Water Oxidation Molecular Assemblies in Dye-Sensitized Photoelectrochemical Cell: An Overview

Dye-sensitized photoelectrochemical cells (DSPECs) are efficient and sustainable approaches for hydrogen production via water splitting, driven by solar energy. Recent advancements have focused on enhancing the performance and stability of photoanodes, which are critical for efficient water oxidation. Herein discussed are the latest innovations including the development of metal-free organic sensitizers, improved chromophore-catalyst assemblies, and core-shell structures. These advances lead to reduced electron-hole recombination, increased light absorption, and enhanced electron transfer efficiency. Pyridine-anchored sensitizers have shown superior stability compared to traditional carboxylate and phosphate anchors in water, while covalently linked chromophores and molecular catalysts provide long-term operational stability. Together, these improvements bring DSPEC technology closer to practical applications in green hydrogen production, addressing key challenges of energy efficiency, scalability, and system durability. These approaches could be explored further toward realizing cost-effective hydrogen production.

Chemical design of metal complexes for electrochemical water oxidation under acidic conditions

The development of viable, stable, and highly efficient molecular water oxidation catalysts under acidic aqueous conditions (pH < 7) is challenging with Earth-abundant metals in the field of renewable energy due to their low stability and catalytic activity. The utilization of these catalysts is generally considered more cost-effective and sustainable relative to conventional catalysts relying on precious metals such as ruthenium and iridium, which exhibit outstanding activities. Herein, we discussed the effectiveness of transition metal complexes for electrocatalytic water oxidation under acidic conditions. We focus on important aspects of 3d first-row metal complexes as they relate to the design of water oxidation systems and emphasize the importance of the fundamental coordination chemistry perspective in this field, which can be applied to the understanding of catalytic activity and fundamental structure-function relationships. Finally, we identified the scientific challenges that should be overcome for the future development and application of water oxidation electrochemical catalysts.

Temperature-Controlled Chemoselective Couplings of Alkyl Halides with Disulfides

An unprecedented, transition metal-free S-alkylation of disulfides with alkyl halides is developed for the first time, providing an efficient and green synthesis of thioethers and even thioesters. Notably, this new method allows the full utilization of both sulfur atoms of disulfides under chemical reductant-free conditions and can be easily scaled up in gram scale, showing good practical value. Control experiments suggested that water, unprecedentedly, serves as the terminal reductant of the whole reaction. Moreover, controllable N-alkyl thiopyridones can be readily obtained at a much lower reaction temperature from the same starting materials, suggesting a kinetic control of N-alkylation vs a thermodynamic control of the S-alkylation process may be involved in the reaction. Interestingly, a halo anion-mediated alkyl group migration from N-alkyl thiopyridones to S-alkyl pyridyl thioethers was observed, suggesting that the reaction may undergo a new and novel reaction mechanism.

Thermodynamic Assessment of Sacrificial Oxidant Potential, H2O/O2 Potential, and Rate-Overpotential Relationship to Examine Catalytic Water Oxidation in Nonaqueous Solvents

The water oxidation reaction (WOR), which is pivotal to storing energy in chemical bonds, requires a catalyst to overcome its inherent kinetic barrier. In bulk solutions, sacrificial oxidants (SOs) can regenerate the catalysts to ensure that the homogeneous WOR can be operated with long-term consistent performance. To implement this strategy for organic WOR systems, we modified four common SOs with tetra-n-butylammonium ([NBu4]+)─[NBu4]2[Ce(NO3)6], [NBu4][IO4], [NBu4][HSO5], and [NBu4]2[S2O8]─and examined their chemical stability and electrochemical behaviors in various organic solvents. We also derived the organic-solvent-associated redox potential of H2O/O2 in organic media (EH2O/O2(org)) using open-circuit potential measurements of the H+/H2 redox couple and the related thermochemical cycle. Our findings indicate that the EH2O/O2(org) varies with solvent identity and can be adjusted by changing the [H2O], [acid], and [base] levels; thus, the SO should be carefully selected for WOR, because the innate redox potentials of SOs are not always higher than EH2O/O2(org) under the studied conditions. Lastly, we obtained catalyst-performance-related insights via a rate-overpotential free-energy relationship by calculating the overpotentials of previously studied WOR systems in organic media.

Promoting Water Oxidation by Proton Acceptable Groups Surrounding Catalyst on Electrode Surface

Large-scale hydrogen production through water splitting represents an optimal approach for storing sustainable but intermittent energy sources. However, water oxidation, a complex and sluggish reaction, poses a significant bottleneck for water splitting efficiency. The impact of outer chemical environments on the reaction kinetics of water oxidation catalytic centers remains unexplored. Herein, chemical environment impacts were integrated by featuring methylpyridinium cation group (Py+) around the classic Ru(bpy)(tpy) (bpy=2,2'-bipyridine, tpy=2,2' : 6',2''-terpyridine) water oxidation catalyst on the electrode surface via electrochemical co-polymerization. The presence of Py+ groups could significantly enhance the turnover frequencies of Ru(bpy)(tpy), surpassing the performance of typical proton acceptors such as pyridine and benzoic acid anchored around the catalyst. Mechanistic investigations reveal that the flexible internal proton acceptor anions induced by Py+ around Ru(bpy)(tpy) are more effective than conventionally anchored proton acceptors, which promoted the rate-determining proton transfer process and enhanced the rate of water nucleophilic attack during O-O bond formation. This study may provide a novel perspective on achieving efficient water oxidation systems by integrating cations into the outer chemical environments of catalytic centers.

Oxyanion-Sensitive Catalytic Activity of Ni(II)/Oxyanion Systems for Heterogeneous Organic Degradation: Differential Oxidizing Capacity of Ni(III) and Ni(IV) as High-Valent Intermediates

This study demonstrated that NiO and Ni(OH)2 as Ni(II) catalysts exhibited significant activity for organic oxidation in the presence of various oxyanions, such as hypochlorous acid (HOCl), peroxymonosulfate (PMS), and peroxydisulfate (PDS), which markedly contrasted with Co-based counterparts exclusively activating PMS to yield sulfate radicals. The oxidizing capacity of the Ni catalyst/oxyanion varied depending on the oxyanion type. Ni catalyst/PMS (or HOCl) degraded a broad spectrum of organics, whereas PDS enabled selective phenol oxidation. This stemmed from the differential reactivity of two high-valent Ni intermediates, Ni(III) and Ni(IV). A high similarity with Ni(III)OOH in a substrate-specific reactivity indicated the role of Ni(III) as the primary oxidant of Ni-activated PDS. With the minor progress of redox reactions with radical probes and multiple spectroscopic evidence on moderate Ni(III) accumulation, the significant elimination of non-phenolic contaminants by NiOOH/PMS (or HOCl) suggested the involvement of Ni(IV) in the substrate-insensitive treatment capability of Ni catalyst/PMS (or HOCl). Since the electron-transfer oxidation of organics by high-valent Ni species involved Ni(II) regeneration, the loss of the treatment efficiency of Ni/oxyanion was marginal over multiple catalytic cycles.

A cyclic trinuclear silver complex for photosynthesis of hydrogen peroxide

The development of metal complexes for photosynthesis of hydrogen peroxide (H2O2) from pure water and oxygen using solar energy, especially in the absence of any additives (e.g., acid, co-catalysts, and sacrificial agents), is a worthwhile pursuit, yet still remains highly challenging. More importantly, the O2 evolution from the water oxidation reaction has been impeded by the classic bottleneck, the photon-flux-density problem of sunlight that could be attributed to rarefied solar radiation for a long time. Herein, we reported synthesis of boron dipyrromethene (BODIPY)-based cyclic trinuclear silver complexes (Ag-CTC), and they exhibited strong visible-light absorption ability, a suitable energy bandgap, excellent photochemical properties and efficient charge separation ability. The integration of BODIPY motifs as oxygen reduction reaction sites and silver ions as water oxidation reaction sites allows Ag-CTC to photosynthesize H2O2 either from pure water or from sea water in the absence of any additives with a high H2O2 production rate of 183.7 and 192.3 μM h-1, which is higher than that of other reported metal-based photocatalysts. The photocatalytic mechanism was systematically and ambiguously investigated by various experimental analyses and density functional theory (DFT) calculations. Our work represents an important breakthrough in developing a new Ag photocatalyst for the transformation of O2 into H2O2 and H2O into H2O2.

Photo-driven water oxidation performed by supramolecular photocatalysts made of Ru(II) photosensitizers and catalysts

Two new supramolecular photocatalysts made of covalently linked Ru(II) polypyridine chromophore subunits ([Ru(bpy)3]2+-type species; bpy = 2,2'-bipyridine) and [RuL(pic)2] (L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline) water oxidation catalyst subunits have been prepared. The new species, 1 and 2, contain chromophore and catalyst subunits in the molecular ratios 1:1 and 1:2, respectively. The model chromophore species [Ru(bpy)2(L1)]2+ (RuP1; L1=4-[2-(4-pyridyl)-2-hydroxyethyl]-4-methyl-2,2'-bipyridine) and [Ru(bpy)2(L2)]2+ (RuP2; L2 = 4,4'-bis[2-(4-pyridyl)-2-hydroxyethyl]-2,2'-bipyridine) have also been prepared. The absorption spectra, oxidation behavior, and luminescent properties of 1 and 2 have been studied, and the results indicate that each subunit largely maintains its own properties in the supramolecular species. However, the luminescence of the chromophore subunits is significantly quenched in 1 and 2 in comparison with the luminescence of the respective model species. Both 1 and 2 exhibit catalytic water oxidation in the presence of cerium ammonium nitrate, exhibiting an I2M mechanism, with a better efficiency than the known catalyst [RuL(pic)2] under the same experimental conditions. Upon light irradiation, in the presence of persulfate as a sacrificial acceptor agent, 1 and 2 are more efficient photocatalysts than a system made of separated [Ru(bpy)3]2+ and [RuL(pic)2] species, highlighting the advantage of using multicomponent, supramolecular species with respect to isolated species. The O-O bond formation step is I2M, even in the photo-driven process. The photocatalytic process of 2 is more efficient than that of 1, with the turnover frequency reaching a value of 1.2 s-1. A possible reason could be an increased local concentration of catalytic subunits in the needed bimolecular assembly required for the I2M mechanism in 2 with respect to 1, a consequence of the presence of two catalytic subunits in each multicomponent species 2.

Why efficient bifunctional hydrogen electrocatalysis requires a change in the reaction mechanism

Hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) are both two-electron processes that culminate in the formation or consumption of gaseous hydrogen in an electrolyzer or a fuel cell, respectively. Unitized regenerative proton exchange membrane fuel cells merge these two functionalities into one device, allowing to switch between the two modes of operation. This prompts the quest for efficient bifunctional electrode materials catalyzing the HER and HOR with reasonable reaction rates at low overpotentials. In the present study using a data-driven framework, we identify a general criterion for efficient bifunctional performance in the hydrogen electrocatalysis, which refers to a change in the reaction mechanism when switching from cathodic to anodic working conditions. The obtained insight can be used in future studies based on density functional theory to pave the design of efficient HER and HOR catalysts by a dedicated consideration of the kinetics in the analysis of reaction mechanisms.

Photophysical Properties of Homo- and Hetero-Aggregate Assemblies Made of N-Annulated Perylene Derivatives

We report the absorption spectra and photophysical properties of homo and hetero-aggregate assemblies of a strongly emissive N-annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P-MV), in ethanol-water solutions. In homo-aggregate assemblies of P, the π-π* fluorescence of the isolated chromophore is replaced by excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo-aggregate assemblies of P-MV, photoinduced charge separation, with formation of P+ -MV- species, occurs in 3 ps with a charge recombination of 20 ps. In hetero-aggregate P/P-MV systems, the light energy absorbed by the P components delocalizes over various P subunits, and when a P-MV unit is reached, charge separation occurs; however, excimer emission is present for P/P-MV ratio larger than 3 : 1, indicating that delocalized excitons within the hetero-aggregate systems extend over a limited number of P chromophores.

New Tridentate Ligand Affords a Long-Lived 3MLCT Excited State in a Ru(II) Complex: DNA Photocleavage and 1O2 Production

Two new complexes, [Ru(tpy)(qdppz)](PF6)2 (1; qdppz = 2-(quinolin-8-yl)dipyrido[3,2-a:2',3'-c]phenazine, tpy = 2,2':6',2″-terpyridine) and [Ru(qdppz)2](PF6)2 (2), were investigated for their potential use as phototherapeutic agents through their ability to photosensitize the production of singlet oxygen, 1O2, upon irradiation with visible light. The complexes exhibit strong Ru(dπ) → qdppz(π*) metal-to-ligand charge transfer (MLCT) absorption with maxima at 485 and 495 nm for 1 and 2 in acetone, respectively, red-shifted from the Ru(dπ) → tpy(π*) absorption at 470 nm observed for [Ru(tpy)2]2+ (3) in the same solvent. Complexes 1 and 3 are not luminescent at room temperature, but 3MLCT emission is observed for 2 with maximum at 690 nm (λexc = 480 nm) in acetone. The lifetimes of the 3MLCT states of 1 and 2 were measured using transient absorption spectroscopy to be ∼9 and 310 ns in methanol, respectively, at room temperature (λexc = 490 nm). The bite angle of the qdppz ligand is closer to octahedral geometry than that of tpy, resulting in the longer lifetime of 2 as compared to those of 1 and 3. Arrhenius treatment of the temperature dependence of the luminescence results in similar activation energies, Ea, from the 3MLCT to the 3LF (ligand-field) state for the two complexes, 2520 cm-1 in 1 and 2400 cm-1 in 2. However, the pre-exponential factors differ by approximately two orders of magnitude, 2.3 × 1013 s-1 for 1 and 1.4 × 1011 s-1 for 2, which, together with differences in the Huang-Rhys factors, lead to markedly different 3MLCT lifetimes. Although both 1 and 2 intercalate between the DNA bases, only 2 is able to photocleave DNA owing to its 1O2 production upon irradiation with ΦΔ = 0.69. The present work highlights the profound effect of the ligand bite angle on the electronic structure, providing guidelines for extending the lifetime of 3MLCT Ru(II) complexes with tridentate ligands, a desired property for a number of applications.

Photocatalyzed CO2 reduction to CO by supramolecular photocatalysts made of Ru(II) photosensitizers and Re(I) catalytic subunits containing preformed CO2TEOA adducts

Two new supramolecular photocatalysts containing Ru(II) polypyridine units as light-harvesting photosensitizers and Re(I) polypyridine subunits as catalytic centers have been prepared. The new species, RuRe2A and Ru2ReA, contain catalytic Re(I) subunits coordinated by the preformed CO₂TEOA adduct (known to be the effective catalytic subunits; TEOA is triethanolamine) and exhibit quite efficient and selective photoreduction of CO₂ to CO, with outstanding TONs of 2368 and 2695 and a selectivity of 99.9% and 98.9%, respectively. Such photocatalytic properties are significantly improved with respect to those of previously studied RuRe2 and Ru2Re parent compounds, containing chloride ligands instead of the CO₂TEOA adduct. Comparison between photocatalytic performance of the new species and their parent compounds allows to investigate the effect of the CO₂TEOA insertion process as well as the eventual effect of the presence of chloride ions in solution on the photocatalytic processes. The improved photocatalytic properties of RuRe2A and Ru2ReA compared with their parent species are attributed to a combined effect of different distribution of the one-electron reduced form of the supramolecular photocatalysts on the Ru-subunit(s) (leading to decreased CO formation due to a poisoning ligand loss process) and on the Re-subunit(s) and to the presence of chloride ions in solution for RuRe2 and Ru2Re, which could interfere with the CO₂TEOA adduct formation, a needed requisite for CO forming catalysis. These results strongly indicate the utility of preparing supramolecular photocatalysts containing preformed adducts.

Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal-oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of CuII and CuIII. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical-oxo coupling pathways, which were previously proposed for Ru-based catalysts. [CuII(HL)(OTf)2] [HL = Hbbpya = N,N-bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [CuII(L-)(H2O)]+ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH⧧ value of 4.49 kcal·mol-1, a ΔS⧧ value of -42.6 cal·mol-1·K-1, and a ΔG⧧ value of 17.2 kcal·mol-1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [CuII(L•)(O•)(H2O)]+. From this intermediate onward, formation of the O-O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the CuII center is proposed to be redox-inactive.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Water oxidation by a dye-catalyst diad in natural sunlight: timing and coordination of excitations and reactions across timescales of picoseconds to hours

The mechanisms of how dyes and catalysts for solar-driven transformations such as water oxidation to form O₂ work have been intensively investigated, however little is known about how their independent photophysical and chemical processes work together. The level of coordination between the dye and the catalyst in time determines the overall water oxidation system's efficiency. In this computational stochastic kinetics study, we have examined coordination and timing for a Ru-based dye-catalyst diad, [P2Ru(4-mebpy-4'-bimpy)Ru(tpy)(OH₂)]⁴⁺, where P2 is 4,4'-bisphosphonato-2,2'-bipyridine, 4-mebpy-4'-bimpy is 4-(methylbipyridin-4'-yl)-N-benzimid-N'-pyridine, a bridging ligand, and tpy is (2,2':6',2''-terpyridine), taking advantage of the extensive data available for both dye and catalyst, and direct studies of the diads bound to a semiconductor surface. The simulation results for both ensembles of diads and single diads show that progress through the generally accepted water oxidation catalytic cycle is not controlled by the relatively low flux of solar irradiation or by charge or excitation losses, rather is gated by buildup of intermediates whose chemical reactions are not accelerated by photoexcitations. The stochastics of these thermal reactions govern the level of coordination between the dye and the catalyst. This suggests that catalytic efficiency can be improved in these multiphoton catalytic cycles by providing a means for photostimulation of all intermediates so that the catalytic rate is governed by charge injection under solar illumination alone.

Self-assembled systems for artificial photosynthesis

The last few decades have seen an impressive development in molecular-based artificial photosynthesis, thanks to the design of integrated light-harvesting antennae, charge separation systems, and catalysts for water oxidation or hydrogen production based on covalently linked subunits. However, in recent years, self-assembly and spontaneous aggregation of components emerged - sometimes also through serendipity - for the preparation of multicomponent systems aimed to perform the basic processes needed for artificial photosynthesis. Here we critically discuss some key articles that have recently shown the potential of self-assembly for artificial photosynthesis, ranging from self-assembly of antennae and charge separation systems to integrated antenna/catalyst assemblies, to planned co-localization of various components into restricted environments. It is evident that self-assembly can generate emerging properties with respect to the non-aggregated species, and such emerging properties can be quite convenient for designing efficient photocatalytic systems.

Photoelectrochemical approaches for the conversion of lignin at room temperature

The selective cleavage of C-C/C-O linkages represents a key step toward achieving the chemical conversion of biomass to targeted value-added chemical products under ambient conditions. Using photoelectrosynthetic solar cells is a promising method to address the energy intensive depolymerization of lignin for the production of biofuels and valuable chemicals. This feature article gives an in-depth overview of recent progress using dye-sensitized photoelectrosynthetic solar cells (DSPECs) to initiate the cleavage of C-C/C-O bonds in lignin and related model compounds. This approach takes advantage of N-oxyl mediated catalysis in organic electrolytes and presents a promising direction for the sustainable production of chemicals currently derived from fossil fuels.

Recent advances in the electrochemical applications of Ni-based metal organic frameworks (Ni-MOFs) and their derivatives

Nickel-based metal-organic skeletal materials (Ni-MOFs) are a new class of inorganic materials that have aroused attention of investigators during past couple of years. They offer advantages such as high specific surface area, structural diversity, tunable framework etc. This assorted class of materials exhibited catalytic activity and electrochemical properties and display wide range of applications in the fields of electrochemical sensing, electrical energy storage and electrocatalysis. In this context, the presented review focuses on strategies to improve the electrochemical performance and stability of Ni-MOFs through the optimization of synthesis conditions, the construction of composite materials, and the preparation of derivatives of precursors. The review also presents the applications of Ni-MOFs and their derivatives as electrochemical sensors, energy storage devices, and electrocatalysts. In addition, the challenges and further electrochemical development prospects of Ni-MOFs have been discussed.

Ligands modification strategies for mononuclear water splitting catalysts

Artificial photosynthesis (AP) has been proved to be a promising way of alleviating global climate change and energy crisis. Among various materials for AP, molecular complexes play an important role due to their favorable efficiency, stability, and activity. As a result of its importance, the topic has been extensively reviewed, however, most of them paid attention to the designs and preparations of complexes and their water splitting mechanisms. In fact, ligands design and preparation also play an important role in metal complexes’ properties and catalysis performance. In this review, we focus on the ligands that are suitable for designing mononuclear catalysts for water splitting, providing a coherent discussion at the strategic level because of the availability of various activity studies for the selected complexes. Two main designing strategies for ligands in molecular catalysts, substituents modification and backbone construction, are discussed in detail in terms of their potentials for water splitting catalysts.

Photoelectrochemical water oxidation improved by pyridine N-oxide as a mimic of tyrosine-Z in photosystem II

Artificial photosynthesis provides a way to store solar energy in chemical bonds with water oxidation as a major challenge for creating highly efficient and robust photoanodes that mimic photosystem II. We report here an easily available pyridine N-oxide (PNO) derivative as an efficient electron transfer relay between an organic light absorber and molecular water oxidation catalyst on a nanoparticle TiO2 photoanode. Spectroscopic and kinetic studies revealed that the PNO/PNO+˙ couple closely mimics the redox behavior of the tyrosine/tyrosyl radical pair in PSII in improving light-driven charge separation via multi-step electron transfer. The integrated photoanode exhibited a 1 sun current density of 3 mA cm-2 in the presence of Na2SO3 and a highly stable photocurrent density of >0.5 mA cm-2 at 0.4 V vs. NHE over a period of 1 h for water oxidation at pH 7. The performance shown here is superior to those of previously reported organic dye-based photoanodes in terms of photocurrent and stability.

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

Molecular Modelling and Simulations of Light-Harvesting Decanuclear Ru-Based Dendrimers for Artificial Photosynthesis

The structure of a decanuclear photo- and redox-active dendrimer based on Ru(II) polypyridine subunits, suitable as a light-harvesting multicomponent species for artificial photosynthesis, has been investigated by means of computer modelling. The compound has the general formula [Ru{(μ-dpp)Ru[(μ-dpp)Ru(bpy)2 ]2 }3 ](PF6 )20 (Ru10; bpy=2,2'-bipyridine; dpp=2,3-bis(2'-pyridyl)pyrazine). The stability of possible isomers of each monomer was investigated by performing classical molecular dynamics (MD) and quantum mechanics (QM) simulations on each monomer and comparing the results. The number of stable isomers is reduced to 36 with a prevalence of MER isomerism in the central core, as previously observed by NMR experiments. The simulations on decanuclear dendrimers suggest that the stability of the dendrimer is not linked to the stability of the individual monomers composing the dendrimer but rather governed by the steric constrains originated by the multimetallic assembly. Finally, the self-aggregation of Ru10 and the distribution of the counterions around the complexes is investigated using Molecular Dynamics both in implicit and explicit acetonitrile solution. In representative examples, with nine and four dendrimers, the calculated pair distribution function for the ruthenium centers suggests a self-aggregation mechanism in which the dendrimers are approaching in small blocks and then aggregate all together. Scanning transmission electron microscopy complements the investigation, supporting the formation of different aggregates at various concentrations.

Molecular Photocatalytic Water Splitting by Mimicking Photosystems I and II

In nature, water is oxidized by plastoquinone to evolve O₂ and form plastoquinol in Photosystem II (PSII), whereas NADP⁺ is reduced by plastoquinol to produce NADPH and regenerate plastoquinone in Photosystem I (PSI), using homogeneous molecular photocatalysts. However, water splitting to evolve H₂ and O₂ in a 2:1 stoichiometric ratio has yet to be achieved using homogeneous molecular photocatalysts, remaining as one of the biggest challenges in science. Herein, we demonstrate overall water splitting to evolve H₂ and O₂ in a 2:1 ratio using a two liquid membranes system composed of two toluene phases, which are separated by a solvent mixture of water and trifluoroethanol (H₂O/TFE, 3:1 v/v), with a glass membrane to combine PSI and PSII molecular models. A PSII model contains plastoquinone analogs [p-benzoquinone derivatives (X-Q)] in toluene and an iron(II) complex as a molecular oxidation catalyst in H₂O/TFE (3:1 v/v), which evolves a stoichiometric amount of O₂ and forms plastoquinol analogs (X-QH₂) under photoirradiation. On the other hand, a PSI model contains nothing in toluene but contains X-QH₂, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photocatalyst, and a cobalt(III) complex as an H₂ evolution catalyst in H₂O/TFE (3:1 v/v), which evolves a stoichiometric amount of H₂ and forms X-Q under photoirradiation. When a PSII model system is combined with a PSI model system with two glass membranes and two liquid membranes, photocatalytic water splitting with homogeneous molecular photocatalysts is achieved to evolve hydrogen and oxygen with the turnover number (TON) of >100.

Water oxidation by Brønsted acid-catalyzed in situ generated thiol cation: dual function of the acid catalyst leading to transition metal-free substitution and addition reactions of S-S bonds

An unprecedented water oxidation reaction by a small organic molecule, i.e., the thiol cation generated in situ by Brønsted acid-catalyzed heterolytic cleavage of S-S bond of a disulfide, is observed...

Ruthenium(II) complexes with phosphonate-substituted 1,10-phenanthroline ligands: synthesis, characterization and use in organic photocatalysis

Ru(II) complexes with polypyridyl ligands play a central role in the development of photocatalytic organic reactions. This work is aimed at the structural modification of such complexes to increase their...

Mechanism of H+ dissociation-induced O-O bond formation via intramolecular coupling of vicinal hydroxo ligands on low-valent Ru(III) centers

The understanding of O-O bond formation is of great importance for revealing the mechanism of water oxidation in photosynthesis and for developing efficient catalysts for water oxidation in artificial photosynthesis. The chemical oxidation of the Ru^II ₂(OH)(OH₂) core with the vicinal OH and OH₂ ligands was spectroscopically and theoretically investigated to provide a mechanistic insight into the O-O bond formation in the core. We demonstrate O-O bond formation at the low-valent Ru^III ₂(OH) core with the vicinal OH ligands to form the Ru^II ₂(μ-OOH) core with a μ-OOH bridge. The O-O bond formation is induced by deprotonation of one of the OH ligands of Ru^III ₂(OH)₂ via intramolecular coupling of the OH and deprotonated O^- ligands, conjugated with two-electron transfer from two Ru^III centers to their ligands. The intersystem crossing between singlet and triple states of Ru^II ₂(μ-OOH) is easily switched by exchange of H⁺ between the μ-OOH bridge and the auxiliary backbone ligand.

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles

The luminophore Ru(bpy)2 (dcbpy)2+ (bpy=2,2'-bipyridine; dcbpy=4,4'-dicarboxy-2,2'-bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru-decorated chitosan fibers (NS-RuCh), with a 20 : 1 ratio between chitosan repeating units and RuII chromophores. The properties of the RuII compound are unperturbed by the chitosan structure, with NS-RuCh exhibiting the typical metal-to-ligand charge-transfer (MLCT) absorption and emission bands of RuII complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2 ⊂NS-RuCh system, in which both RuII photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS-RuCh and IrO2 ⊂NS-RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy-dispersive X-ray analysis, which indicated a 2 : 1 ratio between RuII chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2 ⊂NS-RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O2 with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)3 2+ and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS-RuCh and "free" IrO2 nanoparticles (0.10). A fast hole-scavenging process (rate constant, 7×104  s-1 ) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2 ⊂NS-RuCh system is behind the improved photochemical quantum yield of IrO2 ⊂NS-RuCh.

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.

Facile water oxidation by dinuclear mixed-valence CoIII/CoII complexes: the role of coordinated water

Rational design of a catalyst using earth abundant transition metals that can facilitate the smooth O-O bond formation is crucial for developing efficient water oxidation catalysts. The coordination environment around the metal ion of the catalyst plays a pivotal role in this context. We have chosen dinuclear mixed-valence Co^IIICo^II complexes of the general formula of [Co^IIICo^II(LH₂)₂(X)(H₂O)] (X = OAc or Cl) which bear a coordinated water molecule in the primary coordination sphere. We anticipated that the water molecule in the primary sphere can take part in the proton coupled electron transfer (PCET) mechanism which can accelerate the facile formation of the O-O bond under strong alkaline conditions (1 M NaOH). To understand the role of the coordinated water molecule we have generated an analogous complex, [Co^IIICo^II(LH₂)₂(o-vanillin)] (o-vanillin = 2-hydroxy-3-methoxybenzaldehyde), without coordinated water. Interestingly, we have found that the water coordinated complexes show better oxygen evolution reaction (OER) activity and stability.

Ligand Controls the Activity of Light-Driven Water Oxidation Catalyzed by Nickel(II) Porphyrin Complexes in Neutral Homogeneous Aqueous Solutions

Finding photostable, first‐row transition metal‐based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)₃]²⁺ as photosensitizer and [S₂O₈]²⁻ as sacrificial electron acceptor. Electron‐poorer Ni‐porphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron‐poorer porphyrins afforded 6‐fold higher turnover frequencies (TOFs; ca. 0.65 min⁻¹) than electron‐richer analogues. However, the electron‐poorest Ni‐porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O₂ evolution starts was too high (+1.23 V vs. NHE) to be driven by the photochemically generated [Ru(bpy)₃]³⁺. Critically, these Ni‐porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3 % less O₂ than during the first photocatalytic run.

Switching the O-O Bond Formation Pathways of Ru-pda Water Oxidation Catalyst by Third Coordination Sphere Engineering

Water oxidation is a vital anodic reaction for renewable fuel generation via electrochemical- and photoelectrochemical-driven water splitting or CO₂ reduction. Ruthenium complexes, such as Ru-bda family, have been shown as highly efficient water-oxidation catalysts (WOCs), particularly when they undergo a bimolecular O-O bond formation pathway. In this study, a novel Ru(pda)-type (pda^2- =1,10-phenanthroline-2,9-dicarboxylate) molecular WOC with 4-vinylpyridine axial ligands was immobilized on the glassy carbon electrode surface by electrochemical polymerization. Electrochemical kinetic studies revealed that this homocoupling polymer catalyzes water oxidation through a bimolecular radical coupling pathway, where interaction between two Ru(pda)-oxyl moieties (I2M) forms the O-O bond. The calculated barrier of the I2M pathway by density-functional theory (DFT) is significantly lower than the barrier of a water nucleophilic attack (WNA) pathway. By using this polymerization strategy, the Ru centers are brought closer in the distance, and the O-O bond formation pathway by the Ru (pda) catalyst is switched from WNA in a homogeneous molecular catalytic system to I2M in the polymerized film, providing some deep insights into the importance of third coordination sphere engineering of the water oxidation catalyst.

Electrocatalytic Water Oxidation with α-[Fe(mcp)(OTf)2] and Analogues

The complex α-[Fe(mcp)(OTf)₂] (mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex α-[Fe(mcp)(OTf)₂] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp–iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE ∼ 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET).

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.

Facile Access to an Active γ-NiOOH Electrocatalyst for Durable Water Oxidation Derived From an Intermetallic Nickel Germanide Precursor

Identifying novel classes of precatalysts for the oxygen evolution reaction (OER by water oxidation) with enhanced catalytic activity and stability is a key strategy to enable chemical energy conversion. The vast chemical space of intermetallic phases offers plenty of opportunities to discover OER electrocatalysts with improved performance. Herein we report intermetallic nickel germanide (NiGe) acting as a superior activity and durable Ni-based electro(pre)catalyst for OER. It is produced from a molecular bis(germylene)-Ni precursor. The ultra-small NiGe nanocrystals deposited on both nickel foam and fluorinated tin oxide (FTO) electrodes showed lower overpotentials and a durability of over three weeks (505 h) in comparison to the state-of-the-art Ni-, Co-, Fe-, and benchmark NiFe-based electrocatalysts under identical alkaline OER conditions. In contrast to other Ni-based intermetallic precatalysts under alkaline OER conditions, an unexpected electroconversion of NiGe into γ-NiIII OOH with intercalated OH- /CO3 2- transpired that served as a highly active structure as shown by various ex situ methods and quasi in situ Raman spectroscopy.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Potential- and Buffer-Dependent Catalyst Decomposition during Nickel-Based Water Oxidation Catalysis

The production of hydrogen by water electrolysis benefits from the development of water oxidation catalysts. This development process can be aided by the postulation of design rules for catalytic systems. The analysis of the reactivity of molecular complexes can be complicated by their decomposition under catalytic conditions into nanoparticles that may also be active. Such a misinterpretation can lead to incorrect design rules. In this study, the nickel-based water oxidation catalyst [NiII (meso-L)](ClO4 )2 , which was previously thought to operate as a molecular catalyst, is found to decompose to form a NiOx layer in a pH 7.0 phosphate buffer under prolonged catalytic conditions, as indicated by controlled potential electrolysis, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy measurements. Interestingly, the formed NiOx layer desorbs from the surface of the electrode under less anodic potentials. Therefore, no nickel species can be detected on the electrode after electrolysis. Catalyst decomposition is strongly dependent on the pH and buffer, as there is no indication of NiOx layer formation at pH 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer. Under these conditions, the activity stems from a molecular species, but currents are much lower. This study demonstrates the importance of in situ characterization methods for catalyst decomposition and metal oxide layer formation, and previously proposed design elements for nickel-based catalysts need to be revised.

Long-Lived Mixed 2MLCT/MC States in Antiferromagnetically Coupled d3 Vanadium(II) Bipyridine and Phenanthroline Complexes

Exploration of [V(bpy)₃]²⁺ and [V(phen)₃]²⁺ (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible ⁴MLCT ← ⁴GS (ground-state) absorption band (ε_400-700 nm = 900-8000 M^-1 cm^-1), the ⁴MLCT state rapidly (τ_isc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5-3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy)₃]²⁺ and [V(phen)₃]²⁺, respectively. This stands in stark contrast with isoelectronic [Cr(bpy)₃]³⁺, which rapidly forms a long-lived doublet metal-centered (²MC) state following photoexcitation and lacks strong visible GS absorption character. ²MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy)₃]³⁺, where the GS and ²MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a ²MLCT state at lower energy than the ⁴MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal-ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.

Immobilization of Ir(OH)3 Nanoparticles in Mesospaces of Al-SiO2 Nanoparticles Assembly to Enhance Stability for Photocatalytic Water Oxidation

Iridium hydroxide (Ir(OH)3) nanoparticles exhibiting high catalytic activity for water oxidation were immobilized inside mesospaces of a silica-nanoparticles assembly (SiO2NPA) to suppress catalytic deactivation due to agglomeration. The Ir(OH)3 nanoparticles immobilized in SiO2NPA (Ir(OH)3/SiO2NPA) catalyzed water oxidation by visible light irradiation of a solution containing persulfate ion (S2O82−) and tris(2,2′-bipyridine)ruthenium(II) ion ([RuII(bpy)3]2+) as a sacrificial electron acceptor and a photosensitizer, respectively. The yield of oxygen (O2) based on the used amount of S2O82− was maintained over 80% for four repetitive runs using Ir(OH)3/SiO2NPA prepared by the co-accumulation method, although the yield decreased for the reaction system using Ir(OH)3/SiO2NPA prepared by the equilibrium adsorption method or Ir(OH)3 nanoparticles without SiO2NPA support under the same reaction conditions. Immobilization of Ir(OH)3 nanoparticles in Al3+-doped SiO2NPA (Al-SiO2NPA) results in further enhancement of the catalytic stability with the yield of more than 95% at the fourth run of the repetitive experiments.