Hydrogen-Bonded Metal-Organic Framework Nanosheet as a Proton Conducting Membrane for an H2 /O2 Fuel Cell.

Proton-conducting metal-organic frameworks (MOFs) have attracted attention as potential electrolytes for fuel cells. However, research progress in utilizing MOFs as electrolytes for fuel cells has been limited, mainly due to challenges associated with issues such as the fabrication of MOF membranes, and hydrogen crossover through the MOF's pores. Here, we report proton conductivity and fuel cell performance of a self-standing membrane prepared from of a bismuth subgallate MOF nanosheets with non-porous structure are reported. The fabricated MOF nanosheet membrane with no binding agent exhibitsed structural anisotropy. The proton conductivity in the membrane thickness direction (4.4 × 10-3  S cm-1 ) at 90 °C and RH 100% is observed to be higher than that in the in-plane direction of the membrane (3.3 × 10-5  S cm-1 ). The open circuit voltage (OCV) of a fuel cell with ≈120 µm proton conducting membrane is 1.0 V. The non-porous nature of the MOF nanosheets contributes to the relatively high OCV. A fuel cell using ≈40 µm membrane as proton conducting electrolyte records a maximum of 25 mW cm-2 power density and a maximum of 109 mA cm-2 current density with 0.91 V OCV at 80 °C in humid conditions.

Bandgap Tunable Oxynitride LaNb2 O7-x Nx Nanosheets.

Bandgap tunable lanthanum niobium oxynitride [LaNb2 O7-x Nx ](1+x)- nanosheet is prepared by the delamination of a Ruddlesden-Popper phase perovskite oxynitride via ion-exchange and two-step intercalation processes. The lanthanum niobium oxynitride nanosheets have a homogeneous thickness of 1.6 nm and exhibit a variety of chromatic colors depending on the nitridation temperature of the parent-layered oxynitride. The bandgap energy of the nanosheets is determined by ultraviolet photoemission spectroscopy, Mott-Schottky, and photoelectrochemical measurements and is found to be tunable in the range of 2.03-2.63 eV. Furthermore, the oxide/oxynitride superlattice structures are fabricated by face-to-face stacking of 2D crystals using oxynitride [LaNb2 O7-x Nx ](1+x)- and oxide [Ca2 Nb3 O10 ]- nanosheets as building blocks. Moreover, the superlattices-like restacked oxynitride/oxide nanosheets hybrid exhibits unique proton conductivity and dielectric properties strongly influenced by the oxynitride nanosheets and enhanced photocatalytic activity under visible light irradiation.

Multicolor luminescent material based on interaction between TiNbO5- nanosheets and lanthanide ions for visualization of pH change in inorganic gel electrolyte.

Layered nanosheet materials showing a drastic luminescence change in response to changes in proton concentration (pH) were prepared by sandwiching Eu3+ and Tb3+ cations with anionic TiNbO5- nanosheets using electrostatic interaction. Each trivalent lanthanide ion showed a different response to pH change: a strong red emission from Eu3+ was observed at low proton concentrations (pH: 13) and a green emission from Tb3+ was dominant at high proton concentrations (pH: 1). The photoluminescence intensity was determined by the balance between the photocatalytic activity of TiNbO5- nanosheets and energy transfer from the host layer to the guest lanthanide ions. Moreover, the trivalent lanthanide/TiNbO5- nanosheet hybrid formed a gel-like solid in aqueous solution, which functioned as an inorganic gel electrolyte when mixed with Na2SO4. The multicolor luminescence (red-yellow-green) of the lanthanide/TiNbO5- nanosheet hybrid enabled direct visualization of the diffusion of protons in an inorganic gel electrolyte during water electrolysis.

Infusion of Variable Chemical Structure to Tune Stacking among Metal-Organic Layers in 2D Nano MOFs.

Invited for the cover of this issue is the group of Biplab Manna at the University of Kumamoto. The image depicts various kinds of stacking between the metal-organic layers of MOFs. Read the full text of the article at 10.1002/chem.202201665.

Infusion of Variable Chemical Structure to Tune Stacking among Metal-Organic Layers in 2D Nano MOF.

Thickness of two-dimensional (2D) metal-organic frameworks (MOFs) govern their intriguing functionalities. Primarily this thickness is controlled by the stacking between the metal-organic layers (MOL). It is observed that until now such modulating factors for stacking efficiency of MOL are not well studied. Here, we report a fundamental hypothesis to comprehend regulation of stacking efficiency among MOLs as a function of chemical structure of organic ligands (dicarboxylic acids and pillar linkers). This basically involves a series of isostructural three-dimensional (3D) MOFs which contain linkers of variable chemical nature that could be depillared to generate 2D stacked MOFs of different thickness. Depending on the linkers, we encountered the formation of single MOL to stacked multiple MOLs as evidenced from atomic force microscopic and other experimental analysis. The present study gives a concrete correlation between the stacking within 2D MOFs (from monolayer to multilayers), and their 3D counter parts, which may provide a thickness tuning pathway for 2D MOFs.

Seamless All-Solid-State Supercapacitor Fabricated Using a Proton-Conducting Methanesulfonic Acid-Intercalated Graphene Oxide Film as an Electrolyte.

An all-solid-state supercapacitor with no boundary between the electrode/electrolyte interface is prepared using methanesulfonic acid (MSA)-intercalated graphene oxide (GO)　membranes as a proton-conducting electrolyte. The electrodes (reduced GO) are formed within the surface of the solid GO electrolyte by a combination of self-reduction of the GO under UV-light illumination and electrochemical reduction. In this process, the surface of the GO film is converted to an electrode material with mixed electron/proton conduction, which results in the formation of a seamless capacitor structure. The resultant capacitor shows a large capacitance of 33.8 mF cm-2 , 11.9 F g-1 (g: total weight of device including electrodes, electrolyte, separator and current collector), which is 15 times higher than the capacitance retention of an all-solid-state supercapacitor fabricated using proton-conducting GO film. The seamless structures for the electrode/electrolyte interface suppress the decomposition of the GO electrolyte by the local concentration of applied voltage, resulting in improved cycle stability. The very large capacitance is likely derived not only from the seamless structure but also from the high proton conductivity of the MSA-intercalated GO electrolyte (4.2 × 10-3  S cm-1 ).

Preparation of silicate nanosheets by delaminating RUB-18 for transparent, proton conducting membranes.

The present study demonstrated delamination of the layered silicate RUB-18 with no organo-modification of the silanol group. The obtained nanosheets showed a homogeneous thickness. A sponge-like material and a free-standing transparent, dense membrane were reconstructed using the nanosheets.

Preparation and Properties of Organic-Inorganic Hybrid Antireflection Films Made by a Low-Temperature Process Using Hollow Silica Nanoparticles.

Layers made of hollow silica nanoparticles have potential applications as antireflection films with lower refractive index values compared with existing materials such as silica glass (1.50) and magnesium fluoride (1.38). The advantages of such nanoparticles result from interactions between the solid shell, the cavity phase core, and the voids between particles. To obtain practical antireflection films, it is necessary to control the number of layers of these hollow silica nanoparticles and to fill the gaps between particles with a solid. In the present study, antireflection films were prepared by applying a coating of hollow silica nanoparticles dispersed in a UV-curable monomer solution onto plastic substrates. After film formation and exposure to UV light, the voids between the nanoparticles were completely filled with a polymer matrix. Tuning the particle concentration in the coating solution allowed the formation of antireflection films comprising one to three layers of the hollow silica nanoparticles. The reflectance of the films was dependent on the number of layers, and a 100 nm thick film in which two layers of hollow silica nanoparticles were precisely arranged showed the lowest reflectance of 0.92% at 550 nm wavelength, equivalent to a refractive index of 1.23. Because the voids between particles were filled with the polymer, these films resisted contamination during manual handling and so would be expected to maintain low reflectance during practical applications. This work demonstrates that nanosized inorganic-organic hybrid films composed of hollow silica nanoparticles and a UV-curable resin can exhibit optical properties and structural integrity that cannot be achieved by either substance alone.

Oxygen reduction reaction activity of an iron phthalocyanine/graphene oxide nanocomposite.

Electrocatalysts with metal-nitrogen-carbon (M-N-C) sites have recently attracted much attention as potential catalysts for the oxygen reduction reaction (ORR), and a hybrid of iron phthalocyanine (FePc) and reduced graphene oxide (rGO) is one of the promising candidates. Herein, a FePc/GO nanocomposite was synthesized by electrostatic deposition on the electrode. The electrochemically reduced FePc/GO nanocomposite (ER(FePc/GO)) contained Fe2+ centers in well reduced graphene sites without agglomeration. The ER(FePc/GO) exhibited high ORR activity with an ORR onset (E onset) and half-wave potential (E 1/2) of 0.97 and 0.86 V, respectively. Furthermore, the ORR activity successfully improved by adding an electrolyte such as KCl or KNO3. The small H2O2 yield of 2%, superior tolerance to methanol addition and high-durability indicate that the ER(FePc/GO) is a promising electrocatalyst. Theoretical studies, indicating that the presence of Cl- and NO3 - ions lowered the conversion energy barrier, strongly supported the experimental results.

A perfectly oriented, free-standing and transparent titania nanosheet film with the band gap of a monolayer.

A perfectly oriented, free-standing and transparent titania nanosheet film was prepared using the spin-coating technique. The free-standing film (thickness: ∼210 nm) showed a wide band gap that corresponded to that of the monolayer nanosheet despite the stacking of hundreds of layers.

Ferromagnetic metal conversion directly from two-dimensional nickel hydroxide.

We have demonstrated a direct metallic conversion from nickel hydroxide nanosheets to nickel metal nanostructures by thermal annealing in vacuum. The metal transition of the single-layer nanosheets deposited on a Si substrate was revealed by x-ray absorption near edge structure (XANES) measurements. The XANES signal significantly changed at annealing temperatures above 250 °C. The metal transition temperature coincides with the reported temperatures at which layered nickel hydroxide nanosheets are converted to nickel oxide nanosheets by calcination in air. Auger measurements confirmed that a dissociation of oxygen from the hydroxide nanosheet induces the metallic conversion. The converted nickel metallic structures exhibit ferromagnetic behavior revealed by x-ray magnetic circular dichroism (XMCD) measurement. Atomic force microscopy measurements indicate that diffusions of nickel atoms on the substrates leads to a structural change from a 2D-like structure to a particle-like structure.

Photoelectrochemical properties of a well-structured 1.3 nm-thick pn junction crystal.

A 1.3 nm-thick nickel hydroxide (p-type, 0.5 nm)/titania (n-type, 0.8 nm) pn junction prepared by lamination of nanosheets improved the onset potential for photoelectrochemical oxidation and increased the photooxidation current, indicating that ultrathin pn junctions suppress the recombination of photo-generated carriers.

A Cocatalyst that Stabilizes a Hydride Intermediate during Photocatalytic Hydrogen Evolution over a Rhodium-Doped TiO2 Nanosheet.

The hydrogen evolution reaction using semiconductor photocatalysts has been significantly improved by cocatalyst loading. However, there are still many speculations regarding the actual role of the cocatalyst. Now a photocatalytic hydrogen evolution reaction pathway is reported on a cocatalyst site using TiO2 nanosheets doped with Rh at Ti sites as one-atom cocatalysts. A hydride species adsorbed on the one-atom Rh dopant cocatalyst site was confirmed experimentally as the intermediate state for hydrogen evolution, which was consistent with the results of density functional theory (DFT) calculations. In this system, the role of the cocatalyst in photocatalytic hydrogen evolution is related to the withdrawal of photo-excited electrons and stabilization of the hydride intermediate species; the presence of oxygen vacancies induced by Rh facilitate the withdrawal of electrons and stabilization of the hydride.

Super Dielectric Materials of Two-Dimensional TiO2 or Ca2Nb3O10 Nanosheet Hybrids with Reduced Graphene Oxide.

High dielectric constants (εr) were observed in two-dimensional composites obtained from stacking of reduced graphene oxide (RGO) with Ca2Nb3O10 and with TiO2 nanosheets. The relative dielectric permittivity values of the composites were found to be higher than 105, an amazingly high value compared to that of similar GO composites and other common dielectric materials. As a consequence, we considered application of the hybrids as super dielectric materials in high capacitance supercapacitors. The route to high capacitance involves the variation of oxygen vacancies within the surface and in the closest bulk interior of the hybrids. The effective charges generated throughout the metal oxide and carbon-oxygen polar bonding systems within the graphene skeleton appear to highly influence dielectric polarization. Moreover, the replenishment of oxygen vacancies at the RGO and metal oxide interface also contributes to polarizability.

Ni-Fe Nitride Nanoplates on Nitrogen-Doped Graphene as a Synergistic Catalyst for Reversible Oxygen Evolution Reaction and Rechargeable Zn-Air Battery.

Obtaining bifunctional electrocatalysts with high activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is a main hurdle in the application of rechargeable metal-air batteries. Earth-abundant 3d transition metal-based catalysts have been developed for the OER and ORR; however, most of these are based on oxides, whose insulating nature strongly restricts their catalytic performance. This study describes a metallic Ni-Fe nitride/nitrogen-doped graphene hybrid in which 2D Ni-Fe nitride nanoplates are strongly coupled with the graphene support. Electronic structure of the Ni-Fe nitride is changed by hybridizing with the nitrogen-doped graphene. The unique heterostructure of this hybrid catalyst results in very high OER activity with the lowest onset overpotential (150 mV) reported, and good ORR activity comparable to that for commercial Pt/C. The high activity and durability of this bifunctional catalyst are also confirmed in rechargeable zinc-air batteries that are stable for 180 cycles with an overall overpotential of only 0.77 V at 10 mA-2 .

Application to Photocatalytic H2 Production of a Whole-Cell Reaction by Recombinant Escherichia coli Cells Expressing [FeFe]-Hydrogenase and Maturases Genes.

A photocatalytic H2 production system using an inorganic-bio hybrid photocatalyst could contribute to the efficient utilization of solar energy, but would require the development of a new approach for preparing a H2 -forming biocatalyst. In the present study, we constructed a recombinant strain of Escherichia coli expressing the genes encoding the [FeFe]-hydrogenase and relevant maturases from Clostridium acetobutylicum NBRC 13948 for use as a biocatalyst. We investigated the direct application of a whole-cell of the recombinant E. coli. The combination of TiO2 , methylviologen, and the recombinant E. coli formed H2 under light irradiation, demonstrating that whole cells of the recombinant E. coli could be employed for photocatalytic H2 production without any time-consuming and costly manipulations (for example, enzyme purification). This is the first report of the direct application of a whole-cell reaction of recombinant E. coli to photocatalytic H2 production.

Synthesis and Investigation of the Effect of Substitution on the Structure, Physical Properties, and Electrochemical Properties of Anthracenodifuran Derivatives.

A series of syn/anti mixtures of anthradifuran (ADF) and substituent compounds were systematically synthesized, and the effect of substitution at the 5,11-positions on the neutral and radical states of ADF was investigated. All compounds were measured and analyzed by absorption and fluorescence spectroscopy, cyclic voltammetry, electrochemical absorption spectroscopy, and DFT calculations. The absorption spectra of 5,11-substituent compounds in their neutral state were red-shifted. In addition, the substituted compounds exhibited increased thermal stability with respect to the parent 1a because of elongation of the π-conjugation and an increased steric hindrance effect due to the bulky ethynyl substituent groups. The cyclic voltammograms of all of the compounds exhibited irreversible reduction potentials and irreversible oxidation potentials, except in the case of (trimethylsilyl)silylethynyl-substituted ADF. When the materials were subjected to oxidation/reduction potentials, the radical cation and anion species were generated. The absorption spectra of the radical-cation species of the compounds exhibited similar characteristics and similar absorption ranges (550-1400 nm), whereas the spectra of the radical anion species were blue-shifted (550-850 nm) compared than that of the parent 1a(•-) (550-1100 nm). The DFT computation results suggested that the radical states of lowest energy transitions occurred primarily from π to π(SOMO) or from π(SOMO) to π*.

Discharge performance of solid-state oxygen shuttle metal-air battery using Ca-stabilized ZrO2 electrolyte.

The effects of metal choice on the electrochemical performance of oxygen-shuttle metal-air batteries with Ca-stabilized ZrO2 (CSZ) as the electrolyte and various metals as the anodes were studied at 1073 K. The equilibrium oxygen partial pressure (P O 2) in the anode chamber was governed by the metal used in the anode chamber. A lower-P O 2 environment in the anode decreased the polarization resistance of the anode. The oxidation of oxide ions to oxygen in the anode is drastically enhanced by the n-type conduction generated in the CSZ electrolyte when it is exposed to a reducing atmosphere. A high discharge potential and high capacity can be achieved in an oxygen-shuttle battery with a Li or Mg anode because of the fast anode reaction compared to that of cells with a Zn, Fe, or Sn anode. However, only the mildly reducing metals (Zn, Si, Fe, and Sn) can potentially be used in rechargeable metal-air batteries because the transport number of the CSZ electrolyte must be unity during charge and discharge. Oxygen shuttle rechargeable batteries with Fe, and Sn electrodes are demonstrated.

Photocatalytic reaction centers in two-dimensional titanium oxide crystals.

Co-catalysts play an important role in photocatalytic water splitting. The co-catalyst is generally deposited in the form of nanoparticles on the catalyst surface, and is believed to provide water oxidation and reduction sites. However, the minimum size of a co-catalyst that can function as a reaction site and the detailed local environment of the photocatalytic reaction centers are not yet fully understood. Here, we show that even isolated single-atom Rh dopants in two-dimensional titanium oxide crystals can effectively act as co-catalysts for the water-splitting reaction. At an optimal doping concentration, the hydrogen production rate is increased substantially in comparison to that found with the undoped crystals. We also present first-principles simulations based on density functional theory to provide insights into the atomic-scale mechanism by which the isolated Rh dopants induce changes to the dissociation reaction energy landscape. These results provide new insights for better understanding the role of the co-catalyst in the photocatalytic reaction.

Direct imaging of light emission centers in two-dimensional crystals and their luminescence and photocatalytic properties.

To fully understand the fundamental properties of light-energy-converting materials, it is important to determine the local atomic configuration of photofunctional centers. In this study, direct imaging of one- and two-Tb-atom emission centers in a two-dimensional Tb-doped Ca2Ta3O10 nanocrystal was carried. The emission centers were located at the Ca sites in the perovskite structure, and no concentration-based quenching was observed even when the emission centers were in close proximity to each other. The relative photoluminescence efficiency for green emission of the nanosheet suspension was 38.1%. Furthermore, the Tb-doped Ca2Ta3O10 nanocrystal deposited co-catalyst showed high photocatalytic activity for hydrogen production from water (quantum efficiency: 71% at 270 nm). Tb(3+) dopants in the two-dimensional crystal might have the potential to stabilize the charge separation state.