Single-site pyrrolic-nitrogen-doped sp2-hybridized carbon materials and their pseudocapacitance

Abstract

Integrating nitrogen species into sp 2-hybridized carbon materials has proved an efficient means to improve their electrochemical performance. Nevertheless, an inevitable mixture of nitrogen species in carbon materials, due to the uncontrolled conversion among different nitrogen configurations involved in synthesizing nitrogen-doped carbon materials, largely retards the precise identification of electrochemically active nitrogen configurations for specific reactions. Here, we report the preparation of single pyrrolic N-doped carbon materials (SPNCMs) with a tunable nitrogen content from 0 to 4.22 at.% based on a strategy of low-temperature dehalogenation-induced and subsequent alkaline-activated pyrolysis of 3-halogenated phenol-3-aminophenol-formaldehyde (X-APF) co-condensed resins. Additionally, considering that the pseudocapacitance of SPNCMs is positively dependent on the pyrrolic nitrogen content, it could be inferred that pyrrolic nitrogen species are highly active pseudocapacitive sites for nitrogen-doped carbon materials. This work gives an ideal model for understanding the contribution of pyrrolic nitrogen species in N-doped carbon materials.

Graphitic nitrogen in carbon catalysts is important for the reduction of nitrite as revealed by naturally abundant15N NMR spectroscopy

“…ssNMR. Magic-angle-spinning (MAS) NMR experiments were performed at a magnetic field of 14.1 T (Larmor frequencies of 600.12, 150.92, and 60.83 MHz for 1 H, 13 C, and 15 N, respectively) using a Bruker Avance-III spectrometer with a wide-bore magnet. The 1 H MAS NMR spectrum was acquired using a 1.3 mm probehead and a 60 kHz MAS rate.…”

“…49,60,61 Such a pulse sequence was applied to prevent proton background signals and to achieve a flat baseline in the acquired 1 H MAS NMR spectrum. All pulses operated at a nutation frequency of 200 kHz, and 64 signal transients were acquired using a relaxation delay of 5 s. Directlyexcited 13 C and 15 N MAS NMR spectra were recorded using a 7 mm probehead and a 7 kHz MAS rate. The 13 C acquisition involved a single-pulse (Bloch-decay) protocol with a 90°excitation pulse of 4.0 µs, and 16 384 signal transients were collected using a relaxation delay of 20 s. The 13 C NMR chemical shifts were referenced with respect to TMS.…”

“…Such ring current effects may be responsible for abnormal proton chemical shifts in aromatic systems. 69 The directly-excited 13 C MAS NMR spectrum of N-doped C (NH 3 ) is depicted in Fig. 2b.…”

“…We note that the 15 N MAS NMR spectrum of Fig. 2c might not be quantitative due to the short relaxation interval employed, however, the choice of such compromise is supported by the relatively fast 13 C T 1 relaxation indicating the presence of paramagnetic centers (Fig. S4 †).…”

“…[1][2][3][4][5][6][7][8] These considerations are important for the synthesis of metal-free and carbon-based catalysts, where the activity stems from the enhanced exposure of the structural features being catalytically most active. [9][10][11][12][13][14][15][16][17] The absence of metal-containing entities has intensified the consideration of metal-free carbon catalysts as "green" catalysts. 18,19 The physical and chemical properties of carbon catalysts can be altered by modifying their surface states with carbon-oxygen bonds, 20,21 which in turn modifies their catalytic activity.…”