Solar Hydrogen Evolution with Hydrogenases: From Natural to Hybrid Systems

The crystalline state as a dynamic system: IR microspectroscopy under electrochemical control for a [NiFe] hydrogenase

Controlled formation of catalytically-relevant states within crystals of complex metalloenzymes represents a significant challenge to structure-function studies. Here we show how electrochemical control over single crystals of [NiFe] hydrogenase 1 (Hyd1) from Escherichia coli makes it possible to navigate through the full array of active site states previously observed in solution. Electrochemical control is combined with synchrotron infrared microspectroscopy, which enables us to measure high signal-to-noise IR spectra in situ from a small area of crystal. The output reports on active site speciation via the vibrational stretching band positions of the endogenous CO and CN- ligands at the hydrogenase active site. Variation of pH further demonstrates how equilibria between catalytically-relevant protonation states can be deliberately perturbed in the crystals, generating a map of electrochemical potential and pH conditions which lead to enrichment of specific states. Comparison of in crystallo redox titrations with measurements in solution or of electrode-immobilised Hyd1 confirms the integrity of the proton transfer and redox environment around the active site of the enzyme in crystals. Slowed proton-transfer equilibria in the hydrogenase in crystallo reveals transitions which are only usually observable by ultrafast methods in solution. This study therefore demonstrates the possibilities of electrochemical control over single metalloenzyme crystals in stabilising specific states for further study, and extends mechanistic understanding of proton transfer during the [NiFe] hydrogenase catalytic cycle.

Photocatalyst-enzyme hybrid systems for light-driven biotransformation

Enzymes catalyse target reactions under mild conditions with high efficiency, as well as excellent regional-, stereo-, and enantiomeric selectivity. Photocatalysis utilises sustainable and environment-friendly light power to realise efficient chemical conversion. By combining the interdisciplinary advantages of photo- and enzymatic catalysis, the photocatalyst-enzyme hybrid systems have proceeded various light-driven biotransformation with high efficiency under environmentally benign conditions, thus, attracting unparalleled focus during the last decades. It has also been regarded as a promising pathway towards green chemistry utilising ubiquitous solar energy. This systematic review gives insight into this research field by classifying the existing photocatalyst-enzyme hybrid systems into three sections based on different hybridizing modes between photo- and enzymatic catalysis. Furthermore, existing challenges and proposed strategies are discussed within this context. The first system summarised is the cofactor-mediated hybrid system, in which natural/artificial cofactors act as reducing equivalents that connect photocatalysts with enzymes for light-driven enzymatic biotransformation. Second, the direct contact-based photocatalyst-enzyme hybrid systems are described, including two different kinds of electron exchange sites on the enzyme molecules. Third, some cases where photocatalysts and enzymes are integrated into a reaction cascade with specific intermediates will be discussed in the following chapter. Finally, we provide perspective concerning the future of this field.

Self-powered all weather sensory systems powered by Rhodobacter sphaeroides protein solar cells

Natural photosynthetic proteins can convert solar energy into electrical energy with close to 100% quantum efficiency, and there is increasing interest in their use for sustainable photoelectrochemical devices. The primary processes of photosynthesis remain operational and efficient down to extremely low temperatures, and natural photosystems exhibit a variety of self-healing mechanisms. Herein we demonstrate the use of an amphiphilic triblock copolymer, Pluronic F127, to fabricate a self-healing photosynthetic protein photoelectrochemical cell that operates optimally at sub-zero temperatures. A concentration of 30% (w/w) Pluronic F127 depressed the freezing point of an electrolyte comprising 50 mM ubiquinone-0 in aqueous buffer such that optimal device solar energy conversion was seen at -12 °C rather than at room temperature. Fabrication of the protein photoelectrochemical cells with flexible electrodes enabled the demonstration of self-healing of damage caused by repeated mechanical deformation. Multiple bending cycles caused a marked deterioration of the photocurrent response to around a third of initial levels due to damage to the gel phase of the electrolyte, but this could be restored to ~95% by simply cooling and rewarming the device. This self-recoverability of the electrolyte extended the operational life of the protein cell through a process that increased its photoelectrochemical output during the repair. Utility of the cells as components of a touch sensor operational across a wide temperature range, including freezing conditions, is demonstrated.

Surface-Ligand "Liquid" to "Crystalline" Phase Transition Modulates the Solar H2 Production Quantum Efficiency of CdS Nanorod/Mediator/Hydrogenase Assemblies

This study reports how the length of capping ligands on a nanocrystal surface affects its interfacial electron transfer (ET) with surrounding molecular electron acceptors, and consequently, impact the H₂ production of a biotic-abiotic hybrid artificial photosynthetic system. Specifically, we study how the H₂ production efficiency of a hybrid system, combining CdS nanorods (NRs), [NiFe] hydrogenase, and redox mediators (propyl-bridged 2,2'-bipyridinium, PDQ²⁺), depends on the alkyl chain length of mercaptocarboxylate ligands on the NR surface. We observe a minor decrease of the quantum yield for H₂ production from 54 ± 6 to 43 ± 2% when varying the number of methylene units in the ligands from 2 to 7. In contrast, an abrupt decrease of the yield was observed from 43 ± 2 to 4 ± 1% when further increasing n from 7 to 11. ET studies reveal that the intrinsic ET rates from the NRs to the electron acceptor PDQ²⁺ are all within 10⁸-10⁹ s^-1 regardless of the length of the capping ligands. However, the number of adsorbed PDQ²⁺ molecules on NR surfaces decreases dramatically when n ≥ 10, with the saturating number changing from 45 ± 5 to 0.3 ± 0.1 for n = 2 and 11, respectively. These results are not consistent with the commonly perceived exponential dependence of ET rates on the ligand length. Instead, they can be explained by the change of the accessibility of NR surfaces to electron acceptors from a disordered "liquid" phase at n < 7 to a more ordered "crystalline" phases at n > ∼7. These results highlight that the order of capping ligands is an important design parameter for further constructing nanocrystal/molecular assemblies in broad nanocrystal-based applications.

Electron Transfer from Semiconductor Nanocrystals to Redox Enzymes

This review summarizes progress in understanding electron transfer from photoexcited nanocrystals to redox enzymes. The combination of the light-harvesting properties of nanocrystals and the catalytic properties of redox enzymes has emerged as a versatile platform to drive a variety of enzyme-catalyzed reactions with light. Transfer of a photoexcited charge from a nanocrystal to an enzyme is a critical first step for these reactions. This process has been studied in depth in systems that combine Cd-chalcogenide nanocrystals with hydrogenases. The two components can be assembled in close proximity to enable direct interfacial electron transfer or integrated with redox mediators to transport charges. Time-resolved spectroscopy and kinetic modeling have been used to measure the rates and efficiencies of the electron transfer. Electron transfer has been described within the framework of Marcus theory, providing insights into the factors that can be used to control the photochemical activity of these biohybrid systems. The range of potential applications and reactions that can be achieved using nanocrystal-enzyme systems is expanding, and numerous fundamental and practical questions remain to be addressed.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

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.

Consecutive ligand-based PCET processes affording a doubly reduced nickel pyrazinedithiolate which transforms into a metal hydride required to evolve H2

Consecutive Ligand-based PCET Processes Affording a Doubly Reduced Nickel Pyrazinedithiolate which Transforms into a Metal Hydride Required to Evolve H₂.

Solar Water Splitting with a Hydrogenase Integrated in Photoelectrochemical Tandem Cells

Hydrogenases (H₂ases) are benchmark electrocatalysts for H₂ production, both in biology and (photo)catalysis in vitro. We report the tailoring of a p‐type Si photocathode for optimal loading and wiring of H₂ase through the introduction of a hierarchical inverse opal (IO) TiO₂ interlayer. This proton‐reducing Si|IO‐TiO₂|H₂ase photocathode is capable of driving overall water splitting in combination with a photoanode. We demonstrate unassisted (bias‐free) water splitting by wiring Si|IO‐TiO₂|H₂ase to a modified BiVO₄ photoanode in a photoelectrochemical (PEC) cell during several hours of irradiation. Connecting the Si|IO‐TiO₂|H₂ase to a photosystem II (PSII) photoanode provides proof of concept for an engineered Z‐scheme that replaces the non‐complementary, natural light absorber photosystem I with a complementary abiotic silicon photocathode.

Light-Driven Hydrogen Evolution by Nickel-Substituted Rubredoxin

An enzymatic system for light-driven hydrogen generation has been developed through covalent attachment of a ruthenium chromophore to nickel-substituted rubredoxin (NiRd). The photoinduced activity of the hybrid enzyme is significantly greater than that of a two-component system and is strongly dependent on the position of the ruthenium phototrigger relative to the active site, indicating a role for intramolecular electron transfer in catalysis. Steady-state and time-resolved emission spectra reveal a pathway for rapid, direct quenching of the ruthenium excited state by nickel, but low overall turnover numbers suggest initial electron transfer is not the rate-limiting step. This approach is ideally suited for detailed mechanistic investigations of catalysis by NiRd and other molecular systems, with implications for generation of solar fuels.

A pulse electrosynthesized nanoporous nickel oxyhydroxide-borate thin film in electro- and photoelectro-reforming of methanol for selective production of formate

An nanoporous nickel oxyhydroxide-borate (NiBi) thin film, consisting of an aggregate of spherical particles self-assembled from NiBi nanopetals, has been synthesized using a simple and cost-effective pulse electrodeposition method without the addition of any surfactant, and demonstrated as an efficient, selective, and robust electrocatalyst in the electrochemical and photoelectrochemical reforming of methanol into formate.

Photoelectrocatalytic H2 evolution in water with molecular catalysts immobilised on p-Si via a stabilising mesoporous TiO2 interlayer

A versatile platform for the immobilisation of molecular catalysts on a readily-prepared Si photocathode with a mesoporous TiO₂ layer is reported.

The development of photoelectrodes capable of light-driven hydrogen evolution from water is an important approach for the storage of solar energy in the form of a chemical energy carrier. However, molecular catalyst-based photocathodes remain scarcely reported and typically suffer from low efficiencies and/or stabilities due to inadequate strategies for interfacing the molecular component with the light-harvesting material. In this study, we report the straightforward preparation of a p-silicon|mesoporous titania|molecular catalyst photocathode assembly that is active towards proton reduction in aqueous media with an onset potential of +0.4 V vs. RHE. The mesoporous TiO₂ scaffold acts as an electron shuttle between the silicon and the catalyst, while also stabilising the silicon from passivation and enabling a high loading of molecular catalysts (>30 nmol (geometrical cm)^–2). When a Ni bis(diphosphine)-based catalyst is anchored on the surface of the electrode, a high turnover number of ∼1 × 10³ was obtained from photoelectrolysis under UV-filtered simulated solar irradiation at 1 Sun after 24 h at pH 4.5. Notwithstanding its aptitude for molecular catalyst immobilisation, the p-Si|TiO₂ photoelectrode showed great versatility towards different catalysts and pH conditions, with photoelectrocatalytic H₂ generation also being achieved with platinum and a hydrogenase as catalyst, highlighting the flexible platform it represents for many potential reductive catalysis transformations.

A novel versatile microbiosensor for local hydrogen detection by means of scanning photoelectrochemical microscopy

The development of a versatile microbiosensor for hydrogen detection is reported. Carbon-based microelectrodes were modified with a [NiFe]-hydrogenase embedded in a viologen-modified redox hydrogel for the fabrication of a sensitive hydrogen biosensor By integrating the microbiosensor in a scanning photoelectrochemical microscope, it was capable of serving simultaneously as local light source to initiate photo(bio)electrochemical reactions while acting as sensitive biosensor for the detection of hydrogen. A hydrogen evolution biocatalyst based on photosystem 1-platinum nanoparticle biocomplexes embedded into a specifically designed redox polymer was used as a model for proving the capability of the developed hydrogen biosensor for the detection of hydrogen upon localized illumination. The versatility and sensitivity of the proposed microbiosensor as probe tip allows simplification of the set-up used for the evaluation of complex electrochemical processes and the rapid investigation of local photoelectrocatalytic activity of biocatalysts towards light-induced hydrogen evolution.

Proton Reduction Using a Hydrogenase-Modified Nanoporous Black Silicon Photoelectrode

Metalloenzymes featuring earth-abundant metal-based cores exhibit rates for catalytic processes such as hydrogen evolution comparable to those of noble metals. Realizing these superb catalytic properties in artificial systems is challenging owing to the difficulty of effectively interfacing metalloenzymes with an electrode surface in a manner that supports efficient charge-transfer. Here, we demonstrate that a nanoporous "black" silicon (b-Si) photocathode provides a unique interface for binding an adsorbed [FeFe]-hydrogenase enzyme ([FeFe]-H2ase). The resulting [FeFe]-H2ase/b-Si photoelectrode displays a 280 mV more positive onset potential for hydrogen generation than bare b-Si without hydrogenase, similar to that observed for a b-Si/Pt photoelectrode at the same light intensity. Additionally, we show that this H2ase/b-Si electrode exhibits a turnover frequency of ≥1300 s(-1) and a turnover number above 10(7) and sustains current densities of at least 1 mA/cm(2) based on the actual surface area of the electrode (not the smaller projected geometric area), orders of magnitude greater than that observed for previous enzyme-catalyzed electrodes. While the long-term stability of hydrogenase on the b-Si surface remains too low for practical applications, this work extends the proof-of-concept that biologically derived metalloenzymes can be interfaced with inorganic substrates to support technologically relevant current densities.

Oxygen-Dependent Photocatalytic Water Reduction with a Ruthenium(imidazolium) Chromophore and a Cobaloxime Catalyst

Detailed investigations of a photocatalytic system capable of producing hydrogen under pre-catalytic aerobic conditions are reported. This system consists of the NHC precursor chromophore [Ru(tbbpy)2 (RR'ip)][PF6 ]3 (abbreviated as Ru(RR'ip)[PF6 ]3 ; tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, RR'ip=1,3-disubstituted-1H-imidazo[4,5-f][1,10]phenanthrolinium), the reduction catalyst Co(dmgH)2 (dmgH=dimethylglyoximato), and the electron donor ascorbic acid (AA). Screening studies with respect to solvent, cobaloxime catalyst, electron donor, pH, and concentrations of the individual components yielded optimized photocatalytic conditions. The system shows high activity based on Ru, with turnover numbers up to 2000 under oxygen-free and pre-catalytic aerobic conditions. The turnover frequency in the latter case was even higher than that for the oxygen-free catalyst system. The Ru complexes show high photostability and their first excited state is primarily located on the RR'ip ligand. X-ray crystallographic analysis of the rigid cyclophane-type ligand dd(ip)2 (Br)2 (dd(ip)2 =1,1',3,3'-bis(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(1H-imidazo[4,5-f][1,10]phenanthrolinium)) and the catalytic activity of its Ru complex [{(tbbpy)2 Ru}2 (μ-dd(ip)2 )][PF6 ]6 (abbreviated as Ru2 (dd(ip)2 )[PF6 ]6 ) suggest an intermolecular catalytic cycle.

Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2 -Protected Silicon Electrode

The combination of enzymes with semiconductors enables the photoelectrochemical characterization of electron‐transfer processes at highly active and well‐defined catalytic sites on a light‐harvesting electrode surface. Herein, we report the integration of a hydrogenase on a TiO₂‐coated p‐Si photocathode for the photo‐reduction of protons to H₂. The immobilized hydrogenase exhibits activity on Si attributable to a bifunctional TiO₂ layer, which protects the Si electrode from oxidation and acts as a biocompatible support layer for the productive adsorption of the enzyme. The p‐Si|TiO₂|hydrogenase photocathode displays visible‐light driven production of H₂ at an energy‐storing, positive electrochemical potential and an essentially quantitative faradaic efficiency. We have thus established a widely applicable platform to wire redox enzymes in an active configuration on a p‐type semiconductor photocathode through the engineering of the enzyme–materials interface.

Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2-Protected Silicon Electrode

The combination of enzymes with semiconductors enables the photoelectrochemical characterization of electron‐transfer processes at highly active and well‐defined catalytic sites on a light‐harvesting electrode surface. Herein, we report the integration of a hydrogenase on a TiO₂‐coated p‐Si photocathode for the photo‐reduction of protons to H₂. The immobilized hydrogenase exhibits activity on Si attributable to a bifunctional TiO₂ layer, which protects the Si electrode from oxidation and acts as a biocompatible support layer for the productive adsorption of the enzyme. The p‐Si|TiO₂|hydrogenase photocathode displays visible‐light driven production of H₂ at an energy‐storing, positive electrochemical potential and an essentially quantitative faradaic efficiency. We have thus established a widely applicable platform to wire redox enzymes in an active configuration on a p‐type semiconductor photocathode through the engineering of the enzyme–materials interface.

Recent Developments in Hydrogen Evolving Molecular Cobalt(II)-Polypyridyl Catalysts

The search for efficient noble metal-free hydrogen-evolving catalysts is the subject of intense research activity. A new family of molecular cobalt(II)-polypyridyl catalysts has recently emerged. These catalysts prove more robust under reductive conditions than other cobalt-based systems and display high activities under fully aqueous conditions. This review discusses the design, characterization, and evaluation of these catalysts for electrocatalytic and light-driven hydrogen production. Mechanistic considerations are addressed and structure-catalytic activity relationships identified in order to guide the future design of more efficient catalytic systems.

[NiFeSe]-hydrogenase chemistry

The development of technology for the inexpensive generation of the renewable energy vector H2 through water splitting is of immediate economic, ecological, and humanitarian interest. Recent interest in hydrogenases has been fueled by their exceptionally high catalytic rates for H2 production at a marginal overpotential, which is presently only matched by the nonscalable noble metal platinum. The mechanistic understanding of hydrogenase function guides the design of synthetic catalysts, and selection of a suitable hydrogenase enables direct applications in electro- and photocatalysis. [FeFe]-hydrogenases display excellent H2 evolution activity, but they are irreversibly damaged upon exposure to O2, which currently prevents their use in full water splitting systems. O2-tolerant [NiFe]-hydrogenases are known, but they are typically strongly biased toward H2 oxidation, while H2 production by [NiFe]-hydrogenases is often product (H2) inhibited. [NiFeSe]-hydrogenases are a subclass of [NiFe]-hydrogenases with a selenocysteine residue coordinated to the active site nickel center in place of a cysteine. They exhibit a combination of unique properties that are highly advantageous for applications in water splitting compared with other hydrogenases. They display a high H2 evolution rate with marginal inhibition by H2 and tolerance to O2. [NiFeSe]-hydrogenases are therefore one of the most active molecular H2 evolution catalysts applicable in water splitting. Herein, we summarize our recent progress in exploring the unique chemistry of [NiFeSe]-hydrogenases through biomimetic model chemistry and the chemistry with [NiFeSe]-hydrogenases in semiartificial photosynthetic systems. We gain perspective from the structural, spectroscopic, and electrochemical properties of the [NiFeSe]-hydrogenases and compare them with the chemistry of synthetic models of this hydrogenase active site. Our synthetic models give insight into the effects on the electronic properties and reactivity of the active site upon the introduction of selenium. We have utilized the exceptional properties of the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum in a number of photocatalytic H2 production schemes, which are benchmark systems in terms of single site activity, tolerance toward O2, and in vitro water splitting with biological molecules. Each system comprises a light-harvesting component, which allows for light-driven electron transfer to the hydrogenase in order for it to catalyze H2 production. A system with [NiFeSe]-hydrogenase on a dye-sensitized TiO2 nanoparticle gives an enzyme-semiconductor hybrid for visible light-driven generation of H2 with an enzyme-based turnover frequency of 50 s(-1). A stable and inexpensive polymeric carbon nitride as a photosensitizer in combination with the [NiFeSe]-hydrogenase shows good activity for more than 2 days. Light-driven H2 evolution with the enzyme and an organic dye under high O2 levels demonstrates the excellent robustness and feasibility of water splitting with a hydrogenase-based scheme. This has led, most recently, to the development of a light-driven full water splitting system with a [NiFeSe]-hydrogenase wired to the water oxidation enzyme photosystem II in a photoelectrochemical cell. In contrast to the other systems, this photoelectrochemical system does not rely on a sacrificial electron donor and allowed us to establish the long sought after light-driven water splitting with an isolated hydrogenase.

Carbon nitride-TiO2 hybrid modified with hydrogenase for visible light driven hydrogen production

A system consisting of a [NiFeSe]-hydrogenase (H₂ase) grafted on the surface of a TiO₂ nanoparticle modified with polyheptazine carbon nitride polymer, melon (CN _x ) is reported. This semi-biological assembly shows a turnover number (TON) of more than 5.8 × 10⁵ mol H₂ (mol H₂ase)^-1 after 72 h in a sacrificial electron donor solution at pH 6 during solar AM 1.5 G irradiation. An external quantum efficiency up to 4.8% for photon-to-hydrogen conversion was achieved under irradiation with monochromatic light. The CN _x -TiO₂-H₂ase construct was also active under UV-free solar light irradiation (λ > 420 nm), where it showed a substantially higher activity than TiO₂-H₂ase and CN _x -H₂ase due, in part, to the formation of a CN _x -TiO₂ charge transfer complex and highly productive electron transfer to the H₂ase. The CN _x -TiO₂-H₂ase system sets a new benchmark for photocatalytic H₂ production with a H₂ase immobilised on a noble- and toxic-metal free light absorber in terms of visible light utilisation and stability.

Metagenomic Sequencing Unravels Gene Fragments with Phylogenetic Signatures of O2-Tolerant NiFe Membrane-Bound Hydrogenases in Lacustrine Sediment

Many promising hydrogen technologies utilising hydrogenase enzymes have been slowed by the fact that most hydrogenases are extremely sensitive to O₂. Within the group 1 membrane-bound NiFe hydrogenase, naturally occurring tolerant enzymes do exist, and O₂ tolerance has been largely attributed to changes in iron–sulphur clusters coordinated by different numbers of cysteine residues in the enzyme’s small subunit. Indeed, previous work has provided a robust phylogenetic signature of O₂ tolerance [1], which when combined with new sequencing technologies makes bio prospecting in nature a far more viable endeavour. However, making sense of such a vast diversity is still challenging and could be simplified if known species with O₂-tolerant enzymes were annotated with information on metabolism and natural environments. Here, we utilised a bioinformatics approach to compare O₂-tolerant and sensitive membrane-bound NiFe hydrogenases from 177 bacterial species with fully sequenced genomes for differences in their taxonomy, O₂ requirements, and natural environment. Following this, we interrogated a metagenome from lacustrine surface sediment for novel hydrogenases via high-throughput shotgun DNA sequencing using the Illumina™ MiSeq platform. We found 44 new NiFe group 1 membrane-bound hydrogenase sequence fragments, five of which segregated with the tolerant group on the phylogenetic tree of the enzyme’s small subunit, and four with the large subunit, indicating de novo O₂-tolerant protein sequences that could help engineer more efficient hydrogenases.

Mesoporous materials for clean energy technologies

Alternative energy technologies are greatly hindered by significant limitations in materials science. From low activity to poor stability, and from mineral scarcity to high cost, the current materials are not able to cope with the significant challenges of clean energy technologies. However, recent advances in the preparation of nanomaterials, porous solids, and nanostructured solids are providing hope in the race for a better, cleaner energy production. The present contribution critically reviews the development and role of mesoporosity in a wide range of technologies, as this provides for critical improvements in accessibility, the dispersion of the active phase and a higher surface area. Relevant examples of the development of mesoporosity by a wide range of techniques are provided, including the preparation of hierarchical structures with pore systems in different scale ranges. Mesoporosity plays a significant role in catalysis, especially in the most challenging processes where bulky molecules, like those obtained from biomass or highly unreactive species, such as CO2 should be transformed into most valuable products. Furthermore, mesoporous materials also play a significant role as electrodes in fuel and solar cells and in thermoelectric devices, technologies which are benefiting from improved accessibility and a better dispersion of materials with controlled porosity.

Proton reduction by molecular catalysts in water under demanding atmospheres

The electrocatalytic proton reduction activity of a Ni bis(diphosphine) (NiP) and a cobaloxime (CoP) catalyst has been studied in water in the presence of the gaseous inhibitors O2 and CO. CoP shows an appreciable tolerance towards O2, but its activity suffers severely in the presence of CO. In contrast, NiP is strongly inhibited by O2, but produces H2 under high CO concentrations.

Synthetic Active Site Model of the [NiFeSe] Hydrogenase

A dinuclear synthetic model of the [NiFeSe] hydrogenase active site and a structural, spectroscopic and electrochemical analysis of this complex is reported. [NiFe(‘S₂Se₂’)(CO)₃] (H₂‘S₂Se₂’=1,2-bis(2-thiabutyl-3,3-dimethyl-4-selenol)benzene) has been synthesized by reacting the nickel selenolate complex [Ni(‘S₂Se₂’)] with [Fe(CO)₃bda] (bda=benzylideneacetone). X-ray crystal structure analysis confirms that [NiFe(‘S₂Se₂’)(CO)₃] mimics the key structural features of the enzyme active site, including a doubly bridged heterobimetallic nickel and iron center with a selenolate terminally coordinated to the nickel center. Comparison of [NiFe(‘S₂Se₂’)(CO)₃] with the previously reported thiolate analogue [NiFe(‘S₄’)(CO)₃] (H₂‘S₄’=H₂xbsms=1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) showed that the selenolate groups in [NiFe(‘S₂Se₂’)(CO)₃] give lower carbonyl stretching frequencies in the IR spectrum. Electrochemical studies of [NiFe(‘S₂Se₂’)(CO)₃] and [NiFe(‘S₄’)(CO)₃] demonstrated that both complexes do not operate as homogenous H₂ evolution catalysts, but are precursors to a solid deposit on an electrode surface for H₂ evolution catalysis in organic and aqueous solution.

A Decaheme Cytochrome as a Molecular Electron Conduit in Dye-Sensitized Photoanodes

In nature, charge recombination in light-harvesting reaction centers is minimized by efficient charge separation. Here, it is aimed to mimic this by coupling dye-sensitized TiO₂ nanocrystals to a decaheme protein, MtrC from Shewanella oneidensis MR-1, where the 10 hemes of MtrC form a ≈7-nm-long molecular wire between the TiO₂ and the underlying electrode. The system is assembled by forming a densely packed MtrC film on an ultra-flat gold electrode, followed by the adsorption of approximately 7 nm TiO₂ nanocrystals that are modified with a phosphonated bipyridine Ru(II) dye (RuP). The step-by-step construction of the MtrC/TiO₂ system is monitored with (photo)electrochemistry, quartz-crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM). Photocurrents are dependent on the redox state of the MtrC, confirming that electrons are transferred from the TiO₂ nanocrystals to the surface via the MtrC conduit. In other words, in these TiO₂/MtrC hybrid photodiodes, MtrC traps the conduction-band electrons from TiO₂ before transferring them to the electrode, creating a photobioelectrochemical system in which a redox protein is used to mimic the efficient charge separation found in biological photosystems.

Application of the energetic span model to the electrochemical catalysis of proton reduction by a diiron azadithiolate complex

A method for the computation of TOF of catalysis of electrochemical reaction as a function of the potential was developed.

Photocatalytic hydrogen production using polymeric carbon nitride with a hydrogenase and a bioinspired synthetic Ni catalyst

Solar-light-driven H2 production in water with a [NiFeSe]-hydrogenase (H2ase) and a bioinspired synthetic nickel catalyst (NiP) in combination with a heptazine carbon nitride polymer, melon (CN(x)), is reported. The semibiological and purely synthetic systems show catalytic activity during solar light irradiation with turnover numbers (TONs) of more than 50,000 mol H2(mol H2ase)(-1) and approximately 155 mol H2 (mol NiP)(-1) in redox-mediator-free aqueous solution at pH 6 and 4.5, respectively. Both systems maintained a reduced photoactivity under UV-free solar light irradiation (λ>420 nm).

Photocatalytic Hydrogen Production using Polymeric Carbon Nitride with a Hydrogenase and a Bioinspired Synthetic Ni Catalyst

Solar-light-driven H₂ production in water with a [NiFeSe]-hydrogenase (H₂ase) and a bioinspired synthetic nickel catalyst (NiP) in combination with a heptazine carbon nitride polymer, melon (CN_x), is reported. The semibiological and purely synthetic systems show catalytic activity during solar light irradiation with turnover numbers (TONs) of more than 50 000 mol H₂ (mol H₂ase)⁻¹ and approximately 155 mol H₂ (mol NiP)⁻¹ in redox-mediator-free aqueous solution at pH 6 and 4.5, respectively. Both systems maintained a reduced photoactivity under UV-free solar light irradiation (λ>420 nm).

Redirecting photosynthetic electron flow into light-driven synthesis of alternative products including high-value bioactive natural compounds

Photosynthesis in plants, green algae, and cyanobacteria converts solar energy into chemical energy in the form of ATP and NADPH, both of which are used in primary metabolism. However, often more reducing power is generated by the photosystems than what is needed for primary metabolism. In this review, we discuss the development in the research field, focusing on how the photosystems can be used as synthetic biology building blocks to channel excess reducing power into light-driven production of alternative products. Plants synthesize a large number of high-value bioactive natural compounds. Some of the key enzymes catalyzing their biosynthesis are the cytochrome P450s situated in the endoplasmic reticulum. However, bioactive compounds are often synthesized in low quantities in the plants and are difficult to produce by chemical synthesis due to their often complex structures. Through a synthetic biology approach, enzymes with a requirement for reducing equivalents as cofactors, such as the cytochrome P450s, can be coupled directly to the photosynthetic energy output to obtain environmentally friendly production of complex chemical compounds. By relocating cytochrome P450s to the chloroplasts, reducing power can be diverted toward the reactions catalyzed by the cytochrome P450s. This provides a sustainable production method for high-value compounds that potentially can solve the problem of NADPH regeneration, which currently limits the biotechnological uses of cytochrome P450s. We describe the approaches that have been taken to couple enzymes to photosynthesis in vivo and to photosystem I in vitro and the challenges associated with this approach to develop new green production platforms.

Versatile photocatalytic systems for H2 generation in water based on an efficient DuBois-type nickel catalyst

The generation of renewable H2 through an efficient photochemical route requires photoinduced electron transfer (ET) from a light harvester to an efficient electrocatalyst in water. Here, we report on a molecular H2 evolution catalyst (NiP) with a DuBois-type [Ni(P2(R')N2(R"))2](2+) core (P2(R')N2(R") = bis(1,5-R'-diphospha-3,7-R"-diazacyclooctane), which contains an outer coordination sphere with phosphonic acid groups. The latter functionality allows for good solubility in water and immobilization on metal oxide semiconductors. Electrochemical studies confirm that NiP is a highly active electrocatalyst in aqueous electrolyte solution (overpotential of approximately 200 mV at pH 4.5 with a Faradaic yield of 85 ± 4%). Photocatalytic experiments and investigations on the ET kinetics were carried out in combination with a phosphonated Ru(II) tris(bipyridine) dye (RuP) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of RuP in solution or on the surface of ZrO2 ("on particle" system) or oxidative quenching of RuP when the compounds were immobilized on TiO2 ("through particle" system). Our studies show that NiP can be used in a purely aqueous solution and on a semiconductor surface with a high degree of versatility. A high TOF of 460 ± 60 h(-1) with a TON of 723 ± 171 for photocatalytic H2 generation with a molecular Ni catalyst in water and a photon-to-H2 quantum yield of approximately 10% were achieved for the homogeneous system.

Development and understanding of cobaloxime activity through electrochemical molecular catalyst screening

Electrochemical molecular catalyst screening (EMoCS) has been developed.

Photocatalytic hydrogen evolution with a hydrogenase in a mediator-free system under high levels of oxygen

Take a breath: An oxygen-tolerant hydrogenase can be employed with a dye in a photocatalytic scheme for the generation of H2 . The homogeneous system does not require a redox mediator and visible-light irradiation yields high amounts of H2 even in the presence of air.

Accelerated electron transport from photosystem I to redox partners by covalently linked ferredoxin

Photosystem I is a highly efficient and potent light-induced reductase that is considered to be an appealing target for integration into hybrid solar fuel production systems. However, rapid transport of multiple electrons from the reducing end of photosystem I to downstream processes in vivo is limited by the diffusion of its native redox partner ferredoxin that is a single electron carrier. Here, we describe the design and construction of a faster electron transfer interface based on anchoring ferredoxin to the reducing end of photosystem I thereby confining the diffusion space of ferredoxin to the near vicinity of its photosystem I binding and reduction site. This was achieved by fusing ferredoxin to the PsaE subunit of photosystem I by a flexible peptide linker and reconstituting PSI in vitro with the new fusion protein. A computational algorithm was developed in order to determine the optimal linker length that will confine ferredoxin to the vicinity of photosystem I's reducing end without restricting the formation of electron transfer complexes. According to the calculation, we reconstituted photosystem I with three fusion proteins comprising PsaE and ferredoxin separated by linkers of different lengths, namely 14, 19, and 25 amino acids, and tested their effect on electron transfer rates from photosystem I to downstream processes. Indeed, we found a significant enhancement of light dependent NADPH synthesis using photosystems containing the PsaE-ferredoxin fusion proteins, equivalent to a ten-fold increase in soluble ferredoxin concentration. We propose that such a system could be used for other ferredoxin dependent redox reactions, such as the enzymatic production of hydrogen, a promising alternative fuel. As the system is comprised entirely of natural amino acids and biological cofactors, it could be integrated into the energy conversion apparatus of photosynthetic organisms by genetic engineering.

Cobinamide production of hydrogen in a homogeneous aqueous photochemical system, and assembly and photoreduction in a (βα)8 protein

Components of a protein-integrated, earth-abundant metal macrocycle catalyst, with the purpose of H2 production from aqueous protons under green conditions, are characterized. The cobalt-corrin complex, cobinamide, is demonstrated to produce H2 (4.4 ± 1.8 × 10(-3) turnover number per hour) in a homogeneous, photosensitizer/sacrificial electron donor system in pure water at neutral pH. Turnover is proposed to be limited by the relatively low population of the gateway cobalt(III) hydride species. A heterolytic mechanism for H2 production from the cobalt(II) hydride is proposed. Two essential requirements for assembly of a functional protein-catalyst complex are demonstrated for interaction of cobinamide with the (βα)8 TIM barrel protein, EutB, from the adenosylcobalamin-dependent ethanolamine ammonia lyase from Salmonella typhimurium: (1) high-affinity equilibrium binding of the cobinamide (dissociation constant 2.1 × 10(-7) M) and (2) in situ photoreduction of the cobinamide-protein complex to the Co(I) state. Molecular modeling of the cobinamide-EutB interaction shows that these features arise from specific hydrogen-bond and apolar interactions of the protein with the alkylamide substituents and the ring of the corrin, and accessibility of the binding site to the solution. The results establish cobinamide-EutB as a platform for design and engineering of a robust H2 production metallocatalyst that operates under green conditions and uses the advantages of the protein as a tunable medium and material support.

Influence of the protein structure surrounding the active site on the catalytic activity of [NiFeSe] hydrogenases

A combined experimental and theoretical study of the catalytic activity of a [NiFeSe] hydrogenase has been performed by H/D exchange mass spectrometry and molecular dynamics simulations. Hydrogenases are enzymes that catalyze the heterolytic cleavage or production of H2. The [NiFeSe] hydrogenases belong to a subgroup of the [NiFe] enzymes in which a selenocysteine is a ligand of the nickel atom in the active site instead of cysteine. The aim of this research is to determine how much the specific catalytic properties of this hydrogenase are influenced by the replacement of a sulfur by selenium in the coordination of the bimetallic active site versus the changes in the protein structure surrounding the active site. The pH dependence of the D2/H(+) exchange activity and the high isotope effect observed in the Michaelis constant for the dihydrogen substrate and in the single exchange/double exchange ratio suggest that a "cage effect" due to the protein structure surrounding the active site is modulating the enzymatic catalysis. This "cage effect" is supported by molecular dynamics simulations of the diffusion of H2 and D2 from the outside to the inside of the protein, which show different accumulation of these substrates in a cavity next to the active site.