Electrochemical Cycling Induced Amorphization of Cobalt (II, III) Oxide (Co3O4) for High Surface Area Electrode of Water Oxidation

Abstract

Electrocatalyst’s activity critically depends on the chemical coordination structure around the active sites. Amorphous materials are having different local short-range structure than their counter crystalline materials which can be tuned by the synthesis process. Traditional synthesis of amorphous materials, involving multi-temperature quench processing, is unsuitable as it results in less porosity and surface area. In this context, room temperature synthesis of high surface area amorphous materials with high activity are desirable. Here we have demonstrated the synthesis of high surface area amorphous Co3O4 at room temperature via electrochemical ion intercalation/deintercalation and surface oxidation/reduction cycles and their performance in electrocatalytic oxygen evolution reaction (OER). In Li-ion intercalation/deintercalation (Li/D-Li) cycles in Co3O4 expansion and contraction of the structure induces amorphization of Co3O4 by the pulverization of crystal structure in non-aqueous medium. While during the fast surface oxidation/reduction (Ox/Red) the crystalline nuclei formation and its growth in oxide state is hindered thereby leading to formation of metastable amorphous structure in aqueous medium. The OER specific activity for Li/D-Li-Co3O4 is ~35 times, which is superior to Ox/Red- induced amorphization which is ~25 times higher than their crystalline Co3O4. The superior OER metrics with Li/D-Li-Co3O4 is attributed to Li-ion intercalations inside Co3O4 structure which generates higher bulk-oxygen vacancies upon Li/D-Li- leading to higher conductivity and reduction in overall charge-transport resistance through electrocatalyst. On the other hand, Ox/Red- induced amorphization is restricted to surface or near-surface only with formation of small amount of metallic Co which hampers in increase of active sites for OER. However, in general both amorphization process leads to increase in Co (II) sites which is responsible for increased OER activity over crystalline Co3O4. Additionally, the above study demonstrates the utility of multicycle lithiation/de-lithiation over surface oxidation/reduction technique for obtaining amorphous OER electrocatalysts