Engineering single MnN₄ atomic active sites on polydopamine-modified helical carbon tubes towards efficient oxygen reduction

Among various earth-abundant and noble metal-free catalysts for oxygen reduction reaction (ORR), Mn and N co-doped carbon (Mn-N-C) is highly desirable and promising, which, however, suffer from the limited amount of active sites largely abating its ORR catalytic performance. Herein we demonstrate an effective strategy to elevate the ORR active site density by designing a helical graphitized carbon tubes to highly disperse the single Mn atomic sites coordinated with nitrogen. The obtained polydopamine-modified helical MnNC-PDA-700 catalyst shows excellent ORR electrocatalytic performance with a half-wave potential of 0.87 V and extra-high Zn-air battery power density of 122.7 mW cm⁻², which are comparable to and even higher than those of Pt/C. Such an excellent electrocatalytic performance is attributed to the much enhanced amount of MnN₄ active sites created on the helical graphitized carbon tubes owing to high surface area helical structure of the carbon tubes and the strong bonding of polydopamine molecules onto the tubes. The density functional theory (DFT) calculations further confirm that the MnN₄ sites are the origin of the superior ORR activity via a 4e⁻ pathway in alkaline media.