Tuning Redox Transitions via Inductive Effect in Metal Oxides and Complexes, and Implications in Oxygen Electrocatalysis

The reduction and oxidation (redox) of transition metals allow storing charge/energy in Li-ion batteries and electrochemical capacitors and play an important role in catalysis of electrochemical reactions, such as oxygen reduction reaction (ORR) in fuel cells and metal-air batteries and oxygen evolution reaction (OER) in electrolytic cells. In this review, we present and discuss a universal origin of inductive effect associated with metal substitution in Ni, Co, Fe, Mn-based complexes, (hydr-)oxides, and lithium intercalation compounds by alignment of the electron levels of metal complex redox with partially filled metal d-state and oxygen p-state of oxides on the absolute energy scale. Increased redox potentials of metal complexes and oxides are shown to correlate with the increased electronegativity of the substituting metal, which results in the enhancement of the ORR/OER activity. Such observations provide new insights into potential strategies to optimize ORR/OER catalytic activity by tuning the redox properties of metal sites.