Dye-sensitized photoelectrochemical water oxidation through a buried junction

Electronically Conductive, Multifunctional Polymer Binder for Highly Active, Stable, and Abundant Composite Electrodes for Oxygen Evolution

The electrolysis of water to form hydrogen and oxygen is a promising method to store renewable energy. This method requires electrodes that convert water into protons, electrons, and oxygen. We report a multifunctional polymer that conducts electrons and ions and may coreact with the electrocatalyst in the oxygen evolution reaction (OER). The electrodes were prepared in two steps from off-the-shelf reagents. They operate with low loadings of abundant catalysts and are among the most active (100 mA cm^-2 at 1.43 V vs RHE (1.41 V, iR-corrected)) and stable electrodes, reported to date under harsh conditions (85 °C, 6 M KOH, 120 h (0.69% loss over the first 14.5 h and then 0.61% loss over 105.5 h)). Control experiments on glassy carbon electrodes showed that the polycarbazole system significantly outperformed a Nafion system of the same catalyst loading. This simple strategy can be applied to other types of electrodes.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Photodriven water oxidation initiated by a surface bound chromophore-donor-catalyst assembly

In photosynthesis, solar energy is used to produce solar fuels in the form of new chemical bonds. A critical step to mimic photosystem II (PS II), a key protein in nature's photosynthesis, for artificial photosynthesis is designing devices for efficient light-driven water oxidation. Here, we describe a single molecular assembly electrode that duplicates the key components of PSII. It consists of a polypyridyl light absorber, chemically linked to an intermediate electron donor, with a molecular-based water oxidation catalyst on a SnO₂/TiO₂ core/shell electrode. The synthetic device mimics PSII in achieving sustained, light-driven water oxidation catalysis. It highlights the value of the tyrosine–histidine pair in PSII in achieving efficient water oxidation catalysis in artificial photosynthetic devices.

We describe a single molecular assembly electrode that mimics PSII. Flash photolysis revealed the electron transfer steps between chromophore light absorption and the creation and storage of redox equivalents in the catalyst for water oxidation.

Two-Channel Model for Electron Transfer in a Dye-Catalyst-Dye Supramolecular Complex for Photocatalytic Water Splitting

To improve the performance of dye-sensitized photoelectrochemical cell (DS-PEC) devices for splitting water, the tailoring of the photocatalytic four-photon water oxidation half-reaction represents a principle challenge of fundamental significance. In this study, a Ru-based water oxidation catalyst (WOC) covalently bound to two 2,6-diethoxy-1,4,5,8-diimide-naphthalene (NDI) dye functionalities provides comparable driving forces and channels for electron transfer. Constrained ab initio molecular dynamics simulations are performed to investigate the photocatalytic cycle of this two-channel model for photocatalytic water splitting. The introduction of a second light-harvesting dye in the Ru-based dye-WOC-dye supramolecular complex enables two separate parallel electron-transfer channels, leading to a five-step catalytic cycle with three intermediates and two doubly oxidized states. The total spin S=1 is conserved during the catalytic process and the system with opposite spin on the oxidized NDI proceeds from the Ru=O intermediate to the final Ru-O2 intermediate with a triplet molecular 3 O2 ligand that is eventually released into the environment. The in-depth insight into the proposed photocatalytic cycle of the two-channel model provides a strategy for the development of novel high-efficiency supramolecular complexes for DS-PEC devices with buildup and conservation of spin multiplicity along the reaction coordinate as a design principle.

Methodology for functionalization of water oxidation catalyst IrOx nanoparticles with hydrophobic and multi-functionalized chromophores

We present a straightforward "click chemistry" methodology for the functionalization of water-oxidation catalyst iridium oxide nanoparticles (IrOx-NPs) with a multi-functionalized porphyrin-based photosynthetic model as sensitizer for the preparation of bioinspired photo-catalysts. This efficient method overcomes the usual aggregation issue found when decorating water oxidation nanocolloidal catalysts with hydrophobic sensitizers.

Dye-Sensitized Nonstoichiometric Strontium Titanate Core-Shell Photocathodes for Photoelectrosynthesis Applications

A core-shell approach that utilizes a high-surface-area conducting core and an outer semiconductor shell is exploited here to prepare p-type dye-sensitized solar energy cells that operate with a minimal applied bias. Photocathodes were prepared by coating thin films of nanocrystalline indium tin oxide with a 0.8 nm Al₂O₃ seeding layer, followed by the chemical growth of nonstoichiometric strontium titanate. Films were annealed and sensitized with either a porphyrin chromophore or a chromophore-catalyst molecular assembly consisting of the porphyrin covalently tethered to the ruthenium complex. The sensitized photoelectrodes produced cathodic photocurrents of up to -315 μA/cm² under simulated sunlight (AM1.5G, 100 mW/cm²) in aqueous media, pH 5. The photocurrent was increased by the addition of regenerative hole donors to the system, consistent with slow interfacial recombination kinetics, an important property of p-type dye-sensitized electrodes.

A conductive metal-organic framework photoanode

We report the development of photosensitizing arrays based on conductive metal–organic frameworks (MOFs) that enable light harvesting and efficient charge separation. Zn₂TTFTB (TTFTB = tetrathiafulvalene tetrabenzoate) MOFs are deposited directly onto TiO₂ photoanodes and structurally characterized by pXRD and EXAFS measurements. Photoinduced interfacial charge transfer dynamics are investigated by combining time-resolved THz spectroscopy and quantum dynamics simulations. Sub-600 fs electron injection into TiO₂ is observed for Zn₂TTFTB–TiO₂ and is compared to the corresponding dynamics for TTFTB–TiO₂ analogues that lack the extended MOF architecture. Rapid electron injection from the MOF into TiO₂ is enhanced by facile migration of the hole away from the interfacial region. Holes migrate through strongly coupled HOMO orbitals localized on the tetrathiafulvalene cores of the columnar stacks of the MOF, whereas electrons are less easily transferred through the spiral staircase arrangement of phenyl substituents of the MOF. The reported findings suggest that conductive MOFs could be exploited as novel photosensitizing arrays in applications to slow, and thereby make difficult, photocatalytic reactions such as those required for water-splitting in artificial photosynthesis.

We report the development of photosensitizing arrays based on conductive metal–organic frameworks (MOFs) that enable light harvesting and efficient charge separation.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.

Photodeposition of Ag Nanocrystals onto TiO2 Nanotube Platform for Enhanced Water Splitting and Hydrogen Gas Production

Current work reports the study of Ag nanocrystals (NCs) decorated doubly anodized (DA) TiO2 nanotubes (NTs) thin film as an efficient photoelectrode material for water splitting and photocatalytic hydrogen gas production. DA process has been shown to be capable of producing less defective NTs and creating additional spacious gaps in between NT bundles to allow efficient and uniform integration of Ag NCs. By employing photoreduction method, Ag NCs can be deposited directly onto NTs, where the size and density of coverage can be maneuvered by merely varying the concentration of Ag precursors. Field emission scanning electron microscope (FESEM) images show that the Ag NCs with controllable size are homogeneously decorated onto the walls of NTs with random yet uniform distribution. X-ray diffraction (XRD) results confirm the formation of anatase TiO2 NTs and Ag NCs, which can be well indexed to standard patterns. The decoration of metallic Ag NCs onto the surface of NTs demonstrates a significant enhancement in the photoconversion efficiency as compared to that of pristine TiO2 NTs. Additionally, the as-prepared nanocomposite film also shows improved efficiency when used as a photocatalyst platform in the production of hydrogen gas. Such improvement in the performance of water splitting and photocatalytic hydrogen gas production activity can be credited to the surface plasmonic resonance of Ag NCs present on the surface of the NTs, which renders improved light absorption and better charge separation. The current work can serve as a model of study for designing more advanced nanoarchitecture photoelectrode for renewable energy application.

Proton Acceptor near the Active Site Lowers Dramatically the O-O Bond Formation Energy Barrier in Photocatalytic Water Splitting

The O-O bond formation process via water nucleophilic attack represents a thermodynamic and kinetic bottleneck in photocatalytic water oxidation because of the considerably high activation free energy barrier. It is therefore of fundamental significance and yet challenging to find strategies to facilitate this reaction. The microscopic details of the photocatalytic water oxidation step involving the O-O bond formation in a catalyst-dye supramolecular complex are here elucidated by density functional theory-based Car-Parrinello molecular dynamics simulations in the presence of an extra proton acceptor. Introducing a proton acceptor group (OH-) in the hydration shell near the catalytic active site accelerates the rate-limiting O-O bond formation by inducing a cooperative event proceeding via a concerted proton-coupled electron-transfer mechanism and thus significantly lowering the activation free energy barrier. The in-depth insight provides a strategy for facilitating the photocatalytic water oxidation and for improving the efficiency of dye-sensitized photoelectrochemical cells.

Excitation energy-dependent photocurrent switching in a single-molecule photodiode

The direction of electron flow in molecular optoelectronic devices is dictated by charge transfer between a molecular excited state and an underlying conductor or semiconductor. For those devices, controlling the direction and reversibility of electron flow is a major challenge. We describe here a single-molecule photodiode. It is based on an internally conjugated, bichromophoric dyad with chemically linked (porphyrinato)zinc(II) and bis(terpyridyl)ruthenium(II) groups. On nanocrystalline, degenerately doped indium tin oxide electrodes, the dyad exhibits distinct frequency-dependent, charge-transfer characters. Variations in the light source between red-light (∼1.9 eV) and blue-light (∼2.7 eV) excitation for the integrated photodiode result in switching of photocurrents between cathodic and anodic. The origin of the excitation frequency-dependent photocurrents lies in the electronic structure of the chromophore excited states, as shown by the results of theoretical calculations, laser flash photolysis, and steady-state spectrophotometric measurements.

Metal Oxide/(oxy)hydroxide Overlayers as Hole Collectors and Oxygen-Evolution Catalysts on Water-Splitting Photoanodes

Solar water splitting provides a mechanism to convert and store solar energy in the form of stable chemical bonds. Water-splitting systems often include semiconductor photoanodes, such as n-Fe₂O₃ and n-BiVO₄, which use photogenerated holes to oxidize water. These photoanodes often exhibit improved performance when coated with metal-oxide/(oxy)hydroxide overlayers that are catalytic for the water-oxidation reaction. The mechanism for this improvement, however, remains a controversial topic. This is, in part, due to a lack of experimental techniques that are able to directly track the flow of photogenerated holes in such multicomponent systems. In this Perspective, we illustrate how this issue can be addressed by using a second working electrode to make direct current/voltage measurements on the catalytic overlayer during operation in a photoelectrochemical cell. We discuss examples where the second working electrode is a thin metallic film deposited on the catalyst layer, as well as where it is the tip of a conducting atomic-force-microscopy probe. In applying these techniques to multiple semiconductors (Fe₂O₃, BiVO₄, Si) paired with various metal-(oxy)hydroxide overlayers (e.g., Ni(Fe)O _xH _y and CoO _xH _y), we found in all cases investigated that the overlayers collect photogenerated holes from the semiconductor, charging to potentials sufficient to drive water oxidation. The overlayers studied thus form charge-separating heterojunctions with the semiconductor as well as serve as water-oxidation catalysts.