Some real threats such as global warming, air pollution have made the necessity of thinking about a renewable power resource, among the different methods Microbial fuel cells (MFCs) as a method to produce electricity from the oxidation of organic molecules by biocatalysts, has been proposed. In MFC reactors, electrons generated by biological oxidation reactions are harvested by the anode. Pure culture microbes and mixed communities have been used in the past as biocatalysts. Electron transfer rates as well as electricity production are often significantly improved by the addition of electrocatalytic materials at the surface of the anode, such as carbon nanotubes, graphene derivatives, metals, and conducting polymers. Applications of MFCs are vastly examined in wastewater treatment, greenhouse gas recognizing and mitigating, biosensing, polluted sediment ,and finally surface water remediation. Obstacles of using MFCs, low level of power generation, highly expensive materials such as electrode and the difficultly to scale up MFCs in realm of industrially usage.
Introduction
Nowadays, due to some real issues such as global warming and increasing the level of greenhouse gases in atmosphere, it is urgent to consider another energy resource, which are needed to be green and environmental friendly. Among new technologies fuel cells have some advantages in realm of energy generation [1] (however, there are some obstacles which can restrict the speed of spreading this technology [2]).Fuel cells are electrochemical devices to convert chemical energy into electrical energy, high have a higher electrical efficiency (≥40 %) compared to conventional power generation systems [3]. A microbial fuel cell (MFC) is a hybrid bio-electrochemical device that converts the energy stored in chemical bonds in organic compounds to electrical energy through catalytic reactions that occur in microorganisms [4]. A microbial fuel cell utilizes microbial redox reactions to produce electricity [5], due to the fact that, MFC uses waste organic material to produce electricity and this process is nonpolluting, MFC is highly sustainable energy producing method Microbial fuel cells are a type of fuel cell utilizing microorganisms to produce electricity from organic wastes [6].Microorganisms, such as bacteria, can generate electricity by utilising organic matter and biodegradable substrates such as municipal wastewater [7] Microbial fuel cell (MFC) technology offers numerous opportunities for wastewater treatment [8].Pollutant degradation efficiency and the maximum power density generation are the benefits of MFCs [9]
Applications of Microbial fuel cell (MFC)
In Comparison with conventional aeration technologies for wastewater treatment, MFCs produce less sludge with net energy production. MFCs are vastly examined in wastewater treatment, greenhouse gas recognizing and mitigating, biosensing, polluted sediment ,and finally surface water remediation. Some problems on the way of using MFCs can be considered like, costly electrode material, huge capital cost and inadequate electricity conversion efficiency restricted its large-scale application [10]
Power generation
MFCs utilise the bio-catalytic capabilities of viable microorganisms and are capable of using a range of organic fuel sources, by converting the energy stored in the chemical bonds, to generate an electrical current instead of relying for example, on the use of metal catalysts [7]. Microorganisms, such as bacteria, can generate electricity by utilising organic matter and biodegradable substrates such as wastewater, whilst also accomplishing biodegradation/treatment of biodegradable products, such as municipal wastewater. had been reported that the bioelectric energy efficiency in MFCs is highly dependent on the source and substances of organic matters. Also, it depends on the electron transfer efficiency from anode to cathode.
Education
Soil-based microbial fuel cells serve as educational tools, as they encompass multiple scientific disciplines (microbiology, geochemistry, electrical engineering, etc.) and can be made using commonly available materials, such as soils and items from the refrigerator. Kits for home science projects and classrooms are available.[29] One example of microbial fuel cells being used in the classroom is in the IBET (Integrated Biology, English, and Technology) curriculum for Thomas Jefferson High School for Science and Technology. Several educational videos and articles are also available on the International Society for Microbial Electrochemistry and Technology (ISMET Society)'[30]'.
Biosensor
It’s vital to monitor and detect the toxic compounds and other contaminants for some industries and applications in the field of environmental and clinical production. Therefore, due to the availability and accessibility to biomaterials, using them in the realm of detecting contaminants have been widely recommended [13]. Biosensors are analytical devices used to detect specific analytes. They are analytical devices that convert a biological response into a quantifiable and processable signal [14]. They have umerous existing and prospective applications in various domains [15]. The biological sensing element can be enzymes, antibodies, cell receptors and DNA probes, which interact with a specific analyte. The transducer can be a piezoelectric, optical or physicochemical material, that translates the biological signals to electrical and optical signals. Biosensors can be divided into different categories: (1) amperometric biosensors (measure the electrical current); (2) potentiometric biosensors (measure electrical voltage); (3) conductometric biosensors (measure electrical conductance); (4) optical biosensors (measure the absorbed or emitted light) (5) calorimetric biosensors (measure the change of enthalpy) and (6) piezoelectric biosensors (detect stress) [16]. Microbial fuel cell (MFC) as a biological sensor, widely has been used to detect and monitor toxicity and environmental pollution on industrial applications and other environments, some cases has been presented in table 1.
Wastewater treatment
The traditional methods of wastewater treatment usually use 950 and 2850 kJ/m3 of water treated, in some countries such as United States of America uses 1.5% of all electricity generated in the country[2]. Wastewater treatment can be divided into anaerobic systems (septic tanks, anaerobic ponds and anaerobic digesters), aerobic systems with attached growth (typically trickling filters) and aerobic suspended growth systems (typically activated sludge) [24]. MFCs have advantageous for wastewater treatment over conventional technologies because they enhance conversion efficiency, produce low solid waste and work in ambient temperature [25] Microbial Fuel Cell (MFC) technology presents an appropriate alternative for energy positive wastewater treatment and permits synchronized wastewater treatment, bioelectricity production, and resource recovery via bioelectrochemical remediation mediated by electroactive microbes [26]. Also, removal of biochemical and chemical oxygen demand, nitrification, denitrification, sulfate removal and removal of heavy metals can be done at a same bioreactor. MFCs are used in water treatment to harvest energy utilizing anaerobic digestion. The process can also reduce pathogens. However, it requires temperatures upwards of 30 degrees C and requires an extra step in order to convert biogas to electricity. Spiral spacers may be used to increase electricity generation by creating a helical flow in the MFC. Scaling MFCs is a challenge because of the power output challenges of a larger surface area.
Conclusion
MFC technologies have the potential to play a pivotal role in the transition from fossil fuel based technologies to more renewable energy sources. Research into this area is clearly progressing but there is still much more to do in order for MFC technologies to be routinely adapted into industry and society. This review provides an overview of MFC technologies thus far, whilst benchmarking MFC performance and limitations. Currently the highest power output from an MFC is comparable to that of a PEM hydrogen fuel cell; however, further progression of this field is expected. This expected advancement will be due to the optimisation and tailored development of individual parameters such as, enhanced electrode materials that are more suitable for this application. This, alongside interdisciplinary research intoexoelectrogenic bacteria, their biochemical pathways and the influence of secondary metabolites that underpin electron transfer mechanisms, could lead to power outputs much closer to that of the theoretical limits, as well as furthering the advancing field of electromicrobiology.