Preparation and properties of Raney nickel electrodes on Ni-Zn base for H₂ and O₂ evolution from alkaline solutions Part II: Leaching (activation) of the Ni-Zn electrodeposits in concentrated KOH solutions and H₂ and O₂ overvoltage on activated Ni-Zn Raney electrodes

Abstract

The partial dissolution of zinc from electrodeposited Ni-Zn alloys (withX ⁰_Zn =22–87.3 mol %) was studied, in cold and nearly boiling 10m KOH. It was found that alloys withX ⁰_Zn ≤22 mol % are not dissolved at all. The dissolved zinc fraction,A, increased rapidly with further increase in zinc content and after having passed a maximum withA=82–90% atX ⁰_Zn =55–58 mol % and a sharp minimum withA=52–65% atX ⁰_Zn =65–69 mol %, it asymptotically approached toA → 100% atX ⁰_Zn → 100 mol %. The discontinuous dependence ofA againstX ⁰_Zn may be explained by differences in the crystallographic composition of the alloy deposits. Alloys withX ⁰_Zn <50–60 mol % can be allocated to solid solutions of zinc in the Ni matrix (α-phase); the range of 50–60<X ⁰_Zn <70–80 mol % corresponds to the coexistence of α+γ phases. The pure γ-phase exists within a narrow range atX ⁰_Zn =75–80 mol %. No zinc dissolution from Ni-Zn alloys withX ⁰_Zn ≤22 mol % was explained by extremely low zinc activities in dilute solid solutions of the α-phases shifting the Gibbs energy of the dissolution reaction to very low negative, or even to positive values. The dependence of the hydrogen and oxygen overvoltage atj=0.4 A cm⁻² in 10m, KOH at 100°C on the original zinc contentX ⁰_Zn showed, in both cases, a clear minimum atX ⁰_Zn =75–78 mol %. This points to a practically pure γ-phase in the original Ni-Zn alloy with an approximate composition NiZn₃.