FeNi2S4-A Potent Bifunctional Efficient Electrocatalyst for the Overall Electrochemical Water Splitting in Alkaline Electrolyte.

For a carbon-neutral society, the production of hydrogen as a clean fuel through water electrolysis is currently of great interest. Since water electrolysis is a laborious energetic reaction, it requires high energy to maintain efficient and sustainable production of hydrogen. Catalytic electrodes can reduce the required energy and minimize production costs. In this context, herein, a bifunctional electrocatalyst made from iron nickel sulfide (FeNi2S4 [FNS]) for the overall electrochemical water splitting is introduced. Compared to Fe2NiO4 (FNO), FNS shows a significantly improved performance toward both OER and HER in alkaline electrolytes. At the same time, the FNS electrode exhibits high activity toward the overall electrochemical water splitting, achieving a current density of 10 mA cm-2 at 1.63 V, which is favourable compared to previously published nonprecious electrocatalysts for overall water splitting. The long-term chronopotentiometry test reveals an activation followed by a subsequent stable overall cell potential at around 2.12 V for 20 h at 100 mA cm-2.

Thermodynamically Stable Functionalization of Microporous Aromatic Frameworks with Sulfonic Acid Groups by Inserting Methylene Spacers.

Porous aromatic frameworks (PAFs) are an auspicious class of materials that allow for the introduction of sulfonic acid groups at the aromatic core units by post-synthetic modification. This makes PAFs promising for proton-exchange materials. However, the limited thermal stability of sulfonic acid groups attached to aromatic cores prevents high-temperature applications. Here, we present a framework based on PAF-303 where the acid groups were added as methylene sulfonic acid side chains in a two-step post-synthetic route (SMPAF-303) via the intermediate chloromethylene PAF (ClMPAF-303). Elemental analysis, NMR spectroscopy, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy were used to characterize both frameworks and corroborate the successful attachment of the side chains. The resulting framework SMPAF-303 features high thermal stability and an ion-exchange capacity of about 1.7 mequiv g-1. The proton conductivity depends strongly on the adsorbed water level. It reaches from about 10-7 S cm-1 for 33% RH to about 10-1 S cm-1 for 100% RH. We attribute the strong change to a locally alternating polarity of the inner surfaces. The latter introduces bottleneck effects for the water molecule and oxonium ion diffusion at lower relative humidities, due to electrolyte clustering. When the pores are completely filled with water, these bottlenecks vanish, leading to an unhindered electrolyte diffusion through the framework, explaining the conductivity rise.

Ultrafast Proton Conduction in an Aqueous Electrolyte Confined in Adamantane-like Micropores of a Sulfonated, Aromatic Framework.

Sulfonated, cross-linked porous polymers are promising frameworks for aqueous high-performance electrolyte-host systems for electrochemical energy storage and conversion. The systems offer high proton conductivities, excellent chemical and mechanical stabilities, and straightforward water management. However, little is known about mass transport mechanisms in such nanostructured hosts. We report on the synthesis and postsynthetic sulfonation of an aromatic framework (SPAF-2) with a 3D-interconnected nanoporosity and varying sulfonation degrees. Water adsorption produces the system SPAF-2H20. It features proton exchange capacities up to 6 mequiv g-1 and exceptional proton conductivities of about 1 S cm-1. Two contributions are essential for the highly efficient transport. First, the nanometer-sized pores link the charge transport to the diffusion of adsorbed water molecules, which is almost as fast as bulk water. Second, continuous exchange between interface-bound and mobile species enhances the conductivities at elevated temperatures. SPAF-2H20 showcases how to tailor nanostructured electrolyte-host systems with liquid-like conductivities.

Topochemical Fluorination of LaBaInO4 to LaBaInO3F2, Their Optical Characterization, and Photocatalytic Activities for Hydrogen Evolution.

We report on a nonoxidative topochemical route for the synthesis of a novel indate-based oxyfluoride, LaBaInO3F2, using a low-temperature reaction of Ruddlesden-Popper-type LaBaInO4 with polyvinylidene difluoride as a fluorinating agent. The reaction involves the replacement of oxide ions with fluoride ions as well as the insertion of fluoride ions into the interstitial sites. From the characterization via powder X-ray diffraction (PXRD) and Rietveld analysis as well as automated electron diffraction tomography (ADT), it is deduced that the fluorination results in a symmetry lowering from I4/mmm (139) to monoclinic C2/c (15) with an expansion perpendicular to the perovskite layers and a strong tilting of the octahedra in the ab plane. Disorder of the anions on the apical and interstitial sites seems to be favored. The most stable configuration for the anion ordering is estimated based on an evaluation of bond distances from the ADT measurements via bond valence sums (BVSs). The observed disordering of the anions in the oxyfluoride results in changes in the optical properties and thus shows that the topochemical anion modification can present a viable route to alter the optical properties. Partial densities of states (PDOSs) obtained from ab initio density functional theory (DFT) calculations reveal a bandgap modification upon fluoride-ion introduction which originates from the presence of the oxide anions on the interstitial sites. The photocatalytic performance of the oxide and oxyfluoride shows that both materials are photocatalytically active for hydrogen (H2) evolution.

Nitrogen-doped, proton-exchanged Dion-Jacobson layered niobate perovskites for photocatalytic hydrogen generation in solar light.

Wide band gap semiconductor niobate photocatalysts with Dion-Jacobson layered perovskite structure were nitrogen-doped via simple gas-solid reaction to extend their absorption into the visible light range. Nitrogen doping was performed using ammonia as precursor, resulting in decreased band gaps of doped AB2Nb3O10 compounds (A = Cs, Rb, K; B = Ca, Sr) down to 2.5 eV. The resulting materials were investigated concerning their chemical and electronic structures. Nitrogen-doped AB2Nb3O10 crystals showed a clear red shift in absorption. Photocatalytic performance tests for the doped materials evaluated the capability of H2 production under simulated solar irradiation. The addition of carbonates to the gas-solid reaction turned out to be advantageous for the reduction of defects and the preservation of photocatalytic activity of nitrogen-doped layered niobates AB2Nb3O10.

Self-Assembled Fluorescent Block Copolymer Micelles with Responsive Emission.

Responsive fluorescent materials offer a high potential for sensing and (bio-)imaging applications. To investigate new concepts for such materials and to broaden their applicability, the previously reported non-fluorescent zinc(II) complex [Zn(L)] that shows coordination-induced turn-on emission was encapsulated into a family of non-fluorescent polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles leading to brightly emissive materials. Coordination-induced turn-on emission upon incorporation and ligation of the [Zn(L)] in the P4VP core outperform parent [Zn(L)] in pyridine solution with respect to lifetimes, quantum yields, and temperature resistance. The quantum yield can be easily tuned by tailoring the selectivity of the employed solvent or solvent mixture and, thus, the tendency of the PS-b-P4VP diblock copolymers to self-assemble into micelles. A medium-dependent off-on sensor upon micelle formation could be established by suppression of non-micelle-borne emission background pertinent to chloroform through controlled acidification indicating an additional pH-dependent process.

Magnetic NiFe2 O4 Nanoparticles Prepared via Non-Aqueous Microwave-Assisted Synthesis for Application in Electrocatalytic Water Oxidation.

Phase-pure spinel-type magnetic nickel ferrite (NiFe2 O4 ) nanocrystals in the size range of 4 to 11 nm were successfully synthesized by a fast and energy-saving microwave-assisted approach. Size and accessible surface areas can be tuned precisely by the reaction parameters. Our results highlight the correlation between size, degree of inversion, and magnetic characteristics of NiFe2 O4 nanoparticles, which enables fine-tuning of these parameters for a particular application without changing the elemental composition. Moreover, the application potential of the synthesized powders for the electrocatalytic oxygen evolution reaction in alkaline media was demonstrated, showing that a low degree of inversion is beneficial for the overall performance. The most active sample reaches an overpotential of 380 mV for water oxidation at 10 mA cm-2 and 38.8 mA cm-2 at 1.7 V vs. RHE, combined with a low Tafel slope of 63 mV dec-1 .

Photoinduced Defect and Surface Chemistry of Niobium Tellurium Oxides ANbTeO6 (A = K, Rb, Cs) with Defect-Pyrochlore Structure.

The niobium tellurium oxides ANbTeO6 with varying A cations (A = K, Rb, Cs) and defect-pyrochlore crystal structure were used to investigate the effect of A on crystal-structure deformation and defect-chemistry. We show that the light absorption of these compounds in visible light is due to defects and not the effect of a low band gap. Using the materials in photocatalytic hydrogen generation, the prevailing defects and the surface composition change significantly during photocatalytic hydrogen evolution.

FexNi9-xS8 (x = 3-6) as potential photocatalysts for solar-driven hydrogen production?

The efficient reduction of protons by non-noble metals under mild conditions is a challenge for our modern society. Nature utilises hydrogenases, enzymatic machineries that comprise iron- and nickel- containing active sites, to perform the conversion of protons to hydrogen. We herein report a straightforward synthetic pathway towards well-defined particles of the bio-inspired material FexNi9-xS8, a structural and functional analogue of hydrogenase metal sulfur clusters. Moreover, the potential of pentlandites to serve as photocatalysts for solar-driven H2-production is assessed for the first time. The FexNi9-xS8 materials are visible light responsive (band gaps between 2.02 and 2.49 eV, depending on the pentlandite's Fe : Ni content) and display a conduction band energy close to the thermodynamic potential for proton reduction. Despite the limited driving force, a modest activity for photocatalytic H2 has been observed. Our observations show the potential for the future development of pentlandites as photocatalysts. This work provides a basis to explore powerful synergies between biomimetic chemistry and material design to unlock novel applications in solar energy conversion.

Independent Tailoring of Macropore and Mesopore Space in TiO2 Monoliths.

TiO2 monoliths were synthesized by a partially hindered sol-gel process. Various synthesis parameters like precursor concentrations and gelation temperature were varied to investigate changes in the macroporosity (being in the range of micrometers) and to determine influences on the macropore formation mechanism. Ionic liquids (ILs) were used as templates to vary the mesopore size independently from the macropore size. Depending on the synthesis parameters, TiO2 monoliths with exclusive mesoporosity or with hierarchical meso-/macropore structure were received, and the range of macropores can be shifted between 100 nm and 10 µm without influencing the mesopore diameter. Pore volumes up to 880 mm3/g were achieved, as determined by mercury intrusion porosimetry. The mesopores' diameter can be adjusted between 6 and 25 nm by adding different amounts of IL, and surface areas up to 260 m2/g and mesopore volumes of 0.5 cm3/g were obtained, based on N2-physisorption measurements. The monoliths were cladded by polymer, allowing for studying the flow-through properties depending on the macropore size. This precise control for tailored macropores enables the design of optimized TiO2 monoliths with respect to the desired application requirements.

The Influence of Tin(II) Incorporation on Visible Light Absorption and Photocatalytic Activity in Defect-Pyrochlores.

The defect pyrochlore KTaWO6 has been used to systematically investigate the effect of SnII incorporation conditions on the band structure and subsequent photocatalytic properties. Different tin precursors show varying influence on the resulting band gap. While the optimum conditions diminish the band gap by up to 1.4 eV, the increase in visible light absorption does not correlate with an increase of photocatalytic activity.

Mesoporous ZnFe2 O4 Photoanodes with Template-Tailored Mesopores and Temperature-Dependent Photocurrents.

Mesoporous ZnFe2 O4 photoanodes have been prepared via dip-coating utilizing the evaporation-induced self-assembly of two different block-copolymer templates to investigate the influence of pore geometry on the photoelectrochemcial performance of those earth-abundant photoelectrodes. The use of commercial block copolymers, triblock copolymer Pluronic® F127 and the diblock copolymer PIB3000 as templates, leads to different pore morphologies under identical preparation conditions due to different polymer stabilities. Interestingly, pore morphology in mesoporous ZnFe2 O4 turned out to be less important for the photoelectrochemical performance. Contrary, sufficiently developed crystalline domains gained through optimized temperature treatment resulted in maximum photoelectrochemical performance among the investigated samples. This disproves the necessity of expensive, tailor-made polymer soft templates to synthesize high-performing mesoporous ZnFe2 O4 photoanodes.

Thermal Evolution of ZnS Nanostructures: Effect of Oxidation Phenomena on Structural Features and Photocatalytical Performances.

ZnS nanosystems are being extensively studied for their possible use in a wide range of technological applications. Recently, the gradual oxidation of ZnS to ZnO was exploited to tune their structural, electronic, and functional properties. However, the inherent complexity and size dependence of the ZnS oxidation phenomena resulted in a very fragmented description of the process. In this work, different-sized nanosystems were obtained through two different low temperature wet chemistry routes, namely, hydrothermal and inverse miniemulsion approaches. These protocols were used to obtain ZnS samples consisting of 21 and 7 nm crystallites, respectively, to be used as reference material. The obtained samples were then calcinated at different temperatures, ranging from 400 to 800 °C toward the complete oxidation of ZnO, passing through the coexistence of the two phases (ZnS/ZnO). A thorough comparison of the effects of thermal handling on ZnS structural, chemical, and functional evolution was carried out by TEM, XRD, XAS, XPS, Raman, FT-IR, and UV-Vis. Finally, the photocatalytic activity in the H2 evolution reaction was also compared for selected ZnS and ZnS/ZnO samples. A correlation between size and the oxidation process was observed, as the smaller nanosystems showed the formation of ZnO at lower temperature, or in a larger amount in the case of the ZnS and ZnO co-presence. A difference in the underlying mechanism of the reaction was also evidenced. Despite the ZnS/ZnO mixed samples being characterized by an increased light absorption in the visible range, their photocatalytic activity was found to be much lower.

Heterojunctions in Composite Photocatalysts.

Combining different light-absorbing materials for the formation of semiconductor heterojunctions is a very effective strategy for preparing highly active photocatalyst and photoelectrochemical systems. Moreover, the combination of solid state semiconductors with polymers or molecular absorbers expands the possible combinations of materials to alter light absorption and optimize charge carrier separation. In this chapter, different strategies to prepare such composites are presented, highlighting the necessity of intimate interfacial contact for optimum charge carrier transfer. Moreover, the most recent developments and improvements in the formation of heterojunctions and composite photocatalyst systems based on semiconductor solids are presented.

Improved charge carrier separation in barium tantalate composites investigated by laser flash photolysis.

Charge carrier dynamics in phase pure Ba5Ta4O15 and in a Ba5Ta4O15-Ba3Ta5O15 composite have been studied by means of diffuse reflectance laser flash photolysis spectroscopy in the presence and absence of an electron donor, in order to reveal the reason for the improved photocatalytic performance of the latter. For the first time the transient absorption of trapped electrons with a maximum at around 650 nm and of trapped holes with a transient absorption maximum at around 310 nm is reported for tantalates. The decay kinetics of the photogenerated charge carriers could be fitted by second order reaction kinetics, and the direct recombination of the trapped electrons with the trapped holes was proven. In the absence of an electron donor, no difference in the decay behavior between the phase pure material and the composite material is found. In the presence of methanol, for the pure phase Ba5Ta4O15 the recombination of the charge carriers could not be prevented and the trapped electrons also recombine with the ˙CH2OH radical formed via the methanol oxidation by the trapped holes. However, in the composite, the electron can be stored in the system, the ˙CH2OH radical injects an electron into the conduction band of the second component of the composite, i.e., Ba3Ta5O15. Thus, the electrons are available for an extended period to induce reduction reactions.

Layered Perovskite Nanofibers via Electrospinning for Overall Water Splitting.

The (111)-layered perovskite materials Ba5 Ta4 O15 , Ba5 Ta2 Nb2 O15 and Ba5 Nb4 O15 are prepared with nanofiber morphology via electrospinning for the first time. The nanofibers are built up from small single crystals, with up to several micrometers length even after calcination. The formation mechanism is investigated in detail, revealing an intermediate formation of amorphous barium carbonate strengthening the nanofiber morphology for high temperature treatment. All nanofiber compounds are able to generate hydrogen without any co-catalyst in photocatalytic reformation of methanol. After photodeposition of Rh-Cr2 O3 co-catalysts, the nanofibers show better activity in overall water splitting compared to sol-gel-derived powders.

On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications.

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.

Enhanced photocatalytic hydrogen generation from barium tantalate composites.

(111)-layered Ba5Ta4O15 photocatalysts were synthesised by a solid state reaction route and a citrate synthesis route, and their structural and electronic properties were investigated. After citrate route preparation, the presence of a second phase, namely Ba3Ta5O15, was determined by X-ray powder diffraction and absorption spectroscopy. The existence of this phase had a profound effect on the photocatalytic activity of this Ba5Ta4O15/Ba3Ta5O15 composite in comparison to the pure Ba5Ta4O15 materials. The photocatalytic performance of the barium tantalates was evaluated by investigating the capability in ˙OH radical formation and hydrogen generation. Strongly increased hydrogen evolution rates for the Ba5Ta4O15/Ba3Ta5O15 composite, up to 160% higher than for the pure Ba5Ta4O15, were determined, and only very small amounts of Rh co-catalyst, deposited on the photocatalysts by stepwise reductive photo-deposition, were needed to achieve these results.

Proton transport in functionalised additives for PEM fuel cells: contributions from atomistic simulations.

The conventional polymer electrolyte membrane (PEM) materials for fuel cell applications strongly rely on temperature and pressure conditions for optimal performance. In order to expand the range of operating conditions of these conventional PEM materials, mesoporous functionalised SiO(2) additives are developed. It has been demonstrated that these additives themselves achieve proton conductivities approaching those of conventional materials. However, the proton conduction mechanisms and especially factors influencing charge carrier mobility under different hydration conditions are not well known and difficult to separate from concentration effects in experiments. This tutorial review highlights contributions of atomistic computer simulations to the basic understanding and eventual design of these materials. Some basic introduction to the theoretical and computational framework is provided to introduce the reader to the field, the techniques are in principle applicable to a wide range of other situations as well. Simulation results are directly compared to experimental data as far as possible.

Computerized assessment of pedophilic sexual interest through self-report and viewing time: reliability, validity, and classification accuracy of the affinity program.

Affinity is a computerized assessment tool that combines viewing time and self-report measures of sexual interest. The present study was designed to assess the diagnostic properties of Affinity with respect to sexual interest in prepubescent children. Reliability of both self-report and viewing time components was estimated to be high. The group profile of a sample of pedophilic adult male child molesters (n = 42, all of whom admitted their offenses) differed from the group profiles of male community controls (n = 95) and male nonsexual offenders (n = 27), respectively. More specifically, both ratings and viewing times for images showing small children or prejuvenile children were significantly higher within the child molester sample than in either of the other two groups, attesting to the validity of the measures. Overall classification accuracy, however, was mediocre: A multivariate classification routine yielded 50% sensitivity for child molester status at the cost of 13% false positives. The implications for forensic use of Affinity are discussed.