Effect of Nitrogen Concentration on Capacitance, Density of States, Electronic Conductivity, and Morphology of N-Doped Carbon Nanotube Electrodes

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Abstract

Heteroatom doping (e.g., boron and nitrogen) of graphitic carbon lattices affects various physicochemical properties of sp2 carbon materials. The influence of nitrogen doping in carbon nanotubes (N-CNTs) on their electrochemical and electrical properties such as the differential capacitance, density of states at the Fermi level (D(EF)), bulk conductivity, and work function is presented. Studies were performed on free-standing N-CNTs electrode mats to understand the intrinsic physicochemical properties of the material without relying on the secondary influence of another conductive support. N-Doping levels ranging from 0 to 7.4 atom % N were examined, and electrochemical impedance spectroscopy (EIS) was used to evaluate the differential capacitance and to estimate the effective density of states, D(EF). X-ray photoelectron spectroscopy (XPS) and Raman microscopy were used to assess the compositional and structural properties as a function of nitrogen doping. XPS N1s spectra show three principle types of nitrogen coordination (pyridinic, pyrrolic, and quaternary). Raman was used as diagnostic tool for estimating the amount of disorder by comparing D and G bands. A linear increase in the ratio of integrated D and G band intensities with nitrogen doping indicates that the amount of disorder and number of edge plane sites increase. Furthermore, D(EF) also increases with N doping and the amount of disorder and number of edge plane sites. UV photoelectron spectroscopy (UPS) was used to probe the valence band of N-CNTs in order to estimate the work function of the mats. The work function increased linearly from 4.1 to 4.5 eV for increasing N-doping levels. The bulk electrical conductivity of the N-CNT electrode mats appears to be junction dominated as shown by the relationship between the bulk conductivity and average N-CNT length within the mats determined using high-resolution scanning transmission electron microscopy (STEM).

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