Improving the Electrochemical Performance of Zinc Sulfides by Iron Doping Toward Supercapacitor Applications

The rational design of heterostructured Fe‐doped ZnS microspheres (FeZnS‐MS) and their use as electrode materials for supercapacitors (SCs) is proposed. FeZnS‐MS are mounted on a Ni foam substrate to develop high‐activity positive electrodes for SCs. The morphological and electrochemical performances of the hydrothermally synthesized FeZnS‐MS are investigated, revealing the formation of microspherical Fe‐doped ZnS nanocubes. Different Fe ratios (0.1, 0.2, and 0.3) are successfully introduced to ZnS without impurities, whereas two phases are produced when the percentage is increased to 0.4. Notably, under a current density of 2 A g−1, the Zn0.7Fe0.3S nanocomposite showed an exceptional specific capacitance of 575 F g−1, compared to the moderate value (200 F g−1) achieved by ZnS. At 0.5 and 1 A g−1, the Zn0.7Fe0.3S electrode presented an even better specific capacitance of 700 and 604.5 F g−1, respectively. Using this electrode, a hybrid supercapacitor system is developed and delivered 11 Wh kg−1 and a high specific power of 591.2 W kg−1.


Introduction
[3] The energy squeeze and emissions caused by fossil fuels have encouraged worldwide efforts to seek alternative fresh and green energy sources. [4]Electrical energy storage has emerged as one of the DOI: 10.1002/admi.202300790most critical issues that is expected to contribute to the increased use of renewable energy in the future.Owing to their capability to store and discharge energy instantly, combined with their low cost, long life, high power density, and excellent cycle stability, SCs as electrochemical devices are considered one of the most advantageous energy storage technologies. [5]They are employed in electric cars, consumer electronics, handheld devices, and other applications with high energy demands.SCs store immediately, have long retention periods, and are safer to use. [6]Because of these characteristics, they are highly appealing for use in electronics, electric cars, and other particular applications. [7,8]seudocapacitors, electrochemical double-layer capacitors (EDLCs), and battery-type SCs are examples of electrochemical devices and ultracapacitors.EDLCs can store electric charge in one of two ways: electrostatically or through a non-Faradaic mechanism that prevents the transfer of charge between the electrode and the solution. [9,10]Unlike pseudocapacitors, which store charges via the Faradaic process, charge transfer occurs between the electrode and the electrolyte. [11]Hybrid capacitors combine all EDLC types and battery-type SCs.Hybrid SCs can achieve power densities and high energy with high chemical stability. [12]Transition metal compounds have attracted considerable publicity in recent years [13][14][15][16][17] ; because of their numerous states of oxidation for feasible redox reactions, metal chalcogenides have received notable extensive attention as possible electrode materials in current ultracapacitors. [18]Multi-metal sulfides show more productive redox reactions and higher electronic conductivities than single-metal sulfides, substantially increasing their electrochemical efficiency. [19,20]Doping can boost the properties of conductors and semiconductors, resulting in a powerful method for regulating their critical electronics. [21,22]n-doped Zn chalcogenides have been investigated as possible replacements for CdSe because of their low toxicity and improved chemical and environmental stability. [23]Battery-type materials have excellent potential for SC applications, owing to their good energy density and moderate strength, which meet the increasing request for automobiles and electronic devices.Because of their reversible Faradaic redox reactions, several compositions have been proposed as possible electrodes for electrochemical applications.Transition metal sulfides have been confirmed as electrochemically active materials in the last decade.Because of their high electrical conductivity and numerous oxidation states, some materials have shown good electrochemical efficiency compared with their component oxides and carbonconstructed materials. [24]Metal sulfides are acceptable replacements for metal oxides since sulfur has electronegativity, which is lower than that of oxygen atoms.25] Some scientists reported the solvothermal strategy of ZnS deposited on the surface of graphene (ZnS/G) for new SC electrodes with a high specific capacitance of 197.1 F g −1 . [26]Luo et al. fabricated an ultrathin nanosheet from ZnS decorated on a carbon nanotube hybrid electrode with high specific capacitance (347.3F g −1 ). [27]In an effort to examine and test low-cost electrode materials for SC applications, and driven by the above advancements, we herein present the hydrothermal synthesis of Fedoped ZnS (FeZnS) nanocomposites and systematically discuss their structural and electrochemical properties.Despite extensive studies on ZnS and FeS, no studies have been conducted on the electrochemical performance of Fe-doped ZnS to date.Fe-doped ZnS exhibits intriguing electrochemical characteristics and more active surface centers thanks to the synergistic actions of Zn and Fe.
The preparation of ZnFeS nanocomposites with different elemental (Fe) ratios and without any impurities, along with the measurement of their electrochemical properties as SC electrodes, is a novel aspect of this work.Controlled Fe addition significantly boosted specific capacitance, as evidenced by the blank ZnS sample that achieved 200 F g −1 at 2 A g −1 , versus the Feadded sample (0.3 Fe: 0.7 Zn, Zn 0.7 Fe 0.3 S) that achieved a specific capacitance of 575 F g −1 at 2 A g −1 ; also, it exhibited specific capacitance 700 and 604.5 F g −1 at 0.5 and 1 A g −1 , respectively.Furthermore, apart from its excellent electrochemical efficiency, the hybrid sample demonstrated high stability and electrode energystorage life, with an 87% capacitance retention at a current density of 10 A g −1 after 2000 cycles.Using this hybrid composite, an asymmetric supercapacitor system (ASC) was developed that transferred 11 Wh kg −1 of energy with a high specific strength of 591.2 kW kg −1 at 10 A g −1 .

Characterization
The X-ray diffraction (XRD) patterns of the hydrothermally prepared ZnS, Zn 0.9 Fe 0.1 S, Zn 0.8 Fe 0.2 S, Zn 0.7 Fe 0.3 S, Zn 0.6 Fe 0.4 S, Zn 0.5 Fe 0.5 S, and FeS samples are revealed in Figure 1.The XRD pattern is well indexed to the F-43m cubic space group of FeZnS (JCPDS card no. of 01-089-4938).Doping with Fe until the 0.3 ratio yielded similar patterns, thus revealing the successful doping of Fe into ZnS; on the other hand, at the 0.4 and 0.5 doping ratios, another phase of FeS (orthorhombic phase, JCPDS no.01-076-0964) appeared.Further, the Zn 0.8 Fe 0.2 S and Zn 0.7 Fe 0.3 S patterns also matched the standard patterns of the reference codes 01-089-4936 and 01-089-4938, respectively, which are similar to those of the parent ZnS (JCPDS card no.01-077-2100).The XRD analysis proved the successful Fe doping to ZnS till specific ratios (0.1, 0.2, and 0.3).
In addition, scanning electron microscope (SEM) measurements were conducted to determine the synthesized Fe-doped ZnS microstructure.Figure 2 shows the SEM morphologies of the pure ZnS, FeS, and Zn (1-x) Fe x S samples.The SEM micrographs of the samples exhibited a microspherical structure (the ZnS image is shown in Figure 2a) with different sizes.Notably, by increasing the Fe doping ratio, the size of the microspheres increased (Figure 2b,c), which could be attributed to introduction of Fe atoms into the ZnS crystals.At a Fe doping ratio of 0.4 (Figure 2e), the microspheres returned to a small microhexagonal shape, as observed in a pure FeS sample (Figure 2e).
The elemental distribution throughout the microspherical structure was evaluated by Energy Dispersive -X-ray (EDX) mapping.EDX mapping images of Zn, Fe, and S in the Zn 0.7 Fe 0.  Figure 4 shows Transmission elecron microscopy (TEM) images of the Zn 0.7 Fe 0.3 S sample.As seen in Figure 4a, the microsphere comprised several aggregates with a cube-like nanostructure and a diameter of ≈500 nm (Figure 4b).The corresponding high-resolution TEM (HRTEM) image (Figure 4c) confirms that the lattice spacing is ≈0.275 nm, matching to (200) plane located at the 2 angle of 33°in Zn 0.7 Fe 0.3 S (reference card no.01-089-4938), corresponding with XRD pattern.
Additionally, the Zn 0.7 Fe 0.3 S microspheres' oxidation states are determined using X-ray photoelectron spectroscopy (XPS).Two binding energies at 1045 and 1022 eV, which are attributed to Zn 2p 1/2 and Zn 2p 3/2 , can be seen in the spectra of Zn 2p (Figure 5a), demonstrating the existence of the Zn 2+ ion. [28,29]According to Figure 5b, Fe 2+ is indexed to the appropriate peaks of Fe 2p at 710.4 and 722.2 eV, while Fe 3+ is indexed to the Fe 2p peaks at 716.1 and 726.2 eV, showing the existence of both Fe 3+ and Fe 2+ cations in Zn 0.7 Fe 0.3 S. [30] Two deconvoluted peaks were observed in the S 2p spectrum (Figure 5c).The sulfur-metal bonds (Zn-S and Fe-S) were attributed to the binding energy of S 2p 3/2 (161.2 eV); also, S 2− with low numbers of coordination on the surface were assigned the binding energy of S 2p 1/2 at 162.5 eV. [31]RD, SEM, TEM, and XPS analysis showed that the Fe-doped ZnS samples had been prepared successfully.
The Brunauer-Emmett-Teller (BET)specific surface area of the as-synthesized sample is an essential parameter for the electrochemical efficiency of Zn 0.7 Fe 0.3 S. Figure 6 demonstrates Zn 0.7 Fe 0.3 S's N 2 adsorption-desorption isotherm.Zn 0.7 Fe 0.3 S isotherm reflects the presence of type IV with an H 3 hysteresis loop, indicating a mesoporous structure with a specific surface area (S BET ) of 15.64 m 2 g −1 .Since Zn 0.7 Fe 0.3 S has a high surface area and a mesoporous structure, more active sites are offered in the electrode to facilitate electron transport between the electrode and the electrolyte to increase ion storage capacity.

Electrochemical Activity
To illustrate the behavior of the ZnS and Zn 1-x Fe x S samples for SCs, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), stability, and electrochemical impedance spectroscopy (EIS) measurements were carried out in a KOH solution (6 M) (Figure 7).The CV plots of Zn 1-x Fe x S and ZnS samples at 50 mV s −1 indicated the presence of well-defined peaks of oxidation and reduction reactions, suggesting the influence of Faradaic reactions on the electrode (Figure 7a).For all CV curves, two redox peaks were detected in the 0-0.6 V range that matched each other.The redox peaks were attributed to the reversible redox reaction, which is the principle of typical battery-type behavior.
The CV integrated area increased with increasing the Fe fraction according to the reversible redox reaction mechanism between Fe(II) and Fe(III).According to the CV plots, the Zn 0.7 Fe 0.3 S electrode exhibited the best ratio and capacitance.To quantify the discharge-specific capacity, the electrochemical properties of the Zn 1-x Fe x S and ZnS electrodes are presented through their charge-discharge profiles at a 2 A g −1 current density in the range of potential from 0 to 0.5 V (Figure 7b).The GCD plots show apparent voltage plateaus, indicating that the supercapacitive activity was Faradaic.The existence of these plateaus evidenced the occurrence of reactions of reduction and oxidation, which is in agreement with CV results.C sp was measured from the values of GCD based on the equation S1 (Supporting Information); the considered capacitance values of the ZnS as well as Zn 0.7 Fe 0.3 S electrodes were 200 and 575 F g −1 , respectively, at 2 A g −1 .Additionally, the Zn 0.7 Fe 0.3 S electrode exhibited a specific capacitance of 700 and 604.5 F g −1 at 0.5 and 1 A g −1 , respectively; its GCD at 0.5 and 1 A g −1 is represented in Figure S1 (Supporting Information).Notably, the Zn 0.7 Fe 0.3 S electrode indicated a significant improvement in its electrochemical storage capability.Fe ions are expected to offer richer redox reactions than Zn ions, and the capacity increases due to the contributions from iron ions (the contribution of the Fe 2+ /Fe 3+ redox couple). [32]The reaction of the KOH electrolyte with the ZnFeS electrode can be expressed in the following equations: [33][34][35][36] ZnS + OH − ↔ ZnS∕OH − + e − (1) Accordingly, based on the GCD measurements, Figure 7c exhibits the chemical stability results of the Zn 0.7 Fe 0.3 S electrode at 10 A g −1 .The stability of the Zn 0.7 Fe 0.3 S electrode was evaluated in terms of its capacitance retention (87%) and stability after 2000 cycles.The decreasing capacitance with cycling might be due to the partial delamination collapse of the microsphere-like structure under high current density and extended cycle test.The EIS analysis results of the Zn 0.7 Fe 0.3 S electrode are shown in Figure 7d.The intersection of the curve at the real part (Zr) in the high frequency shows the series resistance (ESR), which was assigned to different resistances, including the solution resistance, internal resistance, and contact resistance between the electrode and the current collector. [37]The plots of the as-prepared Zn 0.7 Fe 0.3 S electrode showed an ESR of 1.3 Ω, which is less than that after the stability test (3.5 Ω).The Nyquist plots display high-frequency and low-frequency regions.Furthermore, the low-frequency area showed good capacitive performance for the Zn 0.7 Fe 0.3 S electrode material.
An asymmetric electrochemical device was tested in a KOH solution (6 M) using two electrodes: Zn 0.7 Fe 0.3 S (anode) and a fabricated activated carbon (AC) (cathode).To estimate the capacitance of this device, the mass ratio was set to 2.7 based on the mass-balance equation (Equation S2, Supporting Information) (C sp -was resulting from the GCD in Figure 5b).The operating potential can be measured from the sum of the different potential windows of Zn 0.7 Fe 0.3 S (0-0.6 V) and AC (−1.0-0V); thus, a potential operating window can be extended to 1.6 V.
CV measured at dissimilar scan rates from 10 to 200 mV s −1 for ASC in the range of 0-1.4 V of the potential is shown in Figure 8a.The CV plots of the symmetrical supercapacitor show redox peaks, demonstrating the battery-like performance of the Zn 0.7 Fe 0.3 S//AC hybrid device (Figure 8a).The GCD ca- pacities of the electrochemical device were measured at 1, 2, 3, 4, 5, and 10 A g −1 of current densities, showing a nonlinear charge-discharge profile (Figure 8b).The device exhibited an Ed of 11 Wh kg −1 and a Pd of 591.2 W kg −1 .Compared to some previously reported devices, the ASC device shows comparable or superior results (ZnS-ZnO/g-C 3 N 4 with polypyrrole [38] exhibited Ed of 11.87 Wh kg −1 , NiCo 2 O 4 -reinforced carbon microfibers (NICAF) [39] displayed Ed 8.32 of Wh kg −1 , NiCo-MOF@f-CNF//AC [40] showed Ed of 8.6 Wh kg −1 , Bi 2 S 3 /Sg-C 3 N 4 //AC [41] showed Ed of 6.2 Wh kg −1 , g-C3N4/ZnS//g-C 3 N 4 /ZnS cell [42] showed Ed of 10.4 Wh kg −1 , CNF-ZrO 2 symmetric supercapacitor [43] reached Ed of 4.86 Wh kg −1 , and porous carbon/KOH/Cu 5 Sn 2 S 7 /ZnS [44] presented Ed of 11.2 Wh kg −1 ), indicating strong energy storage performance.The cycling stability of the as-prepared hybrid device was performed by the same cycles of GCD measurement within 0 to 1.4 V at 10 A g −1 .As shown in Figure 8c, the hybrid device of Zn 0.7 Fe 0.3 S//AC exhibited 80% capacitance retention after 2000 cycles, indicating the excellent reversibility and stability of the device.EIS was conducted (Figure 8d) and showed a lower ESR (1.3 Ω).The ESR slightly changed (1.4 Ω) after 2000 cycles, demonstrating an insignificant ohmic loss upon cycling.Following the stability test, a lightly curved form (semi-circle arc) was noted in the high-frequency zone.This observation corresponded to a slight rise in the charge transfer resistance (Rct) because of the electrode structure's long-term cycling impact.Finally, the electrochemical results suggest that the Zn 0.7 Fe 0.3 S//AC ASC hybrid device functions as a batterytype capacitor and has outstanding potential as an energy storage material.

Conclusion
We prepared Fe-doped ZnS nanocomposites that showed excellent electrochemical efficiency for SC electrodes; particularly, under a current density of 2 A g −1 , the Zn 0.7 Fe 0.3 S electrode attained a significant specific capacitance of 575 F g −1 , while the corresponding value of the plain ZnS electrode was 200 F g −1 .Furthermore, the Fe-doped electrode exhibited exceptional cycling stability, with an 87% capacitance preservation after 2000 cycles at 10 A g −1 .Using this electrode, we developed an ASC (hybrid device) system that delivered 11 Wh kg −1 of energy with a high specific power of 591.2 kW kg −1 .

Experimental Section
All reagents were purchased from Sigma-Aldrich.ZnS, ZnFeS, and FeS nanoparticles were fabricated via a facile hydrothermal method using ZnSO 4 , FeSO 4 , and thioacetamide powders.ZnS, Zn (1-x) Fe x S (x = 0.1, 0.2, 0.3, 0.4, and 0.5), and FeS were prepared by adding the appropriate molar ratios of ZnSO 4 (1 mmol, 0.287 g), FeSO 4 (1 mmol, 0.278 g), and thioacetamide (1 mmol, 0.075 g).Each percentage was added to 35 ml of deionized H 2 O. Heating was performed in an autoclave at 180 °C for 15 h to prepare the powders.The as-prepared precipitates were filtered and washed using a mixture of water and EtOH several times.The products were dried at 90 °C for 1 day to form ZnS, Zn (1-x) Fe x S, and FeS powders.
Materials Characterization and Electrochemical Measurements: More details are written in the Supporting Information.
3 S sample are displayed in Figure 3.The uniform development of Zn 0.7 Fe 0.3 S microspheres was indicated by the even distribution of Zn, Fe, and S, showing that the one-step hydrothermal process successfully produced Fe-doped ZnS microspheres.

Figure 4 .
Figure 4. a, b) TEM and c) HR-TEM images of Zn 0.7 Fe 0.3 S.

Figure 7 .
Figure 7. a) CV at a scan rate of 50 mV s −1 , b) GCD for Zn 1-x Fe x S and ZnS composites at the current density of 2 A g −1 , c) capacitance retention at 10 A g −1 , and d) EIS profile of Zn 0.7 Fe 0.3 S electrode before and after the stability test.

Figure 8 .
Figure 8. Electrochemical measurements of the Zn 0.7 Fe 0.3 S//AC hybrid device: a) CV curves at dissimilar scan rates, b) GCD plots at dissimilar current densities, c) capacitance retention at 10 A g −1 , and d) EIS results for the as-prepared device and after 2K cycles.