Fig. 2: Morphological and electrochemical performance of ordered nitrogen-doped carbon nanotube array on mesoporous carbon (N-CNTs@MC) positive electrode.

a The schematic of the structure of N-CNTs@MC (i) and SEM image of the N-CNTs@MC film (ii). b Optical images of pressed flexible N-CNTs@MC films (i) with different sizes and shapes (ii, iii). c The morphology characterization of CNTs of the N-CNTs@MC: the TEM image (i) and the enlarged TEM image (ii) of a random CNT. d CV curves at scan rates of 10, 20, 50, 100, and 200 mV s⁻¹. e The CV curve at 2 mV s^-1 and the relative contributions of pseudocapacitance (green area) and double layer capacitance (red area). f EIS plots of potassium-ion based half-cell with N-CNT@MC (red) and MC positive electrodes (black) g The corresponding relationship between Z’ and ω^−1/2 for N-CNT@MC (red) and MC positive electrodes (black). h Rate and CE performance of N-CNT@MC positive electrode in potassium-ion based half-cell. i. The cycle stability of N-CNT@MC positive electrode in potassium-ion based half-cell. j Charge-Diff of PF₆− ion on N-CNTs based on DFT calculations.