Abhijit Das Sharma

Synthesis and Characterization of Nanocrystalline MnCo2O4-δ Spinel for Protective Coating Application in SOFC

Nanocrystalline (40-50 nm) powders of MnCo2O4-delta spinel have been synthesized through combustion technique at a relatively low temperature. The fuel to oxidant ratio has been optimized by carrying out the combustion reaction under various redox conditions (i.e. varying fuel : nitrate ratio) using different fuels viz., citric acid and glycine. A controlled combustion of the complex gel is achieved under a fuel lean condition using a mixed fuel of glycine and citric acid. Phase pure materials are obtained upon calcination of the ash at a temperature as low as 650degC and the powder can be consolidated to more than 95% of theoretical density at a temperature of only 1100degC. The bulk samples show a linear thermal expansion behaviour (CTE: 13.1 x 10-6 K-1) and a high electrical conductivity (>100 S/cm at 800degC). Crofer22APU interconnect plates are deposited with the synthesized powder using electrophoretic deposition (EPD) technique for protective coating application.

Use of Electroless Anode Active Layer in Anode-supported Planar SOFC

In SOFC, a novel Ni-YSZ cermet developed through electroless technique is used as an anode as well as anode active layer (AAL). In the present investigation, thickness of such AAL (varied in the range 15 - 140 micrometer) is optimized sequentially for fabricating high performance single cell. The fabrication technique involves tape casting followed by room temperature lamination to form the half cells. Effect of sintering temperature of half cells on the electrochemical performance has been carried out in the range of 1300 degC to 1400 degC. A typical I-V characteristics of coupon cell (active area of ~ 0.3 cm2) sintered at 1400 degC with an optimum AAL thickness show current density of ~ 3 A/cm2 and power density of ~ 2 W/cm2 at 0.7 V and 800 degC. Electrochemical performances of single cells using only electroless anode are also evaluated for comparison. Microstructures of these single cells are correlated with the electrochemical performances.

Multilayered SOFC Anode Structure with Electroless Ni-YSZ for Enhancement of Cell Performance

High performance SOFC is fabricated using multilayer anode support with graded porosity. The graded anode fulfills the requirement of proper fuel oxidation at anode, effective electrochemical reaction at anode/electrolyte interface, thermal compatibility among cell components and long term stability. In such multilayer anode, conventional cermet (40 vol% Ni) helps in proper fuel diffusion and catalytic oxidation. Novel electroless anode (28-32 vol% Ni) is employed adjacent to YSZ electrolyte for effective electrochemical activity and thermal compatibility. Because of the presence of unique core (YSZ)-shell (Ni) microstructure, electroless anode is found to extend the anodic electrochemical reaction from electrolyte/anode interface to the bulk. Significant performance enhancement with current density of ~3.3 Acm -2 is observed for single cells fabricated with multilayer anode at 800 0 C, 0.7V. The multilayer anode is found to reduce the cell degradation from ~11% (conventional anode) to ~1.6% /1000 h during long term performance (~2000 h).

Glass-based Sealants for Application in Planar Solid Oxide Fuel Cell Stack

One of the major challenges for the commercialization of planar anode-supported solid oxide fuel cell (SOFC) is the development of suitable high temperature sealant. The sealants must maintain gas-tightness between the anode and the cathode compartments to avoid intermixing of fuel and oxidant at high operating temperature (700°-800°C), be an electrical insulator and chemically non-reactive with the stack components. Due to the high commercial importance of such sealants, very few sealing compositions are available in the open literature and have to be developed by the individual SOFC developer. In this context the present article gives an overview on such sealant development with particular emphasis on glass-ceramics based sealants. Other than glass-based sealants, very recent approaches such as compressive and self healing seals also show some potential as SOFC sealants. The review of literature is followed by description of the progress of work related to development of glass based sealant at our Institute for application in the developed planar anode-supported SOFC stack. Under this developmental activity, a number of glass systems having SiO2 and B2O3 as glass formers and BaO, MgO and CaO as the alkaline earth metal oxide content have been investigated and thoroughly characterized. Based on the findings, some of these glasses have been used for making seals between different types of SOFC components, e.g. ferritic stainless steel (Crofer22APU) (metallic interconnect) and 8 mol% yttria-stabilized zirconia (YSZ) (electrolyte) under various combinations, viz. metal-metal, metal-ceramic and ceramic-ceramic. A novel concept of “bi-layered sealing” consisting of two different glass compositions having graded CTE values has been implemented for the first time during actual SOFC stack operation. The performance of the developed sealants has been evaluated in several working SOFC short stacks (up to 6-cell) for a maximum period of 20 days. It is observed that the open circuit voltage (OCV) of these stacks is very near to its theoretical value and remains same during the stack testing period and thus ensured the potentiality of the developed sealant for SOFC application.