Introduction

Today, every corner of the world is facing the problem of energy crisis. Hence, there is need of sustainable energy sources and storage devices. The typical non-conventional energy devices such as batteries, fuel cells and supercapacitors are based on the principle of electrochemical energy conversion [1]. As long as the fuel is fed, the electrical energy can be obtained from a fuel cell. Whereas, as per requirement, the stored energy can be drawn from a battery. Supercapacitors are fabricated to bridge the gap between batteries and capacitors to form fast charging energy storage device of intermediate specific energy [2]. Supercapacitor is a device for energy storage which possess high specific capacitance, power density and long cycle life [3]. Electrochemical supercapacitor (EC) stores a lot of energy in a small volume and releases it in the form of power. There are two types of ECs based on the electrochemical mechanisms of energy storage: electric double-layer capacitors (EDLC) and pseudocapacitors or ultracapacitors [1, 4–6]. EDLC is based on the energy storage through accumulation of the charges at the interface of the electrode and the electrolyte. In EDLCs (which include carbon-based materials), have long-term stability and fast charge-discharge. In the second type of supercapacitor, i.e., pseudocapacitor, the energy is stored through redox or faradic reactions occurring between the electrode material and the electrolyte. Generally, pseudocapacitor electrodes include metal oxides and conducting polymers [7]. Among these, transition metal oxides are more advantageous because of low electrochemical series resistance (ESR), more valence states, long cycle life, etc.

Among transition metal oxides, iron oxides are of particular interest due to their easy synthesis, low cost, good structural stability and environmental friendliness. Iron oxides form an important class of materials that are ubiquitous in nature from ancient times [8]. Because of the wide range of their physical and chemical properties, iron oxides are of great interest to a variety of scientific disciplines for various applications that range from opto-electronics, medicine, catalysis, corrosion protection, gas sensing, etc. [9, 10]. Among the different forms of iron oxides, hematite (α-Fe2O3) is the most common polymorph found naturally in soil and rocks because of its high thermodynamic stability and has lot of potential in these applications [11]. It is also one of the promising materials for supercapacitor applications as well [12].

Since supercapacitive performance of α-Fe2O3 depends on phase, crystallinity and morphology, different synthesis methods have been explored to get α-Fe2O3 nanostructures [13–24]. The present investigation aims for the synthesis of iron oxide films using liquid phase deposition (LPD) method to examine its utility as a supercapacitor. LPD is a process in which metal oxide or hydroxide thin films are formed on substrate through the ligand exchanging equilibrium reaction of the metal fluoro complex and the fluorine ion consumption reaction by addition of fluorine ion scavenger [25]. The LPD method has advantages such as easy and exact controllable ability of the thickness and dopant concentration. It is also simple, cost effective, energy efficient and can be used for larger substrates of complex shape [26]. Deki first attempted deposition of β-FeOOH and α-Fe2O3 thin films [27]. The β-FeOOH films deposited by using LPD method have been used in lithium-ion cells [28, 29]. In the present study, we have prepared hematite thin films by LPD method and studied their electrochemical properties for EC applications. To the best of our knowledge, this is for the first time we report the electrochemical properties of α-Fe2O3 thin films deposited by LPD method.

Experimental details

The deposition experiments were performed in open atmosphere at room temperature (300 K). Prior to deposition stainless steel (no-304), substrates (1 × 3 cm2) were polished with zero grade polish paper and washed in detergent solution. These polished substrates were cleaned in 3% HCl for 1 min and then ultrasonically cleaned in double distilled water for 15 min. These cleaned stainless steel substrates were used for the deposition. FeOOH was precipitated from an aqueous solution of ferric nitrate [Fe(NO3)3] by using liquor ammonia. The precipitate was separated by filtration and washed repeatedly by using double distilled water and dried in air. The parent solution FeOOH-NH4F.HF was obtained by dissolving requisite amount (to make concentration 1 M) of precipitate of FeOOH in 0.16 M ammonium bifluoride (NH4F.HF) aqueous solution. Boric acid, H3BO3 (scavenging agent) solution of 1 M was added to this parent solution. Substrates were vertically suspended and soaked in the reaction solution for 20 h. After being removed from the reaction solution the films were rinsed with double distilled water and dried in open atmosphere. In case of LPD, the concentration of scavenging agent has significant influence on morphology of the deposited film [30]. It is noteworthy that supercapacitive performance also critically depends on the morphology of the electrode material [7, 8]. Considering this, we have deposited films by varying concentration of H3BO3. We found that the films deposited with 1 M boric acid exhibit maximum specific capacitance (Supporting fig. S1 ) and hence used it for further studies. To ensure complete crystallization, the films were annealed at 500 °C in air for 1 h followed by natural cooling up to room temperature. These films were further used for characterization. The same deposition process was followed as mentioned above for glass substrates for optical studies. The thickness of the as-deposited film was ~760 nm, whereas after annealing it was reduced to ~500 nm due to removal of water content, phase transformation and crystallization after annealing.

Characterization techniques

Thermogravitometry analysis (TGA) and differential scanning calorimetry (DSC) of the powder (scratched from the as-deposited film) were investigated by using Mettler-Toledo TGA/DSC1 Stare system. It was carried out from room temperature to 600 °C in air atmosphere with 8 °C/min heating rate in alumina crucible. The phase composition and structural characterization of as-deposited and annealed films were recorded using an X-ray diffractometer Bruker D8 Advance instrument in the 2θ range 20–800 scanned with CuK α (λ = 1.54 Å) radiation. Raman spectra were measured on inVia Renishaw instrument in the wavenumber range 100–1000 cm−1 using He-Ne laser excitation source (632 nm, 6 mW) with resolution of 1 cm−1. The surface morphology of the films was observed by using field emission scanning electron microscopy (FE-SEM) on Nova NanoSEM450. The wettability of the films was tested by measuring the contact angle (θ) of a water drop of volume 5 μl placed on the film surface using an optical contact angle goniometer (OCA) at room temperature. Optical properties of the films were studied by using JascoV-670 UV-Visible spectrophotometer in the wavelength range 200–800 nm.

Electrochemical characterizations

The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) studies were carried out using a computer-controlled IVIUM 1A potentiostat with standard three electrodes system. Platinum wire was used as a counter electrode, whereas saturated Ag/AgCl was used as a reference electrode. The α-Fe2O3 (annealed) film deposited on stainless steel substrate acted as a working electrode. Previous electrochemical studies of iron oxides revealed that for obtaining the maximum specific capacitance Na2SO3 electrolyte is better [31, 32, 20, 33–35, 13, 14, 18, 36, 23]. The cyclic voltammograms were recorded between −1.5 to 0.0 V potential window in electrolyte Na2SO3 aqueous solutions of different concentrations 0.025, 0.05, 0.5, 1 and 2 M for different scan rates from 5 to 200 mV s−1. The area of the α-Fe2O3 electrode was (1 cm × 1 cm) and the weight of the active material dipped in the electrolyte was 0.3 mg, which was calculated by the weight difference method. The GCD and EIS studies were performed in 0.5 M Na2SO3 electrolyte, for which best supercapacitive performance was observed in cyclic voltammetry.

Results and discussion

Film formation and reaction mechanism

As proposed by Deki et al. [27] the mechanism of an iron oxyhydroxide film formation by LPD can be explained as follows. In the treatment solution Fe3+ ions are accompanied by F− ions to give [FeF6]3−. F−ions are released in reaction (1 and 2) and the reactions proceed resulting in the film formation.

$$ {\left[{\mathrm{FeF}}_6\right]}^{3-}+\mathrm{n}{\mathrm{H}}_2\mathrm{O}\rightleftarrows {\left[{\mathrm{FeF}}_{6{\textstyle \hbox{-}}\mathrm{n}}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{n}}\right]}^{3-}+\mathrm{n}\mathrm{H}\mathrm{F} $$
(1)
$$ {\left[\mathrm{F}\mathrm{e}{\mathrm{F}}_{6{\textstyle \hbox{-}}\mathrm{n}}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{n}}\right]}^{3-}+{\mathrm{n}\mathrm{H}}_2\mathrm{O}\rightleftarrows {\left[\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_6\right]}^{3-}+\mathrm{n}\mathrm{H}\mathrm{F} $$
(2)
$$ {\mathrm{H}}_3{\mathrm{BO}}_3+4\mathrm{H}\mathrm{F}\rightleftarrows {{\mathrm{BF}}_4}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}+2{\mathrm{H}}_2\mathrm{O} $$
(3)

The ligand-exchange equilibrium reactions (hydrolysis of iron fluoro complex ions) (1 and 2) are shifted to right-hand side by the addition of boric acid to form stable complex ions as in reactions 1 and 3. The consumption of F− ions occurs due to addition of boric acid (scavenging agent of F− ions) to the treatment solution. This accelerates the ligand exchange of [FeF6]3− and [FeF6-n(OH)n]3− as in reaction 1 and 2. The [Fe(OH)6]3− formed in reaction (2) is then dehydrated on the surface of the substrate, which results in the formation of iron oxyhydroxide thin film.

Structural, morphological and wettability analysis

TGA

The decomposition and crystallization temperature of the as-deposited film was confirmed from TGA and DSC measurements as shown in Fig. 1a. A sharp endothermic peak in the DSC curve at 175 °C is seen due to the evaporation of physisorbed water molecules [37]. The second and third endothermic peaks in the DSC curve at 250 and 300 °C are due to the decomposition of FeOOH [38] and the removal of OH−group [39], respectively. As seen in TGA curve, the as-deposited FeOOH mainly undergoes the weight loss in two steps. First, the weight loss of ~8% at 200 °C is attributed to the removal of adsorbed water. Second, the weight loss of 54% from 200 to 360 °C is assigned to the elimination of hydroxyl groups where the decomposition of FeOOH occurs, which is a typical endothermic reaction [39]. The weight loss ceases at ~500 °C and the mass remains same afterwards. The residue can be attributed to the thermodynamically stable hematite phase of iron oxide, α-Fe2O3 [38]. Taking into consideration the TGA results, we conclude that 500 °C is enough for phase conversion of FeOOH to hematite (α-Fe2O3) and hence in the present work the as-deposited films were annealed at 500 °C in air for 1 h in order to get crystalline hematite thin films.

Fig. 1
figure 1

a TGA–DSC curve of the as-deposited sample, b XRD pattern and c Raman spectra of the (A) as-deposited and (B) annealed films on stainless steel substrates, respectively

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XRD and Raman studies

The XRD measurements were performed to determine the crystal structure and the phase of the films. Figure 1b shows the XRD patterns of the as-deposited (A) and annealed (B) films. The peaks found in the XRD pattern of as-deposited film are assigned to FeOOH having orthorhombic crystal structure in correspondence with the Joint Committee on Powder Diffraction Standards (JCPDS), card no. 81-0462. The peaks of stainless steel substrate (denoted as SS in the pattern) are also visible may be due to small thickness of the film. After annealing at 500 °C, the FeOOH structure is destructed and gets transformed to α-Fe2O3. In contrast with earlier report [27] in which glass substrates were used, in present study the phase transformation of FeOOH to α-Fe2O3 is seen at lower temperature probably due to the difference in substrate and concentration of boric acid used. It can be seen from the figure that the crystallinity is improved as the film was annealed at 500 °C in air. The peaks found in the XRD pattern of the annealed film at 500 °C are assigned to α-Fe2O3 with rhombohedral crystal structure in correspondence with JCPDS card no. 33-0664. The average crystallite sizes of the as-deposited (FeOOH) and the annealed film (α-Fe2O3), calculated by the Debye Scherrer relation \( \Big(D=0.9\ \lambda /\beta \cos \theta\ \Big) \) [40] are about 62 and 26 nm, respectively.

Figure 1c shows the Raman spectra of the as-deposited and the annealed films. In Raman spectra, all peaks for the as-deposited film can be attributed to β-FeOOH (akaganeite) [41]. The peaks for the annealed film (B) seen in Raman spectra at 226 and 498 cm−1 are assigned to A1g mode, four peaks 244, 291, 412 and 614 cm−1 are assigned to E g mode of α-Fe2O3 (hematite) [42–44]. The band at 662 cm−1 assigned to E u mode is mostly reported for hematite due to the presence of residual Fe3O4 or disorder in crystal lattice [42, 45]. Thus, Raman spectroscopy results reveal that FeOOH (as-deposited film) is fully transformed to α-Fe2O3 (annealed film) in conformity with XRD results.

Morphology

Figure 2a, b shows the FE-SEM images of the as-deposited and the annealed films on stainless steel substrates. It can be seen from the micrographs that the compact film of spherically shaped densely packed particles are formed. After annealing the nature of the film is porous. In the as-deposited film ~10 at. % fluorine is present and after annealing it evaporated and became zero which may be the reason for the change in morphology from compact to porous (Supporting information fig. S2(a, b)). Also, after annealing phase is changed to α-Fe2O3 from FeOOH as confirmed from XRD and Raman studies. For the as-deposited film cracks are seen (Supporting information fig. S3(a)). This may be due to the internal stress of the film due to the contraction of the film upon drying [30]. After annealing at 500 °C, cracks are seen to increase (Supporting information fig. S3(b)). During the annealing, shrinkage stress is generally dominant which could be the reason for the cracks [46]. The cracks may be useful for electrochemical supercapacitors study due to their high surface area [47, 48, 19, 49].

Fig. 2
figure 2

FE-SEM micrographs a, b and water contact angle images c, d for the as-deposited and annealed films on stainless steel substrates, respectively

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Wettability study

Surface wettability is examined to determine the nature (hydrophilic or hydrophobic) of the thin film surface as shown in Fig. 2c, d. The contact angle (θ) for the as-deposited film is 101° and that of the annealed film is 75°, suggesting that the surface of the film changes from hydrophobic to hydrophilic after annealing. The reason may be that the roughness became less after annealing of the film. The property of α-Fe2O3 film of being hydrophilic is useful in electrolyte/electrode interface for good performances in electrochemical supercapacitors [17].

Optical properties

The variations in transmittance against wavelength for as-deposited and annealed films deposited on glass substrates are shown in Fig. 3a. The transmittance increases in visible range after 356 nm for as-deposited and around 393 nm for annealed films. The theory of optical absorption gives the relationship between the absorption coefficients α and the photon energy (hυ),

$$ \alpha =A{\left({E}_{\mathbf{g}}-h\upsilon \right)}^{\mathbf{n}}/h\upsilon $$
(4)
Fig. 3
figure 3

a Variation of transmittance against wavelength and b variation of (αhυ)2 against (hυ) for the (A) as-deposited and (B) annealed films on glass substrates, respectively

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where hυ is the photon energy, E g is the optical band gap and A is the constant related to the effective masses associated with the valence and conduction bands. For direct allowed transition n = ½ and for allowed indirect transition n = 2 [50]. Figure 3b shows the variation of (αhυ)2 with hυ, which is a straight line in the domain of higher energies, indicating a direct optical transition. Equation (4) gives the band gap (E g) when straight portion of plot is extrapolated to the point α = 0. The optical band gap of the FeOOH was found to be in the range 1.97 to 2.14 eV [51, 37, 52] and that of the α-Fe2O3 given in the literature data is 1.9–2.65 eV [53, 17, 54, 55]. The band gap obtained from Tauc-Mott plots (Fig. 3b) of FeOOH (as-deposited film) is 2.26 eV and that of α-Fe2O3 (annealed film at 500 °C) is 2.19 eV. Phase transition from FeOOH to α-Fe2O3 and increase in particle size may be responsible for decrease in the bandgap due to annealing.

Electrochemical characterizations

Cyclic voltammetry (CV) studies

To determine the interfacial capacitance (Ci) and specific capacitance (Cs) cyclic voltammetry (CV) is usually preferred [56]. From CV curves, specific capacitance associated with the α-Fe2O3 electrode was calculated using the following relations (5, 6 and 7) [56, 57].

$$ C={\displaystyle \int IdV/\left(V\times v\right)} $$
(5)
$$ C\mathrm{i}=C/A $$
(6)
$$ C\mathrm{s}=C\mathrm{i}/m $$
(7)

where,

I :

response current (mA),

V :

potential window (V),

v :

voltage scan rate (mV s−1),

Ci:

interfacial capacitance (F cm−2),

Cs:

specific capacitance (F g−1),

A :

electro-active area of the material dipped in the electrolyte (cm2)

m :

mass of the active electrode material dipped in the electrolyte (g).

The Fig. 4a, b, c, d, e shows CV curves of α-Fe2O3 electrode at scan rates 5–200 mV s−1 in Na2SO3 electrolyte of concentrations 0.025, 0.05, 0.5, 1 and 2 M. The shape of cyclic voltammograms suggests the pseudocapacitance characteristic of α-Fe2O3, which is distinguishable from rectangular shape of the electric double-layer capacitance [58]. The capacitance of α-Fe2O3 in Na2SO3 aqueous solution arises from the surface redox reaction of sulphur in the form of sulphate and sulphite anions [35]. The possible redox reaction between Fe (II) and Fe (III) is accompanied by adsorption of sulphite ions for charge balance on iron oxide [31, 33].

Fig. 4
figure 4

a–e The CV curves of α-Fe2O3 electrode at the scan rates 5–200 mV s−1 in Na2SO3 electrolyte of concentrations a 0.025 M, b 0.05 M, c 0.5 M, d 1 M and e 2 M

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It can be seen from the CV curves that the area under the curve for different scan rates increases from the concentration 0.025 to 0.5 M of the electrolyte. This indicates that there is insufficiency of electrolyte ions to fully utilize the electro-active sites for 0.025 M. Further from 0.5 to 2 M, the area under the curve again decreases which reflects that 0.5 M concentration is sufficient to effectively use all the electro-active surface sites for charge storage. As seen in Fig. 4a, b, c, d, e current density increases with increasing scan rates [59]. It can be clearly seen from CV that there are distinct redox peaks during the anodic and cathodic sweeps [58]. From the Fig. 4a, b, c, d, e it is also observed that anodic peak positions shift towards higher potential with increase in scan rate. Accordingly, cathodic peak positions shift towards lower potential with increase in scan rate. It is noticed that the values of Ci and Cs decrease with increase in scan rate (5–200 mV s−1). The decrease in Ci and Cs can be attributed to the presence of inner active sites that cannot sustain the redox transitions completely at higher scan rates. At higher scan rates, ions do not have enough time to migrate in the double layer and hence the rate of double-layer formation slows down which results in decrease of Cs with increase in scan rate [60]. So, the Cs obtained at the slowest scan rate is believed to be the closest to that of full utilization of the electrode material [56, 61].

Figure 5a, b shows the variation of Ci and Cs with the concentration of the electrolyte for the scan rate 5–200 mV s−1. It is seen that both Ci and Cs are increased from the concentration 0.025 to 0.5 M and then again decreases to 2 M concentration of the electrolyte at different scan rates. It is being reported that the electrolyte concentration has strong influence on the capacitance of a supercapacitor [62, 63]. The ion transport within the electrode layer is easier if the concentration of the electrolyte is high. But if the electrolyte concentration is too high, the ion transport may be reduced because of less water hydration and hence decrease in ion mobility. Thus, in the present study the optimized electrolyte concentration is found to be 0.5 M of Na2SO3 and the highest obtained values of Ci and Cs are 0.175 F cm−2 and 582 F g−1, respectively at the scan rate 5 mV s−1 for 0.5 M aqueous Na2SO3 electrolyte. The Cs value obtained in the present work is higher than the reported values. The high value of specific capacitance may be attributed to the high surface area due to the porous nature of the film. The reported maximum values of Cs for α-Fe2O3 film electrode (binder-free) in different electrolytes calculated from the cyclic voltammograms are given in Table 1.

Fig. 5
figure 5

a Scan rate effect on interfacial capacitance (Ci) and b scan rate effect on specific capacitance (Cs) of the α-Fe2O3 electrode at different concentrations of the electrolyte, respectively

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Table 1 Supercapacitive performance of the α-Fe2O3 thin film electrode based on cyclic voltammetry
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The importance of an electrochemical supercapacitor is examined by its stability and reversibility. As it is seen from the CV studies, the maximum specific capacitance was obtained at 0.5 M aqueous Na2SO3 solution and hence the electrochemical cyclic stability of the α-Fe2O3 electrode was tested in the same electrolyte up to 1000 cycles. Figure 6a shows the CV for 1st, 100th, 500th and 1000th cycles at the scan rate 100 mV s−1 in 0.5 M of aqueous Na2SO3. The area under the curve decreases up to 500th cycle and remains the same afterwards. In Fig. 6b, the variation of specific capacitance with cycle number for α-Fe2O3 electrode at 100 mV s−1 scan rate is shown. It is observed from the figure that initially capacitance decreases rapidly and then steadily with increasing cycle numbers. The decay in the value of capacitance may be due to dissolution of the electrode active material in the electrolyte and capacity imbalance between the electrodes which can cause the electrode potential unstable. As the wide potential window (−1.5 to 0 vs Ag/AgCl) [24] is used for cyclic voltammetry, well-defined redox peaks along with high specific capacitance are obtained but with the poor cycling stability [64–66, 12].

Fig. 6
figure 6

a Electrochemical cyclic stability and b specific capacitance against cycle number of the α-Fe2O3 electrode at the scan rate 100 mV s−1

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Galvanostatic charge-discharge studies

The galvanostatic charge-discharge curves of α-Fe2O3 film were recorded at different current densities of 2, 4, 6, 8 and 10 mA cm−2 in 0.5 M Na2SO3 electrolyte. The charge-discharge curves of α-Fe2O3 film are shown in Fig. 7a. The discharge curves can be divided into three linear regions. First linear region (V 1) represents the sudden iR drop in the potential range 0 to −0.24 V due to internal resistance of the α-Fe2O3 electrode [67]. Second linear region (V 2) shows the nearly linear portion of potential variation with time indicating the double-layer capacitance due to charge separation at the interface of the electrode and the electrolyte [68]. In third region (V 3) variation in slope of the potential with time curve is seen, that refers the pseudo-capacitive behaviour of the α-Fe2O3 electrode [57].

$$ Cs=\left(I\times t\mathrm{d}\right)/\left(V\times m\right) $$
(8)
$$ SP=\left(I\times V\right)/m $$
(9)
$$ SE=\left(I\times V\times t\mathrm{d}\right)/m $$
(10)
Fig. 7
figure 7

a Galvanostatic charge-discharge curves at current densities 2, 4, 6, 8 and 10 mA cm−2, b specific capacitance as a function of discharge current densities and c Ragone plot of the α-Fe2O3 electrode

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where,

Cs:

specific capacitance (F g−1),

I :

current density (mA cm−2),

V :

potential window (V),

td :

discharging time (s),

m :

mass of the active material dipped in the electrolyte (g),

SP :

specific power (kW kg−1),

SE :

specific energy (Wh kg−1),

The specific capacitance, Cs was calculated from the charge-discharge curves by the relation (8) [56, 61]. Figure 7b indicates the specific capacitance as a function of current densities of α-Fe2O3 film electrode. As current density increased from 2 to 10 mA cm−2, the specific capacitance decreased from 179 to 142 F g−1. The maximum Cs value found is 179 F g−1 at current density 2 mA cm−2 which is consistent with the specific capacitance value 167 F g−1 obtained from CV curve at 50 mV s−1 in 0.5 M Na2SO3. Due to the fast charge transfer at the interface of the electrode and electrolyte, α-Fe2O3 electrode shows high capacitance at low current density. The iR drop increases with the increase in current density due to the ohmic resistance of the electrode [69]. The reported maximum values of Cs for α-Fe2O3 film electrode (binder-free) in different electrolytes calculated from the charge-discharge studies are given in Table 2. The specific power (SP) in kW kg−1and the specific energy (SE) in Wh kg−1 were also calculated from the relations (9 and 10) [56, 60]. Figure 7c shows the plot for specific energy verses specific power (Ragone plot) of the α-Fe2O3 electrode. At current densities 2 to 10 mA cm−2 as the SP increases from 6.9 to 29.9 kW kg−1 the SE decreases from 53.4 to 31.8 Wh kg−1, respectively.

Table 2 Preparation and supercapacitive performance of the α-Fe2O3 thin film electrode based on charge-discharge studies
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Electrochemical impedance spectroscopy (EIS) study

A powerful technique to study the fundamental behaviour of an electrode material in an electrolyte is electrochemical impedance spectroscopy (EIS). The EIS measurements were carried out at room temperature to evaluate charge transfer resistance and phase angle. The Nyquist impedance plot of the α-Fe2O3 electrode was examined in the frequency range from 0.01 to 105 Hz of 0.01 V signal amplitude in 0.5 M Na2SO3 electrolyte as shown in Fig. 8a. Figure 8b displays the enlargement of the high frequency region and inset shows the equivalent circuit used to fit the Nyquist impedance plot. The equivalent circuit implies the best fitted impedance data of the α-Fe2O3 electrode.

Fig. 8
figure 8

a Nyquist impedance plot (Z” against Z’) in the frequency range 105–0.01 Hz, b enlarged high-frequency region of Nyquist impedance plot (inset shows the equivalent circuit to fit the electrochemical impedance spectroscopy) and c bode plot of phase angle (φ) and log |Z| against log (f/Hz) of α-Fe2O3 electrode

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The imaginary part of the impedance spectrum sharply increases in lower frequency region indicating the supercapacitive behaviour of the α-Fe2O3 electrode [57]. Nature of the plot was studied at the beginning of the semicircle (high-frequency region) because of the resistance of the electrolyte (R s = 22 Ω) is independent of the frequency. R ct is the charge transfer resistance, which was found to be 18 Ω. The equivalent series resistance (ESR) obtained was 40 Ω. In the intermediate region of frequency, it is noticed that the Nyquist plot intersects the real axis (Z′) at ~45° angle is due to the Warburg (W) component, which is the characteristic of ion diffusion into the electrode materials [70]. In the low frequency part, the straight line bends more towards the imaginary axis which indicates the good capacitive behaviour of the electrode material [71]. Relaxation time constant (τ) was estimated using the expression (11) [70],

$$ \tau =1/\left(2\times \pi \times f\right) $$
(11)

where,

τ :

relaxation time constant (s),

f :

frequency corresponding to the maximum imaginary (Z”) component of the semicircle (Hz).

The calculated relaxation time (τ) was 26 ms, indicating low relaxation time constant ensuring characteristic property of fast charging-discharging of a supercapacitor. Also, the smaller τ of α-Fe2O3 electrode provides high power density due to the capability of faster energy release [70]. Figure 8c shows bode plot of α-Fe2O3 electrode. The phase angle of α-Fe2O3 electrode was about 68° at low frequency (0.017 Hz) which corresponds to contribution of capacitance from pseudocapacitance. The phase angle increases with decrease in frequency. This angle approaches towards 90° and hence the α-Fe2O3 electrode behaves more like a capacitor [70].

Conclusion

In summary, α-Fe2O3 films have been successfully deposited on stainless steel substrates by liquid phase deposition (LPD) method followed by annealing. The XRD and Raman studies reveal the complete phase formation to α-Fe2O3 thin films after annealing at 500 °C. FE-SEM images show the change in morphology from compact to porous after annealing. The wettability test reveals that due to annealing, the hydrophobic film was transformed to hydrophilic. The highest obtained value of specific capacitance found from cyclic voltammetry was 582 Fg−1 at 5 mV s−1 in 0.5 M Na2SO3 electrolyte. The maximum value of specific capacitance was 179 F g−1 from the charge-discharge curve at the current density 2 mA cm−2. The equivalent series resistance (ESR) obtained from Nyquist plot is 40 Ω. Hence, the results obtained reveal that the α-Fe2O3 films prepared by LPD method are affordable and have great potential application as a low cost option for supercapacitor.