Abstract
An in situ electrochemical electron paramagnetic resonance (EPR) spectroscopic study of N-doped reduced graphene oxide (N-rGO) is reported with the aim of understanding the properties of this material when employed as an electrical double-layer capacitor. N-rGO shows a capacitance of 100 F g−1 in 6 M KOH, which is twice that found for reduced graphene oxide (rGO). The temperature dependence of the rGO EPR signal revealed two different components: a narrow component, following the Curie law, was related to defects; and a broad curve with a stronger Pauli law component was attributed to the spin interaction between mobile electrons and localised π electrons trapped at a more extended aromatic structure. The N-rGO sample presented broader EPR signals, indicative of additional contributions to the resonance width. In situ EPR electrochemical spectroscopy was applied to both samples to relate changes in unpaired electron density to the enhanced capacitance. The narrow and broad components increased and diminished reversibly with potential. The potential-dependent narrow feature was related to the generated radical species from corresponding functional groups: e.g. O- and N-centred radicals. Improved capacitance seen for the N-modified basal graphene planes can be accordingly suggested to underlie the enhanced capacitance of N-rGO in basic electrolytes.
Highlights
Electrical energy storage systems include the secondary battery and the supercapacitor
X-ray photoelectron spectroscopy (XPS) was used to confirm the distribution of elements in both reduced graphene oxide (rGO) and N-doped reduced graphene oxide (N-rGO) samples. rGO and N-rGO were prepared via the hydrothermal method by using different reductants, as described above
electron paramagnetic resonance (EPR) measurements identified a composite EPR spectrum for both samples, consisting of two distinct resonance lines: a narrow component arising mainly from localised spins at defects and edge states, and a broad component with strongly temperature dependent linewidth, indicative of conduction electron spins coupled with localised p electrons in more extended sp2 domains
Summary
Electrical energy storage systems include the secondary battery and the supercapacitor. Secondary batteries (such as lithium and sodium ion batteries) have a higher energy density, while supercapacitors have garnered much attention because of their higher power density, long cycling ability and good rate capability [1e3]. Heteroatom doping and functionalization have been shown to increase the capacitance of carbon nanomaterials. Doped graphene (NG) has been shown to have a higher gravimetric capacitance, of up to 300 F gÀ1, as well as a high rate capability [7e10]. This performance enhancement was attributed to increased conductivity (graphitic-N) and pseudo-capacitive behaviour (pyridinic-N and pyrrolic-N functional groups) [7e11]. Density functional theory and classical molecular dynamic studies provide further evidence that N doping increases the quantum capacitance near the Fermi level, detailed experimental studies of the pseudo-capacitive process are still scarce [12e15]
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