Perspectives on Microbial Fuel Cells

Introduction

The rapid industrial development and urban expansion have resulted in concerns relating to threatened energy security, environmental pollution, global warming and climate change both in the developing and developed nations (Nanda et al. 2015a; Nanda et al. 20l6e; Nanda et al. 2017c). Non-renewable sources of energy have quenched the global energy requirement in the manufacturing, transportation, automobile as well as combined heat and pow'er sectors since the industrial revolution (Rana et al. 2018; Rana et al. 2019; Rana et al. 2020). Currently, the w'orld needs further exploration of alternative and renewable energy technologies to suffice the increasing energy demands while synergizing solutions for the above-mentioned environmental concerns (Okolie et al. 2019; Okolie et al. 2020a; Okolie et al. 2020b).

Some renewable sources of energy that are found to be promising are geothermal, wind, solar, tidal, algae, lignocellulosic biomass (e.g. agricultural crop refuse, forestry residues, dedicated energy crops and invasive crops) and organic waste materials (e.g. sewage sludge, municipal solid waste, waste tires, livestock manure, industrial effluents, food waste, etc.) (Nanda et al. 2015b; Nanda et al. 2016a; Nanda et al. 2016b; Nanda et al. 2016c; Reddy et al. 2016;Gong et al. 2017a; Gong et al. 2017b; Nanda et al. 2017b; Parakh et al. 2020; Singh et al. 2020). These waste residues can be transformed to some advanced biofuels such as bio-oil, biodiesel, bioethanol, biobutanol, biogas, biohydrogen, syngas, etc. through a w'ide variety of thermochemical and biological conversion technologies (Nanda et al. 2014; Nanda et al. 2016d; Sarangi and Nanda 2018; Sarangi and Nanda 2019; Sarangi and Nanda 2020; Sarangi et al. 2020). Besides, fuel cells are also some recent options being explored as a renewable source of energy using microorganisms and waste organic matter (Bhatia et al. 2020). Major merit associated with the employment of fuel cells is zero-emission of harmful greenhouse gases (viz. SO_t, NO_t, CO, and CO) (Rahimnejad et al. 2015; Bhatia et al. 2020). This chapter provides an overview of microbial fuel cells for the production of bioelectricity.

Variations of Microbial Fuel Cells

Two forms of biological fuel cells are utilized such as enzymatic fuel cell (EFC) and microbial fuel cell (MFC). Selective enzymes are involved in EFC for the redox reaction. whereas MFC involves microorganisms (or electroactive microorganisms) in an anaerobic anode compartment to generate electricity from organic compounds. The potential of electroactive bacteria (EAB) has been recognized to oxidize a series of organic matter or pollutant serving them with carbon for their metabolism. During this process, EAB transfer the produced electron to anodes (Shen et al. 2014). Hence, MFCs are renewable devices to transform chemical energy into electricity by employing the anaerobic metabolic machinery of EAB.

The MFC technology is highly beneficial for having the capacity of converting waste organic matter and pollutants into electricity using various microorganisms and their enzymes. The MFCs differ from conventional fuel cells in many aspects. For example, MFCs operate at ambient temperature range (approximately 15-45°C), which involves biotic electrocatalyst at the anodic side and neutral pH condition. The MFCs can utilize complex biomass and has less environmental adverse impacts when compared to the traditional fuel cells (He et al. 2005; Larrosa-Guerrero et al. 2010; Borole et al. 2011; Tremouli et al. 2016).

Based on the assembly of the cathode and anode chambers, MFCs can be classified as single-chambered or double-chambered (Figure 8.1). For enhancing the efficiency of MFCs, several modifications have been adopted in their basic design and structure. The electrons generated in the anode chamber by the oxidation of the substrate move towards the cathode, which is managed with the help of a mediator or without a mediator.

Depending on the movement of electrons from the EAB to the anode, MFCs can be classified into two types such as MFCs with mediator and MFCs without a mediator (Pant et al. 2010). In the first category, the MFCs employ mediators that are supplemented to the system. In the latter category, certain microorganisms aid in the transfer of electrons via conductive pili or via cytochromes associated with their membrane and are electrochemically active. In certain instances, the redox-mediating molecules secreted by a microorganism also govern this mechanism. Some metal-reducing bacteria such as Geobacter metallireducens, Rhodoferax ferrireducens, Shewanella putre- faciens, Clostridium butyric and Aeromonas hydrophila exhibit this phenomenon of mediator-less electron transport in MFCs. Soluble redox shuttles play an important role in the power generation when MFCs involve a conglomerate of Alccdigenes faecalis, Enterococcus faecium and Pseudomonas aeruginosa secreting these redox shuttles. The added mediators can sometimes pose the problems of toxicity and instability to limit their applications in MFCs. The employment of microbial-generated native electron shuttles can resolve this issue. It is interesting to employ the secondary metabolites as redox mediators for MFC applications, as their in-situ production curtails the need for adding exogenous redox shuttles to transfer the electrons.

Sediment-type microbial fuel cell (SMFC) is a form of MFC in which anode is submerged in the anaerobic sediment comprising of detritus organic material of plant and

FIGURE 8.1 A typical double-chambered microbial fuel cell

animal and human origin. An electric circuit joins this anode and a cathode electrode dangled in superimposing water (Xu et al. 2015). The feasibility of this design depends on the concept that exoelectrogens can utilize the organic carbon found in these sediments and liberate electrons that are transported outside the cells (Holmes et al. 2004).

New Developments in Microbial Fuel Cells

The implementation of MFC technology aids in the generation of clean energy while providing one of the best platforms for bioremediation of pollutants. The microbial consortia are employed in the anode chamber of the MFC where they oxide the w'aste- water or the pollutant to generate the electrons and protons. Theses microorganisms utilize the electrons by the electron transfer chain for their metabolism after which they travel to the cathode chamber completing the circuit. The protons produced in the anode chamber during the oxidation process also travel via the proton exchange membrane (PEM) to reach the cathode. The cathode chamber is oxygenated w'here the electrons and protons reduce the oxygen to produce water molecules, thereby completing the charge balance w'hile converting the chemical energy to electrical energy (Osman et al.

2010). The extracellular electron transport mechanism of EAB supports the transport of electron, thereby generating voltage (Wang et al. 2014). A study has shown the use of human urine as a source of energy in MFCs (Ieropoulos et al. 2013).

Different mechanisms are adopted for transferring electrons to the solid electrodes. Exoelectrogens in MFCs aid in transferring electrons. For example, pyocyanin or riboflavin is a mediator secreted by Pseudomonas and Shewanella that accelerates the transfer of electrons. Pseudomonas aeruginosa is also a potent secretor of electro- chemically active phenazine derivatives, which plays an important role in anoxygenic conditions allowing the bacteria to produce energy for its growth. Phenazines also help to sustain the redox homeostasis as they act as electron acceptors to re-oxidize the accumulated nicotinamide adenine dinucleotide (NADH) (Jayapriya and Ramamurthy

2012). The chemical variations of the insulating interface across the cellular membrane can enhance the endogenous secretion of pyocyanin mediators in Escherichia coli (Hou et al. 2013) and Pseudomonas aeruginosa (Wang et al. 2013), thereby boosting the power output in the MFCs. Moreover, Pseudomonas-catalyzed MFCs support an exclusive prospect to syndicate the metabolic potential of microorganisms to convert oxidiz- able pollutants and with energy recovery.

Reaction Mechanisms of Microbial Fuel Cells

Cathode and anode are the two chambers in MFCs made up of glass, polycarbonate or Plexiglas. Microorganisms in the anodic chamber generate electrons and protons to metabolize the organic substrates, produce energy and support microbial growth (Das and Mangwani 2010). Protons and electrons travel towards the cathode through the PEM where they cause the reduction of oxygen to water. Oxygen serves as an electron recipient to complete the entire process. Oxygen is a sustainable, non-toxic compound and labeled as an ideal electron acceptor. A practical system is necessary to separate bacteria from oxygen because the latter is inhibitory for electricity production in the anode chamber. Flence, an anaerobic chamber is required for the anodic reaction. The anode is the chamber where bacteria grows and the cathode is the chamber where oxygen reacts with the electrons. A membrane separates biocatalyst from oxygen and allows only charges to be transferred between the anode and the cathode (Das and Mangwani 2010).

Two important factors that influence the functioning of MFCs are biological and electrochemical parameters. In the continuous systems, the rate of substrate loading is a biological parameter but power density and cell voltage are taken as chief electrochemical parameters. The performance of an MFC is also decided by various factors, which include: (i) supply of oxygen with its usage in the cathode chamber, (ii) oxidation of the substrate in the anode chamber, (iii) electron shuttle from anode section to anode surface and (v) penetrability of the PEM (Rahimnejad et al. 2015).

Notable Applications of Microbial Fuel Cells

Electricity Generation and Wastewater Treatment

The major application of MFCs has been realized for bioelectricity generation subsequently treating the wastewater. A wide variety of microorganisms is involved in MFCs either as a single species or in consortia by the virtue of their unique metabolic potentials. Some of the chief substrates used by the microorganisms are sanitary wastes, wastewater generated out of food processing, poultry wastewater and corn stover (Rabaey et al. 2006). The growth promoters proliferate the development of bio- electrochemically active microorganisms during the wastew'ater treatment process. Wastewater treatment is more efficient when the sulfides are removed through microbial metabolism. The energy demands are highly curtailed on the treatment plant along with the reduction in the quantity of unfeasible sludge produced by the predominant anoxic atmosphere. The removal efficiency by MFCs is enhanced when they are connected in series to treat the leachate. The generation of electricity is the additional benefit associated with this process (Galvez et al. 2009).

Biosensors

Organic matter can be monitored online when replaceable anaerobic consortia are used as biosensors in which the biological oxygen demand (BOD) in the wastewater is one of the chief parameters. Most of the methods are not feasible for on-line monitoring and regulation of biological wastewater treatment processes (Chang et al. 2005). The strength of the organic matter and the Coulombic yield of MFCs are linearly correlated, thus making the MFC a feasible BOD sensor. The BOD of a liquid stream can be better perceived by measuring the MFC’s Coulombic yield. This is a feasible approach for a wide concentration range of organic matter in the wastewater (Kumlaghan et al. 2007).

Biofuel Production

Biohydrogen is generated in the MFCs as a biofuel used for the alternative of electricity (Nanda et al. 2017a; Nanda et al. 2017c). Biohydrogen production becomes feasible with minor changes in the MFCs. The MFCs are an alternative producer of biohydrogen when compared to the classical method of its production through photo-fermentation or dark fermentation (Sarangi and Nanda 2020). It is stated that by increasing the external potential at the cathode, microbial electrolytic cells can potentially generate methane and hydrogen. Thus, MFCs can generate biohydrogen and contribute towards fulfilling the energy demands in the future bioeconomy sector (Wagner et al. 2009).