Application of Microbes in Synthesis of Electrode Materials for Supercapacitors

Abstract

The utilization of microorganisms for fabrication of different useful nanomaterials with precise control on their morphologies and micro-architectures has attracted substantial interest owing to their biocompatibility, renewability, sustainability, and cost-effectiveness. High-performance supercapacitors, one of the fast-developing sectors in electrochemical energy storage systems, essentially require active electrode materials with large specific surface area, interconnected uniform porous arrangement, outstanding electronic charge transport, and mechanical flexibility as prime features. In the present chapter, the state-of-the-art research developments in the exploitation of microbial generated materials comprising different carbon nanomaterials with vivid compositions along with their nanocomposites with various counter systems such as high costing synthetic nanocarbons, metallic compounds, conducting polymers, etc. that have been successfully applied as functional supercapacitor electrodes. These microbe-based systems have been mainly applied to fulfill the preferred purposes that include as (a) bio-templates; (b) mechanically flexible, supporting matrix to strongly anchor diverse electroactive materials; and (c) bio-carbonized substances that are highly porous, large surface-based functionalized carbons, which are subsequently also employed in formation of smart nanocomposites. Thus, amalgamation of bio-tools for materials designing in supercapacitor technology will surely open up innovative opportunities for extensive and scalable manufacture of highly proficient energy storage gadgets in the near future.

2.1 Introduction

Highly undesirable and unfavorable ecological impacts imposed by random burning up of non-renewable fossil fuels have triggered serious concerns among scientists to focus on introducing novel, renewable, and sustainable modes of energy production and look over their efficient storage and inter-conversion processes (Larcher and Tarascon 2015; Wang et al. 2017a). This has resulted in fast progression in electrochemical energy storage (EES) devices such as capacitors, rechargeable batteries, supercapacitors, etc., in particular, to counter the sky-high growing demands of uninterrupted power supply (Sumboja et al. 2018; Winter and Brodd 2004). A comparative study on the relative performance of these EES systems has been outline in Table 2.1. Each of the devices has its own merits and limitations. Till date gigantic and voluminous batteries are considered as major power backup devices owing to their large values of specific energy (energy stored per unit mass) (30–300 Wh kg⁻¹) (Winter and Brodd 2004; Jayalakshmi and Balasubramanian 2008; Wang et al. 2017b). However, poor life span, inferior specific power (energy delivered per unit time per unit mass), and environmental toxicity along with safety issues especially related to their recycling and disposal after its life termination are the foremost practical limitations. Nevertheless, lack of suitable substitutes has been the main ground for disregarding these drawbacks. Yet today, eco-friendly, high energy density fuel cells do have less significance for large quantity energy production sectors, primarily, because of their bulk size, large installation costs, and insufficient fuel storage capability (Santoro et al. 2017). Again, conventional capacitors though display inferior specific energy are capable of delivering energy at ultrafast rate along with long cycle life (Poonam et al. 2019; Kotz and Carlen 2000). The above points stimulated researchers to assemble and integrate, as far as possible, all the good qualities of each type of the above EES systems to fabricate novel device with enhanced energy delivering capabilities in the form of supercapacitors, also popularly named as electrochemical capacitors or ultracapacitors (Raza et al. 2018; Conway 1991; Burke 2000). However, both production cost and energy-power efficiency of first-generation supercapacitors are far below to the expectations, and thus, low-priced, handy, portable, miniatured, bendable yet robust, smart future-generation supercapacitors with large energy and power characteristics have been urgently demanded to replace the conventional voluminous, poor responsive, short-lasting batteries soon (Meng et al. 2013; Gidwani et al. 2014).

Table 2.1 Comparative study of various EES devices

Rigorous researches have signalized scrupulous strategies to overcome the issues of poor specific energy of supercapacitors without surrendering their high-power performances and exceptional cyclic efficiency (Xue et al. 2017; Naoi and Simon 2008; González et al. 2016). Experts are targeting to harmonize the energy storing efficiency closer to that of popular rechargeable batteries (Yassine and Fabri 2017; Stoller and Ruoff 2010). On the contrary, extensive scientific explorations are being carried out to improve the power capacity of the prevailing batteries as well (Dupont and Donne 2016; Dong et al. 2016). This has commended in designing next-generation EES possessing elevated energy and power efficiency, strategically, either by (a) employing one of the electrodes as a composite of supercapacitor-type and battery-type ingredients or (b) by devising a setup in combination of supercapacitor electrode with a battery electrode. The so-configured devices with such hybrid electrode materials or electrodes configuration have been commonly termed as “supercapatteries” (Chen 2017; Majumdar et al. 2020; Chae et al. 2012). These devices have been sketched to possess the optimum properties of supercapacitors and rechargeable batteries, all together. A typical Ragone plot (as depicted schematically in Fig. 2.1a) is used to evaluate the relative device performances by considering the specific energies at different specific power values, expressed in logarithmic terms and then correlating them with the performances of other EES, existing in the scientific database (Christen and Carlen 2000; Mei et al. 2018). It clearly indicates the relative position of supercapatteries, bridging the void between the common capacitors and rechargeable batteries in terms of energy and power efficiencies. Although its current position is far to that of theoretically predicted energy-power efficiency of thermodynamically reversible heat engine, the plot signifies that there lies plenty of room for improvement in the supercapattery technology in order to accomplish results that may be very close to the challenging one (Chen 2017; Majumdar et al. 2020).

Fig. 2.1

(a) A characteristic Ragone plot showing relative performance of different energy storage devices (Chen 2017). (b) Schematic illustration of a characteristic supercapacitor device

2.1.1 Basics of Supercapacitors

Thus, before proceeding further, it is worth to have a basic understanding of the fundamental aspects of supercapacitors. Typically, a supercapacitor device (as shown in Fig. 2.1b) comprises the following sections: two electrodes, with positive and negative polarity, possessing large surface area as well as high porosity, connected ionically via electrolytes, partitioned by porous electrolyte-filled separating membrane. The efficiency of various electrochemical energy storage systems is designated by two important parameters, viz., specific energy and specific power, as mathematically expressed through the following Eqs. (2.1) and (2.2), respectively (Majumdar et al. 2020):

$$ \mathrm{Specific}\ \mathrm{energy}(E)=\frac{1}{2}{C}_{\mathrm{s}}{\left(\Delta V\right)}^2 $$

(2.1)

$$ \mathrm{Specific}\ \mathrm{power}\ (P)=\left(E/\Delta t\right) $$

(2.2)

The term C_s stands for the specific or gravimetric capacitance, ΔV corresponds to working potential range, and Δt represents the discharging time period, respectively. Usually, large magnitudes of E and P are ideal for high-performing supercapacitors. The values of power and energy densities can be achieved by considering the volume of the electrode material instead of its mass while computing the capacitances, as applied in the above-stated equations (Zuo et al. 2017a; Akinwolemiwa and Chen 2018). Moreover, some additional crucial parameters also need essential consideration and have been mentioned as follows: Rate capability or capacity, variation of capacitance at different current densities or at varying voltage sweep rates. It judges the ability of the device to generate large power with minimum loss in voltage even at high applied currents. Electrochemical reversibility involves measurement of rates of charge transfer process occurring at the electrode/electrolyte interfaces. Electrochemical window: voltage range within which the system is neither oxidized nor reduced. The electrode as well as electrolyte’s ability to avoid electrochemical decompositions in a given potential range determines its electrochemical stability window. Electrochemical stability: the reversibility in charge storing capacity (capacitance) of the device in a given potential range under constant current or potential scans rates determines its electrochemical stability. It is indicated by high specific capacitance retention efficiency of the device for large galvanostatic charging/discharging cycle numbers. In addition, factors such as mechanical flexibility, environmental stability, and operational temperature range are also analyzed (Afif et al. 2019; Miller et al. 2018; Shi et al. 2013). A number of factors control the electrochemical efficiency of supercapacitor devices, namely, (a) electrode materials, nature, porosity, and morphology, (b) electrolyte ions and their nature, (c) electrode materials’ binder, (d) quantity of electrode material loading, (e) nature of current collector used to hold electroactive materials, (f) separator membrane, (g) sealants used, (h) devise patterning and designing, etc. have been outlined in Fig. 2.2. Each of the components are vital; however detailed discussions on them are beyond the scope of the chapter. The readers can go through the literature for obtaining more information and knowledge about them (Majumdar et al. 2020).

Fig. 2.2

Outline of different parameters that controls the performance of supercapacitors

Electrochemical performances of electrodes in these devices are usually investigated using cyclic voltammetric (CV) analysis, galvanostatic charging-discharging cycling tests (GCD), and electrochemical impedance spectroscopy (EIS) (Majumdar et al. 2020; Wang et al. 2017b).

Specific or gravimetric capacitance (C_s) of electrode materials is commonly calculated using the following equations from the curves and parameters derived from CV or GCD experiments as shown in Eqs. (2.3) and (2.4), respectively (Majumdar et al. 2020):

$$ {C}_{\mathrm{s}}=\left(\frac{1}{ms}\right)\left(\Delta V\right){\int}_{V1}^{Vn}i\mathrm{d}V $$

(2.3)

$$ {C}_{\mathrm{s}}=\kern0.5em \frac{I}{m\ \left(\Delta V/\Delta t\right)}\kern0.75em $$

(2.4)

where the integration ʃ(idV) gives the integrated area under the CV voltammogram curve, s stands for voltage sweep rate, ΔV is the potential range, V_n and V₁ are the terminal voltage limits of voltage scans, and “m” corresponds to the mass of the electrode materials, respectively. I indicates the steady current used for the GCD test. The popular unit for specific or gravimetric capacitance is “F g⁻¹”, while that of areal and volumetric capacitances are “F cm⁻²” and “F cm⁻³”, respectively (Wang et al. 2014a).

Electrochemical impedance spectroscopy (EIS) is used to characterize the resistive as well as capacitive nature of the electrode materials (Majumdar et al. 2020; Obreja 2008).

2.1.2 Electrode Materials for Supercapacitors

Among the above factors, the most decisive one is unambiguously the nature and morphology of electrode materials employed for devising supercapacitors (Iro et al. 2016). The fundamental operational principle for supercapacitors in contrast to batteries encourages judicious designing of electrode materials and electrolyte to undergo faster electron/ions transport in the bulk electrode with special stress on tuning material dimension, surface nature, crystal forms, and electrode/electrolyte interfacial chemistry, respectively, to recognize exceptional capacitances, energy, and power densities (Xiea et al. 2018). Thus, this is certainly a serious and urgent assignment to materialize “smart and efficient” supercapacitor electrodes with outstanding electrochemical features.

Energy storage mechanism in supercapacitors is usually guided by two types of charge storage phenomena exhibited by the electrode materials, specifically, electrical double layer capacitors (EDLCs) and pseudocapacitors, respectively (Simon and Gogotsi 2008; An et al. 2019; Le Fevre et al. 2019; Borenstein et al. 2017; Wang et al. 2015a). EDLC-type electrode materials accumulate charge via rapid ion-adsorption/desorption processes at the electrolyte/electrode boundary during charging/discharging courses as demonstrated schematically in Fig. 2.3. High surface area-containing organic-based materials such as active-carbons, mesoporous carbons, carbon nanotubes/nanofibers, graphenes, carbon foams, carbon aerogels, etc. with stable electrochemical behavior are usually categorized under EDLC materials (Najib and Erdem 2019). EDLC-type materials generally demonstrate low capacitance although survive high figure of fast and stable charging/discharging cycles efficiently (Guan et al. 2016). Pseudocapacitors, on the other hand, illustrate better capacitance values compared to EDLCs but poor charging/discharging efficacy (Wang et al. 2017c; Jiang and Liu 2019; Augustyn et al. 2014). They include mostly transition metal oxides and chalcogenides, and derived compounds along with conducting polymer derivatives involve slow, diffusion-controlled movement of ions to achieve this charge storing capacity, as depicted in Fig. 2.3 (Brousse et al. 2015; Majumdar 2016; Zhan et al. 2018; Majumdar and Bhattacharya 2017). Capacitive faradaic process in these semiconducting materials typically involves incessant electron transfer over comparatively wide potential range owing to activation of delocalized electrons dynamically associated with several redox-active sites thereby establishing groups of energy states (Guan et al. 2016).

Fig. 2.3

Illustration of charging/discharging processes in electrical double-layer capacitor (EDLC) and pseudocapacitor systems

It has been accomplished that hybrids/composite materials are beneficial as they successfully eliminate the individual shortcomings of the components (Majumdar et al. 2019a, 2019b; Majumdar 2018, 2019a, b). In order to devise supercapattery devices, such hybrids materials are fabricated with organic/inorganic nanomaterials with different charge storage mechanisms covering both pseudocapacitance and batteries features that are essential to reach the desired energy efficiency (Chen 2017; Majumdar et al. 2020; Chae et al. 2012). Accordingly, various nanocarbons such as graphene, carbon foams/cloths, etc. have been blended with metal-based compounds, conducting polymers, etc. to obtain electrodes with enhanced specific surface area, high and uniform porosity, as well as improved electrical conductivity so as to overcome the problems of inferior energy density, low cycling rates, and mechanical instability (Majumdar 2019a, 2019b; Chen et al. 2017; Dubey and Guruviah 2019).

In this aspect, it has been spotted that the carbon materials used in designing such composites must satisfy some essential features: (a) easy fabrication from natural abundant resources so as to minimize production cost; (b) large surface area with tunable porosity; (c) easy assembling to form hierarchical three dimensional network structure to facilitate faster transport of ions/electrons; (d) containing adequate surface functionalities to strongly anchor various nanomaterials on their surfaces, thus ensuring large mass loading of electroactive materials as well as inhibit their agglomeration and easy detachment during large charging/discharging cycles; and (e) mechanically flexible with high tensile strength so that they can form efficient matrix for manufacturing flexible energy storage devices (Zhang and Zhao 2009; Borenstein et al. 2017; He et al. 2013).

The synthetic carbons like graphenes, carbon nanotube, carbon foams, activated carbons, carbon aerogels, etc. as illustrated in Fig. 2.4 often require costly instrumentations and rigorous post-synthetic treatments with limited control on surface area, porosity, and electrical conductivity (Hirayama et al. 2017; File:CNTSEM.JPG n.d.; Omidi-Khaniabadi et al. 2015; Ding et al. 2018; Chen et al. 2006). Moreover, they are often susceptible toward agglomeration and restacking that drastically reduces their effective surface area. Again, optimizing of conductivity with functionalization and uniform doping are essentially required as too much surface modification considerably lowers their electronic conductivity (Li and Wei 2013). Such optimization urges several hurdle synthetic steps that considerably limit their practical applications.

Fig. 2.4

Different synthetic carbons such as (a) carbon nanofibers (Hirayama et al. 2017), (b) carbon nanotubes (File:CNTSEM.JPG n.d.), (c) functionalized graphenes, (d) activated carbon (Omidi-Khaniabadi et al. 2015), (e) carbon aerogels (Ding et al. 2018), (f) carbon foams employed in energy storage applications (Chen et al. 2006)

Thus, scientists worldwide have devoted their research in designing electrode materials based on carbons gathered not only from natural resources but successfully able to satisfy all the requisite criteria discussed above (Li and Wei 2013). As an essential aspect of green technology and to save our environment, various bio-masses/bio-wastes as well as microorganisms that are widely available in the nature have been meticulously channelized as effective carbon sources for designing advantageous materials in several technological domains (Yang et al. 2019; Bi et al. 2019; Lyu et al. 2019; Correa and Kruse 2018; Lakshmi et al. 2018).

This book chapter intends to discuss on the application of different microbes in designing and fabrication of electrode materials for flexible supercapacitors/supercapatteries—their current progress and advancements with special emphasis on the role of such microorganisms in structure-designing diversities that led to effective functional modifications so as to improve the overall electrochemical efficiency in these energy storage devices.

2.1.3 Why Microbes in Energy Storage Devices?

Microbes or microorganisms are minute living organisms existing everywhere in the environment that can be detected only under microscopic devices. Microorganisms have been chiefly categorized into different classes such as bacteria, fungi, archaea, algae, virus, etc. (Windt et al. 2005; Shen et al. 2019). The invention and subsequent up-gradations of microscopes have travelled through the skilled hands of the several technologists that have enormously helped the scientific community to recognize and identify the existence of the vast microorganism world. They play crucial role as major foodstuff in the food chain of our ecosystem as well as decomposers in the disintegration of dead organisms that facilitate sustaining of cycle of life (Klaus et al. 1999). With intensive studies, their industrial applications in various important sectors, such as nutrition, pharmaceutical, metallurgy, fuels, chemicals, etc., got revealed (Lahoz and Ibeas 1968; Tasaki et al. 2017; Higgins and Dworkin 2012; Jang et al. 2017). Although many of them are susceptible toward causing diseases to human beings and animals, plenty of their merits that include speedy reproduction rate, biomineralization, genetic revision, self-assembly, variety, and appreciable adaptableness to extreme situations have forced the scientists to show tremendous interest in the past decades (Liu et al. 2016a; Wu et al. 2013; Chen et al. 2010; de Petris 1967; Nam et al. 2006). Some naturally occurring microbes are skilled of fabricating nanoparticles under ambient state without requiring supplementary chemical reagents or physical conditions that have fascinated extensively in promoting green technology over the last few decades (Reverberi et al. 2016; Gopinath et al. 2017; Zhang et al. 2019; Fang et al. 2019). Microbes have proved themselves as efficient precursors for preparing various mesoporous carbons used for various technological applications in the recent past (Deng et al. 2019a). They often possess large surface area-based cellular structures that can result in highly urged porous and large surface-based carbon materials (Dong et al. 2013; Moradi et al. 2015; Gerasopoulos et al. 2012). They can be produced on large industrial scales using mild preparation conditions for serving various purposes from a range of bio-wastes that may otherwise create huge ecological pollutions (Yang et al. 2016; Wang et al. 2015b; Divyashree and Hegde 2015). Further, they possess diverse morphologies which have fruitful implications in natural processes that can be effectively channelized to function analogously in smart energy storage devices (Shen et al. 2019). Thus, microbes can be very effective as bio-templates for controlling morphology during synthesis of nanoscale materials. Again, they can provide efficient matrix for high-density electroactive materials mass loadings; especially their hierarchical three-dimensional interlinked structures promote electronic conductive network pathways along with porous channels that facilitates easy intercalation and approachability of electrolyte ions to the electroactive sites. Their morphology often imparts attractive mechanical tenacity for designing flexible devices. Moreover, these systems can act as precursors of various derived carbons with tunable porosity and surface functionalities. They are also the sources of other elements like nitrogen, phosphorous, and sulfur with trace element amounts that may assist in uniform doping of these derived carbons that may promote adequate density of electrochemical activity sites, which is otherwise difficult to achieve in synthetic carbon analogues. All these unique features have made microbes highly important and indispensable materials in energy storage applications as highlighted in Fig. 2.5 (Ghosh et al. 2012; Pomerantseva et al. 2012).

Fig. 2.5

Role of microbes in fabrication of energy storage devices

2.2 Different Microbes Commonly Used in EES

In the recent years, varieties of bacteria, viruses, and fungi have been employed as precursor for the synthesis of smart electrode materials for high energy storage applications (Table 2.1). This may be in the form of bio-templates or as supporting matrix to hold various electroactive materials or as precursors of various neat as well as heteroatom doped to form a variety of carbon-based composites. Some essential information about a number of popular microbes such as bacteria, fungi, and viruses commonly employed in the preparation of electrodes for flexible supercapacitors have been outlined in Table 2.2, and corresponding images of those microorganisms have been shown in Fig. 2.6 (Micrococcus n.d.; Deinococcus_radiodurans n.d.; Bacillus subtilis n.d.; Geobacter sulfurreducens n.d.; Neurospora crassa n.d.; Cladosporium cladosporioides n.d.; Tobacco mosaic virus n.d.; M13 bacteriophage n.d.; Lee et al. 2014; File:Pseudomonas aeruginosa 01.jpg n.d.; Rhizobia n.d.; Sarcina (bacterium) n.d.; Achromobacter n.d.; Enterobacter n.d.; Escherichia coli n.d.; Dickeya dadantii n.d.; Agaricus n.d.).

Table 2.2 Functional roles of some important microbes in energy storage applications

Fig. 2.6

Various microbes used in energy storage applications (Micrococcus n.d.; Deinococcus_radiodurans n.d.; Bacillus subtilis n.d.; Geobacter sulfurreducens n.d.; Neurospora crassa n.d.; Cladosporium cladosporioides n.d.; Tobacco mosaic virus n.d.; M13 bacteriophage n.d.; Lee et al. 2014; File:Pseudomonas aeruginosa 01.jpg n.d.; Rhizobia n.d.; Sarcina (bacterium) n.d.; Achromobacter n.d.; Enterobacter n.d.; Escherichia coli n.d.; Dickeya dadantii n.d.; Agaricus n.d.; Saccharomyces cerevisiae n.d.; Lingzhi (mushroom) n.d.; Subbiah and Balan 2015; Tremella fuciformis n.d.; Phallus indusiatus n.d.; Xylaria n.d.; File:Hirneola_auricula-judae_(xndr).jpg n.d.)

2.2.1 Bacteria

Bacteria are popularly regarded as the largest variety of living microorganisms that participates actively in material cycling in nature. They display wide range of morphologies, besides being economic, scalable with biomineralization ability and outstanding physicochemical properties. They also exhibit unique electrochemical activity that makes them high-potential candidates in energy-related disciplines (Wilkinson 1963). They can be employed as proficient precursor materials and dopant resources to prepare uniform, in situ singly, as well as multi-element-doped pyrolytically derived carbon nanomaterials after anaerobic thermal decomposition of the proteins, phosphor-lipids, and metal salts in their structure (Shen et al. 2019; Tshikantwa et al. 2018). Remarkable features of such derived carbons along with their nanocomposites have been discussed in subsequent sections. Even one of their synthesized biodegradable, natural cellulose called bacterial cellulose has been effectively employed as well-accepted and efficient flexible electrode material components in high energy supercapacitors for sustaining uninterrupted power in smart wearable electronics (Esa et al. 2014).

2.2.1.1 What so Special About Bacterial Cellulose?

Bacterial cellulose is BC is a special variety of nontoxic, cellulose materials composed of polysaccharides having general chemical formula which is (C₆H₁₀O₅)_n, containing β-1,4-glycosidic linkages, as indicated in Fig. 2.7a, b (Ma et al. 2016a). It is mostly produced in large scale economically by glucose or hexose analogues and fermentation process via several microbes such as Acetobacter, Rhizobium, Pseudomonas, E. coli, etc. as highlighted in Table 2.2. The synthesis of bacterial cellulose involves multistep procedures by means of formation of “uridine diphosphoglucose” from catalytic phosphorylation of hexoses or similar carbon sources followed by isomerization and polymerization steps to form long and un-branched β-1→4 glucan chains by cellulose synthase (Ma et al. 2020). Since the last two decades, BC-based technology has become a fast-developing sector with their extensive usage in biomedical applications, including bio-sensing, biomedical, and tissue-engineering fields, high-quality paper-making industries in addition to the domains of acoustics, optoelectronic usages, food industry, and so on (Lin et al. 2013; Picheth et al. 2017; Stumpf et al. 2018). BC possesses unique properties that are highly urged for manufacturing smart materials for energy applications. These include:

• Simple and cost-effective synthesis as resourced from bio-renewable materials (Luo et al. 2014, 2017).

• Specific ultrafine interconnected networks of bacterial cellulose nanofibers with adequate pore density with high water retention capability (Yano et al. 2005; Li et al. 2014a).

• Good biodegradability with no toxic products (Wang et al. 2016a).

• Flexible, with substantial elastic stretching and bending features along with high tensile strength (Young’s modulus ~138 GPa and tensile strength >2 GPa) in addition to biocompatibility, renewability, and hydrophilicity (Klemm et al. 2011).

• BC fibers being smaller and have distinctive structure than conventional plant cellulose fibers as indicated in Fig. 2.7c, d, the former being more capable of forming smooth and versatile paper-type electrodes for flexible energy storage devices (Ma et al. 2016a).

• Further, dry BC aerogels have self-assembled interlinked nanofibrillar morphology that can be subjected to pyrolysis to achieve 3D carbon-based aerogels that find useful applications in energy storage devices, artificial body parts, sensors, etc. because of light weight, porous nature, enhanced surface-active area, and boosted electrical conductivity along with improved structural flexibility (Ma et al. 2020).

• BC pellicles (films/membranes) can be employed as starting materials for fabricating stretchable conducting systems with amazing electromechanical stability, even on exposure to high stretching and bending conditions (Liang et al. 2012).

• BC gel electrolytes having adequate porosity and hygroscopic and hydrophilic nature can significantly improve electrolyte-ion mobility in high-performance supercapacitors (Zhao et al. 2019).

• High light transparency properties enable their usage in optically visible displays (Ummartyotin et al. 2012).

• Very low coefficient of thermal expansion along the axis, with tunable characteristics, making it eligible for transparent electronic device designs (Yano et al. 2005).

Fig. 2.7

(a) Snap-shot of BC slice held in hand. (b) Chemical structure of bacterial cellulose (BC). (c) Schematic comparative presentation of plant cellulose fibrils (left) with BC microfibers (right). (d) SEM image of the BC fibers (Ma et al. 2016a)

2.2.2 Viruses

Viruses such as M13 bacteriophages and Tobacco mosaic virus (TMV) differ from bacteria and fungi class largely because of their nanoscale dimensions, single DNA or RNA, as well as parasitic behavior (Dong et al. 2013; Gerasopoulos et al. 2012). DNA or RNA genetic modification of such viruses opens up several functional groups (e.g., –COOH, –OH, NH₂, –SH residues) that can effectively anchor various ions as well as nanomaterials (Pires et al. 2016; Heinemann and Walker 2019; Cleaves et al. 2019). Further, they can undergo continuous interlocking web structure or form smooth uniform films by assembling vertically on substrates that serve as effective binder-free building units. Such successful attempts have already been reported for various electronic devices such as micro-batteries, light harvesting systems, etc. (Chaturvedi and Shrivastava 2005; Miller et al. 2007; Tarascon 2009). M13 viruses are filamentous, single circular DNA-containing microorganisms. They only contaminate E. coli bacteria with F+ (F-plasmid) and replicate therein (Lee et al. 2009). TMVs, on the other hand, are cylindrical, rod-type, RNA viruses that predominantly infect specific plants, like tobacco and other nightshade herbs/shrubs, and are therefore considered as pathogens of tobacco mosaic disease (Fan et al. 2013). TMV undergo reproduction at temperatures >60 °C and in pH range of 2–10. However, they are generally nontoxic and, so far, caused no harm for humans and thus can be effective in producing beneficial materials for various technological applications (Ren et al. 2010).

2.2.3 Fungi

Fungi are heterotrophic microorganisms and cannot produce own food by carbon fixation or similar process. They show vast possibilities in energy storage domains in the past few years owing to their high reproductive efficacy, scalability, and varieties (Bhattacharya and Raha 2002; Wang et al. 2014b). Chitin, the chief constituent of the fungi cell wall, is higher polysaccharide macromolecule, containing acetamide functionalities unlike that of plant cellulose with hydroxyl groups. Some of them are toxic as well. Fungi of different types, such as molds, yeasts, mushrooms, etc., have been employed in energy fields (Chang et al. 2010; Krishnan et al. 2009; Li et al. 2019; Campbell et al. 2016). Molds are hyphae-type fungi with branched, filamentous structure, while yeasts possess various shapes such as spherical, oval, ellipsoid, and rod types that can survive in both aerobic and anaerobic environments (Ni et al. 2010). Mushrooms, on the other hand, display various shapes, and often their high porous structure can be useful for energy storage and transfer. Recent research reveals that mushrooms with large K⁺ ion concentration can activate and enhance battery storage capacity considerably (Campbell et al. 2015).

In the following section, the author highlights on the different naturally occurring microbes that have been employed in devising electrode materials in energy storage applications. Such microbes essentially fulfill three essential prospects of material preparation, namely, bio-templates, supporting matrix, and source/precursor for derived porous carbons with superior physicochemical properties.

2.3 Microbes as Bio-templates for Energy Storage Materials

Template-driven strategy has been among the most extensively employed approach for synthesizing wide range of inorganic nanomaterials (Liu et al. 2013). Therefore, in the recent time, there has been an enormous impulse for developing useful, productive, and green methodologies in nanomaterial production using biological systems. Bio-templates provide nanoscale control of synthesis of nanoscale materials similar to that of the existing in natural systems (Singh and Chakarvarti 2016). They also serve as stabilizers and promote uniform dispersion of structures. They provide mild synthetic conditions for synthesis of materials with formation of microscopic to macroscopic hierarchical structures with nanoscale building units and hence open up greater possibilities for effective control on morphology. Large replication rate, self-assembly as well as possess adequate surface charge that behave as nucleation sites for resting particles to form various morphology-based systems, through aid of medium pH and ionic strength maintenance (Stephanopoulos et al. 2013).

2.3.1 Bacteria as Bio-templates

Self-assembled bacterial nanostructures can serve as effective templates using their surface proteins (S-layer proteins) for natural mineralization leading to the production of high-quality nanomaterials as illustrated schematically in Fig. 2.8a (Shim et al. 2013). Several instances have come up that shows that such templates promote porosity and better loading of electroactive inorganic materials thus promoting better capacitive features. For example, Shim et al. fabricated three-dimensional hierarchical porous flower-like Co₃O₄ systems by means of Micrococcus lylae protein serving as bio-template as shown in Fig. 2.8b. The sample electrode showed good capacitive behavior as observed from Fig. 2.8c with good rate capacity and also recorded improved pseudocapacitance of 214 F g⁻¹ (2.04 F cm⁻²) @ 2 Ag⁻¹ (19.02 mA cm⁻²) superior to many reports available in the scientific database, mostly owing to high specific mass loading (≈10 mg cm⁻²). Further, it displayed >95% coulombic efficiency and electrochemical stability as described in Fig. 2.8d (95% specific capacitance retention over 4000 GCD cycles, attributed to the capability of the scaffolds to successfully withstand structural fluctuations during fast and large GCD cycles (Shim et al. 2013).

Fig. 2.8

(a) Schematically shown detailed description of the Micrococcus cell envelope containing thick peptide-glycan, S-layer, and teichoic acid glycopolymers that bind the metal ions. (b) Schematic presentation of the process of single-step synthesis of 3D hierarchical Co₃O₄ nanostructures via assembling of cobalt oxides (green) by the bio-sorption of Co²⁺ directly onto bacterial surface at room temperature followed by subsequent redox reactions. (c) Cyclic voltammograms for the above material recorded at various voltage sweep rates in 3 M KOH at room temperature. (d) Cycling performance of the sample electrode in terms of specific capacitance (SC) and Coulombic efficiency while the inset shows variation of SC retention capacity after 4000 consecutive GCD cycling tests (Shim et al. 2013)

Another report was based on the fabrication of mesoporous NiO micro-ellipsoids obtained in aid of elliptical-shaped Deinococcus radiodurans bacteria as bio-templates, under ordinary reaction conditions, that recorded noticeable gravimetric capacitance of 237 F g⁻¹ @ 0.8 A g⁻¹ specific current in 6 M aqueous KOH electrolyte (Atalay et al. 2015). Bacteria, of the type Bacillus subtilis that are devoid of such S-layer proteins, use peptidoglycans and teichoic acid moieties as metal binding sites on their surfaces (Allred et al. 2005). Lately, rod-type cobalt oxide were obtained using Bacillus subtilis as soft templates under ambient conditions. Furthermore, porous Co₃O₄ hollow rods were produced on annealing at 300 °C that showed exceptional Li storage capability (Shim et al. 2011) .

Bacteria may as well be utilized as bio-templates for fabricating highly porous, large surface-based carbon materials for supercapacitors. Hierarchical porous carbons were produced via freeze-casting technique by assembling graphene oxide on the surface of Escherichia coli (Sun et al. 2012). The so-obtained sample possessed large surface area and porosity and accordingly delivered improved gravimetric capacitance of 327 F g⁻¹ @ 1 A g⁻¹ current density with adequate surface functionalities that promoted sufficient pseudocapacitive contributions besides electrical double layer capacitance. The material also displayed better electrochemical response in aqueous electrolyte in comparison to other popular carbon systems (Zhu et al. 2011). The microorganism Geobacter sulfurreducens containing high number of c-type cytochromes genes coding mainly survives by reducing metals. These c-type cytochromes can serve as effective electron reservoirs and, hence, perform as capacitors. They exhibit an elemental idea for the development of novel methods of energy generation in nature. For instance, it has been suggested that the capacitor behavior plays an important role for bioremediation of uranium (Malvankar et al. 2012).

2.3.2 Fungi as Bio-templates

Fungi like bacteria also demonstrate another effective source of bio-templates owing to their extreme metal bio-accumulation efficiency (Selvakumar et al. 2014). Fungi display a variety of biomineralization effects as well as their filamentous mycelium offering mechanical substructure for efficient mineral loading. For example, the mold Neurospora crassa has been often employed as an effective bio-template for the production of mineral-based composites with carbonized fungal biomass, possessing large charge storing capacity (Zhang et al. 2017). Fungal Mn biomineralization with high gravimetric capacitance >350 F g⁻¹ and good cycling stability has been reported (Li et al. 2016). Further, porous, nickel oxide nano-tubular structures were obtained using cylindrical-shaped Cladosporium cladosporioides fungi as bio-templates via chemical precipitation technique. The material delivered high capacitance value of 334 F g⁻¹ @ 0.8 A g⁻¹ current density, with 95% capacitance retaining efficacy even after 1000 GCD cycles tests (Atalay et al. 2016). In a recent study, La-based nanostructured materials were obtained via chemical precipitation technique using Cladosporium cladosporioides hyphae as bio-template which was then annealed at high temperatures. The resultant porous material displayed good capacitive response recording and very large gravimetric capacitance of 2190 F g⁻¹ @ 2 mV s⁻¹ voltage sweep rates, in 0.5 M Na₂SO₄ aqueous medium (Atalay et al. 2017).

2.3.3 Viruses as Bio-templates

Viruses comprise two chief constituents—core with genetic information and protective shell made of mostly protein moieties (Vilona et al. 2015). Their protein shell often assists biomineralization/bio-metallization processes as the amino acids show large metal ions affinity (Fischlechner and Donath 2007). Thus, tobacco mosaic virus (TMV), M13 bacteriophage, cowpea mosaic virus, etc. because of their non-pathogenic behavior toward humans and other living systems have been widely employed for the purpose (Selvakumar et al. 2014; Douglas and Young 2006). Three-dimensional hierarchical Ni/NiO electrodes were obtained via bio-template mediated electro-less synthesis of Ni-coated TMVs as nano-3D-current collectors, self-assembled on gold-coated Si-micro-pillar arrays as shown in Fig. 2.9a. The large aspect ratio-based morphology confers enhanced surface area, depending on density and height of the columnar arrays. Such unique geometry enabled better mass loading of the electroactive materials as a result of which the rationally designed electrode reported about 32.6 times increase in areal charge capacity in contrast to pure-planar electrode, as depicted in Fig. 2.9b, thus indicating crucial role of surface area and mass loading capacity in promoting electrochemical efficiency (Chu et al. 2016).

Fig. 2.9

(a) Schematic indication of enhanced surface area for the Si-micropillar array structure over planar one. (b) Relative variation of areal discharge capacity for the initial 500 cycles for both the nanostructured—Ni/NiO and planar—Ni/NiO electrodes, respectively (Chu et al. 2016)

Tobacco mosaic virus (TMV) macromolecules show facile behavior of forming intense bio-nano-scaffolds layers within very short time period (Vilona et al. 2015; Lomonossoff and Wege 2018). Thus, with the aid of electro-less plating and thermal oxidation processes, the obtained large surface-based nano-NiO electrodes demonstrated 3.6-fold rise in areal capacitance in comparison to simple NiO planar structures. Hence, easy photolithography and self-assembly techniques stand fine to absolutely reduce the necessity of high costing sophisticated deposition procedures (Zang et al. 2017). Atomic layer deposition of polycrystalline RuO₂ was carried out on TiN/Ni/TMV hetero-structures using genetically modified TMV as bio-template (Gnerlich et al. 2013). The so-obtained composite in combination with nafion as solid proton-conducting electrolyte recorded appreciable charge storage capacity with high capacitance retaining efficacy of 80% yet after undergoing continuous 25,000 GCD cyclic tests (Gnerlich et al. 2015). Genetically engineered M13 bacteriophages have become one of the useful toolkits in fabricating several nano-hybrid materials for EES devices. For example, non-covalently bonded engineered M13 virus and graphene were found to enhance the dispersion character of the graphene at low pH and high ionic strength medium (Oh et al. 2012). Additionally, inorganic materials anchored on such modified graphene sheets increased the overall conductivity and stability of the material considerably. Thus, M13 virus stabilized-graphene with bismuth oxy-fluoride nanocomposites demonstrated superior specific capacity of 131 mA h g⁻¹ @ high current density of 300 mA g⁻¹ (Oh et al. 2012).

2.4 Microbe-Based Carbon Materials as Supporting Matrix

The key issue for devising high-performing supercapacitors is to design flexible electrodes that guarantee outstanding electrochemical features along with mechanical/environmental stability (Wang et al. 2009; Kim et al. 2015; Chen et al. 2013a; Gwon et al. 2011; Maiti et al. 2014). The principal objective lies in optimizing the various determining factors such as reducing nanomaterial agglomeration, minimizing interfacial resistance, promoting faster electron transport, as well as augmenting charge diffusion kinetics along empty, porous channels which may boost the capacitance value closer to the theoretically estimated, besides imparting higher flexibility, robustness, and environmental stability keeping in view of practical usage in portable and wearable electronics, space, defense, as well as biomedical applications (Hyun et al. 2013; Tolle et al. 2012). In the recent past, several carbon nanocomposite materials based on functionalized graphenes and carbon nanotube (CNT) composites film have illustrated remarkable mechanical flexibility for designing robust energy storage devices that are well documented in the literature (Du et al. 2014; Kim et al. 2013). Despite the striking electronic conductivity features in these nanocarbons, their practical usage is limited by high fabrication cost, poor mass loading efficiency, and irretrievable agglomeration issues that results in inferior areal capacitances as well as shortened cycling life span (Long et al. 2014). Additionally, poor scalability and rigorous post-synthetic processes of such nanocarbon systems have largely restricted their commercialization. Thus, scientists have been endlessly devoting their efforts to find out alternative porous, conducting, and flexible carbon materials that would fulfill the criteria of abundant resources, scalability, easy fabrication, reproducibility, and environmentally compatible characteristics. Currently, microbe-based carbon materials have successfully satisfied the above criteria and accordingly have captivated many academicians in this domain of research (Zhou et al. 2012).

Thus, in this context, bacterial cellulose (BC) systems have gained considerable popularity as they offer excellent scaffolds for tailoring hybrid nanomaterials. The inherent surface hydrophilic functional groups such as –OH and –COOH facilitate hydrophilicity and high mass loading along with strong integration with electroactive materials like conductive polymers, metal oxides, and other semiconductors which strengthens their anchoring to the substrate (Kaewnopparat et al. 2008; Li et al. 2014b; Gao et al. 2013; Tang et al. 2015; Jana et al. 2017). Moreover, 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) radical-mediated oxidation of bacterial cellulose fibers has attracted great attention as the obtained products display superior aspect ratio and higher elastic modulus (Isogai et al. 2018). Further, hierarchical morphology facilitates easy accessibility of electrolyte ions to the reactive sites, thereby effectively speeding up diffusion-controlled processes. Compared with other substrates such as graphene paper or CNT films, BC paper provides additional yet sufficient void space and hence enables high electrolyte mass transfer (Deng et al. 2019b). However, BC membrane often possesses relatively low intrinsic electrical conductivity owing to absence of suitable charge carriers in their structure (Hu et al. 2011). To deal with the low conducting and capacitive behavior of BC, they are intimately blended with other conductive carbons such as doped activated carbons, CNTs, graphene nanosheets, as well as pseudocapacitive materials conducting polymers, metal oxides, etc. that considerably accelerate the charge transport in the composites and improve the electrochemical utilization of porous carbon materials (Dutta et al. 2017).

Rigorous investigations have frequently highlighted that conducting polymers though potentially display high pseudocapacitive performance but limited by poor electrochemical stability and structural instability, especially during high current rates and fast voltage scans which can be considerably improved by blending with bacterial cellulose (Meng et al. 2017). Studies reveal that one of the main reasons for such popularization of BC/conducting polymer nanocomposites has been its simplistic synthetic steps as schematically outlined in Fig. 2.10a, which also provides sufficient regulating reaction parameters to tailor diverse morphologies for improved capacitive performances as well as mechanical versatility.

Fig. 2.10

(a) Diagrammatic illustration of generalized fabrication strategy of conducting polymer-based BC binary nanocomposites. (b) Schematic outline of synthesis of 3D inter-connected holey graphene (HRGO)/BC nanocomposite electrodes. (c) Comparative study of tensile strength versus strain for HRGO/BC and pristine-BC air-dried films along with snap shots of 1.0-HRGO/BC composite films at different strained postures. (Reproduced on permission from Guan et al. 2018). (d) Schematically illustrated procedure of fabrication of bacterial cellulose/polypyrrole nanofiber/multi-walled carbon nanotubes ternary composite membrane (Li et al. 2014a)

Such binary nanocomposites have been successfully employed in fabricating binder-free, additive-free, current collector-free, flexible paper like supercapacitor electrodes for energy support to various wearable electronic devices (Luo et al. 2019). Further systematic explorations related to charge transport enhancement supported the fact that core-shell morphology have significant impacts on conductivity improvement in these composites. Accordingly, BC/PPY core-shell nanocomposites were fabricated by in situ oxidative polymerization of self-assembled pyrrole on BC nanofibers surface in dimethylformamide-water medium. The optimum composition of the nanocomposite exhibited appreciable improvement in electrical conductivity and boosted specific capacitance of 316 F g⁻¹ recorded @ 0.2 A g⁻¹current density, the value being much superior to that of plant cellulose-nanocrystal/PPy porous composites. Moreover, the electrochemical stability of the system showed only 10.8% of initial capacitance loss even after completing 1000 GCD cycles. It was concluded that the unique structure promoted better porosity features as well as favored larger active mass loading that resulted in such enhanced electrochemical performance in this nanocomposite (Wang et al. 2013).

Similar strategies were applied to design flexible electrode materials composites with widely explored 2D-graphene systems (Fang et al. 2016). Flexible supercapacitors were designed using combinations of holey reduced graphene oxide (rGO) and BC films by biosynthesis process that produced interesting results. A compact, regular, and aligned honeycomb inter-linked framework of the composite was generated on bacterial culture of the scattered functionalized graphene sheets attached to the BC nanofibers as shown in Fig. 2.10b (Guan et al. 2018). Different HGO concentrations such as 0.8, 1.0, and 2.0 mg mL⁻¹ were mixed with constant BC proportions to prepare the desired nanocomposite named as 0.8-HGO/BC, 1.0-HGO/BC, and 2.0-HGO/BC, respectively. The optimum composition 1.0-HGO/BC sample showed amazing tensile strength under various strained conditions as indicated in Fig. 2.10c, making the nanocomposite highly suitable for powering foldable electronics (Guan et al. 2018). Few more of such interesting results for various BC-based nanocomposites have been demonstrated in Table 2.3 (Bu et al. 2018; Li et al. 2014a, 2017; Cai et al. 2019; Liu et al. 2015a; Wang et al. 2012, 2016b; Xu et al. 2013, 2016).

Table 2.3 Various BC-based nanocomposites for energy storage applications

Further, to pick up the electrochemical efficiency of the BC-based binary nanocomposites, corresponding ternary and quaternary nanocomposites comprising hybrids of bacterial cellulose, conducting polymer, various nanocarbons, and pseudocapacitive metal compounds like oxides, sulfides, etc. with different morphologies were fabricated successfully, some of which have been illustrated in Table 2.3 as well for comparison (Jiao et al. 2019; Zhang et al. 2018; Li et al. 2017; Yao et al. 2018; Liu et al. 2015b, 2016b, c, 2017; Ma et al. 2016b, c; Peng et al. 2016, 2017; Wu et al. 2018; Yuan et al. 2018; Jiajia et al. 2020). Figure 2.10d represents schematically the common strategy for synthesizing ternary composites with BC, conducting polymer, and CNTs, which is rather simple, productive, as well as reproducible with minimum post synthetic hurdles (Li et al. 2014a). The same preparation scheme can be generalized for fabricating several other BC-based ternary nanocomposites too. In most cases, BC serves as porous carbon network matrix with heteroatoms functionalized surfaces that promote easy anchoring of graphenes and conducting polymers to form flexible composites with advanced electrochemical features. Various metal compound-based ternary BC composites have also designed. Especially the metal chalcogenides containing nanocomposites of the composition such as cobalt oxide/graphene/bacterial cellulose and polypyrrole/cobalt sulfide/bacterial cellulose have showed excellent areal capacitances than previous reports with artificially carbons composites (Liu et al. 2015b, 2016b). Herein as expected BC offers highly flexible supporting matrix with large surface area and adequate porosity to hold greater proportions of functionalized materials on its surface and also serve as effective scaffolds to form diversified nanostructures.

In the recent past, a quaternary nanocomposite based on PPy/RGO/CNT/BC was fabricated for symmetric flexible supercapacitor applications. The resultant optimized nanocomposite (PPy/RGO/CNT/BC₂₀)-based symmetric cell indicated superior capacitive signatures recorded at different potential sweep rates and varying current densities as indicated from CV and GCD profiles in Fig. 2.11a, b, respectively. In addition, it also responded to outstanding electrochemical reversibility, achieving ~83% capacitance retention efficiency even after 5000 cycling tests, as well as achieved noticeable areal energy densities at different power densities as specified in Fig. 2.11c, d, respectively. To investigate the mechanical stability of the resultant device, capacitance measurement was carried out at various bending frequencies that indicated a mere loss of 4.6% of original capacitance even after 800 bending cycles (Fig. 2.11e). Even the capacitance loss was negligible for various bending positions (Fig. 2.11f), signifying that the device works with similar efficiency even under deformation and thus ideally suitable for flexible energy storage usage (Bai et al. 2018a).

Fig. 2.11

(a) Cyclic Voltammograms of PPy/RGO/CNT/BC₂₀ symmetric supercapacitor at various voltage sweep rates. (b) GCD profiles of PPy/RGO/CNT/BC₂₀ symmetric supercapacitor at varying current densities. (c) Cycle stability of PPy/RGO/CNT/BC₂₀ symmetric supercapacitor at 1 mA cm⁻² at room temperature. (d) Ragone plot for the above sample at various areal energy-power densities. (e, f) Variation of capacitance retention efficiency and inset showing CV profiles of above device after repeated bent cycles (0th, 100th, 200th, 400th, 600th, and 800th) and at different bending angles (0°, 90°, and 180°), respectively (Bai et al. 2018a)

2.5 Microbe-Derived Carbons for Energy Storage Applications

Popular nanocarbons such as fullerenes, carbon nanotubes, graphenes, etc. have drawn remarkable recognition in energy storage applications because of their exceptional physicochemical characteristics of extensive surface area, advanced electronic charge transport, and outstanding mechanical flexibility (Obreja 2008; Borenstein et al. 2017; Chen et al. 2017; Dubey and Guruviah 2019; Zhang and Zhao 2009). These EDLC-based materials usually display stable and uniform charge-discharge rates, large cycling efficiency, but poor energy storing response owing to rapid agglomeration, layer restacking, and improper pore size distribution (Dubey and Guruviah 2019). Thus, they need to assemble with other suitable pseudocapacitive materials for better device efficiency (Zhang and Zhao 2009). Moreover, working with these materials often experiences low processability issues, high manufacturing costs, and rigorous/inhomogeneous functionalization steps which are mandatory to modify their properties conducive for advanced applications.

Compared to chemical methods, biological methods of synthesis are always preferred owing to their clean, green, and mild behavior along with high yield and reproducibility with special control on the structural and morphological aspects at molecular level (Enock et al. 2017). Therefore, biologically derived carbons using waste-biomasses, microbe-derived ones, etc. are getting importance as fantastic active host materials by virtue of their scalable production, morphological variety, and in situ heteroatoms doping advantages that would promote the material electronic conductivity, chemical adsorption efficiency, wettability, reaction activities, and electrochemical kinetics largely (Shang et al. 2020; Liu et al. 2018b).

One of the most common methods of fabrication of such bio-derived carbons includes biomass carbonization technology that involves thermo-chemical transformation of bio-materials at elevated temperatures under inert atmosphere such that mainly the carbon skeletons are retained while most of other unstable components getting eliminated (Biswal et al. 2013). It is to note that sp² carbon-based structures are advantageous for enhancement of electrical conductivity of the carbon materials. Hence, in this regard, the number of crucial production parameters such as selection of appropriate bio-source with high thermal stability, carbonization temperature and duration, heating rate, etc. along with other factors related to porosity control is important to control in order to derive required micro/nano-structure for desired applications.

Thus to obtain microbe-derived carbons, initially, the microbial cells grown in aqueous culture media are allowed to attain their optimum production yield (Nurfarahin et al. 2018). Then, they are normally harvested via centrifugation and washed thoroughly to eliminate culture medium residues and unwanted by-products formed at some stages in growth process. Subsequently, they are dried under oven-drying or freeze-drying to minimize the water content. The dried cells are then subjected to carbonization under optimum conditions. Often, additional chemical activation steps that are such as treatment with suitable reagents such as steam, supercritical fluids, etc. to introduce porosity as well as H₃PO₄, KOH, etc. to promote exfoliation and surface functionalities of the final products (Ukanwa et al. 2019; Wang and Kaskel 2012). However, chemical complexity of microbial cells limits understanding of detailed mechanisms of such activation procedures, but experimental results suggest that the pore size distribution and carbon yield can be systematically controlled by tuning activation and carbonization parameters. The following section highlights some of the interesting results related to microbe-derived carbons achieved mainly from bacteria and fungi sources that have been fruitfully applied as supercapacitor electrode materials. It is further to note that virus being much smaller in dimensions, very low content of carbon can be derived from them which may be one of the main causes of why virus derived carbons are rare in the scientific literature (Wei et al. 2016).

2.5.1 Bacteria-Derived Carbons for Energy storage applications

There are two varieties of bacterial cellulose (obtained from nata de coco)—one with loose fibrous morphology (freeze-dried) and the other dense paper type on pyrolysis at 950 °C followed by CO₂ activation (Lee et al. 2013). The loose fibrous form resulted in carbon-nanofiber material with too low carbon yield for further activation while the paper-like produced was activated successfully that resulted in activated graphitic carbon as evident from Raman spectroscopy, with peak intensity ratio of the characteristic D- to G-bands in the range of 2.2—2.8, comparable to commercial carbon fibers. The latter material with high surface area demonstrated good EDLC behavior in aqueous K₂SO₄ solution, in the voltage range of −0.2 to +0.2 V, recording gravimetric capacitance of 42 F g⁻¹ and large areal capacitance of 1617 F cm⁻², almost fourfold rise in capacitance than that of commercial carbon nanofibers (365 F cm⁻²), respectively (Lee et al. 2013). In another approach, composites of bacterial cellulose with varying sodium alginate were calcined at 700 °C followed by KOH activation. The resultant three-dimensional interconnected sheet-like hierarchical porous carbon nanomaterial was enriched with oxygen functionalities displaying high percentage of sp² carbons with good electrical conductivity. Systematic analysis revealed that KOH activation was essentially important for upgrading the capacitive behavior in these materials. The pseudo-rectangular CV profiles and very-triangular GCD curves of the optimized composition indicated ideal capacitive response in these activated derived carbons. The optimal composition delivered appreciable gravimetric capacitance of 302 F g⁻¹ @ 0.5 A g⁻¹ current density and high rate capacity of 75.2% recorded at high current density of 10 A g⁻¹, along with outstanding capacitance retaining efficiency of 93.8% beyond 10,000 GCD cycles in 6 M aqueous KOH electrolyte, in the voltage range of −1.0 to 0.0 V (Bai et al. 2018b).

High demands for miniatured kilohertz high-frequency electrochemical capacitors in support of filtering of ripple-current for AC/DC conversions as well as harvesting of natural vibration energy have urged cross-linked carbon nanofiber aero-gel fabrication obtained via fast microwave plasma pyrolysis of bacterial cellulose. To combat the small areal density of previously demonstrated electrodes at 120 Hz owing to thick electrodes, the as-prepared carbon nanofiber aero-gel film electrodes demonstrated appreciable areal capacitance of 4.5 mF cm⁻² at 120 Hz in an aqueous electrolyte. The electrode also showed widespread potential range of greater than 3 V in non-aqueous electrolyte as well (Islam et al. 2017, 2018).

Generally, organic resorcinol-formaldehyde or lignin-resorcinol-formaldehyde aerogels are delicate and brittle, and so they are now being replaced by carbon aerogels made with high aspect ratio carbon nanofibers obtained from carbonization of bacterial cellulose composites. These materials show larger surface area, better crystalline nature, advanced surface functionalities, mechanical flexibility, and charge transport features for superior adsorption and energy storage utilities. Exceedingly graphitized carbon aerogels obtained from BC nanofibers and lignin-resorcinol-formaldehyde polymer composite deliver outstanding areal capacitance with large mesoporous scaffold for electrolyte-ion transportation of ions and reversible deformation due to the interpenetrated networks. Hence, they report themselves as well potential candidates for flexible solid-state energy storage systems (Xu et al. 2015).

It has been well-recognized that heteroatom doping with oxygen, nitrogen, phosphorus, sulfur, etc. in nanocarbon materials has led to exceptional improvement in electronic properties that have urged their impeccable applications in various technological fields (Abbas et al. 2019). However, preparation of 3D porous carbons with uniformly doped heteroatoms is a huge challenge due to lengthy, complicacy, and expensive instrumentations as well as involvement of hazardous and toxic chemicals that acutely constrained their practical applications. Thus, methods involving easy, scalable, green, multifunctional, common strategies to design 3D heteroatom-doped nanocarbons are in the pursue. Chen et al. reported a facile, environmental benign, scalable procedure of fabricating three-dimensional (3D) phosphorus-doped; nitrogen, phosphorus co-doped and boron, phosphorus-co-doped carbon nanofiber networks via pyrolysis of bacterial cellulose treated with aqueous ortho-phosphoric acid (H₃PO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄), and ortho-boric acid/ ortho-phosphoric acid mixture (H₃BO₃/H₃PO₄), respectively. Among them, the P and N-co-doped carbon nanofibers recorded better capacitive signature with notable gravimetric capacitance of 204.9 F g⁻¹ @ 1.0 A g⁻¹ current density in 2 M aqueous H₂SO₄ electrolyte (Chen et al. 2014). Similarly, polypyrrole-coated bacterial cellulose composites on carbonization led to formation of 3D inter-linked large surface area-based N-doped carbon nanofiber networks that illustrated impressive results as both supercapacitor and Li-ion battery electrodes. The unique structure promoted smooth and adequate electrode/electrolyte interface areas, for faster charge transport pathways as well as high electronic conductivity (Lei et al. 2016). In an attempt to design free-standing, N-doped carbon material interlinked frameworks for supercapacitor applications, reduced graphene oxide-embedded bacterial cellulose was pyrolyzed and subsequently treated with urea as nitrogen doping agent. The engineered electrode material recorded utmost capacitance of 216 F g⁻¹ recorded @ current density of 1 A g⁻¹, showing ultimate capacitance loss of only 17% even after 10,000 GCD cycles tests (Chang et al. 2017). Hu et al. also employed NH₄H₂PO₄ impregnated BC pellicles to derive 3D N, P-co-doped porous carbon nanowires network structure via carbonization procedure. The resultant material with synergic doping of N and P heteroatoms reported capacitance of 258 F g⁻¹ recorded at current density of 1 A g⁻¹ with admirable electrochemical reversibility beyond 30,000 cycles. The symmetric supercapacitor with the sample displayed specific energy of 5.4 Wh kg⁻¹ at a specific power of 200 W kg⁻¹ and cyclic stability of 87% after 6000 GCD cycles (Hu et al. 2016). In another attempt, Zhu group demonstrated doping with various salts for encouraging usage in energy conversion devices derived from BC, obtained from Bacillus subtilis precursor. The as-prepared sample indicated improved capacitance than those of commercial carbons even at high current rates (Zhu et al. 2013). Shortly, high-power, flexible symmetrical supercapacitor was designed using nitrogen-doped carbon nanofibers resulting from ammonia-treated pyrolyzed BC source. The resultant device reported utmost specific power of 390.53 kW kg⁻¹ along with outstanding cyclic stability of 95.9% over 5000 GCD cycles (Chen et al. 2013b). Very recently, a novel approach was adopted to obtain large surface area-based, oxygen-doped porous carbon productively made via single-step carbonization cum activation procedure from bacterial cellulose, carboxymethyl cellulose, and citric acid composites. The so-obtained O-enriched carbon electrode recorded substantially improved specific capacitance of 350 F g⁻¹ recorded @ 0.5 A g⁻¹ current density with appreciable rate capability and electrochemical stability of 96% beyond 10,000 GCD cycles tests (Shu et al. 2020).

2.5.2 Fungi-Derived Carbons for Energy Storage Applications

Agaricus, a popular and naturally abundant mushroom variety, on being subjected to carbonization under inert atmosphere and subsequently KOH activation yielded mesoporous carbons. The resultant material displayed large surface area and good capacitance response of 196 F g⁻¹ at potential sweep rate of 5 mV s⁻¹, recording an operating cell voltage of 1 V in aqueous electrolyte along with good electrochemical constancy beyond 1000 GCD cyclic tests (Zhu et al. 2011). Agaricus was further used to obtain N, O-doped, hierarchically porous activated carbon frameworks with ultra-high surface area of 2264 m² g⁻¹, the doping level regulated by varying the mole ratio of KOH and carbon source. The so-prepared electrode material recorded appreciable capacitance of 158 F g⁻¹ in organic electrolyte, achieving a good capacitance retention efficiency of 93% even under 50 times rise in current density as well as outstanding cyclic stability of 92% (capacitance retaining efficiency) after undergoing continuous 10,000 GCD cyclic tests (Wang and Liu 2014). A template-free strategy was employed to design three-dimensional inter-linked porous carbon (as depicted in Fig. 2.12a using yeast (S. cerevisiae) as precursors using dispersion, carbonization at different temperatures, and subsequent KOH activation procedures. The optimized derived carbon material (carbonization temperature at 750 °C) displayed outstanding capacitive response even at high voltage scan rates (Fig. 2.12b) as well as high current densities (Fig. 2.12c), recording utmost capacitance of 330 F g⁻¹ recorded at 1 A g⁻¹ current density along with enhanced electrochemical stability beyond 1000 continuous GCD cycles (Sun et al. 2013).

Fig. 2.12

(a) SEM image and (b) CV profiles of the optimized sample of inter-linked porous carbon using yeast (S. cerevisiae) carbonized at 750 °C at various voltage sweep rates and (c) Galvanostatic Charging/Discharging profiles of the same sample at varying current densities (Sun et al. 2013)

In another approach, systematic fabrication of derived carbons were carried out using the crown-top and stem of two different mushrooms, viz., Ganoderma lucidum and Calocybe indica, from white and brown rot classes, respectively. They were separately subjected to microwave-assisted H₃PO₄ activation, carbonization treatment, and subsequently potassium hydroxide activation to yield activated nanocarbons that largely varied in surface area, pore size, and capacitive properties. Among the two, the former sample showed high BET surface area of 2432.4 m² g⁻¹ with improved thermal properties and utmost specific capacitance of 271.94 F g⁻¹ in addition to good electrochemical stability beyond 10,000 GCD cycles (Gannavarapu et al. 2019). Guo et al. used a very common white fungus called Tremella, composed of heteropolysaccharide varieties to obtain highly activated O-functionalized highly porous nanocarbons with exceptionally large surface area of 3760 m² g⁻¹. The as-prepared material recorded good capacitance, advanced cycle performance in different aqueous electrolytes with wide cell voltage of 1 V in 6 M KOH, 1.6 V in 1 M Na₂SO₄, and high value of 3.0 V in pure ionic electrolyte EMIM BF₄, respectively. The symmetric cell setup using ionic liquid electrolyte recorded fine specific energy of 28 Wh kg⁻¹ even at high specific power value of 19,700 W kg⁻¹ (Guo et al. 2017). Aniline functionalized fungus was used as precursor for preparing N-doped carbon materials prepared by carbonization and then alkali-assisted (KOH) activation processes. The resultant carbon possessed high porosity and inter-linked arrangement that recorded high surface area of 2339 m² g⁻¹ with fast ion diffusion rate. The material displayed appreciable specific capacitance of 218 F g⁻¹ @ 0.1 A g⁻¹ current density in addition to exceptional electrochemical reversibility and rate capacity even beyond 5000 GCD cycles (Wang et al. 2017d). Similarly, S- and N-co-doped carbon fiber networks are designed using the composites formed from bio-concentration of various toxic organic dye pollutants with fungal hyphae as an effective green strategy of converting toxic wastes to important resources. Among them, the N and S-co-doped carbon fiber obtained from methylene blue dye bio-concentration on hyphae displayed utmost specific capacitance of 235 F g⁻¹ @ 1 A g⁻¹ specific current, undergoing a net capacitance loss of only 27.2% on 20-fold rise in current rate (Lei et al. 2018). Again, bamboo fungus as starting component was put through two-step pyrolytic procedures to obtain hierarchical, nitrogen-doped porous carbons with honeycomb structure. The optimized sample showed high surface area (1708 m² g⁻¹) and recorded utmost capacitance value of 228 F g⁻¹, good and even capacitive signatures. The symmetrical cell setup using the same recorded high specific energy of 4.3 Wh kg⁻¹ with almost no capacitance loss even after surviving continuous 10,000 GCD cyclic tests recorded at high current density of 10 A g⁻¹ (Zou et al. 2019). A nitrogen-doped 3D porous activated carbon network was produced on ZnCl₂ activation followed by high-temperature carbonization of mycelium pellets with thread-type chain morphology in presence of ammonium chloride. The obtained mass displayed nearly symmetric rectangular CV curves even at high potential scan rates, recording utmost capacitance of 237.2 F g⁻¹ at the voltage sweep rate of 10 mV s⁻¹, about 1.5 times superior to that of pure and undoped derived carbon analogue. Its unique morphology and surface functionalization synergistically improve the capacitive signature of the doped sample (Hao et al. 2018).

2.5.3 Microbe-Derived Carbon-Based Nanocomposites as Energy Storage Materials

The above discussion clearly indicates that the microbe-derived carbons have shown genuine encouraging results of better processability, charge transport properties, large surface and higher mass loading, porosity, and mechanical flexibility that have motivated their usage in fabrication of carbon-based nanocomposites for energy storage applications. However, to further improve their low theoretical capacitance values of these derived carbon materials, introduction of electroactive components is mandatory in order to achieve high power/energy density devices (Schopf and Es-Souni 2017). Such electroactive materials include semiconducting metallic compounds, conducting polymers, etc. with remarkable theoretical capacitances but limited with low conductivity, large and irreversible volume changes, sluggish charge transfer rates, and poor cycling performance leading to substandard electrochemical stability (Abdah et al. 2020). Thus, synergic cooperation of the components can eliminate their individual shortcomings, promoting higher conductivity and shortened charge transport pathways, as well as introduce improved reaction kinetics and morphological stability. Various nanocomposite materials have been synthesized using these carbons blended with either other carbon nanomaterials, like graphenes, CNTs, carbon nanofibers, and conducting polymers, or metal nanoparticles like Pd, Ag, etc. or metal compounds Fe₃O₄, Co₃O₄, Ni₃S₂, MnO₂, CoFe₂O₄, etc. which has been discussed in the subsequent sections.

In situ growth of polyaniline on bacterial cellulose followed by subsequent pyrolysis and KOH activated yielded N- and O-functionalized carbon powders. The resultant material displayed appreciable volumetric capacitance of 28.3 F cm⁻³, smooth charge transfer rates, and excellent cycle life of 100% over 2500 GCD cycles measured at specific current of 0.1 A g⁻¹ using PVA/H₂SO₄ gel electrolyte (Lv et al. 2017).

Since metallic compounds offer very high pseudocapacitance, a facile designing strategy for obtaining binder-free metal oxides anchored on the carbon papers derived from BC gel was formulated by impregnating desired metal ions within the gel followed by drying and then subsequently carbonizing under suitable conditions to produce the resultant electrode material. Self-supporting three-dimensional bacterial cellulose-derived carbon-fiber network blended with N-doped carbon-coated Fe₃O₄ obtained via combination of hydrothermal and carbonization processes were used for supercapacitor applications. The electrode material displayed large areal as well as volume capacitances of 1.36 F cm⁻² and 2300 F cm⁻³, respectively, @ 3 mA cm⁻² areal current density. In addition, the electrode also responded to appreciable cycle life undergoing only 11.5% capacitance loss beyond 4000 cycles within the working potential range of −1.2 to 0 V in aqueous KOH electrolyte (Lv et al. 2018). An asymmetric cell was assembled using three-dimensional networks of MnO₂ coated bacterial cellulose-derived carbon nanofiber and nitrogen-doped bacterial cellulose nanomaterial as positive and negative electrodes. The optimized gadget displayed a cell output potential of 2.0 V in presence of 1 M aqueous Na₂SO₄ electrolyte. Further, the cell also recorded appreciable specific energy of 32.91 Wh kg⁻¹ along with maximum power output of 284.63 kW kg⁻¹ and cycling efficiency of 95.4% after 2000 nonstop charging/discharging cycles (Chen et al. 2013c). In another report, nitrogen-doped carbon web-like structure was processed via carbonization of polyaniline-coated bacterial cellulose composite that was subsequently blended with MnO₂ (carbon-MnO₂) and assembled to form activated carbon (AC) // carbon-MnO₂ asymmetric cell configuration that displayed high specific energy of 63 Wh kg⁻¹ in 1 M Na₂SO₄ electrolyte, accomplishing a cell output potential of ~1.1 V along with 92% capacitance retaining efficacy even beyond 5000 cycles of GCD tests (Long et al. 2014). Ni₃S₂ nanoparticles were hydrothermally deposited onto carbon nanofibers (CNFs) obtained from carbonized BC, as depicted in the TEM image of Fig. 2.13a, illustrating large capacitance of 883 F g⁻¹ @ 2 A g⁻¹ current density as well as improved good cycle stability compared to its metal sulfide component in alkaline electrolyte. The asymmetric supercapacitor Ni₃S₂@CNFs//CNFs in aqueous KOH electrolyte recorded high operating voltage of 1.7 V in addition to high specific energy of 25.8 Wh kg⁻¹ @ specific power of 425 W kg⁻¹, undergoing only 3% capacitance loss even after continuous 2500 GCD cycles. The Ragone plot for the Ni₃S₂@CNFs//CNFs cell, as reflected in Fig. 2.13b, indicates much superior performance compared to other asymmetric supercapacitors made with synthetic carbon materials. The system was also successful in lighting LED; the corresponding setup has been shown in the inset of Fig. 2.13b, glowed for 3 min on being charged in just 20 s, indicating its capability as high-performance energy storage system (Yu et al. 2014).

Fig. 2.13

(a) TEM image of Ni₃S₂/CNFs sample. (b) Comparison of electrochemical performances based on Ragone plots of Ni₃S₂/CNFs//CNFs, Ni₃S₂/MWCNT-Nitrogen-doped bacterial cellulose pellicles (MWCNTNC)//AC (activated-carbon), graphene-nickel cobaltite nanocomposite (GNCC) //AC, graphene/MnO₂ //graphitic hollow carbon spheres (GHCS), and NiO//carbon asymmetric cell configurations. The inset shows red LED glow for the Ni₃S₂/CNFs//CNFs asymmetric device (Yu et al. 2014)

Lately, it has been perceived that aerogels with large surface area and mesoporous cross-linked morphology work as brilliant electrode materials in supercapacitors. Explorations executed with molybdenum oxides loaded on derived carbon papers obtained from BC gel, as expected, illustrated exceptional redox capacitance along with better charge transport kinetics offered by the interconnected fibrillar carbon matrix of the composite (Miyajima et al. 2016). High pseudocapacitive nickel sulfide was grown in situ on bacterial cellulose-derived carbon sheet aerogels (CA) that recorded not only capacitive performance as high as 1606 F g⁻¹ recorded at specific current of 1 A g⁻¹ but also high capacitive response even at large currents (69% of initial capacitance restored even at current density of 10 A g⁻¹), as well as achieving 91.2% capacitance retaining ability over 10,000 continuous CV cyclic tests, recorded at large potential sweep rate of 100 mV s⁻¹. Furthermore, the asymmetric supercapacitor NiS@CA//CA delivered specific energy of ∼21.5 Wh kg⁻¹ @ specific power of 700 W kg⁻¹ procuring a cell output potential of 1.4 V in aqueous KOH electrolyte, enduring cycling stability of ∼87.1% even after 10,000 CV cycles scans (Zuo et al. 2017b).

2.6 Conclusion and Future Prospects

This book chapter highlights on the fruitful correlation between microbe-derived substances and their electrochemical behavior to be functional for smart energy storage devices. Herein, current advancements on the fabrication and designing of microbe-derived supercapacitor electrodes have been detailed. Several microbes and their by-products have contributed as essential as well sustainable constituents for developing high-performance energy devices. This is made feasible by means of their exclusive capabilities toward large scalability due to fast rate of reproduction, biomineralization, tunable genetic modification, and self-assembling characteristics. Their superior structural stability and interconnected-morphology offer suitable matrix for easy accessibility of intercalating electrolyte ions to numerous electroactive centers; promote superior electronic conductivity; as well as exhibit greater potentiality in electroactive mass loading capacity. In addition, bacteria and fungi can be easily subjected to carbonization to yield mono-or multi-heteroatom-doped carbon compounds with tunable doping quantities that considerably influence as well as alter the composition, electronic properties, and surface characteristics, thus considerably upgrading the supercapacitive signatures. Further, the rationally designed nanocomposites prepared with these microbe-derived heteroatom-doped porous carbons under the influence of synergism report superior electrochemical performance compared to their other synthetic nanocarbon (such as graphene, etc.) material composites that is very impressing especially considering green state-of-the-art research and technology.

Though numerous interesting achievements have come up with this bio-synthetic strategy, till date a number of challenges do exist. Limitations of inferior productivity, poor control on material quality, contaminations, and difficulty in separations still exist in biosynthesis of nanomaterials!

Hence, to further improve the electrochemical performances of these microbe-based energy storage systems, proper selection of microorganisms—having unique characteristics, morphologies, and composition that can come up with desired heteroatoms—is to be carried out. In addition, target-directed synthesis using appropriate bio-compounds with high speculated charge storage capacities, high-quality physicochemical features, and mechanical flexibility can lead to advanced products. Moreover, cost-effective, scalable, and efficient synthesis techniques are urged that would significantly improve the conversion efficiency of the microorganisms to high-quality porous carbon materials. Additional stress has to be imposed on parametric investigations related to the carbonized ash contents, their nature, porosity, etc., and accordingly their influence on electrochemical behavior must be projected in the near future. Further detailed insight on the mechanism and transformation of phases in the biomineralization process that result in the formation of in situ inorganic nanomaterial as well as thorough knowledge on the determining factors that guide their electrochemical behavior, molecular interactions, and chemistries are essential. Again, deep conception of the genetic engineering of viruses is essential that can lead to useful surface functional groups for better binding of electrode materials to the microbe-matrix. Importantly, innovations on the in situ characterization techniques and genetics-related computations are indispensable for finer interpretation of the dynamics and chemistry of self-assembly process, RNA or DNA chains modification strategies, and allied issues. Even though microbe-based materials and their derived products display high potentiality in energy-associated applications, serious issues related to their safety while handling and disposal, environment benignity of the degraded products, production costs in comparison to traditionally employed materials must be considered during commercialization.

Nonetheless, with the pace at which the research areas of microbial electrochemical science and technologies are progressing at present, it is obvious that microbe-based supercapacitor electrode materials will keep long-term promise and successfully address the energy problems of the society soon. Hence, the author hopes that this chapter will dish up as a scaffold for ongoing and energetic thinking in the reading minds that will certainly contribute in shaping and maturation of R&D of this interdisciplinary field of microbiology and electrochemistry for a better tomorrow!