A light-fostered supercapacitor performance of multi-layered ReS2 grown on conducting substrates

The light-fostered supercapacitor performance introduces a new realm in the field of smart energy storage applications. Transition metal dichalcogenides (TMDCs) with direct band gap are intriguing candidates for developing a light-induced supercapacitor that can enhance energy storage when shined with light. Many TMDCs show a transition from a direct to indirect band gap as the layer number increases, while ReS2 possesses a direct band gap in both bulk and monolayer forms. The growth of such multi-layered 2D materials with high surface area on conducting substrates makes them suitable for smart energy storage applications with the ability to tune their performance with light irradiation. In this report, we present the growth of vertically aligned multi-layered ReS2 with large areal coverage on various conducting and non-conducting substrates, including stainless steel via chemical vapor deposition (CVD). To investigate the effect of light illumination on the charge storage performance, electrochemical measurements have been performed in dark and light conditions. Cyclic voltammetry (CV) curves showed an increase in the area enclosed by the curve, manifesting the increased charge storage capacity under light illumination as compared to dark. The volumetric capacitance value calculated from charging–discharging curves has increased from 17.9 F cm−3 to 29.8 F cm−3 with the irradiation of light for the as-grown ReS2 on a stainless steel plate. More than 1.5 times the capacitance enhancement is attributed to excess electron–hole pairs generated upon light illumination, contributing to the charge storage in the presence of light. The electrochemical impedance spectroscopy further augments these results. The high cyclic stability is attained with a capacitance retention value of 81% even after 10 000 repeated charging–discharging cycles.

. The Raman active modes and frequencies of all the ReS 2 samples grown on different substrates in the present work. Table ST2. Proof of concept for the light effect on electrochemical supercapacitor performance of as grown ReS 2 by investigating the temperature effect that might occur on shining the light with time: Table   showing the variation of temperature with time on shining light for 30 minutes.      voltammogram at varied scan rates, (b) Galvanostatic charge-discharge cycles at varied current densities, (c) Variation of specific capacitance with respect to current densities, (d) Capacitive retention plotted against number of cycles showing ~101% capacitance retention over 1,000 charge-discharge cycles in light; corresponding galvanostatic charge-discharge cycles for 10 th and 1000 th cycle are shown in the inset, (e) Cyclic voltammetry curves before and after cyclic stability and (f) Nyquist plot before and after cyclic stability.

Raman Spectroscopy:
ReS 2 grown on all the substrates; conducting as well as non-conducting substrates has been characterized by Raman spectroscopy. The excitation wavelength of laser used for the spectroscopy is 532 nm. We have tabulated the frequencies and Raman active modes of the obtained peaks for all four samples in table ST1

Temperature effect upon light illumination:
To probe the possible effect of temperature rise during light illumination on the electrochemical supercapacitor performance, we have studied the variation of temperature with time on shining the light for 30 minutes as shown in table ST2. We observed a small change of two degrees only and hence, eliminating the temperature effect on electrochemical measurements upon light illumination. Note that the actual electrochemical measurements are done with illumination of light within 2 minutes. Table ST2. Proof of concept for the light effect on electrochemical supercapacitor performance of as grown ReS 2 by investigating the temperature effect that might occur on shining the light with time:

FESEM micrographs of ReS 2 grown on FTO coated glass and ITO coated glass:
The low magnification FESEM micrographs for ReS 2 grown on FTO coated glass and ITO coated glass are presented in figure S1 (a & b) showing the variation in aerial coverage of ReS 2 growth. The corresponding high magnification FESEM micrographs are shown in the inset of figure S1 (a & b). This indicates the substrate dependence on growth of ReS 2 and its controlled growth on four different substrates including both conducting and non-conducting substrates.

TEM micrographs and XRD pattern of ReS 2 grown on Si/SiO 2 substrate:
Bright field TEM micrograph in figure S2 (a) shows the electron transparent ultrathin structure of ReS 2 nanoflowers at low magnification. Figure S2   Si/SiO 2 substrate indicating their crystalline nature with preferred orientation of (h00) family reflections. Figure S3 shows the XPS results which provides a crux of elemental composition and their chemical states in ReS 2 nanoflowers grown on Si/SiO 2 substrate. Figure S3 (a) displays the survey spectrum of as grown ReS 2 nanoflowers, which shows the presence of four elements, Re and S from ReS 2 and Si and O from Si/SiO 2 . The two peaks appearing in XPS spectrum for Re in figure S3 (b) attributes Re 4+ oxidation state. The XPS spectrum of S is shown in figure S3 (c) and possesses two peaks ascribing to sulphide (S -2 ) ions.

Electrochemical measurements for ReS 2 grown on SSP substrate in dark:
The detailed electrochemical measurements for ReS 2 grown on SS plate in dark conditions are shown in figure S4, where figure S4 (a) shows the cyclic voltammograms at varied scan rates of 20, 50, 100, 200 and 500 mV/s. The predominant EDLC behaviour can clearly be observed along with subsidiary characteristics of pseudocapacitive behaviour mainly between -0.2 to -0.4 V potential region. Figure S4 (b) shows the galvanostatic charging-discharging curves at varied current densities ranging from 562.5 μAcm -2 to 937.5 μAcm -2 . These GCD curves has been used to calculate the specific capacitance values by using equation 4 (expressed in main paper). The specific capacitance values thus obtained are plotted with respect to the corresponding scan rate values as represented in figure S4 (c).

Electrochemical active surface area of ReS 2 grown on stainless steel plate:
The cyclic voltammetry experiments were performed in a potential window of -0.1 V to -0.2 V at different scan rates starting from 20 mV/s to 500 mV/s for calculating the Electrochemical active Surface Area (ECSA) as shown in figure S5 (a). The difference in current density (∆j) in middle of the potential range @ 0.15 V divided by 2 is plotted as a function of scan rate. The data points thus obtained were fitted linearly to obtain the value of the slope as shown in figure S5 (b). This value of slope thus obtained is C dl , the double layer capacitance of our working electrode. This double layer capacitance (C dl ) value is further used to calculate the roughness factor (R f ), by using the following equation 3-5 : where Cs is the average double layer capacitance of a smooth metal surface which is assumed to be 20 μFcm -2 . Here, the value of C dl comes out to be 1,410 μFcm -2 . Therefore, the estimated value of roughness factor is found to be 70.5. This roughness factor is then used to estimate the ECSA of our working electrode by using the following equation where S denotes the actual surface area of the smooth metal electrode, which here is the geometric area of our working electrode that was dipped into the electrolyte i.e. 0.8 cm 2 . Therefore, ECSA thus calculated is found to be 56.4 cm 2 . This manifests that a high surface area of 56.4 cm 2 is provided for charge storage by the beautiful vertically aligned microstructure of ReS 2 just within a small geometrical area of 0.8 cm 2 .

Electrochemical measurements for ReS 2 grown on SS plate in light:
The

Galvanostatic charge discharge curves for ReS 2 grown on SS plate in light at lower current density:
The galvanostatic charge discharge curves for ReS 2 grown on SS plate in light at lower current density of 375 μAcm -2 are shown in figure S7 (a). The specific capacitance values calculated from GCD curves found to be 21,066 mFcm -3 in dark and 70,366 mFcm -3 upon illumination with light. This clearly reveals the effect of light on supercapacitor charge storage performance by exhibiting an enhancement of ~3 folds in specific capacitance values upon light illumination. The corresponding five charging-discharging cycle in dark (black) and light (red) are shown in figure S7 (b). But there is a clear signature of pronounced electrode polarization beyond -0.5 V potential, which can have a negative impact on the charge storage performance of the electrode under consideration in terms of efficiency.

Coulombic efficiency of ReS 2 grown on SS plate in dark and light:
The coulombic efficiency was calculated for as grown ReS 2 on SS plate in dark and light. The GCD curves at 562.5 μAcm -2 current density were used for estimating the efficiency. The GCD curves shows the nonlinear characteristics and following expression was employed for the calculation of coulombic efficiency (η): where ∫ E × t dicharging refers to the integral of area under the discharging curve and ∫ E × t charging refers to the integral of area under the charging curve. The coulombic efficiency thus calculated came out to be around 77.8% in dark and around 71.3% in light. This is presented in figure S8. The reason behind such coulombic efficiency is the non-ideal behaviour GCD curves arising due to some faradaic reaction contribution as it is not showing the pure EDL behaviour. Figure S8. Electrochemical supercapacitor performance of as grown ReS 2 on SS plate in dark and light in terms of coulombic efficiency.
As evident from figure 6 (d), a considerable electrode polarization occurs at a potential higher than -0.5 V. This can also result in lower coulombic efficiency. In order to avoid such undesirable and detrimental effect of electrode polarization, we limit the potential range of galvanostatic charge discharge curves from 0 to -0.5 V. The detailed electrochemical measurements are shown in figure S9, which elucidates the disappearance of electrode polarization after reducing the potential window up to -0.5 V. Moreover, the increase in coulombic efficiency after decreasing the potential window was also observed. The coulombic efficiency thus calculated comes out to be ~90 % in dark and 88 % in light. The specific capacitance values calculated at a current density of 562.5 μAcm -2 for dark and light comes out to be 9.4 Fcm -3 and 11.5 Fcm -3 .

Electrochemical measurements for ReS 2 grown on Si/SiO 2 substrate in dark and light:
The detailed electrochemical properties of ReS 2 grown on Si/SiO 2 substrate in dark and light conditions are shown in figure S10 and figure S11, where figure S10 (a) and figure S11 (a) shows the cyclic voltammograms at varied scan rates of 20, 50, 100, 200 and 500 mV/s in dark and light respectively. The cyclic voltammograms in light and dark manifest the contribution from both EDL and pseudocapacitive behaviour of ReS 2 grown on Si/SiO 2 substrates. Figure S10 (b) and figure S11 (b) shows the galvanostatic charge-discharge curves at varied current densities ranging from 600 μAcm -2 to 900 μAcm -2 in dark and light, respectively. The estimated volumetric capacitance is plotted as a function of varying current density for both dark and light in figure S10 (d) and figure S11 (d). The electrochemical impedance spectroscopy has been performed and the corresponding Nyquist plot are represented in figure S10 (c) and figure S11 (c) for dark and light. We performed cyclic stability test upto 1000 charge-discharge cycles for ReS 2 grown on Si/SiO 2 substrate under dark condition as well as upon light illumination. The electrode showed excellent cyclic stability of > 90% in both dark and light conditions as evident from the comparative charge-discharge curves of 10 th and 1000 th cycle, capacitance retention plot as a function of GCD cycle number, CV curves and Nyquist plots taken before and after 1000 GCD cycles for both dark and light conditions as represented in figures S10 (e-h) and figures S11 (e-h).

Electrochemical supercapacitor performance of bare SS plate in dark:
The detailed electrochemical measurements for bare SS plate in dark conditions are shown in figure S12, where figure S12 (a) shows the cyclic voltammograms at varied scan rates of 20, 50, 100, 200 and 500 mV/s. Figure S12 (b) shows the galvanostatic charging-discharging curves at varied current densities ranging from 126.20 μAcm -2 to 584.79 μAcm -2 . It is manifested from figure S12 (b) that the charging and discharging are taking place in fraction of seconds. The electrochemical impedance spectroscopy has been performed and the corresponding Nyquist plot is represented in figure S12 (c).

Electrochemical supercapacitor performance of bare SS plate in light:
The detailed electrochemical measurements for bare SS plate in light conditions are shown in figure S13, where figure S13 (a) shows the cyclic voltammograms at varied scan rates of 20, 50, 100, 200 and 500 mV/s. Figure S13 (b) shows the galvanostatic charging-discharging curves at varied current densities varying from 126.20 μAcm -2 to 584.79 μAcm -2 . Here also, the charging and discharging are taking place very rapidly just in fraction of seconds. The electrochemical impedance spectroscopy has been performed and the corresponding Nyquist plot is represented in figure S13 (c).

Comparative electrochemical supercapacitor performance of bare SS plate under dark and light conditions:
The electrochemical supercapacitor performance of bare SS plate has been compared in dark and light conditions to eliminate the substrate effect in light-induced supercapacitor performance. Figure S14 (a) shows the cyclic voltammograms at 100 mV/s scan rate. Figure S14 (b) shows the galvanostatic chargedischarge curves at 350.88 μAcm -2 current density. The electrochemical impedance spectroscopy has been performed and the corresponding comparative Nyquist plots are represented in figure S14 (c).
There is no prominent difference observed in dark and light conditions in any of the electrochemical measurements, thereby, manifesting that the bare SS plate is not absorbing the light and hence, no light effect can be observed in it. Therefore, eliminating the effect of substrate in light induced supercapacitor performance of ReS 2 grown on SS plate.