**3.3 Phytomass-derived AC for supercapacitor electrodes**

Energy storage devices are the key components for a successful and sustainable world and supercapacitors (SCs) are one among them. SCs are able to supply considerable amount of power over a short time with extended cycle life. They offer a higher specific power density than most batteries and a higher energy density than conventional capacitors [43]. SCs are very useful in load leveling applications where a sudden boost of power is needed in a fraction of a second. More importantly, they do not release any heat during their operation and have a very long lifetime thus reducing the cost of maintenance. Also they do not release any hazardous substances that can damage the environment and their performance does not degrade with time. Hence for these reasons, SC is considered as a versatile technology that plays a prime role in partly fulfilling the energy demands of present and the future.

It is well known that certain physical features of the electrode materials determine the performance characteristics of the energy systems [44]. SCs making use of phytomass-derived porous carbon has the advantages like production of low cost carbon electrode components, environmental friendliness and good capacitive performance. Thus the search and research for advanced electrode materials is sought after and obviously very recently, phytomass-derived AC is providing unprecedented opportunities for researchers to design and fabricate innovative electrode materials for high performing SCs.

As far as research on phytomass-derived AC as electrode material for SCs is concerned, a wealth of information related to its preparation methodology, physical properties and electrochemical properties are available in the open literature in the form of reviews and research communications. The authors of this chapter have reported ample number of interesting work and to cite a few; on papaya seeds [45], onion peels [46] and recently on banyan prop root [47] for the possible application as electrode for SCs.

**Figure 6.** *LSV plot of (a) G-AC and (b) 1% Pt@G-AC electrodes in 1 M H2SO4 [42].*

One important concept in the charge storage mechanism is the pseudocapacitance which is dealt as follows. In an ideal double layer capacitor, energy is stored in electric double layer and no charge transfer occurs across the interface between the electrode and the electrolyte. However, it is possible that some redox reactions (faradaic) can still happen due to the existence of various heteroatoms like O, N & S present in the form of organic functional groups on the phytomassderived AC. The capacitance arising from these faradaic reactions is called pseudocapacitance. Therefore the total capacitance is a combination of capacitance contribution from electrostatic charges and faradaic charge transfer redox reactions [48] which is given in equation 1.

$$\mathbf{C}\_{\text{total}} = \mathbf{C}\_{\text{dl}} + \mathbf{C}\_{\text{f}};\tag{1}$$

where Ctotal is the total capacitance, Cdl is the electrical double layer capacitance, Cf is the pseudocapacitance. Thus by introducing functional groups onto carbon material, pseudocapacitance can be enhanced [49]. The faradaic charge transfer processes at the electrode involving N [50], S [51] and O are given below [52].


where C\* stands for the carbon structure, Ph and R respectively indicate phenyl and aliphatic groups. Therefore an electrochemical capacitor is called "supercapacitor" or "ultracapacitor". Further understanding on this topic can be had from references [53–55].

Having reviewed various aspects of capacitors and significant reported research, it is still relevant to search for newer and cheaper electrode materials and that too if the electrode materials could be derived from greener sources and waste phytomass then it will be a welcoming suggestion for the current scenario of energy crisis.

#### **3.4 Application of phytomass-derived AC as lithium-ion battery anodes**

Research reports on phytomass-derived AC as anodes in lithium-ion batteries seem not very much abundant as available for adsorption and supercapacitor electrodes studies. However, a few noteworthy studies for application as electrodes in lithium-ion battery (LIB) anodes are presented here. Zhang et al. [56] have produced carbons with a high surface area rice straw. They report that the hierarchical porous network with large macroporous channels and micropores within the channel walls enable the porous carbons to provide the pathways for easy accessibility of electrolytes and fast transportation of lithium ions These porous carbons which show a particular large reversible capacity are proved to be promising as anode materials for high rate and capacity LIBs. Bhardwaj et al. [57] synthesized carbon nanomaterials by pyrolysis of tea leaves and used as anode in LIBs. The highest specific capacity reported was 64 mAh g�<sup>1</sup> . Zhang et al. [58] used pinecone hull and activated at 800°C under CO2 atmosphere to obtain microporous carbon. This served as the anode for lithium secondary batteries and retained a discharge capacity of 357 mAh g�<sup>1</sup> and coulombic efficiency of 98.9% was reported to be achieved at higher current density of 10 mA g�<sup>1</sup> .

#### *Phytomass-Derived Multifunctional Activated Carbon as a "Wonder-Material"… DOI: http://dx.doi.org/10.5772/intechopen.99448*

Hwang et al. [59] obtained disordered carbon materials by pyrolysis of coffee shells at 800 and 900°C with KOH and ZnCl2 porogens. The first lithium insertion capacity was 524 & 603 mAh g<sup>1</sup> for the untreated samples pyrolyzed at 800 & 900°C respectively, while obtained 1150 & 1200 mAh g<sup>1</sup> for the KOH treated coffee shells pyrolyzed at the same temperature. Carbon powders of distinct and interesting morphologies were synthesized by pyrolyzing soapnut seeds, jack fruit seeds, date seeds, neem seeds, tea leaves, bamboo stem and coconut fibers, without using any catalyst. These carbon materials were utilized as the anode in LIBs [57]. Amongst the various precursors, carbon fibers obtained from soapnut seeds and bamboo stem, even after 100 cycles, showed the highest capacity of 130 & 93 mAh g<sup>1</sup> respectively. In yet another work, Stephan et al. [60] treated banana fibers with pore forming substances such as ZnCl2 and KOH. The BET surface area of the untreated carbon was 36 m2 g<sup>1</sup> and increased to 686 m2 g<sup>1</sup> & 1097 m2 g<sup>1</sup> for the carbons after treatments with KOH and ZnCl2, respectively. On employing these porogen treated carbons in LIBs, the specific capacities for the ZnCl2 treated sample was found to be 3123 m2 g<sup>1</sup> while it was 921 m2 g<sup>1</sup> for the KOH treated sample and for the untreated carbon, the specific capacity was extremely low as 625 mAh g<sup>1</sup> .

A comprehensive list of phytomass-based AC utilized in LIBs has been provided in Ref. [55]. Thus the foregoing reports clearly suggest that the phytomass-derived AC has ample scope for investigation for anode materials in advanced energy systems like LIBs.

#### **3.5 Electrochemical sensors**

Although a complete description on sensors is out of scope of this chapter, a bird's eye view is worthwhile. A sensor is a device which provides a usable output in response to a physical quantity and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical). Transducer is the active element of a sensor. A biosensor is an analytical device used for the detection of a chemical substance that combines a biological component with a physicochemical detector. The sensitive biological element, for e.g. tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids etc. is a biologically derived material or biomimetic component that interacts with, binds with or recognizes the analyte under study [61]. The biologically sensitive elements can also be created by biological engineering. The transducer or the detector element, which transforms one signal into another one, works in a physicochemical way such as optical, piezoelectric, electrochemical, electrochemiluminescence etc. resulting from the interaction of the analyte with the biological element to easily measure and quantify. A biosensor typically consists of a bio-receptor (enzyme/antibody/cell/nucleic acid/aptamer), transducer component (semi-conducting material/nanomaterial), and electronic system which includes a signal amplifier, processor and a display. There are three main types of electrochemical sensors namely; potentiometric, amperometric and conductometric. **Figures 7** and **8** respectively scheme the basic representation and components of a biosensor.

Electrochemical biosensors are an important type in sensor technology and have electrodes which translate the chemical signal into an electrical signal such as conductance, resistance or capacitance of the biosensor surface. Electrochemical sensors are able to detect many biomolecules in the human body such as glucose, cholesterol, uric acid, lactate, DNA, hemoglobin and blood ketones [64]. Thus they have great potential to detect diseases related to imbalances of biomolecules. Mostly, they are widely used for biosensing applications, however studies on biosensing–drug delivery applications are only limited. Enzyme- or protein-based electrochemical biosensors that have drug-release capability can be useful for the

**Figure 7.**

*(a) Basic schematic representation of a biosensor (b) output signal from a sensor [62] (adapted with modification).*

#### **Figure 8.**

*Components of a biosensor [63] (adapted with modification).*

treatment of various diseases. For example, xanthine oxidase enzyme catalyzes the production of hypoxanthine and xanthine and overproduction of these products cause renal failure [65].

High sensitivity, lower detection limits, automation, reduced costs of testing, and development of disposable devices and methodologies capable of working with very small sample volumes are some of the advantages associated with electrochemical biosensors. Also, electrochemical sensors are unaffected by sample turbidity or interference from absorbing and fluorescing compounds like spectroscopy-based techniques; they require comparatively simple instrumentation that requires low power and is portable. Use of electrochemical techniques, over optical and other transduction techniques, exhibits excellent sensitivity and a large linear detection range in a wide range of solvents, electrolytes, temperatures, etc. Electrochemical biosensors can be classified into voltammetric, amperometric, conductometric, impedimetric and potentiometric. Readers can have a complete understanding of various aspects of sensors, types and their applications from references [66–68].
