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

Context & Scale This review aims to bridge the fields of inorganic molecular chemistry, electrocatalysis, lithium-ion batteries, and chemical physics of oxides by introducing a unifying concept linking the electronic structures and electrochemical properties of transition metal oxides and complexes. In this work, by reviewing broad literature on the redox behavior of a number of Ni, Co, Fe, and Mn (hydr-)oxides, Liintercalation compounds, and 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. organometallic complexes, we demonstrate the effect of the foreign metal substitution on their redox potentials and rationalize the trends by aligning the redox couples of metal complexes with themetal d and oxygen p bands of metal oxides on the absolute electron energy scale. The higher Lewis acidity of the metal substituent compared with parent metal leads to the anodic shift of the redox transition potential due to the inductive effect, which is also shown to correlate with oxygen reduction and evolution activity. This study opens up a new perspective for the design of transition metal compounds with tunable redox properties in order to control the thermodynamics and kinetics of the key Introduction Earth-abundant transition metal oxides are used extensively for storing charge in energy storage devices including Li-ion or Na-ion batteries and electrochemical capacitors, as well as for catalyzing key reactions involved in the chemical transformation of sustainable chemicals and fuels such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in fuel cells, electrolytic cells, metal-air batteries, and devices for solar fuels. In all of these applications, the reduction/oxidation (redox) of transition metal ions plays a critical role. For example, in Li-ion or Na-ion batteries, the charge/discharge (i.e., Li/Na de-intercalation/intercalation) process is accompanied by the redox of transition metal ions in bulk. On the other hand, in fuel cells or electrolytic cells, catalysis of ORR/OER involves the redox of transition metal oxides on or near the surface. Understanding the redox processes in metal oxides could provide a powerful tool for controlling the potentials at which charge/energy can be stored in batteries as well as their capacities, and for enhancing the efficiency of the energy conversion devices by improving the activity of OER/ORR catalysts. One effective way to adjust the redox process in transition metal oxides is through the substitution of metal ions, which is also a common strategy used to tune Li/Na intercalation voltages and to increase the OER/ORR catalytic activity. However, a universal and fundamental understanding of the foreign metal substitution effect to the redox process across these different applications is still missing.