Impedance spectroscopy analysis of La1 − xSritxMnO3-yttria-stabilized zirconia electrode kinetics

Enhancing the Electrochemical Performance in Symmetrical Solid Oxide Cells through Nanoengineered Redox-Stable Electrodes with Exsolved Nanoparticles

Symmetrical solid oxide cells (SSOCs) have recently gained significant attention for their potential in energy conversion due to their simplified cell configuration, cost-effectiveness, and excellent reversibility. However, previous research efforts have mainly focused on improving the electrode performance of perovskite-type electrodes through different doping strategies, neglecting microstructural optimization. This work presents novel approaches for the nanostructural tailoring of (La0.8Sr0.2)0.95Fe1-xTixO3-δ (LSFTx, x = 0.2 and 0.4) electrodes using a single-step spray-pyrolysis deposition process. By incorporating these electrodes into a Ce0.9Gd0.1O1.95 (CGO) porous backbone or employing a nanocomposite architecture with nanoscale particle size, we achieved significant improvements in the polarization resistance (Rp) compared with traditional screen-printed electrodes. To further boost the fuel oxidation performance, a Ni-doping strategy, coupled with meticulous microstructural optimization, was implemented. The exsolution of Ni nanoparticles under reducing conditions resulted in remarkable Rp values as low as 0.34 and 0.11 Ω cm2 in air and wet H2 at 700 °C, respectively. Moreover, an electrolyte-supported cell with symmetrical electrodes demonstrated a stable maximum power density of 617 mW cm-2 at 800 °C. These findings highlight the importance of combining electrode composition optimization with advanced morphology control in the design of highly efficient and durable SSOCs.

Effect of calcium doping on the electrocatalytic activity of the Bi1-x Ca x FeO3-δ oxygen electrode for solid oxide fuel cells

For solid oxide fuel cell (SOFC) applications, there remains a growing interest in developing efficient cathode catalysts. Herein, iron-based Ca-doped Bi_1-x Ca _x FeO_3-δ (BCFx, x = 0.1, 0.2, and 0.3) oxides are evaluated as potential cathode materials for SOFCs. The phase structure, thermal expansion behavior, electrical conductivity, and electrocatalytic properties for the oxygen reduction reaction (ORR) of the BCFx cathodes are systematically characterized. Among all compositions, the Bi_0.8Ca_0.2FeO_3-δ (BCF0.2) cathode exhibits the highest oxygen vacancy concentration and considerable electrocatalytic activity, demonstrating the lowest polarization resistance (0.11 Ω cm²) and largest exchange current density of 41.91 mA cm^-2 at 700 °C. The BCF0.2 cathode-based single cell delivers excellent output performance, yielding a maximum power density of 760 mW cm^-2 700 °C along with exceptional stability over a period of 60 h. This work highlights the Ca-doping strategy for enhancing electrocatalytic activity of the cathode electrocatalysts in SOFCs.

LaCrO3-CeO2-Based Nanocomposite Electrodes for Efficient Symmetrical Solid Oxide Fuel Cells

La_0.98Cr_0.75Mn_0.25O_3-δ-Ce_0.9Gd_0.1O_1.95 (LCM-CGO) nanocomposite layers with different LCM contents, between 40 and 60 wt %, are prepared in a single step by a spray-pyrolysis deposition method and evaluated as both air and fuel electrodes for solid oxide fuel cells (SOFCs). The formation of fluorite (CGO) and perovskite (LCM) phases in the nanocomposite electrode is confirmed by different structural and microstructural techniques. The intimate mixture of LCM and CGO phases inhibits the grain growth, retaining the nanoscale microstructure even after annealing at 1000 °C with a grain size lower than 50 nm for LCM-CGO compared to 200 nm for pure LCM. The synergetic effect of nanosized LCM and CGO by combining their high electronic and ionic conductivity, respectively, leads to efficient and durable symmetrical electrodes. The best electrochemical properties are found for 50 wt % LCM-CGO, showing polarization resistance values of 0.29 and 0.09 Ω cm² at 750 °C in air and H₂, respectively, compared to 2.05 and 1.9 Ω cm² for a screen-printed electrode with the same composition. This outstanding performance is mainly ascribed to the nanoscale electrode microstructure formed directly on the electrolyte at a relatively low temperature. These results reveal that the combination of different immiscible phases with different crystal structures and electrochemical properties could be a promising strategy to design highly efficient and durable air and fuel electrodes for SOFCs.

Chemical Gardens as Electrochemical Systems: In Situ Characterization of Simulated Prebiotic Hydrothermal Vents by Impedance Spectroscopy

In an early earth or planetary chimney systems, hydrothermal fluid chemistry and flow durations play a large role in the chimney's ability to drive electrochemical reactions for the origin of life. We performed continuous electrochemical impedance spectroscopy (EIS) characterization on inorganic membranes representing prebiotic hydrothermal chimney vents in natural seafloor systems, by incorporating an electrode array into a chimney growth experiment. Localized potential and capacitances profiles in the chimney reveal a dynamic system where redox processes are driven by transport phenomena, increasing rapidly due to disequilibrium until achieving equilibrium at about 100 mV and 1000 μF/cm² . The impedance in the chimney interior is three orders of magnitude lower (100 Ohms/cm² vs 100 KOhms/cm² ) than at the ocean or the ocean/chimney interface. The calculated peak dissipation factor (DF) values are more than ten times higher (40.0 vs 3.0) and also confirm the elevated chemical reactivity in the chimney interior.

Highly Stable and Efficient Perovskite Ferrite Electrode for Symmetrical Solid Oxide Fuel Cells

Here, we report a new perovskite oxide with formula Sm_0.8Sr_0.2Fe_0.8Ti_0.15Ru_0.05O_3-δ (SSFTR), which exhibits a great potential as a symmetrical electrode material with satisfying stability in both reducing and oxidizing environments. Moreover, SSFTR exhibits good redox and thermal cycle stability. The electrolyte-supported (Sm_0.2Ce_0.8O_1.9, SDC) symmetrical cell with SSFTR electrodes possesses a peak power density of 271 mW·cm^-2 at 800 °C in wet H₂. Moreover, the peak power density is remarkably improved to 417 mW·cm^-2 when applying A-site-deficient perovskite oxide Sm_0.7Sr_0.2Fe_0.8Ti_0.15Ru_0.05O_3-δ as the symmetrical electrode, benifiting by the in situ-exsolved Ru nanoparticles with excellent electrocatalytic activity, since A-site deficiency can provide additional driving force for the exsolution of B-site cations upon reduction. As an ingenious approach, this exsolution of electrocatalytically active nanoparticles on the surface of electrode may be applicable to the development of other excellent performance electrodes for symmetrical SOFCs and other electrochemical systems.

Biomolecular interfaces based on self-assembly and self-recognition form biosensors capable of recording molecular binding and release

This research proposed to create the next generation of versatile electrochemical-based biosensors capable of monitoring target capture and release as dictated by molecular binding or unbinding. The biosensor integrates cellular machines (i.e., microtubules, structural elements of cells and kinesin molecular motors involved in cellular transport) as functional units; its assembly is based on molecular self-assembly and self-recognition. Our results demonstrate that the designed biosensor was capable of allowing detection of binding and unbinding events based on redox reactions at user-controlled electrode interfaces. The analysis also showed that the sensitivity of the designed biosensor or its ability to record such events could be user-controlled at any given time by adjusting the energy source that "fuels" the system.

Improving the Electrocatalytic Activity and Durability of the La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode by Surface Modification

Electrode materials with high activity and good stability are essential for commercialization of energy conversion systems such as solid oxide fuel cells or electrolysis cells at the intermediate temperature. Modifying the existing perovskite-based electrode surface to form a heterostructure has been widely applied for the rational design of novel electrodes with high performance. Despite many successful developments in enhancing electrode performance by surface modification, some controversial results are also reported in the literature and the mechanisms are still not well understood. In this work, the mechanism of how surface modification impacts the oxygen reduction reaction (ORR) activity and stability of perovskite-based oxides was investigated. We took La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) as the thin-film model system and modified its surface with additive Pr _xCe_{1- x}O₂ layers of different thicknesses. We found a strong correlation between surface oxygen defects and the ORR activity of the heterostructure. By inducing higher oxygen vacancy concentration compared to bare LSCF, PrO₂ coating is proved to greatly facilitate the rate of oxygen dissociation, thus significantly enhancing the ORR activity. Because of low oxygen vacancy density introduced by Pr_0.2Ce_0.8O₂ and CeO₂ coating, on the one hand, it does not boost the rate of ORR but successfully suppresses surface Sr segregation, leading to an enhanced durability. Our findings demonstrate the vital role of surface oxygen defects and provide important insights for the rational design of high-performance electrode materials through surface defect engineering.

On the bulk transport process and its impact on the electrode behavior of mixed conducting electrodes for SOFCs

The contribution of the electrical field and concentration gradient on the bulk transport of oxygen ions in the MIEC electrodes of SOFC is deconvoluted. A case study of LSCF is carried out.

Apparent Oxygen Uphill Diffusion in La0.8Sr0.2MnO3 Thin Films upon Cathodic Polarization

The impact of cathodic bias on oxygen transport in La_0.8Sr_0.2MnO₃ (LSM) thin films was investigated. Columnar‐grown LSM thin films with different microstructures were deposited by pulsed laser deposition. ¹⁸O tracer experiments were performed on thin film microelectrodes with an applied cathodic bias of −300 or −450 mV, and the microelectrodes were subsequently analyzed by time‐of‐flight secondary ion mass spectrometry. The ¹⁸O concentration in the cathodically polarized LSM microelectrodes was strongly increased relative to that in the thermally annealed film (without bias). Most remarkable, however, was the appearance of a pronounced ¹⁸O fraction maximum in the center of the films. This strongly depended on the applied bias and on the microstructure of the LSM thin layers. The unusual shape of the ¹⁸O depth profiles was caused by a combination of Wagner–Hebb‐type stoichiometry polarization of the LSM bulk, fast grain boundary transport and voltage‐induced modification of the oxygen incorporation kinetics,

Oxygen sorption and desorption properties of selected lanthanum manganites and lanthanum ferrite manganites

Temperature-programmed desorption (TPD) with a carrier gas was used to study the oxygen sorption and desorption properties of oxidation catalysts and solid-oxide fuel cell (SOFC) cathode materials (La(0.85) Sr(0.15)0.95 MnO(3+δ) (LSM) and La(0.60) Sr(0.40) Fe(0.80) Mn(0.20) O(3-δ) (LSFM). The powders were characterized by X-ray diffractometry, atomic force microscopy (AFM), and BET surface adsorption. Sorbed oxygen could be distinguished from oxygen originating from stoichiometry changes. The results indicated that there is one main site for oxygen sorption/desorption. The amount of sorbed oxygen was monitored over time at different temperatures. Furthermore, through data analysis it was shown that the desorption peak associated with oxygen sorption is described well by second-order desorption kinetics. This indicates that oxygen molecules dissociate upon adsorption and that the rate-determining step for the desorption reaction is a recombination of monatomic oxygen. Typical problems with re-adsorption in this kind of TPD setup were revealed to be insignificant by using simulations. Finally, different key parameters of sorption and desorption were determined, such as desorption activation energies, density of sorption sites, and adsorption and desorption reaction order.

Novel microstructural strategies to enhance the electrochemical performance of La0.8Sr0.2MnO3-δ cathodes

Novel strategies based on spray-pyrolysis deposition are proposed to increase the triple-phase boundary (TPB) of La0.8Sr0.2MnO3-δ (LSM) cathodes in contact with yttria-stabilized zirconia (YSZ) electrolyte: (i) nanocrystalline LSM films deposited on as-prepared YSZ surface; (ii) the addition of poly(methyl methacrylate) microspheres as pore formers to further increase the porosity of the film cathodes; and (iii) the deposition of LSM by spray pyrolysis on backbones of Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), and Bi1.5Y0.5O3-δ (BYO) previously fixed onto the YSZ. This last method is an alternative to the classical infiltration process with several advantages for large-scale manufacturing of planar solid oxide fuel cells (SOFCs), including easier industrial implementation, shorter preparation time, and low cost. The morphology and electrochemical performance of the electrodes are investigated by scanning electron microscopy and impedance spectroscopy. Very low values of area specific resistance are obtained, ranging from 1.4 Ω·cm(2) for LSM films deposited on as-prepared YSZ surface to 0.06 Ω-cm(2) for LSM deposited onto BYO backbone at a measured temperature of 650 °C. These electrodes exhibit high performance even after annealing at 950 °C, making them potentially suitable for applications in SOFCs at intermediate temperatures.

Ti-doped SrFeO3 nanostructured electrodes for symmetric solid oxide fuel cells

Ti-doped SrFeO_3−δ electrodes, prepared by a novel method based on spray-pyrolysis deposition, exhibit high efficiency as both cathode and anode in SOFCs.

Understanding the processes governing performance and durability of solid oxide electrolysis cells

Operation of a Ni–YSZ electrode supported Solid Oxide Cell (SOC) was studied in both fuel cell mode (FC-mode) and electrolysis cell mode (EC-mode) in mixtures of H₂O/H₂, CO₂/CO, H₂O/H₂O/CO₂/CO at 750 °C, 800 °C and 850 °C. Although the SOCs are reversible, the polarisation characterisation shows that the kinetics for the reduction of H₂O and CO₂ is slower compared to oxidation of H₂ and CO, and that oxidation/reduction in CO₂/CO mixtures is slower than in H₂O/H₂ mixtures. The kinetic differences are partly related to the polarisation heating and the entropy change. Also the diffusion resistance is larger in EC-mode as compared to FC-mode and the low frequency concentration resistance (which is affected by diffusion), is asymmetric around the open circuit voltage (OCV), and is significantly higher in the EC-mode. Both the increased diffusion resistance and the asymmetric low frequency concentration resistance result in a decreased activity in the EC-mode. Changing the porosity of the support structure shows a significant change in both the diffusion resistance and low frequency concentration resistance when applying current, showing that diffusion limitations cannot be neglected for SOCs operated in the EC-mode. Also the Ni–YSZ TPB resistance is affected by changing the support porosity, indicating that kinetic investigations under current and even at OCV, and the chase for a general expression for “all” Ni–YSZ electrodes may be pointless. The diffusion limitations through the support and active electrode structure create an increased reducing atmosphere at the interface which may be related to the degradation of the cells.