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

“…The trend of the potential base-lines for the series of anodes, relating to electrochemical reactivity, was very similar to that previously measured with other anode geometries [39,40]; graphite had the highest potential (correspondingly lowest reactivity), Anodes 1-4 generally showed a small decrease in base-line potential and Anode 5 had the lowest potential. However, although changes in reactivity may be related to anode isotropy or metal impurity content [39,40], 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.…”

“…The contributions of δR s and η h towards total potential change for the anodes are shown in Figure 18. The relative contributions of δR s and η h on the potential increase were similar on all anodes, as expected based on the linear current-voltage characteristics observed for these anodes [40].…”

“…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.…”

“…Figure 7a compares the measured voltage-time characteristics of graphite anodes with horizontal, vertical [40] or 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.…”

“…large potential oscillations were observed during electrolysis. This was verified by comparing the horizontal graphite anode against graphite anodes with vertical sides and a graphite rod, as described in [40] and [39] respectively. Additionally, horizontal graphite anodes with diameter 6, 10 or 14 mm were tested in order to verify that the selected geometry did not affect the results.…”