Diatomite-like KFeS2 for Use in High-Performance Electrodes for Energy Storage and Oxygen Evolution

Bifunctional materials possess remarkable properties that allow them to store and convert electrical energy easily. In this paper, diatomite-like potassium iron disulfide (KFeS2) was synthesized by a multistep sacrificial template method, and its morphological, electrochemical, and oxygen evolution reaction (OER) properties were investigated. KFeS2 was found to be porous, hollow, and cake-like, which suggests a high specific surface area (SSA) and abundant electrochemically active sites. A very high specific capacitance of 651 F g−1 at 1.0 A g−1 was also obtained due to the substance’s unique structure and high porosity. Additionally, the diatomite-like KFeS2 possessed a very low overpotential ƞ10 of 254 mV at a current density of 10 mA cm−2 and a small Tafel slope of about 48.4 mV dec−1. Thus, the diatomite-like KFeS2 demonstrates broad application prospects for both energy storage and conversion.


Introduction
Energy storage and conversion are two key points related to many proposed solutions for serious environmental problems to energy production, and both have already received considerable academic attention over the years [1][2][3][4]. As a type of electrochemical energy storage device, supercapacitors can be widely used in many "new energy" vehicles, camera flashes, and energy back-up systems because of their higher power density, faster charge and discharge rates, and longer cycle lives compared to conventional batteries [5][6][7]. In addition, the electrochemical energy conversion pathway of the oxygen evolution reaction (OER), which is one half of the electrochemical decomposition of water, is environmentally friendly, low cost, emits no carbon, and can be used to produce clean hydrogen energy. OER has also already been widely studied, and the performance of supercapacitance and OER relies on the active materials used. Iron-based compounds have been used as a primary electrode material for supercapacitors due to the various valence states of Fe, as well as their high specific capacitance, wide potential window, low cost, and their own environmental friendliness [8,9]. Among them, iron-based sulfides have become one of the most important electrode materials for supercapacitors due to their high electrical conductivity and unique physicochemical properties [10][11][12][13]. Furthermore, iron-based sulfide nanomaterials are considered to be efficient electrocatalytic materials due to their high abundance, low toxicity, and high electrochemical activity [14][15][16][17][18].
Among all iron-based compounds, ternary KFeS 2 possesses variable physical and chemical properties because of its mixed-valence Fe [19,20]. Thus, ternary KFeS 2 shows broad application prospects in energy storage and electrocatalysis [21][22][23][24][25][26]. However, the electrochemical properties of most KFeS 2 nanostructures are much lower than their theoretical values because of their low specific surface area (SSA) resulting in poor electrochemically active sites [27][28][29]. Additionally, KFeS 2 has also been found to exhibit the phenomenon of easy agglomeration [21]. Therefore, shaping KFeS 2 into a unique morphology that both prevents agglomeration and enhances its SSA would have immediate benefits.
To this end, as a sacrificial template material, diatomite has become one of the most popular templates for preparing porous structures due to its own high porosity, low volumetric weight, high chemical stability, and high SSA [30,31]. In addition, the threedimensional structure of diatomite has been shown to be able to solve the problem of material agglomeration effectively [6,32,33]. Thus, in this work, we propose a simple and controllable method for the synthesis of KFeS 2 with diatomite morphology using a multistep sacrificial template.

Materials and Methods
All chemicals were purchased from Aladdin (Shanghai, China) and were of analytical purity and thus used without further purification. The diatomite material was also supplied from Aladdin.

Synthesis of FeOOH@D Nanorods on Diatomite (D)
MnO 2 @D was prepared by modifying a one-step hydrothermal method that has been previously published by our research group [6]. KMnO 4 solution (70 mL, 0.05 M) and diatomite (100 mg) were placed into a Teflon-lined stainless-steel autoclave (Henan Gongyi Yuhua Instrument Co. LTD, Henan, China) that was subsequently maintained at 160 • C for 24 h. The sample was then removed, washed with distilled water and ethanol, and dried at 60 • C to obtain MnO 2 composites. Subsequently, 150 mg FeSO 4 ·7H 2 O and 80 mg MnO 2 @D were dissolved and dispersed in 70 mL of an ethylene glycol-water solution (ethylene glycol:water = 1:7) successively, and the mixture was transferred to a Teflon-lined stainless-steel autoclave and rotated at 120 • C for 12 h. After cooling to room temperature, the product was washed with de-ionized (DI) water and dried at 60 • C for 12 h.

Synthesis of Diatomite (D)-like KFeS 2
The as-prepared FeOOH@D (100 mg), Na 2 S·9H 2 O (2.88 g), and KOH (19.6 g) were next dispersed and dissolved in 70 mL of the ethanol-water solution (water: ethanol = 1:1). Then, the mixture was transferred into a Teflon-lined stainless-steel autoclave and rotated constantly at 120 • C for 12 h. After cooling to room temperature, the product was washed with DI water and dried at 60 • C for 12 h.

Synthesis of FeS 2 -Modified Diatomite
To create the FeS 2 -modified diatomite, the as-prepared FeOOH@D (100 mg) and Na 2 S·9H 2 O (2.88 g) were dispersed and dissolved in 70 mL of the ethanol-water solution (water:ethanol = 1:1). Then, the mixture was transferred into a Teflon-lined stainless-steel autoclave and constantly rotated at 120 • C for 12 h. After cooling to room temperature, this product was washed with DI water and dried at 60 • C for 12 h.

Synthesis of KFeS 2
The MnO 2 composites were prepared using a one-step hydrothermal method. As is typical, KMnO 4 solution (70 mL, 0.05 M) was placed into a Teflon-lined stainless-steel autoclave at 160 • C for 24 h. The sample was then removed, washed with distilled water and ethanol, and dried at 60 • C to obtain the MnO 2 composites.
Next, 150 mg FeSO 4 ·7H 2 O and 80 mg MnO 2 were then and dispersed in 70 mL of an ethylene glycol-water solution (ethylene glycol:water = 1:7) successively. The mixture was subsequently transferred to a Teflon-lined stainless-steel autoclave and rotated constantly at 120 • C for 12 h. After cooling to room temperature, the product was washed with DI water and dried at 60 • C for 12 h.
Finally, the as-prepared FeOOH (100 mg), Na 2 S·9H 2 O (2.88 g), and KOH (19.6 g) were dispersed and dissolved in 70 mL of the ethanol-water solution (water:ethanol = 1:1), and the mixture was then transferred into a Teflon-lined stainless-steel autoclave and rotated constantly at 120 • C for 12 h. After cooling to room temperature, the product was washed with DI water and dried at 60 • C for 12 h.

Characterization of Materials
The crystal structures of the KFeS 2 and diatomite-like KFeS 2 were determined by X-ray diffraction (XRD) (Panaco, Almelo, The Netherlands) at a scanning angle of 2θ = 5-80 degrees. The chemical constituents of the samples were determined by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Technology, Waltham, MA, USA) as well, and the micromorphology and structure were characterized using scanning electron microscopy (SEM) (Zeiss, Germany) at 5 kV and transmission electron microscopy (TEM) (Zeiss, Germany).

Electrochemical Measurements
A working electrode was prepared by mixing 70 wt% active materials (diatomite-like KFeS 2 ), 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP), and the slurry was spread onto a foam nickel current collector (1 × 1 cm 2 ). This electrode was then heated to 120 • C for 12 h in order to evaporate the solvent and was then uniaxially pressed under 10 MPa. A three-electrode was then used to evaluate the capacitive performance of the electrode materials.
The electrochemical performance of the as-prepared electrode was carried out using the CHI 660E electrochemical station (Shanghai Chenhua Instrument Co., LTD, Shanghai, China). For the three-electrode configuration, the working electrodes (1 × 1 cm 2 ) were the diatomite-like KFeS 2 , a platinum plate was used as the counter electrode, and silver chloride electrode was used as the reference electrode. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and galvanostatic charging/discharging (GCD) were employed to investigate the electrochemical performance of the composites, where the applied potential window ranged from 0 to 0.6 V in a 6 M KOH electrolyte. CVs were recorded at scan rates of 10, 20, 40, 50, 80, and 100 mV s −1 , and GCD curves were obtained at constant current densities of 1, 2, 4, 5, 8, and 10 A g −1 . CV and GCD had potential windows of 0.6 V and 0.45 V, respectively. Finally, electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 100 kHz to 0.01 Hz with a perturbation amplitude of 5 mV versus the open-circuit potential.

Results and Discussion
As shown in Scheme 1, the diatomite-like KFeS 2 was synthesized using a sacrificial template method that combined the etching of sulfide and diatomite into one step. In the first step of the process, MnO 2 @diatomite (MnO 2 @D) was prepared using a hydrothermal method according to the self-decomposition of KMnO 4 . Then, the prepared MnO 2 was replaced by hydroxyl iron oxide (FeOOH) based on the spontaneous redox reaction between Fe 2+ and MnO 2 . After becoming vulcanized by Na 2 S·9H 2 O in the alcohol-water solution under high temperature, FeOOH was transformed into KFeS 2 and the diatomite was dissolved by KOH, resulting in a hollow diatomite-like KFeS 2 structure.

Scheme 1.
Multistep sacrificial template method to prepare biomimetic diatomite structured KFeS2 electrode materials with bifunction of supercapacitance and hydrolysis: (i) Conversion of MnO2 nanosheets to FeOOH nanorods on the surface of diatomite template; (ii) Etching of diatomite in FeOOH@D template and sulfation of FeOOH by KOH and Na2S·9H20 at the same time to prepare diatomite-like KFeS2. Figure 1 shows the morphology of diatomite-like KFeS2. In Figure 1a, after the multistep sacrificial template method, the KFeS2 also kept a diatomite-like round cake shape that consisted of numerous nanosheets that joined together with each other to form a porous structure (Figure 1b,c). This structure could also be observed inform the TEM images (Figure 1d-f). This unique nanostructure definitely increased the SSA of KFeS2, which is very important for improving its electrochemical and OER properties. Moreover, a lattice spacing of 0.564 nm, as shown in Figure 1g, can be indexed to the (0 2 0) plane of KFeS2, and the typical EDS spectrum in Figure 1h reveals that the atomic percentage of elements K, Fe, and S followed a ratio of 1:1:2, which indicates the presence of KFeS2. Furthermore, the element mapping images and scatter superimposed graph in Figure 1i also show that K, Fe, and S were uniformly distributed in the material. These results point to the successful synthesis of KFeS2 with the 3D morphology and dimensions of diatomite. Scheme 1. Multistep sacrificial template method to prepare biomimetic diatomite structured KFeS 2 electrode materials with bifunction of supercapacitance and hydrolysis: (i) Conversion of MnO 2 nanosheets to FeOOH nanorods on the surface of diatomite template; (ii) Etching of diatomite in FeOOH@D template and sulfation of FeOOH by KOH and Na 2 S·9H 2 0 at the same time to prepare diatomite-like KFeS 2 . Figure 1 shows the morphology of diatomite-like KFeS 2 . In Figure 1a, after the multistep sacrificial template method, the KFeS 2 also kept a diatomite-like round cake shape that consisted of numerous nanosheets that joined together with each other to form a porous structure (Figure 1b,c). This structure could also be observed inform the TEM images (Figure 1d-f). This unique nanostructure definitely increased the SSA of KFeS 2 , which is very important for improving its electrochemical and OER properties. Moreover, a lattice spacing of 0.564 nm, as shown in Figure 1g, can be indexed to the (0 2 0) plane of KFeS 2 , and the typical EDS spectrum in Figure 1h reveals that the atomic percentage of elements K, Fe, and S followed a ratio of 1:1:2, which indicates the presence of KFeS 2 . Furthermore, the element mapping images and scatter superimposed graph in Figure 1i also show that K, Fe, and S were uniformly distributed in the material. These results point to the successful synthesis of KFeS 2 with the 3D morphology and dimensions of diatomite.
For comparison, FeS 2 -modified diatomite was produced but without diatomite etching. .304 Å, c = 5.398 Å), demonstrating its high crystallinity, and the average crystallites size is about 27.5 nm. Additionally, the diffraction peaks of purified diatomite could not be observed, which confirmed that the diatomite was completely etched by KOH. Moreover, the sample without etching was also studied and clearly showed that the synthesis of KFeS 2 had occurred as a product of the co-heating of KOH with Na 2 S·9H 2 O and hydroxy-iron oxide. Figure 2b depicts the FTIR spectrum of the diatomite-like KFeS 2 . The peak at 3420 cm −1 corresponds to strong stretching vibrations of the O-H bond (H 2 O), and the bands at 1126 cm −1 and 829 cm −1 correspond to the asymmetric stretching of sulfur functional groups (S-O). Finally, the peaks at 708 cm −1 , 665 cm −1 , 614 cm −1 , and 539 cm −1 correspond to disulfide stretching vibrations (S-S) [34]. For comparison, FeS2-modified diatomite was produced but without diatomite etching. Figure 2a  .304 Å , c = 5.398 Å ), demonstrating its high crystallinity, and the average crystallites size is about 27.5 nm. Additionally, the diffraction peaks of purified diatomite could not be observed, which confirmed that the diatomite was completely etched by KOH. Moreover, the sample without etching was also studied and clearly showed that the synthesis of KFeS2 had occurred as a product of the co-heating of KOH with Na2S·9H2O and hydroxy-iron oxide. Figure 2b   The surface properties of the prepared diatomite-like KFeS 2 were tested by Brunauer-Emmett-Teller (BET) and N 2 adsorption-desorption measurements. Figure 2c shows the corresponding N 2 adsorption-desorption isotherms and the pore size distribution curves of the samples. As shown in Figure 2c, the isotherm surface is that of a typical mesoporous material with a sharp increase in N 2 adsorption near a relative pressure of 1, which indicates the presence of macropores in the diatomite-like KFeS 2 . This sharp increase is due to the presence of macropores on the surface of the diatomite template. According to BET analysis, the synthesized diatomite-like KFeS 2 exhibited a large specific surface area of 42.35 m 2 g −1 , which is much higher than that of the modified diatomite studied in our previous work [35]. From the pore size distribution curves, the diatomite-like KFeS 2 shows a wide pore size spread, and the average pore diameter was 29.03 nm in 4 V A −1 by BET, which formed a layered porous structure possessed by the composite. This type of layered surface morphology with good pore structure is beneficial for enhancing the performance of electrochemical capacitors because the large pore channels allow fast electrolyte transport, and the small pores provide more active sites for chemical reactions to take place. 292.61 eV, corresponding to the K 2p1/2 and K 2p3/2 spin-orbit peaks of diatomite-like KFeS2, respectively. Figure 2e shows the spectra of Fe 2p for the diatomite-like KFeS2, where the peaks located at binding energies 722.53 eV and 708.32 eV belong to Fe 2p1/2 and Fe 2p3/2, respectively, suggesting the presence of Fe 3+ [36,37]. In Figure 2f, the peaks at 161.81 eV and 160.67 eV correspond to S 2p1/2 and S 2p3/2, respectively, and we attribute the additional weak peak at 167.74 eV to a sulfur-oxygen bond that was possibly caused by air contact [38]. To explore the chemical composition of the structure further, X-ray photoelectron spectroscopy (XPS) was performed to examine the elemental states of the diatomite-like KFeS 2 . The corresponding high-resolution spectra of K, Fe, and S are displayed in Figure 2d-f. The XPS spectrum of K 2p in Figure 2d shows two typical peaks at 295.40 eV and 292.61 eV, corresponding to the K 2p 1/2 and K 2p 3/2 spin-orbit peaks of diatomite-like KFeS 2 , respectively. Figure 2e shows the spectra of Fe 2p for the diatomite-like KFeS 2 , where the peaks located at binding energies 722.53 eV and 708.32 eV belong to Fe 2p 1/2 and Fe 2p 3/2 , respectively, suggesting the presence of Fe 3+ [36,37]. In Figure 2f, the peaks at 161.81 eV and 160.67 eV correspond to S 2p 1/2 and S 2p 3/2 , respectively, and we attribute the additional weak peak at 167.74 eV to a sulfur-oxygen bond that was possibly caused by air contact [38].
Next, in order to compare the electrochemical performance of diatomite-like KFeS 2 , FeS 2 @D, and KFeS 2 , the CV curves at a scan rate of 20 mV s −1 with GCD curves at a current density of 4 A g −1 are shown in Figure 3a,b. Evidently, the area surrounded by the curve of the diatomite-like KFeS 2 was larger than the others. The GCD curves of the diatomite-like KFeS 2 , FeS 2 @D, and KFeS 2 are shown in Figure 3b. The discharge time of the diatomite-like KFeS 2 was longer than that of FeS 2 @D or KFeS 2 , which indicates its larger capacitance. In addition, the areas of the CV curves of the diatomite-like KFeS 2 and FeS 2 @D were larger than that of KFeS 2 , and their discharge times were also longer than KFeS 2 , indicating enhancement in electrochemical capacitance from diatomite morphology. Additionally, the CV curves of the diatomite-like KFeS 2 electrodes in 6 M KOH aqueous electrolyte at various scan rates are shown in Figure 3c, where an obvious redox peak can be seen that indicates that a typical faradaic pseudo reaction occurred between the KFeS 2 electrode and the KOH electrolyte. As shown in Figure 3d, stable platforms can also be observed in the charge-discharge curves, indicating a pseudo-reaction response of the active material. The specific capacitance C m (F g −1 ) can be calculated using Equation (1): where I is the discharging current, U is the potential, t is the discharging time, and m is the weight of the active materials. The specific capacitance of diatomite-like KFeS 2 was calculated to be 651 F g −1 at a current density of 1.0 A g −1 , highlighting its unique porous structure and high SSA. High porosity makes it easier for ions to be transferred into the structure, leading to more redox reactions and surface adsorption of electrolyte ions.    Table 1 shows the comparison of iron-based sulfide electrodes and highlights the excellent electrochemical performance of diatomite-like KFeS 2 . In addition, the rate capabilities of diatomite-like KFeS 2 , FeS 2 @D, and KFeS 2 are displayed in Figure 3e. Obviously, the slope of the diatomite-like KFeS 2 was lower than that of FeS 2 @D or KFeS 2 , meaning that the diatomite-like KFeS 2 had a better rate capability. The overall specific capacitance of the diatomite-like KFeS 2 also showed a slightly decreasing trend as current density increased from 1.0 to 10.0 A g −1 . However, a slight increase at low current density (1.0 to 4.0 A g −1 ) can be ascribed to the thermal effect caused by repeated charging and discharging of the electrode [39]. After increasing the current density to 10 A g −1 , about 66.8% of the initial capacitance remained for the diatomite-like KFeS 2 electrode, which serves to highlight is rate capability further. Specifically, this rate in FeS 2 @D was about 70.6% and in KFeS 2 was about 54.6%.  [44] The rate capabilities of the diatomite-like KFeS 2 and FeS 2 @D were both better than KFeS 2 , which indicates improvement from introducing the diatomite morphology. Furthermore, Figure S1 shows the cycle performance of the diatomite-like KFeS 2 at a current density of 4 A g −1 . Here, we see that the impedance of the diatomite-like KFeS 2 electrode was calculated over a frequency range of 100 kHz-0.01 Hz by applying an AC voltage with an amplitude of 5 mV at the open circuit potential. As shown in Figure 3f, the impedance patterns had a half arc at high frequencies and a linear part at low frequencies.
The equivalent circuit of the Nyquist plot is shown in Figure 3f, where we can see that the impedance arc in the high frequencies region can be replaced by an interfacial Faraday charge transfer resistor (R ct ) and a parallel constant phase element (C d ) for the double layer capacitance. A straight line with a slope of 45 • in the mid-frequency region along the arc indicates the finite-length diffusive Warburg impedance (Z w ), which is associated with the diffusion/transport of electrolyte ions in the electrode. The vertical line in the very low frequency region shows the ideal capacitive behavior of the diatomite-like KFeS 2 electrode material, and the EIS results show the pseudo-capacitive properties and porous structure characteristics of the diatomite-like KFeS 2 electrode.
The catalytic performance of the diatomite-like KFeS 2 was also evaluated in 1 M KOH. For comparison, FeS 2 -modified diatomite and iron-based sulfide without diatomite morphology (KFeS 2 ) were set as controls. The electrocatalytic activities of the diatomite-like KFeS 2 , FeS 2 @D, and KFeS 2 nanostructured electrodes were investigated using linear sweep voltammetry (LSV) measurements at a scan rate of 5.0 mA s −1 , and the results are shown in Figure 4a. The overpotential (η) for the diatomite-like KFeS 2 was estimated to be~0.25 mV, which was much lower than both KFeS 2 (~0.29 mV) and FeS 2 @D (~0.37 mV at a current density of 10 mA cm −2 . In addition, the Tafel slopes of the diatomite-like KFeS 2 , KFeS 2 , and FeS 2 @D were estimated as 48.4 mV dec −1 , 52.7 mV dec −1 , and 155.4 mV dec −1 , respectively, as shown in Figure 4b. The Tafel slope of the diatomite-like KFeS 2 was lower than the other two structures, indicating its superior reaction kinetics characteristics and excellent oxygen evolution performance. The EIS impedance spectrum in Figure 4c also shows that the impedance of the diatomite-like KFeS 2 was much lower than that of KFeS 2 or FeS 2 @D, which indicates its superior electrical conductivity. The lowest impedance of diatomite-like KFeS 2 favors the transfer of electrons and results in the lowest overpotential among the three substances as well. This low impedance is also beneficial for the adsorption and desorption of intermediates during the reaction process and contributes to the excellent catalytic performance of diatomite-like KFeS 2 .    Cycling performance was further tested using the time-current method, and the results for the diatomite-like KFeS 2 , KFeS 2 , and FeS 2 @D at an initial potential of 0.53 V, 0.62 V, and 0.67 V for each after 24 h are shown in Figure 4d. Analysis of the time-current curve results shows that the cycling stability of the diatomite-like KFeS 2 and FeS 2 @D were much higher than that of the KFeS 2 without the morphology of diatomite. The cyclic curves of the diatomite-like KFeS 2 and FeS 2 @D were noticeably similar, indicating that the morphology of diatomite made a central contribution to its cycling performance. The electrocatalytic performance of catalysts for hydrolysis is highly dependent on their catalytic activity and the number of active sites, which are closely related to the morphology, size, and structure of the catalyst [45]. Indeed, we found that the extremely low overpotential, good electrical conductivity, and superior cycling stability of the diatomitelike KFeS 2 synthesized in this study were due to its unique diatomite nanostructure, which increased the number of active centers. According to the adsorbate evolution mechanism (AEM), an increased number of metal active sites can adsorb more intermediates and can facilitate the synergistic effect of Fe and S, thus accelerating the electrocatalytic performance of water decomposition [46]. Finally, we provide a table of comparisons (see Table 2) of the results for the diatomite-like KFes 2 with other sulfides that have been reported earlier, and these results show that the prepared diatomite-like KFeS 2 had excellent OER performance.

Conclusions
In summary, diatomite-like KFeS 2 was successfully synthesized using a simple multistep sacrificial template method. The nanostructure exhibited the potential for a broad set of applications, such as in supercapacitor anodes and electrocatalysts for OERs. The diatomitelike KFeS 2 also exhibited extremely high specific capability (651 F g −1 at 1.0 A g −1 ). In addition, as an electrocatalyst, the diatomite-like KFeS 2 possessed a lower overpotential (η 10 of 254 mV at a current density of 10 mA cm −2 ) during OERs, compared to other ironbased sulfide composite materials. These results indicate that diatomite-like KFeS 2 shows promise for use in the assembly of supercapacitors and oxygen precipitation reactions.