Impedance Study of Anodic Properties of Ni-Fe Impregnated SDC

Samaria-doped ceria (SDC) possesses a higher oxide ionic conductivity compared to YSZ, furthermore it exhibits a high electronic conductivity under a reducing condition.  Therefore SDC can form electron conduction paths instead of Ni particles, which means Ni concentration can be suppressed to avoid their aggregation.  As a result of suppressing Ni aggregation, the 20wt% Ni/SDC anode exhibited the higher performance compared to that containing lower or higher concentration of Ni.  This is due to the increase in the number of reaction site, i.e., interfaces between Ni particles and SDC.  Ni-Fe anodes with the high Ni-Fe concentrations have been already reported.  Lu et al. evaluated the 50wt% Ni_1-xFe_x/SDC anode in hydrogen fuel and reported their best performance at x=0.25.  They assigned the modified performance to the electronic interaction between Ni and Fe.  In the present study, we newly prepared Ni-Fe/SDC anode containing 20wt% Ni-Fe alloy, and their anodic properties were investigated by the impedance analyses.  The Ni_1-xFe_x/SDC particles were prepared by the impregnation method using the nitrates of the corresponding metals.  The precursors were calcinated at 700^oC for 10h.  The obtained powers were deposited on the dense 10Sc1CeSZ disk and sintered at 900^oC.  The cathode was formed with Pt paste on the opposite surface of the 10Sc1CeSZ disk.  100%H₂ and air were supplied on the anode and cathode, respectively.  All the impedance spectra of the Ni_1-xFe_x/SDC anodes were measured at 800^oC with the three-terminal configuration under the OCV condition.  The impedance spectra of the anodes exhibited an ohmic component and two depressed arcs.  Therefore, the spectra were fitted by assuming the equivalent circuit of R_ohm(R_HFQ_HF)(R_LHQ_LF), where Q is a phase constant element.  The R_HF component hardly influenced by P_H2, and on the other hand, the R_LF component was proportional to P_H2^0.5.  From these results, we assigned the R_HF and R_LF to charge transfer process of hydrogen and surface diffusion of the hydrogen atoms, respectively.  Furthermore, the R_LF component sensitively decreased by the humidification of hydrogen.  Therefore it was found that the adsorbed water molecule or associated species helped hydrogen atom diffuse on the anode.  The i-v characteristics of the cells with the Ni_1-xFe_x/SDC anodes were resulted in order of 0.5 ≥ 0.2 > 0.8 ≥ 0.  The crystallite sizes of the Ni_1-xFe_x/SDC were dependent on x, and minimum at x=0.5.  The i-v characteristics correlated with the crystallite size.  The impedance spectra of the Ni_1-xFe_x/SDC anodes were also fitted by assuming the equivalent circuit of R_ohm(R_HFQ_HF)(R_LHQ_LF).  The R_LF, which can be assigned to the dissociative adsorption and surface diffusion of hydrogen, was most variable among the resistance components.  The x=0.5 anode exhibited the R_LF minimum value. Therefore we tentatively conclude that Fe addition to Ni reduces the potential for the hydrogen dissociation and the surface diffusion processes.