Andrei Kovalevsky - Academia.edu (original) (raw)

Papers by Andrei Kovalevsky

Journal of Materials Chemistry A, 2020

This work explores how, by redox-sensitive substitutions, the thermoelectric properties of oxides... more This work explores how, by redox-sensitive substitutions, the thermoelectric properties of oxides can be tuned and enhanced.

Chemphyschem : a European journal of chemical physics and physical chemistry, Jan 4, 2016

The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. T... more The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. To clarify the chemical interaction between MgH2 and Nb2 O5 , the mechanochemical reaction products of a composite mixture of MgH2 +0.167 Nb2 O5 was monitored at different time intervals (2, 5, 15, 30, and 45 min, as well as 1, 2, 5, 10, 15, 20, 25, and 30 h). The study confirms the formation of catalytically active Nb-doped MgO nanoparticles (typically Mgx Nby Ox+y , with a crystallite size of 4-8 nm) by transforming reactants through an intermediate phase typified by Mgm-x Nb2n-y O5n-(x+y) . The initially formed Mgx Nby Ox+y product is shown to be Nb rich, with the concentration of Mg increasing upon increasing milling time. The nanoscale end-product Mgx Nby Ox+y closely resembles the crystallographic features of MgO, but with at least a 1-4 % higher unit cell volume. Unlike MgO, which is known to passivate the surfaces in MgH2 system, the Nb-dissolved MgO effectively mediates the Mg-H2...

Owing to their high stability and good bulk proton conductivity yttrium doped barium zirconate-ba... more Owing to their high stability and good bulk proton conductivity yttrium doped barium zirconate-based materials are considered as potential electrolytes for protonic ceramic fuel cell applications. Nonetheless, their refractory nature leads to problematic densification that can necessitate the addition of sintering additives. While these additives assist processing, undesirable, strong, negative impacts on proton conductivity have been regularly reported. The current work assesses the potential sintering additives NiO, BaNiO x and BaY 2 NiO 5 and their influence on subsequent electrochemical properties of BaZ-r 0.85 Y 0.15 O 3-d. All sintering additives allow dense electrolyte materials (>95%) to be formed at temperatures below 1450 C, with enhanced grain growth; with the largest grain growth being offered by the BaNiO x additive. Degradation in overall electrical performances is shown to be bulk related, corresponding to large reductions in bulk conductivity up to two orders of magnitude, whilst grain boundary conductivities are less affected. Most importantly, the current article demonstrates that these high depletions in bulk proton conductivity can be effectively inverted by redox cycling in relatively mild conditions (750 C, cycling from N 2 to H 2 and back to N 2), opening the way to improve processing of these materials whilst maintaining high levels of proton conductivity.

The present work deals with the fabrication and electrochemical characterisation of a potential p... more The present work deals with the fabrication and electrochemical characterisation of a potential protonic ceramic fuel cell based on a Ni-BaZr0.85Y0.15O3-δ anode supported thin film proton conducting BaCe0.4Zr0.4Y0.2O3-δ electrolyte with a Pr2NiO4+δ cathode. Anode and electrolyte materials were prepared by an acetate-H2O2 combustion method. A thin (∼5μm), dense and crack free BaCe0.4Zr0.4Y0.2O3-δ electrolyte film was successfully obtained on a porous anode support by spin coating and firing at 1450 °C. Maximum power densities of 234, 158, 102 and 63 mW/cm2 at 700, 650, 600 and 550 °C, respectively were achieved for the Ni-BaZr0.85Y0.15O3-δ/BaCe0.4Zr0.4Y0.2O3-δ/Pr2NiO4+δ single cell under fuel cell testing conditions. Electrode polarisation resistance was assessed at open circuit conditions by use of electrochemical impedance spectroscopy (EIS) and is shown to dominate the area specific resistance at low temperatures. Postmortem analysis by scanning electron microsocpy (SEM), reveals that no delamination occurs at anode/electrolyte or electrolyte/cathode interfaces upon cell operation.

Journal of The Electrochemical Society

ABSTRACT

Boletin de la Sociedad Espanola de Ceramica y Vidrio

ABSTRACT

Boletin de la Sociedad Espanola de Ceramica y Vidrio

Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both sil... more Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both silica and lanthanum oxide, and with lanthanum silicate, were prepared by the conventional ceramic route, to assess the silica scavenging role of lanthanum oxide additions. Structural, microstructural and electrical characterization of these samples confirmed the formation of one apatite type lanthanum silicate-based phase from reaction of silica with lanthanum oxide. The formation of this phase occurred in parallel with a significant enhancement of the grain boundary conductivity of these composite materials. Further interaction between constituents, involving diffusion of La to CGO, and Ce and Gd to the apatite phase, had no significant consequences on the electrical performance of these materials. Overall, lanthanum oxide was shown to remove the siliceous phases from the grain boundaries of CGO.

ChemInform, 2001

ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance t... more ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Journal of the European Ceramic Society

Composite Ce0.8Gd0.2O2-δ (CGO, solid electrolyte) and La0.7Sr0.3MnO3-δ (LSM, electronic conductor... more Composite Ce0.8Gd0.2O2-δ (CGO, solid electrolyte) and La0.7Sr0.3MnO3-δ (LSM, electronic conductor) ceramics were tested as dual-phase membranes for oxygen separation. Oxygen permeation through CGO-LSM composite ceramics containing similar percentages of both phases is limited by bulk ionic transport. In contrast to electronic transport, oxygen ion transport in these composites depends strongly on processing conditions, decreasing with interdiffusion of components. Oxygen ions are blocked by low ionic conductivity layers formed by diffusion of cations of LSM to the contacts between CGO grains. Testing of CGO-LSM membranes at high oxygen pressures (1-50 atm) showed that the composite ceramics are stable in these conditions and exhibit Wagner-type permeation fluxes. © 2001 Elsevier Science Ltd. All rights reserved.

The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-con... more The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+delta (M = Fe, Co, Cu; x = 0-0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923-1223 K in the oxygen partial pressure range 5 x 10(-4) to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (epsilon(C)) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from -3 x 10(-3) to 6 x 10(-3), which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7-15.1)...

Journal of Solid State Electrochemistry

A gas turbine power plant for CO 2 capture, based on oxygen-permeable membranes with mixed ionic-... more A gas turbine power plant for CO 2 capture, based on oxygen-permeable membranes with mixed ionic-electronic conductivity, was analysed with respect to long-term stability by means of numerical simulation. Due to the attractive transport and physicochemical properties of mixed-conducting La 2 NiO 4+δ , this nickelate was selected as a prototype membrane material for this application. Experiments showed very slow degradation of La 2 NiO 4+δ membranes at oxygen chemical potentials close to atmo-spheric conditions, which are associated with kinetic demixing and other microstructure-related factors. Interac-tion with CO 2 in the intermediate temperature range also leads to lower oxygen permeation, whilst increasing oxygen pressure may cause partial phase decomposition and microstructural changes, thus again limiting the range of possible operation conditions. The relevant operational constraints were included in a detailed membrane-based gas turbine power plant model. The membrane perfor...

Solid State Ionics

The steady-state oxygen permeation through dense La2NiO4+δ ceramics, limited by both surface exch... more The steady-state oxygen permeation through dense La2NiO4+δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La2NiO4+δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in t...

ChemInform

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Comprehensive Membrane Science and Engineering, 2010

ABSTRACT This multivolume work covers all aspects of membrane science and technology--from basic ... more ABSTRACT This multivolume work covers all aspects of membrane science and technology--from basic phenomena to the most advanced applications and future perspectives.

Boletín de la Sociedad Española de Cerámica y Vidrio, 2008

Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both sil... more Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both silica and lanthanum oxide, and with lanthanum silicate, were prepared by the conventional ceramic route, to assess the silica scavenging role of lanthanum oxide additions. Structural, microstructural and electrical characterization of these samples confirmed the formation of one apatite type lanthanum silicate-based phase from reaction of silica with lanthanum oxide. The formation of this phase occurred in parallel with a significant enhancement of the grain boundary conductivity of these composite materials. Further interaction between constituents, involving diffusion of La to CGO, and Ce and Gd to the apatite phase, had no significant consequences on the electrical performance of these materials. Overall, lanthanum oxide was shown to remove the siliceous phases from the grain boundaries of CGO.

Physical chemistry chemical physics : PCCP, Jan 14, 2015

Although steel production by molten oxide electrolysis offers potential economic and environmenta... more Although steel production by molten oxide electrolysis offers potential economic and environmental advantages over classic extractive metallurgy, its feasibility is far from being convincingly demonstrated, mainly due to inherent experimental difficulties exerted by harsh conditions and lack of knowledge regarding relevant mechanisms and physico-chemical processes in the melts. The present work was intended to demonstrate the concept of pyroelectrolysis at very high temperature near the minimum liquidus point of magnesium aluminosilicate, being conducted under electron-blocking conditions using yttria-stabilized zirconia cells, and to provide a new insight into electrochemistry behind this process. Significant current yields are possible for pyroelectrolysis performed in electron-blocking mode using a solid electrolyte membrane to separate the anode and the molten electrolyte. Parasitic electrochemical processes rise gradually as the concentration of iron oxide dissolved in the molt...

Materials Letters, 1999

ABSTRACT

Journal of Materials Chemistry A, 2020

This work explores how, by redox-sensitive substitutions, the thermoelectric properties of oxides... more This work explores how, by redox-sensitive substitutions, the thermoelectric properties of oxides can be tuned and enhanced.

Chemphyschem : a European journal of chemical physics and physical chemistry, Jan 4, 2016

The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. T... more The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. To clarify the chemical interaction between MgH2 and Nb2 O5 , the mechanochemical reaction products of a composite mixture of MgH2 +0.167 Nb2 O5 was monitored at different time intervals (2, 5, 15, 30, and 45 min, as well as 1, 2, 5, 10, 15, 20, 25, and 30 h). The study confirms the formation of catalytically active Nb-doped MgO nanoparticles (typically Mgx Nby Ox+y , with a crystallite size of 4-8 nm) by transforming reactants through an intermediate phase typified by Mgm-x Nb2n-y O5n-(x+y) . The initially formed Mgx Nby Ox+y product is shown to be Nb rich, with the concentration of Mg increasing upon increasing milling time. The nanoscale end-product Mgx Nby Ox+y closely resembles the crystallographic features of MgO, but with at least a 1-4 % higher unit cell volume. Unlike MgO, which is known to passivate the surfaces in MgH2 system, the Nb-dissolved MgO effectively mediates the Mg-H2...

Owing to their high stability and good bulk proton conductivity yttrium doped barium zirconate-ba... more Owing to their high stability and good bulk proton conductivity yttrium doped barium zirconate-based materials are considered as potential electrolytes for protonic ceramic fuel cell applications. Nonetheless, their refractory nature leads to problematic densification that can necessitate the addition of sintering additives. While these additives assist processing, undesirable, strong, negative impacts on proton conductivity have been regularly reported. The current work assesses the potential sintering additives NiO, BaNiO x and BaY 2 NiO 5 and their influence on subsequent electrochemical properties of BaZ-r 0.85 Y 0.15 O 3-d. All sintering additives allow dense electrolyte materials (>95%) to be formed at temperatures below 1450 C, with enhanced grain growth; with the largest grain growth being offered by the BaNiO x additive. Degradation in overall electrical performances is shown to be bulk related, corresponding to large reductions in bulk conductivity up to two orders of magnitude, whilst grain boundary conductivities are less affected. Most importantly, the current article demonstrates that these high depletions in bulk proton conductivity can be effectively inverted by redox cycling in relatively mild conditions (750 C, cycling from N 2 to H 2 and back to N 2), opening the way to improve processing of these materials whilst maintaining high levels of proton conductivity.

The present work deals with the fabrication and electrochemical characterisation of a potential p... more The present work deals with the fabrication and electrochemical characterisation of a potential protonic ceramic fuel cell based on a Ni-BaZr0.85Y0.15O3-δ anode supported thin film proton conducting BaCe0.4Zr0.4Y0.2O3-δ electrolyte with a Pr2NiO4+δ cathode. Anode and electrolyte materials were prepared by an acetate-H2O2 combustion method. A thin (∼5μm), dense and crack free BaCe0.4Zr0.4Y0.2O3-δ electrolyte film was successfully obtained on a porous anode support by spin coating and firing at 1450 °C. Maximum power densities of 234, 158, 102 and 63 mW/cm2 at 700, 650, 600 and 550 °C, respectively were achieved for the Ni-BaZr0.85Y0.15O3-δ/BaCe0.4Zr0.4Y0.2O3-δ/Pr2NiO4+δ single cell under fuel cell testing conditions. Electrode polarisation resistance was assessed at open circuit conditions by use of electrochemical impedance spectroscopy (EIS) and is shown to dominate the area specific resistance at low temperatures. Postmortem analysis by scanning electron microsocpy (SEM), reveals that no delamination occurs at anode/electrolyte or electrolyte/cathode interfaces upon cell operation.

Journal of The Electrochemical Society

ABSTRACT

Boletin de la Sociedad Espanola de Ceramica y Vidrio

ABSTRACT

Boletin de la Sociedad Espanola de Ceramica y Vidrio

Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both sil... more Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both silica and lanthanum oxide, and with lanthanum silicate, were prepared by the conventional ceramic route, to assess the silica scavenging role of lanthanum oxide additions. Structural, microstructural and electrical characterization of these samples confirmed the formation of one apatite type lanthanum silicate-based phase from reaction of silica with lanthanum oxide. The formation of this phase occurred in parallel with a significant enhancement of the grain boundary conductivity of these composite materials. Further interaction between constituents, involving diffusion of La to CGO, and Ce and Gd to the apatite phase, had no significant consequences on the electrical performance of these materials. Overall, lanthanum oxide was shown to remove the siliceous phases from the grain boundaries of CGO.

ChemInform, 2001

ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance t... more ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Journal of the European Ceramic Society

Composite Ce0.8Gd0.2O2-δ (CGO, solid electrolyte) and La0.7Sr0.3MnO3-δ (LSM, electronic conductor... more Composite Ce0.8Gd0.2O2-δ (CGO, solid electrolyte) and La0.7Sr0.3MnO3-δ (LSM, electronic conductor) ceramics were tested as dual-phase membranes for oxygen separation. Oxygen permeation through CGO-LSM composite ceramics containing similar percentages of both phases is limited by bulk ionic transport. In contrast to electronic transport, oxygen ion transport in these composites depends strongly on processing conditions, decreasing with interdiffusion of components. Oxygen ions are blocked by low ionic conductivity layers formed by diffusion of cations of LSM to the contacts between CGO grains. Testing of CGO-LSM membranes at high oxygen pressures (1-50 atm) showed that the composite ceramics are stable in these conditions and exhibit Wagner-type permeation fluxes. © 2001 Elsevier Science Ltd. All rights reserved.

The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-con... more The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+delta (M = Fe, Co, Cu; x = 0-0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923-1223 K in the oxygen partial pressure range 5 x 10(-4) to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (epsilon(C)) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from -3 x 10(-3) to 6 x 10(-3), which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7-15.1)...

Journal of Solid State Electrochemistry

A gas turbine power plant for CO 2 capture, based on oxygen-permeable membranes with mixed ionic-... more A gas turbine power plant for CO 2 capture, based on oxygen-permeable membranes with mixed ionic-electronic conductivity, was analysed with respect to long-term stability by means of numerical simulation. Due to the attractive transport and physicochemical properties of mixed-conducting La 2 NiO 4+δ , this nickelate was selected as a prototype membrane material for this application. Experiments showed very slow degradation of La 2 NiO 4+δ membranes at oxygen chemical potentials close to atmo-spheric conditions, which are associated with kinetic demixing and other microstructure-related factors. Interac-tion with CO 2 in the intermediate temperature range also leads to lower oxygen permeation, whilst increasing oxygen pressure may cause partial phase decomposition and microstructural changes, thus again limiting the range of possible operation conditions. The relevant operational constraints were included in a detailed membrane-based gas turbine power plant model. The membrane perfor...

Solid State Ionics

The steady-state oxygen permeation through dense La2NiO4+δ ceramics, limited by both surface exch... more The steady-state oxygen permeation through dense La2NiO4+δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La2NiO4+δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in t...

ChemInform

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Comprehensive Membrane Science and Engineering, 2010

ABSTRACT This multivolume work covers all aspects of membrane science and technology--from basic ... more ABSTRACT This multivolume work covers all aspects of membrane science and technology--from basic phenomena to the most advanced applications and future perspectives.

Boletín de la Sociedad Española de Cerámica y Vidrio, 2008

Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both sil... more Composite materials based on gadolinium doped ceria (CGO) with additions of silica, with both silica and lanthanum oxide, and with lanthanum silicate, were prepared by the conventional ceramic route, to assess the silica scavenging role of lanthanum oxide additions. Structural, microstructural and electrical characterization of these samples confirmed the formation of one apatite type lanthanum silicate-based phase from reaction of silica with lanthanum oxide. The formation of this phase occurred in parallel with a significant enhancement of the grain boundary conductivity of these composite materials. Further interaction between constituents, involving diffusion of La to CGO, and Ce and Gd to the apatite phase, had no significant consequences on the electrical performance of these materials. Overall, lanthanum oxide was shown to remove the siliceous phases from the grain boundaries of CGO.

Physical chemistry chemical physics : PCCP, Jan 14, 2015

Although steel production by molten oxide electrolysis offers potential economic and environmenta... more Although steel production by molten oxide electrolysis offers potential economic and environmental advantages over classic extractive metallurgy, its feasibility is far from being convincingly demonstrated, mainly due to inherent experimental difficulties exerted by harsh conditions and lack of knowledge regarding relevant mechanisms and physico-chemical processes in the melts. The present work was intended to demonstrate the concept of pyroelectrolysis at very high temperature near the minimum liquidus point of magnesium aluminosilicate, being conducted under electron-blocking conditions using yttria-stabilized zirconia cells, and to provide a new insight into electrochemistry behind this process. Significant current yields are possible for pyroelectrolysis performed in electron-blocking mode using a solid electrolyte membrane to separate the anode and the molten electrolyte. Parasitic electrochemical processes rise gradually as the concentration of iron oxide dissolved in the molt...

Materials Letters, 1999

ABSTRACT