The ageing characteristics of flame-made 2 wt% Pd supported on YFeO 3 were analysed in comparison with a Pd/Al 2 O 3-CeO 2-ZrO 2 three-way catalyst (TWC) with respect to structural changes and catalytic performance for methane oxidation under stoichiometric reaction conditions. Thermal treatment under lean conditions (air, 900°C) resulted in slight decrease in the methane oxidation activity of the TWC. In marked contrast, YFeO 3-supported Pd catalysts exhibit an increase in activity after such treatment. Activity enhancement is even higher when the treatment was performed under stoichiometric conditions (air-fuel equivalence ratio, λ = 1, 900°C). To explain this observation, in-depth characterization (BET, STEM, OSCC, XAS, and CO chemisorption) of fresh and aged catalysts was performed. Both thermal and stoichiometric ageing cause a severe sintering of the support particles and the phase transformation from hexagonal to orthorhombic YFeO 3. Despite the absence of a mixed Pd-YFeO 3 phase, the growth of Pd particles appears to be limited under the λ = 1 atmosphere. In contrast to thermally aged catalysts where large PdO particles are formed, well-defined metallic Pd nanoparticles of 10-20 nm are present after stoichiometric ageing along with higher methane oxidation activity. Although it is tempting to conclude that metallic Pd is active for methane oxidation under the given conditions, reversible and periodic partial oxidation of the large metallic particles is observed in modulation excitation high energy X-ray diffraction (HXRD) experiments designed to simulate the oscillating redox conditions experienced during operation. These results indicate that large Pd particles exhibit improved methane oxidation activity but equally confirm that activity under stoichiometric conditions is the result of a delicate equilibrium dictated by the bulk-Pd/surface-PdO pair.

The two half-cell systems were compared to evaluate the influence of infiltration, overpotential and ageing on their performance and stability. Structure, microstructure and chemical composition were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDX), whereas redox behavior was evaluated through temperature programmed reduction (TPR) and thermal gravimetric analysis (TGA) in combination with electrochemical impedance spectroscopy (EIS), which was also used to test the electrochemical performance. A decrease of polarization resistance for both infiltrated electrodes, compared to the reference ones, was highlighted at open circuit voltage (OCV), although the BSCF-based cathode was more benefitted more from infiltration than the LSCF-based one. Moreover, the application of cathodic overpotentials (−0.1 to −0.3 V) resulted in opposite effects on the LSCF-and BSCF-based cathodes, highlighting a different response of the oxygen vacancies to the applied voltage for the two perovskite compositions. The positive effect of the LSM layer was further confirmed by the observed improvement in long-term stability of both infiltrated perovskite-type systems. 1 They exhibit mixed ionic and electronic conductivity, with excellent charge and mass transfer properties 2 and high oxygen exchange surface coefficients. 3 Their excellent activity for oxygen reduction has been proved widely. [4][5][6][7][8][9][10] Although the structure and electrochemical processes of LSCF and BSCF are quite different, both of them have long-term stability and redox behavior issues to be clarified, in order to be used as cathode materials in commercial devices. Indeed, when used as cathodes in intermediate-temperatures solid oxide fuel cells (IT-SOFCs), they suffer from chemical and structural instability, which causes degradation during operation time. Moreover, the correlation between their redox behavior under electrochemical operating conditions is still unclear. Many studies have been devoted to investigating LSCF degradation and several causes have been reported, among them: i) mutual cations diffusion between interfaces with consequent atoms depletion and phase separation, [11][12][13][14][15][16] ii) LSCF grain coarsening, 17 iii) reactivity with YSZ-based electrolytes, even with ceria-based barrier layer, 18 iv) impurity contamination, namely Cr from metal interconnects and B from sealing when the cell is inside a stack.19-21 Some recent work performed on an LSCF/CGO (Ce 0.9 Gd 0.1 O 2-δ ) composite cathode on a YSZ electrolyte with a CGO barrier layer 11 affirms that Sr depletion, phase separation and cations inter-diffusion occur mainly during the fabrication process, with negligible contributions during long-term * Electrochemical Society Member.e Present Address: R&D Manufacturing Support Dept., Ansaldo Energia, Via Nicola Lorenzi 8, I-16152 Genoa. z E-mail: carpanese@unige.it operation (973 K). On the other hand, some previous works conversely found that LS...

a b s t r a c t Platinum-based catalysts for the room temperature removal of formaldehyde are prepared on ceria andother supports (TiO2, Al2O3and ZrO2) for comparison, starting from chloride and nitrate precursors.The behaviour towards formaldehyde adsorption and oxidation is evaluated by means of temperature-programmed desorption (TPD), temperature-programmed oxidation (TPO) and catalytic tests. The resultsindicate that the catalytic performances are affected by Pt oxidation state, which, in turn, strongly dependson the properties of the support. In order to obtain high HCHO conversions, the support should guaranteethe accessibility to metal active sites as well as affinity for water. Moreover, to avoid Pt partial oxidation,non-reducible oxides are preferable to reducible ones

The reactivity of a ceria-rich Ce0.85Zr0.15O2 solid solution towards the thermochemical water splitting process (TWS) was studied over repeated H2/H2O redox cycles. The structural and surface modifications after treatment at high temperature under air or N2 atmospheres were characterized by High-Resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray Photoemission Spectroscopy (XPS) and Positron Annihilation Lifetime Spectroscopy (PALS). Samples treated under nitrogen resulted more active due to phase segregation with formation of a zirconyl oxynitride phase in catalytic amount. Insertion of N 3into the structure contributes to increase the numbers of oxygen vacancies that preferably arrange in large clusters, and to stabilize Ce 3+ centers on the surface. In comparison, treatment under air resulted in a different arrangement of defects with less Ce 3+ and smaller and more numerous vacancy clusters. This affects charge transfer and H-coupling processes, that play an important role in boosting the rate of H2 production. The behavior is found to be only slightly dependent on the starting ceria-zirconia composition and it is related to the development of a similar surface hetero-structure configuration, characterized by the presence of at least a ceria-rich solid solution and a (cerium-doped) zirconyl oxynitride phase, which is supposed to act as a promoter for TWS reaction. The above findings confirm the importance of a multi-phase structure in the design of ceria-zirconia oxides for water splitting reaction and allow a step forward to find an optimal composition. Moreover, the results indicate that doping with nitrogen might be a novel approach for the design of robust, thermally resistant and redox active materials. All these findings suggest new approaches for the development and design of ceria based materials for the two-step water splitting reaction and highlight the importance of engineering the surface defect structure/configuration of the material to obtain an efficient catalyst. In this regard, the role and the impact of nitridation process need to be further investigated.

A distributed charge transfer model for IT-SOFCs with MIEC electrolyte and composite electrodes is developed. A physically-based description of the electronic leakage current in the electrolyte is included, together with mass and charge conservation equations. The model is applied to simulate experimental polarization curves and impedance spectra collected on IT-SOFCs consisting of SDC electrolytes, Cu-Pd-CZ80 infiltrated anodes and LSCF/GDC composite cathodes. Hydrogen electro-oxidation experiments are examined (H 2 /N 2 humidified mixtures, 700 • C, 30-100% H 2 molar fraction). A significant increase of the ohmic resistance measured in the impedance spectra is revealed at decreasing the H 2 partial pressure or increasing the voltage (from 0.71 cm 2 at 100% H 2 to 0.81 cm 2 at 30% H 2 ). Good agreement between the calculated and experimental polarization and EIS curves is achieved by fitting the exchange current density and the capacitance of each electrode. Model and theoretical analyses allow to rationalize the observed shift of the ohmic resistance, highlighting the key-role played by the electronic leakage current. Overall, the model is able to capture significant kinetic features of IT-SOFCs, and allows to gain insight into relevant parameters for the optimal design of the cell (electrochemically active thickness, current and potential distribution, mass diffusion gradients). Samaria doped Ceria (SDC) and Gadolinia doped Ceria (GDC) are reference electrolyte materials for intermediate temperature solid oxide fuel cell (IT-SOFC) applications, thanks to their high ionic conductivity between 500• C and 700• C. 1-5 Associated to a high ionic conductivity, however, these materials show mixed ionic and electronic conductive (MIEC) properties, and their electronic conductivity increases when exposed to reducing atmospheres. The MIEC character of the electrolyte, in turn, results in the onset of electronic leakage currents (or short circuit currents) and low open-circuit voltage values. The leakage current has a chemical origin and is due to the partial reduction of Ce 4+ ions to Ce 3+ ions, prompted by the different chemical potential at the electrodes: this current is active also in the absence of an electric field applied on the cell, since it stems from the spontaneous transfer of ions across the lattice structure. As a consequence, the effects of the leakage current are not confined to the electrolyte, but extend to the electrodes, and entail a complex relationship with the current drawn from the cell. 6 Deeper insight can therefore be achieved by mathematical modeling.In the literature, comprehensive models can be found, which take into account the MIEC properties of Ce-based electrolytes. Starting from continuum rigorous descriptions, a series of models has been derived for the prediction of impedance spectra collected on MIEC electrolytes, the most important examples being those by Jamnik and Maier, 7 by the group of Haile 8,9 and by Atkinson et al. 10 These models propose closed-form, equivalent circuit-l...

The compositional and structural heterogeneity of a sample of Ce0.15Zr0.85O2 subjected to a two-step thermochemical water splitting reaction was investigated by means of X-ray powder diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS) and High-Resolution Transmission Electron Microscopy (HRTEM) analysis. High temperature treatment under N2 resulted in segregation of a Zr-rich monoclinic phase on one side and a Ce-rich cubic phase on the other. The treatment also led to a higher reducibility of the material compared to similar studies on Ce-rich compositions. HRTEM revealed the presence of a zirconia based superstructure that it is identifiable with an oxynitride phase Zr2ON2, while ceria surface enrichment was detected via XPS. H2 yield, investigated at 800°C by pulsing water over several redox cycles, showed a six-fold increase after the first cycle remaining constant after at least three subsequent cycles. The presence of zirconia oxynitride was found to be beneficial for both the oxidation and reduction steps. Solar thermochemical water splitting cycles (WSC) are an attractive carbon-free approach to H2 production from water and sunlight.1 Two-step metal oxide based cycles generate H2 through high temperature (~1500-2000°C) reduction in inert atmosphere and the subsequent water oxidation at a lower (~400-1300°C) temperature making water splitting (WS) possible at temperatures lower than the thermodynamic value (2300°C). 2 Among many metal oxides investigated in literature, ceria is one of the most viable candidates, 3 and it can deliver pure oxygen and hydrogen according to the following two-step redox cycleThe main drawback of this cycle is that a significant reduction of ceria occurs only at temperatures higher than 1800°C, where sublimation can occur with a decrease of the yield over cycles. 5 It follows that studies on ceria-based systems have been focused on lowering the reduction temperature of the Ce 4+ /Ce 3+ redox couple, while maintaining the high reactivity of reduced Ce 3+ species towards water. 3,6 The addition of high valence dopant cations, such as Zr 4+ , proved effective in increasing the thermodynamic driving force of CeO2 reduction at lower temperatures, 7-9 and the effect of zirconium content in the two step water splitting reaction has been widely studied. [10][11][12][13][14] Its presence favours CeO2 reduction under inert atmosphere at temperatures lower than 1500°C preventing sublimation and the consequent yield loss; moreover, the increased oxygen storage of ceria-zirconia positively affects O2yield. On the other hand, the H2 productivity depends on the number of exposed surface redox sites, and thus on the textural, morphological and structural properties of these materials. It was reported that in these materials the kinetics of water splitting is often hampered by gassolid diffusion limitations due to the simultaneous occurence of sintering processes. Several efforts have been made to overcome this issue, and the change of the morphological and structural properties o...

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-SOFCs). The activation of a co-oxidative route is a most distinguishing feature of Ce-based cells, compared to traditional SOFCs. SDC electrolyte supported ITSOFCs with Cu-Pd-CZ80 composite anodes and LSCF cathodes were tested under a wide range of operating conditions. Polarization and EIS measurements were collected at 600°C and 650°C with syngas mixtures (between 2.3 to 0.4 H2/CO ratio), H2/N2 mixtures (from 97 to 30% H2 v/v) and CO/CO2 mixtures (from 97 to 50% CO v/v). A 1D, dynamic and heterogeneous model of the cell was applied to analyze the polarization and the EIS curves. The kinetics of the reactions of H2 electrooxidation, CO electro-oxidation and O2 reduction were individually investigated and global power law rates were derived. The syngas experiments were simulated on a fully predictive basis and no parameter adjustment, confirming that the polarization behavior could be best reproduced exclusively by assuming the presence of the co-oxidative route. The IT-SOFCs were also exposed to biogas mixtures, revealing that the dry-reforming reaction was active. Milan, August 7 th 2015Dear Editor of the Journal of Power Sources, Please find attached our manuscript, entitled "Experimental and model analysis of the cooxidative behavior of syngas feed in an IT-SOFC" by Alessandro Donazzi, Morteza Rahmanipour, Matteo Maestri, Gianpiero Groppi, Luca Bardini, Alfonsina Pappacena and Marta Boaro, which we would like to submit for publication.In this paper, we present experimental and modeling results concerning the activation of a co-oxidative reaction mechanism in IT-SOFCs exposed to syngas mixtures. Novel, Ni-free, Ce-based IT-SOFCs are tested in a wide range of operative conditions, including biogas, syngas, H 2 /N 2 and CO/CO 2 mixtures. By means of model analysis, we show that the electro-oxidation of H 2 and the electro-oxidation of CO are active in parallel when syngas is fed to the cell, that is, the cell is able to directly oxidize CO in addition to H 2 . This is a most significant and promising result, associated to the presence of Ce in the anode formulation. For the first time in the case of ITSOFCs, the co-oxidative mechanism is numerically demonstrated. To accomplish this task, a onedimensional, dynamic and heterogeneous model is presented, which allows to predict both the polarization curves and the impedance spectroscopy curves. The model is based on fundamental conservation equations and also accounts for the presence of parasitic leakage currents, which characterize the behavior Ce electrolytes. A kinetic analysis is performed and power law rates are extracted for the electrocatalytic processes that are active in the cell.The work has not been previously submitted to the Journal of Power Sources, it has not been published previously in any form and is not under consideration for publication elsewhere. All the authors and the host authorities approve the manuscript.Thank you for your kind consideration. power law rates were derived. The syngas experiments were simulated on a fully predictive bas...