**2. Batteries and super-capacitors**

The desire to increase the energy density of electrical cells or batteries (collection of cells) is what drives research in electrical energy storage systems. From the Zn/ZnCl2 Leclanché cell to the modern Li-ion or Li-polymer batteries, what the scientist and engineers have been seeking are larger capacities (Coulombs, or Ampere-Hours delivered) [3]. All batteries consist of two

© 2013 Cardona and Santiago-Avilés; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. electrodes, a salt-bridge (permeable membrane that allows for the transport of ions), and electrically conductive liquid called the electrolyte. Batteries are very low impedance current sources, meaning that they have a small series resistance, this series resistance is kept small by design in order to reduce the time it takes to discharge the battery or charge the battery in the case of rechargeables [3]. Multiple challenges remain, such as better electrode materials (the density of Li ions "stored" in an electrode can be enhanced by intercalation, a phenomenon occurring in layered materials such as graphite, where the ions penetrate between the material layers), better electrolytes, the use of environmentally friendly materials (such as substituting Li for Mg as the active ion), and the use of inexpensive materials. Although electrical storage systems, such as batteries, are incapable of storing as much energy as liquid fossil fuels, for portable room temperature operations they avoid the burden that fossil fuels imposed as one try to extract the energy content stored in the chemical bonds, which require their oxidation or "burning" at high temperatures. Is evident that for this to happen at room temperature the combustion process must be thermally isolated, not a particularly inexpensive proposition, so under these circumstances batteries are a very good option.

Batteries are not the only charge storage device currently utilized or being researched. A relatively new breed of capacitors, called electrochemical capacitors, or depending on its formulation and operation, also called super-capacitors or pseudo-capacitors are been explored as potential energy storage devices [4]. The difference between these and the usual capacitors utilized in electronics / electrical applications is the capacitance or amount of charge stored per volt. The definition of a capacitor is therefore,

$$C = \frac{\underline{Q}}{V} \tag{1}$$

super-capacitor, and the second is called a redox pseudo-capacitor [4]. The first type, store charges utilizing the physical phenomenon of a double layer, where in the first layer lies negative charges (electrons) placed in the electrodes by a voltage bias, and the second layer is formed by mobile ions in an electrolyte. An EDL is formed when you immerse a conductor in an electrolyte, and proceed to bias the conductor. In that case, electrons are injected into the conductor, producing an attractive electric field to the positive ions in the electrolyte. These ions move to the surface of the conductor in order to minimize the distance between opposite polarity charges, forming the so-called inner Helmholtz layer. Since one can place more electrons in the conductor than there are ions in the electrolyte, not all the electric field from the conductor is cancelled, and more ions are accommodated increasingly farther away from the surface and the inner Helmholtz layer, forming the outer Helmholtz layer and the diffuse layer. The average distance from the conductor, where the diffuse layer ends, is called the

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Debye Length. See figure 1 for a schematic of the EDL.

**Figure 1.** Simplified graphical description of the Electrical Double Layer (EDL).

For the second type, one has one or two electro active polymers (EAP) electro-deposited onto a metallic current collector and bathed by a suitable electrolyte. When you place opposite polarity voltage bias in the two electrodes, the built-on electric field attracts ions from the electrolyte, which undergo redox reactions placing charges in the peripheral chemical groups

Where C is the capacitance value, Q the stored charge in Coulombs, and V the electric potential in Volts. In terms of materials parameters and dimensions, another simple version of this relation is given by:

$$C = \frac{kA}{d} \tag{2}$$

In this case k is a material parameter, namely the product of ε the relative permittivity, and ε0, the permittivity of free space, A, the effective capacitor area and d, the distance between the positive and negative charges. This last relation is fundamental in the understanding of these electrochemical super-capacitors functioning, as it is described in page 3 of this chapter.

#### **3. Electrical double layer and pseudo-capacitors**

There are two distinct types of electrochemical super-capacitors with different phenomena utilized as a charge storage mechanism. The first type is called an electrical double layer (EDL) super-capacitor, and the second is called a redox pseudo-capacitor [4]. The first type, store charges utilizing the physical phenomenon of a double layer, where in the first layer lies negative charges (electrons) placed in the electrodes by a voltage bias, and the second layer is formed by mobile ions in an electrolyte. An EDL is formed when you immerse a conductor in an electrolyte, and proceed to bias the conductor. In that case, electrons are injected into the conductor, producing an attractive electric field to the positive ions in the electrolyte. These ions move to the surface of the conductor in order to minimize the distance between opposite polarity charges, forming the so-called inner Helmholtz layer. Since one can place more electrons in the conductor than there are ions in the electrolyte, not all the electric field from the conductor is cancelled, and more ions are accommodated increasingly farther away from the surface and the inner Helmholtz layer, forming the outer Helmholtz layer and the diffuse layer. The average distance from the conductor, where the diffuse layer ends, is called the Debye Length. See figure 1 for a schematic of the EDL.

electrodes, a salt-bridge (permeable membrane that allows for the transport of ions), and electrically conductive liquid called the electrolyte. Batteries are very low impedance current sources, meaning that they have a small series resistance, this series resistance is kept small by design in order to reduce the time it takes to discharge the battery or charge the battery in the case of rechargeables [3]. Multiple challenges remain, such as better electrode materials (the density of Li ions "stored" in an electrode can be enhanced by intercalation, a phenomenon occurring in layered materials such as graphite, where the ions penetrate between the material layers), better electrolytes, the use of environmentally friendly materials (such as substituting Li for Mg as the active ion), and the use of inexpensive materials. Although electrical storage systems, such as batteries, are incapable of storing as much energy as liquid fossil fuels, for portable room temperature operations they avoid the burden that fossil fuels imposed as one try to extract the energy content stored in the chemical bonds, which require their oxidation or "burning" at high temperatures. Is evident that for this to happen at room temperature the combustion process must be thermally isolated, not a particularly inexpensive proposition, so

Batteries are not the only charge storage device currently utilized or being researched. A relatively new breed of capacitors, called electrochemical capacitors, or depending on its formulation and operation, also called super-capacitors or pseudo-capacitors are been explored as potential energy storage devices [4]. The difference between these and the usual capacitors utilized in electronics / electrical applications is the capacitance or amount of charge

<sup>=</sup> *<sup>Q</sup> <sup>C</sup>*

<sup>=</sup> *kA <sup>C</sup>*

Where C is the capacitance value, Q the stored charge in Coulombs, and V the electric potential in Volts. In terms of materials parameters and dimensions, another simple version of this

In this case k is a material parameter, namely the product of ε the relative permittivity, and ε0, the permittivity of free space, A, the effective capacitor area and d, the distance between the positive and negative charges. This last relation is fundamental in the understanding of these electrochemical super-capacitors functioning, as it is described in page 3 of this chapter.

There are two distinct types of electrochemical super-capacitors with different phenomena utilized as a charge storage mechanism. The first type is called an electrical double layer (EDL)

*<sup>V</sup>* (1)

*<sup>d</sup>* (2)

under these circumstances batteries are a very good option.

stored per volt. The definition of a capacitor is therefore,

**3. Electrical double layer and pseudo-capacitors**

relation is given by:

146 Advances in Nanofibers

**Figure 1.** Simplified graphical description of the Electrical Double Layer (EDL).

For the second type, one has one or two electro active polymers (EAP) electro-deposited onto a metallic current collector and bathed by a suitable electrolyte. When you place opposite polarity voltage bias in the two electrodes, the built-on electric field attracts ions from the electrolyte, which undergo redox reactions placing charges in the peripheral chemical groups of the EAP. If you find that this second type looks a lot like a battery, you are completely correct, it is a battery that can be charged and discharged quickly and for the electric circuit, if it behaves as a capacitor, it must be a capacitor. Since the phenomena mediating in the charge storage for this type of capacitor is chemical, they are generally addressed as pseudo-capacitors. From equation 2, one may note that capacitance increases with the effective area, which in the case of a DLC device, the area can be enhanced by using a high specific surface area (SSA) conductor such as activated carbon or any of a multiplicity of carbon allotropes [5] such as Graphene, carbon nano-tubes, carbide derived carbons, carbon onions and perhaps others.

## **4. Materials and models**

Some of these materials possess SSA as large as 3,000 m2 /g. The other geometrical parameter in equation 2 is d, the separation between oppositely charged particles, in this case, electrons in the electrode and ions in the electrolyte. As electrostatics demands, the mobile ions will lie on the surface of the carbon electrode pores, making d of nanoscopic dimensions. This combination of large numerator and small denominator leads to a large value of the capaci‐ tance, as large as several hundred Farads [4]. Super-capacitor in general consists of two capacitors in series, the one electrode biased negatively will attract the positive ions and the other, biased positively, will attract the counter-ions. For this reason, relation 3 gives the total capacitance, namely:

$$\frac{1}{C\_T} = \frac{1}{C\_1} + \frac{1}{C\_2} \tag{3}$$

electrolyte, and the separating membrane. The structure is symmetric, so the other side is pretty much the same sequence in reverse order. Figure 2 shows a schematic of a super-capacitor configuration and the inner details of its working. The simplest way of modeling a supercapacitor is by an ideal capacitor with a resistor in series. This resistor is a lump parameter, and includes all possible phenomena offering resistance (or impedance) to the flow of current, including the electrolyte, the electrodes, the current collectors, the external contacts, etc. One key aspect in the construction and improvement of EDLC is the ion permeable separator that prevents electrical contact between the conducting electrodes (shorting), but still allows ions from the electrolyte to pass through. Most of the membranes currently used, are made from polypropylene [6], a thermoplastic polymer that among other characteristics, can stand the chemical environment to which they are exposed. However, its use is limited due to UV-Vis

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**Figure 2.** Schematic of the super-capacitor configuration and inner working details (not to scale).

chemically inert to the electrolyte utilized, electrically insulating (R>108

Ideally a good separating membrane will be thin (10-15 µm), mechanically strong (1-4 GPa),

stable. The high resistivity of the composite membrane might be counter intuitive, as the filler are highly conductive carbon nano-tubes (CNT's). Remember that in order for the composite to conduct an electric current effectively (with low resistance), the CNT's must be touching each other making a conductive path from one electrode to the other, in a process called percolation. The amount of CNT's as filler added to a given amount of binder in the composite

Ω), and thermally

degradation and low biocompatibility.

**5. Separating membranes**

Note that some electrolytes are of the aqueous type (a salt dissociated in water) and other use more sophisticated organic liquids or ionic liquids, these last two, organic in origin and capable of sustaining larger voltages before degrading [4] as implied in the following relation (equation 4):

$$E = \frac{1}{2}CV^2\tag{4}$$

Where, E is energy in Joules, C capacitance in Farads and V electrical potential in volts. Both types of super-capacitors have the distinct characteristic that they can be charged and dis‐ charged very quickly (the DLC more than the redox). Since you are doing work in order to load energy into these devices, and how quickly you do work (or use energy) is power, these super-capacitors possess large power densities (power per unit weight, or gravimetric power density, or power per unit volume, or volumetric power density). Most super-capacitors share a common configuration. If we look at one of those devices edgewise, they possess the packaging (usually metallic or polymeric), a metallization as a current collector for one of the electrodes, the electrode material (usually a carbon allotrope or a EAP / composite), the electrolyte, and the separating membrane. The structure is symmetric, so the other side is pretty much the same sequence in reverse order. Figure 2 shows a schematic of a super-capacitor configuration and the inner details of its working. The simplest way of modeling a supercapacitor is by an ideal capacitor with a resistor in series. This resistor is a lump parameter, and includes all possible phenomena offering resistance (or impedance) to the flow of current, including the electrolyte, the electrodes, the current collectors, the external contacts, etc. One key aspect in the construction and improvement of EDLC is the ion permeable separator that prevents electrical contact between the conducting electrodes (shorting), but still allows ions from the electrolyte to pass through. Most of the membranes currently used, are made from polypropylene [6], a thermoplastic polymer that among other characteristics, can stand the chemical environment to which they are exposed. However, its use is limited due to UV-Vis degradation and low biocompatibility.

**Figure 2.** Schematic of the super-capacitor configuration and inner working details (not to scale).

## **5. Separating membranes**

of the EAP. If you find that this second type looks a lot like a battery, you are completely correct, it is a battery that can be charged and discharged quickly and for the electric circuit, if it behaves as a capacitor, it must be a capacitor. Since the phenomena mediating in the charge storage for this type of capacitor is chemical, they are generally addressed as pseudo-capacitors. From equation 2, one may note that capacitance increases with the effective area, which in the case of a DLC device, the area can be enhanced by using a high specific surface area (SSA) conductor such as activated carbon or any of a multiplicity of carbon allotropes [5] such as Graphene,

in equation 2 is d, the separation between oppositely charged particles, in this case, electrons in the electrode and ions in the electrolyte. As electrostatics demands, the mobile ions will lie on the surface of the carbon electrode pores, making d of nanoscopic dimensions. This combination of large numerator and small denominator leads to a large value of the capaci‐ tance, as large as several hundred Farads [4]. Super-capacitor in general consists of two capacitors in series, the one electrode biased negatively will attract the positive ions and the other, biased positively, will attract the counter-ions. For this reason, relation 3 gives the total

1 2

Note that some electrolytes are of the aqueous type (a salt dissociated in water) and other use more sophisticated organic liquids or ionic liquids, these last two, organic in origin and capable of sustaining larger voltages before degrading [4] as implied in the following

> 1 <sup>2</sup> 2

Where, E is energy in Joules, C capacitance in Farads and V electrical potential in volts. Both types of super-capacitors have the distinct characteristic that they can be charged and dis‐ charged very quickly (the DLC more than the redox). Since you are doing work in order to load energy into these devices, and how quickly you do work (or use energy) is power, these super-capacitors possess large power densities (power per unit weight, or gravimetric power density, or power per unit volume, or volumetric power density). Most super-capacitors share a common configuration. If we look at one of those devices edgewise, they possess the packaging (usually metallic or polymeric), a metallization as a current collector for one of the electrodes, the electrode material (usually a carbon allotrope or a EAP / composite), the

*E CV* = (4)

1 11 = + *C CC <sup>T</sup>*

/g. The other geometrical parameter

(3)

carbon nano-tubes, carbide derived carbons, carbon onions and perhaps others.

**4. Materials and models**

148 Advances in Nanofibers

capacitance, namely:

relation (equation 4):

Some of these materials possess SSA as large as 3,000 m2

Ideally a good separating membrane will be thin (10-15 µm), mechanically strong (1-4 GPa), chemically inert to the electrolyte utilized, electrically insulating (R>108 Ω), and thermally stable. The high resistivity of the composite membrane might be counter intuitive, as the filler are highly conductive carbon nano-tubes (CNT's). Remember that in order for the composite to conduct an electric current effectively (with low resistance), the CNT's must be touching each other making a conductive path from one electrode to the other, in a process called percolation. The amount of CNT's as filler added to a given amount of binder in the composite as to provoke this change in resistance is called the percolation threshold (PT). Although the PT for most polymers is small, for PLA at the highest concentration used in this study, the membrane resistance exceeded tens of MegOhms.

Besides polypropylene another common separating membrane is a composite layer of polytetra-fluoro-ethylene (PTFE or Teflon) between nylon or polyester layers. Polypropylene is a low-density linear polymer often used for its reasonable mechanical properties and low cost. The PTFE composite (GoreTexTM) [7] relies on the properties of PTFE whose mechanical properties are defined by the rate of the strain applied during its manufacture. If high enough, it produces billions of slit shaped nanoscopic pores per square inch, allowing the flow of ions but impeding the passage of particles or colloids. The polypropylene membranes are often weaved mechanically, but they can be electro-statically deposited by electro-spinning, a process that will be described shortly. Polypropylene is also notorious for its sensitivity to sunlight, in particular the UV part of the solar spectrum as mentioned above, where carbon bonds in their chain structures are attacked by the UV photons. The ultra-violet rays modify the affected bonds to form highly reactive free radicals, which then further react with atmos‐ pheric oxygen forming carbonyl groups in the main chain. These chemical transformations modify the polymer properties, in particular mechanical properties. The device might lose color and surface cracks will appear often leading to device failure. There are other types of membranes being explored for the particular application of electrochemical super-capacitors. Some work on bipolar membranes constructed using ion-exchange membranes (anions) and a cation exchange solution to form the bipolar structure. This proto-membrane is then coated with a NAFION layer. NAFION has been utilized using other formulations [8]. Sulfonated poly (ether ether ketone, SPEEK) has been used, as a proton conducting polymer membrane in similar applications [9].

optimization of cornstarch bacterial fermentation using a Lactobacillus strain, this is now a days the most often employed method to obtained the monomer. Poly-L-lactic acid (PLLA), the principal product of the reaction under controlled synthetic conditions, is chiral, that is, it rotates the polarization plane of light, that is why the L, for levo-rotation. PLLA has a crystal‐ linity of around 37%, a glass transition temperature between 60-65 °C, a melting temperature between 173-178 °C and a tensile modulus between 2.7-16 GPa. Interestingly, heat resistant PLLA can withstand temperatures of 110 °C (230F) [12]. These relatively high temperatures represent an adequate factor of safety for automotive and other traction applications of super-

O

H3C

O

O

L-Lactide Meso-Lactide D-Lactide

O

CH3

O

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D-Lactic Acid [0.5%]

OH HO

H3C H

O

H3C

O

O

O

CH3

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Besides its synthetic and physical properties advantages, PLLA is a biodegradable and biocompatible polymer approved by the FDA for food packaging and implantable medical

There are several examples in the literature of PLA and other polymers [13] been combined with other materials to form composites. In a composite, one mixes (or react) two or more materials (phases) with diverse properties, to combine the effect of those properties in the product material or phase. In our continual study of materials for permeable membranes, we have combined the ease of fabrication by electro-spinning fibers formation of polymeric materials, with the enhanced mechanical and electrical properties of carbonaceous allotropes materials, and some metallic colloids such as silver. Although diamond and graphite are the most commonly mentioned allotropic phases of carbon, there are multiple members in this family, from graphene through various types of carbon nano-tubes and bucky-balls, to carbon onions, they span a gamma of physical and chemical characteristics of substantial relevance to super / pseudo- capacitor technology [14]. More on polymer based composites will be

devices, allowing it use in EDL applications as for example pacemakers.

O

H CH3

L-Lactic Acid [99.5%]

HO

CH3

O

Dextrose

H3C

O

O

O

**Figure 3.** Synthetic routes for the production of Poly lactic acid

OH

capacitors.

presented latter in the chapter.
