Influence of Alkali Metal Cations on the Rate of Oxygen Evolution from a Mixed Mg−LiCoO2 Oxide

Abstract

O2 evolution from a mixed Mg−LiCoO2 oxide is investigated in base solutions of alkali metal cations (0.1−2 M LiOH, KOH, or CsOH) without added supporting electrolytes. Tafel slopes range 30−50 mV/decade, scarcely related to the used alkali. However, Tafel lines are displaced to more positive potentials in passing from CsOH to KOH and LiOH at the same concentration, and moreover, reaction orders with respect to OH- decrease from ∼3 to ∼1 along the same alkali sequence with fractional rather than integer values. The reaction mechanism is examined on the assumption that Temkin-type adsorption conditions apply to the many reacting intermediates possibly involved. Analytical equations representing Tafel slope and reaction order in Temkin conditions are derived and reported for many frequently adopted O2 evolution pathways, most of them for the first time. From these equations, the experimental behavior is reconciled with a constant mechanism (Kobussen's path) in which the rate-determining step varies depending on the alkali cation, shifting forward in the sequence of elementary steps from early positions in CsOH and KOH to the last one in which molecular O2 is released in LiOH. Cation interactions with reacting surface sites apparently intervene to modify the reaction activation free energy profile, stronger for Li+ than K+ and Cs+. Structural relations with Li+ (and Mg2+) sites in the oxide lattice may be involved. However, the interaction sequence, Li+ > K+ > Cs+, is similar to that observed at the interface of many structurally and chemically unrelated oxides suspended in water and is attributed to entropic contributions from interactions of water molecules bonded in the hydration ion cosphere and at the oxide−solution interface.