Photoelectrochemical water splitting directly converts solar energy to chemical energy stored in hydrogen, a high energy density fuel. Although water splitting using semiconductor photoelectrodes has been studied for more than 40 years, it has only recently been demonstrated using dye-sensitized electrodes. The quantum yield for water splitting in these dye-based systems has, so far, been very low because the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. We show here that the quantum yield is more than doubled by incorporating an electron transfer mediator that is mimetic of the tyrosine-histidine mediator in Photosystem II. The mediator molecule is covalently bound to the water oxidation catalyst, a colloidal iridium oxide particle, and is coadsorbed onto a porous titanium dioxide electrode with a Ruthenium polypyridyl sensitizer. As in the natural photosynthetic system, this molecule mediates electron transfer between a relatively slow metal oxide catalyst that oxidizes water on the millisecond timescale and a dye molecule that is oxidized in a fast light-induced electron transfer reaction. The presence of the mediator molecule in the system results in photoelectrochemical water splitting with an internal quantum efficiency of approximately 2.3% using blue light.artificial photosynthesis | photoelectrochemistry T he design of biomimetic systems for artificial photosynthesis is of fundamental interest in the study of light-driven electron and proton transfer reactions. It also represents a potential route to the efficient conversion of solar energy to energy stored in fuel. System modeling has shown that it should be possible, using complementary dye molecules that absorb in the infrared and the visible, to construct artificial Z-schemes that split water with over 10% energy conversion efficiency (1, 2). It is simpler in many ways to design small molecules with the proper photoredox properties than it is to find a set of semiconductors that can be coupled for visible light water splitting. Nevertheless, the molecular approach has so far lagged behind the semiconductor-based approach where high efficiencies have been realized with expensive materials (3-7).A ubiquitous problem in molecular artificial photosynthesis is back electron transfer, which rapidly thermalizes the energy stored by light-induced charge separation in donor-acceptor pairs. Recently, our group and several others have studied this problem in dye-sensitized solar cells where a molecular dye and a porous TiO 2 electrode act as the donor-acceptor dyad (8-13). The dye is covalently coupled to a colloidal or molecular water oxidation catalyst. Fast back electron transfer, relative to the rate of water oxidation, results in low quantum yields for water splitting in these systems.It is well known that charge-separation lifetimes in molecular donor-acceptor systems can be increased dramatically by adding secondary electron donors or acceptors to form triads, tetrads, and more complex supermolecu...
A hybrid strategy for solar water splitting is exploited here based on a dye-sensitized photoelectrosynthesis cell (DSPEC) with a mesoporous SnO 2 /TiO 2 core/shell nanostructured electrode derivatized with a surface-bound Ru(II) polypyridyl-based chromophore-catalyst assembly. The assembly, [(4, H 2 ) 2 bpy) 2 Ru(4-Mebpy-4'-bimpy)Ru (tpy) (OH 2 dye-sensitized photoelectrosynthesis cell | water oxidation | core/shell A lthough promising, significant challenges remain in the search for successful strategies for artificial photosynthesis by water splitting into oxygen and hydrogen or reduction of CO 2 to reduced forms of carbon (1-5). In a dye-sensitized photoelectrosynthesis cell (DSPEC), a wide band gap, nanoparticle oxide film, typically TiO 2 , is derivatized with a surface-bound molecular assembly or assemblies for light absorption and catalysis (6-8). In a DSPEC, visible light is absorbed by a chromophore, initiating a series of events that culminate in water splitting: injection, intraassembly electron transfer, catalyst activation, and electron transfer to a cathode or photocathode for H 2 production. Sun and coworkers have recently demonstrated visible-light-driven water splitting with a coloading approach combining Ru(II) polypyridyl-based light absorbers and catalysts on TiO 2 (9). The efficiency of DSPEC devices is dependent on interfacial dynamics and competing kinetic processes. A major limiting factor is the requirement for accumulating multiple oxidative equivalents at a catalyst site to meet the 4e − /4H + demands for oxidizing water to dioxygen (2H 2 O -4e − -4H + → O 2 ) in competition with back electron transfer of injected electrons to the oxidized assembly.One approach to achieving structural control of local electron transfer dynamics at the oxide interface in dye-sensitized devices is by use of nanostructured core/shell electrodes (10-12). In this approach, a mesoporous network of nanoparticles is uniformly coated with a thin oxide overlayer prepared by atomic layer deposition (ALD). We have used core/shell electrodes to demonstrate benzyl alcohol dehydrogenation (13). This approach has also been used to enhance the efficiency of dye-sensitized solar cells (14,15). Recently, we described the use of a core/shell consisting of an inner core of a nanoparticle transparent conducting oxide, tin-doped indium oxide (nanoITO), and a thin outer shell of TiO 2 for water splitting by visible light (16 Fig. 1A, provided the basis for a photoanode in a DSPEC application with a Pt cathode for H 2 generation with a small applied bias in an acetate buffer at pH 4.6.Application of the core/shell structure led to a greatly enhanced efficiency for water splitting compared with mesoscopic, nanoparticle TiO 2 but the per-photon absorbed efficiency of the resulting DSPEC was relatively low and problems arose from longterm instability due to loss of the assembly from the oxide surface in the acetate buffer at pH 4.6. The latter is problematic because the rate of water oxidation is enhanced by added buffer bases...
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...
A Ru(ii)-polypyridyl chromophore-catalyst assembly for light-assisted water oxidation is constructed using atomic layer deposition with no covalent bonds between molecules required for bilayer formation.
Widespread implementation of renewable energy technologies, while preventing significant increases in greenhouse gas emissions, appears to be the only viable solution to meeting the wor d"s energy dem nds for sust in b e energy future. The fin energy mix wi inc ude conservation and energy efficiency, wind, geothermal, biomass, and others, but none more ubiquitous or abundant than the sun. Over several decades of development, the cost of photovoltaic cells has decreased significantly with lifetimes that exceed 25 years and there is promise for widespread implementation in the future. However, the solar input is intermittent and, to be practical at a truly large scale, will require an equally large capability for energy storage. One approach involves artificial photosynthesis and the use of the sun to drive solar fuel reactions for water splitting into hydrogen and oxygen or to reduce CO 2 to reduced carbon fuels. An early breakthrough in this area came from an initial report by Honda and Fujishima on photoelectrochemical water splitting at TiO 2 with UV excitation. Significant progress has been made since in exploiting semiconductor devices in water splitting with impressive gains in spectral coverage and solar efficiencies. An alternate, hybrid approach, which integrates molecular light absorption and catalysis with the band gap properties of oxide semiconductors, the dye-sensitized photoelectrosynthesis cell (DSPEC), has been pioneered by the University of North Carolina Energy Frontier Research Center (UNC EFRC) on Solar Fuels. By utilizing chromophore-catalyst assemblies, core/shell oxide structures, and surface stabilization, the EFRC recently demonstrated a viable DSPEC for solar water splitting.
In the photosynthetic photosystem II, electrons are transferred from the manganese-containing oxygen evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680 •+ by a proton-coupled electron transfer process involving a tyrosine-histidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680 •+ and the OEC. We report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF 10 ), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF 10 in benzonitrile is followed by singlet energy transfer to TCNP ( τ = 41 ps), whose excited state decays by photoinduced electron transfer ( τ = 830 ps) to yield . A second electron transfer reaction follows ( τ < 12 ps), giving a final state postulated as BiH + -PhO • -PF 10 -TCNP •- , in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 μs. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production.
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.
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