Water Reduction Catalyst Research Articles

Self-assembled 2D WSe2 thin films for photoelectrochemical hydrogen production.

WSe2—a layered semiconductor that can be exfoliated into atomically thin two-dimensional sheets—offers promising characteristics for application in solar energy conversion. However, the lack of controllable, cost-effective methods to scalably fabricate homogeneous thin films currently limits practical application. Here we present a technique to prepare controlled thin films of 2D WSe2 from dispersions of solvent-exfoliated few-layer flakes. Flake self-assembly at a liquid/liquid interface (formed exceptionally from two non-solvents for WSe2) followed by substrate transfer affords large-area thin films with superior 2D flake alignment compared with traditional (liquid/air) self-assembly techniques. We further demonstrate, for the first time, solar-to-hydrogen conversion from solution-processed WSe2 thin films. Bare photoelectrodes with a thickness of ca. 25 nm exhibit sustained p-type photocurrent under simulated solar illumination, and up to 1.0 mA cm–2 at 0 V versus reversible hydrogen electrode with an added water reduction catalyst (Pt). The importance of the self-assembled morphology is established by photoelectrochemical and conductivity measurements.

Nickel-Containing Keggin-Type Polyoxometalates as Hydrogen Evolution Catalysts: Photochemical Structure-Activity Relationships.

In search of structure-activity relationships for polyoxometalate (POM)-based water reduction catalysts, nickel-monosubstituted Keggin-type POMs ([Ni(H2 O)XW11 O39 ]n- ; XP, Si, Ge) were compared with respect to their activity in photochemical hydrogen evolution. The title compound series was characterized by single-crystal X-ray diffraction methods and a wide range of spectroscopic and electrochemical techniques. Nickel substitution was identified as a crucial feature for catalytic activity through comparison with nickel-free reference POMs. Furthermore, turnover number (TON) and turnover frequency strongly depended on the heteroatom X, and the highest TON among the series was recorded for [Ni(H2 O)GeW11 O39 ]6- . Photochemical hydrogen evolution activity was compared with redox and onset potentials obtained from electrochemical analyses. Furthermore, activity trends were correlated with electronic structure properties derived from density functional theory calculations.

Electro- and photo-chemical driven water reduction catalyzed by a cobalt(III) complex with high turnover number

A new catalyst based on a cobalt complex [LCoCl] 1 is formed by the reaction of dimethylaminoethylamino-N,N-bis(2-methylene-4-tert-butyl-6-methyl) phenol (H2L) with CoCl2·6H2O for both electrolytic and photolytic water reduction. 1 can electrocatalyze hydrogen evolution from water with a turnover frequency (TOF) of 230moles of hydrogen per mole of catalyst per hour at an overpotential of 689mV (pH 7.0). Additionally, under photoirradiation with blue light (λmax=469nm), 1, together with [Ru(bpy)3]Cl2 and ascorbic acid in a pH 6.0 aqueous solution, also produces hydrogen with a remarkable turnover number (TON) of 4130mol of H2 per mol of catalysts. This performance is different from the reported results that the best pH for photocatalytic H2 evolution by photo-catalyst was observed at pH 4.0. This discovery has established a new chemical paradigm for creating water reduction catalyst that is highly active near neutral media.

Closing the pressure gap in x-ray photoelectron spectroscopy by membrane hydrogenation.

Comprehensive studies of gas-solid reactions require the in-situ interaction of the gas at a pressure beyond the operating pressure of ultrahigh vacuum (UHV) X-ray photoelectron spectroscopy (XPS). The recent progress of near ambient pressure XPS allows to dose gases to the sample up to a pressure of 20 mbar. The present work describes an alternative to this experimental challenge, with a focus on H2 as the interacting gas. Instead of exposing the sample under investigation to gaseous hydrogen, the sample is in contact with a hydrogen permeation membrane, through which hydrogen is transported from the outside to the sample as atomic hydrogen. Thereby, we can reach local hydrogen concentrations at the sample inside an UHV chamber, which is equipped with surface science tools, and this corresponds to a hydrogen pressure up to 1 bar without affecting the sensitivity or energy resolution of the spectrometer. This experimental approach is validated by two examples, that is, the reduction of a catalyst precursor for CO2 hydrogenation and the hydrogenation of a water reduction catalyst for photocatalytic H2 production, but it opens the possibility of the new in situ characterisation of energy materials and catalysts.

Unusual stability of dyads during photochemical hydrogen production.

Dyads for photochemical water splitting often suffer from instability during irradiation with visible light. However, the use of bis(bidentate) phosphines forming a five-membered ring enhances their stability. The coordination of these phosphor based chelates to soft metals like Pd(ii) prolongs the photocatalytic activity to 1000 hours. To avoid contribution to hydrogen production by colloidal metal, a small amount of Hg is added to the reaction mixture. In the course of our investigations, it turned out that colloidal palladium was not able to produce hydrogen under our irradiation conditions. As soon as metallic palladium emerged in our reaction vessels, no further hydrogen production was detected. This is confirmed by the observation that the hydrogen production depends on the kind of ancillary ligands present in the dyads. The first dyads of the type [MI(bpy)2(dppcb)MII(bpy)](4+) are presented (MI = Os, MII = Pd (1); MI = Ru, MII = Pd (2); MI = Os, MII = Pt (3); MI = Ru, MII = Pt (4)). In [Os(bpy)2(dppcb)Pd(dppm)](PF6)4 (5) the ancillary ligand is varied. Furthermore, it is also possible to produce hydrogen in an intermolecular way. Using different bidentate diphosphines instead of a bis(bidentate) tetraphosphine leads to this intermolecular approach, where the chromophore and the water reduction catalyst (WRC) belong now to two molecules. In this case the TON is sensitive to the type of diphosphine, which is only possible if intact molecules act as catalysts and no free palladium(0) is formed.

Non-innocent adsorption of Co-pyrphyrin on rutile(110).

Solar-light driven water splitting is a promising way for the sustainable production of molecular hydrogen, the latter representing an efficient carrier for energy storage and conversion into common liquid fuels. In search of novel catalysts for high-performance water splitting devices, Co-pyrphyrin (CoPy) has been recently synthesized and successfully used as a homogeneous water reduction catalyst. We investigate the adsorption of this molecule on the rutile TiO2(110) surface as a possible first step towards the design of a heterogeneous water reduction system. We find that the adsorption of the molecule is stabilized by the interaction of the cyano groups with the under-coordinated Ti centers present at the surface. This interaction induces the rehybridization of the molecular orbitals localized on the cyano groups and the realignment of the lowest unoccupied molecular states. Moreover, the highest occupied molecular orbital of CoPy@rutile(110) is localized on CoPy and the energy gap turns out to be significantly smaller than the gap of pristine rutile(110). This implies that direct or indirect injection of electrons from CoPy to the rutile(110) surface is in principle possible upon the absorption of light in the visible range. On the other hand, the electronic properties of the Co(ii) center are not modified by the adsorption, which suggests that CoPy and its derivatives may be used in water electrolysis for hydrogen production also in the adsorbed state.

DFT analysis into the intermediates of nickel pyridinethiolate catalysed proton reduction.

Nickel pyridine 2-thiolate (Ni(PyS)3(-)) has shown good stability and activity as a H2 generation catalyst for use in solar energy storage. The experimentally proposed catalytic pathway is explored using DFT calculations. Free energy changes along the reaction coordinate, spin states, localization of charge and geometry of the intermediates were explored. Calculations were performed using Gaussian 09 with a B3P86/6-31+G(d) basis set and a CPCM water solvation model. Of particular interest were our findings that the first reduction occurs at the nickel rather than through non-innocent ligands and that water coordination is not favourable although protonation of the pyridyl nitrogen causes dechelation. Sequential and concerted proton coupled electron transfer were considered in the formation of the hydride.

Electrochemical-driven hydrogen evolution from acetic acid and water catalyzed by a mixed-valent CoII–CoI complex with high turnover frequency

The reaction of 2,3-bis(2-hydroxybenzylideneimino)-2,3-butenedinitrile (H2L1) and CoCl2·6H2O affords a mixed-valent dinuclear CoII–CoI complex [CoIL(py)3][CoIIL2] 1 (L = 2-(2-hydroxybenzylideneimino)-butenedinitrile ion), a new molecular electrocatalyst, which has been determined by X-ray crystallography and XPS spectrum. Electrochemical studies indicate that the catalyst is the first CoII–CoI complex species, that is among the most rapid homogeneous water-reduction catalysts, with a turnover frequency (TOF) of 2387 mol of hydrogen per mole of catalyst per hour at an overpotential of 787 mV vs SHE (pH 7.0). Sustained proton reduction catalysis occurs at glassy carbon (GC) to give H2 over a 46 h electrolysis period with 97% Faradaic yield and no observable decomposition of the catalyst.

Mechanism of photocatalytic hydrogen generation by a polypyridyl-based cobalt catalyst in aqueous solution.

The mechanism of photocatalytic hydrogen production was studied with a three-component system consisting of fac-[Re(py)(CO)3bipy](+) (py = pyridine, bipy = 2,2'-bipyridine) as photosensitizer, [Co(TPY-OH)(OH2)](2+) (TPY-OH = 2-bis(2-pyridyl)(hydroxy)methyl-6-pyridylpyridine), a polypyridyl-based cobalt complex, as water reduction catalyst (WRC), and triethanolamine (TEOA) as sacrificial electron donor in aqueous solution. A detailed mechanistic picture is provided, which covers all processes from excited state quenching on the time scale of a few nanoseconds to hydrogen release taking place between seconds and minutes at moderately basic reaction conditions. Altogether these processes span 9 orders of magnitude in time. The following reaction sequence was found to be the dominant pathway for hydrogen generation: After reductive quenching by TEOA, the reduced photosensitizer (PS) transfers an electron to the Co(II)-WRC. Protonation of Co(I) yields Co(III)H which is reduced in the presence of excess Co(I). Co(II)H releases hydrogen after a second protonation step, which is detected time-resolved by a clark-type hydrogen electrode. Aside from these productive steps, the role of side and back reactions involving TEOA-derived species is assessed, which is particularly relevant in laser flash photolysis measurements with significantly larger transient concentrations of reactive species as compared to continuous photolysis experiments. Most notable is an equilibrium reaction involving Co(I), which is explained by a nucleophilic addition of Co(I) to the oxidation product of TEOA, an electrophilic iminium ion. Quantum chemical calculations indicate that the reaction is energetically feasible. The calculated spectra of the adduct are consistent with the spectroscopic observations.

A molecular copper catalyst for electrochemical water reduction with a large hydrogen-generation rate constant in aqueous solution.

The copper complex [(bztpen)Cu](BF4)2 (bztpen=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen-generation rate constant (k(obs)) of over 10000 s(-1). A turnover frequency (TOF) of 7000 h(-1) cm(-2) and a Faradaic efficiency of 96% were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu](2+) in pH 2.5 buffer solution at -0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton-coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu](2+).

A Molecular Copper Catalyst for Electrochemical Water Reduction with a Large Hydrogen‐Generation Rate Constant in Aqueous Solution

AbstractThe copper complex [(bztpen)Cu](BF4)2 (bztpen=N‐benzyl‐N,N′,N′‐tris(pyridin‐2‐ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen‐generation rate constant (kobs) of over 10000 s−1. A turnover frequency (TOF) of 7000 h−1 cm−2 and a Faradaic efficiency of 96 % were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu]2+ in pH 2.5 buffer solution at −0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton‐coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu]2+.

Functionalized membranes for photocatalytic hydrogen production.

Functionalized vesicles for photocatalytic hydrogen production in water have been prepared by co-embedding of amphiphilic photosensitizers and a hydrogen-evolving catalyst in phospholipid membranes. The self-assembly allows a simple two-dimensional arrangement of the multicomponent system with close spatial proximity, which gave turnover numbers up to 165 for the incorporated amphiphilic cobaloxime water reduction catalyst 3 b under optimized conditions in purely aqueous solution. Superior photocatalytic activity in fluid membranes indicates that mobility and dynamic reorganization of catalytic subunits in the membrane promote the visible-light-driven hydrogen production. The functionalized membranes represent nanostructured assemblies for hydrogen production in aqueous solution mimicking natural photosynthesis.

A dinuclear copper(II) electrocatalyst both water reduction and oxidation

Splitting water is a key challenge in the production of chemical fuels from electricity. Although several catalysts have been developed for these reactions, substantial challenges remain towards the ultimate goal of an efficient, inexpensive and robust electrocatalyst. Until now, there is as yet no report on both water oxidation and reduction by identical catalyst. Reported here is the first soluble copper-based catalyst, Cu(Me2oxpn)Cu(OH)2] 1 (Me2oxpn: N,N′-bis(2,2′-dimethyl-3-aminopropyl)oxamido) for both electrolytic water oxidation and reduction. Water oxidation occurs at an overpotential of 636 mV vs SHE to give O2 with a turnover frequency (TOF) of ∼2.14 s−1. Electrochemical studies also indicate that 1 is a soluble molecular species, that is among the most rapid homogeneous water reduction catalysts, with a TOF of 654 mol of hydrogen per mole of catalyst per hour at an overpotential of 789 mV vs SHE (pH 7.0). Sustained water reduction catalysis occurs at glassy carbon (GC) to give H2 over a 32 h electrolysis period with 95% Faradaic yield and no observable decomposition of the catalyst.

Copper-based water reduction catalysts for efficient light-driven hydrogen generation

Although largely neglected, convenient and commercially available copper salts constitute appropriate precursors for efficient and stable water reduction catalysts (WRC). Various copper salts have been applied in the photocatalytic proton reduction together with [Ir(ppy)2(bpy)]PF6 as photosensitizer (PS) and triethylamine (TEA) as electron donor. Among them, copper(I) iodide showed the best productivity, resulting in a TONCu of 260 and a stability of 4 days. The addition of 2,2′-bipyridine (bpy) led to a significant improvement of the catalytic system. Application of 2equiv. of bpy and an increased amount of PS improved the stability as well as the productivity (TONCu 711). Remarkably, this catalytic system was reactivated seven times with only further addition of fresh PS and TEA. Consequently, this copper-based WRC was shown to be active for more than 48 days reaching a maximum TONCu of >3900, which is in the same order of magnitude as previously known noble metal WRCs. Aberration-corrected HAADF-STEM measurements revealed a part transformation of the molecularly-defined copper precursor into Cu containing nanoparticles of 5–10nm size. Additionally, by XAS measurements the conversion of the copper(I) salts into Cu(0) compounds under light irradiation was observed. With respect to low costs and high abundance of copper these WRCs are an appropriate alternative for noble metal based catalysts.

A Bismuth Vanadate–Cuprous Oxide Tandem Cell for Overall Solar Water Splitting

Through examination of the optoelectronic and photoelectrochemical properties of BiVO4 and Cu2O photoelectrodes, we evaluate the feasibility of a BiVO4/Cu2O photoanode/photocathode tandem cell for overall unassisted solar water splitting. Using state-of-the-art photoelectrodes we identify current-matching conditions by altering the photoanode active layer thickness. By further employing water oxidation and reduction catalysts (Co-Pi and RuOx, respectively) together with an operating point analysis, we show that an unassisted solar photocurrent density on the order of 1 mA cm(-2) is possible in a tandem cell and moreover gain insight into routes for improvement. Finally, we demonstrate the unassisted 2-electrode operation of the tandem cell. Photocurrents corresponding to ca. 0.5% solar-to-hydrogen conversion efficiency were found to decay over the course of minutes because of the detachment of the Co-Pi catalyst. This aspect provides a fundamental challenge to the stable operation of the tandem cell with the currently employed catalysts.

Band gap engineering by lanthanide doping in the photocatalyst LaOF: First-principles study

Recent experiment [Xie et al., Catal. Commun., 27, 21 (2012)] reported that LaOF is an active catalyst for water reduction: the catalytic activity per surface area of LaOF is about ten times higher than that of anatase TiO 2. First-principles density functional theory (DFT) calculations have been performed on Ln-doped LaOF (Ln = Ce , Pr , Nd and Pm ) to evaluate the effect of lanthanide doping on the electronic and optical properties. It is found that the lowest conduction band (CB) edge potential of LaOF is less than zero (versus normal hydrogen electrode (NHE)), confirming it has enough driving force for photocatalytic water splitting. The band gap of LaOF could be reduced significantly by lanthanide doping. Electronic structure analysis shows that the impurity states appear deep inside the band gap of LaOF, which is in favor of the separation center of photogenerated carriers due to large effective mass differences between electron and hole. Moreover, doping both Pm and Nd into LaOF is an effective approach to extend the optical absorption edge to the visible light. These findings suggest that LaOF doped with lanthanide element is a promising candidate for the photocatalytic hydrogen generation from water and pollutant decomposition.

Hydrogen Photogeneration Promoted by Efficient Electron Transfer from Iridium Sensitizers to Colloidal MoS2 Catalysts

We report the utilization of colloidal MoS2 nanoparticles (NPs) for multicomponent photocatalytic water reduction systems in cooperation with a series of cyclometalated Ir(III) sensitizers. The effects of the particle size and particle dispersion of MoS2 NPs catalyst, reaction solvent and the concentration of the components on hydrogen evolution efficiency were investigated. The MoS2 NPs exhibited higher catalytic performance than did other commonly used water reduction catalysts under identical experiment conditions. The introduction of the carboxylate anchoring groups in the iridium complexes allows the species to be favorably chem-adsorbed onto the MoS2 NPs surface to increase the electron transfer, resulting in enhancement of hydrogen evolution relative to the non-attached systems. The highest apparent quantum yield, which was as high as 12.4%, for hydrogen evolution, was obtained (λ = 400 nm).

Proton Transport in Clostridium pasteurianum [FeFe] Hydrogenase I: A Computational Study

To better understand the proton transport through the H2 production catalysts, the [FeFe] hydrogenases, we have undertaken a modeling and simulation study of the proton transfer processes mediated by amino acid side-chain residues in hydrogenase I from Clostridium pasteurianum. Free-energy calculation studies show that the side chains of two conserved glutamate residues, Glu-279 and Glu-282, each possess two stable conformations with energies that are sensitive to protonation state. Coordinated conformational changes of these residues can form a proton shuttle between the surface Glu-282 and Cys-299, which is the penultimate proton donor to the catalytic H-cluster. Calculated acid dissociation constants are consistent with a proton relay connecting the H-cluster to the bulk solution. The complete proton-transport process from the surface-disposed Glu-282 to Cys-299 is studied using coupled semiempirical quantum-mechanical/classical-mechanical dynamics. Two-dimensional free-energy maps show the mechanisms of proton transport, which involve Glu-279, Ser-319, and a short internal water relay to connect functionally Glu-282 with the H-cluster. The findings of conformational bistability, PT event coupling with pKa mismatch, and water participation have implications in the design of artificial water reduction or general electrocatalytic H2-production catalysts.

A Noble‐Metal‐Free System for Photocatalytic Hydrogen Production from Water

A series of heteroleptic copper(I) complexes with bidentate PP and NN chelate ligands was prepared and successfully applied as photosensitizers in the light-driven production of hydrogen, by using [Fe3(CO)12] as a water-reduction catalyst (WRC). These systems efficiently reduces protons from water/THF/triethylamine mixtures, in which the amine serves as a sacrificial electron donor (SR). Turnover numbers (for H) up to 1330 were obtained with these fully noble-metal-free systems. The new complexes were electrochemically and photophysically characterized. They exhibited a correlation between the lifetimes of the MLCT excited state and their efficiency as photosensitizers in proton-reduction systems. Within these experiments, considerably long excited-state lifetimes of up to 54 μs were observed. Quenching studies with the SR, in the presence and absence of the WRC, showed that intramolecular deactivation was more efficient in the former case, thus suggesting the predominance of an oxidative quenching pathway.

Bioinspired Photocatalytic Water Reduction and Oxidation with Earth-Abundant Metal Catalysts

The development of efficient water reduction and oxidation catalysts is the key issue for achieving solar energy conversion to sustainable energy sources to replace of fossil fuels. Platinum group metal (PGM) catalysts have been recognized as active catalysts for both water reduction and oxidation. However, it is highly desired to replace precious and scarce PGM catalysts by earth-abundant metal catalysts for water reduction and oxidation. In the past 5 years, there has been significant progress in the development of water reduction and oxidation catalysts based on earth-abundant metals such as iron, nickel, copper, and manganese, which have been combined with organic photocatalysts. This work describes the state of the art and future challenges in bioinspired photocatalytic water reduction and oxidation with earth-abundant metals.