Enhancement Mechanism of Electrochemical Capacitance in Nitrogen-/Boron-Doped Carbons with Uniform Straight Nanochannels

Abstract

Anodic aluminum oxide (AAO) with uniform straight nanochannels was completely coated with pure, N-doped, or B-doped carbon layer. Their electric double layer capacitances are measured in aqueous (1 M sulfuric acid) and organic (1 M Et4NBF4/polypropylene carbonate) electrolyte solutions in order to investigate the capacitance enhancement mechanisms caused by N- or B-doping. Since the three types of carbon-coated AAOs (pure, N-doped, or B-doped) have exactly the same pore structure, the observed capacitance enhancement was ascribable to only the following factors: (i) better wettability, (ii) the decrease of equivalent series resistance, (iii) the contribution of space-charge-layer capacitance, and (iv) the occurrence of pseudocapacitance. From the measurements of the wettability and the electrical resistance of the coated AAOs together with the electrochemical investigation (the cyclic voltammetry, the galvanostatic charge/discharge cycling, and the impedance analysis), it is concluded that the pseudocapacitance through faradic charge transfer (factor iv) is the most important factor to enhance the capacitance by N- or B-doping. This can be applied to not only the present carbon-coated AAOs but also any other porous carbons.