Electrochemical Cycling-Induced Amorphization of Cobalt(II,III) Oxide for Stable High Surface Area Oxygen Evolution Electrocatalysts

Abstract

The activity of electrocatalysts critically depends on the chemical coordination around the active sites. Amorphous materials have short-range atomic ordering while their crystalline counterparts have both short and long-range ordering. Traditional synthesis of amorphous materials, involving quenching from high temperatures is unsuitable as it results in less porosity and surface area. In this context, room-temperature syntheses of high surface area amorphous materials with high activity are desirable. Here, we contrast two electrochemical synthesis procedures for generating high surface area amorphous Co3O4 at room temperature via electrochemical ion intercalation/deintercalation and surface oxidation/reduction cycles and evaluate their performance for electrocatalytic oxygen evolution reaction (OER). In the first approach, Li-ion is used for the intercalation/deintercalation (Li/D-Li) cycles in Co3O4, which leads to expansion and contraction of structure, inducing amorphization of Co3O4 by the pulverization of crystal structure in non-aqueous medium. In the second approach, rapid electrochemical surface oxidation/reduction (Ox/Red) of Co3O4 in the aqueous medium leads to the formation of a metastable amorphous structure. The OER specific activity (activity per unit electrochemical surface area) for Li/D-Li-Co3O4 is similar to 3.5 times and Ox/Red- Co3O4 induced amorphization is similar to 2.5 times higher than their crystalline Co3O4. The superior OER metrics of both the room-temperature amorphization techniques are rationalized via the increase in the ratio of Co2+/Co3+ obtained from the Co-2p XPS spectra. Further, the decrease in overall polarization resistance per site for the OER reaction for both amorphous samples were analyzed from the Tafel plot and electrochemical impedance spectroscopy (EIS). In Li/D-Li-Co3O4, the Li-ion intercalation in bulk Co3O4 structure generates higher bulk-oxygen vacancies leading to higher conductivity and reduction in overall charge-transport resistance for electrocatalyst. On the other hand, Ox/Red- induced amorphization is restricted to the surface or near-surface only with the formation of a small amount of metallic Co which hampers the OER.