Adaptation to high current using low external resistances eliminates power overshoot in microbial fuel cells

One form of power overshoot commonly observed with mixed culture microbial fuel cells (MFCs) is doubling back of the power density curve at higher current densities, but the reasons for this type of overshoot have not been well explored. To investigate this, MFCs were acclimated to different external resistances, producing a range of anode potentials and current densities. Power overshoot was observed for reactors acclimated to higher (500 and 5000 Ω) but not lower (5 and 50 Ω) resistances. Acclimation of the high external resistance reactors for a few cycles to low external resistance (5 Ω), and therefore higher current densities, eliminated power overshoot. MFCs initially acclimated to low external resistances exhibited both higher current in cyclic voltammograms (CVs) and higher levels of redox activity over a broader range of anode potentials (-0.4 to 0 V; vs. a Ag/AgCl electrode) based on first derivative cyclic voltammetry (DCV) plots. Reactors acclimated to higher external resistances produced lower current in CVs, exhibited lower redox activity over a narrower anode potential range (-0.4 to -0.2 V vs. Ag/AgCl), and failed to produce higher currents above ～-0.3 V (vs. Ag/AgCl). After the higher resistance reactors were acclimated to the lowest resistance they also exhibited similar CV and DCV profiles. Our findings show that to avoid overshoot, prior to the polarization and power density tests the anode biofilm must adapt to low external resistances to be capable of higher currents.     

Archaea-based microbial fuel cell operating at high ionic strength conditions

In this work, two archaea microorganisms (Haloferax volcanii and Natrialba magadii) used as biocatalyst at a microbial fuel cell (MFC) anode were evaluated. Both archaea are able to grow at high salt concentrations. By increasing the media conductivity, the internal resistance was diminished, improving the MFC’s performance. Without any added redox mediator, maximum power (P _max) and current at P _max were 11.87/4.57/0.12 μW cm⁻² and 49.67/22.03/0.59 μA cm⁻² for H. volcanii, N. magadii and E. coli, respectively. When neutral red was used as the redox mediator, P _max was 50.98 and 5.39 μW cm⁻² for H. volcanii and N. magadii, respectively. In this paper, an archaea MFC is described and compared with other MFC systems; the high salt concentration assayed here, comparable with that used in Pt-catalyzed alkaline hydrogen fuel cells, will open new options when MFC scaling up is the objective necessary for practical applications.     

Performance of electron acceptors in catholyte of a two-chambered microbial fuel cell using anion exchange membrane

The performance of the cathodic electron acceptors (CEA) used in the two-chambered microbial fuel cell (MFC) was in the following order: potassium permanganate (1.11 V; 116.2 mW/m²) > potassium persulfate (1.10 V; 101.7 mW/m²) > potassium dichromate, K₂Cr₂O₇ (0.76 V; 45.9 mW/m²) > potassium ferricyanide (0.78 V; 40.6 mW/m²). Different operational parameters were considered to find out the performance of the MFC like initial pH in aqueous solutions, concentrations of the electron acceptors, phosphate buffer and aeration. Potassium persulfate was found to be more suitable out of the four electron acceptors which had a higher open circuit potential (OCP) but sustained the voltage for a much longer period than permanganate. Chemical oxygen demand (COD) reduction of 59% was achieved using 10 mM persulfate in a batch process. RALEX™ AEM-PES, an anion exchange membrane (AEM), performed better in terms of power density and OCP in comparison to Nafion®117 Cation Exchange Membrane (CEM).     

Biodegradation and proton exchange using natural rubber in microbial fuel cells

Microbial fuel cells (MFCs) generate electricity from waste but to date the technology’s development and scale-up has been held-up by the need to incorporate expensive materials. A costly but vital component is the ion exchange membrane (IEM) which conducts protons between the anode and cathode electrodes. The current study compares natural rubber as an alternative material to two commercially available IEMs. Initially, the material proved impermeable to protons, but gradually a working voltage was generated that improved with time. After 6 months, MFCs with natural rubber membrane outperformed those with anion exchange membrane (AEM) but cation exchange membrane (CEM) produced 109 % higher power and 16 % higher current. After 11 months, polarisation experiments showed a decline in performance for both commercially available membranes while natural rubber continued to improve and generated 12 % higher power and 54 % higher current than CEM MFC. Scanning electron microscope images revealed distinct structural changes and the formation of micropores in natural latex samples that had been employed as IEM for 9 months. It is proposed that the channels and micropores formed as a result of biodegradation were providing pathways for proton transfer, reflected by the steady increase in power generation over time. These improvements may also be aided by the establishment of biofilms that, in contrast, caused declining performance in the CEM. The research demonstrates for the first time that the biodegradation of a ubiquitous waste material operating as IEM can benefit MFC performance while also improving the reactor’s lifetime compared to commercially available membranes.     

A comparison of membranes and enrichment strategies for microbial fuel cells

The external resistance is perhaps the easiest way to influence the operation of a microbial fuel cell (MFC). In this paper, three enrichment strategies, whereby the external resistance was fixed at: (1) a high value in order to maximize the cell voltage (U strategy); (2) a low value in order to maximize the current (I strategy); and (3) a value equal to the internal resistance of the MFC to maximize the power output (P strategy), were investigated. The I strategy resulted in increased maximum power generation and the likely reason is that electron transfer was facilitated under low external resistance, which in turn, favored the development of an electrochemically active biofilm. This experiment was conducted in a single-chamber MFC system equipped with a membrane electrode assembly, and a comparison of the performance achieved by five different membranes is also provided. Selemion was found to be a suitable alternative to Nafion.     

Effects of biofouling on ion transport through cation exchange membranes and microbial fuel cell performance

This study examines the effects of biofouling on the electrochemical properties of cation exchange membranes (CEMs), such as membrane electrical resistance (MER), specific proton conductivity (SC), and ion transport number (t₊), in addition to on microbial fuel cell (MFC) performance. CEM biofouling using a 15.5 ± 4.6 μm biofilm was found to slightly increase the MER from 15.65 Ω cm² (fresh Nafion) to 19.1 Ω cm², whereas an increase of almost two times was achieved when the electrolyte was changed from deionized water to an anolyte containing a high cation concentration supporting bacterial growth. The simple physical cleaning of CEMs had little effect on the Coulombic efficiency (CE), whereas replacing a biofouled CEM with new one resulted in considerable increase of up to 59.3%, compared to 45.1% for a biofouled membrane. These results clearly suggest the internal resistance increase of MFC was mainly caused by the sulfonate functional groups of CEM being occupied with cations contained in the anolyte, rather than biofouling itself.     

Electroporation of curved lipid membranes in ionic strength gradients

A thermodynamic theory for the membrane electroporation of curved membranes such as those of lipid vesicles and cylindrical membrane tubes has been developed. The theory covers in particular the observation that electric pore formation and shape deformation of vesicles and cells are dependent on the salt concentration of the suspending solvent. It is shown that transmembrane salt gradients can appreciably modify the electrostatic part of Helfrich's spontaneous curvature, elastic bending rigidity and Gaussian curvature modulus of charged membranes. The Gibbs reaction energy of membrane electroporation can be explicitely expressed in terms of salt gradient-dependent contributions of bending, the ionic double layers and electric surface potentials and dielectric polarisation of aqueous pores. In order to cover the various physical contribution to the chemical process of electroporation-resealing, we have introduced a generalised chemophysical potential covering all generalised forces and generalised displacements in terms of a transformed Gibbs energy formalism. Comparison with, and analysis of, the data of electrooptical relaxation kinetic studies show that the Gibbs reaction energy terms can be directly determined from turbidity dichroism (Planck's conservative dichroism). The approach also quantifies the electroporative cross-membrane material exchange such as electrolyte release, electrohaemolysis of red blood cells or uptake of drugs and dyes and finally gene DNA by membrane electroporation.     

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.     

Anodic reactions in microbial fuel cells

Potentiometric and amperometric measurements were made with microbial fuel cells containing E. coli or yeast as the anodic reducing agent and glucose as the oxidizable substrate. The catalytic effects of thionine and resorufin on the anode reaction were investigated. Results on the potentiometry, polarization, and coulombic output of the cells support a mediator-coupled mechanism for the transfer of electrons from the organism to the electrode in preference to a mechanism of "direct" electrochemical oxidation of glucose or its degradation products. Experiments with (14)C-labeled glucose show that when a microbial fuel cell produces a current under load, exogenous glucose is metabolized to produce (14)CO(2). The Coulombic yields of the cells indicate a high degree of energy conversion in these systems.     

Photosynthetic microbial fuel cells with positive light response

The current study introduces an aerobic single‐chamber photosynthetic microbial fuel cell (PMFC). Evaluation of PMFC performance using naturally growing fresh‐water photosynthetic biofilm revealed a weak positive light response, that is, an increase in cell voltage upon illumination. When the PMFC anodes were coated with electrically conductive polymers, the rate of voltage increased and the amplitude of the light response improved significantly. The rapid immediate positive response to light was consistent with a mechanism postulating that the photosynthetic electron‐transfer chain is the source of the electrons harvested on the anode surface. This mechanism is fundamentally different from the one exploited in previously designed anaerobic microbial fuel cells (MFCs), sediment MFCs, or anaerobic PMFCs, where the electrons are derived from the respiratory electron‐transfer chain. The power densities produced in PMFCs were substantially lower than those that are currently reported for conventional MFC (0.95 mW/m² for polyaniline‐coated and 1.3 mW/m² for polypyrrole‐coated anodes). However, the PMFC did not depend on an organic substrate as an energy source and was powered only by light energy. Its operation was CO₂‐neutral and did not require buffers or exogenous electron transfer shuttles. Biotechnol. Bioeng. 2009; 104: 939–946. © 2009 Wiley Periodicals, Inc.     

The association of serum proteins with peripheral blood cells at low ionic strength: Ultrastructural examination

Summary Peripheral white blood cells have been shown to bind serum immunoglobins when prepared in low ionic strength sucrose solutions. The presence and distribution of Ig on neutrophils, monocytes and lymphocytes was described using an immunoferritin probe and the electron microscope. It appears that this approach reveals immunoglobulins associated with peripheral white blood cells that are not observed by the usual methods of examining these cell surfaces after preparation at physiological ionic strength.Supported by Grant Number AI 12750.Presented in part at Thirty-First Annual EMSA Meeting 1974, and again in part at the Seventy Sixth Annual. American Society of Microbiology, 1976.Predoctoral supported on Training Grant No. 1-Tol-CA-052595.     

Kinetics of nucleosome unfolding at low ionic strength

The kinetics of a conformational change which occurs in nucleosome core particles at about 1 mM ionic strength have been studied by observing changes in the fluorescence of labeled histone H3. The unfolding reaction is intramolecular since no concentration dependence is observed. However, the kinetics are unexpectedly complicated and reveal evidence of at least three relaxation times. It is possible to fit the kinetics observed under several conditions to a consistent four-state cyclic mechanism in which folded and unfolded forms can inter-convert by two parallel pathways, each involving a distinct intermediate. While the data are not sufficient to establish this mechanism as a unique choice, they exclude many simpler possibilities. The cyclic mechanism is quite reasonable in view of what is currently known about the structures of the folded and unfolded forms.     

Dynamic changes in the microbial community composition in microbial fuel cells fed with sucrose

The performance and dynamics of the bacterial communities in the biofilm and suspended culture in the anode chamber of sucrose-fed microbial fuel cells (MFCs) were studied by using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified partial 16S rRNA genes followed by species identification by sequencing. The power density of MFCs was correlated to the relative proportions of species obtained from DGGE analysis in order to detect bacterial species or taxonomic classes with important functional role in electricity production. Although replicate MFCs showed similarity in performance, cluster analysis of DGGE profiles revealed differences in the evolution of bacterial communities between replicate MFCs. No correlation was found between the proportion trends of specific species and the enhancement of power output. However, in all MFCs, putative exoelectrogenic denitrifiers and sulphate-reducers accounted for approximately 24% of the bacterial biofilm community at the end of the study. Pareto–Lorenz evenness distribution curves extracted from the DGGE patterns obtained from time course samples indicated community structures where shifts between functionally similar species occur, as observed within the predominant fermentative bacteria. These results suggest the presence of functional redundancy within the anodic communities, a probable indication that stable MFC performance can be maintained in changing environmental conditions. The capability of bacteria to adapt to electricity generation might be present among a wide range of bacteria.