Surface Ce Decorated Exsolved Ni Nanoparticles Pinned Titanate Based Perovskite for Solid Oxide Fuel Cells
Authors: Zhang Yaqian , Sun Yi-Fei , Luo Jing-Li 
Published: February 2020

Tremendous endeavor has been devoted to synthesizing highly active supported nanoparticles (NPs) catalysts for energy conversion and storage devices. Specifically, planting NPs has been proven to be a promising approach to enhance both the catalytic activity and durability of the state-of-the-art perovskite oxide catalysts for solid oxide fuel cells.

Unlike traditional impregnation method, the exsolution process provides a promising pathway to design and synthesize NPs with better thermal stability and dispersion, which can usually drive B-site cations exsolution from their parent lattice. This study demonstrated that the transitional metals exsolved on diverse perovskites exhibited special properties and desirable activities. It is also believed that the functionality of such exsolved particle pinned perovskite can be further optimized and altered through the decoration of other surface promoters.

Herein, we developed a surface Ce decorated exsolved Ni NPs pinned titanate based perovskite (LSCNT) for solid oxide fuel cells via a modified sol–gel method. The exsolution of nano Ni particles with the uniformly dispersed nanoscale Ce over material surface could be obtained by sintering the material in consecutive atmospheres. The SOFC with the prepared anode shows promising electrochemical performance and stability in both sour H2 and dry CH4. In addition, the material also exhibited excellent long term stability with good carbon deposition and sulfur resistance. The detailed characterization results including SEM-EDX, O2 and CH4 pulses, H2-TPR and O2-TPO demonstrate that the presence of exsolved metallic Ni can be regarded as the active sites for the fuel oxidation reaction. Simultaneously, the presence of widely dispersed Ce species enclosing the Ni particles had oxygen storage ability and redox properties. This provided more active oxygen ions to promote the electrochemical reaction and to facilitate the removal of carbon deposits formed on Ni. The synergistic effect between Ni and Ce over LST scaffold enabled the fabricated material to be a potential anode candidate operating in various fuels. The obtained precursor was sintered at 1200 oC for 10 h in consecutive atmospheres of air and 10% H2/Ar for 10 h, respectively.

The SEM image demonstrates the exsolved particles widely dispersed on the surface of the material. The composition of these particles on LSCNT was analyzed by EDX mapping analysis. Here, in order to quantitatively describe the variation in concentration of each element, five zones corresponding to the dramatic changes in the signal intensity for each element (La, Sr, Ce, Ni, Ti) were selected for detailed investigation. The EDX mapping scan results support the conclusion that the exsolved Ni particles with high concentration were well dispersed. And the other elements, including Ce, were also widely and uniformly dispersed.

The voltage-current and power density curves of the single fuel cells with LSCNT anode operating at the temperatures from 800 to 900 oC in 5000 ppm H2S-H2 were shown in Figure 1. As the temperature increased, the maximum power density sharply increased from 260 mW/cm2 to 660 mW/cm2 and the maximum current density from 1100 mA/cm2 to 2600 mA/cm2, respectively. The corresponding electrochemical impedance spectra (EIS) data for LSCNT under OCV condition were demonstrated in the figure below. At 900 oC, for example, the cell with LSCNT anode shows the ohmic resistance of 0.21 Ω cm2 and activation polarization resistance of 0.19 Ω cm2, indicating its comparable performance to the traditional Ni/YSZ cermet anode.

Figure 1. Current density vs. voltage and power density curves and Electrochemical impedance spectra (EIS) of fuel cells with LSCNT in 5000 ppm H2S-H2

Figure 1