Enhancing proton mobility and thermal stability in phosphate glasses with WO3: the mixed glass former effect in proton conducting glasses.

This study aimed to investigate the impact of WO3 on the thermal stability of glass, as measured by the glass transition temperature (Tg), as well as the activation energy (Ea) of proton conduction and proton mobility (µH). These parameters were analyzed based on variations in the glass network structure and the nature of the P-O and O-H bonds in 35HO1/2-xWO3-8NbO5/2-5LaO3/2-(52 - x) PO5/2 (x = 2, 4, 6, and 8) glasses. As previously predicted by a linear regression model, replacing PO5/2 with WO3 resulted in an increase in Tg and µH at Tg. The observed enhancement rates were +9.1 °C per mol% of WO3 for Tg and 0.09 per mol% of WO3 for log(µH at Tg [cm2 V-1 s-1]), which aligned with the predicted values of +6.5 °C and 0.08, respectively, validating the linear regression model. The increased Tg was attributed to the formation of heteroatomic P-O-W linkages that tightly cross-linked the phosphate chains. The decrease in Ea and increase in µH at Tg with increasing WO3 content were attributed to the reduction of the energy barrier for inter-phosphate chain proton migration owing to the increasing proton migration paths through P-O-W linkages. This µH enhancement is distinct from previously reported ones due to the reduction of the energy barrier for proton dissociation from OH groups. This phenomenon can be attributed to the mixed glass former effect in proton conducting glass.

Anhydrous Silicophosphoric Acid Glass: Thermal Properties and Proton Conductivity.

Anhydrous silicophosphoric acid glass with an approximate composition of H5 Si2 P9 O29 was synthesized and its thermal and proton-conducting properties were characterized. Despite exhibiting a glass transition at 192 °C, the supercooled liquid could be handled as a solid up to 280 °C owing to its high viscosity. The glass and its melt exhibited proton conduction with a proton transport number of ∼1. Although covalent O-H bonds were weakened by relatively strong hydrogen bonding, the proton conductivity (4×10-4  S cm-1 at 276 °C) was considerably lower than that of phosphoric acid. The high viscosity of the melt was due to the tight cross-linking of phosphate ion chains by six-fold-coordinated Si atoms. The low proton conductivity was attributed to the trapping of positively charged proton carriers around anionic SiO6 units (expressed as (SiO6/2 )2- ) to compensate for the negative charges.

Proton transport properties of proton-conducting phosphate glasses at their glass transition temperatures.

The proton transport properties of 32 kinds of proton-conducting phosphate glasses with broad ranges of glass transition temperature, proton conductivity, and the proton carrier concentration were studied. Almost constant proton mobility of around 2 × 10-8 cm2 V-1 s-1 at the glass transition temperature, corresponding to a diffusion coefficient of approximately 4 × 10-10 cm2 s-1, was found for the glasses. The reason why the diffusion coefficient of protons is almost constant in various proton-conducting phosphate glasses was discussed based on the role of the protons as a cross-linker within the phosphate framework via hydrogen bonding. We evaluated the highest proton conductivity of the phosphate glasses and melts based on the almost constant mobility at their glass transition temperatures and obtained a highest expected proton conductivity of 7.5 × 10-3 S cm-1 at 300 °C. The potential of proton-conducting phosphate glasses as electrolytes in intermediate temperature fuel cells was also 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.

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.

The mobility of proton carriers in phosphate glasses depends on polymerization of the phosphate framework.

Proton conducting phosphate glasses were prepared by electrochemical substitution of sodium ions with protons applied to glasses with the compositions xNaO1/2-1WO3-8NbO5/2-5LaO3/2-(86 - x)PO5/2 (x = 28, 32, 35, 38, and 40). The mobilities of proton carriers in the glasses were studied in terms of the polymerization degree of the phosphate framework. The proton mobility at 200 °C increased as the depolymerization of the phosphate framework developed up to x = 38, and decreased at x = 40. On the basis of Raman and infrared spectra measurements of the O-H stretching vibration region, the decreasing mobility at x > 38 was attributed to the increasing concentration of protons trapped by non-bridging oxygen in P2O74- ions, owing to strong O-H bonding. We found that the highly polymerized phosphate framework decreased the mobility of proton carriers, not because of suppression of the proton dissociation from oxygen atoms but rather the suppression of the proton migration. The compositions at which the phosphate framework was sufficiently depolymerized and did not contain P2O74- ions as a main component, achieved high mobility of proton carriers in phosphate glasses.

Structural change of NaO1/2-WO3-NbO5/2-LaO3/2-PO5/2 glass induced by electrochemical substitution of sodium ions with protons.

Structural changes of 35NaO1/2-1WO3-8NbO5/2-5LaO3/2-51PO5/2 glass (1W-glass) before and after the electrochemical substitution of sodium ions with protons by alkali-proton substitution (APS) are studied by Raman and (31)P magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies. The glass before APS consists of (PO3(-))8.6(P2O7(4-)) chains on average and the terminal Q(1) units (-O-PO3(3-)) are bound to MO6 octahedra (M denotes niobium or tungsten) through P-O-M bonds. Some non-bridging oxygens (NBOs) in the MO6 octahedra are present in addition to the bridging oxygens (BOs) in P-O-M bonds. APS induces fragmentation of the phosphate chains because the average chain length decreases to (PO3(-))3.7(P2O7(4-)) after APS, despite the total number of modifier cations of sodium and lanthanum ions and protons being unaffected by APS. This fragmentation is induced by some of the NBOs in the MO6 octahedra before APS, changing to BOs of the newly formed M-O-P bonds after APS, because of the preferential formation of P-OH bonds over M-OH ones in the present glass. We show that APS under the conditions used here is not a simple substitution of sodium ions with protons, but it is accompanied by the structural relaxation of the glass to stabilize the injected protons.

Laser Ablation Nanoarchitectonics of Au-Cu Alloys Deposited on TiO2 Photocatalyst Films for Switchable Hydrogen Evolution from Formic Acid Dehydrogenation.

The regulation of H2 evolution from formic acid dehydrogenation using recyclable photocatalyst films is an essential approach for on-demand H2 production. We have successfully generated Au-Cu nanoalloys using a laser ablation method and deposited them on TiO2 photocatalyst films (Au x Cu100-x /TiO2). The Au-Cu/TiO2 films were employed as photocatalysts for H2 production from formic acid dehydrogenation under light-emitting diode (LED) irradiation (365 nm). The highest H2 evolution rate for Au20Cu80/TiO2 is archived to 62,500 µmol h-1 g-1 per photocatalyst weight. The remarkable performance of Au20Cu80/TiO2 may account for the formation of Au-rich surfaces and the effect of Au alloying that enables Cu to sustain the metallic form on its surface. The metallic Au-Cu surface on TiO2 is vital to supply the photoexcited electrons of TiO2 to its surface for H2 evolution. The rate-determining step (RDS) is identified as the reaction of a surface-active species with protons. The results establish a practical preparation of metal alloy deposited on photocatalyst films using laser ablation to develop efficient photocatalysts.

Nanoengineering of cathode layers for solid oxide fuel cells to achieve superior power densities.

Solid oxide fuel cells (SOFCs) are power-generating devices with high efficiencies and considered as promising alternatives to mitigate energy and environmental issues associated with fossil fuel technologies. Nanoengineering of electrodes utilized for SOFCs has emerged as a versatile tool for significantly enhancing the electrochemical performance but needs to overcome issues for integration into practical cells suitable for widespread application. Here, we report an innovative concept for high-performance thin-film cathodes comprising nanoporous La0.6Sr0.4CoO3-δ cathodes in conjunction with highly ordered, self-assembled nanocomposite La0.6Sr0.4Co0.2Fe0.8O3-δ (lanthanum strontium cobalt ferrite) and Ce0.9Gd0.1O2-δ (gadolinia-doped ceria) cathode layers prepared using pulsed laser deposition. Integration of the nanoengineered cathode layers into conventional anode-supported cells enabled the achievement of high current densities at 0.7 V reaching ~2.2 and ~4.7 A/cm2 at 650 °C and 700 °C, respectively. This result demonstrates that tuning material properties through an effective nanoengineering approach could significantly boost the electrochemical performance of cathodes for development of next-generation SOFCs with high power output.

Understanding the effect of oxide components on proton mobility in phosphate glasses using a statical analysis approach.

The models to describe the proton mobility (µ H) together with the glass transition temperature (T g) of proton conducting phosphate glasses employing the glass composition as descriptors have been developed using a statical analysis approach. According to the models, the effects of additional HO1/2, MgO, BaO, LaO3/2, WO3, NbO5/2, BO3/2 and GeO2 as alternative to PO5/2 were found as following. µ H at T g is determined first by concentrations of HO1/2 and PO5/2, and µ H at T g increases with increasing HO1/2 concentration and decreasing PO5/2. The component oxides are categorized into three groups according to the effects on µ H at T g and T g. The group 1 oxides increase µ H at T g and decrease T g, and HO1/2, MgO, BaO and LaO3/2 and BO3/2 are involved in this group. The group 2 oxides increase both µ H at T g and T g, and WO3 and GeO2 are involved in this group. The group 3 oxides increase T g but do not vary µ H at T g. Only NbO5/2 falls into the group 3 among the oxides examined in this study. The origin of the effect of respective oxide groups on µ H at T g and T g were discussed.