Microbial Fuel Cell: Recent Developments in Organic Substrate Use and Bacterial Electrode Interaction

Abstract

A new bioelectrochemical approach based on metabolic activities inoculated bacteria, and the microbial fuel cell (MFC) acts as biocatalysts for the natural conversion to energy of organic substrates. Among several factors, the organic substrate is the most critical challenge in MFC, which requires long-term stability. The utilization of unstable organic substrate directly affects the MFC performance, such as low energy generation. Similarly, the interaction and effect of the electrode with organic substrate are well discussed. The electrode-bacterial interaction is also another aspect after organic substrate in order to ensure the MFC performance. The conclusion is based on this literature view; the electrode content is also a significant challenge for MFCs with organic substrates in realistic applications. The current review discusses several commercial aspects of MFCs and their potential prospects. A durable organic substrate with an efficient electron transfer medium (anode electrode) is the modern necessity for this approach.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Over the past few years, there has been an increase in energy demand. Nonrenewable energy sources, such as fossil fuels and nuclear power, are widely used in the world [1]. When it comes to fossil fuels, this source of energy does more damage to the environment and continuous use of fossil fuels emits carbon dioxide, which becomes toxic when there is too much of it in the air. The rapid depletion of fossil fuels had a significant impact on human life through air pollution and global warming [2–4]. However, many nations worldwide have made outstanding attempts to find a viable alternative to address the energy issue by focusing on renewable sources of energy, such as solar energy, water energy, and wind energy [5–7]. These attempts provided a new way to produce electricity that utilizes a fuel cell through the use of metal catalysts of high value (in the conventional version). As a matter of fact, many benefits can be obtained by using fuel cell compared to other energy producers, such as emission of zero environmentally polluting gases, for example, CO₂, CO, NOx, and SOx, greater efficiency, and the absence of mobile components, resulting in less sonic `pollution [8]. The only shortcomings of these new energy sources, on the other hand, are their high cost and high mass generation [8]. Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell that requires the presence of active bacteria that function as biocatalyst for bioenergy generation in anodic chambers [9, 10]. In the year 1911, Potter [11] reported that bacteria were capable of generating current, but only after a period of 50 years, good results can be observed even they were very low in quantity [12]. Nevertheless, the fuel cell became the center of attention at the beginning of 1990s and as a result, the MFC technology also received attention [13]. In addition, more research was done in the year 1999, as it was known that the presence of a mediator is not a must for an MFC [14]. In general, MFC is composed of the chambers of cathode and anode isolated by the presence of proton exchange membrane (PEM), as shown in Figure 1. Organic substrates will undergo oxidation via the active biocatalyst located within an anode to form protons and electrons [16]. The PEM facilitates the migration of protons towards the cathode, whereas electrons are transferred via an external circuit. Both electrons and protons form a reaction in the cathodic chamber with simultaneous reduction of oxygen to form water. It is the biocatalyst existing in the anode chamber that facilitates the oxidation sources of substrate for the production of protons and electrons. At an anode, electricity generation is prevented by the presence of oxygen; thus, a design of a realistic system that can avoid oxygen from reacting with bacteria should be created, thus the anaerobic condition for the anode chamber [17]. In the past 10 years, the system of MFC has been drastically enhanced but with certain limitations in regard to practicability and scale-up issues, for instance, the resistance of membrane during transportation of protons and problems in both chambers [18, 19]. In addition to the previous issues, MFC is also facing constriction in producing energy because energy generation through the MFC system is dependent on the concentration of the substrate. If the concentration of the substrates present is larger than a certain value, production of power will be obstructed [20, 21]. The organic substrate is the most critical challenge. The lower stability of the organic substrate may have had an impact on remediation efficiency and energy generation because the organic substrate did not provide enough power to the bacterial population, resulting in poor MFC output. The long-term stability of organic substrates in MFC for industrial use should be the subject of future research. According to the literature survey, there is no similar information available on the interaction of electrode-bacteria and electrode effect in the presence of different organic substrates in MFC. This review provides a direction to researchers to improve the organic substrate factor with electrode development to ensure the strong interaction of bacteria with the electrode material. In the present review article, different organic substrates are extensively studied with bacterial interaction. The effects of different previous electrodes and electrode-bacteria interactions are also summarized in the present study.

[figure omitted; refer to PDF]

Indication of direct contact between anode and cells through interaction via c-type cytochromes on the external layers is recognized by analysis of electrochemical and expression of genome-scale gene [130]. The thickness of biofilms formed from G. sulfurreducens is more than 50 μm and the cells are active metabolically and facilitate the generation of energy [131]. The investigation of the expression of genes proposed that for the long-distance transfer of electrons via the biofilms of G. sulfurreducens, it is crucial to form microbial nanowires [132]. Formation of the high value of current is also anticipated by representative studies via biofilm of thick anode if the conductive biofilm is present [133]. The generation of biofilm that is conductive is unlikely because the majority function as insulators [134]. In the field of MFC, it will be a great discovery to explore the conductivity determination, measurement, and the materials that affect the biofilm conductivity.

The focus that energy is gained by bacterial species for the electrode or shuttle of electrons directly is a common misunderstanding. In actuality, energy is received from the activities of protons pumping over the internal membrane that produces a gradient of protons that enhances the production of ATP via ATPase from ADP. In this regard, the external transfer of electron function is to migrate the electron towards the surface of anode, but no energy is obtained during the process for the growth of bacteria; instead, the proton gradient enables the formation of ATP, which in return supplies energy for the bacterial species [135].

The effort has been made to enhance the generation of current by genetic engineering but is unsuccessful [136]. The power generated does not increase, albeit there is an excess of microbial nanowires or cytochromes; similarly, ATP drain formation as forecasted by metabolism representative to raise the metabolic activity also did not have any effect on the current. Based on these investigations, it is clear that the production of current in an MFC is a difficult process that needs more than a few gene modifications or rates of bacterial respiration to have an impact on the current rate of generation. Adaptive selection has shown more positive results in strain production to increase the density of current in MFC. One example is MFC with a low operating potential that was deployed for a period of more than 5 months and resulted in the separation of a G. sulfurreducens variant called KN400 that allows the rise in power density up to 8 times. The strain formed generates biofilm that is thinner along with cytochromes of small external surface area but a large number of nanowires that may enable the examination of the transfer of electrons occurring at increased current densities.

6. MFC Commercialization and Future Perspectives

The commercialization of technology can be deemed successful if more people become aware of the product and it is marketed in large amounts to a wide range of consumers. The commercialization of MFC will bring more benefits due to its function in energy production through the utilization of wastes. For instance, electricity can easily be generated at homes; energy generation with low expense can be accomplished all year long as waste materials. The xenobiotics are abundant in low-income countries like Africa; the MFC will be beneficial because of its low operating cost as compared to large foundations needed to set up energy generation plants that are unavailable [137]. Lastly, MFC could be an alternative for the remediation process to remove hazardous substances from xenobiotics and wastes [138]. The performance of an MFC technology is dependent on the number of variables used to observe the system, for example, the utilized substrates, the setup, microbes present, catalyst, concentration, ideal membrane, and electrode materials. A large number of accessible reports have shown that MFC could be configured from sizes of a few milliliters to a few thousand liters. Based on these reports, the outcome is that the power generated is affected by the MFC scaling, which is a major weakness in marketing MFC. The space between anode and cathode is one of the components that can affect the power production for MFC [139]. With bigger cell size, the size of electrodes used will also increase; however, the separation itself does not change to the same extent to avoid bulky cells and this causes the low generation of power. Another reason that limits MFC scaling is the price of electrodes, which rationally should be inexpensive, but as the electrodes are bought instead of being produced in the industry, the price becomes higher, as well as because of the material itself. The membrane present in MFC is also created using high-cost material such as nylon. Substrates also play an important role for MFC and depending on the type of substrate utilized, the power output will differ. As an example, the pure substrate will result in high power production, but by using wastes as substrates, the value of power generated decreased significantly. The reason for this is that microorganisms are incapable of metabolizing waste and pure sources of carbon [140]. These are some of the limiting factors towards MFC commercialization. MFC is capable of utilizing various organic materials in producing energy. Nevertheless, MFC still has few weaknesses that need to be addressed in order for the technology to be applied in a practical situation. The biggest challenge of an MFC is the poor density of power, which can be sorted out either by separating strong microbes that are capable of transferring electrons towards anode or by synthesizing engineered strains via DNA recombination that provides a higher transfer rate of electrons. Various consortia of bacteria are proven to be more efficient in transferring electrons than pure cultures and more strains of bacteria can generate mediators for a more successful electron transfer. New types of mediators are recognized to enhance MFC performance. The small surface area of an MFC is also a big challenge as a limited number of microorganisms can adhere to it. Further studies reported new techniques that can improve the performance of MFC, which provide a more efficient configuration of small-scale MFC. Some of the techniques are stacked reactors, assemblies of cloth electrodes, and utilization of air cathodes. Utilization of air-cathode MFC is the most efficient method out of all techniques because it utilizes oxygen sources from air efficiently, in return removing the requirement of water aeration and employing chemical catholytes like ferricyanide that need to be regenerated. Distinct cell designs are used in evaluating the consequences of utilizing various shapes and positions for MFC improvement and optimization of air-cathode is also done for MFC utilization. Great results have been achieved through these attempts in which a power output of more than 1000 W m³ was generated from an efficient small-scale MFC (∼20 ml volume of anode) [141]. Nevertheless, producing a large-size MFC that can provide increased energy generation and performance stability is still a difficult task. According to a study done by Liu et al. [142], the highest value of power density 20 W m³ can be achieved by using MFC of 500 ml in volume. The final downside of the MFC system is in regard to wastewater treatment and MFC scaling up. These are crucial issues needing solutions as scaling up can result in MFC application for a large-scale setup that can help in thoroughly improving MFC performance, especially in the case of wastewater treatment that is abundant.

7. Conclusion

Electricity generation through the MFC system by utilizing organic substrates that are oxidized by bacterial species can provide a promising technique for the future. In this review, major waste materials such as acetate, brewery wastewater, synthetic wastewater, inorganic compounds, and azo dyes, which are harmful to the environment and living beings, have been discussed, which can explore new potential through electricity generation via MFC as substrates. In addition, some of the toxic substances can also be treated and converted into less harmful substances that are beneficial for the management of waste and can decrease the amount of environmental pollution. Until recently, a wide range of substrates had been used in MFC for energy production. Even so, major challenges for practical use of MFC need to be addressed and solved, such as the poor output of power and reduction of energy production as a result of scaling up. These factors contribute to difficulties in MFC commercialization. Thus, more efforts are needed to provide a feasible, applicable, and efficient technology that can be approved and acknowledged by the industry.

Acknowledgments

This article was financially supported by Open University of Sudan.

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