- 老爺子Lv 71 decade agoFavorite Answer
When the cell discharges, oxidation occurs at the anode, and reduction occurs at the cathode. When the cell is used for a long time, a lot of reactants are consumed at the anode and the cathode, and a lot of products accumulate simultaneously. Therefore, the concentrations of reactants are very low at the surface of the anode and that of the cathode. This would stop the redox reaction in the cell
When the cell is put aside for a few days, the diffusion of the substances in the electrolyte increases the concentrations of reactants at the anode and cathode, and decreases the concentrations of products. The electrodes can discharge by undergoing oxidation and reduction, because the concentrations of reactants increase to a certain extent.