A molecular catalyst for water oxidation that binds to metal oxide surfaces

Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox-potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH-dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six-coordinate [Ru(IV) ] center to yield an anionic seven-coordinate [Ru(IV) -OH](-) intermediate is one of the key steps of a single-site mechanism in which all species are anionic or neutral. These complexes are among the fastest single-site catalysts reported to date.

Porous LaCo1-xNixO3-δ Nanostructures as an Efficient Electrocatalyst for Water Oxidation and for a Zinc-Air Battery

Perovskites have emerged as promising earth-abundant alternatives to precious metals for catalyzing the oxygen evolution reaction (OER). Herein, we report the synthesis of a series of porous perovskite nanostructures, LaCo0.97O3-δ, with systematic Ni substitution in Co octahedral sites. Their electrocatalytic activity during the water oxidation reaction was studied in alkaline electrolytes. The electrocatalytic OER activity and stability of the perovskite nanostructure was evaluated using the rotating disk electrode technique. We show that the progressive replacement of Co by Ni in the LaCo0.97O3-δ perovskite structure greatly altered the electrocatalytic activity and that the La(Co0.71Ni0.25)0.96O3-δ composition exhibited the lowest OER overpotential of 324 and 265 mV at 10 mA cm(-2) in 0.1 M KOH and 1 M KOH, respectively. This value was much lower than that of the noble metal catalysts, IrO2, Ru/C, and Pt/C. Furthermore, the La(Co0.71Ni0.25)0.96O3-δ nanostructure showed outstanding electrode stability, with no observable decrease in performance up to 114th cycle in the auxiliary linear sweep voltammetry that lasted for 10 h in chronoamperometry studies. The excellent oxygen evolution activity of the La(Co0.71Ni0.25)0.96O3-δ perovskite nanostructure can be attributed to its intrinsic structure, interconnected particle arrangement, and unique redox characteristics. The enhanced intrinsic electrocatalytic activity of the La(Co0.71Ni0.25)0.96O3-δ catalyst was correlated with several parameters, such as the electrochemical surface area, the roughness factor, and the turnover frequency, with respect to variation in the transition metals of the perovskite structure. Subsequently, La(Co0.71Ni0.25)0.96O3-δ was utilized as the air cathode in a zinc-air battery application.

New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis

This paper introduces Ir(I)(CO)2(pyalc) (pyalc = (2-pyridyl)-2-propanoate) as an atom-efficient precursor for Ir-based homogeneous oxidation catalysis. This compound was chosen to simplify analysis of the water oxidation catalyst species formed by the previously reported Cp*Ir(III)(pyalc)OH water oxidation precatalyst. Here, we present a comparative study on the chemical and catalytic properties of these two precursors. Previous studies show that oxidative activation of Cp*Ir-based precursors with NaIO4 results in formation of a blue Ir(IV) species. This activation is concomitant with the loss of the placeholder Cp* ligand which oxidatively degrades to form acetic acid, iodate, and other obligatory byproducts. The activation process requires substantial amounts of primary oxidant, and the degradation products complicate analysis of the resulting Ir(IV) species. The species formed from oxidation of the Ir(CO)2(pyalc) precursor, on the other hand, lacks these degradation products (the CO ligands are easily lost upon oxidation) which allows for more detailed examination of the resulting Ir(pyalc) active species both catalytically and spectroscopically, although complete structural analysis is still elusive. Once Ir(CO)2(pyalc) is activated, the system requires acetic acid or acetate to prevent the formation of nanoparticles. Investigation of the activated bis-carbonyl complex also suggests several Ir(pyalc) isomers may exist in solution. By (1)H NMR, activated Ir(CO)2(pyalc) has fewer isomers than activated Cp*Ir complexes, allowing for advanced characterization. Future research in this direction is expected to contribute to a better structural understanding of the active species. A diol crystallization agent was needed for the structure determination of 3.

Understanding the origin of photoelectrode performance enhancement by probing surface kinetics

Probing the surface kinetics of different hematite electrodes with and without surface passivations.

Photoelectrochemical (PEC) water splitting holds the potential to meet the challenges associated with the intermittent nature of sunlight. Catalysts have often been shown to improve the performance of PEC water splitting, but their working mechanisms are not well understood. Using intensity modulated photocurrent spectroscopy (IMPS), we determined the rate constants of water oxidation and recombination at the surface of three different hematite-based photoanodes. It was found that the best performing electrodes, in terms of photocurrent onset potential, exhibited the slowest water oxidation rate constants, which was a surprise. The performance of these photoelectrodes was enabled by the slow surface recombination. When amorphous NiFeO_x, a water oxidation catalyst, was present, the rate of surface hole transfer actually slowed down; what was slowed more was the recombination rate at the hematite surface, resulting in better water oxidation performance. As such, NiFeO_x primarily serves as a passivation layer rather than a catalytic layer. Together a better understanding of the role of catalytic overlayers for water oxidation has been achieved.

Efficient water oxidation with electromodified Langmuir–Blodgett films of procatalytic [CoIII(N2O3)] metallosurfactants on electrodes

Electromodified Langmuir–Blodgett films of a cobalt-based metallosurfactant support water oxidation at turnover numbers of up to 54 000 h⁻¹.

Tailored design of ruthenium molecular catalysts with 2,2′-bypyridine-6,6′-dicarboxylate and pyrazole based ligands for water oxidation

A series of tailor-designed Ruthenium based water oxidation catalysts have been synthesized. By fine tuning of the catalyst structure, the turnover frequency was increased up to 500 s⁻¹and the stability over 6000 turnovers.

Non-directed, carbonate-mediated C–H activation and aerobic C–H oxygenation with Cp*Ir catalysts

Carbonate additives enhance the activity of [Cp*Ir(H₂O)₃](OTf)₂ for non-directed C–H activations and the aerobic C–H oxygenation of alkyl arenes.

On the mechanism of water oxidation catalyzed by a dinuclear ruthenium complex: a quantum chemical study

The development of efficient and robust catalysts for water oxidation is an essential element in solar water splitting. In the present paper, the reaction mechanism for a dinuclear Ru water oxidation catalyst has been investigated in detail through quantum chemical calculations.

Electrocatalytic water oxidation by a macrocyclic Cu(ii) complex in neutral phosphate buffer

A robust molecular copper catalyst [Cu(TMC)(H₂O)](NO₃)₂ was reported for electrocatalytic water oxidation in phosphate buffer. It exhibits superior activity to the ever reported homogeneous and heterogeneous copper-based WOCs under neutral conditions.

Evaluation of iron-based electrocatalysts for water oxidation – an on-line mass spectrometry approach

Using on-line mass spectrometry in combination with classical electroanalytical techniques makes it possible to reliably determine onset potentials and to distinguish between competing reactions such as oxygen evolution and carbon dioxide formation.

Early stages of catalyst aging in the iridium mediated water oxidation reaction

The structure of an iridium Cp* water oxidation catalyst was evaluated as a function of time and applied potential.

A Stable Coordination Complex of Rh(IV) in an N,O-Donor Environment

We describe facial and meridional isomers of [Rh(III)(pyalk)3], as well as meridional [Rh(IV)(pyalk)3](+) {pyalk =2-(2-pyridyl)-2-propanoate}, the first coordination complex in an N,O-donor environment to show a clean, reversible Rh(III/IV) redox couple and to have a stable Rh(IV) form, which we characterize by EPR and UV-visible spectroscopy as well as X-ray crystallography. The unprecedented stability of the Rh(IV) species is ascribed to the exceptional donor strength of the ligands, their oxidation resistance, and the meridional coordination geometry.

Immobilization of a Molecular Ruthenium Catalyst on Hematite Nanorod Arrays for Water Oxidation with Stable Photocurrent

Photoelectrochemical (PEC) cells for light-driven water splitting are prepared using hematite nanorod arrays on conductive glass as the photoanode. These devices improve the photocurrent of the hematite-based photoanode for water splitting, owing to fewer surface traps and decreased electron recombination resulting from the one-dimensional structure. By employing a molecular ruthenium co-catalyst, which contains a strong 2,6-pyridine-dicarboxylic acid anchoring group at the hematite photoanode, the photocurrent of the PEC cell is enhanced with high stability for over 10 000 s in a 1 m KOH solution. This approach can pave a route for combining one-dimensional nanomaterials and molecular catalysts to split water with high efficiency and stability.

Development of a Dinitrosyl Iron Complex Molecular Catalyst into a Hydrogen Evolution Cathode

Despite extensive efforts, the electrocatalytic reduction of water using homogeneous/heterogeneous Fe, Co, Ni, Cu, W, and Mo complexes remains challenging because of issues involving the development of efficient, recyclable, stable, and aqueous-compatible catalysts. In this study, evolution of the de novo designed dinitrosyl iron complex DNIC-PMDTA from a molecular catalyst into a solid-state hydrogen evolution cathode, considering all the parameters to fulfill the electronic and structural requirements of each step of the catalytic cycle, is demonstrated. DNIC-PMDTA reveals electrocatalytic reduction of water at neutral and basic media, whereas its deposit on electrode preserves exceptional longevity, 139 h. This discovery will initiate a systematic study on the assembly of [Fe(NO)2] motif into current collector for mass production of H2, whereas the efficiency remains tailored by its molecular precursor [(L)Fe(NO)2].

Surfactant-mediated electrodeposition of a water-oxidizing manganese oxide

Use of sodium dodecyl sulfate (SDS) during electrodeposition improves the mechanical stability and catalytic activity of manganese dioxide for electrocatalytic water oxidation.

Iridium-based complexes for water oxidation

Activation of organometallic Ir precatalysts can yield either homogeneous or heterogeneous water-oxidation catalysts with very high activity.

Silatranes for binding inorganic complexes to metal oxide surfaces

Silatranes do not react when synthesizing inorganic complexes and form bonds to metal oxide surfaces that are stable under acidic, neutral, and basic conditions.