Electrochemical Behavior at Porous Electrodes; Applications to Capacitors

Abstract

The point has often been made that electrochemistry is a two-dimensional science since its processes, including charging or discharging of double layers, proceed on surfaces rather than in bulk. When the condition of 2-dimensionality is imposed on an electrode reaction (e.g., at a plane electrode interface), then a coupled condition arises, namely, reagents must diffuse to and products diffuse away from the electrode surface. Mechanical or hydrodynamic assistance to this mass-transfer condition is often provided, (e.g., in electrode-synthetic processes), but the requirement of diffusion or migration to or from the electrode still obtains. Even in charging the capacitance of the double layers at high-area electrodes, there is some reorganization of the distribution of cations and anions of the solution, especially in a two-electrode double-layer capacitor where ions of opposite charge must be differentially accumulated at the two respective electrode interfaces of each cell of the device. Hence there must be electrolytic and diffusional migration of ions during charging or discharging.