Catalytic Water Splitting Research Articles

Probing the Redox States at the Surface of Electroactive Nanoporous NiO Thin Films

Nanoporous NiO thin film electrodes were obtained via plasma-assisted microwave sintering and characterized by means of a combination of electrochemical techniques and X-ray photoelectron spectroscopy (XPS). The aim of this study is the elucidation of the nature of the surface changes introduced by the redox processes of this nanostructured material. NiO undergoes two distinct electrochemical processes of oxidation in aqueous electrolyte with the progress of NiO anodic polarization. These findings are consistent with the sequential formation of oxyhydroxide species at the surface, the chemical nature of which was assessed by XPS. Electronic relaxation effects in the Ni 2p spectra clearly indicated that the superficial oxyhydroxide species resulted to be β-NiOOH and γ-NiOOH. We also show for the first time spectral evidence of an electrochemically generated Ni(IV) species. This study has direct relevance for those applications in which NiO electrodes are utilized in aqueous electrolyte, namely catalytic water splitting or electrochromism, and may constitute a starting point for the comprehension of electronic phenomena at the NiO/organic electrolyte interface of cathodic dye-sensitized solar cells (p-DSCs).

Fabrication of an oxysulfide of bismuth Bi2O2S and its photocatalytic activity in a Bi2O2S/In2O3 composite

Abstract Oxide-chalcogenide semiconductors are proposed in order to attain a smaller band gap and improve efficiencies in comparison to oxides for light induced total water splitting. Dibismuthoxysulfide (Bi2O2S) was synthesized under relatively mild hydrothermal conditions. The synthesized compound was characterized by XRD, SEM and UV–vis DRS techniques. The band gap of 1.5 eV measured is smaller than 2.8 eV of bismuth oxide (Bi2O3). Photoelectrochemical studies showed that it is possible to utilize Bi2O2S and the Bi2O2S/In2O3 composite as n-type semiconductors. Dye modified Bi2O2S/In2O3 showed promise for the catalysis of water splitting.

Efficient photocatalytic hydrogen production from water over a CuO and carbon fiber comodified TiO2 nanocomposite photocatalyst

An efficient composite catalyst, CuO/CF/TiO2 (where CF represents catalytic grown carbon fiber and TiO2 is the commercial product P25), is prepared with a wet impregnation and calcination process and is applied in the photo stimulated catalytic water splitting. The modification of TiO2 with CuO, serving as the H2 evolution cocatalyst, and carbon fiber, behaving as the electron transporter, enhances greatly the overall activity of the material. The activity of CuO/CF/TiO2 with 1 wt% CF is 45 times higher than that of TiO2 and 2 times higher than that of CuO/TiO2. CF and CuO play a synergistic role in decreasing the recombination rate of the photogenerated carriers in TiO2 bulk phase. The CF in the composite catalyst extends the reaction range, and makes the reaction proceeds on both the surface of CuO intimately contacting with TiO2 particles and the surfaces of the CuO particles which do not in contact with TiO2 but in contact with CF.

A New Way To Split Water

To meet future energy needs, scientists would like to find sustainable ways to generate hydrogen for use in fuel cells and to make liquid fuels. As an alternative to other H2 sources, such as processing of biomass and electrolysis of water, scientists are developing a thermal water-splitting method that would use heat from solar arrays and metal oxide catalysts. But the classic thermal approach has required repetitive heating and cooling, which wastes energy. Charles B. Musgrave, Alan W. Weimer, and coworkers at the University of Colorado, Boulder, have now sidestepped the temperature-cycling problem by demonstrating the first constant-temperature version of the technique (Science 2013, DOI: 10.1126/science.1239454). The work “offers the potential for reaching higher solar-to-fuel energy conversion efficiencies” to make solar-based fuels at large scale and competitive costs, says solar technology expert Aldo Steinfeld of ETH Zurich. In temperature-swing catalytic water splitting, a metal oxide catalyst is...

Enhancing the activity of a SiC–TiO2 composite catalyst for photo-stimulated catalytic water splitting

In this work, effort has been made to design an efficient catalyst for the photo-stimulated water splitting reaction, starting with the modification of TiO2 (P25) to enhance its activity. A SiC(1 wt%)–TiO2 composite material shows an activity as high as twice of that of TiO2. NiOx, an electron collector, promotes the activity of TiO2, while IrO2, a hole capturer, enhances the hydrogen evolution rate of SiC. A SiC(1 wt%)–NiOx/TiO2 three-component and an IrO2/SiC(1 wt%)–NiOx/TiO2 four-component composite materials produce 30% and 100% more H2 than the NiOx/TiO2 catalyst during the first 5 h, respectively, with ethanol used as the sacrificial reagent. Furthermore, the SiC(1 wt%)–NiOx/TiO2 catalyst is active under visible light, while the NiOx/TiO2 catalyst shows no activity under the same irradiation condition. 3C-SiC has a narrow band gap and its band edge well compensates that of the TiO2. The enhancing effect of dopants on the SiC(1 wt%)–TiO2 composite material is sensitive to the location of the modifiers, which further proves that an efficient separation of the charge carriers is crucial to the overall activity of the composite catalyst.

Surface‐Immobilized Single‐Site Iridium Complexes for Electrocatalytic Water Splitting

Water into oxygen: Mono-iridium complexes (see picture; L=PO3H2 or COOH) were immobilized on an indium tin oxide (ITO) surface to form a molecular electrocatalytic water oxidation assembly that mimics photosystem II in producing molecular oxygen with high turnover numbers (TONs). The catalyst shows TONs for O2 higher than 210 000 and turnover frequencies higher than 6.7 s−1 during electrochemical catalytic water splitting.

ChemInform Abstract: The Past, Present and Future of Heterogeneous Catalysis

AbstractReview: 459 refs.

Water Adsorption on Na/Cu(111): State-Specific Coupling with Quantum Well States

The adsorption of water molecules on sodium-covered Cu(111) surfaces have been studied using first-principles calculations. Water molecules interact predominantly with the quantum well states of the Na overlayers, leading to charge transfer and reduction of the dissociation barrier. This interaction is state specific. At one monolayer Na coverage, the water molecule shows a maximum ratio between the adsorption energy and dissociation barrier, favoring the catalytic water splitting reaction. This conclusion is in accord with experimental observation and suggests the possibility for state-selective control of water–surface interactions.

Roles of manganese in photosystem II dynamics to irradiations and temperatures

The most amazing chemistry is the light-driven water splitting reaction occurred in the oxygen-evolving complex of phototsystem II in higher plants, green algae, and cyanobacteria. Mn, in the form of Mn4CaO5 cluster in photosystem II, is responsible for the catalytic water splitting reaction as well as plays roles in photosystem II dynamics to irradiation and temperatures. Manganese hypothesis of UV-initiated photoinhibition as a direct target is established, and thermal inactivation of photosystem II involves the valence and structural changes of manganese. Recent progresses in understanding the roles of manganese in photoinhibition especially under UV light and in thermal inactivation including elevated temperatures using synthetic models and native PS II complexes are summarized and evaluated. Potential problems and possible solutions are discussed and presented.

Biomimetic molecular water splitting catalysts for hydrogen generation

During the last few decades, the scientific community has been striving hard to develop new and alternative sources for renewable energy and fuel. Hydrogen or carbon free fuels obtained from catalytic water splitting using sunlight offer an attractive solution for a cleaner and greener future. In this pursuit, to establish effective molecular catalytic systems for efficient water oxidation is considered to be a bottleneck, hampering the design, implementation and exploitation of electrochemical and photo-electrochemical modular devices for light driven energy conversion into hydrogen-based storable fuels. From metal oxides to composite materials, noble metal complexes to transition metal organometallics, multinuclear to mono-site catalysts, various water oxidation complexes (WOCs) have been investigated both in a homogeneous environment and on surfaces in photo- or electrochemical conditions. However, a truly biomimetic catalytic system that matches the performance of photosystem-II for efficient water splitting, operating with four consecutive proton coupled electron transfer (PCET) steps to generate oxygen and hydrogen for hundred thousands of cycles at high rate is yet to be achieved. We here present an overview of biomimetic molecular complexes that have been investigated recently for water oxidation catalysis in homogeneous solution using chemical oxidants, or as heterogeneous species for catalytic electrochemical systems.

Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water

The development of water oxidation catalysts (WOCs) to replace costly noble metals in commercial electrolyzers and solar fuel cells is an unmet need that is preventing the global development of hydrogen fuel technologies. Two of the main challenges in realizing catalytic water splitting are lowering the substantial overpotential that is required to achieve practical operating current densities in the O2-evolving halfreaction at the anode, and the use of earth-abundant elements for the fabrication of inexpensive electrodes that are free from noble metals. To meet these challenges, molecular catalysts that are based upon the cubic CaMn4Ox core within photosystem II in photosynthetic organisms, which is the gold standard of catalytic efficiency, have begun to appear. Among solid-state materials, several noble-metal oxides, which include IrO2 and RuO2, are already in use in industrial electrolyzers, but are not globally scalable. Aqueous solutions of cobalt phosphate form water-oxidation catalysts under electrolysis and photolysis that are suitable for the fabrication of noncrystalline electrode materials. Nanocrystalline spinel-phase metal oxides (AM2O4, M= transition metals) that are comprised of M4O4 cubical subunits and are active water oxidation catalysts have been developed. The catalytic activity of the spinel Co3O4 has been reported for Co3O4 nanorods that are incorporated into SBA-15 silica, as well as Co3O4 nanoparticles that are adsorbed onto Ni electrodes. NiCo2O4 spinel also oxidizes water when the nanoparticles are electrophoretically deposited onto a Ni electrode. Reports that examined the effect of lithium doping on the surface of Co3O4 electrodes in solutions of KOH attributed the higher evolution rate of O2 to better electrical conductivity. However, the oxidation of water by Co3O4 was strongly dependent on crystallite size and surface area and frequently necessitates high overpotentials and alkaline conditions to accelerate the rate of reaction. In contrast, we recently reported that the catalytically inert spinel LiMn2O4 gives spinel l-MnO2, which is an active water oxidation catalyst, upon topotactic delithiation. Thus, the importance of removing the A-site lithium for catalysis by the cubic Mn4O4 core of spinels was revealed. [11]

Potential Pt-complexes for catalytic water splitting

Potential Pt-complexes for catalytic water splitting

Dendron-functionalized multiwalled carbon nanotubes incorporating polyoxometalates for water-splitting catalysis

Carbon nanotubes (CNTs) are versatile nanomaterials with applications spanning from medicinal chemistry and biology, to electronics as field effect transistors or energy as fuel cells. The major drawback stems from the CNT insolubility in most of the organic and aqueous media, which severely hampers the material processability. To overcome this problem, functionalization of CNTs is generally accomplished by either covalent strategies resulting in the modification of the CNT backbone via radical reactions, fluorination, and/or cycloaddition reactions, or noncovalent protocols, exploiting multiple weak interactions (hydrophobic, van der Waals, electrostatic) with suitable reagents. Herein, we highlight that a rewarding approach includes a combination of covalent/noncovalent methods, by a tailored synthetic modification of the CNT surface with polycationic dendrimeric chains, fostering the successive decoration with a multimetallic and polyanionic water oxidation catalyst. The outcome is a hybrid nanomaterial with unperturbed CNT electrical properties, in close contact with a unique multi-electron catalyst enabling electrocatalytic water splitting with high efficiency at low overpotentials.

A homologous series of structures on the surface of SrTiO3(110)

Strontium titanate is seeing increasing interest in fields ranging from thin-film growth to water-splitting catalysis and electronic devices. Although the surface structure and chemistry are of vital importance to many of these applications, theories about the driving forces vary widely. We report here a solution to the 3 x 1 SrTiO(3)(110) surface structure obtained through transmission electron diffraction and direct methods, and confirmed through density functional theory calculations and scanning tunnelling microscopy images and simulations, consisting of rings of six or eight corner-sharing TiO(4) tetrahedra. Further, by changing the number of tetrahedra per ring, a homologous series of n x 1 (n > or = 2) surface reconstructions is formed. Calculations show that the lower members of the series (n < or = 6) are thermodynamically stable and the structures agree with scanning tunnelling microscopy images. Although the surface energy of a crystal is usually thought to determine the structure and stoichiometry, we demonstrate that the opposite can occur. The n x 1 reconstructions are sufficiently close in energy for the stoichiometry in the near-surface region to determine which reconstruction is formed. Our results indicate that the rules of inorganic coordination chemistry apply to oxide surfaces, with concepts such as homologous series and intergrowths as valid at the surface as they are in the bulk.

Coupled electron transfers in artificial photosynthesis

Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET.

PEG–catalytic water splitting in the interface of a bipolar membrane

This paper investigates the effect of polyethylene glycol (PEG) on the water dissociation of bipolar membranes. To do this, bipolar membranes were prepared by immersing anion exchange membranes in different-concentration solutions of different-molecular-weight PEGs and then casting the solutions of sulfonated polyphenylene oxide (SPPO) on the anion exchange membranes. All the bipolar membranes with PEG in the interface are evaluated by current–voltage curves. The experimental results prove that PEG has excellent catalytic function for water dissociation. Furthermore, this function is enhanced by both PEG amount (PEG concentration) and PEG molecular weight in the interface of a bipolar membrane.