Influence of Graphite Pore Forming Agents on the Structural and Electrochemical Properties of Porous Ni-CGO Anode

Abstract

Influence of different graphite pore forming agents on the microstructure and electrochemical properties of supporting Ni - CGO (ceria gadolinia oxide) anodes for intermediate temperature solid oxide fuel cells (SOFC) has been studied by changing the amount and particle size distribution of a pore former in the raw anode material. Single cells were analyzed electrochemically in two electrode setup as well as by different structural analysis methods, like XRD, FIB-SEM and AFM. For identification of characteristic frequencies of limiting processes and corresponding parameters of cathodic and anodic reaction, single cell impedance spectra were recorded and compared analytically as a function of applied potential, partial pressure of hydrogen and oxygen, respectively. Recorded data were analyzed in detail using non-linear regression fitting analysis of theoretically calculated spectra to experimental data and differential derivate of impedance modulus versus frequency dependence methods. Power densities, resistances and other parameters obtained using fitting analysis were correlated with microstructural and porosity characteristics and pore forming agent (PFA) concentrations in raw anode paste. It was detected that medium frequency range anodic processes in impedance spectra, related to the gas phase diffusion, were influenced by the particle size distribution of PFA in the raw anode material. Impedance values at low frequency range of spectra related to the gas conversion and/or H2O desorption from catalytic sites of microporous Ni anode particles, depend on the amount of pore former as well as on the particle size distribution of particles of the PFA in the raw anode material. Highest power density and smallest polarization resistance values were achieved for anodes modified with PFA with smallest average particle size, less than 10 μm, at the 4.0 wt% of PFA in the raw anode material.