J

A

miarc©t

K

1

aia

P

h1C

ARTICLE IN PRESS+ModelTUSCI-334; No. of Pages 6

Journal of Taibah University for Science xxx (2016) xxx–xxx

Available online at www.sciencedirect.com

ScienceDirect

Short Communication

Supercapacitors studies on BiPO4 nanoparticles synthesized via asimple microwave approach

S. Vadivel a,∗, D. Maruthamani a, M. Kumaravel a, B. Saravanakumar b, Bappi Paul c,Siddhartha Sankar Dhar c, K. Saravanakumar d, V. Muthuraj d

a Department of Chemistry, PSG College of Technology, Coimbatore 641004, Indiab Department of Physics, Dr. Mahalingam College of Engineering and Technology, Pollachi 642003, Tamil Nadu, India

c Department of Chemistry, National Institute of Technology, Silchar 788010, Assam, Indiad Department of Chemistry, VHNSN College, Virudhunagar 626 001, Tamil Nadu, India

Received 13 March 2016; received in revised form 21 August 2016; accepted 6 September 2016

bstract

BiPO4 nanomaterial was synthesized using EDTA (ethylene diamine tetra acetic acid) as the surfactant via a simple microwaveethod. The structure and morphology of BiPO4 were systematically characterized by X-ray diffraction (XRD), Fourier-transform

nfrared spectroscopy (FT-IR), and scanning electron microscopy (FE-SEM) studies. The obtained BiPO4 nanoparticles were, onverage, 150–300 nm. The electrochemical results showed that the specific capacitance of BiPO4 obtained using the microwave

−1 −1

oute was up to 104 Fg at a current density of 1 Ag with a large potential window of 1.7 V. The material showed excellentycling stability (92% capacitance retention) after 500 cycles at a current density of 1 Ag−1.

2016 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article underhe CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

apacito

eywords: EDTA; BiPO4; Microwave approach; Microscopy; Superc

. Introduction

Recently, supercapacitors have received significantttention among researchers due to the outstand-ng features of delivering more power than batteriesnd exhibiting higher energy densities than ordinary

Please cite this article in press as: S. Vadivel, et al. Supercapacitomicrowave approach, J. Taibah Univ. Sci. (2016), http://dx.doi.org/

∗ Corresponding author.E-mail address: vlvelu7@gmail.com (S. Vadivel).

eer review under responsibility of Taibah University.

ttp://dx.doi.org/10.1016/j.jtusci.2016.09.007658-3655 © 2016 The Authors. Production and hosting by Elsevier B.V. on

C BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

rs

capacitors [1,2]. They can be used in hybrid elec-tric vehicles with batteries to recover the energy lossduring braking. Based on the energy storage mecha-nism of supercapacitors, they can be classified into twotypes: electric double layer capacitors (EDLC process)and pseudocapacitors (Faradaic process). In EDLC, thecharge storage characteristics are mainly due to theelectrode–electrolyte interface. Carbon materials, suchas activated carbon, carbon nanotubes, and graphene,are used as EDLC-type electrodes. In pseudocapacitors,the charge is stored by means of a Faradic process.

rs studies on BiPO4 nanoparticles synthesized via a simple10.1016/j.jtusci.2016.09.007

behalf of Taibah University. This is an open access article under the

Conducting polymers, metal oxides and sulphides arecommonly utilized as pseudocapacitor electrodes. In thepast decades, RuO2 has been studied extensively as an

IN+Model

Univer

ARTICLEJTUSCI-334; No. of Pages 6

2 S. Vadivel et al. / Journal of Taibah

electrode material for supercapacitors [3]. However, thetoxicity and high cost limits its practical applications.In this context, various metal phosphates such as BiPO4[4] and Co3(PO4)2·8H2O [5] have been studied as pos-sible electrode materials for supercapacitors. BiPO4, anoxoacid salt semiconductor, has been reported to bean active material for photocatalytic disinfection, sen-sors and Li-ion batteries [4]. To date, increasing interesthas focused on BiPO4 in supercapacitors owing to itshigh thermal and chemical stability and inherent redoxactive species [6]. Recently, a few inspiring reportshave been published based on a BiPO4 nanomaterialas an electrode material for supercapacitor applications.For example, Nithya and co-workers (2015) synthe-sized BiPO4 using a hydrothermal method and testedits potential as a supercapacitor electrode in 1 M KOH[4,7]. This BiPO4 electrode material exhibited a spe-cific capacitance of 202 Fg−1. Furthermore, Nithya et al.(2015) synthesized BiPO4 nanoparticles by a simplesonochemical approach [8]. A maximum specific capac-itance of 1052 Fg−1 (pH-7 at 2 mV s−1) was observedfor the BiPO4 prepared from the ultrasonication process.Vadivel et al. (2016) reported a solvothermal route tofabricate a BiPO4/MWCNT composite; a maximum spe-cific capacitance of 504 Fg−1 at a scan rate of 5 mV s−1

was obtained [9]. Thus, the electrochemical propertiesof the BiPO4 nanomaterial were greatly influenced bythe synthesis route. BiPO4 can be synthesized via var-ious methods including hydrothermal and solvothermalmethods, but microwave induced synthesis is a facile andrapid route for obtaining unique nanomaterial morpholo-gies [10,11]. The microwave route has many advantagessuch as a high phase purity and the reproducibility ofthe materials. To the best of our knowledge, the superca-pacitor properties of BiPO4 nanomaterials synthesizedusing the microwave approach has not been reportedso far. In this paper, we report the synthesis of BiPO4nanomaterials using a microwave method employingEDTA as a surfactant. The electrochemical performanceof the BiPO4 nanomaterials prepared via the microwavemethod has been reported and discussed for supercapac-itor applications in a 2 M KOH electrolyte solution.

2. Experimental

2.1. Materials and methods

The bismuth nitrate pentahydrate (Bi(NO3)3·5H2O),

Please cite this article in press as: S. Vadivel, et al. Supercapacitomicrowave approach, J. Taibah Univ. Sci. (2016), http://dx.doi.org/

disodium phosphate (Na2HPO4), ethylene glycol (EG)and ethylene diamine tetra acetic acid (EDTA) were pur-chased from Sisco research laboratories Pvt. Ltd. (SRL),India.

PRESSsity for Science xxx (2016) xxx–xxx

2.2. Synthesis of BiPO4 nanoparticles

The BiPO4 was synthesized through a simplemicrowave route. In a typical synthesis, 1 g ofBi(NO3)3·5H2O and 0.1 g of EDTA were dissolved ina 20 ml EG solution with continuous magnetic stirring(solution A). Meanwhile, 0.35 g of Na2HPO4 was dis-solved in another 20 ml EG solution (solution B). Then,solution B was poured into solution A under vigorousstirring, leading to the formation of a white suspension.Next, the mixture was kept inside a microwave oven(2.45 GHz and 800 W) and heated to 160 ◦C for 3 min.After the completion of the reaction, the obtained dirty-white precipitate was cooled naturally and then washedwith DI water and acetone before it was dried at 70 ◦Covernight. Finally, the obtained sample was analyzed byvarious characterization techniques.

2.3. Characterizations

The crystalline structure of the BiPO4 nanoparticleswas confirmed by X-ray diffractometry (Xpert Pro PANanalytical X-Ray diffractometer) by applying a scanningspeed of 5◦/min with Cu K�1 radiation. The shape andstructure of the BiPO4 nanoparticles was determinedusing a scanning electron microscope (SEM TescanVega3 SBUVG8211771N-Czech Republic). The trans-mission electron microscope (TEM) was recorded usinga Hitachi H7650 electron microscope. The Raman spec-trum of the BiPO4 nanoparticles was recorded at roomtemperature using a nano-photon confocal Raman spec-trometer. The FT-IR spectrum of the BiPO4 was recordedusing a TENSOR 27 Bruker FT-IR spectrophotometer inthe range of 4000–400 cm−1. The BiPO4 samples wereprepared by using KBr to form a pellet. Electrochemi-cal and photoelectrochemical studies were recorded in athree-electrode system with a 2 M KOH electrolyte solu-tion. Pt wire was used as the counter electrode, and acalomel electrode (SCE) was used as the reference elec-trode. BiPO4 coated on a glassy carbon electrode (GC)served as the working electrode. The electrochemicalstudies were recorded using an electrochemical system(CHI-660D). The mass of the active electrode materialwas approximately 1 mg.

3. Results and discussion

Fig. 1(a) shows the XRD spectrum of the as-prepared

rs studies on BiPO4 nanoparticles synthesized via a simple10.1016/j.jtusci.2016.09.007

BiPO4 nanoparticles. It can be seen that all of the diffrac-tion peaks in the XRD pattern of the BiPO4 can beindexed to the monoclinic phase of BiPO4 (JCPDSCard. No. 89-0287) [12]. No characteristic peaks were

ARTICLE IN PRESS+ModelJTUSCI-334; No. of Pages 6

S. Vadivel et al. / Journal of Taibah University for Science xxx (2016) xxx–xxx 3

trum an

ottpO1ttgcvt1atbgRntpttaei

Fig. 1. (a) XRD spectrum, (b) FT-IR spectrum, (c) Raman spec

bserved for other impurities, such as surfactant, fur-her implying the purity of the BiPO4. Fig. 1(b) showshe FT-IR spectrum of BiPO4. The main absorptioneak in the region of 3480 cm−1 is assigned to the

H stretching vibrations, and the peak observed at603 cm−1 is attributed to the O H bending vibra-ions, respectively. The peak at 1020 cm−1 correspondso the �3 asymmetric stretching vibrations of the PO4roups [13]. The peaks observed at 595 and 537 cm−1

orrespond to the � (O P O) and �4 (PO4) bendingibrations of PO4 moieties, respectively. Fig. 1(c) showshe Raman spectrum of the BiPO4. The peaks observed at018 cm−1 and 951 cm−1 correspond to the asymmetricnd symmetric vibration modes of PO4. Furthermore,he peaks observed at 540 cm−1 and 591 cm−1 cane attributed to the �4 bending vibrations of the PO4roups, respectively [14]. Based on the XRD, FT-IR andaman spectroscopy results, the formation of the BiPO4anoparticles was established. Fig. 1(d) shows the pho-ocurrent measurements of the BiPO4 nanoparticles. Thehotocurrent measurement is used to elucidate the elec-ron mobility and interfacial charge transfer dynamics ofhe semiconductor matrix. BiPO4 nanoparticles exhibit

Please cite this article in press as: S. Vadivel, et al. Supercapacitomicrowave approach, J. Taibah Univ. Sci. (2016), http://dx.doi.org/

n obvious photocurrent response, which favours a largerlectrode–electrolyte interface during charge discharg-ng process [15].

d (d) photoelectrochemical spectrum of BiPO4 nanoparticles.

Fig. 2(a) and (b) shows the SEM images of theBiPO4 nanoparticles obtained via the microwave route.It can be observed that the as-synthesized BiPO4 samplecomprises irregular spherically shaped particles with anaverage size between 150 nm and 400 nm. The main rea-son for the formation of the spherically shaped BiPO4could result from the introduction of EDTA as a surfac-tant for capping the bismuth nuclei. The morphologyof the BiPO4 sample was further analyzed by TEM,and it is represented in Fig. 2(c). The BiPO4 nanopar-ticles with severe agglomeration were clearly observed,which are in accordance with the SEM results. Further-more, a 200 nm average size of the BiPO4 nanoparticleshas been observed from TEM analysis. Furthermore, thescheme of the formation of the BiPO4 nanoparticles isrepresented in Fig. 2(d) based on the SEM and TEMobservations.

Electrochemical studies were used to determinethe potential of BiPO4 as a suitable material forsupercapacitor applications. Cyclic voltammetry (CV)studies are a powerful tool to identify the nature ofelectrochemical reactions, whether a Faradaic or non-Faradaic reaction occurs at the electrode/electrolyte

rs studies on BiPO4 nanoparticles synthesized via a simple10.1016/j.jtusci.2016.09.007

interfaces [16,17]. The CV studies were performed ina 2 M KOH electrolyte solution in the potential rangefrom 0.2 V to −1.4 V. The CV curves of the BiPO4

ARTICLE IN PRESS+ModelJTUSCI-334; No. of Pages 6

4 S. Vadivel et al. / Journal of Taibah University for Science xxx (2016) xxx–xxx

nopartic

Fig. 2. (a, b) SEM images, (c) TEM images of BiPO4 na

−1

Please cite this article in press as: S. Vadivel, et al. Supercapacitomicrowave approach, J. Taibah Univ. Sci. (2016), http://dx.doi.org/

nanoparticles at various scan rates (5–100 mV s ) arerepresented in Fig. 3(a). The presence of oxidation andreduction peaks (Faradaic nature) in the BiPO4 nanopar-ticles infers the pseudocapacitive nature of the BiPO4

Fig. 3. (a) CV curve of BiPO4 nanoparticles at different scan rate (5–100 mV sat different current densities, (c) variation of specific capacitance values of BiPof BiPO4 electrode.

les and (d) scheme of formation of BiPO4 nanoparticles.

rs studies on BiPO4 nanoparticles synthesized via a simple10.1016/j.jtusci.2016.09.007

nanoparticles and is attributed to the oxidation andreduction reaction between the Bi(III) and Bi(0) states[8]. The peak current values increased with insignifi-cant changes in the shape of the CV curves and with

−1), (b) galvanostatic charge–discharge curves of BiPO4 nanoparticlesO4 nanoparticles as a function of current density and (d) EIS spectrum

IN+ModelJ

Univer

amotccgBpob

l

S

wcr[spce

R

Hodo

RiaoiiiFt5tfcatovttB

ARTICLETUSCI-334; No. of Pages 6

S. Vadivel et al. / Journal of Taibah

n increase in the scan rate [18]. The CV perfor-ance indicates that the electrochemical performance

f the BiPO4 nanoparticles is quite appreciable inhe 2 M KOH solution. The improved electrochemi-al performance was also confirmed using galvanostaticharge–discharge (GCD) studies. Fig. 3(b) depicts thealvanostatic charge–discharge (GCD) studies of theiPO4 nanoparticles. Consistent with the CV curves, thelateaus in the discharge curves indicate the existencef Faradaic processes. A non-linear type of dischargingehaviour is observed in various current densities.

The specific capacitance (SC) values can be calcu-ated by Eq. (1) [19]:

C = I × Δt

ΔV × mFg−1 (1)

here I is the current density, �t is theharging–discharging time, �V is the potentialange, and m is the mass of the electroactive material20]. BiPO4 nanoparticle electrodes achieved a highpecific capacitance of 104 Fg−1 at 1 Ag−1 with a largeotential window of 1.7 V (Fig. 3(c)). In addition, thealculated internal resistance (R) values of the BiPO4lectrode was based on Eq. (2):

= VIR

2I(2)

ere VIR is the potential drop between the first two pointsf the potential drop at the top cut-off; the applied currentensity is denoted. The calculated internal resistance (R)f the BiPO4 electrode at 1 Ag−1 is 402 �.

From the charge–discharge analysis, a large internal in the electro active species was observed, which may

ncrease the internal resistance of the BiPO4 electrode,nd in turn may produce a lower specific capacitancef the composite [21]. A high potential window, whichmplies high energy density of the electrode material,s an elusive feature. Furthermore, the cyclic stabilitys useful for realizing applications in a supercapacitor.ig. 3(c) shows the cycle performance of a BiPO4 elec-

rode examined using galvanostatic charge–discharge00 cycle tests at a current density of 1 Ag−1. It is clearhat the BiPO4 electrode exhibited a good cycling per-ormance with a small capacitance loss of 8% after 500ycles. For further understanding, the charge transfernd capacitive nature of BiPO4 electrode is evaluatedhrough an EIS measurement in the frequency rangef 0.01–100 kHz. The charge-transfer resistance (Rct)

Please cite this article in press as: S. Vadivel, et al. Supercapacitomicrowave approach, J. Taibah Univ. Sci. (2016), http://dx.doi.org/

alue is calculated from the high frequency region ofhe EIS plot (Fig. 3(d)). The observed Rct value ofhe BiPO4 electrode was 40 �, which revealed that theiPO4 electrode could enhance the electrolyte diffusion

PRESSsity for Science xxx (2016) xxx–xxx 5

into the electrode surface during the Faradaic process[22]. Fig. 3(d) inset depicts the equivalent circuit forthe EIS studies. In that circuit, Rs stands for the solu-tion resistance, Rct is the charge transfer resistance, Cdlis the double layer capacitance and W is the Warburgimpedance.

The superior capacitive performance of the BiPO4electrode is mainly attributed to the following reasons:first, the sphere-like morphology leads to an improvedhigh-current capacitive behaviour; second, the meso-porous structure provides suitable pore volumes, whichcan store more OH ions, resulting in a lower resistancevalue during the redox process; last, nanosized buildingblocks greatly enhance the outer surface areas, achiev-ing a high number of active sites in the supercapacitormatrix [23,24].

4. Conclusion

This study demonstrates a facile approach for thesynthesis of BiPO4 nanoparticles. Microwaves influ-enced the initial nucleation process during the formationof BiPO4. The crystalline nature, structure and purityare clearly investigated via XRD, Raman analysis andSEM analysis. A high specific capacitance (104 Fg−1

at 1 Ag−1) as well as an excellent rate capability andcycling stability (over 500 cycles at 1 Ag−1) wereachieved. Therefore, these results substantiate that con-trolling the particle size and morphology play a vital rolein using BiPO4 for energy storage applications.

References

[1] S. Vijayakumar, S. Nagamuthu, G. Muralidharan, Supercapaci-tor studies on NiO nanoflakes synthesized through a microwaveroute, ACS Appl. Mater. Interface 5 (2013) 2188–2196.

[2] S. Vijayakumar, A.K. Ponnalagi, S. Nagamuthu, G. Muralidha-ran, Microwave assisted synthesis of Co3O4 nanoparticles forhigh-performance supercapacitors, Electrochim. Acta 106 (2013)500–505.

[3] L. Cui, J. Li, X.G. Zhang, Preparation and properties of Co3O4

nanorods as supercapacitor material, J. Appl. Electrochem. 39(2009) 1871–1876.

[4] V.D. Nithya, R. Kalaiselvan, L. Vasylechko, Hexamethylenete-tramine assisted hydrothermal synthesis of BiPO4 and itselectrochemical properties for supercapacitors, J. Phys. Chem.Solids 86 (2015) 11–18.

[5] H. Li, H. Yu, J. Zhai, L. Sun, H. Yang, S. Xie, Self assembled3D cobalt phosphate octahydrate architecture for supercapacitorelectrode, Mater. Lett. 1 (2015) 25–28.

rs studies on BiPO4 nanoparticles synthesized via a simple10.1016/j.jtusci.2016.09.007

[6] C. Yuan, J. Li, L. Hou, J. Lin, G. Pang, L. Zhang, L. Lian,X. Zhang, Template engaged synthesis of uniform mesoporoushollow NiCo2O4 sub-microspheres towards high-performanceelectrochemical capacitors, RSC Adv. 3 (2013) 18573–18578.

IN+Model

Univer

[

[

[

[

[

[

[

[

[

[

[

[

[

[

ARTICLEJTUSCI-334; No. of Pages 6

6 S. Vadivel et al. / Journal of Taibah

[7] Y. Zhang, L. Li, H. Su, W. Huang, X. Dong, Binary metal oxide:advanced energy storage materials in supercapacitors, J. Mater.Chem. A 3 (2015) 43–59.

[8] V.D. Nithya, B. Hanitha, S. Surendran, D. Kalpana, R. Kalai-selvan, Effect of pH on the sonochemical synthesis of BiPO4

nanostructures and its electrochemical properties for pseudoca-pacitors, Ultrason. Sonochem. 22 (2015) 300–310.

[9] S. Vadivel, A.N. Naveen, J. Theerthagiri, J. Madhavan, T. San-thoshini Priya, N. Balasubramanian, Solvothermal synthesis ofBiPO4 nanorods/MWCNT (1D–1D) composite for photocat-alyst and supercapacitor applications, Ceram. Int. 42 (2016)14196–14205.

10] V. Vivier, A. Regis, G. Sagon, J.Y. Nedelec, L.T. Yu, C. Cachet-Vivier, Cyclic voltammetry study of Bi2O3 powder by means ofa cavity microelectrode coupled with Raman microspectrometry,Electrochim. Acta 46 (2001) 907–914.

11] X. Su, H. Chai, D. Jia, S. Bao, W. Zhou, M. Zhou, Effectivemicrowave-assisted synthesis of graphene nanosheets/NiO com-posite for high-performance supercapacitors, New J. Chem. 37(2013) 439–443.

12] L. She, G. Tan, H. Ren, C. Xu, C. Zhao, A. Xia, BiPO4@glucose-based C core–shell nanorod heterojunction photocatalyst withenhanced photocatalytic activity, J. Alloys Compd. 662 (2016)220–231.

13] Y. Zhang, B. Shen, H. Huang, Y. He, B. Fei, F. Lv, BiPO4/reducedgraphene oxide composites photocatalyst with high photocat-alytic activity, Appl. Surf. Sci. 319 (2014) 272–277.

14] H. Lv, X. Shen, Z. Ji, D. Qiu, G. Zhu, Y. Bi, Synthesis of grapheneoxide–BiPO4 composites with enhanced photocatalytic proper-ties, Appl. Surf. Sci. 284 (2013) 308–314.

15] R.F. Dezfuly, R. Yousefi, F.J. Sheini, Photocurrent applications

Please cite this article in press as: S. Vadivel, et al. Supercapacitomicrowave approach, J. Taibah Univ. Sci. (2016), http://dx.doi.org/

of Zn(1−x)CdxO/rGO nanocomposites, Ceram. Int. 42 (2016)7455–7461.

16] Y. Tao, L. Ruiyi, L. Zaijun, Facile construction of three-dimensional NiCo2S4 with tremella-like morphology for

[

PRESSsity for Science xxx (2016) xxx–xxx

high-performance supercapacitors, Mater. Lett. 167 (2016)234–237.

17] G. Srikesh, A.S. Nagaraj, Chemical synthesis of Co and Mnco-doped NiO nanocrystalline materials as high-performanceelectrode materials for potential application in supercapacitors,Ceram. Int. 42 (2016) 276–294.

18] S. Vijayakumar, S.H. Lee, K.S. Ryu, Hierarchical CuCo2O4

nanobelts as a supercapacitor electrode with high arealand specific capacitance, microwave assisted synthesis ofCo3O4 nanoparticles for high-performance supercapacitors, Elec-trochim. Acta 182 (2015) 979–986.

19] S. Vadivel, A.N. Naveen, V.P. Kamalakannan, P. Cao, N.Balasubramanian, Facile large scale synthesis of Bi2S3 nanorods–graphene composite for photocatalytic photoelectrochem-ical and supercapacitor application, Appl. Surf. Sci. 351 (2015)635–645.

20] S. Vijayakumar, S. Nagamuthu, G. Muralidharan, PorousNiO/C nanocomposites as electrode material for electrochemi-cal supercapacitors, ACS Sustain. Chem. Eng. 1 (2013) 1110–1118.

21] B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, Fab-rication of two-dimensional reduced graphene oxide supportedV2O5 networks and their application in supercapacitors, Mater.Chem. Phys. 170 (2016) 266–275.

22] N. Duraisamy, A. Numan, S.O. Fatin, K. Ramesh, S. Ramesh,Facile sonochemical synthesis of nanostructured NiO withdifferent particle sizes and its electrochemical properties forsupercapacitor application, J. Colloids Interface Sci. 471 (2016)136–144.

23] R. Yuksei, C. Durucan, H.E. Unalan, Ternary nanocompositeSWNT/WO3/PANI thin film electrodes for supercapacitors, J.

rs studies on BiPO4 nanoparticles synthesized via a simple10.1016/j.jtusci.2016.09.007

Alloys Compd. 658 (2016) 183–189.24] W. Sun, X. Rui, M. Ulaganathan, S. Madhavi, Q. Yan,

Few-layered Ni(OH)2 nanosheets for high-performance super-capacitors, J. Power Sources 295 (2015) 323–328.