Date on Master's Thesis/Doctoral Dissertation

12-2017

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Grapperhaus, Craig

Committee Co-Chair (if applicable)

Buchanan, Robert

Committee Member

Sumanasekera, Gamini

Committee Member

Zamborini, Francis

Author's Keywords

catalyst; HER

Abstract

Hydrogen fuel is a promising future energy carrier with many applications. Thermodynamically, 1.23 V is needed for the water electrolysis, which is a clean method to generate hydrogen gas. In practice excess potential is required, which is called overpotential, due to kinetic barriers. Catalysts to reduce the overpotential for the hydrogen evolution reaction (HER) include many homogeneous catalysts and heterogeneous catalysts. Heterogenization of homogeneous catalysts on electrode surfaces is an ideal way to study a catalyst by combining homogeneous and heterogeneous study together. In this dissertation, several characterization techniques have been employed, include cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, UV-visible spectroscopy, X-ray photoelectron spectroscopy, chronopotentiometry, and density functional theory. The heterogenization of the rhenium complex of the redox-active ligand 2-diphenylphosphinobenzenethiolate (ReL3)on glassy carbon electrode surfaces generated a stable modified electrode work for HER electrocatalysis in aqueous acidic condition with a relatively large overpotential. The cation form of ReL3, [ReL3]+ has been drop casted on the GC surface to prepare GC-[ReL3]+ which showed improved overpotential. Addition of a carbon black layer to generate GC-CB-ReL3 and GC-CB-[ReL3]+ decreased significantly the charge transfer resistance and overpotential for both catalysts. Tafel slope analysis for all electrodes indicates a Volmer rate determine step. The proposed mechanism is based on the homogeneous mechanism with support of DFT calculations. The redox active bis-thiosemicarbazone ligands ATSM and ATSP and their Cu- and Zn- derivatives were fabricated on GC surface as heterogeneous HER catalysts. The overpotential for the ATSM and ATSP ligands are 1.12 V and 1.09 V, while the overpotential for CuATSM and CuATSP are 774 mV and 745 mV, respectively. The Zn derivatives de-metallation during the electrolysis as observed by UV-vis spectroscopy and XPS analysis. Carbon paste electrodes (CPEs) have been used to improve the charge transfer resistance for the fabricated electrodes to achieve lower overpotential. The CPE-ReL3, CPE-ATSM and CPE-CuATSM electrodes were prepared and compared with GC electrodes. The CPE catalysts show significantly reduced overpotential compared to GC and the charge transfer resistance was decreased over 100 times relative to GC electrodes. Moreover, the CPEs exhibit excellent stability properties during the long-term electrolysis.

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