Solar Driven Water Oxidation by a Bioinspired Manganese Molecular Catalyst

138

107

Solar fuel generation requires the efficient capture and conversion of visible light. In both natural and artificial systems, molecular sensitizers can be tuned to capture, convert, and transfer visible light energy. We demonstrate that a series of metal-free porphyrins can drive photoelectrochemical water splitting under broadband and red light (λ > 590 nm) illumination in a dye-sensitized TiO2 solar cell. We report the synthesis, spectral, and electrochemical properties of the sensitizers. Despite slow recombination of photoinjected electrons with oxidized porphyrins, photocurrents are low because of low injection yields and slow electron self-exchange between oxidized porphyrins. The free-base porphyrins are stable under conditions of water photoelectrolysis and in some cases photovoltages in excess of 1 V are observed.

mentioning

confidence: 99%

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells

Swierk¹,

Méndez-Hernández

McCool³

et al. 2015

138

107

“…Previous reports have appeared describing photocurrents and water oxidation by various combinations of chromophores and catalysts on TiO 2 (26)(27)(28)(29)(30)(31), most notably by Mallouk and coworkers (32), but typically with small photocurrents, low efficiencies, and limited stabilities. The core-shell molecular assembly approach described here offers a general platform for using chromophorecatalyst assemblies and it should open the door to a host of DSPEC applications.…”

Section: Discussionmentioning

confidence: 99%

Solar water splitting in a molecular photoelectrochemical cell

Alibabaei

Brennaman

Norris

et al. 2013

214

232

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...

“…The structure of the water oxidation complex involves a cubic cluster with four Mn atoms bridged via oxygen atoms (8,9). To study the principles governing photosynthesis, molecular systems with structures comparable to the Mn 4 O 5 Ca cluster having a [Mn 4 O 4 ] core have been studied (10,11). Mn is not only an abundant element, but also occurs in easily switchable oxidation states.…”

Section: Artificial Photosynthesis | Water Oxidationmentioning

confidence: 99%

Importance of trivalency and the e _g ¹ configuration in the photocatalytic oxidation of water by Mn and Co oxides

Maitra

Naidu

Govindaraj

et al. 2013

163

134

Prompted by the early results on the catalytic activity of LiMn 2 O 4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn 2 O 3 , LaMnO 3 , and MgMn 2 O 4 are found to be very good catalysts with turnover frequencies of 5 × 10 −4 s −1 , 4.8 × 10 −4 s −1 , and 0.8 ×10 −4 s −1 , respectively. Among the cobalt oxides, Li 2 Co 2 O 4 and LaCoO 3 -especially the latter-exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10 −4 s −1 and 1.4 × 10 −3 s −1 , respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co 3+ in the Co oxides is in the intermediate t 2g 5 e g 1 state whereas Mn 3+ is in the t 2g 3 e g 1 state. The presence of the e g 1 electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.artificial photosynthesis | water oxidation A ny strategy for solving the energy crisis would involve the generation of fuels through artificial photosynthesis, involving the sun as the only source of energy. To complete the solar cycle, water has to act as the source of electrons, either to generate liquid fuels by the reduction of CO 2 or to yield H 2 through a complete cycle of transfer of electrons. Oxidation of water, involving the transfer of four electrons, is energy-intensive. One of the challenges with artificial photosynthesis is the development of cost-effective catalysts made of abundant elements for the efficient oxidation of water to O 2 (1). RuO 2 and IrO 2 are widely used as oxygen evolution catalysts although their availability is limited and are expensive (2-5). Oxidation of water in plants occurs in Photosystem II making use of the Mn 4 O 5 Ca cluster (6, 7) present in the protein. The structure of the water oxidation complex involves a cubic cluster with four Mn atoms bridged via oxygen atoms (8,9 Results and DiscussionWe first studied the catalytic properties of the nanoparticles of the LiMn 2 O 4 spinel, with an average crystallite size of 57 nm [see Fig. S1 for an X-ray diffraction pattern and transmission electron microscope (TEM) image], prepared by the citrate sol-gel method from their corresponding metal precursors. Oxygen evolution was studied under visible light in a standard photoexcitation system consisting of Ru(bpy) 3 2+ as the photosensitizer and Na 2 S 2 O 8 as the sacrificial electron acceptor in a solution buffered at pH = 5.8. Oxygen evolved was quantified both by a Clark-type electrode and gas chromatography. We found reasonable catalytic activity with these nanoparticles as shown in Fig. 1A. Delithiation of LiMn 2 O 4 in dilute nitric acid resulted in a decrease of unit cell volume and an increase in surface area from 20 to 55 m 2 /g (Table S1). The average crystallite size of the delithiated sample (DlLiMn 2 O 4 ) was 46 nm. Delithiation seemed to result in a slight impr...