Neutral water splitting catalysis with a high FF triple junction polymer cell

This document is the Accepted Manuscript version of a Published Work that appeared in final form in CS catalysis, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acscatal.6b01036We report a photovoltaics-electrochemical (PV-EC) assembly based on a compact and easily processable triple homojunction polymer cell with high fill factor (76%), optimized conversion efficiencies up to 8.7%, and enough potential for the energetically demanding water splitting reaction (V-oc = 2.1 V). A platinum-free cathode made of abundant materials is coupled to a ruthenium oxide on glassy carbon anode (GC-RuO2) to perform the reaction at optimum potential (Delta E = 1.70-1.78 V, overpotential = 470-550 mV). The GC-RuO2 anode contains a single monolayer of catalyst corresponding to a superficial concentration (Gamma) of 0.15 nmol cm(-2) and is highly active at pH 7. The PV-EC cell achieves solar to hydrogen conversion efficiencies (STH) ranging from 5.6 to 6.0%. As a result of the solar cell's high fill factor, the optimal photovoltaic response is found at 1.70 V, the minimum potential at which the electrodes used perform the water splitting reaction. This allows generating hydrogen at efficiencies that would be very similar (96%) to those obtained as if the system were to be operating at 1.23 V, the thermodynamic potential threshold for the water splitting reaction.Peer ReviewedPostprint (author's final draft

Catalytic Oxidation of Water to Dioxygen by Mononuclear Ru Complexes Bearing a 2,6‐Pyridinedicarboxylato Ligand

The synthesis, purification, and isolation of mononuclear Ru complexes containing the tridentate dianionic meridional ligand pyridyl‐2,6‐dicarboxylato (pdc2−) of general formula [RuIII(pdc‐κ3‐N1O2)(bpy)Cl] (1III) and [RuII(pdc‐κ2‐N1O1)(bpy)2] (2II) (bpy is 2,2′‐bipyridine) is reported. These two complexes and their derivatives were thoroughly characterized through spectroscopic (UV/Vis, NMR) and electrochemical (cyclic voltammetry, differential pulse voltammetry, and coulometry) analyses, and three of the complexes were analyzed by single‐crystal X‐ray diffraction techniques. Under a high anodic applied potential, both complexes evolve towards the formation of Ru‐aquo/oxo derivative species, namely, [RuIII(pdc‐κ3‐N1O2)(bpy)(OH2)]+ (1‐O) and [RuIV(O)(pdc‐κ2‐N1O1)(bpy)2] (2‐O). These two complexes are active catalysts for the oxidation of water to dioxygen and their catalytic activity was analyzed through electrochemical techniques. A maximum turnover frequency (TOFmax)=2.4–3.4×103 s−1 was calculated for 2‐O

Bridgehead Hydrogen Atoms Are Important: Unusual Electrochemistry and Proton Reduction at Iron Dimers with Ferrocenyl-Substituted Phosphido Bridges

The diphosphido-bridged diiron clusters <i>syn</i>-[{μ_<i>2</i>-P­(CH₂Fc)­H}₂Fe₂(CO)₆] (<b>2a</b>) and <i>anti</i>-/<i>syn</i>-[{μ_<i>2</i>-P­(CH₂Fc)­Me}₂Fe₂(CO)₆] (<b>3</b>), containing covalently linked ferrocenyl (Fc) groups, were synthesized in order to explore the effect of having a redox-active ligand bound to a Fe₂P₂ core as in the covalently linked Fe₄S₄-{μ₂-S­(Cys)}-Fe₂S₂ cofactor of [FeFe]-hydrogenases. The X-ray crystal structure of <b>2a</b> shows an Fe–Fe bond length of 2.630(1) Å and confirms that the two P–H bonds of the bridging 1°-phosphido groups are parallel and are separated by 2.683(2) Å. Cyclic voltammetry and spectroelectrochemistry studies revealed that <b>2a</b>, unusually, undergoes one-electron reduction at −2.18 V (vs Fc⁺/Fc) to give the anion [{μ_<i>2</i>-P­(CH₂Fc)}­{μ_<i>2</i>-P­(CH₂Fc)­H}­Fe₂(CO)₆]^<b>–</b> ([<b>2a</b> – H]^<b>–</b>), which was independently obtained by deprotonation of <b>2a</b> with excess 1,8-diazabicycloundec-7-ene (DBU). The reduction proceeds through the radical anion <b>2a</b>′^{<b>•–</b>} intermediate, which was detected by X-band EPR spectroscopy in situ during electrolysis. The formation of [<b>2a</b> – H]^<b>–</b> from the <b>2a</b>′^{<b>•–</b>} radical formally equates to loss of a hydrogen atom from the bridging P–H group. The result suggests that a new low-energy route for evolution of molecular hydrogen is available in Fe₂E₂ dimers with bridgehead hydrogen atomsi.e. dimers with hydrogen directly bonded to the bridging nonmetal atoms (E = P, S). In contrast to the one-electron reduction behavior of <b>2a</b>, the mixture of dimers <b>3</b> exhibited a two-electron reduction at −2.11 V (vs Fc⁺/Fc) that afforded <b>3</b>^2–. Both dimers catalyze the reduction of protons from <i>p</i>-toluenesulfonic acid, with <b>2a</b> exhibiting higher catalytic currents at lower overpotential

A Ru-bda complex with a dangling carboxylate group: synthesis and electrochemical properties

Ruthenium complexes containing the tetradentate 2,2'-bipyridine-6,6'-dicarboxylato (bda2-) equatorial ligand and ortho-subsituted pyridines in the axial position have been prepared and characterized using spectroscopic, crystallographic and electrochemical techniques. Complexes [Ru(Hbda)(DMSO)(pyC)] (1) and [Ru(bda)(DMSO)(pyA)] (2) (where pyC is 2-pyridinecarboxylate, pyA is pyridine-2-ylmethanol and DMSO is dimethylsulfoxide) have been isolated in moderate to high yields. The solid state structures of (1-H)- and 2 reveal the strong chelate effect of the axial pyridine ligand that coordinates in a bidentate fashion leaving the bda2- equatorial ligand coordinating in a tridentate mode. In solution, compound 2 shows a dynamic equilibrium between different coordination modes of the bda2- and pyA ligands. This phenomenon does not occur for 1 because the carboxylate binds stronger than the labile alcohol in 2. Cyclic voltammetry analysis of 1 reveal a complex behavior with a pH independent wave at E1/2 = 1.12 V that is tentatively associated with the two- electron RuIV/II. In sharp contrast, complex 2 shows a pH dependent one-electron wave at E1/2 = 0.83 V (pH 1), assigned to the proton couple electron transfer process of the RuIII/II couple and a pH independent wave at E1/2 = 1.06 V assigned to the RuIV/III couple. Compound 2 was used to prepare complex [Ru(bda)(pic)(pyA)] (4). This complex is air sensitive and converts to complex [Ru(bda)(pic)(pyE)] (5) (where pyE is methyl 2-pyridine carboxylate) in the presence of methanol. This oxidation also occurs by applying a positive potential to an aqueous solution of 4, producing the derivative [Ru(bda)(pic)(pyC)] (3). Cyclic voltammetry of 3 shows two pH independent one-electron oxidation waves at E1/2 = 0.64 V and E1/2 = 1.0 V, corresponding to the RuIII/II and RuIV/III couples, respectively. In addition, a water oxidation catalytic wave appears at Eonset ≈ 1.4 V. Foot of the wave analysis of this catalytic wave based on a water nucleophilic attack accounts for a TOFmax = 0.63-0.74 s-1

Bridgehead Hydrogen Atoms Are Important: Unusual Electrochemistry and Proton Reduction at Iron Dimers with Ferrocenyl-Substituted Phosphido Bridges

The diphosphido-bridged diiron clusters <i>syn</i>-[{μ_<i>2</i>-P­(CH₂Fc)­H}₂Fe₂(CO)₆] (<b>2a</b>) and <i>anti</i>-/<i>syn</i>-[{μ_<i>2</i>-P­(CH₂Fc)­Me}₂Fe₂(CO)₆] (<b>3</b>), containing covalently linked ferrocenyl (Fc) groups, were synthesized in order to explore the effect of having a redox-active ligand bound to a Fe₂P₂ core as in the covalently linked Fe₄S₄-{μ₂-S­(Cys)}-Fe₂S₂ cofactor of [FeFe]-hydrogenases. The X-ray crystal structure of <b>2a</b> shows an Fe–Fe bond length of 2.630(1) Å and confirms that the two P–H bonds of the bridging 1°-phosphido groups are parallel and are separated by 2.683(2) Å. Cyclic voltammetry and spectroelectrochemistry studies revealed that <b>2a</b>, unusually, undergoes one-electron reduction at −2.18 V (vs Fc⁺/Fc) to give the anion [{μ_<i>2</i>-P­(CH₂Fc)}­{μ_<i>2</i>-P­(CH₂Fc)­H}­Fe₂(CO)₆]^<b>–</b> ([<b>2a</b> – H]^<b>–</b>), which was independently obtained by deprotonation of <b>2a</b> with excess 1,8-diazabicycloundec-7-ene (DBU). The reduction proceeds through the radical anion <b>2a</b>′^{<b>•–</b>} intermediate, which was detected by X-band EPR spectroscopy in situ during electrolysis. The formation of [<b>2a</b> – H]^<b>–</b> from the <b>2a</b>′^{<b>•–</b>} radical formally equates to loss of a hydrogen atom from the bridging P–H group. The result suggests that a new low-energy route for evolution of molecular hydrogen is available in Fe₂E₂ dimers with bridgehead hydrogen atomsi.e. dimers with hydrogen directly bonded to the bridging nonmetal atoms (E = P, S). In contrast to the one-electron reduction behavior of <b>2a</b>, the mixture of dimers <b>3</b> exhibited a two-electron reduction at −2.11 V (vs Fc⁺/Fc) that afforded <b>3</b>^2–. Both dimers catalyze the reduction of protons from <i>p</i>-toluenesulfonic acid, with <b>2a</b> exhibiting higher catalytic currents at lower overpotential

Seven Coordinated Molecular Ru-Water Oxidation Catalysts: a Coordination Chemistry Journey

Molecular water oxidation catalysis is a field that has experienced an impressive development over the last decade mainly fueled by the promise of generation of sustainable carbon neutral fuel society, based on water splitting. Most of these advancements have been possible thanks to the detailed understanding of the reactions and intermediates involved in the catalytic cycles. Today’s best molecular water oxidation catalysts reach turnover frequencies that are orders of magnitude higher than that of natural oxygen evolving center in photosystem II. These catalysts are based on Ru complexes where at some stage, the first coordination sphere of the metal center becomes seven coordinated. The key for this achievement is largely based on the use of adaptative ligands that adjust their coordination mode depending on the structural and electronic demands of the metal center at different oxidation states accessed within the catalytic cycle. This Review covers the latest and most significant developments on Ru complexes that behave as powerful water oxidation catalysts and where at some stage the Ru metal attains coordination number 7. Further it provides a comprehensive and rational understanding of the different structural and electronic factors that govern the behavior of these catalysts

Synthesis, Electrochemical Characterization and Water Oxidation Chemistry of Ru Complexes Containing the 2,6- Pyridinedicarboxylato Ligand

The tridentate meridional ligand pyridyl-2,6-dicarboxylato (pdc2-) has been used to prepare complexes RuII(pdc-ҡ3-N1O2)(DMSO)2Cl] (1II), RuII(pdc-ҡ3-N1O2)(bpy)(DMSO)] (2II) and {[RuIII(pdc-ҡ3-N1O2)(bpy)]2(-O)} (5III,III) where bpy: 2,2’-bipyridine. All complexes have been fully characterized through spectroscopic, electrochemical and single crystal X-ray diffraction techniques. Compounds 1II and 2II show SO linkage isomerization of the DMSO ligand upon oxidation from RuII to RuIII and thermodynamic and kinetic data have been obtained from cyclic voltammetry experiments. Dimeric complex 5III,III is a precursor of the monomeric complex [RuII(pdc-ҡ3-N1O2)(bpy)(H2O)] (4II) which is a water oxidation catalyst. The electrochemistry and catalytic activity of 4II has been ascertained for the first time and compared with related Ru-aquo complexes that are also active for the water oxidation reaction. It shows a TOFmax = 0.2 s-1 and overpotential of 240 mV in pH 1. The overpotential shown by 4II is one of the lowest reported in the literature and is associated to the role of the two carboxylato groups of the pdc ligand, providing high electron density to the ruthenium complex

Can Ni complexes behave as molecular water oxidation catalysts?

The present report uncovers the borderline between homogeneous and heterogeneous water oxidation catalysis using a family of Ni complexes containing oxamidate anionic type of ligands. In particular, the Ni complex [(L1)NiII]2- (12-; L1 = o-phenylenebis(oxamidate)) and its modified analogues [(L2)NiII]2- (22- ;L2 = 4,5-dimethyl-1,2-phenylenebis(oxamidate)) and [(L3)NiII]2- (32- ;L2 = 4- methoxy-1,2-phenylenebis(oxamidate)) have been prepared and evaluated as molecular water oxidation catalysts at basic pH. Their redox features have been analyzed by mean of electrochemical measurements revealing a crucial involvement of the ligand in the electron transfer processes. Moreover, the stability of those complexes has been assessed both in solution and immobilized on graphene-based electrodes at different potentials and pHs. The degradation of the molecular species generates a NiOx layer, whose stability and activity as water oxidation catalyst has also been stablished. Electrochemical methods, together with surface characterization techniques, have shown the complex mechanistic scenario in water oxidation catalyzed by this family of Ni complexes, which consists of the coexistence of two catalytic mechanism: a homogeneous pathway driven by the molecular complex and a heterogeneous pathway based on NiOx. The electronic perturbations exerted through the ligand framework has manifested a strong influence over the stability of the molecular species under turnover conditions. Finally, 12- has been used as a molecular precursor for the formation of NiFeOx anodes that behave as extremely powerful water oxidation anodes

Electrochemically and Photochemically Induced Hydrogen Evolution Catalysis with Co-Tetraazamacrocycles Occur via Different Pathways

Cobalt complexes containing equatorial tetraazamacrocyclic ligands are active catalysts for the hydrogen evolution reaction in pure aqueous conditions. Herein we explore the effect of different groups directly linked to the macrocyclic ligand (- NH-, -NH2+- or –N(CH2OH)-). Electrochemically induced hydrogen evolution catalysis studies at pH 4 invoke a mechanism, in which the rate determining step is the protonation of the reduced CoI species that gives a cobalt hydride (CoIII-H), a key intermediate towards the H- H bond formation. In sharp contrast, under photochemical conditions using [Ru(bpy)3]2+ as a photosensitizer and ascorbate as sacrificial electron donor, the formation of a “Co0” species that quickly protonates to give a CoII-H is proposed. In this scenario, the rate determining step is the H-H bond formation that occurs in an intermolecular fashion from the CoII-H species and a water molecule. Both mechanisms are supported by density functional theory (DFT) calculations that allowed us to estimate the pKa values of the CoIII-H and CoII-H species, as well as transition states based on intramolecular and intermolecular H-H bond formation from CoII-H

A broad view at the complexity involved in water oxidation catalysis based on Ru-bpn complexes

A new Ru complex with the formula [Ru(bpn)(pic)2]Cl2 (where bpn is 2,2′-bi(1,10-phenanthroline) and pic stands for 4-picoline) (1Cl2) is synthesized to investigate the true nature of active species involved in the electrochemical and chemical water oxidation mediated by a class of N4 tetradentate equatorial ligands. Comprehensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV-vis, NMR, and resonance Raman), and kinetic studies are performed. 12+ undergoes a substitution reaction when it is chemically (by using NaIO4) or electrochemically oxidized to RuIII, in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)]2+ (22+). The former complex is in equilibrium with an oxo-bridged species {[Ru(bpn)(pic)]2(μ-O)}4+ (34+) which is the major form of the complex in the RuIII oxidation state. The dimer formation is the rate determining step of the overall oxidation process (kdimer = 1.35 M−1 s−1), which is in line with the electrochemical data at pH = 7 (kdimer = 1.4 M−1 s−1). 34+ can be reduced to [Ru(bpn)(pic)(OH2)]2+ (42+), showing a sort of square mechanism. All species generated in situ at pH 7 have been thoroughly characterized by NMR, mass spectrometry, UV-Vis and electrochemical techniques. 12+ and 42+ are also characterized by single crystal X-ray diffraction analysis. Chemical oxidation of 12+ triggered by CeIV shows its capability to oxidize water to dioxygen