Sihyuk Choi

Highly efficient and robust cathode materials for low-temperature solid oxide fuel cells: PrBa0.5Sr0.5Co2−xFexO5+δ

Solid oxide fuel cells (SOFC) are the cleanest, most efficient, and cost-effective option for direct conversion to electricity of a wide variety of fuels. While significant progress has been made in anode materials with enhanced tolerance to coking and contaminant poisoning, cathodic polarization still contributes considerably to energy loss, more so at lower operating temperatures. Here we report a synergistic effect of co-doping in a cation-ordered double-perovskite material, PrBa0.5Sr0.5Co2−xFexO5+δ, which has created pore channels that dramatically enhance oxygen ion diffusion and surface oxygen exchange while maintaining excellent compatibility and stability under operating conditions. Test cells based on these cathode materials demonstrate peak power densities ~2.2 W cm−2 at 600°C, representing an important step toward commercially viable SOFC technologies.

Chemically Stable Perovskites as Cathode Materials for Solid Oxide Fuel Cells: La‐Doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ

Ba0.5Sr0.5Co0.8Fe0.2O(3-δ) (BSCF) has won tremendous attention as a cathode material for intermediate-temperature solid-oxide fuel cells (IT-SOFC) on the basis of its fast oxygen-ion transport properties. Nevertheless, wide application of BSCF is impeded by its phase instabilities at intermediate temperature. Here we report on a chemically stable SOFC cathode material, La0.5Ba0.25Sr0.25Co0.8Fe0.2O(3-δ) (LBSCF), prepared by strategic approaches using the Goldschmidt tolerance factor. The tolerance factors of LBSCF and BSCF indicate that the structure of the former has a smaller deformation of cubic symmetry than that of the latter. The electrical property and electrochemical performance of LBSCF are improved compared with those of BSCF. LBSCF also shows excellent chemical stability under air, a CO2-containg atmosphere, and low oxygen partial pressure while BSCF decomposed under the same conditions. Together with this excellent stability, LBSCF shows a power density of 0.81 W cm(-2) after 100 h, whereas 25 % degradation for BSCF is observed after 100 h.

A collaborative study of sintering and composite effects for a PrBa0.5Sr0.5Co1.5Fe0.5O5+δ IT-SOFC cathode

Recently, a novel cathode material PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) has been proposed as a solution to overcome the drawbacks of a conventional cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Here we report systematic procedures to optimize the sintering temperature and the composite for PBSCF as an IT-SOFC cathode. For optimization of the heat treatment conditions for a PBSCF composite cathode, the effects of sintering temperature on the microstructure and electrical transport properties of the material are examined. We also suggest the optimization processes to effectively expand the electrochemical reaction zone based on a combination of a mixed ionic and electronic conductor (MIEC) electrode and an ionically conducting phase (PBSCF-Ce0.9Gd0.1O1.95 (GDC)x, x = 0, 20, 40, 50, and 60 wt%). The optimal intersection point between these two processing systems is revealed to be 50 wt% of GDC containing a composite cathode sintered at 950 °C for 4 h. The area specific resistance (ASR) of PBSCF-GDC50 sintered at 950 °C for 4 h reaches a minimum value of 0.052 Ω cm2 at 600 °C, which is consistent with the electrochemical performance results representing peak power density of ∼2.0 W cm−2 at 600 °C.

Correlation between fast oxygen kinetics and enhanced performance in Fe doped layered perovskite cathodes for solid oxide fuel cells

Many researchers have recently focused on layered perovskite oxides as cathode materials for solid oxide fuel cells because of their much higher chemical diffusion and surface exchange coefficients relative to those of ABO3-type perovskite oxides. Herein, we study the catalytic effect of Fe doping into SmBa0.5Sr0.5Co2O5+δ on the oxygen reduction reaction (ORR) and investigate the optimal Fe substitution through an analysis of the structural characteristics, electrical properties, redox properties, oxygen kinetics, and electrochemical performance of SmBa0.5Sr0.5Co2−xFexO5+δ (x = 0, 0.25, 0.5, 0.75, and 1.0). The optimal Fe substitution, SmBa0.5Sr0.5Co1.5Fe0.5O5+δ, enhanced the performance and redox stability remarkably and also led to satisfactory electrical properties and electrochemical performance due to its fast oxygen bulk diffusion and high surface kinetics under typical fuel cell operating conditions. The results suggest that SmBa0.5Sr0.5Co1.5Fe0.5O5+δ is a promising cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs).

The effect of calcium doping on the improvement of performance and durability in a layered perovskite cathode for intermediate-temperature solid oxide fuel cells

Layered perovskite oxides have received extensive attention as promising cathode materials for solid oxide fuel cells (SOFCs) because of their faster diffusion coefficient and oxygen transport kinetics. With the goals of enhancing electrochemical properties and improving the durability, this study focuses on the effect of calcium (Ca) doping in PrBa0.5Sr0.5−xCaxCo2O5+δ (x = 0 and 0.25) layered perovskite oxides through an investigation of their structural characteristics, electrical properties, redox behavior, electrochemical performances, and stability. In the temperature range of 100–750 °C, the electrical conductivity of PrBa0.5Sr0.25Ca0.25Co2O5+δ (PBSCaCO) is higher than that of Ca-free PrBa0.5Sr0.5Co2O5+δ (PBSCO). The area specific resistance (ASR) value of PBSCaCO–GDC (0.079 Ω cm2) is lower than that of PBSCO–GDC (0.093 Ω cm2) at 600 °C, based on a GDC electrolyte. Moreover, PBSCaCO–GDC achieves a good performance of 1.83 W cm−2 at 600 °C. PBSCaCO shows a stable power output without observable degradation for 100 h. On the basis of these results, the PBSCaCO cathode is an excellent candidate for IT-SOFC applications.

Electrical properties, thermodynamic behavior, and defect analysis of Lan+1NinO3n+1+δ infiltrated into YSZ scaffolds as cathodes for intermediate-temperature SOFCs

A solid oxide fuel cell (SOFC) is an electrochemical device for chemical-to-electrical energy conversion with high efficiency, low emissions, and excellent fuel flexibility. The requirement for high operating temperatures (1073–1273 K) of conventional SOFCs, however, leads to notable problems such as high costs and high rates of degradation due to interactions between cell components during cell fabrication and operation. To overcome these problems, much effort has been devoted to lowering the SOFC operating temperature toward an intermediate range (873 to 1073 K). One of the challenges for IT-SOFC is to develop cathode materials with high electrocatalytic activity for oxygen reduction at these temperatures. While La12xSrxMnO3 (LSM) has been widely used as a cathode material for SOFCs based on YSZ electrolytes at high temperatures, it is inadequate for use in an intermediate temperature range due to reduced ionic and electronic conductivity and diminished catalytic activity at lower temperatures. Recently, mixed ionic and electronic conductors (MIECs) have received tremendous attention as potential cathodes for ITSOFCs. MIECs based on transition metal (e.g. Mn, Fe, Co, and Ni) oxides have been extensively investigated. Among various MIECs, cobalt containing oxides showed superior electrocatalytic activity than oxides with predominant electronic conductivity (and little ionic conductivity) such as lanthanum manganese. In particular, La12xSrxCo12yFeyO32d (LSCF)-based cathodes have attracted much attention for IT-SOFCs. However, the long-term stability of LSCF-based cathodes is still a concern. As a mixed conductor derived from the K2NiF4-type materials, La2NiO4 has attracted significant attention for possible application as IT-SOFC cathodes. Its advantages include high oxygen ionic and electronic conductivity, moderate thermal expansion coefficient (TEC), and high electrocatalytic activity toward oxygen reduction under oxidizing conditions. Other materials such as lanthanum cobaltite perovskite also exhibit good conductivities; however, the large thermal expansion mismatch with other cell components may lead to thermomechanical problems. Ruddlesden–Popper compounds are comprised of alternating perovskite and rock-salt layers, as shown in Fig. 1. The number of perovskite layers increases with n in this structure, leading to the formation of higher order Ruddlesden–Popper phases, La3Ni2O7 and La4Ni3O10, which is argued to allow faster ionic and electronic transport. These effects are primarily attributed to increased concentration of Ni–O–Ni bonds, which are responsible for electronic conduction due to progressive delocalization of the p-type electronic charge carriers, and enhanced vacancy-migration or oxygen ion diffusivity. Currently, many of the studies on Lan+1NinO3n+1+d (n = 1, 2, or 3) focused on the structure and electrochemical properties of the bulk phases, with little attention to the basic thermodynamic properties. To date, the characteristics of Lan+1NinO3n+1+d infiltrated into a scaffold of YSZ and the actual configuration of a porous cathode fabricated by infiltration, are still unknown. Because of the unique microstructures and possible interactions between the two phases, the behaviour of this Lan+1NinO3n+1+d– YSZ could be very different from that of a pure Lan+1NinO3n+1+d phase. Further, redox properties related to oxygen thermodynamics such as oxidation enthalpies and entropies of Lan+1NinO3n+1+d (n = 1, 2, and 3), and redox stability have not been reported for the intermediate temperature range. In this study, we characterized non-stoichiometric variations of oxygen and electrical conductivities of Lan+1NinO3n+1+d (n = 1, 2, and 3) infiltrated into porous YSZ as a function of oxygen partial pressure in a temperature range of 923–1023 K. Redox behavior was evaluated using coulometric titration and the electrical conductivity was determined using 4-probe conductivity measurement.

Highly efficient layer-by-layer-assisted infiltration for high-performance and cost-effective fabrication of nanoelectrodes.

We present a novel cathode fabrication technique for improved performance and production efficiency of SOFCs based on an infiltration method assisted by layer-by-layer (LbL) assembly of polyelectrolytes. Preparation of the electrode with LbL-assisted infiltration leads to a 6.5-fold reduction in the electrode fabrication time while providing uniform and small formation of Pr0.7Sr0.3CoO3-δ (PSC) particles on the electrode. The increased surface area by 24.5% and number of active sites of the prepared electrode exhibits superior electrochemical performance up to 36.1% while preserving the electrical properties of the electrode. Because of its versatility and tenability, the LbL-assisted infiltration process may become a new route for fabrication of composite electrodes for other energy storage and conversion devices.

Electrostatic Force Microscopic Characterization of Early Stage Carbon Deposition on Nickel Anodes in Solid Oxide Fuel Cells

Carbon deposition on nickel anodes degrades the performance of solid oxide fuel cells that utilize hydrocarbon fuels. Nickel anodes with BaO nanoclusters deposited on the surface exhibit improved performance by delaying carbon deposition (i.e., coking). The goal of this research was to visualize early stage deposition of carbon on nickel surface and to identify the role BaO nanoclusters play in coking resistance. Electrostatic force microscopy was employed to spatially map carbon deposition on nickel foils patterned with BaO nanoclusters. Image analysis reveals that upon propane exposure initial carbon deposition occurs on the Ni surface at a distance from the BaO features. With continued exposure, carbon deposits penetrate into the BaO-modified regions. After extended exposure, carbon accumulates on and covers BaO. The morphology and spatial distribution of deposited carbon was found to be sensitive to experimental conditions.

Catalytic Dynamics and Oxygen Diffusion in Doped PrBaCo2O5.5+δ Thin Films

The Sr and Fe codoped double perovskites PrBaCo2O5.5+δ (PrBCO) thin films of Pr(Ba0.5Sr0.5)(Co1.5Fe0.5)O5.5+δ (PBSCFO) were epitaxially grown for chemical catalytic studies. The resistance behavior of PBSCFO epitaxial films was monitored under the switching flow of reducing and oxidizing gases as a function of the gas flow time, t, using an electrical conductivity relaxation (ECR) experimental setup. The R(t) vs t relationships determined at various temperatures show the occurrence of two oxidation processes, Co(2+)/Co(3+) ↔ Co(3+) and Co(3+) ↔ Co(3+)/Co(4+). Mathematical fitting of the observed R(t) vs t relationships was carried out using Ficks second law for one-dimensional diffusion of charge carriers to derive the diffusivity D(T) and τ(T) for the two processes at various temperatures, T. The D(T) vs T relationships were analyzed in terms of the Arrhenius relationship to find the activation energies Ea for each process. Oscillations in the dR(t)/dt plots, observed under oxidation reactions, were discussed in terms of a layer-by-layer oxygen vacancy exchange diffusion mechanism. Our work suggests that thin films of LnBCO (Ln = lanthanide) with their A and B sites doped as in PBSCFO are excellent candidates for the development of low or intermediate temperature energy conversion devices and gas sensor applications.