Chemical and electrochemical studies of Leclanché cells
The densities of NH4CI-ZnCI2 solutions were measured at 25°C over a wide range of concentrations and a calculation procedure was derived assuming ideal mixing of solutions of NH4CI, ZnCl2, and the complex (NH4)ZnCI3 which accurately predicted the measured densities within plus/minus 0.7 %. The question of the NH4CI concentration at which the precipitate formed on discharge changes from Zn(NH3)2Cl2 to ZnCI2.4Zn(OH)2.H20 has been clarified and the free energies of formation of both products have been determined, for the first time for ZnCI2.4Zn(OH)2.H20. The zinc electrode potential was measured in solutions of ZnCl2 (0 to 17 molal) and of NH4CI (zero to saturation). The concentrations of the different species were calculated; ZnCI3 appeared to be predominant in all solutions except those with a large excess of NH4Cl. The solubility diagram of the NH4CI-ZnCI2- H20 system was determined for the fIrst time at 25°C. The three stages of the intermittent discharge of a Leclanché cell previously predicted by Tye have been observed and the duration of each stage explained on a theoretical basis. Hetaerolite was formed during intermittent discharge of cells containing the chemically prepared manganese dioxide (CMD) Faradiser M, as a chemical step following the normal reduction of the Mn02. This formation increased the positive electrode potential and regenerated the NH4CI by dissolving the Zn(NH3)2CI2 formed earlier in the discharge. This is the flIst reported observation of the regeneration of NH4CI caused by hetaerolite formation. In zinc chloride electrolyte, the discharge product appeared to be 2ZnC12.5Zn(OH)2.H2O and not ZnCI2.4Zn(OH)2.H20 as previously reported. An interruption technique has been used to study cells undergoing continuous discharges. The reverse reaction rate was negligible during the anodic zinc dissolution and no significant activation overpotential was observed for the manganese dioxide electrode. During these discharges in Leclanché electrolyte, the NH4CI concentration decreased at the zinc electrode interface reducing the activation overpotential and increasing the concentration overpotential. When the NH4CI concentration reached zero at the interface, the concentration proflle (moving boundary) moved toward the cathode. In the zinc chloride electrolyte, the ZnCl2 concentration at the negative electrode increased proportionally with the square root of the time on load until the diffusion layer intercepted the positive electrode. The Mn02 electrode potentials measured against a reference electrode, the Luggin capillary of which was inserted inside the cathode, were very similar for electrodeposited manganese dioxide (EMD) and CMDs throughout the discharge in ZnCl2 but higher for EMD than for a CMD in the Leclanché electrolyte. Towards the end of the discharge in both electrolytes, a large (70-100 mV) diffusion potential was generated in the separator region causing the cell voltage to decrease rapidly. This is the first time that this phenomenon has been reported. The main difference between the various cells in ZnCl2 electrolyte was in the magnitude of this diffusion potential which was significantly decreased by increase of volume of electrolyte in the cell. Although the cells containing EMD lasted longer than those containing CMD on continuous discharges. the specific performances (F mol-1) were very similar for all the materials. The non-uniform reduction rate distribution in the positive electrode has been calculated on the basis of measured potential differences within the mix using a new model of the electrode. The Mn02 potential-composition relationship conformed to the equation derived by Tye for all the dioxides at low degrees of reduction. Beyond about MnOOH0.4 the potential of EMD followed the equation derived assuming independent mobility of inserted protons and electrons while the potential of CMDs suggested permanent association of the inserted species. This difference, which was observed after both chemical and electrochemical reduction, is a new finding.