Effects of Fe doping on enhancing electrochemical properties of NiCo2S4 supercapacitor electrode

Journal Pre-proof Effects of Fe doping on enhancing electrochemical properties of NiCo2S4 supercapacitor electrode Praphaiphon Phonsuksawang, Patcharaporn Khajondetchairit, Teera Butburee, Suchinda Sattayaporn, Narong Chanlek, Pussana Hirunsit, Suwit Suthirakun, Theeranun Siritanon PII:

S0013-4686(20)30331-5

DOI:

https://doi.org/10.1016/j.electacta.2020.135939

Reference:

EA 135939

To appear in:

Electrochimica Acta

Received Date: 15 November 2019 Revised Date:

20 February 2020

Accepted Date: 20 February 2020

Please cite this article as: P. Phonsuksawang, P. Khajondetchairit, T. Butburee, S. Sattayaporn, N. Chanlek, P. Hirunsit, S. Suthirakun, T. Siritanon, Effects of Fe doping on enhancing electrochemical properties of NiCo2S4 supercapacitor electrode, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135939. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement Praphaiphon Phonsuksawang: Methodology, Investigation, Visualization, Writing - Original Draft Patcharaporn Khajondetchairit: Formal analysis, Investigation, Teera Butburee: Investigation, Resources Suchinda Sattayaporn: Investigation, Resources Narong Chanlek: Investigation, Resources Pussana Hirunsit: Resources, Formal analysis Suwit Suthirakun: Validation, Formal analysis, Visualization, Writing - Review & Editing Theeranun Siritanon: Conceptualization, Writing - Review & Editing, Funding acquisition, Supervision, Project administration

Effects of Fe doping on enhancing electrochemical properties of NiCo2S4 supercapacitor electrode Praphaiphon Phonsuksawang,a Patcharaporn Khajondetchairit,a Teera Butburee,b,c Suchinda Sattayaporn,d Narong Chanlek,c,d,e Pussana Hirunsit,b,c Suwit Suthirakun,a,c,e Theeranun Siritanona,c,e* a

School of Chemistry, Institute of Science, Suranaree University of Technology, 111, University Avenue, Nakhon Ratchasima, 30000, Thailand b

National Nanotechnology Center, National Science and Technology Development Agency, 111 Thailand Science Park, Pathum Thani, 12120, Thailand

c

Research Network NANOTEC – SUT on Advanced Nanomaterials and Characterization, School of Chemistry, Suranaree University of Technology d

Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, 30000, Thailand e

Center of Excellent-Advanced Functional Materials, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand Corresponding Author *E-mail address: [email protected]

Keywords: NiCo2S4, Fe-doping, binder-free electrode, supercapacitor

Abstract NiCo2S4 have been widely studied as electrode materials for supercapacitors. Several strategies, including the morphology controls and elemental doping, have been used to improve its performance. This research investigates the effects of FeCl3 addition on structure, morphology, and electrochemical performance of NiCo2S4 prepared by a one-step hydrothermal method. FeCl3 plays two important roles. It affects the synthetic condition which consequently influences the morphology and acts as a source of Fe3+ which is incorporated in NiCo2S4 lattice. Remarkably, the Fe-added electrode shows significantly improved performance with high specific capacity of 167 mAh/g at current density of 10 A/g, which is ~170% enhancement compared with that of the electrode without FeCl3 addition. Detail analyses on electrochemical behavior and theoretical computations reveal that the enhanced performance is caused by the improved electrical conductivity as evidenced by electrochemical impedance spectroscopy (EIS) and the increased density of states (DOS) at the Fermi level. In addition, the doped system could facilitate OH– adsorption on the material surface which benefits the electrochemical reaction. The obtained results give insight to the roles of metal doping in NiCo2S4 electrode which could be helpful in the future development of this material.

1. Introduction Energy is involved in almost all activities in our daily lives. Consequently, energy storage is a crucial tool. Supercapacitors are one of the candidates for next-generation energy storage systems due to their high-power density and long-term stability. With these attractive properties, supercapacitors are widely used in portable electronic devices, backup systems, and electric

vehicles [1]. Nevertheless, applications of supecapacitors are still limited by their low energy density. To overcome this limitation, new electrode materials have been continuously explored [2,3]. Several oxides such as MnO2, NiO, and Co3O4 have been developed as electrode materials because they exhibit good electrochemical behavior during charge-discharge cycles leading to high specific capacities [4-6]. Additionally, their advantages are low cost, environmental friendliness, and structural stability. However, these metal oxides suffer from poor conductivity which limits their performance [4]. Metal sulfides are interesting materials for supercapacitor electrodes. Among all, nickel cobalt sulfides (NiCo2S4) have been widely studied because of their high electrical conductivity and theoretical specific capacity [7]. It is well known that many factors including conductivity, surface area, and morphology could affect the electrochemical performance of electrodes. As a result, NiCo2S4 with various nanostructures and nanoarrays such as hollow microspheres, nanotubes, nanosheets and flower-like structures have been prepared by hydrothermal method [2,6,8]. In the hydrothermal process, the morphologies can be tuned by adding the additives such as ammonium fluoride (NH4F)[1,4,6], sodium acetate (NaAc)[9], polyvinylpyrrolidone (PVP) [10], and sodium dodecyl surfactant (SDS) [8]. These additives act as protective agents and/or chelating agents in particle-formation processes and prevent agglomeration. In addition to morphological control, doping is an effective strategy to improve the electrochemical performance. Lin et al. [11] reported that phosphorous-doped (P-doped) NiCo2S4 showed 60% enhancement in specific capacity compared to that of the pristine NiCo2S4. The improvement was attributed to the higher reactivity and conductivity after P-doping. Similarly, iron-doping significantly improves both cycling performance and specific capacity of NiCo2O4 electrodes [5-12]. While a large number of researches are focusing on preparing nanostructures

and composites of NiCo2S4 [13-18], investigation on doped NiCo2S4 has been less explored. Very recently, it was reported that Fe doping in NiCo2S4 could enhance the specific capacity of the material in ionic liquid-based supercapacitors [19]. Here, FeCl3 is used as an additive during the synthesis to tune the morphology and to improve the electrochemical properties. Fe-added NiCo2S4 binder-free electrodes were prepared by directly growing active materials on a conductive Ni foam substrate via one-step hydrothermal method. The effects of FeCl3 additive on the electrodes’performance are investigated. Interestingly, we found that adding FeCl3 during the hydrothermal preparation alters the electrode morphology and Fe3+ is doped into NiCo2S4 lattice. The obtained Fe-doped NiCo2S4 electrode shows a significant improvement in the capacitive behavior.

2. Experimental 2.1 Preparation All samples were prepared by a simple one-step hydrothermal method. A piece of Ni foam with a size of 1 × 2 × 0.08 mm3 was used as a conductive substrate. In a synthesis of NiCo2S4, 0.5 mmol of Ni(NO3)2•6H2O (99%, Acros Organics), 1 mmol of Co(NO3)2•6H2O (99+%, Acros Organics), and 2 mmol of thiourea (99+%, Carlo Erba) were dissolved in 20 mL of deionized water/ethylene glycol mixture (1:1 by volume) under magnetic stirring for 30 min [3]. The mixed homogeneous solution was then transferred to a Teflon-lined stainless-steel autoclave with a piece of Ni foam immersed in the solution. The autoclave was put in an oven and heated at 90 oC for 8 h. After naturally cooled down to room temperature, the dark black product was deposited on Ni foam (named as 0Fe-NCS). NiCo2S4-coated Ni foam was washed with deionized water for several times. Series of Fe-doped NiCo2S4 samples were synthesized under the similar condition,

except varied amounts of FeCl3, including 0.12 mmol (assigned as 0.25Fe-NCS following the mole ratio of Fe:Ni), 0.25 mmol (0.5Fe-NCS), 0.5 mmol (1Fe-NCS), 0.75 mmol (1.5Fe-NCS), and 1 mmol (2Fe-NCS), were added to the reaction bath. The mass loadings of samples were calculated by the weight difference of Ni foam before and after the reaction. 2.2 Characterization Crystal structures of the samples were characterized by X-ray diffraction (XRD, Bruker D2 Phaser) with Cu Kα radiation (λ = 1.5406 Å) operating at the voltage of 30 kV and the current of 10 mA. The samples’ morphologies were studied by field emission scanning electron microscopy (FESEM, Zeiss AURIGA) and transmission electron microscope (TEM: JEOL2100plus, operated at 200 keV). The oxidation states of Ni and Co were studied by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Japan) with monochromatic X-ray of Al Kα (1486.6 eV). The oxidation states of Fe and S were confirmed by X-ray absorption near edge spectroscopy (XANES). Fe K-edge and S K-edge measurements were performed in fluorescence mode at the SUT-NANOTEC-SLRI beamline 5.2, Synchrotron light research institute (Public Organization), Nakhon Ratchasima, Thailand. Fe content in the samples were determined by ICP-OES, where the electrodes were digested in acidic solution before analysis. The elemental distribution of each electrode was investigated by energy dispersive spectroscopy (EDS). 2.3 Electrochemical measurements The electrochemical measurements were carried out using a standard three-electrode configuration

connected

to an

electrochemical workstation (AUTOLAB

instrument

electrochemical workstation, PGSTAT204). The prepared binder-free electrodes were directly used as a working electrode. Pt plate and Ag/AgCl were used as a counter and a reference electrode, respectively. The electrochemical performances of the electrodes were investigated by

cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 2 M KOH solution. The specific capacity (Q, mAh/g) was obtained from the discharge curves according to equation

Q=

I∆t m

where I is the discharge current (mA), ∆t is discharge time (h), and m is active mass (g). The EIS measurements were performed in a frequency range from 100 kHz to 0.1 Hz with 10 mV amplitude. 2.4 Computation Details All calculations were carried out using the spin polarized density functional theory [20] with periodic boundary conditions as implemented in the Vienna ab-initio simulation package (VASP 5.3) [21-23]. The generalized gradient approximation of Perdew-Burke-Ernzerhof functional was used to describe the exchange correlation term [24]. We used the Grimme’s correction method (DFT-D3) to properly take into account the van der Waals interaction between surface (NiCo2S4) and adsorbates [25]. The nucleus and core electron potentials were treated using the projector augmented wave method [26-27] where the valence electrons 3d4s of Fe Co and Ni and 3s3p of S were described by plane-wave basis at 500 eV cutoff. The convergence criteria of the selfconsistent field are within 1.0 × 10–6 eV whereas the force convergence of ionic relaxation was set to 0.02 eV/ Å. To study the effect of Fe doping on the electronic structures of NiCo2S4, we first optimized the cubic unit cell of NiCo2S4 using the Monkhorst-Pack (MP) sampling [28] of 4×4×4 k-point mesh in the Brillouin zone. As depicted in Fig. S1a, the unit cell contains eight formula units of NiCo2S4 which corresponds to Ni8Co16S32. The calculated lattice parameter of 9.34 Å is

consistent with that of the experimental value (9.38 Å) [29]. Then, we replaced one Co or Ni atom with one Fe atom in the unit cell and computed their projected density of states (PDOS) using the tetrahedron smearing method. To further examine the role of Fe doping on the formation of electrical double layer (EDL) of KOH electrolyte, we used the optimized unit cell to build a slab model of NiCo2S4 (100) surface with eight-atomic layers as shown in Fig. S1 panel b and c. The bottom four layers were kept frozen to mimic a bulk-liked structure while the others were relaxed. A 15 Å vacuum gap was added in the c direction to avoid the interaction between periodic images. We replaced one Fe atom at the Ni site or Co site to obtain the Fe-doped surfaces. Six K+ ions and OH– ions were added to the surface to form the EDL on the surface. Their internal coordinates were relaxed using 4 × 4 × 1 k-point sampling of MP scheme where the interactions, Ead, were calculated according to the given equation Ead = EEDL – (6EKOH + Eslab) where EEDL is the total energy of the system when six KOH are adsorbed on the slab, EKOH and Eslab are the total energy of free adsorbate and the clean or doped (100) surface before adsorption, respectively. 3. Results and discussions 3.1 Preparation Fig. 1 shows XRD patterns of all samples on Ni foam. Note that the electrodes (not the powder) were directly used in XRD measurement, which may cause some shifts in the peak positions. The two strong peaks at 45 o and 52 o belong to Ni foam. The five diffraction peaks at 27o, 31o, 38o, 50o and 55o correspond to (220), (311), (400), (511) and (440) diffraction planes of the cubic NiCo2S4 phase (PDF 43-1477), respectively [3,30]. Some additional peaks at 21.7o and 50.0o

found in 0Fe-NCS can be indexed as Ni3S2 impurity. Ni3S2 may form by the interfacial reaction between the outer surface of Ni foam and sulfur ion from thiourea during the hydrothermal reaction [31,32]. The possible chemical reactions are shown by equation 1-3: NH2CSNH2 + 2H2O H2S + H2

→

3Ni + S2– + O2

→

2NH3 + H2S + CO2

2H3O+ + S2– →

Ni3S2 + OH–

(1) (2) (3)

The XRD patterns demonstrate that NiCo2S4 were successfully synthesized in 0.25Fe-NCS, 0.5Fe-NCS and 1Fe-NCS samples without impurities. Higher Fe loading (>1:1 atomic ratio) causes Ni3S2 impurity (Fig. S2, Supporting information). It should be noted that the mass loading of 0.25Fe-NCS, 0.5Fe-NCS, 1Fe-NCS are in the range of 2-3 mg, while that of 0Fe-NCS is slightly lower (1.5 mg). X-ray photoelectron spectroscopy (XPS) is employed to identify the oxidation states of elements in the samples. The XPS spectra of Ni and Co are fitted by the Gaussian method. As shown in Fig. 2a and 2b, XPS spectra of 0Fe-NCS, 0.25Fe-NCS, 0.5Fe-NCS, and 1Fe-NCS have similar pattern. Considering both the binding energies and the full width at half maximum (FWHM), the Ni 2p spectra are deconvoluted into four peaks with two spin-orbit doublets and two shakeup satellites (marked as “sat.”). The binding energy at around 856 eV in Ni 2p3/2 and 873 eV in Ni 2p1/2 are assigned to Ni2+, while the binding energy at around 857 eV in Ni 2p3/2 and 876 eV in Ni 2p1/2 correspond to Ni3+ [1,3]. These results are in good agreement with the previous report of NiCo2S4 [3]. Moreover, we found that Ni3+ content slightly decreases in Feloaded samples. This suggests that Ni3+ is converted to Ni2+ upon Fe doping, which is similar to the previous study of Fe-substituted NiCo2O4 reported by Yuan et al. [33].

In Fig. 2b, the Co 2p spectrum of 0Fe-NCS sample can be well-fitted with two spin-orbit doublets and two shakeup satellites. The fitting peaks at 779.9 eV and 796.2 eV are attributed to Co3+ [30,34]. On the other hand, the peaks with binding energy of 781.4 eV and 797.0 eV are associated with Co2+ [35]. Moreover, the spin-orbit splitting value of Co 2p1/2 and Co 2p3/2 in 0Fe-NCS sample is 15.6 eV, suggesting the coexistence of Co2+ and Co3+ [1,30,35]. Unfortunately, the Co 2p spectra of the other samples with FeCl3 addition are not clear due to the overlap between photoemission of Co and auger of Fe. Nevertheless, spectra’s positions and shapes are roughly the same. Because Fe 2p and S 2p XPS spectra are complicated by the overlap with Auger of Co and surface oxidation, we employed X-ray absorption spectroscopy (XAS) technique to further clarify the valence states of Fe and S [36]. Fig. 2c shows the Fe K-edge XANES spectra of Fedoped samples, and Fe with different oxidation states such as FeO (Fe2+), Fe2O3 (Fe3+), FeCl3·6H2O (Fe3+) for comparison. The position of the pre-edge, edge energies, and the position of the white line indicate the presence of Fe3+ in the samples [37-39]. The edge energy of S Kedge XANES is proportional to the oxidation states of sulfur, ranging from 2469 eV for S2– to 2483 eV for S6+ [40,41]. Here, the edge energies of S K-edge XANES are between 2469.92470.2 eV (Fig. 2d), which indicate that the oxidation state of sulfur is –2. The effects of FeCl3 addition on morphologies of the samples are clearly seen in Fig. 3. The nanosheet feature of 0Fe-NCS is similar to the previous report [3]. Surprisingly, morphological changes are observed when FeCl3 is added (Fig. 3b-3d). We found that Cl– ion do not have a significant effect on the morphologies (Fig. S3-S4, Supporting Information). The common ionic compound used as an additive in hydrothermal synthesis of NiCo2S4 is NH4F [1,4,42]. It was concluded that NH4F increases the acidity of the system and activates surface of the samples

which benefit the growth of particles with high dimensions (2D-3D). Similarly, Fe3+ is well known to form an aqua acid, [Fe(H2O)6]3+, whose presence would increase the acidity of the system and promote agglomeration of particles (Fig. 3d). The TEM images suggest that the particles become denser with increasing Fe3+ content. In addition, when Fe content is increased, the coral-like feature with denser center and thinner surface is obtained. The high-magnification TEM image shows well-defined lattice fringes in the ‘surface’ region of 1Fe-NCS (Fig. 4b), which suggests a crystalline nature of the sample. Although the morphology of the electrodes is influenced by Fe content, BET surface areas of all electrodes are similar. The surface areas are in the range of 80-110 m2/g (Fig. S5 and Table S1, supporting information), which are similar to other hydrothermally prepared NiCo2S4 electrodes [43-45]. The EDS mapping (Fig. 4c and Fig. S6-S8 in supporting information) indicates a homogeneous distribution of elements, including Fe, in the samples. The Fe contents in 0.25FeNCS, 0.5Fe-NCS and 1Fe-NCS sample were determined by ICP-OES to be 0.89%, 1.16% and 1.61% by mole relative to Ni content, respectively. Although the Fe content slightly increases with added FeCl3 content, the numbers are much lower than the starting Fe : Ni ratio. Fe in the samples can exist in two possible forms. They may form FeCo2S4 which is mixed with NiCo2S4 forming a composite as similarly reported by Zhu and co-workers [13] or they may be doped into the NiCo2S4 lattice. Note that the fine XRD scans (with 0.02o and 2s/step) were collected from 25o - 42o to check the presence of FeCo2S4 phase (Fig. 1b), whose strongest diffraction should be near 31o and clearly separated from that of NiCo2S4 [13,46,47]. However, only one main peak of NiCo2S4 is observed. Although the small amount of FeCo2S4 phase may be too small to be detected by XRD, results from XPS and electrochemical measurements suggest that the latter scenario is much more probable here. As will be discussed later, our Fe-added samples exhibit a

significantly higher specific capacity than NiCo2S4. Such a large effect is unlikely caused by a presence of a small amount of FeCo2S4. Thus, the low Fe content in our samples is an indication of low solubility limit of Fe in the NiCo2S4 lattice under the current preparing condition. 3.2 Electrochemical properties The electrochemical performances of NiCo2S4 electrodes were evaluated by CV and GCD in 2 M KOH solution using a standard three-electrode system. The prepared electrodes were directly used as working electrodes. Clear redox peaks are observed in CV of all samples, corresponding to redox reactions of Ni and Co species as follows [2,35]: CoS + OH–

↔

CoSOH + e–

CoSOH + OH–

↔

NiS + OH–

NiSOH + e–

↔

CoSO + H2O + e–

(4) (5) (6)

In many cases, the three redox reactions are inseparable in the CV curve [30,43]. As shown in Fig. 5a, our 0Fe-NCS electrode shows two pairs of redox peaks at 0.23/0.13 and 0.32/0.22 V which could be assigned to the redox reactions of CoS and NiS, respectively [48-52]. Such clear redox peaks indicate that the prepared electrode is battery-like materials which store charges by means of redox reactions. It is noted that smaller redox peaks at 0.23/0.13 V might be additionally contributed by Ni3S2 impurity as follows [31,32]: Ni3S2 + 3OH– ↔

Ni3S2(OH)3 + 3e–

(7)

Adding Fe in NiCo2S4 lattice does not add extra peaks to the CV curves. In addition, the first redox peaks at 0.23/0.13 V is reduced. The exact cause of such a reduction is unclear but could be a combination of the lower Ni3S2 content, lower Co content (replaced by Fe), and shift of the peak position due to different redox processes, for example [53]. The capacity contribution from

Ni foam which was used as substrate is negligible because the CV curve of a pure Ni is almost a straight line. Nevertheless, comparing the CV curves under the same scan rate, the 1Fe-NCS electrode has more remarkable peaks and larger area within the curves, indicating the higher capacity. This is further confirmed by GCD measurements (Fig. 5b). We measured three electrodes from each sample and the average specific capacity with standard deviation are shown in the bar chart (Fig. 5b inset). The capacity of 0Fe-NCS sample (98 mAh/g or 926 F/g at 10 A/g) is close to the value reported by Wei et al. (898.13 F/g at 10 A/g) where similar one-step synthesis of NiCo2S4 was used [43]. It is clear that the charge storage capacity increases with Fe content. A plot of specific capacity as a function of current density for all samples are shown in Fig. 5c. The rate capabilities of 0Fe-NCS, 0.25Fe-NCS, 0.5Fe-NCS and 1Fe-NCS electrodes are 80%, 71%, 68% and 65%, respectively. With increasing current densities, the specific capacity decreases because some regions become inaccessible by ions, while the ions have enough time to diffuse through the materials at low current density. Obviously, the 1Fe-NCS electrode exhibits the highest specific capacity at all current densities. Although the rate capabilities of 1Fe-NCS is lower than other samples, a specific capacity of 167 mAh/g is achieved at a current density of 10 A/g. Such a number is comparable to other NiCo2S4- based electrodes prepared by a hydrothermal technique (Table 1). The cycling stability is evaluated by repeated charge-discharge measurements. A plot of specific capacity as a function of cycle number at current density of 10 A/g is shown in Fig. 5d. The specific capacity slightly increases in the first 100 cycles as a result of the activation of electrode materials [3,6,30]. Then the value gradually decreases with increasing cycle number and 1Fe-NCS retains 58% of its initial capacity after 1000 cycles. Li et al. reported that

amorphous materials possess better cycle stability because amorphous structure can endure the structural changes during redox reaction better [54]. Thus, the lower cycle stability in 1Fe-NCS sample can be explained by its different morphology and high crystallinity as evidenced by the TEM images. SEM images of 0Fe-NCS and 1Fe-NCS before and after 1000 cycles are shown in Fig. S9. The changes in the electrode morphology is obvious and those of 1Fe-NCS is indeed more significant. On the other hand, the Coulombic efficiency (η) during the cycling test was estimated by divivding discharging time with charging time (η = td/tc). As shown in Fig. 5d (inset), the Coulombic efficiencies remain at ~96% for all samples, suggesting that the redox reactions of our electrodes are highly reversible [48,55]. Though the low cycle stability would limit the applications of 1Fe-NCS electrode at this point, it should be emphasized that the electrode exhibits significantly higher specific capacity. The cycle stability may be further improved by modifying the morphology and compositing with other materials [5,56]. 3.3 The role of Fe To gain insight into the role of Fe in improving electrode performance, we consider several related factors. Since surface areas of the electrodes are not significantly different, we consider other factors that may affect the specific capacity of the materials including conductivity of the electrodes, characteristic redox processes of each component, and reactivity of the active surface [19,63-65]. The electrical conductivities of the electrodes were studied by EIS measurements (Fig. 6a). In general, a Nyquist plot consists of a semicircle and a straight line. At high frequency, the intercept at the real axis demonstrates an internal resistance (Rs) and the diameter of a semicircle represents the charge transfer resistances (Rct). The linear part at low frequency region exhibits diffusive resistance (Walberg impedance), indicating the diffusion rate of electrolyte ions. The

straight line should be perpendicular to real axis for ideal supercapacitors [66]. All samples display very small Rs and small semicircles in high frequency region, indicating very low internal resistance and charge-transfer resistances during electrochemical processes [6,42,67]. Such low resistances result from the metallic conducting behavior of NiCo2S4 itself and from a low contact resistance as a result of the one-step preparation [68]. Based on the EIS results, doping Fe improves the conductivity of NiCo2S4. We additionally calculated and analyzed the PDOS of the pristine and doped systems where one Co or Ni atom was replaced by one Fe atom in the NiCo2S4 unit cell. As shown in Fig. 6b, the calculated PDOS reveal that these materials exhibit metallic character as their Fermi energies lie within the band. Substitution of Fe at the Co site results in higher DOS around the Fermi level which could improve electronic conductivity of the materials. The calculated results are consistent with previous computational works [19] and current experimental observation based on the EIS results. Nevertheless, we found that the increased DOS is marginally small due to a dilute doping concentration. It is worth noting that Fe doping at the Ni site exhibits a negative effect toward the conductivity as the DOS at the Fermi level is decreased upon doping. Both experimental and computational results confirm that Fe doping improves electrical conductivity of NiCo2S4 and the electrical conductivities of the electrodes in the order of 1Fe-NCS > 0.5Fe-NCS > 0.25Fe-NCS > 0Fe-NCS is consistent with the capacity trend. Thus, the improved conductivity is one of the reasons for enhanced capacitive performance [6,42,66] but the effect is unlikely large enough to explain the observed 150% enhancement [64]. It is interesting to note that FeCo2S4 is electrochemically active, with the specific capacity close to that of NiCo2S4 [13]. Although Fe3+ in our samples might be electrochemically active as well, the small amount of Fe3+ could not provide such rich redox reactions that the capacity of

1Fe-NCS electrode with only about 2% Fe content is 1.5 times of that of 0Fe-NCS. Thus, we consider the activity of the materials’ surface, another possible factor that might affect the capacity [19]. We analyzed the electronic structures of electrode surfaces to study the influence of Fe doping toward the charge storage capability. To do this, we created a slab model of NiCo2S4 (100) surface and replace one transition metal (TM) atom with one Fe atom to yield the Fe-doped surfaces. Three different doping sites were considered including Co and Ni sites on the topmost surface and a Ni site at the subsurface as schematically depicted in Fig. S1 panel b and c. The computed Bader charges reveal that doping with relatively less electronegative Fe at a Co site increases the electron densities of the neighboring S atoms as their net charges become more negative (Fig. 6c). On the contrary, doping at Ni sites, either on the topmost or subsurface layer, does not display charge accumulation at the neighboring S atoms. The electron densities are distributed over several Co and S sites on the surface. In addition, the calculated PDOS of the surfaces show that the d-band center shifts closer to the Fermi level upon Fe doping at the Co site (from –1.30 eV to –1.26 eV) suggesting the stronger chemical adsorption capability. In contrast, weaker adsorption was predicted for the Fe doping at Ni sites as their d-band centers shift toward the negative energy from –1.30 eV to –1.34 eV (Fig. S10). Since the redox reactions of NiCo2S4 (equation 4-6) require both active species to react with OH– in the electrolyte, the electrolyte-electrode interactions play an important role in determining the electrochemical properties of the NiCo2S4 electrodes. Thus, the formation of KOH electrical double layer (EDL) at the electrode interface was further explored. Since there are six TM sites on the topmost layer of the surface model, we added six formula units of KOH on the surface where OH– ions were placed on the atop sites of TM atoms. The counter ions, K+,

were added in the second layer next to the OH– ions to model the electrical double layer formed during the supercapacitor operations. Relaxation of internal coordinates yields equilibrium structures of KOH electrolyte on the surfaces. As shown in Fig. 6d, four OH– ions chemically adsorb on the surface where TM–O bonds were formed while the other two OH– ions weakly adsorb on the S sites pointing their H ends toward the surface. The calculated energies reveal that Fe doping at the Co site improves electrolyte interaction as its energy is lower than that of the pristine surface (–2.24 vs –2.07 eV / KOH). On the other hand, doping at Ni sites display higher energy (–1.99 and –1.92 eV / KOH) suggesting a slightly lower stability of KOH on the electrode surfaces. The calculated energies are in agreement with the electronic structure calculations. Only the Fe substituted Co surface displays significant charge alterations with a positive shift of d-band center, suggesting the improved adsorptivity of KOH electrolyte on the surface. The improved EDL-surface interaction suggests that replacing Co with Fe eases the OH– adsorption which improves the reactivity of both metallic ions and consequently increases the capacity. 4. Conclusions We have successfully synthesized series of Fe-doped NiCo2S4, directly grown on Ni foam via a simple one-step hydrothermal method. The performance of Fe-doped NiCo2S4 binder-free electrodes can be improved by increasing the amount of FeCl3. Here, FeCl3 additive plays two different roles in the synthesis. It promotes a 3D growth of highly crystalline materials in hydrothermal reaction and serves as a source providing Fe3+ to NiCo2S4 lattice. The first effect does not have major influence on the capacity but indirectly lowers the cycle stability of the electrode. On the other hand, Fe doping in NiCo2S4 lattice significantly improves the specific capacity as 1Fe-NCS electrode exhibits the highest specific capacity of 167 mAh/g at a current

density of 10 A/g, ~1.7 times higher than that of the pristine NiCo2S4 electrode.

Both

experimental and computational results confirm that Fe improves the electrical conductivity of material. In addition, doping Fe at Co site improves the electrolyte adsorption as it lowers adsorption energies, which benefits charge storage capability. The obtained electrodes exhibit good rate capability and moderate cycle stability. These results suggest that metallic doping can be an interesting strategy to improve the performance of NiCo2S4- based electrodes for supercapacitor applications. Acknowledgements This work is supported by Suranaree University of Technology and the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Higher Education, Science, Research and Innovation (MHESI), Thailand. SUTNANOTEC-SLRI (BL5.2 and BL5.3) Joint Research Facility, the Synchrotron Light Research Institute (SLRI), Thailand, is acknowledged for XANES and XPS facility. We thank W. Meevasana, S. Maensiri and their research groups for the facilities for electrochemical measurements. We would like to thank NSTDA Supercomputer Center (ThaiSC) and Institute of Science, Suranaree University of Technology for computational resources. P. Phonsuksawang acknowledge the the Development and Promotion of Science and Technology Talents Project (DPST) for her study.

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Figure captions Figure 1. XRD patterns of 0Fe-NCS,0.25Fe-NCS, 0.5Fe-NCS, and 1Fe-NCS. Figure 2. (a) Ni 2p XPS spectra (b) Co 2p XPS spectra (c) Fe K-edge XANES spectra and (d) S K-edge XANES spectra of the samples. Figure 3. SEM and TEM images of (a) 0Fe-NCS (1st row), (b) 0.25Fe-NCS (2nd row), (c) 0.5Fe-NCS (3rd row), and (d) 1Fe-NCS (4th row). Figure 4. TEM image (a), a close-up TEM image showing the shell region (b), EDS mapping (c) and EDS spectrum of 1Fe-NCS (d). Figure 5. (a) CV curves at scan rate of 5 mV/s; (b) Galvanostatic discharge curves at current density of 10 A/g and the specific capacity (inset); (c) Specific capacity as a function of current density; (d) Cycle performance at current density of 10 A/g of all samples. Figure 6. a) EIS Nyquist plots of all samples (inset) the enlarge EIS Nyquist plots b) PDOS of pristine, Fe-doped NiCo2S4 bulk with doping sites at Co and Ni c) Bader charges of the two topmost layers of pristine and Fe-doped surfaces where grey, teal, dark blue, and yellow circles represent Ni at topmost layer, Ni at subsurface layer, Co, and S atoms, respectively. The light blue and orange shadow highlight the negative and positive changes of charges upon doping. d) Formation of KOH electrical double layer on the Fe-doped surface at a Co site.

Table 1 NiCo2S4 materials prepared by various methods and their performances Materials Nanosphere-like NiCo2S4 NiCo2S4 arrays

Synthetic technique PVP-assisted hydrothermal method hydrothermal

NiCo2S4 nanoflake Electrodeposition using constant potential mode NiCo2S4 Two-step 3D honeycomb hydrothermal Hierarchical Two-step NiCo2S4 Corehydrothermal Shell Nanowire Arrays Hierarchical One-step porous NiCo2S4 hydrothermal nanostructures MoS2/NiCo2S4@C Hydrothermal (selfhollow template strategy) microspheres NiCo2S4 Two-step microaggregates hydrothermal Fe-added NiCo2S4

One-step hydrothermal

Specific capacity (mAh/g) 70.6 at 4 A/g 77.6 at 2 A/g 82.6 at 1 A/g 76.7 at 6 A/g 77.9 at 4 A/g 79.4 at 2 A/g 80.7 at 1 A/g 154.4 at 4 A/g

Retention

Ref

-

[10]

93% after 5000 cycles at 5 A/g

[57]

80.1% after 5000 cycles at 4 A/g

[58]

-

[59]

93.9% after 5000 cycles at 50 mV/s

[60]

242 at 1 A/g

85.2% after 3000 cycles at 30 A/g

[3]

238 at 10 A/g 250 at 2 A/g

87% after 4000 cycles at 10 A/g

[61]

167 at 10 A/g 238 at 2 A/g 249 at 1 A/g 169.7 at 10 A/g 207.3 at 6 A/g 233.4 at 4 A/g 276.7 at 2 A/g 351.1 at 1 A/g

-

[62]

145.8 at 10 A/g 176.0 at 2 A/g 228 at 10 A/g

58% after 1000 cycles at 10 A/g

This work

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: