Enhancing the Performance of Bi2S3 in Electrocatalytic and Supercapacitor Applications by Controlling Lattice Strain

Lattice‐strained Bi2S3 with 3D hierarchical structures are prepared through a top‐down route by a topotactic transformation. High‐resolution transmission electron microscopy and X‐ray diffraction (XRD) confirm the lattice spacing is expanded by prolonged sulfuration. Performance studies demonstrate that Bi2S3 with the largest lattice expansion (Bi2S3‐9.7%, where 9.7% represents the lattice expansion) exhibits a greater electrocatalytic hydrogen evolution reaction (HER) activity compared to Bi2S3 and Bi2S3‐3.2%. Density functional theory calculations reveal the expansion of the lattice spacing reduces the bandwidth and upshifts the band center of the Bi 3d orbits, facilitating electron exchange with the S 2p orbits. The resultant intrinsic electronic configuration exhibits favorable H* adsorption kinetics and a reduced energy barrier for water dissociation in hydrogen evolution. Operando Raman and post‐mortem characterizations using XRD and X‐ray photoelectron spectroscopy reveal the generation of pseudo‐amorphous Bi at the edge of Bi2S3 nanorods of the sample with lattice strain during HER, yielding Bi2S3‐9.7%‐A. It is worth noting when Bi2S3‐9.7%‐A is assembled as a positive electrode in an asymmetric supercapacitor, its performance is greatly superior to that of the same device formed using pristine Bi2S3‐9.7%. The as‐prepared Bi2S3‐9.7%‐A//activated carbon asymmetric supercapacitor achieves a high specific capacitance of 307.4 F g−1 at 1 A g−1, exhibiting high retention of 84.1% after 10 000 cycles.


Introduction
Bismuth sulfide (Bi 2 S 3 ) is a layered semiconductor with a direct bandgap of 1.3-1.7 eV. [1][2][3] The bismuth and sulfur atoms are connected by covalent bonds in a 2D layer, and 2D layers are connected by weak van der Waals forces. [4,5] The inter layer spacing of Bi 2 S 3 is 0.32 nm, which is about half of that of MoS 2 . [6] Bi 2 S 3 has aroused great interest due to its high photo conductivity, good environmental compat ibility, and abundant natural reserves. [7] However, up till now, Bi 2 S 3 has been used in the field of photoelectrochemistry, [8] photoelectronics, [9] CO 2 reduction, [10,11] and nitrogen fixation, [12][13][14] and there is no research on the application of pure Bi 2 S 3 in electrocatalytic hydrogen evolution reaction (HER). [15,16] Similar to MoS 2 , due to the bismuth monoatomic layer sand wiched between two sulfur monoatomic layers in the 2D planar structure of Bi 2 S 3 , only the unsaturated sulfur edge atoms in Bi 2 S 3 have proved catalytically active in photochemical applications, while the basal plane of the 2D structure is chemically inert. [17][18][19] This prevents Bi 2 S 3 from contacting reactant molecules and greatly impacts its electrochemical performance. [20] Applying lattice strain is an effective method to modify the internal interatomic distance, potentially increasing the con tact with solvent molecules and changing the geometry and electronic structure of the active site, which helps to optimize electrocatalytic activity. [21,22] Due to the complex structure of inorganic metal sulfides and the difficulty of characterization, the lattice strain produced in previous studies of metal sulfides is usually related to structure mismatch, substrates induced or heteroatom substitution, and these factors make it difficult to identify strainactivity correlations. [23][24][25][26] However, the unique periodic layered structure and the loosely stacked framework with large open channels of Bi 2 S 3 provide the opportunity of applying lattice strain without introducing the aforementioned factors.
Herein, a topdown route of topological transformation is described that permits the preparation of Bi 2 S 3 with 3D multi level architectures and different levels of strain. The influ ence of sulfuration time on lattice strain is determined using highresolution transmission electron microscopy (HRTEM) and Xray diffraction (XRD). The HER electrocatalytic activity of Bi 2 S 3 samples with different lattice strain is investigated and rationalized with reference to Xray photoelectron spec troscopy (XPS) and ultravioletvisible spectroscopy (UV-vis) spectroscopy studies and density functional theory (DFT) calculations.

Preparation and Lattice Strain Analysis
Bi 2 S 3 hierarchical structures with lattice strain are synthe sized via a topotactic transformation method, as illustrated in Figure 1a. The hierarchical BiOCOOH was prepared as a pre cursor, and it was sulfurized to yield Bi 2 S 3 with similar archi tectures to the parent material. In this procedure, the crystal structure changes to that of Bi 2 S 3 while the spatial morphology of the precursor is maintained, this is due to ion exchange of S 2− with O 2− and COOH − and lattice matching of BiOCOOH and Bi 2 S 3 . By prolonging the conversion time to Bi 2 S 3 , the interlayer interaction is weakened and the interlayer distance expanded, resulting in increased lattice expansion. Bi 2 S 3 with different lattice strains is obtained by adjusting the sulfuration time. Figure 1b displays a scanning electron microscopy (SEM) image of the hierarchical BiOCOOH assembled from inter laced nanosheets. Figure 1c-e is the SEM and HRTEM images of Bi 2 S 3 with sulfuration times of 10, 12, and 14 h, respectively. It is seen from the SEM images that the Bi 2 S 3 networks are interwoven by crossed nanorods with longrange order. As the sulfuration time increases from 10 to 14 h, the nanorods gradu ally become thinner, while the morphology remains essen tially unchanged. When the sulfuration time is prolonged to 16 h, the interwoven structure of the Bi 2 S 3 nanorods collapses ( Figure S1, Supporting Information). From the HRTEM images of the sample sulfurized for 10 h (Figure 1c), a lattice spacing of 3.1 Å is attributed to the (211) planes of Bi 2 S 3 . When the sul furation time is prolonged to 12 and 14 h, the corresponding lattice spacing is enlarged to 3.2 and 3.4 Å, which corresponds to lattice expansion ratios of 3.2% and 9.7%, respectively. The percentages of S measured by inductively coupled plasma mass spectrometry (ICPMS) in Bi 2 S 3 , Bi 2 S 3 3.2%, Bi 2 S 3 9.7% are 18.3%, 21.9%, and 26.2%, respectively, indicating an increased S 2− percentage in the Bi 2 S 3 samples with the increase of sul furation time, which enhances the interlayer repulsion and weakens the van der Waals forces between Bi 2 S 3 layers, resulting in increased layer mobility and lattice expansion, and decreased crystallinity. In addition, the BiS ratio in Bi 2 S 3 9.7% is significantly lower than that of Bi 2 S 3 from the energy dispersive Xray spectroscopy (STEMEDS), which agrees with the ICP results, indicating an increase in the relative content of sulfur ( Figure S2, Supporting Information). Figure 1g displays the XRD patterns of the precursor and final products after sulfuration for 10 h, in which all the characteristic diffraction peaks are indexed to tetragonal BiO COOH (JCPDS card No. 35-0939) and orthorhombic Bi 2 S 3 (JCPDS card No.170320, a = 11.15 Å, b = 11.30 Å, c = 3.98 Å), respectively, indicating that the BiOCOOH is completely trans formed to pure Bi 2 S 3 crystal without lattice strain. A complete transformation is further confirmed by the disappearance of the peaks of CO, COH, OH, BiO bands and the appear ance of BiS bands in the Fouriertransform infrared (FTIR) spectra after sulfuration (Figure 1h). [27,28] The XRD patterns of the Bi 2 S 3 products with different sul furation times are shown in Figure 1i, the characteristic reflec tion of the (211) plane at 29.1 o of the Bi 2 S 3 sulfurized for 10 h shifts down to 28.8 o and 28.4 o after the Bi 2 S 3 is sulfurized for 12 and 14 h, which is lower than the expansion rate observed from HRTEM. As XRD patterns reflect the lattice strain of the entire bulk phase, while HRTEM images observe the lattice expan sion on the surface of the material, this result indicates that the internal lattice expansion is smaller than the surface lattice expansion. It is also observed from the XRD patterns that the crystal planes at the highdiffraction angle do not change sig nificantly, suggesting that they are less affected by the lattice strain. The HRTEM and XRD results confirm the successful synthesis of Bi 2 S 3 with lattice strain.

Structural Characterization and Energy Level Analysis
The TEM image of Bi 2 S 3 9.7% is shown in Figure 2a, the Bi 2 S 3 superstructure is composed of nanorods with a diameter of ≈10 nm, and the angle between adjacent nanorods is ≈90°. From the HRTEM image (Figure 2b), the crystal planes parallel to the rod axis have a lattice spacing of 0.36 nm, corresponding to that of the (130) plane, which indicates that the nanorods have a [001]directional growth. The corresponding selected area electron diffraction (SAED) pattern ( Figure 2c) displays unique fourstrong ordered electron diffraction spots, similar to singlecrystal diffraction spots, indicating these nanorods have high crystallinity and a preferred growth direction of [001], which is consistent with the HRTEM result. The linearly ordered small spots between strong spots suggest the nanorods are organized to form a structure that has tetragonal symmetry. The fourstrong tetragonal electron diffraction spots are calcu lated to have a dspacing of 5.56 Å, which are from the periodic crystal planes of (200) of the tetragonally arranged Bi 2 S 3 single crystal nanorods. Furthermore, the tendency of the growth of [001]oriented Bi 2 S 3 nanorods along the two perpendicular [100] and [010] directions is due to the close lattice matching between the a or b axis of tetragonal BiOCOOH (a = b = 3.90 Å) and the caxis of orthorhombic Bi 2 S 3 (c = 3.98 Å), which finally leads to the interwoven Bi 2 S 3 networks. The highangle annular darkfield scanning transmission electron microscopy (HAADFSTEM) image and the corresponding element map pings ( Figure 2d) confirm the asprepared Bi 2 S 3 only contains Bi and S. The specific surface area of Bi 2 S 3 9.7% was measured by nitrogen adsorption/desorption isotherms. As displayed in Figure S3a (Supporting Information), the specific surface area of the sample is 179.13 m 2 g −1 , showing a much higher cata lytic surface area than that of the commercial Bi 2 S 3 . [29] The pore volume ( Figure S3b, Supporting Information) of Bi 2 S 3 9.7% calculated by the Barrett-Joyner-Halenda (BJH) method is 0.102 cm 3 g −1 , and the pores are mainly distributed at less than 1, 2.1, and 3.6 nm, confirming the coexistence of micropores www.advancedsciencenews.com and mesopores. The results indicate the 3D hierarchical struc ture greatly enhanced the absorption ability and pore structure of the sample.
The energy level diagrams of Bi 2 S 3 and Bi 2 S 3 9.7% are derived from the XPS valence band and UV-vis absorption spectra, as shown in Figure 2e,f and Figure S4 (Supporting Information). The valence band maxima of Bi 2 S 3 and Bi 2 S 3 9.7% are 1.01 and 1.84 eV, and the bandgaps are 1.66 and 0.64 eV, respectively. From the energy level diagram, it is seen that the bandgap is decreased by interlayer swelling, indicating the lattice strain enhances the conductivity of the sample.

Evaluation of the Hydrogen Evolution Performance
The lattice strain alters the local atomic and electronic envi ronment in the Bi 2 S 3 samples, thereby affecting their catalytic activity. The electrocatalytic HER activities were evaluated in 0.5 m H 2 SO 4 , 1 m KOH, and 1 m PBS electrolyte using a threeelectrode system, all potentials quoted are versus revers ible hydrogen electrode (RHE). As shown in  . c-e) SEM (top) (scale bar: 500 nm) and HRTEM (bottom) (scale bar: 2 nm) images of Bi 2 S 3 without lattice strain and with lattice strains of 3.2% and 9.7%, respectively (insets: SAED patterns corresponding to the HRTEM images). f) Schematic of the crystal structure variation of Bi 2 S 3 . g) XRD patterns of BiOCOOH and Bi 2 S 3 . h) FT-IR spectra of BiOCOOH and Bi 2 S 3 . i) XRD patterns of Bi 2 S 3 , Bi 2 S 3 -3.2% and Bi 2 S 3 -9.7%. respectively, which are much lower than those of Bi 2 S 3 (81, 161, and 220 mV) and Bi 2 S 3 3.2% (52, 118, and 163 mV). The fitted value of Tafel slope for Bi 2 S 3 9.7% is 72.6 mV dec −1 , much smaller than those of Bi 2 S 3 (91.4 mV dec −1 ) and Bi 2 S 3 3.2% (77.2 mV dec −1 ). The observed Tafel slope of Bi 2 S 3 9.7% suggests that the mechanism of HER is Volmer [30,31] Tables S2 and S3, Supporting Information). The similar values of Tafel slopes for the Bi 2 S 3 samples indicate the Volmer-Hey rovsky mechanism across the pH range. The Tafel slopes in acidic media are smaller than those in alkaline and neutral electrolytes because the Volmer step requires a large amount of activation energy in alkaline and neutral solutions, which leads to relatively slower HER reaction kinetics. [32] In view of the good activity in all pH media, the Bi 2 S 3 sam ples were tested for HER performance in seawater (Figure 4a,b; Table S4, Supporting Information). Bi 2 S 3 9.7% exhibits far better catalytic activity than the other samples, showing the lowest overpotentials (η 10 = 126 mV, η 50 = 221 mV, η 100 = 303 mV) and the lowest Tafel slope (85.1 mV dec −1 ), further proving the activity enhancement from lattice strain. The presence of lattice strain also prolongs the electrocatalytic HER stability of Bi 2 S 3 9.7% in seawater, and there is no degradation in performance after 60 h (Figure 4c). The N,Ndiethylpphenylenediamine (DPD) method is used to detect the possible formation of Cl oxidation products (ClO − ) during the test (Figure 4c, inset). No color change indicates no production of the hypochlorite that corrodes the catalyst. [33,34] The intrinsic catalytic activity of the Bi 2 S 3 samples is evaluated by determining the electrochemically active surface area (ECSA) and mass activity. ECSA is propor tional to the doublelayer capacitance (C dl ), which is obtained and calculated from cyclic voltammetry (CV) curves, as shown in Figure 4d and Figure S5   activity of Bi 2 S 3 9.7% at an overpotential of 60 mV in freshwater is 80.3 A g −1 (Figure 4e), which is ≈ five times and two times higher than that of Bi 2 S 3 and Bi 2 S 3 3.2%, respective, which implies that the lattice strain increases the catalytic activity. In addition, the electrolyte contact angle test demonstrates that the Bi 2 S 3 with the largest lattice swelling (Bi 2 S 3 9.7%) has the highest hydrophilicity (Figure 4f), indicating that the lattice expansion effectively increases the contact between reactants and solvent molecules, leading to a better catalytic perfor mance. The remarkable HER activity of the asobtained Bi 2 S 3 9.7% is superior to most of the stateoftheart Bi and Sbased nonnoble metal HER electrocatalysts previously reported in for all pH electrolytes and seawater (Figure 4g-i; Tables S5-S7, Supporting Information).

Interpretation of the Lattice Strain on the Influence of HER Activity
Theoretical simulations were performed to gain further insight into the relationship of the improved electrocatalytic activities and lattice strain. The geometric structures of Bi 2 S 3 samples with different lattice strains are shown in Figure 5a, these struc tures were built in consideration of generated lattice strain, and the corresponding bulk moduli were calculated, which is inversely proportional to the experimental lattice strain. [35][36][37] The calculated density of states (DOS) of Bi 2 S 3 9.7% near the Fermi level is higher and the bandgap is smaller compared with those of the other samples (Figure 5b), implying enhanced elec tron mobility, consistent with the XPS and UV-vis results. The optimized structures and Gibbs free energy diagrams of Bi 2 S 3 samples for HER in acidic solution are shown in Figure 5c. It is observed that the |ΔG H* | of Bi 2 S 3 9.7% (−0.28 eV) is much smaller and closer to the optimal value of zero than those of Bi 2 S 3 (−0.89 eV) and Bi 2 S 3 3.2% (−0.53 eV), revealing the favorable H* adsorption after the introduction of lattice strain. The optimized structures and free energy diagrams in alkaline and neutral solutions are displayed in Figure 5d, in which the dissociation of water is pivotal. The energy barrier for water dissociation in the Volmer reaction on the surface of Bi 2 S 3 9.7% is 0.67 eV, which is lower than those of Bi 2 S 3 (1.09 eV) and Bi 2 S 3 3.2% (0.88 eV), and the barriers on the Heyrovsky step for Bi 2 S 3 9.7%, Bi 2 S 3 3.2%, and Bi 2 S 3 are 1.12, 1.45, and 1.63 eV, respectively. The relatively lower barriers of Bi 2 S 3 9.  indicate the promoted water dissociation kinetics on the sample with high lattice strain. Furthermore, the influence of lattice strain on Bi 3d orbital configuration was investigated. With the increase of the lattice strain, the Bi 3d band width decreases, and the dband center shifts positively compared to the Fermi level, implying that the generated strain greatly alters the elec tronic properties (Figure 5e). A scheme of the changes of the band structures in the samples with lattice strain is shown in Figure 5f. The lattice strain gives rise to a decreased bandwidth and an uplift of the Bi 3dband center, promoting the electron exchange with the S 2p orbits, leading to greater covalencies in the BiS bond and enhancing the binding of hydrogen inter mediates with neighboring Bi atoms, which is conducive to the improvement of hydrogen evolution catalytic activities.

Post-Mortem Investigations after HER
To understand the catalytically active sites that produce the extraordinary HER activity of the Bi 2 S 3 catalyst with lattice strain, the microstructure, chemical composition and valence state evolution of Bi 2 S 3 9.7% after the HER test (Bi 2 S 3 9.7% A) in alkaline electrolyte were studied. The SEM image, TEM image, and corresponding SAED pattern of Bi 2 S 3 9.7% after HER show that the hierarchical structure with interwoven nanorods is maintained, indicating its great structural sta bility (Figure 6a-c). However, it can be seen from the HRTEM images (Figure 6d,e) that a pseudoamorphous structure is formed on the outside of the nanorods after HER, and the lattice spacing is reduced to 3.3 Å, corresponding to a lattice expansion of 6.4%, indicating a decrease of lattice strain during the test. The XRD test was performed on the sample after HER (Figure 6f), by comparing with the XRD pattern of Bi 2 S 3 9.7%, a peak appearing at 30.6° corresponding to Bi, indicating that Bi reduction occurs during the test (JCPDS card No. 51-0765), suggesting that the structure at the edge of the Bi 2 S 3 nanorods observed in postmortem studies is quasicrystalline Bi metal. Shift to high angle in the XRD pattern of Bi 2 S 3 9.7%A is also observed, indicating that the lattice expansion is reduced, con sistent with the HRTEM result. The blueshift of the BiS band  in FTIR reveals that the reduced lattice expansion shortens the bond length of BiS (Figure 6g). [38][39][40] The fitted Bi 4f spectra of Bi 2 S 3 with different lattice strains before and after HER were compared using XPS (Figure 6h; Figure S6, Supporting Infor mation) and found that the chemical composition of the Bi 2 S 3 sample without lattice strain remained basically unchanged before and after the HER test. With the increase of lattice strain, a new metallic Bi 0 peak appeared in the fitted Bi 4f, which is consistent with the results of HRTEM and XRD, con firming the generation of Bi in the samples with lattice strain after HER. [41,42] Operando Raman spectroscopy was further performed on the Bi 2 S 3 catalysts to explore the effect of lattice strain on the change of valence state in the realtime HER process (Figure 7). As the voltage alters from 0 to −0.2 V versus RHE, the strain free catalyst remains stable, and the peaks at 123 and 139 cm −1 are attributed to the E g and A 1g stretching modes of BiS bonds, respectively (Figure 7a). [43][44][45] For the straincontaining Bi 2 S 3 , the characteristic vibration of BiS band gradually weak ened, and a new band appeared at 96 cm −1 , corresponding to the A 1g stretching modes of BiBi bonds (Figure 7b,c). [46,47] In addition, the selfreconstruction of the samples with high strain is more obvious, the time of this process for Bi 2 S 3 9.7% is shorter compared with that of Bi 2 S 3 3.2%, and the new BiBi bands are also stronger for Bi 2 S 3 9.7%, which shows that the lattice strain is beneficial in the selfreconstruction and produc tion of Bi during HER.
By combining ex situ electron microscopy with Operando Raman spectroscopy, it is concluded that lattice strain plays a key role in triggering surface reconstruction and the reduction of Bi 2 S 3 under applied negative potential, and the in situ gener ated Bimetallic phase are the active sites for the HER process.

Charge Storage Kinetics of Bi 2 S 3 -9.7% after HER
Bi 2 S 3 has been widely used as an electrode in supercapacitors due to its low price and high theoretical capacity, however, its supercapacitive performance is limited by the low electrical conductivity and selfaggregation during the test. [48][49][50][51][52][53]   Bi 2 S 3 9.7% catalyst after HER (Bi 2 S 3 9.7%A) demonstrates good structural stability, and its conductivity has been greatly improved due to the introduction of lattice strain and the in situ generation of metallic Bi by reduction during the HER process. In view of the benefits discussed above, the supercapacitive behaviors of the Bi 2 S 3 9.7%A electrode have been investigated in a threeelectrode configuration in 3 m KOH electrolyte. Figure 8a shows the CV curves of Bi 2 S 3 9.7% electrodes in 3 m KOH before and after HER testing. The Bi 2 S 3 9.7%A elec trode exhibits a higher redox current and a larger enclosed area, indicating its enhanced faradic capacitance that results from increased electroactive sites. The redox peaks reveal the revers ible conversion between Bi 2 S 3 and Bi 2 S 3 OH in the electrolyte (Bi 2 S 3 + OH − → Bi 2 S 3 OH + e − ). [54][55][56][57][58] For Bi 2 S 3 9.7%A, the addi tional peak area is attributed to the reversible reaction of Bi and Bi 2 O 3 (2Bi +6OH − → Bi 2 O 3 + 3H 2 O + 6e − ), the electron transfer number of Bi is significantly higher than that of Bi 2 S 3 , greatly increasing the capacitance. Figure 8b displays the CV curves of Bi 2 S 3 9.7%A electrode at scan rates of 5-25 mV s −1 . As the scan rate increases, the peak current response increases, and the oxidation and reduction peaks shift positive and negative, respectively, which is due to the electrochemical polarization and internal resistance of the electrode.
To further study the enhanced supercapacitive behavior of Bi 2 S 3 9.7%A, the CV curve is analyzed. The power law was applied to scrutinize the diffusioncontrolled and sur face capacitive contributions of current, which is expressed as i p = a × ν b , where i p is the peak current (A cm −2 ), ν is the scan rate (V s −1 ), a and b are the adjustable parameters. [47] A bvalue of 0.5 indicates a diffusioncontrolled process, whereas a bvalue of 1 suggests surface capacitive behavior. [55] The bvalues of cathodic and anodic peaks calculated from the slopes of the linear fit of log(i) versus log(ν) are 0.60 and 0.53, respectively (Figure 8c), suggesting the current response of Bi 2 S 3 9.7%A electrode is dominated by diffusioncontrolled behavior. Figures S7 and S8 (Supporting Information)show the linear fit of ν 1/2 versus i/ν 1/2 for Bi 2 S 3 9.7%A electrode. To quantify the relative contribution of the diffusioncontrolled Figure 6. Active sites for hydrogen evolution catalysis. a) SEM, b) TEM, c) corresponding SAED pattern (left) and simulated SAED pattern (right) of Bi 2 S 3 -9.7% after HER in alkaline solution. d,e) HRTEM of Bi 2 S 3 -9.7% after HER in alkaline solution (Inset: corresponding SAED pattern). f) XRD patterns of Bi 2 S 3 -9.7% before and after HER in alkaline solution. g) FT-IR spectra of Bi 2 S 3 -9.7% before and after HER in alkaline solution. h) XPS of Bi 2 S 3 -9.7% before and after HER in alkaline solution. All the spectra were shown with raw data and fitting data derived by Lorentz-Gaussian function. and surface capacitive mechanisms, the power law is modified as i p = k 1 ν + k 2 ν 1/2 , where it is changed as i p /ν 1/2 = k 1 ν 1/2 + k 2 , where i p is the peak current (A cm −2 ), ν is the scan rate (V s −1 ), k 1 and k 2 are current contributions from surface capacitive and diffusioncontrolled behavior, respectively. [59][60][61][62][63] The rela tive current contributions obtained according to the above equations are displayed in Figure 8d. It is observed that at a low scan rate of 5 mV s −1 , the diffusioncontrolled contribu tion (87.4%) is much higher than the capacitive current contri bution (12.6%). With an increase in the scan rate, the surface capacitive contribution of the fast kinetics process gradually increases, reaching 28.3% at a high scan rate of 25 mV s −1 , while it is still lower than the current contribution of the diffusioncontrolled process (71.7%). This indicates that the charge storage capacity of the Bi 2 S 3 9.7%A electrode is mainly derived from faradaic behavior.

Capacitive Performance of Bi 2 S 3 -9.7%-A//AC Asymmetric Supercapacitor
Encouraged by the excellent electrochemical capacitive perfor mance of Bi 2 S 3 9.7%A, an asymmetric supercapacitor (ASC) was assembled using Bi 2 S 3 9.7%A as the positive electrode and activated carbon (AC) as the negative electrode, as shown in Figure 8e. Figure 8f displays the CV curves of the Bi 2 S 3 9.7% A//AC ASC measured at scan rates between 10 and 50 mV s −1 across a potential window of 0-1.5 V. It is observed that the CV curves of Bi 2 S 3 9.7%A //AC ASC exhibit ideal capacitive and faradaic behaviors as the scan rate increases, which shows its good electrochemical reversibility and the benefits of the com bination of doublelayer capacitive and faradaic mechanisms. Figure 8g displays the galvanostatic charge-discharge (GCD) curves performed at different current densities of 1-32 A g −1 . Bi 2 S 3 9.7%A//AC ASC exhibits high gravimetric capacitance of 307 and 133 F g −1 at current densities of 1 and 30 A g −1 , superb capacitance and rate capability, much better than that of Bi 2 S 3 9.7%//AC ASC (Figure 8h; Figure S9, Supporting Infor mation). This demonstrates that the in situ generated Bimetal on the fringe of the Bi 2 S 3 nanorods greatly enhances the charge storage ability. The comparison of energy and power den sity with data previously reported is presented in the Ragone plot (Figure 8i). Bi 2 S 3 9.7%A//AC ASC reaches an excellent energy density of 109 Wh kg −1 at a power density of 984 W kg −1 , and even maintains 47 Wh kg −1 at a highpower density of 24.4 kW kg −1 . The energy storage capacities of Bi 2 S 3 9.7%A// AC ASC are much higher than those of the reported Bi 2 S 3 based ASC devices (Figure 8i,j; Table S8, Supporting Infor mation). The EIS curves and equivalent circuit of Bi 2 S 3 9.7% A//AC ASC are shown in Figure 8k, suggesting a low charge transfer resistance (R ct ), which indicates that the device has good charge transfer kinetics. [52] The longterm cycling perfor mance evaluated at 1 A g −1 shows the device has good durability with a capacitance retention of 84.1% after 10 000 cycles.

Conclusions
In summary, a novel method for the preparation of 3D hierar chical Bi 2 S 3 architectures with tunable lattice strain is reported. The lattice spacing is confirmed to be expanded with the pro longation of sulfuration. The results of XPS valence band spectra and UV-vis show that the bandgap decreases as the lat tice strain increases. The asprepared Bi 2 S 3 electrocatalyst with 9.7%lattice strain exhibits the best HER performance among the Bi 2 S 3 samples, exhibiting 22 and 91, 74 and 143, 96 and 209, 126 and 221 mV in acidic, alkaline, neutral electrolytes and sim ulated seawater, with excellent stability for 60 h test in seawater.
Adv. Funct. Mater. 2022, 32, 2205974 Figure 8. Capacitive Performance of Bi 2 S 3 -9.7% after HER. a) CV curves of Bi 2 S 3 -9.7% before and after HER in alkaline solution. b) CV curves of Bi 2 S 3 -9.7% after HER measured at different scan rates of 5-25 mV s −1 . c) The power-law dependence of cathodic and anodic peak currents of Bi 2 S 3 -9.7% after HER from CV curves. d) Histograms of the capacitive and diffusion-controlled battery-type current contribution of Bi 2 S 3 -9.7% after HER at different scan rates. e) Schematic illustration of the assembled Bi 2 S 3 -9.7%-A//AC ASC. f) CV, and g) GCD curves of Bi 2 S 3 -9.7%-A//AC ASC at various scan rates and current densities. h) Calculated specific capacitance for Bi 2 S 3 -9.7%//AC ASC and Bi 2 S 3 -9.7%-A//AC ASC at various current densities. i) Ragone plot of Bi 2 S 3 -9.7%-A//AC ASC. j) Comparisons of the capacitive performance with some recently reported Bi-based catalysts in alkaline electrolyte. k) Cycling stability of the assembled Bi 2 S 3 -9.7%-A//AC ASC at the constant current density of 1 A g −1 (Inset: Nyquist plots and the corresponding equivalent circuit of Bi 2 S 3 -9.7%-A//AC ASC).
DFT simulations reveal that the lattice strain reduces the bandwidth and upshifts the band center of Bi 3d orbits, which promotes the electron exchange with the S 2p orbits, favoring the adsorption of H* kinetics and reducing the energy barrier of water dissociation for HER activities. Postmortem charac terizations reveal the formation of pseudoamorphous Bi on the boundary of Bi 2 S 3 9.7% nanorods after HER, which signifi cantly enhances its capacitance. Bi 2 S 3 9.7%A is constructed as an electrode in the ASC, and the asprepared Bi 2 S 3 9.7%A//AC ASC displays both high energy and power densities in a wide specific capacitance range and shows good retention of 84.1% after 10 000 cycles at 1 A g −1 . This work provides valuable guid ance for lattice strain regulation of 2D layered sulfide materials for clean energy conversion and storage applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.