Boosting Zinc Hybrid Supercapacitor Performance via Thiol Functionalization of Graphene‐Based Cathodes

Abstract Zinc hybrid supercapacitors (Zn‐HSCs) hold immense potential toward the next‐generation energy storage systems, effectively spanning the divide between conventional lithium‐ion batteries (LIBs) and supercapacitors. Unfortunately, the energy density of most of Zn‐HSCs has not yet rivalled the levels observed in LIBs. The electrochemical performance of aqueous Zn‐HSCs can be enhanced through the chemical functionalization of graphene‐based cathode materials with thiol moieties as they will be highly suitable for favoring Zn2+ adsorption/desorption. Here, a single‐step reaction is employed to synthesize thiol‐functionalized reduced graphene oxide (rGOSH), incorporating both oxygen functional groups (OFGs) and thiol functionalities, as demonstrated by X‐ray photoelectron spectroscopy (XPS) studies. Electrochemical analysis reveals that rGOSH cathodes exhibit a specific capacitance (540 F g−1) and specific capacity (139 mAh g−1) at 0.1 A g−1 as well as long‐term stability, with over 92% capacitance retention after 10 000 cycles, outperforming chemically reduced graphene oxide (CrGO). Notably, rGOSH electrodes displayed an exceptional maximum energy density of 187.6 Wh kg−1 and power density of 48.6 kW kg−1. Overall, this study offers an unprecedented powerful strategy for the design and optimization of cathode materials, paving the way for efficient and sustainable energy storage solutions to meet the increasing demands of modern energy applications.

6. PXRD characterization PXRD diffractograms have been recorded using a D8 Advanced with twin-twin optics (Bruker).The diffraction patterns of the powder sample have been acquired in 5-40 (2θ)   range in Bragg-Brentano configuration, using CuK radiation and a LINXEYE 2 detector.Two 2.5 mm Soller Cu slits have been applied to the primary and secondary optics.In the latter, a Ni stopper has been added to filter Cuk radiations.An automatized blade has been mounted to limit the contribution of air scattering at low angles.The data has been acquired with a step of 0.02° and an acquisition time of 0.2 s per step.

Calculation of the XRD parameters
From the XRD diffractograms the peak position has been calculated using the Bragg's law: Where d(hkl) is the calculated inter planar distance (Å), λ is the wavelength of the XRD source (Å),  is the scattering angle (rad).In the present case λ=1.54 Å.
The crystallite dimension has been derived from the Scherrer formula: Where Lc is the crystallite thickness (Å), K is the shape factor equal to 0.89, [1]  is the FWHM of the (002) peaks and  is the corresponding scattering angles.
The number of layer has been obtained from the following formula: Table S2.The XRD peak position, d-spacing, crystallite thickness (Lc) and average graphene layer number (nc) calculated for the (002) plane for GO, CrGO and rGOSH. [2] BET characterization Table S3.Physical parameters of GO, CrGO and rGOSH.
Figure S9.a) CV curves of CrGO at various scan rates, b) fitting plots between log(i) and log (υ) at various peak currents.

Figure S10 .
Figure S10.Capacitive (contribution) and diffusion-controlled contribution of CrGO at various scan rates

Figure S12 .
Figure S12.Capacitive (contribution) and diffusion-controlled contribution fraction for the CV curves of rGOSH recorded at different scan rates.

Figure S14 .
Figure S14.The equivalent electric circuit models used for fitting the Nyquist plots.Rs: the intrinsic ohmic resistance; Rct: charge transfer resistance; C1: capacitance element; CPEEDL: constant phase element representing the electrical double layer capacitance (EDLC).
Figure S15.a) GCD profiles of CrGO at various current densities, b) specific capacitance (blue points), discharged (white points) and coulombic efficiency (red points) of CrGO at various current densities.Each current density is expressed in A/g.

Figure S16 .
Figure S16.Specific capacity vs voltage for a) CrGO, and b) rGOSH at different current densities.

Figure S17 .
Figure S17.GCD profiles corresponding to four different samples of rGOSH at various current densities.

Figure S18 .
Figure S18.GCD profiles corresponding to four different samples of CrGO at various current densities.

Figure S19 .
Figure S19.Long-term cycling performance of CrGO at 1 A/g.

Figure S20 .S17 9 .
Figure S20.Ragone plot for CrGO (black points) and rGOSH (orange points) as well as other cathode materials employed in Zn-HSCs.

Figure S22 .
Figure S22.Chemical structures employed for the binding energy calculations.

Table S5 .
Parameters obtained from Gaussian for the binding energy calculations of carbonyl groups with Zn ions.

Table S6 .
Parameters obtained from Gaussian for the binding energy calculations of hydroxyl groups with Zn ions.

Table S7 .
Parameters obtained from Gaussian for the binding energy calculations of thiol groups with Zn ions.

Table S8 .
State of the art of the electrochemical performance of various cathode materials in Zn-HSCs.