Spontaneous Delithiation under <i>Operando</i> Condition Triggers Formation of an Amorphous Active Layer in Spinel Cobalt Oxides Electrocatalyst toward Oxygen Evolution

Abstract

Understanding the dynamic surface self-reconstruction of transition metal oxides (TMOs) under operando condition is the key for the rational design of oxygen evolution reaction (OER) electrocatalysts. Herein, we aimed toward spinel Li2Co2O4, as a desirable system because of the pure Co3+ ions in the as-prepared sample, to clarify the origin of its enhanced activity during in situ evolution. It was discovered that spontaneous extraction of lithium occurs upon the electrochemical condition to generate an amorphous active layer Li2–xCo2O4−δ(OH)δ (Co3+/Co4+), resulting in a progressively increased OER reactivity. Additionally, operando X-ray spectroscopy combined with ex situ techniques unraveled that the appearance of Co4+ and oxidized oxygen ions (oxygen sites with electronic hole) is tightly accompanied by this surface reconstruction. Finally, density functional theory calculations further suggested a possible reconstruction pathway and addressed the importance of oxidized oxygen ions that are triggered by Co4+ ions. These new insights are helpful for understanding why high activity occurs in the system with Co4+ ions and thereby developing efficient OER electrocatalysts.