Redox-Mediated ORR and OER Reactions: Redox Flow Lithium Oxygen Batteries Enabled with a Pair of Soluble Redox Catalysts

We demonstrate a redox flow lithium oxygen battery (RFLOB) using a pair of soluble redox catalysts, 2,5-di-tert-butyl-p-benzoquinone (DTBBQ) and tris{4-[2-(2-methoxyethoxy)ethoxy]phenyl}amine (TMPPA). The catalytic effects of DTBBQ on the oxygen reduction reaction (ORR) and TMPPA on the oxygen evolution reaction (OER) were investigated and unambiguously substantiated with various electrochemical, morphological, and spectroscopic characterization methods. It is observed that, upon discharging, oxygen is rapidly reduced by DTBBQ•– and Li2O2 is formed in the presence of Li+; upon charging, Li2O2 is oxidized by TMPPA•+, releasing oxygen. Such redox-mediated ORR and OER reactions enable the formation and oxidation of Li2O2 in a separate gas diffusion tank (GDT) other than on the cathode of the cell, thus obviating surface passivation and pore clogging of the electrode. The cell presents a high energy density with DTBBQ as the ORR redox catalyst and good rechargeability when paired with TMPPA as the OER redox catalyst. While the robustness and redox potential of two molecules are to be further optimized, the RFLOB demonstrated in this study provides an intriguing means for high-density and large-scale energy storage.