A high molar extinction coefficient heteroleptic ruthenium complex, incorporating an electron-rich hexylthio-terminal chain, has been synthesized and demonstrated as an efficient sensitizer for dye-sensitized solar cells. With this new sensitizer excellent power conversion efficiency is 11.5% and 4.7% obtained under an irradiation of full sunlight (air mass 1.5 global) in combination with a volatility electrolyte and solid state hole transporting material, respectively. The devices with low volatility electrolyte showed good stability under visible-light soaking at 60 degrees C during 1000 h of accelerated tests.
The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.
Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core-shell nanostructured photoanode with the core a high surface area conductive metal oxide film--indium tin oxide or antimony tin oxide--coated with a thin outer shell of TiO 2 formed by atomic layer deposition. A "chromophore-catalyst assembly" 1, [(PO 3 H 2 ) 2 bpy) 2 Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH 2 )] 4+ , which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO 2 shell. Visible photolysis of the resulting core-shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.P hotosynthesis uses the energy of the sun with water as the reducing agent to drive the reduction of carbon dioxide to carbohydrates with oxygen as a coproduct through a remarkably complex process. At photosystem II, a subsystem imbedded in the thylakoid membrane where O 2 is produced, light absorption, energy migration, electron transfer, proton transfer, and catalysis are all used in multiple stepwise chemical reactions which are carefully orchestrated at the molecular level (1, 2).Photosynthesis solves the problem of energy storage by biomass production but with low solar efficiencies, typically <1%. In artificial photosynthesis with solar fuels production, the goal is similar but the targets are either hydrogen production from water splitting, Eq. 1, or reduction of carbon dioxide to a carbon-based fuel, Eq. 2 (3, 4). Different strategies for solar fuels have evolved (5, 6). In one, direct bandgap excitation of semiconductors creates electron-hole pairs which are then used to drive separate halfreactions for water oxidation (2H 2 O → O 2 + 4H + + 4e − ) and water/proton reduction (2H + + 2e − → H 2 ) (7-9).Here, we report a hybrid strategy for solar water splitting, the dye sensitized photoelectrosynthesis cell (DSPEC). It combines the electron transport properties of semiconductor nanocrystalline thin films with molecular-level reactions (10). In this approach, a chromophore-catalyst molecular assembly acts as both light absorber and catalyst. It is bound to the surface of a "core-shell," nanostructured, transparent conducting oxide film. The core structure consists of a nanoparticle film of either tin-doped indium oxide (nanoITO), or antimony-doped tin oxide (nanoATO), deposited on a fluoride-doped tin oxide (FTO) glass substrate. The shell consists of a conformal TiO 2 nanolayer applied by atomic layer deposition (ALD). The resulting "photoanode," where water oxidation occurs, is connected to a Pt cathode for proton reduction to complete the water splitting cell. A diagram for the photoanode in the DSPEC device is shown in Fig. 1. It illustrates...
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...
Enhancing the surface binding stability of chromophores, catalysts, and chromophore-catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1-12) by atomic layer deposition of an overlayer of TiO 2 . Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases −H 2 PO − 4 /HPO 2− 4 , B(OH) 3 /B(OH) 2 O − , HPO 2− 4 /PO 3− 4 − and with a pathway identified in which O-atom transfer to OH − occurs with a rate constant increase of 10 6 compared to water oxidation in acid.electrocatalysis | surface stabilization H eterogeneous catalysis plays an important role in industrial chemical processing, fuel reforming, and energy-producing reactions. Examples include the Haber-Bosch process, steam reforming, Ziegler-Natta polymerization, and hydrocarbon cracking (1-8). Research in heterogeneous catalysis continues to flourish (9-15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms. By contrast, synthetic modification of molecular catalysts is possible by readily available routes; a variety of experimental techniques is available for monitoring rates and mechanism in solution for the investigation of homogeneous catalysis (16-23). Transferring this knowledge and the reactivity of homogeneous molecular catalysts to a surface could open the door to heterogeneous applications in fuel cells, dye sensitized photoelectrochemical cells, and multiphase industrial reactions.Procedures are available for immobilization of organometallic and coordination complexes on the surfaces of solid supports. Common strategies include surface derivatization of metal oxides by carboxylate, phosphonate, and siloxane bindings (24-27), carbongrafted electrodes (28-30), and electropolymerization (31-33). These approaches provide a useful bridge to the interface and a way to translate mechanistic understanding and ease of synthetic modification of solution catalysts to heterogeneous applications with a promise of higher reactivity under milder conditions. A significant barrier to this approach arises from the limited stability of surface binding. Surface-bound carboxylates are typically unstable to hydrolysis in water, whereas phosphonates are unstable in neutral or basic...
Interfacial electron transfer at titanium dioxide (TiO2) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru(II)(bpy-A)(L)2(2+), where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO2. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru(III/)* couple, E(+/)*, in 0.1 M perchloric acid (HClO4(aq)) to be tuned from -0.69 to -1.03 V vs NHE. Each dye is excited by a 200 fs pulse of light in the visible region of the spectrum and probed with a time-delayed supercontiuum pulse (350-800 nm). Decay of the MLCT excited-state absorption at 376 nm is observed without loss of the ground-state bleach, which is a clear signature of electron injection and formation of the oxidized dye. The dye-dependent decays are biphasic with time constants in the 3-30 and 30-500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to E(+/)*. The correlation between the exponentially diminishing density of TiO2 sub-band acceptor levels and injection rate is well described using Marcus-Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the (3)MLCT manifold. These results and detailed analyses incorporating molecular photophysics and semiconductor density of states measurements indicate that the multiexponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, this work shows that the kinetic heterogeneity results from competition between excited-state relaxation and injection as the photoexcited dye relaxes through the (3)MLCT manifold to the thermally equilibrated state, underscoring the potential for a simple kinetic model to reproduce the complex kinetic behavior often observed at the interface of mesoporous metal oxide materials.
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.
The oxidative stability of the molecular components of dye-sensitized photoelectrosynthesis cells for solar water splitting remains to be explored systematically. We report here the results of an electrochemical study on the oxidative stability of ruthenium(II) polypyridyl complexes surface-bound to fluorine-doped tin oxide electrodes in acidic solutions and, to a lesser extent, as a function of pH and solvent with electrochemical monitoring. Desorption occurs for the Ru(II) forms of the surface-bound complexes with oxidation to Ru(III) enhancing both desorption and decomposition. Based on the results of long-term potential hold experiments with cyclic voltammetry monitoring, electrochemical oxidation to Ru(III) results in slow decomposition of the complex by 2,2'-bipyridine ligand loss and aquation and/or anation. A similar pattern of ligand loss was also observed for a known chromophore-catalyst assembly for both electrochemical water oxidation and photoelectrochemical water splitting. Our results are significant in identifying the importance of enhancing chromophore stability, or at least transient stability, in oxidized forms in order to achieve stable performance in aqueous environments in photoelectrochemical devices.
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