Metal Oxide-Based Photoelectrochemical Cells for Solar Energy Conversion

Citation

Katz, Jordan E. (2008) Metal Oxide-Based Photoelectrochemical Cells for Solar Energy Conversion. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/29VB-HV45. https://resolver.caltech.edu/CaltechETD:etd-10192007-190231

Abstract

In order to address the need for CO₂-free energy, recent trends in global CO₂ emissions and energy production are analyzed, and the photoelectrochemical properties of two types of metal oxide-based solar cells are presented.

The effects of potential-determining cations (Li⁺, H⁺) in the electrolyte of TiO₂-based dye-sensitized solar cells, using Ru(H₂L’)₂(NCS)₂, where H₂L’ is 4,4’-dicarboxylic acid-2,2’bipyridine, as a sensitizer was investigated using current density vs potential (J-E), spectrochronocoulometric, and spectroscopic methods. Photoelectrochemical cells with lower concentrations of the cations Li⁺ and H⁺ had increased open-circuit voltages (V_oc), and decreased short-circuit current densities (J_sc). Spectrochronocoulometric methods indicated that the energy of states in TiO₂ shifted by approximately -1 V when in contact with electrolytes lacking small cations. Spectral response measurements indicated that the loss of photocurrent was accompanied by a nearly monotonic drop in the external quantum yield across all wavelengths.

Transient absorption spectroscopy was used to measure the kinetics of interfacial electron transfer of the same system. No dependence was observed on the ultrafast dynamics of electron injection on cations used in ClO₄^--based solutions. However, in solutions of TBA⁺ with I₃^-/I^-, femtosecond, but not picosecond, dynamics were observed. In contrast, for solutions with Li⁺ and ClO₄^-, I^- or I^-/I₃^-, both femtosecond and picosecond dynamics were observed. Nanosecond-resolved spectroscopy results show that the absence of small cations did not affect the rate of recombination, while the regeneration rate of [RuIII(H₂L’)₂(NCS)₂]⁺ was decreased. Results indicate that both the ground and excited state reduction potentials of the sensitizer shift as a function of small cations in solution, along with the energy of states in TiO₂. The efficiency of electron injection is thus largely unchanged; rather a decrease in the regeneration rate accounts for the loss of J_sc.

Finally, a novel, high-throughput, combinatorial approach for the synthesis and screening of mixed-metal oxides for use as water-splitting photocatalysts was developed. The methodology relies on inkjet printing to form quantitative mixtures of aqueous metal oxide precursors. After pyrolysis, the photoelectrochemical properties of metal oxides can be fully characterized in an automated high-speed system, including measurement of the V_oc and J-E curves.

Thesis Files