Large stack of MFCs: issues and impedance mismatching

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Bacteria involved in MFCs

The most interesting class of bacteria is one that can produce electricity without requiring the addition of mediators. Known as electroactive species, these bacteria directly transfer electrons to chemical species or a substance that is not an immediate electron-acceptor. Among the major strains of electroactive bacteria, can count Geobacter metallireducens, Geobacter sulfurreducens [10] and Desulfuromonas acetoxidans in the family of Geobacteraceae, while among Shewanella, we mention Shewanella putrefaciens [60] and Shewanella oneidensis [39].
The MFC voltage is determined by the difference in redox potential between the two distinct electrodes. Redox potential discloses the tendency of a chemical species to be reduced that is to acquire electrons. Its value is expressed in volt (V).
Many MFC researchers are evaluating the benefits related to the inoculation of certain species of electroactive bacteria. However, for MFCs that use natural substrates such as soil, sediments or effluents, inoculation is not necessary, since these species of electroactive bacteria are already present in abundance, along with complex sugars and other nutrients that have accumulated during millions of years due to the decay of plants and material of animal origin.
The current research has not yet given results regarding the existence of an isolated bacterial strain able to guarantee a higher energy production compared to that obtained from other strains or compared to mixed cultures.
Identifying a potential strain capable of ensuring higher current densities and understanding the synergy or antagonism taking place between different bacterial strains is the goal of current research.
The following table in Fig 2.3 shows some of the bacterial strains that have been identified in literature within the substrates used in MFCs.

Different typologies of Microbial fuel cell

There are different typologies of microbial fuel cells, depending on reactor configuration, membrane and substrates used. One is named the terrestrial microbial fuel cell and is founded on the same basic MFC principles described above; whereby, standard topsoil is the nutrient-rich anodic medium, the bacteria inoculum and the proton-exchange membrane. Another type of bioreactor is the waste water microbial fuel cell where waste water acts as flow of nutrient and bacteria. Moreover, there are some different types of MFCs, like benthic or plant MFCs.

Benthic Microbial Fuel Cell

Benthic microbial fuel cells (BMFCs) are MFCs that recover energy from marine sediments. BMFCs can be deployed in all types of aquatic environments with a carbon-rich substrate where electroactive bacteria can grow and proliferate.
In the case of the BMFC (Fig. 2.4), the sediment is related to the anode compartment and the supernatant water (on the sediment surface) to the cathode compartment.
The naturally created interface between water and sediments play the role of the proton exchange membrane. This interface thus passes ions and filters oxygen present in the water. This filtration is even more effective because some of the bacteria present in the anodic chamber are aerobic, i.e. they consume oxygen. With a sufficiently deep immersion of the electrode (a minimum of few centimeters), the anode compartment is therefore assumed to be anaerobic.
Oxygen dissolved in water comes from both surface oxygen that diffuses into the surface layers and photosynthesis from nearby plants. The distribution of this oxygen is ensured by the continuous flow of water thanks to the waves and the wind.
The layers close to the surface are thus generally saturated with oxygen. In addition, the ocean floor has a very rich and diverse bacterial flora among which electro-active bacteria necessary for the functioning of the BMFC.
Moreover, it is full of organic matter resulting from animal, plant and bacterial degradation, accumulated and constantly renewed.

Terrestrial Microbial Fuel Cell

The terrestrial microbial fuel cell (TMFC) is founded on the same basic MFC principles described above; whereby, standard topsoil is the nutrient-rich anodic medium, the inoculum and the PEM. As a matter of fact, it is possible to harvest energy from the soil present in the entire earth using proper electrodes, in any moment everywhere and forever.
Microbial fuel cells in configurations such as BMFC or WMFC must work principally in a marine environment or in presence of waste water, which limits the MFCs’ application fields. Terrestrial microbial fuel cells can be inoculated by soil or even work on land, which can overcome the disadvantages and extend the MFCs’ application range. TMFC can be used as a closed reactor with a controlled environment able to self-maintain its functioning for years or installed directly on a land with or without presence of plants.
In a terrestrial microbial fuel cell (Fig. 2.5), the anode is placed at a working depth, generally about 8 cm into the ground, while the cathode is preferable on top of the soil, in order to expose it to the oxygen in the air.
In the classical configuration the two electrodes, the anode and the cathode, of an MFC are made of graphite felt. Besides being an appropriate conductor, graphite is also a very economical material.
Located in the soil, the aerobic microbes act as an oxygen filter, consuming oxygen and, thus, preventing infiltration into the anode compartment.

Waste Water Microbial fuel cell

The waste water fuel cell (WWMFC) is equally based on the same basic MFC principles, where the waste water plays the role of flow of nutrient (biofuel) and inoculum of bacteria (biocatalysts) [132]. These cells are generally is single-chamber configuration (Fig. 2.6). The chamber is filled with waste water and the anode is placed inside water. Usually, an air-cathode is used for WWMFCs, which is useful to eliminate any limitation in oxygen supply to the electrode due to mass transport issues.

Lab-scale MFC prototypes

In this paragraph, an economical way to build MFCs and the specific design lab-scale MFCs is presented. It was used two different kinds of MFC for experimentations: one is a terrestrial microbial fuel cell (TMFC) based on soil that acts as the membrane, inoculum of bacteria and as flow of nutrient, the other one is a waste water microbial fuel cell (WWMFC) where waste water acts as flow of nutrient and bacteria inoculum. Usually in an MFC reactor, the anode is placed at a working distance of about 4 cm up to 8 cm with respect to the cathode. The materials chosen for the anode and the cathode have to be electrically conductive and chemically inert, so normally the electrodes are made of graphite fiber felt and carbon cloth or brush with a diameter that varies from 8 cm to 13 cm. The structure of the reactor is usually made of PVC with cylindrical shape. All the electrical cables are made of titanium wire.

TMFC lab-scale

Lab-scale TMFCs (Fig. 2.8) are built from PCV with a height of 10 cm and a cylindrical shape. The reactors are filled with topsoil, preventing air bubbles inside the ground to avoid oxygen pervading into the anodic chamber. The reactor diameter measures 10 cm.

Energy mechanisms of an ideal MFC

Chemotrophic bacteria gain their energy for life and growth through the coupled mechanisms of nutrient oxidation and reduction (electron donation). Chemoorganotrophs are organisms, which oxidize the chemical bonds in organic compounds as their energy source. So, a chemoorganotrophs organism is one that requires organic substrates to get its carbon for growth and development, and that produces its energy from the oxidation and reduction (redox) of an organic compound.
These mechanisms involve the transformation of reactants (as organic matter and oxygen) into products (as water and carbon dioxide). Oxygen is the electron acceptor of aerobic organisms. Other inorganic compounds, such as nitrate, are the electron acceptors of anaerobic organisms. The theoretical energy available for the bacteria is related to the difference between the energy content of the reactants and the products.
The reaction can be evaluated in terms of Gibbs free energy (G), which is a measure of the maximal work that can be derived from the overall reaction. The Gibbs free energy equation (3) represent the energy content that can be expressed as the product of the electromotive force (EMF) and the corresponding charge flow nF [2]. 𝐺=𝑛𝐹(𝐸𝑅𝐸𝐷−𝐸𝑂𝑋) (3).
where n is the number of electrons involved in the reaction, F is the constant of Faraday, and ERED and EOX are the reduction and oxidation potentials respectively which difference corresponds to the EMF as in equation (4). 𝐸𝑀𝐹= 𝐸𝑅𝐸𝐷−𝐸𝑂𝑋 =𝐸𝐴𝐶𝐶𝐸𝑃𝑇𝑂𝑅−𝐸𝐷𝑂𝑁𝑂𝑅 =𝐸𝑐𝑎𝑡ℎ𝑜𝑑𝑒−𝐸𝑎𝑛𝑜𝑑𝑒 (4).

Table of contents :

1. INTRODUCTION
1.1 Abstract (EN)
1.2 Abstract (IT)
1.3 Abstract (FR)
1.4 Energy harvesting techniques
1.5 Context of Biofuel cell
1.5.1 Enzymatic fuel cell
1.5.2 Mitochondrial fuel cell
1.5.3 Microbial Fuel Cell
1.6 Bioelectricity and a brief history of MFC
2. MICROBIAL FUEL CELL
2.1 MFC principles
2.2 MCF operation and main application
2.2.1 Bio-fuel cell
2.2.2 Bio-sensor
2.2.3 Bio-remediator
2.3 Bacteria involved in MFCs
2.4 Different typologies of Microbial fuel cell
2.4.1 Benthic Microbial Fuel Cell
2.4.2 Terrestrial Microbial Fuel Cell
2.4.3 Waste Water Microbial fuel cell
2.5 Continuous flow and batch mode
2.6 Substrates
2.7 Electrodes
2.8 Lab-scale MFC prototypes
2.8.1 TMFC lab-scale
2.8.2 WWMFC lab-scale
2.9 MFC losses and limitation
2.9.1 Energy mechanisms of an ideal MFC
2.9.2 Voltage losses
2.9.3 Current losses
2.9.4 Load influence
2.9.5 Internal resistance
2.10 Main measurement and electrical issues
2.10.1 Reducing internal load and MFC limitations
2.10.2 Large stack of MFCs: issues and impedance mismatching
2.10.3 Avoid oxygen presence in the anodic chamber
2.10.4 Maximum power point tracking (MPPT)
3. MEASURING BOARD
3.1 Introduction
3.2 Design of a measuring instrument dedicated to MFC
3.2.1 Power supply and accurate voltage reference
3.2.2 Measurement phase selector (Charge, Discharge, Power analysis)
3.2.3 Current and voltage measurements
3.2.4 Offset voltage, amplifier and transimpedance block
3.3 Instrument advantages and GUI features
4. I-V EXPERIMENTAL RESULTS
4.1 TMFC
4.1.1 Influence of pH
4.2 WWMFC
4.2.1 Synthetic wastewater
4.2.2 Single reactor
4.2.3 MFCs arranged in form of a pack
4.2.4 Parallel configuration
4.2.5 Series configuration
4.2.6 Reversal voltage: polarization of MFC
5. MFC APPLICATION
5.1 Field of application
5.2 Energy management system to supply WSN
5.2.1 Comparison of DC/DC converters and transceivers for WSN
5.2.2 WSN powered by TMFC
5.2.3 Energy management system with a Flyback converter
5.2.4 Long range WSN with BQ2550x series
5.2.5 Voltage polarization method for energy management
5.3 Continuous power supply mode
6. CONCLUSIONS AND FUTURE DEVELOPMENT
6.1 Perspectives
7. APPENDIX
7.1 Appendix A – Bibliography
7.2 Appendix B – Board components
7.2.1 Hardware Description
7.2.2 Relays and accurate voltage reference
7.2.3 Microcontroller and ADC (Analog to digital converter)
7.3 Appendix C – Publications List
7.3.1 List of Articles and Publications
7.3.2 Conferences, Symposia and PhD school attended:
7.4 Appendix D – Joint-supervision PhD
7.4.1 PhD organization, future development and European collaboration
7.4.2 Prizes, projects and awards

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