Benjamin D. Sherman

Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator

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.

Visible photoelectrochemical water splitting into H2 and O2 in a dye-sensitized photoelectrosynthesis cell.

Significance Mesoporous SnO2/TiO2 core/shell nanostructured electrodes derivatized with a surface-bound Ru(II) polypyridyl-based chromophore–catalyst assembly are used for water splitting into H2 and O2 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 TiO2 shell by using atomic layer deposition to deposit overlayers of Al2O3 or TiO2 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 SnO2/TiO2 core/shell nanostructured electrode derivatized with a surface-bound Ru(II) polypyridyl-based chromophore–catalyst assembly. The assembly, [(4,4’-(PO3H2)2bpy)2Ru(4-Mebpy-4’-bimpy)Ru(tpy)(OH2)]4+ ([RuaII-RubII-OH2]4+, combines both a light absorber and a water oxidation catalyst in a single molecule. It was attached to the TiO2 shell by phosphonate-surface oxide binding. The oxide-bound assembly was further stabilized on the surface by atomic layer deposition (ALD) of either Al2O3 or TiO2 overlayers. Illumination of the resulting fluorine-doped tin oxide (FTO)|SnO2/TiO2|-[RuaII-RubII-OH2]4+(Al2O3 or TiO2) 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 H2 and O2. The performance of the resulting DSPECs varies with shell thickness and the nature and extent of the oxide overlayer. Use of the SnO2/TiO2 core/shell compared with nanoITO/TiO2 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/cm2 with 445-nm, ∼90-mW/cm2 illumination in a phosphate buffer at pH 7.

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation.

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 (PF10), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF10 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•-PF10-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.

Photochemical Synthesis of a Water Oxidation Catalyst Based on Cobalt Nanostructures

New cobalt-based nanocomposites have been prepared by photoreduction of Co(2+) salts to generate cobalt nanoparticles deposited on carbon-based materials such as nanocyrstalline diamond and carbon felt. Spontaneous air oxidation converts the metal to Co(2)O(3) which has been tested as a water oxidation catalyst. This work demonstrates that the cobalt oxide nanostructures can be deposited on various carbon surfaces and can catalyze the four-electron oxidation of water to oxygen under anodic bias.

Evolution of reaction center mimics to systems capable of generating solar fuel

Capturing and converting solar energy via artificial photosynthesis offers an ideal way to limit society’s dependence on fossil fuel and its myriad consequences. The development and study of molecular artificial photosynthetic reactions centers and antenna complexes and the combination of these constructs with catalysts to drive the photochemical production of a fuel helps to build the understanding needed for development of future scalable technologies. This review focuses on the study of molecular complexes, design of which is inspired by the components of natural photosynthesis, and covers research from early triad reaction centers developed by the group of Gust, Moore, and Moore to recent photoelectrochemical systems capable of using light to convert water to oxygen and hydrogen.

An aqueous, organic dye derivatized SnO2/TiO2 core/shell photoanode

Visible light driven water splitting in a dye-sensitized photoelectrochemical cell (DSPEC) based on a phosphonic acid-derivatized donor–π–acceptor (D–π–A) organic dye (P–A–π–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–π–D] compared to core/shell, FTO|SnO2/TiO2(3 nm)|–[P–A–π–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 mW cm−2) of FTO|SnO2/TiO2(3 nm)|–[P–A–π–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–π–D] due to the core/shell effect. On surfaces co-loaded with both –[P–A–π–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 two-electrode, 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.

A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water

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 I3-/I- redox mediator solution and the other D35 dye with a tris(bipyridine)cobalt ([Co(bpy)3]3+/2+) based mediator. The tandem configuration consisting of the DSPEC and D35/[Co(bpy)3]3+/2+ based DSC gave the best overall performance and demonstrated the production of H2 from H2O with the only energy input from simulated solar illumination.

Carotenoids as electron or excited-state energy donors in artificial photosynthesis: an ultrafast investigation of a carotenoporphyrin and a carotenofullerene dyad

Photophysical investigations of molecular donor-acceptor systems have helped elucidate many details of natural photosynthesis and revealed design principles for artificial photosynthetic systems. To obtain insights into the factors that govern the partition between excited-state energy transfer (EET) and electron transfer (ET) processes among carotenoids and tetrapyrroles and fullerenes, we have designed artificial photosynthetic dyads that are thermodynamically poised to favor ET over EET processes. The dyads were studied using transient absorption spectroscopy with ∼100 femtosecond time resolution. For dyad , a carotenoporphyrin, excitation to the carotenoid S2 state induces ultrafast ET, competing with internal conversion (IC) to the carotenoid S1 state. In addition, the carotenoid S1 state gives rise to ET. In contrast with biological photosynthesis and many artificial photosynthetic systems, no EET at all was detected for this dyad upon carotenoid S2 excitation. Recombination of the charge separated state takes place in hundreds of picoseconds and yields a triplet state, which is interpreted as a triplet delocalized between the porphyrin and carotenoid moieties. In dyad , a carotenofullerene, excitation of the carotenoid in the S2 band results in internal conversion to the S1 state, ET and probably EET to fullerene on ultrafast timescales. From the carotenoid S1 state EET to fullerene occurs. Subsequently, the excited-state fullerene gives rise to ET from the carotenoid to the fullerene. Again, the charge separated state recombines in hundreds of picoseconds. The results illustrate that for a given rate of EET, the ratio of ET to EET can be controlled by adjusting the driving force for electron transfer.

A tandem dye-sensitized photoelectrochemical cell for light driven hydrogen production

Combining a dye-sensitized photoelectrochemical cell for hydrogen production based on a SnO2 photoanode in series with a dye-sensitized photovoltaic solar cell using a TiO2 photoanode yields a tandem system for the generation of H2 from hydroquinone using only light energy and no applied electrical bias. To target distinct portions of the solar spectrum, a more blue-absorbing free base porphyrin and a more red-absorbing Si phthalocyanine chromophore are used at the two photoelectrodes. With incorporation of a suitable water oxidation catalyst, a similar approach could enable tandem photochemical water splitting without the need for p-type semiconductors.

Two Electrode Collector–Generator Method for the Detection of Electrochemically or Photoelectrochemically Produced O2

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.