Dye-Sensitized Photoelectrochemical Cell for Water Splitting

Open Access
- Author:
- Xu, Pengtao
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 06, 2018
- Committee Members:
- Thomas E. Mallouk, Dissertation Advisor/Co-Advisor
Thomas E. Mallouk, Committee Chair/Co-Chair
Raymond E. Schaak, Committee Member
Benjamin J. Lear, Committee Member
Noel C. Giebink, Outside Member - Keywords:
- dye-sensitized photoelectrochemical cells
water splitting
intensity modulated photovoltage spectroscopy
artificial photosynthesis - Abstract:
- Artificial photosynthesis mimics the natural processes of converting solar energy, water, and CO2 into chemical fuels. Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) present a modular approach on a molecular level for solar-to-fuel conversion. WS-DSPECs utilize a high surface area metal-oxide electrode that is sensitized with molecular light absorbers for light harvesting and functionalized with molecular or nanoscopic catalysts for promoting water oxidation and reduction reactions. In an operating photoanode of a WS-DSPEC, light-induced electron injection from sensitizer molecules to the metal oxide support (e.g. TiO2) occurs within one nanosecond. The slow electron recombination from the metal oxide to oxidized sensitizer molecules creates at the semiconductor-sensitizer interface a charge-separated state that persists for microseconds to milliseconds, an adequate time for the subsequent water oxidation reaction. Despite these promising features, WS-DSPECs reported to date still operate at a low energy conversion efficiency, with the main bottleneck being the undesired back electron transfer process. This thesis explores in details the fundamental kinetic processes in WS-DSPECs. Chapter 1 introduces the research background and working principles of WS-DSPECs and summarizes recent research progress. Chapter 2 presents the characterization of the flat-band potentials of two-dimensional metal oxide nanosheets, an initial attempt to use thin metal oxide nanosheets as sensitizer scaffolds. In Chapter 3, we characterize the charge recombination process in WS-DSPECs using a combination of intensity-modulated photovoltage spectroscopy, photoelectrochemical impedance spectroscopy, and time-resolved absorption spectroscopy. The sharp differences in recombination lifetimes as measured by different techniques is rationalized in terms of the experimental conditions. We also formulate the reaction orders for the recombination process at the semiconductor-sensitizer interface. In Chapter 4, we combine numerical modeling with intensity-modulated photocurrent spectroscopy to simulate the charge transport dynamics in WS-DSPECs. This approach outlines how individual processes (such as electron diffusion, electron recombination, and sensitizer regeneration) influence the electrode performance. Chapter 5 demonstrates a buried-junction design of WS-DSPECs for water oxidation. Mummifying a solid-state dye-sensitized solar cell within an atomically thin protecting layer gives improved power-conversion efficiency and greatly enhanced stability. These results help us understand the fundamental mechanisms of electron transfer and catalysis in WS-DSPECs and enable re-design of the photoanode for the creation of more efficient and durable artificial photosynthetic systems.