Suppression of Cation Segregation in (La,Sr)CoO3-δ by Elastic Energy Minimization

Controlling surface cation segregation in a double perovskite for oxygen anion transport in high temperature energy conversion devices

Double perovskite materials have shown promising applications as an electrode in solid oxide fuel cells and Li-air batteries for oxygen reduction, evolution, and transport. However, degradation of the material due to cation migration to the surface, forming secondary phases, poses an existential bottleneck in materials development. Herein, a theoretical approach combining density functional theory and molecular dynamics simulations is presented to study the Ba-cation segregation in a double perovskite NdBaCo2O5+δ. Solutions to circumvent segregation at the molecular level are presented in two different forms by applying strain and introducing dopants in the structure. On applying compressive strain or Ca as a dopant in the NBCO structure, segregation is estimated to reduce significantly. A more direct way of estimating cation segregation is proposed in MD simulations, wherein the counting of the cations migrating from the sub-surface layers to the surface provided a reliable theoretical assessment of the level of cation segregation.

Improving the durability of cobaltite cathode of solid oxide fuel cells - a review

Solid oxide fuel cells (SOFCs) are highly efficient, low-emission, and fuel-flexible energy conversion devices. However, their commercialization has lagged due to the lack of long-term durability. Among several performance degradation mechanisms, cathode degradation and elemental inter-diffusion of the electrolyte and cathode has been identified as the predominant factors. In the most common SOFC systems, a cobalt-based perovskite material is used, for example LSC or LSCF. These cobalt-based materials offer mixed conductivity and higher concentration of oxygen vacancies as compared to LSM at lower operating temperature leading to favorable reduction kinetics. However, the presence of cobalt results in higher cost, higher thermal expansion co-efficient (TEC) mismatch and most importantly leads to rapid degradation. Various elements like strontium, cobalt, cerium, chromium, or zirconium accumulate or deposit at the electrode-electrolyte interface, which results in sluggish reaction kinetics of the oxygen reduction reaction (ORR). These elements react to form secondary phases that have lower ionic and electronic conductivity, cover active reaction sites, and eventually lead to cell and system deterioration. Over the past decade, several studies have focused on preventative and protective measures to prolong SOFC lifetime which includes novel fabrication techniques, introduction of new layers, addition of thin films to block the cation transport. Such efforts to prevent the formation of insulating phases and decomposition of the cathode have resulted in a remarkable improvement in long-term stability. In this review paper, current research on leading mechanisms responsible for the degradation of cobaltite cathode of solid oxide fuel cell has been summarized and durability improvement strategies of cobalt-based SOFC cathodes have been discussed.

In situ electrochemical observation of anisotropic lattice contraction of La0.6Sr0.4FeO3-δ electrodes during pulsed laser deposition

La_0.6Sr_0.4FeO_3-δ (LSF) electrodes were grown on different electrolyte substrates by pulsed laser deposition (PLD) and their oxygen exchange reaction (OER) resistance was tracked in real-time by in situ PLD impedance spectroscopy (i-PLD) inside the PLD chamber. This enables measurements on pristine surfaces free from any contaminations and the direct observation of thickness dependent properties. As substrates, yttria-stabilized zirconia single crystals (YSZ) were used for polycrystalline LSF growth and La_0.95Sr_0.05Ga_0.95Mg_0.05O_3-δ (LSGM) single crystals or YSZ single crystals with a 5 nm buffer-layer of Gd_0.2Ce_0.8O_2-δ for epitaxial LSF film growth. While polycrystalline LSF electrodes show a constant OER resistance in a broad thickness range, epitaxially grown LSF electrodes exhibit a continuous and strong increase of the OER resistance with film thickness until ≈60 nm. In addition, the activation energy of the OER resistance increases by 0.23 eV compared to polycrystalline LSF. High resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) measurements reveal an increasing contraction of the out-of-plane lattice parameter in the epitaxial LSF electrodes over electrode thickness. Defect thermodynamic simulations suggest that the decrease of the LSF unit cell volume is accompanied by a lowering of the oxygen vacancy concentration, explaining both the resistive increase and the increased activation energy.

Sol-Gel Combustion-Assisted Electrostatic Spray Deposition for Durable Solid Oxide Fuel Cell Cathodes

The chemical instability of perovskite oxides containing Sr is a critical issue for the long-term operation of solid oxide fuel cells. In this study, we demonstrate a remarkable improvement in the chemical and electrochemical stability of a heterostructured La_0.6Sr_0.4CoO_3-δ (LSC)-Ce_0.9Gd_0.1O_1.95 (GDC) electrode. Electrostatic spray deposition was employed to fabricate heterostructured nanoparticles in a single step with a coaxial nozzle supplying the LSC powders in the core nozzle and the GDC precursors in the shell nozzle. Moreover, the reducing fuel added to the GDC precursor solution induced the sol-gel combustion reaction in the droplet to form a uniform nanocrystalline GDC coating with high surface coverage. The high surface coverage of GDC on the LSC more significantly improved long-term stability compared with than of the bare LSC cathode at a constant current density of 1 A/cm² at 600°C for 100 h.

Infiltrated thin film structure with hydrogel-mediated precursor ink for durable SOFCs

The hydrogel of biomolecule-assisted metal/organic complex has the superior ability to form a uniform, continuous, and densely integrated structure, which is necessary for fine thin film fabrication. As a representative of nature-originated polymers with abundant reactive side chains, we select the gelatin molecule as an element for weaving the metal cations. Here, we demonstrate the interaction between the metal cation and gelatin molecules, and associate it with coating quality. We investigate the rheological property of gelatin solutions interacting with metal cation from the view of cross-linking and denaturing of gelatin molecules. Also, we quantitatively compare the corresponding interactions by monitoring the absorbance spectrum of the cation. The coated porous structure is systematically investigated from the infiltration of gelatin-mediated Gd_0.2Ce_0.8O_2-δ (GDC) precursor into Sm_0.5Sr_0.5CoO_3-δ (SSC) porous scaffold. By applying the actively interacting gelatin-GDC system, we achieve a thin film of GDC on SSC with excellent uniformity. Compare to the discrete coating from the typical infiltration process, the optimized thin film coated structure shows enhanced performance and stability.

Engineering of Charged Defects at Perovskite Oxide Surfaces for Exceptionally Stable Solid Oxide Fuel Cell Electrodes

Cation segregation, particularly Sr segregation, toward a perovskite surface has a significant effect on the performance degradation of a solid oxide cell (solid oxide electrolysis/fuel cell). Among the number of key reasons generating the instability of perovskite oxide, surface-accumulated positively charged defects (oxygen vacancy, V_o^··) have been considered as the most crucial drivers in strongly attracting negatively charged defects (Sr_A - site^') toward the surface. Herein, we demonstrate the effects of a heterointerface on the redistribution of both positively and negatively charged defects for a reduction of V_o^·· at a perovskite surface. We took Sm_0.5Sr_0.5CoO_3-δ (SSC) as a model perovskite film and coated Gd_0.1Ce_0.9O_2-δ (GDC) additionally onto the SSC film to create a heterointerface (GDC/SSC), resulting in an ∼11-fold reduction in a degradation rate of ∼8% at 650 °C and ∼10-fold higher surface exchange (k^q) than a bare SSC film after 150 h at 650 °C. Using X-ray photoelectron spectroscopy and electron energy loss spectroscopy, we revealed a decrease in positively charged defects of V_o^·· and transferred electrons in an SSC film at the GDC/SSC heterointerface, resulting in a suppression of negatively charged Sr (Sr_Sm^') segregation. Finally, the energetic behavior, including the charge transfer phenomenon, O p-band center, and oxygen vacancy formation energy calculated using the density functional theory, verified the effects of the heterointerface on the redistribution of the charged defects, resulting in a remarkable impact on the stability of perovskite oxide at elevated temperatures.

Understanding the Role of Minor Molybdenum Doping in LiNi0.5Co0.2Mn0.3O2 Electrodes: from Structural and Surface Analyses and Theoretical Modeling to Practical Electrochemical Cells

Doping LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode material by small amount of Mo⁶⁺ ions, around 1 mol %, affects pronouncedly its structure, surface properties, and electronic and electrochemical behavior. Cathodes comprising Mo⁶⁺-doped NCM523 exhibited in Li cells higher specific capacities, higher rate capabilities, lower capacity fading, and lower charge-transfer resistance that relates to a more stable electrode/solution interface due to doping. This, in turn, is ascribed to the fact that the Mo⁶⁺ ions tend to concentrate more at the surface, as a result of a synthesis that always includes a necessary calcination, high-temperature stage. This phenomenon of the Mo dopant segregation at the surface in NCM523 material was discovered in the present work for the first time. It appears that Mo doping reduces the reactivity of the Ni-rich NCM cathode materials toward the standard electrolyte solutions of Li-ion batteries. Using density functional theory (DFT) calculations, we showed that Mo⁶⁺ ions are preferably incorporated at Ni sites and that the doping increases the amount of Ni²⁺ ions at the expense of Ni³⁺ ions, due to charge compensation, in accord with X-ray absorption fine structure (XAFS) spectroscopy measurements. Furthermore, DFT calculations predicted Ni-O bond length distributions in good agreement with the XAFS results, supporting a model of partial substitution of Ni sites by molybdenum.

Molten Salt Flux Synthesis, Crystal Facet Design, Characterization, Electronic Structure, and Catalytic Properties of Perovskite Cobaltite

We present a simple and cost-effective molten salt synthetic route toward phase-pure perovskite cobaltite microcrystallines and successfully regulate different crystal facets for perovskite LaCoO₃ by the strong interaction between Cl^- anions and Sr²⁺ cations in molten salt system and polar plane. We then take LaCoO₃ (100 and 110), LaCoO₃ (111), and La_0.7Sr_0.3CoO₃ (111) as comparison models, and we characterize their crystal structure, morphology, composition, electronic state, and catalytic properties. X-ray photoelectron spectroscopy (XPS) shows that the prepared samples with high-energy (111) crystal facets contain more surface oxygen species and active Co ions than La enrichment perovskite LaCoO₃ (110 and 100) on the surface. Furthermore, combining with ambient-pressure XAS, valence band spectroscopy, and density functional calculations, we find that exposed high-energy (111) crystal facets and doping Sr ions can enhance the hybridization between Co cations and O anions and their O p-band center is closer to the Fermi level, compared with that of LaCoO₃ (100 and 110). As expected, the samples with high-energy (111) crystal facets show better CO oxidation activity than LaCoO₃ (100 and 110), and La_0.7Sr_0.3CoO₃ (111) exhibits the highest catalytic activity. Our findings provide a new avenue to prepare high-energy facet perovskite catalysts and we also clearly reveal the relationship between surface electronic structure and intrinsic CO oxidation activity of perovskite cobaltite.