Photoautotrophic microalgae Scenedesmus obliquus attached on a cathode as oxygen producers for microbial fuel cell (MFC) operation

Photoautotrophic algae Scenedismus obliquus could attach on the surface of a cathode electrode and produced oxygen for electricity generation in microbial fuel cell (MFC). Oxygen concentration by algae aeration in the cathode chamber increased from 0 to 15.7 mg/l within 12-h, and a voltage generation of 0.47 ± 0.03 V was obtained with 1000 Ω external resistance. In polarization test, MFC with algal aeration exhibited the maximum power density of 153 mW/m², which was 32% higher than the value (116 mW/m²) with mechanical aeration at oxygen concentration of 5.9 mg/l. The internal resistance of MFC with algal aeration decreased in ohmic resistance (5.9–5.2 Ω) and charge transfer resistance (9.6–7.2 Ω) over 72-h operation. Cyclic voltammetry of cathode during algal aeration revealed higher reduction current of −9.3 mA compared to mechanical aeration (−4.7 mA).     

Pyrolyzing pyrite and microalgae for enhanced anode performance in microbial fuel cells

Developing low-cost and high-performance anodes is of great significance for wider applications of microbial fuel cells (MFCs). In this study, microalgae and pyrite were co-pyrolyzed (P/MC) and then coated on carbon felt (CF) with PTFE as a binder. P/MC modification resulted in increased electroactive surface area, superhydrophilicity and higher biocompatibility. Besides, the P/MC-CF anode reduced the charge transfer resistance from 35.1 Ω to 11.4 Ω. The highest output voltage and the maximum power density of the MFC equipped with the P/MC-CF anode were 657 mV and 1266.7 mW/m², respectively, which were much larger than that of the MFC with the CF anode (530 mV, 556.7 mW/m²). The P/MC-CF anode also displayed higher columbic efficiency (39.41%) than the CF anode (32.37%). This work suggests that pyrolyzing microalgae with pyrite is a promising method to enhance the performance of MFCs.     

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P_max = 13 mW; large: ESR = 4.2 Ω and P_max = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm⁻²) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm⁻² and 633 μW cm⁻², respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL⁻¹, 265 μW mL⁻¹ and 249 μW cm⁻¹, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g⁻¹-C- and 3270 μW g⁻¹-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.     

A graphite-granule membrane-less tubular air-cathode microbial fuel cell for power generation under continuously operational conditions

An air-cathode microbial fuel cell (MFC) is an efficient and sustainable MFC configuration for recovering electrical energy from organic substances. In this paper, we developed a graphite-granule anode, tubular air-cathode MFC (GTMFC) capable of continuous electricity generation from glucose-based substrates. This GTMFC produced a maximum volumetric power of 50.2 W m⁻³ at current density of 216 A m⁻³ (R_EX = 22 Ω). Electrochemistry impedance spectroscopy (EIS) measurements demonstrated an overall internal resistance of 27 Ω, consisting of ohmic resistance of R_ohm = 13.8 Ω (51.1%), a charge-transfer resistance of R_c = 6.1 Ω (22.6%) and a diffusion resistance of R_d = 7.2 Ω (26.3%). Power generation with respect to initial chemical oxygen demand (COD) concentration was described well by an exponential saturation model. Recirculation was to found to have a significant effect on electrochemical performance at low COD concentrations, while such effect was absent at high COD concentrations. This study suggests a feasible and simple method to reduce internal resistance and improve power generation of sustainable air-cathode MFCs.     

Microbial fuel cell constructed with micro-organisms isolated from industry effluent

Three types of aerobic bacteria such as Citrobacter freundii, Proteus mirabilis and Bacillus subtilis were evaluated in terms of bioelectricity production using double chambered microbial fuel cell (MFC) with graphite cloth as anode and cathode and Nafion membrane as proton exchange membrane (PEM). Performance of MFC was studied with addition of glucose. Cyclic voltammetry (CV) experiments showed the presence of peaks at −92 and −163 mV vs Ag/AgCl for C. freundii and P. mirabilis indicating their electrochemical activity without an external mediator. Potential time experiments showed the potential of MFC solely depend on change in anode potential rather than cathode potential. The internal resistance of MFC containing B. subtilis was lower than C. freundii and P. mirabilis. Fuel cell performance was evaluated employing polarization curve and power output along with cell potentials. MFC containing B. subtilis with neutral red mediator showed current output of 112 mA m⁻² at external resistance of 0.3 kΩ which is higher than the current outputs from MFC containing C. freundii and P. mirabilis. The relative efficiency of power generation observed in aerobic microenvironment may be attributed to the effective substrate oxidation and good biofilm growth observed on the anodic surface.     

Bioelectricity production on xylose with a compost enrichment culture

An exoelectrogenic culture was enriched on 1.0 g/L xylose from a compost sample in two-chamber microbial fuel cells (MFCs). Electricity production was optimized by changing mixing type, external resistance, xylose concentration and pH. Furthermore, the changes in microbial communities after each optimization step were monitored with PCR-DGGE. Electricity production was highly dependent on operational conditions that affected power densities (PD), Coulombic efficiencies (CE), substrate degradation, utilization of soluble metabolites for electricity production and stability of MFC performance. The optimum operational conditions for electricity production were without mixing, 100 Ω external resistance, 0.5 g/L xylose and pH 7. With optimized operational conditions PD of 590 mW/m² and CE of 82% were obtained. Microbial community composition, consisting mainly of Geobacter sulfurreducens, Escherichia coli, Sphaerochaeta sp. TQ1 and Bacteroides species, was mainly affected by MFC configuration, i.e. electrical connections, which likely affected the anode potential.     

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).     

Decomposition characteristics of propionate when changing the electrode material,external resistance and reactor temperature of microbial fuel cells

Assuming a series-type microbial fuel cell (MFC) that sequentially consumes organic acid components, the effects of the electrode material, external resistance, and temperature of MFC on the decomposition characteristics of acetate and propionate were investigated. As to electrode materials, propionate decomposition required less time in carbon cloth (CC) than that in carbon felt (CF), and maximum power was produced higher in CC than that in CF as well as acetate substrate. When the external resistance of 1000 Ω was replaced with 100 Ω or 10 Ω, both the decomposition rate and maximum power in propionate were lower than those in acetate, respectively. The time required for acetate decomposition at the temperature of 30 °C and 37 °C was 14.8 and 19.4 h, while 25.3 and 17.2 h in propionate at 30 °C and 37 °C, respectively. The microbial community changed significantly between 30 °C and 37 °C of the temperature.     

Occurrence of power overshoot for two-chambered MFC at nearly steady-state operation

At high electrical current regime power overshoot may occur in microbial fuel cell (MFC) operation with both cell voltage and electrical current declining at reduced external loads. This study demonstrated the power overshoot in a two-chamber MFC using acetate and oxygen respectively as anodic and cathodic fuels. The cell worked well until reaching 0.51 V and 790 mA/m² at power density of 400 mW/m²; further reducing external load leads to decrease in both cell voltage and generated current. During the noted power overshoot regime the internal resistance of MFC increased monotonically with decreasing external load. Based on the electrochemical analysis of anodic and cathodic losses, the occurrence of power overshoot is proposed to be determined by the combined resistance of intracellular loss and of extracellular electron transfer (EET) loss.     

Improved performance of a tubular microbial fuel cell with a composite anode of graphite fiber brush and graphite granules

In this study, a composite electrode combined of a graphite fiber brush and carbon granules (MFC-GFB/GG) was adopted as the anode of a tubular microbial fuel cell (MFC). Compared with an MFC with graphite granules (MFC-GG) and an MFC with a graphite fiber brush (MFC-GFB), MFC-GFB/GG showed a longer lag time during the start-up process, while it reached the highest operating voltage at 50 Ω. Furthermore, during the stable operation, the MFC-GFB/GG achieved the highest power density of 66.9 ± 1.6 W m⁻³, which was about 5.3 and 1.2 times as that of MFC-GG and MFC-GFB, respectively. The highest performance of the MFC-GFB/GG can be attributed to the highest active biomass content on the electrode and the smallest internal resistance of the MFC. The optimum COD concentration was found to be 500 mg COD L⁻¹ for MFC-GFB/GG.     

Optimization of culture conditions and electricity generation using Geobacter sulfurreducens in a dual-chambered microbial fuel-cell

The promise of generating electricity from the oxidation of organic substances using metal-reducing bacteria is drawing attention as an alternate form of bio-technology with positive environmental implications. In this study, we examined various experimental factors to obtain the maximum power output in a dual-chamber mediator-less microbial fuel-cell (MFC) using Geobacter sulfurreducens and acetate as an electron donor in a semi-continuous mode. The G. sulfurreducens culture conditions were optimized in a nutrient buffer containing 20 mM of acetate and 50 mM of fumarate at pH 6.8 and 30 °C. For use in the MFC system, electrodes were made with carbon paper (area: 11.5 cm²) and spaced 1.5 cm apart. Once the MFC was inoculated with the pre-cultured G. sulfurreducens in the anode chamber and while air was continuously sparged to the cathode chamber, the cells produced electricity stably over 60 days with the regular addition of 20 mM acetate, generating the maximum power density of 7 mW/m² with a 5000 Ω load. The current output was significantly increased, by 1.6 times after 20 days of incubation under the same experimental conditions, when the carbon-paper anode was coated with carbon nanotubes.     

Power recovery with multi-anode/cathode microbial fuel cells suitable for future large-scale applications

Multi-anode/cathode microbial fuel cells (MFCs) incorporate multiple MFCs into a single unit, which maintain high power generation at a low cost and small space occupation for the scale-up MFC systems. The power production of multi-anode/cathode MFCs was similar to the total power production of multiple single-anode/cathode MFCs. The power density of a 4-anode/cathode MFC was 1184 mW/m³, which was 3.2 times as that of a single-anode/cathode MFC (350 mW/m³). The effect of chemical oxygen demand (COD) was studied as the preliminary factor affecting the MFC performance. The power density of MFCs increased with COD concentrations. Multi-anode/cathode MFCs exhibited higher power generation efficiencies than single-anode/cathode MFCs at high CODs. The power output of the 4-anode/cathode MFCs kept increasing from 200 mW/m³ to 1200 mW/m³ as COD increased from 500 mg/L to 3000 mg/L, while the single-anode/cathode MFC showed no increase in the power output at CODs above 1000 mg/L. In addition, the internal resistance (R_in) exhibited strong dependence on COD and electrode distance. The R_in decreased at high CODs and short electrode distances. The tests indicated that the multi-anode/cathode configuration efficiently enhanced the power generation.     

Modified graphite electrode by polyaniline/tourmaline improves the performance of bio-cathode microbial fuel cell

Bio-cathode which uses microorganisms as catalyst can reduce MFC cost and sustain similar power output compared to noble metal catalyst. Thereby, looking for a cathode material which is high conductivity, good biocompatibility and even can stimulate and enhance activity of bio-catalyst is of great interest. In this paper, modified electrode by tourmaline and polyaniline (reactor 3) was used as cathode. The output power density was improved by 492.6% and 192.8% compared to reactor 1 (unmodified cathode) and reactor 2 (cathode modified only by polyaniline) (54 mW m⁻² for reactor 1, 138 mW m⁻² for reactor 2 and 266 mW m⁻² for reactor 3, respectively). When the external resistance was 800Ω, output voltages of reactor 1, 2 and 3 were kept at 0.20 ± 0.005 V, 0.26 ± 0.005 V and 0.37 ± 0.005 V, respectively. Cyclic voltammetry curves showed that reductive current of reactor 3 was higher than those of reactor 1 and 2, indicating that the cathode of reactor 3 had the strongest catalytic activity which was due to that tourmaline could help the interfacial electron transfer, and thereby facilitate the reduction of oxygen at the cathode. Results demonstrated that the tourmaline modified electrode could effectively improve the reduction reaction and enhance the performance of the whole MFC system.     

Effects of mediator producer and dissolved oxygen on electricity generation in a baffled stacking microbial fuel cell treating high strength molasses wastewater

The effects of Pseudomonas aeruginosa, pyocyanin, and influent dissolved oxygen (DO) on the electricity generation in a baffled stacking microbial fuel cell (MFC) treating high strength molasses wastewater were investigated. The result shows that the influent chemical oxygen demand (COD) of 500–1000 mg l⁻¹ had the optimal substrate-energy conversion rate. The addition of a low density of P. aeruginosa (8.2 mg l⁻¹) or P. aeruginosa with pyocyanin improved the COD removal and power generation. This improvement could be attributed to the enhancement of electron transfer with the help of redox mediators. Influent DO at a concentration of up to 1.22 mg l⁻¹ did not inhibit the electricity generation. Large proportions of COD, organic-N and total-N were removed by the MFC. The MFC effluent was highly biodegradable. Denaturing gradient gel electrophoresis analysis shows that the added pyocyanin resided in the MFC for up to 14 days. An analysis of anode voltage reveals that microbial proton transport to the cathode was importantly responsible for the internal resistance.     

Power generation from organic substrate in batch and continuous flow microbial fuel cell operations

Microbial fuel cells (MFCs) are biochemical-catalyzed systems in which electricity is produced by oxidizing biodegradable organic matters in presence of either bacteria or enzyme. This system can serve as a device for generating clean energy and, also wastewater treatment unit. In this paper, production of bioelectricity in MFC in batch and continuous systems were investigated. A dual chambered air–cathode MFC was fabricated for this purpose. Graphite plates were used as electrodes and glucose as a substrate with initial concentration of 30 g l⁻¹ was used. Cubic MFC reactor was fabricated and inoculated with Saccharomyces cerevisiae PTCC 5269 as active biocatalyst. Neutral red with concentration of 200 μmol l⁻¹ was selected as electron shuttle in anaerobic anode chamber. In order to enhance the performance of MFC, potassium permanganate at 400 μmol l⁻¹ concentration as oxidizer was used. The performance of MFC was analyzed by the measurement of polarization curve and cyclic volatmmetric data as well. Closed circuit voltage was obtained using a 1 kΩ resistance. The voltage at steady-state condition was 440 mV and it was stable for the entire operation time. In a continuous system, the effect of hydraulic retention time (HRT) on performance of MFC was examined. The optimum HRT was found to be around 7 h. Maximum produced power and current density at optimum HRT were 1210 mA m⁻² and 283 mW m⁻², respectively.     

A non-noble V2O5 nanorods as an alternative cathode catalyst for microbial fuel cell applications

This study assessed the feasibility of vanadium pentoxide (V₂O₅) as a novel cathode catalyst material in air-cathode single chamber microbial fuel cells (SCMFCs). The V₂O₅ nanorod catalyst was synthesized using a hydrothermal method. MFCs with different cathode catalyst loadings were studied. Cyclic voltammetry (CV) was used to examine the electrochemical behavior of the catalysts in the MFCs. The V₂O₅ cathode catalyst constructed with a double loading MFC exhibited the highest maximum power density of 1073 ± 18 mW m⁻² (OCP; 691±4 mV) compared with 447 ± 12 mW m⁻² (OCP; 594 ± 5 mV) and 936 ± 15 mW m⁻² (OCP; 647±5 mV) for the single loading MFC and triple loading MFC, respectively. The power density of MFC with double loaded V₂O₅ is comparable to the traditional Pt/C cathode (2067 ± 25 mW m⁻², OCP; 821 ± 4 mV), which covers up to 55% of the performance of Pt/C. This finding highlights the potential of the V₂O₅ cathode as an inexpensive catalyst material for MFCs that may have commercial applications.     

Biomass pectin-derived N,S-enriched carbon with hierarchical porous structure as a metal-free catalyst for enhancing bio-electricity generation

Heteroatoms-doped carbon-based materials (with non-precious metals or no metals) with porous structure have already shown high catalytic activities for oxygen reduction reaction (ORR), especially in microbial fuel cells (MFCs). Here, we use pectin extracted from pomelo peels as carbon source to prepare metal-free and sulphur/nitrogen co-doped partially-graphitized carbon (HP-SN-PGCs) by using silica nanospheres as sacrificial templates. Single-chamber MFC (SC-MFC) with HP-SN-PGC-0.5 (0.5 g of silica) cathode has the shortest start-up time (45 h) and lowest charge transfer resistance (19.3 Ω). The maximum power density of HP-SN-PGC-0.5 (1161.34 mW m⁻²) cathode is higher than that of Pt/C (1116.90 mW m⁻²) at the initial cycle. After 75 d operation, power density of HP-SN-PGC-0.5 cathode only declines 4.6%, which is more stable than that of Pt/C (37.69%). HP-SN-PGC-0.5 has a highly porous structure (869.25 m² g⁻¹) by removal of templates and Fe species (as the graphitization catalyst) to facilitate exposure of active sites and diffusion of ORR-related intermediates (OH⁻ and HO₂⁻, etc) to accessible active sites. N and S species provide highly active sites to enhance OH⁻ generation to conduct the 4e⁻ ORR process. Thus, this study presents a viable ORR catalyst with high activity and long-term stability for bio-electricity generation from organic wastewater in SC-MFCs.     

Performance comparison of up-flow microbial fuel cells fabricated using proton exchange membrane and earthen cylinder

Continuous bioelectricity generation was studied in a novel up-flow bio-cathode microbial fuel cell (MFC). The performance of MFC-1, employing commercially available proton exchange membrane (PEM), was evaluated under different organic loading rates (OLRs). Maximum volumetric power density of 10.04 W m⁻³ was obtained in MFC-1 at the OLR of 0.923 kg COD m⁻³ d⁻¹. Overall chemical oxygen demand (COD) removal efficiency more than 90% was achieved under all the OLRs. The performance of MFC-1 was compared with MFC-2, in which the inner anode chamber was made up of earthen cylinder, without employing polymer membrane. MFC-2 generated maximum volumetric power density of 14.59 W m⁻³ at OLR of 0.923 kg COD m⁻³ d⁻¹, which was 46% higher than that produced in MFC-1. The internal resistance of MFC-1 (96 Ω) was higher than MFC-2 (69 Ω). The earthen cylinder MFC demonstrated better COD removal and power generation than the MFC employing PEM.     

Microbial fuel cell – A novel self-powered wastewater electrolyser for electrocoagulation of heavy metals

This paper describes the suitability of the Microbial Fuel Cell (MFC) for generation of electrical power with a simultaneous synthesis of active catholyte in the form of caustic solution. The active solution formed inside a terracotta based MFC reactor was a product of self-powered wastewater electrolysis utilizing i) wastewater with added sodium acetate as a carbon source and ii) neat urine. The catholyte solution that has been actively synthesized was harvested and used for precipitation of heavy metals such as: iron, copper and zinc showing its suitability for use in electro-coagulation (electro-flocculation). This proposed alternative approach to self-powered electrocoagulation is based on electrochemically formed caustic catholyte within the inner cathode chamber of the MFC and then used ex situ to form metal hydroxides that precipitate out from heavy metal solutions.     

Microbial fuel cell (MFC) using commercially available unglazed ceramic wares: Low-cost ceramic separators suitable for scale-up

This study aimed to evaluate the influence of commercially available unglazed wall ceramic (UGWC) and unglazed floor ceramic (UGFC) separators with different thickness and porosity on the performance of dual-chamber microbial fuel cells (MFCs). These MFCs were operated under continuous condition using domestic wastewater. The UGWC-based MFC produced higher maximum power density (321 mW/m² with a thickness of 9 mm) than UGFC-based MFC (106.89 mW/m² with a thickness of 3 mm) due to lower internal resistance. Power generation using both types of separators was lower than that of obtained using the Nafion 117 membrane as control (602 mW/m²). The maximum average coulombic efficiencies (CE) of the UGWC-based MFCs (with thickness levels of 6 and 9 mm) were 58% and 68%, respectively, which was more than that of UGFC-based MFCs and also control MFC (53%). Voltammetric analysis revealed that the maximum peak current of 6 mA was obtained for UGWC-based MFC which was in the order of control MFC (5.9 mA). The UGWC separators exhibited smaller ohmic and diffusion resistances of 57, 65 and 87 Ω in MFCs at the thickness levels of 3, 6 and 9 mm, respectively, compared to the UGFC separators with that of 164.27 and 366.23 Ω in MFCs at the thickness levels of 3 and 6 mm, respectively. UGWC separators because of their low production cost, high mechanical strength and increased output power density of the MFC proved to be a suitable alternative to replace with a costly polymeric membrane such as Nafion 117.