Quinone-Mediated Electrochemical O2 Reduction Accessing High Power Density with an Off-Electrode Co-N/C Catalyst

New methods for interconversion of electrical and chemical energy are crucial to addressing emerging challenges in renewable energy storage and conversion. Dissolved redox mediators provide an effective means to transport electrons (and protons) between an electrode and catalysts that are not in direct physical contact with an electrode. Mediators are widely implemented in biofuel cells, but such devices typically exhibit low power densities. Here, we report the development of a tetrasubstituted quinone mediator that exhibits excellent stability under strongly acidic conditions and supports electrochemical reduction of O2 at an off-electrode heterogeneous cobalt catalyst contained within a packed-bed reactor. Efficient oxidation of the hydroquinone by O2 in the reactor provides the basis for mediator regeneration, and an integrated “flow-cathode” fuel cell system supports high power densities. These results establish the combination of organic mediators with off-electrode heterogeneous catalysts as a versatile platform for development of new electrode-driven chemical redox transformations. [Display omitted] •Highly stable quinone designed and used as an electrochemical mediator•Hydroquinone paired with heterogeneous Co-N/C catalyst enables efficient O2 reduction•Flow-cathode system accesses power densities much higher than mediated biofuel cells•Strategy provides foundation for diverse electrode-driven redox transformations The increasing incorporation of renewable energy sources into the grid is motivating efforts to identify new strategies for the interconversion of electrical and chemical energy. Fuel cells, which convert chemical energy into electricity, will play an important role in this transformation. Herein, we report a new strategy for electrochemical O2 reduction in a fuel cell that uses an organic mediator to shuttle electrons and protons between the electrode and a heterogeneous catalyst that is contained within an off-electrode packed-bed reactor where O2 reduction occurs. Design and synthesis of a robust quinone mediator provided the basis for this advance. Further development of this fuel cell design has the potential to achieve higher power densities than current state-of-the-art non-PGM fuel cells. This strategy has potential use in various other electrode-driven chemical redox transformations. Dissolved organic redox mediators enable transport of electrons and protons between an electrode and off-electrode heterogeneous catalysts. A new