Pyrrole electropolymerization

Recently the mechanisms of pyrrole electropolymerization have been reviewed in Ref. [9b]. By the anodic reaction, an electron is withdrawn from the pyrrole monomers and cationic radicals are formed. The cationic radicals undergo a series of chemical-electro-chemical-chemical reactions and, as the result, the polymerization proceeds. If the cationic... 

A nitrate-selective potentiometric MIP chemosensor has been devised [197, 198]. For preparation of this chemosensor, a polypyrrole film was deposited by pyrrole electropolymerization on a glassy carbon electrode (GCE) in aqueous solution of the nitrate template. Potentiostatic conditions of electropolymerization used were optimized for enhanced affinity of the resulting MIP film towards this template. In effect, selectivity of the chemosensor towards nitrate was much higher than that to the interfering perchlorate ( o3 cio4 = 5.7 x 10-2) or iodide ( N03, r = x 10 2) anion. Moreover, with the use of this MIP chemosensor the selectivity of the nitrate detection has been improved, as compared to those of commercial ISEs, by four orders of magnitude at the linear concentration range of 50 pM to 0.5 M and LOD for nitrate of (20 10) pM [197]. 

Analytical application of oligopyrrole macrocycles 01CCC693. Mechanisms of pyrrole electropolymerization 00CSR283. Organometallic compounds of pyrroles 01AHC(79)115. 

After that, the use of oxalate salts was adopted, and in spite of a slight dissolution of the metal during the electropolymerization, very adherent (11.5 MPa adherence) and homogeneous films were obtained by Beck and Michaelis [14,15]. Some time later, the study was taken up again by Su and Iroh who studied the effects of various electrochemical process parameters on the synthesis and the properties of PPy electrodeposited on iron [89-90]. They confirmed that in acidic medium the pyrrole electrochemical polymerization takes place on the passivated electrode coated with crystalhne iron(ll) oxalate, and that PPy is deposited after the desorption of oxalate. As mentioned previously, the work of Shaftinghen, Deslouis et al. [91 ] showed that the protection properties of the material were dependent on the pyrrole electropolymerization conditions. 

M. Bazzaoui, J.I. Martins, E.A. Bazzaoui, T.C. Reis, and L. Martins, Pyrrole Electropolymerization on Copper and Brass in a Single-Step Process from Aqueous Solution, J. ofAppl. Electrochem., 34, 815-822 (2004). 

Pyrrole electropolymerization on noninert metallic substrates has been specially optimized for aluminum [33,34] and stainless steel [35,36]. In the case of aluminum electrodes, highly conductive polypyrrole films are obtained from solutions of t-butylammonium p-toluenesulfonate in acetonitrile [37]. Their conductivities range between 10 and 350 S cm", as a function of the electrical and chemical variables of synthesis. On stainless steel, highly conductive polypyrrole films can be obtained by means of square waves of potential [36]. In this case, charging of the electrical double layers, oxidation of pyrrole molecules, and formation of a porous oxide layer occur during the application of the anodic step and promote the polymerization process. The application of the cathodic potential seems to avoid corrosion... 

FIGURE 2 Possible reactions during pyrrole electropolymerization. 

The second problem associated with electrochemical synthesis of polypyrroles seems to be related to die difficulties found in correlating the polymer s properties with the conditions of synthesis. This has been derived from the widely spread idea of an overall understanding of the electrochemical mechanism. However, even the most simple electrochemical process of pyrrole electropolymerization involves different experimental variables in order to optimize polymer properties. These variables can be chemical, such as solvent or reactants (monomer and dopant salt), or physical, such as temperature, nature and shape of die electrodes, cell geometry or electrical conditions during synthesis. In addition, commonly the effects of all these variables are interdependent. 

From a mechanistic point of view, the most important feature of pyrrole electropolymerization (on inert electrodes and at low potentials where no parallel reactions coexist) is that it proceeds through an electrochemical stoichiometry, with n values in the range 2.0-2.7 Faraday/mol of reacting monomer [45-7]. From elementary analysis it has been deduced that the film-forming process needs 2 Faraday/mol, that it, 2 electrons per molecule. The excess of charge corre-... 

Table 10.3. Different reaction orders obtained by several methods, corresponding to pyrrole electropolymerization from acetonitrile solutions...

From kinetic studies we have experimental evidence to accept the coexistence of several processes during pyrrole electropolymerization. The relative rate of the different reactions, and thus the final composition and properties of the electrogenerated polypyrroles, will depend on the chosen parameters of synthesis. The reverse reasoning always is true and fundamental from a technological point of view defining a property, conditions of synthesis can be selected in order to optimize it. 

At present, pyrrole electropolymerization on noninert metallic substrates is specially optimized on A1 and stainless steel. In the case of A1 electrodes, Beck and co-workers have made a systematic study of... 

Figure 10.11. Storage efficiency (electrical charge stored in the film/electrical charge consumed during electropolymerization) versus the percentage of water content of acetonitrile solutions, during pyrrole electropolymerization at 800 mV (versus SCE).

Figure 10.16. Proposed mechanism for the effect of acetonitrile-water interactions on the mechanism of pyrrole electropolymerization.

Reaction 4 attempts to recover the considerable influence of electrolytes in empirical kinetics, through its discharge on the polymer. The formed radical is immediately transferred to a monomer molecule. This indirect initiation can explain the behaviour of pyrrole electropolymerization at high potentials in water [51]. The other electrolyte influences act on the metal oxide production [49], by stabilization of the monomeric radical cations [130] and through polymer oxidation. 

Alvarez-Romero, G. A., S. M. Lozada-Ascencio, J. A. Rodriguez-Avila, C. A. Galan-Vidal, and M. E. Paez-Hernandez. 2010. Potentiometric quantification of saccharin by a selective membrane formed by pyrrole electropolymerization. Food Chem. 

Pyrrole was first electropolymerized onto the bare Pt electrode to form PPy matrix by following the reported procedure using cycHc voltammetry (CV). The optimum of 10 cycles for pyrrole electropolymerization was used owing to its increased conductivity and large surface area as reported earHer for the effective incorporation of GNP. 

An interesting battery system which appears to be a non-Li system but on closer examination reveals the participation of the Li/Li redox couple is that described by Killian et al. [738]. This uses pyrrole electropolymerized into graphite fibers as both the anode and cathode, with a LiC104-EC-PAN-PC-acetonitrile gelled electrolyte. The reactions during battery discliarge are ... 

Fan, F.-R. R, Bard, A. J. In situ scanning tunneling microscopy of polycrystalline platinum electrodes under potential control. Copper electrodeposition and pyrrole electropolymerization, J. Electrochem. Soc. 1989, 136, 3216-3222. 

Sadki S, Schottland P, Brodie N, Sabouraud G. The mechanisms of pyrrole electropolymerization. Chem Soc Rev 2000 29 283-293. 

---