Yusuke Daiko

Ionic and Electronic Conductivities and Fuel Cell Performance of Oxygen Excess-Type Lanthanum Silicates

Highly dense pellets of an oxygen excess-type lanthanum silicate (La 9.333+x Si 6 O 26+1.5x , x > ca. 0.3, OE-LSO) were successfully fabricated, and their electrical conducting properties were studied. The replacement of Si by Al enhanced its conductivity, and the slightly Al-doped OE-LSO specimen [La 9.62 (Si 5.79 Al 0.21 )O 26.33 ] had excellent features as a solid electrolyte; that is, it had high ionic conductivity and was highly stable under reducing as well as oxidizing conditions at 873-1073 K. In addition, the ionic transference number was higher than 0.99. In the fuel cell utilizing this electrolyte (0.72 mm thick), (La 0.6 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3―δ cathode, and Ni―Ce 0.9 Gd 0.1 O 1.95―δ anode, good performance with the maximum power density of ca. 0.25 W cm ―2 was obtained at 1073 K. In addition, this electrolyte also had high compatibility with these conventional mixed conducting electrodes, according to an analysis near the electrode/electrolyte interfaces after the fuel cell test.

Sol-Gel Preparation of Fast Proton-Conducting P2O5-SiO2 Glasses

We have investigated the proton conductivities of the sol-gel-derived P2O5-SiO2 glass at −50 to 120°C. The obtained glass is porous, where the surface area, pore volume and pore diameter are 740 m2/g, 0.5 cm3/g and <5 nm, respectively. The freezing temperature of water molecules adsorbed in the pores was −20°C, which is much lower than that of free liquid water due to the quantum size effect of the water confined in the pores. The electrical conductivities followed the Arrhenius equation in the temperatures between −20 and 120°C. Below −20°C, the adsorbed-water molecules were frozen, resulting in a rapid decrease of the proton conductivity. Considering the high conductivity, chemical and thermal stability, this oxide glass membranes have potential for the fuel cell membrane.

Characterization of (Bi3.25Nd0.75)Ti3O12 Thin Films with a- and b-Axis Orientations Deposited on Nb:TiO2 Substrates by High-Temperature Sputtering

a- and b-axis-oriented (Bi3.25Nd0.75)Ti3O12 (BNT-0.75) films, 3.0 µm thick, were fabricated on conductive Nb:TiO2(101) substrates with 0.001–0.79 mass % Nb at 650 °C by high-temperature sputtering. All the films had a mostly single-phase orthorhombic structure and a- and b-axis orientations. The degree of a- and b-axis orientations was high, with values of ≥96%. BNT-0.75 films grown heteroepitaxially on Nb:TiO2(101) substrates containing 0.79 mass % Nb were comprised of nanoplate-like crystals and exhibited the best hysteresis loop shapes, with a remanent polarization (2Pr) of 29 µC/cm2 and a coercive field (2Ec) of 297 kV/cm.

Deposition of Ultrathin Nafion Layers on Sol–Gel-Derived Phenylsilsesquioxane Particles via Layer-by-Layer Assembly

Ultrathin layers of oppositely charged poly(diallyldimethylammonium chloride) (PDDA) and Nafion were alternately deposited on negatively charged phenylsilsesquioxane (PhSiO 3/2 ) microparticles via layer-by-layer assembly. An extremely small amount of Nafion was used for the layers. The average thickness of a Nafion layer was estimated to be 2.6 nm from the amount of deposited Nafion and the density. A monolithic sheet was obtained from PhSiO 3/2 particles with PDDA and the Nafion multilayer when the particles were pressed at 70 MPa. Fentons test, differential thermal analysis, and thermogravimetry revealed that PhSiO 3/2 particles are chemically and thermally stable. The monolithic samples prepared using PDDA/Nafion-multilayer-coated PhSiO 3/2 particles showed proton conductivities ∼4 orders of magnitude higher than those of samples without multilayers, and their conductivity reached about 10 -5 S/cm at 80°C and 80% relative humidity.

Preparation of P 2 O 5 ­ SiO2 Glasses with Proton Conductivity of ∼100 mS/cm at Room Temperature

Sol-gel-derived P 2 O 5 -SiO 2 glasses were investigated to prepare high proton conductors for fuel cell electrolyte. Glasses with a pore size of 2-30 nm diam were prepared using Si(OC 2 H 5 ) 4 , colloidal silica, PO(OCH 3 ) 3 , H 3 PO 4 , and POCl 3 , the pore properties of which were discussed to relate with the water absorption and proton conduction from conductivity measurements and relaxation of proton transfer. In the glasses with pores larger than 10 nm, high humidity is needed to absorb the water in pores, resulting in low proton conductivity. As the pore size decreases, the pores are filled with the water molecules at low humidity, which act to form the pathways of the proton transfer. The P 2 O 5 -SiO 2 glasses having 2 nm diam pores exhibited high conductivity of ∼100 mS/cm at 50°C and above 50% relative humidity.

Fabrication and Characterization of Nd-Substituted Bi4Ti3O12 Thin Films with

a- and b-axis-oriented (Bi4-xNdx)Ti3O12 (BNT; x = 0.5–1.0) films with 0.3 and 3.0 µm thicknesses, respectively, were fabricated on conductive IrO2(101)/Al2O3(012) substrates at 650 °C by high-temperature sputtering. The BNT films on the IrO2 electrode were preferentially a- and b-axis-oriented. The 3.0-µm BNT samples with x = 0.75–1.0 maintained a stable leakage current density range of 2.8 ×10-10–6.5 ×10-8 A/cm2 in the wide field range of 25–160 kV/cm. The remanent polarization (Pr) exhibited maxima (0.3 µm; 2Pr = 36 µC/cm2, 3.0 µm; 2Pr = 53 µC/cm2) at x = 0.75, regardless of film thickness. It is shown that the samples with x = 0.75–1.0 have relatively superior fatigue endurance due to a firm Bi/Nd–O bonding state formed with increasing Nd substitution.

a

Nanoporous glasses and nanoporous thin films were prepared using sol–gel method, and proton conductivities in nanopores of sol–gel-derived porous glasses and thin films are overviewed in this paper. Proton motions inside nanopores were monitored by impedance and nuclear magnetic resonance (NMR) spectroscopies. The impedance data is correlated with the proton motion in bulk scale, whereas NMR data is correlated with that in nanometer scale, respectively. From the comparison of the activation energies obtained from impedance and NMR spectroscopies, percolation of proton conducting path and its relation to the amount of absorbed water molecules are shown. In the case of nanoporous thin films, directions of pores can be controlled by using cationic and non-ionic surfactants. Relationship between direction of pores and proton conductivity is discussed based on impedance test results.

- and

A high proton-conducting P2O5-SiO2 nanoporous glass rod was prepared via sol–gel technique, and its tip was sharpened by a meniscus-etching method. The glass rod shows proton conductivity of 1 × 10−3 at room temperature after absorption of water molecules. A palm-sized proton gun was prepared by utilizing the glass rod as a H+ emitter. A high voltage (~2.5 kV) was applied between the tip of glass rod and an extraction electrode, and a high ionic current was successfully observed even under non-vacuum atmosphere at room temperature. Protonation reaction for polyaniline was confirmed from the structural changes of C=N in quinone to protonated C–N+. New applications of proton implantation will be expected especially in bioscience and medical technology.Graphical Abstract

b

Solid-state potentiometric thin film hydrogen gas sensors were successfully fabricated using a sol-gel-derived high proton-conducting P2O5-SiO2 glass films. Manganese oxide thin film coated on an indium tin oxide (ITO)-coated glass substrate was used for reference electrode. The sensor exhibited high speed responsibility within 10 s and 120 s at 30 oC and -30oC, respectively, for 1 vol.% hydrogen gas. A linear relationship between the electromotive force (EMF) and the logarithmic hydrogen concentration of 0.1~1 vol.% was obtained in the temperature ranging from -30 to 30 oC. The sensing mechanism was also discussed to improve the sensitivity and sensing speed against low H2 concentration at low temperatures.

-Axis Orientations by High-Temperature Sputtering

Polyalkoxysilsesquiazanes ([ROSi(NH)1.5]n, ROSZ, R = Et, nPr, iPr, nBu, sBu, nHex, sHex, cHex, decahydronaphthyl (DHNp)) were synthesized by ammonolysis at −78 °C of alkoxytrichlorosilane (ROSiCl3), which was isolated by distillation as a reaction product of SiCl4 and ROH. The simultaneous thermogravimetric and mass spectrometry analyses of the ROSZs under helium revealed a common decomposition reaction, the cleavage of the oxygen–carbon bond of the RO group to evolve alkene as a main gaseous species formed in-situ, leading to the formation of microporous amorphous Si–O–N at 550 °C to 800 °C. The microporosity in terms of the peak of the pore size distribution curve located within the micropore size range (<2 nm) and the total micropore volume, as well as the specific surface area (SSA) of the Si–O–N, increased consistently with the molecular size estimated for the alkene formed in-situ during the pyrolysis. The CO2 capture capacity at 0 °C of the Si–O–N material increased consistently with its SSA, and an excellent CO2 capture capacity of 3.9 mmol·g−1 at 0 °C and CO2 1 atm was achieved for the Si–O–N derived from DHNpOSZ having an SSA of 750 m2·g−1. The CO2 capture properties were further discussed based on their temperature dependency, and a surface functional group of the Si–O–N formed in-situ during the polymer/ceramics thermal conversion.