Sputter deposition of cermet fuel electrodes for solid oxide fuel cells

Abstract

We describe the structure and electrochemical properties of cermet thin films composed of 50 vol % Ni and Y-stabilized ZrO2 (YSZ), of interest for solid-oxide fuel cell (SOFC) electrode applications. The Ni–YSZ films—deposited by dc reactive magnetron sputtering of Ni–Zr–Y targets in Ar–O2 mixtures—were porous, two-phase, and exhibited an equiaxed structure with grain sizes of ≊35 nm. The typical conductivity of 103 Ω−1 cm−1 was similar to that of bulk Ni–YSZ. Impedance spectroscopy was used to characterize reactions of H2/H2O fuel gases at the interfaces between Ni–YSZ films and YSZ electrolytes. The spectra typically showed two arcs, attributed to charge transfer and gas diffusion. Either high sputter gas pressures of 40 mTorr or roughened YSZ substrate surfaces were necessary to obtain sufficient film porosity. Electrochemical resistance values of 0.15 to 0.35 Ω cm2 were measured at 750 °C in 97% H2+3% H2O, lower than those for bulk Ni–YSZ and low enough for high-power-density SOFC operation. The low resistance values were probably due to a small grain size. Using a 40 mTorr pressure and a roughened substrate led to film cracking, apparently due to tensile stresses, while using 20 mTorr and a polished substrate led to a too-dense film. Either change increased the electrochemical resistance by ≳100 times.