As the world’s population increases, the U.S. Department of Energy has estimated that energy consumption will increase with it, increasing from 5.7 × 1020 J in 2012 to 8.1 × 1020 J in 2040. This ...projection takes into account population growth, average gross domestic product per capita, and globally averaged energy intensity. At the same time, there is an increased understanding of the importance of integrating good environmental practices with stimulating economic growth and leadership provided by international organizations, such as the Organization for Economic Cooperation and Development. Most of the increases in energy consumption will come from emerging economies in Asia, Africa, and South America where goals, priorities, and challenges are diverse. To meet these demands there is a need to develop alternate energy sources. Although renewable and nuclear energy are growing by 2.5% a year, fossil fuels are still projected to make up at least 80% of the global energy supply in 2040.
Nitrogen-doped carbon nanotubes are selective and robust electrocatalysts for CO2 reduction to formate in aqueous media without the use of a metal catalyst. Polyethylenimine (PEI) functions as a ...co-catalyst by significantly reducing catalytic overpotential and increasing current density and efficiency. The co-catalysis appears to help in stabilizing the singly reduced intermediate CO2 •– and concentrating CO2 in the PEI overlayer.
Significance Mesoporous SnO ₂/TiO ₂ core/shell nanostructured electrodes derivatized with a surface-bound Ru(II) polypyridyl-based chromophore–catalyst assembly are used for water splitting into H ₂ ...and O ₂ with visible light in a dye-sensitized photoelectrosynthesis cell. Photocurrents with a small applied bias are among the highest reported. Stabilization of the assembly on the surface of the TiO ₂ shell by using atomic layer deposition to deposit overlayers of Al ₂O ₃ or TiO ₂ results in long-term water splitting even in a phosphate buffer at pH 7.
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, (4,4’-(PO ₃H ₂) ₂bpy) ₂Ru(4-Mebpy-4’-bimpy)Ru(tpy)(OH ₂) ⁴⁺ (Ru ₐᴵᴵ-Ru bᴵᴵ-OH ₂ ⁴⁺, combines both a light absorber and a water oxidation catalyst in a single molecule. It was attached to the TiO ₂ shell by phosphonate-surface oxide binding. The oxide-bound assembly was further stabilized on the surface by atomic layer deposition (ALD) of either Al ₂O ₃ or TiO ₂ overlayers. Illumination of the resulting fluorine-doped tin oxide (FTO)|SnO ₂/TiO ₂|-Ru ₐᴵᴵ-Ru bᴵᴵ-OH ₂ ⁴⁺(Al ₂O ₃ or TiO ₂) photoanodes in photoelectrochemical cells with a Pt cathode and a small applied bias resulted in visible-light water splitting as shown by direct measurements of both evolved H ₂ and O ₂. The performance of the resulting DSPECs varies with shell thickness and the nature and extent of the oxide overlayer. Use of the SnO ₂/TiO ₂ core/shell compared with nano ITO/TiO ₂ with the same assembly results in photocurrent enhancements of ∼5. Systematic variations in shell thickness and ALD overlayer lead to photocurrent densities as high as 1.97 mA/cm ² with 445-nm, ∼90-mW/cm ² illumination in a phosphate buffer at pH 7.
In order for solar energy to serve as a primary energy source, it must be paired with energy storage on a massive scale. At this scale, solar fuels and energy storage in chemical bonds is the only ...practical approach. Solar fuels are produced in massive amounts by photosynthesis with the reduction of CO
by water to give carbohydrates but efficiencies are low. In photosystem II (PSII), the oxygen-producing site for photosynthesis, light absorption and sensitization trigger a cascade of coupled electron-proton transfer events with time scales ranging from picoseconds to microseconds. Oxidative equivalents are built up at the oxygen evolving complex (OEC) for water oxidation by the Kok cycle. A systematic approach to artificial photo-synthesis is available based on a “modular approach” in which the separate functions of a final device are studied separately, maximized for rates and stability, and used as modules in constructing integrated devices based on molecular assemblies, nanoscale arrays, self-assembled monolayers, etc. Considerable simplification is available by adopting a “dye-sensitized photoelectrosynthesis cell” (DSPEC) approach inspired by dye-sensitized solar cells (DSSCs). Water oxidation catalysis is a key feature, and significant progress has been made in developing a single-site solution and surface catalysts based on polypyridyl complexes of Ru. In this series, ligand variations can be used to tune redox potentials and reactivity over a wide range. Water oxidation electrocatalysis has been extended to chromophore-catalyst assemblies for both water oxidation and DSPEC applications.
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 ₃H ₂) ₂bpy) ₂Ru(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.
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 CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 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/CO2 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.
Splitting CO₂ into CO and O₂ by a single catalyst Chen, Zuofeng; Concepcion, Javier J; Brennaman, M. Kyle ...
Proceedings of the National Academy of Sciences - PNAS,
09/2012, Letnik:
109, Številka:
39
Journal Article
Recenzirano
The metal complex (tpy)(Mebim-py)Ru ᴵᴵ(S) ²⁺ (tpy = 2,2 ′ : 6 ′,2 ′′-terpyridine; Mebim-py = 3-methyl-1-pyridylbenzimidazol-2-ylidene; S = solvent) is a robust, reactive electrocatalyst toward both ...water oxidation to oxygen and carbon dioxide reduction to carbon monoxide. Here we describe its use as a single electrocatalyst for CO ₂ splitting, CO ₂ → CO + 1/2 O ₂, in a two-compartment electrochemical cell.
Photoanodes in dye-sensitized photoelectrosynthesis cells integrate molecular chromophore/catalyst assemblies on mesoporous n-type metal oxide electrodes for light-driven water oxidation. One ...limitation for sustainable photoanodes is the stability of chromophore/catalyst assembly on electrode surfaces for long periods. Progress has been made in stabilizing chromophores based on atomic layer deposition, polymer dip coating, C–C cross-coupling by electropolymerization, and silane surface binding, but little progress has been made on catalyst stabilization. We report here the silane-derivatized catalyst, Ru(bda)(L)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = 4-(6-(triethoxysilyl)hexyl)pyridine), catalyst 1, which is stabilized on metal oxide electrode surfaces over an extended pH range. A surface stabilization study shows that it maintains its reactivity on the electrode surface toward electrochemical oxidation over a wide range of conditions. Its electrochemical stability on electrode surfaces has been systematically evaluated, and its role as a catalyst for water oxidation has been explored. On surfaces of mesoporous nanostructured core/shell SnO2/TiO2, with a TiO2 stabilized inner layer of the Ru(II) polypyridyl chromophore, Ru(4,4′-(PO3H2)2bpy)(bpy)22+ (RuP 2+ ; bpy = 2,2′-bipyridine), highly efficient photoelectrochemical water oxidation catalysis occurs to produce O2 with a maximum efficiency of ∼1.25 mA/cm2. Long-term loss of catalytic activity occurs with time owing to catalyst loss from the electrode surface by axial ligand dissociation in the high oxidation states of the catalyst.
The photostability of Ru(II)(bpy)(2)(4,4'-(PO(3)H(2))(2)bpy)Cl(2) (bpy = 4,4'-bipyridine) on nanocrystalline TiO(2) and ZrO(2) films was investigated using a standard measurement protocol. Stability ...was evaluated by monitoring visible light absorbance spectral changes, in real time, during 455 nm photolysis (30 nm fwhm, 475 mW/cm(2)) in a variety of conditions relevant to dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. Desorption (k(des)) and photochemical (k(chem)) processes were observed and found to be dependent upon solvent, anion, semiconductor, and presence of oxygen. Both processes are affected by oxygen with k(des) and k(photo) noticeably smaller in argon saturated solution. Desorption was strongly solvent and pH dependent with desorption rates increasing in the order: methanol (MeOH) ≈ acetonitrile (MeCN) < propylene carbonate (PC) < pH 1 ≪ pH 7. Photochemistry occurred in MeOH and PC but not in aqueous, 0.1 M HClO(4) and MeCN. The anion and solvent dependence of k(photo) strongly suggests the photoreaction involves ligand substitution initiated by population of metal centered d-d states. The relative stability of -PO(3)H(2)- versus -COOH-substituted Ru(II)(bpy)(3)(2+) was also quantitatively established.
Synthesis and photophysical properties of the highly emissive complex Ir(Fppy)2(dmb)+ are reported along with those of additional heteroleptic cyclometalated Ir(III) complexes, Ir(ppy)2(NN)(PF6): ...FppyH = 2-(2,4-difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN = 4,4′-dimethyl-2,2′-bipyridine (dmb), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph 2 phen). TD-DFT calculations and Franck–Condon emission spectral band shape analyses show that the broad and structureless emission from Ir(Fppy)2(dmb)+ in acetonitrile at 298 K mainly arises from a triplet metal-to-ligand charge-transfer excited state, 3MLCTIr(ppy)→NN. The emission maximum varies systematically with variations in electron-donating or -withdrawing substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive ϕ ≈ 1 for Ir(Fppy)2(dmb)+. At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from Ir(Fppy)2(dmb)+ is narrow and highly structured consistent with a triplet ligand-centered transition (3LCNN) and an inversion in excited-state ordering between the 3MLCTIr(ppy)→NN and 3LCNN states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol spacers, PEG-DMA550, emission from Ir(Fppy)2(dmb)+ is MLCT-based. The thermal sensitivity of the photophysical properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-emitting devices.
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