Supercapacitors are promising energy storage devices due to their high life cycle and high power density. However, supercapacitors also known as Electric Double Layer Capacitor (EDLCs) are affected by high leakage or self discharge. The leakage is mainly dominated by redox reactions at the electrode surface through which electrons cross the double layer. Experiments have shown that a very thin blocking layer coating on the electrodes improves the energy storage capability by reducing the leakage, but it reduces the specific capacitance. The detrimental effect of the blocking layer on the capacitance requires the study of the blocking layer thickness' impact on the performance of the supercapacitor. In this work we have developed a mathematical model to study such effects. The model is a combination of simple quantum mechanical and electrochemical phenomena that occur during the self-discharge process. Using the developed model, a computational method was applied to simulate the discharge profile and specific energy in devices with various thicknesses in the blocking layer. The simulation approach can be used to study supercapacitor performances for practical applications.