Protonated C3N4 Nanosheets for Enhanced Energy Storage in Symmetric Supercapacitors through Hydrochloric Acid Treatment

Abstract

Next-generation electrochemical energy storage materials are essential in delivering high power for long periods of time. Double-layer carbonaceous materials provide high power density with low energy density due to surface-controlled adsorption. This limitation can be overcome by developing a low-cost, more abundant material that delivers high energy and power density. Herein, we develop layered C₃N₄ as a sustainable charge storage material for supercapacitor applications. It was thermally polymerized using urea and then protonated with various acids to enhance its charge storage contribution by activating more reaction sites through the exfoliation of the C-N framework. The increased electron-rich nitrogen moieties in the C-N framework material lead to better electrolytic ion impregnation into the electrode, resulting in a 7-fold increase in charge storage compared to the pristine material and other acids. It was found that C₃N₄ treated with hydrochloric acid showed a very high capacitance of 761 F g^-1 at a current density of 20 A g^-1 and maintained 100% cyclic retention over 10,000 cycles in a three-electrode configuration, outperforming both the pristine material and other acids. A symmetric device was fabricated using a KOH/LiI gel-based electrolyte, exhibiting a maximum specific capacitance of 175 F g^-1 at a current density of 1 A g^-1. Additionally, the device showed remarkable power and energy density, reaching 600 W kg^-1 and 35 Wh kg^-1, with an exceptional cyclic stability of 60% even after 5000 cycles. This study provides an archetype to understand the underlying mechanism of acid protonation and paves the way to a metal-carbon-free environment.

Figure 1

X-ray diffraction patterns of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl.

Figure 2

Raman spectra of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl.

Figure 3

FTIR spectra of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl.

Figure 4

(a) CV profiles of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl at a scan rate of 100 mV s^–1. (b) Charge–discharge profiles of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl at a current density of 20 A g^–1. (c) Plot of the specific capacity vs current density. (d) Plot of the specific capacity vs cycle number at various current densities.

Figure 5

Trasatti analyses of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl. (a, c, e, g) Plots of the areal capacitance vs the reciprocal of the square root of the scan rate (ν^–1/2) for C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl, respectively. (b, d, f, h) Plots of the areal capacitance vs the square root of the scan rate (ν^1/2) for C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl, respectively. (i) Overall capacitance contributions of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl.

Figure 6

(a) Nyquist plots of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl. (b) Bode phase angle vs frequency plots of C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl. (c) Electrochemical cyclic stability as a function of the cycle number for C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl. (d) Coulombic efficiency as a function of the cycle number for C₃N₄-B, C₃N₄-H₂SO₄, C₃N₄-HNO₃, and C₃N₄-HCl. Note that the sphere represents experimental data, and the straight line represents the fitted data presented in the Nyquist plot of (a).

Figure 7

SEM images of C₃N₄-B at scales of (a) 2 μm and (b) 200 nm. SEM images of C₃N₄–HCl at scales of (c) 2 μm and (d) 200 nm.

Figure 8

HRTEM images of (a) C₃N₄-B and (d) C₃N₄-HCl at 20 nm scale. SAED patterns of (b, c) C₃N₄-B and (e, f) C₃N₄-HCl.

Figure 9

HR-XPS spectra of C₃N₄-B and C₃N₄-HCl. (a) Full scan spectra of C₃N₄-B and C₃N₄-HCl. (b, d) C 1s region of C₃N₄-B and C₃N₄-HCl. (c, e) N 1s region of C₃N₄-B and C₃N₄-HCl.

Figure 10

N₂ adsorption–desorption isotherms of (a) C₃N₄-B and (b) C₃N₄-HCl (insets show the pore diameter distribution profiles).

Figure 11

Reproducibility measurement of C₃N₄-HCl. (a) Charge–discharge profile of C₃N₄-HCl at the current density of 20 A g^–1. (b) Plot of specific capacity vs repeatability of C₃N₄-HCl.

Figure 12

Electrochemical performance of the fabricated symmetric HCl-treated C₃N₄ with the PEO/PEGDME/KOH/LiI-based gel electrolyte. (a) CV profile of the symmetric device in various voltage windows at a scan rate of 100 mV s^–1. (b) Charge–discharge profile of the device in various voltage windows at a current density of 5 A g^–1. (c) Charge–discharge profile of the symmetric device at various current densities at 1.8 V potential window. (d) Charge–discharge profile of the symmetric device at various current densities at 1.2 V potential window. (e) Electrochemical cyclic stability and Coulombic efficiency as a function of the cycle number of the fabricated symmetric device. (f) Ragone plot of the fabricated symmetric device at 1.2 and 1.8 voltage window. (g) Nyquist plot of the fabricated symmetric device.