Doctor of Philosophy in Chemical and Biological Engineering (PhD)
Optimization of Electrolyzers for CO2 Reduction
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The conversion of electrical energy derived from clean, renewable, and intermittent sources such as wind and solar into transportable and storable fuels is a means of matching energy supply and demand. Effective electrocatalysts can facilitate these conversions in an economical manner. Our group has developed photodeposition techniques for synthesizing amorphous thin-film metal oxide electrocatalysts. The thicknesses of amorphous metal oxide films were determined by cross-sectional scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (XRF). XRF measurements recorded on the films provided a strong linear correlation with the thicknesses determined by cross-sectional SEM. The electrochemical surface area (ECSA) determined by double-layer capacitance measurements did not universally show a linear relationship with film thicknesses. These results highlight the limitations of using ECSA to determine electrocatalyst film thickness. The noninvasive XRF technique is demonstrated to be a superior method for reporting on the thickness and loadings of thin metal oxide films. XRF measurements were made on iron-nickel oxy/hydroxide (FeNiOx) films that are widely known to mediate the oxygen evolution reaction at modest current densities (10 mA cm-²). These measurements enabled the determination of the electrochemical stability and metal composition of these electrocatalyst films when subjected to sustained electrolysis in strong base at a current density J = 200 mA cm-². Most of the iron in the film was liberated during the first 24 h of electrolysis and deposited on the cathode. These results show that one must account for the instability of this mixed-metal composition when drawing structure-property relationships and when considering the scale-up of electrocatalysts.Finally, modifications to the photodeposition technique are demonstrated that enables access to metal and metal alloy thin films. Silver and copper are widely studied metals for catalyzing the CO₂ reduction reaction (CO₂RR), yet studies of Ag-Cu alloys are rare due to the immiscibility of the metals. I report that our photodeposition procedure provides access to Ag-Cu alloys at ambient pressures and temperatures. Our photodeposition procedure is shown to furnish metastable alloys with ~10 atomic weight % (at-%) copper incorporated into the silver lattice. These results provide proof that photodeposition can be used to access kinetic phases of alloys.
Dye-sensitized solar cells (DSSCs) are promising, cost-effective technologies used to harness solar energy for electricity. Previous efforts to improve the solar-to-electricity conversion efficiency have primarily focused on sensitizer engineering and photocurrent generation. Alternatively, the efficiency can be increased by tuning the redox potential of the charge mediator to maximize the photovoltage in the device. This work describes the implementation of a new cobalt mediator (Co-bpm) with an exceptionally positively shifted redox potential of 1.07 V vs NHE in the DSSC. The best-performing device showed one of the highest reported DSSC photovoltages. The poor solubility of Co-bpm in MeCN was a major obstacle that was overcome by testing a variety of electrolyte solvent systems and counterions. Notwithstanding, Co-bpm mediator-based devices exhibited low photocurrents and low power conversion efficiencies despite the high voltages.A comparative study was then performed to elucidate how the positively shifted redox potential affect the photocurrent in Co-bpm mediator-based devices. Three cobalt analogs [Co-(bpm-DTB), Co-bpy and Co-(bpy-DTB)] of varying redox potentials were studied alongside Co-bpm to determine the trend between redox potential, device performance, and recombination lifetime. The redox potentials of the cobalt analogs were tuned by installing tert-butyl substituents and varying the number of nitrogen atoms in the ligand. A positive shift in the redox potentials correlated to a linear increase in photovoltage and non-linear decrease in photocurrent in DSSCs. A low quasi-Fermi level (EF,n) at open-circuit conditions and a short electron lifetime (Tn) in device containing Co-bpm indicate that a significant loss of electrons from TiO₂ via recombination pathways is one key factor that contribute to the poor photocurrent and overall device performance.
A series of donor-bridge-acceptor (D-π-A) compounds, differing only by the identity of two halogen atoms substituted on the triphenylamine (TPA, donor), were synthesized and characterized for insight into the regeneration reactions within dye-sensitized solar cells (DSSCs) [Dye-X⁺/TiO₂(e-) + I− → Dye-X/TiO₂ + I₂•−]. The structures of each series conformed to a molecular scaffold bearing a TPA donor, thiophene spacer, and acrylic acid unit as the anchoring group. In Chapter 2, each Dye-X (X = F, Cl, Br, and I) was immobilized on a TiO₂ surface to investigate how the halogen substituents affect the reaction rate between the light-induced charge-separated state, TiO₂(e−)/Dye-X⁺, with iodide in solution. Transient absorption spectroscopy showed progressively faster reactivity towards nucleophilic iodide with more polarizable halogen substituents: Dye-F
The splitting of water into hydrogen and oxygen is widely viewed as the most sustainable option for storing energy produced by intermittent renewable energy sources such as solar or wind. Economically feasible large-scale deployment of this type of system requires the discovery of efficient electrocatalysts, particularly for the kinetically slow oxygen evolution reaction (OER). Transition metal oxides are the most durable and active water oxidation catalysts, and there is a growing body of evidence showing amorphous metal oxide films mediate the OER more efficiently than the crystalline phases of the same compositions. Notwithstanding, there is a limited set of fabrication methods available for making amorphous films, particularly in the absence of a conducting substrate. I introduce herein a scalable preparative method for accessing oxidized and reduced phases of amorphous films that involves the efficient decomposition of molecular precursors, including simple metal salts, by exposure to near-infrared (NIR) radiation. The NIR-driven decomposition process provides sufficient localized heating to trigger the liberation of the ligand from solution-deposited precursors on substrates, but insufficient thermal energy to form crystalline phases. This method provides access to state-of-the-art electrocatalyst films, as demonstrated herein for the electrolysis of water, and extends the scope of usable substrates to include non-conducting and temperature-sensitive platforms. Because crystalline ruthenium oxide is one of the most efficient electrocatalysts in acidic media, it would be highly advantageous to be able to readily access the amorphous phase of the material. I also document two facile preparation techniques for accessing amorphous ruthenium oxide, a state-of-the-art electrocatalyst. The formation of amorphous ruthenium oxide films is triggered by the decomposition of a film of spin-cast molecular ruthenium precursors on conducting glass by either ultraviolet (UV) and near infrared (NIR) light.
A series of three bis-tridentate ruthenium(II) complexes containing one cyclometalating ligand with terminal triphenylamine (TPA) substituents have been synthesized and characterized for insight into electron transfer reactions at TiO₂ surfaces. The structure of each complex conforms to a molecular scaffold formulated as [Ru(II)(TPA-2,5-thiophene-pbpy)(H₃tctpy)] (pbpy = 6-phenyl-2,2’-bipyridine; H₃tctpy = 4,4’,4”-tricarboxy-2,2’:6’,2”-terpyridine), where an electron-donating group (EDG) or an electron-withdrawing group (EWG) is installed about the anionic ring of the pbpy ligand and methyl groups surrounding the TPA-thiophene bridge. Modification of the anionic ring of the pbpy chelated with EDGs and EWGs enables the modulation of the Ru(III)/Ru(II) redox potential over 140 mV. This property offers the opportunity to turn on and off intramolecular hole transfer. Pulsed light laser excitation of the sensitized thin film resulted in rapid excited state injection and in some cases hole transfer to TPA [TiO₂(e⁻)/Ru(III)−TPA → TiO₂(e⁻)/Ru(II)−TPA･⁺. The rate constants for charge recombination of [TiO₂(e⁻)/Ru(III)−TPA → TiO₂/Ru(II)−TPA and TiO₂(e⁻)/Ru(II)−TPA･⁺ → TiO₂/Ru(II)−TPA] were drastically affected by modification of the bridging unit and can be modulated over 5.2 – 6.2×10⁵ s ⁻¹ and 1.7 – 5.1×10⁴ s⁻¹ respectively.