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Title:Investigation of electrochemical interfaces at the molecular level
Author(s):Ha, Yeyoung
Director of Research:Gewirth, Andrew A
Doctoral Committee Chair(s):Gewirth, Andrew A
Doctoral Committee Member(s):Murphy, Catherine J; Nuzzo, Ralph G; Vura-Weis, Josh
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):electrochemical interfaces
fuel cells
surface stress
density functional theory
Abstract:This dissertation focuses on the investigation of electrochemical interfaces at the molecular level. Various systems including fuel cells, batteries, and electroplating are examined with in situ surface measurement techniques and density functional theory calculations. Experimental and computational studies presented herein will provide a fundamental understanding of the electrochemical systems examined and guide the field toward practical improvements. Electrochemical Surface Stress Development during CO and NO Oxidation on Pt: Dynamic processes on the surface can be monitored through the use of in situ electrochemical surface stress measurements. With this technique, electrochemical oxidation of CO and NO on Pt is examined and the experimentally measured surface stress behavior is successfully explained at a molecular level through DFT calculations. The changes in the surface stress response, Δstress, demonstrate the interplay between the adsorbed species during the oxidation process, which is determined by the coverage and the nature of the adsorbates. During the oxidation of CO on Pt, COads reacts with adjacent OHads and removal of COads results in a tensile surface stress (surface contraction). Concurrently, adsorption of OHads induces a compressive surface stress (surface expansion). This opposite Δstress from the removal of COads and the adsorption of OHads give rise to an inflection point in the stress profile during the oxidation. On the other hand, oxidation of another strongly bound diatomic adsorbate NOads shows a continuous compressive Δstress throughout the oxidation process. This difference in the stress profile is attributed to replacement of NOads by the oxidation product, NO3, instead of complete desorption as NO3-, and therefore compressive Δstress from the oxide and hydroxide on the surface dominates. In Situ Surface Stress Measurement and Computational Analysis Examining the Oxygen Reduction Reaction on Pt and Pd: Dynamic electrochemical surface stress response during the oxygen reduction reaction (ORR) on Pt and Pd cantilever electrodes in HClO4 and KOH was examined to elucidate surface binding configurations during O2 reduction electrocatalysis. Upon reduction of O2, the surface of Pt exhibits a compressive surface stress response, ΔStress, in both acid and base electrolytes due to adsorption of the ORR reactant and intermediates (O2, O, and OH). The magnitude of compressive ΔStress on Pt is greater in acid relative to base. On the other hand, the surface of Pd exhibits a negligible ΔStress in acid and a slight compressive ΔStress in base. Thus, magnitudes of the compressive ΔStress (surface expansion) during the ORR follow the order of Pt (acid) > Pt (base) > Pd (base) > Pd (acid) ~ 0. Density functional theory (DFT) calculations of adsorbate-induced excess surface stress on Pt(111) and Pd(111) surfaces imply a greater compressive surface stress induced on Pt(111) for nearly all adsorbate geometries examined. This trend, which agrees with the experimental observations, can be correlated to a greater tensile intrinsic surface stress of Pt(111) relative to Pd(111) resulting from difference in bond strength and bulk modulus of two metals. On stepped Pt(221) and Pd(221) surfaces, both the intrinsic tensile stress of the clean surface and the adsorbate-induced excess compressive stress are significantly reduced due to the presence of less coordinated, flexible step sites. Moreover, this difference between surface stress at terrace and step sites is more pronounced on Pt, which exhibits a greater intrinsic surface stress. Dynamic Surface Stress Response during Reversible Mg Electrodeposition and Stripping: Behavior of Mg battery electrolytes at the anode/electrolyte interface during Mg deposition and stripping process is studied using the electrochemical surface stress measurements. Four electrolytes electrolytes are examined in this section: PhMgCl+AlCl3/THF, (DTBP)MgCl–MgCl2/THF, MgCl2+AlCl3/THF, and Mg(BH4)2+LiBH4/diglyme. Each of these electrolytes exhibits common surface stress response features, indicating that the mechanisms of Mg deposition and stripping are similar among the different electrolytes. Combining the measurements with density functional theory calculations, each part of the stress-potential curve is assigned to steps in the deposition and stripping reactions. The analysis suggests the following mechanism: (1) Mg2+/anion/solvent complexes adsorb on the substrate prior to the deposition, reaching a saturation coverage; (2) Mg deposits as random nuclei and the deposition continues without a recrystallization process; (3) during the initial stage of Mg stripping, less coordinated Mg(0) is converted to soluble Mg(II) species and/or to partially oxidized species, MgOx; (4) as the anodic reactions proceed further, less reactive Mg planes are oxidized and MgOx species are removed via chemical processes; (5) due to the strong interaction between Mg and the noble metal substrate atoms, the Mg layer directly bound to the substrate are the last to be anodically converted (and desorb). Glycerol Oxidation Products on Silver Probed using In Situ Surface-Enhanced Raman Spectroscopy and Two-Dimensional Correlation Spectroscopy: Electrochemical oxidation of glycerol on Ag catalyst and the oxidation products are evaluated using in situ surface-enhanced Raman spectroscopy (SERS). The SER spectra exhibit following vibrational modes during glycerol oxidation: (1) presence of carboxylate group; (2) presence of ν(C–C) along with ν(C–COO-), suggesting presence of three carbon atoms in the oxidation product molecule; (3) presence of methylene group; and (4) absence of ν(C=O) around 1700 cm-1, and hence absence of aldehydes and ketones. From these considerations, the major oxidation product candidates can be narrowed down to glyceric acid and glycolic acid. Moreover, by analyzing the potential dependent intensity variations of the peaks in SER spectra and their correlations using two-dimensional correlation spectroscopy (2DCOS), changes in the adsorbate configurations are examined. At negative potentials (< 0.3 V), interactions between Ag and alkyl groups in neutral glycerol molecules dominate on the electrode surface. The C–H vibrational modes exhibit a considerable redshift, indicating a strong interaction between the alkyl groups and Ag surface, possibly involving charge transfer. As the electrode potential becomes more positive, direct C–H∙∙∙Ag interactions are removed initially and all alkyl groups near the surface are eventually replaced with carboxylate groups from oxidation products as glycerol oxidation begins ca. 0.7 V. These carboxylate groups exhibit different binding configurations upon varying electrode potentials: at less positive potentials C=O∙∙∙Ag interaction is favored and at more positive potentials C–O∙∙∙Ag and COO-∙∙∙Ag interactions are observed.
Issue Date:2017-06-02
Rights Information:Copyright 2017 Yeyoung Ha
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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