Joosun Kim

The impact of anode microstructure on the power generating characteristics of SOFC

It is normally required that the anode materials should have high electrical conductivity and gas permeability to reduce the polarization loss of the cell. In this study, we made anode substrates of SOFC with two different methods, which resulted in different anode microstructures, especially different pore structures. We investigated the effect of microstructure of anode substrate on the unit cell performance in terms of its electrical conductivity, gas permeability and power-generating characteristics. According to the analysis, the anode substrate with higher inactive pore volume resulted in significantly reduced cell performance by decreasing the number of electrical conduction paths, the effective gas diffusion paths and the electrochemically active sites for the anodic reaction.

Electrospun Ni-added SnO2-carbon nanofiber composite anode for high-performance lithium-ion batteries.

The SnO(2) anode is a promising anode for next-generation Li ion batteries because of its high theoretical capacity. However, it exhibits inherent capacity fading because of the large volume change and pulverization that occur during the charge/discharge cycles. The buffer matrix, such as electrospun carbon nanofibers (CNFs), can alleviate this problem to some extent, but SnO(2) particles are thermodynamically incompatible with the carbon matrix such that large Sn agglomerates form after carbonization upon melting of the Sn. Herein, we introduce well-dispersed nanosized SnO(2) attached to CNFs for high-performance anodes developed by Ni presence. The addition of Ni increases the stability of the SnO(2) such that the morphologies of the dispersed SnO(2) phase are modified as a function of the Ni composition. The optimal adding composition is determined to be Ni:Sn = 10:90 wt % in terms of the crystallite size and the distribution uniformity. A high capacity retention of 447.6 mA h g(-1) after 100 cycles can be obtained for 10 wt % Ni-added SnO(2)-CNFs, whereas Ni-free Sn/SnO(2)-CNFs have a capacity retention of 304.6 mA h g(-1).

Single-Chamber Solid Oxide Fuel Cell with Micropatterned Interdigitated Electrodes

A single-chamber solid oxide fuel cell (SC-SOFC) with micropatterned electrodes on the electrolyte has been fabricated by a microfluidic lithography technique. Microchannels formed by contacting the polydimethylsiloxane mold with a polished substrate of YSZ disk were filled with the well-dispersed suspensions of the cathode and the anode. The suspensions solidified upon solvent evaporation through the mold, resulting in the SC-SOFC with interdigitated micropatterned electrodes in an area of 5 X 5 mm in which each electrode had 100 μm line width with anode-to-cathode distances of 50 μm. I-V characteristics of the SC-SOFC were analyzed using a dc source meter.

SOFCs with Sc-Doped Zirconia Electrolyte and Co-Containing Perovskite Cathodes

Recently, much attention has been directed toward the study of Sc-doped zirconia electrolyte for intermediate-temperature SOFCs (IT-SOFCs) due to its fairly good ionic conductivity compared with conventional Y-doped zirconia, which can reduce the internal ohmic loss of the cell. For improving unit cell performance, another important point to be considered is the selection of appropriate electrode materials to reduce the polarization loss at the electrode. In this study, we fabricated anode-supported SOFCs with a 1 mol % CeO 2 codoped 10 mol % Sc 2 O 3 -ZrO 2 (CeSSZ) electrolyte. Various kinds of cathode materials such as La-Sr-Mn-O, La-Sr-Co-O (LSCo), La-Sr-Co-Fe-O, and Sm-Sr-Co-O (SSCo) have been applied in order to identify the most suitable cathode material for Sc-doped zirconia. We evaluated the power-generating characteristics of 5 X 5 cm scaled unit cells as well as the cathode polarization effect via a dc current-voltage measurement, a dc current interruption method, and ac impedance spectroscopy. We also thoroughly investigated the interfacial reaction between the electrolyte and the cathode in order to identify appropriate heat-treatment conditions for each candidate cathode material on the CeSSZ electrolyte. According to the investigation, unit cells with a SSCo and LSCo cathode showed superior power density of 1.13 and 1.33 W/cm 2 , respectively, at 700°C and fairly stable cell performance without any serious interfacial reaction.

Characterization of Thin-Film YSZ Deposited via EB-PVD Technique in Anode-supported SOFCs

An anode-supported solid oxide fuel cell (SOFC) based on 8 mol % yttria-stabilized zirconia (8YSZ) electrolyte that was deposited via an electron-beam physical vapor deposition (EB-PVD) technique, was investigated. A NiO-8YSZ plate was used as both an anode substrate and as a substrate for the deposition of 8YSZ. An additional buffering layer was coated on the anode substrate via a screen printing before the deposition of YSZ film in order to enhance the surface smoothness for the film deposition. In this study, we controlled various experimental variables for the film deposition and subsequent heat treatment process to obtain highly dense and impervious electrolyte film. Consequently, we were able to obtain a large area (12 X 12 cm) and dense YSZ film with a uniform thickness of ∼ 8 μm. The surface and cross-sectional microstructure and the corresponding phase of deposited film were thoroughly analyzed by scanning electron microscopy and X-ray diffraction. The electrochemical performance of the EB-PVD unit cell was investigated using dc current-voltage load testing, a dc current interruption technique, and an ac-impedance spectroscopy. Peculiar microstructural features of the deposited film, such as the columnar structure of grains along the growing axis, and the films microstructural effect on unit cell performance, were addressed.

A nanocomposite material for highly durable solid oxide fuel cell cathodes

We have synthesized well-engineered nanocomposite particles for the achievement of highly durable solid oxide fuel cell cathodes. The use of a dual-composite approach in which both LSM and YSZ phases are placed on a YSZ grain allows for the development of an ideal cathode microstructure with improved phase contiguity and interfacial coherence. Microstructural observations in conjunction with impedance analysis during accelerated lifetime tests clearly reveal that three-dimensionally interconnected percolative networks of both electronic and ionic conduction give cells based on the dual-composite cathode better long-term stability. A cell with such a well-controlled cathode maintains nearly constant power density over 500 h. In contrast, a cathode prepared by mechanical mixing undergoes significant degradation during the stability test due to thermochemically and electrochemically driven coarsening and shrinkage of the LSM phase.

Oxidation Behavior of Tungsten in H2O2- and Fe ( NO3 ) 3-Base Aqueous Slurries

By using potentiodynamic scanning and cyclic voltametry, the oxidation and dissolution kinetics of tungsten/tungsten-oxides were investigated in the chemical mechanical planarization (CMP) slurries containing either H 2 O 2 or Fe(NO 3 ) 3 as an oxidant. The microstructure of the tungsten/tungsten-oxide surface and its temporal and depth variation of the chemical state were also examined by scanning electron microscopy and X-ray photoelectron spectroscopy. Through the analysis, it is shown that the oxide layer formed in the slurry containing Fe(NO 3 ) 3 is relatively dense and therefore passive, whereas that formed in the H 2 O 2 slurry is a porous tungsten oxide film which would not protect the underlying tungsten from further chemical etching. The electrochemical responses of tungsten surfaces in the two different oxidant conditions were compared, and a significant difference in the CMP performance due to the microstructure and chemical state variation of the tungsten surface was observed.

Self-oriented Sb2Se3 nanoneedle photocathodes for water splitting obtained by a simple spin-coating method

Synthesis of one-dimensional nanostructured chalcogenide compounds using nontoxic and abundant constituents provides an important pathway to the development of commercially feasible photoelectrochemical water splitting. In this study, grass-like Sb2Se3 nanoneedle arrays are successfully fabricated on a substrate via a facile spin-coating method without any complicated processes such as templating, seed formation, or use of a vapor phase. Preferential [001] growth of the initial single-crystalline Sb2Se3 occurs during the first spin-coating, but interfacial defects are generated upon subsequent spin-coating iterations, resulting in annual-ring-like growth of Sb2Se3 nanoneedles. After sequential surface modification with TiO2 and Pt, the resistance to charge transfer from the photoelectrode to the electrolyte decreases significantly, yielding a remarkable record-high photocurrent of 2 mA cm−2 at 0 VRHE (4.5 mA cm−2 at −0.2 VRHE).

Characterization of Sputter-Deposited LiCoO2 Thin Film Grown on NASICON-type Electrolyte for Application in All-Solid-State Rechargeable Lithium Battery

All-solid-state Li-rechargeable batteries using a 500 nm-thick LiCoO2 (LCO) film deposited on two NASICON-type solid electrolyte substrates, LICGC (OHARA Inc.) and Li1.3Al0.3Ti1.7(PO4)3 (LATP), are constructed. The postdeposition annealing temperature prior to the cell assembly is critical to produce a stable sharp LCO/electrolyte interface and to develop a strong crystallographic texture in the LCO film, conducive to migration of Li ions. Although the cells deliver a limited discharge capacity, the cells cycled stably for 50 cycles. The analysis of the LCO/electrolyte interfaces after cycling demonstrates that the sharp interface, once formed by proper thermal annealing, will remain stable without any evidence for contamination and with minimal intermixing of the constituent elements during cycling. Hence, although ionic conductivity of the NASICON-type solid electrolyte is lower than that of the sulfide electrolytes, the NACSICON-type electrolytes will maintain a stable interface in contact with a LCO cathode, which should be beneficial to improving the capacity retention as well as the rate capability of the all-solid state cell.

Fabrication of Gd2O3-Doped CeO2 Thin Films for Single-Chamber-Type Solid Oxide Fuel Cells and Their Characterization

Gd 2 O 3 -doped CeO 2 (GDC) thin-film electrolytes with lateral-type electrode geometry for integrated single-chamber solid oxide fuel cells (SOFCs) were prepared by dc/radio-frequency reactive magnetron sputtering. The morphological and electrical properties of the GDC films with various electrode geometries were characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy. The electrolyte films had an average grain size of ∼50 nm and exhibited a conducting behavior similar to bulk GDC. The activation energy for electrical conductivity was estimated to be 0.77 eV along the direction normal to the film surface. The electrical conductivity along both the lateral and vertical directions was similar to those of bulk GDC. This suggests that a GDC thin film with such a small average grain size has negligible deleterious effects originating from the grain boundaries. It was confirmed from the oxygen partial pressure-independent electrical conductivity of the films that the electrical conduction originated mainly from oxygen ions, not electrons. Overall, the fabricated cell with the interdigitated electrode geometry might be suitable for the single-chamber SOFC cell.