Intermetallic compounds as oxygen evolving anodes for metal electrowinning: Electrochemical dealloying and effects of scale in practical electrochemistry

Electrochemical dealloying has recently been highlighted as a promising technique for developing active electrodes for water electrosplitting. Intermetallic compounds of a base metal with passive film-forming elements are effective oxygen evolution anodes for metal electrowinning. Cobalt-manganese-silicon alloys and titanium-nickel intermetallics are demonstrated as examples related to the proposed paradigm of electrochemical dealloying. These systems illustrate the effects of scale in practical electrochemistry, where 'scale' has multiple meanings: the transition to practice, the formation of surface deposits, and the evolution of the interface in extended use. In the case of cobalt-silicon-manganese alloys, a highly porous surface layer is developed, within which the active phase consists of 'nanostrands' of cobalt metal in 'nanoconfinement' within a slowly-dissolving silicide matrix. Within this confined environment, a saturated solution of cobalt salt causes a salt film over the cobalt metal, under which an oxygen-evolving cobalt anodic oxide is stabilized. For TiNi, a nickel-rich surface forms over a thin titanium anodic oxide, where oxygen evolution occurs by field-assisted electron tunneling to the surface nickel titanium oxide states. The research indicates a useful composition range between 51 and 55 wt% Ni that balances ductility against dissolution rate.