1. Introduction
It is well known that excessive burning of fossil fuels contributes to global warming and leads to the ever-existing energy crisis. Therefore, it has become an inevitability to develop renewable sources of energy that are eco-friendly and durable. In addition, wastewater emanating from various industries contains significant amounts of inorganic substances such as solvents and insoluble materials causing contamination of the environment if not well-treated [
1]. As a result, and to track together the issues of the energy crisis and water pollution, fuel cell technology is a promising alternative to fossil fuels that could lead to a cleaner environment and reduced CO
2 emissions [
2]. Power devices convert the vitality contained in electron producing organic materials into electricity utilizing a chemical catalyst. With the help of biocatalysts, biological fuel cells (biofuel cells) can achieve the anodic, the cathodic or both reactions. This results in enzymatic power devices or Microbial Fuel Cells (MFCs).
A Microbial Fuel Cell (MFC) is a developing technique for the production of power via biodegradable organic matter using microorganisms [
3]. Even though the concepts of creating electricity from microorganisms have been prevalent since 1911, there were not many practical applications of this process for around 55 years [
3]. MFC is a device that uses bacteria, as a fuel source, to breakdown organic materials and thereby generate electricity. Living creatures metabolize food to provide them with energy [
4]. In a single-celled microorganism, this process is rather an intricate one involving many enzyme-catalyzed reactions [
5]. It involves successive oxidation-reduction changes, and in this manner resembles an electrochemical process [
6]. The studies showed that electron transports (mediators) are required to convey electrons from inside the cell to the anode [
7]. A simple schematic representation of an MFC is shown in
Figure 1.
The MFC is composed of two compartments, the cathode and the anode, separated by a conducting membrane. The microorganisms fed on the carbon substrate within the anode chamber. The electrons at the anode move through an external circuit to the cathode. To tolerate this process, electro-neutrality needs to be observed, i.e., transport of electrons to the cathode needs to be compensated by transport of an equal amount of positive charge to the cathode chamber [
9]. The potential difference between the respiratory system and the cathode chamber produces a voltage and current which in turn gives bioelectricity [
10].
The electrons produced during the respiratory cycle move through a sequence of respiratory enzymes in the cell, thus producing power in the form of ATP. These electrons within the cell are transferred and made available to the anode by mediators. Performance of MFC using three different mediators was investigated in terms of power density and internal resistance by Taskan et al. [
11], where they used methylene blue (MB), neutral red (NR) and 2-hydroxy-1,4-naphthoquinone (HNQ) mediators at different concentrations and found that internal resistance changed with mediator concentration. However, certain bacteria called exoelectorgenic bacteria are capable of directly transferring the electrons outside the cell [
7] without the mediator, in spite of the fact that they have been depicted utilizing different terms, for example, electrochemically dynamic microorganisms [
12], anode respiring microscopic organisms and electricigens. A mediator-less microbial fuel cell was developed by Herrero-Hernandez et al. [
13] using
Escherichia coli bacteria and platinized titanium mesh as electrodes, producing a maximum power density of 627 mW m
−2.
Though various kinds of microorganisms produce electrical current in MFCs, huge numbers of these strains show low power densities when developed as pure organisms [
3]. It is thusly indistinct whether these microorganisms exist as exoelectrogenic oligotrophs among more quickly developing contenders, or whether a low degree of current generation gives some different advantages through interactions [
14]. For instance, a Gram-positive bacterium (
Brevibacillus sp. PTH1) that was copious in a blended network in an MFC created little force as an unadulterated culture except if a
Pseudomonas sp. was additionally present or supernatant from an MFC with this bacterium was included [
5]. Toczyłowska-Mamińska et al. [
15] studied the bacterial communities that evolve in single-chamber air-cathode MFCs fed cellulose; they examined the changes in the bacterial consortium in an MFC fed cellulose over time and found that the most predominant bacteria shown to be capable electron generation was
Firmicutes, with the fermenters decomposing cellulose
Bacteroidetes.
It is trusted that one day MFC will be utilized as an individual technique for power generation; however, its latest applications are that of energy recovery during wastewater treatment and remote power production [
16]. Through this energy recovery processes from wastewater, it is likely that this will guarantee the energy sustainability of the water foundation. It is assessed that local wastewater contains 9.3 times as much energy as that right now used to treat the wastewater through energy-intensive aeration-based processes [
17]. The development of these MFCs, however, has its limitations. Practical full-scale MFC application in wastewater treatment and electricity production design has been long delayed by the instability of full-scale engineered systems, low achieved power densities and output voltages practically achievable so far [
18]. Huggins et al. [
19] attempted to directly compare the wastewater treatment efficiency and energy consumption and generation among three reactor systems—a traditional aeration process, a simple submerged MFC configuration and a control reactor acting similar as natural lagoons.
Many parameters can influence the efficiency of a microbial fuel cell. These may include, the rate of fuel oxidation and electron transfer to the anode by the microorganisms, the circuit resistance, the oxygen supply and the proton transport to the cathode through the membrane [
20]. Furthermore, the type of electrode has significant effect of the performance of MFC [
21,
22,
23]. The present work attempts to consider various cathodic and anodic materials, with two different MFC sizes, and test their influence on the generated power density of the MFC. The use of the different combinations of copper, aluminum, carbon cloth, steel and brass for the anode and cathode was also considered. The use of different oxidizing agents such as copper sulphate and potassium hexacyanoferrate is also investigated. Furthermore, the size and distance between the electrodes was investigated. The power outputs between electrochemical and microbial cells were studied.