Keywords

14.1 Introduction

The main challenges for the globally developed world economies are increasing oil consumption and global warming. Approximately 86% of the world’s energy is currently derived from fossil fuels. The use of non-renewable resources like fossil fuel, however, has increased the impact of greenhouse gases (GHGs) that raise the global mean temperature with possible adverse effects on the atmosphere, humans and other living forms present on earth. This has encouraged researchers to pursue new sources of renewable energy to supplement fossil fuels (Watson et al. 2015). In the United States, primary energy usage in 2016 was almost 96 quadrillion BTUs (U.S. Energy Information Administration, Monthly Energy Review, April 2017). Most of the developed nations heavily depend on the non-renewable sources of energy for the economic as well as societal development. Major burdens on nuclear plants have incurred negative effects on environmental niche as well as on human life (Kyne et al. 2016). Energy-generating plants running on the non-renewable sources like coal and petroleum products are found to be responsible for many cardiovascular and adverse disorders (Pandit et al. 2011). The non-renewable sources are posing great challenges with respect to its rapid exhaustion, health risks and global warming (Navanietha Krishnaraj et al. 2015; Yu et al. 2017).

Microbial electrolysis cell (MEC) is a young-generation bioenergy technology that has a huge potential for the generation of hydrogen and other value-added chemicals (methane, formic acid and hydrogen peroxide) along with the treatment of waste water, aside from the traditional approach of discovering green energy sources (Escapa et al. 2016; Zhang et al. 2019a, b).

Bioelectrochemical systems (BES) offer a novel and attractive option for electricity production using biological entities (Pant et al. 2012). BES has become an attractive tool utilising the power of the microbes to catalyse the reaction and mediate the process of biological electrocatalysis (Fig. 14.1).

Fig. 14.1
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The schematic view of bioelectrochemical systems (BES)

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The microorganisms or enzymes work as catalysts for electricity generation and are called the microbial and enzyme electrocatalysts. Apart from the bioelectricity production, the electrocatalytic property of microbes is used in the biofuel processing like generation of biohydrogen, alcohols and its derivatives, biodiesel production, wastewater treatment and production of value-added compounds by BES (Sleutels et al. 2012).

Moreover, in comparison to the non-renewable energy sources the BES offers advantages like low cost, cheap raw materials and efficient operational systems. Many reports show the efficient usage of the bioelectrochemical cells as biological fuel cells, bioelectrocatalytic cells, bioelectrosynthetic cells and biological sensors (Logan et al. 2006a, b; Navanietha Krishnaraj et al. 2015) (Fig. 14.2).

Fig. 14.2
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Applications of BES

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14.2 Microbial Electrolytic Cells (MECs)

Microbial electrolytic cells (MECs) are electrochemical devices operating with the major application as catalysis in microbial fuel cells (MFCs). The major difference with respect to the microbial fuel cell is the way it generates and converts electrical energy to chemical energy. The movement of the electrons and its operating principle are different with objectives of energy generation. The reverse flow of electrons has led to the generation of carbon dioxide and also biohydrogen. With the help of the external voltage series, electricity is generated due to microbial and enzymatic capacity of the microbes. Substrate is oxidised at the anode compartment through the electrigens (electroactive microbes) leading to the release of electrons and protons (H+ ions). The electrons are initially arrested at anodes with further extension joining to the cathode via an external circuit. With the presence of mediator electrolytes, the protons produced at anode get passed through the cathode. This sequential reaction leads to hydrogen production by a combination of H+ ions. The electrochemical potential of the catalytic reaction is usually found to be insufficient for the production of hydrogen via oxidation reaction at anode. The reaction requires additional power supply for significant electricity generation. The voltage usually in the range of 0.2–1.0 V is required for hydrogen production (Logan et al. 2008). However to the merits, MECs required very less voltage supply from the external source when compared with the classical approach in the water electrolysis process. The MECs and MFCs offer many advantages in terms of substrate usage and product formation (Fig. 14.3).

Fig. 14.3
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Applications of MFCs and MECs

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The important parameter for designing the process is about the selection of the microorganism. The strain and the parameters tend to be very specific and also work under the limited set of operating conditions. The microbes are very sensitive to the external environmental factors like temperature, pH, and pressure. The selective and specific binding of the enzyme at the substrate site is also playing a crucial role in designing operational parameters for the microbial catalysis. In this case, the microbes surviving at extreme conditions are very useful for the generation of bioelectricity as its natural environment is unusual. The extremozymes can also restore the limitation of the mesophilic microbes. To add on, extremophiles can easily grow on the many substrates and lignocellulose biomass (Turner et al. 2007; Bhalla et al. 2014a, b). The chapter explains the use of extremophiles in MECs and in renewable energy production.

14.3 MECs General Concepts

Depending on the substrate-binding efficiency and the type of substrate and enzyme, the substrate is typically oxidised by a chemical or microbial catalysed reaction in order to form one or more items. In MECs, the derived electrons are further passed either directly or indirectly to the conductors (anode). To form the desired product, electrons will be combined at the other end (cathode). This reaction can occur at the anode through microbes or metals that act as catalysts.

In MECs, at the anode site, microorganisms which serve as catalysts are known as electricigens. The microbes dwelling on this anode site are also known as ‘electrogens’ or ‘anophiles’. The extremophiles from many phyla are known to work as electricigens; however in particular phyla representing members of the Proteobacteria and Firmicutes are most favourable for the process. Among the group members of this family, Geobacter sulfurreducens and Shewanella oneidensis are reported to be the most studied and used for the biocatalysis process (Caccavo et al. 1994; Venkateswaran et al. 1999). Under ideal conditions, the anode usually does not use any of the external alternative electron sources and also cell transmission mechanisms. The direct mechanism involves the transfer of electrons anaerobically via electron transport chain using the insoluble electron acceptor (the anode). The transfer of electrons is processed by the c-type of the cytochromes and proteins loaded with iron sulphur to be found on the cell surface (Liu et al. 2014).

Recently, as reported by Reguera et al. (2005), the electrons can be transferred directly through electricigens using nanowires. The study on the ways and pathways used by these electricigens to transfer electrons from the anode site using the mediators like cytochromes in composition of nanowires is going on (Lovley and Malvankar 2015; Malvankar et al. 2015). In the mechanisms of indirect electron transfer to the anode, the mediators like redox shuttles are extensively used. Mediators can be either organic like humic acid or organic like SHS shuttle. In the indirect mechanism, the movement of electrons can also mediate the use of the planktonic cells which can form biofilms on the surface of the anode to facilitate the electron transfer and movement to the other site. The other type of the diversion of electrons was observed in the methanogenesis process using methanogens in the system.

14.4 Cathode’s Reactions

Electrons move from the anode to cathode site to complete the reduction reaction for generation of bioelectricity. The final electron acceptor differs as per the system types and also about its operations. For example, oxygen was used for electricity generation as the electron acceptor due to its wide and abundant presence and also working with water as the reducing element (Logan et al. 2006a, b; Clauwaert et al. 2007). On the contrary, MECs work on the requirement of additional external energy to complete the power supply for production of valuable resources like cathode hydrogen. The main advantage of biohydrogen production through the MECs is that higher production can be achieved compared to the conventional classical method of water electrolysis for energy consumption.

14.5 Separators

As indicated in many reports, electrons are separated by the membranes preferably by the ion-exchange one for the separation of anolytes and catholytes having varied compositions. The ion-exchange membrane also aids in stopping the phenomena of crossing over of the substrate and product. This step helps in creation of the pure product facilitating the transport of the unique ionic loads and refusal of the opposed ionic loads. This ion-exchange membrane (anion exchange) allows the flow and transfer of the negative ions with refusal of the positive ones. On the contrary, the cation-exchange membrane allows the positive ion movement with refusal of the negative ions. These applications and selective ion-exchange movement help in optimising conditions for maximum production. For example, just a simple optimisation in the bipolar movement aids in maintaining the pH gradient between the anode and the cathode (Ter Heijne et al. 2006; Harnisch et al. 2008; Harnisch and Schroder 2009).

14.6 Couplings of MFC-MEC

In theory, an applied voltage of 0.14 V is necessary for driving hydrogen power in MECs. In practice, a tension of 0.6 V or more for a high-efficiency hydrogen production is required because of the overpotential (Call and Logan 2008). A typical MFC open-circuit voltage can in particular reach up to 0.8 V (Min et al. 2008) and can thus be done by the use of an MFC to power a MEC, providing a combined MFC-MEC device, to achieve the high-efficiency processing of hydrogen. This method can harvest hydrogen from substrates and does not need an external power supply. Min et al., who combined a single-chamber MFC with an air cathode and a dual-chamber MEC, reported the first demonstration of an MFC-MEI connector. As an electron donor for both the MFC and MEC, the device’s hydrogen output rate approaches 2.2 0.2 mL L−1d−1 with acetate (0.1 g L−1). Cathode hydrogen recovery and Columbia’s systemic overall performance were, respectively, 88–96% and 28–33%. The systemic hydrogen production was 1.21 mol-H/mol-acetate in average (Min et al. 2008). The performance of the coupling system was tested in different configurations: the results showed that the hydrogen output values of 2.9 ± 0.2–0.2 ± 0.0 ml L−1d−1 were variable when the resistor shifted from 10X to 100X. The hydrogen production rate grew dramatically when the MFCs were connected in one line, while when connected in parallel, it slowed slightly (Sun et al. 2009).

14.7 Microbial Photo-Coupled Device

The direct use of renewables such as solar is a visible, but still difficult, solution to hydrogen production in an environmentally friendly way. A MEC (dye-sensitised solar cell (DSSC)) system where an external solar cell replaces the electrical partition was combined with the MEC device to provide the additional power required (Ajayi et al. 2010; Chae et al. 2009; Jeon and Kim 2016) as reported in the literature. Furthermore a solar MEC device combining the microbe anode and the photocathode of a semiconductor has been shown to produce effective hydrogen. The material preparation and development costs of a DSSC-powered MEC can be reduced.

14.8 MECs and Fermentation

Due to thermodynamical limitations, many organic compounds produced by dark fermentation cannot further degrade to hydrogen via fermentation (Call and Logan 2008). A MEC can be combined with fermentation to further degrade these dead-end products. In an ethanol-type fermentation reactor, for example, Lu et al. (2009) fed effluent into a single-chamber MEC. The MEC has achieved a hydrogen production rate of 1.41 ± 0.08 m3 L−3d−1 with a voltage of 0.6 V significantly higher than that of the fermenting reactor (0.70 m3L−3d−1) (Lu et al. 2009). The recalcitrant substrate fermentation effluents, including lignocellulose and cellobiose, have also been degraded with MECs. Lalaurette et al. (2009) achieved the hydrogen output rate for the lignocellulose and cellobiose fermentation effluent at MECs of 0.96 ± 0.16 L (cellobiose) and 1.00 ± 0.19 L−1d−1 (lignocellulose) (Lalaurette et al. 2009). Yan et al. (2015) fed MFCs fermentation effluent of xylose and corncob hydrolysate. When a current was formed, MFCs were used for hydrogen production. For xylose and corncob hydrolysate effluent the production rates for hydrogen were 41.7 and 23.3 mmol per mol acetate, respectively. Fermentation effluents were also employed in MECs in the form of cellulose (Wang et al. 2011) and glycerol (Selembo et al. 2009).

14.9 Wastewater-Processing MECs

7.6 kJ L−1 energy from household waste water has been reported by Heidrich et al. (2011), indicating the abundance of energy in waste water. Often used for the recovery of energy from waste water are the MFCs and MECs. MECs have many advantages over MFCs from the economic and environmental points of view (Sleutels et al. 2012; Zhang and Angelidaki 2014). Some MEC reactors have been designed for the treatment of waste water. Ditzig et al. (2007) developed the first MEC to use domestic waste water as a substratum.

The domestic waste water in the anode chamber was treated with a double-chamber reactor with applied voltages of 0.2–0.6 V. The MEC was run in fed-batch mode, with COD removed almost completely (87–100%). The hydraulic yield (approximately 10% theoretical value) was poor because of low substrate conversion and loss of hydrogen. Laboratory results on the pilot scale must be used in order to assess the functional application of MECs and to evaluate the durability of their critical components, such as electrodes and membranes. Cusick et al. (2011) designed the first pilot MEC to handle real waste water from a winery. The MEC was a 1000 L volume single-chamber reactor using graphite fibre brushes as anode and SS mesh as cathode. The MEC achieved a 0.2 L−1d−1 hydrogen output rate and a mean soluble COD removal of 62%. The emissions of the gas were, however, mainly CH4 (86%) and CO, with trace quantities of hydrogen, as methanogens converted hydrogen to CH2 further. A 120-L volume MEC device consists of six separate modules of MEC for domestic wastewater treatment using a stainless steel cathode and low-cost microporous membrane was developed by Heidrich et al. (2011). The MEC system produces mainly pure hydrogen gas (100 ± 6.4%) for more than 3 months with an average efficiency of 34% with a hydrogen output rate of 0.015 L d−1.

The use of electron acceptors in cathodes can also minimise the recalcitrant pollutants (such as nitrobenzene and 4-chlorophenol) because the possible cathode of MECs can be regulated by electricity. Compared to the conventional electrochemical reduction, removal of these pollutants in MECs needs much less energy. In addition, the excess potential of electrochemical reactions can significantly be decreased by electroactive microorganisms in the anode or cathode, thus achieving improved effectiveness and removal rate.

14.10 Electrochemical Constraints

14.10.1 Electrodes

Electrodes are the main sources for generation of electrons and are also the active sites for biochemical reactions. Electrodes also support the formation of biofilms for electrogenic/electroactive interface between the surface of the electrodes and microbial species. Many carbon-based and metal-based electrodes have been used for the production of the novel material for achieving more significant production. The use of different materials will lead to increase in energy production with cheap raw sources and also better chemical treatment costs. The use of low-conductivity material needs to be avoided as it might cause problems in large-scale energy production. The porous nature of electrodes provides higher and better prospects in energy generation and also in long-term industrial economical applications. In comparison of the porous electrodes, the anodes made of carbon- and metal-dependent anodes face major issues due to the poor strength and corrosive nature (Butti et al. 2016). The use of energy production with combination of microbes and heavy materials like brass and aluminium can result in significant production. The inputs to increase durability, reliability and material composition for the anode production are in progress (Navaneeth et al. 2015). With inputs on the anode material, it is important that the biocompatibility of the material with the microbes is well studied. The more the biocompatibility, the more chances of biofilm production which in turn increases energy production.

14.10.2 Design

Design of the electrodes and the vessel makes it important for initialising the pilot-level studies. The raw material choice, composition, construction and efficiency play crucial roles in designing single cell or stacked cells (Oliveira et al. 2013). Apart from raw materials, the cost of the cells is driven by the reactor volume. The identical cathode system was found to be insufficient for power supply without the membrane separators. The other factors found to lower the efficiency include electrical configuration and also distance between the electrodes. The less distance between electrodes proved better for system performance (Liu et al. 2008). Electrolyte strength in the membrane-free reactors was reported to have lower flow of ions and can also result in the increase in internal resistance to 38–60% due to resistance (Ohm) (Fan et al. 2008). Advancement in the designing and type of materials for surface electrodes makes it an ideal candidate for better output of microbial fuel cells. The reports published by Hsu et al. (2013) show that the intensity of power is not always found to be linear and it can change exponentially. The research by Cheng and Logan (2011) reports that increasing the size of the cathode leads to significantly more than 60% of electricity. On the contrary, the output in power decreases to 12% when the process is initiated to the anode. This shows that changes in the anode site of cells will not make a process efficient for the energy production and also a factor to be taken care of is greater performance of the cathode electrode which can make cathode site a rate-limiting factor for energy generation.

14.10.3 Connectivity

The arrangement of individual MFC units often explicates electric communication in scalable systems. These scalable units are linked mainly by potential means of a serial or analogue combination of both current and voltage necessities (Liu et al. 2008). The power system usually calculates the total end product and a potential reduction during the process of ion crossover (Kim et al. 2012; Zhuang and Zhou 2009). The reports by Galvez et al. (2009) indicate that at the sites where leaching is prominent, the multiple-series MFCs were found to produce significant energy output. In the study by Ewing et al. (2014), the relation between single MFC and multiple MFCs was found to be significant and upscaling in the process has led to greater energy generation. The combination has delivered more than 3% of the output compared to the single microbial fuel cells. Similar studies conducted by Zhuang et al. (2012b) suggest that the output generation of a combination of multiple MFCs yields better energy generation. Based on all these reports, it clearly points out that the connected MFCs can achieve high electrical power in comparison to the analogue system for energy production.

14.10.4 Operational Restrictions

Effective functioning of the catalyst acts as the main factor in the designing and performance of the catalyst. The role of catalyst is the most crucial when MECs are used in the large-volume wastewater treatment facility. However, it can become a rate-limiting factor when it is quantified with the starting line-up of energy production. The value can vary in between hundreds and thousands depending on its operation conditions as well as set-up (Feng et al. 2010; Liu et al. 2008). The type of substrate and inoculum concentration affect the operation of the MFC (Aelterman et al. 2006). Additionally the selection of the type of microbial strain also makes the system more viable and operationally substantial. The pH load also affects biological film creation and hence its commencement (Patil et al. 2011). Although this is feasible for better start-ups, large volumes (>1 mm3) of MFCs are anticipated to function on an industrial scale with the enriched substrate/culture (~10–20 m3 reactor volume), thereby restricting its usefulness. Fe (III) or fumarate was reported to be the fastest for effective functional MFC before the inoculation with culture of Geobacter sulfurreducens (Torres et al. 2009). Connectivity at the anode site also regulates the intensity of the current and the formation and configuration of biofilm at the anode site of the cell. The preservation of desired conductivity levels can thus inhibit non-exoelectrogen growth and thus promote start-ups. The regulation of potential at the anode site can also offer a promising solution in designing efficient operational parameters (Wang et al. 2010).

14.10.5 Loading of Substrates

The loading rate (OLR) and load rate of sludge (SLR) are important for bulk-scale systems during the initialisation process. Those parameters calculate the strength of the reactor per volume per unit and the number of microbes in organic substrates (Oliveira et al. 2013). Numerous studies have reported about the overall influence of OLR and SLR on productivity of microbial fuel cells, and clinched that these factors are directly proportionate to biomass yield and degradation and reversely to efficiency of the cell and internal and external resistance (Martin et al. 2010).

The use of waste water was not beneficial because the system was improved with regard to cathode surface, and increased power loss was observed with significantly lower charge rate (Cheng and Logan 2011). It was reported that the OLR can cause a direct effect on the anode-site oxidation and also an increase in substrate depletion (Martin et al. 2010). On the contrary, the decreased or lower OLR can increase cell efficiency with lower methane activity with significant rise in resistance in the system that lowers the OLR. The balanced approach of recovering the electron at optimum levels at both sites is a prerequisite for scale-up industrial operations. In this context, the MFC needs to be operational at optimum level at both SLR- and OLR-level values for better biomass generation and also efficient power production.

14.10.6 Economics

The MEC proficiency can guarantee an independent biological process for simultaneous waste depletion; however the overall usefulness is largely restricted by fiscal dynamics. Overall, the laboratory installation cost with a planned 10-year life cycle leads to the investment cost of around US$ 3 to 1 k (Fan et al. 2008). When considering large-scale operations, the economic importance of this aspect will vary. A qualitative analysis of traditional treatment schemes may also include a techno-economic viewpoint. An MFC shows an advantage over conventional sludge therapy where additional benefit is expanded (Liu and Cheng 2014). MECs also offer other economic advantages as compared to the other classical as well as advanced treatment technologies. The main advantages include (1) minor biomass and no requirement for the aeration and very minor temperature regulation requirements and (2) high convertible energy values (Liu and Cheng 2014).

Several other published reports showed encouraging consequences in the development of economically feasible MEC applications. But on the other hand, long-term consumer acceptance is not feasible. The material and production costs of the MFC are a significant impediment to overcoming (Zhuang et al. 2012a, b). The major costs are associated with pretreatment by chemical electrodes, the use of valuable and costly metallic elements, and the composition and specificity of electrodes as current producer and collector (Seelam et al. 2015). Electrode constituents, accumulators, promoters (catalyst) and membrane exchangers/separators are too costly and also contribute to the overall economy of the system development. Although the price of anode products has fallen, as an important site for energy generation the different types of cathodes are still overpriced. To reduce the price, the utilisation of anodes made up of graphite material would be preferable to the economical one (Feng et al. 2010). Usually, cathodes account for 75% of the overall price of cells (Rozendal et al. 2008). Reduced cathode content costs alone will reduce capital costs and also make the system more economic and viable (Fan et al. 2012). During modification and designing of cathodes, when the base material is of iron and nickel the power generation and intensity were found to be around 23–36 W/m 3 (Aelterman et al. 2006). To prevent any damage and to increase shelf life of cathode in the cell, the base material like stainless steel can be used. Stainless steel is a cost-efficient, non-corrosive and efficient binder. The different separators also add to the cost of the cells. In order to reduce the cost, the distance between electrodes and also separators is narrowed down to make sure that efficient power generation is possible with less volume in the reactor (Liu and Cheng 2014).

Although the use of individual-compartment MECs will require a smaller amount of investment than double/multi-compartment arrangement because there is no separator, the efficiency of bioelectricity is normally not affected (Butti et al. 2016). It can be beneficial to have separators because they prevent short circuits and have more potential for gaps in electrode arrangement (anode and cathode). This type of arrangement boosts electrical efficiency on a measurable quantity. Inexpensive substitute separators are under evaluation, although reliability and quality of their long-term operation have yet to be assessed (Butti et al. 2016). Biofeuling and scaling adversely impact MECs in the operating front, ultimately impacting its shelf life, strength and power competence (Liu and Cheng 2014). During continuous process, membrane cleansing may be vital in a two/multi-chambered facility. Published reports have recommended the importance of bio-based cathodes in a MEC set-up. However, to set up bio-based cathodes can also be a pricey concern. As the biological entity is bound to cathode, aeration plays a crucial role in the designing of a biological cathode and in the supplying and transferring of oxygen (Cheng and Logan 2011). The use of activated carbon in combination with a wire mesh collector can be a potential sturdy substitute as a catalyst for oxygen reduction. Another encouraging solution is to use air cathodes. The single-chamber MECs with air cathode and fabricated electrode assembly can be created for attainment of more efficiency. Such designs can provide comparatively fine return since it is cheaper, and easy to manufacture, control and produce elevated power. For ecological considerations as well as fiscal viability, MECs necessitate lower power operational needs. Waste water usually works as precious feedstock and supplies extra financial gain to the MEC skeleton. Fornero et al. (2010) found poor electrode competence using waste water and reported that this technology is not viable for the production of electricity. Wang and Ren (2013) reported that MEC technology continues to be an expensive variant in waste flows as the cost of the cell-grade electrode material is also too high. The MEC is a silent budding technology, but these financial limitations signify that expenditure matter remains oversized. However, it can be assumed that technological drift in science advancement can most likely solve this barrier for its use in industrial application.

14.10.7 Extremophiles

Electroactive microorganisms are cells that can display electrocatalytic action. Electrons can be produced/consumed and the electrons pass through the electrode-electrolyte interfaces after oxidation/reduction from the electrode donor and electron acceptor. They are the key actors and can serve as electrocatalysts in electrochemical response in any bioelectrochemical process. Electron transfer becomes troublesome when microbial electrocatalysis happens deeper within the cell. Electron transfer is difficult. However the redox positions of the enzymes/microorganisms cannot be transferred to an electrode surface. For example, the Gram-positive bacteria have a layer of peptidoglycan and a periplasmic intermembrane distance. However, this trouble can be evaded by cautiously amassing electrically active microbes. Additionally, the microbes possess excellent oxidation-reduction potential and surface-reactive proteins on the cell wall, making the system better for electron transmission characteristics to the electrodes. Further to the merits, activity over broad environmental parameters like pH, temperature, metal tolerance and toxin resistance is also useful for energy generation. Microorganisms can carry electrons either via direct transport of electrons or through electron shuttle compounds. The direct transmission of electrons is conceded by microbes via mediators like different cytochromes and pilis or using extracellular proteins. Reguera et al. (2005) reported the mechanism underlying the electron transfer in Geobacter sulfurreducens through pili.

Recent studies have reported about the use similar to the metal conductance of these microbial nanowires. This microbial assembly will facilitate the guided electron transmission across syntonic species, in addition to being able to pass electrons among acceptors and donors of electrons (Malvankar and Lovley 2012). For example, the species Shewanella oneidensis can directly transfer electrons via mediators like c-type cytochromes which are found to be entrenched in periplasmic membranes (Schuetz 2009). Apart from the mediators certain intracellular and extracellular proteins were found to have compounds which can transfer/mediate the electron transmission at electrode interfaces. Few microbes were found to generate mediators or shuttles like compounds of class quinine which can mediate electron transfer (Schuetz 2009; Rabaey 2004). Analysis at gene level indicates that proteins and gene expressions are regulated and usually produced at the surface of microbes during its interaction/attachment on the electrode surface (Holmes 2006). The morphology and conductivity of certain electroactive microorganisms in pili nanowires are also stated in a report by Malvankar et al. (2015). Vargas (2013) has revealed that Geobacter sulfurreducens shows good conductivity through pili due to the presence of essential aromatic amino acids.

Due to the presence of catalytic activity and adaptivity at harsh environments, the use of extremophiles will benefit for better performance in bioelectrochemical system designing. Extremophiles represent species which can tolerate extreme salt, pH, temperature, pressure, saline, metals, etc. Additionally, this group of microbes are found to be good mediators of the oxidation-reduction potential and also possess proteins having good electron transfer abilities. As substrate, extremophiles can use many recalcitrant, xenobiotic as well as lignocellulosic biomass for energy generation. The additional benefit is that extremophiles can be used for the development of a safe, highly advantageous system for the efficient commercialisation of any microbial/enzymatic technology. For example, numerous thermophiles were isolated and reported due to the presence of its cellulose/hemicellulose-degrading ability (Rastogi et al. 2010). The sequencing results of the isolated culture demonstrated that the sequences were related to the species of Actinobacteria, Bacteroidetes, Chloroflexi and Deinococcus-Thermus. Several isolates belonging to the family Bacillus and Cohnella showed prospective for depletion of cellulose and also raw sawdust. Among many important species, Geobacillus sp. is one of the species which were widely studied by many researchers. Among that the strain WSUCF1 was studied due to its high cellulose-degrading efficiency. The optimal pH and temperature were 5.0 and 70 °C. The genome of whole-Geobacillus strains was sequenced. The annotated results indicate that the polysaccharide degradation was correlated with 70 open reading frames, 3 cellulose degradation ORFs and 13 ORFs among the 865 carbohydrate metabolism enzymes that were annotated as xylan-degrading ones. The strains found to degrade polysaccharides were also found to be significant producers of endoglucanase, xylanase and beta xylosidase (Bhalla et al. 2014a; Bhalla et al. 2015). Recent research has revealed that the WSUCF1 thermophile strain can be used in the carbon-free electrode and can conduct direct transmission reactions. The results indicate the impact of species for creation of new MEC systems utilising lignocellulosic substrates. Any bioelectrochemical approach will considerably reduce running costs and ensure that these wastes obtained in large quantities from different environmental sources are disposed of in an inexpensive and abundant lignocellulosic biomass.

Most studies reported attributes that extremophiles are paving way for improvement in the electrocatalytic process and are also known to boost for forming electrocatalytic films in the sense of severe conditions (e.g. high heat). Thermincola ferriacetica, for instance, has had an exceptionally high current concentration in comparison to mesophile species. Due to operating high temperature adaptability and surface characteristics these thermopiles are working well in electrical operation. However, it cannot be predicted that whether the particular biological engineering further leads to enhanced catalytic activity in thermophiles.

Extremophiles’ electrocatalytic behaviour has been studied in depth with the intention of mediating electrooxidation/mediate/dons/electron acceptors and electron transmission property (Hawkins et al. 2011; Sokolovskaya et al. 2015). Extremophile catalytic action is used in many bioelectrochemical applications (Dopson et al. 2016). Bioelectricity from various substances, such as glucose, xyloses, cellulose, acetate, lignocelluloses, waste water and heavy metals, has been produced using the extremophiles for bioelectricity processing in anode or cathode compartments (Wrighton et al. 2008; Jong et al. 2006; Lusk et al. 2015). A thermophile possessing similar cellulose-degrading ability and acetate production ability was reported to be closely related to Thermincola carboxydophila (Mathis et al. 2008).

Microbes residing in deep-sea sediment barophile microorganisms were isolated and used as electron donors for microbial fuel cells. Extremophile electrochemical activity is also reported in the literature for the development of biohydrogen in microbial electrolysis cells (MECs) (Lu et al. 2011). Lusk et al. (2007) reported about the designing of microbial electrolytic cell using a combination of two thermophilic species, i.e. Thermoanaerobacter and Thermincola, member of phylum Firmicutes. Using cellulose as substrate, the developed MECs work in the anodic compartment at 60 °C and have provided a current intensity of 6.5A m2 and a coulombic (CE) effectiveness of 84% without CH production (Lusk et al. 2007). Further, the other thermophilic species Moorella thermoautotrophica growing at 60 °C was reported for bioelectrosynthesis and the simultaneous reduction in formate and acetate (Yu et al. 2017).

14.11 Extremophiles and Its Types

14.11.1 Acidophiles

In conditions with low pH (<7.0) the microbes which survive are known as acidophiles. The extreme acidophiles (Acidithiobacillus sp.) grow well under extremely low pH conditions (<3.0). In a pH range of 3–5, mild Bacillus acidoterrestris sp. live (Gerday and Glansdorff 2007). All three realms, viz. Archaea, Bacteria and Eukarya, are representative of some acid-tolerant bacteria. The mesophilic microbes grow in a range of 20–40 °C, moderate microbes live at 40–60 °C and thermophilic/extreme microbes thrive at >60 °C (Gerday and Glansdorff 2007). Acidophiles can thrive in acidic conditions with notable examples of Metallosphaera sp. and Acidithiobacillus ferrooxidans. The acidophiles are usually aerobic heterotrophs which can oxidise iron and sulphur and also species like Acidianus sp. that minimise iron concentration. Acidophiles can handle acidic waste (3–5 pH) due to the presence of biological processes (e.g. nitrification and fermentation), from volcanic areas and from the mining industries of geothermal and coastal coal and steel (Johnson et al. 1992). A. cryptum was the first acidophile used in MEC experiments (Borole et al. 2008). Acidophiles are used in a number of further subsequent experiments for the treatment of mine sulphide waste (Ni et al. 2016), leachate food waste (Li et al. 2013) and waste distillery (Kim et al. 2014). Usage of Ferroplasma sp., Desulfovibrio sp. and Acidithiobacillus sp. has also shown its application in MECs (Rojas et al. 2017; Sulonen et al. 2015; Sulonen et al. 2016).

In acid conditions, acidophiles used in the MES anode act and function as follows: (1) inhibit methanogenic activity; (2) increase the pH gradient between the anode and the cathode body; and (3) promote proton transfer through the cation-exchange membranes. In addition, acidophiles are more commonly treated with sulphides, minerals and acidic wastewater by-products. More anodic overpotential is nevertheless sustained, which offsets the metabolic losses needed to maintain neutral conditions of cytoplasm (Dopson et al. 2016).

14.11.2 Challenges in Design

Electrode and membrane materials are vulnerable to substrate degradation issues, including corrosion, drowning and oxidation when exposed to acidic circumstances (Martínez-Huerta and Lázaro 2017; Yi et al. 2017). Under acidic conditions, glassy carbon electrodes are highly vulnerable to oxidation, especially with higher electrode potential (more positive potential), and glassy carbon electrodes are affected by problems linked to agglomeration, reduced porosity and increased mass transport limitations (Yi et al. 2017). The lower pH reduces proton-exchange durability for normal membranes. Acidic conditions also promote deposition of electrode products on the membrane surface, resulting in increased resistance to diffusion and a decrease in MES’s electrochemical efficiency. It is necessary to choose oxidation and corrodible materials but without sacrificing biocompatibility, conductivity and large specific surface area desirable features.

14.11.3 Alkaliphiles

The alkali-tolerant bacteria (pH 8–9) or alkali-resistant bacteria (pH >10) are known as alkaliphiles (pH 8–9), e.g. Bacillus alcalophilus. Alkaliphiles are divided into subclasses depending on the tolerance towards pH. Usually, depending on the pH range, they are categorised into moderate pH of around 9 and extreme alkaliphiles with pH range of 12–13 (Horikoshi 1999). All three cellular life classes comprise alkaliphiles, such as prokaryotes, eukaryotes and archaea. Alkaliphiles can use a diverse source of energy and can be found in different ecological niches such as aerobic as well as anaerobic conditions. The alkalophilic group of organisms can include phototrophic, fermentative, sulphur-oxidising (e.g. Thioalkalimicrobium), acetogenic and methanogenic (e.g. Methylomicrobium) groups.

In the MES operations alkaliphiles are advantageous. They allow anodic reactions, increase the anode potential to more negative values first under higher pH conditions and improve overall electrochemical conversion efficiency. Secondly, the formation of alkaline environments reduces accidental emissions of methanogens. Finally, alkaline situations produce a greater number of electroactive moieties in anodic biofilms. In a recent study by (Zhang et al. in 2016), the benefits from alkaline conditions (pH 1/4 10) in MFCs were seen in the mixed microbial population from a brewing waste treatment plant-activated sludge. At the end of the study, high-performance sequencing studies found that most group of microbes include Firmicutes (88.14%), Alkalibacter (5.14%), Bacillus (2.14%), Alkaliflexus (2.107%), Anoxyna tronum (0.48%) and Alkaliphilus (0.09%). Among different microbe groups, Corynebacterium sp. were the dominant species of alkaline-enriched biofilm. The methods used in the processing of paper and cement from livestock can be used to process alkaline waste streams from the electroplating industry and to treat potato lye.

Enzymes and useful by-products for their processing capability (starch degradation enzymes, cellulases, lipases, xylanases, pectinases, chitinases) can be further explored (2-phenylamine, carotenoids, siderophores, and cholic acid derivatives).

14.11.4 Design and Challenges

Graphite components are inert under alkaline conditions (Iken et al. 2007). However, metal compounds (e.g. stainless steel) are susceptible to consistent corrosion and stress corrosion cracking at high levels of caustic soda and potash used in experiments with MEC to protect under alkaline conditions (Iken et al. 2007). Due to stress, corrosion cracking problems can be worsened by the increased impact of MECs under thermophilic and high pressures. One way to overcome these challenges is the use of compatible materials such as nickel alloys and stainless steel withstanding hot (>95 °C) and strong 50% alkaline conditions (Jones 1992). To avoid stress corrosion, it is better to minimise copper electrode sensitivity towards MEC ammonia-based alkalis (Fontana 2005).

14.11.5 Thermophiles

The operation of MES is activated at 45 °C for most thermophiles. The literature suggests that extremophiles isolated from the coal waste piles tolerating temperature of about 65 °C can be employed for the method of MESs (Beffa et al. 1996). The use of such thermophiles can be used in compost and silage piles for household and industrial applications (Singh 2012). Archaea species tolerate an exceptional temperature of 100 °C between prokaryotes (such as methanogenesis, decreased sulphate, sulphur oxidation, nitrate reduction and hydrogen oxidation with little effect on their metabolic chemolithotrophic activities) (Gerday and Glansdorff 2007). Thermophilic conditions speed up the kinetics of waste oxidation in MECs, through enhanced substrate solubility, mass transfer characteristics and microbial behaviour.

They can also kill pathogens and reduce other infection hazards. Recent studies have demonstrated the use in thermophilic MECs, viz. Thermincola ferriacetica and Thermoanaerobacter pseudethanolicus (Dopson et al. 2016; Lusk et al. 2015). Thermophiles were also used for the processing of energy supplies in multiple electrolytic microbial cell (MEC) systems, such as hydrogen (Dopson et al. 2016; Dai et al. 2017; Fu et al. 2015).

14.11.6 Design and Challenges

Thermophilic MECs require thermally jacketed cells or advanced autoclave systems in order to withstand redox reactions at desired temperatures. It is a daunting task to enforce thermal conditions in the MECs (Wildgoose et al. 2004). In system, the MEC reactors and their components (electrodes and membranes) are built to resist the high temperatures and pressures faced. Specifically, the metallic electrodes are susceptible to corrosion and high temperature. High temperatures minimise the solubility of standard electrolyte gases (e.g. CO2). Higher temperatures (>45 °C) may lead to problems with degradation of electrode materials, membrane surface agglomeration and a resulting loss of resilience (Chandan et al. 2013). For instance, Nafion membrane seems to be degraded at temperatures above 100 °C (Rahimnejad et al. 2015). The literature shows that sulphonated hydrocarbon polymers, composite membranes (e.g. doped polymers with graphene oxide) and solid acid membranes are ideal for thermophilic conditions (Park et al. 2011).

14.11.7 Halophiles

Halophiles are a group of microbes that include a group of oxygenic and anoxygenic phototrophs, anaerobic heterotrophs, fermenting and denitrifying agents, sulphate reducers and methanogens, as well (Crowley 2017; Oren 2002). Mixed cultures and pure halophile cultures have been studied in MFCs, MCDCs and MECs.

14.11.8 Design and Challenges

Fouling and corrosion problems can arise during exposure of the MES components to saline waste streams. Fouling is an inherently difficult issue with the typical elements of electrodes and membrane that are exposed to salt water primarily due to alkalisation, evaporation, sorption and crystallisation (Jiang et al. 2011; Zhuang et al. 2012a; Thomas 2003). Corrosion processes result in skewed electrochemical signal measurements, improved impedance to acceptable faradaic processes and lowered MES performance.

14.11.9 Biohydrogen Production

Hydrogen (H2) is a desirable alternative fuel for the future, due to its non-polluting and non-exhaustible nature. Global research supports growth in processing biological hydrogen (biohydrogen), in order to mitigate the stresses produced by the emissions of carbon dioxide and the lack of fossil fuel. Biohydrogen will replace current hydrogen-processing technologies that are heavily dependent upon fossil fuels with electricity generation.

The use of non-renewable fossil fuels leads to the emission of greenhouse gases. On the other hand, biohydrogen (BioH2) production is environmentally friendly (non-polluting in nature) and renewable, since it can be extracted from biomass, and no GHGs are generated. H2 microbial generation would also meet the requirements for a workable prospect for biofuels and provide a renewable alternative to traditional production practises which is environmentally free and energy saving. Several options for the biological production of H2 are well studied, such as microalgae and cyanobacteria water biophotolysis, use of photo-fermentation photosynthetic bacteria for organic substances and dark fermentation by anaerobic organisms using organic substances. The final solution is typically preferred, i.e. dark fermentation, as the provision of light sources does not depend on this.

The major benefit in the fermentation (dark) process includes (1) ease of fermentor (reactor) unit, (2) known biochemical pathways and biochemical activity, (3) broader substrate range and (4) elevated H2 production compared to classical methods (Kumar et al. 2015; Saripan and Reungsang 2013).

14.11.10 Hydrogen Production Pathway

The Embden-Meyerhof (EMP) pathway is followed by the formation of BioH2 through the decarboxylation leading to formation of acetyl-CoA. During this pathway, the protons are reduced to hydrogen using the protein ferredoxin (Verhaart et al. 2010). Routinely, in aerobic conditions, the pyruvate generated at the end of the glycolysis pathway is further reduced to the form lactate by enzyme lactate dehydrogenase. In anaerobic conditions, the enzyme pyruvate oxidoreductase reacts with pyruvate and acetyl-CoA. The product acetic acid is formed depending on the environmental conditions. For formation/generation of biohydrogen, in strict anaerobes two pathways are followed; pathway follows the use of nicotinamide adenine dinucleotide phosphate, glyceraldehyde 3-phosphate dehydrogenase and pyruvate ferredoxin-oxidoreductase (PFOR) (Akhtar and Jones 2008). The reaction for production of the biohydrogen in anaerobic condition was found to be thermodynamically unfavourable (Akhtar and Jones 2008; Hallenbeck and Ghosh 2009). However, among the classes of extremophiles, methanogens were found to be better suited for biohydrogen production. Methanogens have the ability to reduce sulphur and in turn lower the partial pressure for biohydrogen formation. The resultant lower biohydrogen pressure favours the acetate formation. In case of methanogens, it became evident that the partial pressure and high temperature will not affect the hydrogen generation. Using the methanogens and optimum conditions, biohydrogen up to 4 moles can be generated along with side production of 2 moles of acetate.

Hydrogenase is the main BioH2 synthesis enzyme with two forms depending on the metal content. During the fermentative EMP route, H2 is generated when pyruvates are transformed into acetyl-CoA.

The thermophilic bacteria can generate elevated H2 quantity compared to mesophiles (Van Niel et al. 2003; Pradhan et al. 2015). The fermentation process of hyperthermophilic bacteria at high temperatures has several advantages. For example, other microorganisms (Pradhan et al. 2015; Mohan 2010) would be at lower risk for infection and improvement of pathogens (Sahlstrom 2003), due to higher temperature, during the procedure. Furthermore the process will benefit from a higher degree of reaction, lower viscosity and improved mixing. Compared to mesophilic reactions, reaction susceptibility is less selectively at high H2 partial pressure (Van Niel et al. 2003). In the presence of different feedstocks, bacterial growth in combination with the metabolism of production of H2 can vary, apart from the benefits above (Pradhan et al. 2016). Thermotogales were seen as the preferred alternative to biofuel production and for industrial applications with a form of hyperthermophilic bacteria because they have a capacity to produce high H2 yields (1.5–3.85 mol H2/1 hexoses) from various cellulosic waste materials (Cappelletti et al. 2012). The total potential gain of intensive thermophilic bacteria (4 mol hydrogen/mol glucose) yields 83–100%. Bear in mind that H2 also depends on many variables, including substrate concentration, immobilised cells, anaerobic conditions, etc. Various factors producing H2, such as glycerol levels, medium-growth composition, pH and temperature, are studied in a wide range (Lo et al. 2013).

14.11.11 Thermophilic Species

A wide variety of sugar compounds can be used and fermented into acetate, CO2 and H2 by Thermotogales roots to intense hyperthermophiles with the potential to expand at a temperature of 80 °C. Thermotoga species are Gram-negative bacteria within Thermotogales, which are mainly fermentative anaerobes with optimum rising temperatures in the 75–80 °C range and found primarily in geothermally heated sediment regions (Huber and Stetter 1992).

Many complex sugars can be reduced to acetate, carbon dioxide and hydrogen belonging to the group of Thermotogales hyperthermophilic microbes which can resist temperature up to 80 °C. Thermotoga species belongs to a group of Gram-negative bacteria. The species belonging to the class of Thermotogales are facultatively fermentative anaerobes with optimum temperature of about 75–80 °C. The habitat for the Thermotoga species is usually the geothermal sediments or hot springs.

14.11.12 Thermotoga Species

Thermotoga species represents the microorganisms which have large classes of hydrolysis enzymes which are suitable for biomass fermentation but majorly lack the exoglucanase enzyme which plays a vital role in the degradation of cellulose (Han et al. 2014). Over the last few years, Thermotoga maritima was the sort of strain that researchers have been most interested in (optimum growth at 80 °C). Due to its enormous ability, T. maritima has been chosen to produce hydrogen from simple and complex carbohydrates. The genome of the species shows that 7% of the coding sequences were functioning for monosaccharide and polysaccharide metabolism (Nelson et al. 1999; Chhabra et al. 2003; Conners et al. 2005). Other species of T. maritima catabolise sugar and polymer results during anaerobic cellular respiration generating hydrogen and carbon dioxide. These species can be predicted as future sources for replacement of non-renewable sources. These can also be reported to utilise cellulose and xylan. This species also represents anaerobic respiration by decreasing Fe (III) metabolites.

Another species in the same community that researchers primarily use for biohydrogen production is T. neapolitana. Thermotoga neapolitana is also a hyperthermophilic organism that grows on a large variety of substrates, such as glycerol, glucose, xylose and starch (Pradhan et al. 2016). These microorganisms can display fast kinetic growth (Pradhan et al. 2016), oxygen tolerance (Pradhan et al. 2015) and low contamination risk (Nguyen et al. 2010). It was claimed by D’Ippolito et al. (2010) that anaerobic growth of species is highly influenced by parameters like pH buffering, culture/headspace volume ratio, stirring and N2 sparging for hydrogen production. During their analysis, the maximum hydrogen yield was 3.85 mol H2/mol glucose and the development rate was 51 ml L−1h.

14.12 Conclusion

The use of extremophiles in MESs has presently commenced and the oxidising potential of the proteins along with the ability to form biofilms will reveal some novel uses for energy generation and remediation of polluted/degraded sites. The bioremediation of waste water, which reflects a combination of many recalcitrant factors, proffers substantial potential for the crucial role of microbes in designing MESs. In order to treat the desired waste water, the identification of extreme microbes will be a crucial research requirement which involves sampling of different extreme conditions and optimising of the concentration of microorganisms used in MESs. However other unresolved issues still remain, including the pollutants used as electron donors, as well as the energy requirement that is available at the same location to both donors and acceptors. In all cases, electron acceptors are needed to provide a sufficient power gain when coupled with those donors.

MESs that have advantages for electricity generation can also be used to treat high-pH waste water. Alkaliphiles are very less reported in MESs and very limited numbers have been studied in pure and mixed cultures. New electricigens are required for better on-site performance in different extreme conditions like high-pH conditions. Additional research on cathodes, anodes, economic considerations and membranes, and especially their selectivity, may also determine the potential large-scale application of MES. Biohydrogen generation utilising the agricultural substrates is however at its initial phase because of the unanswered problems at some stage in the fermentation process.