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 element are effective oxygen evolution anodes for metal electrowinning. Cobalt-silicon alloys and titanium-nickel intermetallics are demonstrated as examples and 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 practise; formation of surface deposits; and the evolution of the interface in extended use. In the case of cobalt-silicon alloys, the microstructure of the metal is critical. A highly porous surface layer is developed, within which the active phase is ‘nanostrands’ of cobalt metal in ‘nanoconfinement’ within a slowly-dissolving silicide matrix. Within the 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 stabilised. In the case of TiNi, a nickel-rich surface forms over a thin titanium anodic oxide. Oxygen evolution occurs by field-assisted electron tunnelling to the surface nickel titanium oxide states. Field-driven ion migration both leads to these active states and leads to a slow dissolution of the metal. There is a useful range of composition between 51 and 55 wt% Ni (46 – 50 at %) which balances ductility against dissolution rate: excess titanium leading to a continuous phase of Ti2Ni results in a very brittle material; separation of TiNi3 leads to more rapid dissolution. In practise however, intrusion of oxygen during casting of large plates segregated Ti as the oxide Ti4Ni2O leading to separation of TiNi3 . The more rapid dissolution of this phase, in association with formation of MnO2 from Mn salts present in plant electrolyte, led in association with oxidation of cobalt salts to deposition of an adherent surface scale under which the solution became strongly acidic. The accelerated anode dissolution led to an under-scale of TiO2 which further increased acidification. Impractically rapid destruction of the anode resulted. The work illustrates how mass-transport and microstructure in the evolving interphase between electrode and electrolyte can interact in subtle ways important for practical application.