Microbial fuel cells converting chemical energy into electrical energy

Microbial fuel cell – for conversion of chemical energy to electrical energy

Category : General Chemicals
Published by : Data Research Analyst, Worldofchemicals.com

Abstract

A microbial fuel cell (MFC) is a bio-electrochemical system that converts the chemical energy in the organic compounds/renewable energy sources to electrical energy/bio-electrical energy through microbial catalysis at the anode under anaerobic conditions. This process is becoming the attractive and alternative methodology for the generation of electricity.

MFC is even considering as the completely new approach to wastewater treatment and electricity generation. MFC performed well for chemical oxygen demand (COD) and biological oxygen demand (BOD) removal from the wastewater. MFC has the capability of production of the maximum power of 6.73mW/m2 and it is a cost-effective process.

Newly emerging concepts with alternative materials for electrodes and catalysts as well as innovative designs have made MFCs a promising technology. In this context, the present article will explain the MFC’s concept, operation methodology, and applications in various fields.

Introduction

MFC is considered to be a promising sustainable technology to meet increasing energy needs, especially by using wastewaters as substrates, resulting in electricity and clean water as final products.

MFC can convert biomass spontaneously into electricity through the metabolic activity of the microorganisms. In a MFC, microorganisms interact with electrodes using electrons, which are either removed or supplied through an electrical circuit.

Microbes used in MFCs

Many microorganisms possess the ability to transfer electrons derived from the metabolism of organic matters to the anode.

Example

Microbes Substrate
Actinobacillus succinogenes Glucose
Aeromonas hydrophila Acetate
Clostridium butyricum Starch, Glucose, Lactate, Molasses
Desulfovibrio desulfuricans Sucrose
Escherichia coli Glucose, Sucrose

 

Substrates and their performance values

The substrates which are used in MFC for the production of electricity are

Substrates Power produced in mA/cm2
Acetate 0.8
Arabitol 0.68
Azo dye with glucose 0.09
Carboxymethyl cellulose 0.05
Cysteine 0.0186
1,2-Dichloroethane 0.008
Ethanol 0.025
Furfural 0.17
Galactitol 0.78
Glucose 0.70
Glucuronic acid 1.18
Lactate 0.005
Mannitol 0.58
Phenol 0.1
Propionate 0.035
Ribitol 0.73
Sodium formate 0.22
Sorbitol 0.62
Starch 0.62
Sucrose 0.19
Xylitol 0.71
Xylose 0.74

 

In the past two decades, high rate anaerobic processes are finding increasing application for the treatment of domestic as well as industrial wastewaters.  The major advantages these systems offer over conventional aerobic treatment are no energy requirement for oxygen supply, less sludge production, and recovery of methane gas.

MFC can convert chemical energy directly into electricity without an intermediate conversion into mechanical power.

Benefits

  • Clean; Safe and quiet performance
  • High energy efficiency and
  • It is easy to operate.

MFC Configuration

MFC is being constructed using a variety of materials. These systems are operated under a range of conditions that include differences in temperature, pH, electron acceptor, electrode surface areas, and reactor size and operation time.

Types of MFCs

  • Single - Chamber MFC
  • Two-Chambered MFC

Single - Chamber MFC

Fig. 1) Schematic diagram of single chamber MFC

A simpler and more efficient MFC can be made by omitting the cathode chamber and placing the cathode electrode directly onto the proton exchange membrane (PEM). Single chamber MFC avoids the need to aerate water because the oxygen in air can be directly transferred to the cathode. It offers simpler designs and cost savings.

Single-chambered MFCs are quite attractive for increasing the power output because they can be run without artificial aeration in an open-air cathode systems and can reduce the internal ohmic resistance by avoiding the use of a catholyte as a result of combining two chambers.

Graphite rods were placed inside the anode chamber and these rods extended outside of the anode chamber and were connected to the cathode via an external circuit.

Two-Chambered MFC

Fig 2). Schematic diagram of two-chambered MFC

Two-compartment MFCs are typically run in batch mode often with a chemically defined medium such as glucose or acetate solution to generate energy. They are currently used only in laboratories.

A typical two-compartment MFC has an anodic chamber and a cathodic chamber connected by a PEM, or sometimes a salt bridge, to allow protons to move across to the cathode while blocking the diffusion of oxygen into the anode.

Procedure

MFC catalyzes the conversion of organic matter into electricity by transferring electrons to the circuit with the aid of bacteria. Further the microorganisms can transfer electrons to the anode electro in three ways, firstly by using exogenous mediators such as potassium ferricyanide, thonine or natural red; secondly by using mediators produced by the bacteria and lastly by direct transfer of electrons from the respiratory enzymes to the electrodes.

The mediator and micro-organism, in this case, yeast, are mixed together in a solution to which is added a suitable substrate such as glucose. This mixture is placed in a sealed chamber to stop oxygen from entering, thus forcing the micro-organism to use anaerobic respiration. An electrode is placed in the solution that will act as the anode as described previously.

In the second chamber of the MFC is another solution and electrode (cathode). The cathode is positively charged and is the equivalent of the oxygen sink at the end of the electron transport chain. The solution is an oxidizing agent that picks up the electrons at the cathode.

Two electrodes are connected by the salt bridge or PEM or ion-exchange membrane to allow protons to move across to the cathode while blocking the diffusion of oxygen into the anode.

In a microbial fuel cell operation, the anode is the terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore, the microbial activity is strongly dependent on the redox potential of the anode. A critical anodic potential exists at which a maximum power output of a microbial fuel cell is achieved.

The basic reactions are presented below; when microorganisms consume a substrate such as sugar in the aerobic condition they produce CO2 and H2 O. However when oxygen is not present i.e. under the anaerobic condition they produce CO2, H+ and e- .

Anodic reaction

C12H22O11 +13H2O → 12CO2 + 48H+ + 48e

Cathodic reaction

O2 + 4e + 4H+ → 2H2O

Applications

  • Electricity generation
  • Biohydrogen production
  • Wastewater treatment
  • Bioremediation

Wastewater treatment

Municipal wastewater contains a multitude of organic compounds that can fuel MFCs. The amount of power generated by MFCs in the wastewater treatment process can potentially reduce the electricity needed in a conventional treatment.

MFCs using certain microbes have a special ability to remove sulfides as required in wastewater treatment. MFCs can enhance the growth of bioelectrochemically active microbes during wastewater treatment thus they have good operational stabilities.

Continuous flow and single-compartment MFCs and membrane-less MFCs are favored for wastewater treatment due to concerns in scale-up. Sanitary wastes, food processing wastewater, swine wastewater, and corn stover are all great biomass sources for MFCs because they are rich in organic matters. It can even break the organic molecules such as acetate, propionate, and butyrate to CO2 and H2O.

MFC can remove the COD and BOD of wastewater of about 90 percent. MFCs yield 50-90 percent less excess sludge, which eventually reduces the sludge disposal cost. This shows the effectiveness of MFC performance in wastewater treatment.

Biohydrogen

MFCs can be readily modified to produce hydrogen instead of electricity. This modified system, which was recently suggested and referred to as biocatalyzed electrolysis or a bio-electrochemically assisted microbial reactor (BEAMR) process or electrohydrogenesis, has been considered an interesting new technology for the production of biohydrogen from organics.

However, hydrogen generation from the protons and electrons produced by the anaerobic degradation of a substrate by electrochemically active bacteria in a modified MFC is thermodynamically unfavorable. this thermodynamic barrier can be overcome by applying an external potential. In this system, the protons and electrons produced by the anodic reaction migrate and combine at the cathode to form hydrogen under anaerobic conditions.

The potential for the oxidation of acetate (1M) at the anode and the reduction of protons to hydrogen at the cathode are -0.28 and -0.42 V (NHE), respectively.

Current research work

Dr. Orianna Bretschger, from the J. Craig Venter Institute, Maryland, USA, and her team has made improvements to one version of the MFC.

"We've improved its energy recovery capacity from about two percent to as much as thirteen percent, which is a great step in the right direction. That actually puts us in a realm where we could produce a meaningful amount of electricity if this technology is implemented commercially. Eventually, we could have wastewater treatment for free."

- Dr. Orianna Bretschger

MFC also removes organic material from sewage and prevents bad microbes that can spread diseases. Dr. Orianna Bretschger team’s MFC can remove around 97 percent of organic materials and it is converting around 13 percent of slurry's energy into electricity.

Conclusion

The achievable power output from MFCs has increased remarkably over the last decade, which was obtained by altering their designs, such as optimization of the MFC configurations, their physical and chemical operating conditions, and their choice of the biocatalyst.

MFCs are capable of converting biomass at temperatures below 20 °C and with low substrate concentrations, both of which are problematic for methanogenic digesters.

A major disadvantage of MFCs is their reliance on biofilms for mediator-less electron transport, while anaerobic digesters such as up-flow anaerobic sludge blanket reactors eliminate this need by efficiently reusing the microbial consortium without cell immobilization. Another limitation is the inherent naturally low catalytic rate of the microbes.

Although some basic knowledge has been gained in MFC research, there is still a lot to be learned in the scaleup of MFC for large-scale applications. However, the recent advances might shorten the time required for their large-scale applications for both energy harvesting and wastewater treatment systems and for the scale-up process.

Reference

[1] Liliana Alzate-Gaviria, Microbial Fuel Cells for Wastewater Treatment, Available from - http://cdn.intechopen.com/pdfs/14554/InTech-Microbial_fuel_cells_for_wastewater_treatment.pdf

[2] Microbial fuel cell, Eco-friendly sewage treatment, Orianna Bretschger - correction Available from - http://www.earthtimes.org/energy/microbiol-fuel-cell-eco-friendly-sewage-treatment/1900/

[3] Zhuwei Du, Haoran Li, Tingyue Gu, A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy, 10 May 2007, Biotechnology Advances 25 (2007) 464–482, Available from - http://132.235.17.4/Paper-gu/MFCreview.pdf

[4] B.K. Pandey  , V. Mishra  , S. Agrawal, Production of bio-electricity during wastewater treatment using a single chamber microbial fuel cell, Vol. 3, No. 4, 2011, pp. 42-47, Available from - http://www.ajol.info/index.php/ijest/article/viewFile/68540/56618

[5] Deepak Pant, Gilbert Van Bogaert, Ludo Diels, Karolien Vanbroekhoven, A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production, 7 October 2009, Available from - http://www.microbialfuelcell.org/Publications/2010-Pant-Areviewofthesubstratesusedinmicrobialfuelcellsforsustainableenergyproduction.pdf

[6] In S. Kim, Kyu-Jung Chae, Mi-Jin Choi, and Willy Verstraete, Microbial Fuel Cells: Recent Advances, Bacterial Communities and Application Beyond Electricity Generation, Vol. 13, No. 2, pp. 51-65, 2008, Available from - http://www.eer.or.kr/home/pdf/In%20S.%20Kim.pdf

Image Reference

Fig 1) Schematic diagram of single chamber MFC - Deepak Pant , Gilbert Van Bogaert, Ludo Diels, Karolien Vanbroekhoven, A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production, 4 October 2009, Available from - http://www.microbialfuelcell.org/Publications/2010-Pant-Areviewofthesubstratesusedinmicrobialfuelcellsforsustainableenergyproduction.pdf

Fig 2) Schematic diagram of two-chambered MFC - Deepak Pant, Gilbert Van Bogaert, Ludo Diels, Karolien Vanbroekhoven, A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production, 4 October 2009, Available from - http://www.microbialfuelcell.org/Publications/2010-Pant-Areviewofthesubstratesusedinmicrobialfuelcellsforsustainableenergyproduction.pdf

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