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, 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 exhibits 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.

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.

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.

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 ).

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.

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.

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.

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.

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*.

Potential gradient and photocatalytic activity of an ultrathin p-n junction surface prepared with two-dimensional semiconducting nanocrystals.

The creation of p-n junction structure in photocatalysts is a smart approach to improve the photocatalytic activity, as p-n junctions can potentially act to suppress the recombination reaction. Understanding the surface conditions of the junction parts is one of the biggest challenges in the development of photocatalyst surface chemistry. Here, we show a relationship between the photocatalytic activity and potential gradient of the junction surface prepared from two-dimensional crystals of p-type NiO and n-type calcium niobate (CNO). The ultrathin (ca. 2 nm) junction structure and the surface potential were analyzed using low energy ion scattering spectroscopy and Kelvin probe force microscopy. The photocatalytic H2 production rate for the n-p (CNO/NiO) junction surface was higher than those for p-n (NiO/CNO) junction, p, and n surfaces. The surface potential of the CNO/NiO junction part (surface: CNO) was lower than that of the CNO crystals in the same CNO crystal surface. These potential gradients result in specially separated reaction sites, which suppress the recombination reaction in the CNO nanosheet. Photo-oxidation and photoreduction sites in the junction structure were confirmed using the photodeposition reaction of MnO(x) and Ag.

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.

Preparation of tantalum-based oxynitride nanosheets by exfoliation of a layered oxynitride, CsCa2Ta3O(10-x)N(y), and their photocatalytic activity.

Calcium tantalum oxynitride [Ca(2)Ta(3)O(9.7)N(0.2)](-) nanosheets were prepared by exfoliating a layered perovskite oxynitride (CsCa(2)Ta(3)O(9.7)N(0.2)) via proton exchange and two-step intercalation of ethylamine and tetrabutylammonium ions. Monolayer nanosheet was prepared by the above processes, although some bilayer or trilayer nanosheets were also produced. The [Ca(2)Ta(3)O(9.7)N(0.2)](-) nanosheets exhibited photocatalytic activity for H(2) evolution from water under visible light irradiation. In contrast, CsCa(2)Ta(3)O(9.7)N(0.2) exhibited very low photocatalytic activity for H(2) evolution under the visible light irradiation, even when methanol was added to water as a sacrificial agent. The improved photocatalytic activity originates from the characteristics of nanosheets such as their molecular thickness and large surface area. Further, the Rh-loaded [Ca(2)Ta(3)O(9.7)N(0.2)](-) nanosheets restacked with protons exhibited photocatalytic activity for H(2) and O(2) evolution from pure water under UV-light irradiation. The ratio of H(2)/O(2) evolved was around 3. The ratio of N/O in the catalyst remained the same after the photocatalytic reaction, signifying that there was no decomposition of the catalyst during the reaction. This indicates that the present N-doped nanosheet is stable in the photocatalytic reaction.

Self-recovery of Pd nanoparticles that were dispersed over La(Sr)Fe(Mn)O3 for intelligent oxide anodes of solid-oxide fuel cells.

Self-recovery is one of the most-desirable properties for functional materials. Recently, oxide anodes have attracted significant attention as alternative anode materials for solid-oxide fuel cells (SOFCs) that can overcome reoxidation, deactivation, and coke-deposition. However, the electrical conductivity and surface activity of the most-widely used oxide anodes remain unsatisfactory. Herein, we report the synthesis of an "intelligent oxide anode" that exhibits self-recovery from power-density degradation in the redox cycle by using a Pd-doped La(Sr)Fe-(Mn)O(3) cell as an oxide anode for the SOFCs. We investigated the anodic performance and oxidation-tolerance of the cell by using Pd-doped perovskite as an anode and fairly high maximum power densities of 0.5 and 0.1 W cm(-2) were achieved at 1073 and 873 K, respectively, despite using a 0.3 mm-thick electrolyte. Long-term stability was also examined and the power density was recovered upon exposure of the anode to air. This recovery of the power density can be explained by the formation of Pd nanoparticles, which were self-recovered through reoxidation and reduction. In addition, the self-recovery of the anode by oxidation was confirmed by XRD and SEM and this process was effective for improving the durability of SOFC systems when they were exposed to severe operating conditions.

High capacity of an Fe-air rechargeable battery using LaGaO3-based oxide ion conductor as an electrolyte.

Rapid growth and improved functions of mobile equipment present the need for an advanced rechargeable battery with extremely high capacity. In this study, we investigated the application of fuel cell technology to an Fe-air rechargeable battery. Because the redox potential of Fe is similar to that of H(2), the combination of H(2) formation by the oxidation of Fe with a fuel cell has led to a new type of metal-air rechargeable battery. By decreasing the operating temperature, a deep oxidation state of Fe can be achieved, resulting in enlarged capacity of the Fe-air battery. We found that the metal Fe is oxidized to Fe(3)O(4) by using H(2)/H(2)O as mediator. The observed discharge capacity is 817 mA h g(-1)-Fe, which is approximately 68% of the theoretical capacity of the formation of Fe(3)O(4), 1200 mA h g(-1)-Fe, at 10 mA cm(-2) and 873 K. Moreover, the cycle stability of this cell is examined. At 1073 K, the cell shows a discharge capacity of ca. 800 mA h g(-1)-Fe with reasonably high discharge capacity sustained over five cycles.

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.

Doped CeO2-LaFeO3 composite oxide as an active anode for direct hydrocarbon-type solid oxide fuel cells.

Direct utilization of hydrocarbon and other renewable fuels is one of the most important issues concerning solid oxide fuel cells (SOFCs). Mixed ionic and electronic conductors (MIECs) have been explored as anode materials for direct hydrocarbon-type SOFCs. However, electrical conductivity of the most often reported MIEC oxide electrodes is still not satisfactory. As a result, mixed-conducting oxides with high electrical conductivity and catalytic activity are attracting considerable interest as an alternative anode material for noncoke depositing anodes. In this study, we examine the oxide composite Ce(Mn,Fe)O(2)-La(Sr)Fe(Mn)O(3) for use as an oxide anode in direct hydrocarbon-type SOFCs. High performance was demonstrated for this composite oxide anode in direct hydrocarbon-type SOFCs, showing high maximum power density of approximately 1 W cm(-2) at 1073 K when propane and butane were used as fuel. The high power density of the cell results from the high electrical conductivity of the composite oxide in hydrocarbon and the high surface activity in relation to direct hydrocarbon oxidation.

Synthesis and photocatalytic activity of rhodium-doped calcium niobate nanosheets for hydrogen production from a water/methanol system without cocatalyst loading.

Rhodium-doped calcium niobate nanosheets were synthesized by exfoliating layered KCa(2)Nb(3-x)Rh(x)O(10-δ) and exhibited high photocatalytic activity for H(2) production from a water/methanol system without cocatalyst loading. The maximum H(2) production rate of the nanosheets was 165 times larger than that of the parent Rh-doped layered oxide. The quantum efficiency at 300 nm was 65%. In this system, the methanol was oxidized to formaldehyde (main product), formic acid, and carbon dioxide by holes, whereas electrons cause the reduction of water to H(2). The conductivity of the parent layered oxide was decreased by doping, which indicates the octahedral RhO(6) unit in the lattice of the nanosheet functions as an electron trap site. The RhO(6) units in the nanosheet probably also act as reaction sites for H(2) evolution.

Long-time charge separation in porphyrin/KTa(Zr)O3 as water splitting photocatalyst.

Artificial photosynthesis is one of the chemists' dreams and the separation of charges with long lives is fundamental for achieving artificial photosynthesis. In actual photosynthesis, Z-scheme excitation separates electronic charge with high efficiency using solar light. Here we report that photo-excitation in Cr-tetraphenylporphyrin chroride (Cr-TPPCl)/Zr doped KTaO(3) (KTa(Zr)O(3)) is analogous to the Z-scheme in photosynthesis, and that we achieve complete charge separation at room temperature. Photovoltaic decay and transient fluorescence spectra measurements showed that the photo-excited charge in KTa(Zr)O(3) transferred to the HOMO of Cr-TPPCl within a few hundred pico-seconds on charge transfer. In contrast, the reduced state of the Cr-TPPCl species that was formed by the electronic injection from KTa(Zr)O(3) was observed for more than 0.5 s at room temperature in the transient decay of the absorption spectra change after the initial excitation of KTa(Zr)O(3). The formed reduced state of Cr-TPPCl was highly stable and was detected by static ESR measurements.