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Open Pengtao Xu Thesis V5.Pdf The Pennsylvania State University The Graduate School Eberly College of Science DYE-SENSITIZED PHOTOELECTROCHEMICAL CELL FOR WATER SPLITTING A Dissertation in Chemistry by Pengtao Xu © 2018 Pengtao Xu Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2018 The dissertation of Pengtao Xu was reviewed and approved* by the following: Thomas E. Mallouk Evan Pugh University Professor of Chemistry, Biochemistry and Molecular Biology, Physics, and Engineering Science and Mechanics Dissertation Advisor Chair of Committee Raymond E. Schaak DuPoint Professor of Materials Chemistry Benjamin J. Lear Associate Professor of Chemistry Noel C. Giebink Assistant Professor of Electrical Engineering Scott A. Showalter Associate Professor of Chemistry, and Biochemistry and Molecular Biology Graduate Program Chair *Signatures are on file in the Graduate School ii 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 iii 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. iv Table of Contents List of Figures ............................................................................................................................... viii List of Tables ................................................................................................................................. xv Acknowledgements ....................................................................................................................... xvi Water-splitting Dye-sensitized Solar Cells .................................................................... 1 1.1. Introduction ...................................................................................................................... 2 1.2. Dye sensitized solar cells: regenerative and photosynthetic ............................................ 5 1.3. Dye-sensitized photoanodes for water-splitting cells ...................................................... 8 1.3.1. Electron and proton transfer at dye-sensitized photoanodes ..................................... 8 1.3.2. Core-shell photoanode architectures ....................................................................... 20 1.4. Photocathodes for water splitting dye cells .................................................................... 29 1.4.1. General principles ................................................................................................... 29 1.4.2. p-Type semiconductors for water splitting photocathodes ..................................... 35 1.4.3. Photocathode sensitizers ......................................................................................... 38 1.4.4. Photocathode catalysts ............................................................................................ 40 1.4.5. Conclusions ............................................................................................................. 42 Flat-band Potential in Molecularly Thin Metal Oxide Nanosheets .............................. 44 2.1. Introduction .................................................................................................................... 45 2.2. Experimental Section ..................................................................................................... 47 2.2.1. Preparation of layered metal oxide nanosheets ....................................................... 47 2.2.2. Layer-by-layer (LBL) assembly of the nanosheets ................................................. 48 2.2.3. Electrochemical measurements ............................................................................... 49 2.2.4. Characterization ...................................................................................................... 50 2.2.5. Electronic structure calculations ............................................................................. 50 2.3. Results and Discussion .................................................................................................. 51 2.3.1. Preparation of nanosheets ....................................................................................... 51 2.3.2. Layer-by-Layer Assembly ...................................................................................... 52 2.3.3. Band gap determination .......................................................................................... 53 2.3.4. Mott-Schottky Experiments .................................................................................... 55 2.4. Conclusions .................................................................................................................... 61 v Charge Recombination with Fractional Reaction Orders in Water-Splitting Dye- sensitized Photoelectrochemical Cells ........................................................................ 63 3.1. Introduction .................................................................................................................... 64 3.2. Theory ............................................................................................................................ 67 3.3. Experimental Section ..................................................................................................... 69 3.4. Results and Discussion .................................................................................................. 71 3.4.1. Photoelectrochemical impedance spectroscopy ...................................................... 72 3.4.2. Intensity-modulated photovoltage spectroscopy..................................................... 74 3.4.3. Transient absorption spectroscopy .......................................................................... 80 3.5. Conclusions .................................................................................................................... 83 Charge Transport Dynamics in Dye-sensitized Photoelectrochemical Cells ............... 85 4.1. Introduction .................................................................................................................... 86 4.2. Theory for Numerical Modeling of IMPS ..................................................................... 88 4.3. Experimental Section ..................................................................................................... 90 4.4. Results and Discussion .................................................................................................. 91 4.4.1. Simulation parameters ............................................................................................ 91 4.4.2. Steady-state concentration profile .......................................................................... 94 4.4.3. Influence of 푰ퟎ, 푫풏, 풌ퟐ, and 풌ퟑ on IMPS............................................................. 95 4.5. Conclusions .................................................................................................................
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