An Operando Fluorescence XAS Study of a Ni/YSZ Anode in a Solid Oxide Fuel Cell

Abstract

Introduction Solid oxide fuel cells (SOFC) have high energy conversion efficiencies because of their ability to directly convert the chemical energy stored in fuels into electrical energy. Ni/YSZ (yttria-stabilized zirconia) cermet is commonly used for SOFC anodes, because Ni provides excellent catalytic activity and current collectability and YSZ provides good ionic conductivity and structural support. However, Ni/YSZ facies several problems such as instability upon redox cycling, nickel coarsening, carbon deposition and so on. Therefore, designing anode materials with high durability is a key objective in the development of SOFC technology. Especially, it is essential to understand the redox behavior of the Ni electrocatalysts at work. Operando X-ray absorption spectroscopy (XAS) is a powerful tool to investigate the relationship between the oxidation state of active site and cell performance under an operating condition. Hence in this work, we developed an operando fluorescence XAS measurement system for SOFC to study the oxidation state of the working Ni electrocatalyst at temperatures ranging from 973 to 1073 K. Experimental Figure 1a depicts a two-chamber SOFC for operando fluorescence XAS measurement developed in the present work. An anode-supported cell having a groove for the investigation of the interfacial region between the Ni/YSZ anode and the YSZ electrolyte was installed to the operando XAS measurement system. The lidded XAS instrument was heated up to 1073 K in the air to melt the seal material (Pyrex glass), and subsequently NiO in the anode was reduced in a gas flow consisting of 4% H2, 3% H2O, and N2 balance. After completing gas seal and NiO reduction, the reactor temperature was lowered to 973 K for operation. The Ni K-edge XAS measurement was implemented at the BL33XU (the Toyota beamline) of SPring-8 in fluorescence mode by irradiating the bottom center of the groove at the anode and detecting the emitted X-ray with a four-element silicon drift detector. The anode potential was measured or adjusted with reference to the potential of lanthanum strontium cobalt ferrite cathode. Results The operando XAS technique was applied to the reduction of NiO at the Ni/YSZ anode at 1073 K. Figure 1b shows a chronopotentiogram of the Ni/YSZ anode and corresponding change in average Ni valence estimated by the height of the main peak at the Ni K-edge. The average Ni valence and the anode potential exhibited different changing trends in the reduction process. There seemed to exist four regions for anode potential (i.e., two potential transition and two plateau regions) and two regions for average Ni valence. In situ observation with a CCD camera clarified that the reduction of NiO around the anode–electrolyte interface started in region 2 and continued until region 4. Operando XAS measurement was also conducted to examine the oxidation state of Ni in a high-power operation at 973 K. As a time-averaged oxidation state, Ni at the Ni/YSZ anode kept its metallic state even though the anode potential was so high as −0.4 V. The oxidation behavior of Ni at the anode as mentioned above and its relationship with degradation in the high-power operation will be further discussed in the conference. Acknowledgements The authors are grateful to Prof. Koji Amezawa of Tohoku University for his fruitful advices and discussion on the development of the operando fluorescence XAS measurement system for SOFC. All the XAFS measurements were performed at the BL33XU (the Toyota beamline) of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) with the proposal numbers of 2014B7024, 2015A7024, and 2015B7024. Figure 1