Synthesis of bimetallic iron ferrite Co0.5Zn0.5Fe2O4 as a superior catalyst for oxygen reduction reaction to replace noble metal catalysts in microbial fuel cell

A low-cost electrochemically active oxygen reduction reaction (ORR) catalyst is obligatory for making microbial fuel cells (MFCs) sustainable and economically viable. In this endeavour, a highly active surface modified ferrite, with Co and Zn bimetal in the ratio of 1:1 (w/w), Co_0.5Zn_0.5Fe₂O₄ was synthesised using simple sol-gel auto combustion method. Physical characterisation methods revealed a successful synthesis of nano-scaled Co_0.5Zn_0.5Fe₂O₄. For determination of ORR kinetics of cathode, using Co_0.5Zn_0.5Fe₂O₄ catalyst, electrochemical studies viz. cyclic voltammetry and electrochemical impedance spectroscopy were conducted, which demonstrated excellent reduction current response with less charge transfer resistance. These electrochemical properties were observed to be comparable with the results obtained for cathode using 10% Pt/C as a catalyst on the cathode. The MFC using Co_0.5Zn_0.5Fe₂O₄ catalysed cathode could produce a maximum power density of 21.3 ± 0.5 W/m³ (176.9 ± 4.2 mW/m²) with a coulombic efficiency of 43.3%, which was found to be substantially higher than MFC using no catalyst on the cathode 1.8 ± 0.2 W/m³ (15.2 ± 1.3 mW/m²). Also, the specific power recovery per unit cost for MFC with Co_0.5Zn_0.5Fe₂O₄ catalysed cathode was found to be 4 times higher as compared to Pt/C based MFC. This exceptionally low-cost cathode catalyst has enough merit to replace costly cathode catalyst, like platinum, for scaling up of the MFCs.     

Enhanced bioelectrochemical performance by NiCoAl-LDH/MXene hybrid as cathode catalyst for microbial fuel cell

In this work, NiCoAl-layered double hydroxide (LDH)/MXene was successfully prepared through straightforward hydrothermal method. NiCoAl-LDH was tightly and uniformly coated on MXene, forming a kind of porous structure. NiCoAl-LDH/MXene exhibited the (002) (012) (105) (100) crystal planes of hydrotalcite reflection. NiCoAl-LDH/MXene also showed superior catalytic oxygen reduction reaction (ORR) in response current according to electrochemical test (cyclic voltammetry (CV) etc.). The maximum power density and output voltage of NiCoAl-LDH/MXene as cathode in microbial fuel cell (MFC) was 362.404 mW/m² and 450 mV, respectively, which was 1.54 times of MXene-MFC (234.256 mW/m²) and 1.71 times of NiCoAl-LDH-MFC (211.56 mW/m²). The results indicated that NiCoAl-LDH/MXene was a kind of potential cathode catalyst for MFC and was full of future application.     

Evaluation of TiO2 as catalyst support in Pt-TiO2/C composite cathodes for the proton exchange membrane fuel cell

Anatase TiO₂ is evaluated as catalyst support material in authentic Pt-TiO₂/C composite gas diffusion electrodes (GDEs), as a different approach in the context of improving the proton exchange membrane fuel cell (PEMFC) cathode stability. A thermal stability study shows high carbon stability as Pt nanoparticles are supported on TiO₂ instead of carbon in the Pt-TiO₂/C composite material, presumably due to a reduced direct contact between Pt and C. The performance of Pt-TiO₂/C cathodes is investigated electrochemically in assembled membrane-electrode assemblies (MEAs) considering the added carbon fraction and Pt concentration deposited on TiO₂. The O₂ reduction current for the Pt-TiO₂ alone is expectedly low due to the low electronic conductivity in bulk TiO₂. However, the Pt-TiO₂/C composite cathodes show enhanced fuel cell cathode performance with growing carbon fraction and increasing Pt concentration deposited on TiO₂. The proposed reasons for these observations are improved macroscopic and local electronic conductivity, respectively. Electron micrographs of fuel cell tested Pt-TiO₂/C composite cathodes illustrate only a minor Pt migration in the Pt-TiO₂/C structure, in which anatase TiO₂ is used as Pt support. On the whole, the study demonstrates a stable Pt-TiO₂/C composite material possessing a performance comparable to conventional Pt–C materials when incorporated in a PEMFC cathode.     

Assessment of immobilized cell reactor and microbial fuel cell for simultaneous cheese whey treatment and lactic acid/electricity production

Two biological methods for treatment of cheese whey and concentrated cheese whey were investigated in this research. As the first method, fermentation of cheese whey for production of lactic acid, in an immobilized cell reactor (ICR) was successfully carried out. The immobilisation of Lactobacillus bulgaricus was performed by the enriched cells cultured media harvested at exponential growth phase. Furthermore, the FTIR analysis has been done to prove the production of lactic acid. The COD removal during the continuous process for both whey and concentrated whey was above 70% which showed the capability of reaction for wastewater treatment. The cells were immobilised by sodium alginate as a perfect polymer in this regard. The maximum produced lactic acid from whey was 10.7 g l^?1 at 0.125 h^?1 and 19.5 g l^?1 from concentrated whey at 0.063 h^?1. Finally it can be concluded that the process is efficient for lactic acid production and COD removal simultaneously. As the second studied method, whey and concentrated cheese whey were used as the sources of carbon in a microbial fuel cell. The power densities of 188.8 and 288.12 mW m^?2 were recorded for whey-fed and concentrated whey-fed MFCs while the COD removal were 95% and 86% respectively. Biological wastewater treatment can be a very efficient alternative for traditional wastewater treatment which selecting any and or integrating of them depends on specific applications needed to be achieved.     

Carbon supported nickel-phthalocyanine/MnOx as novel cathode catalyst for microbial fuel cell application

Performance of microbial fuel cells (MFCs) with carbon supported nickel phthalocyanine (NiPc)MnO_x composite (MFC-1) and nickel phthalocyanine (MFC-2) incorporated cathode was compared with a control MFC with non-catalysed carbon felt as cathode (MFC-3) and MFC-4 having Pt on cathode (as benchmark reference control). MFC-1 exhibited power density of 8.02 Wm^?3, which was four folds higher than control MFC-3 (2.08 Wm^?3) and 1.14 times higher than MFC-2 (6.97 Wm^?3). Coulombic efficiency of 30.3% obtained in MFC-1 was almost double of that obtained for control MFC-3 and it was 5.4% lesser as compared to MFC-4 (35.7%). Linear sweep voltammetry study of cathodes revealed that NiPc-MnO_x could enhance the electrocatalytic activity of oxygen reduction reaction (ORR) in comparison to control cathode. However, the power recovery from MFC-1 was noted little lower than what obtained from MFC-4 (10.58 Wm^?3), however the cost normalized power was two times higher than Pt catalyst on cathode. Thus, NiPc-MnO_x based catalyst developed in this study has potential to enhance ORR in cathodes of MFCs in order to harvest more power.     

Novel multi walled carbon nanotube based nitrogen impregnated Co and Fe cathode catalysts for improved microbial fuel cell performance

For application in a microbial fuel cell (MFC), transition metal and nitrogen co-doped nanocarbon catalysts were synthesised by pyrolysis of multi-walled carbon nanotubes (MWCNTs) in the presence of iron- or cobalt chloride and nitrogen source. For the physicochemical characterisation of the catalysts, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) was used. The results obtained by rotating disk electrode (RDE) method showed an extraordinary electrocatalytic activity of these catalysts towards oxygen reduction reaction (ORR) in neutral media, which was also confirmed by the MFC results. The Co-N-CNT and Fe-N-CNT cathode catalysts exhibited maximum power density of 5.1 W m^?3 and 6 W m^?3, respectively. Higher ORR activity and improved electric output in the MFC could be attributed to the formation of the active nitrogen-metal centers. All findings suggest that these materials can be used as potential cathode catalysts for ORR in MFC to replace expensive noble-metal based materials.     

Activated carbon nanofibers as an alternative cathode catalyst to platinum in a two-chamber microbial fuel cell

This paper evaluated the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) system by using chemically and physically activated electrospun carbon nanofibers (ACNFs) in an MFC and comparing their performance with that of plain carbon paper. The chemical and physical activation was carried out by KOH reagents and CO₂ gas to increase the electrode surface area and the catalytic activity. As a result, it was found that the MFC with the chemically activated carbon nanofibers (ACNFs) exhibited better catalytic activity than that of the physically activated ACNFs. Chemically ACNFs with 8 M KOH were found to be one of the most promising candidates for the ORR and could generate up to 3.17 times more power than that of the carbon paper. The ACNFs with 8 M KOH exhibited 78% more power generation than that of the physically activated ACNFs and exhibited 16% more power generation than the chemically activated ACNFs with 4 M KOH. The power per cost of ACNFs with 8 M KOH is 2.65 times greater than that of the traditionally used platinum cathode. Thus, ACNFs are a good alternative catalyst to Pt for MFCs.     

LaNi0.8Co0.2O3 as a cathode catalyst for a direct borohydride fuel cell

A perovskite-type oxide LaNi_0.8Co_0.2O₃ is prepared as a direct borohydride fuel cell (DBFC) cathode catalyst. Its electrochemical properties are studied by cyclic voltammetry. The results demonstrate that LaNi_0.8Co_0.2O₃ exhibits excellent electrochemical activity with respect to the oxygen reduction reaction (ORR) and good tolerance of BH₄⁻ ions. Maximum power densities of 114.5 mW cm⁻² at 30 °C and 151.3 mW cm⁻² at 62 °C are obtained, and good stability (300-h stable performance at 20 mA cm⁻²) is also exhibited, which shows that such perovskite-type oxides as LaNi_0.8Co_0.2O₃ can be excellent catalysts for DBFCs.     

Comparing the performance of microbial fuel cell with mechanical aeration and photosynthetic aeration in the cathode chamber

In this study, the cathode chamber of the microbial fuel cell (MFC) was aerated using the photosynthetic aeration method and was compared with the mechanical aeration method in terms of power generation. Light energy for photosynthetic aeration was supplied in different regimes. The dissolved oxygen of 8.0–9.0 mg/L in the cathode chamber under mechanical aeration was increased to 10.0–11.0 mg/L under photosynthetic aeration of continuous light supply. The maximum power density obtained in the mechanical aeration was increased by 34.5% when photosynthetic aeration employed. Also, the algal assisted cathodic reaction under photosynthetic aeration reduced the internal resistance of the cell by 13.7%. To simulate with natural sun lighting conditions, 12/12 h of dark/light regime was tested to observe the stability of the MFC. During the light phase, microalgae carried out photosynthesis and provided the oxygen required for the cathodic reaction while in the dark phase, the voltage in the cell dropped due to the respiration by microalgae. The dissolved oxygen level dropped to 6.5–7.5 mg/L during the dark phase. Cathode chamber was also operated under 32/32 h of light/dark regime to observe the voltage variation under longer cycles. A higher voltage drop was observed in 32/32 h light/dark regime, as compared with 12/12 h of light/dark regime. Biomass productivity of 0.58 g/L/day was achieved in the MFC conducted under continuous light. It was reduced to 0.51, and 0.39 g/L/day under 12/12 h, and 32/32 h of light/dark regimes, respectively. The results suggested that photosynthetic aeration has the potential to achieve better power generation in the cell than mechanical aeration with an additional advantage of simultaneous microalgal biomass production.     

Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: A review

Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.     

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO₂/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO₂ is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO₂/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO₂ powder, suggesting that the surface coating of MnO₂ onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m⁻² produced from the MFC with in situ MnO₂/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO₂/CNTs (93 mW m⁻²), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m⁻²). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO₂ on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO₂ and that in situ MnO₂/CNTs offer a good alternative to Pt/C for practical MFC applications.     

Polyaniline/carbon black composite-supported iron phthalocyanine as an oxygen reduction catalyst for microbial fuel cells

Polyaniline/carbon black (PANI/C) composite-supported iron phthalocyanine (FePc) (PANI/C/FePc) has been investigated as a catalyst for the oxygen reduction reaction (ORR) in an air-cathode microbial fuel cell (MFC). The electrocatalytic activity of the PANI/C/FePc toward the ORR is evaluated using cyclic voltammogram and linear scan voltammogram methods. In comparison with that of carbon-supported FePc electrode, the peak potential of the ORR at the PANI/C/FePc electrode shifts toward positive potential, and the peak current is greatly increased, suggesting the enhanced activity of FePc absorbed onto PANI/C. Additionally, the results of the MFC experiments show that PANI/C/FePc is well suitable to be the cathode material for MFCs. The maximum power density of 630.5 mW m⁻² with the PANI/C/FePc cathode is higher than that of 336.6 mW m⁻² with the C/FePc cathode, and even higher that that of 575.6 mW m⁻² with a Pt cathode. Meanwhile, the power per cost of the PANI/C/FePc cathode is 7.5 times greater than that of the Pt cathode. Thus, the PANI/C/FePc can be a potential alternative to Pt in MFCs.     

An analysis of the performance of an anaerobic dual anode-chambered microbial fuel cell

The performance of a dual anode-chambered microbial fuel cell (MFC) inoculated with Shewanella oneidesis MR-1 was evaluated. This reactor was constructed by incorporating two anode chambers flanking a shared air cathode chamber in an electrically parallel, geometrically stacked arrangement. The device was shown to have the same maximum power density (approximately 24 W m⁻³, normalized by the anode volume) as a single anode-, single cathode-chambered MFC. The dual anode-chambered unit generated a maximum current of 3.66 mA (at 50 Ω), twice the value of 1.69 mA (at 100 Ω) for the single anode-chambered device at approximately the same volumetric current density. Increasing the Pt-coated cathode surface area by 100% (12 to 24 cm²) had no significant effect on the power generation of the dual anode-chambered MFC, indicating that the performance of the device was limited by the anode. The medium recirculation rate and substrate concentration in the anode were varied to determine their effect on the anode-limited power density. At the highest recirculation rate, 5 ml min⁻¹, the power density was about 25% higher than at the lowest recirculation rate, 1 ml min⁻¹. The dependence of the power density on the lactate concentration showed saturation kinetics with a half-saturation constant K_s on the order of 4.4 mM.     

Impact of salinity on cathode catalyst performance in microbial fuel cells (MFCs)

Several alternative cathode catalysts have been proposed for microbial fuel cells (MFCs), but effects of salinity (sodium chloride) on catalyst performance, separate from those of conductivity on internal resistance, have not been previously examined. Three different types of cathode materials were tested here with increasingly saline solutions using single-chamber, air-cathode MFCs. The best MFC performance was obtained using a Co catalyst (cobalt tetramethoxyphenyl porphyrin; CoTMPP), with power increasing by 24 ± 1% to 1062 ± 9 mW/m² (normalized to the projected cathode surface area) when 250 mM NaCl (final conductivity of 31.3 mS/cm) was added (initial conductivity of 7.5 mS/cm). This power density was 25 ± 1% higher than that achieved with Pt on carbon cloth, and 27 ± 1% more than that produced using an activated carbon/nickel mesh (AC) cathode in the highest salinity solution. Linear sweep voltammetry (LSV) was used to separate changes in performance due to solution conductivity from those produced by reductions in ohmic resistance with the higher conductivity solutions. The potential of the cathode with CoTMPP increased by 17–20 mV in LSVs when the NaCl addition was increased from 0 to 250 mM independent of solution conductivity changes. Increases in current were observed with salinity increases in LSVs for AC, but not for Pt cathodes. Cathodes with CoTMPP had increased catalytic activity at higher salt concentrations in cyclic voltammograms compared to Pt and AC. These results suggest that special consideration should be given to the type of catalyst used with more saline wastewaters. While Pt oxygen reduction activity is reduced, CoTMPP cathode performance will be improved at higher salt concentrations expected for wastewaters containing seawater.     

Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO2/C catalysts

Pt/TiO₂/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO₂/C catalysts are conducted with the physical and electrochemical techniques.After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO₂/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density () of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10⁻⁹ to 1.32 × 10⁻¹¹ A cm⁻² after the AAT. And the value of of the ORR at the Pt/TiO₂/C catalysts is 2.88 × 10⁻⁹ A cm⁻² before the AAT and 2.51 × 10⁻⁹ A cm⁻² after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO₂/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO₂/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO₂/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.     

Performance of plug flow microbial fuel cell (PF-MFC) and complete mixing microbial fuel cell (CM-MFC) for wastewater treatment and power generation

Two flow patterns (plug flow (PF) and complete mixing (CM)) of microbial fuel cells (MFCs) with multiple anodes–cathodes were compared in continuous flow mode for wastewater treatment and power generation. The results indicated that PF-MFCs had higher power generation and columbic efficiency (CE) than CM-MFCs, and the power generation varied along with the flow pathway in the PF-MFCs. The gradient of substrate concentrations along the PF-MFCs was the driving force for the power generation. In contrast, the CM-MFCs had higher wastewater removal efficiency than PF-MFCs, but had lower power conversion efficiency and power generation. This work demonstrated that MFC configuration is a key factor for enhancing power generation and wastewater treatment.     

A dual photoelectrode-based double-chambered microbial fuel cell applied for simultaneous COD and Cr (VI) reduction in wastewater

Cerium oxide (CeO₂) and cuprous oxide (Cu₂O) were used for the first time as photoanode and photocathode, respectively, in a microbial fuel cell (MFC) for simultaneous reduction of chemical oxygen demand (COD) and Cr(VI) in wastewater. The photoelectrodes, viz. Photoanode and photocathode were separately prepared by impregnating activated carbon fiber (ACF) with the respective metal oxide nanoparticles, followed by growing carbon nanofibers (CNFs) on the ACF substrate using catalytic chemical vapor deposition. The MFC, operated under visible light irradiation, showed reduction in COD and Cr(VI) by approximately 94 and 97%, respectively. The MFC also generated high bioelectricity with a current density of ~6918 mA/m² and a power density of ~1107 mW/m². The enhanced performance of the MFC developed in this study was attributed to the combined effects of the metal oxide photocatalysts, the graphitic CNFs, and the microporous ACF substrate. The MFC based on the inexpensive transition metal oxides-based photoelectrodes developed in this study has a potential to be used at a large scale for treating the industrial aqueous effluents co-contaminated with organics and toxic Cr(VI).     

Direct CH4 fuel cell using Sr2FeMoO6 as an anode material

A double-perovskite Sr₂FeMoO₆ (SFMO) has been synthesized with a combined citrate-EDTA complexing method. The material shows a double-perovskite structure after reduction in 5% H₂/Ar at 1100 °C for 20 h. A single fuel cell using this material as anode is constructed with the configuration of SFMO?La_0.8Sr_0.2Ga_0.83Mg_0.17O₃?Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. The cell exhibits a remarkable electrochemical activity in both H₂ and dry CH₄, respectively. With Oxygen as oxidant, the maximum power density is 863.7 mW cm⁻² with H₂ as the fuel and 604.8 mW cm⁻² with dry CH₄ as the fuel at 850 °C, respectively. SFMO has an almost linear thermal expansion coefficient from 30 to 900 °C and is very close to that of La_0.8Sr_0.2Ga_0.83Mg_0.17O₃. A durability test of the single cell indicates that SFMO is stable in dry CH₄ operation. Therefore SFMO can be recommended as a promising anode material for LaGaO₃-based solid oxide fuel cells operating with both H₂ and dry CH₄.     

Transition metal nanoparticles doped carbon paper as a cost-effective anode in a microbial fuel cell powered by pure and mixed biocatalyst cultures

The poor wettability and high cost of the carbonaceous electrodes materials prohibited the practical applications of microbial fuel cells (MFCs) on large scale. Here, a novel nanoparticles of metal sheathed with metal oxide is electrodeposited on carbon paper (CP) to introduce as high-performance anodes of microbial fuel cell (MFC). This thin layer of metal/metal oxide significantly enhance the microbial adhesion, the wettability of the anode surface and decrease the electron transfer resistance. The investigation of the modified CP anodes in an air-cathode MFCs fed by various biocatalyst cultures shows a significant improving in the MFC performance. Where, the generated power and current density was 140% and 210% higher as compared to the pristine CP. Mixed culture of exoelectrogenic microorganism in wastewater exhibited good performance and generated higher power and current density compared to yeast as pure culture. The excellent capacitance with a distinctive nanostructure morphology of the modified-CP open an avenues for practical applications of MFCs.     

Evaluating the performance of microbial fuel cells powering electronic devices

A microbial fuel cell (MFC) is capable of powering an electronic device if we store the energy in an external storage device, such as a capacitor, and dispense that energy intermittently in bursts of high-power when needed. Therefore its performance needs to be evaluated using an energy-storing device such as a capacitor which can be charged and discharged rather than other evaluation techniques, such as continuous energy dissipation through a resistor. In this study, we develop a method of testing microbial fuel cell performance based on storing energy in a capacitor. When a capacitor is connected to a MFC it acts like a variable resistor and stores energy from the MFC at a variable rate. In practice the application of this method to testing microbial fuel cells is very challenging and time consuming; therefore we have custom-designed a microbial fuel cell tester (MFCT). The MFCT evaluates the performance of a MFC as a power source. It uses a capacitor as an energy storing device and waits until a desired amount of energy is stored then discharges the capacitor. The entire process is controlled using an analog-to-digital converter (ADC) board controlled by a custom-written computer program. The utility of our method and the MFCT is demonstrated using a laboratory microbial fuel cell (LMFC) and a sediment microbial fuel cell (SMFC). We determine (1) how frequently a MFC can charge a capacitor, (2) which electrode is current-limiting, (3) what capacitor value will allow the maximum harvested energy from a MFC, which is called the “optimum charging capacitor value,” and (4) what capacitor charging potential will harvest the maximum energy from a MFC, which is called the “optimum charging potential.” Using a LMFC we find that (1) the time needed to charge a 3-F capacitor from 0 to 500 mV is 108 min, (2) the optimum charging capacitor value is 3 F, and (3) the optimum charging potential is 300 mV. Using a SMFC we find that (1) the time needed to charge a 3-F capacitor from 0 to 500 mV is 5 min, (2) the optimum charging capacitor value is 3 F, and (3) the optimum charging potential is 500 mV. Our results demonstrate that the developed method and the MFCT can be used to evaluate and optimize energy harvesting when a MFC is used with a capacitor to power wireless sensors monitoring the environment.