Optimisation of the electrochemical performance of (Nd,Gd)1/3Sr2/3CoO3−d cathode for solid oxide fuel cells via spray-pyrolysis deposition and decoration with Ag nanoparticles

Abstract

Single-step synthesis and deposition of composite solid oxide fuel cell cathodes of composition (Nd,Gd)1/3Sr2/3CoO3−δ : Ce0.9(Gd,Nd)0.1O1.95 (70 : 30 w : w) with and without the addition of Ag nanoparticles (Ag NPs) was achieved employing spray pyrolysis and low-temperature sintering at 900 °C. Spray-pyrolysis-prepared materials offered improved microstructure with smaller particle size and higher porosity (extended triple phase boundary) in comparison to slurry-coated electrodes of analogous composition. Impedance spectroscopy of symmetrical cells of the composite air electrodes indicated a lower resistive response of the spray-pyrolysis-prepared electrodes with lower overall polarisation resistance (Rp), attributable to improved oxygen surface exchange and oxide-ion diffusion processes. Further reduction in Rp was achieved on decoration of the composite air electrodes with Ag NPs; the decorated composites were characterised by high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Ag K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) of Ce0.9Gd0.1O2−δ:Ag spray-pyrolysed composites indicated that the Ag nanoparticles exhibit a core–shell structure with a Ag2O outer layer calculated to be two unit cells in thickness. The outer particle shell is suggested to participate in the enhanced electrochemical response, lowering the electrode polarisation response of symmetrical cells below 700 °C. Anode-supported single-cell fuel cells corresponding to the composite air electrode with Ag NPs and a thin YSZ-CGO electrolyte provided a maximum power density of 0.6 W cm−2 at 700 °C.