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Title:Investigating electrochemical reduction processes via in situ enhanced Raman spectroscopy for carbon dioxide conversion and copper deposition
Author(s):Schmitt, Kevin G
Director of Research:Gewirth, Andrew A.
Doctoral Committee Chair(s):Gewirth, Andrew A.
Doctoral Committee Member(s):Murphy, Catherine J.; Kenis, Paul J. A.; Rodríguez-López, Joaquin
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Raman spectroscopy
Abstract:Increased understanding of electrochemical processes contributes to the development of many fields of technology, including energy storage, catalysis, sensing, and metal plating. In order to probe the behavior of chemical species at the electrochemical interface, in situ spectroelectrochemical methods have been developed. In particular, in situ surface-enhanced and shell-isolated, nanoparticle-enhanced Raman spectroscopy (SERS and SHINERS, respectively), can obtain high signal intensity for the vibrations of surface-bound species in a complete electrochemical cell simultaneously with voltammetric analysis. In this dissertation, these methods are applied to examine the effects of additive species on the reduction reactions involved in CO2 conversion and Cu metal plating. Using renewable energy sources such as wind and solar, CO2 conversion has the potential to sustainably recycle waste CO2 into valuable fuels or commodity chemicals. Nitrogen-containing ligands have been found to increase the formation rate of reduction products on various metal electrocatalysts. In this work, SERS was employed to examine the interfacial behavior of one such ligand, 3,5-diamino-1,2,4-triazole (DAT), on Ag and Cu electrodes. On Ag, SERS revealed that DAT influenced the strength of adsorption between the product CO and the Ag surface. CO normally occupies highly-coordinated bridge and 3-fold hollow sites; however, the addition of DAT promotes adsorption to more weakly-coordinated atop and physisorbed sites. By decreasing the adsorption strength of the product, the overall catalytic rate is increased through faster turnover. Cu is unique among CO2 electroreduction catalysts, as it enables production of C2 products such as ethanol and ethylene with relatively high Faradaic efficiency. Again, the addition of DAT increased overall reduction rates for carbon-containing products, and especially for ethylene. The SER spectra show that competitive adsorption of the DAT ligand on Cu is capable of blocking chemisorption of formate and ethoxy products, without impeding the formation of C2H4. Moreover, a Cu-C-O bending mode in adsorbed carbon monoxide was only observed in the presence of both CO2 and DAT. This bending vibration suggests that hydrogen bonding by DAT may be influencing the geometry of adsorbed carbon monoxide. This influence could increase the rate of C-C coupling, which is believed to be the rate-determining step for ethylene production. The latter half of this work examines the structure and behavior of a commonly used additive in the electroplating of Cu, 3-mercapto-1-propanesulfonate (MPS). However, to avoid the loss of signal enhancement as continued Cu deposition converts the substrate from rough to smooth, SHINERS was employed as the signal enhancement method. SHINERS spectra of MPS analogues containing either the thiol or sulfonate functional groups confirmed that the thiol moiety is required for surface tethering of the additive, and that surface interactions of the sulfonate are negligible. The strength of the Cu-thiol interaction was further supported by DFT calculations identifying the degree of depolarization that thiol adsorption has on neighboring Cu-Cl bonds. Indeed, SHINER spectra confirmed weaker Cu-Cl stretching vibrations in the prescence of thiol-containing additives. In addition to examination of the relevant functional groups, SHINERS illustrated the significance of the additive’s alkyl chain length by examining the relative ratio of the peak intensities representing C-Sthiol bonds in gauche and trans conformations. In the absence of chloride, shorter chain derivatives presented higher gauche/trans ratios and greater sensitivity of this ratio to potential changes. However, in the presence of chloride, MPS demonstrated higher gauche character than two- and four-carbon analogues across all measured potentials. This indicated that the three-carbon backbone is uniquely able to position the sulfonate group close to the Cu electrode surface under typical plating conditions, and therefore enable faster Cu deposition rates.
Issue Date:2016-07-05
Rights Information:Copyright 2016 Kevin G. Schmitt
Date Available in IDEALS:2016-11-10
Date Deposited:2016-08

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