Hyperpolarization at gas evolving electrodes—II. Hall/heroult electrolysis

✍ Scribed by James A. Leistra; Paul J. Sides

• Book ID: 103062729
• Publisher: Elsevier Science
• Year: 1988
• Tongue: English
• Weight: 529 KB
• Volume: 33
• Category: Article
• ISSN: 0013-4686
• DOI: 10.1016/0013-4686(88)85011-4

✦ Synopsis

Hyperpolarization, a voltage component at gas evolving electrodes due to the masking of the electrode by bubbles, was investigated in aluminum electrolysis at three types of anodes : graphite, baked carbon, and vitreous carbon . Rotation of a cylindrical electrode separated the hyperpolarization from the other components of the voltage balance . Hyperpolarization was measured as a function of rotation rate and current density. The average amount of hyperpolarization at a current density of 1 Acm -r was approximately 20 mV. A substantial hyperpolarization of 60 mV appeared at current densities between I and 10 mA crtr -', a result of large slowly growing bubbles on the electrode . Laboratory measurements of the kinetics of gas evolving electrode reactions at low current densities should include motion to avoid hyperpolarization .

NOMENCLATURE

A

superficial electrode area, cm' A s average electrode area free of bubbles, cm°b Tafel slope, mV AID total potential between an electrode and a reference electrode of the same kind some distance from the electrode, mV An potential difference obtained by subtracting ohmiccorrected measured potential during rotation from ohmic-corrected measured potential without rotation, mV concentration overpotential due to concentration polarization of the reacting ion, mV the back emf due to supersaturation of the electrolyte with dissolved product gas, mV amount of voltage due to an effectively higher current density that must exist because the bubbles cause a nonuniform current distribution on the electrode surface, mV the surface overpotential due to the electrode reaction if no bubbles were present, mV