Yoshitaka Aoki

Phase Transformation and Capacitance Enhancement of Anodic ZrO2 – SiO2

Title Phase Transformation and Capacitance Enhancement of Anodic ZrO2-SiO2 Author(s) Koyama, S.; Aoki, Y.; Sakaguchi, N.; Nagata, S.; Habazaki, H. Citation Journal of the Electrochemical Society, 157(12): C444-C451 Issue Date 2010-10-22 Doc URL http://hdl.handle.net/2115/44391 Rights

Enhanced hydrogen permeability of hafnium nitride nanocrystalline membranes by interfacial hydride conduction

Hydrogen permeability based on mixed hydride ion electron conduction was demonstrated for hafnium nitride HfNx (film thickness of 100–500 nm, x = 0.8 and 1.0) nanocrystalline membranes. Nanocrystalline films with a (100) orientation and crystallite sizes of a few tens of nanometers were prepared on porous alumina supports by radio frequency (RF) reactive sputtering. Combined spectroscopic, permeability, and microbalance analysis suggests that the nanocrystalline matrices were readily hydrogenated by the formation of Hf–H terminal groups on the internal grain surfaces at ambient temperature and thus efficient hydrogen permeation took place due to an enhanced diffusion of hydridic defects through the grain boundaries; this was further aided by the Hf–H bond exchange process. Hence, membranes with an average crystallite size of 11 nm yielded a hydrogen flux of 6 × 10−7 mol cm−2 s−1 at 25 °C at an applied hydrogen partial pressure of 50 kPa; this value is higher than those exhibited by the current state-of-the-art Pd membranes. These findings establish a new concept for Pd alternatives based on the pronounced hydric conductivity of transition metal nitride nanomaterials.

High-Efficiency Direct Ammonia Fuel Cells Based on BaZr0.1Ce0.7Y0.2O3− δ /Pd Oxide-Metal Junctions

Abstract A direct ammonia‐type intermediate temperature fuel cell is examined by means of a hydrogen membrane fuel cell (HMFC) comprising 1‐µm‐thick BaZr0.1Ce0.7Y0.2O3− δ (BZCY) thin‐film electrolyte and Pd solid anode. It generates the maximum power density of 0.58 W cm−2 at 600 °C with ammonia fuels, and this value is found to be three times larger than the champion data of the recently reported direct ammonia‐type proton‐conducting ceramic fuel cells (PCFCs). AC impedance spectroscopy is performed to determine the interfacial polarization resistances, disclosing that the anodic overpotentials of HMFCs are at least one order of magnitude smaller than those of anode‐supported PCFC under relatively high DC outputs. The anode reactions are driven by the oxidation of monoatomic hydrogen dissolving at the BZCY/Pd solid–solid interface, mediated via proton transfer from Pd to BZCY. The electrochemical analysis reveals that the BZCY/Pd junction forms Ohmic contact without growth of wide depletion layer and thus facilitates the proton transfer reactions because the interfacial region beneath Pd electrode can accommodate amounts of protonic defects as well as the bulk of BZCY due to the small depletion of holes under hole–proton thermodynamic equilibrium.