Solid phase microbial fuel cell (SMFC) for harnessing bioelectricity from composite food waste fermentation: influence of electrode assembly and buffering capacity

Solid phase microbial fuel cells (SMFC; graphite electrodes; open-air cathode) were designed to evaluate the potential of bioelectricity production by stabilizing composite canteen based food waste. The performance was evaluated with three variable electrode-membrane assemblies. Experimental data depicted feasibility of bioelectricity generation from solid state fermentation of food waste. Distance between the electrodes and presence of proton exchange membrane (PEM) showed significant influence on the power yields. SMFC-B (anode placed 5 cm from cathode-PEM) depicted good power output (463 mV; 170.81 mW/m²) followed by SMFC-C (anode placed 5 cm from cathode; without PEM; 398 mV; 53.41 mW/m²). SMFC-A (PEM sandwiched between electrodes) recorded lowest performance (258 mV; 41.8 mW/m²). Sodium carbonate amendment documented marked improvement in power yields due to improvement in the system buffering capacity. SMFCs operation also documented good substrate degradation (COD, 76%) along with bio-ethanol production. The operation of SMFC mimicked solid-sate fermentation which might lead to sustainable solid waste management practices.     

Evaluation of hydrolysis and fermentation rates in microbial fuel cells

This study determined the influence of substrate degradation on power generation in microbial fuel cells (MFCs) and microbial community selection on the anode. Air cathode MFCs were fed synthetic medium containing different substrates (acetate, glucose and starch) using primary clarifier sewage as source of electroactive bacteria. The complexity of the substrate affected the MFC performance both for power generation and COD removal. Power output decreased with an increase in substrate complexity from 99 ± 2 mW m⁻² for acetate to 4 ± 2 mW m⁻² for starch. The organic matter removal and coulombic efficiency (CE) of MFCs with acetate and glucose (82% of COD removal and 26% CE) were greater than MFCs using starch (60% of COD removal and 19% of CE). The combined hydrolysis–fermentation rate obtained (0.0024 h⁻¹) was considerably lower than the fermentation rate (0.018 h⁻¹), indicating that hydrolysis of complex compounds limits current output over fermentation. Statistical analysis of microbial community fingerprints, developed on the anode, showed that microbial communities were enriched according to the type of substrate used. Microbial communities producing high power outputs (fed acetate) clustered separately from bacterial communities producing low power outputs (fed complex compounds).     

Long-term performance of a plant microbial fuel cell with Spartina anglica

The plant microbial fuel cell is a sustainable and renewable way of electricity production. The plant is integrated in the anode of the microbial fuel cell which consists of a bed of graphite granules. In the anode, organic compounds deposited by plant roots are oxidized by electrochemically active bacteria. In this research, salt marsh species Spartina anglica generated current for up to 119 days in a plant microbial fuel cell. Maximum power production was 100 mW m⁻² geometric anode area, highest reported power output for a plant microbial fuel cell. Cathode overpotential was the main potential loss in the period of oxygen reduction due to slow oxygen reduction kinetics at the cathode. Ferricyanide reduction improved the kinetics at the cathode and increased current generation with a maximum of 254%. In the period of ferricyanide reduction, the main potential loss was transport loss. This research shows potential application of microbial fuel cell technology in salt marshes for bio-energy production with the plant microbial fuel cell.     

A multi-electrode continuous flow microbial fuel cell with separator electrode assembly design

Scaling up microbial fuel cells (MFCs) requires the development of compact reactors with multiple electrodes. A scalable single chamber MFC (130 mL), with multiple graphite fiber brush anodes and a single air-cathode cathode chamber (27 m²/m³), was designed with a separator electrode assembly (SEA) to minimize electrode spacing. The maximum voltage produced in fed-batch operation was 0.65 V (1,000 Ω) with a textile separator, compared to only 0.18 V with a glass fiber separator due to short-circuiting by anode bristles through this separator with the cathode. The maximum power density was 975 mW/m², with an overall chemical oxygen demand (COD) removal of >90% and a maximum coulombic efficiency (CE) of 53% (50 Ω resistor). When the reactor was switched to continuous flow operation at a hydraulic retention time (HRT) of 8 h, the cell voltage was 0.21 ± 0.04 V, with a very high CE = 85%. Voltage was reduced to 0.13 ± 0.03 V at a longer HRT = 16 h due to a lower average COD concentration, and the CE (80%) decreased slightly with increased oxygen intrusion into the reactor per amount of COD removed. Total internal resistance was 33 Ω, with a solution resistance of 2 Ω. These results show that the SEA type MFC can produce stable power and a high CE, making it useful for future continuous flow treatment using actual wastewaters.     

Treatment of biodiesel production wastes with simultaneous electricity generation using a single-chamber microbial fuel cell

Biodiesel production through transesterification of lipids generates large quantity of biodiesel waste (BW) containing mainly glycerin. BW can be treated in various ways including distillation to produce glycerin, use as substrate for fermentative propanediol production and discharge as wastes. This study examined microbial fuel cells (MFCs) to treat BW with simultaneous electricity generation. The maximum power density using BW was 487 ± 28 mW/m² cathode (1.5 A/m² cathode) with 50 mM phosphate buffer solution (PBS) as the electrolyte, which was comparable with 533 ± 14 mW/m² cathode obtained from MFCs fed with glycerin medium (COD 1400 mg/L). The power density increased from 778 ± 67 mW/m² cathode using carbon cloth to 1310 ± 15 mW/m² cathode using carbon brush as anode in 200 mM PBS electrolyte. The power density was further increased to 2110 ± 68 mW/m² cathode using the heat-treated carbon brush anode. Coulombic efficiencies (CEs) increased from 8.8 ± 0.6% with carbon cloth anode to 10.4 ± 0.9% and 18.7 ± 0.9% with carbon brush anode and heat-treated carbon brush anode, respectively.     

Microorganism-immobilized carbon nanoparticle anode for microbial fuel cells based on direct electron transfer

A fast and convenient bacterial immobilization method was proposed as an attempt to improve the anode efficiency of a microbial fuel cell, in which bacteria were entrapped into carbon nanoparticle matrix. The direct electron transfer from the entrapped bacterial cells to the anode was verified using cyclic voltammogram (CV). Using the immobilized bioanode, the start-up time of the MFC was greatly reduced. Meanwhile, the maximum power density of 1,947 mW m⁻² with the modified anode was much higher than that with the biofilm-based carbon cloth anode (1,479 mW m⁻²). Impedance measurements suggested that performance improvement resulted from the decrease in charge transfer and diffusion resistances. The results demonstrated that bacteria immobilization using carbon nanoparticle matrix was a simple and efficient approach for improving the anodes performances in MFCs.     

Improved performance of membrane free single-chamber air-cathode microbial fuel cells with nitric acid and ethylenediamine surface modified activated carbon fiber felt anodes

Surface modifications of anode materials are important for enhancing power generation of microbial fuel cell (MFC). Membrane free single-chamber air-cathode MFCs, MFC-A and MFC-N, were constructed using activated carbon fiber felt (ACF) anodes treated by nitric acid and ethylenediamine (EDA), respectively. Experimental results showed that the start-up time to achieve the maximum voltages for the MFC-A and MFC-N was shortened by 45% and 51%, respectively as compared to that for MFC-AT equipped with an unmodified anode. Moreover, the power output of MFCs with modified anodes was significantly improved. In comparison with MFC-AT which had a maximum power density of 1304 mW/m², the MFC-N achieved a maximum power density of 1641 mW/m². The nitric acid-treated anode in MFC-A increased the power density by 58% reaching 2066 mW/m². XPS analysis of the treated and untreated anode materials indicated that the power enhancement was attributable to the changes of surface functional groups.     

Microbial fuel cell with an azo-dye-feeding cathode

Microbial fuel cells (MFCs) were constructed using azo dyes as the cathode oxidants to accept the electrons produced from the respiration of Klebsiella pneumoniae strain L17 in the anode. Experimental results showed that a methyl orange (MO)-feeding MFC produced a comparable performance against that of an air-based one at pH 3.0 and that azo dyes including MO, Orange I, and Orange II could be successfully degraded in such cathodes. The reaction rate constant (k) of azo dye reduction was positively correlated with the power output which was highly dependent on the catholyte pH and the dye molecular structure. When pH was varied from 3.0 to 9.0, the k value in relation to MO degradation decreased from 0.298 to 0.016 μmol min⁻¹, and the maximum power density decreased from 34.77 to 1.51 mW m⁻². The performances of the MFC fed with different azo dyes can be ranked from good to poor as MO > Orange I > Orange II. Furthermore, the cyclic voltammograms of azo dyes disclosed that the pH and the dye structure determined their redox potentials. A higher redox potential corresponded to a higher reaction rate.     

Simultaneous carbon and nitrogen removal from piggery wastewater using loop configuration microbial fuel cell

Simultaneous carbon and nitrogen removal using loop configuration microbial fuel cell (MFC) with relatively large size of 5 L was investigated in this study. Four MFC reactors were constructed with a loop configuration to eliminate the pH gradient, and the reactor performance was examined with different separators and cathode materials. The performance of the reactors in terms of electricity generation and contaminant removal rate was examined. Results showed that a maximum power density of 1415.6 mW/m³ (The empty bed volume of anode chamber) was obtained at a current density of 3258.5 mA/m³ with cation exchange membrane as separator and graphite felt (Pt coated) as cathode using the piggery wastewater as feed, and the organic removal rate obtained was approximately 0.523 kg COD/m³/d (total anode chamber) with nitrogen removal rate of 0.194 kg N/m³/d (total cathode chamber).     

Proton exchange membrane and electrode surface areas as factors that affect power generation in microbial fuel cells

Power generation in microbial fuel cells (MFCs) is a function of the surface areas of the proton exchange membrane (PEM) and the cathode relative to that of the anode. To demonstrate this, the sizes of the anode and cathode were varied in two-chambered MFCs having PEMs with three different surface areas (A _PEM=3.5, 6.2, or 30.6 cm²). For a fixed anode and cathode surface area (A _An=A _Cat=22.5 cm²), the power density normalized to the anode surface area increased with the PEM size in the order 45 mW/m² (A _PEM=3.5 cm²), 68 mW/m² (A _PEM=6.2 cm²), and 190 mW/m² (A _PEM=30.6 cm²). PEM surface area was shown to limit power output when the surface area of the PEM was smaller than that of the electrodes due to an increase in internal resistance. When the relative cross sections of the PEM, anode, and cathode were scaled according to 2A _Cat=A_PEM=2A _An, the maximum power densities of the three different MFCs, based on the surface area of the PEM (A _PEM=3.5, 6.2, or 30.6 cm²), were the same (168±4.53 mW/m²). Increasing the ionic strength and using ferricyanide at the cathode also increased power output.     

Enhancement in current density and energy conversion efficiency of 3-dimensional MFC anodes using pre-enriched consortium and continuous supply of electron donors

Using a pre-enriched microbial consortium as the inoculum and continuous supply of carbon source, improvement in performance of a three-dimensional, flow-through MFC anode utilizing ferricyanide cathode was investigated. The power density increased from 170 W/m³ (1800 mW/m²) to 580 W/m³ (6130 mW/m²), when the carbon loading increased from 2.5 g/l-day to 50 g/l-day. The coulombic efficiency (CE) decreased from 90% to 23% with increasing carbon loading. The CEs are among the highest reported for glucose and lactate as the substrate with the maximum current density reaching 15.1 A/m². This suggests establishment of a very high performance exoelectrogenic microbial consortium at the anode. A maximum energy conversion efficiency of 54% was observed at a loading of 2.5 g/l-day. Biological characterization of the consortium showed presence of Burkholderiales and Rhodocyclales as the dominant members. Imaging of the biofilms revealed thinner biofilms compared to the inoculum MFC, but a 1.9-fold higher power density.     

Litre-scale microbial fuel cells operated in a complete loop

Using the anode effluent to compensate the alkalinization in a bio-cathode has recently been proposed as a way to operate a microbial fuel cell (MFC) in a continuous and pH neutral way. In this research, we successfully demonstrated that the operation of a MFC without any pH adjustments is possible by completing the liquid loop over cathode and anode. During the complete loop operation, a stable current production of 23.2 ± 2.5 A m⁻³ MFC was obtained, even in the presence of 3.2–5.2 mg O₂ L⁻¹ in the anode. The use of current collectors and subdivided electrical circuitries for relative large 2.5-L-scale MFCs resulted in ohmic cell resistances in the order of 1.4–1.7 mΩ m³ MFC, which were comparable to values of ten times smaller MFCs. Nevertheless, the bio-cathode activity still needs to be improved significantly with a factor 10–50 in order achieve desirable current densities of 1,000 A m⁻³ MFC. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Evaluation of biocathodes in freshwater and brackish sediment microbial fuel cells

Biofilms on biocathodes can catalyze the cathodic oxygen reduction and accordingly guarantee high cathode redox potentials. The present research assessed the use of biocathodes in full-sediment microbial fuel cells. Carbon felt-based biocathodes were evaluated in freshwater systems, and an extension of their application to brackish systems and/or stainless steel webs as base material was considered. Efficient biocathodes could be developed within days through inoculation with active microorganisms. Carbon felt was found most suited for the biocathodes in freshwater with increased performance at salinities around 80–250 mM. Maximum long-term performance reached 12.3 μW cm⁻² cathode. The relative benefit of stainless steel seemed to increase with increasing salinity. A combination of stainless steel cathodes with biofilms could, however, also result in decreased electrical performance. In an efficiently catalyzing cathodic biofilm, an enrichment with an uncultured Proteobacterium—previously correlated with steel waste—was observed.     

The effect of flavin electron shuttles in microbial fuel cells current production

The effect of electron shuttles on electron transfer to microbial fuel cell (MFC) anodes was studied in systems where direct contact with the anode was precluded. MFCs were inoculated with Shewanella cells, and flavins used as the electron shuttling compound. In MFCs with no added electron shuttles, flavin concentrations monitored in the MFCs' bulk liquid increased continuously with FMN as the predominant flavin. The maximum concentrations were 0.6 μM for flavin mononucleotide and 0.2 μM for riboflavin. In MFCs with added flavins, micro-molar concentrations were shown to increase current and power output. The peak current was at least four times higher in MFCs with high concentrations of flavins (4.5–5.5 μM) than in MFCs with low concentrations (0.2–0.6 μM). Although high power outputs (around 150 mW/m²) were achieved in MFCs with high concentrations of flavins, a Clostridium-like bacterium along with other reactor limitations affected overall coulombic efficiencies (CE) obtained, achieving a maximum CE of 13%. Electron shuttle compounds (flavins) permitted bacteria to utilise a remote electron acceptor (anode) that was not accessible to the cells allowing current production until the electron donor (lactate) was consumed.     

Application of Co-naphthalocyanine (CoNPc) as alternative cathode catalyst and support structure for microbial fuel cells

Co-naphthalocyanine (CoNPc) was prepared by heat treatment for cathode catalysts to be used in microbial fuel cells (MFCs). Four different catalysts (Carbon black, NPc/C, CoNPc/C, Pt/C) were compared and characterized using XPS, EDAX and TEM. The electrochemical characteristics of oxygen reduction reaction (ORR) were compared by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The Co-macrocyclic complex improves the catalyst dispersion and oxygen reduction reaction of CoNPc/C. The maximum power of CoNPc/C was 64.7 mW/m² at 0.25 mA as compared with 81.3 mW/m² of Pt/C, 29.7 mW/m² of NPc/C and 9.3 mW/m² of carbon black when the cathodes were implemented in H-type MFCs. The steady state cell, cathode and anode potential of MFC with using CoNPc/C were comparable to those of Pt/C.     

Electricity generation at high ionic strength in microbial fuel cell by a newly isolated Shewanella marisflavi EP1

Increasing the ionic strength of the electrolyte in a microbial fuel cell (MFC) can remarkably increase power output due to the reduction of internal resistance. However, only a few bacterial strains are capable of producing electricity at a very high ionic strength. In this report, we demonstrate a newly isolated strain EP1, belonging to Shewanella marisflavi based on polyphasic analysis, which could reduce Fe(III) and generate power at a high ionic strength of up to 1,488 mM (8% NaCl) using lactate as the electron donor. Using this bacterium, a measured maximum power density of 3.6 mW/m² was achieved at an ionic strength of 291 mM. The maximum power density was increased by 167% to 9.6 mW/m² when ionic strength was increased to 1,146 mM. However, further increasing the ionic strength to 1,488 mM resulted in a decrease in power density to 5.2 mW/m². Quantification of the internal resistance distribution revealed that electrolyte resistance was greatly reduced from 1,178 to 50 Ω when ionic strength increased from 291 to 1,488 mM. These results indicate that isolation of specific bacterial strains can effectively improve power generation in some MFC applications.     

The anode potential regulates bacterial activity in microbial fuel cells

The anode potential in microbial fuel cells controls both the theoretical energy gain for the microorganisms as the output of electrical energy. We operated three reactors fed with acetate continuously at a poised anode potential of 0 (R ₀), −200 (R ₋₂₀₀) and −400 (R ₋₄₀₀) mV versus Ag/AgCl and investigated the resulting bacterial activity. The anode potential had no influence on the start-up time of the three reactors. During a 31-day period, R ₋₂₀₀ produced 15% more charge compared to R ₀ and R ₋₄₀₀. In addition, R ₋₂₀₀ had the highest maximal power density (up to 199 W m⁻³ total anode compartment during polarization) but the three reactors evolved to the same power density at the end of the experimental period. During polarization, only the current of R ₋₄₀₀ levelled off at an anode potential of −300 mV versus Ag/AgCl. The maximum respiration rate of the bacteria during batch tests was also considerably lower for R ₋₄₀₀. The specific biomass activity however, was the highest for R ₋₄₀₀ (6.93 g chemical oxygen demand g⁻¹ biomass-volatile suspended solids (VSS) d⁻¹ on day 14). This lowered during the course of the experiment due to an increase of the biomass concentration to an average level of 578 ± 106 mg biomass-VSS L⁻¹ graphite granules for the three reactors. This research indicated that an optimal anode potential of −200 mV versus Ag/AgCl exists, regulating the activity and growth of bacteria to sustain an enhanced current and power generation.     

Air-cathode structure optimization in separator-coupled microbial fuel cells

Microbial fuel cells (MFC) with 30% wet-proofed air cathodes have previously been optimized to have 4 diffusion layers (DLs) in order to limit oxygen transfer into the anode chamber and optimize performance. Newer MFC designs that allow close electrode spacing have a separator that can also reduce oxygen transfer into the anode chamber, and there are many types of carbon wet-proofed materials available. Additional analysis of conditions that optimize performance is therefore needed for separator-coupled MFCs in terms of the number of DLs and the percent of wet proofing used for the cathode. The number of DLs on a 50% wet-proofed carbon cloth cathode significantly affected MFC performance, with the maximum power density decreasing from 1427 to 855 mW/m² for 1–4 DLs. A commonly used cathode (30% wet-proofed, 4 DLs) produced a maximum power density (988 mW/m²) that was 31% less than that produced by the 50% wet-proofed cathode (1 DL). It was shown that the cathode performance with different materials and numbers of DLs was directly related to conditions that increased oxygen transfer. The coulombic efficiency (CE) was more affected by the current density than the oxygen transfer coefficient for the cathode. MFCs with the 50% wet-proofed cathode (2 DLs) had a CE of >84% (6.8 A/m²), which was substantially larger than that previously obtained using carbon cloth air-cathodes lacking separators. These results demonstrate that MFCs constructed with separators should have the minimum number of DLs that prevent water leakage and maximize oxygen transfer to the cathode.     

Increased Power in Sediment Microbial Fuel Cell: Facilitated Mass Transfer via a Water-Layer Anode Embedded in Sediment

We report a methodology for enhancing the mass transfer at the anode electrode of sediment microbial fuel cells (SMFCs), by employing a fabric baffle to create a separate water-layer for installing the anode electrode in sediment. The maximum power in an SMFC with the anode installed in the separate water-layer (SMFC-wFB) was improved by factor of 6.6 compared to an SMFC having the anode embedded in the sediment (SMFC-woFB). The maximum current density in the SMFC-wFB was also 3.9 times higher (220.46 mA/m²) than for the SMFC-woFB. We found that the increased performance in the SMFC-wFB was due to the improved mass transfer rate of organic matter obtained by employing the water-layer during anode installation in the sediment layer. Acetate injection tests revealed that the SMFC-wFB could be applied to natural water bodies in which there is frequent organic contamination, based on the acetate flux from the cathode to the anode.     

Investigating microbial fuel cell bioanode performance under different cathode conditions

A compact, three‐in‐one, flow‐through, porous, electrode design with minimal electrode spacing and minimal dead volume was implemented to develop a microbial fuel cell (MFC) with improved anode performance. A biofilm‐dominated anode consortium enriched under a multimode, continuous‐flow regime was used. The increase in the power density of the MFC was investigated by changing the cathode (type, as well as catholyte strength) to determine whether anode was limiting. The power density obtained with an air‐breathing cathode was 56 W/m³ of net anode volume (590 mW/m²) and 203 W/m³ (2160 mW/m²) with a 50‐mM ferricyanide‐based cathode. Increasing the ferricyanide concentration and ionic strength further increased the power density, reaching 304 W/m³ (3220 mW/m², with 200 mM ferricyanide and 200 mM buffer concentration). The increasing trend in the power density indicated that the anode was not limiting and that higher power densities could be obtained using cathodes capable of higher rates of oxidation. The internal solution resistance for the MFC was 5–6 Ω, which supported the improved performance of the anode design. A new parameter defined as the ratio of projected surface area to total anode volume is suggested as a design parameter to relate volumetric and area‐based power densities and to enable comparison of various MFC configurations. Published 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009