Bilge Yildiz

Oxygen diffusion in solid oxide fuel cell cathode and electrolyte materials: mechanistic insights from atomistic simulations

Solid oxide fuel cells are of technological interest as they offer high efficiency for energy conversion in a clean way. Understanding fundamental aspects of oxygen self-diffusion in solid state ionic systems is important for the discovery of next-generation electrolyte and cathode material compositions and microstructures that can enable the operation of SOFCs at lower temperatures more efficiently, durably, and economically. In the present perspective article, we illustrate the important role of modelling and simulations in providing direct atomic scale insights on the oxygen ion transport mechanisms and conduction properties in the cathode and electrolyte materials, and in accelerating the progress from old materials to new concepts. We first summarize the ionic transport mechanisms in the traditional cathode and electrolyte materials which have been widely studied. We then pay our attention to the non-traditional materials and their oxygen transport paths from recent studies, focusing on structural and transportanisotropy and lattice dynamics. Lastly, we highlight the new developments in the potential to increase the ionic conductivity of the traditional materials through external mechanical stimuli, bringing about the mechano-chemical coupling to drive fast ionic transport.

Nanoscale cation motion in TaOx, HfOx and TiOx memristive systems

A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

Oxygen ion diffusivity in strained yttria stabilized zirconia: where is the fastest strain?

We present the mechanism and the extent of increase in the oxygen anion diffusivity in Y2O3 stabilized ZrO2 (YSZ) under biaxial lattice strain. The oxygen vacancy migration paths and barriers in YSZ as a function of lattice strain was assessed computationally using density functional theory (DFT) and nudged elastic band (NEB) method. Two competing and non-linear processes acting in parallel were identified to alter the migration barrier upon applied strain: (1) the change in the space, or electronic density, along the migration path, and (2) the change in the strength of the interatomic bonds between the migrating oxygen and the nearest neighbor cations that keep the oxygen from migrating. The increase of the migration space and the weakening of the local oxygen–cation bonds correspond to a decrease of the migration barrier, and vice versa. The contribution of the bond strength to the changes in the migration barrier is more significant than that of the opening of migration space in strained YSZ. A database of migration barrier energies as a function of lattice strain for a set of representative defect distributions in the vicinity of the migration path in YSZ was constructed. This database was used in kinetic Monte Carlo (KMC) simulations to estimate the effective oxygen diffusivity in strained YSZ. The oxygen diffusivity exhibits an exponential increase up to a critical value of tensile strain, or the fastest strain. This increase is more significant at the lower temperatures. At the strain states higher than the critical strain, the diffusivity decreases. This is attributed to the local relaxations at large strain states beyond a limit of elastic bond strain, resulting in the strengthening of the local oxygen–cation bonds that increases the migration barrier. The highest enhancement of diffusivity in 9%-YSZ compared to its unstrained state is 6.8 × 103 times at 4% strain and at 400 K. The results indicate that inducing an optimal strain state by direct mechanical load or by creating a coherent hetero-interface with lattice mismatch can enable desirably high ionic conductivity in YSZ at reduced temperatures. The insights gained here particularly on the nonlinear and competing consequences of lattice strain on the local bonding structure and charge transport process are of importance for tuning the ionic transport properties in a variety of solid-state conducting material applications, including but not limited to fuel cells.

Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites

Cation segregation on perovskite oxide surfaces affects vastly the oxygen reduction activity and stability of solid oxide fuel cell (SOFC) cathodes. A unified theory that explains the physical origins of this phenomenon is therefore needed for designing cathode materials with optimal surface chemistry. We quantitatively assessed the elastic and electrostatic interactions of the dopant with the surrounding lattice as the key driving forces for segregation on model perovskite compounds, LnMnO3 (host cation Ln = La, Sm). Our approach combines surface chemical analysis with X-ray photoelectron and Auger electron spectroscopy on model dense thin films and computational analysis with density functional theory (DFT) calculations and analytical models. Elastic energy differences were systematically induced in the system by varying the radius of the selected dopants (Ca, Sr, Ba) with respect to the host cations (La, Sm) while retaining the same charge state. Electrostatic energy differences were introduced by varying the distribution of charged oxygen and cation vacancies in our models. Varying the oxygen chemical potential in our experiments induced changes in both the elastic energy and electrostatic interactions. Our results quantitatively demonstrate that the mechanism of dopant segregation on perovskite oxides includes both the elastic and electrostatic energy contributions. A smaller size mismatch between the host and dopant cations and a chemically expanded lattice were found to reduce the segregation level of the dopant and to enable more stable cathode surfaces. Ca-doped LaMnO3 was found to have the most stable surface composition with the least cation segregation among the compositions surveyed. The diffusion kinetics of the larger dopants, Ba and Sr, was found to be slower and can kinetically trap the segregation at reduced temperatures despite the larger elastic energy driving force. Lastly, scanning probe image contrast showed that the surface chemical heterogeneities made of dopant oxides upon segregation were electronically insulating. The consistency between the results obtained from experiments, DFT calculations, and analytical theory in this work provides a predictive capability to tailor the cathode surface compositions for high-performance SOFCs.

Glassy Nature of Water in an Ultraconfining Disordered Material: The Case of Calcium−Silicate−Hydrate

We present the structural and dynamic nature of water ultraconfined in the quasi-two-dimensional nanopores of the highly disordered calcium-silicate-hydrate (C-S-H), the major binding phase in cement. Our approach is based on classical molecular simulations. We demonstrate that the C-S-H nanopore space is hydrophilic, particularly because of the nonbridging oxygen atoms on the disordered silicate chains which serve as hydrogen-bond acceptor sites, directionally orienting the hydrogen atoms of the interfacial water molecules toward the calcium-silicate layers. The water in this interlayer space adopts a unique multirange structure: a distorted tetrahedral coordination at short range up to 2.7 Å, a disordered structure similar to that of dense fluids and supercooled phases at intermediate range up to 4.2 Å, and persisting spatial correlations through dipole-dipole interactions up to 10 Å. A three-stage dynamics governs the mean square displacement (MSD) of water molecules, with a clear cage stage characteristic of the dynamics in supercooled liquids and glasses, consistent with its intermediate-range structure identified here. At the intermediate time scales corresponding to the β-relaxation of glassy materials, coincident with the cage stage in MSD, the non-Gaussian parameter indicates a significant heterogeneity in the translational dynamics. This dynamic heterogeneity is induced primarily because of the heterogeneity in the distribution of hydrogen bond strengths. The strongly attractive interactions of water molecules with the calcium silicate walls serve to constrain their motion. Our findings have important implications on describing the cohesion and mechanical behavior of cement from its setting to its aging.

Tensile Lattice Strain Accelerates Oxygen Surface Exchange and Diffusion in La1–xSrxCoO3−δ Thin Films

The influence of lattice strain on the oxygen exchange kinetics and diffusion in oxides was investigated on (100) epitaxial La1–xSrxCoO3−δ (LSC) thin films grown by pulsed laser deposition. Planar tensile and compressively strained LSC films were obtained on single-crystalline SrTiO3 and LaAlO3. 18O isotope exchange depth profiling with ToF-SIMS was employed to simultaneously measure the tracer surface exchange coefficient k* and the tracer diffusion coefficient D* in the temperature range 280–475 °C. In accordance with recent theoretical findings, much faster surface exchange (∼4 times) and diffusion (∼10 times) were observed for the tensile strained films compared to the compressively strained films in the entire temperature range. The same strain effect—tensile strain leading to higher k* and D*—was found for different LSC compositions (x = 0.2 and x = 0.4) and for surface-etched films. The temperature dependence of k* and D* is discussed with respect to the contributions of strain states, formation enthalpy of oxygen vacancies, and vacancy mobility at different temperatures. Our findings point toward the control of oxygen surface exchange and diffusion kinetics by means of lattice strain in existing mixed conducting oxides for energy conversion applications.

Surface Electronic Structure Transitions at High Temperature on Perovskite Oxides: The Case of Strained La0.8Sr0.2CoO3 Thin Films

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La(0.8)Sr(0.2)CoO(3) (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10(-3) mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8-1.5 eV at room temperature to a metallic-like state with no energy gap at 200-300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces.

Degradation Mechanism in La0.8Sr0.2CoO3 as Contact Layer on the Solid Oxide Electrolysis Cell Anode

Detailed chemical and structural analyses are presented for the degradation mechanism of La 0.8 Sr 0.2 CoO 3 (LSC) as the contact layer of solid oxide electrolysis cell (SOEC) anodes. SOEC stack cells, which were operated in the presence of Cr-containing interconnects, and reference half-cells, which were tested with Pt interconnects, were investigated. The as-prepared surface chemistry of LSC showed a spatially uniform A-site (La and Sr) enrichment. Undesirable secondary phases of Cr 2 O 3 , LaCrO 3 , La 2 CrO 6 , and Co 3 O 4 were identified in the contact layer of the SOEC stack cells, which had significantly reduced electrochemical performance after long-term testing. Auger electron spectroscopy and analytical transmission electron microscopy showed the presence of Cr throughout the layer cross section on the surface and in the bulk, respectively, with significant variations in the local chemistry at the micro- to nanoscale. Particularly, a long-range transport of Sr and Co cations out of the LSC phase to the top of the contact layer was evident. However, when tested with electrolytic potential and current without a Cr environment, the LSC contact layer composition remained stable. The dissociation of the LSC in the SOEC stack cells can be, most probably, driven by the La-Cr-O related thermodynamics under the electrolytic potential and oxygen pressure at the anode.

Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi1−xFexO3 surfaces

The correlation between the surface chemistry and electronic structure is studied for SrTi1−xFexO3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi0.95Fe0.05O3 (STF5), SrTi0.65Fe0.35O3 (STF35) and SrFeO3 (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved X-ray photoelectron spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy/spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in an oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 °C and 10−3 mbar of oxygen, the surface band gap increased with Fe content, 2.5 ± 0.5 eV for STF35 and 3.6 ± 0.6 eV for STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on the Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrOx on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms – inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface and the smaller concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice.

Electron tunneling characteristics on La0.7Sr0.3MnO3 thin-film surfaces at high temperature

We report on the electron tunneling characteristics on La0.7Sr0.3MnO3 (LSM) thin-film surfaces up to 580 °C in 10−3 mbar oxygen pressure, using scanning tunneling microscopy/spectroscopy (STM/STS). A thresholdlike drop in the tunneling current was observed at positive bias in STS, which is interpreted as a unique indicator for the activation polarization in cation-oxygen bonding on LSM cathodes. Sr-enrichment was found on the surface at high temperature using Auger electron spectroscopy, and was accompanied by a decrease in tunneling conductance in STS. This suggests that Sr-terminated surfaces are less active for electron transfer in oxygen reduction compared to Mn-terminated surfaces on LSM.