Nature of the two semi-circles observed on the complex plane plots on porous electrodes in the presence of a concentration gradient

Electrochemical reduction of electroactive species on a porous electrode composed of cylindrical pores was studied. In the absence of the ohmic drop in pores, analytical solutions for the steady state current and the electrode impedance were found. They predict the formation of two semicircles on the complex plane plots for a simple red-ox system. The properties of these semicircles as functions of different experimental parameters (electrode potential, porosity, concentration, etc.) were studied. Only one semicircle is obtained for semi-infinite porous or flat electrodes. These semicircles are caused by the concentration gradient in pores. Their nature is discussed.