Photoelectrochemical hydrogen production in water using a layer-by-layer assembly of a Ru dye and Ni catalyst on NiO

A Multi-Pyridine-Anchored and -Linked Bilayer Photocathode for Water Reduction

The development of efficient photocathodes is of critical importance for the constructions of promising tandem photo-electrochemical cells. Most known dye-sensitized photocathodes are prepared with the conventional carboxylic or phosphonic acid anchors and require the presence of other terminal linking groups to connect catalysts; they suffer from high synthetic difficulty and low adsorption stability in aqueous media. Here, a compact bilayer photocathode has been prepared by using a pyrene-based photosensitizer with multiple terminal pyridine moieties as both the anchoring and linking groups to connect a Co hydrogen-evolution catalyst to the NiO substrate. The catalyst and dye molecule are assembled in a layer-by-layer manner on NiO through the metal-pyridine coordination. This photocathode exhibits good dye adsorption stability in aqueous media. A stable cathodic photocurrent of 70 μA cm-2 was achieved, with H2 being generated at the photocathode under the visible-light irradiation. The Faraday efficiency of H2 evolution was estimated to be 9.1 %. Transient absorption spectral studies suggest that the interfacial hole transfer occurs within a few picoseconds. The integration of the organic photosensitizer with pyridine anchoring and linking groups is expected to provide a simple method for the fabrication of stable and efficient photocathodes.

Mapping the Ultrafast Mechanistic Pathways of Co Photocatalysts in Pure Water through Time-Resolved X-ray Spectroscopy

Nanosecond time-resolved X-ray (tr-XAS) and optical transient absorption spectroscopy (OTA) are applied to study 3 multimolecular photocatalytic systems with [Ru(bpy)3 ]2+ photoabsorber, ascorbic acid electron donor and Co catalysts with methylene (1), hydroxomethylene (2) and methyl (3) amine substituents in pure water. OTA and tr-XAS of 1 and 2 show that the favored catalytic pathway involves reductive quenching of the excited photosensitizer and electron transfer to the catalyst to form a CoII square pyramidal intermediate with a bonded aqua molecule followed by a CoI square planar derivative that decays within ≈8 μs. By contrast, a CoI square pyramidal intermediate with a longer decay lifetime of ≈35 μs is formed from an analogous CoII geometry for 3 in H2 O. These results highlight the protonation of CoI to form the elusive hydride species to be the rate limiting step and show that the catalytic rate can be enhanced through hydrogen containing pendant amines that act as H-H bond formation proton relays.

Optimized H2-evolving dye-sensitized LaFeO3 photocathodes prepared via the layer-by-layer assembly of dyes and catalysts

A molecular dye and a molecular catalyst were loaded onto the surface of a mesoporous LaFeO₃ (LFO) film via layer-by-layer assembly relying on the coordination of phosphates and Zr⁴⁺. After assembling six layers of the dye and four layers of the catalyst, the (NiP-4 + PQA-6)@LFO photocathode exhibited a significant photocurrent for light-driven H₂ generation.

Schottky-functionalized Z-scheme heterojunction: Improved photoelectric conversion efficiency and immunosensing

Designing photovoltaic materials with good photoelectric activity is the crucial to boost the sensitivity of photoelectrochemical (PEC) biosensors. To meet this concern, a Schottky-functionalized direct Z-scheme heterojunction photovoltaic material was proposed by electrodeposition of gold nanoparticles on two kinds of bismuth oxyhalide composites surface (bismuth oxybromide and bismuth oxyiodide with different but matched band gaps) (depAu/BiOI/BiOBr). Specifically, synergistic effect was achieved through the direct Z-scheme heterojunction formed by BiOBr and BiOI as well as the gold Schottky junction, resulting in the enhanced light harvest and photoelectric conversion efficiency. Meanwhile, combined with sandwich immunotechnology, a "signal-off" PEC biosensor was fabricated for highly sensitive detection of carcinoembryonic antigen (CEA). In which, using depAu/BiOI/BiOBr modified glassy carbon electrodes both as the photoactive sensing interface and capture antibody loading matrix, polyethyleneimine copper complex encapsulated gold nanoclusters labeled detection antibody (Ab₂-Au@PEI-Cu) as the quencher, the photocurrent decreased with the increasing target CEA introduced by sandwich immune reaction. The proposed smart PEC immunoassay platform exhibited a wide detection range (1.0 fg/mL-2.0 ng/mL) and a detection limit as low as 0.11 fg/mL with favorable selectivity and stability. In addition, this PEC sensing strategy can be easily extended for other tumor marker analysis, which offers a new perspective of using multiple bismuth oxyhalide as photoactive materials for early diseases diagnosis.

Silver nanostructure-modified graphite electrode for in-operando SERRS investigation of iron porphyrins during high-potential electrocatalysis

In-operando spectroscopic observation of the intermediates formed during various electrocatalytic oxidation and reduction reactions is crucial to propose the mechanism of the corresponding reaction. Surface-enhanced resonance Raman spectroscopy coupled to rotating disk electrochemistry (SERRS-RDE), developed about a decade ago, proved to be an excellent spectroscopic tool to investigate the mechanism of heterogeneous oxygen reduction reaction (ORR) catalyzed by synthetic iron porphyrin complexes under steady-state conditions in water. The information about the formation of the intermediates accumulated during the course of the reaction at the electrode interface helped to develop better ORR catalysts with second sphere residues in the porphyrin rings. To date, the application of this SERRS-RDE setup is limited to ORR only because the thiol self-assembled monolayer (SAM)-modified Ag electrode, used as the working electrode in these experiments, suffers from stability issues at more cathodic and anodic potential, where H₂O oxidation, CO₂ reduction, and H⁺ reduction reactions occur. The current investigation shows the development of a second-generation SERRS-RDE setup consisting of an Ag nanostructure (AgNS)-modified graphite electrode as the working electrode. These electrodes show higher stability (compared to the conventional thiol SAM-modified Ag electrode) upon exposure to very high cathodic and anodic potential with a good signal-to-noise ratio in the Raman spectra. The behavior of this modified electrode toward ORR is found to be the same as the SAM-modified Ag electrode, and the same ORR intermediates are observed during electrochemical ORR. At higher cathodic potential, the signatures of Fe(0) porphyrin, an important intermediate in H⁺ and CO₂ reduction reactions, was observed at the electrode-water interface.

Gold nanoparticle-based supramolecular approach for dye-sensitized H2-evolving photocathodes

Solar conversion of water into the storable energy carrier H₂ can be achieved through photoelectrochemical water splitting using light adsorbing anodes and cathodes bearing O₂ and H₂ evolving catalysts, respectively. Herein a novel photocathode nanohybrid system is reported. This photocathode consists of a dye-sensitized p-type nickel oxide (NiO) with a perylene-based chromophore (PCA) and a tetra-adamantane modified cobaloxime reduction catalyst (Co) that photo-reduces aqueous protons to H₂. An original supramolecular approach was employed, using β-cyclodextrin functionalized gold nanoparticles (β-CD-AuNPs) to link the alkane chain of the PCA dye to the adamantane moieties of the cobaloxime catalyst (Co). This new architecture was investigated by photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy. The results show that irradiation of the complete NiO|PCA|β-CD-AuNPs|Co electrode leads to ultrafast hole injection into NiO (π = 3 ps) from the excited dye, followed by rapid reduction of the catalyst, and finally H₂ evolution.

Evaluation of Pt Deposition onto Dye-Sensitized NiO Photocathodes for Light-Driven Hydrogen Production

The design of photocathodes for the hydrogen evolution reaction (HER), which suitably couple dye-sensitized p-type semiconductors and a hydrogen evolving catalyst (HEC), currently represents an important target in the quest for artificial photosynthesis. In the present manuscript, we report on a systematic evaluation of simple methods for the deposition of Pt metal onto dye-sensitized NiO electrodes. The standard P1 dye was taken as the chromophore of choice and two different NiO substrates were considered. Both potentiostatic and potentiodynamic procedures were evaluated either with or without the inclusion of an additional light bias. Photoelectrochemical characterization of the resulting electrodes in an aqueous solution at pH 4 showed that all the methods tested are effective to attain photocathodes for hydrogen production. The best performances (maximum photocurrent densities of −40 µA·cm−2, IPCE of 0.18%, and ~60% Faradaic yield) were achieved using appreciably fast, light-assisted deposition routes, which are associated with the growth of small Pt islands homogenously distributed on the sensitized NiO.

Photochemical H2 Evolution from Bis(diphosphine)nickel Hydrides Enables Low-Overpotential Electrocatalysis

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H₂ evolution, leveraging visible-light-driven photochemical H₂ evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H₂ evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni-H bond. Thermodynamic analyses provide a basis for understanding and predicting the observed photoelectrocatalytic H₂ evolution by a 3d transition metal based catalyst. Of particular note is the dramatic change in the electrochemical overpotential: in the dark, the nickel complexes require strong acids and therefore high overpotentials for electrocatalysis; but under illumination, the use of weaker acids at the same applied potential results in a more than 500 mV improvement in electrochemical overpotential. New insight into first-row transition metal hydride photochemistry thus enables photoelectrocatalytic H₂ evolution without electrochemical overpotential (at the thermodynamic potential or 0 mV overpotential). This catalyst system does not require sacrificial chemical reductants or light-harvesting semiconductor materials and produces H₂ at rates similar to molecular catalysts attached to silicon.

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.

Hydrogen Production at a NiO Photocathode Based on a Ruthenium Dye-Cobalt Diimine Dioxime Catalyst Assembly: Insights from Advanced Spectroscopy and Post-operando Characterization

The production of hydrogen by efficient, low-cost, and integrated photoelectrochemical water splitting processes represents an important target for the ecological transition. This challenge can be addressed thanks to bioinspired chemistry and artificial photosynthesis approaches by designing dye-sensitized photocathodes for hydrogen production, incorporating bioinspired first-row transition metal-based catalysts. The present work describes the preparation and photoelectrochemical characterization of a NiO photocathode sensitized with a phosphonate-derivatized ruthenium tris-diimine photosensitizer covalently linked to a cobalt diimine dioxime hydrogen-evolving catalyst. Under simulated AM 1.5G irradiation, hydrogen is produced with photocurrent densities reaching 84 ± 7 μA·cm^-2, which is among the highest values reported so far for dye-sensitized photocathodes with surface-immobilized catalysts. Thanks to the unique combination of advanced spectroscopy and surface characterization techniques, the fast desorption of the dyad from the NiO electrode and the low yield of electron transfer to the catalyst, resulting in the Co demetallation from the diimine dioxime framework, were identified as the main barriers limiting the performances and the stability of the system. This work therefore paves the way for a more rational design of molecular photocathodes for solar fuel production and represents a further step toward the development of sustainable processes for the production of hydrogen from sunlight and water.

Influence of Surface and Structural Variations in Donor-Acceptor-Donor Sensitizers on Photoelectrocatalytic Water Splitting

Conjugated organic chromophores composed of linked donor (D) and acceptor (A) moieties have attracted considerable attention for photoelectrochemical applications. In this work, we compare the optoelectronic properties and photoelectrochemical performance of two D-A-D structural isomers with thiophene-X-carboxylic acid (X denotes 3 and 2 positions) derivatives and 2,1,3-benzothiadiazole as the D and A moieties, respectively. 5,5'-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-3-carboxylic acid), BTD1, and 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-2-carboxylic acid), BTD2, were employed in the study to understand how structural isomers affect surface attachments within chromophore-catalyst assemblies and their influence on charge-transfer dynamics. Crystal structures revealed that varying the position of the -COOH anchoring group causes the molecules to either contort out of a plane (BTD1) or adopt a near-perfect planar conformation (BTD2). BTD1 and BTD2 were co-loaded with either a water oxidation catalyst, [Ru(2,6-bis(1-methylbenzimidazol-2-yl)pyridine)-(4,4'-((HO)₂OPCH₂)₂-2,2'-bipyridine)(OH₂)]², RuCt²⁺, or proton reduction catalyst [Ni(P₂^PhN₂^{C₆H₄CH₂PO₃H₂})₂]²⁺, NiCt²⁺, on oxide electrodes to facilitate photodriven water splitting reactions. Emission quenching measurements indicate that both BTD1 and BTD2 inject electrons into n-type SnO₂|TiO₂ electrodes and holes into p-type NiO semiconductors from their respective excited states at high efficiencies >60%. Photocurrent densities of chromophore-catalyst assemblies obtained using linear sweep voltammetry (LSV) show that BTD2-sensitized photoanodes generate significantly more photocurrent than BTD1-sensitized electrodes; however, both exhibit similar performances at the photocathode. Photoelectrocatyltic measurements demonstrate that both BTD1 and BTD2 performed similarly, generating Faradaic efficiencies of 39 and 38% at the anode or 61 and 79% at the cathode. Transient absorption measurements suggest that the differences between the LSV and photoelectrocatalytic measurements result from the differences in quantum yields of the photogenerated redox equivalents, which is also a reflection of the varying metal oxide surface conformation. Our findings suggest that BTD2 should be investigated further in photocathodic studies since it has the structural advantage of being incorporated into diverse types of chromophore-catalyst assemblies.

An Integrated Photoelectrochemical Nanotool for Intracellular Drug Delivery and Evaluation of Treatment Effect

With reduced background and high sensitivity, photoelectrochemistry (PEC) may be applied as an intracellular nanotool and open a new technological direction of single-cell study. Nevertheless, the present palette of single-cell tools lacks such a PEC-oriented solution. Here a dual-functional photocathodic single-cell nanotool capable of direct electroosmotic intracellular drug delivery and evaluation of oxidative stress is devised by engineering a target-specific organic molecule/NiO/Ni film at the tip of a nanopipette. Specifically, the organic molecule probe serves simultaneously as the biorecognition element and sensitizer to synergize with p-type NiO. Upon intracellular delivery at picoliter level, the oxidative stress effect will cause structural change of the organic probe, switching its optical absorption and altering the cathodic response. This work has revealed the potential of PEC single-cell nanotool and extended the boundary of current single-cell electroanalysis.

Unraveling the Mechanisms of Beneficial Cu-Doping of NiO-Based Photocathodes

Dye-sensitized photoelectrochemical (DSPEC) water splitting is an attractive approach to convert and store solar energy into chemical bonds. However, the solar conversion efficiency of a DSPEC cell is typically low due to a poor performance of the photocathode. Here, we demonstrate that Cu-doping improves the performance of a functionalized NiO-based photocathode significantly. Femtosecond transient absorption experiments show longer-lived photoinduced charge separation for the Cu:NiO-based photocathode relative to the undoped analogue. We present a photophysical model that distinguishes between surface and bulk charge recombination, with the first process (∼10 ps) occurring more than 1 order of magnitude faster than the latter. The longer-lived photoinduced charge separation in the Cu:NiO-based photocathode likely originates from less dominant surface recombination and an increased probability for holes to escape into the bulk and to be transported to the electrical contact of the photocathode. Cu-doping of NiO shows promise to suppress detrimental surface charge recombination and to realize more efficient photocathodes.

Sustainable hydrogen production from water using tandem dye-sensitized photoelectrochemical cells

If generated from water using renewable energy, hydrogen could serve as a carbon-zero, environmentally benign fuel to meet the needs of modern society. Photoelectrochemical cells integrate the absorption and conversion of solar energy and chemical catalysis for the generation of high value products. Tandem photoelectrochemical devices have demonstrated impressive solar-to-hydrogen conversion efficiencies but have not become economically relevant due to high production cost. Dye-sensitized solar cells, those based on a monolayer of molecular dye adsorbed to a high surface area, optically transparent semiconductor electrode, offer a possible route to realizing tandem photochemical systems for H₂ production by water photolysis with lower overall material and processing costs. This review addresses the design and materials important to the development of tandem dye-sensitized photoelectrochemical cells for solar H₂ production and highlights current published reports detailing systems capable of spontaneous H₂ formation from water using only dye-sensitized interfaces for light capture.

Supramolecular strategies in artificial photosynthesis

Artificial photosynthesis is a major scientific endeavor aimed at converting solar power into a chemical fuel as a viable approach to sustainable energy production and storage. Photosynthesis requires three fundamental actions performed in order; light harvesting, charge-separation and redox catalysis. These actions span different timescales and require the integration of functional architectures developed in different fields of study. The development of artificial photosynthetic devices is therefore inherently complex and requires an interdisciplinary approach. Supramolecular chemistry has evolved to a mature scientific field in which programmed molecular components form larger functional structures by self-assembly processes. Supramolecular chemistry could provide important tools in preparing, integrating and optimizing artificial photosynthetic devices as it allows precise control over molecular components within such a device. This is illustrated in this perspective by discussing state-of-the-art devices and the current limiting factors - such as recombination and low stability of reactive intermediates - and providing exemplary supramolecular approaches to alleviate some of those problems. Inspiring supramolecular solutions such as those discussed herein will incite expansion of the supramolecular toolbox, which eventually may be needed for the development of applied artificial photosynthesis.

Visible-light-driven CO2 reduction on dye-sensitized NiO photocathodes decorated with palladium nanoparticles

The thin-layer-stacked dye-sensitized NiO photocathodes decorated with palladium nanoparticles (nPd) can be used for the visible-light-driven selective reduction of CO₂, mostly to CO, at potentials starting as low as 0 V vs. RHE (compared to −0.6 V in the dark for electrocatalysis). The photosensitization of NiO by the organic dye P1, with a surface coverage of 1.5 × 10⁻⁸ mol cm⁻², allows the hybrid material to absorb light in the 400–650 nm range. In addition, it improves the stability and the catalytic activity of the final material decorated with palladium nanoparticles (nPd). The resulting multi-layered-type photocathode operates according to the electron-transfer-cascade mechanism. On the one hand, the photosensitizer P1 plays a central role as it generates excited-state electrons and transfers them to nPd, thus producing the catalytically active hydride material PdH_x. On the other hand, the dispersed nPd, absorb/adsorb hydrogen and accumulate electrons, thus easing the reductive electrocatalysis process by further driving the separation of charges at the photoelectrochemical interface. Surface analysis, morphology, and roughness have been assessed using SEM, EDS, and AFM imaging. Both conventional electrochemical and photoelectrochemical experiments have been performed to confirm the catalytic activity of hybrid photocathodes toward the CO₂ reduction. The recorded cathodic photocurrents have been found to be dependent on the loading of Pd nanoparticles. A sufficient amount of loaded catalyst facilitates the electron transfer cascade, making the amount of dye grafted at the surface of the electrode the limiting parameter in catalysis. The formation of CO as the main reaction product is postulated, though the formation of traces of other small organic molecules (e.g. methanol) cannot be excluded.

(A) Cross-section view of the stack of active layers constituting a hybrid photocathode for CO₂ reduction. (B) Structure of dye P1 sensitizing the NiO semiconductor. (C) Energy-level matching between components of the modified photocathode.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye

Mesoporous NiO photocathodes containing the push-pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye-NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H₂ evolution was not observed. Based on UV-vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Non-Catalytic Benefits of Ni(II) Binding to an Si(111)-PNP Construct for Photoelectrochemical Hydrogen Evolution Reaction: Metal Ion Induced Flat Band Potential Modulation

We report here the remarkable and non-catalytic beneficial effects of a Ni(II) ion binding to a Si|PNP type surface as a result of significant thermodynamic band bending induced by ligand attachment and Ni(II) binding. We unambiguously deconvolute the thermodynamic flat band potentials (V_FB) from the kinetic onset potentials (V_on) by synthesizing a specialized bis-PNP macrochelate that enables one-step Ni(II) binding to a p-Si(111) substrate. XPS analysis and rigorous control experiments confirm covalent attachment of the designed ligand and its resulting Ni(II) complex. Illuminated J-V measurements under catalytic conditions show that the Si|BisPNP-Ni substrate exhibits the most positive onset potential for the hydrogen evolution reaction (HER) (-0.55 V vs Fc/Fc⁺) compared to other substrates herein. Thermodynamic flat band potential measurements in the dark reveal that Si|BisPNP-Ni also exhibits the most positive V_FB value (-0.02 V vs Fc/Fc⁺) by a wide margin. Electrochemical impedance spectroscopy data generated under illuminated, catalytic conditions demonstrate a surprising lack of correlation evident between V_on and equivalent circuit element parameters commonly associated with HER. Overall, the resulting paradigm comprises a system wherein the extent of band bending induced by metal ion binding is the primary driver of photoelectrochemical (PEC)-HER benefits, while the kinetic (catalytic) effects of the PNP-Ni(II) are minimal. This suggests that dipole and band-edge engineering must be a primary design consideration (not secondary to catalyst) in semiconductor|catalyst hybrids for PEC-HER.

Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications

A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push-pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy demonstrating the possibility for application in photocathodes.

Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst

Dye-sensitized photoelectrochemical (DSPEC) cells are an emerging approach to producing solar fuels. The recent development of delafossite CuCrO2 as a p-type semiconductor has enabled H2 generation through the coassembly of catalyst and dye components. Here, we present a CuCrO2 electrode based on a high-surface-area inverse opal (IO) architecture with benchmark performance in DSPEC H2 generation. Coimmobilization of a phosphonated diketopyrrolopyrrole (DPP-P) or perylene monoimide (PMI-P) dye with a phosphonated molecular Ni catalyst (NiP) demonstrates the ability of IO-CuCrO2 to photogenerate H2. A positive photocurrent onset potential of approximately +0.8 V vs RHE was achieved with these photocathodes. The DPP-P-based photoelectrodes delivered photocurrents of -18 μA cm-2 and generated 160 ± 24 nmol of H2 cm-2, whereas the PMI-P-based photocathodes displayed higher photocurrents of -25 μA cm-2 and produced 215 ± 10 nmol of H2 cm-2 at 0.0 V vs RHE over the course of 2 h under visible light illumination (100 mW cm-2, AM 1.5G, λ > 420 nm, 25 °C). The high performance of the PMI-constructed system is attributed to the well-suited molecular structure and photophysical properties for p-type sensitization. These precious-metal-free photocathodes highlight the benefits of using bespoke IO-CuCrO2 electrodes as well as the important role of the molecular dye structure in DSPEC fuel synthesis.

A stable dye-sensitized photoelectrosynthesis cell mediated by a NiO overlayer for water oxidation

In the development of photoelectrochemical cells for water splitting or CO₂ reduction, a major challenge is O₂ evolution at photoelectrodes that, in behavior, mimic photosystem II. At an appropriate semiconductor electrode, a water oxidation catalyst must be integrated with a visible light absorber in a stable half-cell configuration. Here, we describe an electrode consisting of a light absorber, an intermediate electron donor layer, and a water oxidation catalyst for sustained light driven water oxidation catalysis. In assembling the electrode on nanoparticle SnO₂/TiO₂ electrodes, a Ru(II) polypyridyl complex was used as the light absorber, NiO was deposited as an overlayer, and a Ru(II) 2,2'-bipyridine-6,6'-dicarboxylate complex as the water oxidation catalyst. In the final electrode, addition of the NiO overlayer enhanced performance toward water oxidation with the final electrode operating with a 1.1 mA/cm² photocurrent density for 2 h without decomposition under one sun illumination in a pH 4.65 solution. We attribute the enhanced performance to the role of NiO as an electron transfer mediator between the light absorber and the catalyst.

Dye-sensitized photocathodes for oxygen reduction: efficient H2O2 production and aprotic redox reactions

Dye-sensitized photoelectrochemical cells (DSPECs) can be used to store solar energy in the form of chemical bonds. Hydrogen peroxide (H2O2) is a versatile energy carrier and can be produced by reduction of O2 on a dye-sensitized photocathode, in which the design of dye molecules is crucial for the conversion efficiency and electrode stability. Herein, using a hydrophobic donor-double-acceptor dye (denoted as BH4) sensitized NiO photocathode, hydrogen peroxide (H2O2) can be produced efficiently by reducing O2 with current density up to 600 μA cm-2 under 1 sun conditions (Xe lamp as sunlight simulator, λ > 400 nm). The DSPECs maintain currents greater than 200 μA cm-2 at low overpotential (0.42 V vs. RHE) for 18 h with no decrease in the rate of H2O2 production in aqueous electrolyte. Moreover, the BH4 sensitized NiO photocathode was for the first time applied in an aprotic electrolyte for oxygen reduction. In the absence of a proton source, the one-electron reduction of O2 generates stable, nucleophilic superoxide radicals that can then be synthetically utilized in the attack of an available electrophile, such as benzoyl chloride. The corresponding photocurrent generated by this photoelectrosynthesis is up to 1.8 mA cm-2. Transient absorption spectroscopy also proves that there is an effective electron transfer from reduced BH4 to O2 with a rate constant of 1.8 × 106 s-1. This work exhibits superior photocurrent in both aqueous and non-aqueous systems and reveals the oxygen/superoxide redox mediator mechanism in the aprotic chemical synthesis.

Molecular Catalysts Immobilized on Semiconductor Photosensitizers for Proton Reduction toward Visible-Light-Driven Overall Water Splitting

Photocatalytic or photoelectrochemical hydrogen production by water splitting is one of the key reactions for the development of an energy supply that enables a clean energy system for a future sustainable society. Utilization of solar photon energy for the uphill water splitting reaction is a promising technology, and therefore many systems using semiconductor photocatalysts and semiconductor photoelectrodes for the reaction producing hydrogen and dioxygen in a 2:1 stoichiometric ratio have been reported. In these systems, molecular catalysts are also considered to be feasible; recently, systems based on molecular catalysts conjugated with semiconductor photosensitizers have been used for photoinduced hydrogen generation by proton reduction. Additionally, there are reports that the so-called Z-scheme (two-step photoexcitation) mechanism realizes the solar-driven uphill reaction by overall water splitting. Although the number of these reports is still small compared to those of all-inorganic systems, the advantages of molecular cocatalysts and its immobilization on a semiconductor are attractive. This Minireview provides a brief overview of approaches and recent research progress toward molecular catalysts immobilized on semiconductor photocatalysts and photoelectrodes for solar-driven hydrogen production with the stoichiometric uphill reaction of hydrogen and oxygen generation.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

A donor-chromophore-catalyst assembly for solar CO2 reduction

We describe here the preparation and characterization of a photocathode assembly for CO₂ reduction to CO in 0.1 M LiClO₄ acetonitrile.

We describe here the preparation and characterization of a photocathode assembly for CO₂ reduction to CO in 0.1 M LiClO₄ acetonitrile. The assembly was formed on 1.0 μm thick mesoporous films of NiO using a layer-by-layer procedure based on Zr(iv)–phosphonate bridging units. The structure of the Zr(iv) bridged assembly, abbreviated as NiO|-DA-RuCP₂²⁺-Re(i), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH₂)₃PO₃H₂)₄-4,4′-dianiline), RuCP²⁺ is the light absorber [Ru((4,4′-(PO₃H₂CH₂)₂-2,2′-bipyridine)(2,2′-bipyridine))₂]²⁺, and Re(i) is the CO₂ reduction catalyst, Re^I((4,4′-PO₃H₂CH₂)₂-2,2′-bipyridine)(CO)₃Cl. Visible light excitation of the assembly in CO₂ saturated solution resulted in CO₂ reduction to CO. A steady-state photocurrent density of 65 μA cm^–2 was achieved under one sun illumination and an IPCE value of 1.9% was obtained with 450 nm illumination. The importance of the DA aniline donor in the assembly as an initial site for reduction of the RuCP²⁺ excited state was demonstrated by an 8 times higher photocurrent generated with DA present in the surface film compared to a control without DA. Nanosecond transient absorption measurements showed that the expected reduced one-electron intermediate, RuCP⁺, was formed on a sub-nanosecond time scale with back electron transfer to the electrode on the microsecond timescale which competes with forward electron transfer to the Re(i) catalyst at t_1/2 = 2.6 μs (k_ET = 2.7 × 10⁵ s^–1).

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Photoelectrocatalytic H2 evolution from integrated photocatalysts adsorbed on NiO

A new approach to increasing the faradaic efficiency of dye-sensitised photocathodes for H2 evolution from water, using integrated photocatalysts, furnished with ester groups on the peripheral ligands, [Ru(decb)2(bpt)PdCl(H2O)](PF6)2 (1) and [Ru(decb)2(2,5-bpp)PtI(CH3CN)](PF6)2 (2), (decb = 4,4'-diethylcarboxy-2,2'-bipyridine, bpp = 2,2':5',2''-terpyridine, bpt = 3,5-bis(2-pyridyl)-1,2,4-triazole) is described. Overall, 1|NiO is superior to previously reported photocathodes, producing photocurrent densities of 30-35 μA cm-2 at an applied bias of -0.2 V vs. Ag/AgCl over 1 hour of continuous white light irradiation, resulting in the generation of 0.41 μmol h-1 cm-2 of H2 with faradaic efficiencies of up to 90%. Furthermore, surface analysis of the photocathodes before and after photoelectrocatalysis revealed that the ruthenium bipyridyl chromophore and Pd catalytic centre (1) were photochemically stable, highlighting the benefits of the approach towards robust, hybrid solar-to-fuel devices.

Homogeneous vs. heterogeneous catalysis for hydrogen evolution by a nickel(ii) bis(diphosphine) complex

A nickel(ii) bis(diphosphine) complex bearing carboxylic acid groups has been tested as a catalyst for hydrogen evolution under different conditions.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution

Utilization of DPPC vesicles allows water-insoluble photoactive Ir(iii) complexes to be dispersed in bulk aqueous solution.

Dye-sensitized LaFeO3 photocathode for solar-driven H2 generation

A mesoporous visible-light-absorbing LaFeO₃ semiconductor adsorbed an organic dye and a molecular catalyst on its surface for solar-driven H₂ generation.

La5 Ti2 Cu0.9 Ag0.1 S5 O7 Modified with a Molecular Ni Catalyst for Photoelectrochemical H2 Generation

The stable and efficient integration of molecular catalysts into p-type semiconductor materials is a contemporary challenge in photoelectrochemical fuel synthesis. Here, we report the combination of a phosphonated molecular Ni catalyst with a TiO2 -coated La5 Ti2 Cu0.9 Ag0.1 S5 O7 photocathode for visible light driven H2 production. This hybrid assembly provides a positive onset potential, large photocurrents, and high Faradaic yield for more than three hours. A decisive feature of the hybrid electrode is the TiO2 interlayer, which stabilizes the oxysulfide semiconductor and allows for robust attachment of the phosphonated molecular catalyst. This demonstration of an oxysulfide-molecular catalyst photocathode provides a novel platform for integrating molecular catalysts into photocathodes and the large photovoltage of the presented system makes it ideal for pairing with photoanodes.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode

Co-grafting of a cobalt diimine–dioxime catalyst and push–pull organic dye on NiO yields a photocathode evolving hydrogen from aqueous solution under sunlight, with equivalent performances compared to a dyad-based architecture using similar components.

Dye-sensitized photo-electrochemical cells (DS-PECs) form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and ease of processing of their constitutive photoelectrode materials. Preparing such molecular photocathodes requires a well-controlled co-immobilization of molecular dyes and catalysts onto transparent semiconducting materials. Here we used a series of surface analysis techniques to describe the molecular assembly of a push–pull organic dye and a cobalt diimine–dioxime catalyst co-grafted on a p-type NiO electrode substrate. (Photo)electrochemical measurements allowed characterization of electron transfer processes within such an assembly and to demonstrate for the first time that a Co^I species is formed as the entry into the light-driven H₂ evolution mechanism of a dye-sensitized photocathode. This co-grafted noble-metal free H₂-evolving photocathode architecture displays similar performances to its covalent dye–catalyst counterpart based on the same catalytic moiety. Post-operando time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of these photoelectrodes after extensive photoelectrochemical operation suggested decomposition pathways of the dye and triazole linkage used to graft the catalyst onto NiO, providing grounds for the design of optimized molecular DS-PEC components with increased robustness upon turnover.