In situ Scanning Tunnelling Microscopy studies of the interfacial structure between Bi(111) electrode and a room temperature ionic liquid
Date
2015-08-04
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Abstract
Viimastel aastatel on uuesti populaarsust kogunud ioonsed vedelikud kui vesilahustele alternatiivsed elektrolüüdid. Neid rakendatakse näiteks elektrilise kaksikkihi kondensaatorite korral. On teada, et vesilahustes on elektrokeemilised protsessid sageli limiteeritud ja kasulikkude protsesside suunamiseks tuleb kasutada suuri ülepingeid. Ioonse vedeliku keskkonnas on seevastu võimalik opereerida madalama energiakuluga, näiteks väärismetallide ja nende segude sadestamine. Lisaks on ioonsed vedelikud väga heade lahusti omadustega ja nende keskkonnas võimalik sünteesida uusi aineid.
Tänaseni puudub üks-ühene arusaam elektrood | ioonne vedelik piirpinna ehitusest ja seal toimuvatest protsessidest elektrokeemilise polarisatsiooni tingimustes.
Skaneeriva tunnelmikroskoopia (STM) ja aatomjõumikroskoopia (AFM) mõõtmistest on teada, et Bi(111) elektroodid on vesilahustes ja mitte-vesilahustes ajaliselt väga stabiilsed negatiivse potentsiaali tingimustes. Atomaarselt sileda pinna tõttu Bi(111) elektroodid heaks alternatiiviks kullast või süsinikust elektroodidele. Lisaks on teada, et ioonse vedeliku keskkonnas on võimalik uurida neid potentsiaale ja seega protsesse, mis vesikeskkonnas samadel elektroodidel tihti ei ole võimalik.
Käesoleva doktoritöö põhiteemaks on Bi(111) ja ioonse vedeliku vahelise piirpinna uuringud in situ STM meetodil. Katseks valiti kolm ioonset vedelikku: 1-etüül-3-metüülimidazolium tetrafluoroboraat (EMImBF4), 1-etüül-4-metüülpüridinium tetrafluoro-boraat ja EMImBF4 + 1% 1-etüül-3-metüülimidazolium iodiidi segu. Doktoritöös kasutati lisaks kahte võrdlevat meetodit: tsükliline voltamperomeetria ja elektrokeemiline impedantsspektroskoopia.
Töö käigus näidati, et Bi(111) elektrood on atomaarselt siledate platoodega ka ioonse vedeliku tingimustes. Kõigi kolme ioonse vedeliku tingimustes saadi Bi(111) aatomlahutused ja mõõdeti ajalised sõltuvused potentsiaali tsükleerimise tingimustel. Ioonsete vedelikkude segu korral saavutati kõrglahutuslikud in situ STM pildid, kus visualiseeriti adsorbeerunud jodiidi ioonide paiknemine Bi(111) pinnal.
Käesoleva doktoritöö raames uuriti lisaks teralise struktuuriga tahkeoksiidse kütuseelemendi elektroodide pinda AFM meetodiga. Spetsiaalse tarkvara abil arvutati kütuseelemendi katoodide, anoodide ja elektrolüüdi pindade karedused. Lisaks täiendati magistritöös alustatud terade suuruse modelleerimise meetodit, et hinnata terade suurust ja suuruste jaotust uuritud pindadel.
Välja töötatud terade suuruse modelleerimise meetodit rakendati Bi(111) | ioonse vedeliku segu piirpinnal adsorbeerunud ioonide visualiseerimiseks. Sama meetodit kasutati ka Bi(111) elektroodi pinna muutuste modelleerimiseks potentsiaalide alas, kus toimub Bi(111) elektroodi anisotroopne lahustumine ja sadenemine ioonse vedeliku keskkonna tingimustes.
Ionic liquids as electrolytes alternative to aqueous solution have gained attention in the recent years. It is well-known that the electrochemical processes in aqueous medium are often limited and much energy is lost, when overpotentials are applied, for example, in the field of electrosynthesis. In the medium of ionic liquids it is often possible to operate with the lower energy costs. For example, the deposition of noble metals and their alloys. In addition, ionic liquids are excellent candidates as solvents. Today, there is no clear understanding about the structure at electrode | ionic liquid interface. Also, the interfacial processes in the conditions of electrochemical polarization require further studies, especially in the field of microscopy. From the prvious works using Scanning Probe Microscopy (STM) and Atomic Force Microscopy (AFM) it is known that Bi(111) electrode is very stable under negative potential conditions in aqueous as well in non-aqueous medium. The Bi(111) electrode is a viable alternative to Au(hkl) or carbon electrodes. Additionally, in the medium of ionic liquid, it is usually possible to study potentials which are not possible in aqueous solutions. As a result, new processes can be studied on the Bi(111) electrode. Main aim of the present PhD thesis is to study the interface between Bi(111) electrode and an ionic liquid usin in situ STM method. Three ionic liquids were chosen: 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-4-methylpyridinium tetrafluoro-borate, and a mixture of 1-ethyl-3-methylimidazolium tetrafluoro-borate + 1-ethyl-3-methylimidazolium iodide. In present PhD thesis, cyclic voltammetry and electrochemical impedance spectroscopy were used as complementary methods. In the PhD thesis, it was demonstrated that Bi(111) electrode in ionic liquids studied has atomically flat surface, comparable with the results obtained for Bi(111) electrode in aqueous solutions. For all three ionic liquids atomic resolution for Bi(111) electrode was established. It was also proven that Bi(111) electrode is time stable under changing negative potential conditions during hours in ionic liquid medium. In addition, the adsorption of iodide anions was visualized at Bi(111) electrode surface from a mixture of ionic liquids. In the PhD thesis, the surfaces of cathodes, anodes and electrolytes of Solid Oxide Fuel Cell (SOFC) were studied with AFM method. The grainy structure of SOFC electrodes was modelled with special hardware and the following characteristics were obtained: medium grain diameter, grain distribution on surface and and Root Mean Square roughness. In addition, the method of grain analysis was further elaborated and has been applied for Bi(111) | ionic liquid interface. The adsorbed anions on Bi(111) surface from a mixture of 1-ethyl-3-methylimidazolium tetrafluoro-borate + 1-ethyl-3-methyl-imidazolium iodide, and the anisotropic dissolution of Bi(111) from 1-ethyl-3-methylimidazolium tetrafluoroborate were modelled. The average ion diameters, and the average diameter dissolution / deposition islands on Bi(111) electrode surface were measured, respectively.
Ionic liquids as electrolytes alternative to aqueous solution have gained attention in the recent years. It is well-known that the electrochemical processes in aqueous medium are often limited and much energy is lost, when overpotentials are applied, for example, in the field of electrosynthesis. In the medium of ionic liquids it is often possible to operate with the lower energy costs. For example, the deposition of noble metals and their alloys. In addition, ionic liquids are excellent candidates as solvents. Today, there is no clear understanding about the structure at electrode | ionic liquid interface. Also, the interfacial processes in the conditions of electrochemical polarization require further studies, especially in the field of microscopy. From the prvious works using Scanning Probe Microscopy (STM) and Atomic Force Microscopy (AFM) it is known that Bi(111) electrode is very stable under negative potential conditions in aqueous as well in non-aqueous medium. The Bi(111) electrode is a viable alternative to Au(hkl) or carbon electrodes. Additionally, in the medium of ionic liquid, it is usually possible to study potentials which are not possible in aqueous solutions. As a result, new processes can be studied on the Bi(111) electrode. Main aim of the present PhD thesis is to study the interface between Bi(111) electrode and an ionic liquid usin in situ STM method. Three ionic liquids were chosen: 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-4-methylpyridinium tetrafluoro-borate, and a mixture of 1-ethyl-3-methylimidazolium tetrafluoro-borate + 1-ethyl-3-methylimidazolium iodide. In present PhD thesis, cyclic voltammetry and electrochemical impedance spectroscopy were used as complementary methods. In the PhD thesis, it was demonstrated that Bi(111) electrode in ionic liquids studied has atomically flat surface, comparable with the results obtained for Bi(111) electrode in aqueous solutions. For all three ionic liquids atomic resolution for Bi(111) electrode was established. It was also proven that Bi(111) electrode is time stable under changing negative potential conditions during hours in ionic liquid medium. In addition, the adsorption of iodide anions was visualized at Bi(111) electrode surface from a mixture of ionic liquids. In the PhD thesis, the surfaces of cathodes, anodes and electrolytes of Solid Oxide Fuel Cell (SOFC) were studied with AFM method. The grainy structure of SOFC electrodes was modelled with special hardware and the following characteristics were obtained: medium grain diameter, grain distribution on surface and and Root Mean Square roughness. In addition, the method of grain analysis was further elaborated and has been applied for Bi(111) | ionic liquid interface. The adsorbed anions on Bi(111) surface from a mixture of 1-ethyl-3-methylimidazolium tetrafluoro-borate + 1-ethyl-3-methyl-imidazolium iodide, and the anisotropic dissolution of Bi(111) from 1-ethyl-3-methylimidazolium tetrafluoroborate were modelled. The average ion diameters, and the average diameter dissolution / deposition islands on Bi(111) electrode surface were measured, respectively.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
ioonsed vedelikud, elektroodid, faasieralduspinnad, skaneeriv teravikmikroskoopia, ionic liquids, electrodes, interfaces (chemistry), scanning probe microscopy