Towards Elimination of the Anode Effect and Perfluorocarbon Emissions from Hall-Héroult Cells

Cyclic voltammetry, stepped-current chronopotentiometry, and steppedpotential chronoamperometry on stationary and rotating-disk carbon electrodes have shown that at potentials exceeding ~3.5 V vs Al/Al in a laboratory-scale Hall-Héroult cell the current drops off precipitously. Furthermore, if the potential on the anode falls below ~3.0 V vs Al/Al the current is quickly restored to values it held previously. The mechanism at work is speculated to be a highly resistive surface film whose formation is strictly potential-dependent. To demonstrate the relevance of these findings to industrial operations, controlled-current electrolysis was conducted in a laboratory-scale cell where it was found that the anode effect and, hence, PFC generation can be completely avoided by simply stepping down cell current in small increments, i.e., by several per cent, whenever cell voltage reaches a critical value.