Intrinsic Carbon Defects in Nitrogen and Sulfur Doped Porous Carbon Nanotubes Accelerate Oxygen Reduction and Sulfur Reduction for Electrochemical Energy Conversion and Storage

Defects and morphology engineering is a serviceable strategy to boost the electrochemical energy conversion and storage performance of carbon-based materials. In this study, nitrogen/sulfur codoped carbon nanotubes (NS-CNTs) were first obtained via the pyrolysis of presynthesized polyaniline nanotubes with micelles composed of methyl orange and ferric chloride acting as the soft template. Furthermore, intrinsic carbon defects and mesopores were introduced to obtain etched NS-CNTs (ENS-CNTs) composites by ammonia etching. The rational combination of intrinsic/extrinsic defects and porous nanotube morphology features is beneficial to the oxygen reduction reaction (ORR) and sulfur reduction reaction (SRR) performances of the ENS-CNTs electrode. The coexistence of intrinsic carbon defects and extrinsic N/S dopants can create massive catalytically active sites for electrochemical processes, while the porous one-dimensional nanotube-like carbon framework is responsible for accessibility of catalytic active sites, species hosting, electrical conductivity, mass transport, and stability. Consequently, the ENS-CNTs-30 (where 30 represents the corresponding etching time in minutes) electrode for ORR displayed a high half-wave potential of 859 mV vs RHE, a diffusion limiting current density of 6.65 mA cm–2, admirable stability, and methanol tolerance. The solid Zn–air battery (ZAB) assembled with ENS-CNTs-30 as the active material for the air cathode revealed remarkable power density (137 mW cm–2) and specific capacity (1467.4 mAh g–1Zn). Meanwhile, the ENS-CNTs-30 electrode for SRR also demonstrated ameliorative lithium–polysulfide (LiPS) trapping capability and Li2S deposition kinetics. The lithium–sulfur battery (LSB) with ENS-CNTs-30 as sulfur host material unfolded initial capacities of 1100 and 883 mAh g–1 at 0.2 and 2 C, respectively, and a capacity retention ratio of 82.0% after 200 cycles at 0.2 C. This work provides a feasible strategy for defects and morphology engineering of multifunctional carbon-based catalysts in electrochemical energy conversion and storage fields.