X-CoOTe (X = S, Se, and P) with Oxygen/Tellurium Dual Vacancies and Banana Stem Fiber-Derived Carbon Fiber as Battery-Type Cathode and Anode Materials for Asymmetric Supercapacitor

In this work, we demonstrated the synthesis of anions (X = selenium (Se), sulfur (S), and phosphorus (P)) doped cobalt oxytelluride (X-CoOTe) with oxygen and tellurium dual vacancies using hydrothermal methods, followed by selenization, sulfurization, and phosphorization reactions. Especially, the Se-CoOTe-modified nickel foam (Se-CoOTe/NF) electrode delivered a higher specific capacity (752.95 C/g) and an extremely lower charge transfer resistance (0.87 Ω) than S-CoOTe/NF and P-CoOTe/NF due to the higher metallic conductivity of Se. Both oxygen and tellurium vacancies facilitate higher charge transfer conductivity, specific capacity, and stability. On the other hand, banana stem core fiber-derived activated carbon fiber (AC) with exfoliated carbon sheet, cracked surface, and corresponding high surface area boosts the excellent cycle stability up to 4000 cycles with capacitance retention of 100.29%. Thus, the asymmetric device (Se-CoOTe/NF//AC/NF) exhibited an extendable cell voltage (1.55 V), higher energy density (155.6 W h kg–1) at a power density (1356.2 W kg–1), and generous long-term stability (100% retention up to 10 000 cycles) in a liquid alkaline electrolyte. In the practicability test, the proposed asymmetric device mutually showed an increased operating voltage from 1.55 to 4.65 V for a three-series connection. In a three-series connection, a single white LED and an LED string glowed efficiently. This new finding will be very useful to develop tellurium-based chalcogenides and biowaste-derived carbon for energy storage applications.

As the result of first step hydrothermal process, CoOOH shows the sea urchin-like structure (Figure S1 (A, B)).In the resultant urchin-like structure, the length of the needle (~ 3.52 µm and 3.32 µm), the thickness of the needle (~ 202.3 nm and 206.0 nm), and the diameter of the sphere (~ 3.14 µm) were measured as shown in Figure S1 (C, D).Moreover, Figure S1E shows the growth of the needle on the surface of the sphere, which is also made up of a bundle of ribbon-like structures.Due to the effect of NaOH in the second step hydrothermal process, CoOTe shows the micro flower-like structure made by the convergent of the hexagonal rods with cone-like edges.It can be seen that all the hexagonal rods are grown from a single center and exhibit micro flower-like morphology (Figure S2).The hexagonal rod consisting different geometry sizes including lengths in the range from 20.41 to 21.83 µm (Figure S2A), side wall width from 590.9 to 937.7 nm, and height of cone-like tip ~ 2.62 µm (Figure S2B).According to the previous literature 1,2 , the NaOH induced the growth in the side wall of the rod only, but no changes in the primary spherical core.Therefore, the spherical structure in CoOTe sample (Figure S2C) also shows a similar diameter as that of the CoOOH (Figure S1C).
The preliminary FESEM image and EDX spectra with the quantitative result (Figure S3 (A, B)), mapping (Figure S3 (C, D)), and line profile mapping (Figure S3 (E-H)) also confirmed the formation of CoOOH urchin-like structure with equal distribution of main constituent elements (Co and O).Furthermore, the EDX elemental mapping (Figure S2 (D-F)), the quantitative results (Figure S4 (A-C)), and the line mapping profile (Figure S4 (D-H           Figure S11 (A-F) demonstrates the different magnified FESEM images of pre-carbonized_C, which exhibits a fiber-like structure with a longer length and ununiform surface.Meanwhile, the AC (Figure S12 (A-F)) displays a similar fiber-like structure with shorter length, exfoliated graphene sheets with thickness from ~ 45.6 to 46.1 nm, and cracked texture, which is more favorable for high surface area, good solubility, and better hydrophilicity properties.The TEM images (Figure S12 (G, H)) also mimic the similar results of the above FESEM analysis.As seen in HRTEM (Figure S12I), the AC is the amorphous hard carbon with the presence of a rich amount of super microporosity.The hard carbon and super microporosity natures facilitate more ion diffusion and subsequent interlayer formation during the charge/discharge reaction.It can be further evidenced by observing the two rings in SAED pattern (Figure S12J) for ( 002) and (100) planes, which are well associated with the amorphous hard carbon.Figure S13 and Figure S14 show the uniform elemental distribution, line mapping, and quantitative result of C and O in both pre-carbonized_C and AC respectively.It suggests that pre-carbonized_C and AC formed without any other impurity.Figure S12K shows the photographic representation of banana core fiberderived carbon with exfoliated graphene sheets by using a chemical activation process.Figure S15B shows the XRD pattern of pre-carbonized_C and AC with major characteristic peaks of graphitic structure at 24.38º and 42.61º for planes (002) and (100) respectively.The peak of (002) plane exhibits broader and shifts to a lower angle in the XRD pattern of AC, which indicates the formation of disordered amorphous carbon due to the strong KOH chemical activation process.
As seen in high-resolution XPS spectra, the Co 2p element exhibits the two major peaks at 781.22 eV and 797.20 eV for Co 2p    As seen in Figure S17 (A-C), the Se-CoOTe exhibits a higher specific surface area (A) of 32.92   Figure S19A shows the rectangular CV curves of pre-carbonized_C/NF and AC/NF with an applied potential window in the range from 0 to -0.96 and a fixed scan rate of 50 mV s -1 .In general, the rectangular CV curve refers to a non-faradic or electrostatic charge storage reaction.As seen in this result, the AC/NF exhibits a higher integrate CV area of 0.018, while pre-carbonized_C/NF shows about 0.005.The higher integrate CV area indicates the better charge storage performance of AC/NF compared to pre-carbonized_C/NF.And also, the CV curves for different scan rates (10 to 100 mV s -1 ) at pre-carbonized_C/NF and AC/NF are demonstrated in Figure S19(B, C).In these CV results, the integrated area increased for increasing scan rate, which implies the ideal electric double layer characteristic and capacitive behaviors.The GCD curve of pre-carbonized_C/NF and AC/NF (Figure S19D) delivered the symmetric triangular charge/discharge response.It also confirms the doublelayer charge storage property of the proposed electrodes.From this plot, it can be observed that AC/NF exhibits the larger charge/discharge time and corresponding higher specific capacitance (158.7 F/g), which is 0.8 fold higher than pre-carbonized_C (88.7 F/g).Moreover, Figure S19E shows the zoomed image of Figure S19D to estimate the IR drop of the active materials.As seen in this data, the IR drop was calculated to be 0.01 V and 0.07 V for AC/NF and pre-carbonized_C/NF respectively.
It suggests higher conductivity and less energy loss at AC/NF electrode during the charge/discharge process.The GCD curves of AC/NF and pre-carbonized_C/NF for different current densities (0.5, 1, 1.5, 2, 2.5, and 3 A/g) are given in Figure S19(F, G).Both AC/NF and pre-carbonized_C/NF show decreased charge/discharge time by increasing the applied current density.By using these data, the calibration plot for current density vs. specific capacitance (Figure S19H) and current density vs.
coulombic efficiency (CE) (Figure S19I) were plotted.In the power law approach, the relationship of scan rate (ν) and peak current (i) can be written as the following equation (Eq.S1) i = aν b (Eq.S1) where, both a and b are appropriate constant values.The b value can be calculated by taking the logarithm on both sides of the above equation and thus obtaining the linear calibration plot for the logarithm of scan rate vs. logarithm of oxidation peak current as shown in Figure S22A.The b values of 0.5 and 1 are the ideal range for diffusion and surface-controlled process respectively.
According to the theory of Dunn and coworkers, the diffusion and capacitive contributions can be quantitatively separated from total charge storage by using the below equations (Eq.S2) and (Eq.S3) Equation (S2) can be altered as below where, i represents the current (A) at a certain potential, and ν refers to the scan rate (mV s -1 ).The k 1 ν and k 2 ν 1/2 represents capacitive and diffusion contributions respectively.The k 1 and k 2 are both adjustable parameters which can be found from the slope and y-axis intercept point of the linear plot (v 1/2 vs. i/v 1/2 ).Herein, the linear calibration plot (v 1/2 vs. i/v 1/2 ) should be plotted at different potentials and sweep rates varied between 10 and 50 mV/s.By following the above procedure, the fraction of the current from capacitive effects and the diffusion-controlled faradaic process can be quantitatively estimated.After performing this calculation under different scan rates, plot the k 1 v (shaded area) as well as the experimental currents (solid line area) as shown in Figure S22.
In general, the total charge storage of the electrode is described from the total contribution of diffusion and capacitive mechanism.There are two kinds of methods to evaluate the diffusion and capacitive percentages such as the power law relationship and Dunn and coworkers reported approach 8, 9 .
Based on power law equation, the b value can be calculated by taking the logarithm on both      The CV shape of AC/NF remains similar without wide deviation compared to pre-carbonized_C/NF.The corresponding CV curves for before and after 1000 cycles confirm the 100% higher stability of AC/NF (Figure S26B), while pre-carbonized_C/NF (Figure S26F) delivered 89%.The comparison of GCD curves before and after 1000 cycles are revealed in Figure S26 (C, G).From this result, the AC/NF delivered an almost analogous GCD curve and retains 104.4% discharge time, while pre-carbonized_C/NF delivered only 75.5%.For additional validation, the EIS study also executed and presented the corresponding Nyquist plot before and after 1000 cycles.The R ct value of AC/NF (Figure S26D) decreased from 2.44 to 1.13 Ω after the cycle test.As given in Figure S26H, the R ct value of pre-carbonized_C/NF alternatively increases

Figure S3 .
Figure S3.(A) FESEM image, (B) EDX spectra and corresponding quantitative result, (C, D) elemental mapping, (E) FESEM image with line mapping data and (F-H) corresponding line mapping spectra of CoOOH.

Figure S4 .
Figure S4.(A) FESEM image, (B, C) EDX spectra and corresponding quantitative result, (D) FESEM image with line mapping data, and (E-H) corresponding Line mapping spectra of CoOTe.
)) confirmed the equal distribution of Co, O, and Te elements over the surface and thickness of the CoOTe rod-like structure.

Figure S6 .
Figure S6.(A) FESEM image, (B, C) EDX spectra and corresponding quantitative result, (D) FESEM image with line mapping spectra, and (E-I) corresponding Line mapping spectra of Se-CoOTe.

Figure S7 .
Figure S7.(A) FESEM image, (B, C) EDX spectra and corresponding quantitative result, (D) FESEM image with line mapping spectra, and (E-I) corresponding Line mapping spectra of S-CoOTe.

Figure S8 .
Figure S8.(A) FESEM image, (B, C) EDX spectra and corresponding quantitative result, (D) FESEM image with line mapping spectra, and (E-I) corresponding Line mapping spectra of P-CoOTe.

Figure S13 .
Figure S13.(A, B) EDX mapping, (C) FESEM image with line mapping data, (D-F) corresponding line mapping spectra, and (G) EDX spectra with quantitative result of pre-carbonized_C.

Figure S14 .
Figure S14.(A, B) EDX mapping, (C) FESEM image with line mapping data, (D-F) corresponding line mapping spectra, and (G) EDX spectra with quantitative result of AC.

oxidation states 3 .
photoionization of the ejected electron from the core of an atom.In general, the core level of O 1s spectrum is divided into four categories such as O1, O2, O3, and O4 for metal-to-oxygen bonding, oxygen vacancy, hydroxyl species, and adsorobed H 2 O molecule respectively 4 .The O 1s spectrum of CoOOH shows peaks at 529.58 eV, 531.42 eV and 533.20 eV for O1, O3 and O4 respectively.Herein, the high-intensity peak of O3 represents the hydroxide environment on the surface of CoOOH 5 .

m 2 /
g and pore diameter (D) of 107 Å.It is comparatively higher than those of S-CoOTe (A = 15.37 m 2 /g and D = 89.15Å) and P-CoOTe (A = 11.37 m 2 /g and D = 34.76Å).Herein, Se-CoOTe, S-CoOTe, and P-CoOTe exhibit the Type-IV isotherm and mesoporous characteristics.On the other hand, FigureS17Dshows the BET isotherm of AC with higher A = 972 m 2 /g and D = 33.68Å, which is 48.71 fold higher surface area than those of pre-carbonized_C (A = 19.55 m 2 /g and D = 159.15Å) (FigureS16C).Based on BHJ pore distribution analysis, both pre-carbonized_C and AC exhibit the mesoporous characteristic.In general, the G band represents the sp 2 -bonded carbon atom in a graphitic structure, while the D band denotes the defect/disorder in a hexagonal graphitic structure.Herein the I D /I G ratio is calculated to evaluate the degree of graphitization and defect formation.According to this hypothesis, the Raman spectra were recorded for pre-carbonized_C and AC and demonstrated in FigureS16D.In this spectra, the intensity of the D band is slightly higher for AC with the I D /I G ratio of 1, meanwhile the pre-carbonized_C exhibits the I D /I G ratio of 0.98.It again confirmed that the KOH activation creates the defective structure in AC.

Figure S19 .
Figure S19.CV curves for (A) comparison of different modified electrodes and varying scan rate at (B) pre-carbonized_C/NF and (C) AC/NF.GCD for (D, E) comparison of different modified electrodes and varying current densities at (F) pre-carbonized_C/NF and (G) AC/NF.Corresponding calibration plot for (H) specific capacitance of various electrodes vs. current density, and (I) Coulombic efficiency vs. current density.

Figure S21 .
Figure S21.Comparison CV curves of AC/NF, pre-carbonized_C/NF, and commercial CNF/NF electrodes at a scan rate of 50 mV s -1 .
sides of the above equation and thus obtaining the linear calibration plot for the logarithm of scan rate vs. logarithm of oxidation peak current as shown in Figure S22A.The b value of 0.5 indicates that the total charge storage process follows a semi-infinite diffusion process, while the b value of 1 indicates the complete surface-controlled mechanism.As seen in the calibration plot, CoOOH/NF and CoOTe/NF electrodes exhibit the b value of 0.75 and 0.87 respectively, which are suggesting both diffusion and surface-controlled mechanisms involved in the charge storage process 10 .

Figure S24 .
Figure S24.(A) CV curves of AC/NF modified electrode for capacitive and diffusion ratio and (B) corresponding bar diagram for scan rate vs. contribution ratio.(C) CV curves of pre-carbonized_C/NF modified electrode for capacitive and diffusion ratio and (D) corresponding bar diagram for scan rate vs. contribution ratio.

Figure S25 .
Figure S25.(A) Calibration plot for long term stability and Coulombic efficiency of Se-CoOTe/NF.(B) Comparison of CV curves for before and after long term stability test.(C, D) GCD curves of first 10 cycles and last 10 cycles from 4,000 cycles.

Figure S26 .
Figure S26.(A) CV curve for continuous 1,000 cycles, comparison of (B) CV, (C) GCD, (D) EIS curves for before and after 1,000 cycles of AC/NF.(E) CV curve for continuous 1000 cycles, comparison of (F) CV, (G) GCD, (H) EIS curves for before and after 1,000 cycles of pre-carbonized C/NF.

Figure S27 .
Figure S27.(A) Calibration plot for long term stability and Coulombic efficiency of AC/NF.(B, C) GCD curves of first 10 cycles and last 10 cycles from 4,000 cycles.

Figure S28 .
Figure S28.A) Calibration plot for long term stability and Coulombic efficiency of pre-carbonized C/NF.(B, C) GCD curves of first 10 cycles and last 10 cycles from 4,000 cycles.

from 3 .
35 to 6.83 Ω.It strongly recommends the superior stability and reversibility of AC/NF compared to pre-carbonized_C/NF.The GCD cycle stability experiment was performed for AC/NF and pre-carbonized_C/NF electrodes by applying the fixed current density of 3 A/g and 4000 continuous charge/discharge cycles.The AC/NF delivered almost 100% of capacitance retention and coulombic efficiency (FigureS27A) owing to higher stability.At the same time, the data point for capacitance retention and coulombic efficiency of pre-carbonized_C/NF (FigureS28A) show more fluctuation.In fact, the GCD curves for the first and last 10 cycles of both AC/NF (FigureS27 (B, C)) and pre-carbonized_C/NF (FigureS28 (B, C)) retain a similar shape due to the better reversibility.The obtained GCD cycle stability results also mimic the CV cycle stability test results.

Figure S29 .
Figure S29.GCD curves of first 10 cycles and last 10 cycles from 10000 cycles of asymmetric device.

Figure S30 .
Figure S30.SEM images of (A, B) Se-CoOTe/NF and (C, D) AC/NF for before and after long-term stability test.

Table S1 .
Concentration of major elements in the prepared samples.