Surface Charge Regulation of Graphene by Fluorine and Chlorine Co-Doping for Constructing Ultra-Stable and Large Energy Density Micro-Supercapacitors

Abstract

Settling the structure stacking of graphene (G) nanosheets to maintain the high dispersity has been an intense issue to facilitate their practical application in the microelectronics-related devices. Herein, the co-doping of the highest electronegative fluorine (F) and large atomic radius chlorine (Cl) into G via a one-step electrochemical exfoliation protocol is engineered to actualize the ultralong cycling stability for flexible micro-supercapacitors (MSCs). Density functional theoretical calculations unveiled that the F into G can form the "ionic" C─F bond to increase the repulsive force between nanosheets, and the introduction of Cl can enlarge the layer spacing of G as well as increase active sites by accumulating the charge on pore defects. The co-doping of F and Cl generates the strong synergy to achieve high reversible capacitance and sturdy structure stability for G. The as-constructed aqueous gel-based MSC exhibited the superb cycling stability for 500,000 cycles with no capacitance loss and structure stacking. Furthermore, the ionic liquid gel-based MSC demonstrated a high energy density of 113.9 mW h cm^-3 under high voltage of up to 3.5 V. The current work enlightens deep insights into the design and scalable preparation of high-performance co-doped G electrode candidate in the field of flexible microelectronics.

Keywords: chlorine; co‐doped graphene; electrochemical exfoliation; flexible supercapacitors; fluorine.

Figure 1

The preparation process of F/Cl co‐doped G.

Figure 2

a) SEM, b) TEM, c) HRTEM, and d) AFM images of the F/Cl‐G nanosheets. e–i) TEM and EDS mapping images of the F/Cl‐G nanosheets. j) AC HAADF‐TEM image of the F/Cl‐G nanosheets. k–n) Focuses on the four different regions of 1–4 in Figure 2j. o) C 1s, p) O 1s, q) F 1s, and r) Cl 2p spectrum of the F/Cl‐G nanosheets.

Figure 3

a) Schematic diagram of the metal template. b) Electrode on PET substrate. c) Cross‐sectional SEM image of electrode. d) G‐based flexible PVA/H₂SO₄‐MSC. Electrochemical test results of PVA/H₂SO₄‐MSCs. e) CV curves of MSCs constructed with different components of F/Cl‐G at the scan rate of 50 mV s⁻¹. f) The areal specific capacitance of MSCs as a function of different components of F/Cl‐G at the scan rate of 50 mV s⁻¹. g) CV curves of F₆Cl₄‐G‐MSC at the scan rate of 5–50 mV s⁻¹. h) CV curves of F₆Cl₄‐G‐MSC at the scan rate of 100–1000 mV s⁻¹. i) The areal specific capacitance of F₆Cl₄‐G‐MSC. j) GCD profiles of F₆Cl₄‐G‐MSC at the current density of 0.005–0.05 mA cm⁻². k) GCD profiles of F₆Cl₄‐G‐MSC at the current density of 0.1–1 mA cm⁻². l) Cycle stability of F₆Cl₄‐G‐MSC at the current density of 0.2 mA cm⁻² for 500,000 uninterrupted charge‐discharge cycles (insert images: schematic diagram of electrolyte ion migration and SEM image of electrode after cycles).

Figure 4

a) The flexible PVA/H₂SO₄‐MSCs with different bending angles. b) CV curves under different bending angles at the scan rate of 50 mV s⁻¹. c) MSCs can connected with co‐doped G rather than conductive metal wire or binder. d) CV curves at the scan rate of 50 mV s⁻¹ and e) GCD profiles at the current of 0.1 mA of 1–12 F₆Cl₄‐G‐MSCs in series. f) The relationship between the total voltage of an integrated circuit and the number of MSCs used for integration. g) The complex plane plot of 1–12 F₆Cl₄‐G‐MSCs in series. h) CV curves at the scan rate of 50 mV s⁻¹ and i) GCD profiles at the current of 0.1 mA of 1–12 F₆Cl₄‐G‐MSCs in parallel. j) The relationship between the discharging time of an integrated circuit and the number of MSCs utilized for integration at the same current. k) The complex plane plot of 1–12 F₆Cl₄‐G‐MSCs in parallel.

Figure 5

Electrochemical performance of EMIMBF₄/PVDF‐HFP‐MSCs. a) CV curves at the scan rate of 5–500 mV s⁻¹. b) The areal specific capacitance calculated from CV curves. c) GCD profiles at a current density of 0.03–0.3 mA cm⁻². d) The areal specific capacitance calculated from GCD profiles. e) The complex plane plot of F₆Cl₄‐G based EMIMBF₄/PVDF‐HFP‐MSC. f) Energy density of F₆Cl₄‐G based EMIMBF₄/PVDF‐HFP‐MSC and other reported MSCs as a function of power density. g) Cycling stability of F₆Cl₄‐G based EMIMBF₄/PVDF‐HFP‐MSC for 20,000 cycles at a current density of 0.3 mA cm⁻² (Insets of Figure 5g). Demonstration of LED lighting by a single EMIMBF₄/PVDF‐HFP‐MSC. h) CV curves and i) GCD profiles of the two F₆Cl₄‐G based EMIMBF₄/PVDF‐HFP‐MSCs in series. j) CV curves and k) GCD profiles of the two F₆Cl₄‐G‐based EMIMBF₄/PVDF‐HFP‐MSCs in parallel.

Figure 6

a–c) The optimized layer distances, d–f) charge density difference, and g‐i) Bader charge of F/F‐G, F/Cl‐G, and Cl/Cl‐G with the zigzag‐edge. C: brown, F: blue, Cl: green. The calculated charge density difference of F/F‐G, F/Cl‐G, and Cl/Cl‐G at isovalue = 0.005, which was calculated by the following equation: Δρ = ρ(total) – ρ(G) – ρ(Cl or F). The yellow region represents the electron accumulation, while the blue region represents the electrons depletion.