This contribution introduces how the integration of biomass as fuel in power plants would balance... more This contribution introduces how the integration of biomass as fuel in power plants would balance CO2 emissions and the related role of oxygen transport membranes (OTM) on it. CO2 capture techniques could be introduced to minimize CO2 emissions at the cost of a substantial energy penalty in the overall process. Among the different approaches, the use of pure O2 and/or N2-free oxidation gases for combustion and/or for gasification leads to promising energy efficiencies. Ceramic OTM membranes could be successfully integrated in such thermal processes, which enable to increase the net plant efficiency when CO2 capture is implemented. Further, this work reviews how selected ceramic materials and membrane architectures behave under CO2 containing atmospheres at high temperatures above 700 ºC. These conditions have been selected for checking the viability of these compositions and configurations to fit in an oxy-co-gasification process, involving coal and biomass. The tested asymmetric membranes present promising oxygen fluxes in the range 0.6-1.2 ml•min-1 •cm-2 when using 100% CO2 as sweep gas at 850 ºC (optimal membrane operation conditions in oxyfuel power plant) and stable oxygen production up to 100 hours of continuous operation in similar conditions. Specifically, La0.6Sr0.4Co0.2Fe0.8O3-δ and NiFe2O4-Ce0.8Tb0.2O2-δ composite materials showed the best results for oxygen permeation and time stability under CO2-rich atmospheres.
La0.8Sr0.2MnO3- (LSM) and Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) composite anodes deposited on proton-c... more La0.8Sr0.2MnO3- (LSM) and Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) composite anodes deposited on proton-conducting BaCe0.2Zr0.7Y0.1O3- (BCZY27) electrolytes were studied as steam electrolysis anodes in symmetric cells. The effect of the electrode composition and microstructure on the electrochemical behavior was investigated using impedance spectroscopy in the 800-500 ºC range under 3 bar of pressure of wet air (75% of steam). The first screening revealed that 50-50 (vol.%) LSCF/BCZY27 composite anodes show the best performance, reaching polarization resistances < 0.68 Ω•cm 2 at 700 °C and high steam pressure (0.75 bar of air and 2.25 bar of steam). The performance of the LSCF/BCZY27 composite was further improved by changing the ratio of the different phases. Finally the anode operation conditions (steam, oxygen and total pressures) were systematically varied in order to identify and characterize the different electrochemical processes that take place in the anode under realistic operation.
Proton Transport through Robust CPO-27-type Metal Organic Frameworks
Journal of Physical Chemistry C, 2014
ABSTRACT `In this work we have studied the robustness of Ni-CPO-27 and Mg-CPO-27 metal organic fr... more ABSTRACT `In this work we have studied the robustness of Ni-CPO-27 and Mg-CPO-27 metal organic frameworks upon cold uniaxial pressing and thermal cycling in dry and wet Ar/H2. The preparation and operation limits for each material are found to be 225 MPa and 150 MPa and temperatures of 250 and 150 ºC for Ni-CPO-27 and Mg-CPO-27, respectively. The electrochemical AC conductivity measurements performed up to 250 ºC showed conductivity values ranging from 10 6-10-8 S/cm, depending on the material, temperature and atmosphere. The protonic nature of the electrochemical transport phenomena was unambiguously confirmed via proton/deuteron isotopic and transient hydration studies. The study reveals high reproducibility and stability of the electrochemical measurements upon cycling in different atmospheres. Meanwhile, the crystallinity of the sample was preserved after the conductivity study and three weeks on stream, which demonstrates the mid-term stability and robustness of this MOF.
Catalytic layer optimization for hydrogen permeation membranes based on La5.5WO11.25-δ/La0.87Sr0.13CrO3-δ composites
ACS Applied Materials & Interfaces
(LWO/LSC) composite is one of the most promising mixed ionic-electronic conducting materials for ... more (LWO/LSC) composite is one of the most promising mixed ionic-electronic conducting materials for hydrogen separation at high temperature. However, these materials present limited catalytic surface activity toward H2 activation and water splitting, which determines the overall H2 separation rate. For the implementation of these materials as catalytic membrane reactors, effective catalytic layers have to be developed that are compatible and stable under the reaction conditions. This contribution presents the development of catalytic layers based on sputtered metals (Cu and Pd), electrochemical characterization by impendace spectroscopy, and the study of the H2 flow obtained by coating them on 60/40-LWO/LSC membranes. Stability of the catalytic layers is also evaluated under H2 permeation conditions and CH4-containing atmospheres.
The electrochemical properties of mixed-conducting ceramic-ceramic (cer-cer) composites for proto... more The electrochemical properties of mixed-conducting ceramic-ceramic (cer-cer) composites for proton-conducting solid oxide fuel cells (PC-SOFC) based on La 0.995 Ca 0.005 NbO 4-δ (LCN) have been investigated. Different ratios of La 0.8 Sr 0.2 MnO 3-δ / La 0.995 Ca 0.005 NbO 4-δ (LSM/LCN) composites have been tested as cathodes in symmetrical cells based on La 0.995 Ca 0.005 NbO 4-δ dense electrolytes while two different electrode sintering temperatures (1050 and 1150 ºC) have been studied. Additionally, different LCN doped materials (Pr, Ce and Mn), which present a different conduction behavior, have been used as components in composite cathodes (mixtures of LSM/doped-LCN 50/50 vol.%). Electrochemical impedance spectroscopy analysis has been carried out in the temperature range 700-900 ºC under moist (2.5%) atmospheres.
Electrochemical properties of composite cathodes for La 0.995Ca 0.005NbO 4- d -based proton conducti
Lancet, 2011
The electrochemical properties of mixed-conducting ceramic–ceramic (cer–cer) composites for proto... more The electrochemical properties of mixed-conducting ceramic–ceramic (cer–cer) composites for proton-conducting solid oxide fuel cells (PC-SOFCs) based on La0.995Ca0.005NbO4−δ (LCN) have been investigated. Different ratios of La0.8Sr0.2MnO3−δ/La0.995Ca0.005NbO4−δ (LSM/LCN) composites have been tested as cathodes in symmetrical cells based on La0.995Ca0.005NbO4−δ dense electrolytes while two different electrode sintering temperatures (1050 and 1150 °C) have been studied. Additionally, different LCN doped materials (Pr, Ce and Mn), which present a different conduction behavior, have been used as components in composite cathodes (mixtures of LSM/doped-LCN 50/50 vol.%). Electrochemical impedance spectroscopy analysis has been carried out in the temperature range 700–900 °C under moist (2.5%) atmospheres. Different oxygen partial pressures (pO2) have been employed in order to characterize the processes (surface reaction and charge transport) occurring at the composite electrode under oxidizing conditions. The main outcome of the present study is that the mixture of LSM (electronic phase) and LCN (protonic phase) enables to decrease substantially the electrode polarization resistance. This is ascribed to the increase in the three-phase-boundary length and therefore it allows electrochemical reactions to occur in a larger region (thickness) of the electrode.► The incorporation of the protonic phase LCN in the LSM electrode allowed improving the electrochemical performance. ► The best cathode was the LSM/LCN 50/50 vol.% sintered at 1150 °C. ► LSM/LCN cathode operation is limited by medium to high frequencies, related to protonic transport through the electrode and through the electrolyte–electrode interface. ► The use of composites based on LSM/doped-LCN does not enable the improvement of the cathode performance.
The aim of the present work is to evaluate the influence of La 6 W 2 O 15 secondary phase on the ... more The aim of the present work is to evaluate the influence of La 6 W 2 O 15 secondary phase on the properties and integrity of La 6-x WO 12-δ -based membranes. Structural, microstructural and thermo-chemical study was carried out evidencing significant crystallographic and thermal expansion anisotropy: the reason for poor thermo-mechanical stability of La 6 W 2 O 15 . Conductivity of La 6 W 2 O 15 was one to two orders of magnitude lower compared to the phase pure La 6-x WO 12-δ in the range of 300 to 900 °C. The relaxation study showed that the hydration process was faster for the La 6 W 2 O 15 compared to the LWO phase, due to the higher electronic contribution to the total conductivity. Short-term stability tests in H 2 at 900 °C and in a mixture of CO 2 and CH 4 at 750 °C were conducted and material remained stable. Remarkable reactivity with NiO and YSZ at elevated temperatures was further evidenced compared to the relative inert behavior towards MgO and CGO.
The conduction properties of La 5.5 WO 12-δ have been studied by using the electrochemical conduc... more The conduction properties of La 5.5 WO 12-δ have been studied by using the electrochemical conductivity relaxation technique. Exchange and diffusion coefficients have been obtained from relaxation experiments including measurements of conductivity transients from oxidation-reduction under dry and wet oxygen; and (2) hydration-dehydration for both H 2 Osaturated and D 2 O saturated oxygen at atmospheric pressure. The evolution of the total conductivity as a function of temperature, oxygen partial pressure and the presence of H 2 O or D 2 O has moreover been characterized. The exchange coefficient for oxygen and water incorporation was calculated from the relaxation experiments. It was also possible to determine the coefficients for oxygen-vacancy/oxygen-ion (in dry and hydrated state) diffusion and for ambipolar diffusion of water. Oxygen vacancy / oxide ion diffusion is apparently not affected by the hydration state within the studied temperature range, water and oxygen partial pressures. The ambipolar water diffusion coefficient was significantly smaller than the one for oxygen ion.
Solid oxide fuel cells Proton conductivity Compatibility AC impedance a b s t r a c t The study p... more Solid oxide fuel cells Proton conductivity Compatibility AC impedance a b s t r a c t The study presents the chemical and mechanical compatibility of the proton conducting electrolyte La 0.995 Ca 0.005 NbO 4 (LCNO) with the LSM, LSCM and BSCF cathodes and the electrochemical performance of symmetrical cells based on LCNO. After annealing at high temperature the electrolyte-cathode mixtures in air and wet air, the obtained products were analyzed by X-ray powder diffraction (XRPD). The microstructure of the cathode and electrolyte materials and the interfaces were observed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX). The results show that LSCM cathode is chemically and mechanically stable with the LCNO electrolyte although the BSCF cathode reacts with it. Cation diffusion was observed between LSM cathode and LCNO electrolyte after the heat treatment of their mixture at T ¼ 1150 C. The electrochemical study performed on symmetrical cells revealed that the LSCM cathode presents the lowest value of area specific resistance (ASR) compared to the ones of the LSM and BSCF cathodes: ASR LSCM ¼ 35 U cm 2 ; ASR LSM ¼ 57 U cm 2 ; ASR BSCF ¼ 416 U cm 2 (in humidified air at 750 C).
The search for new Solid Oxide Fuel Cells (SOFC) cathodes with mixed ionic and electronic conduct... more The search for new Solid Oxide Fuel Cells (SOFC) cathodes with mixed ionic and electronic conductivity (MIEC) has achieved high interest during the last years. These MIEC cathodes allow the enlargement of the three phase boundary (TPB) area to cover the whole electrode surface, thus increasing the number of reaction sites and the electrochemical performance. The oxygen reduction reaction is improved. As a consequence, the SOFC operation temperature can be reduced up to the intermediate temperature range (IT-SOFC) and then the cost of the whole system. The present work is focused on the study of different cathodes for IT-SOFC based on the La n+1 Ni n O 3n+1 (n=1, 2 and 3) Ruddlesden-Popper series. La 2 NiO 4+δ consists of alternating perovskite and rock-salt layers and shows high electronic and ionic conductivity, appropriate thermal matching with common electrolytes and good stability in CO 2 -bearing atmospheres in contrast to well-known Ba or Sr bearing MIEC perovskites, e.g., Ba ...
ABSTRACT Composite cathodes based on an electronic conductor and a protonic conductor show advant... more ABSTRACT Composite cathodes based on an electronic conductor and a protonic conductor show advantages for protonic ceramic fuel cells. In this work, the performance of a La5.5WO11.25−δ/La0.8Sr0.2MnO3+δ (LWO/LSM) composite cathode in a fuel cell based on an LWO protonic conducting electrolyte is shown and catalytically improved. The limiting processes were first unambiguously associated to surface steps, and the reaction rate was significantly enhanced by 1) tailoring the catalytic activity through electrode surface impregnation with intrinsically catalytic nanoparticles and 2) electrochemical activation of surface species by imposing a net current through the electrode.
Currently investigated cathodes in proton conducting solid oxide fuel cells (PC-SOFC) are princip... more Currently investigated cathodes in proton conducting solid oxide fuel cells (PC-SOFC) are principally based on materials employed in oxygen-ion conducting SOFC cathodes. Recently, materials based on ceramic-ceramic composites (cercer) [1-4], combining a proton conducting phase and an electronic conducting phase, have shown appealing electrochemical results. This work presents the electrochemical properties of different mixed-conducting cercer composites as PC-SOFC cathodes for two different kinds of protonic electrolytes:
Lanthanum niobates with general formulas of La 0.99 X 0.01 Nb 0.99 Al 0.01 O 4−δ and La 0.99 X 0.... more Lanthanum niobates with general formulas of La 0.99 X 0.01 Nb 0.99 Al 0.01 O 4−δ and La 0.99 X 0.01 Nb 0.99 Ti 0.01 O 4−δ (X = Mg, Ca, Sr or Ba) were synthesized via the conventional solid state reaction. Specimens with relative density above 96% were produced after sintering. Structural and phase composition studies revealed predominant monoclinic Fergusonite structure for the majority of samples. SEM and TEM studies elucidated the effect of the used dopant combinations on grain growth, micro-crack formation and secondary phase formation. Results from microstructural study were correlated to the grain interior and grain boundary conductivities for selected samples La 0.99 Sr 0.01 Nb 0.99 Al 0.01 O 4−δ and La 0.99 Sr 0.01 Nb 0.99 Ti 0.01 O 4−δ ). The majority of co-doped niobates exhibited appreciable protonic conductivity under humid atmospheres at intermediate temperatures. Sr-or Ca-doped compounds displayed the highest total conductivities with values for LSNA equal to 6 × 10 −4 S/cm and 3 × 10 −4 S/cm in wet air and in wet 4% H 2 -Ar (900 • C), respectively. Additionally, thermal expansion was studied to complete functional characterization of co-doped LaNbO 4 . (M.E. Ivanova).
We report the high temperature electronic transport properties of SrFeO3−δ epitaxial thin films o... more We report the high temperature electronic transport properties of SrFeO3−δ epitaxial thin films obtained by pulsed laser deposition on NdGaO3(110) substrates. The films show total conductivity higher than the bulk material and apparent activation energy of about 0.12 eV in O2, lower than reported values for SrFeO3−δ films. The conductivity dependence with oxygen partial pressure shows a power dependence with an
Structural characterization and transport properties of A-(Ce and Pr) and Bsite (Mn) doped La 0.9... more Structural characterization and transport properties of A-(Ce and Pr) and Bsite (Mn) doped La 0.995 Ca 0.005 NbO 4-δ (LCN) have been investigated. Conductivity data have been analyzed in the temperature range from 400 to 800 ºC as a function of the oxygen partial pressure under dry atmospheres and under different wet atmospheres. All compounds present changes in the activation energy due to a phase transformation at ca. 550 ºC. Under wet conditions the conductivity is predominantly protonic in all compounds except for the Ce doped sample under oxidizing atmospheres. The conductivity of Ce doped sample is principally electronic at high pO 2 and high temperatures while it becomes more ionic at low pO 2 and temperatures. The combination of n-type electronic and protonic conductivity for Ce doped LCN under reducing atmospheres suggests its potential application as an anode component for LCN-based proton conducting fuel cells (PCFC). Moreover, Pr doped LCN shows remarkable p-type electronic conductivity at high pO 2 in combination with protonic conductivity, which are interesting properties for LCN-based PCFCs cathode components. Furthermore, Ce and Pr addition have allowed increasing the sintering activity with respect to LCN powders. The activation energies and the preexponential factors under wet atmospheres have been studied and correlated. It was found that they obey the Meyer-Neldel law and an estimation of the activation energy of protons in both crystal structures has been derived.
Ceramic mixed ionic-electronic conducting (MIEC) membranes enable the very selective oxygen separ... more Ceramic mixed ionic-electronic conducting (MIEC) membranes enable the very selective oxygen separation from air at high temperatures. Two major potential applications of oxygen-transport membranes are (1) oxygen production for oxyfuel power plants and (2) integration within high-temperature catalytic membrane reactors for methane or alkane upgrading by selective oxidative conversions. However, these applications involve the contact with carbon-bearing atmospheres and most of state-ofthe-art highly-permeable MIEC membranes do not tolerate operation under CO 2 -rich environments due to carbonation processes. The present contribution shows our first attempts in the development of ceria-based protective thin layers on monolithic LSCF membranes. Gd-doped ceria (CGO) deposition was carried out by air blast spray pyrolysis on mirror polished LSCF disc membranes. The layer thickness was maintained below 0.4 µm in order to prevent the formation of cracks during thermal cycling and minimize the limitations caused by the reduced oxygen permeability through the ceria layer. After optimization of the spraying process, smooth crack-free dense coatings were obtained with high crystallinity in the as-deposited state. The layers were characterized by XRD, SEM, AFM, DC-conductivity measurements, interferometry and optical microscopy. Oxygen separation was studied on coated LSCF using air as the feed and argon/CO 2 mixtures as the sweep gas in the temperature range 650-1000ºC. The protected membrane exhibited a higher stability than the un-coated LSCF membrane, although the nominal oxygen flux was slightly reduced at temperatures below 850ºC due to the limited ambipolar conductivity of doped ceria in the range of oxygen partial pressures that were investigated. Moreover, the protective layer (250 nm thickness) remained stable after the permeation testing.
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Papers by Cecilia Solis