Thermodynamic studies of cobalt and cadmium additions to nickel hydroxide as material for positive electrodes

Nickel oxide is used as the positive electrode in nickel–cadmium and nickel–metal-hydride batteries. The physical and the electrochemical effects due to the addition of cobalt and cadmium to nickel hydroxide powders are investigated. X-ray diffraction analysis shows that the d001 value (interlayer distance in the nickel hydroxide) decreases with increase in cobalt content, while the d100 value increases with increase in cadmium content. The open-circuit potential in discharge follows an S-shaped curve in 3 M KOH electrolyte. The results are discussed with respect to thermodynamic theory. The experimentally obtained OCP variation is in good agreement with calculated data using thermodynamic theory with one fitting parameter, φ/RT; φ is an interaction energy parameter (J mol−1) and RT is the product of the gas constant and absolute temperature. The φ/RT and E0 (standard electrode potential) values are found to decrease with increase in cobalt content, while the addition of cadmium has little effect on the φ/RT and E0 values. A difference between the OCP data and the calculated value occurs at the end of discharge; this difference tends to increase with increase in cobalt content.