Tomohiro Ishiyama

Electrochemical substitution of sodium ions with protons in phosphate glass to fabricate pure proton conducting glass at intermediate temperatures

Electrochemical substitution involving electrochemical oxidation of hydrogen and proton injection into oxide glass accompanied by electrochemical reduction of alkali ions and discharge of metallic alkali out of the glass has recently been proposed as a proton injection technique. Herein, this electrochemical substitution technique was applied to phosphate glass with a composition of WO3–35NaO1/2–8NbO5/2–5LaO3/2–51PO5/2 (1W-glass). Temporal evolution of the substitution of sodium ions with protons was studied using a range of techniques. The concentration depth profiles of sodium ions and protons were mirror images of each other and the amount of injected protons quantitatively agreed with that of the decrease of sodium ions. The concentration of W5+ ions formed after substitution was only 3 ppm of the amount of injected protons, so reduction of W6+ ions in glass is not essential for proton injection. Raman spectra of the glasses indicated that the glass network structure did not change during electrochemical substitution; therefore, the protons substitute sodium ions not only quantitatively but also structurally. The glass after substitution attained a high proton concentration of 4.6–6.6 × 1021 cm−3 and pure proton conduction with a conductivity of 4.0 × 10−4 S cm−1 at 250 °C. A test fuel cell using electrochemically substituted 1W-glass as a solid electrolyte generated a maximum power density of 0.35 mW cm−2 operating at 250 °C.

Elucidating the origin of oxide ion blocking effects at GDC/SrZr(Y)O3/YSZ interfaces

Significant attention has been directed towards the enhancement of the efficiency of solid oxide fuel cells (SOFCs) for energy applications. One of the main issues of an industry-applicable cathode material, (La0.6Sr0.4)(Co0.2Fe0.8)O3−δ, used in conjunction with an yttria-stabilized zirconia (YSZ) electrolyte, is the formation of a SrZrO3 (SZ) phase which results from the chemical reaction between these two materials at high operating temperatures, notwithstanding the utilization of a diffusion barrier layer such as gadolinia-doped ceria (GDC). To gain a better understanding of the effect of the SZ phase on the oxide ion transport through SOFC heterostructures, dense GDC/SZ/YSZ and GDC/SZY(Y-doped SrZrO3)/YSZ multilayer films with well-defined interfaces are prepared using pulsed laser deposition (PLD). Isotope exchange depth profiling with secondary ion mass spectrometry (SIMS) is employed to probe oxide ion transport through the multilayer films, and advanced analytical techniques including scanning/transmission electron microscopy (S/TEM) and spatially resolved electron energy loss spectroscopy (EELS) are used to examine the heterointerfaces. A steep drop in the concentration of the 18O tracer SIMS depth profile reveals the existence of an oxide ion blocking effect occurring at the GDC/SZ(Y) and SZ(Y)/YSZ interfaces. By correlating this result with the EELS analysis of the O K-edge at SZ(Y)/YSZ interfaces, it was found that an interface-induced oxygen disorder possibly arising from Y diffusion across the interface could account for this behavior. Thermodynamic considerations have been made on the construction of plausible phase diagrams for the Sr–Y–Zr–O system to analyze the stability of interfaces. The implications of this interfacial disorder on the performance of SOFCs are discussed.

Correlation between Dissolved Protons in Nickel-Doped BaZr0.1Ce0.7Y0.1Yb0.1O3−δ and Its Electrical Conductive Properties

The electrical conductivity of nickel (2 wt %)-doped BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) acceptor-doped perovskite oxide was evaluated under air and a 1% H2 atmosphere. The partial conductivity was calculated from the total conductivity and the transport number of each carrier (tH+, tO2-, and th+) obtained using the concentration cell method. Its correlation with the dissolution state of the protons in the oxide as studied by in situ diffuse reflection Fourier transform infrared spectroscopy is discussed. When the concentration of protons that dissolved in BZCYYb-Ni was high, the proton partial conductivity was also high. An increase in hole conductivity in the high-temperature region in an air atmosphere was observed, suggesting that dissociation of protons strongly correlates with such a dominant carrier change. The dissociation of protons should be determined by the stability of protons in the oxide by the interaction with the lattice oxygen, and it was suggested that the dissolution state of protons can be controlled by modifying such stability in the oxide.

Effect of alkaline-earth species in phosphate glasses on the mobility of proton carriers

In this work, the effect of alkaline-earth species used in phosphate glasses on the mobility of proton carriers was investigated. Initially, 35NaO1/2–5RO–3NbO5/2–3LaO3/2–2GeO2–2BO3/2–50PO5/2 (R: Mg and Ba) glasses were subjected to the electrochemical substitution of Na+ ions with H+ at intermediate temperatures, producing proton conductors with high proton carrier concentrations (>8 × 1021 cm−3). The mobility of the proton carriers in the Ba glass was higher than that in the Mg glass. We studied the origin of this phenomenon in terms of the O–H and P–O bonding features using infrared spectroscopy and X-ray photoelectron spectroscopy, respectively. The higher mobility of proton carriers in Ba glass was attributed to weaker O–H bonding, which originated from the highly ionic character of Ba–O bonding compared with the comparatively covalent Mg–O bonding. Our findings indicate that a higher mobility of proton carriers can be achieved for glasses containing less electronegative glass network modifiers. The application of this strategy would be beneficial for the development of proton-conducting glass electrolytes for intermediate-temperature fuel cells.

Formation of Amorphous H3Zr2Si2PO12 by Electrochemical Substitution of Sodium Ions in Na3Zr2Si2PO12 with Protons

The sodium ions in Na3Zr2Si2PO12 (NASICON) were substituted with protons using an electrochemical alkali-proton substitution (APS) technique at 400 °C under a 5% H2/95% N2 atmosphere. The sodium ions in NASICON were successfully substituted with protons to a depth of <400 μm from the anode. Completely protonated NASICON, i.e., H3Zr2Si2PO12, was obtained to a depth <40 μm from the anode, although complete protonation of NASICON cannot be achieved by ion exchange in aqueous acid. H3Zr2Si2PO12 was amorphous, whereas the partially protonated NASICON was crystalline, and its unit cell volume decreased with an increase in the extent of substitution. Amorphous H3Zr2Si2PO12 was prepared by pressure-induced amorphization of the NASICON framework, in which an internal pressure of ∼3.5 GPa was induced by the substitution of large sodium ions with small protons during APS at 400 °C.