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The focus of this study was on the electrochemical synthesis and characterisation of polyaniline-modified electrodes.

The discovery of semi-conducting organic materials has opened new possibilities for electronic devices and systems because of their solution processibility, lightweight and flexibility compared to inorganic semiconductors. The combination of semiconductors with electrolytes, and more especially organic semiconductors and solid electrolytes has attracted the attention of researchers because of the multiple phenomena originating from the simultaneous motion of electrons and ions. This thesis deals with organic-based devices whose working mechanism involves electrolytes. By measuring electrochromism induced by the field in isolated segments of conjugated polymer films, which is in contact with an electrolyte, the direction and the magnitude of the electric field along an electrolyte is quantified (paper I). In addition, using a polyanionic proton conductor in organic field-effect transistor (OFET) as gate dielectric results in low operation voltage and fast response thanks to the high capacitance of the electric double layer (EDLC) that is formed at organic semiconductor/ polyelectrolyte interface (paper III). Because an electrolyte is used as a gate insulator, the effect of the ionic currents on the performance of an EDLC-OFET has been investigated by varying the relative humidity of the device ambience (paper IV). Since the EDLC-OFET and the electrochromic display cell both are operated at low voltages, the transistor has been monolithically integrated with an electrochromic pixel, i.e. combining a solid state device and an electrochemical device (paper V). Further, a theoretical study of the electrostatic potential within a so called pen-heterojunction made up of two semi-infinite, doped semiconductor media separated by an electrolyte region is reported (paper II).

<p>The behaviour of sulphur containing species has been investigated during the last few decades in various molten chloride and fluoride electrolytes but their effect on the performance of the cells producing aluminium still remains a subject of controversy. In the present work the electrochemical behaviour of sulphur containing species was studied in various molten chlorides and fluorides with the aim to contribute to a better understanding of the behaviour of sulphur impurities in the Hall-Héroult process.</p><p>The sulphur electrochemistry and chemistry in the systems containing molten electrolytes are of interest for aluminium electrowinning because of the serious impact of the sulphur containing gases on the environment and the effect of sulphur compounds on the efficiency of the electrolysis. The complexity of the sulphur chemistry is briefly summarised in chapter 2.1.</p><p>The literature review focused on the electrochemistry of sulphur species in molten salts is discussed in chapter 2 while chapter 3 is dealing with the description of the electrochemical and analytical methods used in this study. </p><p>Preliminary experiments in electrolytes consisting of molten NaCl and mixtures of NaF-AlF₃ revealed the necessity to find an inert electrode material for electrochemical studies. Platinum in the chloride melts and gold in the fluoride systems were selected as the most suitable materials for the working electrodes.</p><p>The behaviour of the anions containing sulphur in various oxidation states was investigated by means of cyclic voltammetry and chronoamperometry and is described in chapter 4.3. The behaviour of sulphate anions was studied in a single NaCl melt and compared with the molten CaCl₂-NaCl (10-90 mol%) mixture at 840 ºC. It was found that the sulphate reduction proceeds differently in these two electrolytes which was ascribed to the difference in the oxoacidity of the melts. The number of electrons transferred during sulphate (S^VI) reduction in pure NaCl melt was found to be two with probable sulphite formation which possibly decomposed to sulphur oxide. In the molten CaCl₂-NaCl mixture sulphate seemed to be transformed to SO₃which is further reduced in a one-electron exchange. The electrochemical signals recorded in the voltammograms also suggested that the character of the reduction products involved in those two melts is different. However, the “ec” mechanism, where a charge transfer is followed by a chemical reaction seemed to be the common feature in both electrolytes. The sulphate reduction was also studied in the eutectic LiCl-KCl mixture in the range of temperatures from 450 ºC to 840 ºC. In spite of the fact that the sulphate solubility seemed to rapidly decrease at temperatures below 700 ºC, the possible presence of a chemical reaction taking place after the reduction process of sulphate anions was suggested to be very similar to that found in molten NaCl.From the analysis of the data obtained by cyclic voltammetry followed that the reduction of sulphate anions to a lower oxidation state is probably diffusion controlled in all the three above mentioned systems at the sweep rates where the effect of the coupled chemical reaction can be neglected. Chapter 4 also includes an additional investigation of the behaviour of the sulphite and sulphide anions in the eutectic LiCl-KCl mixture.</p><p>Chapter 5 is devoted to the investigation of the sulphate species in the NaF-AlF₃ mixtures saturated by Al₂O₃ with varying cryolite ratio, i.e. CR = 3, 2, 1.5 and 1.2. It contains a description of sulphate modification at various experimental conditions and revealed its complicated and complex electrochemical behaviour which was monitored by cyclic voltammetry, chronoamperometry and square wave voltammetry. For the electroactive species in the electrolyte with CR equal to 3 two reduction steps were suggested involving three and two electrons respectively. The “ce” reaction mechanism was assumed where a chemical reaction seems to precede a charge transfer. The chemical step might be the partial decomposition of sulphate to SO₂. The sulphate reduction in the electrolyte with CR = 2 appeared to be considerably different. Since the temperature difference in these two systems is only 40 ºC the reason was likely to be due to the higher concentration of AlF₃ in the electrolyte. In this electrolyte only one cathodic process was observed and the reaction mechanism involving three electrons with a preceeding chemical reaction was suggested. Because the production of aluminium metal in the electrolysis cells operating with inert anodes is an attractive idea and have opened a question about the behaviour of sulphur species in very acidic electrolytes (high AlF₃ content) a part of the present studies was devoted to the study of the electrochemical behaviour of the sulphate anions in the electrolytes with CR = 1.5 and 1.2. Cyclic voltammetry and square wave voltammetry revealed that the sulphate is then reduced in two cathodic steps. Simulations and the obtained square wave voltammograms indicated that the cathodic process probably involves more electrons in the very acidic NaF-AlF₃ mixtures than in electrolytes with higher CR equal to 3 or 2. Observation of volatile species formed in the NaF-AlF₃ mixtures with CR = 1.5 and 1.2 was revealed as another important difference. This could be due to the formation of sulphur which at high temperature is present in the gaseous form and thus likely to escape from the laboratory furnace. The difference in the electrochemical behaviour of the sulphate anions in the cryolitic based melts was related to the difference in the bath composition and experimental temperature.</p><p>The present study shows the complexity and variability in the electrochemical behaviour of sulphur containing species as well as the difficulties and limitations resulting from the sulphur chemistry in the molten salts at high temperatures.</p>

The behaviour of sulphur containing species has been investigated during the last few decades in various molten chloride and fluoride electrolytes but their effect on the performance of the cells producing aluminium still remains a subject of controversy. In the present work the electrochemical behaviour of sulphur containing species was studied in various molten chlorides and fluorides with the aim to contribute to a better understanding of the behaviour of sulphur impurities in the Hall-Héroult process. The sulphur electrochemistry and chemistry in the systems containing molten electrolytes are of interest for aluminium electrowinning because of the serious impact of the sulphur containing gases on the environment and the effect of sulphur compounds on the efficiency of the electrolysis. The complexity of the sulphur chemistry is briefly summarised in chapter 2.1. The literature review focused on the electrochemistry of sulphur species in molten salts is discussed in chapter 2 while chapter 3 is dealing with the description of the electrochemical and analytical methods used in this study. Preliminary experiments in electrolytes consisting of molten NaCl and mixtures of NaF-AlF3 revealed the necessity to find an inert electrode material for electrochemical studies. Platinum in the chloride melts and gold in the fluoride systems were selected as the most suitable materials for the working electrodes. The behaviour of the anions containing sulphur in various oxidation states was investigated by means of cyclic voltammetry and chronoamperometry and is described in chapter 4.3. The behaviour of sulphate anions was studied in a single NaCl melt and compared with the molten CaCl2-NaCl (10-90 mol%) mixture at 840 ºC. It was found that the sulphate reduction proceeds differently in these two electrolytes which was ascribed to the difference in the oxoacidity of the melts. The number of electrons transferred during sulphate (SVI) reduction in pure NaCl melt was found to be two with probable sulphite formation which possibly decomposed to sulphur oxide. In the molten CaCl2-NaCl mixture sulphate seemed to be transformed to SO3 which is further reduced in a one-electron exchange. The electrochemical signals recorded in the voltammograms also suggested that the character of the reduction products involved in those two melts is different. However, the “ec” mechanism, where a charge transfer is followed by a chemical reaction seemed to be the common feature in both electrolytes. The sulphate reduction was also studied in the eutectic LiCl-KCl mixture in the range of temperatures from 450 ºC to 840 ºC. In spite of the fact that the sulphate solubility seemed to rapidly decrease at temperatures below 700 ºC, the possible presence of a chemical reaction taking place after the reduction process of sulphate anions was suggested to be very similar to that found in molten NaCl.From the analysis of the data obtained by cyclic voltammetry followed that the reduction of sulphate anions to a lower oxidation state is probably diffusion controlled in all the three above mentioned systems at the sweep rates where the effect of the coupled chemical reaction can be neglected. Chapter 4 also includes an additional investigation of the behaviour of the sulphite and sulphide anions in the eutectic LiCl-KCl mixture. Chapter 5 is devoted to the investigation of the sulphate species in the NaF-AlF3 mixtures saturated by Al2O3 with varying cryolite ratio, i.e. CR = 3, 2, 1.5 and 1.2. It contains a description of sulphate modification at various experimental conditions and revealed its complicated and complex electrochemical behaviour which was monitored by cyclic voltammetry, chronoamperometry and square wave voltammetry. For the electroactive species in the electrolyte with CR equal to 3 two reduction steps were suggested involving three and two electrons respectively. The “ce” reaction mechanism was assumed where a chemical reaction seems to precede a charge transfer. The chemical step might be the partial decomposition of sulphate to SO2. The sulphate reduction in the electrolyte with CR = 2 appeared to be considerably different. Since the temperature difference in these two systems is only 40 ºC the reason was likely to be due to the higher concentration of AlF3 in the electrolyte. In this electrolyte only one cathodic process was observed and the reaction mechanism involving three electrons with a preceeding chemical reaction was suggested. Because the production of aluminium metal in the electrolysis cells operating with inert anodes is an attractive idea and have opened a question about the behaviour of sulphur species in very acidic electrolytes (high AlF3 content) a part of the present studies was devoted to the study of the electrochemical behaviour of the sulphate anions in the electrolytes with CR = 1.5 and 1.2. Cyclic voltammetry and square wave voltammetry revealed that the sulphate is then reduced in two cathodic steps. Simulations and the obtained square wave voltammograms indicated that the cathodic process probably involves more electrons in the very acidic NaF-AlF3 mixtures than in electrolytes with higher CR equal to 3 or 2. Observation of volatile species formed in the NaF-AlF3 mixtures with CR = 1.5 and 1.2 was revealed as another important difference. This could be due to the formation of sulphur which at high temperature is present in the gaseous form and thus likely to escape from the laboratory furnace. The difference in the electrochemical behaviour of the sulphate anions in the cryolitic based melts was related to the difference in the bath composition and experimental temperature. The present study shows the complexity and variability in the electrochemical behaviour of sulphur containing species as well as the difficulties and limitations resulting from the sulphur chemistry in the molten salts at high temperatures.

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Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Allen J. Bard. Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Allen J. Bard. Vita. Includes bibliographical references. Also available from UMI.

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