Single fuel cell with supported LSM cathode

Single fuel cells with bilayer supported cathodes are manufactured and tested. The cathodes consist of a high-porous La_0.6Sr_0.4MnO₃ support with the thickness of approximately 1 mm and a functional composite layer with the thickness of 13?C15 ??m made of La_0.75Sr_0.2MnO₃ and 8YSZ. Voltammetric and power characteristics of single fuel cells with a supported cathode, thin-film YSZ electrolyte, and platinum cathode are determined. The conclusion as to the significant contribution into the polarization overpotential losses on the cathode is made on the basis of the measurements of electric fuel cell characteristics. It decreases significantly as a result of the supported cathode modification by praseodymium oxide. At 850°C and voltage of 0.81 V, electric power density of a fuel cell was 1.65 W/cm².     

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     

Preparation of La0.75Sr0.25MnO3 Nano-Phase Powders and Films from Alkoxide Precursors

The first purely alkoxide-based sol-gel route to nano-phase powders and thin films of perovskite La_0.75Sr_0.25MnO₃ is described. The phase and microstructure evolution on heat treatment of free gel films to form the target nano-phase oxide were investigated by TGA, IR spectroscopy, powder XRD, SEM and TEM-EDS. The xerogel consisted of a hydrated oxo-carbonate, without remaining alkoxo groups or solvent. Heating at 5°C·min^–1 decomposed the carbonate groups and yielded the pure perovskite La_0.75Sr_0.25MnO₃ at 760°C. The cell dimensions were virtually unchanged from the first observation of perovskite at 680°C, to 1000°C, 4 h. The monoclinic cell of La_0.75Sr_0.25MnO₃ obtained at 1000°C, 4 h, had the dimensions a = 5.475(1), b = 5.504(2), c = 7.771(1) Å, = 90.50(2), fitting the literature data quite well. Crack-free, homogenous, 150 nm thick La_0.75Sr_0.25MnO₃ films were prepared by spin-coating Si/SiO₂/TiO₂/Pt and polycrystalline -Al₂O₃ substrates with a 0.6 M alkoxide solution, followed by heating at 5°C·min^–1 to 800°C, 30 min.     

固-溶法制备中温固体氧化物燃料电池高性能La0.8Sr0.2MnO3-Ba0.5Sr0.5Co0.8Fe0.2O3阴极

研究了新型固溶法合成La_0.8Sr_0.2MnO₃（LSM）包覆Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃（BSCF）复合粉体（LSM-BSCF）,并探讨了其作为中温固体氧化物燃料电池阴极材料的电化学性能。LSM-BSCF阴极结合了LSM和BSCF阴极的优点,不仅增大了三相界面,而且稳定了微观结构。当温度为600-750℃时,其极化阻抗为0.61-0.09 Ω·cm²。与溶液注入法制备的高性能电极相比,极大地提高了性能稳定性。     

中温固体氧化物燃料电池La1.6Sr0.4Ni1-xCuxO4阴极材料的制备及电化学性能

采用甘氨酸-硝酸盐法合成了中温固体氧化物燃料电池阴极材料La_1.6Sr_0.4Ni_1-xCu_xO₄ (x=0.2, 0.4, 0.6,0.8), 利用X射线衍射(XRD)和扫描电子显微镜(SEM)对其结构和微观形貌进行了表征. 结果表明, 该阴极材料与固体电解质Ce_0.9Gd_0.1O_1.95(CGO)在1000 °C烧结时不发生化学反应, 且烧结4 h 后, 二者之间可形成良好的接触界面. 利用电化学交流阻抗谱技术对阴极材料的电化学性能进行研究, 结果显示, 当Cu离子掺杂量(x)为0.6 时, La_1.6Sr_0.4Ni_0.4Cu_0.6O₄阴极具有最小的极化电阻, 在空气中当测试温度为750 °C时, 极化电阻为0.35 Ω·cm². 在不同氧分压条件下电化学阻抗谱分析结果表明, 电极上的两个氧还原反应主要包含氧离子从三相界面向电解质CGO 转移的过程和电荷的迁移过程, 其中电荷的迁移过程为电极反应的速率控制步骤.La_1.6Sr_0.4Ni_0.4Cu_0.6O₄电极在空气中700 °C和阴极电流密度为45 mA·cm^-2时, 阴极过电位为45 mV. 本研究的初步结果表明La_1.6Sr_0.4Ni_1-xCu_xO₄材料是一种电化学性能较为优良的新型中温固体氧化物燃料电池(IT-SOFC)阴极材料.     

Temperature dependence of the polarization resistance of the O<Subscript>2</Subscript>,Au/La<Subscript>0.88</Subscript>Sr<Subscript>0.12</Subscript>Ga<Subscript>0.82</Subscript>Mg<Subscript>0.18</Subscript>O<Subscript>2.85</Subscript> electrode system

The impedance of a porous gold electrode in contact with solid electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 and the effect of the manufacture conditions on its polarization resistance are studied at 600–800°C in air. The overall oxygen reaction rate on a gold electrode is described as the sum of two partial constituents, namely, the oxygen exchange at the gas/electrolyte interface at the gold/gas/electrolyte triple-phased boundary.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 190–197.Original Russian Text Copyright © 2005 by Shkerin, Sokolova, Khlupin, Beresnev.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Kinetics of oxygen reduction on porous mixed conducting (La0.85Sr0.15)0.9MnO3 electrode by ac-impedance analysis

The oxygen reduction reaction on mixed conducting (La_0.85Sr_0.15)_0.9MnO₃ electrodes with various porosities was investigated by analysis of the ac-impedance spectra. To attain a mixed electronic/ionic conducting state of (La_0.85Sr_0.15)_0.9MnO₃ with high oxygen vacancy concentration, the electrode specimen was purposely subjected to cathodic polarisation. The ac-impedance spectrum clearly showed a straight line inclined at a constant angle of 45° to the real axis in the high-frequency range, followed by an arc in the low-frequency range, i.e. it exhibited the Gerischer behaviour. This strongly indicates that oxygen reduction on the mixed conducting electrode involves diffusion of oxygen vacancy through the electrode coupled with the electron exchange reaction between oxygen vacancies and gaseous oxygen (charge transfer reaction) at the electrode/gas interface. It was further recognised that the two-dimensional electrochemical active region for oxygen reduction extends from the origin of the three-phase boundaries (TPBs) among electrode, electrolyte and gas into the electrode/gas interface segments, which is on average approximately 0.7 to 1.1 μm in length below the electrode porosity 0.12. Based from the fact that the ac-impedance spectrum deviated more significantly from the Gerischer behaviour with increasing electrode porosity above 0.22, it is proposed that due to the increased length of TPBs, the rate of the overall oxygen reduction on the highly porous electrode is mainly determined by the charge transfer reaction at the TPBs, and the subsequent diffusion of oxygen vacancy occurs facilely through the electrode.     

Oxide cathodes for electrochemical devices made with the use of a nanostructured composition material

The possibility of using the LnO _x mischmetal (Ln = Ce, La, Nd, Pr, Sm) for preparation of cathodes for solid-oxide fuel cells with the supported YSZ electrolyte is studied. The electrical and electrochemical characteristics of Ln-Mn-O electrodes with the ratio of all lanthanides contained in the mischmetal except for cerium to manganese Ln: Mn = 1: 1 and also of a material comprised of Ln-Mn-O and La_0.8Sr_0.2MnO₃ are studied. The latter electrode material that contains 35?C40 wt % of Ln-Mn-O and was sintered at 1200°C has the specific ohmic resistance of 0.1 ?? cm at 800°C. The polarization conductivity is compared for electrodes made of 100% Ln-Mn-O, 40 wt % Ln-Mn-O + 60 wt % La_0.8Sr_0.2MnO₃, and 100% La_0.8Sr_0.2MnO₃ in the initial state and after their modification through the introduction of an electrocatalyst (PrO_{2 ? x} ). The highest polarization conductivity is typical of Ln-Mn-O + (La, Sr)MnO₃ electrodes containing 40 wt % Ln-Mn-O and PrO_{2 ? x} . The polarization conductivity of these electrodes is found to be 25 S/cm² at 800°C.     

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

Low temperature prepared(La_(0.8)Sr_(0.2))_(0.9)MnO_3-δ-Y_(0.15)Zr_(0.85)O_(1.93)(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 10 nm, is introduced by spinning coating hydrolyzed YSZ sol solution on electrolyte and sintering at 800 °C. The thickness of the interlayer is about 150 nm. 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 0.21 W/cm~2 under 0.7 V 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 nanocomposite cathode, contributing to improved electrochemical performance of the cell with interlayer.     

Silver incorporation into cathodes for solid oxide fuel cells operating at intermediate temperature

Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La_0.65Sr_0.3MnO_3– perovskite phase is already formed at 900 °C. The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen.     

Electrochemical characteristics of Ce0.8Gd0.2O1.9|La0.6Sr0.4CoO3-δ + Ce0.8Gd0.2O1.9 half-cell

The analysis of the medium temperature half-cell Ce_0.8Gd_0.2O_1.9|70 wt% La_0.6Sr_0.4CoO_3- (LSCO) + 30 wt % Ce_0.8Gd_0.2O_1.9 (CGO) has been made by electrochemical impedance, cyclic voltammetry and chronoamperometry. The shape of complex impedance plots depends on temperature and cathodic polarisation of the electrode. Nyquist (Z, Z-) plots were fitted by equivalent circuit taking into account the electrolyte properties (at very high frequencies), charge transfer process at grain boundaries (at high frequencies), and medium and low frequency O₂ reduction process at the cathode surface and inside the porous cathode material. Two different time constants have been obtained for the cathode process, i.e. for electroreduction of oxygen. It was found that the addition of CGO into the cathode material (LSCO) only somewhat decreases the surface catalytic activity but the noticeably higher low-frequency resistance (i.e. mainly diffusion-like mass transfer resistance R_D) values at lower temperatures have been calculated. It was found that the mainly bulk diffusion-limited process at T773 K deviates toward the kinetically mixed process (diffusion + charge transfer) with increasing temperature.     

Assessment of doped ceria as electrolyte

A model describing the performance of a fuel cell based on 10 mol% gadolinia-doped ceria, Ce_0.9Gd_0.1O_1.95−x (CG10), was formulated. The total electrical conductivity of CG10 was measured under very reducing conditions in the temperature range of 753 K to 948 K. Oxygen permeation experiments were carried out to measure the leak current through a ceria electrolyte. The results of the measurements are compared with predictions of the formulated model. Furthermore, the response of a fuel cell to changing operating conditions such as external load, temperature, electrode polarization resistances, and defect chemistry is investigated using the model. It is found that the maximum achievable efficiency of a CG10-based fuel cell is increased when (1) the temperature is decreased, when (2) the electrolyte thickness is increased, or when (3) the cathode polarization resistance is decreased. The efficiency can also in certain circumstances be increased by an increase of anode polarization resistance. Finally, the efficiency is reduced if the vacancy formation enthalpy is decreased to the level of fine-grained CG10. The performance of a CG10-based cell is evaluated by comparing it with a state-of-the-art zirconia-based cell. At 873 K, the efficiency of a fuel cell with a 10-μm CG10 electrolyte was limited to 0.74, whereas a cell with a perfect electrolyte would have an efficiency of 1. The power output of the CG10 cell at this efficiency is, however, four times larger than the zirconia-based cell at the same efficiency. This is due to the much lower cathode polarization resistance of -CG10 cathodes on CG10 compared to the (La_0.75Sr_0.25)_0.95MnO₃ cathodes on stabilized zirconia.     

Formation of NiO/YSZ functional anode layers of solid oxide fuel cells by magnetron sputtering

The decrease in the polarization resistance of the anode of solid-oxide fuel cells (SOFCs) due to the formation of an additional NiO/(ZrO₂ + 10 mol % Y₂O₃) (YSZ) functional layer was studied. NiO/YSZ films with different NiO contents were deposited by reactive magnetron sputtering of Ni and Zr–Y targets. The elemental and phase composition of the films was adjusted by regulating oxygen flow rate during the sputtering. The resulting films were studied by scanning electron microscopy and X-ray diffractometry. Comparative tests of planar SOFCs with a NiO/YSZ anode support, NiO/YSZ functional nanostructured anode layer, YSZ electrolyte, and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were performed. It was shown that the formation of a NiO/YSZ functional nanostructured anode leads to a 15–25% increase in the maximum power density of fuel cells in the working temperature range 500–800°C. The NiO/YSZ nanostructured anode layers lead not only to a reduction of the polarization resistance of the anode, but also to the formation of denser electrolyte films during subsequent magnetron sputtering of electrolyte.     

Fabrication and performance test of solid oxide fuel cells with screen-printed yttria-stabilized zirconia electrolyte membranes

Yttria-stabilized zirconia (YSZ) membranes were deposited onto porous NiO–YSZ anode supports by screen printing. Combined with La_0.7Sr_0.3MnO₃–YSZ composite cathode, the prepared anode-supported solid oxide fuel cells (SOFCs) were electrochemically tested. A typical SOFC with a 30-μm-thick YSZ electrolyte membrane gave the maximum power densities (MPDs) of 0.26, 0.53, 0.78, and 1.03 W/cm² at 650, 700, 800, and 850 °C, respectively, using hydrogen as fuel and stationary air as oxidant. Replacement of stationary air with pure oxygen flow exerted a significant positive effect on the MPDs of the cell. Using 100- and 200-ml/min oxygen as oxidants, the MPDs of the cell were enhanced 35.3% and 68.6%, respectively. Polarization analysis indicated that, at the MPD points, the electrode polarization resistances accounted for 80% of the cell total resistances.

     

Cathodes based on (La,Sr)MnO<Subscript>3</Subscript> modified with PrO<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>

The electrochemical characteristics of composite cathodes made of (La, Sr) MnO₃-(Zr, Sc)O₂ (LSM-SSZ), modified with PrO_{2 − x} additive, and designed for application in solid oxide fuel cells at moderately high temperatures were studied. The relationship between activity of catalytically modified composite LSM-SSZ cathodes and dispersity of electrocatalyst was revealed. The boundaries of the temperature range with the maximum dispersity of electrocatalyst and electrochemical activity of cathodes were found. The composite LSM-SSZ cathodes modified with PrO_{2 − x} were shown inert with respect to oxidation reactions of hydrocarbon fuel (methane) and highly active electrochemically with respect to oxygen reaction in non-equilibrium gas mixture of CH₄ and O₂. In cells with (Ce, Sm)O₂ (SDC) and (Zr, Y)O₂ (YSZ) electrolytes, their overvoltage is below 80 mV at the current density about 0.5 A/cm² and temperature of 600°C. These electrodes can be used as cathodes in single-chamber fuel cells. Long-term experiments were carried out to study time stability of characteristics of the said composite cathodes. The studied electrodes show parabolic or damped exponential time curves of polarization resistance if contacting with YSZ or SDC electrolyte, respectively. According to the forecast based on the experimental regularities, the polarization resistance of LSM-SSZ cathodes in 10,000 h will not exceed 0.4 or 0.13 Ohm cm², respectively, if YSZ or SDC electrolyte is used.     

The defect chemistry of La1−ΔMnO3+δ

The effects of oxygen reduction treatments on the magnetic properties of La-deficient manganites, La_1−ΔMnO_3+δ and Sr- and Ca-doped manganites, La_1−xM_xMnO_3+δ (M: Sr, Ca) have been investigated to confirm the contrasting oxygen reduction effects on the magnetization properties. It is found that oxygen reduction treatments in reduced oxygen pressures of 10³- for La_1−ΔMnO_3+δ result in a continuous change in the magnetization but the reduction treatments for La_1−xM_xMnO_3+δ result in a negligible change under the same reduction conditions. To interpret the contrasting behavior of the La-deficient manganites, several possible models have been discussed. Among the models, the most probable model is that vacancies generated by the La deficiency Δ are partially replaced by Δ₂(=Δ−Δ₁?Δ₁) Mn ions to give both La and Mn site vacancies according to the formula La_1−ΔV_ΔMnO_3+δ→{La_1−ΔMn_Δ2V_Δ1}{Mn_1−Δ2V_Δ2}O_3+δ. Details of thermodynamic basis of this model have been presented.     

Electrocatalytic properties of an Sr0.25Bi0.5FeO3– δ/LSGM interface

Sr_0.25Bi_0.5FeO_{3– δ} (SBF) has been studied as a cathode material for low- and intermediate-temperature (600–850 °C) solid oxide fuel cells (SOFCs) based on the La_0.9Sr_0.1Ga_0.9Mg_0.1O₃ (LSGM) electrolyte. The observed cathodic current density passing through an SBF/LSGM interface at 840 °C in pure oxygen is about 1 A·cm^–2 at an overpotential of 40 mV, much higher than that for an La_xSr_1–xMnO₃ electrode under similar conditions reported in the literature. Analysis indicates that the electrode kinetics is controlled primarily by mass transfer at high temperatures and by charge transfer at low temperatures. The inductive loops of the impedance spectra further suggest that the adsorption of intermediate species is involved in the interfacial reaction. Electronic Publication     

Fabrication and performance test of solid oxide fuel cells with screen-printed yttria-stabilized zirconia electrolyte membranes

Yttria-stabilized zirconia (YSZ) membranes were deposited onto porous NiO?CYSZ anode supports by screen printing. Combined with La_0.7Sr_0.3MnO₃?CYSZ composite cathode, the prepared anode-supported solid oxide fuel cells (SOFCs) were electrochemically tested. A typical SOFC with a 30-??m-thick YSZ electrolyte membrane gave the maximum power densities (MPDs) of 0.26, 0.53, 0.78, and 1.03?W/cm² at 650, 700, 800, and 850?°C, respectively, using hydrogen as fuel and stationary air as oxidant. Replacement of stationary air with pure oxygen flow exerted a significant positive effect on the MPDs of the cell. Using 100- and 200-ml/min oxygen as oxidants, the MPDs of the cell were enhanced 35.3% and 68.6%, respectively. Polarization analysis indicated that, at the MPD points, the electrode polarization resistances accounted for 80% of the cell total resistances.     

Solid-state metal hydride secondary batteries using heteropolyacid hydrate as an electrolyte

In order to enhance the performance of a solid-state MnO₂-metal hydride battery using H₃PMo₁₂O₄₀ · 20H₂O as an electrolyte, a moderate amount of the electrolyte was added to both positive and negative electrodes. The high rate characteristics of the battery were improved significantly by optimizing the electrolyte content in the electrodes; the resulting battery was able to operate over 140 cycles, even at a current density of 20 mA/g alloy, which is large enough for the batteries using inorganic solid electrolytes, and keep the discharge efficiency about 90%. The improvement of battery performance appears to be caused by an increase in electrode-electrolyte interface area. The AC impedance analyses revealed that the resistance of interface is decreased by the addition of a suitable amount of the electrolyte, suggesting an increase in the interface area.     

The Reactivity of Manganites in 18O Isotope Exchange Reactions

The effect of the structural properties and the oxidation state of Mn on the ¹⁸O isotope exchange behaviour of ternary manganites (La_1–xSr_xMnO₃, La_0.5Sr_1.5MnO₄ and SrMnO₃) has been studied. All types of ¹⁸O isotope exchange homomolecular, partially and completely heteromolecular) take place on the very active manganites with perovskite (LaMnO₃ and La_0.7Sr_0.3MnO₃) and perovskite-like (SrMnO₃) structure, but not on the less active K₂NiF₄-structure (La_0.5Sr_1.5MnO₄). The highest ¹⁸O exchange activity is observed for La_0.7Sr_0.3MnO₃, for which the completely heteromolecular ¹⁸O exchange starts to occur at 520 K, already, a T_on which is typical for excellent redox catalysts. The influence of the structural properties on the ¹⁸O exchange and oxygen diffusion behaviour of the manganites is much more pronounced than that of the Mn³⁺/Mn⁴⁺ ratio. The different reduction behaviour of the manganites with perovskite and K₂NiF₄-structure can be explained by means of the bond-valence model.This revised version was published online in November 2005 with corrections to the Cover Date.