Showing papers by "Javier J. Concepcion published in 2013"

Solar water splitting in a molecular photoelectrochemical cell

Abstract: Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core–shell nanostructured photoanode with the core a high surface area conductive metal oxide film––indium tin oxide or antimony tin oxide––coated with a thin outer shell of TiO2 formed by atomic layer deposition. A “chromophore–catalyst assembly” 1, [(PO3H2)2bpy)2Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH2)]4+, which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO2 shell. Visible photolysis of the resulting core–shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.

Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

Abstract: Enhancing the surface binding stability of chromophores, catalysts, and chromophore–catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1–12) by atomic layer deposition of an overlayer of TiO2. Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases and with a pathway identified in which O-atom transfer to OH− occurs with a rate constant increase of 106 compared to water oxidation in acid.

Synthesis of phosphonic acid derivatized bipyridine ligands and their ruthenium complexes.

Abstract: Water-stable, surface-bound chromophores, catalysts, and assemblies are an essential element in dye-sensitized photoelectrosynthesis cells for the generation of solar fuels by water splitting and CO2 reduction to CO, other oxygenates, or hydrocarbons. Phosphonic acid derivatives provide a basis for stable chemical binding on metal oxide surfaces. We report here the efficient synthesis of 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine and 4,4′-bis(diethylphosphonate)-2,2′-bipyridine, as well as the mono-, bis-, and tris-substituted ruthenium complexes, [Ru(bpy)2(Pbpy)]2+, [Ru(bpy)(Pbpy)2]2+, [Ru(Pbpy)3]2+, [Ru(bpy)2(CPbpy)]2+, [Ru(bpy)(CPbpy)2]2+, and [Ru(CPbpy)3]2+ [bpy = 2,2′-bipyridine; Pbpy = 4,4′-bis(phosphonic acid)-2,2′-bipyridine; CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine].

Water oxidation and oxygen monitoring by cobalt-modified fluorine-doped tin oxide electrodes.

Abstract: Electrocatalytic water oxidation occurs at fluoride-doped tin oxide (FTO) electrodes that have been surface-modified by addition of Co(II). On the basis of X-ray photoelectron spectroscopy and transmission electron microscopy measurements, the active surface site appears to be a single site or small-molecule assembly bound as Co(II), with no evidence for cobalt oxide film or cluster formation. On the basis of cyclic voltammetry measurements, surface-bound Co(II) undergoes a pH-dependent 1e(-)/1H(+) oxidation to Co(III), which is followed by pH-dependent catalytic water oxidation. O2 reduction at FTO occurs at -0.33 V vs NHE, allowing for in situ detection of oxygen as it is formed by water oxidation on the surface. Controlled-potential electrolysis at 1.61 V vs NHE at pH 7.2 resulted in sustained water oxidation catalysis at a current density of 0.16 mA/cm(2) with 29,000 turnovers per site over an electrolysis period of 2 h. The turnover frequency for oxygen production per Co site was 4 s(-1) at an overpotential of 800 mV at pH 7.2. Initial experiments with Co(II) on a mesoporous, high-surface-area nanoFTO electrode increased the current density by a factor of ~5.

Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation

Abstract: Water oxidation is the key half reaction in artificial photosynthesis. An absence of detailed mechanistic insight impedes design of new catalysts that are more reactive and more robust. A proposed paradigm leading to enhanced reactivity is the existence of oxyl radical intermediates capable of rapid water activation, but there is a dearth of experimental validation. Here, we show the radicaloid nature of an intermediate reactive toward formation of the O-O bond by assessing the spin density on the oxyl group by Electron Paramagnetic Resonance (EPR). In the study, an 17O-labeled form of a highly oxidized, short-lived intermediate in the catalytic cycle of the water oxidation catalyst cis,cis-[(2,2-bipyridine)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ was investigated. It contains Ru centers in oxidation states [4,5], has at least one RuV = O unit, and shows |Axx| = 60G 17O hyperfine splittings (hfs) consistent with the high spin density of a radicaloid. Destabilization of π-bonding in the d3 RuV = O fragment is responsible for the high spin density on the oxygen and its high reactivity.

Low-Overpotential Water Oxidation by a Surface-Bound Ruthenium-Chromophore–Ruthenium-Catalyst Assembly†

Abstract: When anchored to nanoITO (indium tin oxide), the ruthenium chromophore-catalyst assembly shown acts as an electrocatalyst for water oxidation, with O2 evolution occurring at an overpotential of 230 mV in 0.1 M HClO4 . The potential response of the electrode points to 3 e(-) /2 H(+) oxidized [Rua (III) Rub (IV) O](5+) as the active form of the assembly.

Redox mediator effect on water oxidation in a ruthenium-based chromophore-catalyst assembly.

Abstract: The synthesis, characterization, and redox properties are described for a new ruthenium-based chromophore–catalyst assembly, [(bpy)2Ru(4-Mebpy-4′-bimpy)Ru(tpy)(OH2)]4+ (1, [RuaII-RubII-OH2]4+; bpy = 2,2′-bipyridine; 4-Mebpy-4′-bimpy = 4-(methylbipyridin-4′-yl)-N-benzimid-N′-pyridine; tpy = 2,2′:6′,2″-terpyridine), as its chloride salt. The assembly incorporates both a visible light absorber and a catalyst for water oxidation. With added ceric ammonium nitrate (CeIV, or CAN), both 1 and 2, [Ru(tpy)(Mebim-py)(OH2)]2+ (Mebim-py = 2-pyridyl-N-methylbenzimidazole), catalyze water oxidation. Time-dependent UV/vis spectral monitoring following addition of 30 equiv of CeIV reveals that the rate of CeIV consumption is first order both in CeIV and in an oxidized form of the assembly. The rate-limiting step appears to arise from slow oxidation of this intermediate followed by rapid release of O2. This is similar to isolated catalyst 2, with redox potentials comparable to the [-Rub-OH2]2+ site in 1, but 1 is more rea...

Accumulation of multiple oxidative equivalents at a single site by cross-surface electron transfer on TiO2.

Abstract: The photodriven accumulation of two oxidative equivalents at a single site was investigated on TiO2 coloaded with a ruthenium polypyridyl chromophore [Ru(bpy)2((4,4'-(OH)2PO)2bpy)](2+) (Ru(II)P(2+), bpy = 2,2'-bipyridine, ((OH)2PO)2-bpy = 2,2'-bipyridine-4,4'-diyldiphosphonic acid) and a water oxidation catalyst [Ru(Mebimpy) ((4,4'-(OH)2PO-CH2)2bpy)(OH2)](2+) (Ru(II)OH2(2+), Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, (4,4'-(OH)2PO-CH2)2bpy) = 4,4'-bis-methlylenephosphonato-2,2'-bipyridine). Electron injection from the metal-to-ligand charge transfer (MLCT) excited state of -Ru(II)P(2+) (-Ru(II)P(2+)*) to give -Ru(III)P(3+) and TiO2(e(-)) was followed by rapid (<20 ns) nearest-neighbor -Ru(II)OH2(2+) to -Ru(III)P(3+) electron transfer. On surfaces containing both -Ru(II)P(2+) and -Ru(III)OH2(3+) (or -Ru(III)OH(2+)), -Ru(II)OH2(2+) was formed by random migration of the injected electron inside the TiO2 nanoparticle and recombination with the preoxidized catalyst, followed by relatively slow (μs-ms) non-nearest neighbor cross-surface electron transfer from -Ru(II)OH2(2+) to -Ru(III)P(3+). Steady state illumination of coloaded TiO2 photoanodes in a dye sensitized photoelectrosynthesis cell (DSPEC) configuration resulted in the buildup of -Ru(III)P(3+), -Ru(III)OH(2+), and -Ru(IV)═O(2+), with -Ru(IV)═O(2+) formation favored at high chromophore to catalyst ratios.

A Sensitized Nb2O5 Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Abstract: Orthorhombic Nb2O5 nanocrystalline films functionalized with [Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key properties—conduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiency—were undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb2O5 nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO2. Anatase TiO2 has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb2O5 is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale,...

Spectroscopy and Dynamics of Phosphonate-Derivatized Ruthenium Complexes on TiO2

Abstract: Knowledge of electronic structures and transport mechanisms in dye-sensitized semiconductors is motivated by their ubiquity in photoelectrochemical cells. In this work, optical spectroscopies are used to uncover the elementary dynamics initiated by light absorption at such molecule–semiconductor interfaces (e.g., electron transfer and nuclear relaxation). These processes are explored in a family of ruthenium bipyridyl complexes in aqueous solutions, wherein phosphonate groups are used to bind the molecules to TiO2 nanocrystalline films. The complexes differ in (i) the number of phosphonate groups and (ii) the presence (or absence) of a methylene bridge between the molecule and the TiO2 surface. A resonance Raman intensity analysis suggests that the electronic excitations possess very little charge transfer character for all complexes. That is, the electronic orbitals involved in light absorption are essentially localized to the molecules. Because the electronic resonances are molecular in character, the p...

Application of the rotating ring-disc-electrode technique to water oxidation by surface-bound molecular catalysts.

Abstract: We report here the application of a simple hydrodynamic technique, linear sweep voltammetry with a modified rotating-ring-disc electrode, for the study of water oxidation catalysis. With this technique, we have been able to reliably obtain turnover frequencies, overpotentials, Faradaic conversion efficiencies, and mechanistic information from single samples of surface-bound metal complex catalysts.

Inverse kinetic isotope effect in the excited-state relaxation of a Ru(II)-aquo complex: revealing the impact of hydrogen-bond dynamics on nonradiative decay.

Abstract: Photophysics of the MLCT excited-state of [Ru(bpy)(tpy)(OH2)]2+ (1) and [Ru(bpy)(tpy)(OD2)]2+ (2) (bpy = 2,2′-bipyridine and tpy = 2,2′:6′,2″-terpyridine) have been investigated in room-temperature H2O and D2O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the 3MLCT state that initiates H-bond dynamics with the solvent and the 3MC state that serves as the principal pathway for nonradiative decay.

Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC)

Abstract: Time-resolved, UV-vis spectroscopic measurements of Li+ diffusion in mesoscopic TiO2 photoanodes were conducted in dye sensitized photoelectrosynthesis cells (DSPECs) under operating conditions. In these experiments the spectral response of TiO2 derivatized with [Ru(bpy)2(4,4′-((HO)2PO)2bpy)]2+ (RuP, where bpy is 2,2′-bipyridine, (4,4′-((HO)2PO)2bpy) is [2,2′-bipyridine]-4,4′-diphosphonic acid) arises from electric field (Stark) effects on the metal-to-ligand-charge transfer (MLCT) absorption spectrum of RuP, which is screened by cation intercalation. These results verify that Li+ diffusion is coupled to electron injection and to electron recombination/extraction at the TiO2 interface. Li+ doping levels depend on the competition between dynamics of its intercalation and electron recombination/transport. For a DSPEC operating in aqueous solution at pH 4.5, the observed rate constants for Li+ intercalation and release were 0.22 s−1 and 0.014 s−1, respectively. Both processes were considerably slower in the more viscous solvent propylene carbonate with Li+ release rate constants <2 × 10−4 s−1. Accumulation of Li+ under these conditions shifts conduction band/trap states to less negative potentials, increasing electron lifetime in TiO2.

Mechanism of Catalytic Water Oxidation by the Ruthenium Blue Dimer Catalyst: Comparative Study in D2O versus H2O

Abstract: Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)₂(H₂O)RuIIIORuIII(OH₂)(bpy)₂]4+ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D₂O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO₃, as well as further oxidation of BD[4,5] are slower in D₂O by 2.1-2.5. Ce(IV) oxidation of BD[4,5] and reaction with H₂O result in formation of an intermediate, BD[3,4]', which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]' is a rate limiting step in water oxidation with the BD catalyst.

Electrocatalysis on Oxide-Stabilized, High-Surface Area Carbon Electrodes

Abstract: A procedure is described for preparing and derivatizing novel, high surface area electrodes consisting of thin layers of nanostructured ITO (Sn(IV)-doped indium tin oxide, nanoITO) on reticulated vitreous carbon (RVC) to give RVC|nanoITO. The resulting hybrid electrodes are highly stabilized oxidatively. They were surface-derivatized by phosphonate binding of the electrocatalyst, [Ru(Mebimpy)(4,4′-((HO)2OPCH2)2bpy)(OH2)]2+ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2′-bipyridine) (1-PO3H2) to give RVC|nanoITO-RuII-OH22+. The redox properties of the catalyst are retained on the electrode surface. Electrocatalytic oxidation of benzyl alcohol to benzaldehyde occurs with a 75% Faradaic efficiency compared to 57% on nanoITO. Electrocatalytic water oxidation at 1.4 V vs SCE on derivatized RVC|nanoITO electrode with an internal surface area of 19.5 cm2 produced 7.3 μmoles of O2 in 70% Faradaic yield in 50 min.

Coordination chemistry of single-site catalyst precursors in reductively electropolymerized vinylbipyridine films

Abstract: Reductive electropolymerization of [RuII(PhTpy)(5,5′-dvbpy)(Cl)](PF6) and [RuII(PhTpy)(5,5′-dvbpy)(MeCN)](PF6)2 (PhTpy is 4′-phenyl-2,2′:6′,2″-terpyridine; 5,5′-dvbpy is 5,5′-divinyl-2,2′-bipyridine) on glassy carbon electrodes gives well-defined films of poly{[RuII(PhTpy)(5,5′-dvbpy)(Cl)](PF6)} (poly-1) or poly{[RuII(PhTpy)(5,5′-dvbpy)(MeCN)](PF6)2} (poly-2). Oxidative cycling of poly-2 with added NO3– results in the replacement of coordinated MeCN by NO3– to give poly{[RuII(PhTpy)(5,5′-dvbpy)(NO3)]+}, and with 0.1 M HClO4, replacement by H2O occurs to give poly{[RuII(PhTpy)(5,5′-dvbpy)(OH2)]2+} (poly-OH2). Although analogous aqua complexes (e.g., [Ru(tpy)(bpy)(OH2)]2+) undergo rapid loss of H2O to MeCN in solution, poly-OH2 and poly-OH2+ are substitutionally inert in MeCN. The substitution chemistry is reversible, with reductive scans of poly-1 or poly-OH2 in MeCN resulting in poly-2, although with some loss of Faradaic response.

Accelerating slow excited state proton transfer.

Abstract: Visible light excitation of the ligand-bridged assembly [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH(2))(4+)] (bpy is 2,2'-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L(-•))Ru(b)(III)-OH(2) with an excited-state lifetime of 13 ± 1 ns. Near-diffusion-controlled quenching of the emission occurs with added HPO(4)(2-) and partial quenching by added acetate anion (OAc(-)) in buffered solutions with pH control. A Stern-Volmer analysis of quenching by OAc(-) gave a quenching rate constant of k(q) = 4.1 × 10(8) M(-1) • s(-1) and an estimated pK(a)* value of ~5 ± 1 for the [(bpy)(2)Ru(a)(II)(L(•-))Ru(b)(III)(bpy)(OH(2))(4+)]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(II)(bpy)(OH)(3+)] in a H(2)PO(4)(-)/HPO(4)(2-) buffer, back proton transfer occurs from H(2)PO(4)(-) to give [(bpy)(2)Ru(a)(II)(L)Ru(b)(bpy)(OH(2))(4+)] with k(PT,2) = 4.4 × 10(8) M(-1) • s(-1). From the intercept of a plot of k(obs) vs. [H(2)PO(4)(-)], k = 2.1 × 10(6) s(-1) for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK(a) values intermediate between pK(a)(H(3)O(+)) = -1.74 and pK(a)(H(2)O) = 15.7.

Electronic Structure Assessment: Combined Density Functional Theory Calculations and Ru L2,3-Edge X-ray Absorption Near-Edge Spectroscopy of Water Oxidation Catalyst

Abstract: Density functional theory (DFT) is now widely used for analysis of the electronic structure and reactivities of transition metal complexes. However, large variability in how well different combinations of exchange-correlation potentials/basis sets reproduce real molecular geometries and electronic configurations remains a problem. Experimental X-ray absorption near-edge structure (XANES) spectra directly reflect the electronic structure of transition metal complexes. Combined analysis of theoretical calculations and experimental data is highly beneficial for DFT validation as well as for understanding limitations of the DFT. Ruthenium-based molecular water oxidation catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+ is a complex coordination compound with two Ru centers in different oxidation states bound by μ-oxo bridge. Multiple DFT calculations of this catalyst in different oxidation states have been reported previously but it was never clear whether DFT is truly capable of describing its geometry a...

Water Electrolysis with a Homogeneous Catalyst in an Electrochemical Cell

Abstract: Renewable-powered electrolysis is a near-term technology for liquid solar fuels. Alkaline electr olyzers use heterogeneous catalysts (e.g., Raney nickel) an d corrosive electrolytes (e.g., KOH), which can have drawbacks. Specifically, replacement of spent cata lyst requires shutdown of the electrolyzers. Furthermor e, electrolyzer components that withstand the harsh operating conditions are costly. A homogeneous cat alyst that functions at near-neutral pH has the potential to overcome these shortcomings, since it can be replen ish d without unit disassembly and operated with low-cost electrolyzer materials.