Understanding Li diffusion in solid conductors is essential for the next generation Li batteries.... more Understanding Li diffusion in solid conductors is essential for the next generation Li batteries. Here we show that density-based clustering of the trajectories computed using molecular dynamics simulations helps elucidate the Li diffusion mechanism within the Li7La3Zr2O12 (LLZO) crystal lattice. This unsupervised learning method recognizes lattice sites, is able to give the site type, and can identify Li hopping events. Results show that, while the cubic LLZO has a much higher hopping rate compared to its tetragonal counterpart, most of the Li hops in the cubic LLZO do not contribute to the diffusivity due to the dominance of back-and-forth type jumps. The hopping analysis and local Li configuration statistics give evidence that Li diffusivity in cubic LLZO is limited by the low vacancy concentration. The hopping statistics also shows uncorrelated Poisson-like diffusion for Li in the cubic LLZO, and correlated diffusion for Li in the tetragonal LLZO in the temporal scale. Further a...
Small (Weinheim an der Bergstrasse, Germany), Jan 15, 2017
The addition of transition metals, even in a trace amount, into heteroatom-doped carbon (M-N/C) i... more The addition of transition metals, even in a trace amount, into heteroatom-doped carbon (M-N/C) is intensively investigated to further enhance oxygen reduction reaction (ORR) activity. However, the influence of metal decoration on the electrolysis of the reverse reaction of ORR, that is, oxygen evolution reaction (OER), is seldom reported. Moreover, further improving the bifunctional activity and corrosion tolerance for carbon-based materials remains a big challenge, especially in OER potential regions. Here, bimetal-decorated, pyridinic N-dominated large-size carbon tubes (MM'-N/C) are proposed for the first time as highly efficient and durable ORR and OER catalysts. FeFe-N/C, CoCo-N/C, NiNi-N/C, MnMn-N/C, FeCo-N/C, NiFe-N/C, FeMn-N/C, CoNi-N/C, MnCo-N/C, and NiMn-N/C are systematically investigated in terms of their structure, composition, morphology, surface area, and active site densities. In contrast to conventional monometal and N-decorated carbon, small amounts of bimetal...
Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite... more Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied at the sub-10 nm level. Comparison between electrochemical strain microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggests that small-angle grain boundaries act as regions with enhanced electrochemical activity. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study potentially paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single extended structural defects such as grain boundaries and dislocations.
Samarium Doped Ceria (SDC) electrodes are currently of great interest for solid oxide fuel cells ... more Samarium Doped Ceria (SDC) electrodes are currently of great interest for solid oxide fuel cells (SOFC) applications. For example, ceria-containing anodes can be operated directly on hydrocarbons without coking, and in addition can be used at lower temperatures than Ni/YSZ. In order to design, optimize, and characterize electrodes, it is very useful to have models to aid in interpreting experimental results. In this work, we present a non-linear, time-dependent model for the study of SDC. This model allows us to compute species concentrations, electric potential and currents under medium bias conditions. A regular perturbation of the drift diffusion equations and Poisson's equation is used to derive the model for the behavior of bulk of the material. We also include the kinetics of reactions occurring at the SDC-gas surface where the SDC is exposed to a spatially uniform hydrogen-water-argon mixture at fixed total pressure. The numerical procedure allows for fast computations and for the direct determination of fast and rate limiting steps. Impedance spectra are computed in the 2D case and a quantitative comparison between experimental (symmetric cell) and numerical results is presented. Our model can be naturally extended to the non-symmetric case, i.e. the case under which the two sides of the SDC assembly are exposed to different atmospheres.
A two-dimensional small bias model has been developed for a patterned metal current collector $|$... more A two-dimensional small bias model has been developed for a patterned metal current collector $|$ mixed oxygen ion and electronic conductor (MIEC) $|$ patterned metal current collector electrochemical cell in a symmetric gas environment. Specifically, we compute the electrochemical potential distributions of oxygen vacancies and electrons in the bulk and near the surface for $\text{Pt} | \text{Sm}_{0.15}\text{Ce}_{0.85}\text{O}_{1.925} | \text{Pt}$ symmetric cell in a $\text{H}_2-\text{H}_2\text{O}-\text{Ar}$ (reducing) atmosphere from 500 to $650^o C$. Using a two-dimensional finite-element model, we show that two types of electronic current exist within the cell: an in-plane drift-diffusion current that flows between the gas $|$ ceria chemical reaction site and the metal current collector, and a cross-plane current that flows between the two metal electrodes on the opposite side of the cell. By fitting the surface reaction constant $\tilde k_f^0$ to experimental electrode resistance values while fixing material properties such as bulk ionic and electronic equilibrium defect concentrations and mobilities, we are able to separate the electrode polarization into the surface reaction component and the in-plane electron drift-diffusion component. We show that for mixed conductors with a low electronic conductivity (a function of oxygen partial pressure) or a high surface reaction rate constant, the in-plane electron drift-diffusion resistance can become rate-limiting in the electrode reaction.
ABSTRACT This work reports on the adhesion enhancement effects of self-assembled organothiol trea... more ABSTRACT This work reports on the adhesion enhancement effects of self-assembled organothiol treatment on copper/epoxy interfaces, as well as a significant reduction in treatment time under the influence of electric potential. The interfacial adhesion has a 20-fold enhancement through the treatment due to improved linkage between the copper substrate and epoxy layer by chemisorbed organothiol molecules. The treatment time was greatly reduced by a factor of 32 from 16 hours to 30 minutes, thanks to the electrical field assisted method. This was achieved without compromising the maximum adhesion strength, which was shown to be in the order of 97.2 Jm−2.
Physical chemistry chemical physics : PCCP, Jan 27, 2015
BaFeO3 (BFO) is a promising parent material for high-temperature oxygen catalysis. The effects of... more BaFeO3 (BFO) is a promising parent material for high-temperature oxygen catalysis. The effects of La substitution on the oxygen ion diffusion and oxygen catalysis in A-site La-substituted BFO are studied by combining data-driven molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The data-driven MD simulations are capable of providing atomic level information regarding oxygen jumps at different sites, bridging the resolution gap of analysis between MD and DFT. The simulations identify several effects due to the introduction of La. First, according to simple electroneutrality considerations and DFT calculations, La tends to decrease the concentration of oxygen vacancies in BFO. Second, La substitution lowers the activation energy of local oxygen migration, providing faster paths for oxygen diffusion. The MD analysis predicts a higher hopping rate through La-containing bottlenecks as well as easier oxygen jumps from the La-rich cages and lower dwell t...
Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction ... more Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction reaction. Pristine SrCoO3-δ is a widely used parent oxide for the development of highly active mixed conductors. Doping a small amount of redox-inactive cation into the B site (Co site) of SrCoO3-δ has been applied as an effective way to improve physico-chemical properties and electrochemical performance. Most findings however are obtained only from experimental observations and no universal guidelines have been proposed. In this article, combined experimental and theoretical studies are conducted to obtain fundamental understanding of the effect of B-site doping concentration with redox-inactive cation (Sc) on the properties and performance of the perovskite oxides. The phase structure, electronic con-ductivity, defect chemistry, oxygen reduction kinetics, oxygen ion transport and electrochemical reactivity are experi-mentally characterized. In-depth analysis of doping level effect is a...
Understanding Li diffusion in solid conductors is essential for the next generation Li batteries.... more Understanding Li diffusion in solid conductors is essential for the next generation Li batteries. Here we show that density-based clustering of the trajectories computed using molecular dynamics simulations helps elucidate the Li diffusion mechanism within the Li7La3Zr2O12 (LLZO) crystal lattice. This unsupervised learning method recognizes lattice sites, is able to give the site type, and can identify Li hopping events. Results show that, while the cubic LLZO has a much higher hopping rate compared to its tetragonal counterpart, most of the Li hops in the cubic LLZO do not contribute to the diffusivity due to the dominance of back-and-forth type jumps. The hopping analysis and local Li configuration statistics give evidence that Li diffusivity in cubic LLZO is limited by the low vacancy concentration. The hopping statistics also shows uncorrelated Poisson-like diffusion for Li in the cubic LLZO, and correlated diffusion for Li in the tetragonal LLZO in the temporal scale. Further a...
Small (Weinheim an der Bergstrasse, Germany), Jan 15, 2017
The addition of transition metals, even in a trace amount, into heteroatom-doped carbon (M-N/C) i... more The addition of transition metals, even in a trace amount, into heteroatom-doped carbon (M-N/C) is intensively investigated to further enhance oxygen reduction reaction (ORR) activity. However, the influence of metal decoration on the electrolysis of the reverse reaction of ORR, that is, oxygen evolution reaction (OER), is seldom reported. Moreover, further improving the bifunctional activity and corrosion tolerance for carbon-based materials remains a big challenge, especially in OER potential regions. Here, bimetal-decorated, pyridinic N-dominated large-size carbon tubes (MM'-N/C) are proposed for the first time as highly efficient and durable ORR and OER catalysts. FeFe-N/C, CoCo-N/C, NiNi-N/C, MnMn-N/C, FeCo-N/C, NiFe-N/C, FeMn-N/C, CoNi-N/C, MnCo-N/C, and NiMn-N/C are systematically investigated in terms of their structure, composition, morphology, surface area, and active site densities. In contrast to conventional monometal and N-decorated carbon, small amounts of bimetal...
Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite... more Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied at the sub-10 nm level. Comparison between electrochemical strain microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggests that small-angle grain boundaries act as regions with enhanced electrochemical activity. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study potentially paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single extended structural defects such as grain boundaries and dislocations.
Samarium Doped Ceria (SDC) electrodes are currently of great interest for solid oxide fuel cells ... more Samarium Doped Ceria (SDC) electrodes are currently of great interest for solid oxide fuel cells (SOFC) applications. For example, ceria-containing anodes can be operated directly on hydrocarbons without coking, and in addition can be used at lower temperatures than Ni/YSZ. In order to design, optimize, and characterize electrodes, it is very useful to have models to aid in interpreting experimental results. In this work, we present a non-linear, time-dependent model for the study of SDC. This model allows us to compute species concentrations, electric potential and currents under medium bias conditions. A regular perturbation of the drift diffusion equations and Poisson's equation is used to derive the model for the behavior of bulk of the material. We also include the kinetics of reactions occurring at the SDC-gas surface where the SDC is exposed to a spatially uniform hydrogen-water-argon mixture at fixed total pressure. The numerical procedure allows for fast computations and for the direct determination of fast and rate limiting steps. Impedance spectra are computed in the 2D case and a quantitative comparison between experimental (symmetric cell) and numerical results is presented. Our model can be naturally extended to the non-symmetric case, i.e. the case under which the two sides of the SDC assembly are exposed to different atmospheres.
A two-dimensional small bias model has been developed for a patterned metal current collector $|$... more A two-dimensional small bias model has been developed for a patterned metal current collector $|$ mixed oxygen ion and electronic conductor (MIEC) $|$ patterned metal current collector electrochemical cell in a symmetric gas environment. Specifically, we compute the electrochemical potential distributions of oxygen vacancies and electrons in the bulk and near the surface for $\text{Pt} | \text{Sm}_{0.15}\text{Ce}_{0.85}\text{O}_{1.925} | \text{Pt}$ symmetric cell in a $\text{H}_2-\text{H}_2\text{O}-\text{Ar}$ (reducing) atmosphere from 500 to $650^o C$. Using a two-dimensional finite-element model, we show that two types of electronic current exist within the cell: an in-plane drift-diffusion current that flows between the gas $|$ ceria chemical reaction site and the metal current collector, and a cross-plane current that flows between the two metal electrodes on the opposite side of the cell. By fitting the surface reaction constant $\tilde k_f^0$ to experimental electrode resistance values while fixing material properties such as bulk ionic and electronic equilibrium defect concentrations and mobilities, we are able to separate the electrode polarization into the surface reaction component and the in-plane electron drift-diffusion component. We show that for mixed conductors with a low electronic conductivity (a function of oxygen partial pressure) or a high surface reaction rate constant, the in-plane electron drift-diffusion resistance can become rate-limiting in the electrode reaction.
ABSTRACT This work reports on the adhesion enhancement effects of self-assembled organothiol trea... more ABSTRACT This work reports on the adhesion enhancement effects of self-assembled organothiol treatment on copper/epoxy interfaces, as well as a significant reduction in treatment time under the influence of electric potential. The interfacial adhesion has a 20-fold enhancement through the treatment due to improved linkage between the copper substrate and epoxy layer by chemisorbed organothiol molecules. The treatment time was greatly reduced by a factor of 32 from 16 hours to 30 minutes, thanks to the electrical field assisted method. This was achieved without compromising the maximum adhesion strength, which was shown to be in the order of 97.2 Jm−2.
Physical chemistry chemical physics : PCCP, Jan 27, 2015
BaFeO3 (BFO) is a promising parent material for high-temperature oxygen catalysis. The effects of... more BaFeO3 (BFO) is a promising parent material for high-temperature oxygen catalysis. The effects of La substitution on the oxygen ion diffusion and oxygen catalysis in A-site La-substituted BFO are studied by combining data-driven molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The data-driven MD simulations are capable of providing atomic level information regarding oxygen jumps at different sites, bridging the resolution gap of analysis between MD and DFT. The simulations identify several effects due to the introduction of La. First, according to simple electroneutrality considerations and DFT calculations, La tends to decrease the concentration of oxygen vacancies in BFO. Second, La substitution lowers the activation energy of local oxygen migration, providing faster paths for oxygen diffusion. The MD analysis predicts a higher hopping rate through La-containing bottlenecks as well as easier oxygen jumps from the La-rich cages and lower dwell t...
Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction ... more Mixed conducting perovskite oxides are promising catalysts for high-temperature oxygen reduction reaction. Pristine SrCoO3-δ is a widely used parent oxide for the development of highly active mixed conductors. Doping a small amount of redox-inactive cation into the B site (Co site) of SrCoO3-δ has been applied as an effective way to improve physico-chemical properties and electrochemical performance. Most findings however are obtained only from experimental observations and no universal guidelines have been proposed. In this article, combined experimental and theoretical studies are conducted to obtain fundamental understanding of the effect of B-site doping concentration with redox-inactive cation (Sc) on the properties and performance of the perovskite oxides. The phase structure, electronic con-ductivity, defect chemistry, oxygen reduction kinetics, oxygen ion transport and electrochemical reactivity are experi-mentally characterized. In-depth analysis of doping level effect is a...
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Papers by Francesco Ciucci