High Frequency Arc Research Articles

Characterization of A-site doped PrBaCo2O5+δ perovskites as cathode materials for IT-SOFCs

Creating highly efficient and durable cathodes persists as a formidable task in the realm of intermediate temperature solid oxide fuel cells (IT-SOFCs). Hence, we present a double perovskite oxide, Pr0.6Sr0.4BaCo2O5+δ (PS0.4BC), and its electrocatalytic activity is thoroughly investigated. The XRD test is carried out and a notable transformation in the phase structure is observed upon the introduction of Sr2+ ions into the PBC matrix. That is, the phase structure of the material changes from double perovskite structure to single perovskite structure. Through the TG and TEC test that the introduction of Sr2+ ions into the lattice can create additional sites for oxygen vacancy formation and reduce TEC values. Further verify that Sr doping can enhance the concentration of oxygen vacancies of the material. The PS0.4BC cathode material exhibits remarkably low polarization resistance, achieving an impressive value of 0.027 Ω cm2 at an operating temperature of 800 °C. Oxygen partial pressure test shows that as the oxygen partial pressure decreases, there is a notable increase in impedance, particularly evident in the low frequency region of the Nyquist plots. This indicates that the oxygen content has obvious effects on the low-frequency processes. When the oxygen partial pressure exceeds 0.05 atm, the oxygen adsorption-dissociation and diffusion process is the rate-limiting step in the ORR. However, when the oxygen partial pressure is less than 0.05 atm, the formation of lattice oxygen through the combination of oxygen ions and oxygen vacancies processes is the rate-limiting step. Compared PBC cathode material, the more pronounced changes in the high frequency arc of the PS0.4BC cathode at the same oxygen content. It shows that Sr doping significantly changes the high frequency process. After doping, the oxygen vacancy concentration increases, which facilitates the occurrence of the charge transfer process at high frequency.

Electrical Characterization and Modeling of High Frequency Arcs for Higher Voltage Aerospace Systems

Arcing in future high-voltage aerospace systems could occur more frequently and cause irreversible damage to electrical components, system structure and increase the risk of fire. While arcs seen in low-voltage aerospace systems tend to be long-duration and low-energy events, higher power but short-duration arcs may occur in high-voltage aerospace systems if they are readily detectable by system protection. This paper investigates the characteristics of high current arc faults generated at the AC frequencies expected in future rotating machines used for higher voltage aerospace systems. As such, arcs with a peak current up to 4.6 kA are generated at frequencies in the range of 0.5-2 kHz using an underdamped RLC circuit, under pressures of 0.2-1 bar absolute. High frequency arcs exhibit a similar characteristic to lower frequency arcs. A reduction in pressure results in lower arc voltage and arc power. Arcing tests at atmospheric pressure may therefore represent a worst-case scenario and the development of a low-pressure test environment may not be necessary. A black box model is developed to provide good agreement with experimental arc voltage waveforms for different parameters investigated in this study. This is a generalized modeling approach to estimate high-frequency high-voltage arcing characteristics without recourse to experiment.

Investigation of optical, electrical and dielectric properties of pyrophosphates LixCu2-x/2 P2O7 (x = 0.0; 0.05; 0.1)

The objective of this work is to study the electrical/dielectric and optical properties of new pyrophosphates LixCu2-x/2P2O7 (x ​= ​0, 0.05 and 0.1) prepared by the ceramic method. The desired phases are identified by X-ray diffraction (XRD) and by IR spectroscopy. All compounds crystallize in the monoclinic system with space group C2/c. Infrared spectroscopy shows all the expected bands, in particular those of the P2O74− groups. The study of electrical and dielectric properties is carried out at room temperature by impedance spectroscopy. The Nyquist diagram shows a high frequency arc corresponding to grain effects and a medium frequency arc relating to grain boundary effects, followed by a straight line at a low frequency attributed to the effects of electrode polarization. The modeling of experimental data is by using an equivalent electrical circuit, allows us to determine the electrical parameters of these pyrophosphates. The evolution of the conductivity (σac) is adjusted by Jonscher's double power law. The insertion of lithium improves the conductivity of LixCu2-x/2P2O7. Indeed, the σdc passes from 1.14.10-6 ​S/m for 0% Li to 1.07.10-5 ​S/m for 10% Li. Thus, the insertion of lithium increases the permittivity at low frequencies. Conductivity values are characteristic of semiconductors. These compounds are also characterized by UV spectroscopy, which confirmed that the three compounds are semiconductors with a decrease in the gap energy, which shifted from 3.91 to 3.86 ​eV the lithium rate increasing from 0 to 10%.

Sress-corrosion cracking under cathodic protection of low alloy steel joints with high frequency weld and arc weld

According to the results of complex electrochemical, corrosion-mechanical and fractographic studies, the existence of three potential regions, in which the stress-corrosion cracking (SCC) of 17G1S (17G1S-U) steel in the NS4 model soil electrolyte occurs by different mechanisms was established and experimentally confirmed: at potentials positively than -0.8 V – by the mechanism of local anodic dissolution, at potentials region from -0.8 V to -0.98 V – by the mixed mechanism, at potentials less than -0.98 V by hydrogen breaking mechanism. The susceptibility to SCC of high-frequency weld joints, estimated by the coefficient of KS, in the potential range from the corrosion potential to -1.2 V increases (KS increases from 1.1 to 1.8), which is less intense than for steel 17G1S/17G1S-U (KS increases from 1.1 to 2.8), for arc weld joints – does not change much enough (KS increases from 1.1 to 1.3). The validity of KS coefficient introduced for the base metal for comparative assessment of the susceptibility to SCC of welded joints, is provided in case that there are no defects in the welds and SCC occurs on base metal.

Learning approach based DC arc fault location classification in DC microgrids

A very challenging problem in direct current (DC) microgrid (μG) systems are the arc faults due to small air gaps caused by loose connections or degraded insulation. Generally, the voltage difference across these gaps initiates a plasma arcing channel with extremely high temperature. If not detected and extinguished in time, arc faults could endanger adjacent circuits and eventually cause fire hazards. Hence, to minimize the impact of DC arc faults through timely detection, this work proposes a novel detection scheme to improve the detection accuracy and to reduce unwanted false tripping. This is achieved by investigating the physical characteristics of the DC arc and by analyzing the arc current signals. The voltage-current characteristics of the DC arc as well as the high frequency arc noise are modeled and analyzed. This model enables an accurate simulation of the DC arc and its impact to a larger system is analyzed, i.e., a DC μG with multiple voltage sources and multiple resistive loads, and interactions between DC arc faults and two typical μG control strategies are carried out. A new detection scheme based on arc signatures from both time domain and time-frequency domain are proposed and verified through multi-layer perceptron neural networks (MLPNN) in numerical simulations. This strategy increases the detection accuracy and reduces the possibility of false trip. The training process of the proposed approach showed 97% accuracy and the testing process indicated 93.3% accuracy for a large sample of data.

Multi-objective mathematical modeling and optimization of delta straight InterPulse-GTCAW parameters to predict and maximize performance of high temperature aerospace alloy sheets for structural applications

Superalloy 718 is a high temperature nickel (Ni) based alloy, widely used in aeroengines due to its superior mechanical performance at high temperature. It is vulnerable to alloying segregation and Laves phase precipitation in fusion zone (FZ) which deteriorates the tensile properties of Superalloy 718 joints. Hence a newly emerged InterPulse GTCAW process featured by high frequency arc constriction was attempted to fabricate the joints of Superalloy 718. The main objective of this research work is mathematical modeling, prediction and optimization of delta straight GTCAW (DS-GTCAW) parameters for joining Superalloy 718 using response surface methodology (RSM). The process parameters were optimized for maximizing the tensile properties of joints and attain desired FZ microstructure. The tensile properties of Superalloy 718 joints were predicted from the mathematical relationships of DS-GTCAW parameters. The optical and scanning electron microscopy was used to characterize the microstructural features and tensile fracture surface morphology of high strength and low strength joints of Superalloy 718. The energy dispersive spectroscopy was used to characterize the alloying segregation of FZ. The mathematical equations were also formulated to predict the tensile properties of Superalloy 718 joints from secondary dendritic arm spacing (SDAS) of FZ microstructure. The joint developed using optimized DS-GTCAW parameters disclosed 99.20% joint efficiency which is enhanced by 30% than GTAW and comparable to electron beam welded (EBW) and laser beam welded (LBW) joints of Superalloy 718. It refers to the refining of dendritic FZ microstructure and Laves phase precipitation in finer form.

Maximizing strength and corrosion resistance of InterPulsed TIG welded Superalloy 718 joints by RSM for aerospace applications

Inconel 718 (IN-718) is a nickel-based alloy, mostly used in the aeroengine applications due to its good mechanical strength and resistance to corrosion. However, it is amenable to alloying segregation and Laves phase growth in fusion zone (FZ) which severely deteriorates the strength and corrosion resistance of IN-718 joints. To overcome this problem, InterPulsed TIG (IP-TIG) welding process, characterized by high frequency current pulsation and arc constriction, was attempted to join thin sheets of IN-718 alloy. The 3D graphs of response surfaces were built to establish the optimized welding parameters for maximizing the joint strength. The parametric mathematical equation was constructed to predict the strength of IN-718 joints. The mathematical equation was also built to predict the tensile strength from Laves phase. The potentiodynamic corrosion test was done in a 3.56 wt.% NaCl solution for the selected joints to determine the corrosion behavior of IN-718 joints. The IP-TIG welded IN-718 joints exhibited 99.20% of base metal strength and superior corrosion resistance at optimized condition of welding parameters. This refers to the refining of FZ dendritic structure which aids in minimizing the Laves phase. The IN-718 joints developed using optimized IP-TIG parameters exhibited 22–32% higher strength and superior corrosion resistance than TIG welded IN-718 joints and comparable to EBW and LBW IN-718 joints.

Corrosion assessment of metals in bioethanol-gasoline blends using electrochemical impedance spectroscopy

Due to the increased use of environmentally friendly fuels, it becomes imperative to evaluate the impact of biofuels on the performance of materials used in auto parts. The corrosive effects of biofuels are important in terms of durability of auto-parts since there is an evidence of the increasing deterioration in automobile parts with the long-term use of biofuels. In this research, the behavior of metals, used in the manufacturing of auto parts, in pure bioethanol (E100) and bioethanol-gasoline blends with 30% (E30), 50% (E50), and 85% (E85) of ethanol content were evaluated. Electrochemical impedance Spectroscopy (EIS) curves were done under static conditions at 45 °C. The metallic materials evaluated were stainless steel, tin, carbon steel and copper. The EIS diagrams showed two capacitive arcs for all materials. The high frequency arc was related to the dielectric response of the fuel blends, while the second one shows the characteristics and activity of the metal/fuel interface. According to the transfer resistance (Rt) obtained from the second arc of the EIS measurements, copper and carbon steel exhibited corrosion susceptibility in all fuel blends, while stainless steel and tin showed good anticorrosion behavior by showing high Rt values. The highest charge transfer resistance was showed by tin, followed by stainless steel, carbon steel and copper. Except for carbon steel and copper, the other set samples were compatible with the evaluated blends.

Differentiation of SCC Susceptibility with EIS of Alloy 182 in High Temperature Water

Electrochemical Impedance Spectroscopy (EIS) measurements were carried out in high temperature water with Ni-based Alloy-182. The aim was to correlate the EIS results with differences in Stress Corrosion Cracking (SCC) susceptibility that is present around the Ni-NiO transition. There was a clear difference between the EIS results at and away from the Ni-NiO transition. To make a more quantitative correlation a simple equivalent circuit was used to fit the experimental data. A clear correlation between the CPE exponent (n) and the SCC susceptibility could be obtained. Additionally, it was shown that the high frequency arc of the EIS data was related to the diffuse double layer

Short-Term Strong Cathodic Polarization of Ni/YSZ and Pt/YSZ

The initial stages of YSZ reduction and formation of intermetallic phases at Ni∣YSZ interfaces on strongly cathodically polarized electrodes were studied by a number of potential sweep and impedance techniques. The measurements were carried out in the potential range −1.0 V to −3.0 V vs E°(O2) in H2/H2O at 650 and 750 °C. Below −1.7 V the cathodic current increases almost exponentially, mainly due to electronic conductivity in the YSZ. Reduction and oxidation peaks develop below and above −1.9 V, respectively. The peaks reveal a simultaneous reduction of YSZ at the Ni∣YSZ interface and reoxidation of a Ni-Zr phase. Charges calculated from the reoxidation peaks indicate a thickness of 200 nm for the layer formed by conditioning the electrode for 300 s at −2.6 V vs E°(O2). Impedance measurements show a suppressed arc decreasing with increasing polarization. Below −2.3 V vs E°(O2) a capacitive high frequency arc segment and an inductive low frequency loop develop. Both are ascribed to electronic conductivity. SEM/EDS microscopy on cross sections of samples cooled with and without polarization showed the formation of uniform and homogeneous intermetallic reaction layers which after reoxidation resulted in a two-phased nanostructures.

고주파 아크 금속용사 장치의 시공성능 평가에 관한 실험적 연구

아크 금속용사 공법을 활용한 EMP 차폐기술을 개발하기 위하여 국내에서 사용되는 3종류의 금속용사 장치의 시공성능을 평가하였다. MS, KMS, HMS에서 용사금속의 부착량은 각각 58.32%, 64.66%, 87.62%로 나타났다. 또한, 1시간당시공면적(m2/h)은 MS가 14.3, KMS가 19.25, HMS가 22.16으로서 나타났으며, HMS공법의 경우 Φ1.6mm의 선재 사용, 용사면적 80mm 형성, 반선속도 18m/min 사용과 함께 금속용사 시공시의 비산량이 대폭 작아졌기 때문으로 판단된다. 따라서, 시공성능 측면에서 HMS공법이 EMP차폐용 아크 금속용사공법으로 가장 적합하다고 판단된다.

The impact of spiro-OMeTAD photodoping on the reversible light-induced transients of perovskite solar cells

Hole transporting materials (HTMs) play essential roles in facilitating hole extraction and suppressing recombination in lead halide perovskite solar cells (PSCs). High levels of p-doping in HTMs is necessary for achieving high device performance, attributed to an increased electrical conductivity. In this work, we provide evidences that the poor performance of PSCs with low levels of doping (i.e., 4 mol% spiro-OMeTAD+) in spiro-OMeTAD is mainly caused by the presence of a Schottky barrier at the perovskite/spiro-OMeTAD interface, hampering hole injection. Under continuous illumination at open-circuit condition, the barrier gradually diminishes, increasing the PSC power conversion efficiency by 70-fold after 7 h. This process is completely reversible, returning to the initial poor performance after dark storage. We attribute this improvement in performance to a gradual photodoping of spiro-OMeTAD, triggered by the transfer of photogenerated holes and mediated by the slow migration of halide anions from perovskite to compensate the newly formed spiro-OMeTAD+. In-situ parallel analyses with impedance spectroscopy (IS) and photoluminescence are employed to gain insights into the charge dynamics along with light soaking. We find that the Schottky barrier resistance overlays with the recombination signal at the high frequency arc of IS, having important implications for the IS data analysis for PSCs. The work elucidates a major mechanism causing the slow efficiency variations during light/dark cycling, commonly observed in PSCs, which complicates the determination of long-term stability.

Filtration process combined with mechanical action, as a method for efficient extraction of endohedral metallofullerenes from carbon soot

The paper presents results of fullerenes and endohedral metallofullerenes extracts studies, isolated from the graphite rods carbon soot spray, and containing Y2O3 in a high frequency arc discharge. Two ways of extraction were applied and compared – (1) the classic method of Soxhlet extraction, and (2) the one developed by our team – extraction based on mechanical action combined with filtration. To implement the method, we used a laboratory version of installation, embodying technical solutions for rapid extraction. Chromatographic and mass spectrometry studies of fullerene extracts obtained by these methods revealed that by combining mechanical action with simultaneous filtration, we can significantly intensify and reduce the process of extracting fullerenes and endohedral metallofullerenes compared to the Soxhlet extraction method. This is especially evident in the release of endohedral metallofullerenes. Our method allows to reduce the release time of fullerenes from 10 g of carbon soot on laboratory installation up to 15 minutes, against the Soxhlet extraction method taking 18 hours. Whilst, the total number of fullerenes extracted by both methods almost coincides (the extraction method using mechanical action allowed us to extract 0.2-0.4 wt. % more), the composition of the isolated fullerene mixtures is different. The relative content of higher fullerenes and endohedral metallofullerenes exceeds when the mechanical action-based extraction method applied.

Physical modeling of the electrochemical impedance spectra for the O2 reduction reaction in HTPEM fuel cells’ cathodic electrochemical interface

In this study we propose a microkinetic transition state pathway for the oxygen reduction reaction, (ORR), for the high temperature polymer electrolyte (HTPEM) fuel cell cathodic electrode, focusing on the importance of the cathodic impedance spectrum features, in the kinetic low current density, (lcd), regime of operation when ORR activation power losses are dominant. The ORR impedance spectrum involves a linear high frequency part feature and two arcs depending on the double layer capacitance, (Cdl), value. The high frequency linear part of the spectrum stems from the finite ionic, (H+), resistance in the catalyst layer. Our simulations revealed two arcs: all charge transfer reaction steps appear under the same high frequency arc directly related to the Cdl of the electrochemical interface, (EI), while the low frequency, (LF), arc originates from the relaxation of the adsorbed surface reaction intermediates on the catalyst surface, which is caused by the depletion of OHad on the surface. The kinetic model provides two Tafel slopes in the low and high current density (hcd) regime equal to 60 (mV/dec) and 180 (mV/dec) respectively.

Simulation of HT-PEMFC AC Impedance Spectra: Relaxation Impedance and Identification of Oxygen Reduction Reaction Mechanism

High Temperature Polymer Electrolyte Membrane Fuel Cells, (HT-PEMFCs), are one of the most promising alternatives of clean power production by converting chemical energy to electrical energy. Even though the operating principles of an HT-PEMFC are understood, the overall system’s behavior is determined by a number of strongly coupled processes, each proceeding at a different rate.In a HT-PEMFC the elusive multistep Oxygen Reduction Reaction, (ORR), taking place at the cathodic electrode takes a major fraction of the open circuit potential. Thus, in order to optimize fuel cell performance by minimizing ORR power losses, an in depth understanding of the various processes and their interactions is necessary. One of the most powerful electrochemical characterization methods is the Electrochemical Impedance Spectroscopy technique, (EIS). It is a very powerful and sensitive in situ technique that has the ability to separate potential losses corresponding to different processes which proceed at different rates, (have different timescales), [1]. Our research revealed that ORR impedance spectrum consists of two arcs, a high frequency arc attributed to the charge transfer reactions across the interface and a low frequency arc due to the hysteresis phenomena between the oxygenated intermediate species coverage and the current dynamic response, (ORR relaxation impedance), [2]. Thus, by simulating the AC impedance spectra through the development of an appropriate physical model, valuable information on the electrochemical and chemical process at the electrode can be drawn. In order to demystify the multistep ORR kinetics a 1-D dynamic mathematical model has been developed in order to simulate AC impedance spectra at the low current linear activation region while at the same time a Monte Carlo type algorithm was used to estimate the values of the respective kinetic constants by simultaneously fitting polarization-dc bias and the corresponding galvanostatic AC Spectra. The model incorporates a three-step proposed mechanism for the ORR as well as a detailed macroscopic description of mass and charge transfer across the various parts of the Membrane Electrode Assembly, (MEA). The low current activation regime of operation was chosen because there the ORR overpotential dominates the AC spectra, the effect of the finite electrolyte conductivity and mass transport limitations are minimized and the adsorbed intermediate relaxation impedance is prominent while it diminishes at higher current densities as can be seen in Figure 1. As a result, the cathodic overpotential is constant throughout the cathodic electrode which may be well considered to operate under ideal kinetic conditions allowing thus an analytic expression for the HT-PEMFC impedance to be developed and used in the identification of the ORR kinetics. As far as validation of our approach is concerned, synthetic AC Impedance Spectra with known kinetic constants where produced and utilized by the Monte Carlo fitting algorithm at the low current operation regime. Figure 2 shows the histograms of the fitting algorithm for the OH intermediate reduction kinetic constant, by employing different number of bias points (I-V) and their respective AC spectra the algorithm successfully converges to the set value for k3+ , (k3+ =2*10-6 mol/(m2s)). By decoupling the various processes of the simulated AC impedance spectra, information was successfully extracted on the various kinetic and equilibrium constants related with the interaction of the reacting gases in the catalytic layer and the electrochemical interface. [1] O. Antoine, Y. Bultel, and R. Durand, Journal of Electroanalytical Chemistry, vol. 499, no. 1, pp. 85–94, Feb.2001. [2] X. Yuan, H. Wang, J. Colinsun and J. Zhang. 2007. International Journal of Hydrogen Energy, Volume:32, 4365-4380. This work has been conducted and supported within the framework of DeMStack, (DEMSTACK 325368, FCH-JU-2012-1). Figure 1

Cell performance, impedance, and various resistances measurements of an anode-supported button cell using a new pressurized solid oxide fuel cell rig at 1–5 atm and 750–850 °C

We report a novel pressurized solid oxide fuel cell (SOFC) rig that overcomes the stagnation flow problem in classic button cell setups, allowing fuel and air to distribute uniformly over anode and cathode surfaces. Power and impedance measurements of an anode-supported full button cell (ASC; NiO-YSZ/YSZ/LSM-GDC-LSM) operated at p = 1–5 atm and T = 750–850 °C show that both pressurization and increasing temperature enhance the cell performance, which are explained by the Nyquist plots consisted of a high frequency arc with characteristic frequencies (fc) around 100–1000 Hz relating to the cathode activation overvoltage and a low frequency arc with fc around 10–30 Hz corresponding to the anode concentration overvoltage. The activation overvoltage decreases with increasing p and/or T, while the concentration overvoltage decreases with increasing p but rather insensitive to T. We find that the activation overvoltage is the major source for the polarization resistances contributing 63%–82% depending on pressure, temperature, and current density, while the concentration overvoltage is a minor one contributing 18%–37% for the present ASC. These results and the rig should be useful for our understanding and further studies of pressurized SOFCs.

Study of the Rate Limiting Steps and Degradation of a GDC Impregnated La0.6Sr0.4Co0.8Fe0.2O3-δ Cathode

Reducing fuel cells degradation and cathode polarization losses are currently two of the most important challenges in order to achieve more efficient and cost competitive SOFC's. One strategy has been to reduce operation temperatures to the range between 600 ºC and 800 ºC (IT-SOFC), which reduces degradation rates but increases activation over potentials as a consequence (mainly the cathode polarization resistance Rp,c, due to the oxygen reduction reaction -ORR-). Several approaches have been taken to counteract this problem and, recently, promising results [1] have been obtained by modifying the electrode surface using the impregnation technique. This technique consists of synthesizing catalyst nanoparticles on the electrode surface through a liquid solution-based process [2, 3]. The mechanisms through which the impregnated catalysts reduce the polarization resistance and even enhance durability are not completely well understood yet, especially when non conventional catalysts are impregnated (i.e.: electrolyte nanoparticles into mixed ionic and electronic conductors –MIECs- electrodes), as the performance improvement relies upon the microstructure and coupled interactions between the materials. In this work, a cathode surface modification by nanoparticle impregnation was carried out. La0.6Sr0.4Co0.8Fe0.2O3-δ –LSCF- ceramic cathodes impregnated with Ce0.8Gd0.2O3-δ –GDC- nanoparticles were characterized with electronic scanning –SEM- and transmission –TEM- microscopy. The electrochemical response as a function of temperature, oxygen partial pressure -pO2-(10-4 < pO2< 1 atm) and time was evaluated by electrochemical impedance spectroscopy –EIS. Reproducible and homogeneous impregnation were obtained, which reduce the Rp,c around 30% in the temperature range between 400ºC and 800ºC and during 500 h at 700 ºC. Fig. 1 shows a SEM image of the LSCF sample with (Fig 1b) and without impregnation (Fig 1a), the time evolution of the Rp,c for both samples (Fig 1c) and the EIS spectra of both samples at pO2 = 10-2 atm and at 700 ºC (Fig 1d). Through the pO2 measurements, the effect of the impregnation on the rate-limiting steps involved was analyzed. The EIS spectra show at least two arcs, where the pO2 dependence of the low frequency arc (µ(pO2)-n, n ~ 0.7) suggests a complex mechanism where O2-gas diffusion is not the unique driving process. The complexity of this mechanism is also verified by comparing EIS spectra collected at several pO2 by using Ar and He as gas carriers, where the observed reduction of impedance as a consequence of lower diffusion of O2 in He, was lower than expected. On the other hand, the high frequency arc, with a Gerisher-type impedance, is almost independent of pO2. Fig. 1d shows how that the surface modification by GDC-impregnation mainly affects the low frequency contribution. Although a clear effect has been observed in the Rp,c values as a consequence of impregnation, no significant difference was detected in the time evolution behavior at 700 C in air. This means that GDC impregnation reduces the cathode polarization resistance but seems not to alter significantly the origin of the degradation processes.

Electrochemical Performance and Stability of (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ Cathode Powders Modified by Atomic Layer Deposition

For solid oxide fuel cell (SOFC) systems to become economically competitive, continuous development of new materials and processes is required not only to enhance SOFCs durability but also to reduce their cost. The cathode electrode is known to contribute the most to SOFC performance degradation and efficiency loss. In the case of strontium doped lanthanum cobalt iron oxide, it has been frequently suggested that strontium migration to the cathode surface under operating conditions plays an important role in the degradation. Recently, there have been extensive efforts to modify the surface of the cathode in order to improve the reliability. Most of these attempts have been focused on surface modification subsequent to the cell fabrication (e.g. infiltration). An attractive alternative is to modify the cathode powder surface prior to incorporating the powder into the SOFC. This approach would simplify the manufacturing flow, allowing a “drop-in replacement” of existing cathode powders, while potentially providing similar or even better performance enhancements. In this work, commercial (La0.6Sr0.4)0.95Co0.2Fe0.8O3- d (LSCF) powders have been modified by atomic layer deposition (ALD) using zirconia-, ceria- and hafnia-based oxides. The effect of coating thickness (number of ALD cycles) and dopant addition have been investigated. Morphology and porosity of sintered samples based on ALD coated powders and the pristine powder were found to be rather similar. On the other hand, the electrochemical performance of cathodes based on the ALD treated powders was significantly different from that of the unmodified cathode. This paper examines the details of electrochemical behavior of ALD modified and unmodified powders. Electrochemical impedance spectroscopy (EIS) analysis of symmetrical cells under air at 600-800 °C temperature range showed that high and medium frequency arcs were highly affected in ALD treated samples whereas the low frequency region stayed unchanged. The effect of surface modification of LSCF powders on durability of the LSCF-based cathode is under comprehensive assessment.

LSM-YSZ nano-composite cathode with YSZ interlayer for solid oxide fuel cells

Low temperature prepared (La0.8Sr0.2)0.9MnO3-δ-Y0.15Zr0.85O1.93 (LSM-YSZ) nano-composite cathode has high three-phase boundary (TPB) density and shows higher oxygen reduction reaction (ORR) activity than traditional LSM-YSZ cathode at reduced temperatures. But the weak connection between cathode and electrolyte due to low sintering temperature restrains the performance of LSM-YSZ nano-composite cathode. A YSZ interlayer, consisted of nanoparticles smaller than 10nm, is introduced by spinning coating hydrolyzed YSZ sol solution on electrolyte and sintering at 800°C. The thickness of the interlayer is about 150nm. The YSZ interlayer intimately adheres to the electrolyte and shows obvious agglomeration with LSM-YSZ nano-composite cathode. The power densities of the cell with interlayer are 0.83, 0.46 and 0.21W/cm2 under 0.7V at 800, 700 and 600°C, respectively, which are 36%, 48% and 50% improved than that of original cell. The interlayer introduction slightly increases the ohmic resistance but significantly decreases the polarization resistance. The depressed high frequency arcs of impedance spectra suggest that the oxygen incorporation kinetics are enhanced at the boundary of YSZ interlayer and LSM-YSZ nano-composite cathode, contributing to improved electrochemical performance of the cell with interlayer.

Characterization of Proton-Conducting Solid Oxide Electrolysis Cells Using High Valence State Manganate(V) Ba3(MnO4)2 As an Oxygen Evolution Reaction Electrocatalyst

There have been strong concerns in clean and renewable energy production due to finite fossil fuel sources, increasing oil prices and environmental concerns. Hydrogen is regarded as the leading candidate fuel, because it releases H2O alone during combustion and it is compatible to use in high efficiency fuel cell system. Steam reforming of hydrocarbon gas is currently the main way to produce hydrogen but still relies on fossil fuel consumption. On the contrary, water electrolysis using electric power generated by renewable energy is attractive as sustainable hydrogen production method. Especially, steam electrolysis using solid electrolyte cells is promising because high-temperature heat partly offers the energy for water electrolysis, leading favorable kinetics and thermodynamics. Hence, it is motivated to investigate on solid oxide electrolysis cell (SOEC) using proton-conducting ceramics to achieve highly efficient conversion from electrical power into chemical fuel gas directly. However, sufficient performance has not be achieved yet in the current system because large overpotential is needed for oxygen evolution reaction at anode owing to the relatively slow kinetics and the limited active zone in the anode/electrolyte interfaces due to the mismatch of ionic carriers, Accordingly, it is a great challenge to develop high performance oxygen electrode with efficient electrocatalytic ability for 4 electron transfer oxygen evolution reaction. Recently, it is reported that high valence state metal oxide reveal superior electrocatalytic activity for water oxidation because the energy levels between the occupied metal orbital and the O 2p orbital are very close, causing a strong hybridization and facilitating O-O bond formation [1]. Herein, we examined the electrocatalytic performance of high valence state Mn(V) oxide Ba3(MnO4)2as an anode for SOEC. This oxide has been reported to be very stable at elevated temperatures in oxidative conditions. Proton-conducting BaZr0.4Ce0.4Y0.2O3- δ (BZCY) was used as proton conducting electrolyte. Bulk electrolyte cell was constructed with a BZCY disc which was prepared by solid state reactive sintering (SSRS) method. The electrolyte precursor powder was prepared by mixing proper amount of BaCO3, CeO2, ZrO2, and Y2O3 according to the desired stoichiometry with the addition of 2.0 wt.% NiO as a sintering aid. This mixture was ball-milled for 48 h and uniaxially pressed under 20 MPa for 1 min and then cold-isostatic-pressed under 100 MPa for 1 min. Finally, the green pellets were calcined at 1500°C for 8 h so as to obtain dense electrolyte disc (2 mmd, 9 mmΦ). Pt paste was applied at one side of the surface as a cathode. LSCF or LSCF/Ba3(MnO4)2mixed ink were screen-printed at the other side of the surface as anode materials. Samples were evaluated by XRD and XAS. The electrochemical impedance spectroscopy and I-V measurements were carried out to evaluate the SOEC properties. Two kinds of anode materials were examined in this research, namely, the cell-1: Pt | BZCY | LSCF and cell-2: Pt | BZCY | Ba3(MnO4)2/LSCF. The cell-2 showed superior steam electrolytic performance compared to cell-1. The current density of steam electrolysis of cell-2 was 145 mA cm-2 meanwhile cell-1 was 95 mA cm-2 in bias voltage of 1.5 V at 600°C. Impedance spectroscopy was conducted to evaluate the anodic polarization resistance. LSCF anode gives 6 Ω cm2, however, the Ba3(MnO4)2/LSCF composite anode gives reduced resistances, 3 Ω cm2. Furthermore, the spectral features were completely different between both. The spectrum of LSCF anode had three semi-circles: high frequency arc (105-104 Hz), middle frequency arc (104-102 Hz) and low frequency arc (102-10-1 Hz). On the other hand, Ba3(MnO4)2/LSCF involves only two semi-circles: high frequency arc (104-102 Hz) and low frequency arc (102-10-1 Hz). These results indicate Ba3(MnO4)2 changes the reaction pathway of water oxidation electrode/solid electrolyte interface. The oxygen evolution rate was measured by gas chromatography when electrolysis was performed at a constant current density of 100 mA, 200 mA, and 300 mA. The flux of oxygen from anode side was corresponding to the calculated from the current density, indicating that the faradaic efficiency was almost 100%. XRD pattern of the sample after electrolysis showed that there were no secondary phases, indicating Ba3(MnO4)2 is stable in SOEC anodic condition. The above results suggest the Ba3(MnO4)2is promising for oxygen electrode material in proton-conducting solid oxide electrolysis cell. [1] Zhiyi Lu et al. Nature Communications 5, 4345, 2014