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The tailoring of multi-tasking perovskite oxide-based electrodes for solid oxide cells has shown growing interest. The development of flexible structures represents a crucial step towards the design of symmetric and possibly SOFC/SOEC reversible systems. In this work, low (0.5 mol%) B-site Pt-doping in a lanthanum strontium ferrite is presented as a successful approach to enhance the parent perovskite properties as both SOC air and fuel electrode. Structural, morphological and electrochemical characterizations of La0.6Sr0.4Fe0.995Pt0.005O3-δ (LSFPt005) are provided and compared to the undoped compound. LSFPt005-symmetric devices are tested as CO-SOFCs and CO2-SOECs at 850 °C, respectively, obtaining a maximum power density of 301 mW/cm2 and a current density of 0.82 A/cm2 at 1.5 V. Insights of cell operating mechanisms are provided through electrochemical impedance spectroscopy.

Solid oxide fuel cells (SOFCs) have a great potential among the emerging energy conversion technologies because they can efficiently convert various types of fuel (hydrogen, methane, CO, biogas) into electrical energy in the 750-850 °C temperature range. The choice of electrodes remains a challenge because most metal oxides suffer from low chemical stability and low electrocatalytic activity toward oxygen reduction reaction (ORR) in the intermediate temperatures (IT) range (500-700 °C) [1]. In this view, perovskite oxides (ABO3) have shown excellent properties in terms of thermal stability, compatibility with other cell components, low production cost and, most importantly, great flexibility [1]. La0.6Sr0.4FeO3-δ, (LSF) perovskite oxide have been widely investigated for the high electrical conductivity and the electrochemical properties [2]. However, the main drawback is the poor activity towards ORR in the IT range, as most iron-based perovskite oxides [3]. The B-site doping with noble metal catalysts can sensitively improve the properties and make the compounds suitable for electrode application. The addition of small amounts of Ru or Pt into LSF structure can be an effective approach to improve the electrocatalytic properties [4,5]. Specifically, we synthesized B-site doped LSF powders with very low amounts (15 m2/g). All compounds showed a single perovskite phase. As reported in Fig. (a), the addition of Ru and Pt in the structure causes a progressive shift of the XRD peaks to lower 2θ angles indicating the perovskite lattice expansion due to the substitution of Fe4+/ Fe3+ sites with larger Ru and Pt ions. To assess the electrocatalytic activity, the compounds were investigated in oxidizing conditions using electrochemical impedance spectroscopy (EIS) analysis. Fig. (b) shows the Arrhenius plot of area specific resistance (ASR) values of LSFRu and LSFPt perovskite oxides. The ASR values were measured on LSFM/La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM)/LSFM (M=Ru, Pt) symmetric cells in the temperature range 550-800 °C. The insertion of a very low amount (1%mol) of Pt or Ru significantly reduces the polarization resistance, confirming the beneficial effect of noble metal ions on ORR activity. All these features suggest that very low Pt and Ru doping positively affects the structural and electrocatalytic properties valuably for cathodic applications. [1] Hwang J, Rao R.R. (2017). Science 358:751-756 [2] Hansen KK, Mogensen M (2008). ECS Trans 13:153–160 [3] Chavan SV, Singh RN (2013). J Mater Sci 48:6597–6604 [4] Fan W, Sun Z. (2016) RSC Adv 6:34564 [5]Shaoli G., Hongjing Wu, (2015),Catalysts, 5, 366-391 Figure 1

The coupling of semiconductors with organic molecules results in a class of sensors whose chemoresistive properties are dictated by the nature of dyes. Organic molecules generally reduce conductivi...

Recognition of enantiomers is one of the most arduous challenges in chemical sensor development. Although several chiral systems exist, their effective exploitation as the sensitive layer in chemical sensors is hampered by several practical implications that hinder stereoselective recognition in solid state. In this paper, we report a new methodology to efficiently prepare chiral solid films, by using a hybrid material approach where chiral porphyrin derivatives are grafted onto zinc oxide nanoparticles. Circular dichroism (CD) evidences that the solid-state film of the material retains supramolecular chirality due to porphyrin interactions, besides an additional CD feature in correspondence of the absorbance of ZnO (375 nm), suggesting the induction of chirality in the underlying zinc oxide nanoparticles. The capability of hybrid material to detect and recognize vapors of enantiomer pairs was evaluated by fabricating gas sensors based on quartz microbalances. Chiral films of porphyrin on its own were used for comparison. The sensor based on functionalized nanostructures presented a remarkable stereoselectivity in the recognition of limonene enantiomers, whose ability to intercalate in the porphyrin layers makes this terpene an optimal chiral probe. The chiroptical and stereoselective properties of the hybrid material confirm that the use of porphyrin-capped ZnO nanostructures is a viable route for the formation of chiral selective surfaces.

Structural and microstructural investigations of Pd-doped lanthanum strontium ferrite with stoichiometry La0.6Sr0.4Fe0.95Pd0.05O3-? (LSFPd) were carried out under oxidizing and reducing conditions using XRD, Rietveld refinement, XPS, SEM, and TEM analyses. LSFPd exhibited a smart behaviour with a reversible redox structural transformation occurring upon switching from air to hydrogen and then back to air. Exsolution of nanometric metal particles was observed after reduction, and reincorporation of metal cations onto the perovskite structure was confirmed after reoxidation. The electrochemical performances were evaluated on electrolyte supported cells using LSFPd as symmetric electrodes. The self-regenerating redox process largely improved the anodic performance of LSFPd-based SOFCs because it mitigates the coarsening of metallic electrocatalysts at high temperature promoting a redox stable behaviour. Fuel cell tests revealed promising performance in terms of maximum power output and redox stability.

Intermittency of renewable energy sources can be profitably faced using efficient energy storage systems. Reversible solid oxide cells (RSOCs) able to operate with carbon-containing species are likely among the most appealing choices. Energy can be obtained by natural gas and/or biogas (SOFC mode), with useful recovery of CO2 in the exhausts. Besides, if the electrode is also active towards CO2 electrolysis (SOEC mode), CO2 is reduced to CO and O2. In this work a composite material with in-situ formed Ni-Fe alloy catalyst consisting of La1.2Sr0.8Fe0.6Mn0.4O4 Ruddlesden-Popper perovskite and Ni-Ce0.85Sm0.15O2-δ fluorite was developed as a multi-functional fuel-electrode for RSOCs. The composite electrode was tested in SOFC mode as anode for hydrogen, dry methane and carbon monoxide oxidation and showed power density outputs of 657, 668 and 527 mW/cm2 at 850 °C, respectively, together with redox stability and coking tolerance for over 120 h. In SOEC mode, it was tested as cathode and delivered 2.66 A/cm2 at 2 V in a 95:5 CO2:CO mixture, retaining a current density of 1 A/cm2 for more than 40 h.

Perovskite oxides such as ferrites have been widely investigated for their remarkable electrochemical activity as SOFC electrodes. However, their phase instability in reducing conditions remains an issue for anode application. The role of Mn substitution into B-site of La0.6Sr0.4FeO3-δ (LSF) perovskite oxide was investigated. New insights on the structural evolution of La0.6Sr0.4Fe0.8Mn0.2O3-δ (LSFMn) upon high temperature reduction were revealed. In oxidizing atmosphere, Mn substitution reduces the oxygen vacancy concentration while, switching to reducing conditions, it drives the transition from rhombohedral perovskite to single Ruddlesden-Popper phase, affecting the Fe0 exsolution. Redox-cycles of LSFMn were investigated and the properties of re-oxidized compounds were highlighted. The effect of Mn substitution on perovskite conductivity was also evaluated both in oxidizing and reducing conditions.

A structured catalyst for the dry reforming of methane (DRM) was investigated as a biogas pre-reformer for indirect internal reforming solid oxide fuel cell (IIR-SOFC). For this purpose, a NiCrAl open-cell foam was chosen as support and Ni-based samarium doped ceria (Ni-SmDC) as catalyst. Ni-SmDC powder is a highly performing catalyst showing a remarkable carbon resistance due to the presence of oxygen vacancies that promote coke gasification by CO2 activation. Ni-SmDC powder was deposited on the metallic support by wash-coating method. The metallic foam, the powder, and the structured catalyst were characterized by several techniques such as: N2 adsorption-desorption technique, X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), focused ion beam (FIB), temperature programmed reduction (H2-TPR), and Raman spectroscopy. Catalytic tests were performed on structured catalysts to evaluate activity, selectivity, and stability at SOFC operating conditions.

Carbon dioxide collected from outdoor atmosphere or produced in aerospace cabin, may be recycled to produce methane and water by methanation reaction (Sabatier reaction). This reaction needs a heterogeneous catalyst to decrease the reaction temperature and increase the selectivity. A proper catalyst for in-situ CO 2 recycling into methane requires: i) high CO 2 conversion with high CH 4 selectivity at low operating temperature, ii) high stability towards degradation, being required that catalyst remains highly active during the entire production process. This paper presents a preliminary study of a catalyst based on Ni supported on cerium oxide, prepared by one-pot hydrothermal method, leading to a nanorod morphology with high and uniform nickel dispersion. Morphology and chemical properties of the material were characterized by BET, H 2 -TPR, XRD and FE-SEM/EDS. The catalyst was tested in a flow system at atmospheric pressure, using reagents with stoichiometric ratio (CO 2 :H 2 =1:4) or enriched in hydrogen. The Ni/r-CeO 2 catalyst showed high stability over time on stream, in oxidizing condition and after switch off-switch on test.

Reversible solid oxide cells (RSOCs) represent a promising technology for the efficient exploitation of intrinsically intermittent renewable energy sources. RSOCs allow to derive fuel and chemicals from power (power-to-gas technology, P2G) and power from fuel and chemicals (gas-to-power technology, GTP) and can be interchangeably operated either as a solid oxide fuel cell (SOFC) or as a solid oxide electrolyzer cell (SOEC). The key aspect to render these devices competitive on a market scale is the development of multi-tasking, reliable, cost-effective and long-lasting electrodes (1,2). Besides, to overcome the issues related to hydrogen production, storage and distribution (e.g. impractical conversion of the existing grid to hydrogen-based infrastructures), the fuel-electrode of RSOCs should ensure high catalytic activity and coking resistance toward carbon-containing species (3-4). Using a hydrocarbon-tolerant fuel electrode, energy can be obtained by natural gas and biogas (SOFC-mode), with useful recovery of CO2 in the exhausts (carbon capture and storage, CCS). Besides, if the electrode is also active towards CO2 electrolysis (SOEC mode), CO2 is reduced to CO and O2 (carbon capture and utilization, CCU). Ni-YSZ is the reference fuel-electrode material for both H2 fed SOFC and for CO2 electrolysis in SOEC. Nevertheless, Ni-based cermets cannot be used in SOFCs fed with methane-containing fuels, as they suffer from two main drawbacks: mechanical instability upon NiO-Ni redox cycles (5) and passivation due to coking, being Ni a catalyst for methane cracking (3, 4). Moreover, during Ni-YSZ operation in CO2-SOEC mode, ZrO2 reduction can occur at high cathodic potential, resulting in Ni-Zr compounds formation (6). In this work, a recently developed (7, 8) composite material containing La0.6Sr0.4Fe0.8Mn0.2O3-δ (LSFMn) and 5wt% Ni-containing Ce0.58Sm0.15O2-δ (NiSDC) is tested as fuel electrode for LSGM-electrolyte supported cells. In reducing conditions, Fe exsolved from the LSFMn perovskite forms a Fe-Ni alloy with Ni present on SDC. In SOFC mode, the composite is designed to operate in dry methane: Fe-Ni catalytic sites activate CH4, which is successively oxidized on Mn-containing LSFMn, while SDC increases the O2- supply at the anode to get rid of any carbonaceous deposits. As Fe-Ni alloy was reported to be highly active for CO2 reduction (9), the composite was also tested as SOEC cathode in different CO2:CO ratios. LSFMn+NiSDC was tested in SOFC-mode as anode for hydrogen, dry methane and carbon monoxide oxidation and showed power density outputs of 657, 668 and 527 mW/cm2, respectively (Fig. a), a redox stable behavior and coking resistance for over 120 h. LSFMn+NiSDC in SOEC-mode delivered 2.66 A/cm2 at 2 V in 95:5 CO2:CO mixture (Fig. b), keeping 1 A/cm2 of current density output for over 40 h. (1) - M. Mogensen, M. Chen, H. Frandsen, C. Graves, J. Hansen, K. Hansen, A. Hauch, T. Jacobsen, S. Jensen, T. Skafte, Reversible solid-oxide cells for clean and sustainable energy, Clean Energy, 3 (2019) 175-201. (2) - M.B. Mogensen, Materials for Reversible Solid Oxide Cells, Current Opinion in Electrochemistry, (2020). (3) - M. Mogensen, K. Kammer, Conversion of hydrocarbons in solid oxide fuel cells, Annual Review of Materials Research, 33 (2003) 321-331. (4) - S. McIntosh, R.J. Gorte, Direct hydrocarbon solid oxide fuel cells, Chemical reviews, 104 (2004) 4845-4866. (5) - J. Malzbender, R. Steinbrech, Advanced measurement techniques to characterize thermo-mechanical aspects of solid oxide fuel cells, Journal of Power Sources, 173 (2007) 60-67 (6) - A. Hauch, K. Brodersen, M. Chen, M.B. Mogensen, Ni/YSZ electrodes structures optimized for increased electrolysis performance and durability, Solid State Ionics, 293 (2016) 27-36 (7) - L. Duranti, I. Luisetto, S. Licoccia, C. Del Gaudio, E. Di Bartolomeo, Electrochemical performance and stability of LSFMn+ NiSDC anode in dry methane, Electrochimica Acta, (2020) 137116. (8) - L. Duranti, I.N. Sora, F. Zurlo, I. Luisetto, S. Licoccia, E. Di Bartolomeo, The role of manganese substitution on the redox behavior of La0.6Sr0.4Fe0.8Mn0.2O3-δ, Journal of the European Ceramic Society, (2020) ( 9 ) - Liu, S., Liu, Q., & Luo, J. L. (2016). Highly stable and efficient catalyst with in situ exsolved Fe–Ni alloy nanospheres socketed on an oxygen deficient perovskite for direct CO2 electrolysis. ACS Catalysis, 6(9), 6219-6228. Figure 1

Dry reforming (DRM) is as an efficient way for CH 4 and CO 2 valorisation because the produced syn -gas has an H 2 /CO ratio equal to that suitable for the synthesis of oxygenated hydrocarbons and synthetic fuels. The development of Ni (10 wt%) based structured and unstructured catalysts promoted by a small amount of Ru (0.5 wt%) for DRM has been investigated. Unstructured catalysts were prepared by wet impregnation method and a combination of wash coating-wet impregnation methods was used for cordierite monoliths. Samples were characterized by XRD, BET, H 2 -TPR, TEM, FE-SEM, XPS techniques and the catalytic activity for DRM was evaluated in the temperature range 600–800 °C. The catalyst stability was followed at 800 °C during time on stream. Ru promoted catalyst (Ni-Ru) was remarkable active and stable whereas Ni catalyst deactivated due to the formation of Ni 2 + -containing inactive phases. Ni-Ru monolith was initially much more active than monometallic Ni stating the positive effect of Ru on maintaining Ni reduced. Reaching steady state condition, Ni rapidly deactivated due to carbon formation, whereas Ni-Ru monolith remained stable confirming that Ru behaves as an efficient and cheap promoter of Ni for DRM.

Pd-doped lanthanum strontium ferrite powders La0.6Sr0.4Fe1-xPdxO3-δ with x = 0.05, 0.1 (LSFPd05 and LSFPd10) are synthesized by combustion synthesis method and investigated as electrocatalysts for fuel and air electrodes of symmetric solid oxide fuel cells (SSOFCs). X-ray diffraction (XRD), Rietveld refinement, surface area (BET), temperature programmed reaction (TPR) and temperature programmed oxidation (TPO) analyses are performed on LSFPd powders. Electrical measurements both in oxidizing and reducing conditions are carried out on sintered pellets. Electrolyte (La0.8Sr0.2Ga0.8Mg0.2O3-δ) supported cells are prepared using LSFPd powders as electrodes. The electrochemical performances are evaluated in hydrogen and methane mixtures and promising performance are revealed in terms of large power output density and low polarization resistance. © 2019 Acta Materialia Inc.

In the present work, 5wt% Ni-Ce0.85Sm0.15O2-δ powders were synthesized by citrate-nitrate auto-combustion method and structured catalysts based on NiCrAl alloy foam were prepared by wash-coating technique. The open-pores of the foam enhance the heat and mass transfer. Besides, the oxygen vacancies of SDC promote the CH4 and CO2 activation for carbon suppression. Microstructural analysis of Ni-SDC/NiCrAl faom structured catalyst displayed a homogeneous distribution of the catalytic layer. Catalytic tests for DRM at 800 °C showed that the CH4 conversion was 87%, close to thermodynamic equilibrium, and the yield of H2 was 76%, indicating that the structured catalyst is suitable for indirect internal reforming in SOFCs.

Solid oxide fuel cells (SOFCs) have received considerable attention mainly for the high conversion efficiency and the capability of being powered by methane or biogas. Among the different routes for methane-syngas conversion, the dry reforming of methane (DRM) shows the combined advantages of syngas production and CO2 utilization. Besides it is extremely endothermic and it requires temperatures as high as those of operating SOFCs. Thus, SOFCs can operate in indirect internal reforming mode: simultaneously performing DRM reaction in their anode sides and, partly, compensating the energy necessary for the reforming with the heat released during the electro-oxidation. Nickel supported on samarium doped ceria (SDC) is a highly performing anodic material showing a remarkable electro-catalytic activity due to the presence of oxygen vacancies that promote the CH4 and CO2 activation for carbon suppression

The role of dopants on the catalytic activity for the dry reforming of methane (DRM) was investigated on nickel supported on metal doped ceria (Me-DC) catalysts, Ni/Me0.15Ce0.85O2-δ with Me = Zr4+, La3+ or Sm3+, prepared by citric acid synthetic route. The catalytic activity was in the order Ni/Zr-DC Zr-DC > CeO2. The extra oxygen vacancies introduced by La3+ and Sm3+ were not capable to split CO2. La-DC and Sm-DC showed significant differences in the numbers and strength of surface basic sites. Therefore, other phenomena rule the formation/removal of carbon on these catalysts. The nature of dopants influenced the Ni-supports interaction and the electronic state of the metal catalyst.

Copper and gadolinium doped ceria (GDC) anode supported fuel cells were co-sintered at relatively low temperature (900 °C) and successfully tested in the intermediate temperature (IT) range. The GDC electrolyte densification was promoted by a compressive strain induced by increasing the anodic thickness and was evaluated by SEM investigation. Instead of more commonly used La0.8Sr0.2Fe0.6Co0.4O3-δ, strontium and copper-doped lanthanum ferrite La0.8Sr0.2Fe0.8Cu0.2O3-δ (LSFCu) mixed with 30 wt% GDC (LSFCu-GDC) was employed as cathodic material. Preliminary tests on Cu-GDC/GDC/LSFCu-GDC single cells showed promising results at temperature as low as 650 °C using hydrogen as fuel.

In in-situ resource utilization and in atmosphere revitalization processes, CO 2 may be recycled to produce methane and water by Sabatier reaction. The traditional Ni/γ-Al 2 O 3 catalyst for Sabatier reaction was modified by addition of rare earth oxides La 2 O 3 and/or CeO 2 in order to improve activity, selectivity and stability. Preliminary results showed that La addition improved activity and CH 4 selectivity against CO more than Ce addition. Catalytic activity was tested using stoichiometric flow (CO 2 :H 2 = 1:4) without dilution.

Doped perovskite oxides have been widely studied in recent years as proton conducting solid electrolytes for different electrochemical devices, e.g. Solid Oxide Fuel Cells (SOFCs). By using the proton conductor as the electrolyte in a SOFC, the fuel dilution issue can be avoided due to the water formation at the cathode side instead of the anode. Among all the alkaline-earth cerates and zirconates, Y doped BaZrO3 (BZY) has been considered one of the most promising electrolyte for the SOFCs applications for its significant proton conductivity as well as chemical stability.[1] However, most reported fuel cell performance values for this material is much lower than oxide ion conducting ceramics such as YSZ owing to an unavoidable microstructure change, i.e. high grain boundary density and low degree of the crystallinity. In order to correlate the microstructure with the conductivity, we prepared BZY thin films by Pulsed Laser Deposition and discussed the effect of the microstructure on the proton conductivity. In this presentation I will show the conductivity behavior as a function of temperature and thickness by Electrochemical Impedance Spectroscopy (EIS).[2] I will focus on the interface between BZY and substrate NdGaO3 and show the cross-sectional electrochemical activity change with nanoscale resolution by Electrochemical Strain Microscopy (ESM). [3] I will highlight the interplay between the interface defects and the electrochemical performance by comparing the EIS, ESM results with Scanning Transmission Electron microscopy (STEM). I will further show the complementary study by bulk sensitive hard x-ray photoemission spectroscopy measurements which, together with Density Functional Theory calculations, demonstrate that Y and, to some extent, also Zr ions, substitute for Ba near the interface with the substrate. [1] A. D’Epifanio, E. Fabbri, E. Di Bartolomeo, S. Licoccia, E. Traversa, Fuel Cells, 1,69-762008 (2008). [2] V. Foglietti, Nan Yang, A. Tebano, C. Aruta, E. Di Bartolomeo, S. Licoccia, C. Cantoni, G. Balestrino, Applied Physics Letters 104 , 081612 (2014). [3] Nan Yang, C. Cantoni, V. Foglietti, A. Tebano, A. Belianinov, E. Strelcov, S. Jesse, D. Di Castro, E. Di Bartolomeo, S. Licoccia, S. V. Kalinin, G. Balestrino and C. Aruta, Nano Letters, 10.1021/acs.nanolett.5b00698, (2015).

Composite anodes containing La0.6Sr0.4CrO3-δ (LSCr) and Ce0.9Gd0.1O2 (GDC) infiltrated with 5 or 10 wt% Ni were investigated as potential catalysts for dry reforming of methane (DRM) reaction in methane rich mixtures. Powders were characterized and tested as catalysts for DRM reaction. Ni-infiltrated composite anodes were used for fuel cell tests and compared to Ni-free anodes. Electrochemical tests were performed in different gas mixtures to evaluate the anodic performance in externally reformed biogas (external reforming mode). The design of an external layer of composite anode achieved the internal DMR reaction (internal reforming mode).

La0.8Sr0.2Ga0.8Mg0.2O3- d (LSGM) based porous/dense structures were optimized for infiltration of different metal catalysts. A solution of metal (Ni, Co, Cu and their 1:1 mixtures) salts was used for infiltration. Multiple infiltrations were necessary to get enough metal catalyst amount to get performing electrocatalytic performance. For the initial catalyst screening, ex-situ catalyst activity measurements were performed on impregnated LSGM powders with the same amount of metal catalyst infiltrated in the scaffolds. Electrochemical measurements were performed on infiltrated cells with the same amount of Ni and Ni-Co in the temperature range between 650 and 750°C and the results discussed and compared with catalytic measurements.

The role of trivalent rare-earth dopants on the cerium oxidation state has been systematically studied by in situ photoemission spectroscopy with synchrotron radiation for 10 mol % rare-earth doped epitaxial ceria films. It was found that dopant rare-earths with smaller ionic radius foster the formation of Ce3+ by releasing the stress strength induced by the cation substitution. With a decrease of the dopant ionic radius from La3+ to Yb3+, the out-of-plane axis parameter of the crystal lattice decreases without introducing macroscopic defects. The high crystal quality of our films allowed us to comparatively study both the ionic conductivity and surface reactivity ruling out the influence of structural defects. The measured increase in the activation energy of films and their enhanced surface reactivity can be explained in terms of the dopant ionic radius effects on the Ce4+ -> Ce3+ reduction as a result of lattice relaxation. Such findings open new perspectives in designing ceria-based materials with tailored properties by choosing suitable cation substitution.

Dense micrometric La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) films were deposited by spin-coating on porous LSGM scaffolds characterized by homogeneous pore structure. Porous anodes were infiltrated with aqueous nickel and nickel/copper nitrate solutions, dried and fired at 700 °C. Homogeneous metal coating with proper interconnection was observed by SEM, chemical stability confirmed by XRD, and electrical characterization of anodic substrates was performed. Catalytic activity of different anodes was evaluated ex-situ in a quartz micro-reactor fed with CH4:CO2 mixture at range 650 and 700 °C. To investigate the redox properties of the metallic phases, the anodic substrates were subjected to redox ageing cycles and characterized by H2-TPR.

a b s t r a c t The effects of electrical contact configuration for La2CuO4 sensing electrodes for a planar-based poten- tiometric gas sensor were studied in order to further quantify the effects of processing on sensor performance. Five configurations of La2CuO4 were used for the sensing electrode. The La2CuO4 sens- ing electrode was screen printed opposite a Pt counter electrode on a tape cast YSZ electrolyte. Three sensors were prepared for each configuration to test repeatability. Each sensor was tested at tempera- tures from 400 to 700 ◦C at various concentrations of NO2, NO, and CO in an environment of 3% O2 with a balance of N2. Results show that these sensors exhibit suitable sensitivity to all the gases tested. More importantly, it was shown that the sensitivity, selectivity/cross-sensitivity, and repeatability of these sensors are dependent on the sensing electrode configuration.

Cermet anode substrates Ni–BaCe 0.9 Y 0.1 O 3− δ (Ni–BCY10) with varied ratios between Ni and BCY10 were prepared. BCY10 powders were prepared by the citrate–nitrate auto-combustion method and anode substrates were prepared by the method of evaporation and decomposition of solutions and suspensions. Sintered anode substrates were reduced and their properties were examined before and after reduction as a function of the ratio between Ni and BCY10. Microstructural properties of the pellets were investigated using X-ray diffraction analysis and field emission scanning electron microscopy. The influence of the Ni:BCY10 ratio on the microstructure of conducting paths through ceramic and metal parts was discussed. Impedance spectroscopy measurements were used for evaluation of electrical properties of the anode pellets. The high conductivity values of reduced anodes confirmed percolation through Ni particles even for an anode with a reduced amount of nickel. Fuel cell tests were carried out in order to examine the influence of the Ni:BCY10 ratio on fuel cell performance and to compare characteristics of cermet anodes with platinum electrodes. The ratio between Ni and BCY10 did have a slight influence on the power output of fuel cells. Fuel cells with a cermet anode demonstrated a higher power output compared to fuel cells with a platinum electrode.

Electrochemical planar sensors using Nb2O5 powders to fabricate sensing electrodes were tested both in laboratory and "in-situ", i.e. downstream a three-way catalytic converter of a FIAT fire 1242 c.c. spark ignition engine. Two cell configurations were compared, one with and the other without Pt paste under the metal oxide electrode. The best sensing performance both in laboratory and at the engine bench was obtained for the Pt | YSZ | Nb2O5 cell, without Pt below the oxide electrode. For this sensor configuration, very large EMF response values were obtained for hydrocarbon exposure, even at temperatures as high as 700{degree sign}C.

In this work, high temperature planar sensors based on tape-cast YSZ layers with Nb2O5 n-type semiconducting oxide as sensing electrode were studied. The investigated gases were CO and different saturated and unsaturated hydrocarbons. A study on the presence of the metallic electrode under the metal oxide and on the nature of that metallic (Pt or Au) electrode was performed. Thus, different types of sensors were prepared and investigated. The sensors were tested in potentiometric devices at various concentrations of gases in the 500–700 °C temperature range. The most promising sensor in terms of selectivity, sensitivity and stability was Pt/YSZ/Nb2O5, without any metallic electrode under the Nb2O5 powder. In fact, this sensor was found totally selective to propylene with very large EMF response even at temperature as high as 700 °C (−115 mV/decade).

The discussion about the sensing mechanism of non-Nernstian electrochemical gas sensors with a semiconducting sensing electrode is still open. This work reports a study of the influence of the metallic electrode under the semiconducting metal oxide, Nb 2 O 5 , on the sensor response. Planar sensors based on tape-cast YSZ layers were fabricated. Two Pt or Au finger electrodes were deposited on one side of the layers. One of the electrodes was covered with a Nb 2 O 5 thick film. The response to CO was studied performing electromotive force (EMF) measurements in the 450–650 °C temperature range. In order to better understand the sensing mechanism, the electrical measurements were correlated to catalytic measurements performed on both electrodes. With no catalytic electrodes, the adsorption-Fermi level related mechanism seems to prevail, while in the case of sensors with Nb 2 O 5 |Pt|YSZ electrode, the mixed potential theory is more adapted to explain the results.

The phase relations and crystal structures of La 1−x Sr x Fe 1−y Ta y O 3 in the compositional range x = 0–0.60, y = 0–0.20 were investigated by powder X-ray diffraction (XRD). The formal concentration of Fe 4+ in the La 1−x Sr x Fe 1−y Ta y O 3±w (LSFT) system was calculated and related to the electrical/electrochemical properties. To investigate the electrochemical behaviour as cathode material for SOFCs, impedance measurements were performed on LSFT/electrolyte/LSFT symmetrical cells by using La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 , (LSGM) as electrolyte material. The lowest area specific resistance (ASR), derived from the polarization resistance (R p ), 1 Ω cm 2 at 750 °C, was measured for the LSFT compound doped with x = 0.40 and y = 0.05.

Yttrium-doped barium zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here, we use electrochemical strain microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross-sectional measuring setup allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network that introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.

Planar sensors based on tape-cast YSZ layers with two parallel Pt (or Au) finger electrodes, one covered with Ta (10at.%)-doped nanosized titania powders, were prepared and studied. The sensors were tested at various concentrations of NO2, CO, and different hydrocarbons in the 450-650°C temperature range. The sensitivity to different target gases was found to be strongly dependent on the operating temperature. The replacement of Pt electrodes with Au was effective in promoting the response to propylene. Moreover, an inversion of the polarity of the electromotive force response to NO2 gas was observed in comparison with Pt-based sensors. The sensing mechanism is discussed. © 2006 The American Ceramic Society.

Solid electrolyte based sensors have been widely studied for the detection of CO/ HC and NOx at high temperature. Nevertheless, the discussion about the sensing mechanism of non-nernstian electrochemical gas sensors, with a semiconducting sensing electrode, is still open. In this work, a study of the influence of the metallic electrode under the semiconducting metal oxide Nb2O5 on the sensor response was performed. Planar sensors based on tape-cast YSZ layers were fabricated. Two Pt or Au finger electrodes were deposited in a parallel arrangement on a single side of YSZ. One of these electrodes was covered with Nb2O5 thick film. In order to better understand the sensing mechanism, the electrical measurements were correlated with catalytic measurements performed on both sensing and reference electrodes

Planar electrochemical sensors based on tape-cast yttria stabilized zirconia (YSZ, 8 wt.% of Y2O3) layers with two Pt parallel electrodes, one coated with WO3 powders were prepared. The WO3 powders were synthesized in the laboratory using different chemical routes. The NO2 and CO potentiometric response of the sensors was measured at high temperature and compared to the response of the sensors fabricated using commercial WO3 powders. Promising results in terms of selectivity, sensitivity, stability, reproducibility and response time were obtained for synthesized WO3-based sensors in comparison to the response of commercial WO3-based sensors.

Electrochemical devices based on coupling an oxygen ionic conductor (Y2O3-stabilized ZrO2: YSZ) with semiconducting oxides as sensing electrodes were investigated. LaFeO3, a p-type semiconductor, and WO3, an n-type semiconductor, were chosen because of their known good NO2-sensing properties as resistive-type sensors. The entire sensors were wholly exposed to the same atmosphere. The electromotive force (EMF) was measured for different NO2 concentrations and at fixed temperature. The EMF values of both sensors increased in NO2 atmosphere with a fast response time. The best performance was observed around 450°C for LaFeO3-based and 650°C for WO3-based sensors. Moreover, the response of the LaFeO3-based sensors was found to be strongly affected by the grain size of the powder used for the preparation of the sensing electrode. The polarization curves of all sensors showed a nonlinearity; nevertheless an opposite behavior in the presence of NO2 gas was observed for the p- and n- type-based oxide sensors. The nonlinearity was attributed to the potential drop for LaFeO3 or increase for WO3 at the electrode interface. Amperometric measurements, performed at a fixed potential of 1 V, were found in accordance with polarization curves. The electrochemical impedance spectroscopy (EIS) analysis showed that the resistance and reactance at the electrolyte/electrode interface decreased (for LaFeO3)/increased (for WO3) on introduction of NO2 gas, because of the lowering/increasing of the electrode overvoltage.

In the present work, monometallic Co/CeO2 (Co 7.5 wt.%) and Ni/CeO2 (Ni 7.5 wt.%) catalysts, were compared with the bimetallic Co-Ni/CeO2 (Co 3.75 wt.%, Ni 3.75 wt.%) catalyst. CeO2 has been selected for its redox properties that may mitigate the carbon deposition. The synthesis by surfactant assisted co-precipitation method allowed us to obtain powders with high surface area and homogeneous dispersion of cobalt and nickel nanometric particles. Samples were characterized by XRD, BET, TPR, TG-DTA and tested for DRM with CH4:CO2:Ar 20:20:60 vol.% (GHSW = 30000 mL g-1 h-1). After activation in H2 at 800°C, Co-Ni/CeO2 clearly showed the presence of a cobalt and nickel alloy phase. The bimetallic Co-Ni/CeO2 catalyst was more active and more selective for the DRM reaction than monometallic Co/CeO2 and Ni/CeO2 samples. In particular the CH4 conversion on Co-Ni/CeO2 was 50% at 600°C, and 97% at 800°C, with a H2/CO ratio in the range 0.9-1.0. After 16 hours streaming, Ni/CeO2 showed 25 wt.% of carbonaceous deposit and a catalytic activity decrease of 8%, while both cobalt containing catalysts showed 6 wt.% of carbon on the surface and a decrease of activity less than 2%. Theses features suggest that cobalt-nickel alloy combines the high activity of nickel with the carbon suppression property of cobalt.

Electrochemical planar sensors based on yttria-stabilized zirconia (YSZ) with semiconducting oxides (WO 3 and LaFeO 3 ) and mixed conductors (La 0.8 Sr 0.2 FeO 3 ) as sensing electrodes were investigated. The electromotive force (EMF) of the sensors was measured at fixed temperature (450–700 °C) and different concentrations of NO 2 and CO in air in the range 20–1000 ppm. The sensors were wholly exposed to the same gas atmosphere, alternatively air and different gases in air, without using reference air. Fast and stable response was measured for all the different sensors. The EMF of p- and n-type semiconducting-based sensors was measured in opposite directions upon the same gas exposure. These results obtained for the response of planar sensors with different semiconductor sensing electrodes showed that the sensing mechanism cannot be explained by mixed potential theory. The response of these sensors is better described by a differential electrode equilibria sensing mechanism.

The heat treatment of commercial zinc oxide based varistor ceramics at temperatures of 200–300 °C in air with 1000 ppm of NO 2 causes strong shift of voltage-current characteristic (VCC) to low voltages. Subsequent exposure to pure air leads to partial restoration of VCC. Scanning electron microscopy analysis showed microstructural changes of samples after heat treatment in air with 1000 ppm of NO 2 . Chemical reactions between NO 2 molecules and oxygen chemisorbed or Bi-rich oxide phases at grain boundaries followed by the lowering of grain boundary potential barriers can be the reason of observed degradation.

Solid-state NO 2 sensors based on yttria stabilized zirconia (YSZ), an O 2- conductor, combined either with an n- (WO 3 ), or a p-type semiconducting oxide (LaFeO 3 ), or a mixed electronic and ionic conductor (La 0.8 Sr 0.2 FeO 3 ) were investigated. Platinum parallel finger electrodes were applied on the surface of tape-casted YSZ layers and attached with gold wires for current collection. Nanocrystalline perovskite powders were prepared using different chemical methods: LaFeO 3 by the thermal decomposition of the LaFe-hexacyanide complex, and La 0.8 Sr 0.2 FeO 3 by a sol-gel route. A sub-micrometric commercial WO 3 powder was used. The oxide powders were mixed with a screen-printing oil and deposited on one Pt finger electrode. The presence of the oxide powder makes one of the electrodes different from the other in terms of catalytic activity, specific surface area, gas adsorption and reaction kinetics. Both electrodes were wholly exposed to the same gas atmosphere, without using reference air. The sensors were investigated at fixed temperature (450-700 °C) by measuring the electromotive force (EMF) at different concentrations of NO 2 and CO in air in the range 20-1000 ppm. A fast and stable response was measured for all the tested sensors. An EMF of opposite sign was measured for p- and n-type semiconducting based sensors upon exposure to the same gas. After increasing the grain size of the nano-structured La 0.8 Sr 0.2 FeO 3 powder by a heat treatment at 900 °C for 4 h, the response to NO x became small, slow and unstable.

Na2O·x Al2O3 (x= 9, 11)films have been obtained by sol–gel method. Crystallization processes during heat treatments have been investigated by X–ray diffraction analysis. A metastable phase with the mullite structure, λ–Na2O·xAl2O3, has been observed starting from 800°C. Films remained stable after a heat treatment at 1000°C for 250 h. Impedance spectroscopy measurements showed that the films of λ-Na2O·x Al2O3 possess a large three–dimensional ionic conductivity at 400°C.

Dense micrometric La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) films were deposited by spin-coating on porous LSGM scaffolds characterized by an homogeneous pore structure. Porous anodes were infiltrated with aqueous nickel nitrate solutions, dried and fired at 700 °C. Homogeneous metal coating with proper interconnections was observed by SEM, chemical stability was confirmed by XRD. Fuel cell tests and electrochemical impedance spectroscopy (EIS) were performed and discussed.

A “cobalt free” cathode material with stoichiometric composition La0.8Sr0.2Fe0.8Cu0.2O3 (LSFCu) was specifically developed for La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) electrolyte. The chemical stability with LSGM electrolyte was investigated by structural and morphological analysis. The electrochemical properties of LSFCu dense pellets were investigated in the temperature range 600–750°C by electrochemical impedance spectroscopy (EIS). LSFCu/LSGM/LSFCu symmetrical cells were prepared and Area Specific Resistance (ASR) values, directly depending on the rate limiting step of the oxygen reduction reaction, were evaluated. Fuel cells were prepared using LSFCu as cathode material on LSGM pellet and electrochemical tests were performed and compared to similar fuel cells prepared by using commercial La0.6Sr0.4Fe0.8Co0.2O3(LSFCo). The maximum current density and power density recorded for LSFCu and LSFCo were comparable demonstrating that Cu can be used as substitutes Co.

A “cobalt-free” cathode material with stoichiometric composition La 0.8 Sr 0.2 Fe 0.8 Cu 0.2 O 3− δ (LSFCu) was specifically developed for use with La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3− δ (LSGM) electrolyte in intermediate temperature solid oxide fuel cell (IT-SOFC) systems. The chemical stability of LSFCu in contact with LSGM electrolyte was investigated by structural and morphological analysis. The electrochemical properties of LSFCu dense pellets were investigated in the temperature range 600–750 °C by electrochemical impedance spectroscopy (EIS). LSFCu|LSGM|LSFCu symmetrical cells were prepared and area specific resistance (ASR) values, directly depending on the rate limiting step of the oxygen reduction reaction, were evaluated. Fuel cells were prepared using LSFCu as cathode material on a LSGM pellet and electrochemical tests were performed in the 700–800 °C temperature range and compared to similar fuel cells prepared by using commercial La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3− δ (LSFCo) as a cathode. The maximum current density and power density recorded for LSFCu and LSFCo were similar. This fact demonstrates that Cu can be used as Co substitute in perovskite cathode materials.