9.69	52.1	4.2
4	HCl	2,200	475.55	0.22	1.78	3.75	79.8	0.9
5	DW	10	8.09	0.81	0.11	14.04	81.7	6.4
6	DW	100	76.49	0.76	1.01	13.27	75.5	6.0
7	DW	1,000	347.00	0.35	2.09	6.02	81.5	2.9
8	DW	2,200	421.22	0.19	1.40	3.32	86.2	1.2

Sc=Scenario, DW=deionized water.

Figure 6.

Potential and current production in DC-MFC for different external resistance (a) potential (mV) and (b) current (mA) when HCl is used as a catholyte; (c) potential (mV) and (d) current (mA) in control.

Similar studies, such as, Ni et al. [43] and Sulonen et al. [48] evaluated a MFCs at pH 2 and pH below 2.5, operated both systems with 1.0 kΩ and They reported a maximum power density of 2.5 $mW\cdot m^{-2}$ and 13.9 $mW\cdot m^{-2}$ respectively. This demonstrates that the maximum power density (32.05 $mW\cdot m^{-2}$) obtained in this study from the scenario operated with HCl as the catholyte was higher than these results obtained under similar conditions (acidic pH).

On the other hand, in the scenarios operated with deionized water (Fig. 6c), a clear decrease in potential was observed as the electrical resistance decreased. Other studies have also reported this behavior. For instance, Pasternak et al. [49] indicated that MFCs operated with low external resistance improved energy production and decreased internal resistance. Menicucci et al. [19] evaluated a DC-MFC using external resistance from 6 Ω to 0.125 KΩ and observed that the voltage decreased when the external resistance also decreased. Ghangrekar and Shinde [50] also showed that the voltage decreased when the external resistance decreased from 4,000 Ω to 1.0 kΩ.

The scenarios operated with HCl in this work (Fig. 6a) also showed minimal change in the open circuit potential and potential values after connecting the external circuit. The open circuit potentials were 540 mV, 548 mV, and 468 mV, which changed to potentials of 430 mV, 558.4 mV, and 475.6 mV after connecting the 100 Ω, 1.0 kΩ, and 2.2 kΩ external resistor, respectively. In contrast to the control scenarios in which the open circuit of 610 mV, 570 mV, 643 mV, and 358 mV changed to 4.09 mV, 60 mv, 338.32 mV, and 85.97 mV, after connecting the external resistance of 10 Ω, 100 Ω, 1.0 kΩ, and 2.2 kΩ, respectively (Fig. 6c). Therefore, it is possible to indicate that the use of HCl as a catholyte also provides an advantage over the use of deionized water in the cathode chamber. This could be linked to the observation that HCl dissociates almost entirely in water, making it an effective electrical conductor and a suitable electrolyte [51]. Consequently, this fact enhances the conductive properties of DC-MFCs, allowing them to maintain a potential differential between the chambers and promoting electron flow, higher conductivity, reduction of ohmic losses and increased energy production [52]. Conversely, a correlation between external resistance and electrical current was observed in all scenarios, indicating that as the electrical resistance decreased, the electrical current increased (Fig. 6b and 6d). This can be attributed to the fact that lower electrical resistance facilitates higher electron transfer to the cathode, resulting in faster cathode reactions and a higher level of electrogenic activity [2], [53].

The lowest power densities obtained in this investigation were 1.78 $mW\cdot m^{-2}$ for the scenario with 2.2 kΩ of external resistance and HCl as catholyte, and values below 2.09 $mW\cdot m^{-2}$ of the control scenarios. These conditions seem to be more susceptible to the adverse effects of losses than the scenario in which 100 Ω external resistance and HCl were used. In accordance with the present results, similarities have also been demonstrated in previous studies. Pasternak et al. [49] indicated that MFCs operated with low external resistance improved energy production and decreased internal resistance. Menicucci et al. [19] evaluated a DC-MFC using external resistance from 6 Ω to 0.125 KΩ and observed that the voltage decreased when the external resistance also decreased. Ghangrekar and Shinde [50] also showed that the voltage decreased when the external resistance decreased from 4,000 Ω to 1.0 kΩ. The behavior of decreasing potential as the external resistance decreases can be due to the modifications of metabolic activity in microorganisms [54], [55], and the presence of overpotentials in the MFCs. Contrary, higher external resistances can reduce electron transfer by decreasing the activation of exoelectrogenic microorganisms [35].

Variation in energy production of DC-MFCs with different external resistance are also linked to the activation losses at the anode. It can be inferred that the use of lower external resistance reduces the ohmic losses compared to higher external resistances [39], [54]. This includes variations in activation losses, which represent the energy (heat) used by active microorganisms to initiate oxidation and reduction reactions, as well as electron transfer to the anode surface. The optimal external resistance is the one that allows active microorganisms to balance the reduction kinetics with the potential energy gain, resulting in increased energy production in MFCs [2].

Based on these findings, it is stated that using HCl as a catholyte and an external resistance of 100 Ω, can effectively reduce ohmic losses in DC-MFCs. This leads to a decrease in the resistance exerted by the anolyte (wastewater) and the catholyte (HCl or deionized water) to ion flow, as well as to the electron transfer through the electrodes and electrical connections [15], [56], [55]. Moreover, these parameters have the potential to promote the growth of electrogenic microorganisms, thereby enhancing the power density [2], [57].

These observations make evident the influence of external resistance on the anode potential, electrical potential and current, power densities and coulombic efficiency in MFCs [57]. To maximize the production of bioelectricity in the MFCs, it is suggested to identify the external resistance that allows operating at the lowest potential at the anode and the highest potential at the cathode [2], [58].

4. Conclusion

The assessments conducted in this study highlight the potential of scaling up DC-MFCs to medium-scale systems (3.85 L), and operated with HTG plate electrodes, an external resistance of 100 Ω, and HCl as a catholyte, were shown to maintain and improve the efficiency of domestic wastewater treatment and bioenergy production. It is confirmed, as reported in the previous study, that HTG material as electrodes favors the performance of MFCs. Moreover, in comparison with literature results that report a trend in which better efficiencies are obtained in small scale MFCs, and that on the other hand efficiencies are affected or decrease when MFCs increase size. It can be highlighted that the medium-scale MFCs evaluated in this study together with an adequate selection of parameters such as electrodes, external resistance and catholyte demonstrate that it is possible to increase the size in MFCs without sacrificing efficiency. In addition, the potential of wastewater as a valuable resource for generating energy and obtaining reusable water is also confirmed, challenging the traditional notion of it being a harmful waste. The limitation of this study lies in the sensitivity of cell efficiencies to small changes in operational parameters and scale increments. Therefore, it is recommended to conduct further studies focused on the effects of parameters such as variation in external resistance on the MFCs performance at large scale. This will complement and generate valuable information to identify factors that can decrease load losses and those that improve the ability of MFCs to treat wastewater and produce bioenergy.

CRediT authorship contribution statement

Fátima Borja-Maldonado: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Miguel Ángel López Zavala: Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All relevant data are included within this article, tables and references.