Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides.

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy1-3. This task is extremely challenging because thermodynamically stable Am(III) and Ln(III) ions have nearly identical ionic radii and coordination chemistry. Oxidization of Am(III) to Am(VI) produces AmO22+ ions distinct with Ln(III) ions, which has the potential to facilitate separations in principle. However, the rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional separation protocols including solvent and solid extractions hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu and 243Am) over trivalent lanthanides in nitric acid media. To our knowledge, this cluster is the most stable Am(VI) species in aqueous media observed so far. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient and rapid, does not involve any organic components and requires minimal energy input.

A stable dye-sensitized photoelectrosynthesis cell mediated by a NiO overlayer for water oxidation.

In the development of photoelectrochemical cells for water splitting or CO2 reduction, a major challenge is O2 evolution at photoelectrodes that, in behavior, mimic photosystem II. At an appropriate semiconductor electrode, a water oxidation catalyst must be integrated with a visible light absorber in a stable half-cell configuration. Here, we describe an electrode consisting of a light absorber, an intermediate electron donor layer, and a water oxidation catalyst for sustained light driven water oxidation catalysis. In assembling the electrode on nanoparticle SnO2/TiO2 electrodes, a Ru(II) polypyridyl complex was used as the light absorber, NiO was deposited as an overlayer, and a Ru(II) 2,2'-bipyridine-6,6'-dicarboxylate complex as the water oxidation catalyst. In the final electrode, addition of the NiO overlayer enhanced performance toward water oxidation with the final electrode operating with a 1.1 mA/cm2 photocurrent density for 2 h without decomposition under one sun illumination in a pH 4.65 solution. We attribute the enhanced performance to the role of NiO as an electron transfer mediator between the light absorber and the catalyst.

A molecular tandem cell for efficient solar water splitting.

Artificial photosynthesis provides a way to store solar energy in chemical bonds. Achieving water splitting without an applied external potential bias provides the key to artificial photosynthetic devices. We describe here a tandem photoelectrochemical cell design that combines a dye-sensitized photoelectrosynthesis cell (DSPEC) and an organic solar cell (OSC) in a photoanode for water oxidation. When combined with a Pt electrode for H2 evolution, the electrode becomes part of a combined electrochemical cell for water splitting, 2H2O → O2 + 2H2, by increasing the voltage of the photoanode sufficiently to drive bias-free reduction of H+ to H2 The combined electrode gave a 1.5% solar conversion efficiency for water splitting with no external applied bias, providing a mimic for the tandem cell configuration of PSII in natural photosynthesis. The electrode provided sustained water splitting in the molecular photoelectrode with sustained photocurrent densities of 1.24 mA/cm2 for 1 h under 1-sun illumination with no applied bias.

CO2 reduction to acetate in mixtures of ultrasmall (Cu) n ,(Ag) m bimetallic nanoparticles.

Monodispersed mixtures of 6-nm Cu and Ag nanoparticles were prepared by electrochemical reduction on electrochemically polymerized poly-Fe(vbpy)3(PF6)2 film electrodes on glassy carbon. Conversion of the complex to poly-Fe(vbpy)2(CN)2 followed by surface binding of salts of the cations and electrochemical reduction gave a mixture of chemically distinct clusters on the surface, (Cu) m ,(Ag) n |polymer|glassy carbon electrode (GCE), as shown by X-ray photoelectron spectroscopy (XPS) measurements. A (Cu)2,(Ag)3|(80-monolayer-poly-Fe(vbpy)32+|GCE electrode at -1.33 V vs. reversible hydrogen electrode (RHE) in 0.5 M KHCO3, with 8 ppm added benzotriazole (BTA) at 0 °C, gave acetate with a faradaic efficiency of 21.2%.

Stabilized photoanodes for water oxidation by integration of organic dyes, water oxidation catalysts, and electron-transfer mediators.

Stabilized photoanodes for light-driven water oxidation have been prepared on nanoparticle core/shell electrodes with surface-stabilized donor-acceptor chromophores, a water oxidation catalyst, and an electron-transfer mediator. For the electrode, fluorine-doped tin oxide FTO|SnO2/TiO2|-Org1-|1.1 nm Al2O3|-RuP2+-WOC (water oxidation catalyst) with Org1 (1-cyano-2-(4-(diphenylamino)phenyl)vinyl)phosphonic acid), the mediator RuP2+ ([Ru(4,4-(PO3H2)2-2,2-bipyridine)(2,2-bipyridine)2]2+), and the WOC, Ru(bda)(py(CH2)(3or10)P(O3H)2)2 (bda is 2,2-bipyridine-6,6-dicarboxylate with x = 3 or 10), solar excitation resulted in photocurrents of ∼500 µA/cm2 and quantitative O2 evolution at pH 4.65. Related results were obtained for other Ru(II) polypyridyl mediators. For the organic dye PP (5-(4-(dihydroxyphosphoryl)phenyl)-10,15,20-Tris(mesityl)porphyrin), solar water oxidation occurred with a driving force near 0 V.

Electron Beam Irradiation as a General Approach for the Rapid Synthesis of Covalent Organic Frameworks under Ambient Conditions.

Crystalline porous materials such as covalent organic frameworks (COFs) are advanced materials to tackle challenges of catalysis and separation in industrial processes. Their synthetic routes often require elevated temperatures, closed systems with high pressure, and long reaction times, hampering their industrial applications. Here we use a traditionally unperceived strategy to assemble highly crystalline COFs by electron beam irradiation with controlled received dosage, contrasting sharply with the previous observation that radiation damages the crystallinity of solids. Such synthesis by electron beam irradiation can be achieved under ambient conditions within minutes, and the process is amendable for large-scale production. The intense and targeted energy input to the reactants leads to new reaction pathways that favor COF formation in nearly quantitative yield. This strategy is applicable not only to known COFs but also to new series of flexible COFs that are difficult to obtain using traditional methods.

Ethynylphenyl-Derivatized Free Base Porphyrins: Anodic Oxidation Processes and Covalent Grafting onto Glassy Carbon Electrodes.

In six of seven cases, direct anodic oxidation of the ethynyl group of an ethynylphenyl-derivatized free-base porphyrin gave modified glassy carbon electrodes in which the porphyrin was strongly surface-bound, most likely in a perpendicular geometry through covalent attachment of the ethynyl group to a surface carbon atom. The porphyrins each contained an ethynylphenyl group in one meso position and varied in the groups present in the other three meso positions. Electrografted 5,10,15,20-tetrakis(ethynylphenyl)porphyrin, H21, which has ethynyl moieties in all four meso positions, has well-defined surface voltammetry and grows to multilayer levels upon repeated cyclic voltammetry (CV) deposition scans. Multilayering was not observed to the same degree for monoethynylphenyl-substituted porphyrins and became progressively less for porphyrins having groups in the 15-meso position that were more protective against ethynyl radical attack. Clean molecular monolayer-level coverage was observed for 5-ethynylphenyl-10,20-bis(3-methoxyphenyl)-15-hexylporphyrin, H25. Owing to the fact that the ethynyl oxidation potential (1.1 to 1.5 V vs ferrocene) is more positive than that of the second macrocycle oxidation, the longevities and follow-up reactions of the porphyrin dications were also studied by CV, chemical oxidation, and optical spectroscopy in homogeneous solution. The primary follow-up products of the doubly oxidized porphyrins, whether surface-bound or in solution, were pyrrole-protonated species that were easily reduced back to the neutral porphyrin.

Plasmon-enhanced light-driven water oxidation by a dye-sensitized photoanode.

Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a flexible approach for solar water splitting based on the integration of molecular light absorption and catalysis on oxide electrodes. Recent advances in this area, including the use of core/shell oxide interfacial structures and surface stabilization by atomic layer deposition, have led to improved charge-separation lifetimes and the ability to obtain substantially improved photocurrent densities. Here, we investigate the introduction of Ag nanoparticles into the core/shell structure and report that they greatly enhance light-driven water oxidation at a DSPEC photoanode. Under 1-sun illumination, Ag nanoparticle electrodes achieved high photocurrent densities, surpassing 2 mA cm-2 with an incident photon-to-current efficiency of 31.8% under 450-nm illumination.

Molecular Photoelectrode for Water Oxidation Inspired by Photosystem II.

In artificial photosynthesis, the sun drives water splitting into H2 and O2 or converts CO2 into a useful form of carbon. In most schemes, water oxidation is typically the limiting half-reaction. Here, we introduce a molecular approach to the design of a photoanode that incorporates an electron acceptor, a sensitizer, an electron donor, and a water oxidation catalyst in a single molecular assembly. The strategy mimics the key elements in Photosystem II by initiating light-driven water oxidation with integration of a light absorber, an electron acceptor, an electron donor, and a catalyst in a controlled molecular environment on the surface of a conducting oxide electrode. Visible excitation of the assembly results in the appearance of reductive equivalents at the electrode and oxidative equivalents at a catalyst that persist for seconds in aqueous solutions. Steady-state illumination of the assembly with 440 nm light with an applied bias results in photoelectrochemical water oxidation with a per-photon absorbed efficiency of 2.3%. The results are notable in demonstrating that light-driven water oxidation can be carried out at a conductive electrode in a structure with the functional elements of Photosystem II including charge separation and water oxidation.

Interfacial Deposition of Ru(II) Bipyridine-Dicarboxylate Complexes by Ligand Substitution for Applications in Water Oxidation Catalysis.

Water oxidation is a critical step in artificial photosynthesis and provides the protons and electrons used in reduction reactions to make solar fuels. Significant advances have been made in the area of molecular water oxidation catalysts with a notable breakthrough in the development of Ru(II) complexes that use a planar "bda" ligand (bda is 2,2'-bipyridine-6,6'-dicarboxylate). These Ru(II)(bda) complexes show lower overpotentials for driving water oxidation making them ideal for light-driven applications with a suitable chromophore. Nevertheless, synthesis of heterogeneous Ru(II)(bda) complexes remains challenging. We discuss here a new "bottom-up" synthetic method for immobilizing these catalysts at the surface of a photoanode for use in a dye-sensitized photoelectrosynthesis cell (DSPEC). The procedure provides a basis for rapidly screening the role of ligand variations at the catalyst in order to understand the impact on device performance. The best results of a water-oxidation DSPEC photoanode based on this procedure reached 1.4 mA/cm2 at pH 7 in 0.1 M [PO4H2]-/[PO4H]2-solution with minimal loss in catalytic behavior over 30 min, and produced an incident photon to current efficiency (IPCE) of 24.8% at 440 nm.

Effect of Large Electrolyte Anions on the Sequential Oxidations of Bis(fulvalene)diiron Attached to Glassy Carbon by an Ethynyl Linkage.

Two ethynyl-derivatized isomers of bis(fulvalene)diiron (BFD, 1,1'-biferrocenylene) were prepared and covalently attached to glassy carbon electrodes through their ethynyl group by three different electrode modification methods. Cyclic voltammetry and square wave (SW) voltammetry were used to characterize surface coverages of 1.4-5.5 × 10-10 mol cm-2, the higher of these corresponding to roughly a monolayer, based on computation of an idealized close-packing structure for ethynylbis(fulvalene)diiron (E-BFD) on a solid surface. In a dichloromethane solution containing a smaller electrolyte anion such as [PF6]- or [ClO4]-, the E-BFD-modified electrodes exhibited two quasi-Nernstian one-electron oxidations. In contrast, the current for the second oxidation process, [E-BFD]+/2+, was diminished in electrolytes containing one of the large fluoroaryl borate anions, [B(C6F5)4]- or [B(C6H3(CF3)2)4]-. The effect was enhanced for electrodes having higher surface coverages being probed at shorter voltammetric time scales. SW voltammetry showed that the diminished currents for [E-BFD]+/2+ in large-anion electrolytes are not caused by slow electron transfer. Rather, they are attributed to mixed diffusivity of the counter-anions at the electrode/solution interface, as [E-BFD]+ and the anion form the optimum (lowest-energy) configuration of a 1:1 ion pair. The interior transport of the anion required to reach this configuration may be sterically encumbered, accounting for the diminished charge transfer observed with electrolytes containing large anions.

Mechanisms of molecular water oxidation in solution and on oxide surfaces.

Initial experiments on water oxidation by well-defined molecular catalysts were initiated with the goal of finding solutions to solar energy conversion. This account is a summary of research in this area by the T. J. Meyer research group. It begins with the design and characterization of the first catalyst, the blue Ru dimer, to current applications with surface-bound complexes on photoanodes for water oxidation in Dye Sensitized Photoelectrosynthesis Cells.

All-in-One Derivatized Tandem p+n-Silicon-SnO2/TiO2 Water Splitting Photoelectrochemical Cell.

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 µm thick SnO2 films with an outer 4.3 nm shell of TiO2 added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p+n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru(bda)(4-O(CH2)3PO3H2)-pyr)2], 1, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate), and chromophore, [Ru(4,4'-PO3H2-bpy) (bpy)2]2+, RuP2+, (bpy = 2,2'-bipyridine), which form 2:1 RuP2+/1 assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm2; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm2 with an 88% Faradaic efficiency (FE) for O2 production at an applied bias of 600 mV versus RHE ( ACS Energy Lett. , 2016 , 1 , 231 - 236 ). The SnO2/TiO2-chromophore-catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO2 (Ti/a-TiO2) coating as an interconnect. In the integrated electrode, p+n-Si-Ti/a-TiO2-SnO2/TiO2|-2RuP2+/1, the p+n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 µA/cm2 were obtained for water splitting, 2H2O → 2H2 + O2. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode.

Light-Driven Water Splitting Mediated by Photogenerated Bromine.

Light-driven water splitting was achieved using a dye-sensitized mesoporous oxide film and the oxidation of bromide (Br- ) to bromine (Br2 ) or tribromide (Br3- ). The chemical oxidant (Br2 or Br3- ) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)2 ], (1; pic=picoline, bda=2,2'-bipyridine-6,6'-dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br2 /Br- couple or 1.05 V vs. NHE for the Br3- /Br- couple, which is sufficient to drive water oxidation at 1 (RuV/IV ≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4'-PO3 H2 -bpy)(bpy)2 ]2+ (RuP2+ , bpy=2,2'-bipyridine) chromophore dye on a SnO2 /TiO2 core-shell electrode resulted in a photocurrent density of around 1.2 mA cm-2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution.

Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition.

In a dye sensitized photoelectrosynthesis cell (DSPEC), the relative orientation of the catalyst and chromophore plays an important role in determining the device efficiency. Here we introduce a new, robust atomic layer deposition (ALD) procedure for the preparation of molecular chromophore-catalyst assemblies on wide bandgap semiconductors. In this procedure, solution deposited, phosphonate derivatized metal complexes on metal oxide surfaces are treated with reactive metal reagents in the gas phase by ALD to form an outer metal ion bridging group, which can bind a second phosphonate containing species from solution to establish a R1-PO2-O-M-O-PO2-R2 type surface assembly. With the ALD procedure, assemblies bridged by Al(III), Sn(IV), Ti(IV), or Zr(IV) metal oxide units have been prepared. To evaluate the performance of this new type of surface assembly, intra-assembly electron transfer was investigated by transient absorption spectroscopy, and light-driven water splitting experiments under steady-state illumination were conducted. A SnO2 bridged assembly on SnO2/TiO2 core/shell electrodes undergoes light-driven water oxidation with an incident photon to current efficiency (IPCE) of 17.1% at 440 nm. Light-driven water reduction with a ruthenium trisbipyridine chromophore and molecular Ni(II) catalyst on NiO films was also used to produce H2. Compared to conventional solution-based procedures, the ALD approach offers significant advantages in scope and flexibility for the preparation of stable surface structures.

A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water.

Tandem junction photoelectrochemical water-splitting devices, whereby two light absorbing electrodes targeting separate portions of the solar spectrum generate the voltage required to convert water to oxygen and hydrogen, enable much higher possible efficiencies than single absorber systems. We report here on the development of a tandem system consisting of a dye-sensitized photoelectrochemical cell (DSPEC) wired in series with a dye-sensitized solar cell (DSC). The DSPEC photoanode incorporates a tris(bipyridine)ruthenium(II)-type chromophore and molecular ruthenium based water oxidation catalyst. The DSPEC was tested with two more-red absorbing DSC variations, one utilizing N719 dye with an I3-/I- redox mediator solution and the other D35 dye with a tris(bipyridine)cobalt ([Co(bpy)3]3+/2+) based mediator. The tandem configuration consisting of the DSPEC and D35/[Co(bpy)3]3+/2+ based DSC gave the best overall performance and demonstrated the production of H2 from H2O with the only energy input from simulated solar illumination.

Two Electrode Collector-Generator Method for the Detection of Electrochemically or Photoelectrochemically Produced O2.

A dual working electrode technique for the in situ production and quantification of electrochemically or photoelectrochemically produced O2 is described. This technique, termed a collector-generator cell, utilizes a transparent fluorine doped tin oxide electrode to sense O2. This setup is specifically designed for detecting O2 in dye sensitized photoelectrosynthesis cells.

Anodic Methods for Covalent Attachment of Ethynylferrocenes to Electrode Surfaces: Comparison of Ethynyl Activation Processes.

The electrochemical oxidation of ferrocenes having an H- or Li-terminated ethynyl group has been studied, especially as it relates to their covalent anchoring to carbon surfaces. The anodic oxidation of lithioethynylferrocene (1-Li) results in rapid loss of Li(+) and formation of the ethynyl-based radical FeCp(η(5)-C5H4)(C≡C), (1, Cp = η(5)-C5H5), which reacts with the electrode. Chemically modified electrodes (CMEs) were thereby produced containing strongly bonded, ethynyl-linked monolayers and electrochemically controlled multilayers. Strong attachments of ethynylferrocenes to gold and platinum surfaces were also possible. The lithiation/anodic oxidation process is a mirror analogue of the diazonium/cathodic reduction process for preparation of aryl-modified CMEs. A second method produced an ethynylferrocene-modified electrode by direct anodic oxidation of the H-terminated ethynylferrocene (1-H) at a considerably more positive potential. Both processes produced robust modified electrodes with well-defined ferrocene-based surface cyclic voltammetry waves that remained unchanged for as many as 10(4) scans. Ferrocene derivatives in which the ethynyl moiety was separated from the cyclopentadienyl ring by an ether group showed very similar behavior. DFT calculations were performed on the relevant redox states of 1-H, 1-Li, and 1, with emphasis on the ferrocenyl vs ethynyl character of their high valence orbitals. Whereas the HOMOs of both 1-H and 1-Li have some ethynyl character, the SOMOs of the corresponding monocations are strictly ferrocenium in makeup. Predominant ethynyl character returns to the highest valence orbitals after loss of Li(+) from [1-Li](+) or loss of H(+) from [1-H](2+). These anodic processes hold promise for the controlled chemical modification of carbon and other electrode surfaces by a variety of ethynyl or alkynyl-linked organic and metal-containing systems.

Analysis of Homogeneous Water Oxidation Catalysis with Collector-Generator Cells.

A collector-generator (C-G) technique has been applied to determine the Faradaic efficiencies for electrocatalytic O2 production by the homogeneous water oxidation catalysts Ru(bda)(isoq)2 (1; bda = 2,2'-bipyridine and isoq = isoquinoline) and [Ru(tpy)(bpz)(OH2)](2+) (2; tpy = 2,2':6',2″-terpyridine and bpz = 2,2'-bipyrazine). This technique uses a custom-fabricated cell consisting of two fluorine-doped tin oxide (FTO) working electrodes separated by 1 mm with the conductive sides facing each other. With a catalyst in solution, water oxidation occurs at one FTO electrode under a sufficient bias to drive O2 formation by the catalyst; the O2 formed then diffuses to the second FTO electrode poised at a potential sufficiently negative to drive O2 reduction. A comparison of the current versus time response at each electrode enables determination of the Faradaic efficiency for O2 production with high concentrations of supporting electrolyte important for avoiding capacitance effects between the electrodes. The C-G technique was applied to electrocatalytic water oxidation by 1 in the presence of the electron-transfer mediator Ru(bpy)3(2+) in both unbuffered aqueous solutions and with the added buffer bases HCO3(-), HPO4(2-), imidazole, 1-methylimidazole, and 4-methoxypyridine. HCO3(-) and HPO4(2-) facilitate water oxidation by atom-proton transfer (APT), which gave Faradaic yields of 100%. With imidazole as the buffer base, coordination to the catalyst inhibited water oxidation. 1-Methylimidazole and 4-methoxypyridine gave O2 yields of 55% and 76%, respectively, with the lower Faradaic efficiencies possibly due to competitive C-H oxidation of the bases. O2 evolution by catalyst 2 was evaluated at pH 12 with 0.1 M PO4(3-) and at pH 7 in a 0.1 M H2PO4(-)/HPO4(2-) buffer. At pH 12, at an applied potential of 0.8 V vs SCE, water oxidation by the Ru(IV)(O)(2+) form of the catalyst gave O2 in 73% yield. In a pH 7 solution, water oxidation at 1.4 V vs SCE, which is dominated by Ru(V)(O)(3+), gave O2 with an efficiency of 100%. The lower efficiency for Ru(IV)(O)(2+) at pH 12 may be due to competitive oxidation of a polypyridyl ligand.

An anodic method for covalent attachment of molecules to electrodes through an ethynyl linkage.

Electroactive organometallic molecules have been covalently attached to electrode surfaces through an ethynyl linkage. The process takes advantage of ethynyl-based radicals generated by anodic oxidation of a lithio-activated terminal ethynyl group. Electrophores containing redox-active ferrocene, cymantrene, or cobaltocenium moieties have been deposited at the one-to-three monolayer level. Both metal-based and ligand-based chemical reactions have been carried out on the surface-modified systems.