La0.8Sr0.2MnO3/YSZ电极氧电化学还原反应动力学

用线性极化、循环伏安、电位阶跃等方法详细研究了Ｌａ０．８Ｓｒ０．２ＭｎＯ３／ＹＳＺ高温电极上进行的氧化学还原反应。实验结果表明，该反应存在两条路径：低温下氧还原反应主要发生在气相－ＬＳＭ电极－ＹＳＺ电解质接触的三相界面（ＴＰＢ），速度控制步骤为氧原子在ＬＳＭ表面的浓差扩散，高温下由于氧空位在ＬＳＭ表面的形成，氧还原反应区扩展至ＬＳＭ电极表面，速度控制步骤为氧的电荷转移反应，实验同时发现：氧空位的形     

Atomic force microscopy study on the stability of a surface film formed on a graphite negative electrode at elevated temperatures

The stability at elevated temperatures of a solid electrolyte interphase (SEI) formed on a graphite negative electrode in lithium ion batteries was investigated by storage tests and in situ atomic force microscopy (AFM) observation. When the fully discharged graphite electrode was stored at elevated temperatures, the irreversible capacity in the following cycle increased remarkably. On the other hand, when the electrode was stored at the fully charged state at elevated temperatures, it was severely self-discharged during storage. AFM observation of the SEI layer formed on a model electrode of highly oriented pyrolytic graphite revealed two important facts on the stability of the SEI at elevated temperatures: (i) dissolution and agglomeration of the SEI layer at the discharged state and (ii) serious SEI growth at the charged state. These phenomena well explain the results of the charge and discharge tests. It was also shown that the addition of vinylene carbonate greatly improves the stability of the SEI at elevated temperatures, and gives good charge and discharge performance after storage.     

Oxygen Evolution Reaction over the Au/YSZ Interface at High Temperature

The oxygen evolution reaction (OER) is a sluggish electrocatalytic reaction in solid oxide electrolysis cells (SOECs) at high temperatures (600–850 °C). Perovskite oxide has been widely investigated for catalyzing the OER; however, the formation of cation‐enriched secondary phases at the oxide/oxide interface blocks the active sites and decreases OER performance. Herein, we show that the Au/yttria‐stabilized zirconia (YSZ) interface possesses much higher OER activity than the lanthanum strontium manganite/YSZ anode. Electrochemical characterization and density functional theory calculations suggest that the Au/YSZ interface provides a favorable path for OER by triggering interfacial oxygen spillover from the YSZ to the Au surface. In situ X‐ray photoelectron spectroscopy results confirm the existence of spillover oxygen on the Au surface. This study demonstrates that the Au/YSZ interface possesses excellent catalytic activity for OER at high temperatures in SOECs.     

Electrocatalytic properties of La0.9Sr0.1MnO3-based electrodes for oxygen reduction

Electrochemical reduction of oxygen at the interface between a La_0.9Sr_0.1MnO₃ (LSM)-based electrode and an electrolyte, either yttria-stabilized-zirconia (YSZ) or La_0.8Sr_0.2Ga_0.9Mg_0.1O₃ (LSGM), has been investigated using DC polarization, impedance spectroscopy, and potential step methods at temperatures from 1053 to 1173 K. Results show that the mechanism of oxygen reduction at an LSM/electrolyte interface changes with the type of electrolyte. At an LSM/YSZ interface, the apparent cathodic charge transfer coefficient is about 1 at high temperatures, implying that the rate-determining step (r.d.s.) is the diffusion of partially reduced oxygen species, while at an LSM/LSGM interface the cathodic charge transfer coefficient is about 0.5, implying that the r.d.s. is the donation of electrons to atomic oxygen. The relaxation behavior of the LSM/electrolyte interfaces displays an even more dramatic dependence on the type of electrolyte. Under cathodic polarization, the current passing through an LSM/YSZ interface increases with time whereas that through an LSM/LSGM interface decreases with time, further confirming that it is the triple phase boundaries (TPBs), rather than the surface of the LSM or the LSM/gas interface, that dominate the electrode kinetics when LSM is used as an electrode. Electronic Publication     

La0.8Sr0.2MnO3／YSZ高温电极交流阻抗研究

用交流阻抗方法研究了Ｌａ０．８Ｓｒ０．２ＭｎＯ３电极上进行的氧化电化学还原反应。实验表明反应速度控制步骤随反应温度，氧分压及过电位发生显著变化，近平衡下反应的ｒｄｓ为氧的解离吸附过程。强阳极极化下，电解质表面产生大量电子空穴；强阴极极化下，ＬＳＭ电极表面形成大量氧空位，二者的结果均使界面电导增加，电化学反应区扩展。     

Estimation of oxygen ionic conductivity in nickel-zirconia composite by oxygen permeation method

The oxygen transport in the nickel-zirconia composite was investigated using the oxygen permeation method. A disk-shaped sample made of nickel (40 vol%) and yttria-stabilized zirconia (YSZ) was used to construct a permeation cell. By exposing one side of the sample to a CO₂ gas stream and the other side to a CO stream at elevated temperatures, oxide ions were extracted from CO₂ and transported to the other side to oxidize CO. The oxygen permeation flux through the composite was determined by analyzing the effluent from the permeation cell, and the oxygen ionic conductivity of the composite was derived from the permeation data and the oxygen partial pressures. It was shown that the oxygen ionic conductivity of the composite YSZ fraction was about one third of that for the single-phase zirconia ceramic, and the activation energy associated with the transport of oxide ions in the composite is somewhat greater than that of the single-phase zirconia.     

Remarkable Enhancement of O2 Activation on Yttrium‐Stabilized Zirconia Surface in a Dual Catalyst Bed

Yttrium‐stabilized zirconia (YSZ) has been extensively studied as an electrolyte material for solid oxide fuel cells (SOFC) but its performance in heterogeneous catalysis is also the object of a growing number of publications. In both applications, oxygen activation on the YSZ surface remains the step that hinders utilization at moderate temperature. It was demonstrated by oxygen isotope exchange that a dual catalyst bed system consisting of two successive LaMnO₃ and YSZ beds without intimate contact drastically enhances oxygen activation on the YSZ surface at 698 K. It can be concluded that LaMnO₃ activates the triplet ground‐state of molecular oxygen into a low‐lying singlet state, thereby facilitating the activation of the O₂ molecule on the YSZ oxygen vacancy sites. This phenomenon is shown to improve the catalytic activity of the LaMnO₃‐Pd/YSZ system for the partial oxidation of methane.     

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.     

Co-synthesis of nano-sized LSM–YSZ composites with enhanced electrochemical property

Nano-sized LSM–YSZ composite was co-synthesized by a glycine–nitrate process (GNP). Transmission electron microscopy revealed that the as-prepared LSM–YSZ particles consist of nano-sized powders with a dominant YSZ phase. Backscatter electron image shows that LSM and YSZ phases were regularly dispersed within the composite. Alternating current impedance measurement revealed that the co-synthesized LSM–YSZ electrode shows lower polarization resistance and activation energy than the physically mixed LSM–YSZ electrode. This electrochemical improvement would be attributed to the increase in three-phase boundary and good dispersion of LSM and YSZ phases within the composite. This paper is dedicated to Professor Su-Il Pyun on the occasion of his 65th birthday.     

Electrochemical activation of molecular nitrogen at the Ir/YSZ interface

Nitrogen is often used as an inert background atmosphere in solid state studies of electrode and reaction kinetics, of solid state studies of transport phenomena, and in applications e.g. solid oxide fuel cells (SOFC), sensors and membranes. Thus, chemical and electrochemical reactions of oxides related to or with dinitrogen are not supposed and in general not considered. We demonstrate by a steady state electrochemical polarisation experiments complemented with in situ photoelectron spectroscopy (XPS) that at a temperature of 450 °C dinitrogen can be electrochemically activated at the three phase boundary between N(2), a metal microelectrode and one of the most widely used solid oxide electrolytes--yttria stabilized zirconia (YSZ)--at potentials more negative than E = -1.25 V. The process is neither related to a reduction of the electrolyte nor to an adsorption process or a purely chemical reaction but is electrochemical in nature. Only at potentials more negative than E = -2 V did new components of Zr 3d and Y 3d signals with a lower formal charge appear, thus indicating electrochemical reduction of the electrolyte matrix. Theoretical model calculations suggest the presence of anionic intermediates with delocalized electrons at the electrode/electrolyte reaction interface. The ex situ SIMS analysis confirmed that nitrogen is incorporated and migrates into the electrolyte beneath the electrode.     

An EMF cell with a nitrogen solid electrolyte--on the transference of nitrogen ions in yttria-stabilized zirconia

The mobility and electrochemical activity of nitrogen inside and/or at the surface of ionic compounds is of fundamental, as well as of possibly practical, relevance. In order to better understand the role of nitrogen anions in solid electrolytes, we measured the transference number of nitrogen in yttria-stabilized zirconia (YSZ) by a concentration cell technique as a function of oxygen activity at different temperatures in the range of 1023 ≤T/K≤ 1123. YSZ doped with 1.9 wt% of N (YSZ:N) turned out to have an appreciable nitrogen transference number, which increased from 0 to 0.1 with decreasing oxygen activity in the range of -20 < log a(O(2)) < -14. The stability of N in YSZ:N, however, has yet to be elucidated under oxidizing conditions.     

YSZ传导膜H_2S固体氧化物燃料电池

研究了Y2O3稳定的ZrO2(YSZ)氧离子传导膜H2S固体氧化物燃料电池性能。掺杂NiS、电解质、Ag粉和淀粉制备了双金属复合MoS2阳极催化剂,掺杂电解质、Ag粉和淀粉制备了复合NiO阴极催化剂,用扫描电镜对YSZ和膜电极组装(MEA)进行了表征,比较了不同电极催化剂的性能和极化过程,考察了不同温度对电池性能的影响。结果表明,双金属复合MoS2/NiS阳极催化剂在H2S环境下比Pt和单金属MoS2催化剂稳定,复合NiO阴极催化剂比Pt性能好,在电极催化剂中加入Ag可显著提高电极的导电性;与Pt电极相比,复合MoS2阳极和复合NiO阴极催化剂的过电位较小,阳极的极化比阴极侧小;温度升高,电池的电流密度与功率密度增加,电化学性能变好。在750℃、800℃、850℃和900℃及101.13 kPa时,结构为H2S、(复合MoS2阳极催化剂)/YSZ氧离子传导膜/(复合NiO阴极催化剂)、空气的燃料电池最大功率密度分别为30 mW/cm2、70 mW/cm2、155 mW/cm2及295 mW/cm2、最大电流密度分别为120 mA/cm2、240 mA/cm2、560 mA/cm2和890 mA/cm2。     

Operando Isotopic Exchange in Solid Oxide Fuel Cells: Oxygen-Transport Dependency on Applied Potential

The oxygen isotopic exchange technique is a powerful tool to investigate the oxygen transport kinetics in an oxide solid. In a solid oxide fuel cell, isotopic surface exchange and diffusion coefficients are classically determined by using the Isotopic Exchange Depth Profiling method followed by ex situ SIMS characterizations. Despite its relevance, the utilization of in situ or operando techniques to measure the isotopic exchange under an electrical bias remains marginal. We developed here a set-up which enables operando monitoring of oxygen exchange in SOFC type cells under polarization. The system has been used for studying the oxygen mobility dependency upon polarization on a symmetrical Pt/YSZ/Pt cell (YSZ: yttria-stabilized zirconia). Homomolecular and heterolytic exchange reactions were undertaken to investigate the oxygen activation step and discriminate the limiting step among the sequence of elementary steps which constitute the oxygen transport process in the SOFC system. Oxygen ions incorporation into the dense ionic conductor was identified to be the rate determining step, and its first order rate constant dependency on applied potential was established.     

Thermodynamic Foundation for High Temperature Electrochemistry

Thermodynamic concepts required for the thermodynamic calculation of the potentials of electrodes for high temperature applications are briefly reviewed. A thermodynamic approach to the calculation of half cell potentials and the standard chemical potential of an electron at high temperatures which are related to the Standard Hydrogen Electrode(SHE) is discussed. As examples, an external Ag/AgCl reference electrode and a YSZ(Ag|O2) pH sensor for high temperature applications are analyzed by using the thermodynamic ap-proach to derive a high temperature pH measurement equation. The two electrodes are employed to measure high temperature pH and the measured pH was compared with the calculated pH by using a solution chem-istry method. Concepts and principles for electrode kinetics are also briefly introduced and a modification to the Tafel equations is suggested.     

Cathode performance and oxygen-ion transport mechanism of copper oxide for solid-oxide fuel cells

Various copper oxide cathode materials were studied over a YSZ tube at 800 °C and an oxygen partial pressure of 0.21 atm. It was found that the cathode performance of CuO may be improved by doping Ag metal into it. However, an optimal doping content exists and is around 50 mol% Ag. It was also found that copper oxide itself possess enough oxygen vacancies needed for the role of a mixed conductor. The activation energy for the lattice-oxygen reduction and migration has been calculated to be 55.4 kJ/mol. By the use of electrochemical measurements over Ag-YSZ/CuO electrodes, models for the two-layer electrode have been proposed and justified for oxygen-ion transport mechanisms at low and high overpotentials, respectively; thus, the roles of CuO on the cathode behavior of the electron and oxygen-ion conductivities were well identified. Electronic Publication     

Impedance-based total-NOx sensor using stabilized zirconia and ZnCr2O4 sensing electrode operating at high temperature

We report here a new-type zirconia-based sensor that can detect total NO_x content at high temperatures such as 700 °C. A closed-one-end yttria-stabilized zirconia (YSZ) tube was used as a base sensor material. An oxide sensing electrode (SE) and a Pt counter electrode (CE) were formed on the outer and inner surfaces of the YSZ tube, respectively. The complex impedance of the device using a ZnCr₂O₄-sensing electrode was measured with an impedance analyzer in the frequency and the temperature ranges 0.1 Hz–100 kHz and 600–700 °C, respectively. A large semicircular arc was observed in complex impedance plots (Cole–Cole plots) in the lower frequency range examined and it seemed to correspond to the electrolyte/electrode interface. The impedance value at 1 Hz of the present device was found to vary almost linearly with the concentration of NO (or NO₂) from 50 to 400 ppm in the sample gas at 600–700 °C. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO₂. This means that the present device can detect the total NO_x at higher temperatures.     

Electrical properties of the YSZ/STO/YSZ-STO superlattice electrolyte film at low temperatures

This study is focused on characterization of the low temperature properties of the YSZ/STO/YSZ superlattice film deposited onto unilateral polished SrTiO₃ (STO) monocrystalline substrates using pulsed laser deposition (PLD). The phase composition, structure, surface morphology and electrical properties of the oxygen ion conducting electrolyte YSZ/STO/YSZ multilayers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The minimum conductivity activation energy of YSZ/STO/YSZ is 0.76 eV at 300–500°C. The YSZ/STO/YSZ superlattice film shows an enhancement in conductivity by three orders of magnitude compared to bulk YSZ at a temperature of 300°C.     

Development and characterization of Au-YSZ surface plasmon resonance based sensing materials: high temperature detection of CO

Au-YSZ nanocomposite films exhibited a surface plasmon resonance absorption band around 600 nm that underwent a reversible blue shift and narrowed upon exposure to CO in air at 500 degrees C. A linear dependence of the sensing signal was observed for CO concentrations ranging between 0.1 and 1 vol % in an air carrier gas. This behavior of the SPR band, upon exposure to CO, was not observed when using nitrogen as the carrier gas, indicating an oxygen-dependent reaction mechanism. Additionally, the SPR band showed no measurable signal change upon exposure to CO at temperatures below approximately 400 degrees C. The oxygen and temperature-dependent characteristics, coupled with the oxygen ion formation and conduction properties of the YSZ matrix, are indicative of charge-transfer reactions occurring at the three-phase boundary region between oxygen, Au, and YSZ, which result in charge transfer into the Au nanoparticles. These reactions are associated with the oxidation of CO and a corresponding reduction of the YSZ matrix. The chemical-reaction-induced charge injection into the Au nanoparticles results in the observed blue shift and narrowing of the SPR band.     

In situ electrochemical X-ray absorption spectroscopy of oxygen reduction electrocatalysis with high oxygen flux

An in situ electrochemical X-ray absorption spectroscopy (XAS) cell has been fabricated that enables high oxygen flux to the working electrode by utilizing a thin poly(dimethylsiloxane) (PDMS) window. This cell design enables in situ XAS investigations of the oxygen reduction reaction (ORR) at high operating current densities greater than 1 mA in an oxygen-purged environment. When the cell was used to study the ORR for a Pt on carbon electrocatalyst, the data revealed a progressive evolution of the electronic structure of the metal clusters that is both potential-dependent and strongly current-dependent. The trends establish a direct correlation to d-state occupancies that directly tracks the character of the Pt-O bonding present.     

Oxygen isotope exchange between the gas-phase and the electrochemical cell O<Subscript>2</Subscript>, Pt | YSZ | Pt,O<Subscript>2</Subscript> under conditions of applied potential difference

The kinetics of oxygen isotope exchange between gas-phase oxygen and the electrochemical cell O₂, Pt | ZrO₂ + 10 mol % Y₂O₃ (YSZ) | Pt, O₂ with applied potential difference (ΔU = ±1.2 V) is studied in the temperature range of 600–800°С and the oxygen pressure interval of 3–13 kPa. An original design of a vacuum electrochemical cell with the separated gas space is put forward for studying how the potential difference on the electrochemical cell influences the kinetics of interaction of gas-phase oxygen with the gas electrode O₂, Pt | YSZ in the electrochemical cell. It is shown that the oxygen interphase exchange rate is the higher the more negative the charge on the electrode studied; moreover, the mechanism of gas-phase oxygen exchange with the gas electrode O₂, Pt | YSZ in the electrochemical cell depends fundamentally on the electrode charge sign. The possible reasons for the revealed differences are discussed; the corresponding models are proposed.