Double layer structure and adsorption kinetics of ions at metal electrodes in room temperature ionic liquids
Date
2012-06-28
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Abstract
Toatemperatuuril vedelas olekus ioonsete vedelike (RTIL, Room Temperature Ionic Liquids) uurimine on üks populaarsemaid kaasaegse keemia suundi. Lai ideaalse polariseeritavuse ala, hea ioonjuhtivus, aktsepteeritav viskoossus, kõrge termiline stabiilsus, väga madal aururõhk ning võime lahustada paljusid keemilisi aineid teeb RTIL-idest huvitava elektrolüüdi elektrokeemiliste rakenduste jaoks. Võrreldes levinud orgaaniliste ühenditega on neil madal toksilisus ja nad ei ole kergesti süttivad. Peale selle võimaldab ioonide keemilise koostise või isegi ainult alküülrühmade pikkuse varieerimine muuta RTIL-ide füüsikalis-keemilisi omadusi nagu viskoossus, juhtivus, katalüütiline aktiivsus, sulamistemperatuur jne. Seega saab RTIL-e strateegiliselt sünteesida eri rakenduste jaoks. Detailne arusaamine RTIL-ide struktuurist, termodünaamikast ja kineetikast elektroodi pinnal on ülioluline niisuguste kaasaegsete elektrokeemilisete seadmete konstrueerimisel nagu superkondensaatorid, tehislihased, akud, päikesepatareid jne. Tartu Ülikoolis viiakse läbi kondensaatorite, tehislihaste ja sensorite uuringuid. Antud töös uuriti jodiidiooni adsorptsioonikineetikat ja 1-etüül-3-metüülimidasoolium tetrafluoroboraadi elektrokeemilise käitumise seaduspärasusi Cd(0001) elektroodil tsüklilise voltamperomeetria ja elektrokeemilise impedantsspektroskoopia meetoditega. Lisaks alustastati süstemaatilisite kvantkeemiliste uuringutega Tiheduse Funktsionaali Teooria tasemel, mis lõpuks andis parema arusaamise eksperimentaalsetest iodiid-ioonide adsorbeerumisest Cd(0001) ja Bi(111) elektroodidel. Halogeniidioonide adsorptsioonikineetika uuringud monokristallilistel elektroodidel oli elektrokeemia üks tähtsamatest suundadest veel aastakümmend tagasi. Siinkohal on huvitav märkida, et nii vesilahuses kui ioonses vedelikus toimub adsorptsiooniprotsess sarnaseid seaduspärasusi järgides, kuigi keemiline koostis ning vedeliku ruumilised omadused on nendel süsteemidel täiesti erinevad. Seda tõestab diferentsiaalmahtuvuse sõltuvused temperatuurist, mis olid kahe elektrolüüdi jaoks erinevad piirpinna struktuuride erinevuste tõttu. Cd(0001) / elektorlüüdi vesilahuse piirpinna jaoks rakendati mudelit, mis efektiivselt koosnes metall-dipool monokihist ja metall-laetud ioonide kihist. Vaatamata lihtsusele, mudeli kirjeldamisel on kasutatud põhjalikke Tiheduse Funktsionaali Teooria arvutustulemusi. Cd(0001)/RTIL piirpinna puhul esitatud teoreetiline mudel arvestab EDL-I mitmekihilist struktuuri. Järk-järgult „lahustudes“, muutub piirpinna struktuur koos temperatuuri tõusuga, põhjustades diferentsiaalmahtuvuse vähenemise. Eksperimentaalsestes uuringutes oleme omandanud uusi teadmisi temperatuursest sõltuvusest ning vaatluse all oli adsorptioonikineetika ja elektrilise kaksikkihi struktuur elektrood | RTIL piirpinnal. Antud töös esitati nimetatud süsteemide struktuuri ja omaduste võrdlev ülevaade ja tehti sellest lähtuvalt olulisemad järeldused.
Room Temperature Ionic Liquids (RTILs) chemistry are one of the popular trends in modern chemical research. Wide electrochemical potential and thermal windows, good ionic conductivity, acceptable viscosity, high thermal stability, extremely low vapour pressure and the ability to solubilise many chemical species make RTILs interesting electrolytes for electrochemical applications. Compared with commonly used organic compounds, they have low toxicity and are non-flammable. Furthermore, variation of the chemical composition of ions or even just the length of the alkyl groups allows fine tuning of the physicochemical properties of RTILs, such as viscosity, conductivity, catalytic activity, melting point etc. Thus, RTILs can be strategically designed for different applications. A detailed understanding of the structure, thermodynamics and kinetics of RTILs at the electrode surfaces is crucial for design of modern electrochemical devices, such as supercapacitors, actuators, batteries, solar cells, etc. Studies of RTIL based super capacitors, actuators and sensors are performed in the University of Tartu. In this study we applied Electrochemical Impedance spectroscopy and cyclic voltammetry methods for characterizing the processes at the interface between Cd(0001) electrode and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImFB4) as well as adsorption of I− anion on the electrochemically polished Cd(0001) electrode from aqueous electrolyte solutions. We also started a series of systematic quantum chemical studies at Density Functional Theory level, which eventually lead us to a better understanding of our previous experimental studies of iodide ion adsorption at Cd(0001) and Bi(111) electrodes. Studies of the adsorption kinetics of halide ions at single-crystal electrodes was a hot trend in electrochemistry just a decade ago. Interestingly, the adsorption process(es) in aqueous solution and room temperature ionic liquid were found to follow similar patterns, although the chemical composition and bulk properties of the liquids are completely different. We provide explanations to the differential capacitance dependences on temperature, which indeed appeared to be different for two electrolytes due to the difference in the interface structures. Namely, in order to explain the observations found for Cd(0001)/aqueous electrolyte solution interface, we were able to employ a model effectively consisting of metal–dipole monolayer and metal–charged wall of the ions. Despite the simplicity the model employ the results of extensive Density Functional Theory based calculations. In case of Cd(0001)/RTIL interface the presented theoretical model accounts multilayered structure of the EDL. Gradually “dissolving” the interfacial structure changes with the rise of temperature causing the decrease of the differential capacitance. In our experimental research we acquired new insights to the existing knowledge on the temperature dependences as well as made new observation on adsorption kinetics and structure at the electrode | RTIL interface. In this thesis we provide a comparative overview of the results for the different systems named, presenting our most important conclusions as one logical conception of structure–property correlation
Room Temperature Ionic Liquids (RTILs) chemistry are one of the popular trends in modern chemical research. Wide electrochemical potential and thermal windows, good ionic conductivity, acceptable viscosity, high thermal stability, extremely low vapour pressure and the ability to solubilise many chemical species make RTILs interesting electrolytes for electrochemical applications. Compared with commonly used organic compounds, they have low toxicity and are non-flammable. Furthermore, variation of the chemical composition of ions or even just the length of the alkyl groups allows fine tuning of the physicochemical properties of RTILs, such as viscosity, conductivity, catalytic activity, melting point etc. Thus, RTILs can be strategically designed for different applications. A detailed understanding of the structure, thermodynamics and kinetics of RTILs at the electrode surfaces is crucial for design of modern electrochemical devices, such as supercapacitors, actuators, batteries, solar cells, etc. Studies of RTIL based super capacitors, actuators and sensors are performed in the University of Tartu. In this study we applied Electrochemical Impedance spectroscopy and cyclic voltammetry methods for characterizing the processes at the interface between Cd(0001) electrode and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImFB4) as well as adsorption of I− anion on the electrochemically polished Cd(0001) electrode from aqueous electrolyte solutions. We also started a series of systematic quantum chemical studies at Density Functional Theory level, which eventually lead us to a better understanding of our previous experimental studies of iodide ion adsorption at Cd(0001) and Bi(111) electrodes. Studies of the adsorption kinetics of halide ions at single-crystal electrodes was a hot trend in electrochemistry just a decade ago. Interestingly, the adsorption process(es) in aqueous solution and room temperature ionic liquid were found to follow similar patterns, although the chemical composition and bulk properties of the liquids are completely different. We provide explanations to the differential capacitance dependences on temperature, which indeed appeared to be different for two electrolytes due to the difference in the interface structures. Namely, in order to explain the observations found for Cd(0001)/aqueous electrolyte solution interface, we were able to employ a model effectively consisting of metal–dipole monolayer and metal–charged wall of the ions. Despite the simplicity the model employ the results of extensive Density Functional Theory based calculations. In case of Cd(0001)/RTIL interface the presented theoretical model accounts multilayered structure of the EDL. Gradually “dissolving” the interfacial structure changes with the rise of temperature causing the decrease of the differential capacitance. In our experimental research we acquired new insights to the existing knowledge on the temperature dependences as well as made new observation on adsorption kinetics and structure at the electrode | RTIL interface. In this thesis we provide a comparative overview of the results for the different systems named, presenting our most important conclusions as one logical conception of structure–property correlation
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
ioonsed vedelikud, metallelektroodid, elektriline kaksikkiht, adsorptsioon, ionic liquids, metal electrodes, electric double layer, adsorption