Self-accelerating chlorine evolution on porous anodes of finite thickness

Abstract

Operation of a finite-thickness porous electrode under the chlorine evolution conditions is analyzed in the framework of the convective diffusion model. According to calculations, the gas evolution in the anode’s porous coating is a necessary but insufficient condition for the self-acceleration of the electrode process. Despite the gas evolution process in the anode pores, the anodic process on the low-activity electrodes decelerates, which is externally manifested in an increased Tafel slope of the polarization curve as compared with that for a smooth electrode. Self-acceleration of the anodic chlorine evolution takes place only on electrodes with true exchange currents in excess of 10^-4 A cm^-2. Externally, the self-acceleration effect manifests itself in the emergence of a low-polarizability portion in the high-current region of the polarization curve. Such a different effect of the gas evolution process on the chlorine reaction kinetics at porous electrodes of different catalytic activity is due to an altered balance between the diffusive and convective current constituents in the overall process rate following a change in the exchange current for either electrode.