Enhancing Energy Harvesting in Plant Microbial Fuel Cells with SnS-Coated 304 Stainless Steel Electrodes

Abstract

Plant microbial fuel cells (PMFCs) represent an eco-friendly solution for generating clean energy by converting biological processes into electricity. This work presents the first integration of tin sulfide (SnS)-coated 304 stainless steel (SS304) electrodes into Aloe vera-based PMFCs for enhanced energy harvesting. SnS thin films were obtained via chemical bath deposition and screen printing, followed by thermal treatment. X-ray diffraction (XRD) revealed a crystal size of 15 nm, while scanning electron microscopy (SEM) confirmed film thicknesses ranging from 3 to 13.75 µm. Over a 17-week period, SnS-coated SS304 electrodes demonstrated stable performance, with open circuit voltages of 0.6–0.7 V and current densities between 30 and 92 mA/m², significantly improving power generation compared to uncoated electrodes. Polarization analysis indicated an internal resistance of 150 Ω and a power output of 5.8 mW/m². Notably, the system successfully charged a 15 F supercapacitor with 8.8 J of stored energy, demonstrating a practical proof-of-concept for powering low-power IoT devices and advancing PMFC applications beyond power generation. Microbial biofilm formation, observed via SEM, contributed to enhanced electron transfer and system stability. These findings highlight the potential of PMFCs as a scalable, cost-effective, and sustainable energy solution suitable for industrial and commercial applications, contributing to the transition toward greener energy systems. These incremental advances demonstrate the potential of combining low-cost electrode materials and energy storage systems for future scalable and sustainable bioenergy solutions.

1. Introduction

Recent advancements in plant microbial fuel cells (PMFCs) have opened new pathways for sustainable, low-carbon energy solutions [1,2]. PMFCs utilize the synergistic relationships between plants and electrogenic bacteria to convert organic matter into electricity, offering a carbon-neutral technology aligned with global goals to reduce greenhouse gas emissions. These systems typically consist of an anode where exoelectrogenic microorganisms degrade organic compounds, releasing electrons. These electrons travel through external electrical connections from the anode to the cathode, where they combine with protons and an electron acceptor—typically oxygen—to form water or other compounds.

PMFCs present significant advantages as renewable energy systems. These devices leverage the natural ability of plants to continuously produce biomass, thereby eliminating the need for external fuel sources. This characteristic positions PMFCs as a promising technology for isolated areas or environments with limited energy infrastructure, contributing to reduced reliance on fossil fuels [3]. Moreover, PMFCs can be integrated with the Internet of Things (IoT) to power sensors, devices for environmental monitoring, and applications in smart cities, all of which benefit from the continuous low-power energy output of PMFCs [4,5,6].

Factors such as light intensity, plant species, rhizodeposition, soil composition, and electrode materials significantly influence PMFC efficiency. Optimizing these factors can enhance both the power output and overall efficiency of PMFCs [7]. For instance, the use of silicone as an oxygen diffusion layer has proven to be a viable and more environmentally friendly alternative to materials like polytetrafluoroethylene (PTFE). This technique facilitated oxygen reduction in biocathodes without the need for active air injection, continuously generating electricity [8].

The selection of high-performance electrode materials plays a pivotal role in determining the efficiency of PMFC performance. Graphite and activated carbon have been widely used due to their high conductivity and porosity, which promote biofilm formation and efficient electron transfer [9,10]. Recent studies have demonstrated the potential of cobalt oxide (Co₃O₄) nanowires as cathode material in PMFCs. This material not only improves chrome (Cr) (VI) reduction but also enhances electricity generation due to the increased electron transfer and biofilm formation on the cathode surface [11]. In addition, stainless steel has been investigated for its durability and relatively low cost, although it may require modifications to improve electrochemical compatibility [12,13].

Several electrode materials have been tested in PMFCs, each with clear advantages and drawbacks. Carbon-based electrodes, such as graphite felt and carbon cloth, provide high conductivity and a large surface area, which promote microbial adhesion; however, their fragility, relatively high cost, and limited long-term stability constrain large-scale deployment [9,10,14]. Activated carbon has been used for its large surface area but shows unstable electron transfer kinetics and performance losses during extended operation [15]. Stainless steel, on the other hand, is inexpensive, durable, and easy to fabricate, yet its low electrochemical activity and tendency to corrode reduce its efficiency in bioelectrochemical environments [13]. Cobalt oxide (Co₃O₄) nanowires, for example, have demonstrated high electrochemical activity in bioelectrochemical systems, reaching current densities of 60–80 mA/m² and power densities up to 4.5 mW/m², although their use in PMFCs remains limited [11].

In contrast, tin sulfide (SnS) coatings offer a promising alternative by combining the mechanical robustness and affordability of stainless steel with the semiconductive and electroactive properties of SnS, which enhance charge transfer processes and support biofilm formation [16,17,18,19]. To the best of our knowledge, this is the first study reporting SnS-coated stainless steel electrodes in a PMFC system, directly addressing the shortcomings of conventional electrode materials and advancing the development of cost-effective and durable bioelectrodes for sustainable energy applications.

One emerging material of interest is tin sulfide (SnS), a semiconductor with high electrochemical activity, specific capacity, and notable stability, all of which are critical for long-term PMFC performances. SnS thin films have been successfully implemented in energy storage technologies, such as lithium-ion batteries, demonstrating their potential to improve power output and efficiency in PMFC applications [16,17,18,19]. Given the demand for cost-effective, durable, and high-efficiency materials in sustainable energy systems, SnS thin film electrodes could provide significant improvements over traditional material in PMFC technologies.

This study explores, for the first time, the integration of SnS thin films on 304 stainless steel (SS304) electrodes within Aloe vera-based PMFCs, aiming to enhance energy harvesting and enable energy storage applications. By combining the durability and affordability of stainless steel with the semiconductive and electroactive properties of SnS, this approach addresses the limitations of conventional electrode materials and establishes a proof-of-concept for scalable and sustainable bioelectrochemical systems suitable for IoT and environmental monitoring applications.

2. Experimental Details

2.1. SnS Film Obtention

SnS thin films were deposited on glass to carry out the characterization of the material; they were also deposited on 304 stainless steel to be placed as electrodes in Aloe vera plants. The deposition was from 3 h to 9 h at 40 °C. All films, both deposited on glass and steel, were heat treated at 400 °C at 20 mTorr. To carry out the formation of microbial fuel cells using SnS deposited on 304 stainless steel, tin chloride (SnCl₂) (98% Baker brand, Mexico City, Méxcio), sodium citrate (Na₃C₆H₅O₇) (98% Fermont brand), ammonium hydroxide (NH₄OH) (30% Baker brand), and sodium thiosulfate (Na₂S₂O₃) (98% Fermont brand, Monterrey, México) were used, and stainless steel 304 brand 10 × 10 cm stainless steel substrates were cleaned according to ASTM standard [20]. The SnS thin film chemical bath was carried out for 3, 4, 5, 6, 7, 8, and 9 h of deposition at 40 °C. Afterwards, the samples were thermally treated at 400 °C for 1 h at 20 mTorr. The deposition procedure was conducted following established protocols for SnS thin films via chemical bath deposition, as reported by Peña-Méndez et al. (2024), ensuring reproducibility and alignment with validated methodologies [21].

2.2. PMFC Construction

To build the PMFC cells, we use Aloe vera plants. A. vera plants have a CAM-type photosynthetic metabolism, which allows them to have high efficiency in water use, a high photosynthetic rate even under conditions of water and thermal stress [22]. Its root exudates serve as a substrate for electrochemically active bacteria, optimizing electricity production [23]. Additionally, Aloe vera’s natural antimicrobial properties can maintain a healthy microbial community in PMFCs, enhancing their efficiency [24]. Its ability to grow in poor soils and with little water makes it a viable option for applications in areas with limited resources [25]. The PMFCs were assembled using a commercial sandy-loam potting soil (pH ~6.5) obtained from a local supplier. The soil contained organic matter suitable for Aloe vera cultivation and was used as received, without further treatment.

For the construction of the PMFCs, pots with dimensions of 15 cm in height and 12 cm in diameter were used, with one Aloe vera per pot. All plants were obtained from the same local nursery to minimize variability in age, size, and growing conditions. At the time of assembly, the plants were approximately 6–8 months old and 25–30 cm in height. No biological replicates were performed per variant, as the focus of this study was to evaluate the performance of the electrode material under realistic working conditions. One electrode was the 10 × 10 cm 304 stainless steel with the SnS thin film deposit varying in thickness, and the other electrode was the zinc mesh, with a separation between them of 5 cm to optimize electron transfer. The zinc mesh (anode) was selected for its high electrical conductivity and favorable oxidation potential (−0.76 V), which facilitates electron release. While acknowledging concerns regarding potential Zn²⁺ release into the soil, no phytotoxic effects on Aloe vera plants were observed during the 120-day experiment. Phytotoxicity was monitored qualitatively throughout the 17-week operational period by visual inspection of the Aloe vera plants. No symptoms such as chlorosis, necrosis, wilting, or stunted growth were observed, and all plants maintained normal appearance during the experiment. Future studies will explore environmentally benign anodic materials. After planting the Aloe vera and placing the electrodes, the pot soil was filled to 15 cm.

Two reference configurations were also included to provide comparative baselines. The first was the GEO configuration, consisting of copper wire and zinc mesh electrodes. The second was the TA304 configuration, consisting of stainless steel electrodes without SnS thin film deposition. Both GEO and TA304 were used as reference setups for evaluating voltage, current, and power performance in comparison with the SnS-coated electrode configurations. During the entire 17-week experimental period, soil moisture was maintained at approximately 95% to ensure stable operating conditions and to minimize variability among the different PMFC setups.

Figure 1 illustrates the experimental setup, showing the placement of the electrodes and the associated chemical reactions. The SnS thin film-coated SS304 electrode, positioned as the cathode, facilitates oxygen reduction, enhancing electron flow efficiency. At the anode, the predominant reaction involves the oxidation of zinc metal (Zn(s) → Zn²⁺ + 2e⁻), serving as a sacrificial electron source. Additionally, a minor contribution is expected from microbial oxidation of root exudates and soil organic matter (Root exudates + H₂O → CO₂ + H⁺ + e⁻). These processes collectively generate electron flow through the external circuit, enabling energy storage and powering of connected IoT devices.

2.3. Evaluation of PMFC Energy Storage Using SnS Thin Film-Coated Electrodes

To evaluate the energy storage capacity of the PMFC with SnS thin film-coated electrodes, the system was adapted to a BQ25570 energy harvesting board and an energy storage element. The BQ25570 board is specifically designed to efficiently harvest and manage the power generated by low-output energy sources [26]. The electrodes were connected to the BQ25570 board, which was configured to optimize the energy harvesting process. Harvested energy was then stored in a capacitor 15 F to evaluate the amount of energy the PMFCs could store over time. This setup allowed for real-time monitoring and assessment of the PMFC’s performance, demonstrating the potential of SnS thin film-coated electrodes in enhancing the efficiency and storage capacity of microbial fuel cells in practical applications.

The equation of the energy storage in the capacitor is the following:

$$E={\displaystyle \frac{1}{2}}C{\int }_0^t{V}_{Cap}^{2}dt$$

(1)

$$V_{Cap}^2=V_{PMFC}(1-e^{-R_{int}Ct})$$

(2)

where V_cap is the voltage across the capacitor as a function of time within a specified time window, V_PMFC is the open-circuit voltage of the PMFC, R_int is the internal resistance of the PMFC, t is the time at 3600 s, C is the capacitance of the capacitor measured in farads (F), and dt denotes the differential element of time.

2.4. Characterization Techniques

The XRD analysis was performed on a PAnalitycal brand (X’Pert PRO model, Almelo, The Netherlands) diffractometer model XPERT PRO with Cu-Kalfa (lamda = 1.5406 Å) in standard mode from 10 to 70 degrees for the samples with 3, 4, and 6 h of deposition at 40 °C heat treated at 400 °C for 1 h at 20 mTorr. To carry out the morphological analysis of the surface, a JEOL scanning electron microscope (SEM) (Tokyo, Japan) was used, with an acceleration voltage of 25 kV, and an Au/Pd coating was applied to the samples to increase their conductivity for their respective analysis. An Agilent U1252A multimeter (California, USA) was used for electrical evaluation of the short-circuit voltage and current PMFCs. To generate the polarization curves, the Metrohm potentiostat/galvanostat model autolab PGSTAT302N (Herisau, Suiza) was used. The internal resistance (Rint) of the PMFC system was determined from the polarization curve. It should be noted that Rint is a dynamic parameter influenced by plant growth, soil moisture, and microbial activity. According to the maximum power transfer theorem, maximum energy delivery occurs when the external load resistance (Rload) matches the internal resistance of the source. In this study, Rload was applied during external resistance experiments.

To power low-consumption devices using a PMFC, the charging time of an energy storage element was evaluated using a signal acquirer (DAQ-BNC-6216, National Instruments, Austin, TX, USA) [27]. During the evaluation, the signal acquirer was connected to the PMFC system to capture real-time data on the energy generated and transferred to the storage element. The employed configuration ensured accurate measurement of the time required to charge the energy storage element under different PMFC configurations.

2.5. Data Treatment

The experimental setup involved constructing PMFCs with SnS thin films as electrodes, utilizing Aloe vera plants. The V_oc and J_sc performance of each PMFC configuration were evaluated using an Agilent U1252A multimeter. Measurements were taken every hour for 8 h a day over a period of 17 weeks (120 days). To ensure data reliability and accuracy, the hourly measurements taken each day were processed to calculate their mean and standard deviation. This statistical treatment of the data helped in assessing the consistency and stability of the PMFCs’ performance, providing a robust analysis of their energy harvesting capabilities over the experimental period.

3. Results and Discussions

3.1. Structural and Morphology Analysis

The structural and morphological properties of SnS thin films play a significant role in their electrochemical performance in PMFCs. Figure 2 shows the X-ray diffraction peaks of the SnS thin film treated at 40 °C for 60 min. The diffraction peaks corresponding to the crystallographic planes (101), (111), (131), (002), (160), (042), and (251) confirm the presence of crystalline SnS thin film in its orthorhombic phase, consistent with the PDF Card 39-0352. This orthorhombic phase is critical for improving electron transport, enhancing the electrochemical performance in PMFCs by facilitating efficient charge transfer.

Figure 3 shows the SEM cross-sectional images of SnS thin films on stainless steel with different annealing treatment times. In Figure 3a, the 4 h deposition reveals a thick SnS film with a varied thickness ranging from 3.47 µm to 7.18 µm, indicating uneven deposition. In contrast, Figure 3b (8 h of annealing) shows a thinner, more uniform film, with thicknesses between 4.17 µm and 6.23 µm, suggesting improved layer consistency. Figure 3c (9 h of annealing) exhibits the most uniform deposition, with thicknesses ranging from 8.30 µm to 13.78 µm, reflecting optimal thermal treatment. Figure 3d further confirms the structural integrity of the SnS films after 9 h of deposition.

The improved uniformity and thickness consistency observed at longer annealing times are critical parameters for optimizing the SnS electrodes. These morphological features can increase the effective surface area and improve electrical connectivity, both of which are favorable for enhanced energy harvesting performance in PMFCs.

In addition to their structural integrity, the semiconductive properties of SnS thin films and their porous morphology are expected to support microbial adhesion and facilitate electron exchange at the bioelectrode interface, as reported for other semiconductor coatings [11]. These attributes suggest a promising pathway for the development of low-cost, high-efficiency PMFC systems.

3.2. Electrical Measurements of SnS Thin Films in PMFC

Building on the structural advantages of the SnS thin films described in Section 3.1, their electrochemical performance was evaluated in PMFCs configured with Aloe vera plants. Seven PMFCs were assembled using SnS thin films prepared via chemical bath deposition with varying deposition times (3, 4, 5, and 9 h), as detailed in Section 2. Two reference setups were included: one with only soil and another with uncoated stainless steel electrodes. V_oc and J_sc measurements were recorded using an Agilent U1252A multimeter at regular intervals over 17 weeks. The values reported in Figure 4 and Figure 5 correspond to the mean ± standard deviation calculated from the weekly measurements of each PMFC during the 17-week experimental period.

Figure 4a presents the V_oc readings, where PMFCs with SnS thin film-coated electrodes demonstrated voltages ranging from 0.6 V to 0.8 V, significantly outperforming the reference cells (GEO and TA304). Figure 4b shows the current density (J_sc), with the SnS-coated electrodes achieving values between 3 mA/m² and 35 mA/m², well above the ranges typically reported for carbon-based electrodes (1–50 mA/m²) [28] and uncoated stainless steel (V_oc: 0.052–0.4 V) [13]. Notably, the voltage remained stable even under high current outputs, highlighting the improved charge transfer characteristics of these electrodes.

The enhanced performance of the SnS thin films can be attributed to their uniformity and crystalline structure, which provide increased surface area and efficient charge transport pathways while minimizing interfacial resistance. This is consistent with previous studies on advanced electrode materials for PMFCs, including graphene-based systems with Voc values around 0.5–0.536 V [23].

These findings underscore the potential of SnS thin film-coated electrodes as high-efficiency bioelectrodes for energy harvesting applications. Their ability to sustain stable voltages under demanding current loads suggests suitability for integration into real-world energy harvesting systems, such as IoT devices and small-scale industrial applications.

3.3. Polarization Curves of the PMFC

The polarization curves were measured on day 120 of PMFC operation. Figure 5a shows the polarization curves, while Figure 5b presents the power density profiles.

The PMFC equipped with SnS thin film-coated electrodes achieved a maximum open-circuit voltage (Voc) of 0.582 V, a current density of 92.8 mA/m², a power density of 5.8 mW/m², and an Rint of 150 Ω. It should be noted that the Rint reported here was derived exclusively from the polarization curve on day 120. Consequently, Rint should be interpreted as an intrinsic indicator of the electrochemical losses within the system rather than as a direct reflection of load-dependent performance. Future work will combine polarization analysis with controlled load resistance experiments to provide a more comprehensive evaluation of PMFC behavior. These performance metrics indicate a notable enhancement compared to uncoated stainless steel electrodes and align with previous studies where semiconductive coatings improved electron flow in PMFCs [13]. Future work will employ advanced techniques to further elucidate the electrochemical behavior of SnS-coated electrodes in these systems.

This improved performance is attributed to the synergistic interaction between the SnS thin film and the microbial communities in the rhizosphere. The exudates and rhizodeposition, composed of organic acids, carbohydrates, proteins, and other compounds, act as natural mediators for electron transfer from microorganisms to the electrode surface. This coupling mechanism facilitates efficient electron flow, supporting the observed increases in current and power output.

Table 1. Comparative electrical characteristics of the PMFC.

Electrode Material Cathode/Anode	Open-Circuit Voltage (V)	Current Density (mA/m2)	Power Density (mW/m2)	Evaluation Time (Days)	Reference
Carbon/Carbon	0.06 to 0.72	Not specified	Not specified	14	[29]
Carbon/Copper	0.02 to 0.67	Not specified	Not specified	14	[29]
Carbon/Zinc	0.11 to 0.78	Not specified	Not specified	14	[29]
Carbon felts	0.220	Not specified	Not specified	Varies	[14]
Carbon cloth	0.114 ± 5.89	Not specified	Not specified	Varies	[14]
Graphite felt	0.0614	Not specified	121.7	Varies	[14]
Carbon plate	0.543	Not specified	Not specified	40	[14]
Activated Carbon/TiO2 composite	0.536 ± 25	Not specified	1.02 W/m3	200	[15]
Hydrated carbon cloth/carbon brush	Not specified	Not specified	69.32 to 222.54	Not specified	[30]
Manganese-based catalyzed carbon/carbon felt	0.600	90	1.5	114	[28]
Graphite felts	0.050 to 0.391	Not specified	Not specified	32	[23]
Graphite granules	Not specified	Not specified	15.84	367	[14]
Graphite	0.400	20.4	37	20	[13]
Co3O4 nanowires	Not specified	Not specified	75.12 ± 2.9	Not specified	[11]
Stainless Steel 316	0.150	5.33	23.0	25	[13]
Stainless Steel 436	0.106	2.2	10.5	25	[13]
Stainless Steel 304	0.600	92.8	5.8	120	In this study

compares the electrical performance of SnS thin film-coated stainless steel 304 electrodes with other materials reported in the literature. The Voc of 600 mV achieved in this study is significantly higher than traditional graphite and stainless steel electrodes, which exhibit Voc values ranging from 50.84 mV to 150 mV [13,29]. Similarly, the current density of 92.8 mA/m² outperforms stainless steel 316 and 436 electrodes, which achieved only 5.33 mA/m² and 2.2 mA/m², respectively [13]. This represents a ~17-fold increase over uncoated stainless steel electrodes. Furthermore, the power density recorded surpasses values reported for conventional materials, underscoring the efficiency of SnS as a semiconductive bioelectrode material.

These findings demonstrate the potential of SnS thin film-coated SS304 electrodes as highly effective bioelectrodes for PMFC applications. Their enhanced electrical performance, combined with their ability to facilitate efficient microbial electron transfer, positions them as a promising candidate for future large-scale and industrial energy harvesting systems.

3.4. Applied Energy of the PMFC Configurations

In this section, we explore the energy harvesting capabilities of the plant microbial fuel cell (PMFC) system, as shown in Figure 6. The PMFC setup is integrated with various components that facilitate the conversion, storage, and management of the energy generated by the microbial activity in the soil. The system includes a DC/DC converter (Figure 6a), which regulates the voltage output, and a supercapacitor (Figure 6b) for energy storage. The core PMFC is connected to a data acquisition system (DAQ), allowing real-time monitoring and analysis of the energy parameters (Figure 6c). The harvested energy is used to power small-scale IoT devices, demonstrating the potential of this bioenergy system for sustainable applications.

Figure 7a,b provide crucial insights into the performance of the PMFC in terms of capacitor charging times and energy storage. The variants evaluated included the reference systems GEO and TA304 as well as SnS-coated SS304 electrodes prepared with deposition times of 4 h (ASnS-4h), 8 h (ASnS-8h), and 9 h (ASnS-9h). In addition, a combined serial configuration (Series_489h), consisting of three PMFC units connected in series with electrodes treated for 4, 8, and 9 h, was tested.

As shown in Figure 7a, charging times decreased with longer thermal treatment durations. Electrodes treated for 9 h achieved the shortest charging time, reaching ~0.9 V, while ASnS-4h electrodes required a longer charging time, achieving only ~0.7 V. The reference systems GEO and TA304 displayed lower voltage outputs compared to the SnS-coated variants, confirming the positive effect of the thin film coatings. The Series_489h configuration demonstrated the best overall performance, reaching ~1.1 V, which can be attributed to the complementary electrochemical properties of the combined electrodes.

In Figure 7b, the energy stored in a capacitor increases with treatment duration. Electrodes treated for 9 h stored 6 J, while 4 h-treated electrodes stored 4 J. The Series_489h configuration achieved the highest energy storage, reaching 8.8 J. These results suggest that PMFC systems with SnS thin film-coated SS304 electrodes are suitable for powering low-power devices continuously [31].

Series configurations are an effective strategy for enhancing PMFC performance. In such setups, voltages of individual PMFC units add up, nearly tripling the total output compared to single-unit PMFCs [9]. Previous studies reported voltage increases up to 3.2 V in serial-parallel configurations [32,33]. In [34], a PMFC array successfully powered a LoRa-based wireless sensor node, demonstrating real-world applicability. These findings align with the observed improvements in voltage and energy storage efficiency in this study, supporting the scalability of SnS thin film-coated electrodes for industrial applications. Although serial connection is theoretically expected to produce additive voltages, the Series_489h configuration in this study reached only ~1.1 V. This reduced output can be explained by mismatched internal resistances and the occurrence of reversible potentials in PMFC systems [9,33].

Figure 8a–c present SEM micrographs of SnS thin film-coated electrodes treated for 9 h (ASnS-9h) after 120 days of operation. Figure 8a shows rough surfaces with agglomerated particles and high porosity. Figure 8b reveals laminar and compact structures, suggesting environmental interactions. Figure 8c highlights porous regions and biological-like particles, indicating microbial colonization. Although quantitative biofilm analysis was not performed, this qualitative evidence aligns with previous findings where biofilm development on conductive surfaces facilitated electron transfer and system stability [1]. These observations reinforce the dual functionality of the SnS-coated SS304 electrode, combining improved oxygen reduction kinetics with a supportive environment for electrogenic microbial communities.

The demonstrated performance of SnS thin film-coated SS304 electrodes in PMFCs highlights their potential for scalable and sustainable energy applications. The Series_489h configuration, capable of storing up to 8.8 J, illustrates how multi-electrode systems can be optimized for higher energy outputs, supporting continuous operation of low-power devices such as IoT sensors [31]. From a techno-economic perspective, the use of SS304 stainless steel as a substrate offers a cost-effective platform due to its wide availability and corrosion resistance, while the SnS thin film fabrication via chemical bath deposition ensures a scalable and low-energy production process. Furthermore, integrating PMFCs into modular arrays, as evidenced by the voltage gains reported in serial-parallel configurations [9,32,34], provides a viable pathway for meeting diverse energy demands in rural and off-grid communities. The sustainability of this approach is reinforced by the ability of PMFCs to harvest energy directly from plant–soil systems without external input, minimizing environmental impact and promoting greener energy solutions. These attributes collectively position SnS thin film-coated SS304 electrodes as promising candidates for next-generation bioelectrochemical systems designed for industrial and commercial applications.

4. Conclusions

This study demonstrates the significant impact of optimizing electrode materials and treatment processes on the efficiency of PMFCs. The fabrication of SnS thin films on 304 stainless steel (SS304) anodes, achieved through chemical bath deposition and screen printing followed by thermal treatments, improved energy harvesting capabilities.

The SnS thin film-coated electrodes reached a current density of 92.8 mA/m² and a power density of 5.8 mW/m², values significantly higher than those of uncoated stainless steel electrodes. In particular, the current density of the SnS thin film-coated electrodes was approximately 17 times higher than that of stainless steel 316 electrodes, which showed a current density of 5.33 mA/m², and more than 40 times higher than that of stainless steel 436 electrodes, which presented a current density of 2.2 mA/m².

The SnS thin film-coated electrodes showed consistent electrical generation and enhanced energy storage, with SEM analysis confirming the presence of microorganisms that contribute to the bioelectrochemical processes. These findings highlight the potential for practical implementation of PMFCs in industrial and commercial sectors, though further development is required for large-scale deployment.

It should also be noted that, in this study, phytotoxicity was assessed only through visual monitoring, without morphometric or biochemical analyses. Future work should incorporate quantitative physiological and biochemical measurements to better characterize plant responses to electrode integration. Furthermore, work should include post-operational characterization of SnS thin films to better understand their long-term stability under PMFC conditions.