Double layer capacity of graphite in cryolite-alumina melts and surface area changes by electrolyte consumption of graphite and baked carbon

Abstract

The double layer (d.l.) capacity of pyrolytic graphite in cryolite-alumina melts at 1010°C was found to exhibit a minimum of 20μF cm−2 at 0·9 V positive to the aluminium electrode. The d.l. capacity attained a plateau of ∼60μF cm−2 at 1·1–1·4 V, while it rose steeply at potentials below 0·7 V. During electrolytic consumption involving CO2 evolution the d.l. capacity of pyrolytic graphite remained unaffected, while that of baked carbon was changed, reflecting changes in surface area. At low current densities (cds) the surface area increased substantially and the surface was noticeably roughened, while the opposite behaviour was observed at above 2.5 A cm−2.