Redox-Mediated Oxygen Electrochemistry and the Materials Aspects

Conspectus High-energy density, efficient, robust, and environmentally friendly electrical energy storage and conversion systems are crucial for ensuring the resilience of power grids and enabling effective use of intermittent renewable energy sources like solar and wind power. These systems have also been gaining more and more attention for applications in electric vehicles. Metal–air/O2 batteries, particularly zinc–air (aqueous) and lithium–air/O2 (nonaqueous) rechargeable batteries, are showing great promise as the next generation of energy storage systems due to their considerably higher energy density compared to the conventional lithium-ion batteries (1080–3500 vs 200–250 Wh/kg). In addition, there are ongoing studies of hydrogen economy exploring hydrogen gas as a carbon-free energy carrier. This encompasses various aspects such as hydrogen production and its conversion into electricity or other valuable chemical commodities through advanced systems, such as water splitting and fuel cells. However, these systems often suffer from large overpotentials upon operation due to the sluggish oxygen reduction and evolution reactions, requiring the use of catalysts. To address these issues, the introduction of redox mediators (RMs) via redox targeting (RT) reactions provides an elegant solution for improving the efficiency of energy storage and conversion systems. This Account consolidates our research efforts and other notable studies on the use of RMs as charge carriers in energy storage and conversion systems through RT reactions, with a primary focus on redox-mediated oxygen electrochemistry. We begin by presenting the concept and history of the RT reactions, from their initial form utilizing two RMs to the current iteration utilizing a single RM driven by the Nernstian potential difference between the RM and energy storage materials (EnSMs) during charging and discharging. We then provide a detailed review of the redox-mediated oxygen electrochemistry and its applications in both nonaqueous and aqueous energy storage and conversion systems. This includes an exploration of the mechanisms behind the redox-mediated oxygen evolution and reduction reactions (OER/ORR) and battery design approaches in lithium–air/O2 batteries to address issues related to low discharge capacity caused by the insulating nature of Li2O2. In aqueous systems, the redox-mediated oxygen electrochemistry is more extensively discussed, covering both 2-electron- and 4-electron-based rea