Seonyoung Yoo

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.

Development of Double-Perovskite Compounds as Cathode Materials for Low-Temperature Solid Oxide Fuel Cells

A class of double-perovskite compounds display fast oxygen ion diffusion and high catalytic activity toward oxygen reduction while maintaining excellent compatibility with the electrolyte. The astoundingly extended stability of NdBa(1-x)Ca(x)Co2O(5+δ) (NBCaCO) under both air and CO2-containing atmosphere is reported along with excellent electrochemical performance by only Ca doping into the A site of NdBaCo2O(5+δ) (NBCO). The enhanced stability can be ascribed to both the increased electron affinity of mobile oxygen species with Ca, determined through density functional theory calculations and the increased redox stability from the coulometric titration.

A tailored double perovskite nanofiber catalyst enables ultrafast oxygen evolution

Rechargeable metal–air batteries and water splitting are highly competitive options for a sustainable energy future, but their commercialization is hindered by the absence of cost-effective, highly efficient and stable catalysts for the oxygen evolution reaction. Here we report the rational design and synthesis of a double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ nanofiber as a highly efficient and robust catalyst for the oxygen evolution reaction. Co-doping of strontium and iron into PrBaCo2O5+δ is found to be very effective in enhancing intrinsic activity (normalized by the geometrical surface area, ∼4.7 times), as validated by electrochemical measurements and first-principles calculations. Further, the nanofiber morphology enhances its mass activity remarkably (by ∼20 times) as the diameter is reduced to ∼20 nm, attributed to the increased surface area and an unexpected intrinsic activity enhancement due possibly to a favourable eg electron filling associated with partial surface reduction, as unravelled from chemical titration and electron energy-loss spectroscopy.

Thermodynamic and electrical characteristics of NdBaCo2O5+δ at various oxidation and reduction states

A solid oxide fuel cell is a device that converts the chemical energy in fuel to the electrical energy directly, efficiently and eco-friendly. The cation ordered perovskite-related oxide, NdBaCo2O5+δ (NBCO), is known as one of the best cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs) because of their superior electrochemical properties. The aim of this work is to identify the crucial characteristics of NdBaCo2O5+δ/YSZ composites, such as the redox properties, the structures, and the electrical conductivities for a new cathode material for IT-SOFCs. Thermodynamic redox properties including oxidation enthalpies and entropies were investigated by coulometric titration. The isotherms were obtained at 923, 973 and 1023 K over a wide range of p(O2). Oxygen non-stoichiometries have shown that higher temperatures lead the composites to a deeper reduction state rapidly near the p(O2) of 10−5 atm and at 1023 K, which can be the sign of the decomposition. Therefore, the NBCO-YSZ composites may not be suitable for the cathodes of SOFCs above operating temperatures of 1023 K due to the decomposition. The oxidation enthalpies and entropies were determined from the isotherms and the electrical conductivities measured by the 4-probe method were high enough for IT-SOFC cathode applications.

Antimony-doped graphene nanoplatelets.

Heteroatom doping into the graphitic frameworks have been intensively studied for the development of metal-free electrocatalysts. However, the choice of heteroatoms is limited to non-metallic elements and heteroatom-doped graphitic materials do not satisfy commercial demands in terms of cost and stability. Here we realize doping semimetal antimony (Sb) at the edges of graphene nanoplatelets (GnPs) via a simple mechanochemical reaction between pristine graphite and solid Sb. The covalent bonding of the metalloid Sb with the graphitic carbon is visualized using atomic-resolution transmission electron microscopy. The Sb-doped GnPs display zero loss of electrocatalytic activity for oxygen reduction reaction even after 100,000 cycles. Density functional theory calculations indicate that the multiple oxidation states (Sb3+ and Sb5+) of Sb are responsible for the unusual electrochemical stability. Sb-doped GnPs may provide new insights and practical methods for designing stable carbon-based electrocatalysts.

A robust and active hybrid catalyst for facile oxygen reduction in solid oxide fuel cells

The sluggish oxygen reduction reaction (ORR) greatly reduces the energy efficiency of solid oxide fuel cells (SOFCs). Here we report our findings in dramatically enhancing the ORR kinetics and durability of the state-of-the-art La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode using a hybrid catalyst coating composed of a conformal PrNi0.5Mn0.5O3 (PNM) thin film with exsoluted PrOx nanoparticles. At 750 °C, the hybrid catalyst-coated LSCF cathode shows a polarization resistance of ∼0.022 Ω cm2, about 1/6 of that for a bare LSCF cathode (∼0.134 Ω cm2). Further, anode-supported cells with the hybrid catalyst-coated LSCF cathode demonstrate remarkable peak power densities (∼1.21 W cm−2) while maintaining excellent durability (0.7 V for ∼500 h). Near Ambient X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-Ray Absorption Fine Structure (NEXAFS) analyses, together with density functional theory (DFT) calculations, indicate that the oxygen-vacancy-rich surfaces of the PrOx nanoparticles greatly accelerate the rate of electron transfer in the ORR whereas the thin PNM film facilitates rapid oxide-ion transport while drastically enhancing the surface stability of the LSCF electrode.

Fe@N-Graphene Nanoplatelet-Embedded Carbon Nanofibers as Efficient Electrocatalysts for Oxygen Reduction Reaction

An activated carbon nanofiber (CNF) is prepared with incorporated Fe‐N‐doped graphene nanoplatelets (Fe@NGnPs), via a novel and simple synthesis approach. The activated CNF–Fe@NGnP catalysts exhibit substantially improved activity for the oxygen reduction reaction compared to those of commercial carbon blacks and Pt/carbon catalysts.

Thermodynamic and Electrical Properties of Layered Perovskite NdBaCo2−xFexO5+δ−YSZ (x = 0, 1) Composites for Intermediate Temperature SOFC Cathodes

The thermodynamic and electrical characteristics of the NdBaCo 2 O 5+δ (NBCO) have been studied as a competitive candidate of cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). NBCO has got extensive attention because it shows rapid oxygen diffusion, fast surface exchange kinetics, and high electrical conductivity, but a lack of long term stability is a concern. We investigated Fe doped NBCO based on the fact that stability of the NBCO-YSZ composite was increased by Fe doping. As expected, our results confirm that NdBaCoFeO 5+δ (NBCF) is more stable than the NBCO because the partial molar enthalpies of oxidation for the NBCF are higher than those for the NBCO. The higher partial molar entropies of oxidation for the NBCF is attributed to the fact that excess oxygen can be added into the interstitial lattice more freely in the NBCF than in the NBCO. However, the electrical conductivity of the NBCF (∼1 S cm ―1 ) is about one order of magnitude lower than that of the NBCO (∼10 S cm ―1 ), but it is still manageable if proper current collectors are used. The maximum power density obtained for a single cell with NBCF-YSZ and NBCO-YSZ composites is 1.034 and 0.866 W cm ―2 at 1023 K, respectively : .

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).

Nanostructured Double Perovskite Cathode With Low Sintering Temperature For Intermediate Temperature Solid Oxide Fuel Cells

This study focuses on reducing the cathode polarization resistance through the use of mixed ionic electronic conductors and the optimization of cathode microstructure to increase the number of electrochemically active sites. Among the available mixed ionic electronic conductors (MIECs), the layered perovskite GdBa0.5 Sr0.5 CoFeO5+δ (GBSCF) was chosen as a cathode material for intermediate temperature solid oxide fuel cells owing to its excellent electrochemical performance and structural stability. The optimized microstructure of a GBSCF-yttria-stabilized zirconia (YSZ) composite cathode was prepared through an infiltration method with careful control of the sintering temperature to achieve high surface area, adequate porosity, and well-organized connection between nanosized particles to transfer electrons. A symmetric cell shows outstanding results, with the cathode exhibiting an area-specific resistance of 0.006 Ω cm(2) at 700 °C. The maximum power density of a single cell using Ce-Pd anode with a thickness of ∼80 μm electrolyte was ∼0.6 W cm(-2) at 700 °C.