**2.2. n-doping**

284 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**2.1. p-doping** 

electrons (*e-*

where MA-

(Pol0), *n* anions (A-

1986; Otero, 1999; Tsai et al., 1988).

of anion occurs during reaction:

in order to keep the electroneutrality.

**2. Electrochemical reactions in conducting polymers** 

Once generated (chemical or electrochemically), conducting polymers can be considered as both, classical unreactive materials having either a constant composition and a constant value of the magnitude of its physical and chemical properties; or new reactive materials varying their composition (polymeric chains, ions and solvent in different ratios) and properties along several orders of magnitude in a reversible way under electrochemical reaction (Otero, 1999), or promoted by redox agents present in solution (Kuttel et al., 2009).

When the polymer chains are oxidized, consecutive electrons are removed from each chain generating an excess of positive charges (holes) along the chains. This excess of positive charges (lack of electrons) promotes the repulsion between the polymeric chains and the generation of free volume between them. This free space is occupied by anions arriving from the solution to compensate the emerging positive charges (keeping the electroneutrality) and solvent molecules to keep osmotic pressure balance (Huang et al.,

When the polymer is generated in the presence of small anions, they can be exchanged by other small anions present in solution by electrochemical reactions so a prevailing exchange

> ( ) () ( ) ( )( ) ( ) ( ) <sup>0</sup> *<sup>n</sup> <sup>s</sup> sol s n <sup>m</sup> gel metal Pol n A m Solv Pol A Solv n e* <sup>−</sup> + − <sup>−</sup> + + <sup>+</sup> ←⎯⎯⎯⎯→

molecules of solvent (*Solv*) required to keep osmotic pressure balance.

where the different subscripts mean: *s,* solid; *sol*, in solution; *gel* indicates that the material is a gel formed by oxidized polymeric chains (Poln+) generated after the extraction of *n*

When the polymer is generated in the presence of a macroanion, due to its volume and the interaction with polymer chains, this macroanion cannot be exchanged by the electrochemical reaction keeping trapped inside the polymer film. So, in order to keep the electroneutrality, smaller cations are exchanged with the solution during the reaction:

( )( ) ( ) ( ) ( )( ) ( ) ( ) ( ) <sup>0</sup> *<sup>n</sup>*

Usually, the real redox process is not as easy as expressed by reactions (1) and (2): anions and cations are exchanged simultaneously (Hillman et al., 1989; Inzelt, 2008). Usually one of the previous interchanges prevails supporting the greater percentage of charge balance (Kim

et al., 2010; Lyutov et al., 2011; Orata & Buttry, 1987; Torresi & Maranhao, 1999).

) through the metal (indicated by subscript *metal*) from neutral polymer chains

*<sup>m</sup> n n gel <sup>n</sup> gel sol metal Pol MA C Solv Pol MA n C m Solv n e* − + + − <sup>+</sup> <sup>−</sup> ←⎯⎯⎯⎯→ +++ (2)

is the macroanion trapped inside the polymer film and C+ are cations exchanged

) coming from the solution to keep the gel electroneutrality and m

(1)

Some CP such as PEDOT (Ahonen et al., 2000), polythiophene (Arbizzani et al., 1995) or polyfluorenes (Ranger & Leclerc, 1998) have an electronic affinity high enough to allow transitions from the neutral state to a reduced state, storing negative charges (by electron injection) on the chains at high cathodic potentials. In this case, very stable solvents and salts are required, as electrolytes, to perform this redox reaction:

$$n\left(\operatorname{Pol}^{\circ}\right)\_{\boldsymbol{s}} + n\left(\operatorname{C}^{+}\right)\_{\boldsymbol{s}d} + m\left(\operatorname{Solv}\right) + n\left(\operatorname{e}^{-}\right)\_{\operatorname{mod}} \xleftarrow{} \xleftarrow{} \operatorname{Ind}\left[\left(\operatorname{Pol}^{\circ -}\right)\_{\boldsymbol{s}}\left(\operatorname{C}^{+}\right)\_{\boldsymbol{s}}\left(\operatorname{Solv}\right)\_{\boldsymbol{m}}\right]\_{\mathcal{g}d} \tag{3}$$

where Poln- represents the reduced polymer chains after insertion of n electrons. Here, in an analogous way compared to reaction (1), an excess of negatives charges promotes repulsion between polymeric chains generating the free volume that will be occupied by cations (exchanged with solution to keep electroneutrality) and solvent (exchanged with solution to keep pressure and osmotic balance).
