Unveiling the Origin of Fast Hydride Ion Diffusion at Grain Boundaries in Nanocrystalline TiN Membranes.

Nanocrystalline titanium nitride (TiN) has been determined to be a promising alternative to noble metal palladium (Pd) for fabricating base membranes for the energy-efficient production of pure hydrogen. However, the mechanism of transport of hydrogen through a TiN membrane remains unclear. In this study, we established an atomistic model of the transport of grain boundary hydride ions through such a membrane. High-resolution transmission electron microscopy and X-ray reflectivity confirmed that a nanocrystalline TiN1.0 membrane with a (100) preferred growth orientation retained about 4 Å-wide interfacial spaces along its grain boundaries. First-principles calculations based on the density functional theory showed that these grain boundaries allowed the diffusion of interfacial hydride ion defects with very small activation barriers (<12 kJ mol-1). This was substantiated by the experiment. In addition, the narrow boundary produced a sieving effect, resulting in a selective H permeation. Both the experimental and theoretical results confirmed that the granular microstructures with the 4 Å-wide interlayer enabled the transition metal nitride to exhibit pronounced hydrogen permeability.

Controlling Dielectric Film Defects to Increase the Breakdown Voltage of Conductive Polymer Solid Capacitors.

Aluminum solid polymer capacitors are promising devices for the increased demand for power electronics applications. Nonetheless, the low breakdown voltage of commercially available catalysts (∼100 V) limits their applications. In this study, a hydroxide-film-covered high-purity aluminum was anodized at 700 V in boric acid at 85 °C, and the effect of a second hot water immersion (posthydration treatment) after anodizing on the breakdown voltage was studied as a possible future treatment to enhance the withstand voltages of solid electrolytic capacitors. The dielectric breakdown voltage of the anodized aluminum with a PEDOT:PSS coating was ∼500 V, being ∼200 V less than the anodizing voltage; however, the dielectric breakdown voltage was increased above 700 V by introducing the posthydration treatment due to the formation of a nanovoid layer above the dielectric alumina film. Our research suggests that the highly dispersed nanovoids incorporated with PEDOT:PSS avoid the current concentration at some local regions, effectively increasing the dielectric breakdown voltage. The posthydration treatment increased the leakage current by introducing physical defects in the dielectric film. However, the leakage current was reduced by a voltage sweep below the breakdown voltage after the PEDOT:PSS coating or a second anodizing process before the coating, keeping the breakdown voltage above 600 V. A promising processing route to obtain aluminum solid capacitors with high withstand voltage (600 V) found in our research is, first, dipping in hot water; second, anodizing at 700 V; then a second hot water treatment; and a second anodizing at 400 V, which keeps the capacitance invariable with a breakdown voltage enhanced.

Lithiation of Anodic Magnetite-Hematite Nanotubes Formed on Iron.

Electrochemically active iron oxide nanotubes formed by anodization are of high interest as battery components in various battery systems due to their 1D geometry, offering high volume expansion tolerance and applications without the use of binders and conductive additives. This work takes a step forward toward understanding lithium-ion storage in 1D nanotubes through the analysis of differential capacity plots d(Q - Q0)·dE-1 supported by in situ Raman spectroscopy observations. The iron oxide nanotubes were synthesized by anodizing polycrystalline iron and subsequently modified by thermal treatment in order to control the degree of crystallinity and the ratio of hematite (Fe2O3) to magnetite (Fe3O4). The electrochemical fingerprints revealed a quasi-reversible lithiation/delithiation process through Li2O formation. Significant improvement in electrochemical performance was found to be related to the high degree of crystallinity and the increase of the hematite (Fe2O3) to magnetite (Fe3O4) ratio. In situ mechanistic studies revealed a reversible reduction of iron oxide to metallic iron simultaneously with Li2O formation.

Facile synthesis approach of bifunctional Co-Ni-Fe oxyhydroxide and spinel oxide composite electrocatalysts from hydroxide and layered double hydroxide composite precursors.

Zinc-air batteries (ZABs) are promising candidates for the next-generation energy storage systems, however, their further development is severely hindered by kinetically sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Facile synthesis approaches of highly active bifunctional electrocatalysts for OER and ORR are required for their practical applications. Herein, we develop a facile synthesis procedure for composite electrocatalysts composed of OER-active metal oxyhydroxide and ORR-active spinel oxide containing Co, Ni and Fe from composite precursors consisting of metal hydroxide and layered double hydroxide (LDH). Both hydroxide and LDH are simultaneously produced by a precipitation method with a controlled molar ratio of Co2+, Ni2+ and Fe3+ in the reaction solution, and calcination of the precursor at a moderate temperature provides composite catalysts of metal oxyhydroxides and spinel oxides. The composite catalyst shows superb bifunctional performances with a small potential difference of 0.64 V between a potential of 1.51 V vs. RHE at 10 mA cm-2 for OER and a half-wave potential of 0.87 V vs. RHE for ORR. The rechargeable ZAB assembled with the composite catalyst as an air-electrode exhibits a power density of 195 mA cm-2 and excellent durability of 430 hours (1270 cycles) of a charge-discharge cycle test.

Enhanced Performance of Protonic Solid Oxide Steam Electrolysis Cell of Zr-Rich Side BaZr0.6Ce0.2Y0.2O3-δ Electrolyte with an Anode Functional Layer.

Proton-conducting solid oxide electrolysis cells (H-SOEC) containing a 15-µm-thick BaZr0.6Ce0.2Y0.2O3-δ (BZCY622) electrolyte thin film, porous cathode cermet support, and La0.6Sr0.4Co0.2Fe0.8O3-δ anodes were fabricated using a reactive cofiring process at approximately 1400 °C. Steam electrolysis was conducted by supplying wet air to the anode at a water partial pressure of 20 kPa. The performance was evaluated using electrochemical measurements and gas chromatography. At 600 °C, the cells generated an electrolysis current of 0.47 A cm-2 at a 1.3 V bias while the Faradaic efficiency reached 56% using 400 mA cm-2. The electrolysis performance was efficiently improved by introducing a 40-nm-thick La0.5Sr0.5CoO3-δ (LSC) nanolayer as an anode functional layer (AFL). The cells with LSC AFL produced an electrolysis current of 0.87 A cm-2 at a 1.3 V bias at 600 °C, and the Faradaic efficiency reached 65% under 400 mA cm-2. Impedance analysis showed that the introduction of the AFL decreased the ohmic resistances and improved interfacial proton transfer across the anode/electrolyte interface and polarization resistances related to the anode reaction. These results demonstrate opportunities for future research on AFL to improve the performance of H-SOECs with Zr-rich BaZr x Ce1-x-y Y y O3-δ electrolytes.

Slippery Liquid-Infused Porous Surfaces on Aluminum for Corrosion Protection with Improved Self-Healing Ability.

Slippery liquid-infused porous surfaces (SLIPSs) can be formed by impregnating lubricants in porous surfaces with low surface energy. In this study, SLIPSs have been obtained on practically important aluminum with a porous anodic alumina layer by impregnating lubricants containing organic additives. The additive-containing lubricants change the surface slippery even without prior organic coating of the porous alumina surface. The additive-containing SLIPSs reveal a low water sliding angle of <5° and markedly improved corrosion resistance in an acetic acid solution containing chloride. The SLIPSs are formed by the in situ adsorption of the organic additives on the porous alumina surface. The scratched defects induce corrosion of the organic coating-type SLIPSs, whereas the additive-containing SLIPSs sustain high corrosion resistance even after introducing scratch defects. The adsorption of the organic additive in lubricants and refilling of the lubricant are responsible for the self-healing of the corrosion resistance. Thus, the additive-containing SLIPSs are promising self-healing corrosion-resistant surfaces.

In Situ Activation of a Manganese Perovskite Oxygen Reduction Catalyst in Concentrated Alkaline Media.

The reaction pathway of the oxygen reduction reaction (ORR) is strongly affected by the electrolytic environment. Meanwhile, the ORR mechanism on transition-metal oxide catalysts has not been studied intensely in very concentrated alkaline solutions that are used in practical metal-air batteries. Herein, we report the in situ activation of ORR catalysis on manganese perovskite in a concentrated alkaline solution, mediated by the spontaneous formation of oxygen vacancy sites. Electrochemical analyses of the (100) epitaxial film electrodes reveal that the exchange current and electron number of the ORR on La0.7Sr0.3Mn0.9Ni0.1O3 significantly increase with the duration of the ORR when the KOH concentration is greater than 4 M. However, these values remain unchanged with time at less than 1 M KOH concentration. Operando synchrotron X-ray spectroscopy of the (100) epitaxial film confirmed that La0.7Sr0.3Mn0.9Ni0.1O3 involves the oxygen vacancy sites with the reduction of Mn atoms in concentrated KOH solution via the hydroxylation decomposition of perhydroxyl intermediates. Hence, the O2 adsorption switched from an end-on to a bidentate mode because the cooperative active sites of the oxygen vacancy and neighboring Mn allow bidentate adsorption of the dissolved O2. Due to the simultaneous interaction with the oxygen vacancy and Mn sites, the O-O bonds are activated and the potential barrier for the electron transfer to adsorbed O2 is lowered, resulting in a shift in the reaction mechanism from that involving an indirect "2 + 2" transfer pathway to a direct 4-electron pathway.

Activation of Catalytically Active Edge-Sharing Domains in Ca2FeCoO5 for Oxygen Evolution Reaction in Highly Alkaline Media.

Rechargeable zinc-air batteries are considered as one of the possible candidates to replace conventional lithium-ion batteries. One of the requirements for effective battery operation is an oxygen evolution reaction (OER) that needs to be generated in a highly alkaline electrolyte. The A2BB'O5 brownmillerite-type Ca2FeCoO5 electrocatalyst having a 57 Pbcm symmetry exhibits very high electrocatalytic activity toward OER in 4 mol dm-3 KOH. Our studies show that the electrocatalyst undergoes bulk amorphization upon OER and adequately activates catalytically active domains. The synchrotron radiation studies using the extended X-ray absorption fine structure (EXAFS) technique show that the central structural unit found in the polarized Ca2FeCoO5 is a cluster of edge-sharing CoO6 octahedra. The electrochemical data indicate that OER preferentially takes place on the edge-sharing CoO6 octahedra catalytic centers reconstructed in the brownmillerite-type electrocatalyst. The EXAFS second shell peaks at an interatomic distance of 2.8 Å are the fingerprints of the catalytically active domains.

High dispersion and oxygen reduction reaction activity of Co3O4 nanoparticles on platelet-type carbon nanofibers.

In this study, platelet-type carbon nanofibers prepared by the liquid phase carbonization of polymers in the pores of a porous anodic alumina template were used to prepare the Co3O4/carbon electrocatalysts. For comparison, Co3O4 nanoparticles were also deposited on multiwall carbon nanotubes (MWCNTs). Both the nitrogen-free platelet-type carbon nanofibers (pCNFs) and the nitrogen-containing analogue (N-pCNFs) exhibited better dispersion and higher amount of deposited Co3O4 nanoparticles compared to the MWCNTs. In addition, many individual Co3O4 nanoparticles were deposited separately on pCNF and N-pCNF, whereas aggregated deposition was commonplace on MWCNTs. The results indicated that the side wall of the pCNFs, which consisted of carbon edge planes, was the preferential nucleation site of Co3O4 nanoparticles rather than the basal planes of carbon that predominated the surface of the MWCNTs. The oxygen reduction reaction (ORR) activity of the Co3O4/pCNF composite in 0.1 mol dm-3 KOH solution was better than that of Co3O4/MWCNTs. The N-pCNF further enhanced the ORR activity of the Co3O4/pCNFs even though the dispersion and supported amount of Co3O4 nanoparticles were negligibly affected by the presence of the nitrogen species. Synergistic interactions of the Co3O4 nanoparticles with N-doped CNFs contributed to the increased ORR activity.

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

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.

Evaluation of thin film fuel cells with Zr-rich BaZr x Ce0.8-x Y0.2O3-δ electrolytes (x ≥ 0.4) fabricated by a single-step reactive sintering method.

This paper reports a survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich BaZr x Ce0.8-x Y0.2O3-δ (x = 0.4, 0.6, 0.7, and 0.8) proton-conducting electrolytes, which were fabricated by single step co-firing with Zn(NO3)2 additives at a relatively low temperature (1400 °C). The grain sizes significantly increased to several µm for x = 0.4 and 0.6, whereas the grain sizes remained in the sub-µm ranges for x = 0.7 and 0.8, which resulted in large gaps of the fuel cell performances at x over and below 0.6. The cells for x = 0.4 and 0.6 exhibited efficient power generation, yielding peak powers of 279 and 336 mW cm-2 at 600 °C, respectively, which were higher than those of the corresponding cells previously reported. However, the performances abruptly deteriorated with the increasing x to more than 0.7 because the electrolyte films were highly resistive due to the coarse-grained microstructures. Impedance spectroscopy for the dense sintered BaZr x Ce0.8-x Y0.2O3-δ discs confirmed that the total proton conductivity of BaZr0.6Ce0.2Y0.2O3-δ was higher than that of BaZr0.4Ce0.4Y0.2O3-δ at temperatures above 500 °C despite relatively small grain sizes. In addition, BaZr0.6Ce0.2Y0.2O3-δ cells could gain a stable current throughout a continuous run for a few days under CO2-containing fuel supply, which was due to high fraction of thermodynamically stable BaZrO3 matrices. It was demonstrated that BaZr0.6Ce0.2Y0.2O3-δ is a promising electrolyte for proton-conducting ceramic fuel cells with excellent proton conductivity and CO2 tolerance at intermediate temperatures.

Low-Temperature Oxygen Storage of CrIV-CrV Mixed-Valence YCr1-xPxO4-δ Driven by Local Condensation around Oxygen-Deficient Orthochromite.

The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by heating-cooling cycles in the temperature range from 50 to 600 °C. YCr1-xPxO4 started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of CrV to CrIV, while the reduced YCr1-xPxO4-δ phase was significantly reoxidized via absorbing oxygen by cooling to 50 °C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-xPxO4-δ phases were stabilized by forming linking polyhedral CrIV2O76- via corner sharing between oxygen-deficient CrIVO32- and adjacent CrIVO44-. YCr1-xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol-1 probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.

Brownmillerite-type Ca2 FeCoO5 as a Practicable Oxygen Evolution Reaction Catalyst.

Here, we report remarkable oxygen evolution reaction (OER) catalytic activity of brownmillerite (BM)-type Ca2 FeCoO5 . The OER activity of this oxide is comparable to or beyond those of the state-of-the-art perovskite (PV)-catalyst Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-δ (BSCF) and a precious-metal catalyst RuO2 , emphasizing the importance of the characteristic BM structure with multiple coordination environments of transition metal (TM) species. Also, Ca2 FeCoO5 is clearly advantageous in terms of expense/laboriousness of the material synthesis. These facts make this oxide a promising OER catalyst used in many energy conversion technologies such as metal-air secondary batteries and hydrogen production from electrochemical/photocatalytic water splitting.

Corrosion protection of iron using porous anodic oxide/conducting polymer composite coatings.

Conducting polymers (CPs), including polypyrrole, have attracted attention for their potential in the protection of metals against corrosion; however, CP coatings have the limitation of poor adhesion to metal substrates. In this study, a composite coating, comprising a self-organized porous anodic oxide layer and a polypyrrole layer, has been developed on iron. Because of electropolymerization in the pores of the anodic oxide layer, the composite coating showed improved adhesion to the substrate along with prolonged corrosion protection in a NaCl aqueous corrosive environment. The anodic oxide layers are formed in a fluoride-containing organic electrolyte and contain a large amount of fluoride species. The removal of these fluoride species from the oxide layer and the metal/oxide interface region is crucial for improving the corrosion protection.

Percolative proton conductivity of sol-gel derived amorphous aluminosilicate thin films.

The finite size effect of proton conductivity of amorphous aluminosilicate thin films, a-Al(n)Si(1-n)O(x) (n = 0.07, 0.1, 0.2, 0.3 and 0.45), prepared by a sol-gel process was investigated by experimental and numerical techniques. High-resolution TEM clarified that a-Al(n)Si(1-n)O(x) films had the heterogeneous nanoscale microstructures comprised of the ion-conducting, condensed glass microdomain and the poor-conductive, uncondensed glass microdomain. σ of the films with n≤0.1 exponentially increased upon decreasing thickness in the sub-100 nm range because the volume fraction of conductive domains was less than the percolation threshold and cluster size scaling of the conductive domain was operative. The numerical simulation suggested that conductance of the condensed domain was higher than that of the uncondensed domain by 2 orders of magnitude. Volume fractions of the condensed domain increased with increasing Al/Si molar ratio and were over the percolation threshold (24.5%) with n≥0.2. However, conductance of the condensed domain decreased with increasing Al/Si ratio with n≥0.2 because the aluminosilicate glass framework made of 4-fold-connected MO(4) tetrahedra was deformed by forming the octahedral AlO(6) moieties, as checked by Al K-edge XAS. It was found that the optimal Al/Si composition in terms of the conductance of the condensed domain is not in coincidence with that in terms of the average conductivity of the films.

Fabrication of super-oil-repellent dual pillar surfaces with optimized pillar intervals.

Hierarchical dual pillar surfaces with optimized pillar intervals are fabricated by a novel combined process of the oblique angle magnetron sputtering deposition of Al-Nb alloys and their anodizing. The pillar intervals are controlled by the deposition angle and cell size of a scalloped substrate for oblique angle deposition. Anodizing of the deposited pillar surfaces develops a nanopillar oxide layer, producing the hierarchical dual pillar surfaces. After being coated with a fluoroalkyl phosphate layer to reduce the surface free energy, hierarchical surfaces with submicrometer pillar intervals greater than 400 nm show super liquid repellency even for hexadecane with a low surface tension of 27.5 mN m(-1), although the submicrometer pillar surfaces with smaller submicrometer pillar intervals and without nanopillars were not super-oil-repellent. In contrast, the dual pillar surfaces show superhydrophobicity regardless of the submicrometer pillar intervals. Thus, the present study demonstrates the importance of the pillar intervals (gap size between pillars) to realize the superoleophobicity.

Enhanced capacitance of composite anodic ZrO2 films comprising high permittivity oxide nanocrystals and highly resistive amorphous oxide matrix.

Anodic oxide films with nanocrystalline tetragonal ZrO(2) precipitated in an amorphous oxide matrix were formed on Zr-Si and Zr-Al alloys and had significantly enhanced capacitance in comparison with those formed on zirconium metal. The capacitance enhancement was associated with the formation of a high-temperature stable tetragonal ZrO(2) phase with high relative permittivity as well as increased ionic resistivity, which reduces the thickness of anodic oxide films at a certain formation voltage. However, there is a general empirical trend that single-phase materials with higher permittivity have lower ionic resistivity. This study presents a novel material design based on a nanocrystalline-amorphous composite anodic oxide film for capacitor applications.

Finite size effect of proton-conductivity of amorphous silicate thin films based on mesoscopic fluctuation of glass network.

The finite size effect of proton conductivity of amorphous silicate thin films, a-M(0.1)Si(0.9)O(x) (M = Al, Ga, Hf, Ti, Ta, and La), was investigated. The proton conductivity across films, σ, was measured in dry air by changing the thickness in the range of 10-1000 nm. σ of the films with M = Al, Ga, and Ta was elevated in a power law by decreasing thickness into less than a few hundred nanometers, and the increment was saturated at a thickness of several 10's of nanometers. On the other hand, σ of the films with M = Hf, Ti, and La was not related to the decrease of the thickness in the range of >10 nm. Thickness-dependent conductivity of the former could be numerically simulated by a percolative resistor network model that involves the randomly distributed array of two kinds of resistors R(1) and R(2) (R(1) > R(2)) in the form of a simple cubic-type lattice. High-resolution TEM clarified that a-M(0.1)Si(0.9)O(x) films involved heterogeneous microstructures made of the condensed domain and the surrounding uncondensed matrix due to the fluctuation of glass networks on the nanometer scale. The condensed domain had a wormlike shape with an average length of several 10's of nanometers and performed the role of the proton conduction pathway penetrating through the poorly conducting matrix. It was concluded that the thickness-dependent conductivity could be identical to finite-size scaling of the percolative network of the interconnected domains in the nanometer range.