Hanlin Luo

Interfacial Dynamics and Solar Fuel Formation in Dye-Sensitized Photoelectrosynthesis Cells

Dye-sensitized photoelectrosynthesis cells (DSPECs) represent a promising approach to solar fuels with solar-energy storage in chemical bonds. The targets are water splitting and carbon dioxide reduction by water to CO, other oxygenates, or hydrocarbons. DSPECs are based on dye-sensitized solar cells (DSSCs) but with photoexcitation driving physically separated solar fuel half reactions. A systematic basis for DSPECs is available based on a modular approach with light absorption/excited-state electron injection, and catalyst activation assembled in integrated structures. Progress has been made on catalysts for water oxidation and CO(2) reduction, dynamics of electron injection, back electron transfer, and photostability under conditions appropriate for water splitting. With added reductive scavengers, as surrogates for water oxidation, DSPECs have been investigated for hydrogen generation based on transient absorption and photocurrent measurements. Detailed insights are emerging which define kinetic and thermodynamic requirements for the individual processes underlying DSPEC performance.

Controlled Electropolymerization of Ruthenium(II) Vinylbipyridyl Complexes in Mesoporous Nanoparticle Films of TiO2

Surface-initiated, oligomeric assemblies of ruthenium(II) vinylpolypyridyl complexes have been grown within the cavities of mesoporous nanoparticle films of TiO2 by electrochemically controlled radical polymerization. Surface growth was monitored by cyclic voltammetry as well as UV/Vis and X-ray photoelectron spectroscopy. Polymerization occurs by a radical chain mechanism following cyclic voltammetry scans to negative potentials where reduction occurs at the π* levels of the polypyridyl ligands. Oligomeric growth within the cavities of the TiO2 films occurs until an average of six repeat units are added to the surface-bound initiator site, which is in agreement with estimates of the internal volumes of the pores in the nanoparticle films.

Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC)

Time-resolved, UV-vis spectroscopic measurements of Li+ diffusion in mesoscopic TiO2 photoanodes were conducted in dye sensitized photoelectrosynthesis cells (DSPECs) under operating conditions. In these experiments the spectral response of TiO2 derivatized with [Ru(bpy)2(4,4′-((HO)2PO)2bpy)]2+ (RuP, where bpy is 2,2′-bipyridine, (4,4′-((HO)2PO)2bpy) is [2,2′-bipyridine]-4,4′-diphosphonic acid) arises from electric field (Stark) effects on the metal-to-ligand-charge transfer (MLCT) absorption spectrum of RuP, which is screened by cation intercalation. These results verify that Li+ diffusion is coupled to electron injection and to electron recombination/extraction at the TiO2 interface. Li+ doping levels depend on the competition between dynamics of its intercalation and electron recombination/transport. For a DSPEC operating in aqueous solution at pH 4.5, the observed rate constants for Li+ intercalation and release were 0.22 s−1 and 0.014 s−1, respectively. Both processes were considerably slower in the more viscous solvent propylene carbonate with Li+ release rate constants <2 × 10−4 s−1. Accumulation of Li+ under these conditions shifts conduction band/trap states to less negative potentials, increasing electron lifetime in TiO2.

High Surface Area Antimony-Doped Tin Oxide Electrodes Templated by Graft Copolymerization. Applications in Electrochemical and Photoelectrochemical Catalysis

Mesoporous ATO nanocrystalline electrodes of micrometer thicknesses have been prepared from ATO nanocrystals and the grafted copolymer templating agents poly vinyl chloride-g-poly(oxyethylene methacrylate). As-obtained electrodes have high interfacial surface areas, large pore volumes, and rapid intraoxide electron transfer. The resulting high surface area materials are useful substrates for electrochemically catalyzed water oxidation. With thin added shells of TiO2 deposited by atomic layer deposition (ALD) and a surface-bound Ru(II) polypyridyl chromophore, they become photoanodes for hydrogen generation in the presence of a reductive scavenger.