**4.2.2 Oxygen Reduction Reaction (ORR)**

As early as in 1983, Bull et al. observed that Fe tetrasulfonated phthalocyanines-doped PPy film on GC electrode catalyzes the reduction of O2 at potentials 250-800 mV less negative than at bare GC or at PPy-coated GC electrode. Co-doped PPy films on metal electrodes also showed the electrocatalytic activity for O2 reduction (Ikeda et al., 1983). Osaka et. al. (1984)

Polypyrrole Composites: Electrochemical Synthesis, Characterizations and Applications 145

environmental friendliness. As S is an insulating material, the cathode material is combined with 30-55% carbon black (Jin et al., 2003; Song et al., 2004) of the total weight of the electrode materials. Recently, PPy has been used as an additive to improve the performance of anode and cathode materials in Li-ion batteries (Guo et al., 2005; Pasquier et al., 1999;

Very recently, Liang et al., (2010) observed that the morphology of the PPy combined with S shows a significant effect on the dispersion status and electrochemical behaviour of S. When S was highly combined with two types of PPy, G-PPy and T-PPy, by in-situ oxidation and co-heating methods, the cycle durability of the composite was more favourable with the T-PPy matrix in comparison with G-PPy matrix. Guo et al. (2005), for the first time, prepared a series of novel Si/PPy composites by high-energy mechanical milling techniques. These anode materials had high capacities characteristic of the Li-Si alloy sytem but substantially

Metal oxide powders such as LiCoO2 (Ohzuku & Ueda, 1994), LiMn2O4 (Tarascon et al., 1994), LiNiO2 (Kanno et al., 1994), and V2O5 (Leroux et al, 1996) have been considered as promising candidates for positive electrode materials in rechargeable Li batteries having high energy densities. In order to prepare these oxide electrodes, a conducting matrix and a binder are usually mixed with the oxide powder. PPy can serve as both as the conductor and binder. In view of this, composite electrodes of LiMn2O4 and PPy were prepared and studied for the charge-discharge properties of electrodes for 3V class (Gemeay et al., 1995) and also 4V class (Kuwabata et al., 1999) of Li-batteries. Results show that the PPy works well as a conducting matrix for the redox reaction of LiMn2O4 ↔ Li1-x Mn2O4 + x Li+ + x e- as well as like a capacitor and contributed to the capacity of the LiMn2O4/PPy composite. Recently, LiFePO4 has emerged as an important cathode material for lithium-ion batteries due to its high theoretical capacity (170 mAh/g), high potential (3.4 V versus Li/Li+), low cost, natural abundance and environmental friendliness of Fe. Bare LiFePO4 is an insulator with an electrical conductivity of about ~10-11 S/cm. To improve the electrical conductivity efforts have been made. Since PPy is a conductive polymer and also has lithium storage capacity in lithium-ion cells, a coating of PPy on the LiFePO4 particles would increase the electrical conductivity of the LiFePO4. With this idea, Wang et al (2005) prepared a series of PPy-LiFePO4 composite materials. The PPy-LiFePO4 composite electrodes demonstrated an increased reversible capacity and better cyclability, compared to the bare LiFePO4 electrode. The thin PPy film has also been used to make Li-batteries lighter and more flexible than the existing ones for portable electronic equipments. Wang et al. (2008) prepared highly flexible, paper-like, free-standing PPy and PPy-LiFePO4 composite film electrodes and observed that

LiFePO4 composite film had a higher discharge capacity beyond 50 cycles

To ensure long cycle life and safety, the use of graphite, among carbon materials, as anode for the Li-ion battery is favoured, however carbonaceous anodes exhibit capacity loss during the first intercalanation step. To minimize the capacity loss the graphite electrode was coated by a thin PPy film, the later is found to decrease the initial Erev capacity loss of the graphite anode (Veeraraghavan et al., 2002). The decrease in the Erev capacity loss has been ascribed to the reduction in the thickness of the solid electrolyte interface (SEI) layer. PPy/C (7.8%) gives the optimum performance based on the Erev capacity loss and the discharge

The research work on the use of PPy as electrode material of the aqueous based power sources has recently been also commenced. Grgur et al. (2008) obtained PPy thin film on

Veeraraghavan et al., 2002; Wang et al., 2006).

improved cyclability compared to bare Si anodes.

(80 mAh/g) than that of the cell with pure PPy (60 mAh/g).

the cell with PPy-

capacity of the composite.

studied the ORR at the PPy film electrode containing Co(III) tetrakis(sulfophenyl)porphine and observed a 4 electron reduction of O2 to H2O in H2SO4, contrary to the 2 electron reduction which usually occurs in this system in homogeneous state. The ORR at the GC/PPy/CoTSP ((tetrasulfonatopthalocyaninato)cobalt) electrode in 0.05 M H2SO4 showed apprx. 0.5 V more anodic onset potential than the value obtained with a GC electrode in 0.05 M H2SO4 containing 10-3 M CoTSP (Osaka et al., 1986). The electrodeposited Pt particles on PPy films were also active for the ORR (Vork and Barendrecht, 1989). PPy films containing nanometer-sized Pt particles (PPy/Pt), electrosynthesized from a solution containing Py and colloidal Pt particles, exhibited high catalytic activity towards the ORR (Bose & Rajeshwar, 1992).The ORR study has also been carried out on PPy films doped with anionic Co-species (Seeliger & Hamnett, 1992), tungstophosphate anions (Dong & Liu, 1994), Fe/Co phthalocyanines (Coutanceau et al., 1995), ferriporphyrin (FeTPPS) (Wu et al., 1999), mesotetra(4-sulfonatophenyl)porphine (TPPS4) (Johanson et al., 2005), and anthraquinone-2,7 disulfonate (AQDS) (Zhang et al., 2007). These modified electrodes exhibited good activity towards the ORR. The PPy-cobalt complex-modified C particles displayed electrocatalytic activity for four-electron reduction of O2 and the catalyst showed high stability against degradation after use for several hours (Yuasa et al., 2005). The catalytic reduction of molecolar oxygen on the PPy-Mn phthalocyanine film (GC/ITO-coated glass support) indicated the participation of the Mn center of the PPy in the reduction of molecular oxygen (Rodrigues et al., 2005). The ORR displayed a pathway of irreversible 2-electron reduction to form H2O2 on the PPy/AQDS (anthraquinonedisulfonate) composite film on GC at all electrolyte pH employed, the pH 6.0 buffer solution being a more suitable medium for the reduction of dioxygen (Zhang & Yang, 2007).

Recently, it has shown (Ding & Cheng, 2009) that electrochemically produced MnO2-PPy composite material is ORR active in 0.5M H2SO4 when compared to the pure PPy. Templatesynthesized Co porphyrin/PPy nanocomposite in a neutral medium catalyzes the ORR mainly through a 4-electron pathway, exhibiting excellent electrocatalytic activity (Zhou et al., 2007). Also, Pd-PPy/C nanocomposite efficiently catalyzes reduction of oxygen, with resistance to methanol oxidation, directly to water through a four-electron pathway (Jeyabharathi et al., 2010). C-supported CoPPy composite material exhibits the Tafel slopes, - 110 -120 mV (Millan et al., 2009). Very recently, Zhao et al. (2011) observed an enhanced electrocatalytic performance for the ORR on the AQS (anthraquinone-2-sulfonate)/PPy composite modified graphite electrode.

Investigations on the ORR at sandwich composite electrodes of PPy and Cu1.4Mn1.6O4 (Nguyen Cong et al., 2000; 2002a), PPy and NixCo3-xO4 (Gautier et al., 2002; Nguyen Cong et al. 2002b), PPy and CoFe2O4 (Singh et al., 2004), PPy and LaNiO3 (Singh et al., 2007a), PPy and La1-xSrxMnO3 (0≤x≤0.4) (Singh et al., 2007b) and La1-xSrxCoO3 (Singh et al., 2007c) were also carried out. The results have shown that the composite electrode had excellent catalytic activities as well as remarkable stability even in acid solutions wherin mixed oxide cathodes normally undergo deactivation.

#### **4.3 Rechargeable batteries**

Among the various types of rechargeable batteries, the Li/S battery system is a very attractive candidate for rechargeable Li-batteries due to its high theoretical specific capacity of 1672 mAhg-1 and theoretical power density of 2600 Wh kg-1 based on S active materials. The use of S as a cathode material is advantageous because of its abundance, low cost, and

studied the ORR at the PPy film electrode containing Co(III) tetrakis(sulfophenyl)porphine and observed a 4 electron reduction of O2 to H2O in H2SO4, contrary to the 2 electron reduction which usually occurs in this system in homogeneous state. The ORR at the GC/PPy/CoTSP ((tetrasulfonatopthalocyaninato)cobalt) electrode in 0.05 M H2SO4 showed apprx. 0.5 V more anodic onset potential than the value obtained with a GC electrode in 0.05 M H2SO4 containing 10-3 M CoTSP (Osaka et al., 1986). The electrodeposited Pt particles on PPy films were also active for the ORR (Vork and Barendrecht, 1989). PPy films containing nanometer-sized Pt particles (PPy/Pt), electrosynthesized from a solution containing Py and colloidal Pt particles, exhibited high catalytic activity towards the ORR (Bose & Rajeshwar, 1992).The ORR study has also been carried out on PPy films doped with anionic Co-species (Seeliger & Hamnett, 1992), tungstophosphate anions (Dong & Liu, 1994), Fe/Co phthalocyanines (Coutanceau et al., 1995), ferriporphyrin (FeTPPS) (Wu et al., 1999), mesotetra(4-sulfonatophenyl)porphine (TPPS4) (Johanson et al., 2005), and anthraquinone-2,7 disulfonate (AQDS) (Zhang et al., 2007). These modified electrodes exhibited good activity towards the ORR. The PPy-cobalt complex-modified C particles displayed electrocatalytic activity for four-electron reduction of O2 and the catalyst showed high stability against degradation after use for several hours (Yuasa et al., 2005). The catalytic reduction of molecolar oxygen on the PPy-Mn phthalocyanine film (GC/ITO-coated glass support) indicated the participation of the Mn center of the PPy in the reduction of molecular oxygen (Rodrigues et al., 2005). The ORR displayed a pathway of irreversible 2-electron reduction to form H2O2 on the PPy/AQDS (anthraquinonedisulfonate) composite film on GC at all electrolyte pH employed, the pH 6.0 buffer solution being a more suitable medium for the

Recently, it has shown (Ding & Cheng, 2009) that electrochemically produced MnO2-PPy composite material is ORR active in 0.5M H2SO4 when compared to the pure PPy. Templatesynthesized Co porphyrin/PPy nanocomposite in a neutral medium catalyzes the ORR mainly through a 4-electron pathway, exhibiting excellent electrocatalytic activity (Zhou et al., 2007). Also, Pd-PPy/C nanocomposite efficiently catalyzes reduction of oxygen, with resistance to methanol oxidation, directly to water through a four-electron pathway (Jeyabharathi et al., 2010). C-supported CoPPy composite material exhibits the Tafel slopes, - 110 -120 mV (Millan et al., 2009). Very recently, Zhao et al. (2011) observed an enhanced electrocatalytic performance for the ORR on the AQS (anthraquinone-2-sulfonate)/PPy

Investigations on the ORR at sandwich composite electrodes of PPy and Cu1.4Mn1.6O4 (Nguyen Cong et al., 2000; 2002a), PPy and NixCo3-xO4 (Gautier et al., 2002; Nguyen Cong et al. 2002b), PPy and CoFe2O4 (Singh et al., 2004), PPy and LaNiO3 (Singh et al., 2007a), PPy and La1-xSrxMnO3 (0≤x≤0.4) (Singh et al., 2007b) and La1-xSrxCoO3 (Singh et al., 2007c) were also carried out. The results have shown that the composite electrode had excellent catalytic activities as well as remarkable stability even in acid solutions wherin mixed oxide cathodes

Among the various types of rechargeable batteries, the Li/S battery system is a very attractive candidate for rechargeable Li-batteries due to its high theoretical specific capacity of 1672 mAhg-1 and theoretical power density of 2600 Wh kg-1 based on S active materials. The use of S as a cathode material is advantageous because of its abundance, low cost, and

reduction of dioxygen (Zhang & Yang, 2007).

composite modified graphite electrode.

normally undergo deactivation.

**4.3 Rechargeable batteries** 

environmental friendliness. As S is an insulating material, the cathode material is combined with 30-55% carbon black (Jin et al., 2003; Song et al., 2004) of the total weight of the electrode materials. Recently, PPy has been used as an additive to improve the performance of anode and cathode materials in Li-ion batteries (Guo et al., 2005; Pasquier et al., 1999; Veeraraghavan et al., 2002; Wang et al., 2006).

Very recently, Liang et al., (2010) observed that the morphology of the PPy combined with S shows a significant effect on the dispersion status and electrochemical behaviour of S. When S was highly combined with two types of PPy, G-PPy and T-PPy, by in-situ oxidation and co-heating methods, the cycle durability of the composite was more favourable with the T-PPy matrix in comparison with G-PPy matrix. Guo et al. (2005), for the first time, prepared a series of novel Si/PPy composites by high-energy mechanical milling techniques. These anode materials had high capacities characteristic of the Li-Si alloy sytem but substantially improved cyclability compared to bare Si anodes.

Metal oxide powders such as LiCoO2 (Ohzuku & Ueda, 1994), LiMn2O4 (Tarascon et al., 1994), LiNiO2 (Kanno et al., 1994), and V2O5 (Leroux et al, 1996) have been considered as promising candidates for positive electrode materials in rechargeable Li batteries having high energy densities. In order to prepare these oxide electrodes, a conducting matrix and a binder are usually mixed with the oxide powder. PPy can serve as both as the conductor and binder. In view of this, composite electrodes of LiMn2O4 and PPy were prepared and studied for the charge-discharge properties of electrodes for 3V class (Gemeay et al., 1995) and also 4V class (Kuwabata et al., 1999) of Li-batteries. Results show that the PPy works well as a conducting matrix for the redox reaction of LiMn2O4 ↔ Li1-x Mn2O4 + x Li+ + x e- as well as like a capacitor and contributed to the capacity of the LiMn2O4/PPy composite.

Recently, LiFePO4 has emerged as an important cathode material for lithium-ion batteries due to its high theoretical capacity (170 mAh/g), high potential (3.4 V versus Li/Li+), low cost, natural abundance and environmental friendliness of Fe. Bare LiFePO4 is an insulator with an electrical conductivity of about ~10-11 S/cm. To improve the electrical conductivity efforts have been made. Since PPy is a conductive polymer and also has lithium storage capacity in lithium-ion cells, a coating of PPy on the LiFePO4 particles would increase the electrical conductivity of the LiFePO4. With this idea, Wang et al (2005) prepared a series of PPy-LiFePO4 composite materials. The PPy-LiFePO4 composite electrodes demonstrated an increased reversible capacity and better cyclability, compared to the bare LiFePO4 electrode. The thin PPy film has also been used to make Li-batteries lighter and more flexible than the existing ones for portable electronic equipments. Wang et al. (2008) prepared highly flexible, paper-like, free-standing PPy and PPy-LiFePO4 composite film electrodes and observed that the cell with PPy-LiFePO4 composite film had a higher discharge capacity beyond 50 cycles (80 mAh/g) than that of the cell with pure PPy (60 mAh/g).

To ensure long cycle life and safety, the use of graphite, among carbon materials, as anode for the Li-ion battery is favoured, however carbonaceous anodes exhibit capacity loss during the first intercalanation step. To minimize the capacity loss the graphite electrode was coated by a thin PPy film, the later is found to decrease the initial Erev capacity loss of the graphite anode (Veeraraghavan et al., 2002). The decrease in the Erev capacity loss has been ascribed to the reduction in the thickness of the solid electrolyte interface (SEI) layer. PPy/C (7.8%) gives the optimum performance based on the Erev capacity loss and the discharge capacity of the composite.

The research work on the use of PPy as electrode material of the aqueous based power sources has recently been also commenced. Grgur et al. (2008) obtained PPy thin film on

Polypyrrole Composites: Electrochemical Synthesis, Characterizations and Applications 147

1.5 irradiation (100 mW/cm2), which was 92% of the energy conversion efficiency of the

Photoelectrochemical and electrochemical behavior of gold electrode modified with bilayers of PPy and PANI have been investigated in acid solutions (Upadhyay et al., 1995). Both PPy and PANI films on gold exhibit photo electrochemical activity, with the former showing a considerably high activity than the latter. PEC solar cells based on nanostructured ZnO/dye/PPy/ film electrode display excellent properties as anode in the conversion process of light to electricity (Hao et al., 2000). PPy was prepared on Ru-dye sensitized TiO2 nanoporous film and solar cell was constructed using gold as the counter electrode with PPy acting as the hole conductor (Cervini et al., 2004). Photodevices comprising covalently grafted PPy films on surface modified mesoporous TiO2 substrates via 3-(trimethoxysilyl) propyl methacrylate were fabricated and tested their performances with a counter electrode having a thin layer of gold (Senadeera et al., 2006). Significant enhancements in photoresponses were observed with the above additives in PPy than the reported devices comprising TiO2/PPy. Hybrid Cu-In disulphide/PPy photovoltaic structures prepared by electrodeposition exhibited (Bereznev et al., 2005) significant photovoltage and photocurrent under standard white light illumination. Electrodeposited Cu-In-Se/PPy PV structures exhibited the formation of a n-p barrier between the n-CuInSe and p-PPy layers (Bereznev et al., 2006). The PPy encapsulated TiO2 nanotube array (PPy/TiO2 NTs) electrode was also synthesized (Zhang et al., 2008) by electropolymerization to encapsulate PPy inside the TiO2

nanotube channels and walls in order to enhance the photocurrent density.

PPy films are being used in other areas such as sensors, bio-fuel cells, etc.

Different electrochemical methods for obtaining pure PPy, functionalized PPy and PPy composites are briefly described. Structural and electrochemical characterizations of new conductive materials and their application in solar cells, fuel cells, batteries, super capacitors and in corrosion protection have been highlighted. Studies available in literature have shown that the PPy has been used as carbon-substitute, particularly in fuel cells and batteries and found to greatly improve their performances.The use of active films or composites of PPy is very effective in enhancement of the capacitance of the supercapacitors. The PPy and its composites such as PPy/graphite have also shown favourable catalytic activity for I2/I¯ redox reaction. The PPy film has also improved the efficiency of the photo anodes in dye-sensitized solar cells significantly. PPy and PPy-based coatings are proved to be very effective inhibitors in the corrosion of oxidizable metals and alloys, stainless steels, mild steel, etc. Besides these,

Authors gratefully acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support through the research project (01/2320/09-EMR-II).

Abrantes, L. M.; Cordas, C. M.; Correia, J. P.; Montforts, F-P. & Wedel, M. (2000).

Electropolymerization of pyrrole substituted metalloporphyrins-synthesis and characterization. *Portu. Electrochim. Acta,* Vol.18, No.1, pp. 3-12, ISSN 1647-1571

DSSCs with Pt electrode.

**5. Conclusion** 

**6. Acknowledgement** 

**7. References** 

graphite electrode in 0.1M HCl galvanostatically and characterized as cathode material for the aqueous based rechargeable zinc batteries. Results have shown that Zn/PPy cell have potentially promising characteristic.
