Lack of anodic capacitance causes power overshoot in microbial fuel cells

Fabrication of 3D graphene anode for improving performance of miniaturized microbial fuel cells

A three-dimensional graphene (3D GR) grown by chemical vapor deposition method was used as the anode of a miniaturized microbial fuel cell (mini-MFC), which was to be embedded in a 56-μL anode chamber for the formation of a thicker biofilm from Shewanella bacterial culture to promote high efficient extracellular electron transfer. Such 3D GR structure had fewer defects with few layers, and the framework showed significant high REDOX peak current density, high charge storage and low charge transfer resistance. Besides, the electron transport rate of 3D GR electrode was 0.0176 s-1, which was about two times faster than that of GR electrode with nickel foam substrate (GR/NF). Benefiting from the macroporous networks, high electron transfer rate and electrocatalytic activity, 3D GR anode facilitated efficient mass transfer and effective electron transport, further forming denser biofilm on the 3D GR. The maximum output voltage and power density of this mini-MFC were 820 mV and 23.8 mW/m2, which were much higher than those of the GR/NF anode at 590 mV and 12.8 mW/m2 and the bare NF anode at 450 mV and 4.6 mW/m2. The study demonstrated that 3D GR can be a promising anode material for improving MFC performance.

Review on microbial fuel cells applications, developments and costs

The microbial fuel cell (MFC) technology has attracted significant attention in the last years due to its potential to recover energy in a wastewater treatment. The idea of using an MFC in industry is very attractive as the organic wastes can be converted into energy, reducing the waste disposal costs and the energy needs while increasing the company profit. However, taking aside these promising prospects, the attempts to apply MFCs in large-scale have not been succeeded so far since their lower performance and high costs remains challenging. This review intends to present the main applications of the MFC systems and its developments, particularly the advances on configuration and operating conditions. The diagnostic techniques used to evaluate the MFC performance as well as the different modeling approaches are described. Towards the introduction of the MFC in the market, a cost analysis is also included. The development of low-cost materials and more efficient systems, with high higher power outputs and durability, are crucial towards the application of MFCs in industrial/large scale. This work is a helpful tool for discovering new operation and design regimes.

The influential role of external electrical load in microbial fuel cells and related improvement strategies: A review

The scope of the currentreviewis to discuss and evaluate the role of the external electrical load/resistor (EEL) on the overall behavior and functional properties of microbial fuel cells (MFCs). In this work, a comprehensive analysis is made by considering various levels of MFC architecture, such as electric and energy harvesting efficiency, anode electrode potential shifts, electro-active biofilm formation, cell metabolism and extracellular electron transfer mechanisms, as a function of the EEL and its control strategies. It is outlined that taking the regulation of EEL into account at MFC optimization is highly beneficial, and in order to support this step, in this review, a variety of guidelines are collected and analyzed.

Electrogenic Biofilm Development Determines Charge Accumulation and Resistance to pH Perturbation

The electrogenic biofilm and the bio-electrode interface are the key biocatalytic components in bioelectrochemical systems (BES) and can have a large impact on cell performance. This study used four different anodic carbons to investigate electrogenic biofilm development to determine the influence of charge accumulation and biofilm growth on system performance and how biofilm structure may mitigate against pH perturbations. Power production was highest (1.40 W/m3) using carbon felt, but significant power was also produced when felt carbon was open-circuit acclimated in a control reactor (0.95 W/m3). The influence of carbon material on electrogenic biofilm development was determined by measuring the level of biofilm growth, using sequencing to identify the microbial populations and confocal microscopy to understand the spatial locations of key microbial groups. Geobacter spp. were found to be enriched in closed-circuit operation and these were in close association with the carbon anode, but these were not observed in the open-circuit controls. Electrochemical analysis also demonstrated that the highest mid-point anode potentials were close to values reported for cytochromes from Geobacter sulfurreductans. Biofilm development was greatest in felt anodes (closed-circuit acclimated 1209 ng/μL DNA), and this facilitated the highest pseudo-capacitive values due to the presence of redox-active species, and this was associated with higher levels of power production and also served to mitigate against the effects of low-pH operation. Supporting carbon anode structures are key to electrogenic biofilm development and associated system performance and are also capable of protecting electrochemically active bacteria from the effects of environmental perturbations.

Evaluation of degradation behavior over tetracycline hydrochloride by microbial electrochemical technology: Performance, kinetics, and microbial communities

Tetracycline hydrochloride (TCH), as a typical antibiotic-pollutant, is desired to enhance its removal from public environment, due to its toxicity and persistence. Microbial electrochemical technology (MET) is a series complex microorganisms-driven processes with characteristics of simultaneous wastewater treatment and electricity generation. The study was presented to evaluate the TCH removal behavior and power generation performance through the co-metabolism under constant glucose with different TCH concentrations using MET. It was found that the TCH removal efficiency arrived at 40% during the first 6 h, when TCH concentrations ranged from 1 to 50 mg/L. It was interesting that TCH degradation rate increased to a maximum of 4.15 × 10^-2 h^-1 with its concentrations varying from 1 to 20 mg/L, however, the further increase to 50 mg/L in TCH concentration resulted in a reverse 66% reduction. In the meantime, the generated bioelectricity declared a similar fluctuation trend with a maximum power density of 600 mW/m² under the condition of 20 mg/L TCH co-degradation with glucose. What's more, the TCH inhibition effect fitted well with Haldane's model, indicating that the microbial electrochemical system had a better potency toward TCH toxicity than that reported (EC₅₀ = 2.2 mg/L). Thauera as mainly functional aromatics-degrading bacteria and Bdellovibrio against bacterial pathogens, only existed in the mixed cultures with TCH and glucose, indicating extremely remarkable changes in bacterial community with TCH addition. In summary, a new approach for the anaerobic biodegradation of TCH was explored through co-metabolism with glucose using MET. The results should be useful for antibiotics wastewater disposal of containing TCH.

Degradation mechanisms of sulfamethoxazole and its induction of bacterial community changes and antibiotic resistance genes in a microbial fuel cell

In this study, more than 85.1% of sulfamethoxazole (SMX) could be degraded within 60 h. The strengthening of microbial metabolisms and the sustainment of electrical stimulation contributed to the rapid removal of SMX in microbial fuel cells (MFCs). High-performance liquid chromatography identified that SMX could be thoroughly degraded into less harmful alcohols and methane after the MFC processing. In addition, the major role of Shewanella sp. and Geobacteria sp. in power generation, and the promotion of Alcaligenes, Pseudomonas and Achromobacter in SMX degradation have been demonstrated. Moreover, this study further proved that the copy numbers of targeted antibiotic resistance genes and integrons produced in MFCs were much lower than those found in conventional wastewater treatment plants; MFCs seem to be a promising alternative to reduce antibiotics in wastewater treatment and water purification.

Accelerated removal of high concentration p-chloronitrobenzene using bioelectrocatalysis process and its microbial communities analysis

p-Chloronitrobenzene (p-CNB) is a persistent refractory and toxic pollutant with a concentration up to 200 mg/L in industrial wastewater. Here, a super-fast removal rate was found at 0.2-0.8 V of external voltage over a p-CNB concentration of 40-120 mg/L when a bioelectrochemical technology is used comparing to the natural biodegradation and electrochemical methods. The reduction kinetics (k) was fitted well according to pseudo-first order model with respect to the different initial concentration, indicating a 1.12-fold decrease from 1.80 to 0.85 h^-1 within the experimental range. Meanwhile, the highest k was provided at 0.5 V with the characteristic of energy saving. It was revealed that the functional bacterial (Propionimicrobium, Desulfovibrio, Halanaerobium, Desulfobacterales) was selectively enriched under electro-stimulation, which possibly processed Cl-substituted nitro-aromatics reduction. The possible degradation pathway was also proposed. This work provides the beneficial choice on the rapid treatment of high-concentration p-CNB wastewater.

Electrochemistry and microbiology of microbial fuel cells treating marine sediments polluted with heavy metals

Novel laboratory-designed aerated and non-aerated sediment microbial fuel cell (SMFC) models were constructed for power generation and heavy metal bioremediation.

Self-recoverable voltage reversal in stacked microbial fuel cells due to biofilm capacitance

In order to assess the effects of biofilm capacitance on self-recovering voltage reversals, the restored current is determined and compared with the measured biofilm capacitance by analyzing the results of electrochemical impedance spectroscopy. This comparison demonstrates that self-recovering voltage reversals are caused by temporary damage to, and the recovery of, biofilm capacitance which arises due to the ability of redox enzymes in the electron transfer system to temporarily store electrons. Thus, the development of procedures for voltage reversal control and for the maintenance of serially connected microbial fuel cells (MFCs) should take into account such temporary voltage reversal phenomenon. This discovery and characterization of self-recovering voltage reversals is expected to be practically useful to enhance the reliability of MFCs to be scaled up and implemented in practical systems.

Elimination of Power Overshoot at Bioanode through Assistance Current in Microbial Fuel Cells

The power overshoot generated by electron depletion in microbial fuel cells (MFCs) was characterized in this study. Various causes of power overshoot, identified in previous studies, are discussed in terms of their plausible contributions to electron depletion. We found that power overshoot occurred if the anodic overpotential generated by electron depletion exceeded the cathodic overpotential. The introduction of assistance current from anode connections, which ameliorated the electron depletion in the MFCs, immediately eliminated the power overshoot. As a result, if the electron production at the anode exceeded electron reduction at the cathode, a power overshoot was not generated. The results revealed that introducing assistance current supplied from an additional anode to the limited anode eliminated power overshoot. The power overshoot is not generated by kinetic limitation at the cathode; it is only generated by the kinetic limitation at the anode. The mechanism underlying power overshoot should be considered in the design of MFCs to improve reliability, particularly in scaled-up plant applications. The proposed technique is more practical than previously proposed methods.

Bio-power performance enhancement in microbial fuel cell using Ni–ferrite decorated anode

Ni–ferrite-decorated anode enhanced the MPD by 26% to 806.4 mW m⁻².

Surface-to-surface biofilm transfer: a quick and reliable startup strategy for mixed culture microbial fuel cells

The startup of microbial fuel cells (MFCs) is known to be prone to failure or result in erratic performance impeding the research. The aim of this study was to advise a quick launch strategy for laboratory-scale MFCs that ensures steady operation performance in a short period of time. Different startup strategies were investigated and compared with membraneless single chamber MFCs. A direct surface-to-surface biofilm transfer (BFT) in an operating MFC proved to be the most efficient method. It provided steady power densities of 163 ± 13 mWm(-2) 4 days after inoculation compared to 58 ± 15 mWm(-2) after 30 days following a conventional inoculation approach. The in situ BFT eliminates the need for microbial acclimation during startup and reduces performance fluctuations caused by shifts in microbial biodiversity. Anaerobic pretreatment of the substrate and addition of suspended enzymes from an operating MFC into the new MFC proved to have a beneficial effect on startup and subsequent operation. Polarization methods were applied to characterize the startup phase and the steady state operation in terms of power densities, internal resistance and power overshoot during biofilm maturation. Applying this method a well-working MFC can be multiplied into an array of identically performing MFCs.

A monolithic three-dimensional macroporous graphene anode with low cost for high performance microbial fuel cells

Monolithic 3D-G which is inflexible and has a macroporous structure, crumpled matrix, good conductivity and low cost enhanced the electrogenesis of a MFC.

Bifunctional Manganese Ferrite/Polyaniline Hybrid as Electrode Material for Enhanced Energy Recovery in Microbial Fuel Cell

Microbial fuel cells (MFCs) are emerging as a sustainable technology for waste to energy conversion where electrode materials play a vital role on its performance. Platinum (Pt) is the most common material used as cathode catalyst in the MFCs. However, the high cost and low earth abundance associated with Pt prompt the researcher to explore inexpensive catalysts. The present study demonstrates a noble metal-free MFC using a manganese ferrite (MnFe2O4)/polyaniline (PANI)-based electrode material. The MnFe2O4 nanoparticles (NPs) and MnFe2O4 NPs/PANI hybrid composite not only exhibited superior oxygen reduction reaction (ORR) activity for the air cathode but also enhanced anode half-cell potential upon modifying carbon cloth anode in the single-chambered MFC. This is attributed to the improved extracellular electron transfer of exoelectrogens due to Fe(3+) in MnFe2O4 and its capacitive nature. The present work demonstrates for the first time the dual property of MnFe2O4 NPs/PANI, i.e., as cathode catalyst and an anode modifier, thereby promising cost-effective MFCs for practical applications.

Biological capacitance studies of anodes in microbial fuel cells using electrochemical impedance spectroscopy

It is known that cell potential increases while anode resistance decreases during the start-up of microbial fuel cells (MFCs). Biological capacitance, defined as the apparent capacitance attributed to biological activity including biofilm production, plays a role in this phenomenon. In this research, electrochemical impedance spectroscopy was employed to study anode capacitance and resistance during the start-up period of MFCs so that the role of biological capacitance was revealed in electricity generation by MFCs. It was observed that the anode capacitance ranged from 3.29 to 120 mF which increased by 16.8% to 18-20 times over 10-12 days. Notably, lowering the temperature and arresting biological activity via fixation by 4% para formaldehyde resulted in the decrease of biological capacitance by 16.9 and 62.6%, indicating a negative correlation between anode capacitance and anode resistance of MFCs. Thus, biological capacitance of anode should play an important role in power generation by MFCs. We suggest that MFCs are not only biological reactors and/or electrochemical cells, but also biological capacitors, extending the vision on mechanism exploration of electron transfer, reactor structure design and electrode materials development of MFCs.

Improvement of power generation using Shewanella putrefaciens mediated bioanode in a single chambered microbial fuel cell: effect of different anodic operating conditions

Three different approaches were employed to improve single chambered microbial fuel cell (sMFC) performance using Shewanella putrefaciens as biocatalyst. Taguchi design was used to identify the key process parameter (anolyte concentration, CaCl₂ and initial anolyte pH) for maximization of volumetric power. Supplementation of CaCl₂ was found most significant and maximum power density of 4.92 W/m(3) was achieved. In subsequent approaches, effect on power output by riboflavin supplementation to anolyte and anode surface modification using nano-hematite (Fe₂O₃) was observed. Volumetric power density was increased by 44% with addition of 100 nM riboflavin to anolyte while with 0.8 mg/cm(2) nano-Fe₂O₃ impregnated anode power density and columbic efficiency increased by 40% and 33% respectively. Cyclic voltammetry revealed improvement in electrochemical activity of Shewanella with nano-Fe₂O₃ loading and electrochemical impedance depicted inverse relationship between charge transfer resistance and nano-Fe₂O₃ loading. This study suggests anodic improvement strategies for maximization of power output.

Anode modification with capacitive materials for a microbial fuel cell: an increase in transient power or stationary power

The discharge of bio-electrons stored in the capacitive anode of an MFC significantly contributes to the measured power density.

Time behavior and capacitance analysis of nano-Fe3O4 added microbial fuel cells

The addition of nano Fe3O4 is beneficial to boost the transient charge storage of the anode accompanying with the enhancement of power performance in microbial fuel cells (MFCs) in our previous study. Here we found that both the anodic open circuit potential and the current increased when comparing the AcFeM (Fe3O4 added activated carbon anode) with the AcM (activated carbon anode), indicating that the Fe3O4 dynamically accelerated the anodic electron transfer although it thermodynamically limited the anode potential. The net storage capacity initially increased followed by a decrease with the maximum capacitance of 574.6 C m(-2) (AcFeM) and 459 C m(-2) (AcM) under 20 min of open circuit interval. The Fe3O4/Fe(II) possibly stored charges temporarily as a solid-state electron shuttle.

Acidic and alkaline pretreatments of activated carbon and their effects on the performance of air-cathodes in microbial fuel cells

Activated carbon (AC) is a high performing and cost effective catalyst for oxygen reduction reactions (ORRs) of air-cathodes in microbial fuel cells (MFCs). Acidic (HNO3) and alkaline (KOH) pretreatments on AC at low temperature (85°C) are conducted to enhance the performance of MFCs. The alkaline pretreatment increased the power density by 16% from 804±70 to 957±31 mW m(-2), possibly due to the decrease of ohmic resistance (from 20.58 to 19.20 Ω) and the increase of ORR activities provided by the adsorbed hydroxide ion and extra micropore area/volume after alkaline pretreatment. However, acidic pretreatment decreased the power output to 537±36 mW m(-2), which can be mainly attributed to the corrosion by adsorbed proton at the interface of AC powder and stainless steel mesh and the decreased pore area.