Binder-free fabricated CuFeS₂ electrodes for supercapacitor applications

The chemicals utilized to make CuFeS₂ thin films were analytical grade and had not been refined before use. Copper chloride dehydrate (CuCl₂.2H₂O), iron chloride tetrahydrate (FeCl₂.4H₂O), sodium sulfide (Na₂S), and Triethanolamine (TEA) were utilized to make CuFeS₂ thin films, with distilled water used throughout.

1.0 × 5.0 cm sizes of stainless steel, polished with zero grade polish paper, sonicated with the glass slides for 15 min in acetone and 15 min in distilled water at 30 °C, and rinsed with distilled water before being dried in the oven. 0. 5 g of CuCl₂.2H₂O and 0.3 g of FeCl₂ were dissolved in 100 ml distilled water, then stirred for 15 min. The cationic bath was maintained at pH 4 by adding 1.0 ml of TEA drop by drop under steady stirring for another 15 min. Separately, 0.8 g Na₂S was mixed in 100 ml distilled water and agitated for 15 min before being utilized as an anionic bath with a pH of 11. The treated stainless steel and glass were each inserted into the bath containing cations for 30 seconds. Cu²⁺ and Fe^2+, present in the bath are adsorbed on the substrates' surfaces. Cu^2+/and Fe²⁺ ions that were not well adsorbed were washed for 10 seconds in distilled water. The Cu²⁺/Fe²⁺ carrying substrates were then submerged in a Na₂S bath for sulfur adsorption on the substrates for 30 seconds. The anions which were not well adsorbed were washed off for 10 seconds in distilled water. The process was continued until ten, twenty, thirty, and forty cycles had been accomplished. The deposition process is depicted schematically in figure 1.

Zoom In Zoom Out Reset image size

The samples are labelled CFS 1, CFS 2, CFS 3 and CFS 4 for sampled deposited at 10, 20, 30 and 40 cycles.

X-ray diffraction (XRD) was used to characterize the structure and phase of CuFeS₂ thin films (Bruker AXS D8 diffractometer coupled with copper anode at 1.540A). The morphology of the films was examined using a KFU-Sci machine with a voltage of 15.0 kV for scanning electron microscopy, and the optical characteristics of the CuFeS₂ thin films were determined using a 756S UV-Vis spectrophotometer.

BioLogic Potentiostat was used to examine the electrochemical characteristics of CuFeS₂ thin films. The studies were done in a three-electrode configuration. The electrolyte used was 2.0 M of KOH at a temperature of 303 K. The working electrode was the as-synthesized thin-film electrodes, the working electrode is the Ag/AgCl and the counter electrode is the graphite. The capability of CuFeS₂ thin-film electrodes was tested in the 0 to 1.3 V potential range. The electrochemical test employed was cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS).

The specific capacitance of electrodes was determined from CV according to using equation (2) [9],

$$\begin{eqnarray}&&{C}_{\mathrm{sp}\,}=\displaystyle \frac{{C}_{T}}{M}\end{eqnarray} \tag{ 1 }$$

where C_T is

$$\begin{eqnarray}&&{C}_{T\,}=\displaystyle \frac{{\int }_{V1}^{V2}IdV}{S\,\times \,V}\end{eqnarray} \tag{ 2 }$$

${\int }_{V2}^{V1}IdV\,$ = integration of the CV curve area, $s\,$ = scan rate, and $V\,$ = potential used.

The specific capacitance of the CuFeS₂ thin-film electrodes was also calculated from the GCD profile using equation (3),

$$\begin{eqnarray}&&{C}_{{\rm{sp}}\,}=\displaystyle \frac{I{\rm{\Delta }}t}{{\rm{m}}\,V}\end{eqnarray} \tag{ 3 }$$

where I = current, m = mass (mg), $V\,$ = potential, and ${\rm{\Delta }}t\,$ = time of discharge.

The XRD patterns of produced CuFeS₂ thin films placed on glass substrates at 10–40 cycles are shown in figure 2. The reflection peaks at 29.4°, 48.7 °, and 58.7^o corresponding to (112), (220) and (303) planes. The peaks correspond to tetragonal structure (space group: I-42d) with a lattice constant of 5.2864 _Å (JCPDS no: 83–0983). There were no other diffraction peaks as seen in figure 2 indicating the purity of the CuFeS₂ thin films.

Zoom In Zoom Out Reset image size

The average crystallite of the CuFeS₂ thin films was calculated using Debyer-Scherrer's formula (equation (7))

$$\begin{eqnarray}&&{\rm{D}}\,=\displaystyle \frac{0.9\lambda }{\beta \,\cos \,\theta }\end{eqnarray} \tag{ 4 }$$

where $\lambda \,$ = wavelength, $\beta \,$ = full width at half maximum (in radians) and $\theta$ = the angle of diffraction. The most intense peak (112) was used to determine the FWHM. The average crystallite sizes of 16.4 nm, 16.96 nm, 17.12 nm, and 18.07 nm correspond to CuFeS₂ thin films at deposition cycles 10–40, respectively. A close look at the XRD pattern shows that the increase in deposition cycles relatively dropped the peaks. This could be due to the size growth as the number of deposition cycles increases. The result is consistent with the findings of [16, 19–21].

Figure 3 depicts the surface morphologies of the CFS films. The films were rather spherical and densely spread on the substrate, according to SEM micrographs. The micrograph of the CFS 1 film revealed a spherical but fleecy-like shape with particle aggregation at higher deposition cycles as in CFS 2 and CFS 3, and eventually, an irregular platelet-like nanostructure in CFS 4. The platelets have irregular thicknesses and a narrow gap between them. This narrow gap lessens pores and limits ions intercalation for redox reactions [7], hence the low capacitance of CFS 4. The fleecy-like nature and smaller particle size of CFS 1 encourage easy ion intercalation for faster redox reactions and hence an appreciably high capacitance measured. The measured average particle sizes using Image J software are approximately 35.5 nm, 51.7 nm, 54.9 nm and 83.3 nm for CFS 1, CFS 2, CFS 3 and CFS 4 respectively. The bigger particles came from crystallite aggregation.

Zoom In Zoom Out Reset image size

Energy dispersive x-ray spectroscopy gave the elemental composition of the CFS thin films. Figure 4 indicates that our film contains Copper, Iron, and Sulfur, in the atomic percentage ratios of 1:1:2. Also found in the spectrum were Sodium and Calcium peaks from the used glass substrate and Oxygen from water used in the synthesis.

Zoom In Zoom Out Reset image size

From the photon energy extrapolation in figure 5(a), the bandgap energy of the range 1.08–1.22 eV were recorded for CFS1 - CFS4. The narrow bandgap of the films complements the small particle size and favours photovoltaic applications, while their unique small bandgap boosts electrical conductivity and is a bonus in enhancing electrochemical performance. The range of values for electrical conductivity against wavelength ranged between 6.9 ×10–3–6.3 ×10^–3 Sm⁻¹ for CFS 1—CFS 4 within the Vis region as shown in figure 5(b). According to Teranishi and Sato [16], it diminishes as the wavelength increases. The CFS films have a high electrical conductivity that decreases as the deposition cycle increases.

Zoom In Zoom Out Reset image size

The CuFeS₂ electrodes' CV profile is shown in figure 6. The CV profile shows few redox peaks, signifying that the CuFeS₂ electrode's capacitance is largely driven by a faradaic mechanism. The invariance in the redox peaks even at a scan rate of 100 mV s⁻¹ implies that the CuFeS₂ electrodes have good rate capability [12]. The shape of the CV profile is 'Type B' CV profile as suggested by [22] categorically. The calculated specific capacitance recorded at 10 mV s⁻¹ for CFS 1, CFS 2, CFS 3, and CFS 4 is 584.2, 493.4, 279.7 and 139.8 F g⁻¹ respectively using equation (2). The increase in specific capacitance is inversely proportional to scan rate because, at a higher scan rate, there is restricted movement of the electrolyte ions. The trend with which the specific capacitance varies with the scan rate is shown in figure 8(c).

Zoom In Zoom Out Reset image size

The C_s obtained for the electrodes is considerably higher than the previous reports of Deen et al [23], Sahoo et al [12], Zardkhoshoui et al [13]and slightly lower than the report of Lohkande et al [9] as shown in table 1. Table 1 shows the summary of specific capacitance (using three-electrode configuration) of CuFeS₂ and some copper-based electrode materials.

The CD measurements were also carried out at current densities 1.0 A/g—3.0 A/g shown in figure 7. The analysis was done in the potential window of 0 to 1.4 V as depicted in figure 6. The nonlinear nature of the CD profile of CuFeS₂ confirms its pseudocapacitance nature. From figure 6, the maximum specific capacitance for each of the electrodes CFS1, CFS 2, CFS 3, and CFS 4 is 537.0, 384.02, 215.1 and 93.17 F g⁻¹ respectively using equation (3) at current density 0.5 A g⁻¹.

Zoom In Zoom Out Reset image size

Figure 8 show the capacitance retention 8(a), EIS 8(b) and specific capacitance variation 8(c). Our work's better electrochemical performance is ascribed to the electrodes' unique structure and morphology, as well as the electrode material's direct deposition on the substrate without the use of a binder, which greatly reduced electrical resistance and superfluous weight.

In order to check the reaction kinetics and resistances of CuFeS₂ electrodes in detail, EIS tests were carried out in the frequency range of 1.0 Hz—100.0 kHz. It is shown in figure 8(b) using the Nyquist plot. Two unique features can be seen on the electrodes. The first is a small, less prominent arc in the high-frequency zone (showing lower charge transfer resistance) with linear vertical spikes in the low-frequency region, peculiar to pseudocapacitors.

Zoom In Zoom Out Reset image size

As seen from the bloated area of the Nyquist plot, the CFS 1 electrode exhibits the lowest ESR ∼ 0.44 Ω, and R_ct of 1.8 Ω showing good conductivity. Smaller ESR amounts to better electrochemical performance [3, 27]. Smaller resistance maintained by these electrodes could be as a result of the direct deposition done without additional inactive material in the form of binder posing more resistance to the ion intercalation [28–30]. R_ct for the films increased as the deposition cycles increased [7, 31]. This is a result of the increase in the electrode thickness [32].