One-Pot Synthesis of Bismuth Sulﬁde Nanostructures as an Active Electrode Material for Aqueous Hybrid Capacitors

: The high theoretical capacity of Bi 2 S 3 shows high promise as a negative electrode material for energy storage devices. Herein, we investigate a facile, one-step chemical precipitation method using common organic solvents, such as acetone, ethanol, and isopropanol, for the synthesis of Bi 2 S 3 nanostructures. The nanospherical Bi 2 S 3 from acetone (Bi 2 S 3 -A) presents the most balanced electrochemical properties, exhibiting a high speciﬁc capacity of 181 mAh g − 1 at 1 A g − 1 and decent rate capability. Additionally, Bi 2 S 3 -A is used as a negative electrode in an aqueous hybrid system with an activated carbon positive electrode, demonstrating a capacitance of 86 F g − 1 , a speciﬁc energy of 7.6 Wh kg − 1 , and an initial capacity retention of 74% after 1000 cycles.


Introduction
The development of energy storage systems is currently flourishing due to the growing demand for systems that can deliver high amounts of energy. Lithium-ion batteries, electrochemical capacitors, supercapacitors, and their combination as supercapatteries are used in many portable devices, and it is necessary to develop these devices to deliver high power/energy densities in a relatively short period of time. Among the many materials that are used as electrode materials in the discussed devices, the most known are activated carbons (ACs) [1][2][3]. Other carbon materials, such as carbon nanofibers and nanotubes or graphene and reduced graphene oxide, have been increasingly considered for energy storage devices [4][5][6]. However, the capacitance of carbon materials is limited by the electric double layer (EDL) due to the specific surface area, which is a crucial factor [7][8][9]. In addition to carbon materials, materials with pseudocapacitive properties as transition metal oxides/hydroxides have been intensively considered for application in pseudocapacitors due to their fast redox reactions and high specific capacitance. Transition metal oxides and/or hydroxides are characterized by high resistance values, which are undesirable in energy storage devices [10,11]. To manage the current requirements, innovative materials with high specific capacitance and low resistance have been increasingly studied. A novel group of electrode materials are transition metal sulfides, which combine the most important features for being competitive in comparison to carbon materials and metal oxides/hydroxides. Transition metal sulfides are characterized by extremely high specific capacitance values and semiconducting properties. Among various transition metal sulfides, such as CuS, CoS, Ni 2 S 3 , MnS, and MoS 2 , Bi 2 S 3 seems to be the most attractive material for energy storage and conversion devices [12][13][14][15][16][17]. The extraordinary features of bismuth sulfide include its unique electric properties resulting from its semiconductor behavior with a direct band gap of 1.3 eV and a high capacitance value; thus, Bi 2 S 3 shows tremendous promise. Recently, a large amount of effort has been exerted to explore a new concept of electrochemical capacitors called hybrid capacitors based on electrodes with EDL or pseudocapacitive properties that are combined with The specific capacity was expressed in mAh per mass of active material in one electrode. The specific capacity values (Qs/mAh g −1 ) were calculated from the cyclic voltammetry curves and galvanostatic discharge profiles using Equations (1) and (2), respectively.
where I is the current (A), ∆t is the discharge time (s), ν is the scan rate (V s −1 ), and m el is the mass of the active material in the electrode (g). For the two-electrode Swagelok™ cell, a hybrid device with a Bi 2 S 3 negatrode and pitch-based AC positrode (Bi 2 S 3 -A/AC) was assembled at a negative electrode:positive electrode mass ratio of 1:2 (4-5 and 9-10 mg, respectively). The gravimetric cell capacitance (C cell ) was calculated using Equation (3) [26]: where I is the discharge current (A), Vdt is the integrated area under the galvanostatic discharge curve, M is the total mass of the electrodes (g), and V i -V f is the operating discharge voltage from initial to final potential values. Additionally, electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 400 kHz to 10 mHz at an AC amplitude of 5 mV were performed for the assembled device along with cycling stability tests at a current density of 1 A g −1 .
The specific energy (E in Wh kg −1 ) and specific power delivered (P in kW kg −1 ) by a hybrid capacitor were calculated using Equations (4) and (5), respectively:

Results and Discussion
The porous structure of the synthesized Bi 2 S 3 using various solvents was recognized by N 2 sorption at 77 K. The mesoporous character of all bismuth sulfides are corroborated by the hysteresis loops ( Figure 1). The BET surface area of obtained samples was relatively low, as suspected for pristine inorganic materials. The highest BET value of 32 m 2 g −1 was noticed for Bi 2 S 3 -E, while the lowest value of 17 m 2 g −1 for Bi 2 S 3 -I. The structural parameters of the obtained samples are included in Table 1. Moreover, among the four discussed samples, two groups with comparable porous structure parameters can be distinguished. The first group consists of the Bi 2 S 3 samples synthesized using acetone and ethanol as solvents, while the second group consists of bismuth sulfides obtained in isopropanol and water environments. Notably, within both groups, it is clearly seen that the porous texture of Bi 2 S 3 is more developed when the synthesis is carried out using acetone and ethanol as solvents. The V T of the samples ranges from 0.080 cm 3 g −1 to 0.121 cm 3 g −1 for Bi 2 S 3 -W and Bi 2 S 3 -A, respectively. Some differences are observed in the case of V mes , which varies from 0.067 cm 3 g −1 for Bi 2 S 3 -I to 0.088 cm 3 g −1 for Bi 2 S 3 -A.  The XRD patterns ( Figure 2) present different crystalline structures depending on the synthesis solvent. An amorphous phase is recorded for Bi2S3-A and Bi2S3-E. The XRD pattern of Bi2S3-I reveals the appearance of some characteristic of a primitive orthorhombic crystalline structure of Bi2S3 (JCPDS 17-0320). Results can suggest that when isopropanol is applied as the synthesis solvent, crystalline structure is not fully formed and isopropanol led to intermediate form between crystalline and amorphous phase. Moreover, the crystalline phase of Bi2S3 is clearly observed when water is the synthesis environment, and the diffraction peaks correlate with the Bi2S3 orthorhombic phase. The diffraction planes are assigned in agreement with the standard diffraction pattern of Bi2S3 (JCPDS 65-2435). The different crystalline structures of the obtained bismuth sulfides indicate that the kind of solvent strictly affects the crystalline structure. In addition to the influence of the solvent type, the formation of crystalline structure of bismuth sulfide can be also achieved by the Bi2S3 treatment at a growing temperature [27]. No impurities or Bi2O3 phases are observed in the materials synthesized via the proposed precipitation method with ammonium sulfide.  The XRD patterns ( Figure 2) present different crystalline structures depending on the synthesis solvent. An amorphous phase is recorded for Bi 2 S 3 -A and Bi 2 S 3 -E. The XRD pattern of Bi 2 S 3 -I reveals the appearance of some characteristic of a primitive orthorhombic crystalline structure of Bi 2 S 3 (JCPDS 17-0320). Results can suggest that when isopropanol is applied as the synthesis solvent, crystalline structure is not fully formed and isopropanol led to intermediate form between crystalline and amorphous phase. Moreover, the crystalline phase of Bi 2 S 3 is clearly observed when water is the synthesis environment, and the diffraction peaks correlate with the Bi 2 S 3 orthorhombic phase. The diffraction planes are assigned in agreement with the standard diffraction pattern of Bi 2 S 3 (JCPDS 65-2435). The different crystalline structures of the obtained bismuth sulfides indicate that the kind of solvent strictly affects the crystalline structure. In addition to the influence of the solvent type, the formation of crystalline structure of bismuth sulfide can be also achieved by the Bi 2 S 3 treatment at a growing temperature [27]. No impurities or Bi 2 O 3 phases are observed in the materials synthesized via the proposed precipitation method with ammonium sulfide.  The XRD patterns ( Figure 2) present different crystalline structures depending on the synthesis solvent. An amorphous phase is recorded for Bi2S3-A and Bi2S3-E. The XRD pattern of Bi2S3-I reveals the appearance of some characteristic of a primitive orthorhombic crystalline structure of Bi2S3 (JCPDS 17-0320). Results can suggest that when isopropanol is applied as the synthesis solvent, crystalline structure is not fully formed and isopropanol led to intermediate form between crystalline and amorphous phase. Moreover, the crystalline phase of Bi2S3 is clearly observed when water is the synthesis environment, and the diffraction peaks correlate with the Bi2S3 orthorhombic phase. The diffraction planes are assigned in agreement with the standard diffraction pattern of Bi2S3 (JCPDS 65-2435). The different crystalline structures of the obtained bismuth sulfides indicate that the kind of solvent strictly affects the crystalline structure. In addition to the influence of the solvent type, the formation of crystalline structure of bismuth sulfide can be also achieved by the Bi2S3 treatment at a growing temperature [27]. No impurities or Bi2O3 phases are observed in the materials synthesized via the proposed precipitation method with ammonium sulfide.  FESEM was employed to compare and contrast the surface morphologies of the bismuth sulfides synthesized using various solvents. The observations reveal discernible differences in the Bi 2 S 3 morphologies ( Figure 3). The use of distinct solvents leads to Bi 2 S 3 with different particle shapes. Coral-like particles of Bi 2 S 3 are obtained when ethanol or acetone is used as the solvent. However, the Bi 2 S 3 -E particles are significantly smaller than the Bi 2 S 3 -A particles and do not exceed 100 nm, whereas the Bi 2 S 3 -A particles can have a diameter of 250 nm. Moreover, the size of particles can affect the porous structure of Bi 2 S 3 . It was observed that the size of bismuth sulfide particles increases in the sequence Bi 2 S 3 -E < Bi 2 S 3 -A < Bi 2 S 3 -W < Bi 2 S 3 -I, which is in accordance with the decreasing BET surface area of bismuth sulfides. FESEM was employed to compare and contrast the surface morphologies of the bismuth sulfides synthesized using various solvents. The observations reveal discernible differences in the Bi2S3 morphologies ( Figure 3). The use of distinct solvents leads to Bi2S3 with different particle shapes. Coral-like particles of Bi2S3 are obtained when ethanol or acetone is used as the solvent. However, the Bi2S3-E particles are significantly smaller than the Bi2S3-A particles and do not exceed 100 nm, whereas the Bi2S3-A particles can have a diameter of 250 nm. Moreover, the size of particles can affect the porous structure of Bi2S3. It was observed that the size of bismuth sulfide particles increases in the sequence Bi2S3-E < Bi2S3-A < Bi2S3-W < Bi2S3-I, which is in accordance with the decreasing BET surface area of bismuth sulfides. To further analyze the surface chemical composition and oxidation state of the Bi2S3 samples synthesized in different solvents, XPS was performed, and the results are shown in Figure 4. The XPS survey spectra presented in Figure 4a explicitly reveal the presence of only Bi and S, suggesting the purity of the obtained bismuth sulfides. The Bi and S peak positions agree well with those in other papers [23,24]. The high-resolution Bi 4f and S 2p spectra of the obtained bismuth sulfides are shown in Figure 4b-e. The two strong peaks at binding energies of 158.9 and 164.2 eV correspond to the Bi 4f7/2 and Bi 4f5/2 in Bi2S3, respectively [28]. The bismuth sulfides synthesized using ethanol, isopropanol, and acetone as solvents are characterized by the nearly symmetrical shape of their peaks, which is attributed to the pure phase of the Bi2S3 surface. In the case of bismuth sulfide obtained in water, two extra signals, leading to some distortions on the shoulders of the Bi 4f7/2 and Bi 4f5/2 peaks, are observed. This feature can be attributed to the heterogeneous composition on the surface of Bi2S3. Moreover, the two peaks with lower intensity located between Bi 4f7/2 and Bi 4f5/2 can be ascribed to S 2p3/2 (161.1 eV) and S 2p1/2 (162.2 eV), confirming the valence state of sulfur as S 2- [29]. To further analyze the surface chemical composition and oxidation state of the Bi 2 S 3 samples synthesized in different solvents, XPS was performed, and the results are shown in Figure 4. The XPS survey spectra presented in Figure 4a explicitly reveal the presence of only Bi and S, suggesting the purity of the obtained bismuth sulfides. The Bi and S peak positions agree well with those in other papers [23,24]. The high-resolution Bi 4f and S 2p spectra of the obtained bismuth sulfides are shown in Figure 4b-e. The two strong peaks at binding energies of 158.9 and 164.2 eV correspond to the Bi 4f 7/2 and Bi 4f 5/2 in Bi 2 S 3 , respectively [28]. The bismuth sulfides synthesized using ethanol, isopropanol, and acetone as solvents are characterized by the nearly symmetrical shape of their peaks, which is attributed to the pure phase of the Bi 2 S 3 surface. In the case of bismuth sulfide obtained in water, two extra signals, leading to some distortions on the shoulders of the Bi 4f 7/2 and Bi 4f 5/2 peaks, are observed. This feature can be attributed to the heterogeneous composition on the surface of Bi 2 S 3 . Moreover, the two peaks with lower intensity located between Bi 4f 7/2 and Bi 4f 5/2 can be ascribed to S 2p 3/2 (161.1 eV) and S 2p 1/2 (162.2 eV), confirming the valence state of sulfur as S 2− [29].  The results of the CV measurements, presented in Figure 5, show distinguishable redox peaks related to the faradaic reactions of the Bi2S3 nanostructures. At a low scan rate, where the current response is diffusion controlled, two pairs of anodic/cathodic peaks appear [30]. The first pair is attributed to the reaction of Bi2S3 with OH-ions, and this pair originates from the change in the bismuth valence state (−0.6 and −0.45 V, Figure  5a) [23]. Moreover, Bi2S3 prepared from isopropanol presents a peak shift towards more negative values compared with the rest of the synthesized materials. The second pair is related to the adsorption of hydrogen in the lamellar structure of Bi2S3 (−0.85 V), while the anodic peak at −0.3 V is related to hydrogen oxidation during the electrochemical measurements. [25,31] As the scan rate increases to 100 mV s −1 (Figure 5b,c), the cathodic peaks become very wide, and a shift of both the anodic and cathodic redox peaks is observed,  The results of the CV measurements, presented in Figure 5, show distinguishable redox peaks related to the faradaic reactions of the Bi 2 S 3 nanostructures. At a low scan rate, where the current response is diffusion controlled, two pairs of anodic/cathodic peaks appear [30]. The first pair is attributed to the reaction of Bi 2 S 3 with OH-ions, and this pair originates from the change in the bismuth valence state (−0.6 and −0.45 V, Figure 5a) [23]. Moreover, Bi 2 S 3 prepared from isopropanol presents a peak shift towards more negative values compared with the rest of the synthesized materials. The second pair is related to the adsorption of hydrogen in the lamellar structure of Bi 2 S 3 (−0.85 V), while the anodic peak at −0.3 V is related to hydrogen oxidation during the electrochemical measurements. [25,31] As the scan rate increases to 100 mV s −1 (Figure 5b,c), the cathodic peaks become very wide, and a shift of both the anodic and cathodic redox peaks is observed, indicating the indicating the change in the internal resistance of the electrode and the quasi-reversible nature of the faradaic reaction. The faradaic reaction of bismuth sulfide is described according to the following Equation: Bi2S3 + H2O +e -⇄ (Hads + Bi2S3) + OH - In accordance with the CV results, the GCD discharge curves of the Bi2S3 electrodes ( Figure 6a) show a distinct plateau at a potential of approximately −0.6 V, indicating the battery-like behavior of the charge storage mechanism. At a current density of 0.5 A g −1 , a smaller plateau, which is related to hydrogen adsorption and is observable in the CV measurements at a potential of −0.85 V, does not appear due to diffusion limitations [31]. At a higher current density of 10 A g −1 , changes in the charging curve occur in the Bi2S3 electrodes as the slope becomes more capacitive in nature. The longest charge-discharge time among all materials at 0.5 A g −1 is observed for Bi2S3-I; however, under increasing current regimes, its charge storage capability decreases compared with other materials (Figure 6b). These results indicate that the formed crystalline structure and hexagonal nanoplatelet morphology lead to the highest specific capacity values, but in terms of rate capability, the more amorphous structure and spherical-like particles of Bi2S3-A are more favorable traits. These observations can be related to the degree of crystallinity of the material; for instance, more amorphous/enriched defects in the structure of a material exhibiting Nernstian features allows for the distribution of redox reactions over a wider potential range, thereby translating to improved electrochemical performance at higher scan rates [32]. indicating the change in the internal resistance of the electrode and the quasi-reversible nature of the faradaic reaction. The faradaic reaction of bismuth sulfide is described according to the following Equation: Bi2S3 + H2O +e -⇄ (Hads + Bi2S3) + OH -(6) In accordance with the CV results, the GCD discharge curves of the Bi2S3 electrodes ( Figure 6a) show a distinct plateau at a potential of approximately −0.6 V, indicating the battery-like behavior of the charge storage mechanism. At a current density of 0.5 A g −1 , a smaller plateau, which is related to hydrogen adsorption and is observable in the CV measurements at a potential of −0.85 V, does not appear due to diffusion limitations [31]. At a higher current density of 10 A g −1 , changes in the charging curve occur in the Bi2S3 electrodes as the slope becomes more capacitive in nature. The longest charge-discharge time among all materials at 0.5 A g −1 is observed for Bi2S3-I; however, under increasing current regimes, its charge storage capability decreases compared with other materials (Figure 6b). These results indicate that the formed crystalline structure and hexagonal nanoplatelet morphology lead to the highest specific capacity values, but in terms of rate capability, the more amorphous structure and spherical-like particles of Bi2S3-A are more favorable traits. These observations can be related to the degree of crystallinity of the material; for instance, more amorphous/enriched defects in the structure of a material exhibiting Nernstian features allows for the distribution of redox reactions over a wider potential range, thereby translating to improved electrochemical performance at higher scan In accordance with the CV results, the GCD discharge curves of the Bi 2 S 3 electrodes (Figure 6a) show a distinct plateau at a potential of approximately −0.6 V, indicating the battery-like behavior of the charge storage mechanism. At a current density of 0.5 A g −1 , a smaller plateau, which is related to hydrogen adsorption and is observable in the CV measurements at a potential of −0.85 V, does not appear due to diffusion limitations [31]. At a higher current density of 10 A g −1 , changes in the charging curve occur in the Bi 2 S 3 electrodes as the slope becomes more capacitive in nature. The longest charge-discharge time among all materials at 0.5 A g −1 is observed for Bi 2 S 3 -I; however, under increasing current regimes, its charge storage capability decreases compared with other materials (Figure 6b). These results indicate that the formed crystalline structure and hexagonal nanoplatelet morphology lead to the highest specific capacity values, but in terms of rate capability, the more amorphous structure and spherical-like particles of Bi 2 S 3 -A are more favorable traits. These observations can be related to the degree of crystallinity of the material; for instance, more amorphous/enriched defects in the structure of a material exhibiting Nernstian features allows for the distribution of redox reactions over a wider potential range, thereby translating to improved electrochemical performance at higher scan rates [32]. The relationships between the specific capacity and scan rate are presented in Figure  7a, while the specific capacity vs. discharge current density relationship is shown in Figure 7b. The highest specific capacity at a scan rate of 1 mV s −1 is achieved by Bi2S3-I, with a value of 247 mAh g −1 , while the lowest value of 169 mAh g −1 is recorded for Bi2S3-W. From the galvanostatic charge-discharge measurements, Bi2S3-I exhibits a specific capacity of 204 mAh g −1 at a current density of 0.5 A g −1 , followed by Bi2S3-E, Bi2S3-A and Bi2S3-W, which present specific capacities of 189, 187, and 135 mAh g −1 , respectively. The fully crystalline structure of Bi2S3-W and its nanorod-like morphology does not provide enough active sites for redox reactions; thus, its charge storage capability is the lowest among all materials. Moreover, bismuth sulfide prepared in acetone and water presents the best rate capability among all samples, maintaining 25 and 29% of the initial capacity at a high current density of 20 A g −1 , respectively. Bi2S3-I exhibits the highest recorded capacity, while Bi2S3-A is a more rational choice for electrochemical applications, as its charge storage properties do not decrease as fast as the material obtained from isopropanol. Among other reported bismuth sulfides, Bi2S3 synthesized from organic solvents such as acetone, isopropanol, and ethanol exhibits higher specific capacities due to its stable electrochemical performance over a wide working potential window ( Table 2). The relationships between the specific capacity and scan rate are presented in Figure 7a, while the specific capacity vs. discharge current density relationship is shown in Figure 7b. The highest specific capacity at a scan rate of 1 mV s −1 is achieved by Bi 2 S 3 -I, with a value of 247 mAh g −1 , while the lowest value of 169 mAh g −1 is recorded for Bi 2 S 3 -W. From the galvanostatic charge-discharge measurements, Bi 2 S 3 -I exhibits a specific capacity of 204 mAh g −1 at a current density of 0.5 A g −1 , followed by Bi 2 S 3 -E, Bi 2 S 3 -A and Bi 2 S 3 -W, which present specific capacities of 189, 187, and 135 mAh g −1 , respectively. The fully crystalline structure of Bi 2 S 3 -W and its nanorod-like morphology does not provide enough active sites for redox reactions; thus, its charge storage capability is the lowest among all materials. Moreover, bismuth sulfide prepared in acetone and water presents the best rate capability among all samples, maintaining 25 and 29% of the initial capacity at a high current density of 20 A g −1 , respectively. Bi 2 S 3 -I exhibits the highest recorded capacity, while Bi 2 S 3 -A is a more rational choice for electrochemical applications, as its charge storage properties do not decrease as fast as the material obtained from isopropanol. Among other reported bismuth sulfides, Bi 2 S 3 synthesized from organic solvents such as acetone, isopropanol, and ethanol exhibits higher specific capacities due to its stable electrochemical performance over a wide working potential window (Table 2). capability among all samples, maintaining 25 and 29% of the initial capacity at a high current density of 20 A g −1 , respectively. Bi2S3-I exhibits the highest recorded capacity, while Bi2S3-A is a more rational choice for electrochemical applications, as its charge storage properties do not decrease as fast as the material obtained from isopropanol. Among other reported bismuth sulfides, Bi2S3 synthesized from organic solvents such as acetone, isopropanol, and ethanol exhibits higher specific capacities due to its stable electrochemical performance over a wide working potential window (Table 2).  For further evaluation of the application potential of bismuth sulfides, the material with the most balanced specific capacity and rate capability was selected (Bi 2 S 3 -A) as a negatrode for an asymmetric device with an activated carbon positrode. The assembled hybrid capacitor with a battery-type oxidation-reduction reaction and AC with an electrochemical double layer mechanism was investigated in an aqueous 6 M KOH electrolyte. As shown in Figure 8a, the CV curves present an irregular shape with visible peaks, which can be attributed to the combination of the faradaic and non-faradaic capacitance from the transition metal sulfide and carbon electrodes. The quasi-triangular shape of the GCD profile further confirms the capacitive nature of the hybrid capacitor (Figure 8b), indicating its reversible charge-discharge capability [35]. Furthermore, both the negatrode and positrode potential ranges are balanced while the cell is operating, with a clear distinction between the EDL-type GCD curve of the carbon electrode and the battery-like plot of Bi 2 S 3 -A. The assembled hybrid capacitor exhibits a specific capacitance of 104 F g −1 at scan rates of 1 mV s −1 and 86 F g −1 and a current density of 0.2 A g −1 , as calculated from the CV and GCD measurements, respectively (Figure 8c). The rate capability at higher scan rates and current regimes is approximately 20-33% due to diffusion-controlled processes at the negatrode [18].
The EIS measurement results of the hybrid capacitor are presented in the form of Nyquist plots, Figure 8d. The small semicircle in the high-frequency region is attributed to the charge transfer resistance (R ct ), which is related to the faradaic reactions of Bi 2 S 3 [23]. The R ct value of the Bi 2 S 3 /AC capacitor is 0.25 Ω, implying fast charge kinetics. In the lowfrequency region, the more inclined the plot is towards the -Z" axis, the lower the resistance attributed to the diffusion of ions at the electrode/electrolyte interface, which for the hybrid device shows certain distortions in the ion movement at both the electrode/electrolyte interfaces; thus, this resistance may explain the capacitive performance drop at higher current densities [36]. The investigated asymmetric system presents an equivalent circuit of the solution resistance and two parallel ladders, one for each electrode [37] (Figure 8d). The fitting of the impedance spectrum presents components such as charge transfer resistance, Warburg impedance related with diffusion of ions and constant phase element (CPE) attributed to the double layer capacitance at the electrodes interface and faradaic reactions of bismuth sulfide [38]. The long-term performance of the hybrid capacitor is an important factor in the evaluation of its applicability for energy storage devices (Figure 9a). The initial drop of 35% is related to the formation of a surface passivation layer and the side reactions related to bismuth sulfide [38]; however, after the 125th cycle, the assembled Bi 2 S 3 -A/AC capacitor shows greatly improved stability, retaining 74% of the initial capacitance after 1000 charge-discharge cycles. The EIS measurement results of the hybrid capacitor are presented in the form of Nyquist plots, Figure 8d. The small semicircle in the high-frequency region is attributed to the charge transfer resistance (Rct), which is related to the faradaic reactions of Bi2S3 [23]. The Rct value of the Bi2S3/AC capacitor is 0.25 Ω, implying fast charge kinetics. In the lowfrequency region, the more inclined the plot is towards the -Z'' axis, the lower the resistance attributed to the diffusion of ions at the electrode/electrolyte interface, which for the hybrid device shows certain distortions in the ion movement at both the electrode/electrolyte interfaces; thus, this resistance may explain the capacitive performance drop at higher current densities [36]. The investigated asymmetric system presents an equivalent circuit of the solution resistance and two parallel ladders, one for each electrode [37] (Figure 8d). The fitting of the impedance spectrum presents components such as charge transfer resistance, Warburg impedance related with diffusion of ions and constant phase element (CPE) attributed to the double layer capacitance at the electrodes interface and faradaic reactions of bismuth sulfide [38]. The long-term performance of the hybrid capacitor is an important factor in the evaluation of its applicability for energy storage devices (Figure 9a). The initial drop of 35 % is related to the formation of a surface passivation layer and the side reactions related to bismuth sulfide [38]; however, after the 125 th cycle, the assembled Bi2S3-A/AC capacitor shows greatly improved stability, retaining 74 % of the initial capacitance after 1000 charge-discharge cycles.  The energy storage capability of the Bi2S3-A/AC hybrid capacitor was evaluated in the form of a Ragone plot (Figure 9b). The assembled device can reach a maximum specific energy density of 7.6 Wh kg -1 at a specific power density of 0.09 kW kg -1 , which is in the medium range/slightly improved compared with the previously reported polyaniline-copyrrole/thermally reduced graphene oxide composite capacitor [39], Bi2S3/CoNi-LDH hybrid capacitor or polyimidazole/CuS/CNT composite [40], and on pair with commercially available devices [41]. However, due to the decreasing capacitive performance at higher current densities, the Bi2S3-A/AC hybrid capacitor shows approximately 1 Wh kg -1 of specific energy at a high power density of 5 kW kg -1 .

Conclusions
In summary, we showed that the one-pot synthesis of bismuth sulfide nanostructures is a promising and economical approach for preparing negative electrodes for hybrid aqueous capacitors. The amorphous coral-like structure of Bi2S3 synthesized from an ace- The energy storage capability of the Bi 2 S 3 -A/AC hybrid capacitor was evaluated in the form of a Ragone plot (Figure 9b). The assembled device can reach a maximum specific energy density of 7.6 Wh kg −1 at a specific power density of 0.09 kW kg −1 , which is in the medium range/slightly improved compared with the previously reported polyaniline-copyrrole/thermally reduced graphene oxide composite capacitor [39], Bi 2 S 3 /CoNi-LDH hybrid capacitor or polyimidazole/CuS/CNT composite [40], and on pair with commercially available devices [41]. However, due to the decreasing capacitive performance at higher current densities, the Bi 2 S 3 -A/AC hybrid capacitor shows approximately 1 Wh kg −1 of specific energy at a high power density of 5 kW kg −1 .

Conclusions
In summary, we showed that the one-pot synthesis of bismuth sulfide nanostructures is a promising and economical approach for preparing negative electrodes for hybrid aqueous capacitors. The amorphous coral-like structure of Bi 2 S 3 synthesized from an acetone solution provided active sites for faradaic reactions at the electrode/electrolyte interface, yielding high capacity values and reasonable rate capability. Moreover, a hybrid aqueous capacitor assembled with Bi 2 S 3 as a negatrode and activated carbon as a positrode exhibited a specific energy density of 7.6 Wh kg −1 at a power density of 0.9 W kg −1 . The current work provides directions for the successful design of Bi 2 S 3 nanostructures to explore the use of high-performance anode materials in potential energy-related applications such as hybrid supercapacitors or aqueous lithium or sodium batteries.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.