Potential Dependent Reorientation Controlling Activity of a Molecular Electrocatalyst.

The activity of molecular electrocatalysts depends on the interplay of electrolyte composition near the electrode surface, the composition and morphology of the electrode surface, and the electric field at the electrode-electrolyte interface. This interplay is challenging to study and often overlooked when assessing molecular catalyst activity. Here, we use surface specific vibrational sum frequency generation (VSFG) spectroscopy to study the solvent and potential dependent activation of Mo(bpy)(CO)4, a CO2 reduction catalyst, at a polycrystalline Au electrode. We find that the parent complex undergoes potential dependent reorientation at the electrode surface when a small amount of N-methyl-2-pyrrolidone (NMP) is present. This preactivates the complex, resulting in greater yields at less negative potentials, of the active electrocatalyst for CO2 reduction.

Pulsed Electrolysis with a Nickel Molecular Catalyst Improves Selectivity for Carbon Dioxide Reduction.

Pulsed electrolysis can significantly improve carbon dioxide reduction on metal electrodes, but the effect of short (millisecond to seconds) voltage steps on molecular electrocatalysts is largely unstudied. In this work, we investigate the effect pulse electrolysis has on the selectivity and stability of the homogeneous electrocatalyst [Ni(cyclam)]2+ at a carbon electrode. By tuning the potential and pulse duration, we achieve a significant improvement in CO Faradaic efficiencies (85%) after 3 h, double that of the system under potentiostatic conditions. The improved activity is due to in situ catalyst regeneration from an intermediate that occurs as part of the catalyst's degradation pathway. This study demonstrates the wider opportunity to apply pulsed electrolysis to molecular electrocatalysts to control activity and improve selectivity.

Design principles for a nanoconfined enzyme cascade electrode via reaction-diffusion modelling.

The study of enzymes by direct electrochemistry has been extended to enzyme cascades, with a key development being the 'electrochemical leaf': an electroactive enzyme is immobilized within a porous electrode, providing in situ cofactor (NADP(H)) regeneration for a co-immobilized downstream enzyme. This system has been further developed to include multiple downstream enzymes, and it has become an important tool in biocatalysis, however, the local environment within the porous electrode has not been investigated in detail. Here, we constructed a 1D reaction-diffusion model, comprising the porous electrode with 2 kinds of enzymes immobilized, and an enzyme-free electrolyte diffusion layer. The modelling results show that the rate of the downstream enzyme is a key parameter, and that substrate transport within the porous electrode is not a main limiting factor. The insights obtained from this model can guide future rational design and improvement of these electrodes and immobilized enzyme cascade systems.

From Protein Film Electrochemistry to Nanoconfined Enzyme Cascades and the Electrochemical Leaf.

Protein film electrochemistry (PFE) has given unrivalled insight into the properties of redox proteins and many electron-transferring enzymes, allowing investigations of otherwise ill-defined or intractable topics such as unstable Fe-S centers and the catalytic bias of enzymes. Many enzymes have been established to be reversible electrocatalysts when attached to an electrode, and further investigations have revealed how unusual dependences of catalytic rates on electrode potential have stark similarities with electronics. A special case, the reversible electrochemistry of a photosynthetic enzyme, ferredoxin-NADP+ reductase (FNR), loaded at very high concentrations in the 3D nanopores of a conducting metal oxide layer, is leading to a new technology that brings PFE to myriad enzymes of other classes, the activities of which become controlled by the primary electron exchange. This extension is possible because FNR-based recycling of NADP(H) can be coupled to a dehydrogenase, and thence to other enzymes linked in tandem by the tight channelling of cofactors and intermediates within the nanopores of the material. The earlier interpretations of catalytic wave-shapes and various analogies with electronics are thus extended to initiate a field perhaps aptly named "cascade-tronics", in which the flow of reactions along an enzyme cascade is monitored and controlled through an electrochemical analyzer. Unlike in photosynthesis where FNR transduces electron transfer and hydride transfer through the unidirectional recycling of NADPH, the "electrochemical leaf" (e-Leaf) can be used to drive reactions in both oxidizing and reducing directions. The e-Leaf offers a natural way to study how enzymes are affected by nanoconfinement and crowding, mimicking the physical conditions under which enzyme cascades operate in living cells. The reactions of the trapped enzymes, often at very high local concentration, are thus studied electrochemically, exploiting the potential domain to control rates and direction and the current-rate analogy to derive kinetic data. Localized NADP(H) recycling is very efficient, resulting in very high cofactor turnover numbers and new opportunities for controlling and exploiting biocatalysis.

Zero-Gap Bipolar Membrane Electrolyzer for Carbon Dioxide Reduction Using Acid-Tolerant Molecular Electrocatalysts.

The scaling-up of electrochemical CO2 reduction requires circumventing the CO2 loss as carbonates under alkaline conditions. Zero-gap cell configurations with a reverse-bias bipolar membrane (BPM) represent a possible solution, but the catalyst layer in direct contact with the acidic environment of a BPM usually leads to H2 evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO2 reduction can be achieved in a zero-gap BPM device using a pure water and CO2 feed. At a higher current density (100 mA cm-2), CO selectivity decreases, but was still >30%, due to reversible product inhibition. This study demonstrates the importance of developing acid-tolerant catalysts for use in large-scale CO2 reduction devices.

Manganese Carbonyl Complexes as Selective Electrocatalysts for CO2 Reduction in Water and Organic Solvents.

The electrochemical reduction of CO2 provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO2 reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO)3Br] and its derivatives (bpy = 2,2'-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO)3Br] was first shown to be an active and selective catalyst for reducing CO2 to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism.This class of Mn catalysts only shows CO2 reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO2 electrolyzers.Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO2 binding mechanism, where for the acids widely studied, lower pKa values actually favor CO2 binding over Mn-H formation, a precursor to H2 evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO)3Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO2 binding operates in both the "dimer" and primary (via [Mn(bpy)(CO)3]-) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO2 reduction catalysts.

Electron flow between the worlds of Marcus and Warburg.

Living organisms are characterized by the ability to process energy (all release heat). Redox reactions play a central role in biology, from energy transduction (photosynthesis, respiratory chains) to highly selective catalyzed transformations of complex molecules. Distance and scale are important: electrons transfer on a 1 nm scale, hydrogen nuclei transfer between molecules on a 0.1 nm scale, and extended catalytic processes (cascades) operate most efficiently when the different enzymes are under nanoconfinement (10 nm-100 nm scale). Dynamic electrochemistry experiments (defined broadly within the term "protein film electrochemistry," PFE) reveal details that are usually hidden in conventional kinetic experiments. In PFE, the enzyme is attached to an electrode, often in an innovative way, and electron-transfer reactions, individual or within steady-state catalytic flow, can be analyzed in terms of precise potentials, proton coupling, cooperativity, driving-force dependence of rates, and reversibility (a mark of efficiency). The electrochemical experiments reveal subtle factors that would have played an essential role in molecular evolution. This article describes how PFE is used to visualize and analyze different aspects of biological redox chemistry, from long-range directional electron transfer to electron/hydride (NADPH) interconversion by a flavoenzyme and finally to NADPH recycling in a nanoconfined enzyme cascade.

Electrocatalytic Volleyball: Rapid Nanoconfined Nicotinamide Cycling for Organic Synthesis in Electrode Pores.

In living cells, redox chains rely on nanoconfinement using tiny enclosures, such as the mitochondrial matrix or chloroplast stroma, to concentrate enzymes and limit distances that nicotinamide cofactors and other metabolites must diffuse. In a chemical analogue exploiting this principle, nicotinamide adenine dinucleotide phosphate (NADPH) and NADP+ are cycled rapidly between ferredoxin-NADP+ reductase and a second enzyme-the pairs being juxtaposed within the 5-100 nm scale pores of an indium tin oxide electrode. The resulting electrode material, denoted (FNR+E2)@ITO/support, can drive and exploit a potentially large number of enzyme-catalysed reactions.

The value of enzymes in solar fuels research - efficient electrocatalysts through evolution.

The reasons for using enzymes as tools for solar fuels research are discussed. Many oxidoreductases, including components of membrane-bound electron-transfer chains in living organisms, are extremely active when directly attached to an electrode, at which they display their inherent catalytic activity as electrical current. Electrocatalytic voltammograms, which show the rate of electron flow at steady-state, provide direct information on enzyme efficiency with regard to optimising use of available energy, a factor that would have driven early evolution. Oxidoreductases have evolved to minimise energy wastage ('overpotential requirement') across electron-transport chains where rate and power must be maximised for a given change in Gibbs energy, in order to perform work such as proton pumping. At the elementary level (uncoupled from work output), redox catalysis by many enzymes operates close to the thermodynamically reversible limit. Examples include efficient and selective electrocatalytic reduction of CO2 to CO or formate - reactions that are very challenging at the chemistry level, yet appear almost reversible when catalysed by enzymes. Experiments also reveal the fleeting existence of reversible four-electron O2 reduction and water oxidation by 'blue' Cu oxidases, another reaction of great importance in realising a future based on renewable energy. Being aware that such enzymes have evolved to approach perfection, chemists are interested to know the minimal active site structure they would need to synthesise in order to mimic their performance.

A hydrogen fuel cell for rapid, enzyme-catalysed organic synthesis with continuous monitoring.

A one-pot fuel cell for specific, enzyme-catalysed organic synthesis, with continuous monitoring of rate and reaction progress, combines an electrode catalysing rapid, reversible and diffusion-controlled interconversion of NADP+ and NADPH with a Pt electrode catalysing 2H+/H2 interconversion. This Communication demonstrates its performance and characteristics using the reductive amination of 2-oxoglutarate as a test system.

Transfer of photosynthetic NADP+/NADPH recycling activity to a porous metal oxide for highly specific, electrochemically-driven organic synthesis.

In a discovery of the transfer of chloroplast biosynthesis activity to an inorganic material, ferredoxin-NADP+ reductase (FNR), the pivotal redox flavoenzyme of photosynthetic CO2 assimilation, binds tightly within the pores of indium tin oxide (ITO) to produce an electrode for direct studies of the redox chemistry of the FAD active site, and fast, reversible and diffusion-controlled interconversion of NADP+ and NADPH in solution. The dynamic electrochemical properties of FNR and NADP(H) are thus revealed in a special way that enables facile coupling of selective, enzyme-catalysed organic synthesis to a controllable power source, as demonstrated by efficient synthesis of l-glutamate from 2-oxoglutarate and NH4+.

SrNbO2N as a water-splitting photoanode with a wide visible-light absorption band.

Strontium niobium oxynitride (SrNbO(2)N) particles were coated on fluorine-doped tin oxide (FTO) glass and examined as a photoelectrode for water splitting under visible light in a neutral aqueous solution (Na(2)SO(4), pH ≈ 6). SrNbO(2)N, which has a band gap of ca. 1.8 eV, acted as an n-type semiconductor and generated an anodic photocurrent assignable to water oxidation upon irradiation with visible-light photons with wavelengths of up to 700 nm, even without an externally applied potential. Under visible light (λ > 420 nm) with an applied potential of +1.0-1.55 V vs RHE, nearly stoichiometric H(2) and O(2) evolution was achieved using a SrNbO(2)N/FTO electrode modified with colloidal iridium oxide (IrO(2)) as a water oxidation promoter. This study presents the first example of photoelectrochemical water splitting involving an n-type semiconductor with a band gap smaller than 2.0 eV that does not require an externally applied potential.

Synthesis and photocatalytic activity of perovskite niobium oxynitrides with wide visible-light absorption bands.

Photocatalytic activities of perovskite-type niobium oxynitrides (CaNbO2N, SrNbO2N, BaNbO2N, and LaNbON2) were examined for hydrogen and oxygen evolution from water under visible-light irradiation. These niobium oxynitrides were prepared by heating the corresponding oxide precursors, which were synthesized using the polymerized complex method, for 15 h under a flow of ammonia. They possess visible-light absorption bands between 600-750 nm, depending on the A-site cations in the structures. The oxynitride CaNbO2N, was found to be active for hydrogen and oxygen evolution from methanol and aqueous AgNO3, respectively, even under irradiation by light at long wavelengths (λ<560 nm). The nitridation temperature dependence of CaNbO2N was investigated and 1023 K was found to be the optimal temperature. At lower temperatures, the oxynitride phase is not adequately produced, whereas higher temperatures produce more reduced niobium species (e. g., Nb³(+) and Nb4(+)), which can act as electron-hole recombination centers, resulting in a decrease in activity.