1 Introduction
The constant demand for energy and the limited supply of fossil fuels and their
impact in the environment have required the development of alternative energy
sources. Among the next generation of energy sources, microbial fuel cells (MFCs)
continue to attract wide attention due to their ability to recover energy in the form of
electricity using microorganisms as catalysts. This technology is among the most
studied bioelectrochemical systems (BES), with these devices also being used for
other eco-friendly purposes such as the production of biofuels and chemicals,
biosensors, bioremediation, wastewater treatment and desalination [1–5].
MFCs employ microorganisms to produce electrical current while metabolizing
nutrients available in the medium [6, 7]. The capacity of using organic waste (e.g.,
wastewater) as substrate has opened the possibility of producing electricity in a way
that is close to carbon neutral [8, 9]. These cells consist of an anode that is kept
under anoxic conditions and receives electrons from the bioenergetic metabolism of
the microorganisms growing on its surface. MFCs also contain a cathode that
transfers electrons to the terminal electron acceptor. It is the electron flow from the
anode to the cathode through an external circuit that allows the production of
electrical current. Nowadays, there are a wide variety of designs, where the anode
and the cathode may coexist in a single compartment (single-chamber) or can be
separated by a physical barrier that is permeable to ions (dual-chamber) (Fig. 1)
[10].
In BES, electron transfer efficiency depends on several parameters, with the
electron transfer processes performed by the microorganisms among the core factors that affect power generation performance [11, 12]. Microorganisms that oxidize
organic compounds and transfer electrons to the anodes of BES are called electroactive but are also known under several other names in the literature, such as,
electricigens, exoelectrogenic, anode-respiring or anodophilic microorganisms [13].
These electroactive microorganisms are united in their ability to perform extracellular electron transfer (EET), directly and/or mediated, to the electrode.
The concept of electric current generation by microorganisms is not new and
was reported over 100 years ago [14], with research on MFCs making several
advances in the last decade [7, 15]. These include different MFC architectures and
construction materials for the anode and cathode, diverse microbial communities
and knowledge on the biochemical characteristics of the EET performed by the
microorganisms [7, 16]. Nevertheless, the commercialization of MFCs is still
limited due to low performance, expensive core parts and materials, and bottlenecks
in scale-up [15, 17]. Therefore, many challenges and room for improvement remain
in BES, including the identification of new electroactive microorganisms with high
electrochemical activities and the characterization of the electron transfer process
between cells and electrodes. This has been a crucial aspect in the enhancement of
MFC performance and paramount in promoting their future applications [18–20].
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B. M. Fonseca et al.
The constant demand for energy and the limited supply of fossil fuels and their
impact in the environment have required the development of alternative energy
sources. Among the next generation of energy sources, microbial fuel cells (MFCs)
continue to attract wide attention due to their ability to recover energy in the form of
electricity using microorganisms as catalysts. This technology is among the most
studied bioelectrochemical systems (BES), with these devices also being used for
other eco-friendly purposes such as the production of biofuels and chemicals,
biosensors, bioremediation, wastewater treatment and desalination [1–5].
MFCs employ microorganisms to produce electrical current while metabolizing
nutrients available in the medium [6, 7]. The capacity of using organic waste (e.g.,
wastewater) as substrate has opened the possibility of producing electricity in a way
that is close to carbon neutral [8, 9]. These cells consist of an anode that is kept
under anoxic conditions and receives electrons from the bioenergetic metabolism of
the microorganisms growing on its surface. MFCs also contain a cathode that
transfers electrons to the terminal electron acceptor. It is the electron flow from the
anode to the cathode through an external circuit that allows the production of
electrical current. Nowadays, there are a wide variety of designs, where the anode
and the cathode may coexist in a single compartment (single-chamber) or can be
separated by a physical barrier that is permeable to ions (dual-chamber) (Fig. 1)
[10].
In BES, electron transfer efficiency depends on several parameters, with the
electron transfer processes performed by the microorganisms among the core factors that affect power generation performance [11, 12]. Microorganisms that oxidize
organic compounds and transfer electrons to the anodes of BES are called electroactive but are also known under several other names in the literature, such as,
electricigens, exoelectrogenic, anode-respiring or anodophilic microorganisms [13].
These electroactive microorganisms are united in their ability to perform extracellular electron transfer (EET), directly and/or mediated, to the electrode.
The concept of electric current generation by microorganisms is not new and
was reported over 100 years ago [14], with research on MFCs making several
advances in the last decade [7, 15]. These include different MFC architectures and
construction materials for the anode and cathode, diverse microbial communities
and knowledge on the biochemical characteristics of the EET performed by the
microorganisms [7, 16]. Nevertheless, the commercialization of MFCs is still
limited due to low performance, expensive core parts and materials, and bottlenecks
in scale-up [15, 17]. Therefore, many challenges and room for improvement remain
in BES, including the identification of new electroactive microorganisms with high
electrochemical activities and the characterization of the electron transfer process
between cells and electrodes. This has been a crucial aspect in the enhancement of
MFC performance and paramount in promoting their future applications [18–20].
216
B. M. Fonseca et al.