Wenjing Song

Solar water splitting in a molecular photoelectrochemical cell.

Significance Solar water splitting into H2 and O2 with visible light has been achieved by a molecular assembly. The dye sensitized photoelectrosynthesis cell configuration combined with core–shell structures with a thin layer of TiO2 on transparent, nanostructured transparent conducting oxides (TCO), with the outer TiO2 shell formed by atomic layer deposition. In this configuration, excitation and injection occur rapidly and efficiently with the injected electrons collected by the nanostructured TCO on the nanosecond timescale where they are collected by the planar conductive electrode and transmitted to the cathode for H2 production. This allows multiple oxidative equivalents to accumulate at a remote catalyst where water oxidation catalysis occurs. 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 TiO2 formed by atomic layer deposition. A “chromophore–catalyst assembly” 1, [(PO3H2)2bpy)2Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH2)]4+, which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO2 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.

Making solar fuels by artificial photosynthesis

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 CO2 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.

Visible Light Driven Benzyl Alcohol Dehydrogenation in a Dye-Sensitized Photoelectrosynthesis Cell

Light-driven dehydrogenation of benzyl alcohol (BnOH) to benzaldehyde and hydrogen has been shown to occur in a dye-sensitized photoelectrosynthesis cell (DSPEC). In the DSPEC, the photoanode consists of mesoporous films of TiO2 nanoparticles or of core/shell nanoparticles with tin-doped In2O3 nanoparticle (nanoITO) cores and thin layers of TiO2 deposited by atomic layer deposition (nanoITO/TiO2). Metal oxide surfaces were coderivatized with both a ruthenium polypyridyl chromophore in excess and an oxidation catalyst. Chromophore excitation and electron injection were followed by cross-surface electron-transfer activation of the catalyst to -Ru(IV)═O(2+), which then oxidizes benzyl alcohol to benzaldehyde. The injected electrons are transferred to a Pt electrode for H2 production. The nanoITO/TiO2 core/shell structure causes a decrease of up to 2 orders of magnitude in back electron-transfer rate compared to TiO2. At the optimized shell thickness, sustained absorbed photon to current efficiency of 3.7% was achieved for BnOH dehydrogenation, an enhancement of ~10 compared to TiO2.

Interfacial Dynamics and Solar Fuel Formation in Dye-Sensitized Photoelectrosynthesis Cells

Dye-sensitized photoelectrosynthesis cells (DSPECs) represent a promising approach to solar fuels with solar-energy storage in chemical bonds. The targets are water splitting and carbon dioxide reduction by water to CO, other oxygenates, or hydrocarbons. DSPECs are based on dye-sensitized solar cells (DSSCs) but with photoexcitation driving physically separated solar fuel half reactions. A systematic basis for DSPECs is available based on a modular approach with light absorption/excited-state electron injection, and catalyst activation assembled in integrated structures. Progress has been made on catalysts for water oxidation and CO(2) reduction, dynamics of electron injection, back electron transfer, and photostability under conditions appropriate for water splitting. With added reductive scavengers, as surrogates for water oxidation, DSPECs have been investigated for hydrogen generation based on transient absorption and photocurrent measurements. Detailed insights are emerging which define kinetic and thermodynamic requirements for the individual processes underlying DSPEC performance.

Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC)

Time-resolved, UV-vis spectroscopic measurements of Li+ diffusion in mesoscopic TiO2 photoanodes were conducted in dye sensitized photoelectrosynthesis cells (DSPECs) under operating conditions. In these experiments the spectral response of TiO2 derivatized with [Ru(bpy)2(4,4′-((HO)2PO)2bpy)]2+ (RuP, where bpy is 2,2′-bipyridine, (4,4′-((HO)2PO)2bpy) is [2,2′-bipyridine]-4,4′-diphosphonic acid) arises from electric field (Stark) effects on the metal-to-ligand-charge transfer (MLCT) absorption spectrum of RuP, which is screened by cation intercalation. These results verify that Li+ diffusion is coupled to electron injection and to electron recombination/extraction at the TiO2 interface. Li+ doping levels depend on the competition between dynamics of its intercalation and electron recombination/transport. For a DSPEC operating in aqueous solution at pH 4.5, the observed rate constants for Li+ intercalation and release were 0.22 s−1 and 0.014 s−1, respectively. Both processes were considerably slower in the more viscous solvent propylene carbonate with Li+ release rate constants <2 × 10−4 s−1. Accumulation of Li+ under these conditions shifts conduction band/trap states to less negative potentials, increasing electron lifetime in TiO2.