Complex Precursors for Doped Lanthanum Chromite Synthesis

Doped lanthanum chromites are investigated as ceramic interconnect materials to be used in high-temperature solid oxide fuel cells (SOFCs). This paper deals with the preparation of LaCr_1–x M _x O₃ (M = Ni, Cu, x = 0 – 0.2) from complex precursors isolated in the La(NO₃)₃-Cr(NO₃)₃-M(NO₃)₂-urea system. The isolated complex precursors were characterized by atomic absorption spectrometry, electronic, and FT-IR spectra, as well as, by thermal analysis. Doped lanthanum chromites, LaCr_1–x M _x O₃ (M = Ni, Cu) were prepared by calcining of corresponding urea-based precursors at 1000-1200°C for 2 h. Doped mixed-oxide samples were characterized by X-ray diffraction (XRD), Scanning electron-microscopy-energy-dispersive X-ray analysis (SEM-EDX), electrical conductivity, as well as magnetic measurements.     

Infiltrated NiCo Alloy Nanoparticle Decorated Perovskite Oxide: A Highly Active,Stable, and Antisintering Anode for Direct‐Ammonia Solid Oxide Fuel Cells

Direct ammonia solid oxide fuel cell (DA‐SOFC) is superior to low‐temperature direct ammonia fuel cell using anion exchange membrane because of much improved anode reaction kinetics at elevated temperature. However, significant performance degradation due to severe sintering of conventional nickel cermet anode under operating conditions is a big challenge for realizing its practical use. Herein, a high‐performance anode based on La_0.55Sr_0.30TiO_3?δ (LST) perovskite substrate with its surface decorated with in situ exsolved and strongly coupled NiCo alloy nanoparticles (NPs) is designed and fabricated for DA‐SOFCs, exhibiting superior catalytic activity for NH₃ decomposition reaction due to balanced NH₃ adsorption and N₂ desorption processes. An electrolyte‐supported single cell with infiltrated NiCo/LST on Sm_0.2Ce_0.8O_1.9 scaffold anode delivers a maximum power density of 361 mW cm^?2 at 800 °C in NH₃ fuel, superior to similar SOFCs with Ni or Co NP‐decorated LST based anodes (161 and 98 mW cm^?2). Furthermore, the SOFC with this newly developed anode displays favorable operational stability without obvious performance degradation at 700 °C for a test period of ≈120 h, attributed to its high antisintering capability. This study provides some strategies to develop highly active, stable, and antisintering perovskite‐based nanocomposite for DA‐SOFCs, facilitating the practical use of this technology.     

Shape‐Dependent Activity of Ceria for Hydrogen Electro‐Oxidation in Reduced‐Temperature Solid Oxide Fuel Cells

Single crystalline ceria nanooctahedra, nanocubes, and nanorods are hydrothermally synthesized, colloidally impregnated into the porous La_0.9Sr_0.1Ga_0.8Mg_0.2O_3‐δ (LSGM) scaffolds, and electrochemically evaluated as the anode catalysts for reduced temperature solid oxide fuel cells (SOFCs). Well‐defined surface terminations are confirmed by the high‐resolution transmission electron microscopy — (111) for nanooctahedra, (100) for nanocubes, and both (110) and (100) for nanorods. Temperature‐programmed reduction in H₂ shows the highest reducibility for nanorods, followed sequentially by nanocubes and nanooctahedra. Measurements of the anode polarization resistances and the fuel cell power densities reveal different orders of activity of ceria nanocrystals at high and low temperatures for hydrogen electro‐oxidation, i.e., nanorods > nanocubes > nanooctahedra at T ≤ 450 °C and nanooctahedra > nanorods > nanocubes at T ≥ 500 °C. Such shape‐dependent activities of these ceria nanocrystals have been correlated to their difference in the local structure distortions and thus in the reducibility. These findings will open up a new strategy for design of advanced catalysts for reduced‐temperature SOFCs by elaborately engineering the shape of nanocrystals and thus selectively exposing the crystal facets.     

Electrical Conductivity and Thermodynamic Properties of Some Alkaline Earth-Doped Lanthanum Chromites

Alkaline-earth doped lanthanum chromites are currently the interconnecting materials of choice for solid oxide fuel cells (SOFCs). Since these materials in SOFC operating conditions are under a large oxygen potential gradient and at high temperature (1273 K), a thorough knowledge of their physical and thermochemical properties is very important. In the present study, the alkaline-earth doped lanthanum chromites La_1-xSr_xCrO₃ (x=0–0.3) and La_0.7Ca_0.3CrO₃ were prepared from complex precursors isolated from the La(NO₃)₃–Cr(NO₃)₃–urea system. The oxide powders were characterized by means of X-ray diffraction (XRD). The DC electrical conductivities of the samples were measured in the temperature range of 295–1273 K in air. The thermodynamic properties represented by the relative partial molar free energies, enthalpies, and entropies of oxygen dissolution in the perovskite phase, as well as the partial pressures of oxygen, have been investigated by the solid electrolyte galvanic cells method coupled with the solid-state coulometric titration technique, within the temperature range of 1073–1273 K and in a reducing atmosphere (10^–5 Pa). The variation of the electrical conductivities and thermodynamic properties with changing oxygen stoichiometry is discussed. The study demonstrates new correlations existing between the structural, electrical, and thermodynamic properties in the doped lanthanum chromites.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

Alloy Nanocatalysts for the Electrochemical Oxygen Reduction (ORR) and the Direct Electrochemical Carbon Dioxide Reduction Reaction (CO2RR)

In the face of the global energy challenge and progressing global climate change, renewable energy systems and components, such as fuel cells and electrolyzers, which close the energetic oxygen and carbon cycles, have become a technology development priority. The electrochemical oxygen reduction reaction (ORR) and the direct electrochemical carbon dioxide reduction reaction (CO₂RR) are important electrocatalytic processes that proceed at gas diffusion electrodes of hydrogen fuel cells and CO₂ electrolyzers, respectively. However, their low catalytic activity (voltage efficiency), limited long‐term stability, and moderate product selectivity (related to their Faradaic efficiency) have remained challenges. To address these, suitable catalysts are required. This review addresses the current state of research on Pt‐based and Cu‐based nanoalloy electrocatalysts for ORR and CO₂RR, respectively, and critically compares and contrasts key performance parameters such as activity, selectivity, and durability. In particular, Pt nanoparticles alloyed with transition metals, post‐transition metals and lanthanides, are discussed, as well as the material characterization and their performance for the ORR. Then, bimetallic Cu nanoalloy catalysts are reviewed and organized according to their main reaction product generated by the second metal. This review concludes with a perspective on nanoalloy catalysts for the ORR and the CO₂RR, and proposes future research directions.     

Rational Design of a Novel Catalyst Cu‐SAPO‐42 for NH3‐SCR Reaction

Cu‐exchanged LTA‐type aluminosilicate catalyst has been considered as an efficient catalyst for the selective catalytic reduction of NO_x with ammonia (NH₃‐SCR). However, expensive organic structure‐directing agents (OSDAs) and the corrosive fluoride medium are inevitably used to synthesize LTA‐type molecular sieve (high‐silica LTA‐type aluminosilicate and its analogue LTA‐type silicoaluminophosphate SAPO‐42). Herein, a series of cheap and commercialized OSDAs, which are successfully applied for the targeted synthesis of SAPO‐42 in the fluoride‐free system, are identified by a novel RSS (refine, summarize, and search) approach. Furthermore, Cu‐SAPO‐42 catalysts are utilized for NH₃‐SCR. Among these catalysts, Cu‐SAPO‐42 prepared with 2‐(butylamino)ethanol (BAEA) as OSDA demonstrates the excellent activity even after hydrothermal aging at 800 °C for 16 h, which shows much better hydrothermal stability than the commercialized Cu‐SAPO‐34 catalyst with comparable Si and Cu contents. Electron paramagnetic resonance (EPR) spectroscopy and Rietveld refinement are performed to identify the locations of active Cu²⁺ ions. It turns out that the active Cu²⁺ ions are distributed near the center of single 6‐rings of the lta cage.     

Fabrication of Sr- and Co-doped lanthanum chromite interconnectors for SOFC

Many studies have been performed dealing with the processing conditions of electrodes and electrolytes in solid oxide fuel cells (SOFCs). However, the processing of the interconnector material has received less attention. Lanthanum chromite (LaCrO₃) is probably the most studied material as SOFCs interconnector. This paper deals with the rheology and casting behaviour of lanthanum chromite based materials to produce interconnectors for SOFCs. A powder with the composition La_0.80Sr_0.20Cr_0.92Co_0.08O₃ was obtained by combustion synthesis. Aqueous suspensions were prepared to solids loading ranging from 8 to 17.5 vol.%, using ammonium polyacrylate (PAA) as dispersant and tetramethylammonium hydroxide (TMAH) to assure a basic pH and providing stabilization. The influence of the additives concentrations and suspension ball milling time were studied. Suspensions prepared with 24 h ball milling, with 3 wt.% and 1 wt.% of PAA and TMAH, respectively, yielded the best conditions for successful slip casting. Sintering of the green discs was performed in air at 1600 °C for 4 h leading to relatively dense materials.     

Solution combustion synthesis of MgAl2O4 using fuel mixtures

The paper reveals a new perspective concerning the rational fuel selection and the logical elaboration of the recipes for the MgAl₂O₄ solution combustion synthesis. It was shown that Mg(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O exhibit different behavior with respect to urea, glycine and β-alanine. Urea proved to be the most adequate fuel for Al(NO₃)₃·9H₂O, while β-alanine proved to be the most appropriate fuel for Mg(NO₃)₂·6H₂O. Considering that there is a predilection of metal nitrates with respect to these fuels, in the case of MgAl₂O₄ combustion synthesis best results were achieved when fuel mixtures (urea and β-alanine, urea and glycine) were used. The use of fuel mixtures allowed the formation of pure, nanocrystalline MgAl₂O₄ directly from the combustion reaction, without any subsequent annealing step. The use of a single fuel (urea, glycine or β-alanine) led to the formation of an amorphous powder, which required further annealing in order to achieve the formation of crystalline MgAl₂O₄.     

La2Zr2O7 formed at ceramic electrode/YSZ contacts

Strontium-doped LaCoO₃ or LaMnO₃ materials have been studied for use as cathodes for solid oxide fuel cells (SOFCs). This choice relies on the required properties and competitive cost. However, formation of reaction products under typical electrode-firing conditions may affect the performance of SOFCs. La₂Zr₂O₇ was detected at YSZ/electrode interfaces. This reaction product was synthesized from powders and characterized to obtain a better understanding of its effects on cell performance. Its structural, thermal, and electrical properties are reported.     

Cu doped ZnO nanoparticle sheets

Cu doped ZnO nanoparticle sheets were synthesized via a proposed solution route with mixed Zn(NO₃)₂ and Cu(NO₃)₂ precursors at a low temperature of 95 °C. Scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectrometry results demonstrate that the nanostructues synthesized by solutions with higher Cu(NO₃)₂ concentration are nanoparticle sheets comprised of uniform Cu doped ZnO nanoparticles with diameters around 20 nm. Room-temperature photoluminescence spectra of the nanoparticle sheets show tunable near band emissions centered at 390–405 nm and strong yellow emissions at 585–600 nm. Absorbance spectra show gradual redshift in the UV range with the increase of Cu concentrations in the ZnO nanomaterials. The study provides a simple and efficient route to prepare Cu doped ZnO nanomaterials at low temperature. The as-synthesized products with both violet and yellow emissions are promising for white light-emitting diode applications.     

Synthesis and characterisation of infinite coordination networks with 1,6-bis(4-pyridyl)hexane and copper nitrate

Crystallisation of 1,6-bis(4-pyridyl)hexane (Py₂C₆H₁₂) with copper nitrate gives two different phases. Phase 1 of composition [Cu(Py₂C₆H₁₂)₃(NO₃)₂]·2[Cu(Py₂C₆H₁₂)₂(H₂O)(NO₃)]·2(NO₃)·EtOH consists of two different infinite chains in a 1:2 ratio that are interlocked. Hydrogen bonds link chains I to II and chains II to II. In contrast phase 2 of composition [Cu₂(Py₂C₆H₁₂)₄(H₂O)₂]·(NO₃)₄·(Py₂C₆H₁₂)·(EtOH)·2(H₂O) is based upon an infinite 3D framework. It consists of four interpenetrating 3D networks that are crystallographically equivalent.     

Safety‐by‐Design of Metal Oxide Nanoparticles Based on the Regulation of their Energy Edges

The safety of metal oxide (MOx) nanoparticles (NPs) has been highly concerned because of their wide application and potential toxicological injury. The safe‐by‐design strategy is usually developed to make safer MOx NPs based on regulation of their physicochemical properties. In the present study, manganese oxide (Mn₃O₄) NPs, as a representative of insoluble toxic MOx NPs, are doped with a series of transition metal to regulate their conduction band energy (E_c) out of biological redox potential range (BRPR) or Fermi energy (E_f) far away from valence band energy (E_v), aiming at completely eliminating the toxicity or significantly reducing the toxicity. It is found that all these M‐doping cannot move E_c of Mn₃O₄ NPs out of the BRPR but zinc (Zn)‐, copper (Cu)‐, and chromium (Cr)‐doping do move E_f far away from E_v, where Zn‐doping results in the largest |E_f ? E_v| value. Various abiotic, in vitro and in vivo assessments reveal that Zn‐, Cu‐, and Cr‐doped Mn₃O₄ NPs can generate lower amount of ?OH and trigger weaker injury than Mn₃O₄ NPs, where Zn‐doped Mn₃O₄ NPs show the lowest toxicity. Regulating E_f far away from E_v becomes a feasible safe‐by‐design approach to achieve safe MOx NPs.     

Photo‐/Thermal‐Responsive Field‐Effect Transistor upon Blending Polymeric Semiconductor with Hexaarylbiimidazole toward Photonically Programmable and Thermally Erasable Memory Device

It is shown that the semiconducting performance of field‐effect transistors (FETs) with PDPP4T (poly(diketopyrrolopyrrole‐quaterthiophene)) can be reversibly tuned by UV light irradiation and thermal heating after blending with the photochromic hexaarylbiimidazole compound (p‐NO₂‐HABI). A photo‐/thermal‐responsive FET with a blend thin film of PDPP4T and p‐NO₂‐HABI is successfully fabricated. The transfer characteristics are altered significantly with current enhanced up to 10⁶‐fold at V_G = 0 V after UV light irradiation. However, further heating results in the recovery of the transfer curve. This approach can be extended to other semiconducting polymers such as P3HT (poly(3‐hexyl thiophene)), PBTTT (poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b] thiophene)) and PDPPDTT (poly(diketopyrrolopyrrole‐dithienothiophene)). It is hypothesized that TPIRs (2,4,5‐triphenylimidazolyl radicals) formed from p‐NO₂‐HABI after UV light irradiation can interact with charge defects at the gate dielectric–semiconductor interface and those in the semiconducting layer to induce more hole carriers in the semiconducting channel. The application of the blend thin film of PDPP4T and p‐NO₂‐HABI is further demonstrated to fabricate the photonically programmable and thermally erasable FET‐based nonvolatile memory devices that are advantageous in terms of i) high ON/OFF current ratio, ii) nondestructive reading at low electrical bias, and iii) reasonably highly stable ON‐state and OFF‐state.     

Anwendung dilatometrischer Messungen bei der Entwicklung von Glasloten und reaktiven Metallloten zum Fügen von Hochtemperaturbrennstoffzellen

The principle of operation of solid oxide fuel cells (SOFCs) is very simple. However, the fact that very different materials are used for the individual components requires advanced thermal joining techniques to join them in a functional manner. Two very distinct designs have established themselves for the two different intended applications: decentralised power generation (stationary SOFCs) on the one hand, and power converters for vehicles (mobile SOFCs) on the other hand. As a consequence, alternative techniques for joining the individual components are also required. The principal joining process for the stationary SOFC design consists of joining individual steel plates with a glass sealant in an electrically insulating way so that they form an SOFC stack. For the mobile fuel cell design, the SOFC stack consists of individual thin steel cassettes. The window frame of the cassettes, which is made of ferritic chromium steel, is brazed to the ceramic layer of the zirconium oxide solid electrolyte using a filler metal. The material used is a silver‐based brazing filler metal which contains only small amounts of copper oxide (CuO) and titanium hydride (TiH₂) as wetting agents. Both joining processes must be applicable in normal atmospheric air, i. e. under oxidative conditions. R&D activities continue for improving the efficiency and long‐term operational stability of the technology to such an extent that SOFCs will become ready for the energy sector market. The two joining techniques described cannot yet be considered standard processes. They, too, will require continuous improvement with respect to reproducibility, endurance and strength of the joints. The Special Joining Techniques working group at Forschungszentrum Jülich uses specially modified dilatometric techniques as suitable quick replacement methods for studying and measuring the joining characteristics of the materials without having to manufacture complex and expensive SOFC stacks. The shrinkage processes in the glass sealant joints are simulated and measured in the μm range using a special dilatometer. In this way, the amount of glass sealant – which is decisive for tightness and bonding – and the process parameters can be determined in advance. With a vertical dilatometer, the melting behaviour of the reactive silver filler metals is examined with respect to melting point shift, viscosity and void ratio, and as a function of the metal additives (Al) and the process atmosphere.     

N-Coordinated Cu–Ni Dual-Single-Atom Catalyst for Highly Selective Electrocatalytic Reduction of Nitrate to Ammonia

As a traditional method of ammonia (NH₃) synthesis, Haber–Bosch method expends a vast amount of energy. An alternative route for NH₃ synthesis is proposed from nitrate (NO₃⁻) via electrocatalysis. However, the structure–activity relationship remains challenging and requires in-depth research both experimentally and theoretically. Here an N-coordinated Cu–Ni dual-single-atom catalyst anchored in N-doped carbon (Cu/Ni–NC) is reported, which has competitive activity with a maximal NH₃ Faradaic efficiency of 97.28%. Detailed characterizations demonstrate that the high activity of Cu/Ni–NC mainly comes from the contribution of Cu–Ni dual active sites. That is, (1) the electron transfer (Ni → Cu) reveals the strong electron interaction of Cu–Ni dual-single-atom; (2) the strong hybridizations of Cu 3d—and Ni 3d—O 2p orbitals of NO₃⁻ can accelerate electron transfer from Cu–Ni dual-site to NO₃⁻; (3) Cu/Ni–NC can effectively decrease the rate-limiting step barriers, suppress N–N coupling for N₂O and N₂ formation and hydrogen production.     

Solvent-free organic syntheses on mineral supports using microwave irradiation

 An expeditious and solvent-free approach for selective organic synthesis is described which involves simple exposure of neat reactants to microwave (MW) irradiation. The coupling of MW irradiation with the use of catalysts or mineral supported reagents, under solvent-free conditions, provides clean chemical processes with special attributes such as enhanced reaction rates, higher yields, greater selectivity and the ease of manipulation. Our recent results on this eco-friendly approach utilizing recyclable inorganic oxides or supported reagents such as Fe(NO₃)₃-clay (clayfen), Cu(NO₃)₂-clay (claycop), NH₄NO₃-clay (clayan), NH₂OH-clay, PhI(OAc)₂-alumina, NaIO₄-silica, CrO₃-alumina, MnO₂-silica, and NaBH₄-clay etc. are exemplified in MW-assisted deprotection, condensation, cyclization, oxidation and reduction reactions including the efficient one-pot assembly of heterocyclic molecules from in situ generated intermediates. Received: 20 October 1998 / Accepted: 27 December 1998     

Gas-Phase Volume Oxidation of Uranium Mononitride

Gas-phase volume oxidation (voloxidation) of UN in various atmospheres was studied. Oxidation of compact UN samples under the conditions characteristic of the voloxidation of the oxide fuel leads to the formation of uranium oxides. The use of the oxygen-containing atmosphere leads to the predominant formation of U₃O₈, and the use of water vapor, to the formation of UO₂. The major gaseous nitrogen-containing conversion product is, apparently, N₂. The use of the alternative oxidizing atmosphere based on NO_x gases does not allow the conversion to be performed at a lower temperature. In this case, both UO₃ and UO₂(NO₃)₂ hydrates are formed. The maximal degree of the UN conversion to water-soluble compounds, equal to ～80%, is reached at the process temperature of ～565 K.

     

Modifying Commercial Carbon with Trace Amounts of ZIF to Prepare Derivatives with Superior ORR Activities

Reducing costs while maintaining high activities and stabilities of oxygen reduction reaction (ORR) catalysts has long been pursued for applications to membrane fuel cells. Here, an absorption‐reaction method is used to prepare a zeolitic imidazolate framework coated commercial carbon, and the pyrolysis of such material brings about impressive ORR activities and stabilities with large diffusion‐limited current density, half‐wave potential, and no obvious decay after 10 000 cyclic voltammetry cycles, which is even better than that of the commercial Pt/C catalysts. The absorption‐reaction method is realized by simply soaking the commercial carbon black sequentially in Co(NO₃)₂ and 2‐methylimidazole solutions at ambient conditions. The detailed analysis on such carbon materials reveals that both Co and N are essential to activities, even the amount of N and Co species is very low. The reduction of raw materials and simplified handling procedures result in well‐controlled costs in applications.     

Sorption of CH<Subscript>3</Subscript><Superscript>131</Superscript>I from water vapor-air medium on porous inorganic sorbents containing triethylenediamine and <Emphasis Type="Italic">d</Emphasis> element nitrates

Sorption of CH₃ ¹³¹I from a water vapor-air medium on porous inorganic sorbents based on silica gel of KSKG grade and containing triethylenediamine (CH₂-CH₂)₃N₂ and d element nitrates was studied. The sorbents prepared by impregnation with (CH₂-CH₂)₃N₂ and Zn, Ni, and Cu nitrates from aqueous solution recover CH₃ ¹³¹I from a water vapor-air flow poorly (degree of recovery <10%). Calcination of the sorbents at temperatures exceeding 250°C does not noticeably affect their sorption power. Heating of the complex Ag(NO₃)(OH)·(CH₂-CH₂)₃N₂H to 160°C causes its exothermic decomposition with a large heat release and formation of metallic silver. Thermal decomposition of the complex of Cu²⁺ with (CH₂-CH₂)₃N₂, synthesized from an aqueous solution at the molar ratio Cu(NO₃)₂: (CH₂-CH₂)₃N₂ = 1: 2, occurs similarly.     

Crystal Structure of K[UO2(NO3)3] and Some Features of Compounds M[UO2(NO3)3] (M = K,Rb, and Cs)

Greenish-yellow transparent crystals of K[UO₂(NO₃)₃] were prepared from aqueous solutions as by-product in synthesis of potassium chromatouranylates. The crystal structure was solved by the direct methods and refined to R ₁ = 0.037 (wR ₂ = 0.070) for 1452 reflections with |F _hkl| 4|F _hkl|. Mono- clinic system, space group C2/c, a = 13.4877(10), b = 9.5843(7), c = 7.9564(6) Å, = 116.124(2)°, V = 923.45(12) ų. The structure of K[UO₂(NO₃)₃] contains isolated complex ions [UO₂(NO₃)₃]^- whose uranyl groups are aligned parallel to the [-101] plane. The K⁺ cations, coordinated by twelve oxygen atoms, are located between the complex anions. Comparison of the structure with known data on M[UO₂(NO₃)₃] compounds (M = K, Rb, Cs) suggests the possibility of phase transitions due to relatively small displacements of [UO₂(NO₃)₃]^- anions and K⁺ cations, retaining the general structural motif.