In this paper, the effects of ion irradiation at the cathode-electrolyte interface of solid oxide fuel cells (SOFCs) are studied. In particular, the current-voltage performance is studied as a function of irradiation energy for both xenon and sodium ions at a dose of 1 x 10(16) ions/cm(2). Impedance spectroscopy is used to determine the type of loss most affected by the irradiation. Results indicate that xenon ion irradiation at a low energy (70 keV and below) produces an increase of more than 100% in the peak power density at 310 degrees C by reducing the activation losses of the SOFCs, while sodium irradiation produces little effect. Both xenon and sodium ion irradiation produces a significantly worse performance at a high irradiation energy (150 keV and above). Secondary-ion mass spectrometry was used to verify the depth profile of the implanted ions, which compared favorably with simulations performed using a kinetic Monte Carlo model. Additionally, transmission electron microscopy is used to visualize the damage created in the electrolyte substrates under various irradiation conditions. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3256137] All rights reserved.