Biological Fuel Cells: Fundamental to Applications

By Mostafa Rahimnejad

Biological Fuel Cells: Fundamental to Applications offers a comprehensive update on the latest microbial fuel cells technologies and their systems development and implementation. Taking a practical approach to MFCs, the book provides guidance on analytical methods and tools, economic and performance analyses of various technologies and systems, and engineering aspects. Established and newly developed technologies are presented alongside their applications within the context of cost, practicality and future technologies, which are discussed within the context of other renewable energy systems. This book is a comprehensive reference for users working in the field of fuel cells, microbial fuel cells and bioenergy.

• Presents lab-scale case studies and real-world application on microbial fuel cells
• Provides the fundamental theories and concepts behind MFCs, along with the latest technologies
• Offers guidance on economic and cost analyses for technologies and systems within each chapter

• Language: English
• Publisher: Elsevier Science
• Release date: Mar 15, 2023
• ISBN: 9780323857123

Mostafa Rahimnejad

Mostafa Rahimnejad is a full professor in Biochemical engineering, Department of Chemical Engineering, Babol Noshirvani University of Technology (BNUT), Babol, Iran. In addition, he is Head of Renewable Energies and Biofuels Research Centre. He received BS from Tehran University, Tehran, Iran; MS from University of Mazandaran, Iran; and completed a PhD in biotechnology-chemical engineering from University of Mazandaran, Iran. His research subject was “Fabrication and Optimization of Biological Fuel Cell.” He spent his sabbatical leave at Kangwon National University, Chuncheon, South Korea. Measurable outcomes of his research are 160 research papers and review articles published in leading peer-reviewed international journals, 7 book chapters contributed to renowned publishers, and 8 filed patents. Dr. Rahimnejad has attended many invited lectures and plenary/keynote presentations at several state, national, and international conferences, seminars, and symposiums. His 12 years of expertise and practical experience cover various fields of environmental biotechnology, biosynthetic technology, and energy biosciences, focusing on biological fuel cell, sensor and biosensor, purification of wastewater, fermentation, bioproducts, etc. His awards and honors include “Young Research Award for BIOVISION,” “Top Elite Researcher,” “Top Professor,” “Top Researcher,” and “Top Scientist in Biotechnology” (Top 2% in the world, 2020).

Book preview

Part 1

Constituents, structure, materials and measurement with conceptual, practical and economical views

Chapter 1: Introduction to biological fuel cell technology

Mostafa Rahimnejad    Biofuel and Renewable Energy Research Center, Chemical Engineering Department, Babol Noshirvani University of Technology, Babol, Mazandaran, Iran

Abstract

Considerable research over the past decade has established that biological fuel cell (BFC) constructs has the potential to provide significant economic opportunities for energy conservation. BFCs are known as economically feasible and an eco-friendly bioelectrochemical system in which microorganisms/enzymes actively participate in bioconversion of chemical energy embedded within organic materials. BFCs can be broadly classified into microorganism-based BFCs and enzyme-based fuel cells. BFCs are widely used for pollutant removal from soil, power generation, wastewater treatment, freshwater generation, in situ electrochemical biosensing, and for the production of value-added bioproducts. A wide range of waste materials including solid wastes, liquid wastewater, and synthetic wastewater can be employed as substrates for BFCs. In this chapter, we review the basic principles of energy recovery from organic waste materials through microbial/enzymatic electrochemical reactions and summarize the effect of potential feedstocks on BFC performance. Furthermore, we describe microorganism-based BFCs and enzyme-based fuel cells and examine directions for future research.

Keywords

Bioconversion of chemical energy; Biological fuel cells; Photosynthetic microbial fuel cells; Microbial desalination cells; Sediment microbial fuel cells; Enzyme-based fuel cells

1.1: Background

Energy consistently impacts the quality of life and plays a crucial role in the development of modern society. Energy can be obtained from renewable or nonrenewable sources (Fig. 1.1). Nonrenewable energy possesses cumulative share of world energy consumption including fossil fuels (oil, coal, and natural gas) and nuclear energy. As reported in Statistical Review of World Energy, fossil fuels provide more than 80% of total energy on the Earth (Fig. 1.2). Although fossil fuels are prime energy providers, the risks of carbon dioxide emission, air pollution, global warming, and health problems restrict their applications [1,2]. Furthermore, due to population growth and energy crisis [3] there is an immense need to develop alternative energy sources.

Fig. 1.1

Fig. 1.1 Types of energy sources.

Fig. 1.2

Fig. 1.2 Primary global energy consumption 2019.

Renewable energy sources comprising solar energy, wind energy, biomass energy, geothermal energy, wave energy, and hydro energy are established around the world as green and cost-effective power suppliers (Fig. 1.3). However, there are some drawbacks in reliability of the generated power as to meteorological conditions and financial constraints.

Fig. 1.3

Fig. 1.3 Renewable electricity generation 2017. (Reported by International Renewables Energy Agency (IRENA).)

From feasibility assessment provided in the literature, it can be concluded that biomass has the potential to provide significant economic opportunities for energy conservation. Biomass is one of the most abundant renewable energy available. Chemical energy embedded within bulk biomass can be converted into electricity using bio-electrochemical systems. Agricultural residue, animal waste, energy crops, and food processing wastewater are well-known biomass options for electricity production.

Interest in biological fuel cells (BFCs) increased when NASA utilized biocatalytic devices to turn organic waste into electrical power on space missions.

BFCs transform electrical energy of organics into electrical energy carried out either with enzymes or microorganisms as biocatalysts. As a matter of fact, enzymatic-based fuel cells (EFCs) have some advantages such as biocompatibility, good occurring under mild conditions as well as being specific and hierarchical in performance. However, enzymatic life time is limited and there are some practical problems associated with enzyme operating conditions. Enzyme stability enhancement is possible, although it requires additional operating costs.

Microbial fuel cells (MFCs) are known as bioelectrochemical systems in which microorganisms actively participate in direct conversion of chemical energy embedded within organic materials into sustainable electrical energy. MFCs are primarily utilized for bioelectricity production. Being economically feasible and an eco-friendly approach, MFCs provide immense potential as an alternative to the uncontrolled use of fossil fuels (Fig. 1.4). There are a wide variety of carbonaceous substrates for MFC operation such as pure organic compounds, synthetic feedstocks, biomass, and domestic and industrial wastewater.

Fig. 1.4

Fig. 1.4 Comparison of operational characteristics between MFCs and chemical fuel cells (CFCs).

Recent studies have developed MFC constructs for simultaneous wastewater treatment and power generation (Fig. 1.5). The electrical power is prognosticated to partially cover energy demand of wastewater treatment plants [4]. The process includes microbial decomposition of substrates, aiming at sustained electron flows to provide a valuable green electrical current. Chapter 12 deals with applications and challenges of MFCs as novel wastewater treatment systems.

Fig. 1.5

Fig. 1.5 MFC a green energy supplier of wastewater treatment plants.

In situ electrochemical biosensing is another potential application of MFCs. Recently, significant progress has been made in the development of MFC-based biosensors for monitoring biochemical oxygen demand (BOD) and toxicity in the environment. The application of MFCs as biosensors are discussed Chapter 16. Despite significant advancement in MFC technology, its commercialization remained a challenge due to its low revenue per unit energy [5]. Chapter 6 provides an overview of practical designs of MFCs, scale-up parameters, and limitations in large-scale application of MFCs.

This chapter presents an introduction to MFCs, their basic principles, classifications, and a brief evaluation of potential feedstocks.

1.2: Basic principles

1.2.1: Microbial decomposition of organic materials

Decomposition means biocatalytic breakdown of raw organic materials into CO2, water, mineral nutrients, and energy. In the past 10–15 years, global energy crisis has captured the attention of researchers for energy recovery from organic waste through microbial/enzymatic electrochemical reactions, among which microbial catalysts are preferred due to their economic feasibility, and more sustainable biocatalytic activity of microorganisms.

In MFCs, microbes are used as catalysts for electrochemical reactions. Microbes have excellent capacity to generate electricity. The choice of microbial community is dependent on the type of substrate and operating conditions [6,7].

MFCs cover a broad range of biodegradable organic substrates that alter the degradation kinetics and affect the operational conditions [8]. Until now, various groups of microorganisms (such as bacteria, microalgae, fungi, and yeast) have been shown to be capable of biocatalytic function in MFC systems. Different kinds of microorganisms used in MFC operation are discussed in detail in Chapter 2.

1.2.2: Working principles of MFCs

As a biological technology, MFCs integrate cathode and anode materials with microorganisms to achieve electricity generation. MFCs utilize a strategy that includes the microbial conversion of chemical energy of organic and inorganic compounds into electricity [9,10].

MFCs require four components, an anodic electrode within which electrons, protons, and other metabolites are produced through anaerobic oxidation of substrates, a cathodic electrode for collecting the electrons by an electron acceptor material, a membrane for physical separation of the electrodes and mediating proton diffusion to the cathode electrodes, and an external circuit allowing electron flow from anode to cathode. Cathodic and anodic chambers contain electrolytes that play a vital role in electron and proton transportation within MFCs. Chapters 3–5 discuss recent advances in anode, cathode, and separator materials used in MFC systems.

Microbes decompose organic substrates in anodic chamber to provide electron and proton flows. Electrons migrate toward the cathode via an external circuit, while the protons are transported from anode to cathode through electrolytes. At cathodic chamber, the collected particles are combined with oxygen to produce water. Oxygen is a conventional electron acceptor capable of providing high oxidation potential and offering a clean final product (water).

MFC technology has a high energy conversion efficiency and a reduced level of undesired by-products [11,12]. However, there are some disadvantages of using MFCs. For instance, one drawback is that there has been electron mediating limitation between microbial cells and exogenous electrodes, restricting the choice of available materials. Thus, further studies are required to improve the performance of MFCs.

Following the use of galvanic cells as a way of electricity production, it is necessary to look at standard cell potential of electrodes involved in MFCs. To date various redox reactions have been coupled to generate electron flows (Table 1.1). Studies show that a wide range of biodegradable organic substances can be oxidized within the anode chambers. The substrates can be of natural or synthetic origin, including sugars, organic acids [8] or even domestic and industrial waste.

Table 1.1

Oxygen reduction reaction (ORR) is the most prevalent redox couple in MFCs. Generally, ORR is a logical way to achieve positive redox potential at cathodes because of limitless availability of oxygen in the air. However, ORR has some unappealing performances, such as poor oxygen solubility in electrolytes. Alternative materials such as hexacyanoferrate and ferri-/ferrocyanide do not have solubility issues and possess faster reduction kinetics, but they are not ideal candidates as cathode materials due to the toxicity and lack of sustainability [13].

A simple example of cathodic and anodic reactions is as follows:

si1_e

   (1.1a)

si2_e

   (1.1b)

si3_e

   (1.1c)

The Gibbs free energy confirms that the process is thermodynamically favorable [14]. The overall standard potential is thermodynamically predictable using the Nernst equation (Eq. 1.2).

si4_e    (1.2)

si5_e    (1.3)

A positive standard cell potential (Ecell⁰) indicates the need for an external power source. Contrary to this is when possessing a positive cell potential. Galvanic cells have spontaneous release of electrons and, therefore, negative Gibbs free energy (ΔG, J/mol).

si6_e    (1.4)

The Gibbs free energy resulting from material oxidation of microbes is ultimately converted into electric power. However, the power production of MFCs is strongly dependent on the types of the microorganisms and operation conditions, where a part of the Gibbs free energy can be used for catabolic activities of microbes and other undesired losses of energy [15,16]. The rest of the free energy may be used for power generation.

Findings suggest that sustainable function of MFCs is correlated with the balance between the amount of energy that is consumed for catabolic activities of microbes and energy being converted into electrical energy. The results from the studies confirm the involvement of respiration and fermentation pathways in microbes’ energy conversion. In respiratory cycle, fuel glucose undergoes glycolysis, Krebs cycle, electron transport chain, and oxidative phosphorylation steps under aerobic conditions. Besides respiration, fermentation has also demonstrated great potential in improving electricity production under anaerobic conditions.

MFC requires an ideal working volume (VMFC, m³) to promote voltage generation. The required volume can be easily calculated using the following equation:

si7_e    (1.5)

where Q is organic material flow rate (m³/h) and tHRT is the time required to achieve desired power density.

1.3: Potential feedstocks for MFCs

The MFC is an electro-biological cell that fulfills its function through microbial oxidation of substrates. Different substrates ranging from pure compounds to complex mixture of organics can be employed as feasible power sources in MFCs (Table 1.2). Carbohydrates, sugar acids, organic acids, esters, alcohols, and amino acids are the most widely used organic compounds for power production. However, the use of pure compounds is limited to laboratory-scale investigation [77]. High production costs and limited availability of pure organics have restricted their pilot-scale and large-scale applications.

Table 1.2

A wide range of waste materials including solid waste, liquid wastewater, and synthetic wastewater can be used as low-cost substrates. Compared with pure compounds, the objective of the process is simultaneous water treatment and energy production [78]. The treating process produces less sludge (anaerobic macerate) which can be used for soil enrichment. Furthermore, MFC exhibits a high performance in COD removal without using chemicals and, as the most important features, in case of civil and industrial wastewater the produced energy can cover waste treatment costs [79–81]. The excellent power density is closely correlated with operational conditions, such as pH, temperature, substrate concentration, organic loading rate (OLR), hydraulic retention time (HRT), type of microorganism, parallel or serial connection, and static magnetic field [82–85].

1.3.1: Pure organics

Accumulating evidence demonstrates that organic compounds are potential substrates of power generation in MFCs. Oxidation of these complex materials have been linked to decreased pollutant load, enhanced microbial growth, improved water purity, and electricity generation [86,87]. Compared with complex organic substrates, simple defined substrates, such as monomers of carbohydrate, protein, and lipid, are required for fundamental MFC studies [8].

Glucose, fructose, and sucrose are simple materials that can be used as a source of electricity. Glucose mediates higher electricity production as compared to other simple organics, although up to 5 g/L glucose concentration correlates with an inhibitory effect on metabolic activity of microbes [88]. Proteus vulgaris [89], Rhodoferax ferrireducens [90]. Escherichia coli [91], Geobacter sulfurreducens [92], and Saccharomyces cerevisiae [93] have been shown as effective microbes that oxidize glucose into electricity.

Compared with glucose, fructose produces relatively lower electrical power [7]. Furthermore, it was demonstrated that aerobic oxidation of fructose and other substrates mediates higher electricity generation other than anaerobic condition [94] due to high level of key proteins regulating Krebs cycle and Entner-Doudoroff (ED) pathways.

Sucrose is an additional carbon source for MFCs [95–97]. The effectiveness of sucrose in power generation depends on the status of microorganisms. For instance, power density of 0.52 mW/m² was observed for dual-chambered membraneless anaerobic cathode [96], while 10.13 mW/m²[95] and 148.76 mW/m²[97] have been reported for aerobic cathode.

As a conventional end product of various microbial pathways, acetate induces alternative metabolic conversions [97]. Such conversions suppress current generation and have been confirmed to decrease the resulted power density from 309 to 220 mW/m²[98]. However, there are some evidence on power density increment up to 362 mW/m² from acetate conversion to bioethanol in MFC apparatus [99]. Ironically, when comparing glucose and acetate as substrates for bioelectricity generation, higher power densities have been reported with acetate [98,100].

Besides simple defined substrates, several complex synthetic materials have shown potential for power production in MFCs. The substrates have been used to determine whether or not natural complex substrates available such as starch, cellulose/lignocelluloses, butyrate/propionate, methanol/ethanol, glycerol, and phenol can be utilized for microbial electricity production. For instance, 1.16 g of starch residue have exhibited a high capacity in power density generation (502 mW/m²) using E. coli as a biocatalyst [101], and up to 143 mW/m² power density has been reported from cellulosic substrate mediated by Clostridium cellulolyticum [102]. Butyrate and propionate exhibited less power generation capacity as compared with glucose [103]. Pure glycerol resulted in a 0.06 mW/cm² maximum power density in one culture system [104]. However, maximum power density of 2.15 mW/m² has been reported for Shewanella oneidensis and Klebsiella pneumoniae coculture system [105].

1.3.2: Solid wastes

Simultaneous electricity production supplies energy-intensive removal of organics. MFC-based technologies can effectively remove solid wastes that are difficult to be recycled by conventional methods. Specifically, the objective of the process is organic loading removal for high power consumption rate. Cellulose and hemicellulose are the major components of the solid waste and particularly active participants of bioelectricity generation. Although, the presence of such complex materials can be detrimental to electrogenic bacteria, multiple pretreatment methods such as mechanical, thermal, chemical, and biological strategies are required to hydrolyze them into low-molecular weight compounds [106]. The performance of the operation is determined via volumetric power densities and COD removal efficiencies. Regarding the substrate chemistry, energy-generating yield of MFCs varies from 2 to 100 W/m³ with the COD removal capacities in the range 40%–90%.

1.3.3: Organic materials

MFCs are known to reduce operational costs of the wastewater treatment plants [107]. The technology provides high energy generation from a wide range of wastewater [49] and less secondary sludge production during the process.

Preventing from stringent aseptic conditions and using multiple substrates, mixed microbial cultures are utilized to inoculate MFCs. A range of 3–5 g/L is reported as an effective COD range for MFCs’ function. Municipal wastewater is characterized as low energy-content feedstock and is relatively easy to treat. However, high BOD content of 2000 mg/L observed in industrial and agricultural wastewater can be treated under anaerobic conditions. Accumulating evidence demonstrates that for treatment of high-strength wastewater, integration of MFCs with other wastewater treatment technologies have been known to improve energy recovery by 30%–40% and removal efficiencies by 70%–90% [108–110]. Observations suggest that energy recovery from wastewater might be interrupted due to the presence of alternative electron acceptors, such as nitrate, nitrite, and sulfate. On the other hand, high carbohydrate content of feedstocks as well as low concentration of ammonia nitrogen are known to be involved in high power density production. For instance, high carbohydrate content and low concentrations of organic nitrogen have been reported for food processing wastewater [111,112]. Electricity production from this type of wastewater lies in the range of 2–260 kWh/ton depending on different BOD contents of the feedstock. It has been demonstrated that 46 and 1960 MW of electricity can be produced from milk dairy farms and high-strength dairy industry, respectively [113]. Livestock-related wastewater is another class of MFC feedstock with high COD content and nitrogen-containing components [114]. Because of high level of hardly biodegradable components, the efficacy of power production in this class of wastewater is limited.

1.4: BFC's classification

BFCs can be broadly classified into those related to the microorganisms and those related to concentrated enzymes. Microorganism-based BFCs include photosynthetic MFCs (PMFCs), microbial desalination cells (MDCs), and sediment microbial fuel cells (SMFCs) (Fig. 1.6).

Fig. 1.6

Fig. 1.6 Classification of MFCs based on their working principles: (A) the overall design of microorganism-based BFCs, (B) algal MFC, (C) MDC, (D) SMFC, and (E) plant MFC.

1.4.1: MFCs

1.4.1.1: Photosynthetic MFCs

In photosynthetic MFCs (PMFCs), sunlight is converted into electricity through metabolic reactions. The innovative technologies based on a combination of sunlight and microbial powers have been an effective strategy for improved voltage generation. The most attention has been focused on microscopic photosynthetic organisms, because they can grow in a broad range of aquatic habitats. Several microalgae, cyanobacteria, and plant species have exhibited remarkable capacity to optimize voltage generation in PMFCs. The main limitation of the light-enhanced MFCs is overcoming the photosynthetic dark reactions which convert the products of the light reaction into carbohydrates.

Plant MFCs

Plant MFC is a sustainable technology aimed at bioelectricity generation through synergistic interaction between photosynthetic organism and heterotrophic bacteria [115]. PMFCs have been known as renewable energy sources because they have a clean conversion and circumvent the impacts of competition for arable land [116]. Vascular plants, rhizosphere electrochemical active bacteria, and electrically connected anodes and cathodes are the main components of PMFCs [117,118]. Around 70% of the fixed carbon resulted from photosynthesis process is extruded into rhizosphere in terms of rhizodeposits [119]. Rhizosphere-resident heterotrophic microorganisms oxidize the excreted materials into carbon dioxide and protons and transfer electrons to the anode [116]. The selection of plant type is dependent on the aim of the research. Among the plethora of plants, Spartina anglica generated the highest power output (679 mW/m²). The anode of a PMFC is suitable for the growth of bacteria that don’t produce electricity. This can suppress power production capacity of PMFCs.

Algal MFCs

In the past decade, algal MFC has been characterized as a novel technology to which a succession of solar irradiance conversion (up to 9%) has been applied. Given that algae have high growth rate, can be harvested all the year round, and have no competition with food and feed, studies suggest that algae should be sustainable feedstock for MFCs. In algal MFCs, proton induction of algal cells mediates CO2 fixation into a variety of cellular compounds. Case series have reported that algae can be applied at both cathode and anode for providing oxygen and substrate for bacterial growth, respectively. Algae have two different types of glucose metabolism: autotrophic and heterotrophic [120]. The heterotrophic growth mode poses some advantages over the autotrophic growth including high growth rate of the biomass due to high level of ATP production and high content of nitrogen and lipid contents. Furthermore, there is no requirement of specify bioreactor design. Although, for heterotrophic growth model the number of microalgae species available is limited, energy expenses are high, and there are some risks of contamination with other microorganisms [121].

Mixotrophic growth can be also exerted to combine photosynthetic metabolism and respiratory metabolism in order to assimilate organic carbon and carbon dioxide [122]. Heterotrophic organisms oxidize organic materials in anodic chamber to create electron flow and produce carbon dioxide, which are utilized by photosynthetic organisms in cathodic chamber to generate biomass, water, and oxygen [123]. During dark phase oxygen is consumed to oxidize organic materials and to produce energy [123]. Following reactions describe the anodic and cathodic biochemical reactions involved in algal MFCs [124]:

si8_e

   (1.6a)

si9_e

   (1.6b)

si10_e

   (1.6c)

1.4.1.2: Microbial desalination cells (MDCs)

The focus on absolute demand of drinking water because of rapid industrialization and population growth has increased awareness among researchers in determining sustainable and cost-effective technologies for freshwater production. International Desalination Association has reported active 18,426 desalination plants worldwide with operational capacity of 86.8 million m³per day. Conventional destination plants are energy intensive and consume 3.7–650 kWh energy per m³ of water desalination [125]. Moreover, since up to 60% of the desalination plants use fossil fuels as energy source [126], the emission of greenhouse gases and global warming could be additional drawbacks of conventional desalination plants. Microbial desalination cell (MDC) is important for simultaneous organic waste and wastewater treatment, water desalination, and electricity generation through oxidation of organics. The main evidence for advantages of MDCs comes from a study that confirms 90% desalination efficiency and 1.8 kWh energy production per m³ of water treatment [127]. Additionally, MDCs have been successfully employed for the production of valuable chemicals along with wastewater treatment [128,129], water softening [130], and denitrification [131].

Preacclimated inoculum provided from existing MFCs and inoculum from local wastewater treatment plants are usually required as microorganism seeds of MDCs [132,133]. During the desalination process increment in electrolyte conductivity and overall system performance depend on diffusion of cations and anions from desalination chamber toward cathodic and anodic chambers [134]. However, increase in Cl− concentration may disturb microbial activities. Studies have demonstrated that exoelectrogenic community tolerates salinity of 41 g/L TSD though permanently loses its functionality at salinity of 46 g/L TDS [135]. Today, longer period of acclimation has been known as a possible technique for increment of salinity tolerance in microbes.

Air cathode, biocathode, capacitive, photosynthetic, osmotic, stacked, resin packed, and upflow configurations are the most widely used laboratory-scale designs of MDCs [136,137]. To date, the use of MDCs in pilot- or full-scale studies have not been investigated and a more fundamental understanding of MDC process is required to circumvent design and operational challenges.

1.4.1.3: Sediment microbial fuel cells (SMFCs)

Eutrophication significantly increases the risk to environmental health. With an upward deposition of nutrient pollutants such as nitrogen and phosphorus into aquatic environment, the worldwide incidence of eutrophication is expected to increase. Eutrophication can have devastating consequences and is often associated with water blooms and bioturbation [138]. An improvement in the quality of eutrophication water can be achieved by removing pollutants from sediments. Sediment microbial fuel cells (SMFCs) have drawn a lot of attention as promising techniques that provide simultaneous heavy metal bioremediation and bioenergy production. Studies demonstrated the potential involvement of SMFCs in contaminant removal from submerged soil using clean energy [139]. Furthermore, findings suggest that SMFCs can simultaneously generate sustainable electrical energy along with wastewater decontamination. Low-cost operation and long-term power generation have been known as the most important advantages of SMFCs [140].

A simple SMFC consists of an anode electrode buried in an anaerobic sediment including exoelectrogens [141,142] and a cathode electrode immersed in the overlying water [143]. SMFC is a membrane-free technology in which electrons flow in an external circuit, while water enables proton transport between electrodes [144]. Poor organic content of sediments could be the sole obstacle of the method [145,146] which can be overcome through the use of sediment from eutrophic water body [147].

Electron transfer via conductive pili is a prominent microbial mechanism in SMFCs [148]. Firmicutes, Acidobacteria, Proteobacteria phylum, and yeast, fungi, and microalgae are five major groups of microorganisms capable of electricity generation in SMFCs [149]. It was anticipated that iron-reducing species including Shewanella spp. [150], Aeromonas hydrophila [151], Clostridium butyricum [152], Geobacter spp. [153], Rhodoferax ferrireducens [154], and Enterococcus gallinarum [155] have excellent electron exchange with electrodes. Additionally, some bacterial species such as Geobacter display supercapacitor and transistor properties due to their structural conductive polymers [156].

The integration of SMFCs with photosynthetic organisms, plant, and algae has increased the effectiveness of sediment remediation technique. The photosynthetic organism can produce oxygen and organics during the process which serve as electron acceptors and donors during the operation [157]. The incorporation of plants can improve the mass transfer rate between the anode and the electron donors, which subsequently increase the harvested energy by 7-fold and 18-fold [158,159]. Importantly, SMFCs have shown significant scale-up potential, although further studies are needed to develop this technology and intensify its applications.

1.4.2: Enzyme-based fuel cells (EFCs)

Enzyme-based fuel cells (EFCs) are bioelectrochemical devices that convert organics into energy through catalyzing the oxidant-reductive reaction (Fig. 1.7). The main difference between MFCs and EFCs lies in dissimilar enzyme concentration by which the efficacy of electricity production is determined. In MFCs, enzyme concentration is limited and their activity is correlated with microbial metabolism. Extraction of enzymes and using them with high degree of purity, therefore, have an important role in the increment of power output. Some studies have reported that the use of pure enzymes in either cathodes or anodes improves the diffusion of reactants and products.

Fig. 1.7

Fig. 1.7 A schematic representation of enzyme-based fuel cell.

Additionally, enzymes possess high selectivity and compatibility, making them potential candidates for micropower suppliers of implanted medical devices [160,161]. However, there are some limitations in enzyme stability and survival [162]. Moreover, enzyme selectivity imposes additional operation costs for providing multiple sequential enzymes which are essential for the multistep oxidation of organic substrates and mediators that are necessary for electron transfer and product release [163,164]. Bilirubin oxidase [165] and laccase [166] are the enzymes most frequently used at cathode, and sugar-based enzymes like glucose oxidase [167] have been introduced as highly efficient enzymes employed at anodic electrode.

1.5: Conclusions

MFCs offer real potential for green power generation, which can reduce global energy demand and solve the health-care crisis. Simultaneous wastewater treatment and power production is intended for this application. Despite considerable advances in the development of MFCs, there are several needs and challenges for improvement in power output from potential substrates. One such challenge is limited understanding of the involved microbial mechanisms and deprived progression in scale-up of the process. Addressing these challenges requires further studies on operating conditions, including influent COD concentration, HRT, feed pH, and specific organic loading rate. Nevertheless, the studies are often hindered by microbiological, technological, and economic challenges of the process. This book can serve as a tool for researchers to facilitate right direction of MFC processing toward large-scale industrial applications.

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