Porous conducting polymer films

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

Given the nature of the polymer and the conduction pathway, a simple homogeneous model cannot be applied to thin conducting polymer film-electrolyte systems [27,28,31]. For thin films (< lOOnm) with pore sizes estimated to range from 1 to 4 nm, the porous surface-electrolyte interface will dominate the electrical and physical properties of the sensor. Since the oxidation of the porous surface occurs first, the interface properties play a major role in determining device response. To make use of this information for the immunosensor response, the appropriate measurement frequency must be chosen to discriminate between bulk and interface phenomena. To determine the optimum frequency to probe the interface, the admittance spectra of the conducting polymer films in the frequency range of interest are required. 

Li K., Diaz D. C., He Y., Campbell J. C., and Tsai C., Electroluminescence from porous silicon with conducting polymer film contacts, Appl. Phys. Lett., 64, 2394-2396,... 

As mentioned, Go can be determined separately performing the same step experiment in pure supporting electrolyte. It is of importance to note that adsorption usually influences the interfacial capacitance thus if R or O is adsorbed, the capacitive components determined in the absence or presence of the adsorbing species, respectively, will differ from each other. For films - especially in the case of porous conducting polymer - Cd evaluated for metal/electrolyte and metal/film/electrolyte systems, respectively, may differ by orders of magnitude. 

A number of biomolecules have been physically immobilised on conducting polymers [66,112, 116-119]. This is the simplest method of enzyme immobilisation. Since the binding forces involved are hydrogen bonds, van der Waals forces, etc., porous conducting polymer surfaces are most commonly used. The pre-adsorption of an enzyme monolayer prior to the electrodeposition of the polymer, [120] and two-step enzyme adsorption on the bare electrode surface and then on PPy film [121] have also been investigated. 

Other templates have also been used. Polypyrrole nanowires have been produced by growing the polymer in porous alumina. Highly porous conducting polymers have also been produced using the inverse opal method [38], in which the polymer is deposited around a matrix of tightly packed spheres, which form the synthetic opal. When the spheres are removed, a highly porous film is left with... 

Lauerhaas JM, Sailor MJ (1993) The effects of halogen exposure on the photoluminescence of porous silicon. Mater Res Soc Symp Proc (USA) 298 259-263 Lees IN, Lin H, Canaria CA, Miskelly GM et al (2003) Chemical stability of porous silico surfaces electrochemically modified with functional alkyl species. Langmuir 19 9812-9817 Li K, Diaz DC, He Yet al (1994) Electroluminescence Ifom porous silicon with conducting polymer film contacts. Appl Phys Lett 64(18) 2394-2396... 

Fletcher proposes adopting a porous electrode model, considering the conductive polymer film in contact with an aqueous electrolyte solution as consisting of a large number of identical, noninterconnected pores. The electrolyte solution is contained within the pores. The analysis then considers a single pore of uniform cross section. Three general impedance elements are considered the solution impedance x within the pore the interfacial impedance y between the solution within the pore and the pore wall and z, the internal impedance of the polymer. The latter quantities are assumed not to vary with distance inside the pore. 

In addition to the criticisms from Anderman, a further challenge to the application of SPEs comes from their interfacial contact with the electrode materials, which presents a far more severe problem to the ion transport than the bulk ion conduction does. In liquid electrolytes, the electrodes are well wetted and soaked, so that the electrode/electrolyte interface is well extended into the porosity structure of the electrode hence, the ion path is little affected by the tortuosity of the electrode materials. However, the solid nature of the polymer would make it impossible to fill these voids with SPEs that would have been accessible to the liquid electrolytes, even if the polymer film is cast on the electrode surface from a solution. Hence, the actual area of the interface could be close to the geometric area of the electrode, that is, only a fraction of the actual surface area. The high interfacial impedance frequently encountered in the electrochemical characterization of SPEs should originate at least partially from this reduced surface contact between electrode and electrolyte. Since the porous structure is present in both electrodes in a lithium ion cell, the effect of interfacial impedances associated with SPEs would become more pronounced as compared with the case of lithium cells in which only the cathode material is porous. 

The novel porous materials have many potential applications including ultrafiltration, conductive polymers, polymer composites, etc. As reported in the previous section, preliminary experiments on characterization of deposited film on graphite electrode shows promising results. Work is currently in progress to characterize such films and study the effect of the incorporated surfactant on different electrochemical reactions on a variety of electrodes. 

A third category comprises conducting polymers. The film-forming anodic polymerization of monomers, e.g., pyrrole, leads in the majority of cases to porous or even biporous [28] polymer layers, adhering to the substrate. The porosity can be improv at higher current densities, but the overoxidation limit must be considered. Another improvement is possible in terms of the application of graphite felt as a substrate [28, 58, 455]. Last but not least, the co-deposition of dispersed c.b.s in the electrolyte leads to composites with up to 65 wt% c.b. in the polymer layer for... 

The corresponding time constant mil be T(2 = 9 x 10 s, and the corresponding characteristic frequency mil be f(2 = 7 x 10 Hz or 700 MHz. This frequency is well above the capabilities of electrochemical impedance instrumentation. Thus, the capacitive loop corresponding to the outer layer will not be observed experimentally. The resistance of the layer influences measurements at all frequencies thus, the presence of a growing layer thickness will be manifested as an apparent increase cf the Ohmic resistance. For the situation described in this example, the circuit shown in Figure 9.5 should be amended as shown in Figure 9.6P- The ability to measure the capacitive loop associated with the outer porous layer does not depend on layer thickness, but it is sensitive to the effectixje conductivity of the layer. The effective conductivity of paints and polymer films is much... 

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. 

---