An extended simulation method for electrochemical systems was proposed based on kinetic Monte Carlo method. The new kinetic Monte Carlo method adopts an open boundary condition for electrochemistry, and therefore the new kinetic Monte Carlo can be executed in a grand canonical ensemble characterized with electrochemical potential. In addition, a direct counting approach was introduced in the new kinetic Monte Carlo to calculate ionic current in the atomistic point of view, which will enable us to design solid oxide fuel cells with an extremely fine view. In this study, the new kinetic Monte Carlo was applied for the calculations of electromotive force of an oxygen concentration cell as the first target. The results on the calculated electromotive force were compared with those given by Nernst's equation for electrode potential and quantitative correspondences were confirmed, which clearly verifies the applicability of the new kinetic Monte Carlo with direct counting approach for electrochemical systems.