Electrocrystallisation studies on cadmium

Abstract

An investigation into the electrocrystallisation deposition of cadmium has been carried out. The work presented here was followed with the aim of increasing the understanding of cadmium electrodeposition processes, with relevance to general metal deposition. In particular the cadmium deposition behaviour that leads to dendritic growth has been studied. This is particularly relevant in the nickel/cadmium battery industry, where cadmium dendritic growth is a frequently attributed cause for cell failure. The investigative approach has been from both the theoretical and direct experimental sides. The main experimental techniques involved, a. c. impedance, electron microscopy, rotating disc and potentiostatic studies. The usage of a. c. impedance to study double layer capacity changes, proved to be a very accurate method of detecting dendritic growth (through surface area changes). From a more theoretical angle, extensive use has been made of computer simulation, in order to follow the initial stages of deposition of hexagonal close-packed atoms. Experiments involving cadmium dissolved as the species Cd(OH)42 , in 10.00M KOH (saturation limit 0.00035M), have revealed that only grainy cadmium is deposited at timescales of <20 Hours (-400mV overpotential). Continued deposition in 10.00M KOH + 0.00028M Cd(II) at -300mV overpotential, has revealed dendrites of length 30μm can be grown after 6 days. This deposition behaviour remains largely unaffected by changes in surface roughness, electrode pretreatment and the presence of oxygen. However, cadmium deposition behaviour is highly dependent on the presence of small quantities of cadmium salts in suspension. As little as 1x 10-6M of Cd(OH)2 in suspension will dramatically lower the time required for dendrite deposition (25μm dendrites can be grown within 6 hours). This finding is of importance to the battery industry, since the negative plate in the nickel cadmium cell, consists of powdered Cd(OH)2 contained within a nickel-plated steel support. In the absence of suspension, cadmium dendritic growth was found to follow along conventional lines, such that the growth time required for a particular dendrite is given by; At = ln[h/ho ]prb MDCQ Observed dendritic growth times quite closely fitted the calculated values. Studies involving deposition from acidic cadmium sulphate (0.001 - 0.1M) solutions, revealed a similar agreement with the calculated dendritic growth times. However, these times are considerably lower than for the alkaline solutions, primarily due to the concentration increase in Cd(II). In 0.1M CdSO4 + 0.5M H2SO4 the growth time for a 25fLm dendrite is reduced to -60 s. Applying an adapted "Monte Carlo' method, computer simulation of multilayer electrodeposition onto perfect hexagonal close-packed surfaces has been simulated. it is shown that under diffusion-independent conditions the shape of the computer-generated current/time curve is dependent on the size of the lattice used and the trueness" of the random site selection. When diffusion is allowed to become important in the simulation, the deposit shows a dramatic change in morphology, with some clear parallels to observed deposits of cadmium. It is shown that even in the absence of surface defects and impurities, grainy microcrystallites can be simulated under the linear diffusion conditions. This implies that, contrary to the established belief, surface abnormalities are not necessary precursors for dendritic growth.