In situ growing catalytic sites on 3D carbon fiber paper as self-standing bifunctional air electrodes for air-based flow batteries

Noble metal-free bifunctional catalysts are highly desirable for both oxygen reduction and evolution reactions in air-based flow batteries, but remain a big challenge. Herein, micropores dominated nitrogen and sulfur codoped carbon (NSC) film is directly growing on conductive carbon fiber paper (CFP) as self-standing air electrode through the electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole and followed pyrolysis process. NSC film provides abundant active sites, while conductive CFP with 3D macro-networks offers affluent channels for electron and mass transfer. Benefit from the strong interaction between NSC and CFP, the resultant CFP@NSC displays highly catalytic activity for both oxygen reduction (comparable half-wave potential to commercial Pt/C) and oxygen evolution (lower overpotential than commercial IrO2), along with robust stability. As a proof of practical usage, zinc air flow battery with CFP@NSC displays lower charge voltage and higher discharge voltage, compared with commercial Pt/C and IrO2 coated CFP electrode. Importantly, zinc air flow battery also exhibits excellent cycling stability including nearly 100% retention of coulombic efficiency and energy efficiency over 3400 min, representing the most efficient bifunctional oxygen electrode reported so far.