Manganese-rich SmBaCo2−x−yMnxMgyO5+δ (x = 0.5, 1, 1.5 and y = 0.05, 0.1) with stable structure and low thermal expansion coefficient as cathode materials for IT-SOFCs

SmBaCo_2−x−yMn_xMg_yO_5+δ (x = 0.5, 1, 1.5 and y = 0.05, 0.1) samples are synthesized by sol-gel method. The influence of different substitution of Mn and Mg for Co on crystal structures, thermal expansion coefficient (TEC), electrical conductivities and electrochemical performances have been investigated. The generation of the secondary phase BaMnO₃ is suppressed with Mg²⁺ increasing. Demonstrated by temperature-dependent X-ray diffraction from 25 °C to 700 °C, the structure of SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ in high temperature is stable. The TEC of SmBaCo_1.45Mn_0.5Mg_0.05O_5+δ, SmBaCo_0.95MnMg_0.05O_5+δ, SmBaCo_0.45Mn_1.5Mg_0.05O_5+δ and SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ are 15.77 × 10⁻⁶ K⁻¹, 16.20 × 10⁻⁶ K⁻¹, 12.19 × 10⁻⁶ K⁻¹ and 12.58 × 10⁻⁶ K⁻¹, respectively, which are much lower than those of cobalt-based layered perovskites and more compatible with the thermal expansion of SDC electrolyte. Although the electrochemical performances of SmBaCo_2−x−yMn_xMg_yO_5+δ (x = 0.5, 1, 1.5 and y = 0.05, 0.1) decrease slightly with Mn increasing, the polarization resistances of the SmBaCo_1.45Mn_0.5Mg_0.05O_5+δ and SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ are 0.17 Ω cm² and 0.30 Ω cm² at 800 °C, respectively, which can meet the electrochemical performance requirements of cathode materials. Among the samples, the SmBaCo_1.45Mn_0.5Mg_0.05O_5+δ and SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ show better tradeoff properties between TEC and electrochemical performance as cathode materials for IT-SOFCs.     

Experimental investigations of the microscopic features and polarization limiting factors of planar SOFCs with LSM and LSCF cathodes

The paper deals with the microscopic and polarization evaluation of planar circular-shaped seal-less SOFC cells from InDEC^® with an outline of performance limiting factors at reduced temperature. The cells consist of porous NiO–YSZ anode as mechanical support, NiO–YSZ anode active layer, yttria-stabilized zirconia (YSZ) electrolyte, and only differ for the cathode design. A first design (ASC1) with strontium doped lanthanum manganate LSM–YSZ cathode functional layer (CFL) and LSM cathode current collector layer (CCCL); the second design (ASC2) with yttria doped ceria (YDC) barrier layer and lanthanum strontium cobalt ferrite oxide (LSCF) cathode.     

Electrochemical performance of LiCoO2 cathodes by surface modification using lanthanum aluminum garnet

LiCoO₂ particles were coated with various wt.% of lanthanum aluminum garnets (3LaAlO₃:Al₂O₃) by an in situ sol–gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirmed the formation of a 3LaAlO₃:Al₂O₃ compound and the in situ sol–gel process synthesized 3LaAlO₃:Al₂O₃-coated LiCoO₂ was a single-phase hexagonal α-NaFeO₂-type structure of the core material without any modification. Scanning electron microscope (SEM) images revealed a modification of the surface of the cathode particles. Transmission electron microscope (TEM) images exposed that the surface of the core material was coated with a uniform compact layer of 3LaAlO₃:Al₂O₃, which had an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt.% 3LaAlO₃:Al₂O₃-coated LiCoO₂ cathode showed excellent cycle stability of 182 cycles, which was much higher than the 38 cycles sustained by the pristine LiCoO₂ cathode material when it was charged at 4.4 V.     

Structural and electrical properties of thin films of Pr0.8Sr0.2Fe0.8Ni0.2O3−δ

Pr_0.8Sr_0.2Fe_0.8Ni_0.2O_3−δ (PN22) films have been deposited at different temperatures on yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by X-ray diffraction (XRD) and atomic force microscopy (AFM). A marked influence in the structural properties of the substrate temperature has been found but not of the composition. Samples deposited at temperatures below 700 °C are amorphous, with granular aspect, and with decreasing roughness with the temperature. Meanwhile, the films at 700 °C are polycrystalline and exhibit a needle-shaped surface, with the highest roughness observed. Additionally, the conducting behaviour of the films has been studied by electrochemical impedance spectroscopy (EIS) and their cathodic area specific resistance (ASR) was determined. The main part of the impedance of the testing cells is due to the electrode. The ASR values of the films of PN22 are lower than those of Pr_0.9Sr_0.1Fe_0.8Ni_0.2O_3−δ (PN12), being the lowest 0.5 Ω cm² at 850 °C for the sample PN22 deposited at room temperature.     

Characterization of (Y1-xCax)BaCo4-yZnyO7 as cathodes for intermediate temperature solid oxide fuel cells

The effects of Ca and Zn substitution, respectively, for Y and Co in (Y_1-xCa_x)BaCo_4-yZn_yO₇ (0.25 ≤ x ≤ 0.75 and 1.0 ≤ y ≤ 1.75) on the structure, high-temperature phase stability, thermal expansion coefficient (TEC), and electrochemical performance for intermediate temperature solid oxide fuel cells (IT-SOFC) have been investigated. The (Y_1-xCa_x)BaCo_4-yZn_yO₇ oxides crystallize in a trigonal P31c symmetry similar to YBaCo₄O₇. The substitution of Zn for Co improves the long-term phase stability at high temperatures, but at the expense of the electrochemical performance. In contrast, the substitution of Ca for Y is improves electrochemical performances, but deteriorates the long-term phase stability at high temperatures at high Ca contents (x = 0.75 and 1.0). Among the various chemical compositions investigated in the (Y_1-xCa_x)BaCo_4-yZn_yO₇ system, the (Y_0.5Ca_0.5)BaCo_2.5Zn_1.5O₇ composition offers a combination of good electrochemical performance and low TEC, while maintaining the phase stability at 600-800 °C for 120 h. The (Y_0.5Ca_0.5)BaCo_2.5Zn_1.5O₇ + GDC (50 : 50 wt. %) composite cathodes exhibit a maximum power density of ∼ 450 mW cm⁻² at 700 °C in anode-supported single SOFC.     

A review on the stability and surface modification of layered transition-metal oxide cathodes

An ever-increasing market for electric vehicles (EVs), electronic devices and others has brought tremendous attention on the need for high energy density batteries with reliable electrochemical performances. However, even the successfully commercialized lithium (Li)-ion batteries still face significant challenges with respect to cost and safety issues when they are used in EVs. From a cathode material point of view, layered transition-metal (TM) oxides, represented by LiMO₂ (M = Ni, Mn, Co, Al, etc.) and Li-/Mn-rich xLi₂MnO₃·(1–x)LiMO₂, have been considered as promising candidates because of their high theoretical capacity, high operating voltage, and low manufacturing cost. However, layered TM oxides still have not reached their full potential for EV applications due to their intrinsic stability issues during electrochemical processes. To address these problems, a variety of surface modification strategies have been pursued in the literature. Herein, we summarize the recent progresses on the enhanced stability of layered TM oxides cathode materials by different surface modification techniques, analyze the manufacturing process and cost of the surface modification methods, and finally propose future research directions in this area.     

Electrochemical studies on LiCoO2 surface coated with Y3Al5O12 for lithium-ion cells

Synthesized yttrium aluminum garnet (YAG) sol was coated on the surface of the LiCoO₂ cathode particles by an in situ sol–gel process, followed by calcination at 923 K for 10 h in air. Based on XRD, TEM, and ESCA data, a compact YAG kernel with an average thickness of ∼20 nm was formed on the surface of the core LiCoO₂ particles, which ranged from ∼90 to 120 nm in size. The charge–discharge cycling studies for the coated materials suggest that 0.3 wt.% YAG-coated LiCoO₂ heated at 923 K for 10 h in air, delivered a discharge capacity of 167 mAh g⁻¹ and a cycle stability of about 164 cycles with a fading rate of 0.2 mAh cycle⁻¹ at a 0.2C-rate between 2.75 and 4.40 V vs. Li/Li⁺. The differential capacity plots revealed that impedance growth was slower for YAG surface treated LiCoO₂, when cells were charged at 4.40 V. DSC results exemplified that the exothermic peak at ∼468 K corresponded to the release of much less oxygen and greater thermal-stability.     

Chromium deposition and poisoning of cathodes of solid oxide fuel cells – A review

Intermediate temperature solid oxide fuel cells (IT-SOFCs) using chromia-forming alloy interconnect requires the development of cathode not only with high electrochemical activity but also with the high resistance or tolerance towards Cr deposition and poisoning. This is due to the fact that, at SOFC operating temperatures, volatile Cr species are generated over the chromia scale, poisoning the cathodes such as (La,Sr)MnO₃ (LSM) and (La,Sr)(Co,Fe)O₃ (LSCF) and causing a rapid degradation of the cell performance. Thus, a fundamental understanding of the interaction between the Fe–Cr alloys and SOFC cathode is essential for the development of high performance and stable SOFCs. The objective of this paper is to critically review the progress and particularly the work done in the last 10 years in this important area. The mechanism and kinetics of the Cr deposition and Cr poisoning process on the cathodes of SOFCs are discussed. Chromium deposition at SOFC cathodes is most likely dominated by the chemical reduction of high valence Cr species, facilitated by the nucleation agents on the electrode and electrolyte surface and/or at the electrode/electrolyte interface, i.e., the nucleation theory. The driving force behind the nucleation theory is the surface segregation and migration of cationic species on the surface of perovskite oxide cathodes. Overwhelming evidences indicate that the surface segregation plays a critical role in the Cr deposition. The prospect of the development in the Cr-tolerant cathodes for SOFCs is presented.     

Mn1.5Co1.5O4−δ infiltrated yttria stabilized zirconia composite cathodes for intermediate-temperature solid oxide fuel cells

The composite cathodes of yttria stabilized zirconia (YSZ) and Mn_1.5Co_1.5O₄ (MCO) are prepared by infiltration of the MCO oxides into porous YSZ backbones using aqueous solutions of the corresponding nitrate salts. Calcinations at 850 °C promote the formation of the MCO spinel oxide and yield nano-scale catalyst coatings on the YSZ pore walls. Impedance measurements on the symmetric MCO–YSZ cathode fuel cells show that the lowest polarization resistance in air at 800 °C is 0.43 Ω cm² for the MCO impregnated YSZ composite at the MCO volume loading of 13.5%. Analysis of the impedance spectra suggest that the oxygen reduction kinetics is probably limited by double ionization of the adsorbed oxygen atoms or charge transfer at the triple-phase boundaries. Furthermore, introducing the oxide ion conductor of samarium-doped ceria as a second component in the coated catalysts yields much lower polarization resistances, e.g., 0.15 Ω cm² at 800 °C.