Zinc oxide nanoflakes supported copper oxide nanosheets as a bifunctional electrocatalyst for OER and HER in an alkaline medium.

Bifunctional electrocatalysts are the attractive research in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in the overall water-splitting reactions. The design and development of the cost-effective OER/HER bifunctional electrocatalysts with superior catalytic activity are still remaining as the big challenges. Herein, we have developed the CuO-ZnO nanocomposite as a bifunctional OER/HER electrocatalyst via simple chemical precipitation method. The nanocomposite was investigated for its crystalline structure, surface morphology and the functions of elements using XRD, FT-IR, SEM, TEM and XPS characterization techniques, respectively. The nanocomposite exhibited the excellent activity for the overall water-splitting in an alkaline medium. The CuO-ZnO nanocomposite showed the less onset potential of 1.4 and 0.15 V versus RHE in 1M KOH (Tafel slopes value of 0.180 and 0.400 V dec-1) for OER and HER, respectively. Hence, the as-prepared bifunctional electrocatalyst displayed the high stability for 10 h in the water electrolysis processes.

Ultrafine Kaolinite Removal in Recycled Water from the Overflow of Thickener Using Electroflotation: A Novel Application of Saline Water Splitting in Mineral Processing.

The presence of ultrafine clay particles that are difficult to remove by conventional filtration creates many operational problems in mining processing systems. In this work, the removal of clay suspensions has been investigated using an electroflotation (EF) process with titanium electrodes. The results show that EF is a viable and novel alternative for removing ultrafine particles of kaolinite-type clay present in sedimentation tank overflows with low salt concentrations (<0.1 mol/L) in copper mining facilities based on the saline water splitting concept. Maximum suspended solid removal values of 91.4 and 83.2% in NaCl and KCl solutions, respectively, were obtained under the experimental conditions of the constant applied potential of 20 V/SHE, salinity concentration of 0.1 mol/L, and electroflotation time of 10 and 20 min in NaCl and KCl solutions, respectively. Furthermore, the visual evidence of particle aggregation by flocculation during the experiments indicates a synergy between EF and electrocoagulation (EC) that enhances the removal of ultrafine particles of kaolinite.

CVD Growth of Hematite Thin Films for Photoelectrochemical Water Splitting: Effect of Precursor-Substrate Distance on Their Final Properties.

The development of photoelectrode materials for efficient water splitting using solar energy is a crucial research topic for green hydrogen production. These materials need to be abundant, fabricated on a large scale, and at low cost. In this context, hematite is a promising material that has been widely studied. However, it is a huge challenge to achieve high-efficiency performance as a photoelectrode in water splitting. This paper reports a study of chemical vapor deposition (CVD) growth of hematite nanocrystalline thin films on fluorine-doped tin oxide as a photoanode for photoelectrochemical water splitting, with a particular focus on the effect of the precursor-substrate distance in the CVD system. A full morphological, structural, and optical characterization of hematite nanocrystalline thin films was performed, revealing that no change occurred in the structure of the films as a function of the previously mentioned distance. However, it was found that the thickness of the hematite film, which is a critical parameter in the photoelectrochemical performance, linearly depends on the precursor-substrate distance; however, the electrochemical response exhibits a nonmonotonic behavior. A maximum photocurrent value close to 2.5 mA/cm2 was obtained for a film with a thickness of around 220 nm under solar irradiation.

A New Supramolecular Tetraruthenated Cobalt (II) Porphyrazine Displaying Outstanding Electrocatalytical Performance in Oxygen Evolution Reaction.

A new supramolecular electrocatalyst for Oxygen Evolution Reaction (OER) was synthesized from a central multibridging cobalt tetrapyridylporphyrazine (CoTPyPz) species by attaching four [Ru(bpy)2Cl]+ groups. Both CoTPyPz and the tetraruthenated cobalt porphyrazine species, TRuCoTPyPz, form very homogenous molecular films just by dropcasting their methanol solutions onto GCE electrodes. Such films exhibited low overpotentials for O2 evolution, e.g., 560 e 340 mV, respectively, displaying high stability, typically exceeding 15 h. The kinetic parameters obtained from the Tafel plots showed that the peripheral complexes are very important for the electrocatalytic activity. Hyperspectral Raman images taken along the electrochemical process demonstrated that the cobalt center is the primary active catalyst site, but its performance is enhanced by the ruthenium complexes, which act as electron-donating groups, in the supramolecular system.

Ternary Oxide CuWO4/BiVO4/FeCoOx Films for Photoelectrochemical Water Oxidation: Insights into the Electronic Structure and Interfacial Band Alignment.

Photoelectrochemical (PEC) water oxidation using ternary oxide systems has been considered a promising approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxide-based CuWO4/BiVO4/FeCoOx film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO4/BiVO4 photoanode exhibits a significant photocurrent density of 0.82 mA cm-2 at 1.23 V vs RHE under AM 1.5G illumination, which is a record 382% increase compared to that of the bare CuWO4 film. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoOx cocatalyst, resulting in a triple CuWO4/BiVO4/FeCoOx heterojunction with a significant reduction in onset potential and a 500% increase in the photocurrent density of bare CuWO4. Experimental and theoretical approaches were used to provide insights into the interfacial band alignment and photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO4/BiVO4 heterojunction, potentially limiting its application in tandem systems. Conversely, the deposition of the FeCoOx ultrathin layer over the CuWO4/BiVO4 heterojunction induces a p-n junction on the BiVO4/FeCoOx interface, which, when combined with the abundant FeCoOx oxygen vacancies, results in improved charge separation and transport as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunction systems and introduces simple tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.

Biomimetic Catalysts Based on Au@TiO2-MoS2-CeO2 Composites for the Production of Hydrogen by Water Splitting.

The photocatalytic hydrogen evolution reaction (HER) by water splitting has been studied, using catalysts based on crystalline TiO2 nanowires (TiO2NWs), which were synthesized by a hydrothermal procedure. This nanomaterial was subsequently modified by incorporating different loadings (1%, 3% and 5%) of gold nanoparticles (AuNPs) on the surface, previously exfoliated MoS2 nanosheets, and CeO2 nanoparticles (CeO2NPs). These nanomaterials, as well as the different synthesized catalysts, were characterized by electron microscopy (HR-SEM and HR-TEM), XPS, XRD, Raman, Reflectance and BET surface area. HER studies were performed in aqueous solution, under irradiation at different wavelengths (UV-visible), which were selected through the appropriate use of optical filters. The results obtained show that there is a synergistic effect between the different nanomaterials of the catalysts. The specific area of the catalyst, and especially the increased loading of MoS2 and CeO2NPs in the catalyst substantially improved the H2 production, with values of ca. 1114 µm/hg for the catalyst that had the best efficiency. Recyclability studies showed only a decrease in activity of approx. 7% after 15 cycles of use, possibly due to partial leaching of gold nanoparticles during catalyst use cycles. The results obtained in this research are certainly relevant and open many possibilities regarding the potential use and scaling of these heterostructures in the photocatalytic production of H2 from water.

Jahn-Teller Effects in a Vanadate-Stabilized Manganese-Oxo Cubane Water Oxidation Catalyst.

Heuristic rules that allow identifying the preferred mixed-valence isomers and Jahn-Teller axis arrangements in the water oxidation catalyst [(Mn4 O4 )(V4 O13 )(OAc)3 ]n- and its activated form [(Mn4 O4 )(V4 O13 )(OAc)2 (H2 O)(OH)]n- are derived. These rules are based on computing all combinatorially possible mixed-valence isomers and Jahn-Teller axis arrangements of the MnIII atoms, and associate energetic costs with some structural features, like crossings of multiple Jahn-Teller axes, the location of these axes, or the involved ligands. It is found that the different oxidation states localize on different Mn centers, giving rise to clear Jahn-Teller distortions, unlike in previous crystallographic findings where an apparent valence delocalization was found. The low barriers that connect different Jahn-Teller axis arrangements suggest that the system quickly interconverts between them, leading to the observation of averaged bond lengths in the crystal structure. We conclude that the combination of cubane-vanadate bonds that are chemically inert, cubane-acetate/water bonds that can be activated through a Jahn-Teller axis, and low activation barriers for intramolecular rearrangement of the Jahn-Teller axes plays an essential role in the reactivity of this and probably related compounds.

Spectral Signatures of Oxidation States in a Manganese-Oxo Cubane Water Oxidation Catalyst.

We report IR and UV/Vis spectroscopic signatures that allow discriminating between the oxidation states of the manganese-based water oxidation catalyst [(Mn4 O4 )(V4 O13 )(OAc)3 ]3- . Simulated IR spectra show that V=O stretching vibrations in the 900-1000 cm-1 region shift consistently by about 20 cm-1 per oxidation equivalent. Multiple bands in the 1450-1550 cm-1 region also change systematically upon oxidation/reduction. The computed UV/Vis spectra predict that the spectral range above 350 nm is characteristic of the managanese-oxo cubane oxidation state, whereas transitions at higher energy are due to the vanadate ligand. The presence of absorption signals above 680 nm is indicative of the presence of MnIII atoms. Spectroelectrochemical measurements of the oxidation from [Mn 2 III Mn 2 IV ] to [Mn 4 IV ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.

Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films.

With a view to developing multimetallic molecular catalysts that mimic the oxygen-evolving catalyst (OEC) in Nature's photosystem II, the synthesis of various dicubanoid manganese clusters is described and their catalytic activity investigated for water oxidation in basic, aqueous solution. Pyridinemethanol-based ligands are known to support polynuclear and cubanoid structures in manganese coordination chemistry. The chelators 2,6-pyridinedimethanol (H2 L1 ) and 6-methyl-2-pyridinemethanol (HL2 ) were chosen to yield polynuclear manganese complexes; namely, the tetranuclear defective dicubanes [MnII 2 MnIII 2 (HL1 )4 (OAc)4 (OMe)2 ] and [MnII 2 MnIII 2 (HL1 )6 (OAc)2 ] (OAc)2 ⋅2 H2 O, as well as the octanuclear-dicubanoid [MnII 6 MnIII 2 (L2 )4 (O)2 (OAc)10 (HOMe/OH2 )2 ]⋅3MeOH⋅MeCN. In freshly prepared solutions, polynuclear species were detected by electrospray ionization mass spectrometry, whereas X-band electron paramagnetic resonance studies in dilute, liquid solution suggested the presence of divalent mononuclear Mn species with g values of 2. However, the magnetochemical investigation of the complexes' solutions by the Evans technique confirmed a haphazard combination of manganese coordination complexes, from mononuclear to polynuclear species. Subsequently, the newly synthesized and characterized manganese molecular complexes were employed as precursors to prepare electrode-deposited films in a buffer-free solution to evaluate and compare their stability and catalytic activity for water oxidation electrocatalysis.

Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt.

Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrites such as Cu(In, Ga)(Se,S)2 (CIGS) and CuIn(Se,S)2 (CIS) present excellent semiconductor characteristics for this purpose, but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demanding improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material, but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present Minireview highlighted the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.

The Use of Anodic Oxides in Practical and Sustainable Devices for Energy Conversion and Storage.

This review addresses the main contributions of anodic oxide films synthesized and designed to overcome the current limitations of practical applications in energy conversion and storage devices. We present some strategies adopted to improve the efficiency, stability, and overall performance of these sustainable technologies operating via photo, photoelectrochemical, and electrochemical processes. The facile and scalable synthesis with strict control of the properties combined with the low-cost, high surface area, chemical stability, and unidirectional orientation of these nanostructures make the anodized oxides attractive for these applications. Assuming different functionalities, TiO2-NT is the widely explored anodic oxide in dye-sensitized solar cells, PEC water-splitting systems, fuel cells, supercapacitors, and batteries. However, other nanostructured anodic films based on WO3, CuxO, ZnO, NiO, SnO, Fe2O3, ZrO2, Nb2O5, and Ta2O5 are also explored and act as the respective active layers in several devices. The use of AAO as a structural material to guide the synthesis is also reported. Although in the development stage, the proof-of-concept of these devices demonstrates the feasibility of using the anodic oxide as a component and opens up new perspectives for the industrial and commercial utilization of these technologies.

Unveiling the Occurrence of Co(III) in NiCo Layered Electroactive Hydroxides: The Role of Distorted Environments.

Co- and Ni-based layered hydroxides constitute a unique class of two-dimensional inorganic materials with exceptional chemical diversity, physicochemical properties and outstanding performance as supercapacitors and overall water splitting catalysts. Recently, the occurrence of Co(III) in these phases has been proposed as a key factor that enhance their electrochemical performance. However, the origin of this centers and control over its contents remains as an open question. We employed the Epoxide Route to synthesize a whole set of α-NiCo layered hydroxides. The PXRD and XAS characterization alert about the occurrence of Co(III) as a consequence of the increment in the Ni content. DFT+U simulation suggest that the shortening of the Co-O distance promotes a structural distortion in the Co environments, resulting in a double degeneration in the octahedral Co 3d orbitals. Hence, a strong modification of the electronic properties leaves the system prone to oxidation, by the appearance of Co localized electronic states on the Fermi level. This work combines a microscopic interpretation supported by a multiscale crystallochemical analysis, regarding the so-called synergistic redox behavior of Co and Ni, offering fundamental tools for the controllable design of highly efficient electroactive materials. To the best of our knowledge, this is the first computational-experimental investigation of the electronic and structural details of α-NiCo hydroxides, laying the foundation for the fine tuning of electronic properties in layered hydroxides.

NiMo-NiCu Inexpensive Composite with High Activity for Hydrogen Evolution Reaction.

In this work, the effect of copper addition on NiMo coating is evaluated in regard to the hydrogen evolution reaction (HER). NiMo and NiMo-NiCu composites are prepared by a simple coelectrodeposition process. The effect of Cu on deposit characters were tested by varying it in the range of 0.06-0.20 molar ratio. Copper addition promotes the growth of a new crystalline phase: NiCu. Also, the copper addition changed the composite surface. NiMo-NiCu0.12 shows a surface roughness 30 times higher than the NiMo material. NiMo-NiCu materials present higher activity toward HER, larger electroactive area, and higher stability in continuous water electrolysis than NiMo catalysts, as demonstrated by Tafel curves, electrochemical impedance spectroscopy measurements, and polarization tests. The combination of the large electroactive area due to the copper addition, the synergism between Ni-Mo, and the presence of Ni and Mo oxides on the surface results in catalyst with excellent features for HER application.

Tailoring a Zinc Oxide Nanorod Surface by Adding an Earth-Abundant Cocatalyst for Induced Sunlight Water Oxidation.

Herein, a detailed investigation of the surface modification of a zinc oxide (ZnO) nanorod electrode with FeOOH nanoparticles dispersed in glycine was conducted to improve the water oxidation reaction assisted by sunlight. The results were systematically analysed in terms of the general parameters (light absorption, charge separation, and surface for catalysis) that govern the photocurrent density response of metal oxide as photoanode in a photoelectrochemical (PEC) cell. ZnO electrodes surface were modified with different concentration of FeOOH nanoparticles using the spin-coating deposition method, and it was found that 6-layer deposition of glycine-FeOOH nanoparticles is the optimum condition. The glycine plays an important role decreasing the agglomeration of FeOOH nanoparticles over the ZnO electrode surface and increasing the overall performance. Comparing bare ZnO electrodes with the ones modified with glycine-FeOOH nanoparticles an enhanced photocurrent density can be observed from 0.27 to 0.57 mA/cm2 at 1.23 VRHE under sunlight irradiation. The impedance spectroscopy data aid us to conclude that the higher photocurrent density is an effect associated with more efficient surface for chemical reaction instead of electronic improvement. Nevertheless, the charge separation efficiency remains low for this system. The present discovery shows that the combination of glycine-FeOOH nanoparticle is suitable and environmentally-friend cocatalyst to enhance the ZnO nanorod electrode activity for the oxygen evolution reaction assisted by sunlight irradiation.

Bioinspired cobalt cubanes with tunable redox potentials for photocatalytic water oxidation and CO2 reduction.

The development of efficient, robust and earth-abundant catalysts for photocatalytic conversions has been the Achilles' heel of solar energy utilization. Here, we report on a chemical approach based on ligand designed architectures to fabricate unique structural molecular catalysts coupled with appropriate light harvesters (e.g., carbon nitride and Ru(bpy)32+) for photoredox reactions. The "Co4O4" cubane complex Co4O4(CO2Me)4(RNC5H4)4 (R = CN, Br, H, Me, OMe), serves as a molecular catalyst for the efficient and stable photocatalytic water oxidation and CO2 reduction. A comprehensive structure-function analysis emerged herein, highlights the regulation of electronic characteristics for a molecular catalyst by selective ligand modification. This work demonstrates a modulation method for fabricating effective, stable and earth-abundant molecular catalysts, which might facilitate further innovation in the function-led design and synthesis of cubane clusters for photoredox reactions.

Bismuth Vanadate Photoelectrodes with High Photovoltage as Photoanode and Photocathode in Photoelectrochemical Cells for Water Splitting.

Using dual-photoelectrode photoelectrochemical (PEC) devices based on earth-abundant metal oxides for unbiased water splitting is an attractive means of producing green H2 fuel, but is challenging, owing to low photovoltages generated by PEC cells. This problem can be solved by coupling n-type BiVO4 with n-type Bi4 V2 O11 to create a virtual p/n junction due to the formation of a hole-inversion layer at the semiconductor interface. Thus, photoelectrodes with high photovoltage outputs were synthesized. The photoelectrodes exhibited features of p- and n-type semiconductors when illuminated under an applied bias, suggesting their use as photoanode and photocathode in a dual-photoelectrode PEC cell. This concept was proved by connecting a 1 mol % W-doped BiVO4 /Bi4 V2 O11 photoanode with an undoped BiVO4 /Bi4 V2 O11 photocathode, which produced a high photovoltage of 1.54 V, sufficient to drive stand-alone water splitting with 0.95 % efficiency.

In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts.

The Co4O4 cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV)2 state may be captured in a Co(III)2(IV)2 cubane. We demonstrate that the Co(III)2(IV)2 cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III)2(IV)2 cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV)2 dimer. The exchange coupling in the cofacial Co(IV)2 site allows for parallels to be drawn between the electronic structure of the Co4O4 cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)2 center on O-O bond formation.