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Electrochemical Methods

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Electrochemical Methods Zenobi Group Department of Chemistry and Applied Biosciences https://zenobi.ethz.ch/ 24/09/2020 Electrochemical methods Dr. Stamatios Giannoukos 1 Electrochemistry The relationship between chemical reactions and electricity Ø Certain chemical reactions can produce electricity, e.g. in a battery Ø Electricity can make certain chemical reactions (that would NOT happen otherwise) happen Electricity: movement of electrons e- e- e- e- e- Chemical reactions: oxidation reduction reactions à electrons move between atoms A B 2 Electrochemistry e- A B A e- e- B wire e- X X pull push C D C e- Battery e- D 3 Electrochemical techniques Techniques in analytical chemistry that measure potential, charge, or current to: a) determine an analyte's concentration or b) characterize an analyte's chemical reactivity Ø Provide qualitative and quantitative analysis Bulk à measurement of a property of the solution in an electrochemical cell, e.g. solution conductivity (proportional to the Ctotal of dissolved ions) Interfacial à potential, charge, or current depend on the interface between a solution and an electrode e.g. determination of the pH using a pH electrode 4 Interfacial electrochemical techniques 5 1. Potentiometry • Measures the potential of an electrochemical cell. • The potential is then related to the concentration of one or more analytes. • Selective sensitive to the ion of interest electrodes. • e.g. glass-membrane electrode in a pH meter. Zn(s)+2Ag+(aq)⇋2Ag(s)+Zn2+(aq) 6 2. Voltammetry • Measures the current as a function of a fixed or variable potential. • In voltammetry, a time-dependent potential is applied to an electrochemical cell and we measure the resulting current with a 3 electrode system as a function of that potential. Types of voltammetry (rep. examples) linear staircase cyclic 7 2. Cyclic Voltammetry SV = Sisal-like V2O5 BV = bulk V2O5 Cyclic voltammetry of the SV and BV electrodes at a scanning rate of 0.1 mV s−1 between 2.0 and 4.0 V (vs Li/Li+); (c) Nyquist plots of the BV and SV electrodes after 400 cycles (inset showing the equivalent circuit for the Nyquist plots); and (d) XRD patterns of BV and SV electrodes after 400 cycles at 1 C. 8 3. Coulometry • Measures an unknown concentration of an analyte in solution by completely converting the analyte from one oxidation state to another. • It is based on an the exhaustive electrolysis of the analyte. • Forms of coulometry: i. controlled-potential à a constant potential is applied to the electrochemical cell, ii. controlled-current à a constant current pass through the electrochemical cell. During the electrolysis, the total charge Q passing through the electrochemical cell is proportional to the absolute amount of analyte: Faraday law: Q = nFNA n is the number of electrons per mole of analyte, F is Faraday’s constant (96487 C mol–1), NA is the moles of analyte 9 4. Conductometry • Conductometry à measurement of the electrolytic conductivity to monitor the progress of a chemical reaction. Conductivity of an electrolyte solution is a measure of its ability to conduct electricity. It is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance. Example à Conductometric titration in which the electrolytic conductivity of a reaction mixture is continuously monitored as one reactant is added. 10 5. Dielectrometry • Measures the dielectric constant (or relative permittivity) of a substance resulting from the orientation of particles (molecules or ions) that have a dipole moment in an electric field. • It is the absolute permittivity expressed as a ratio relative to the vacuum permittivity. • Used to monitor the purity of dielectrics, e.g. detection of small amounts of moisture, as an epoxy adhesive cure monitoring method, to identify pharmaceutical ingredients, etc. 11 6. X-Ray Diffraction v Operando XRD, both reflection and transmission modes, can be used to monitor the crystal structure of electrode material v Special sources (synchrotron) and special window materials (Be) may be necessary v change and phase transitions of electrodes during the electrochemical process XRD spectra collected during the first galvanostatic cycle between 4.0 and 2.9 V. T:LiFePO4; H:FePO4; *: cell package 7. Raman Spectroscopy v Vibrational spectroscopy v Molecular Analysis, works for analyzing solids (phonons) v Works in aqueous environments v Operando spectroscopy LiNi0.33Co0.33Mn0.33O2 8. FTIR Spectroscopy v Vibrational spectroscopy v Molecular Analysis, works for analyzing solids (phonons) v Aqueous environments: problematic => special cells FTIR Spectroscopy / ATR 9. ICP-MS v Elemental analysis v Suitable for trace detection v Can analyze the electrolyte v Can analyze the solid (sample preparation = acid digestion) 10. Scanning Electron Microscopy v High-resolution imaging method, down to the nm range v Elemental analysis with EDX possible v Works in vacuum v Operando / in-situ EM only with very special cells possible SEM images of double-shelled o LiNi0.5Co0.2Mn0.3O2 prepared at (a) 800 C, (c) 850 oC, (e) 900 oC and normal o LiNi0.5Co0.2Mn0.3O2 at (b) 800 C, (d) 850 oC, (f) 900 oC.. Transmission Electron Microscopy v Extremely high resolution method (atomic resolution) – very small part of the sample analyzed v Analysis of various phases v Pinpoints defects v ED allows local order to be established v Expensive HRTEM image of phase boundary (marked by red arrows) between LiFePO4 and FePO4 (= charging end product) along the [010] direction during lithiation; time = 0 s. (c) At 176 s, SAED the thickness of LFP layer increased. (d) Inverse fast Fourier transform (IFFT) image of (b), showing the distribution of pattern in dislocations near the phase boundary. LiFePO4 Operando TEM ? Liquid-cell configuration with battery relevant liquid electrolyte: Li metal counter electrode, Si nanowire, and silicon nitride membranes are deposited on a Si chip..
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