Reversible and Selective Interconversion of Hydrogen and Carbon Dioxide into Formate by a Semiartificial Formate Hydrogenlyase Mimic.

The biological formate hydrogenlyase (FHL) complex links a formate dehydrogenase (FDH) to a hydrogenase (H2ase) and produces H2 and CO2 from formate via mixed-acid fermentation in Escherichia coli. Here, we describe an electrochemical and a colloidal semiartificial FHL system that consists of an FDH and a H2ase immobilized on conductive indium tin oxide (ITO) as an electron relay. These in vitro systems benefit from the efficient wiring of a highly active enzyme pair and allow for the reversible conversion of formate to H2 and CO2 under ambient temperature and pressure. The hybrid systems provide a template for the design of synthetic catalysts and surpass the FHL complex in vivo by storing and releasing H2 on demand by interconverting CO2/H2 and formate with minimal bias in either direction.

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.

Wiring of Photosystem II to Hydrogenase for Photoelectrochemical Water Splitting.

In natural photosynthesis, light is used for the production of chemical energy carriers to fuel biological activity. The re-engineering of natural photosynthetic pathways can provide inspiration for sustainable fuel production and insights for understanding the process itself. Here, we employ a semiartificial approach to study photobiological water splitting via a pathway unavailable to nature: the direct coupling of the water oxidation enzyme, photosystem II, to the H2 evolving enzyme, hydrogenase. Essential to this approach is the integration of the isolated enzymes into the artificial circuit of a photoelectrochemical cell. We therefore developed a tailor-made hierarchically structured indium-tin oxide electrode that gives rise to the excellent integration of both photosystem II and hydrogenase for performing the anodic and cathodic half-reactions, respectively. When connected together with the aid of an applied bias, the semiartificial cell demonstrated quantitative electron flow from photosystem II to the hydrogenase with the production of H2 and O2 being in the expected two-to-one ratio and a light-to-hydrogen conversion efficiency of 5.4% under low-intensity red-light irradiation. We thereby demonstrate efficient light-driven water splitting using a pathway inaccessible to biology and report on a widely applicable in vitro platform for the controlled coupling of enzymatic redox processes to meaningfully study photocatalytic reactions.

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.

Enhancing the water splitting efficiency of Sn-doped hematite nanoflakes by flame annealing.

The effect of flame annealing on the water-splitting properties of Sn decorated hematite (α-Fe2O3) nanoflakes has been investigated. It is shown that flame annealing can yield a considerable enhancement in the maximum photocurrent under AM 1.5 (100 mW cm(-2)) conditions compared to classic furnace annealing treatments. Optimizing the annealing time (10 s at 1000 °C) leads to a photocurrent of 1.1 mA cm(-2) at 1.23 V (vs. RHE) with a maximum value 1.6 mA cm(-2) at 1.6 V (vs. RHE) in 1 M KOH. The improvement in photocurrent can be attributed to the fast direct heating that maintains the nanoscale morphology, leads to optimized Sn decoration, and minimizes detrimental substrate effects.

Mediator enhanced water oxidation using Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2] film modified electrodes.

The water insoluble complex Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2], ([Ru(II)bpy]5[Ru(III)4POM]), was synthesized from Rb8K2[{Ru(IV)4O4(OH)2(H2O)4}(γ-SiW10O36)2] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [Ru(II)bpy]5[Ru(III)4POM] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru(bpy)3](3+)([Ru(III)bpy]) acting as the electron transfer mediator and [Ru(V)4POM] as the species releasing O2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s(-1) was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiClO4 as an additional electrolyte includes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [Ru(II)bpy]5[Ru(III)4POM] → [Ru(III)bpy]5[Ru(V)4POM] + 13 e(-) and [Ru(III)bpy]5[Ru(V)4POM] + 2H2O → [Ru(III)bpy]5[Ru(IV)4POM] + O2 + 4H(+). The voltammetry of related water insoluble [Ru(II)bpy]2[S2M18O62] (M = W and Mo) and [Fe(II)Phen]x[Ru(III)4POM] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [Ru(III)bpy] is the electron transfer mediator and [Ru(V)4POM] is the species responsible for oxygen evolution.

Photoelectrochemical reduction of aqueous protons with a CuO|CuBi2O4 heterojunction under visible light irradiation.

A p-type heterojunction photoelectrode consisting of platinized CuBi2O4 layered on a CuO film was prepared. The CuO|CuBi2O4|Pt electrode photo-generates H2 in pH neutral aqueous solution during visible light irradiation and exhibits a substantially enhanced photocurrent compared to CuO|Pt and CuBi2O4|Pt electrodes. Reduced electron-hole recombination by the band offsets in the heterostructure is responsible for the improved photoelectrochemical performance of CuO|CuBi2O4 with a small band-gap of approximately 1.5 eV.

Copper/Polyaniline Interfaces Confined CO2 Electroreduction for Selective Hydrocarbon Production.

Polyaniline (PANI) provides an attractive organic platform for CO2 electrochemical reduction due to the ability to adsorb CO2 molecules and in providing means to interact with metal nanostructures. In this work, a novel PANI supported copper catalyst has been developed by coupling the interfacial polymerization of PANI and Cu. The hybrid catalyst demonstrates excellent activity towards production of hydrocarbon products including CH4 and C2H4, compared with the use of bare Cu. A Faradaic efficiency of 71.8 % and a current density of 16.9 mA/cm2 were achieved at -0.86 V vs. RHE, in contrast to only 22.2 % and 1.0 mA/cm2 from the counterpart Cu catalysts. The remarkably enhanced catalytic performance of the hybrid PANI/Cu catalyst can be attributed to the synergistic interaction between the PANI underlayer and copper. The PANI favours the adsorption and binding of CO2 molecules via its nitrogen sites to form *CO intermediates, while the Cu/PANI interfaces confine the diffusion or desorption of the *CO intermediates favouring their further hydrogenation or carbon-carbon coupling to form hydrocarbon products. This work provides insights into the formation of hydrocarbon products on PANI-modified Cu catalysts, which may guide the development of conducting polymer-metal catalysts for CO2 electroreduction.

Theoretical analysis of the two-electron transfer reaction and experimental studies with surface-confined cytochrome c peroxidase using large-amplitude Fourier transformed AC voltammetry.

A detailed analysis of the cooperative two-electron transfer of surface-confined cytochrome c peroxidase (CcP) in contact with pH 6.0 phosphate buffer solution has been undertaken. This investigation is prompted by the prospect of achieving a richer understanding of this biologically important system via the employment of kinetically sensitive, but background devoid, higher harmonic components available in the large-amplitude Fourier transform ac voltammetric method. Data obtained from the conventional dc cyclic voltammetric method are also provided for comparison. Theoretical considerations based on both ac and dc approaches are presented for cases where reversible or quasi-reversible cooperative two-electron transfer involves variation in the separation of their reversible potentials, including potential inversion (as described previously for solution phase studies), and reversibility of the electrode processes. Comparison is also made with respect to the case of a simultaneous two-electron transfer process that is unlikely to occur in the physiological situation. Theoretical analysis confirms that the ac higher harmonic components provide greater sensitivity to the various mechanistic nuances that can arise in two-electron surface-confined processes. Experimentally, the ac perturbation with amplitude and frequency of 200 mV and 3.88 Hz, respectively, was employed to detect the electron transfer when CcP is confined to the surface of a graphite electrode. Simulations based on cooperative two-electron transfer with the employment of reversible potentials of 0.745 ± 0.010 V, heterogeneous electron transfer rate constants of between 3 and 10 s(-1) and charge transfer coefficients of 0.5 for both processes fitted experimental data for the fifth to eighth ac harmonics. Imperfections in theory-experiment comparison are consistent with kinetic and thermodynamic dispersion and other nonidealities not included in the theory used to model the voltammetry of surface-confined CcP.

Detailed electrochemical studies of the tetraruthenium polyoxometalate water oxidation catalyst in acidic media: identification of an extended oxidation series using Fourier transformed alternating current voltammetry.

The electrochemistry of the water oxidation catalyst, Rb(8)K(2)[{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)] (Rb(8)K(2)-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A'/A to H/H') that are readily detected under dc voltammetric conditions. The six most positive processes (A'/A to F/F'), each involve a one electron ruthenium based charge transfer step (A'/A, B'/B are Ru(IV/V) oxidation and C/C' to F/F' are Ru(IV/III) reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B'/B, E/E', and F/F' processes are quasi-reversible, while the others are close to reversible. A third Ru(IV/V) oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb(8)K(2)-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G') assigned to Ru(III/II) reduction and the multiple electron transfer reduction process (H/H') that arise from the tungstate polyoxometalate framework. A significant shift of E°' to a more positive potential value for process H/H' was observed on removal of K(+) (~100 mV in H(2)SO(4) and ~50 mV in HCl).

A high-performance capillary-fed electrolysis cell promises more cost-competitive renewable hydrogen.

Renewable, or green, hydrogen will play a critical role in the decarbonisation of hard-to-abate sectors and will therefore be important in limiting global warming. However, renewable hydrogen is not cost-competitive with fossil fuels, due to the moderate energy efficiency and high capital costs of traditional water electrolysers. Here a unique concept of water electrolysis is introduced, wherein water is supplied to hydrogen- and oxygen-evolving electrodes via capillary-induced transport along a porous inter-electrode separator, leading to inherently bubble-free operation at the electrodes. An alkaline capillary-fed electrolysis cell of this type demonstrates water electrolysis performance exceeding commercial electrolysis cells, with a cell voltage at 0.5 A cm-2 and 85 °C of only 1.51 V, equating to 98% energy efficiency, with an energy consumption of 40.4 kWh/kg hydrogen (vs. ~47.5 kWh/kg in commercial electrolysis cells). High energy efficiency, combined with the promise of a simplified balance-of-plant, brings cost-competitive renewable hydrogen closer to reality.

Boosting Formate Production from CO2 at High Current Densities Over a Wide Electrochemical Potential Window on a SnS Catalyst.

The flow-cell design offers prospect for transition to commercial-relevant high current density CO2 electrolysis. However, it remains to understand the fundamental interplay between the catalyst, and the electrolyte in such configuration toward CO2 reduction performance. Herein, the dramatic influence of electrolyte alkalinity in widening potential window for CO2 electroreduction in a flow-cell system based on SnS nanosheets is reported. The optimized SnS catalyst operated in 1 m KOH achieves a maximum formate Faradaic efficiency of 88 ± 2% at -1.3 V vs reversible hydrogen electrode (RHE) with the current density of ≈120 mA cm-2 . Alkaline electrolyte is found suppressing the hydrogen evolution across all potentials which is particularly dominant at the less negative potentials, as well as CO evolution at more negative potentials. This in turn widens the potential window for formate conversion (>70% across -0.5 to -1.5 V vs RHE). A comparative study to SnOx counterpart indicates sulfur also acts to suppress hydrogen evolution, although electrolyte alkalinity resulting in a greater suppression. The boosting of the electrochemical potential window, along with high current densities in SnS derived catalytic system offers a highly attractive and promising route toward industrial-relevant electrocatalytic production of formate from CO2 .

A comparison of the higher order harmonic components derived from large-amplitude Fourier transformed ac voltammetry of myoglobin and heme in DDAB films at a pyrolytic graphite electrode.

A debate as to whether heme remains bound or is released in myoglobin molecules incorporated into a didodecyldimethylammonium bromide (DDAB) film adhered to a pyrolytic graphite electrode has prompted a comparison of their electrochemistry by the highly sensitive large-amplitude Fourier transformed ac voltammetric method. The accessibility of third, fourth, and higher harmonic components that are devoid of background current and the enhanced resolution relative to that available in dc voltammetry have allowed a detailed comparison of the Fe(III)/Fe(II) and Fe(II)/Fe(I) redox processes of myoglobin and heme molecules to be undertaken as a function of buffer composition and pH and in the presence and absence of NaBr in the buffer and/or film. Under most conditions examined, only very subtle differences, in the Fe(III)/Fe(II) process were found, implying this process cannot be used to indicate the intactness or otherwise of myoglobin in myoglobin-DDAB films. In contrast, higher order ac harmonics obtained from myoglobin-DDAB and heme-DDAB films reveal pH dependent differences with respect to the Fe(II)/Fe(I) couple. Analysis of the ac harmonics, and with the hypothesis that the Fe(II)/Fe(I) process reflects the myoglobin state, suggests that the majority of the iron heme is released from myoglobin-DDAB (pH 5.0, no NaBr) films in contact with pH 5.0 (0.1 M sodium acetate) buffer solution devoid of or containing NaBr. However, myoglobin films prepared with pH 5.0 buffer containing NaBr shows significant difference in the higher harmonic shapes and midpoint potentials in the Fe(II)/Fe(I) process relative to the case when heme molecules are used, although as noted in other studies, a significant fraction of the Mb is rendered electroinactive in the presence of NaBr. The voltammetric responses of myoglobin and heme-DDAB (pH 5.0) films in contact with pH 7.0 (0.1 M) phosphate buffer solution also exhibit significant differences in the Fe(II)/Fe(I) redox couple in the higher harmonics in contrast to a report [de Groot, M.T.; Merkx, M.; Koper, M. T. M. J. Am. Chem. Soc. 2005, 127, 16224] that claimed identical midpoint potentials apply to both films under conditions of dc cyclic voltammetry. The FT-ac voltammetric data therefore suggest that a substantial fraction of myoglobin in myoglobin-DDAB (pH 5.0) films in contact with pH 7.0 phosphate buffer solution remains intact. No evidence of a catalytic effect that enhanced the released of heme from myoglobin was found at the pyrolytic graphite electrode surface. In summary, higher harmonic ac voltammetric data indicate that the Fe(II)/Fe(I) process but not the Fe(III)/Fe(II) reflects the state of myoglobin in DDAB films. On this basis, films prepared at pH 5.0 should include NaBr, or else films should be prepared at neutral pH to achieve films with myoglobin remains in its intact near native state when a myoglobin-DDAB film is confined to a graphite electrode surface. Otherwise, the release of heme in myoglobin molecules incorporated into a DDAB film is likely to be a dominant reaction pathway.

Revelation of multiple underlying RuO2 redox processes associated with pseudocapacitance and electrocatalysis.

Advances in basic knowledge relevant to the pseudocapacitive and electrocatalytic properties of RuO(2) materials require a detailed understanding of the redox chemistry that occurs at the electrode interface. Although several redox processes have been identified via dc cyclic voltammograms derived from surface-confined RuO(2) materials, mechanistic details remain limited because the faradaic signals of interest are heavily masked by the background current. Here, it is shown that the underlying electron transfer reactions associated with the VI to II oxidation states of surface-confined RuO(2) materials in acidic medium are far more accessible in the background current free fourth and higher harmonic components available via large-amplitude Fourier transformed ac voltammetry. Enhanced resolution and sensitivity to both electron transfer and protonation processes and discrimination against solvent and background capacitance are achieved so that the Ru(V) to Ru(VI) process can be studied for the first time. Thus, kinetic and thermodynamic information relevant to each ruthenium redox level is readily deduced. The relative rate of electron transfer and the impact of protonation associated with Ru(VI) to Ru(II) redox processes are found to depend on the nature of the RuO(2) materials (extent of crystallinity and hydration) and concentration of sulfuric acid electrolyte. In the electrocatalytic oxidation of glucose in alkaline medium, access to the underlying electron transfer processes allows ready detection of the redox couple associated with the catalysis. Thus, application of an advanced ac electroanalytical technique is shown to provide the methodology for enhancing our understanding of the charge transfer processes of RuO(2), relevant to pseudocapacitance and electrocatalysis.

Effects of coupled homogeneous chemical reactions on the response of large-amplitude AC voltammetry: extraction of kinetic and mechanistic information by Fourier transform analysis of higher harmonic data.

Large-amplitude ac voltammograms contain a wealth of kinetic information concerning electrode processes and can provide unique mechanistic insights compared to other techniques. This paper describes the effects homogeneous chemical processes have on ac voltammetry in general and provides experimental examples using two well-known chemical systems: one simple and one complex. Oxidation of [Cp*Fe(CO)(2)](2) (Cp* = η(5)-pentamethylcyclopentadienyl) in noncoordinating media is a reversible one-electron process; in the presence of nucleophiles, however, the resulting ligand-induced disproportionation changes the process to a multiple step regeneration. The chemical kinetic parameters of the regeneration mechanism were discerned via analysis of the third and higher harmonics of Fourier-transformed ac voltammetry data. Comparison of experimental data to digital simulations provides clear evidence that the reaction proceeds via a rapid pre-equilibrium between the electrogenerated monocation and the coordinating ligand; simultaneous fitting of the first nine harmonics indicates that k(f) = 7500 M(-1) s(-1) and k(r) = 100 s(-1), and that the unimolecular decomposition of the corresponding intermediate occurs with a rate constant of 2.2 s(-1). The rapid cis(+) → trans(+) isomerization of the electrogenerated cis-[W(CO)(2)(dpe)(2)](+), where dpe = 1,2-diphenylphosphinoethane, was examined to illustrate the effects of a simpler EC mechanism on the higher harmonics; a rate constant of 280 s(-1) was determined. These results not only shed new light on the chemistry of these systems, but provide a clear demonstration that the higher harmonics of ac voltammetry provide mechanistic insights into coupled homogeneous processes far more detailed than those that are readily accessible with dc techniques.

A Self-Assembled CO2 Reduction Electrocatalyst: Posy-Bouquet-Shaped Gold-Polyaniline Core-Shell Nanocomposite.

Here it was demonstrated that the decoration of gold (Au) with polyaniline is an effective approach in increasing its electrocatalytic reduction of CO2 to CO. The core-shell-structured gold-polyaniline (Au-PANI) nanocomposite delivered a CO2 -to-CO conversion efficiency of 85 % with a high current density of 11.6 mA cm-2 . The polyaniline shell facilitated CO2 adsorption, and the subsequent formation of reaction intermediates on the gold core contributed to the high efficiency observed.

Evaluation of levels of defect sites present in highly ordered pyrolytic graphite electrodes using capacitive and faradaic current components derived simultaneously from large-amplitude Fourier transformed ac voltammetric experiments.

The level of edge plane defect sites present in highly ordered pyrolytic graphite (HOPG) electrodes has been evaluated via analysis of dc, ac fundamental, and higher-order ac harmonics available from a single large-amplitude Fourier transformed (FT) ac voltammetric experiment. Deliberate introduction of a low level of edge plane defect was achieved by polishing, with a higher level being introduced via electrochemical pretreatment. Kinetics regimes associated with fast electron transfer on the edge plane defect sites and slow electron transfer on the basal plane surface are resolved under ac conditions when using the surface-sensitive [Fe(CN)(6)](3-/4-) redox probe. However, because of their insensitivity to slow electron transfer, higher-order ac faradaic harmonics almost exclusively detect only the much faster processes that emanate from edge plane defect sites. Thus, detection of fourth- and higher-order ac Faradaic harmonic components that are devoid of background capacitive current is possible at freshly cleaved HOPG in the region near the reversible potential for the [Fe(CN)(6)](3-/4-) process. Under these circumstances, dc cyclic voltammograms exhibit only reduction and oxidation peaks separated by more than 1 V. The fundamental ac harmonic provides detailed information on the capacitive current, which increases with the level of edge plane defect sites. Apparent charge transfer rate constants also can be derived from peak-to-peak separations obtained from the dc aperiodic component. Estimates of the percentage of edge plane defect sites based on ac higher harmonics, capacitance, and dc aperiodic component that are available from a single experiment have been compared. The edge plane defect levels deduced from capacitance (fundamental harmonic ac component) and higher harmonic Faradaic currents are considered to be more reliable than estimations based on apparent rate constants derived from the dc aperiodic component or conventional dc cyclic voltammogram.