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Solid oxide fuel cells (SOFCs) are promising energy conversion devices because of their high electrical efficiency, even for small power systems. However, when the anode is exposed to reduction and oxidation (redox) cycles, the Ni phase causes a large microstructural change as a result of its chemical expansion and contraction. This negatively affects the electrochemical performance. However, most studies have focused on the redox cycling behaviors of SOFCs at high operation temperatures (≥ 800 °C). Therefore, in this study, we investigate the degradation behavior of the SOFC anode during redox cycles at 500 °C. To identify the individual steps of the electrochemical processes of the anode, in-situ monitored impedance spectra were analyzed using the distribution of relaxation time method at various oxygen and hydrogen partial pressures. Consequently, the electrode polarization process was deconvoluted into five sub-processes. During the redox cycles, three major peaks were altered: gas phase diffusion in the anode substrate (10–1–101 Hz), gas diffusion coupled with charge transfer reaction and ionic transport (102–103 Hz) and charged species across the Ni–yttria stabilized zirconia interface at the anode (103–104 Hz). The major degradation of the electrode performance at 500 °C was attributed to the increase in gas phase diffusion resistance due to Ni phase aggregation and the decrease in porosity in the anode during the redox cycles. This was confirmed by microstructural analysis. By contrast, the other two processes (102–103 and 103–104 Hz) compensated each other, thus having negligible effect on performance degradation.

Solid oxide cells (SOCs) are mutually convertible energy devices capable of generating electricity from chemical fuels including hydrogen in the fuel cell mode and producing green hydrogen using electricity from renewable but intermittent solar and wind resources in the electrolysis cell mode. An effective approach to enhance the performance of SOCs at reduced temperatures is by developing highly active oxygen electrodes for both oxygen reduction and oxygen evolution reactions. Herein, highly conductive Sm3+ and Nd3+ double-doped ceria (Sm0.075Nd0.075Ce0.85O2-δ, SNDC) is utilized as an active component for reversible SOC applications. We develop a novel La0.6Sr0.4Co0.2Fe0.8O3 -δ (LSCF)-SNDC composite oxygen electrode. Compared with the conventional LSCF-Gd-doped ceria oxygen electrode, the LSCF-SNDC exhibits ∼35% lower cathode polarization resistance (0.042 Ω cm2 at 750 °C) owing to rapid oxygen incorporation and surface diffusion kinetics. Furthermore, the SOC with the LSCF-SNDC oxygen electrode and the SNDC buffer layer yields a remarkable performance in both the fuel cell (1.54 W cm-2 at 750 °C) and electrolysis cell (1.37 A cm-2 at 750 °C) modes because the incorporation of SNDC promotes the surface diffusion kinetics at the oxygen electrode bulk and the activity of the triple phase boundary at the interface. These findings suggest that the highly conductive SNDC material effectively enhances both oxygen reduction and oxygen evolution reactions, thus serving as a promising material in reversible SOC applications at reduced temperatures.

Reversible solid oxide cells (SOCs) are unique devices that perform interconversion between chemical energy (particularly hydrogen) and electricity, providing efficient energy storage for site-specific and weather-dependent solar and wind resources. One of the key requirements for achieving high-performance reversible SOCs is the development of highly active bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we investigate a La0.2Sr0.8Co0.8Fe0.2O3−δ (LSCF2882) material as a novel oxygen electrode for reversible SOCs at intermediate temperatures. Unlike most widely used La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF6428) with a rhombohedrally distorted perovskite structure, LSCF2882 possesses a simple cubic perovskite structure with a symmetric BO6 octahedral network. Furthermore, the 3D bond valence sum calculation of the LSCF2882 structure suggests a reduction in oxygen ion conduction barrier energy. Oxygen surface exchange (kchem) and diffusion (Dchem) coefficients of LSCF2882 determined by electrical conductivity relaxation are consistently remarkably higher by >2 and 20 times compared to those of LSCF6428 at 700 °C, respectively. This result is further supported by a 43% reduction in the oxygen vacancy formation energy of LSCF2882 determined from density functional theory calculations. The reversible SOCs with LSCF2882 oxygen electrodes greatly outperform LSCF6428 cells in both fuel cell (2.55 W cm−2) and electrolysis mode (2.09 A cm−2 at 1.3 V) at 800 °C, with excellent reversible cycling stability. Our findings strongly suggest that LSCF2882 is a promising candidate as a bifunctional oxygen electrode for high performance reversible SOCs at reduced temperatures.

We demonstrate ultra-fast fabrication of anode-supported solid oxide fuel cells (SOFCs) using microwave-assisted sintering technology. Due to the nature of microwaves that transfers heat directly into the material, the SOFC sintering process was completed within 8 h, ∼ six times faster compared to a conventional sintering process (∼47 h). Despite extremely rapid processing time, the microstructure of the SOFC fabricated by microwave-assisted sintering (M-SOFC) was almost identical to that of the conventionally sintered SOFC. Moreover, the electrochemical performance of the M-SOFC at 750 °C was 0.52 W/cm2 in peak power density, which is even higher than that of the conventionally sintered sample (0.49W/cm2). Thus, our results demonstrate that the ultra-fast microwave-assisted sintering process is a highly effective and practically promising technology for fabricating high performance SOFCs.

The Sr segregation at the surface of a perovskite La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) oxygen electrode is detrimental to the electrochemical performance and durability of energy conversion devices such as solid oxide fuel cells. However, a quantitative correlation of degradation of the oxygen surface exchange kinetics with Sr precipitation formation at the LSCF surface is not clearly understood yet. Herein, the correlation of the time-dependent degradation mechanisms of the LSCF catalysts with respect to Sr segregation phenomenon at the surface were investigated at 800 °C for a prolonged annealing time (∼800 h) by combining in situ electrochemical measurements, and ex situ chemical and structural analyses at the multiscale. The in situ monitored surface exchange coefficient (kchem) was found to drastically drop by ∼86% over the 800 h, and it was accompanied by the formation of Sr-containing secondary phases on the bulk LSCF surface, as expected. However, the estimated coverage of Sr segregation on the LSCF surface was only ∼15%, even after 800 h of aging time, showing significant deviation from the kchem degradation rate (∼86%). The surface chemistry evolution at the clean surface area, which is believed to be electrochemically active, was further analyzed on the nanoscale. The quantified results showed that the Sr elemental fraction of the A-site at the outermost surface of the LSCF samples was becoming deficient from ∼4.0 at 0 h to ∼0.27 at 800 h annealing. Interestingly, the time-dependent behavioral tendencies between kchem degradation and surface Sr fractional changes were highly analogous. Thus, our results suggest that this Sr deficiency at the clean surface region more dominantly impacts the degradation process rather than an electrochemical activity passivation by the SrOx precipitates, which has been shown to be a major degradation mechanism of LSCF performance.

Herein, for the first time, we demonstrate ultra-fast fabrication of a tape casted NiO-yttria stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFCs) using resonant acoustic mixing (RAM) technology. Due to its characteristics of non-contact and high-intensity acoustic field-assisted mixing, NiO-YSZ tape-cast slurry is prepared via a RAM process within ∼0.5 h, > 140 times faster than use of a conventional ball-milling (BM) process (∼72 h). During the RAM process, liquid binders more effectively penetrate into soft agglomerated ceramic powders and covered larger surface area than the case of BM process. The optimization of RAM procedure requires more subdivided mixing sequence and higher content of binders and plasticizers compared to that of the BM. Despite drastically reduced mixing time, the microstructures of RAM Ni-YSZ anode, quantified via a 3D reconstruction, are statistically identical to that of BM Ni-YSZ. The SOFC employing RAM Ni-YSZ anode support achieves 0.55 W/cm2 at 750 °C in peak power density and exhibits high durability for 300 h without noticeable degradation. Thus, our results demonstrate that the RAM process is a highly effective and ultra-fast mixing technology to produce high performance SOFC components.

A unique nanostructured NiO-yttria-stabilized zirconia (YSZ) composite is synthesized in-situ via a water-in-oil microemulsion technique for the solid oxide fuel cell (SOFC) anode. Thermogravimetric analysis and X-ray diffraction confirm that as-synthesized powders are crystallized in-situ at ∼500 °C as the distinct NiO and YSZ phases without any impurities. Moreover, transmission electron microscopy analysis reveals that the as-synthesized primary particles via microemulsion are ∼40 nm in size and have a characteristic structure in which NiO and YSZ nanograins are heterogeneously distributed. The electrochemical activity of the nanostructured NiO-YSZ composite is evaluated using an YSZ supported cell with a La0.8Sr0.2MnO3-δ-YSZ (50:50 wt.%) cathode. The maximum power density of the SOFC employing the microemulsion-mediated NiO-YSZ anode is 2.2 times greater than that of the SOFC with the conventionally ball-milled nano-sized NiO-YSZ anode. The higher performance with our nanocomposite NiO-YSZ anode is primarily attributed to its heterogeneous nanograin structure, thus leading to a significant increase in triple phase boundary densities.

Solid oxide cells (SOCs) have been reported as promising energy production and storage devices due to their high efficiency and low pollutant emissions. A key barrier to achieve high performance of SOCs is high electrode polarization resistance which attributes to the sluggish kinetics of oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) at reduced temperatures. One of the potential oxygen electrodes is a perovskite La0.6Sr0.4Co0.2Fe0.8O3- δ(LSCF6428) with mixed ionic and electronic conduction (MIEC) because of its considerable conductivities and good chemical compatibility with conventional electrolyte materials such as ceria-based oxides. However, upon long-term operation under fuel cell conditions, the LSCF6428 electrode is usually subjected to severe degradation of ORR kinetics owing to chemical instability. It has been well known that the tailoring the chemical composition of the perovskite material strongly modulates their electrochemical activities and oxygen ion transport properties. In this study, we synthesized the La0.2Sr0.8Co0.8Fe0.2O3- δ (LSCF2882) and systematically investigated its characteristics on intrinsic properties. In addition, we evaluated oxygen transport kinetics and electrochemical performance of LSCF2882 as an oxygen electrode for reversible SOC applications.

Ultra-fine Gd-doped ceria (GDC) powders were synthesized via co-precipitation using ammonium carbonate as the precipitant. The crystallite size of the resultant GDC powders was measured as ∼33 nm. The dilatometry test of the powder compacts and the relative density measurement of sintered pellets with various sintering temperatures revealed the synthesized nano-GDC powders had superior sinterability compared to commercial GDC powders (e.g., 96% vs 78% in relative density at 1300 °C, respectively). Based on the total conductivity measurement of the co-precipitated GDC via electrochemical impedance spectroscopy, we found there was an optimum sintering temperature range (1300–1400 °C) to achieve both high density and high conductivity due to significant increase in grain boundary resistance at higher temperature (1500 °C). Moreover, the nano-sized and highly sinterable co-precipitated GDC effectively enhanced oxygen reduction reaction at the La0.6Sr0.4Co0.2Fe0.8O3−δ/GDC composite cathode due to increase in active reaction sites as well as enhanced phase connectivity in 3D-bulk at lower sintering temperatures.

We develop a novel nanocomposite electrolyte, consisting of yttria-stabilized zirconia (YSZ) and erbia-stabilized bismuth oxide (ESB). The 20 mol% ESB-incorporated YSZ composite (20ESB-YSZ) achieves the high density (>97%) at the low sintering temperature of 800 °C. The microstructural analysis of 20ESB-YSZ reveals the characteristic nanocomposite structure of the highly percolated ESB phase at the YSZ grain boundaries (a few ∼ nm thick). The ionic conductivity of 20ESB-YSZ is increased by 5 times compared to that of the conventional YSZ due to the fast oxygen ion transport along the ESB phase. Moreover, this high conductivity is maintained up to 580 h, indicating high stability of the ESB-YSZ nanocomposite. In addition, the oxygen reduction reaction at the composite electrolyte/cathode interface is effectively enhanced (∼70%) at the temperature below 650 °C, mainly due to the fast dissociative oxygen adsorption on the ESB surface as well as the rapid oxygen ion incorporation into the ESB lattice. Thus, we believe this ESB-YSZ nanocomposite is a promising electrolyte for high performance solid oxide fuel cells at reduced temperatures.

A functionally graded Bi1.6Er0.4O3 (ESB)/Y0.16Zr0.84O1.92 (YSZ) bilayer electrolyte is successfully developed via a cost-effective screen printing process using nanoscale ESB powders on the tape-cast NiO-YSZ anode support. Because of the highly enhanced oxygen incorporation process at the cathode/electrolyte interface, a novel bilayer solid oxide fuel cell (SOFC) yields extremely high power density of ∼2.1 W cm–2 at 700 °C, which is a 2.4 times increase compared to that of the YSZ single electrolyte SOFC.