Polypyrrole/electrolyte interface

Ewing et al. did work (47) showing oxidation of ascorbic acid and dopamine occur at the polypyrrole-electrolyte interface. They discuss possible electrostatic interactions between the anionic solutes in polypyrrole and the negative functional groups of the sub strates. 

The broad nature of the current peaks in the voltammogram of conducting polymers such as poly pyrrole has been interpreted in a number of w one of which was to attribute it to the movement of anions across the polymei, electrolyte interface, a vital process if the overall charge neutrality of the film is to be maintained. The participation of the electrolyte in the electrochemistry of the polymer film is easily seen by comparing the response of polypyrrole in a variety of different electrolytes (see Figure 3.74). 

The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig. 10. 

Admittance spectra have been used to characterize conducting polymer-electrolyte interfaces for poly aniline [32], polypyrrole [33], and poly thiophene... 

Atomic force microscopy (AFM) and electrochemical atomic force microscopy (ECAFM) have proven usefiil for the study of nucleation and growth of electrodeposited CP films on A1 alloy [59]. AFM was used to study adhesion between polypyrrole and mild steel [60], whereas electric force microscopy (EFM) has been used to study local variations in the surface potential (work function) of CP films [61]. AFM with a conductive tip permits a nanoscale AC impedance measurement of polymer and electrolyte interfaces, permitting differentiation between highly conductive amorphous regions and less-conductive crystalline regions of the CP film [62]. 

In conclusion, the study of the overall process of electrochemical formation of polypyrrole must include not only the simple oxidation of monomer and the coupling of the charged species to produce the polymer chains, but also the nature, kinetics and effects on polymer structure and properties of all the parallel electrochemical and chemical processes which accompany it. Models of interfacial reactions, including the different processes taking place at the electrode/ electrolyte interface, must be developed showing the possibilities for the use of electrochemical methods of synthesis to obtain specific polymer films for each technological application. 

By comparing impedance results for polypyrrole in electrolyte-polymer-electrolyte and electrode-polymer-electrolyte systems, Des-louis et alm have shown that the charge-transfer resistance in the latter case can contain contributions from both interfaces. Charge-transfer resistances at the polymer/electrode interface were about five times higher than those at the polymer/solution interface. Thus the assignments made by Albery and Mount,203 and by Ren and Pickup145 are supported, with the caveat that only the primary source of the high-frequency semicircle was identified. Contributions from the polymer/solution interface, and possibly from the bulk, are probably responsible for the deviations from the theoretical expressions/45... 

In the course of original investigations, Pyo et al. investigated the effect of pH, rather than that of an electrochemical procedure, on the ion transport at the perchlorate-doped polypyrrole film/electrolytic solution interface [150]. Preliminary EC-AFM investigations allowed them to detect a 12.6% volume increase when the solution pH varied from 1.0 to 11.3 in a 50 mM NaC104 aqueous solution. 

Zhao, C., C. Wang, Z. Yue, K. Shu, and G. G. Wallace. 2013. Intrinsically stretchable supercapacitors composed of polypyrrole electrodes and highly stretchable gel electrolyte. ACS Applied Materials Interfaces 5 9008-9014. 

Interestingly, when the potential is made more positive so that the film becomes oxidized, the observed force-distance data remain the same. Similar effects were observed with polypyrrole films. This suggests that the positive charge in the film is fully compensated by negatively charged solution ions that can penetrate the polymer matrix either within the polymer strands or within pores in the polymer, as depicted in Fig. 11. Thus, at the conductive polymer-solution interface, no DL exists. This is in stark contrast to the behavior of metal electrodes in contact with aqueous electrolytes. 

The second important interface is found at the cathode, where the triiodide is reduced to iodide. While older cell types applied platinized indium tin oxide (ITO)-coated glass, novel concepts are using polypyrrol-coated ITO [7], because the catalytic activity of platinum led also to slow, but nonnegUgible degradation of the electrolytes. 

These films have also been used to improve the charge transfer efficiency of photovoltaic cells employing solid state electrolyte Polypyrrole was used to modify the interface between n-Si and polyethylene oxide-KI/I solid electrolyte in a thin film photoelectrochemical cell (see Fig. 11). This cell has an open circuit potential of 320 mV when irradiated with 100 mW/cm tungsten-halogen light. The photocurrent response and the stability of the cell was improved by an order of magnitude when the n-Si surface was precoated with 1--2 nm of platinum. 

Ruangchuay L, Sirivat A, Schwank J (2004) Electrical conductivity response of polypyrrole to acetone vapor effect of dopant anions and interaction mechanisms. Synth Met 140 15-21 Sakthivel M, Weppner W (2006a) Response behaviour of a hydrogen sensor based on ionic conducting polymer-metal interfaces prepared by the chemical reduction method. Sensors 6 284-297 Sakthivel M, Weppner W (2006b) Development of a hydrogen sensor based on solid polymer electrolyte membranes. Sens Actuators B 113 998-1004... 

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