A three-dimensional finite element method (FEM) model simulating the electrochemical behaviour of Pt coplanar thin-film electrodes used for liquid cell transmission electron microscopy measurements was developed. The model included reaction kinetics and it was applied for the oxygen evolution reaction (OER). Kinetic parameters of OER in the liquid cell were experimentally acquired and applied to the FEM model. Comparison between the experimental and simulated polarization curves demonstrated the reliability of the FEM predictions. The simulations were used to produce maps of the potential and current density distributions of the working and counter electrodes as well as for calculating the distribution of the current density in the liquid electrolyte. Two distinctive electrode geometries were evaluated with the FEM model. It was predicted that non-symmetrical electrode designs can cause unexpected electrochemical behaviour with respect to the electrolyte current density between working and counter electrodes accompanied by the presence of hot spots. The findings suggest that FEM simulations could be key to designing well-performing electrochemical microcells for liquid phase electron microscopy experiments.