Improve 3D electrode materials performance on electricity generation from livestock wastewater in microbial fuel cell

Swine wastewater that is collected from animal husbandry has organic high ammonia nitrogen. In this study, swine wastewater is converted into electrical energy using microbial fuel cells (MFCs). Carbon fibers are respectively combined with zinc-coated metallic wires or stainless steel wires in order to form different laminated electrodes, whose influence on the electricity generation of MFCs is then examined. The 3D laminated FN/carbon composites are used as electrodes, the stable electricity voltage is 291 mV and the COD removal efficiency reaches 81％. In contrast, SS/carbon composites only contribute to a stable electricity voltage of 12.3 mV and COD removal efficiency of 33%. Based on the surface contact angle test and the scanning electron microscopy (SEM) observation, the laminated FN/carbon composites have greater hydrophilicity and wettability than the laminated SS/carbon composites, and thus have a positive influence on the electricity generation of MFCs.     

Electricity generation and COD removal of microbial fuel cells (MFCs) operated with alkaline substrates

The characteristics of electricity generation and COD removal of dual-chamber microbial fuel cells (MFCs) operated with alkaline substrates were studied. Substrates with constant pH of either 7 or 9 as well as varying pH in a cycle of 7-8-9-8-7 were used. MFCs operated with these substrates were denoted as MFC-pH7, MFC-pH9 and MFC-pHV, respectively. The experimental results indicate that the MFC-pHV can generate the highest performance of 2554 ± 159 mW/m². Cyclic voltammetry (CV), active biomass and electrochemical impedance spectroscopy (EIS) measurements were conducted and these results suggested that the MFC-pHV had the highest electrochemical activity per unit biomass and the lowest internal resistance, which together contributed to the improved power output of the MFC-pHV. In addition, compared with the other two MFCs operated at fixed pH values, the COD removal efficiency of the MFC-pHV was improved due to the stronger adaptation to the varying pH-environment.     

Effects of furan derivatives and phenolic compounds on electricity generation in microbial fuel cells

Lignocellulosic biomass is an attractive fuel source for MFCs due to its renewable nature and ready availability. Furan derivatives and phenolic compounds could be potentially formed during the pre-treatment process of lignocellulosic biomass. In this study, voltage generation from these compounds and the effects of these compounds on voltage generation from glucose in air-cathode microbial fuel cells (MFCs) were examined. Except for 5-hydroxymethyl furfural (5-HMF), all the other compounds tested were unable to be utilized directly for electricity production in MFCs in the absence of other electron donors. One furan derivate, 5-HMF and two phenolic compounds, trans-cinnamic acid and 3,5-dimethoxy-4-hydroxy-cinnamic acid did not affect electricity generation from glucose at a concentration up to 10 mM. Four phenolic compounds, including syringaldeyhde, vanillin, trans-4-hydroxy-3-methoxy, and 4-hydroxy cinnamic acids inhibited electricity generation at concentrations above 5 mM. Other compounds, including 2-furaldehyde, benzyl alcohol and acetophenone, inhibited the electricity generation even at concentrations less than 0.2 mM. This study suggests that effective electricity generation from the hydrolysates of lignocellulosic biomass in MFCs may require the employment of the hydrolysis methods with low furan derivatives and phenolic compounds production, or the removal of some strong inhibitors prior to the MFC operation, or the improvement of bacterial tolerance against these compounds through the enrichment of new bacterial cultures or genetic modification of the bacterial strains.     

Microbial fuel cells for bioelectricity generation through reduction of hexavalent chromium in wastewater: A review

The study proposes the use of microbial fuel cell (MFC) technology to reduce toxic Cr(VI) present in industrial wastewater to less toxic trivalent chromium [Cr(III)], while generating electricity through a bioelectrochemical oxidation-reduction process. Factors influencing the treatment process and electricity generation include the concentration of Cr(VI) in wastewater, substrate types used for anodes, types of microorganisms involved, types of cathode and anode, surface area of the cathode and anode, and pH and temperature of cathodic and anodic solutions. While other heavy metals in wastewater may be removed by MFC technology, Cr(VI) removal is more efficient in terms of electricity generation. Previous research indicated that the maximum electrical power generated by Cr(VI) removal through the use of MFCs is 1600 mW/m², which is expected to increase as the factors affecting this process are optimized. Based on current data, MFC-based electricity generation along with Cr(VI) removal is a potential future source of sustainable energy. However, research priorities need to focus on reducing the cost of MFC technology by using economical and effective materials and increasing electricity production.     

A monetary comparison of energy recovered from microbial fuel cells and microbial electrolysis cells fed winery or domestic wastewaters

Microbial fuel (MFCs) and electrolysis cells (MECs) can be used to recover energy directly as electricity or hydrogen from organic matter. Organic removal efficiencies and values of the different energy products were compared for MFCs and MECs fed winery or domestic wastewater. TCOD removal (%) and energy recoveries (kWh/kg-COD) were higher for MFCs than MECs with both wastewaters. At a cost of $4.51/kg-H₂ for winery wastewater and $3.01/kg-H₂ for domestic wastewater, the hydrogen produced using MECs cost less than the estimated merchant value of hydrogen ($6/kg-H₂). 16S rRNA clone libraries indicated the predominance of Geobacter species in anodic microbial communities in MECs for both wastewaters, suggesting low current densities were the result of substrate limitations. The results of this study show that energy recovery and organic removal from wastewater are more effective with MFCs than MECs, but that hydrogen production from wastewater fed MECs can be cost effective.     

Three-dimensional graphitic mesoporous carbon-doped carbon felt bioanodes enables high electric current production in microbial fuel cells

Microbial fuel cells (MFCs) represent a new approach that can simultaneously enhance the treatment of waste streams and generate electricity. Although MFCs represent a promising technology for renewable energy production, they have not been successfully scaled-up mainly due to the relatively-low electricity generation and high cost associated with MFCs operation. Here, we investigated whether graphitic mesoporous carbon (GMC) decoration of carbon felt would improve the conductivity and biocompatibility of carbon felt anodes, leading to higher biomass attachment and electricity generation in MFCs fed with an organic substrate. To test this hypothesis, we applied 3 different GMC loading (i.e., 2, 5, and 10 mg/cm² of anode surface area) in MFCs compared to control MFCs (with pristine carbon felt electrodes). We observed that the internal resistances of modified anodes with GMC were 1.2–2.3-order of magnitude less than pristine carbon felt anode, leading to maximum power densities of 70.3, 33.3, and 9.8 mW/m² for 10, 5, and 2 mg/cm²-doped anode, respectively compared to only 3.8 mW/m² for the untreated carbon felt. High-throughput sequencing revealed that increasing the GMC loading rate was associated with enriching more robust anode-respiring bacteria (ARB) biofilm community. These results demonstrate that 3-D GMC-doped carbon felt anodes could be a potential alternative to other expensive metal-based electrodes for achieving high electric current densities in MFCs fed with organic substrates, such as wastewater. Most importantly, high electron transfer capability, strong chemical stability, low cost, and excellent mechanical strength of 3-D GMC-doped carbon felt open up new opportunities for scaling-up of MFCs using cheap and high-performance anodes.     

Design and research progress of nano materials in cathode catalysts of microbial fuel cells: A review

With the advantages of clean, efficient and energy-saving, microbial fuel cells (MFCs) were characterized with perfect significance in the field of degrading environmental pollutants and generating electricity meanwhile. The cathode materials affected the activity of oxygen reduction reaction (ORR), and affected the power generation performance for MFCs. There were many kinds of nano materials played an important role in the field of cathode catalysis. The advantages of metal and non-metal composites were easy to obtain and low cost; layered double hydroxide (LDH) was easy to control and compound, and could be fully realized functionalization; metal organic frameworks (MOFs) were widely used since their porosity, high specific surface area and high activity; covalent organic frameworks (COFs) were low density and easy to be modified, so as to modify and realize functionalization; MXene was an excellent two-dimensional material, which could provide more channels for the movement of ions. The nano materials formed by the composite of various materials combined the advantages of various materials and played key role in improving ORR performance of MFCs.     

Electricity generation from rice straw using a microbial fuel cell

This study demonstrated electricity generation from rice straw without pretreatment in a two-chambered microbial fuel cell (MFC) inoculated with a mixed culture of cellulose-degrading bacteria (CDB). The power density reached 145 mW/m² with an initial rice straw concentration of 1 g/L; while the coulombic efficiencies (CEs) ranged from 54.3 to 45.3%, corresponding to initial rice straw concentrations of 0.5–1 g/L. Stackable MFCs in series and parallel produced an open circuit voltage of 2.17 and 0.723 V, respectively, using hexacyanoferrate as the catholyte. The maximum power for serial connection of three stacked MFCs was 490 mW/m² (0.5 mA). In parallelly stacked MFCs, the current levels were approximately 3-fold (1.5 mA) higher than those produced from the serial connection. These results demonstrated that electricity can be produced from rice straw by exploiting CDB as the biocatalyst. Thus, this method provides a promising way to utilize rice straw for bioenergy production.     

Microbial fuel cell coupled ecological floating bed for enhancing bioelectricity generation and nitrogen removal

An ecological floating bed-microbial fuel cell (EFB-MFC) coupled system was constructed in this study. With windmill grass, goldfish algae, water hyacinth and water spinach chosen as the cathodic floating bed plants, performance with regard to electricity generation and nitrogen removal was investigated. For electricity generation, introducing plants into an MFC reduced the internal resistance of the system by 21.23%–67.66% and increased the average voltage by 26.26%–62.63% compared to the system with no plants. In addition, the coupled system improved the removal efficiency of NH₄⁺-N and TN by 2.54%–16.40% and 2.91%–16.86%, respectively. The water spinach system achieved the best performance for electricity generation and nitrogen removal. However, the extra NH₄⁺-N released by rotten roots of windmill grass impaired the nitrogen removal. Radial oxygen loss and root exudates played significant roles in enhancing electricity generation and nitrogen removal. Moreover, electricity generation can stimulate the activity of enzymes related to nitrogen removal and drive the migration of NH₄⁺-N thereby enhancing nitrogen removal. This study also first revealed that an EFB-MFC showed greater capability than a nonplanted group for protecting enzyme activity related to nitrogen removal from excessive high water temperature (over 30 °C).     

Effect of hydrogen producing mixed culture on performance of microbial fuel cells

This study examined the influence of H₂-producing mixed cultures on improving power generation using air-cathode microbial fuel cells (MFCs) inoculated with heat-treated anaerobic sludge. The MFCs installed with graphite brush anode generated higher power than the MFCs with carbon cloth anode, regardless heat treatment of anaerobic sludge. When the graphite brush anode-MFCs were inoculated selectively with H₂-producing bacteria by heat treatment, power production was not improved (about 490 mW/m²) in batch mode operation, but for slightly increased in carbon cloth anode-MFCs (from 0.16 to 2.0 mW/m²). Although H⁺/H₂ produced from H₂-producing bacteria can contribute to the performance of MFCs, suspended biomass did not affect the power density or potential, but the Coulombic efficiency (CE) increased. A batch test shows that propionate and acetate were used effectively for electricity generation, whereas butyrate made a minor contribution. H₂-producing mixed cultures do not affect the improvement in power generation and seed sludge, regardless of the pretreatment, can be used directly for the MFC performance.     

An overview of electrode materials in microbial fuel cells

Electrode materials play an important role in the performance (e.g., power output) and cost of microbial fuel cells (MFCs), which use bacteria as the catalysts to oxidize organic (inorganic) matter and convert chemical energy into electricity. In this paper, the recent progress of anode/cathode materials and filling materials as three-dimensional electrodes for MFCs has been systematically reviewed, resulting in comprehensive insights into the characteristics, options, modifications, and evaluations of the electrode materials and their effects on different actual wastewater treatment. Some existing problems of electrode materials in current MFCs are summarized, and outlooks for future development are also suggested.     

Effect of ammonium and COD concentrations on the performance of fixed-bed air-cathode microbial fuel cells treating reject water

In this study, a bio-electrochemical reactor comprising of anaerobic and aerobic chambers with filled granular activated carbon as biocarrier and third electrode was developed to investigate the effect of ammonium and COD concentrations on the power generation and COD removal. Two important operating parameters including initial COD concentrations (50–2000 mg/L) and initial ammonium concentrations (40–1000 mg/L) were optimized to attain the best response of electricity generation and COD removal using response surface methodology (RSM) through an experiments design software. A total of thirteen runs of experiments were employed for statistical analysis of data and developing empirical model, which has ability to predict optimum condition. Based on the results, the maximum COD removal efficiency was 96.8%, when the produced current at the maximum level was 1.149 mA. It was found that main variables as solely or combination effects significantly affect the efficiency of MFCs.     

Research progress in the application of metal organic frameworks and its complex in microbial fuel cells: Present status,opportunities, future directions and prospects

Metal organic frameworks (MOFs) could greatly improve the power generation and degradation performance of microbial fuel cells (MFCs). MOFs and their compound derivatives played key role in cathode, anode and proton exchange membrane of MFCs, which greatly promoted the power generation of MFC and the degradation efficiency of various pollutants. However, MOFs were still possessed some defects, such as complex synthesis process, difficult regulation, instability, etc. Moreover, the application of MFC was limited in low power density, system internal resistance, microbial consumption, etc. Which further limited the degradation of pollutants by MFC. The existing problems and various improvement schemes of MOFs for MFCs were further summarized, which would provide references for promoting the application of MOFs materials in MFC system. It was expected to enhance the application of MOFs materials and promote the performance of MFC.     

Electricity generation and microbial community structure of air-cathode microbial fuel cells powered with the organic fraction of municipal solid waste and inoculated with different seeds

The organic fraction of municipal solid waste (OFMSW), normally exceeding 60% of the waste stream in developing countries, could constitute a valuable source of feed for microbial fuel cells (MFCs). This study tested the start-up of two sets of OFMSW-fed air-cathode MFCs inoculated with wastewater sludge or cattle manure. The maximum power density obtained was 123 ± 41 mW m⁻² in the manure-seeded MFCs and 116 ± 29 mW m⁻² in the wastewater-seeded MFCs. Coulombic efficiencies ranged between 24 ± 5% (manure-seeded MFCs) and 23 ± 2% (wastewater-seeded MFCs). Chemical oxygen demand removal was >86% in all the MFCs and carbohydrate removal >98%. Microbial community analysis using 16S rRNA gene pyrosequencing demonstrated the dominance of the phylum Firmicutes (67%) on the anode suggesting the possible role of members of this phylum in electricity generation. Principal coordinate analysis showed that the microbial community structure in replicate MFCs converged regardless of the inoculum source. This study demonstrates efficient electricity production coupled with organic treatment in OFMSW-fueled MFCs inoculated with manure or wastewater.     

Electricity harvest from nitrate/sulfide-containing wastewaters using microbial fuel cell with autotrophic denitrifier, Pseudomonas sp. C27

Toxicity prevents the bioenergy content of certain industrial effluents from being recovered. In operation of microbial fuel cell (MFC), microorganisms can be inhibited with high levels of sulfide. This study applied a pure culture, an autotrophic denitrifier, Pseudomonas sp. C27, to start up a two-chambered MFC using sulfide as the sole electron donor. The MFC can successfully convert sulfide to elementary sulfur with electricity generation at a maximum power density of 40 mW m⁻². The addition of acetate interfered biofilm activity to convert sulfide to electricity. Nitrate was revealed as the more powerful electron acceptor than anode in the MFC. The present device introduces a route for treating sulfide laden wastewaters with electricity harvest.     

Electricity generation by Shewanella sp. HN-41 in microbial fuel cells

Microbial fuel cells (MFCs) provide new opportunities for energy generation through conversion of organic matter to electricity by electricity-generating bacteria. In this study, Shewanella sp. strain HN-41 was described as an exoelectrogen that had the ability of extracellular electron transfer in MFCs fed with lactate or glucose. The maximum power density produced by the strain HN-41 in lactate- and glucose-fed single-chamber MFCs reached 71.6 and 18.2 mW m⁻², respectively. The strain showed strong capability to reduce Fe(III) with lactate or glucose as electron donor during the initial incubation period, and secreted flavin mononucleotide (FMN), riboflavin, and traces of flavin adenine dinucleotide in MFCs. Addition of riboflavin and FMN as electron mediators contributed to 2–5 folds increase in power density. These findings on the ability of Shewanella sp. HN-41 to couple oxidation of glucose contributed to the expansion of our knowledge on utilization of carbon source by Shewanella sp.     

A review on carbon and non-precious metal based cathode catalysts in microbial fuel cells

Microbial fuel cells, an emerging technology has been paid a great attention in recent years, due to its unique advantages in treating wastewater to portable water, together with the generation of useful electricity, with the help of bio-active anodes and electrochemical cathodes, simultaneously. When applying this technology in a practical scale, the indigenous bacteria present in the wastewater catalyze the breakdown of organic matter in the anode compartment, with generation of electrons and in the cathode compartment an oxidant, usually the oxygen present in the air, take the electron and reduce to water (oxygen reduction reaction, ORR). An ideal ORR catalyst should be highly active, durable, scalable, and most importantly it should be cost effective. Generally, platinum-based catalyst is utilized, however, due to the high cost of Pt based catalysts, many cheap, cost effective catalyst have been identified as efficient ORR catalyst. Carbon based catalysts known to possess good electronic conductivity, desirable surface area, high stability, together when doped with heteroatoms and cheap metals is found to remarkably enhance the ORR activity. Although a lot of research has been done in view of developing carbon based cheap, cost-effective catalysts, still their collective information has not been reviewed. In this article we anticipate reviewing various non-precious metal and metal-free catalysts that are synthesized and investigated for MFCs, factors that affect the ORR activity, catalyst designing strategies, membranes utilized for MFCs, together with the cost comparison of non-precious and metal-free catalysts with respect to Pt based catalysts have been summarized. We anticipate that this review could offer researchers an overview of the catalyst developed so far in the literatures and provides a direction to the young researchers.     

Power generation and organics removal from wastewater using activated carbon nanofiber (ACNF) microbial fuel cells (MFCs)

Carbon-based materials are the most commonly used electrode material for anodes in microbial fuel cell (MFC), but are often limited by their surface areas available for biofilm growth and subsequent electron transfer process. This study investigated the use of activated carbon nanofibers (ACNF) as the anode material to enhance bacterial biofilm growth, and improve MFC performance. Qualitative and quantitative biofilm adhesion analysis indicated that ACNF exhibited better performance over the other commonly used carbon anodes (granular activated carbon (GAC), carbon cloth (CC)). Batch-scale MFC tests showed that MFCs with ACNF and GAC as anodes achieved power densities of 3.50 ± 0.46 W/m³ and 3.09 ± 0.33 W/m³ respectively, while MFCs with CC had a lower power density of 1.10 ± 0.21 W/m³ In addition, the MFCs with ACNF achieved higher contaminant removal efficiency (85 ± 4%) than those of GAC (75 ± 5%) and CC (70 ± 2%). This study demonstrated the distinct advantages of ACNF in terms of biofilm growth and electron transport. ACNF has a potential for higher power generation of MFCs to treat wastewaters.     

Voltage reversal during microbial fuel cell stack operation

Microbial fuel cells (MFC) can be used to directly generate electricity from organic matter, but the voltage produced by a single reactor is only ∼0.5 V. Voltage can be increased by stacking cells, i.e. by linking individual reactors in series, as is commonly done with hydrogen fuel cells, to provide a higher voltage output. A two-cell air-cathode MFC stack tested here produced a working voltage of 0.9 V (external load 500 Ω) and had an open circuit voltage (OCV) of 1.3 V when operated in fed batch mode under substrate-sufficient conditions. When multiple cells are stacked together, however, charge reversal can result in the reverse polarity of one or more cells and a loss of power generation. We investigated the causes of charge reversal and the impact of prolonged reversal on power generation using a two air-cathode MFCs stack. When voltage began to decline at the end of a fed batch cycle, we observed voltage reversal with one cell producing a working voltage of 0.6 V, and the other cell having a reversed voltage of −0.58 V, producing only a minimal stack voltage of 0.02 V. The reason for the voltage reversal was shown to be fuel starvation, resulting in a loss of bacterial activity. Voltage reversal adversely affected bacteria on the anode of the affected cell, as shown by a relative decrease in cell performance following a cycle of starvation (no feeding). The control of voltage reversal will be crucial for long-term operation of MFCs in series. Rapid feeding of a cell can restore positive voltage generation, but the long-term impact of charge reversal will be inactivation of bacteria and it will require that the affected cells be short-circuited to maintain stack power production. A better understanding of the long term effects of voltage reversal on power generation by MFC stacks is needed in order to efficiently increase voltage production by using stacked MFC systems.     

Miniature microbial fuel cells and stacks for urine utilisation

MFCs are becoming a stronger contender in the area of alternative energy sources and show great promise in utilising a wide variety of organic sources. This paper describes the utilisation of neat undiluted urine as the main feedstock for different types of individual MFCs and stacks of small-scale MFCs, for direct electricity production, with conversion efficiencies of >50%. The smallest MFC (1.4 mL total volume) produced equal amounts of power to that produced by larger MFCs (6.25 mL), resulting in increased power densities. Power densities of 4.93 mW/m² (absolute power of 1.5 mW) were recorded when 48 small-scale MFCs were connected as a stack and fed with urine. This study demonstrates the feasibility of using urine as an untreated fuel and that improved power outputs can be achieved through MFC miniaturisation and multiplication into stacks.