Reactions at the interface La0.5Ca0.5MnO3-YSZ/Al2O3 under anodic current

The reaction at the interface of a solid electrolyte cell between air electrode (La_0.5Ca_0.5MnO₃) and YSZ-electrolyte with different Al₂O₃-contents was investigated by electron microscopy (SEM) and x-ray diffraction (XRD). Formation of MnAl₂O₄ was detected in a 5 μm diffusion zone within the electrolyte. MnAl₂O₄ formation can be explained by diffusion of Mn-ions into the electrolyte and subsequent reaction with the α-Al₂O₃ grains during sintering. Cell performance and long-term stability in SOFC operation are not negatively affected by MnAl₂O₄ formation. However a rise in electrode resistance and slow delamination of perovskite oxide electrode were observed after some hours of electrolysis. This reaction is the consequence of oxygen gas pressure at the electrolyte in the MnAl₂O₄ diffusion zone. It is caused by local increase of electronic conductivity by MnAl₂O₄ formation. Long-term stability also for electrolysis conditions has been achieved by an additional intermediate YSZ-layer between air electrode and electrolyte. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Nanostructured cathode thin films with vertically-aligned nanopores for thin film SOFC and their characteristics

Nanostructured cathode thin films with vertically-aligned nanopores (VANP) were processed using a pulsed laser deposition technique (PLD). These VANP structures enhance the oxygen-gas phase diffusivity, thus improve the overall thin film SOFC performance. La_0.5Sr_0.5CoO₃ (LSCO) and La_0.4Sr_0.6Co_0.8Fe_0.2O₃ (LSCFO) were deposited on various substrates (YSZ, Si and pressed Ce_0.9Gd_0.1O_1.95 disks). Microstructures and properties of the nanostructured cathodes were characterized by TEM, HRTEM, SEM and electrochemical measurements. Additionally these well-aligned VANP structures relieve or partially relieve the internal thermal stress and lattice strain caused by the differences of thermal expansion coefficients and lattice mismatch between the electrode and the electrolyte.     

Frustration driven magnetic states of A-site spinels probed by <Emphasis Type="Italic">μ</Emphasis>SR

The normal A-site spinels MnAl₂O₄, FeAl₂O₄, CoAl₂O₄, as well as related mixed (Mn_0.5Fe_0.5Al₂O₄) and partially inverted (Fe_1.4Al_1.6O₄) spinels have been studied by μSR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the μSR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T _M is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl₂O₄, Fe_1.4Al_1.6O₄) or the formation of a spin liquid phase (FeAl₂O₄, CoAl₂O₄, Mn_0.5Fe_0.5Al₂O₄). The magnetic ground state of MnAl₂O₄ shows coexistence of antiferromagnetic and spin liquid phases. In FeAl₂O₄ and CoAl₂O₄ long-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn_0.5Fe_0.5Al₂O₄ prevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe_1.4Al_1.6O₄ strengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (~100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.     

Demonstration of medium temperature,one atmosphere reversible H2/O2 fuel and steam electrolysis cell operation using polycrystalline H3O+β/β″ -Al2O3

The development of a reversible fuel and steam electrolysis cell based on an H₃O⁺β/β″ -Al₂O₃ solid electrolyte is described. The unit has been operated between 100 and 300°C at one atmosphere steam pressure. The major limitations are discussed with reference to the solid electrolyte and the method of electrode preparation.     

Hexagonal-like NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanostructure based high-performance supercapacitor electrodes

A novel approach of double hydroxide-mediated synthesis of nickel cobaltite (NiCo₂O₄) electro-active material by the hydrothermal method is reported. The obtained NiCo₂O₄ electro-active material displays the spinel cubic phase and hexagonal-like morphology. Thermogravimetry analysis confirms the thermal stability of the electrode material. The functional groups and phase formation of NiCo₂O₄ have been confirmed by FT-IR and Raman spectral analysis. The modified NiCo₂O₄ electrode exhibits the highest specific capacitance of 767.5 F g^?1 at a current density of 0.5 A g^?1 in 3 M KOH electrolyte and excellent cyclic stability (94 % capacitance retention after 1000 cycles at a high current density of 5 A g^?1). The excellent electrochemical performance of the electrode is attributed to the hexagonal-like morphology, which contributes to the rich surface electro-active sites and easy transport pathway for the ions during the electrochemical reaction. The attractive Faradic behavior of NiCo₂O₄ electrode has been ascribed to the redox contribution of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ metal species in the alkaline medium. The symmetrical two-electrode cell has been fabricated using the NiCo₂O₄ electro-active material with excellent electrochemical properties for supercapacitor applications.     

Evaluation of a cobalt-free Ba0.5Sr0.5Fe0.9Nb0.1O3?δ cathode material for intermediate-temperature solid oxide fuel cells

A cobalt-free Ba_0.5Sr_0.5Fe_0.9Nb_0.1O_3??? (BSFNb) perovskite-type oxide was investigated as the cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. XRD results showed that BSFNb cathode was chemically compatible with the electrolyte SDC up to 1,000?°C. The maximum output of anode-supported thin-film SOFC reached 503?mW?cm^?2 at 650?°C when employing humidified H₂ as fuel and static air as oxidizer. The electrode polarization resistance was low as 0.078????cm² at 650?°C, and the activation energy of the electrode polarization resistance was 129.72?kJ?mol^?1. The experimental results indicated that the cobalt-free BSFNb was a promising cathode candidate for IT-SOFCs.     

Post-test analysis of electrode-supported solid oxide electrolyser cells

Three solid oxide cells have been investigated after long-term high temperature electrolysis to explain the phenomena of accelerated degradation. These cells contain a Ni-YSZ cermet (Ni-yttria-stabilised-zirconia) as hydrogen electrode (cathode), yttria-stabilised-zirconia (YSZ) as electrolyte, Ce_0.8Gd_0.2O_1.9 (CGO) as diffusion barrier layer and La_0.58Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) as oxygen electrode (anode). Cell 1, cell 2 and cell 3 were tested continuously at about 770 °C, with a current density of ?1 A cm^?2 and 80 % H₂O of absolute humidity for 9000, 1770 and 1460 h, respectively. It was found that in cell 1, the degradation rate was about 2.2 % per 1000 h, in cell 2 the degradation rate increased to 3.4 % per 1000 h and in cell 3 the degradation rate was 2.6 % per 1000 h. The mode of cell degradation was also investigated as a function of the cell fabrication in the four layers system (anode/diffusion barrier layer/electrolyte/cathode). An intergranular fractured surface along the grain boundaries of the electrolyte, and the formation of porous structures throughout the thickness of the electrolyte were observed in cell 1. LSCF, as the oxygen electrode, showed compositional fluctuations with a changed perovskite composition and formation of cobalt oxide. This phenomenon reduces the electrical conductivity and, probably, also the catalytic properties. The hydrogen electrode did not show major changes in all the three cells tested. Cells 2 and 3 showed similar features as observed for cell 1, except the fact that they retained the electrolyte structure without intergranular fracture and formation of porosity after continuous testing for long duration.     

Evaluation of Sr- and Mn-substituted LaAlO3 as potential SOFC anode materials

Lanthanum–aluminate-based oxides, (La_0.8Sr_0.2)_1−yAl_1−xMn_xO_3−δ (x = 0, 0.3, 0.5; y = 0 or 0.06) (LSAM), were synthesized and evaluated in detail as potential anode materials for solid oxide fuel cells (SOFCs). The electrical conductivity of LSAM (Mn ≥ 30 mol%) is dominated by p-type electronic conduction and can be treated as a diluted system of lanthanum manganites, (La,Sr)MnO₃. At 810 °C, the electrical conductivity of (La_0.8Sr_0.2)_0.94Al_0.5Mn_0.5O_3−δ (LSAM8255b) reaches 12 S/cm in air and 2.7 S/cm in humidified Ar/4% H₂ (p(O₂) ≈ 10^− 18 bar). The thermal expansion coefficients of LSAM8255a and LSAM8255b match YSZ very well and no chemical reaction was observed between these two perovskite materials and YSZ up to at least 1400 °C. Fairly good electrochemical performance was observed for an LSAM8255b–YSZ composite anode. At 850 °C, the polarization resistances are only 0.34 and 0.50 Ω cm² in wet (∼3% H₂O) Ar/20% H₂ and wet Ar/20% CH₄, respectively. In addition, an exposure to Ar/20% CH₄/3% H₂O for 35 h did not cause any apparent carbon deposition on the electrode. However, the chemical stability of LSAM8255a and LSAM8255b in a typical anode environment under open circuit conditions does not seem sufficient, leading to performance degradation with time in wet Ar/20% H₂ or wet Ar/20% CH₄. Furthermore, relatively large chemical expansion (0.3–0.5%) was observed when the atmosphere was switched from air to wet Ar/4% H₂, which might cause intolerable stress on the thin film electrolyte layer for a large-area anode-supported planar SOFC, but which might be tolerable for small geometries or electrolyte-supported SOFCs.     

A lithium salt additive Li<Subscript>2</Subscript>ZrF<Subscript>6</Subscript> for enhancing the electrochemical performance of high-voltage LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode

In this work, Li₂ZrF₆, a lithium salt additive, is reported to improve the interface stability of LiNi_0.5Mn_1.5O₄ (LNMO)/electrolyte interface under high voltage (4.9 V vs Li/Li⁺). Li₂ZrF₆ is an effective additive to serve as an in situ surface coating material for high-voltage LNMO half cells. A protective SEI layer is formed on the electrode surface due to the involvement of Li₂ZrF₆ during the formation of SEI layer. Charge/discharge tests show that 0.15 mol L^?1 Li₂ZrF₆ is the optimal concentration for the LiNi_0.5Mn_1.5O₄ electrode and it can improve the cycling performance and rate property of LNMO/Li half cells. The results obtained by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) demonstrate that Li₂ZrF₆ can facilitate the formation of a thin, uniform, and stable solid electrolyte interface (SEI) layer. This layer inhibits the oxidation decomposition of the electrolyte and suppresses the dissolution of the cathode materials, resulting in improved electrochemical performances.     

Characterization of the surface of positive electrodes for Li-ion batteries using 7Li MAS NMR

The growth and evolution of the interphase, due to contact with the ambient atmosphere or electrolyte, are followed using ⁷Li magic-angle spinning nuclear magnetic resonance (MAS NMR) in the case of two materials amongst the most promising candidates for positive electrodes for lithium batteries: LiFePO₄ and LiMn_0.5Ni_0.5O₂. The use of appropriate experimental conditions to acquire the NMR signal allows observing only the «diamagnetic» lithium species at the surface of the grains of active material. The reaction of LiMn_0.5Ni_0.5O₂ with the ambient atmosphere or LiPF₆ (1 M in Ethylene Carbonated/DiMéthyl Carbonate (EC/DMC)) electrolyte is extremely fast and leads to an important amount of lithium-containing diamagnetic species compared to what can be observed in the case of LiFePO₄. The two studied materials display a completely different surface chemistry in terms of reactivity and/or kinetics of the surface towards electrolyte. Moreover, these results show that MAS NMR is a very promising tool to monitor phenomena taking place at the interface between electrode and electrolyte.     

Cells studies on PEO/PEG/NaClO<Subscript>3</Subscript> thin-film electrolyte system based on composite V<Subscript>2</Subscript>O<Subscript>5</Subscript> electrode

The thin-film solid polymer electrolyte based on polyethylene oxide (PEO) with sodium chlorite (NaClO₃) has been prepared by a solution-cast technique. The electrolyte was characterized by X-ray diffraction (XRD), infrared (IR), cyclic voltammetry, alternating current conductivity, and Wagner’s polarization studies. The complexation of NaClO₃ with PEO was confirmed through the XRD and IR studies. The transference number measurement has shown that the ion transport is predominant over electrons in the polymer electrolytes (t _ions ≈ 0.94). The conductivity enhancement was observed in the case of the PEO/NaClO₃ system with the addition of plasticizers (low-molecular-weight polyethylene glycol, organic solvents propylene carbonate and dimethyl formamide. Cyclic voltammetry analysis showed the stability and redox character of the electrolyte and electrode. Finally, polymer electrolyte systems were examined by electrochemical cell studies using V₂O₅ and composite V₂O₅ cathode at temperature of 35 °C. Overall, the plasticized electrolyte shows a better electrochemical performance, and a higher discharge capacity was observed in composite V₂O₅-based cells over V₂O₅-based cells.     

氧化还原可逆Nb2TiO7复合阴极电解池高温水蒸气电解研究

系统地研究Nb₂TiO₇与Nb_1.33Ti_0.67O₄材料相互转变的氧化还原循环可逆性能,同时研究Nb2TiO7和Nb_1.33Ti_0.67O₄样品随温度和氧分压变化的电导率,并与复合电极对称电池和电解池的电化学性能相关联. 在830 oC下,对Nb_1.33Ti_0.67O₄复合电极电解池进行水蒸气的电解研究测试. 电流电压曲线和电解池短期性能测试表明在低电压下主要为电极的还原和活化过程;而在高电压下主要为水蒸气的电解. 当3%H₂O/Ar/4%H_{2气体通入阴极时电解池水蒸气电解的法拉第效率为98.9%;而当通入气体转换为3%H2O/Ar时效率为89%.}     

Mixed conductive electrode materials for sensors and SOFC

Modified active electrode materials based upon rare earth manganites were developed for different solid electrolyte electrochemical cells. The preparation, structure, thermal expansion, the state of oxygen on the surface, the electronic and ionic conductivity of the perovskites Ln_1−xCa(Sr)_xMn_1−y(Co, Ni)_yO_3−δ with various compositions and electrode kinetics on the manganite electrode/solid electrolyte interfaces were investigated. The value of the bulk conductivity was larger than 150 S/cm (at 1100 K) and increased significantly with increasing contents of Ni or Co. The thermal expansion coefficients of rare earth manganites were close to those of ZrO₂ based solid electrolytes. The expansion coefficients of Co or Ni subsituted lanthanum manganites increase with Co or Ni substitution and are over 12•10⁻⁶K⁻¹. The ionic conductivities were determined using encapsulated zirconia microelectrodes based on a Hebb-Wagner analysis of the currentvoltage curves. The relatively high oxide ion conductivity of 10⁻⁵ S/cm at 900...1000 K was found by Ni or Co doped manganites. Studies of the electrode kinetics using complex impedance spectroscopy show that Co and Ni doped manganites have advantages if used as electrodes as compared with these for noble metals. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994     

An application of oxide and silver electrode on the BaO-doped Bi2O3 electrolyte

Oxide and silver paste were applied on the BaO-doped Bi₂O₃ electrolyte and their behavior was studied as a function of temperature and oxygen partial pressure. Interface resistance of most oxide/electrolyte were of the same order of magnitude with those of Ag paste/electrolyte in air (300–500°C). A high electrode capacitance of (0.8–1.7)×10^?2 F/cm² was observed for the silver electrode at 450°C in the P_O2 region of 1–10^?5 atm.     

Studies on the percolation limit of Ce0.9Gd0.1O1.95 in La0.6Sr0.4Co0.2Fe0.8O3−δ–Ce0.9Gd0.1O1.95 nanocomposites for solid oxide fuel cells application

A large difference in thermal expansion coefficient of electrode and electrolyte leads to imperfect electrode/electrolyte interface and hence significant polarization losses in solid oxide fuel cells. To overcome the difficulties associated with electrode and electrode/electrolyte interface, there is need to fabricate the composite cathode. Thus the present paper deals with study of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ(LSCF)–Ce_0.9Gd_0.1O_1.95(GDC) nanocomposite with different fractions of GDC obtained by physical mixing of combustion synthesized nanopowders. No secondary phases were observed upon sintering at 1100 °C for 2 h affirming the chemical compatibility between LSCF and GDC. The composites with relatively high GDC% have higher density as a consequence of rapid grain growth and less conductivity. The nanocomposite with 50% of GDC showed electric conductivity of 30 Scm⁻¹ at 500 °C and low area specific resistance of 106 Ω cm² with 10 μs relaxation time at 200 °C.     

Structural and compositional modification of a barium boroaluminosilicate glass surface by thermal poling

In addition to inducing second-order nonlinear properties, significant structural and compositional alteration can be imparted to glass surfaces during the process of thermal poling. In this work, we focus on how thermal poling affects a structurally complex, nominally alkali-free boroaluminosilicate display glass composition. We provide evidence for electrolysis of the glass network, characterized by the migration of both cations (Ba²⁺, Na⁺) and anions (O^?, F^?) towards opposing electrode interfaces. This process results in oxidation of the positively biased electrode and forms a network-former rich, modifier-depleted glass surface layer adjacent to the anodic interface. The modified glass layer thickness is qualitatively correlated to the oxidation resistance of the electrode material, while extrinsic ions such as H⁺/H₃O⁺ at not found in the depletion layer to compensate for the migration of modifier cations out of the region. Rather, FTIR spectroscopy suggests a local restructuring of the B₂O₃–Al₂O₃–SiO₂ network species to accommodate the charge imbalance created by the exodus of network-modifying cations, specifically the conversion of tetrahedral B(4) to trigonal B(3) as Ba or Na ions are removed from B-related sites in the parent network. The resultant poling-induced depletion layer exhibits enhanced hydrolytic resistance under acidic conditions, and the IR spectra are substantially unlike those produced by acid leaching the same glass.     

Carbon based double layer capacitors with aprotic electrolyte solutions: the possible role of intercalation/insertion processes

The extraordinary stability and cycle life performance of today’s electrochemical double-layer capacitors (EDLCs) are generally ascribed to the fact that charge storage in activated carbon (AC) is based on pure double-layer charging. In contrast, Faradaic charge-transfer reactions like those occurring in batteries are often connected with dimensional changes, which can affect the cycle life of these storage devices. Here we report the charge-induced height change of an AC electrode in an aprotic electrolyte solution, 1 mol/l (C₂H₅)₄NBF₄ (TEABF₄) in acetonitrile. The results are compared with those obtained for a graphite electrode in the same electrolyte. For both electrodes, we observe an expansion/contraction of several percent for a potential window of ±2 V vs. the immersion potential (ip). For the EDLC electrode, significant expansion starts at about 1 V remote from the ip and hence is well within the normal EDLC operation range. For the graphite electrode, the height changes are unambiguously caused by intercalation/deintercalation of both anions and cations. The close analogies between the graphite and the EDLC electrode suggest that ion intercalation or insertion processes might play a major role for charge storage, self discharge, cyclability, and the voltage limitation of EDLCs. PACS 82.47.Uv; 82.45.Fk; 82.45.Gj; 82.80.Fk; 81.05.Uw     

A review of recent developments in the surface modification of LiMn2O4 as cathode material of power lithium-ion battery

LiMn₂O₄ (LMO) is a very attractive choice as cathode material for power lithium-ion batteries due to its economical and environmental advantages. However, LiMn₂O₄ in the 4-V region suffers from a poor cycling behavior. Recent research results confirm that modification by coating is an important method to achieve improved electrochemical performance of LMO, and the latest progress was reviewed in the paper. The surface treatment of LMO by coating oxides and nonoxide systems could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling test. At present, LiMn₂O₄ is the mainstreaming cathode material of power lithium-ion battery, and, especially the modified LMO, is the trend of development of power lithium-ion battery cathode material in the long term.     

Evaluation of the electrical capacitance of M,O2/O2− interfaces (M=Pt, Au, Pd, In2O3; O2− = zirconia based electrolyte) exhibiting constant phase element behavior

The behavior of the electrode systems M,O₂/O² (M = porous Pd, Pt, A and dense In₂O₃; O²⁻ = ZrO₂-based single-crystal solid electrolyte) was studied by means of impedance measurements. The examination of the Pt,O₂/O²⁻ electrode system showed that the constant phase element (CPE) can be attributed to a nonuniform distribution of current at the electrode surface. It was observed that the CPE parameters n and B in the expression Y_CPE = B (jω)ⁿ may be related by B=(C_dl)ⁿ (R_Ω)_n-1, where C_dl is the double layer capacitance and R_Ω the resistance of the electrolyte in the cell. Then, C_dl of the electrode - electrolyte interface could be determined. The specific C_dl of the oxidized noble metals and india electrodes is nearly one order of magnitude lower than C_dl of the electrodes in the metallic state. The C_dl value of all the electrodes studied depends little or is independent of temperature and oxygen pressure. It is concluded that the Helmholtz model of double layer structure does not contradict the C_dl behavior.     

(La0.75Sr0.25)(Cr0.5Mn0.5)O3/YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells

The synthesis and performance of (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃/Y₂O₃–ZrO₂ (LSCM/YSZ) composites are investigated as alternative anodes for the direct utilization of methane (i.e., natural gas) in solid oxide fuel cells. Addition of YSZ phase greatly improves the adhesion and reduces the electrode polarization resistance of the LSCM/YSZ composite anodes. LSCM/YSZ composite anodes show reasonably good performance for the methane oxidation reaction in wet CH₄ and the best electrode performance was achieved for the composite with LSCM contents of 50–60 wt.% with polarization resistances of 2–3 Ω cm² in 97% CH₄/3% H₂O at 850 °C. The electrode impedance for the methane oxidation in wet CH₄ on the LSCM/YSZ composite anodes was characterized by three separable arcs and the electrode behavior could be explained based on the ALS model for the reaction on the MIEC electrode. The results indicate that electrocatalytic activity of the LSCM/YSZ composite anodes for the methane oxidation is likely limited by the oxygen vacancy diffusion in the substituted lanthanum chromite-based materials.