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Electrochimica Acta 111 (2013) 869– 875

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

nvestigation on photoelectrochemical and pseudo-capacitanceroperties of the non-stoichiometric hematite �-Fe2O3 elaborated byol–gel

bderrezak Abdia,∗, Mohamed Trarib

Laboratoire d’Electrochimie et de Corrosion, Ecole Militaire Polytechnique, BP17, BEB 16111 Algiers, AlgeriaLaboratory of Storage and Valorization of Renewable Energies, (Faculty of Chemistry) USTHB, BP 32, 16111 Algiers, Algeria

r t i c l e i n f o

rticle history:eceived 18 June 2013eceived in revised form 13 August 2013ccepted 13 August 2013vailable online xxx

eywords:ematite �-Fe2O3

uper capacitorol–gelhoto-electrochemistry

a b s t r a c t

The hematite �-Fe2O3 is prepared by sol–gel route at 900 ◦C. Structural and morphological characteri-zations are investigated by X-ray diffraction and scanning electron microscopy. The SEM image shows amonophasic structure with a chemical composition uniformly distributed, based on sintered agglomer-ates particles with no particular orientation. The EDS characterization shows wide non-stochiometry(O/Fe = 0.72). The X-ray diffraction indicates an orthorhombic crystalline structure with an averagecrystallite size of 87 nm and a surface specific area of 13.3 m2 g−1. The hematite exhibits n-type con-duction with direct (1.99 eV) and indirect (1.75 eV) optical transitions. The electrochemical propertiesare investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and constantcharge–discharge methods in Na2SO3 (1 M) and NaOH (1 M) electrolytes. The CV characteristics show atypical pseudo-capacitive electrode behavior, associated with a faradic mechanism. The electrochemi-

cal impedance spectroscopy carried out over the frequency range (1 mHz–100 kHz) exhibits a dominantpseudo-capacitive behavior at low frequencies. The pseudo-capacitive phenomenon is preceded by diffu-sion process, implying a blocking behavior at high frequencies. The super capacitive properties of Fe2O3 ata scan rate of 5 mV s−1 show a maximum super capacitance of 4.915 and 48.21 F g−1 in NaOH and Na2SO3,respectively. A typical charge–discharge behavior at 5 mA over a potential window of 1.2 V is obtainedwith coulombic efficiency approaching 100% in both electrolytes.

. Introduction

Among iron oxides, the hematite �-Fe2O3 has attracted increas-ng intention since it is an excellent candidate for applications likenode in Li-ion batteries [1], dye solar cells [2], magnetic appli-ations [3], alcohol-sensing material [4], H2S decomposition [5]nd water splitting [6]. In addition, �-Fe2O3 is an interesting semi-onducting material owing to its low cost, high resistance againstorrosion [7] and its moderate gap (∼2 eV) allowing the capturef ∼40% of sunlight [8]. Depending on the elaborating conditions,-Fe2O3 occurs in n- as well as p-type. Its crystallizes in the corun-um structure like a majority of binary oxides M2O3, where M is 3detal, and exhibits a high ionicity (60%) generating great interac-

ion anion–cation. The corundum is a hard solid since it reaches 9

egree on the Mohs scale, just after diamant (degree 10). The crys-al lattice is formed by hexagonal packing of oxygen atoms in whichron occupies two thirds of octahedral sites giving a rhombohedric

∗ Corresponding author. Tel.: +213 553 45 31 68; fax: +213 21 24 80 08.E-mail address: abdi.abderrezak@gmail.com (A. Abdi).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.08.076

© 2013 Elsevier Ltd. All rights reserved.

symmetry. The coordinances of iron and oxygen are: Fe/O = [6] andO/Fe = [4]. Moreover, the hematite is abundant, non-toxic, inex-pensive and easy to prepare. These properties make it potentiallyuseful for large scale applications. The main goal of this work is theevaluation of the photo-electrochemical and the pseudo capacitiveproperties of the non-stoichiometric hematite �-Fe2O3 elaboratedby sol–gel. �-Fe2O3 for super capacitor application was widelydeveloped as thin layer [9] or mixed with other materials in com-posite systems [10] and only fewer papers were devoted to the bulkmaterial. With this in mind, this work aims to enrich the literatureon �-Fe2O3 where the electrochemical properties have not beenfully investigated.

2. Experimental

2.1. Elaboration

The synthesis of the hematite by sol–gel is carried out from purematerials (Prolabo 99%). It is prepared from Fe(NO3)3 (0.05 M) usingethylene glycol (60 mL) in a flask with tri-neck under reflux andmoderate agitation. The molar ratio of Fe(III) to ethylene glycol is

8 imica Acta 111 (2013) 869– 875

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O : 2p orbital while the conduction band involving the emptydFe3+ orbitals represents the lower unoccupied molecular orbitals(LUMO) [11]. The internal Fe3+ d–d transition is not allowed bythe Laporte rules occurring only between transitions in which the

70 A. Abdi, M. Trari / Electroch

xed at 0.003:1. The reaction mixture is heated for 1 h at 70◦ C andhen for 6 h at 120 ◦C, at heating rate of approximately 2 ◦C min−1.he mixture is then placed in a beaker and subjected to moderateeating until elimination of the liquid phase. The recovered solid

s finely ground in an agate mortar and heat-treated in air oven at00 ◦C for 6 h (10 ◦C min−1). The cooling rate is not controlled.

.2. Material characterization

Various characterization methods are used to highlight theormation of the hematite Fe2O3 and to give informationn the structure and morphology, including X-ray diffractionXRD), scanning electron microscopy (SEM) and UV–vis-NirReflectance spectroscopy). To have more information on thehoto-electrochemical effect of �-Fe2O3, the intensity–potentialurves J(V) are plotted with a BioLogic SP-50 potentiostat both inhe dark and under illumination in a three-electrode cell in Na2SO31 M, pH ∼7) and NaOH (1 M, pH ∼13) electrolytes at a scan rate of

mV s−1 and under magnetic stirring. The light source consists of aungsten lamp (Osram, 200 W). The powder is pressed under 70 kNnto pellets (∅ = 1.3 cm, thickness ∼1 mm) and sintered at 800 ◦C.opper wires are attached to the back pellets with silver paint to

nsure ohmic contact. The pellets are encapsulated in glass holderssing epoxy resin. A Pt-foil serves as the counter electrode and aaturated calomel electrode (SCE) as reference electrode (0.242 Vs. NHE).

.3. Electrochemical capacitance measurements

The pseudo-capacitance investigation is performed in the samelectrochemical set up. The working electrode is prepared by mix-ng the active material (�-Fe2O3, 75%) with graphite (Prolabo,5%) and polytetrafluoroethylene (10%), pressed under 3 kN on

stainless steel foil (1 cm2) and dried at 70 ◦C. Cyclic voltam-etry (CV) is recorded from −1 to 0.2 V using Na2SO3 (1 M)

nd NaOH (1) as electrolytes at 5, 25, 50 and 100 mV s−1. Theharge–discharge cycle tests are plotted at 5 mA with cutoff volt-ges of −1 and 0.2 V. The electrochemical impedance spectroscopyEIS) and the Mott–Schottky curves are performed in a.c.-frequencysing BioLogic SP-150 Potentiostat, from 100 kHz to 1 mHz with 5 mVmplitude around the open circuit potential (OCP).

. Results and discussion

.1. Material characterization

The XRD pattern (Fig. 1) confirms the formation of the hematite-Fe2O3. All peaks are assigned to the rhombohedral unit cell

space group: R3c) with the lattice parameters: a = 5.039 A and = 13.770 A, in agreement with the JCPDS card N◦ 89-0598. Theverage crystallite size (87 nm) is evaluated from the empiricalormula {D = 0.9�( cos �)−1} where ˇ(rd.) is the full width at half

aximum (FWHM) and � the diffraction angle. Assuming sphericalnd nonporous crystallites, the specific surface area (13.3 m2 g−1) iseduced from the relation {Sspec = 6/�ex D}, �ex = 5.10 g cm−3}. TheEM image (Fig. 2) shows a monophasic structure with uniformhemical composition, based on sintered agglomerates particlesith no particular orientation. The EDS characterization (Fig. 3)

f the chemical composition indicates a large oxygen deficiency44.88% O, 58.12% Fe). The atomic ratio (O/Fe = 0.72) strongly dif-ers from the stoichiometric one (1.5). This result is directly relatedo the thermally treatment at 900 ◦C where a substantial part of

xygen is removed, thus yielding n-type semiconductor, accordingo the following reaction:

e2O3 → Fe2O3−x + x••

Vo + 2x [e−CB] + x/2 O2 (1)

2θ (degrees)

Fig. 1. X-ray diffraction patterns of the hematite �-Fe2O3.

It is well known that the deviation from the stoichiometry inducesan enhancement of the electrical properties. The room temperatureconductivity measured by the four probe method is found to be1.68 × 10−2 (� cm)−1. The optical band gap is determined from thediffuse reflectance spectroscopy by plotting (F(R∞)h�)m versus theexcitation energy (h�) and extrapolating the linear portion of thecurve (Fig. 4) at h� = 0 according to the relation:

(F(R∞)h�)m = A(h� − Eg) (2)

The exposing m is equal to 1/2 or 2, respectively, for indirect anddirect optical transition. F(R∞) is the Kubelka–Munk function:

F(R∞) = (1 − R∞)22R∞

(3)

where R∞ (I/Io)diff is the diffuse reflectivity. In Fe2O3, the elec-tronic charge transfer inter-bands occurs between the valenceband representing the high occupied molecular orbital (HOMO)and consisting of empty Fe3+ 3d orbital with some admixture of

2−

Fig. 2. SEM micrograph of the hematite �-Fe2O3.

A. Abdi, M. Trari / Electrochimica Acta 111 (2013) 869– 875 871

stacasftdL

bit(poiib

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-6.0

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-2.5

-2.0

log

i / A

.cm

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ec-1

Pote ntial / VSCE

under ligh t

in the dark

Fig. 3. EDS spectra of �-Fe2O3.

econdary quantum number (l) changes by ±1 [12]. On the con-rary, the 2p–d transition associated with the tail of O2− Fe3+ isllowed by the Laporte rules and is classified as ligand-to-metalharge transition type (LMCT). The gap (Eg) between the valencend conduction bands depends on both the purity and the particleize of the semiconductor. The optical transitions of �-Fe2O3 areound to be 1.75 eV (indirect) and 1.99 eV (direct) (Fig. 4). The exis-ence of both transitions is attributed to the possibility of two-foldegeneracy of the valence band at the wave vector k = 0 and k /= 0,–S coupling and in some cases to strong local electric fields [13].

As mentioned in introduction, �-Fe2O3 exhibits a chemical sta-ility over a fair pH range. The semilogarithmic plot log J(V) (Fig. 5)

n Na2SO3 acting as hole scavenger, in the dark and under illumina-ion, exhibits clearly an anodic photocurrent Jph starting at −0.88 Vphotocurrent onset potential Eon) in the positive potential sweeproviding unambiguous evidence of n-type character. It is alsobserved that the plateau region in the large range (−0.8 to +0.1 V)ndicates that the recombination rate of electron/hole (e−/h+) pairs

s kept at a low level. The process of the electron transfer occursetween the occupied quantum state including the reduced species

Fig. 4. Plot of (F(R∞)h�)2 and (F(R∞)h�)1/2 aginst h� of the hematite �-Fe2O3.

Fig. 5. The log J versus potential characteristics of �-Fe2O3 plotted both in the darkand under illumination in (1 M Na2S2O3), scan rate 5 mV s−1.

SO32− in the electrolyte and the vacant state involving the valence

band (Fe2O3-VB), described by [17]:

�-Fe2O3 + h� → e− (CB) + h+ (VB) (4)

SO32− + 2OH− + 2 h+(VB) → SO4

2− (5)

SO32− + 2 h+(VB) → S2O6

2− + 2 H2O (6)

2H2O + 2e− (CB) → H2 + 2 OH− (7)

The Mott–Schottky characteristic i.e. the plot of the reciprocalsquare capacitance (C−2) vs. the applied potential (E) using Eq. (8)[14] fits well with the experimental data with a good linearity overthe potential range (0–0.3 V) (Fig. 6).

C−2 = 2 × (εε0eND)−1(

E − Efb − kT

e

)(8)

where ε is the permittivity of Fe2O3, ε0 the permittivity of the vac-uum, ND the donor density, kT the thermal energy (∼26 meV at300 K) and e the electron charge. The positive slope confirms the n-type character of Fe2O3. By extrapolating the curve to C−2 = 0, the

potential Efb is found to be −0.70 V. The potential (Eon = −0.9 V) ismore negative than the potential Efb and this indicates that somesurface states are present within the gap region.

C-2 /

μF2 c

m-4

Potential / VSCE

Fig. 6. Mott–Schottky plot of the hematite �-Fe2O3 in Na2S2O3 (1 M) electrolyte.

872 A. Abdi, M. Trari / Electrochimica Acta 111 (2013) 869– 875

F�s

3

cp5pi(ppsbttcpfooot

F�s

ig. 7. Cyclic voltammograms of �-Fe2O3 recorded in Na2S2O3 (1 M) at sweep rate = 5, 25, 50 and 100 mV s−1. Inset: current density versus sweep (�) rate and squareweep rate (

√�).

.2. Electrochemical capacitance

The cyclic voltammetry is a powerful tool for the pseudo-apacitance investigation. The voltammograms (Figs. 7 and 8)lotted from 0.2 down to −1 V at different sweep rates (v = 5, 25,0 and 100 mV s−1 in both Na2SO3 and NaOH solutions show aseudo-capacitive behavior identified by a visible proportional-

ty of the area under the curves with increasing the sweep ratev). The CV profiles plotted at 5 mV s−1 (Fig. 9) exhibit four peakotentials of oxidation and reduction in NaOH and only two peakotentials in Na2SO3. The peaks potentials (Table 1) are widelyeparated with a difference (�E > 59 mV) indicating non-Nernstianehavior with quasi-reversible systems. Therefore, the peak poten-ial depends on the scan rate and occurs at potential higher thanhe standard potential (E◦). The current is controlled by both theharge transfer and mass transport. It is well known that for typicalseudo-capacitive electrode, two mechanisms charge–dischargearadic processes can be involved namely the insertion–deinsertionr redox reactions. They are clearly identified by the appearance of

ne or more peaks current on the CV curves and the magnitudef the peak current is linearly proportional to the square root ofhe sweep rate (

√v). Assuming semi-infinite linear diffusion, the

ig. 8. Cyclic voltammograms of �-Fe2O3 recorded in NaOH (1 M) at sweep rate = 5, 25, 50 and 100 mV s−1. Inset: current density versus sweep (�) rate and squareweep rate (

√�).

Fig. 9. Peak potentials of redox process exhibited by CV curve plotted at 5 mV s−1

in 1 M NaOH and in 1 M Na2S2O3 (inset).

peak current versus v for irreversible and quasi-reversible systemsis given by [15]:

Jp = (2.99 × 105) n (˛na)1/2AC∗D1/2�1/2 (9)

where n, na ˛, D, C* are the number of electrons exchanged in a givenreaction, the number of electrons involved in the charge-transferstep, the transfer coefficient, the diffusion coefficient (m2 s−1) andinitial concentration of oxidized or reduced species (mol m−3)respectively. R, T and F have their usual meaning. Eq. (9) can besimplified as:

Jp = K√

v (10)

K is the proportionality constant. As shown in Figs. 7 and 8 (inset),the clear linearity of the current density versus

√v in both elec-

trolytes, confirms that the charge–discharge phenomena at theelectrode Fe2O3 is mainly associated with the faradic mechanism.Despite the symmetry of the CV curves in Na2SO3 (Fig. 7), there is agreat dissimilarity from de ideal rectangular shape with increasingthe sweep rate, illustrating a high ohmic resistance of the elec-trolyte motion in the mesopores of Fe2O3. This may be attributedto the gap between the rate of ion insertion–deinsertion processand the rate of potential change. Concerning the same phenom-ena occurring in Fe2O3 thin film as super capacitor electrode, Kulalet al. [16] have suggested the inaccessibility to ions of a part ofthe electrode surface at high charge–discharge rate. In addition,a significantly shift in the anodic and cathodic peaks at posi-tive and negative potential respectively is observed. Moreover,Yuan et al. [17] have attributed a similar shift in Bi2O3 to themass transfer limitation in the crystalline oxide framework. There-fore, we suggest two different charge–discharge mechanisms:insertion–deinsertion in Na2SO3 according to Eqs. (11) and (12) inagreement with Ref. [18] and redox reactions in NaOH according toEqs. (13)–(16).

(Fe2O3)surf + Na+ + e− ↔ (Fe2O3−, Na+)surf (11)

(Fe2O3−, Na+)surf + Na+ + e− ↔ (Fe2O3

2−, 2Na+)surf (12)

3Fe2O3 + H2O + 2e− ↔ 2Fe3O4 + 2OH− (13)

Fe3O4 + 4H2O + 2e− ↔ 3Fe(OH)2 + 2OH− (14)

Fe(OH)2 + e− ↔ Fe(OH)ads + OH− (15)

Fe(OH)ads + e− ↔ Fe + OH− (16)

A. Abdi, M. Trari / Electrochimica Acta 111 (2013) 869– 875 873

Cur

rent

den

sity

/ A

cm

-2

Potential / VSCE

α

Fo

Eatemti

cdrcridvil

0 10 0 20 0 300 400 500 6000

200

400

600

800

Im(Z

) /cm

2

Re(Z) / .cm2

Fig. 11. The nyquist diagram of the hematite �-Fe2O3 in the range of 100 kHz–1 mHzat the open circuit potential in NaOH (1 M) and Na2SO3 (1 M). Inset: log(Z) vs log(ω).

ig. 10. Test of stability of the electrolyte Na2SO3 performed by cyclic voltammetryn a catalytic platinum electrode in the range of 0.2 to −1 V.

qs. (15) and (16) are in agreement with previous works [19]pproving the reduction of Fe(II) to Fe(0) in alkaline solution inwo steps involving adsorption of OH− ions. The stability of thelectrolyte Na2SO3 from 0.2 to −1 V is verified by cyclic voltam-etry on a catalytic platinum electrode (Fig. 10). After 10 cycles,

here is no occurrence of oxidation or reduction peak which maynterfere with those of the charge–discharge cycle process.

The EIS is a powerful technique for investigating the electro-hemical process with different time constants in the frequencyomain. Fig. 11 shows the EIS plot carried out over the frequencyange (1 mHz–100 kHz) with 5 mV amplitude around the openircuit potential (OCP). From the Nyquist diagram of �-Fe2O3 cor-esponding to the real part of the impedance (Re(Z)) versus themaginary part (Im(Z)), it can be easily seen at low frequencies a

ominant pseudo-capacitive behavior with a slight deviation fromertical straight line with phase angles of ∼80◦ and ∼75◦ obtainedn Na2SO3 and NaOH electrolytes respectively. The examination ofog (Z) versus log (ω) (Fig. 11) establishes that the pseudo-capacitive

0 100 20 0 300 400 5

0

100

200

300

400

500

600

700

800

Im(Z

) /cm

2

Re(Z) / .c m2

d1

Fig. 12. The fitting of the data to the R

behavior at the interface of Fe2O3/NaOH starts at 0.24 Hz with aslope of −0.9, close to −1, and slightly ahead of that in Na2SO3starting only at 0.03 Hz with slope = −0.9. In addition, the pseudo-capacitive phenomenon in NaOH is preceded by a net diffusionprocess with a slope of −0.44 (close to −0.50), thus implying thatthe super-capacitor electrode has a blocking behavior at high fre-quencies and a super-capacitive behavior at low frequencies. TheEIS data are well fitted by the Randles equivalent circuit model(Fig. 12). It combines the high frequency resistance solution, theionic charge transfer resistance, and the impedance restricted lin-ear diffusion (ZRLD) of the inserted species in parallel with anelement phase constant CPE. The impedance ZCPE = [Q(jω)ˇ]−1,(−1 ≤ ≤ 1), represents the pseudo-capacitance transfer of ions in�-Fe2O3 framework at low frequencies, where j2 = −1 and Q a con-

stant (Fsn−1). The impedance restricted linear diffusion is due to the

00 600 70 0 80 0

andles equivalent circuit model.

874 A. Abdi, M. Trari / Electrochimica

F1

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Fe

ig. 13. The specific capacitance versus the sweep rate of the hematite �-Fe2O3: in M Na2SO3 and in 1 M NaOH (inset).

bstruction to species to material transport by diffusion in a thinayer thickness ı given by [20]:

RLD = Rd coth (dp)1/2

(dp)1/2(17)

here Rd and d are a scale factor (˝) and time constant (s), respec-ively. ZRLD is equivalent, at high frequencies (f 3.88/2�d) to a

arburg impedance (case of insertions reactions for example), andorresponds to a capacitor (d/Rd) and a resistor (Rd/3) in seriest low frequencies. The calculated parameters (Table 2) confirmhe results mentioned above and allow us to conclude that the ionharge transfer at high frequencies is by far better in NaOH thann Na2SO3 owing to the difference in the mobility between OH−

2.05 × 10−3 cm2 V−1 s−1) and Na+ (5.19 × 10−4 cm2 V−1 s−1). Onhe contrary, at low frequencies, the behavior in Na2SO3 is that

f pseudo-capacitive, evidenced from = 0.89, (close to unity). Itorresponds to pure capacity and this means that the �-Fe2O3lectrode can store more charge with Na2O3 than with NaOH. Theseudo-capacitance and the specific pseudo-capacitance of Fe2O3

ig. 14. Galvanostatic charge–discharge curves of the hematite �-Fe2O3 electrode at 5 mvaluate the stability of the electrode.

Acta 111 (2013) 869– 875

are quantitatively evaluated versus the scan rate from CV curvesusing the following equation:

Csp = (w�V)−1∫

Jdt (18)

where, J [A cm−2] is the current density integrated in the poten-tial window �V and w is the active weight of the electrodematerial (≈0.059 g cm−2). Fig. 13 shows the variation of specificpseudo-capacitance with the scan rate. A decrease in the specificcapacitance is observed with a scan rate from 4.911 to 3.073 F g−1

in NaOH and from 48.19 to 17.96 F g−1. All the calculated propertiesare summarized in Table 3.

The specific capacities are low compared to those reported inthe literature. We attributed this result to the treatment temper-ature of Fe2O3 (900 ◦C). It confirms the existence of an inverserelationship between the treatment temperature and the spe-cific capacity as evidenced by previous works [21]. The authorshave studied the binary Mn–Fe oxide material electroplated ontographite substrates as a function of the annealing temperature forsuper capacitors applications. They have attributed the regressionof the specific capacitance to the shrinkage in ionic conductiv-ity, due to the decrease in hydrous state, as well as the decreaseof both the accessibility of electrolyte to the pores and the reac-tivity of the oxide. Similar explanations were given by Zhanget al. [22] on the annealing temperature on the capacity of porousCo3O4 nano/micro superstructures. The authors indicate a prob-able increase of the crystal size at high temperatures, generatingconsequently a decrease of both the specific surface area and thechemical reactivity. Nevertheless, it should be noted that the resultsobtained in this study are of the same order of magnitude thanthose obtained on various electrode materials for super capacitorsapplications [23,24].

Typical chrono-potentiograms at a constant current of 5 mAover a potential window of 1.2 V are shown in Fig. 14 andthe charge curves in both Na2SO3 and NaOH electrolytes are

symmetric to their corresponding discharge counterparts. Thisdemonstrates the usual charge–discharge behavior displayedby the super capacitors. The coulombic efficiency (�), calcu-lated from the charge–discharge curve using Eq. (19) [25],

A in the range of 0.2 to −1 V. Inset: The charge–discharge cycling test in order to

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A. Abdi, M. Trari / Electroch

orks out to be 99% for both electrolytes, indicating a goodeversibility.

=(

tdischarge

tcharge

)× 100% (19)

here tcharge and tdischarge are charge and discharge times, respec-ively. Furthermore, Fig. 14 (inset) shows ideally, a . . . shapedoltage response and this indicates the feasibility of Fe2O3 electrodeor the development of super capacitors.

. Conclusion

In summary, non-stoichiometric hematite �-Fe2O3 material isrepared by sol gel. The XRD pattern confirms the phase formationnd the average crystallite size and the specific surface area arevaluated. The SEM characterization shows a monophasic struc-ure with chemical composition uniformly distributed, based onintered agglomerates particles with no particular orientation. TheDS characterization shows a great oxygen deficiency. The semi-ogarithmic and the Mott–Schottky electrochemical test approve-type semi-conductivity of the material with both direct and indi-ect optical transitions. The CV voltamograms plotted within −1o 0.2 V in both NaOH and Na2SO3 solutions show a typical aseudo-capacitive behavior associated with a faradic process. TheIS measurement carried out over a large frequency range con-rms the dominant pseudo-capacitive behavior at low frequenciesreceded by a net diffusion process implying a blocking electrodet high frequencies. The maximum specific pseudo-capacitance ofhe elaborated material in both NaOH and Na2SO3 electrolytes arebtained at 5 mV s−1 and were respectively 4.915 and 48.21 F g−1.he coulombic efficiency (�) calculated from charge–dischargeurve approaches 100% for both electrolytes indicating a goodeversibility.

cknowledgments

The authors thank Dr R. Mellouki for providing XRD analysis

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[

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