Reduced Methyl Viologen Research Articles

Transparent Conductive Encapsulants for Photoelectrochemical Applications

AbstractInvited for this issue's Front Cover is a group of researchers from the Materials, Chemistry, and Computational Science Directorate at the National Renewable Energy Laboratory, led by Dr. Ann L. Greenaway. The front cover picture shows the surface of a TCE sheet, with the metallized spheres that act as conductive pathways from the semiconductor surface, through the polymer matrix, to the electrochemical interface. The rainbow represents the light going through the TCE layer to the semiconductor below. The primary electrochemical reaction used in this work was the first reduction of methyl viologen, as illustrated with the floating chemical structures. Cover design by Talysa Klein (https://www.tk2.design/). Read the full text of the Research Article at 10.1002/celc.202300209.

Utilizing a Transparent Conductive Encapsulant to Protect Photoelectrodes during Solar Fuel Formation

Carbon-neutral electricity is rapidly becoming available worldwide as solar and wind technologies advance, but storing energy in chemical bonds will remain a critical need for the transportation sector, as planes and other energy intensive processes will still require liquid fuels. Solar fuels, utilizing sunlight to directly convert CO2 into useful chemicals, are a renewable and net carbon-neutral way to produce needed liquid fuels. However, a common problem with photoelectrochemical solar fuel production is semiconductor degradation from submersion in aqueous environments. An ideal protective layer should 1) prevent solution from reaching the semiconductor, 2) maintain charge transfer to and from the solution, and 3) be transparent to light above the semiconductor band gap. While there presently are protective layer options that meet all three requirements, such as leaky TiO2 and MoS2, they are not easily adaptable to new semiconductor surfaces and/or to new electrochemical reactions. This can make protection difficult for newly developed photoabsorbers and catalytic reaction pairings. In this work, we demonstrate the use of transparent conductive encapsulants (TCEs) to meet these three requirements while also allowing for photoelectrode- and catalysis-agnostic adaptability. TCEs are composed of an ethyl vinyl acetate (EVA) matrix with embedded conductive metal-coated poly(methyl methacrylate) (PMMA) microspheres that can be attached to substrates through a lamination process. First, we characterize the electrochemical behavior of TCE-coated electrodes using the reduction of methyl viologen, demonstrating electrical conduction through the TCE layer. Results from a pinhole detection apparatus suggest the TCE is initially defect free and thus able to prevent solution from reaching the substrate. Then, we perform photoelectrochemical measurements of TCE-covered semiconductors to demonstrate the flexibility of this protection scheme for multiple materials. We also show the results of long-term photoelectrochemical measurements designed to probe the efficacy of TCEs as protective layers. These findings demonstrate that TCEs are an effective protective layer for a variety of photoelectrochemical applications.

(Invited) New Approaches to Photoelectrochemical Cascade Reactions and Protection of Photoelectrodes

A half-century of research has positioned direct photoelectrochemical (PEC) fuel generation as a promising technology that still requires substantial development. Work on the hydrogen evolution reaction (HER) has provided a strong basis for realizing the generation of more complex fuels through the carbon dioxide reduction reaction (CO2RR), but multiple challenges remain. The CO2RR requires larger driving forces and integration of multiple catalysts in order to selectively generate multi-carbonproducts, while still presenting many unsolved issues from HER, such stabilizing photoelectrodes under operation. In this talk I will discuss advances in the integration of multiple catalytic microenvironments in a single photoelectrochemical device, and progress toward the stabilization of photoelectrodes for fuel generation.First, I will highlight the adaptation of three-terminal tandem (3TT) photovoltaic technology to photoelectrochemical applications. In our 3TT devices, a two-junction III-V solar cell device has an additional contact, enabling two unique catalyst sites operating at different voltages under the same illumination. Idealized circuit modeling shows the promise of these 3TT devices compared to traditional two-terminal, two-junction devices, particularly with respect to spectral tolerance. We have adapted the structure of 3TT photovoltaics to function as PEC devices and developed multiple catalysts for integration into the two solution contact sites. We demonstrate progress toward light-driven cascade catalysis for multi-carbon CO2RR products.Second, I will highlight our approaches to protective schemes for photoelectrodes: one which is independent of semiconductor chemistry, and one which is driven by that chemistry. In the first approach, transparent conductive encapsulants (TCEs) are demonstrated as photoabsorber-agnostic protective layers. Unlike many protection schemes, these TCEs can be applied to semiconductors post-processing and without substantial modification, providing facile and robust protection. We have characterized the electrochemical performance of TCEs and demonstrate their integration with multiple semiconductors for the reduction of methyl viologen as a proxy for PEC fuel formation. In the second approach, we use the knowledge generated over fifty years of photoelectrode research to develop a new material, ZnTiN2, which can be directly integrated with established semiconductors and which will degrade under PEC conditions. We leverage this degradation to create protective layers which may be self-healing, and demonstrate rapid refinement of ZnTiN2 optoelectronic properties enabling integration with other semiconductors in tandem configurations.

Transparent Conductive Encapsulants for Photoelectrochemical Applications

AbstractUtilizing sunlight to directly perform photoelectrochemical reactions is a promising route to renewable, net carbon‐neutral fuels. However, a common problem with solar fuel production is semiconductor degradation in aqueous environments. An ideal protection layer should (1) prevent solution from reaching the semiconductor, (2) maintain charge transfer to and from solution, and (3) be transparent to light above the semiconductor band gap. While there have been substantial advances toward layers that meet these requirements, they are not easily adapted to new surfaces or new reactions, which can make protection difficult for newly developed photoabsorbers and (photo)electrochemical reaction pairings. In this work, we demonstrate the use of transparent conductive encapsulants (TCEs) to meet these requirements while also allowing for photoelectrode‐ and reaction‐agnostic adaptability. TCEs are composed of an ethyl‐vinyl acetate matrix with embedded conductive metal‐coated microspheres that can be laminated to semiconductors. First, the electrochemical behavior of TCE‐coated electrodes for the reduction of methyl viologen is characterized, demonstrating through‐TCE electrical conduction. Then, photoelectrochemical measurements on TCE‐protected semiconductors demonstrate the flexibility of this protection scheme. Finally, long‐term photoelectrochemical measurements probe the efficacy of TCEs as protection layers. These findings demonstrate the potential of TCEs as adaptable protection layers in various photoelectrochemical applications.

Comprehensive structural, infrared spectroscopic and kinetic investigations of the roles of the active-site arginine in bidirectional hydrogen activation by the [NiFe]-hydrogenase 'Hyd-2' from Escherichia coli.

The active site of [NiFe]-hydrogenases contains a strictly-conserved pendant arginine, the guanidine head group of which is suspended immediately above the Ni and Fe atoms. Replacement of this arginine (R479) in hydrogenase-2 from E. coli results in an enzyme that is isolated with a very tightly-bound diatomic ligand attached end-on to the Ni and stabilised by hydrogen bonding to the Nζ atom of the pendant lysine and one of the three additional water molecules located in the active site of the variant. The diatomic ligand is bound under oxidising conditions and is removed only after a prolonged period of reduction with H2 and reduced methyl viologen. Once freed of the diatomic ligand, the R479K variant catalyses both H2 oxidation and evolution but with greatly decreased rates compared to the native enzyme. Key kinetic characteristics are revealed by protein film electrochemistry: most importantly, a very low activation energy for H2 oxidation that is not linked to an increased H/D isotope effect. Native electrocatalytic reversibility is retained. The results show that the sluggish kinetics observed for the lysine variant arise most obviously because the advantage of a more favourable low-energy pathway is massively offset by an extremely unfavourable activation entropy. Extensive efforts to establish the identity of the diatomic ligand, the tight binding of which is an unexpected further consequence of replacing the pendant arginine, prove inconclusive.

Effect of charge selective contacts on the quasi Fermi level splitting of CuGa3Se5 thin film photocathodes for hydrogen evolution and methylviologen reduction

The defect-copper chalcopyrite CuGa3Se5 is a promising photocathode material for solar hydrogen generation. Here we assess its performance with photoelectrochemical measurements and vibrating Kelvin probe surface photovoltage spectroscopy.

Adapting confocal Raman microscopy for in situ studies of redox transformations at electrode-electrolyte interfaces

Confocal Raman microscopy was applied to quantify redox species present within the diffusion layer adjacent to an electrode surface under potentiostatic control. A glass microscope coverslip with a thin indium tin oxide (ITO) coating served as both the working electrode and optical window for a microscope-stage mountable spectroelectrochemical cell. A high numerical aperture objective mounted in an inverted microscope frame just below the stage brought excitation radiation through the coverslip window and to a tight focus a few micrometers above the ITO film surface. Species diffusing into the confocal probe volume defined by the excitation beam focus and the collected light region were detected, identified and quantified based on their Raman scattering frequencies and intensities. In measurements that interrogated the interconversion of ferrocyanide and ferricyanide ions as a function of applied voltage, least-squares regression analysis of spectral datasets predicted the formal potential and relative surface concentrations of the ions in good agreement with the expected Nernstian response. Preliminary studies of methyl viologen reduction at an ITO film/Nafion membrane interface were conducted and show the possibility for estimation of mass transport coefficients of redox species within ionic polymer materials.

Electrochemical Trimethylamine N-Oxide Biosensor with Enzyme-Based Oxygen-Scavenging Membrane for Long-Term Operation under Ambient Air

An amperometric trimethylamine N-oxide (TMAO) biosensor is reported, where TMAO reductase (TorA) and glucose oxidase (GOD) and catalase (Cat) were immobilized on the electrode surface, enabling measurements of mediated enzymatic TMAO reduction at low potential under ambient air conditions. The oxygen anti-interference membrane composed of GOD, Cat and polyvinyl alcohol (PVA) hydrogel, together with glucose concentration, was optimized until the O2 reduction current of a Clark-type electrode was completely suppressed for at least 3 h. For the preparation of the TMAO biosensor, Escherichia coli TorA was purified under anaerobic conditions and immobilized on the surface of a carbon electrode and covered by the optimized O2 scavenging membrane. The TMAO sensor operates at a potential of −0.8 V vs. Ag/AgCl (1 M KCl), where the reduction of methylviologen (MV) is recorded. The sensor signal depends linearly on TMAO concentrations between 2 µM and 15 mM, with a sensitivity of 2.75 ± 1.7 µA/mM. The developed biosensor is characterized by a response time of about 33 s and an operational stability over 3 weeks. Furthermore, measurements of TMAO concentration were performed in 10% human serum, where the lowest detectable concentration is of 10 µM TMAO.

Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals.

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr6-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Zinc porphyrin/mesoporous titania thin film electrodes: a hybrid material nanoarchitecture for photocatalytic reduction.

In this work, photocatalytic reduction of methyl viologen is achieved using zinc tetra(4-N-methylpyridyl)porphine (ZnP) functionalized mesoporous titania thin films (MTTF). Metalloporphyrins are the core of natural systems that harvest energy from the sun. Thus, a bioinspired approach is used, taking advantage of ZnP sensitizing capabilities and MTTF organized structure.

Photocatalytic activity of the light-harvesting complex of photosystem II (LHCII) monomer

Light-harvesting complex of photosystem II (LHCII) is the most abundant membrane protein-chlorophyll complex in chloroplasts. Here, we evaluated the photocatalytic activity of the native trimer and the enzymatically treated monomer forms of LHCII. Upon white light irradiation using a solar simulator, photocatalytic reduction of methylviologen revealed that the activity of monomeric LHCII was higher than that of the trimer form. Fluorescence of the monomeric LHCII was more significantly quenched by methylviologen than that of the trimer form. In the presence of Pt nanoparticles in a catalytic solution system, photoinduced hydrogen production was observed for the LHCII monomer. The difference in photocatalytic activities of LHCII trimer and monomer are briefly discussed in terms of an interaction between LHCII and methylviologen.

CdS-coated thin plastic films for visible-light photocatalysis

The preparation of a visible light-absorbing, very thin (2.5 μm), flexible CdS nanoparticle coated polystyrene (PS) film, CdS-PS, with a 3D-printed backing is described. Scanning electron microscopy confirms that the CdS-PS film comprise a thin layer of CdS nanoparticles (26 ± 4 nm) on just one side of the PS film, with no nanoparticles on the other side. When irradiated with 420 nm or 365 nm radiation, in air-saturated water, the CdS film is photobleached, and dissolved O2 consumed, due to the photoinduced oxidative corrosion of the CdS by O2. In contrast, under the same aerobic conditions, the CdS-PS film is very stable, when a sacrificial electron donor (SED) is present, such as EDTA or ascorbate/ascorbic acid, with the latter appearing the most effective. In the presence of an SED, the CdS-PS film photocatalyses the reduction of the dyes, methyl orange and crystal violet, and the electron-relay, methyl viologen, by different SEDs, using visible and UV light. In the photocatalysed reduction of methyl viologen by EDTA, colloidal Pt reacts with the highly coloured blue methyl viologen radicals generated to produce H2. Visible light irradiation of the CdS-PS/MV2+/EDTA/colloidal Pt system promotes the reduction of water to H2 by the SED, EDTA, mediated by methyl viologen. A colourless, TiO2-PS film, made using P25 TiO2, is used to effect the same photocatalytic reduction reactions as the CdS-PS film, but only when irradiated with UV (365 nm) radiation. In both cases the films are used repeatedly with no evidence of deterioration in activity or film stability. This is the first example of the preparation and testing of a visible light absorbing photocatalytic, i.e. CdS, thin plastic film, the preparation of which is very simple and inexpensive and may prove invaluable for the production of thin, flexible plastic photocatalytic films for solar research.

Photoinduced Charge Accumulation and Prolonged Multielectron Storage for the Separation of Light and Dark Reaction.

The utilization of solar energy is restricted by the intermittent nature of solar influx. We present novel noble-metal free complexes that can be photochemically charged in the presence of sacrificial electron donors and remain stable in its charged form for over 14 h. This allows the doubly reduced Cu(I) 4H-imidazolate complex to be stored after photochemical charging and used as a reagent in dark reactions, such as the reduction of methyl viologen or oxygen. Combined UV-vis/EPR spectroelectrochemistry indicates that a two-electron reduction is induced by introducing sacrificial electron donors that facilitate proton-coupled electron transfer. Repeated photochemical reduction and chemical oxidation reveals that the complex retained a charging capacity of 72% after four cycles. We demonstrate a chemical system that can decouple photochemical processes from the day-night cycle, which has been a barrier to realizing utilization of solar energy in photochemical processes on a global scale.

Sulfite oxidation by the quinone-reducing molybdenum sulfite dehydrogenase SoeABC from the bacterium Aquifex aeolicus

The microaerophilic bacterium Aquifex aeolicus is a chemolitoautotroph that uses sulfur compounds as electron sources. The model of oxidation of the energetic sulfur compounds in this bacterium predicts that sulfite would probably be a metabolic intermediate released in the cytoplasm. In this work, we purified and characterized a membrane-bound sulfite dehydrogenase, identified as an SoeABC enzyme, that was previously described as a sulfur reductase. It is a member of the DMSO-reductase family of molybdenum enzymes. This type of enzyme was identified a few years ago but never purified, and biochemical data and kinetic properties were completely lacking. An enzyme catalyzing sulfite oxidation using Nitro-blue tetrazolium as artificial electron acceptor was extracted from the membrane fraction of Aquifex aeolicus. The purified enzyme is a dimer of trimer (αβγ)2 of about 390 kDa. The KM for sulfite and kcat values were 34 μM and 567 s−1 respectively, at pH 8.3 and 55 °C. We furthermore showed that SoeABC reduces a UQ10 analogue, the decyl-ubiquinone, as well, with a KM of 2.6 μM and a kcat of 52.9 s−1. It seems to specifically oxidize sulfite but can work in the reverse direction, reduction of sulfur or tetrathionate, using reduced methyl viologen as electron donor. The close phylogenetic relationship of Soe with sulfur and tetrathionate reductases that we established, perfectly explains this enzymatic ability, although its bidirectionality in vivo still needs to be clarified. Oxygen-consumption measurements confirmed that electrons generated by sulfite oxidation in the cytoplasm enter the respiratory chain at the level of quinones.

A ferredoxin-dependent dihydropyrimidine dehydrogenase in Clostridium chromiireducens.

Dihydropyrimidine dehydrogenase (PydA) catalyzes the first step of the reductive pyrimidine degradation (Pyd) pathway in bacteria and eukaryotes, enabling pyrimidines to be utilized as substrates for growth. PydA homologs studied to date catalyze the reduction of uracil to dihydrouracil, coupled to the oxidation of NAD(P)H. Uracil reduction occurs at a flavin mononucleotide (FMN) site, and NAD(P)H oxidation occurs at a flavin adenine dinucleotide (FAD) site, with two ferredoxin domains thought to mediate inter-site electron transfer. Here, we report the biochemical characterization of a Clostridial PydA homolog (PydAc) from a Pyd gene cluster in the strict anaerobic bacterium Clostridium chromiireducens. PydAc lacks the FAD domain, and instead is able to catalyze uracil reduction using reduced methyl viologen or reduced ferredoxin as the electron source. Homologs of PydAc are present in Pyd gene clusters in many strict anaerobic bacteria, which use reduced ferredoxin as an intermediate in their energy metabolism.

Nanoscale integration of porphyrin in GroEL protein cage: Photophysical and photochemical investigation.

In this paper, we introduce a new type of functional, supramolecular porphyrin conjugate created using the bacterial GroEL protein cage based on non-specific hydrophobic interaction. The synthesis, structure and property of the porphyrin conjugate were characterized by dynamic light scattering, UV-vis spectroscopy and fluorescence spectroscopy. We observed that the model zinc-tetraphenylporphyrin (Zn-TPP) with high hydrophobicity can be well-dispersed in aqueous solutions with the aid of GroEL open chamber, which is known to be a favorable nanocompartment for aggregation-prone molecules. The maximal encapsulation efficiency of Zn-TPP in GroEL was determined to be ~98%. It is further seen that the constructed double Zn-TPP-GroEL complex exhibited good photocatalytic activity in the model reactions of the production of singlet oxygen and the reduction of methyl viologen under illumination with visible light. Moreover, we found that GroEL can significantly improve the photostability of Zn-TPP molecules as a result of nanoscale assembly within its hydrophobic chamber. Hence enhanced water solubility and photostability of Zn-TPP, which are considered as the first two hurdles for the wide usage of porphyrins, were achieved simultaneously by the development of GroEL cage as a building block. Supramolecular nanostructures formed from porphyrins (or related molecules) and GroEL for photocatalysis would greatly simplify applications of such structures.

Synthesis and Characteristics of Ag–Pd Nanoparticles: Inhibition of Palladium Surface Catalytic Activity by Silver

Palladium nanoparticles catalyze the reduction of Ag+ ions with hydrogen on their surface, thereby leading to the formation of bimetallic particles with the “core–shell” structure. The catalytic reaction of methylviologen reduction with hydrogen at ratio [Ag] : [Pd] > 1 is suppressed by silver atoms located on the surface of palladium nanoparticles. At [Ag] : [Pd] ≤ 1, the reduction takes place; however, it is preceded by an induction period, and, the higher the silver content on the nanoparticle surface, the longer the induction period. The rate constant of the catalytic reduction, which begins after the end of the induction period, decreases with increasing silver content on the surface of palladium nanoparticles.

Evaluation of electron-transferring cofactor mediating enzyme systems involved in urolithin dehydroxylation in Gordonibacter urolithinfaciens DSM 27213

The gut bacterium Gordonibacter urolithinfaciens DSM 27213 metabolizes ellagic acid into three polyphenol compounds, namely, urolithin M5, urolithin M6, and urolithin C, which are collectively called urolithin. The key reactions of this metabolic pathway are the dehydroxylation of the phenolic hydroxy group, i.e., conversion of urolithin M5 to urolithin M6, and successive conversion of urolithin M6 to urolithin C. By testing the effects of various electron-transferring compounds on the dehydroxylation reactions, methylviologen was found to effectively support the dehydroxylation catalyzed by the cell free extracts. The urolithin dehydroxylating enzymes were found in the soluble fraction of the cell free extracts. The urolithin dehydroxylation was found to be coupled with reduction of dicationic methylviologen to a cation radical form catalyzed by enzymes with hydrogen as an electron donor, which was also found with the soluble fraction. Further investigation of the reaction in the presence of natural cofactors with or without methylviologen and hydrogen revealed the involvement of NADPH and FAD in the electron transportation systems of the urolithin dehydroxylation.

Structural and Photochemical Properties of Zn(II) Phenanthroline-Derived Complexes: From Mononuclear to Bimetallic and Circular-Trimetallic Helicates.

In the design of self-assembled compounds, small variations in the linkers connecting the coordinating moieties can produce large differences in the obtained structures. Here, we report three novel zinc(II) complexes with phenanthroline-derived ligands as building blocks (L1-L3): A mononuclear complex, a bimetallic helicate, and a trimetallic circular helicate. The even-number spacer in L2 promotes the formation of a bimetallic helicate stabilized by π-π interactions of adjacent phenanthrolines. The addition of an extra methylene in L3 increases the distance between where the phenanthrolines can stack, and CH-π noncovalent interactions give stability to the circular helicate. When irradiated at 308 nm in acetonitrile, long-lived excited states are formed with all three complexes, which are able to participate in oxidation of 2-propanol and in reduction of methylviologen, MV2+. While the overall behavior of the three complexes is similar, the bimetallic helicate is able to form a ground-state adduct with MV2+, while the trimer reaches the excited state to form an exciplex with MV2+.

Triplet photosensitizer-nanotube conjugates: synthesis, characterization and photochemistry of charge stabilizing, palladium porphyrin/carbon nanotube conjugates.

The ability of a triplet photosensitizer to generate long-lived charge separated states, in contrast to traditionally used singlet photosensitizers, in covalently functionalized single-walled carbon nanotube hybrids has been investigated. Enriched single-walled carbon nanotubes with two diameters, namely (6,5) and (7,6), were covalently modified to carry a charge-stabilizing triplet photosensitizer derived from a palladium porphyrin. The nanohybrids were fully characterized and the presence of intramolecular interactions between the porphyrin and nanotubes was established from various spectroscopic, imaging, electrochemical and thermochemical studies. Photoluminescence of palladium porphyrin was found to be quantitatively quenched in the presence of covalently appended SWCNTs and this quenching is due to excited state charge separation and has been established by femtosecond transient absorption studies. Owing to the presence of the triplet photosensitizer, the charge separated states lasted over 3 ns, i.e., much longer than those reported earlier for singlet photosensitizer-derived nanotube hybrids. The nanohybrids also exhibited efficient photocatalytic behavior in experiments involving electron pooling of one-electron reduced methyl viologen in the presence of a sacrificial electron donor. Higher yields of photoproducts were achieved from the present donor-acceptor nanohybrids when compared with those of singlet photosensitizer-derived nanohybrids, more so for (6,5) nanotube derived hybrids compared to (7,6) nanotube derived hybrids. The present findings highlight the importance of triplet photosensitizer derived nanohybrids in artificial photosynthesis of charge separation and photocatalytic applicatons.