Understanding Anode Overpotential

Abstract

In aluminium electrolysis cells the anodic process is associated with a substantial overpotential. Industrial carbon anodes are produced from a blend of coke materials, but the effect of coke type on anodic overpotential has not been well studied. In this work, lab-scale anodes were fabricated from single cokes and electrochemical methods were subsequently used to determine the overpotential trend of the anode materials. Attempts were then made to explain these trends in terms of both the physical and chemical characteristics of the baked anodes themselves and their raw materials. Routine coke and anode characterisation methods were used to measure properties such as impurity concentrations and reactivity (to air and CO 2 ), while non-routine characterisation methods were applied to study other surface and structural properties. It was found that the overpotential trend of the anodes correlated well with many of the properties studied, and explanations for these observed correlations are suggested. These findings offer exciting possibilities for reducing the energy demand of the anodic process.

Bubble Evolution and Anode Surface Properties in Aluminium Electrolysis

“…It is also apparent that the more isotropic anodes (Anodes 3-5) had lower potential oscillation, and therefore lower bubble-related voltage losses, associated with bubble formation and release. The trend of the potential base-lines for the series of anodes was similar to that previously measured with other anode geometries; 39,40 graphite had the highest potential, Anodes 1-4 generally showed a small decrease in base-line potential and Anode 5 had the lowest potential. Changes in potential may be related to changes in anode reactivity due to varying anode properties.…”

“…Therefore, it may be more likely that variation in wetting is due to variation in isotropy or possibly impurity concentrations, as also suggested by Qiu et al 47 The wetting angle of graphite with cryolite electrolyte in this study (∼100 • ) was slightly lower than that recorded in other studies (120-130 •16 ), possibly due to material or electrolyte composition differences. Electrochemical measurements of bubble evolution.-Figure 7a compares the measured voltage-time characteristics of graphite anodes with horizontal, vertical 40 or mixed (rod) 39 geometry. When a vertical anode was used, only very small oscillations were observed, possibly due to the evolution of small bubbles from the surface.…”

“…Changes in potential may be related to changes in anode reactivity due to varying anode properties. 39,40 However, as this paper focused on measuring the change in anode potential with bubble release (rather than absolute potential), potentials were not IR corrected and therefore cannot give information on reactivity.…”

“…This was verified by comparing the horizontal graphite anode against graphite anodes with vertical sides and a graphite rod, as described in Ref. 40 and Ref. 39 respectively.…”

“…The anodes used in this study comprised of pilot scale anodes produced by Norsk Hydro ASA from five single source cokes that varied in isotropy (particle size 2-0 mm), as described previously. 39,40 Anodes 1-4 were made from petroleum cokes and Anode 5 from a coal tar pitch based coke. Aside from coke type, all other production parameters were constant.…”