Nano iron pyrite (FeS₂) exhibits bi-functional electrode character

Abstract

Sustainable charge storage devices require materials that are environmentally benign, readily moldable, easily synthesizable, and profitable for applications in the electronics industry. Nano iron pyrite (FeS₂) is one such material, which is applicable in diverse areas like photovoltaic devices to seed dressing in agriculture. In this work, we propose an innovative application of nano FeS₂viz., as a symmetric charge storage device that is flexible, portable, and lightweight; along with its fabrication details. The device consists of a (H₃PO₄)/polyvinyl alcohol (PVA) electrolyte gel sandwiched between two similar electrodes made up of FeS₂/poly-aniline (PA), upon which graphite sheets are used as current collectors. Electrodes were characterized by XRD, FTIR and SEM. The device was calibrated by cyclic voltammetry and charge–discharge cycle. In its present laboratory prototype form, it powers solid-state electronic devices and electric motors. Further refinements of this device will open up new avenues in the field of sustainable charge storage devices and low power electronics.

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1. Introduction

There is an ever-increasing demand in the energy sector for compact, lightweight, portable, economical and environmental friendly charge storage devices. Charge storage devices are broadly classified as batteries, capacitors and super-capacitors.^1,2 The basic differences between these devices are in their energy storage potential, power density and cycle time.³

Current battery and super-capacitor manufacturers use a wide range of materials in combination. This includes sulfur-rich cathodes,⁴ lithium polysulfide,⁴ lithium nitrate,⁵ sodium sulfide,⁵ composite electrodes,^3,5–7 conducting polymers,⁸ solid polymer electrolytes,^8–10 organic electrolytes^10,11etc. Many of these materials are inherently toxic, highly reactive and cost prohibitive.^11,12 Thus there is always a quest for cheap, abundant and non-toxic materials for developing next generation of charge storage devices.^13–17

Here we have developed a flexible, portable, light-weight charge storage device using nano iron pyrite (FeS₂). FeS₂ in its bulk form is an earth abundant mineral.^18–21 It is an environmentally benign compound.²¹ It is readily and economically synthesized in nano-dimension, at a relatively low temperature.^22–24 Nano FeS₂ has been explored as a potential photo-voltaic material and as a seed dressing agent in agriculture.^{19,21–23,25} Nano FeS₂ has an effective electron density around 3 × 10¹⁸ cm⁻³, intrinsic resistivity around 0.18 Ω cm, carrier mobility around 300 cm² V⁻¹ s⁻¹, atomic packaging density around 7.55 × 10²² atoms per cm³ and charge storage capacity of around 900 mA h g⁻¹.^{18–21,26–28} Lithium ion battery manufacturers use it as a counter electrode.²⁹ In quest for cheap, abundant and non-toxic materials for developing next generation of charge storage devices,^13–17 we explore the possibility of developing a charge storage device using nano FeS₂. In this work, we are reporting the stepwise protocol for fabrication and characterization of a new kind of a flexible, symmetric charge storage device using nano FeS₂, where (FeS₂) exhibits bi-functional electrode character.

Why we used nano FeS₂? FeS₂ as such comprise of one electro-positive metal (Fe) with highly polarizable soft electro-negative sulfur atom. This will lead to a residual charge separation showing appreciable electro-positive character on iron and electro negative character on sulfur as well. Alignment of this charge distribution is selective, where if it function as a nucleophile then under a nano-surface the sulfur part will act as a nucleophile. Similarly its role of electrophile will be automatically decided by the iron atom to embrace the approaching nucleophile. Thus under optimum condition an electrolyte can impose such polarization so that the two faces of nano FeS₂, may develop amphiphilic character. So a dissociable ionic electrolyte may allow its two ends, that is a cation and an anion to separate accordingly utilizing the nucleophilic and electrophilic heads of the nano FeS₂. Thus we propose that this unique feature of nano FeS₂ can be exploited to develop a bi-functional electrode to construct an electromotive cell. Fig. 1 describes the proposed device architecture.

Fig. 1 Architecture and assembly of the prototyped charge storage device. (a) Preparation flow of charge storage device. (i) Two graphite sheets as current collectors. (ii) FeS₂/Poly-aniline composite spray painted using an air brush onto the graphite sheets for making the electrodes. (iii) The electrolyte gel (phosphoric acid + polyvinyl alcohol) is placed on top of the electrodes. (iv) Charge storage device assembly having a semi-permeable membrane between the gel faces. (b) A schematic representation of a functional charge storage device with symmetric nano FeS₂/PA electrodes connected to an LED. (c) Demonstrating flexibility property of the prototype. (d) Size comparison of actual charge storage device (three devices connected in series) with a standard coin.

2. Experimental section

2.1. Synthesis of FeS₂ nanoparticles

The major raw material for the device is nano iron pyrite. In this Section we described the nano FeS₂ synthesis. Multiple approaches are available for synthesis of FeS₂, where almost all of them require high temperature, yet few synthesis techniques utilize low temperature strategy.^22–24 A recent hot-injection based approach has successfully demonstrated FeS₂ synthesis at a temperature of 200 °C.²⁴ We developed a two-step method, which is slightly more complicated than the hot injection method, but can reduce the synthesis temperature to 100 °C. In the first step, we synthesized the polysulfide solution. In the second step, we use the polysulfide to synthesize the FeS₂ particles. Briefly the synthesis was based upon the nucleophilic reaction between polysulfide and FeCl₃ in an inert atmosphere at 100 °C, which resulted in the formation of FeS₂ nanoparticles with uniform morphology. During the course of this reaction, FeS and FeSH⁺ species were formed, which reacted with polysulfide nucleophile thus leading to the formation of FeS₂.^22,23 The complete synthesis of nano FeS₂ is given in ESI section (Fig. S1†).

2.2. Electrolyte preparation and electrode fabrication

The synthesized nano FeS₂ particle powder color was grayish black. Now the base material is ready to make electrode material. So next, we developed the FeS₂/poly-aniline (PA) composite electrode by chemically polymerizing the monomers of aniline in the presence of FeS₂ nanoparticles in an aqueous suspension.⁵ The detailed description of preparation is given in ESI and illustrated in Fig. S2.† After synthesis, we got the brownish precipitate, which was the composite of FeS₂ and poly-aniline (PA). PA electrodes were prepared by the same process as described above, without addition of FeS₂.^5,30,31 The electrolyte chosen for our charge storage device was a bio-compatible one, viz., phosphoric acid (H₃PO₄). H₃PO₄ is a weak acid that disassociates into H⁺ and H₂PO₄⁻ ions. Poly-vinyl alcohol (PVA) was added to H₃PO₄ to create the polymeric-gel matrix that enables maximum entrapment of H₃PO₄ molecules; allowing ionic transportation with long time sustainability.^8–10 Gel electrolyte offers flexibility by retaining maximum fluid in its pores, thereby significantly reducing the evaporation rate; prolonging the functional life of the prototype. The detailed preparation schematic is provided in the ESI section (Fig. S2†).

2.3. Assembling the device

Fig. 1a details the assembly of the device. The outer current collectors are made of two identical graphite sheets, which are flexible, having high thermal and electrical conductivity, possess chemical inertness, and has high refractory index. The dried FeS₂/PA which was synthesized for electrodes was deposited on one side of graphite sheets using spray painting guns. To facilitate the spraying process, 30 mg of FeS₂/PA was taken and mixed with 3 ml of isopropyl alcohol and 30 μl diluted (6×) Nafion solution, which was sonicated for four hours. The coating surface area was 2 cm² of the graphite sheets, as shown in Fig. 1a(i and ii). The electrolyte gel formed from phosphoric acid + polyvinyl alcohol gel (PVA/H₃PO₄) was poured and casted on top of the electrodes and left undisturbed for duration of four hours at a temperature around 25 °C. Two such electrodes and electrolyte assembly were prepared. Then these graphite sheets were assembled with the gel layers facing each other, while being separated by a cellulosic semi-permeable membrane, as shown in Fig. 1a(iii). The final assembly of the device is depicted in Fig. 1a(iv). Initially the device had no polarity, and it acquired its positive and negative terminals after its first charging. The terminal that was connected to the positive terminal of the source becomes positive and vice versa.

2.4. Structural characterization of the electrodes

The FeS₂/PA electrodes were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and scanning electron microscopy (SEM) techniques. XRD pattern of FeS₂ exhibited the diffraction peaks at 28.71, 33.05, 37.09, 40.73, 47.40, 56.18, 61.53 and 64.22; which were indexed to (111), (200), (210), (211), (220), (311), (222) and (023) planes respectively (Fig. 2a). This was consistent with single phase of FeS₂ (JCPDS card no. 42-1340) and indexed to space group Pa3 [205], that showed the cubic structure. No additional peak was observed in the XRD pattern thereby confirming the purity of the synthesized FeS₂ sample.^{22,23,32–35} Similar peaks were observed in the diffraction pattern of FeS₂/PA, thus indicating that the while the polymerization of poly-aniline took place, the FeS₂ particles were not dissolved in the acidic medium. By the Scherrer's equation (D = Kλ/B cos θ), (where λ = wave length of X-ray, θ the diffraction angle, K the shape factor and B = B_actual = √(B_observed² − B_resolution²) is the correct half width of the observed half width) the average crystalline size was calculated to be 35 nm. In FTIR spectra (Fig. 2b), the FeS₂ powder exhibited a broad band centered at 3415 cm⁻¹ and at 1615 cm⁻¹, which could be assigned to the O–H stretching and bending modes of water, respectively. The bands at 1420 cm⁻¹ and 1355 cm⁻¹ corresponds to the asymmetric S–O stretching of the sulfate species while the peaks at 612 cm⁻¹ and 410 cm⁻¹ were characteristic of the disulfide stretch (S–S). For the FeS₂/PA composite, band at 3380 cm⁻¹ and 1530 cm⁻¹ could be corresponding to O–H stretching and bending mode of water respectively. The band at 1420 cm⁻¹ could be S–O stretching of sulfite species and 403 cm⁻¹ would be disulfide (S–S) stretch. The band at the characteristic peaks at 1295 cm⁻¹ and 1225 cm⁻¹ could be corresponding to the quinoid ring (Q) and the benzene ring, respectively. The characteristic band of poly-aniline base will be the N=Q=N stretching band at 1128 cm⁻¹.^5,32–35 Raman spectrum was recorded in wave numbers ranging from 200 cm⁻¹ to 500 cm⁻¹ (ESI Fig. S3†). Sharp peaks were observed at 337 cm⁻¹, 378 cm⁻¹, and 431 cm⁻¹; which is in agreement with published Raman spectroscopic results for pure FeS₂ crystals.^32–35 From the XRD, Raman Spectroscopy and FTIR it could be concluded that synthesized FeS₂ possess single-phase pyrite without detectable marcasite, greigite, pyrrhotite, or other impurities. In SEM analysis of FeS₂, it exhibited a pseudo octahedral or flowery morphology with an average particle diameter of 400–500 nm (Fig. 2c). We observed similar morphologies of FeS₂ nanostructures in our earlier work.^22,23Fig. 2d provides the SEM image of the poly-aniline. The poly-aniline exhibited a rod or tubular morphology with an average thickness of 20–30 nm. Fig. 2e showed the representative SEM picture of FeS₂/PA composite. We observed that the sharp feature of FeS₂ is missing. Instead we observed a more globular morphology. It seems as if the poly-aniline has encapsulated the FeS₂ particles and offered them a more smooth and rounded texture and morphology respectively. Thus we have speculated that PA is forming a coating on the FeS₂ surface thereby enhancing the electrochemical activity/conductivity of the composite (Fig. 2f).

Fig. 2 Characterization of the electrode. (a) XRD of FeS₂ and FeS₂/PA. (b) FTIR of FeS₂ and FeS₂/PA. (c) SEM images of FeS₂ showed pseudo octahedral flowery morphology. (d) SEM image of PA showed rod or tubular morphology. (e) SEM images of FeS₂/PA have missing sharp feature of FeS₂, instead showed more globular morphology. (f) Poly-aniline encapsulation on the FeS₂ particles.

2.5. Electrochemical characterization of the device

The electrochemical characterization of the device was performed using electrochemical work station ‘Epsilon BASi Cell Stand C3’. The charge–discharge measurements were performed using AutolabB.V (Metrohm). The working electrode was of FeS₂/PA with 1 cm² area which was sprayed on one side of the graphite sheet. Only FeS₂/PA area was activated as an electrode, remaining graphite sheet was kept as electrochemically insulated. The reference electrode was an Ag/AgCl (in 3 mol l⁻¹ NaCl) electrode and the counter electrode was a platinum wire. CV was done at different scan rate from 5 mV s⁻¹ to 300 mV s⁻¹ at 0 to 0.8 V voltage window in aqueous 2 M H₃PO₄ solution.

3. Results and discussion

3.1. Rational for developing FeS₂/PA composite electrode and gel electrolyte

The two electrodes were prepared by a FeS₂/poly-aniline (PA) composite. Our rational for developing FeS₂/PA composite electrode is the following: PA has been extensively studied as cathode material for electrochemical capacitor due to its excellent conductivity, redox reversibility and its stability towards environment.^3,5,36–43 But one of the major drawbacks of using PA as stand alone electrode in the charge storage device is its poor discharge capacity.^38,39,42 On the other hand iron disulfide (FeS₂), one of the most abundant metal sulfide found on earth's crust, have three basic properties which are of immense significance for developing charge storage devices^{19–21,40,41,44,45}viz., (a) mediating electron-transfer (b) electrical-charge separation, (c) capability to store energy temporarily or in the form of stable chemical products. Here we have shown that using a low temperature strategy we could obtain uniform size particles of FeS₂. Thus marrying FeS₂ with PA in the form of a composite could offer added benefit in terms of charge storage and discharge potential. That inspired us to create FeS₂/poly-aniline composite. Further CV analysis showed that this strategy indeed mutually improved the charge storage and discharge capacity of both FeS₂ and PA. Further it has been observed that the presence of nanoparticles in the matrix of the conducting polymer results in enhanced electrochemical activity/conductivity.⁴³ The rational for choosing gel electrolyte was to ensure that the solution does not dry out easily and offer more flexibility to the device. The H₃PO₄ is reasonably biocompatible and also functions as an ionic liquid. Poly-vinyl alcohol (PVA) acts as a polymeric matrix thus simultaneously allowing ionic transport and functioning as a separator layer between the electrodes.⁴⁶

3.2. Cyclic voltammetry (CV) analysis of the FeS₂, PA & FeS₂/PA electrodes and reason to select FeS₂/PA for developing the device

To evaluate the electrochemical performance of the FeS₂, PA and FeS₂/PA electrodes, cyclic voltammetry was carried out using Epsilon Basi C3 cell stand with conventional three electrode configuration in 2 M aqueous solution of H₃PO₄. CV was done at different scan rate from 5 mV s⁻¹ to 300 mV s⁻¹ at 0 to 0.8 V voltage window. Cyclic voltammogram of FeS₂/PA had approximately rectangular shape as compared to the FeS₂ and PA alone (Fig. 3a–c). FeS₂/PA showed higher specific capacitance, as compared to FeS₂ and PA alone. When the integrated area from CV from the voltammogram of FeS₂, PA and FeS₂/PA, we observed that FeS₂/PA covers 14 times higher area as compared to FeS₂ and 7 times higher area as compared to PA, thus highlighting the fact that FeS₂/PA has highest specific capacitance as compared to the individual components (Fig. 3d). Correspondingly the current density was almost 15 times higher in FeS₂/PA as compared to FeS₂ alone and 2 times higher than PA (at a scan rate of 300 mV s⁻¹) (Fig. 3d). Another interesting aspect of the CV of FeS₂ is the following: in Fig. 3a we observed a significant hump at 0.2 and 0.4 V thus indicating that certain redox reaction is taking place, thus resulting in the pseudo capacitance as well as double layer capacitance. In Fig. 3b, in CV of PA, we observed that there is no hump but the area of curve and peak current is significantly higher than FeS₂. Surprisingly in phosphoric acid environment, PA is not exhibiting observable pseudo-capacitance, whereas double layer capacitance is distinctly observable.^39,42,46 Now in the Fig. 3c and d, CV of FeS₂/PA showed small humps at 0.2 and 0.4 V, thus hinting at the occurrence of redox reaction. The FeS₂/PA has specific capacitance approximately 362 F g⁻¹ m⁻¹ whereas the FeS₂ have 112 F g⁻¹ m⁻¹. Thus addition of PA along with FeS₂ improves the electrochemical properties of the electrode. Now, on the basis of the CV analysis, it was clear that FeS₂/PA could perform as a better charge storage module as compared to FeS₂ and PA alone. Thus we fabricated a FeS₂/PA hybrid device and performed an electrochemical and charge/discharge characterization of the device (Fig. 4).

Fig. 3 CV analysis of electrode material. (a) CV of FeS₂ at different scan rates. (b) & (c) CV of PA and FeS₂/PA at same scan rates as seen in FeS₂. (d) Comparison of the specific capacitance of FeS₂ and FeS₂/PA through CV at 300 mV s⁻¹ scan rate.

Fig. 4 Device characterization (a) CV of the device at different scan rate. (b) Relation between scan rate and specific capacitance. (c) Charge–discharge cycle of FeS₂/PA device at different current density. (d) Percentage capacitance retention vs. number of cycles to validate stability and performance of the prototype device till 10 000 charge–discharge cycles.

3.3. Electrochemical analysis of charge storage device

Further, cyclic voltammogram the charge storage device of FeS₂/PA was performed. CV at different scan rate from 100 mV s⁻¹ to 300 mV s⁻¹ and 0 to 0.8 V voltage window showed that the graph was almost rectangular in all scan rates and current density was approximately 0.025A cm⁻² for 300 mV s⁻¹ (Fig. 4a). The specific capacitance (C_s) were calculated from the CV curves by equation

(C_s = ∫I(u)dt/m × v × ΔV)

where, m indicates the mass of the active electrode material, I is the oxidation or reduction current, v is the scan rate, dt is time differential, and ΔV indicates the voltage range of one sweep segment.⁶ It is observed that the specific capacitance decreased with an increase in the scan rate from 5 mV s⁻¹ to 500 mV s⁻¹ (Fig. 4b). This is possibly due to the insufficient time available for ion diffusion at faster scan rate. Galvanostatic charge–discharge characteristics of the device were performed using Autolab (NOVA 1.10 interface) at different current densities (1 mA cm⁻², 2 mA cm⁻², 4 mA cm⁻² and 5 mA cm⁻²). The observed discharge curves of the device exhibited an almost mirror symmetry with its charge counterpart at higher current density such as 5 mA cm⁻² (Fig. 4c). Interestingly as the current density decreased, the charge time became less and discharge time became high as could be seen in 1 mA cm⁻² curve. In terms of values, the charge and discharge time is same for 5 mA cm⁻² that is 20 s but for 1 mA cm⁻² the charging time is 160 seconds but the discharging time 310 seconds. This means that the device could store charges for longer period of time. The thing which is noteworthy was that for low current density (4 mA cm⁻² downwards), the discharge curve shows some flatness after 0.25 to 0.3 V. This flatness indicates that the charge storage device could hold charge in usable voltage range for a significant amount of time. The specific capacitance for the electrodes could be obtained from charge–discharge data by equation

(C = I × Δt/m × ΔV),

where C (F g⁻¹) is the specific capacitance, I (A) is the constant discharging current, Δt (s) is the discharging time, ΔV (V) is the potential window, and m (g) is the mass loading of the active material in the working.⁶ The specific capacitances of the FeS₂/PA electrode are 390, 220, 125 and 113 F g⁻¹ at the current densities of 1, 2, 4 and 5.0 mA cm⁻², respectively. It can be seen here, that the specific capacitance values decreased with increasing current density.

3.4. Evaluating the functional stability of the electrodes and the device

Three methods were employed to verify the long term stability of the device. First, a 10 000 charge/discharge cycles at a fixed current density of 1 mA cm⁻² was used to verify the long term capacitance retention property of the device (Fig. 4d) and cyclic voltammetry of FeS₂/PA electrode at 300 mV s⁻¹ scan rate to verify the long term cycling stability (Fig. 5a). We observed that after 10 000 charge/discharge cycles, the charge storage in the device was at 97% of the maximum storage capability. Similar electrochemical stability was observed after conducting 10 000 cycles of CV using the FeS₂/PA electrode material. It should be noted from Fig. 5b that the current density was still at 40 mA cm⁻², which is about 95% of the initial value; demonstrating the long term cyclic stability of the device prototype. Further we made a comparative CV analysis of fresh FeS₂/PA electrode with FeS₂/PA electrodes treated continuously with H₃PO₄ (pH 1.4, 2 M H₃PO₄ aqueous solution) for two, four and seven days respectively demonstrating similar current density and shape (Fig. 5c).

Fig. 5 Functional stability experiment. (a) CV cycles comparison for shape and storage capability retention from first to ten thousand cycle indicating that current density of 0.04 A cm⁻² is maintained throughout. (b) Peak current density following 10 000 cycles was maintained at 0.04 A cm⁻² c⁻¹. CV comparison of fresh FeS₂/PA electrode with FeS₂/PA electrodes treated with H₃PO₄ for two, four and seven days respectively demonstrating similar current density and shape.

3.5. Evaluating the structural stability of the electrode material used in the device

We conducted two experiments to establish the structural stability of FeS₂/PA material in H₃PO₄ environment (pH 1.21–1.4). First, freshly prepared samples of FeS₂ (100 mg each) was kept in 1.4 pH 5 ml 2 M H₃PO₄ for two, four, and seven days in three different vials. After completing the second, fourth and seventh day respectively; FeS₂ was taken out from the vials, and conducted XRD and SEM investigation on those samples. Same experiment was conducted for the FeS₂/PA material. The XRD graph in Fig. 6 exhibits the same peaks for all samples for each specific day for both FeS₂ and FeS₂/PA materials. We did not observe any difference in the XRD peaks and further we observed similar morphology in the SEM analysis as shown in Fig. 7; thus demonstrating the stability of the material even after seventh day in H₃PO₄ environment.

Fig. 6 Structural comparison (XRD). (a) XRD of FeS₂ at second, fourth and seventh day of H₃PO₄ treatment. (b) XRD of FeS₂/PA at second, fourth and seventh day of H₃PO₄ treatment.

Fig. 7 SEM images of FeS₂ and FeS₂/PA nanoparticles after 2, 4 and 7 days of H₃PO₄ treatment (a) FeS₂ at day 2. (b) FeS₂ at day 4. (c) FeS₂ at day 7. (d) FeS₂/PA at day 2. (e) FeS₂/PA at day 4. (f) FeS₂/PA at day 7.

3.6. Testing the efficacy of the device

Initially the device had to be charged using a 2 V DC power source. For optimal efficiency of the device, the first charging should be done approximately for 1 hour. Then onward, subsequent charging need not require more than 5 minute. Once the device was fully charged, three devices are connected in a series circuit, which is sufficient to glow an light emitting diode (LED) device for more than 1 hour (ESI video file 1†) and could operate a simple electrical motor (ESI video file 2†). The minimum voltage requirement of the LED was 1.6 V and 20 μA current. The device had a charge storage capacity of approximately 20 mA h and the maximum discharging current is 30 mA (device dimension 2 cm²). If the size of the device was increased, then the discharge carrying capacity would subsequently increase, and it could run a DC motor, stepper motor etc. The device could be more extremely flexible and foldable by coating of graphite on a cello-tape.

3.7. Proposed model of charge transfer and the overall discussion

The key question is how the proposed device stored electrical energy? There could be multiple strategies by which charge could be stored viz., electrochemical double-layer capacitance, pseudo-capacitance, battery effects and other small capacitive effects.^47,48 The charge accumulation at the electrode–electrolyte interface generates electrochemical double-layer capacitance and possibly some pseudo-capacitance was generated due to certain faradic reaction on the electrode surface^6,8,49 (Fig. 8). Thus the total capacitance produced by the device was due to the addition of two capacitances along with the other small capacitive effects. Further the property of sulfur ions to work as electron sink and thus causing delocalized electronic states, might help in efficient electron transfer and optimal kinetics to run the device efficiently.^{15,26–28,41,44} Further, the entrapment of FeS₂ nano particles in the conducting polymer matrix of PA possibly offers better electrochemical activity/conductivity.⁴³ The device which had been developed here is in its crudest form. If this is refined further, this could find several applications in the emerging domain of sustainable energy storage devices for flexible electronics.

Fig. 8 Proposed charge storage mechanism of the device. (a) Electrical double layer formation during charging. The electrolyte (H₃PO₄) dissociates and H⁺ ions migrate towards the cathode and H₂PO₄⁻ ions migrate towards the anode. (b) Possible pseudo capacitive effects occur at the electrode surface. Since sulfur atom is a nucleophile having enough electron density, which will easily create an adduct with H⁺ and thus possibly forming H–S hydrogen bonding. Whereas Fe center has enough positive character, so it can act as an electrophile, where it can create a loose electrostatic bonds with oxo donor group of phospho ion. In summary these features of nano FeS₂ offers it a bi-functional electrode character.

4. Conclusions

In this work, we propose an innovative and novel application of nano FeS₂viz., as a symmetric charge storage device that is flexible, portable, light weight; along with its fabrication details. In its present laboratory prototype form; it powers solid-state electronic devices and electric motors. Further refinements of this device will open new avenues in the field of sustainable charge storage devices and low power electronics.

Acknowledgements

Acknowledgement This work is part of AD's doctoral thesis. AD is supported by MHRD, GOI. MR is supported by SB/FT/CS-199/2013 (DST- SERB, GOI).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01973k

‡ SS and MD are co-corresponding authors.

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