Reversible water uptake and release of pseudo-cubic type La0.7Sr0.3Mn1- xNixO3 at intermediate temperatures

Solid oxide fuel cells (SOFCs) based on oxide-ion conducting electrolytes possess several attractive advantages such as high energy conversion, low pollutant emission and fuel flexibility. However, SOFCs suffer from the high operating temperatures 800-1000 °C; such high temperature operations result in the increase of costs and lessened lifetimes of materials. Hence, there exists a strong demand to decrease the working temperature into intermediate temperature (IT) region below 600 °C. Proton conducting ceramic fuel cells (PCFCs) is a kind of promising IT-fuel cells operating at around 400-600 °C because of lower activation energies of proton conductivity than oxide-ion conductivity. Recently Choi et al [1] reported that PCFC with BaZr0.4Ce0.4Y0.1Yb0.1O3 electrolyte exceeds 500 mW cm-2 at 500 °C, however, the performance still lags far behind the predicted values that is over 1.0 W cm-2 at 500°C. There are two major challenges, one is big ohm resistance of Zr-rich Ba(Zr, Ce, Y)O3 (BZCY) electrolyte, and the other one is lack of highly efficient cathode specially designed for PCFCs [2]. Since most of the cobaltite base cathodes are oxide-ion conductors, the mismatch of main ionic carriers between cathode and electrolytes limits the efficient cathodic reaction area into cathode-electrolyte-gas triple boundaries. Hence, it is motivated to develop cathode catalysts which exhibit sufficient proton conductivity in order to extend the efficient reaction zone and thus reduce cathode overpotentials and finally increase reaction efficiency. The protonic defects are incorporated into oxides via hydration reaction, whereas, many oxides do not have enough large hydration enthalpy [3-5] and thus, the reaction is less-pronounced at elevated temperatures. Please click Additional Files below to see the full abstract

The Corrosion Behavior of Sputter-Deposited Magnesium-Valve Metal Alloys

An attempt was made for preparation of magnesium alloys with valve metals, such as titanium, zirconium, niobium and tantalum whose melting points far exceed the boiling point of magnesium. These alloys became single phase solid solutions in wide composition ranges, but were crystalline in contrast to the fact that other alloys with valve metals such as nickel-, copper-and aluminum-base alloys were amorphous in wide composition ranges. The alloys containing sufficient amounts of valve metals showed high corrosion resistance due to spontaneous passivation in 1 M HCl at 30℃. The high corrosion resistance was attributed to the formation of passive oxyhydroxide films in which valve metal cations were remarkably concentrated. However, because of crystalline alloys and because of the presence of active magnesium, their corrosion resistance is lower than that of valve metals

Manganese oxide base electrocatalysts for proton-conducting ceramic cells

There has been a strong interest in clean and renewable energy sources due to finite fossil fuel sources, increasing oil prices and environmental concerns. Hydrogen is regarded as the leading candidate fuel, because it releases only H2O during combustion and it is compatible to use in high efficiency fuel system. Steam reforming of hydrocarbon gas is currently the main way to produce hydrogen but still relies on fossil fuel consumption. On the contrary, water electrolysis using electric power generated by renewable energy is attracted as sustainable hydrogen production method. Especially, steam electrolysis using solid electrolyte cells is promising for efficient hydrogen production because high-temperature heat partly offers the energy for water electrolysis, leading favorable kinetics and thermodynamics. Hence, it is motivated to investigate on solid oxide electrolysis cell (SOEC) using proton-conducting ceramics to achieve highly efficient conversion from electrical power into chemical fuel gas directly. However, sufficient performance has not be achieved yet in the current system because large overpotential is needed for oxygen evolution reaction at anode owing to the relatively slow kinetics and the limited active zone in the anode/electrolyte interfaces due to the mismatch of ionic carries, Accordingly, it is a great challenge to develop high performance oxygen electrode with efficient electrocatalytic ability for 4 electron transfer oxygen evolution reaction. Recently, it is reported that high valence state metal oxide reveal superior electrocatalytic activity for water oxidation s because the energy levels between the occupied metal orbital and the O 2p orbital are very close, causing a strong hybridization and facilitating o-o bond formation. Herein, we examined electrocatalytic performance of high valence state Mn(V) oxide Ba3(MnO4)2 as an anode for SOEC This oxide has been reported to be very stable at elevated temperatures in oxidative conditions. Proton-conducting BaZr0.4Ce0.4Y0.2O3-δ (BZCY) was used as proton conducting electrolyte. Bulk electrolyte cell were constructed with a BZCY disc which were prepared by solid state reactive sintering (SSRS) method. The electrolyte precursor powder was prepared by mixing proper amount of BaCO3, CeO2, ZrO2, and Y2O3 according to the desired stoichiometry with the addition of 1.0wt.% NiO as a sintering aid. This mixture was ball-milled for 48 h and uniaxially pressed under 20 MPa for 1 min and then cold-isostatic-pressed under 100 MPa for 1 min. Finally, green pellets were calcined at 1500°C for 10 h so as to obtain dense electrolyte disc (2 mmd, 9 mmf)Pt paste was applied at one side of the surface as a cathode. LSCF or LSCF/Ba3(MnO4)2 mixed ink were screen-printed at the other side of the surface as anode materials. Samples are evaluated by XRD and XAS. The electrochemical impedance spectroscopy and I-V measurements were carried out to evaluate the SOEC properties. Two kinds of anode materials were examined in this research, namely, the cell-1: Pt | BZCY | LSCF and cell-2: Ba3(MnO4)2/LSCF mixed anode cell. The cell-2 showed superior steam electrolytic performance compared to cell-1. The current density of steam electrolysis of cell-2 was 145 mA cm-2 meanwhile cell-1 was 145 mA cm-2 in bias voltage of 1.5 V at 600°C. Impedance spectroscopy was conducted to evaluate the anodic polarization resistance. LSCF anode gives 6 Ω cm2, however, the Ba3(MnO4)2/LSCF composite anode gives 3 Ω cm2. Furthermore, the spectral features were completely different between both. The spectrum of LSCF anode had three semi-circles: high frequency arc (10x-10y Hz), middle frequency arc (10zz-10zy Hz) and low frequency arc (10zz-10zy Hz). On the other hand, Ba3(MnO4)2/LSCF involves only two semi-circles: high frequency arc (10x-10y Hz) and low frequency arc (10zz-10zy Hz). These results indicate Ba3(MnO4)2 changes the reaction pathway of water oxidation electrode/solid electrolyte interface. .The oxygen evolution rate was measured by gas chromatography when electrolysis was performed at a constant current density of 100 mA, 200 mA, and 300 mA. The flax of oxygen from anode side is corresponded to that of calculated from the current density, indicating that the faradaic efficiency was almost 100%. XRD pattern of the sample after electrolysis showed that there were no secondary phases, indicating stability of Ba3(MnO4)2 is enough to use in SOEC anodic condition. The above results suggest the Ba3(MnO4)2 is promising for OER electrocatalysts for SOEC

CO_2 Methanation Catalysts Prepared from Amorphous Ni-Valve Metal Alloys Containing Platinum Group Elements

The amorphous Ni-valve metal (Ti, Zr, Nb and Ta) alloys containing a few at% of platinum group elements were activated by immersion into hydrofluoric acid and used for hydrogenation of carbon dioxide at 100-300℃. This surface activation led to formation of nanocrystalline surface alloys with high surface area, and to surface enrichment of platinum group elements on the titanium-, niobium- and tantalum-containing alloys, but not on the zirconium-containing alloys. The surface of the latter alloys was mainly composed of nickel. The activity and selectivity for methane formation on the titanium-, niobium- and tantalum-containing alloys were significantly affected by the difference in the platinum group elements; the ruthenium- and rhodium-containing alloys showed higher activity and selectivity for methane formation while the platinum-containing alloys exhibited the lowest activity for methane formation and produced mainly carbon monoxide. The zirconium-containing alloys showed the one order of magnitude higher activity for methanation of carbon dioxide in comparison with the titanium-, niobium- and tantalum-containing alloys and produced exclusively methane independent of platinum group elements contained. The alloying with zirconium seems very important to prepare the alloy catalysts having the extremely high activity

The Electrocatalytic Oxidation of Ethylene and Methane, and Reduction of Oxygen on Gas-Diffusion Electrodes Made of Amorphous Nickel-Valve Metal-Platinum Group Metal Alloys

Exploratory work has been done on the performance of electrocatalytic reduction of oxygen and anodic oxidation of ethylene and methane on the gas-diffusion electrodes prepared from amorphous alloys containing one atomic percent platinum group elements. Gas-diffusion electrodes were made by coating the mixture of catalysts prepared by immersion in 46% HF from melt-spun ribbon shaped amorphous alloys, carbon black, polytetrafluoroethylene and sugar, and subsequent baking in nitrogen gas. The electrode made of catalyst prepared from amorphous nickel-niobium alloy containing platinum and ruthenium was the most active for electrocatalytic reduction of oxygen. For electro-oxidation of ethylene and methane, amorphous nickel-value metal alloy containing only platinum possesses higher activity in comparison to the electrode made of platinum black powder

Influence of Metallic Powder Contents on Corrosion Resistance of Galvanized Steel with Metal Powder-containing Organic Coatings for Automobile Fuel Tanks

Pb- and Cr(VI)-free galvanized steel sheets for fuel tanks are coated with epoxy-resin films (thickness: approx. 3 µm) containing particulate Ni powder and flaky Al powder to provide a combination of weldability and degraded gasoline resistance (sour gasoline resistance). The corrosion behavior of galvanized steel specimens coated with epoxy resin containing different amounts of the two types of metal powders was investigated in a solution containing acetic acid, formic acid and NaCl at pH 3.2 and 40°C to elucidate the mechanism of corrosion protection by the coatings. The oxygen gas permeability and water vapor permeability of the coatings were also examined. The results indicated that the addition of particulate Ni powder promoted galvanic corrosion between the Ni and the Zn coating. Voids generated around the embedded Ni powder particles also appeared to accelerate the penetration of the corrosive solution through the coating. On the other hand, the addition of the flaky Al powder improved corrosion resistance. This improved corrosion resistance is associated with the suppression of direct contact between the Ni powder and the Zn coating and also with increased barrier properties, which could be confirmed from oxygen gas and water vapor permeation measurements

Synthesis of single- and multi-component carbides utilizing exfoliated graphite

Different metal alkoxides were impregnated into exfoliated graphite (EG) by sorption and hydrolyzed with steam. Thus formed precursors were pyrolyzed at 1500-1700°C for 1-10h in argon to form single- and multi-component carbides, such as ZrC, TiC, (Ti, Zr) C solid solutions, TiC-ZrC composites, and TiC-Fe composites. In this process, hydrolyzed metal alkoxides were converted mainly to oxides and/or oxyhydroxides on the surface of graphite sheet composing EG, and eventually they are reduced to carbide or metal by graphite carbon at elevated temperatures. The small reaction spaces in EG produce fine metal carbide particles only by the pyrolysis. The process is simple and low cost, and possible to be developed to synthesize other carbides and composites

Effect of Pb-underpotential deposition on anodic dissolution and passivation of pure Fe and Fe-Ni alloys in acidic perchlorate solution

The potentio-dynamic polarization curves of pure Fe, Fe-30 Ni, and Fe-70 Ni alloys in acidic perchlorate solutions (pH 1.9) without and with 10(-3) M Pb2+ were measured to investigate the effect of Pb-underpotential deposition (Pb-UPD) on anodic dissolution and passivation in relation to Pb-induced stress corrosion cracking (Pb-SCC) of Ni base alloys. The addition of 10(-3) M Pb2+ shifts the open circuit potentials of pure Fe and Fe-Ni alloys toward noble direction to inhibit the anodic dissolution and promote the passivation, which results from Pb-UPD on substrate metals. The electro-desorption of Pb proceeds with anodic potential sweep and the anodic dissolution is enhanced when the surface coverage of Pb is reduced to a critical level. Tafel slopes (b (+) = 8.5 similar to 15 mV decade(-1)) of anodic dissolution for pure Fe and Fe-Ni alloys in the presence of Pb2+ are significantly low as compared with those (b (+) = 34 similar to 40 mV decade(-1)) in the absence of Pb2+, which reflects on the rapid enhancement in surface reactivity as a result of electro-desorption of Pb. It is found that the potential region in which anodic dissolution is inhibited by Pb-UPD is located within the potential window of Pb-UPD estimated from the differences in work-function between substrate metals and Pb

Element Distribution in Porous Ga Oxide Obtained by Anodizing Ga in Phosphoric Acid

A STEM/EDS study of a porous Ga oxide film formed by an anodization process was conducted in this study to examine the crystalline structure of the film and the elemental distribution in the oxide film before and after heat treatment. The as-formed anodic film with a morphology resembling the well-known porous anodic Al oxide film was amorphous, crystallizing after heat treatment at 600 degrees C without changing the morphology and elemental distribution. The EDS elemental maps disclosed the duplex nature of the pore wall oxide; the phosphate anion was contaminated in the outer oxide layer next to the pores, and the inner layer consisted of relatively pure Ga oxide, practically free from phosphate. The similarity of morphology and elemental distributions between the porous anodic Al and Ga oxides suggests that the growth of both anodic oxide films proceeds under the same mechanism. In addition, crystallized porous Ga oxides are expected to be applied to fabricate various functional devices requiring geometrically controlled semiconductor nanohole arrays, such as devices for hydrogen formation. (c) 2023 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited