In the present article, a phase field model is used to study cation interdiffusion across electrolyte-electrode interface of a solid oxide fuel cell incorporating the finite size effect of the ions. The chemical potential of the ions is modified to incorporate their finite size effect. A slight difference is observed in the cation interdiffusion profiles with and without considering the finite size effect of ions. However, interestingly the consideration of the finite size effect of ions results in significant variations of the electric potential which first decreases and then increases sharply at the electrode end away from the interface. These investigations are explained on the basis of the concentration gradients which turn out to be non-trivial despite relatively small changes in the concentration profiles themselves. Furthermore, the finite size effect of ions is captured in terms of an effective activity coefficient for respective ions, and this provides an estimate of non-ideality arising due to the ion size. It is expected that such considerations of non-ideality may help the development of more realistic models of electrode-electrolyte interface which in turn may contribute to a better design of solid oxide fuel cells and accurate prediction of their performance. (C) 2018 The Electrochemical Society.