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.

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 ).

Production of water-dispersible reduced graphene oxide without stabilizers using liquid-phase photoreduction.

We successfully produced water-dispersible reduced graphene oxide (rGO) by pH tuning liquid-phase photoreduction. In this method, the stabilizers and chemical modification usually used for dispersing rGO are not required. The stable carboxyl groups continue to ionize throughout the photoreduction process under alkaline conditions and continue to provide water-dispersible rGO. Moreover, the decomposition of GO into CO2 is prevented, and the production of defects is largely avoided. This is because the epoxide groups on the GO nanosheets that lead to decomposition are converted into hydroxide groups under alkaline conditions. Thus, this simple aggregation-, defect-, and stabilizer-free method is potentially important for the future application of rGO.

Reversibly tunable upconversion luminescence by host-guest chemistry.

Tuning upconversion (UPC) luminescence using external stimuli and fields, as well as chemical reactions, is expected to lead to novel and efficient optical sensors. Herein, highly tunable UPC luminescence was achieved through a host-guest chemistry approach. Specifically, interlayer ion exchange reactions reversibly tuned the emission intensity and green-red color of Er/Yb-codoped A2La2Ti3O10 layered perovskite, where A corresponds to proton and alkali metal ions, enabling the visualization of host-guest interactions and reactions.

Proton conductivities of graphene oxide nanosheets: single, multilayer, and modified nanosheets.

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop-cast method) are larger than those of single-layer GO (prepared by either the drop-cast or the Langmuir-Blodgett (LB) method). At 60% relative humidity (RH), the σ value increases from 1×10(-6) S cm(-1) in single-layer GO to 1×10(-4) and 4×10(-4) S cm(-1) for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.

Graphene oxide nanosheet with high proton conductivity.

We measured the proton conductivity of bulk graphite oxide (GO'), a graphene oxide/proton hybrid (GO-H), and a graphene oxide (GO) nanosheet for the first time. GO is a well-known electronic insulator, but for proton conduction we observed the reverse trend, as it exhibited superionic conductivity. The hydrophilic sites present in GO as -O-, -OH, and -COOH functional groups attract the protons, which propagate through hydrogen-bonding networks along the adsorbed water film. The proton conductivities of GO' and GO-H at 100% humidity were ∼10(-4) and ∼10(-5) S cm(-1), respectively, whereas that for GO was amazingly high, nearly 10(-2) S cm(-1). This finding indicates the possibility of GO-based perfect two-dimensional proton-conductive materials for applications in fuel cells, sensors, and so on.

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.

Slide-Ring Material/Highly Dispersed Graphene Oxide Composite with Mechanical Strength and Tunable Electrical Conduction as a Stretchable-Base Substrate.

The development of stretchable elastomer composites with considerable mechanical strength and electrical conductivity is desired for future applications in communication tools, healthcare, and robotics. Herein, we have developed a novel stretchable elastomer composite by employing a slide-ring (SR) material as a matrix for restoration and graphene oxide (GO) as a precursor for a conductive filler. Highly dispersed GO in an organic solvent, prepared via a new method developed by the authors, allowed the uniform dispersion of GO into the matrix by simply mixing the solvent and SR. The resultant SR/GO composite exhibited considerably high mechanical toughness and cyclic durability. These properties were approximately maintained after pulse laser irradiation to add electrical conductivity on the composite by photoreducing of the dispersed GO, and its electrical conductivity was higher than that of the SR/graphene, carbon nanotubes, or graphite composites. The potential of the SR/GO composite as a stretchable base substrate for wearable devices was demonstrated by producing a prototype humidity sensor, a human motion monitoring sensor, and an electrical heater based on the composite with conductive circuits drawn using pulse laser patterning.

Solid Electrolyte Gas Sensor Based on a Proton-Conducting Graphene Oxide Membrane.

Graphene oxide (GO) is an ultrathin carbon nanosheet with various oxygen-containing functional groups. The utilization of GO has attracted tremendous attention in a number of areas, such as electronics, optics, optoelectronics, catalysis, and bioengineering. Here, we report the development of GO-based solid electrolyte gas sensors that can continuously detect combustible gases at low concentrations. GO membranes were fabricated by filtration using a colloidal solution containing GO nanosheets synthesized by a modified Hummers' method. The GO membrane exposed to humid air showed good proton-conducting properties at room temperature, as confirmed by hydrogen concentration cell measurements and complex impedance analyses. Gas sensor devices were fabricated using the GO membrane fitted with a Pt/C sensing electrode. The gas-sensing properties were examined by potentiometric and amperometric techniques. The GO sensor showed high, stable, and reproducible responses to hydrogen at parts per million concentrations in humid air at room temperature. The sensing mechanism is explained in terms of the mixed-potential theory. Our results suggest the promising capability of GO for the electrochemical detection of combustible gases.

All-Graphene Oxide Flexible Solid-State Supercapacitors with Enhanced Electrochemical Performance.

The rapid development of flexible and wearable electronics has led to an increase in the demand for flexible supercapacitors with enhanced electrochemical performance. Graphene oxide (GO) and reduced GO (rGO) exhibit several key properties required for supercapacitor components. Although solid-state rGO/GO/rGO supercapacitors with unique structures are promising, their moderate capacitance is inadequate for practical applications. Herein, we report a flexible solid-state rGO/GO/rGO supercapacitor comprising H2SO4-intercalated GO electrolyte/separator and pseudocapacitive rGO electrodes, which demonstrate excellent electrochemical performance. The resulting supercapacitor delivered an areal capacitance of 14.5 mF cm-2, which is among the highest values achieved for any rGO/GO/rGO supercapacitor. High ionic concentration and fast ion conduction in the H2SO4-intercalated GO electrolyte/separator and abundant CH defects, which serve as pseudocapacitive sites on the rGO electrode, were responsible for the high capacitance of this device. The rGO electrode, well separated by the H2SO4 molecular spacer, supplied highly efficient ion transport channels, leading to excellent rate capability. The highly packed rGO electrode and high specific capacitance resulted in a high volumetric energy density (1.24 mWh cm-3) observed in this supercapacitor. The structure, without a clear interface between GO and rGO, provides extremely low resistance and flexibility for devices. Our device operated in air (25 °C 40%) without the use of external electrolytes, conductive additives, and binders. Furthermore, we demonstrate a simple and versatile technique for supercapacitor fabrication by combining photoreduction and electrochemical treatment. These advantages are attractive for developing novel carbon-based energy devices with high device performance and low fabrication costs.

All-graphene oxide device with tunable supercapacitor and battery behaviour by the working voltage.

We propose a new type of all-graphene oxide device. Reduced graphene oxide (rGO)/graphene oxide (GO)/rGO functions as both a supercapacitor and a battery, depending on the working voltage. The rGO/GO/rGO operates as a supercapacitor until 1.2 V. At greater than 1.5 V, it behaves as a battery using redox reaction.

Thermally Stable Super Ionic Conductor from Carbon Sphere Oxide.

A highly stable proton conductor has been developed from carbon sphere oxide (CSO). Carbon sphere (CS) generated from sucrose was oxidized successfully to CSO using Hummers' graphite oxidation technique. At room temperature and 90 % relative humidity, the proton conductivity of thin layer CSO on microsized comb electrode was found to be 8.7×10(-3)  S cm(-1) , which is higher than that for a similar graphene oxide (GO) sample (3.4×10(-3)  S cm(-1) ). The activation energy (Ea ) of 0.258 eV suggests that the proton is conducted through the Grotthuss mechanism. The carboxyl functional groups on the CSO surface are primarily responsible for transporting protons. In contrast to conventional carbon-based proton conductors, in which the functional groups decompose around 80 °C, CSO has a stable morphology and functional groups with reproducible proton conductivity up to 400 °C. Even once annealed at different temperatures at high relative humidity, the proton conductivity of CSO remains almost unchanged, whereas significant change is seen with a similar GO sample. After annealing at 100 and 200 °C, the respective proton conductivity of CSO was almost the same, and was about ∼50 % of the proton conductivity at room temperature. Carbon-based solid electrolyte with such high thermal stability and reproducible proton conductivity is desired for practical applications. We expect that a CSO-based proton conductor would be applicable for fuel cells and sensing devices operating under high temperatures.

Coal Oxide as a Thermally Robust Carbon-Based Proton Conductor.

Inexpensive solid proton conducting materials with high proton conductivity and thermal stability are necessary for practical solid state electrochemical devices. Here we report that coal oxide (CO) is a promising carbon-based proton conductor with remarkable thermal robustness. The CO produced by simple liquid-phase oxidation of coal demonstrates excellent dispersibility in water owing to the surface carboxyl groups. The proton conductivity of CO, 3.9 × 10(-3) S cm(-1) at 90% relative humidity, is as high as that of graphene oxide (GO). Remarkably, CO exhibits much higher thermal stability than GO, with CO retaining the excellent proton conductivity as well as the capacitance performance even after thermal annealing at 200 °C. Our study demonstrates that the chemical modification of the abundant coal provides proton conductors that can be used in practical applications for a wide range of energy devices.

Optimization of proton conductivity in graphene oxide by filling sulfate ions.

Graphene oxide (GO) walled channels filled by sulfate ions exhibit an optimized proton conductivity, which is higher than the proton conductivity of all other forms of GO. The sulphate ion increases the water absorbing capacity and hydrogen bond reformation process in GO.

Metal permeation into multi-layered graphene oxide.

Understanding the chemical and physical properties of metal/graphene oxide (M/GO) interfaces is important when GO is used in electronic and electrochemical devices because the metal layer must be firmly attached to GO. Here, permeation of metal from the surface into GO paper bulk at the M/GO interface was observed at room temperature for metals such as Cu, Ag, Ni, Au, and Pt. Cu, Ag, and Ni quickly permeated GO as ions into the bulk under humid conditions. At first, these metals changed to hydrated ions as a result of redox reactions (with reduction of GO) at the surface, and then permeated the interlayers. Au and Pt were observed to permeate GO as atoms into the GO bulk at room temperature, although the permeation rates were low. These surprising results are considered to be due to the presence of many defects and/or edges with oxygenated groups in the GO paper.

DNA analysis based on toehold-mediated strand displacement on graphene oxide.

Fluorescent dye-labeled probe DNA was immobilized on fluorescence-quenching graphene oxide (GO) through a capture DNA. When targets were added, the probes were released from the GO through toehold-mediated strand exchange. Higher emission recovery and more signal contrast were achieved relative to conventional methods that are based on direct adsorption of probes.

Intense photoluminescence from ceria-based nanoscale lamellar hybrid.

Nanosheets, which are ultrathin inorganic crystals, have the potential to exhibit unique surface states and quantum effects. These nanosheets can be further manipulated to form lamellar structures for the fabrication of advanced hybrid nanomaterials. Here we report that conventionally nonluminescent ceria yields intense UV photoluminescence with an internal quantum yield (QY) of 59% when self-organized into a nanosheet lamellar architecture with dodecyl sulfate (DS) bilayers. The origin of luminescence exist at the organic/inorganic interfaces, where surface Ce(3+) ions of ceria nanosheet layers graft with DS anions to activate radiative 5d → 4f transition.

Simple photoreduction of graphene oxide nanosheet under mild conditions.

Graphene oxide (GO) nanosheets were reduced by UV irradiation in H2 or N2 under mild conditions (at room temperature) without a photocatalyst. Photoreduction proceeded even in an aqueous suspension of nanosheets. The GO nanosheets reduced by this method were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. It was found that epoxy groups attached to the interiors of aromatic domains of the GO nanosheet were destroyed during UV irradiation to form relatively large sp2 islands resulting in a high conductivity. I-V curves were measured by conductive atomic force microscopy (AFM; perpendicular to a single nanosheet) and a two-electrode system (parallel to the nanosheet). They revealed that photoreduced GO nanosheets have high conductivities, whereas nonreduced GO nanosheets are nearly insulating. Ag+ adsorbed on GO nanosheets promoted the photoreduction. This photoreduction method was very useful for photopatterning a conducting section of micrometer size on insulating GO. The developed photoreduction process based on a photoreaction will extend the applications of GO to many fields because it can be performed in mild conditions without a photocatalyst.