Initial experiments on water oxidation by well-defined molecular catalysts were initiated with the goal of finding solutions to solar energy conversion. This account is a summary of research in this area by the T. J. Meyer research group. It begins with the design and characterization of the first catalyst, the blue Ru dimer, to current applications with surface-bound complexes on photoanodes for water oxidation in Dye Sensitized Photoelectrosynthesis Cells.
An electrochemical procedure for preparing chromophore-catalyst assemblies on oxide electrode surfaces by reductive vinyl coupling is described. On core/shell SnO 2 /TiO 2 nanoparticle oxide films, excitation of the assembly with 1 sun (100 mW cm −2 ) illumination in 0.1 M H 2 PO 4 − /HPO 4 2− at pH 7 with an applied bias of 0.4 V versus SCE leads to water splitting in a DSPEC with a Pt cathode. Over a 5 min photolysis period, the core/shell photoanode produced O 2 with a faradaic efficiency of 22%. Instability of the surface bound chromophore in its oxidized state in the phosphate buffer leads to a gradual decrease in photocurrent and to the relatively modest faradaic efficiencies.T he dye-sensitized photoelectrosynthesis cell (DSPEC), which incorporates electrode architectures similar to those used in dye-sensitized solar cells (DSSCs), 1,2 integrates molecular chromophores and catalysts with a high band gap semiconductor oxide electrode for water splitting into O 2 and H 2 or for CO 2 reduction to a reduced carbon fuel. 3−7 In exploiting the initial, seminal work of Fujishima and Honda, 8 a DSPEC integrates molecular light absorption and catalysis with the bandgap properties of oxide semiconductors to extend light absorption into the visible and utilize chemical catalysis of solar fuel half reactions. A variety of DSPEC configurations have appeared in the literature, 9−15 but low overall solar energy conversion efficiencies as well as poor long-term stability remain a central challenge.Examples of DSPEC water splitting have been reported based on carboxylate-, 13 phosphonate-, 14,16 or siloxyl-derivatized 12 surface binding and by preformed, covalently linked chromophore-catalyst assemblies. 9 Additional surface binding strategies have been explored including embedding molecular components in polymer film coatings 10,15 and "layer-by-layer" assemblies with Zr(IV)-phosphonate bridges. 17,18 Use of preformed assemblies offers synthetic control and well-defined structures but, typically, requires laborious multiple-step synthetic procedures resulting in low overall yields. Based on earlier procedures for preparing cross-linked electropolymerized films by reductive coupling of vinylderivatized polypyridyl complexes, 19 electroassembly offers the advantage of on-surface synthesis without prior covalent bond formation.Electropolymerization has been used to form electroactive thin films on a variety of conducting and semiconducting substrates, including metal oxides. 20,21 It provides a basis for preparing controlled surface coverages, 22,23 multicomponent and layered structures, 19,22,23 and electrocatalytic films, 21,24,25 including films for electrocatalytic water oxidation. 22 Photoelectrochemical oxidation of iodide and hydroquinone in electropolymerized Ru(II) polypyridyl films has also been reported. 26 In a recent report, we described an extension of the vinyl reduction/C−C coupling chemistry used in cross-linked films to the preparation of electroassemblies within the cavities of nanoparticle and mesoscopi...
Visible light driven water splitting in a dye-sensitized photoelectrochemical cell (DSPEC) based on a phosphonic acidderivatized donor-p-acceptor (D-p-A) organic dye (P-A-p-D) is described with the dye anchored to an FTO|SnO2/TiO2 core/shell photoanode in a pH 7 phosphate buffer solution. Transient absorption measurements on FTO|TiO2|-[P-A-p-D] compared to core/shell, FTO|SnO2/TiO2(3nm)|-[P-A-p-D], reveal that excitation of the dye is rapid and efficient with a decrease in back electron rate by a factor of ~10 on the core/shell. Upon visible, 1 sun excitation (100 mWcm -2 ) of FTO|SnO2/TiO2(3nm)|-[P-A-p-D] in a phosphate buffer at pH 7 with 20 mM added hydroquinone (H2Q), photocurrents of ~2.5 mA/cm 2 are observed which are sustained over >15 min photolysis periods with a current enhancement of ~30-fold compared to FTO|TiO2|-[P-A-p-D] due to the core/shell effect. On surfaces co-loaded with both -[P-A-p-D] and the known water oxidation catalyst, Ru(bda)(pyP)2 (pyP = pyridin-4-methyl phosphonic acid), maximum photocurrent levels of 1.4 mA/cm 2 were observed which decreased over an 10 min interval to 0.1 mA/cm 2 . O2 was measured by use of a twoelectrode, collector-generator sandwich cell and was produced in low Faradaic efficiencies with the majority of the oxidative photocurrent due to oxidative decomposition of the dye. D] 2+ , inset, Fig. 1a. In acetonitrile, no significant reduction in current was observed even after 50 CV scan cycles at 20 mV/s, Fig. S1, highlighting the relative stability of the mono-and dicationic forms of the dye in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAP) as the electrolyte.
Tandem junction photoelectrochemical water-splitting devices, whereby two light absorbing electrodes targeting separate portions of the solar spectrum generate the voltage required to convert water to oxygen and hydrogen, enable much higher possible efficiencies than single absorber systems. We report here on the development of a tandem system consisting of a dye-sensitized photoelectrochemical cell (DSPEC) wired in series with a dye-sensitized solar cell (DSC). The DSPEC photoanode incorporates a tris(bipyridine)ruthenium(II)-type chromophore and molecular ruthenium based water oxidation catalyst. The DSPEC was tested with two more-red absorbing DSC variations, one utilizing N719 dye with an I/I redox mediator solution and the other D35 dye with a tris(bipyridine)cobalt ([Co(bpy)]) based mediator. The tandem configuration consisting of the DSPEC and D35/[Co(bpy)] based DSC gave the best overall performance and demonstrated the production of H from HO with the only energy input from simulated solar illumination.
A dual working electrode technique for the in situ production and quantification of electrochemically or photoelectrochemically produced O2 is described. This technique, termed a collector-generator cell, utilizes a transparent fluorine doped tin oxide electrode to sense O2. This setup is specifically designed for detecting O2 in dye sensitized photoelectrosynthesis cells.
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