Processing, microstructural evolution and electrochemical performance relationships in solid oxide fuel cells

The relationships between the processing parameters, microstructures and electrochemical performance of solid oxide fuel cell (SOFC) components were investigated. The operating regimes (i.e., reducing vs. oxidizing) as well as the elevated temperatures (e.g. 800°C) for their operation introduce several material challenges. Therefore, composite materials are employed to withstand operating conditions while providing sufficient electrochemical performance for fuel cell operation. Analyses on lanthanum-strontium manganite (LSM) - yttria stabilized zirconia (YSZ) compositions (45 vol%-55 vol%) by impedance spectroscopy demonstrated that two competing polarization mechanisms (i.e. charge-exchange and surface adsorption-diffusion of oxygen) limit performance. Optimization of microstructures resulted in total resistances as low as 0.040 Ohm cm². Studies on Ag composites revealed that incorporation of up to 25 vol% oxide particles (LSM and YSZ) with sizes comparable to the Ag grains (~0.5 μm) can minimize the densification and coarsening of the Ag matrix. While the powder based oxide additions increased the stability limit of porous Ag composites from \u3c550°C to 800°C, the use of nanostructured coatings increased the stability limit to 900°C for cathodes and current collectors. Investigations of Ni-YSZ anode microstructures demonstrated that uniform distribution of percolating isometric pores (\u3e5 μm) allows forming desired continuous percolation of all phases (Ni, YSZ and pores) lowering activation polarization below 0.100 Ohm cm² and maintaining significant electrical conductivity (\u3e1000 S/cm). Identification of polarization mechanisms by deconvolution of impedance spectra and tailoring the corresponding microstructures was demonstrated as an effective method for optimization of SOFC components --Abstract, page v

A comprehensive overview of radioguided surgery using gamma detection probe technology

The concept of radioguided surgery, which was first developed some 60 years ago, involves the use of a radiation detection probe system for the intraoperative detection of radionuclides. The use of gamma detection probe technology in radioguided surgery has tremendously expanded and has evolved into what is now considered an established discipline within the practice of surgery, revolutionizing the surgical management of many malignancies, including breast cancer, melanoma, and colorectal cancer, as well as the surgical management of parathyroid disease. The impact of radioguided surgery on the surgical management of cancer patients includes providing vital and real-time information to the surgeon regarding the location and extent of disease, as well as regarding the assessment of surgical resection margins. Additionally, it has allowed the surgeon to minimize the surgical invasiveness of many diagnostic and therapeutic procedures, while still maintaining maximum benefit to the cancer patient. In the current review, we have attempted to comprehensively evaluate the history, technical aspects, and clinical applications of radioguided surgery using gamma detection probe technology

Effect of Various Pore Formers on the Microstructural Development of Tape-Cast Porous Ceramics

Various types of pore formers have been used for the fabrication of ceramics with controlled porosity. This study addresses a detailed and systematic comparison of different pore formers (e.g. graphite, polymethyl methacrylate, sucrose and polystyrene) with distinct features such as size, distribution and morphology of particles and decomposition/oxidation behavior. Investigations also involve their effect on the rheological properties of the slurries and the microstructural development of laminated porous ceramic tapes. Morphological features of the pore former particles were characterized using laser diffraction, B.E.T. surface area measurement and scanning electron microscopy (SEM) techniques as their thermal decomposition/oxidation behavior were determined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods. Tape compositions were developed and optimized in order to incorporate identical volumetric loadings of the materials in the tape formulations with different pore formers for a reliable comparison of their pore forming characteristics. Porous yttria stabilized zirconia (YSZ) ceramics were fabricated without macroscopic defects (e.g. cracks, warpage and delamination) by developing heating profiles based on the identified thermal properties of the pore formers. Characterization of the sintered porous ceramics by SEM and mercury intrusion porosimetry techniques revealed novel relationships between the physical properties of the utilized pore formers, processing parameters and final pore structures

Development of a Silver Based Stable Current Collector for Solid Oxide Fuel Cell Cathodes

Long term stability has been a crucial issue for the future applications of the solid oxide fuel cells (SOFCs). Current collectors for the cathodes have been among the most vulnerable components of the SOFCs due to their operation in oxidizing atmospheres at relatively high temperatures. Ag and Ag based LSM (lanthanum-strontium manganite) composites were studied to develop highly stable and low-cost current collectors compatible with other fuel cell components. In this study, no degradation was observed in the electrical conductivity and the porous microstructure of the Ag-LSM composite current collectors after 600 hours of operation at 800oC in air

Effect of the Anode Microstructure on the Enhanced Performance of Solid Oxide Fuel Cells

Anode microstructure has a vital effect on the performance of anode supported solid oxide fuel cells. High electrical conductivity, gas permeability and low polarization are the required features of anodes to achieve high power densities. The desired properties of the anodes were obtained by modifying their microstructural development using pyrolyzable pore former particles without introducing any functional layers and compositional modifications. The microstructures of fabricated anodes were characterized using scanning electron microscopy and mercury intrusion porosimetry techniques while their electrochemical properties were identified using impedance spectroscopy and voltammetric measurements. Detailed investigations demonstrated that the pore structure has a major impact on the electrical conductivity, polarization and gas permeability of the anodes. Through tailoring of conventional anode microstructures, a significantly high power density of 1.54 W/cm2 was achieved at 800 °C using diluted hydrogen (10% H2 in argon) as fuel

Effect of Microstructural Evolution on the Electrochemical Properties of High Performance SOFCs

Microstructures of the component layers play a critical role in the performance of anode supported solid oxide fuel cells (SOFCs). Conventional SOFC configuration composed of laminated and co-sintered NiO-YSZ (yttria stabilized zirconia) anode and YSZ electrolyte and screen printed YSZ-LSM (lanthanum-strontium manganite) was used to investigate the performance of the tailored microstructures. Identification of the polarization mechanisms in half and complete cells allowed improving the microstructural development of the electrodes. Polystyrene demonstrated striking features among the pore formers evaluated to optimize the microstructure of Ni-YSZ anodes. The effects of the composition and sintering temperature on the polarization of YSZ-LSM cathodes were investigated to optimize the cathode performance. The contribution of the electrolyte to the ohmic polarization was lowered by decreasing its thickness. Power densities over 1.75 W/cm2 were measured at 800oC in flowing 10% hydrogen - 90% argon gas mixture on the anode side and air on the cathode side

Silver Based Perovskite Nanocomposites as Combined Cathode and Current Collector Layers for Solid Oxide Fuel Cells

Properties of silver at elevated temperatures limit its use as a porous electrode or current collector in solid oxide fuel cell (SOFC) configurations. Its relatively low melting temperature (962°C) leads to accelerated densification of silver at operating temperatures of intermediate temperature SOFCs (550°C-800°C). In previous studies, powder based silver composites demonstrated stable microstructures with desirable porosity and electrical conductivity during long-term testing. Although the characterized features of the silver based composites allowed their use as efficient and stable cathode current collectors, they demonstrated limited electrochemical performance as a cathode layer. In this study, nanostructured coatings of various perovskite based cathode materials (lanthanum-strontium manganite (LSM), lanthanum-strontium ferrite (LSF), lanthanum-strontium cobalt-ferrite (LSCF)) were deposited into porous silver composites by infiltration of polymeric precursors to enhance their electrode efficiencies. As a result, novel metal-matrix perovskite nanocomposites were obtained. The properties of the Ag based nanocomposites allowed their use as a combined cathode and current collector layer with significantly improved electrochemical performance and long term stability at 800°C in air

Electrochemical Properties of Silver Based Composite Electrodes for SOFC Cathodes and Current Collectors

Performance of the cathode and cathode current collecting layers is of vital importance for development of stable solid oxide fuel cells (SOFCs) during long-term operational conditions. Although pure silver electrodes densify and become impermeable for gas transport at operating temperatures (e.g. 800oC) of SOFCs, incorporation of oxide particles introduces open porosity to silver composites. Previously, it was demonstrated that stable porosity and electrical properties of porous silver composites were achieved at 800oC in air for \u3e5000 hours. Stable open porosity of obtained silver current collectors was utilized to fabricate LSM (lanthanum-strontium manganite), LSF (lanthanum-strontium ferrite), and LSCF (lanthanum-strontium-cobalt ferrite) composites of silver for SOFC cathode applications. Impedance spectroscopy techniques were applied to reveal polarization mechanisms of fabricated cathode/current collector layers. While infiltration of LSM gave rise to the performance of porous silver as cathode, infiltration of LSF and LSCF lowered the cathode polarizations further due to their enhanced ionic conductivities

High Temperature Stability of Silver Based Porous Nanocomposites for Electrochemical Devices

High temperature (\u3e550°C) applications of silver based porous composites have been limited due to relatively low melting temperature (962°C) of Ag. Incorporation of oxide particles was demonstrated as an effective approach for stabilization of the porous Ag microstructures. This study aims developing an understanding based on the relationships between the properties of the incorporated YSZ (yttria-stabilized zirconia), the developed porous microstructures and the electrochemical response of their electrodes. Minimum degradation was observed with the composite microstructure based on comparable Ag and YSZ particle sizes. The results demonstrated that YSZ incorporation into Ag matrix can increase the stable application temperature to 800°C

Silver Composites As Highly Stable Cathode Current Collectors for Solid Oxide Fuel Cells

Time stability of the solid oxide fuel cells (SOFCs) has been a significant concern toward realization of their practical applications. Its operation at elevated temperatures and in oxidizing atmospheres makes the cathode current collector one of the most vulnerable components of the SOFCs. Silver and silver-based metal oxide [lanthanum-strontium manganite (LSM) and yttria-stabilized zirconia] composites were investigated for the development of low-cost current collectors with long-term stability. While densification of pure silver limited its use as current collector, incorporation of oxide particles to the silver matrix led to formation of porous composites. However, addition of YSZ particles did not result in a stable porosity. Analysis of the impedance spectra allowed further investigations on the obtained microstructures and the formed contacts. No microstructural degradation has been observed in the porous Ag-LSM composite current collector and its electrical properties remained stable for over 5000 h of measurements at 800 °C in air