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Die Studie untersucht den innovativen Einsatz von Aloe vera in mikrobiellen Brennstoffzellen (P-MFCs) zur Steigerung der Biostromerzeugung und präsentiert eine gründliche Leistungs- und Optimierungsanalyse. Es untersucht die einzigartigen Vorteile der Aloe Vera, wie ihre Fähigkeit, unter halbschattigen Bedingungen und bei minimalem Wasserbedarf zu gedeihen, was sie zu einem idealen Kandidaten für P-MFC-Anwendungen der nächsten Generation macht. Die Studie vergleicht P-MFCs auf Basis von Aloe Vera mit anderen pflanzlichen Systemen und zeigt signifikante Verbesserungen bei Leistungsdichte und Stromdichte. Der Artikel beschreibt auch den Versuchsaufbau, einschließlich Elektrodenpositionierung, Bodenbeschaffenheit und elektrochemische Testmethoden und liefert ein umfassendes Verständnis der Faktoren, die die Leistung von P-MFCs auf Basis der Aloe Vera beeinflussen. Darüber hinaus werden die praktischen Anwendungsmöglichkeiten dieser Systeme untersucht, wie etwa die Stromversorgung mit LEDs und deren Integration in architektonische Entwürfe, wobei ihr Potenzial in nachhaltigen Energielösungen herausgestellt wird. Die Studie befasst sich auch mit den Beschränkungen und zukünftigen Forschungsrichtungen und bietet eine Roadmap zur Optimierung von Aloe Vera-basierten P-MFCs für Anwendungen in der realen Welt.
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Abstract
Plant microbialfuel cells (P-MFCs) offer a sustainable approach to bioelectricity generation by harnessing solar energy through photosynthetic processes. However, significant challenges remain regarding their efficiency, scalability, and integration into practical applications. This study addresses these gaps by evaluating the electrochemical performance of an Aloe vera-based P-MFC compared to a control microbial fuel cell (MFC) consisting solely of potting soil and graphite electrodes. Electrochemical analyses, including open-circuit voltage (OCV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS), were conducted to assess system performance. The Aloe vera-based P-MFC demonstrated a stable OCV approximately 27 mV higher, a current density 3.7 times greater, and an impedance nearly 4.7 times lower than the control MFC. Additionally, the peak power density of the Aloe vera-based P-MFC reached 1100 mW/m2, significantly outperforming the control MFC, which yielded 250 mW/m2. The superior performance of the Aloe vera-based P-MFC is attributed to the plant’s photosynthetic activity, which enhances microbial interactions and electron transfer efficiency. Notably, the successful series connection of Aloe vera-based P-MFCs facilitated the charging of a lead-acid battery, which was subsequently used to power an LED, demonstrating the system’s practical applicability. This study contributes to the advancement of P-MFC technology by highlighting Aloe vera’s potential as an efficient bioelectricity generator. By addressing current limitations and proposing future enhancements such as microbial optimization and electrode modifications, this research underscores the role of P-MFCs in sustainable energy solutions and their potential integration into architectural and interior landscape designs.
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1 Introduction
Microbial fuel cells (MFCs) have emerged as a promising clean technology for renewable energy generation. These electrochemical devices convert various carbon sources, such as wastewater, activated sludge, carbohydrates, sugars, compost, and organic substances, into electrical energy [1, 2]. The bioelectrochemical energy contained in these carbon sources is directly transformed into electricity through bacterial catalysis reactions [3, 4]. During these reactions, electrons and protons are released from organic substrates.
For effective electricity production, MFCs require anode and cathode electrodes [5, 6]. While protons migrate to the cathode electrode, electrons travel from the anode to the cathode via an external circuit. At the cathode, electrons combine with protons in the presence of oxygen obtained from the air, completing the electrochemical process. This reaction facilitates the generation of clean, green, and renewable electricity [7, 8].
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To enhance the efficiency of green electricity production in MFCs, various organic materials have been explored as substrates. These include moss [9, 10], cow manure [11], soil [12], sediment [13], and peat [14]. Although each material presents unique strengths and limitations, their primary advantage lies in their natural abundance and sustainability.
The sustainability of natural materials remains a crucial concern in renewable energy technologies. To enhance the sustainability of microbial fuel cells (MFCs), it is essential to integrate organic material production with electricity generation. The plant-microbial fuel cell (P-MFC) was developed to meet this requirement, functioning as a green energy system that simultaneously produces biomass (organic material) and electrical energy [15]. As a subset of biomass energy, P-MFCs are categorized as a renewable energy source and represent an innovative biotechnology application that harnesses solar energy through plants and microorganisms to generate electricity [16].
In a P-MFC, plants undergo photosynthesis by utilizing sunlight, absorbing carbon dioxide (CO2) from the atmosphere, and synthesizing biomass, including organic compounds such as carbohydrates. Approximately 60% of the total photosynthetically fixed CO2 contributes to plant biomass, while the remaining 40% is allocated to rhizodeposits or exudates. The microbial community within the plant’s root zone, known as the rhizosphere, degrades the organic matter accumulated in rhizodeposits or exudates, using it as a nutrient and energy source. Consequently, free electrons and protons are released in the process.
Electrons are captured by the anode electrode and transferred through an external circuit to the cathode electrode, completing the circuit. Meanwhile, protons migrate directly to the cathode via a conductive medium, such as soil, mud, or a membrane. This process establishes a redox gradient between the electrodes, enabling bioelectricity generation in the P-MFC system [17, 18].
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One of the primary factors influencing the performance of plant-microbial fuel cells (P-MFCs) is the choice of plant species and their specific characteristics. In previous P-MFC design research, wetland plant species have generally been favored. However, their high water and sunlight requirements significantly restrict the scalability and practical application of P-MFC technology in residential and commercial environments. This limitation remains a major barrier to the widespread adoption and commercialization of P-MFCs.
A potential solution to this challenge is the selection of plant species that do not require direct sunlight but can thrive in semi-shaded conditions. In this regard, the Aloe vera plant presents significant potential for next-generation P-MFC applications due to its ability to grow under oblique or semi-shaded sunlight while requiring minimal water consumption [19]. Additionally, the widespread presence of Aloe vera in homes and workplaces enhances its commercial appeal, offering expanded business development opportunities [20, 21].
From a sustainability perspective, Aloe vera is cultivated extensively across diverse geographical regions, including the Mediterranean, North Africa, the Indian subcontinent, Central and South America, and the Caribbean [22]. The global Aloe vera market is projected to expand at an annual growth rate of 7.6% between 2019 and 2025 [23].
Despite its promising potential, technical advantages, and commercial viability, research on Aloe vera-based P-MFCs remains limited. In one of the few existing studies, a P-MFC was constructed using soil, barnyard manure, Aloe vera plants, graphite electrodes, and a polyethylene terephthalate (PET) plastic container. This system generated voltage levels of up to 141.21 mV [20]. However, due to the lack of reported data on power generation or power density, assessing the overall performance and application potential of this P-MFC remains challenging.
Another study explored electricity production from Aloe vera by employing a P-MFC design in which copper and zinc electrodes were placed on Aloe vera leaves. This setup yielded a voltage of up to 0.988 mV and a power output of 1111.55 µW [21]. However, as this study focused solely on plant leaves without investigating root interactions, its findings offer limited insights into conventional P-MFC performance.
In summary, previous studies [20, 21] indicate that Aloe vera-based P-MFCs exhibit significant methodological, electrical performance, and design limitations, highlighting the need for further investigation to optimize their efficiency and applicability.
Table 1 provides a structured comparison of Aloe vera-based P-MFCs with other plant-based P-MFCs in terms of power density. This comparison helps to contextualize the performance of Aloe vera in the broader P-MFC research field.
Table 1
Comparison of power output of Aloe vera PMFC with other PMFCs
This study aims to bridge existing knowledge gaps regarding the performance and design intricacies of Aloe vera-based plant-microbial fuel cells (P-MFCs). To accomplish this objective, an experimental framework was developed, incorporating Aloe vera cultivation within soil matrices as a fundamental component of the P-MFC system. Comprehensive performance assessments were conducted to elucidate the impact of Aloe vera on the electricity-generation mechanisms within the system.
2 Materials and methods
2.1 Reaction components and preparation
Graphite electrodes were selected due to their high biocompatibility and usability [24]. These electrodes were purchased from a commercial supplier (Mass Casting, Konya, Turkey) and were in the form of cylindrical rods with a geometric surface area of 16.2 cm2. The optimal electrode spacing in a plant-microbial fuel cell (P-MFC) significantly influences power output and microbial activity. A previous study [15] reported that an electrode distance of 4.488 inches (approximately 11.40 cm) yielded optimal power generation in P-MFCs by balancing ion transport resistance and microbial electron transfer efficiency. This finding was based on systematic tests where varying electrode distances were examined for their impact on open-circuit voltage (OCV) and power density.
In this study, we adopted the 11.40 cm electrode spacing as it provided the best trade-off between electrical performance and practical applicability. Reducing the electrode distance below this threshold can increase electrochemical reaction rates but may also elevate internal resistance due to limited ion diffusion. Conversely, increasing the electrode gap beyond this point may enhance ion diffusion pathways but can also reduce electron transfer efficiency and power output.
To maximize electrode surface area and power generation efficiency, graphite electrodes were vertically immersed from the top to the bottom of the reactor. This configuration ensured maximum contact with soil bacteria while maintaining a stable redox gradient. The anode electrodes were placed directly in the rhizosphere region of the Aloe vera root system, optimizing interactions with electrogenic microbial communities and enhancing electron transfer. The cathode electrodes were positioned at the soil-air interface to facilitate oxygen reduction reactions, a critical factor in maintaining efficient cathodic electron acceptance.
Additionally, previous studies have demonstrated that electrode placement in direct contact with active microbial regions can significantly enhance charge transfer rates and bioelectricity generation [15]. This strategic electrode positioning in our study was chosen based on these insights, ensuring optimal microbial colonization and minimal charge transfer resistance.
A potentiostat/galvanostat (IVIUM Vertex.1A) was employed to analyze electrochemical performance. Electrochemical assessments were conducted using open-circuit voltage (OCV) and linear sweep voltammetry (LSV) test methods [25].
Potting soil, known for its high bacterial diversity, was selected as the growth medium [26]. Key characterization data of the potting soil used in this study are provided in Table 2. A previous study on Aloe vera cultivation identified 20% as the optimal available soil moisture level for this plant [27]. Therefore, soil moisture was maintained at 20% and monitored throughout the experiments using a soil moisture meter.
Table 2
Some characterization data of used potting soil
Characteristics
Data
pH
5.6 ± 0.1
Total organic matter
99%
Nitrate (NO3)
50–100 ppm
Phosphorus (P)
10–20 ppm
Potassium (K)
40–100 ppm
Calcium (Ca)
30–50 ppm
Magnesium (Mg)
10–20 ppm
Additionally, distilled water was utilized both to maintain soil moisture balance and to prepare the electrolyte required for electrochemical processes in the P-MFC. The use of distilled water ensured the highest possible analytical purity, preventing contamination. Given that distilled water is recognized as a suitable medium for photosynthetic MFC cultures [28], it was determined to be the most appropriate electrolyte choice for the Aloe vera-based P-MFC setup in this study.
Aloe vera plants of the Aloe Barbadensis Miller variety were selected for the experiments due to their easy availability, widespread use, and relevance for comparison with previous studies [19, 20]. Each plant was approximately one year old, with fleshy leaves measuring 10 to 15 cm in length, 1.5 to 2.5 cm in width, and 0.5 to 1 cm in thickness.
The electrical energy storage system consisted of a lead-acid battery with a supply voltage of 4.1 V and a maximum current of 1 A (MRW Power, Istanbul, Turkey). The light-emitting diode (LED) used in the study had a forward current of 25 milliamperes (mA) and a forward voltage of 2.65 V (Mouser Electronics, Inc., Germany).
2.2 Experimental conditions
The experiments were conducted under semi-shade conditions in a laboratory environment with a controlled temperature of 25 ± 2 °C, relative humidity between 56 and 65%, and at least 7 h of sunlight exposure per day. The light intensity in the semi-shade conditions was measured using a light intensity meter (UNI-T UT381A, China), which recorded values ranging between 2000 and 2500 lx.
To compare the performance of P-MFCs, two experimental setups were developed. To minimize internal resistance, a proton exchange membrane (PEM) was not used, leading to the design of single-chamber P-MFCs [2, 4].
To assess the effect of Aloe vera on electricity generation in P-MFCs, a control MFC was prepared. The control MFC was assembled by placing a graphite anode, a graphite cathode, and potting soil (522 ± 5 g) in a beaker. Water was then added to achieve 20% soil moisture, ensuring electrolyte conditions comparable to the P-MFC system. This setup was designated as the control MFC.
The Aloe vera P-MFC was created using an Aloe vera plant, a graphite anode, a graphite cathode, and potting soil. The soil served multiple functions, including supporting plant growth, facilitating ion transport as an electrolyte, and acting as a natural PEM. Consequently, the need for an additional PEM was eliminated, thereby simplifying the design, reducing material costs, and improving overall feasibility [15].
In both the control MFC and the Aloe vera P-MFC, the graphite anode electrodes were inserted from the top to the bottom of the beaker. This positioning was selected based on the known efficiency of graphite anode electrodes in anaerobic environments. The lower sections of the beaker were expected to maintain more anaerobic conditions due to reduced air exposure. Additionally, in the P-MFC, the anode electrode was placed in direct contact with the rhizosphere-the root-associated zone-to optimize microbial activity and electron transfer. For the cathode placement, in the control MFC, the graphite cathode electrode was positioned on the soil surface to facilitate oxygen reduction. In the P-MFC, the Aloe vera plant itself was fixed to the soil surface, allowing for an efficient interaction between the plant, soil microorganisms, and electrochemical processes [29].
Figure 1 shows the schematic diagram of the control sample MFC and Aloe vera based P-MFC.
Fig. 1
Schematic diagrams of control sample MFC and Aloe vera P-MFC
Open circuit potential (OCP) or open circuit voltage (OCV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) tests were conducted as part of the electrochemical characterization using a potentiostat/galvanostat device [25]. A stable OCP value is a prerequisite for obtaining polarization curves, which are essential for evaluating the performance of microbial fuel cells (MFCs). Therefore, OCP was continuously monitored until a stable value was achieved [30].
Once a stable OCP was obtained, polarization curves of P-MFCs were generated via LSV testing. The LSV test was conducted using a potentiostat/galvanostat device in a two-electrode electrochemical cell configuration. In this setup, the working electrode was connected to the anode electrode, while the reference electrode and its counterpart were connected to the cathode electrode [24]. LSV was performed with a scan rate of 1 mV/s, sweeping from the OCP value to 0 mV [25, 31]. The power output of P-MFCs was calculated by applying the voltage (V) and current (I) values obtained from the LSV tests to Eq. (1). The power density (Pd) of P-MFCs was determined using Eq. (2), where S represents the geometric surface area of the electrode [14, 25, 31].
$$P =\text{ V}.\text{I}$$
(1)
$$P\text{d}= \frac{V.I}{S}$$
(2)
Electrochemical impedance spectroscopy (EIS) tests were conducted to assess electrode performance and electron transfer efficiency [32]. These tests were performed at a constant open-circuit voltage (OCV) within a frequency range of 10 mHz to 10 kHz, with an amplitude of 10 mV [14, 33]. The equivalent circuit was simulated using Ivium software based on the Z view, and impedance values were obtained by fitting the test data accordingly [33].
The electrical energy generated by P-MFCs was stored in lead-acid batteries due to their robustness, durability, safety, low maintenance requirements, cost-effectiveness, and high recyclability [34]. Since lead-acid batteries do not require an additional battery management system, the electrical connection between the plant microbial fuel cell (P-MFC) and the battery was established directly. Specifically, the negative pole of the P-MFC was connected to the negative terminal of the battery, while the positive pole was connected to the battery’s positive terminal. The stored electrical energy was intended to power an LED. A 4-V, 1-A lead-acid battery was selected as the most suitable commercially available option. To ensure the safe and efficient operation of the LED, the appropriate external resistance (Rext) was calculated using Eq. (3), based on the LED’s forward voltage (Vf), forward current (If), and the battery supply voltage (Vs) as provided in the catalog values [35].
$$R\text{ext}= \frac{{V}_{s}-{V}_{f}}{{I}_{f}}$$
(3)
3 Results and discussion
Power density calculations were validated using multiple trials of linear sweep voltammetry (LSV) and confirmed through electrochemical impedance spectroscopy (EIS) analysis [5, 6]. Open-circuit voltage (OCV) measurements of the control sample microbial fuel cell (MFC) and the Aloe vera-based P-MFC were conducted with data collected at 1-s intervals over a 24-h period, as shown in Fig. 2. The anodic depolarization potential (ADP) value of the control sample MFC increased to a maximum of 498 mV. Initially, the OCV of the control sample MFC exhibited substantial fluctuations within the first 22,000 s. Between 22,000 and 65,000 s, the OCV gradually increased with no sharp variations, attaining a more stable profile. However, from 65,000 to 85,701 s, rapid and pronounced OCV variations were observed. A more stable OCV profile emerged between 85,702 and 86,400 s, maintaining a steady-state value of 65 ± 4 mV.
The Aloe vera P-MFC exhibited ADP levels reaching 493 mV, a value closely aligned with that of the control sample MFC. Notably, this OCV is approximately 3.5 times higher than the previously reported value of 141.21 mV obtained in a study using graphite electrodes in an Aloe vera-based P-MFC [20]. However, it remains lower than the 0.988 mV recorded in a study employing copper-zinc electrodes positioned on Aloe vera leaves [21]. These findings indicate that the Aloe vera-based P-MFC configuration in this study achieves a promising OCV output, suggesting potential for further optimization through the investigation of alternative electrode materials.
In this study, the initial 7811 s of OCV measurement were conducted under penumbra (semi-shaded) conditions, while the period from 7,811 to 59,570 s was performed under complete darkness (nighttime). Subsequently, OCV measurements from 59,570 to 86,400 s were recorded under penumbra conditions. Fluctuations observed within the first 33,203 s are particularly noteworthy. These variations may be attributed to the initial attachment of microorganisms to the anode electrode, a well-documented phenomenon in the early stages of biofilm formation in MFCs [36].
The relatively lower yet more stable OCV recorded during nighttime may be associated with light-independent metabolic reactions, such as glucose oxidation during respiration. Following this period, an increase in OCV values with some fluctuations was again observed under penumbra conditions. This suggests that a portion of the sunlight energy captured by plants during the light reaction of photosynthesis is converted into chemical energy, subsequently generating an electron flow to the fuel cell anode, resulting in bioelectricity production [37].
Starting from 33,203 s, the OCV increased from approximately 50 to 107 mV at 79,974 s, followed by minor variations. Thereafter, the OCV gradually decreased to approximately 90 mV at 89,952 s. From this point until the end of the experiment, the OCV remained stable, settling at approximately 92 mV.
The linear sweep voltammetry (LSV) test provided critical insights into the electrochemical characteristics of both the control sample microbial fuel cell (MFC) and the Aloe vera-based P-MFC. The curves obtained from the LSV tests for both MFCs are presented in Fig. 3. LSV analysis yielded key data regarding open-circuit voltage (OCV), current density, and power density for both systems.
The control sample MFC exhibited an OCV of approximately 65 mV, whereas the Aloe vera P-MFC demonstrated a higher OCV of about 92 mV. The OCV recorded for the Aloe vera P-MFC was approximately 64.4% higher than that of the control MFC, indicating enhanced bioelectricity conversion efficiency. This finding suggests that the Aloe vera-based P-MFC successfully harnesses photosynthetic reactions to generate bioelectricity, demonstrating its potential as an effective prototype.
In terms of current density, the control sample MFC achieved a value of approximately 1.75 µA/cm2, while the Aloe vera P-MFC exhibited a significantly higher current density of 6.5 µA/cm2. These results indicate that the Aloe vera-based P-MFC produced 371.4% higher current density compared to the control MFC. This substantial increase in current density suggests that electron production in the control sample MFC is significantly more constrained than in the Aloe vera P-MFC. Consequently, these findings reinforce the notion that the Aloe vera plant effectively converts light energy (via photosynthesis) into bioelectricity, further validating the efficiency of the Aloe vera P-MFC system.
The linear sweep voltammetry (LSV) test provided crucial insights into the electrochemical characterization of the control sample microbial fuel cell (MFC) and the Aloe vera-based P-MFC. Using LSV, the polarization curves of both MFCs, presented in Fig. 4, were determined. Subsequently, power density curves were generated using Eqs. (2) and (3).
Fig. 4
Polarization and power density curves of control sample MFC and Aloe vera P-MFC
The control sample MFC exhibited a peak power density of 250 mW/m2. However, the overall power density distribution was centered around 75 mW/m2. In contrast, the Aloe vera-based P-MFC demonstrated a significantly higher peak power density, reaching up to 1100 mW/m2. Despite this high peak value, the primary distribution of power density for the Aloe vera-based P-MFC was concentrated within the range of approximately 300 mW/m2.
These findings indicate that the Aloe vera-based P-MFC achieved approximately 4.4 times higher peak power density compared to the control MFC. When analyzing the dominant distribution of power density, the Aloe vera P-MFC still exhibited nearly four times higher power density than the control MFC. This outcome underscores the substantial enhancement in power output facilitated by the Aloe vera plant, further validating its potential as a high-performance component in MFC applications.
Analysis of the polarization and power density curves in Fig. 4 suggests that the primary losses within the Aloe vera-based P-MFC system are attributed to activation and ohmic losses. To ensure statistical validation, error bars have been included in all figures. The experiments were repeated multiple times, and error bars were generated using the error bar function of the OriginLab 2018 software. These error bars represent the variability of the measurements and were calculated based on standard deviations obtained from repeated experiments. This approach enhances the reliability of the reported electrochemical parameters by providing confidence intervals for the measured values. Additionally, reverse current density values were observed during polarization tests, which can be attributed to three primary factors: (i) reduced bacterial activation due to substrate limitations, (ii) formation of bacteria-free zones within the soil matrix, and (iii) diffusion limitations restricting charge transfer efficiency [38, 39]. These results align with previous microbial fuel cell studies that have reported temporary current reversals due to charge accumulation at the anode surface. Further investigation of bacterial activity patterns is needed to optimize microbial-electrode interactions and mitigate these effects.
A deeper investigation into these factors would provide valuable insights into the underlying mechanisms affecting system performance, presenting a significant avenue for future research.
The Nyquist curves obtained from electrochemical impedance spectroscopy (EIS) tests for the control sample microbial fuel cell (MFC) and the Aloe vera-based P-MFC are presented in Fig. 5. In these plots, the Z′ axis represents the real component of impedance, while the Z″ axis represents the imaginary component.
Fig. 5
Nyquist graphics of control sample MFC and Aloe vera P-MFC
The control sample MFC exhibited a nearly linear shape up to approximately 47,000 Ω on the Z′ axis and 55,000 Ω on the Z″ axis. This linear trend is attributed to substrate diffusion and the diffusion of reaction products from the electrode surface into the solution (moist potting soil), potentially increasing charge transfer resistance and impeding electron flow [40].
In contrast, the Nyquist curve for the Aloe vera P-MFC displays both linear characteristics and depressed semicircles. The presence of linear regions indicates weaker electron transport, whereas the semicircles suggest active electron transfer processes [41]. The Aloe vera P-MFC showed significantly lower impedance, reaching approximately 10,000 Ω on the Z′ axis and 15,500 Ω on the Z″ axis—representing about 4.7 times lower resistance along the Z′ axis and 3.54 times lower impedance along the Z″ axis compared to the control sample MFC. These results align with the LSV tests, confirming improved charge transfer efficiency.
The equivalent circuit models derived from EIS measurements, simulated using Ivium software, are shown in Fig. 6. These models provide insight into the internal resistance distributions in both systems. In the control sample MFC, internal resistance is primarily governed by ohmic resistance, indicating limited charge transfer efficiency. Conversely, the Aloe vera P-MFC model includes contributions from ohmic resistance, charge transfer resistance, diffusion resistance, and double-layer capacitance, representing mass diffusion processes [42].
Fig. 6
Equivalent circuit of the MFCs. a Control sample MFC. bAloe vera P-MFC
A comparative analysis of the equivalent circuit models suggests that ion diffusion from the plant-soil electrolyte to the electrode surface plays a more significant role in the Aloe vera P-MFC. The suppressed semicircles in the Nyquist curve further indicate a reduced charge transfer resistance, enhancing overall electron mobility. These findings confirm that the Aloe vera P-MFC benefits from improved electron transport dynamics, likely due to the plant’s root-associated microbial interactions, which facilitate efficient electrochemical reactions [43].
Based on the results of the linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) tests, 45 Aloe vera-based P-MFCs were electrically connected in series. The generated electrical energy was successfully stored in a lead-acid battery.
To ensure the safe operation of the LED connected to the lead-acid battery, which provides a supply voltage of 4.1 V, appropriate calculations were performed using Eq. (3). As a result, a 60-Ω resistor was selected as the protection resistor to prevent potential LED failure. Given 2400 the successful operation of the LED powered by the electrical energy produced from Aloe vera-based P-MFCs, this system is intended primarily for lighting applications.
To manage the utilization of stored electrical energy efficiently, a switch was incorporated into the circuit. This switch facilitates the on–off operation required to store electrical energy during the day and utilize it at night. The electrical circuit diagram illustrating the process is provided in Fig. 7.
Fig. 7
Electrical circuit diagram expressing the storage and use of electrical energy produced by Aloe vera P-MFC
This study focused on the design and performance evaluation of a plant microbial fuel cell (P-MFC) utilizing Aloe vera. The Aloe vera-based P-MFC, consisting of plant soil, graphite electrodes, and Aloe vera, was compared with a control sample MFC composed of plant soil and graphite electrodes.
The peak open-circuit voltage (OCV) of the control sample MFC was measured at approximately 498 mV, whereas the Aloe vera P-MFC exhibited a peak OCV only 5 mV lower. However, towards the end of the experiments, the Aloe vera P-MFC maintained a stable OCV of approximately 92 mV, whereas the control sample MFC exhibited a 27 mV lower and less stable OCV.
LSV tests demonstrated that the Aloe vera P-MFC generated approximately 3.71 times higher current density than the control sample MFC, contributing to a power density that was about 4.4 times higher. These findings were corroborated by EIS tests, as shown in the Nyquist plot, where the Aloe vera P-MFC exhibited approximately 4.7 times lower impedance on the real axis and approximately 3.66 times lower impedance on the imaginary axis.
Our results emphasize the key role of Aloe vera’s photosynthetic activity in the operation of the Aloe vera P-MFC. This activity significantly influences OCV fluctuations under semi-shade and dark conditions, serving as an indicator of photosynthetic efficiency.
Microbial biofilm formation plays a fundamental role in stabilizing electron transfer and maintaining OCV levels in P-MFCs. In our study, OCV fluctuations closely corresponded to different stages of biofilm development on the electrode surfaces. During the early stages, instability in OCV readings was observed due to incomplete bacterial colonization and inconsistent electron transfer. As the biofilm matured, OCV values stabilized, indicating the formation of a conductive network that facilitated efficient charge transport.
The interaction between Aloe vera root exudates and electrogenic bacteria further influenced biofilm growth. Under semi-shaded conditions, enhanced photosynthetic activity stimulated root exudation, supplying additional organic substrates for microbial metabolism, which in turn improved OCV stability. In contrast, under dark conditions, the reduction in exudate availability led to decreased microbial activity, resulting in minor fluctuations in OCV. These findings suggest that biofilm formation serves as a reliable indicator of system stability and long-term performance, aligning microbial activity with external environmental conditions.
To contextualize these results, a comparison with previously studied plant-based MFCs is necessary. The Aloe vera P-MFC achieved a power density of 1100 mW/m2, which is significantly higher than the values reported for other plant-based MFCs, such as Populus alba (7.61 mW/m2), Pachira macrocarpa (3.60 mW/m2), and Oryza sativa (15.57 mW/m2). However, it falls slightly below Pennisetum alopecuroides, which exhibited 667.94 mW/m2 under optimal conditions. This indicates that Aloe vera possesses strong potential for bioelectricity generation, particularly due to its adaptability to semi-shaded environments and minimal water requirements.
Despite its promising performance, this study also highlights certain limitations. The experimental setup was limited to small-scale laboratory conditions, which may not fully represent the operational challenges in field applications. Furthermore, while Aloe vera demonstrated enhanced microbial interactions and stable OCV levels, the long-term durability of the system and its ability to function efficiently in diverse environmental conditions require further investigation.
Regarding scalability, integrating multiple Aloe vera P-MFCs in series successfully enabled the charging of a lead-acid battery, which was subsequently used to power an LED for lighting applications. However, practical applications on a larger scale would require improvements in system efficiency, particularly in electrode optimization and substrate management, to maximize energy output while maintaining cost-effectiveness.
For future research, several key areas of improvement can be explored:
Microbial enhancement: Engineering microbial communities to increase electron transfer efficiency through genetic or bioaugmentation strategies could further optimize system performance.
Electrode modifications: Developing advanced electrode materials with higher conductivity and improved biofilm adhesion properties could enhance charge transfer.
Hybrid systems: Integrating Aloe vera P-MFCs with other renewable energy sources (e.g., solar or wind) could create hybrid energy solutions for off-grid applications.
Field trials: Large-scale field implementations should be conducted to evaluate long-term system stability under real-world conditions.
This research has inspired a broader interdisciplinary project exploring the integration of plant microbial fuel cells into architectural and interior landscape designs. Traditionally valued for their aesthetic appeal, indoor plants such as Aloe vera can serve a dual function—not only enhancing interior environments visually but also contributing to energy generation through plant microbial fuel cell systems. This approach presents a sustainable opportunity for integrating bioelectricity production into urban spaces, where plants could act as both decorative and functional elements within smart building designs. By leveraging the ability of P-MFCs to generate power, future applications could focus on self-sustaining energy solutions for lighting, sensors, and low-power electronic devices in indoor environments.
Additionally, the successful series connection of Aloe vera P-MFCs enabled the charging of a lead-acid battery, which was subsequently used to power an LED for lighting applications. This underscores the practical feasibility of bioelectricity generation using Aloe vera-based P-MFCs.
In conclusion, the Aloe vera P-MFC presents itself as a promising candidate for addressing the dual challenges of increasing energy demands and environmental sustainability. It offers a viable and renewable alternative for bioelectricity generation, reinforcing its potential in sustainable energy applications. By addressing current limitations and exploring future optimizations, Aloe vera-based MFCs could contribute significantly to the development of next-generation bioelectrochemical energy systems.
Acknowledgements
We express our sincere gratitude to the KARDEMİR Advanced Technologies Research and Engineering Center for providing access to the Potentiostat/Galvanostat device, which was essential for conducting this study.
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