DISSERTATIONES CHIMICAE UNIVERSITATIS TARTUENSIS 148 ERIK ANDERSON In situ Scanning Tunnelling Microscopy studies of the interfacial structure between Bi(111) electrode and a room temperature ionic liquid Tartu 2015 ISSN 1406-0299 ISBN 978-9949-32-893-2 DISSERTATIONES CHIMICAE UNIVERSITATIS TARTUENSIS 148 DISSERTATIONES CHIMICAE UNIVERSITATIS TARTUENSIS 148 ERIK ANDERSON In situ Scanning Tunnelling Microscopy studies of the interfacial structure between Bi(111) electrode and a room temperature ionic liquid Institute of Chemistry, Faculty of Science and Technology, University of Tartu, Estonia Dissertation is accepted for the commencement of the degree of Doctor of Philosophy in Chemistry on June 25th, 2015 by the Council of Institute of Chemistry, University of Tartu. Supervisors: Prof. Enn Lust Institute of Chemistry, University of Tartu, Estonia Opponent: Prof. Pawel J. Kulesza Department of Chemistry, University of Warsaw, Poland Commencement: August 28th, 2015, at 16:00 14a Ravila Street, Tartu (Chemicum), auditorium 1021 Publication of this thesis is granted by the Institute of Chemistry, University of Tartu ISSN 1406-0299 ISBN 978-9949-32-893-2 (print) ISBN 978-9949-32-894-9 (pdf) Copyright: Erik Anderson, 2015 University of Tartu Press www.tyk.ee TABLE OF CONTENTS TABLE OF CONTENTS ............................................................................ 5 LIST OF ORIGINAL PUBLICATIONS .................................................... 6 ABBREVIATIONS AND SYMBOLS USED ............................................ 7 I INTRODUCTION .................................................................................... 9 II LITERATURE OVERVIEW .................................................................. 10 2.1 General background ........................................................................ 10 2.2 Cyclic Voltammetry ....................................................................... 13 2.3 Electrochemical Impedance Spectroscopy ..................................... 15 2.4 Scanning Probe Microscopy ........................................................... 17 2.5 In situ STM in modern electrochemistry ........................................ 19 III EXPERIMENTAL ................................................................................. 20 IV RESULTS AND DISCUSSION ............................................................ 22 4.1 Analysis of cyclic voltammetry data .............................................. 22 4.2 Analysis of electrochemical impedance data .................................. 23 4.3 Fitting and analysis of Nyquist plots data ...................................... 27 4.4 Analysis of the in situ STM data .................................................... 31 4.4.1 Stability of the Bi(111) | RTIL interface and its main characteristics under cathodic polarisation ........................... 31 4.4.2 The Bi(111) | RTIL interface under the changing conditions of electrochemical polarisation ............................................ 34 4.4.2.1 Bi(111) | EMImBF4 interface.................................. 34 4.4.2.2 Bi(111) | BMPyBF interface .................................. 38 4 4.4.2.3 Bi(111) | EMImBF + 1wt% EMImI interface ........ 44 4 4.4.3 High-resolution in situ STM data for Bi(111) | RTIL interface ................................................................................ 46 4.4.3.1 Atomic resolution data ............................................ 46 4.4.3.2 Ionic resolution data ................................................ 49 4.4.4 Influence of the in situ STM scanning parameters on the Bi(111) | RTIL interfacial structure ...................................... 53 4.4.5 Comparative fitting of Bi(111) | RTIL interface and solid oxide fuel cell materials by applying selective grain analysis method .................................................................................. 56 V SUMMARY ............................................................................................ 60 REFERENCES ............................................................................................ 62 SUMMARY IN ESTONIAN ...................................................................... 67 AKNOWLEDGEMENTS ........................................................................... 69 PUBLICATIONS ........................................................................................ 71 CURRICULUM VITAE ............................................................................. 105 ELULOOKIRJELDUS ................................................................................ 108 5 LIST OF ORIGINAL PUBLICATIONS 1. E. Lust, R. Küngas, I. Kivi, H. Kurig, P. Möller, E. Anderson, K. Lust, K. Tamm, A. Samussenko, G. Nurk, Electrochemical and gas phase parameters of cathodes for intermediate temperature solid oxide fuel cells, Electrochim. Acta 55, 7669–7678 (2010). 2. E. Anderson, V. Grozovski, L. Siinor, C. Siimenson, V. Ivaništšev, K. Lust, S. Kallip, E. Lust, Influence of the electrode potential and in situ STM scanning conditions on the phase boundary structure of the single crystal Bi(111) | 1-butyl-4-methylpyridinium tetrafluoroborate interface, J. of Electroanal. Chem. 709, 46–56 (2013). 3. E. Anderson, V. Grozovski, L. Siinor, C. Siimenson, E. Lust, In situ STM studies of Bi(111) | 1-ethyl-3-methyl-imidazolium tetrafluoroborate + 1-ethyl- 3-methylimidazolium iodide interface, J. of Electrochem. Commun. 46, 18–21 (2014). Author's contribution: 1. Performed all scanning probe microscopy and cyclic voltammetry measure- ments and results analysis of Papers 1.–3. 2. Attended to text preparations, except the measurements of electrochemical impedance spectra of Papers 1.–3. 6 ABBREVIATIONS AND SYMBOLS USED in situ measurement under specific and / or unique or changing conditions STM scanning tunnelling microscopy AFM atomic force microscopy C(0001) basal plane of a graphite electrode EDL electrical double layer EDLC electrical double layer capacitor HSC hybrid supercapacitor RTIL room temperature ionic liquid ∆E potential region of ideal polarisability NAS non-aqueous solvent E maximum specific energy max P maximum specific power max (hkl the notation of the Bi or Au crystallographic plane (index) EIS electrochemical impedance spectroscopy CV cyclic voltammetry HOPG highly oriented pyrolytic graphite HOPG(0001) highly oriented pyrolytic graphite basal plane E electrode potential RMS root mean square roughness BMPyBF 1-butyl-4-methylpyridinium tetrafluoroborate 4 EMImBF 1-ethyl-3-methylimidazolium tetrafluoroborate 4 EMImBF + EMImI a mixture of EMImBF + 1-ethyl-3-methylimidazolium 4 4 iodide σ surface charge density of an electrode ε dielectric constant R(σ) Debye-length-dependent electrochemical roughness t experimental time E potential of zero charge σ=0 υ potential sweep rate i current in an electrochemical system q electrode charge R solution resistance or high-frequency series resistance s C capacitance of an electrical double layer d R current resistance I flow of an electrical current Z complex impedance ac alternating current E value of potential E at time t t E amplitude of a signal 0 ω radial frequency f frequency 7 Φ phase shift I value of current I at time t t I amplitude of an ac current signal 0 Z magnitude of impedance 0 j imaginary unit, (j = √-1) Z ' real part of impedance Z '' imaginary part of impedance EC equivalent circuit Z Warburg impedance w δ thickness of Nernstian diffusion layer D average value of diffusion coefficients V bias between tip and a substrate x distance between tip and a sample R resistance of a tunnelling gap tunnel E bias between tip and a substrate tip WE working electrode CE counter electrode RE reference electrode E potential of a working electrode WE i tunnelling current tunnel υ image scanning rate scan j current density C series capacitance s R mass transfer resistance d R charge transfer resistance ct C high-frequency capacitance dl C low-frequency capacitance ad ε dielectric constant of vacuum 0 d effective thickness of an electrical double layer region R high frequency series resistance 1 C parallel capacitance p τ characteristic time constant ch S(ω) complex power P(ω) active power (i.e. real) component of the complex power Q(ω) reactive (i.e. imaginary) component of the complex power t polarisation time pol FTT fast Fourier transform filtering technique SOFC solid oxide fuel cell GD grain distribution calculation method T sintering temperature sint 8 I INTRODUCTION Ionic liquids are electrolytes with the growing potential, being widely applied in many areas of science: in synthesis, deposition of noble metals and galvanic coatings, as electrolytes in supercapacitors, dye-sensitized solar cells, batteries and electrolysers, as well as solvents for various technological processes etc. Nowadays there are an infinite number of combinations of cations and anions that can be combined as novel ionic liquids with tuneable characteristics. The surface properties of ionic liquids have been studied only in the case of a few metals, and therefore, the classical theories about the electrical double layer structure formation and their properties in the conditions of changing electrical field on atomically flat electrodes are often incomplete. Scanning probe microscopy is a very informative tool for studying the solid electrode | electrolyte interface in general. High resolution in situ scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are the most powerful but simple analytical methods for investigation of the interfacial structure at electrode | electrolyte interface in real time and at the atomic or molecular level. However, only the modest number of studies which combine the results obtained with in situ STM and other surface sensitive electrochemical or crystallographic methods have been published so far. During the past 30 years, single crystal and polycrystalline bismuth electrodes and their application in electrochemical kinetics and electro analysis studies have been of a great interest for the scientists at University of Tartu. Due to quick technological progress in the materials purification and production of metal single crystals it is nowadays essentially possible to obtain nearly perfectly atomically flat crystal surfaces, including Bi(111) electrodes. The main aim of this work was to study the electrochemical behaviour and interfacial structure of three ionic liquids at Bi(111) electrodes with the in situ STM method. Cyclic voltammetry and electrochemical impedance methods have been used as complementary methods for a detailed analysis of the electrical double layer structure, adsorption kinetics and specific adsorption of I– ions at the electrochemically polished and cleaved Bi(111) surface. 9 II LITERATURE OVERVIEW 2.1 General background The structure of electrical double layer (EDL) at metal | electrolyte interface influences many important applications of electrochemistry, including design of electrical double layer capacitors (EDLC) and hybrid supercapacitors (HSC), corrosion-protective layers, fuel cells, battery-type devices, electrolysers, dye- sensitised solar cells and CO electro reduction devices [1–18]. In the recent 2 years, ionic liquids have been extensively studied as electrolytes applicable in these various modern technological devices. Use of the non-aqueous electrolytes and room temperature ionic liquids (RTIL) [19] in EDLCs has been initiated by the wider region of ideal polarisability of carbon electrodes (cell potential ∆E up to 3.6 V [15–21] compared to aqueous solutions). Application of RTILs with small additions of the non-aqueous solvents (NAS) (so-called RTIL + NAS mixtures) as an electrolyte has been initiated by the lower viscosity of RTIL + NAS mixtures, increase in ∆E, if compared with those based on the aqueous electrolytes [22–24]. Thus, for exponential increase in the maximum energy-density, E , and power-density, P , being both max max proportional to ∆E2 of the EDLC. Our recent research, therefore, has been focused on using the RTILs, non-aqueous electrolytes and the RTIL + NAS mixtures as electrolytes for the high energy- and power density EDLCs based on the specially designed microporous–mesoporous carbon electrodes with hierarchical porous structure [15–21]. The high resolution microscopy studies combined with electrochemical measurements of a porous carbon electrode | RTIL interface are more complicated, time consuming and expensive, thus, some testing measurements have been carried out using the cleaved and electrochemically polished atomically flat Bi(111) electrodes [25,26]. Due to the absence of fundamental information on the influence of surface roughness of the carbon electrodes on the energy density and power density of supercapacitors, fuel cells and battery- type systems, the new experimental studies analysing the EDL structure at metal | ionic liquid interface, including analysis of the nano-roughness effects, are essential. Due to the technological progress, in the production and purification of single elements in recent years, sufficient prerequisites are met to produce nearly ideal single crystals for almost each element with highest purity (up to 99.999% and even higher). Therefore, the EDL structure at cleaved and electrochemically polished Bi(111) electrode | RTIL interface within wide region of electrode potentials has been studied in this work. Wider application of RTILs is still hindered by limited fundamental understanding of the structural characteristics of interface between the electrode material and RTIL [27–35]. Only a few complex studies combining the electro- chemical impedance spectroscopy (EIS) [1,3,36] and in situ scanning tunnelling microscopy (STM) [34,37] have been published so far. Influence of temperature and crystallographic structure of the metal single crystal plane electrode on the 10