Electrodes polymer films

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

A second major event in the saga of polymer conductors was the discovery that the doping processes of polyacetylene could be promoted and driven electrochemically in a reversible fashion by polarising the polymer film electrode in a suitable electrochemical cell (MacDiarmid and Maxfield, 1987). Typically, a three-electrode cell, containing the (CH) film as the working electrode, a suitable electrolyte (e.g. a non-aqueous solution of lithium perchlorate in propylene carbonate, here abbreviated to LiC104-PC) and suitable counter (e.g. lithium metal) and reference (e.g. again Li) electrodes, can be used. 

Fig. 9.11 Schematic of the electrochemical p-doping process of polymer film electrodes, involving transfer of electrolyte anions A".

Ghosh PK, Bard AJ (1983) Qay-modified electrodes. J Am Chem Soc 105 5691-5693. Domenech A, Domenech-Carbo MT, Moya M, Gimeno JV, Bosch F (2000) Identification of inorganic pigments from paintings and polychromed sculptures immobilized into polymer film electrodes by stripping differential pulse voltammetry. Anal Chim Acta 407 275-289. 

Domenech A, Domenech-Carbo MT, Gimeno JV, Bosch F, Sauri MC, Casas MJ (2001) Electrochemical analysis using charge transfer coefficient/peak potential diagrams of the alteration in copper pigments from microsamples of baroque wall paintings attached to polymer film electrodes. Fresenius J Anal Chem 369 576-581. 

The relative fragility and preparative difficulty associated with monolayer-modified electrode surfaces hampered significant analytical progress for some time, and it was not until polymer-film electrodes were developed that the utility of modified electrodes in analysis could be demonstrated. 

Electrochemical polymerization is preferred to chemical polymerization, especially if the polymeric product is intended to be used as a polymer film electrode, thin layer sensor, in microtechnology etc., because the potential control is a precondition of the production of good-quality material and the polymer film is formed at the desirable spot that serves as an anode during the synthesis. 

Polymer-modified electrodes — A polymer-modified electrode or polymer film electrode can be defined as an - electrode in which at least three phases are contacted successively in such a way that between a first-order - conductor (usually a metal) and a second-order conductor (usually an -> electrolyte solution) is an elec-trochemically active polymer layer this polymer is, in general, a mixed (electronic and ionic) conductor [i], A transfer of electrons to solution species may occur at the two - interfaces (phase boundaries) and as a mediated reaction inside the film. The essential characteristic of polymer-modified electrodes is the mechanism of the... 

Polymer film electrodes are prepared either by evaporation technique or by -> electropolymerization. Redox polymers that are usually synthesized chemically are dissolved in a suitable solvent, placed as a droplet on the surface of a metal (or dip-coating, spin-coating techniques are applied) and the solvent is subsequently left to evaporate. The electrode can be used in other solvents in which the polymer is insoluble. Conducting polymer layers are usually developed by electropolymerization directly on the surface of the metal. 

FIG U RE 5.5 CV of a polymer film electrode modified with Cu-MOF immersed into 0.25 M HAc 4- 0.25 M NaAc, pH 4.85. Potential scan rate, 50 mV/sec. (From Domenech et al., 2007d. J. Phys. Chem. Clll, 13701-13711, with permission.)... 

Figure 5.6 shows CVs of (a) polymer film electrode (PFE) modified with Zn-MOF immersed into acetate buffer and (b) an unmodified GCE in contact with a 1.0 mM solution of Zn(NO3)2 in the above electrolyte. In both cases, although with different profiles, a reduction wave at highly negative potentials is recorded, followed, in the subsequent anodic scan, by a prominent stripping peak. 

FIGURE 8.22 Square-wave voltammograms for (a) as-synthesized, (b) calcined, and (c) steamed FeZSM-5 samples attached to polymer film electrodes immersed into 0.10 M HCl. Potential scan initiated at -1-0.85 V vs. AgCl/Ag in the negative direction. Potential step increment, 4 mV square-wave amplitude, 25 mV frequency, 5 Hz. (From Domenech et al., 2002e. J. Electroanal. Chem. 519, 72-84, with permission.)... 

FIGURE 9.12 SQWVs at monoclinic Ceo.ioZr,9oO,-modified polymer film electrodes deposited over glassy carbon for (a) 0.14 mM MDA plus 0.30 mM dopamine and (b) 0.14 mM MDA plus 1.28 mM ascorbate, in 0.50 M potassium phosphate buffer, pH 7.0. Potential step increment 4 mV square-wave amplitude 25 mV frequency 15 Hz. (From Domenech et al., 2004b. Electrochem. Comm. 6, 719-723, with permission.)... 

In addition to the modified electrodes described in the previous sections, which usually involve a conductive substrate and a single film of modifying material, more complicated structures have been described. Typical examples (Figure 14.2.4) include multiple films of different polymers (e.g., bilayer structures), metal films formed on the polymer layer (sandwich structures), multiple conductive substrates under the polymer film (electrode arrays), intermixed films of ionic and electronic conductor (biconductive layers), and polymer layers with porous metal or minigrid supports (solid polymer electrolyte or ion-gate structures) (6,7). These often show different electrochemical properties than the simpler modified electrodes and may be useful in applications such as switches, amplifiers, and sensors. 

The long term slope of the response to ImM acetaminophen was used to evaluate the various eleotropolymerized films formed on platinized RVC electrodes. The parameter of slope x 100 (m X 100) is used for convenience because some of the slopes were so small. The number of days that each film was studied is indicated by a minimum and maximum time for each type of polymer film electrode. At least three electrodes with each polymer film were studied, except for resorcinol (2 electrodes). A value of 2.0 or less was arbitrarily assigned to be an acceptable value for m X 100 because this is a clear Improvement over the 1,2-DAB polymer film. Also, a narrow range for these values is desirable as this indicates reproducibility of the polymer film. Resorcinol, despite the narrow range of slope is not a suitable polymer film (m X 100 values of 8.5 and 9.0) because the slopes are much greater than 2. 

Figure 11.40 Electrochemical cell for a separate investigation of the reactions on an iron electrode and on a conducting polymer film electrode. WEI, working iron/steel electrode WE2, polythiophene covered platinum electrode CE, counter electrode RE, reference electrode and P, potentiostat.

The adsorbed reactant will be oxidized or reduced essentially instantaneously, because it is already present on the electrode surface. It is the situation when mono-layer or submonolayer adsorption prevails F < 10 mol cm ). On the other hand, in the case of a large pseudocapacitance (e.g., any conductor covered with a molecular layer that contains more than one monolayer-equivalent of electroactive centers typical examples are the polymer film electrodes), Qc may be rather high and the current-time decay can reflect the diffusion rate of the charge carriers through the surface layer, thus at shorter times the decay of the current should conform to the Cottrell equation. 

II.10.3.3 Polymer Film Electrodes Electrodeposition and Ionic Exchange Processes... 

Lang G. and Inzelt G., Some problems connected with impedance analysis of polymer film electrodes effect of the film thickness and the thickness distribution, Electrochim. Acta, 1991,36,847-854. 

Fig. 11 Schematic of the electrode-electrolyte interface for a conductive polymer film electrode (after Ref. [47]).

Mizutani, F., S.I. Lijima, Y. Tanabe, and K. Tsuda (1985). Conducting polymer film electrodes with immobilized catalytic sites. J. Chem. Soc., Chem. Commun. 1728-1729. 

Oguro, K., Kawami, Y. and Takenaka, H. (1992). Bending of an ion-conducting polymer film-electrode composite by an electric stimulus at low voltage. Journal of Micromachine Society 5, 1, pp. 27-30. 

To prepare polymer film electrodes there are different techniques like dip coating, spin coating, electropolymerization from monomers, and molecularly imprinted polymers (MIPs) developed very recently. They are often used for both potentiometric and amperometric ion sensors. Some feature of different types of ion sensors based on polymer CMEs are shown in Table 1. Conducting polymer-based CMEs... 

The complete expression of the impedance contains 11 parameters. Based on the mathematical structure of (3.32), the parameters are expected to be strongly correlated. It was therefore indeed found that the number of parameters was decreased basis on reasonable assumptions. However, this was achieved in such a way that the contributions of the individual branches to the total capacity of the film could be determined. Figure 3.13 illustrates the goodness-of-fit. It was concluded that the low-lfequency distortion effect (CPE behavior) is most likely connected with the film s nonuniformity however, the surface roughness of the underlying metal substrate influences the ratio of the long to the short polymer chains. At low frequencies the characteristics of the impedance spectra are mainly determined by the long polymer chains. With the help of these models, reasonable values for the different parameters that characterize the polymer film electrodes can be derived. 

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