Performances of Metal Particle-Dispersed Ceria Hydrogen Electrodes in Reversible SOFCs

Abstract

Solid oxide fuel cells (SOFCs) can be operated in reverse as solid oxide electrolysis cells (SOECs) to generate pure hydrogen from water vapor with the highest possible efficiency, due to favorable thermodynamic and kinetic conditions. Thus, the so-called reversible SOFC is regarded as a reciprocal direct energy converter between hydrogen and electricity. The operating temperature of SOEC is, so far, restricted to about 1000 o C because of insufficient performance of the state-of-the-art electrolyte and electrodes at low temperatures. Lowering the operating temperature is a good option to overcome a limited choice of the component materials, as well as to utilize waste heat from various sources. It is essential to develop high performance electrodes for the reversible SOFCs operated at reduced temperatures. We have developed a mixed conducting samaria-doped ceria (SDC) porous electrode with highly dispersed Ni catalysts in the reversible SOFCs. 1-5 Recently, we have succeeded to synthesize nanometer-sized Ni catalysts supported on hollow spherical particles of SDC (Ni/SDC) by spraying a mixed solution of nickel, samarium, and cerium nitrates into an atmospheric pressure plasma. The SSP apparatus consisted of an ultrasonic mist generator for the catalyst precursor solution, a plasma torch reactor, and a catalyst particle collector with a water shower. 6, 7 A mixture of Ar and O 2 was used as the carrier gas for the mist and the plasma-forming gas. A mixed solution of Ce and Sm nitrates was prepared to form (CeO 2 ) 0.8 (SmO 1.5 ) 0.2 . Ni and Co acetates were added in the solution so as to control the total metal content at 17 vol.% in the composite after the reduction, which corresponds to the optimal one for Ni/SDC in our SOEC. 5 In the case of Ni/SDC (X = 0), the mist in the plasma was found to be decomposed completely within ca. 0.2 s, resulting in mixed oxides of NiO and SDC. 6 From X = 10 to 50, the formation of solid solution Ni 100 -X Co X O was identified by XRD. A representative SEM image of the Ni 100 -X Co X O-SDC (X=20) particles is shown in Onto an 8 mol% YSZ electrolyte disk, a porous Ni 100 -X Co X /SDC electrode was prepared. A porous Pt counter electrode was formed on the backside of the YSZ disk