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Cite this: Dalton Trans., 2015, 44,20108

Received 1st September 2015,Accepted 18th October 2015

DOI: 10.1039/c5dt03394b

www.rsc.org/dalton

Synthesis, and crystal and electronic structure ofsodium metal phosphate for use as a hybridcapacitor in non-aqueous electrolyte

Manickam Minakshi Sundaram,*a Teeraphat Watcharatharapong,b

Sudip Chakraborty,b Rajeev Ahuja,b Shanmughasundaram Duraisamy,c

Penki Tirupathi Raoc and Nookala Munichandraiahc

Energy storage devices based on sodium have been considered as an alternative to traditional lithium

based systems because of the natural abundance, cost effectiveness and low environmental impact of

sodium. Their synthesis, and crystal and electronic properties have been discussed, because of the impor-

tance of electronic conductivity in supercapacitors for high rate applications. The density of states of a

mixed sodium transition metal phosphate (maricite, NaMn1/3Co1/3Ni1/3PO4) has been determined with the

ab initio generalized gradient approximation (GGA)+Hubbard term (U) method. The computed results for

the mixed maricite are compared with the band gap of the parent NaFePO4 and the electrochemical

experimental results are in good agreement. A mixed sodium transition metal phosphate served as an

active electrode material for a hybrid supercapacitor. The hybrid device (maricite versus carbon) in a non-

aqueous electrolyte shows redox peaks in the cyclic voltammograms and asymmetric profiles in the

charge–discharge curves while exhibiting a specific capacitance of 40 F g−1 and these processes are

found to be quasi-reversible. After long term cycling, the device exhibits excellent capacity retention

(95%) and coulombic efficiency (92%). The presence of carbon and the nanocomposite morphology,

identified through X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM)

studies, ensures the high rate capability while offering possibilities to develop new cathode materials for

sodium hybrid devices.

I. Introduction

Recently, supercapacitors (also named electrochemical capaci-tors or ultracapacitors) have been extensively studied as powersources for many electronic devices. Supercapacitors storelarger amounts of energy than traditional dielectric capacitorsand deliver energy far faster than batteries. Electrochemicalcapacitors have a higher power density with an excellent cou-lombic efficiency over a longer life time, nevertheless, theyprovide a lower energy density than their battery counter-parts.1,2 Despite the lower energy density, electrochemicalcapacitors provide an opportunity for load levelling in energystorage. Supercapacitors can be classified as symmetric,asymmetric or hybrid systems. The most traditional type is asymmetric system consisting of two identical carbon

electrodes3–5 relying on a double layer charging mechanism;this system is called an electrical double layer capacitor(EDLC). Asymmetric capacitors incorporate dissimilar electro-des exhibiting non-faradaic (EDLC) behaviour, while hybridsystems are composed of a porous carbon “capacitor type”electrode and a metal oxide “battery type” electrode. The asym-metric hybrid system is characterized by faradaic reactions(pseudo-capacitance) and non-faradaic charge storage behav-ior (electrical double layer capacitance) and is a new class ofenergy storage device developed in recent years. The classes ofmaterials undergoing pseudocapacitance reactions are mainlyelectronically conducting polymers6–8 and metal oxides.9–12

Among the several oxides studied, asymmetric capacitorscoupled with iron oxide (Fe3O4) and MnO2

7 seem to be themost promising. Until now, lithium (Li) based energy storagedevices, including both batteries and capacitors, have beenextensively studied.9–14 In terms of cost, availability and safetyissues the most appealing alternative to lithium is to usesodium because the resource is abundant in nature. In thisregard, the results for sodium batteries show their promise asstationary storage devices for load levelling.15 The success of

aDepartment of Chemistry, Murdoch University, Murdoch, WA 6150, Australia.

E-mail: m.minakshi-sundaram@murdoch.edu.aubDepartment of Physics and Astronomy, Uppsala University, SwedencInorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012,

India

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MnO2 as a cathode material for aqueous sodium batteries andmaricite NaMPO4 (where M = Co, Ni or Mn) for aqueoushybrid capacitors inspired the present investigation in non-aqueous solvents. The substitution of mixed metal cations inthe parent NaFePO4 maricite framework employed here is toovercome the poor electronic conductivity of iron containingphosphates and to obtain a high rate capability.

Although supercapacitors are now available on the market,they still require improvements, in terms of energy and powerdensities, governed by the specific capacitance of the activeelectrode material and the cell voltage. Extensive research hasbeen done on metal oxides such as manganese dioxide(MnO2),

16 iron oxide (Fe2O3),17 tin oxide (SnO2),

18 cobalt mol-ybdenum oxide (CoMoO4),

19 and vanadium oxide (V2O5)12 but

to the best of our knowledge no work has been reported onolivine or Na analogue materials in non-aqueous electrolytes.Hence, with these factors as objectives, various strategies suchas in situ carbon coating on the active maricite electrode whileusing a polymer as a chelating agent in the sol–gel syntheticroute, the use of an in-expensive and environmentally friendlyNa analogue maricite electrode, and the use of non-aqueoussolvent to increase the cell voltage, have been reported in thiswork. The use of aqueous solution (i.e. KOH, LiOH, NaOH orNa2SO4) as an electrolyte limits the cell potential to 2 V,beyond which water splitting occurs and hence a lower energydensity (20 Wh kg−1) is obtained. To overcome this, non-aqueous solution has been used as an electrolyte to increasethe voltage window to 3 V, resulting in a higher energy density(50 Wh kg−1). To our knowledge, no relevant research hasbeen published in the literature on sodium phosphate basedelectrode materials suitable for hybrid devices in non-aqueoussolutions. Another objective of this study, because of thepotential interest in this material, is to understand the impor-tance of electronic conductivity through the theoretical studyof the material’s electronic structure using density functionaltheory (DFT) and the generalised gradient approximation(GGA)+U method. The computed results show that the parentNaFePO4 in its pure form has very poor conductivity; however,with mixed sodium transition metals the conductivity hasbeen significantly improved and found suitable for high-rateapplications. The outline of the paper is as follows. Firstly, wedetail the computational methods used in this study includingthe structural parameters, density of states and relativeenergies. Secondly, the experimental methods employed inthis study including the physical and electrochemical resultsare presented.

II. Computational methods

The simulation and calculations were performed using theprojector-augmented wave (PAW) method implemented in theVienna Ab-initio Simulation Package (VASP), which is based onDensity Functional Theory (DFT) formalism. The plane wavecutoff of 500 eV is reasonably sufficient to describe the elec-tronic properties and has been tested well with the conver-

gence criteria. In this work, all the optimized structures arefully relaxed by a conjugate-gradient (CG) algorithm so as toobtain the minimal total energy. The optimization isconverged when the energy difference in each electronic stepis less than 0.01 meV and the residue Hellmann–Feynmanforces also reach 0.01 eV Å−1 in ionic relaxation. 6 × 4 × 2 and1 × 4 × 2 k-point grids generated by the Monkhorst–Packscheme20 are used for the pure and mixed sodium iron phos-phate respectively, as a consequence of the k-point conver-gence. Throughout the calculation, for the transition metals(TM) chosen (Fe, Co, Mn and Ni), the 3p, 3d- and 4s-electronsare considered as valence electrons while for alkali metal (Na)atoms the 2p-electron has been taken as a valence electron.The electron configurations of P and O are [Ar] 3s2, 3p3 and[He] 2s2, 2p4 in their PAW potentials, respectively. For theexchange correlational functional, a Perdew Burke Ernzerhof(PBE) type Generalized Gradient Approximation (GGA) isused21,22 to approximate the non-linear exchange correlationterm. The van der Waals correction proposed by Grimme23 isalso taken into consideration for incorporating the dispersioneffect. Owing to the strong d-electron localization of transitionmetals, particularly in phosphate materials, using pure DFTcould result in self-interaction errors. In order to treat the loca-lized 3d-electron accurately, the standard Hubbard model(DFT+U approach) proposed by Dudarev et al.24 is an impor-tant requirement and hence GGA+U has been employed in thecalculation. The chosen U parameters are from ref. 25 for theolivine structure LiMPO4, where M = Fe, Mn, Co, and Ni, (UFe =3.71 eV, UMn = 3.92 eV, UCo = 5.05, UNi = 5.26 eV) in which theeffective U parameters were determined from a linear responseapproach, and yielded an improvement in lattice parametersand cell volume corresponding to the experimental results. Tosimulate the mixed sodium transition metal phosphateNaMn1/3Co1/3Ni1/3PO4 structure, the unit cell of the parentsodium iron phosphate (NaFePO4) has been enlarged by 3 × 1 × 1and the preferential site for the substitution atoms is likely tobe the same site as the iron atom. There are a number of dis-tinct structures that have been simulated using the criteria ofdifferent TM–TM displacements and those results arediscussed in the Results and discussion.

III. Experimental

A sol–gel synthesis of NaMn1/3Co1/3Ni1/3PO4 was performedthrough mixing a stoichiometric amount of precursors con-taining sodium acetate, cobalt acetate, manganese acetate,nickel acetate and ammonium dihydrogen phosphate indouble distilled water with effective stirring. A unique syn-thetic approach is chosen to carbon coat the phosphate (mari-cite) particles using PVP as a source. The metal and phosphatereactants were dissolved in water at around 80 °C and withconstant stirring, polyvinylpyrrolidone (PVP) was added to thesolution as a chelating agent in a 1 : 1 weight ratio to the metalions. The pH of the solution was adjusted to 3.5 by addingnitric acid. The resulting sol underwent continued heating at

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80 °C for an hour to obtain a viscous gel. Continuous stirringand heating of the slurry resulted in the complete evaporationof water and the formation of a thick gel. Then, the thick trans-parent gel was dried at 110 °C in a hot air oven for 12 h. Theobtained powder was calcined at 300 °C for 8 h and at 550 °Cfor 6 h in air with intermittent grinding. The resultant furnacecooled “maricite” powder was ground for further analysis.

When PVP is added to the colloidal dispersion, the imidegroup (N and O atoms) establishes a strong affinity to themetal colloidal dispersion. This bonding controls the particlegrowth and with further heating of this product at a highertemperature, the polymer decomposes leaving carbon andnitrogen residues on the surface of the mixed maricite. Thepresence of these residues reduces the particle size and conse-quently the reduction of the diffusion length enhances theperformance of the mixed maricite. The synthesized mixedmaricite, i.e. sodium phosphate (NaMn1/3Co1/3Ni1/3PO4),powders were subject to systematic physical and electrochemi-cal studies. Powder X-ray diffraction (XRD) patterns wererecorded on an X-ray diffractometer (Bruker D8 advance diffr-actometer) with a Cu Kα radiation (λ = 1.5418 Å) source. Themicroscopy studies were carried out using scanning electronmicroscopy (FEI Co. Sirion) and transmission electronmicroscopy (JEOL TEM 2100F). The surface area and pore sizedistribution of the samples were measured using a Micromeri-tics surface area analyser (model ASAP 2020). The surfaceelements on the maricite were examined with X-ray photo-electron spectroscopy (XPS) using a SPECS GmBH spectro-meter (Phoibos100MCD Energy Analyser) with MgKα (1253.6eV) radiation. The peak of C (1s) at 286 eV was taken as thereference energy position.

All electrochemical studies were carried out in a coin cell(CR2032, Hohsen Corporation, Japan). Maricite powder coatedon a stainless steel disc, as the current collector, was used ascathode. The “commercial carbon powder” (YEC8) used in thisstudy was in its “as-received” form, purchased from Fuzhou YiHuan Carbon Co. Ltd, China. Carbon powder or sodium metalfoil (Aldrich) was used for the counter and reference electro-des. The stainless steel disc was polished with successivegrades of emery, degreased, etched in dilute 10% HNO3 and10% HCl, washed with detergent, and rinsed with distilledwater and acetone followed by drying in air. The active material(maricite powder, 75 wt%), conductive material (Ketjen black,15 wt%) and polyvinylidene fluoride (PVDF, 10 wt%) weremixed and ground in a mortar. A few drops of n-methyl pyrrol-idinone (NMP) were added to form a slurry. The slurry wascoated on the pre-treated stainless steel foil until the requiredloading level was reached and the coated disc was dried at100 °C under reduced pressure for 12 h. A carbon electrodedisc was prepared in an identical manner except for theinclusion of the conductive material. The coin cell was con-structed with the sodium phosphate disk as the cathode,carbon as the anode and a Celgard porous propylene mem-brane (2400) as the separator. The optimal mass ratio betweenthe sodium phosphate (NaMn1/3Co1/3Ni1/3PO4) and commer-cial carbon was determined to be 1.5 for the fabricated hybrid

(NaMn1/3Co1/3Ni1/3PO4||commercial carbon) capacitor. Themass balance was calculated using the following eqn (1)

mþ=m� ¼ ðC� � ΔE�Þ=ðCþ � ΔEþÞ ð1Þwhere C is the specific capacitance and ΔE is the potentialwindow for the charge/discharge processes. The mass of thepositive and negative electrode materials was 2.0 and 3.0 mg,respectively. The coin cells were assembled in an argon filledglove box (MBraun model UNILAB). A non-aqueous solution of1 M NaPF6 dissolved in ethylene carbonate, diethyl carbonateand dimethyl carbonate (2 : 1 : 2 v/v) electrolyte was used as anelectrolyte. An aliquot (5 µL) of non-aqueous electrolyte wasadded on to the separator. The cyclic voltammetry (CV) andgalvanostatic charge–discharge cycling were conducted using aBiologic SA multichannel potentiostat/galvanostat modelVMP3. The electrochemical impedance spectroscopy (EIS)measurements were carried out on the maricite electrodesbefore and after electrochemical cycling experiments at anopen-circuit potential in the frequency ranging from 0.01 Hzto 100 kHz with an a. c. excitation amplitude of 5 mV using aSolatron frequency response analyser model 1255B and Sola-tron electrochemical interface model 1287.

IV. Results and discussion1. Crystal structure and density of states

(a) Pristine m-NaFePO4. The maricite-NaFePO4

(m-NaFePO4) structure containing 28 atoms is constructed andoptimized using the convergence criteria mentioned earlier inthe introduction section. The crystal structural parameters ofpure NaFePO4 are shown in Table 1, obtained from Fig. 1, forboth the effective UFe and an arbitrary value of 5.3 eV. Theexperimental values are also compared in Table 1. We havefound a good correspondence between the calculated resultsand the experimental values after the Hubbard U was intro-duced. It is worth mentioning that there is a substantialvolume expansion when a higher U value is considered. In thiswork, UFe = 3.71 eV is the value that we have mainly taken intoaccount for the electronic structure calculations as it yields cellparameters which are close to the experimental ones.

The density of states calculations as depicted in Fig. 2 showthe significant effect of onsite coulomb interactions on the

Table 1 The crystal structural parameters of m-NaFePO4 calculated byusing different values of U parameters and compared with thoseobtained from experimental values

Cellparameter U = 0 eV (GGA)

U = 3.71 eV(ref. 24)

U = 5.3eV

Experimentalvalues fromref. 24

a 5.024 5.038 5.047 5.0434b 6.847 6.845 6.846 6.8679c 8.977 9.013 9.021 8.9773Eg 0 2.99 3.84 —Volume/cell 308.802 310.832 311.704 310.951

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electronic structure, which leads to a more reasonable bandgap. The m-NaFePO4 is known as an indirect gap semiconduc-tor, but it can be distinctly seen that the conduction banddrops and crosses the fermi level, indicating that the PerdewBurke Ernzerhof (PBE) functional cannot predict the electronicproperties correctly due to the under estimated localization ofthe Fe 3d-electrons. We have found the band gaps (Eg) for theU values 3.71 and 5.3 eV to be 2.99 and 3.84 eV respectively.Although a larger Eg can be obtained using a higher U para-meter, the density of states (DOS) results (shown in Fig. 2)appear to be still identical. Fig. 2b shows that the valence andconduction bands are mostly attributed to the hybridization ofthe Fe-3d and O-2p orbitals. In the experimental study byAvdeev et al.,26 for an identical material, the local structure ofthe FeO6 octahedra was found to exhibit the elongation Jahn–Taller effect leading to longer Fe–O axial bonds. Moreover, wetheoretically found that m-NaFePO4 is characterized by an anti-

ferromagnetic ground state with an Fe local magnetic momentof 3.71μB, in good agreement with the experimental value of3.89μB.

26

(b) Mixed sodium transition metal phosphate (m-NaMn1/3Co1/3Ni1/3PO4). To determine the density of states for mixedmaricite NaMn1/3Co1/3Ni1/3PO4, the pristine m-NaFePO4 ismixed, with Mn, Co and Ni atoms replacing the Fe sitesindividually with a fractional occupancy of 0.33 and the iso-structures are shown in Fig. 3. Here, the three differentm-NaMn1/3Co1/3Ni1/3PO4 structures are termed as the C1, C2and C3 configurations. The three configurations have beendifferentiated by the magnitude of the average TM–TM (tran-sition metal) distance (tabulated in Table 2), e.g. Mn–Mndistance, where the C1 and C3 structures have the shortest andlongest distance, respectively, as noticed in Fig. 3.

Among the three different configurations, in Fig. 3, the C3structure has the lowest total energy compared to C2 and C1.

Fig. 1 Illustration of the m-NaFePO4 (maricite) structure from a side and bird’s eye view perspective respectively. Atomic species are labeled corres-ponding to the ball colors as displayed. Na, Fe, P and O spheres are represented with spheres of decreasing size.

Fig. 2 The density of states of m-NaFePO4 using (a) PBE type GGA (UFe = 0 eV) and (b) PBE type GGA+U calculations (UFe = 3.71 eV). The fermi levelhas been set at 0 eV.

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This indicates that the TM (Ni) atoms prefer to reside apartfrom each other and the consequent configuration is energeti-cally favorable. This can also be explained by the charge distri-bution. Since the number of electrons to be donated isdifferent, the way the TM atom resides on a specific siteshould also influence the electron distribution within the

respective structure. The homogeneity of the charge distributionof the C3 configuration makes the structure more stable com-pared to the C1 and C2 configurations. Based on the relation ofthe total energy and structural stability, the C1 and C3 configur-ations are energetically favorable in the ferromagnetic groundstate with the difference in the total energies between the twodistinct magnetic orderings being 0.77 and 13.97 meV higherthan for antiferromagnetic spin ordering, respectively. Whereas,the C2 configuration is likely to be antiferromagnetic and poss-ibly manifests ferromagnetism owing to the very small value ofsuch energy (0.12 meV). This shows the magnetic ordering isalso related to the stability of the respective structures. Even so,in order to perceive the difference between them, the results oftheir DOS are all plotted in Fig. 4. The fermi level (Ef ) of all theDOS has been set at 0 eV and we have found that the main con-tributions of the DOS in mixed NaMn1/3Co1/3Ni1/3PO4 originatesfrom the O-2p, Mn-3d, Co-3d and Ni-3d orbitals and the Na andP atoms have a weak density of states in the energy rangebetween −10.5 and 10.5 eV. The conduction band occurs fromthe 3d-electrons of the TM atoms. As seen in Fig. 4, indepen-dent of the TM atom location, the Ni-3d electrons play an essen-tial role on the edge of the conduction band as well as thevalence band, which is mostly dominated by the 2p-electrons ofO and has hybridized O-2p and Mn-3d character at the edge ofthe valence band. The observed smaller Eg (∼2.24–2.32 eV) ofm-NaMn1/3Co1/3Ni1/3PO4 suggests that the electrical conductivityis improved after mixing with various TMs such as Mn, Co, andNi. Moreover, at the near core energy states, small peaks fromNi, Co and Mn atoms, corresponding to their 3p-states, appearin the energy order of Ni < Co < Mn. The relatively smaller bandgap observed for the mixed sodium transition metal phosphateis consistent with its improved electrochemical activity, dis-cussed in the electrochemical characterization section. TheX-ray photoelectron spectroscopy (XPS) spectrum describing theactivation energy required to excite electrons in the core region(detailed in the physical characterization section) shows asimilar order of energy states as we found in the lower energyrange of the DOS for the mixed metal phosphate. Comparingthe DOS of each configuration, in Fig. 4, they look quite similarbut have different spin-up and down. The computed methodled to comparable results between the experimental XPS spec-trum and the calculated total DOS of the mixed sodium tran-sition metal phosphate.

2. Experimental measurements of m-NaMn1/3Co1/3Ni1/3PO4

(a) Physical characterization. The X-ray diffraction analysis(XRD) carried out on the synthesized mixed maricite NaMn1/3Co1/3Ni1/3PO4 powder evidences well-defined diffraction lineswith high diffraction intensities ascertaining the crystallinestructure of the material. Fig. 5 shows the XRD pattern of themaricite; all peaks can be indexed to the orthorhombicNaMPO4 analogue (JCPDS card no. 71-5040, ICDD no. 29-1216;Pmna space group) based on the maricite mineral. It is alsoimportant to note that the synthesized so-called maricitephase exhibits lattice parameters higher than those of theLi analogue.27 This difference is attributed to the larger ionic

Fig. 3 The optimized iso-structures of mixed maricite, m-NaMn1/3Co1/3Ni1/3PO4, obtained from three different configurations, namely (a) C1 (b) C2and (c) C3 that we simulated in this work by considering the difference inTM–TM distances.

Table 2 Average TM–TM distances of m-NaMn1/3Co1/3Ni1/3PO4 inangstroms

Mn–Mn Co–Co Ni–Ni TM–TM_avg

C1 3.376 3.384 3.377 3.379C2 3.383 3.383 3.384 3.383C3 5.965 5.864 5.846 5.891

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ratio of Na+ with respect to the Li+ counterpart i.e. 1.02 Å vs.0.76 Å, respectively.28,29 As each of the metal cation dopants (Mn,Co and Ni) has an ionic radius in an octahedral coordinationcomparable to that of the parent Fe2+,30 a preference for this siteis expected and the material crystallises in a single phase withoutany detectable impurities or a second phase. To aid this fact,X-ray photoelectron spectroscopy (XPS) analysis is also performedon the maricite powder (discussed later in this section).

The N2 adsorption isotherms and pore size distributioncurves of the mixed maricite NaMn1/3Co1/3Ni1/3PO4 powder arepresented in Fig. 6. The surface area of the sample was calcu-lated using the Brunauer–Emmett–Teller (BET) method, whilethe pore size distribution and pore volume were calculated

from the desorption based on the Barrett–Joyner–Halenda(BJH) equation. The surface area measured from the BET iso-therm for the mixed maricite was found to be 5 m2 g−1. Thetypical nitrogen sorption isotherms exhibit type IV isothermswith a typical hysteresis loop in the intermediate relativepressure range (0.7–0.9). The pore size (shown in the inset,Fig. 6) shows a uniform distribution of around 60 nm, illustrat-ing the formation of a mesoporous material. Although thesurface area (5 m2 g−1) of the synthesized maricite is muchlower than that of reported materials for its oxide, phosphateand molybdate counterparts, i.e., MnO2 (16 m2 g−1),16 reducedgraphene oxide/MnO2 (49 m2 g−1),31 TiP2O7 (12 m2 g−1),32

Fig. 4 The total DOS (dash line) and orbital-decomposed DOS for each element (solid line) for the (a) C1, (b) C2, and (c) C3 configuration ofm-NaMn1/3Co1/3Ni1/3PO4 obtained from GGA+U calculations.

Fig. 5 X-ray diffraction (XRD) pattern of the as-synthesized mariciteNaMn1/3Co1/3Ni1/3PO4 powder with the hkl indexes of major reflectionpeaks marked with reference to the available database; NaMPO4 (M =Fe).

Fig. 6 Nitrogen adsorption–desorption isotherms of the as-syn-thesized m-NaMn1/3Co1/3Ni1/3PO4 powder. Pore size distributions ofthese composites are shown in the inset.

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NiMoO4 (15 m2 g−1),33 and graphene added MnMoO4

(∼10 m2 g−1),34 it is comparable to that of other olivine/maricite materials, e.g., Na2FePO4F (8.7 m2 g−1)35 and LiFePO4/C (5.2 m2 g−1).36 The surface area for the commercial carbonelectrode was observed to be 1450 m2 g−1 with a high porosity of0.76 cm3 g−1. The high surface area and controlled distributionof pores obtained for both electrodes determine the electrode/electrolyte interface for supercapacitor applications that facilitatesthe EDL capacitance and faradaic capacity.37,38 Hence, a highspecific area together with a narrow pore size distribution ofcarbon coupled with the crystalline structure of maricite (battery-like electrode) is sufficient to achieve a high capacitive perform-ance, as demonstrated in the following section.

The scanning electron microscope images in Fig. 7 showthat the morphology of the synthesized mixed mariciteNaMn1/3Co1/3Ni1/3PO4 powder appears to be porous with voidshaving an individual grain size of 0.4–0.6 µm. At the macro-scopic scale, the electrode homogeneity and integrity havebeen important in ensuring the active material performance.Energy dispersive X-ray Spectroscopy (EDS) analysis carried outat various locations on the maricite powder showed very

similar spectra confirming the high homogeneity of thematerial. A representative spectrum from this analysis isreported in Fig. 8. EDS analysis confirms the elements presentin the material. The presence of all transition metal cationsand the sodium peak position supports the XRD pattern inshowing that the substitution of multiple dopants crystallisesas a single phase compound. The quantitative amount ofcarbon (from CHNS analysis) in the synthesized maricite wasfound to be 2.15 wt% of the material. This confirms that thecarbon present on the synthesized maricite particles couldprovide the electronic conductivity and act as a percolatorbetween the grains. The evidence for carbon coating on themaricite material, and the surface morphology and crystallinenature of the synthesized maricite powder were also investi-gated by X-ray photoelectron spectroscopy (XPS), transmissionelectron microscopy and high resolution (TEM & HRTEM)measurements. An XPS survey scan of the surface (0–1200 eV)of the synthesized mixed maricite powder is shown in Fig. 9a.The important feature to notice in the wide spectrum is thehighly dominant O (1s) and Na (1s) peaks, while the contri-butions of the other elements of interest (Mn, Co, Ni and P)corresponding to mixed transition metal phosphates are alsoseen but are weak in their intensities. To confirm the presenceof a carbon coating on the surface of the as-synthesized mari-cite, XPS spectra over C (1s) were recorded over five sweeps andare shown in Fig. 9b. Based upon the binding energiesreported in the literature,39–41 the C (1s) peaks at 286 eV wereassigned to hydrocarbons present in the XPS chamber (takenas charge referencing) and the peak at 282 eV was assigned tocarbon present in the maricite material itself, coming fromPVP. The peak fitting was done by applying multiple Gaus-sians. The composition of the surface elements from the XPSprofiles further confirms the role of PVP and its carboncoating on the maricite surface. While the XPS analysis pro-vides an indication of the composition of the elements presenton the surface including carbon, CHNS measurements are anindication of the concentration of the C element in the wholemixed maricite powder. We found that both techniques arecomplimentary and quantitatively the amount of C is found tobe 2.15 wt% of the material. Fig. 10 exhibits TEM images ofthe maricite nanocomposites showing a carbon coating on theactive material, seen as clusters. The polymer polyvinylpyrrol-idone (PVP) from the chelating agent during the sol–gel syn-thesis allows unique control over particle growth and thecomposites in the nanometre range; having small dimensionsfacilitates electron and ion transport. PVP, an electrically con-ducting polymer, can form a conducting matrix, having par-ticle to particle interaction, and acts as a backbone in thepoorly conducting maricite material. PVP also acts as a tem-plate for the formation of the unique (flexible-type) shape ofthe particles as shown in the TEM images.42 The presence of acarbon coating on the maricite surface (as evidenced from theXPS and EDS analyses) may accelerate the diffusion reactionon the surface of the maricite material.43 The TEM imagesalso demonstrate the porous nature, confirming the SEMmorphology, which will enhance the electron/ion trans-

Fig. 7 Scanning electron microscopy (SEM) images of the as-synthesized m-NaMn1/3Co1/3Ni1/3PO4 powder samples with differentmagnifications.

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portation for better electrochemical reactions. The HRTEMimage further confirms the crystallinity of the obtainedproduct and carbon particles on the surface are evidencedthrough a series of rings and diffraction spots. The carbonlayer may improve the electronic conductivity and shorten thesodium-ion diffusion paths.

(b) Electrochemical characterization. An asymmetricalhybrid device is constructed with mass loadings, as specifiedin the experimental section, of sodium phosphate nanocompo-sites versus commercial carbon (NaMn1/3Co1/3Ni1/3PO4 ||carbon) as the positive and negative electrodes, respectively.They were characterised using cyclic voltammetry, galvano-static charge–discharge and electrochemical impedance spectro-scopy. In the case of conventional symmetric capacitors1,3

(activated carbon vs. activated carbon), during charging anddischarging, an electrochemical double layer is found to bereversibly formed on the electrodes. The anionic and cationicspecies (Na and PF6

−) from the electrolyte adsorb on the posi-tive and negative electrodes, respectively, during the chargingprocess. Such non-faradaic processes lead to a low capacitanceof only 10 F g−1.1 In the hybrid device, on the other hand, themixed maricite electrode exhibits battery-type behavior. Thehybrid capacitor (mixed maricite vs. carbon) utilises double-layer capacitance on the carbon, which occurs in parallel witha faradaic process at the maricite.44 The faradaic processinvolves redox reactions of electrochemically active maricitespecies at the electrode particle surface.45 Intercalation anddouble-layer adsorption are the two reactions which we haveobserved in our asymmetrical hybrid device, totalling a capaci-tance of 40 F g−1, which is explained in the following discus-sion on the galvanostatic and potentiostatic experiments. Thecapacitance (C) is calculated from the discharge profilesaccording the equation C = IΔt/mΔV, where I is the constantdischarge current, Δt is the discharge time, m is mass of theactive maricite material and ΔV is the potential window fordischarge.

Cyclic voltammograms recorded for the cathode, i.e. mixedmaricite NaMn1/3Co1/3Ni1/3PO4 electrode versus commercialcarbon, at two different potential sweep rates of 1 and100 mV s−1 in the potential window between 0 and 3 V are pre-sented in Fig. 11. Initially, the electrode was swept in an anodic(oxidation) direction until the cut-off voltage and then reversedback in the cathodic (reduction) direction to the initial voltage.

Fig. 8 Energy dispersive X-ray spectroscopy (EDS) analysis of the as-synthesized m-NaMn1/3Co1/3Ni1/3PO4.

Fig. 9 XPS profiles showing a (a) wide scan and (b) narrow scan (C1score level) of the as-synthesized m-NaMn1/3Co1/3Ni1/3PO4 powdersamples.

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The current response of the higher scan rate is superior whencompared to the sample at a lower scan rate indicating that thesynthesised maricite has better capacitance behavior. At a lowscan rate (Fig. 11a), the voltammogram shows a weak anodicpeak around 2.5 V during the forward scan, relating to the oxi-

dation of a redox couple. In the reverse scan, two clearreduction current peaks are observed at 1.7 and 0.4 V, respect-ively. From one of our earlier studies46 based on NaMPO4

(where M = Co, Ni or Mn) it is inferred that the weak reduction

peak at 1.7 V (Fig. 11a) is due to the reduction of Co3+ and theintense peak at 0.4 V is due to the reduction of Ni3+ and Mnhasn’t taken part in the redox activity. On the basis of the cyclicvoltammetric data, the reaction proposed for the charge/dis-charge processes of maricite is as follows:

The proposed reaction for the commercial carbon is

Cþ xNaþ þ xe� $ Cx�jjxNaþ ðcapacitor typeÞ ð3Þ

Fig. 10 Transmission electron microscopy (TEM) images of the as-synthesized m-NaMn1/3Co1/3Ni1/3PO4 powder samples under different magnifi-cations. The high-resolution TEM (HRTEM) image shows lattice fringes indicating a crystalline nature. The bottom image (on the left) at a differentlocation represents the carbon particles on the surface evidenced by the selected area diffraction pattern (on the right) indicating series of rings andbright spots.

NaMn1=3Co1=3Ni1=3PO4 ���*)���charge

Na1�xMn1=3Co1=3Ni1=3PO4 þ xNaþ þ xe� ðbattery typeÞ ð2Þ

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In addition to the proposed Na absorption reaction in eqn(3), an electric double layer with the anion PF6

− from the elec-trolyte could not be ruled out.

The redox peaks observed in Fig. 11a are found to fullyreversible for successive cycles indicating reversible dominantfaradaic behavior. As the scan rate was increased to 100 mV s−1

(Fig. 11b), a clear deviation from battery type (faradaic) behaviorto capacitive (non-faradaic) EDL behavior is observed.47

No distinct redox peaks were observed for higher sweep rates.This suggests that at low scan rates the diffusion controlledprocess can proceed with sodium-ion de-insertion and inser-tion into almost all of the available pores in the maricite struc-ture (shown in the SEM and TEM images, Fig. 7 and 10)resulting in pseudo-capacitive behavior but with less capaci-tance. As higher scan rates were achieved (in Fig. 12), thesurface of the electrode results in a large reduction and hencethe observed high current response. The absence of well-defined redox peaks could be related to difficulties in iondiffusion inside the pores at high current densities≥20 mV s−1 (Fig. 12). Hence, at very low scan rates, the faradaiccharacter is clearly indicated by the redox peaks on the CV

while a quasi-rectangular shape close to that expected for apure electrochemical double layer capacitor with some contri-bution from the faradaic process is observed for very high scanrates.37,48 From these results, it is established that highercurrent may lead to pure capacitive behavior without a pseudo-capacitive contribution and hence only an optimal lowercurrent was selected for further charge–discharge studies.

The supercapacitive performance was demonstrated by gal-vanostatic charge–discharge experiments and the profiles formultiple cycles are shown in Fig. 13a and b. The operatingvoltage range (0–3.0 V) is in agreement with the cyclic voltam-metry data. The device exhibits charge–discharge profiles (inFig. 13a and b) slightly deviating from linearity as a result ofthe pseudo-faradaic processes for the phosphate electrodes.The bent-like curve (instead of straight lines) at higher andlower potentials suggests that partial Na+ ion insertion/extrac-tion occurs49 in the maricite electrode. The overall capacitancecontribution of the maricite electrode comes from both thecapacitive and battery-like behaviour, totalling 40 F g−1. Theobtained capacitance values are comparable to the reportedvalues for sodium and lithium-ion capacitors.4,31,35,50,51 Thestability upon cycling was investigated and Fig. 13b shows thatreversibility can be achieved with the NaMn1/3Co1/3Ni1/3PO4

versus commercial carbon electrochemical supercapacitor.Fig. 13c shows the cycleability and coulombic efficiency of thehybrid device. The available capacitance remained constantthroughout the tested cycling period of 1000 cycles exhibitinga discharge capacitance of 38 F g−1 with an excellent coulom-bic efficiency of 92%. The exhibited excellent cyclability couldbe attributed to the role of Mn in stabilising the structure forlongevity. As reported by Amine et al.52 for manganese layeredoxide and Kang et al.53 for manganese mixed phosphate, Mnachieves cycling stability at the expense of low capacitance.This is explained as being due to the unique Jahn–Tellerdistortion of Mn3+, which mitigates the diffusion channel for

Fig. 11 Cyclic voltammograms (CV) of as-synthesized olivinem-NaMn1/3Co1/3Ni1/3PO4 electrode versus commercial carbon in 1 MNaPF6 dissolved in ethylene carbonate, diethyl carbonate and dimethylcarbonate (2 : 1 : 2 v/v) non-aqueous electrolyte at a scan rate of (a) 1,and (b) 100 mV s−1. Y-axes in the figures are different.

Fig. 12 The rate capability behaviour of the as-synthesized m-NaMn1/3-Co1/3Ni1/3PO4 electrode versus commercial carbon (two electrodeconfiguration) in 1 M LiPF6 dissolved in ethylene carbonate, diethylcarbonate and dimethyl carbonate (2 : 1 : 2 v/v) non-aqueous electrolyteat different sweep rates (1, 2, 5, 10, 20, 50 and 100 mV s−1) from top tobottom in the curve.

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reversibility. The designed hybrid device can deliver an energydensity of 50 Wh kg−1 at a power density of 180 W kg−1. Thepower density (Pd) and energy density (Ed) of the hybrid deviceare calculated from the capacitance value obtained from thecharge–discharge profiles using the relations Pd (W kg−1) =(ΔE·I/m) and Ed (Wh kg−1) = (Pd·t/3600), where ΔE = (Emax +Emin)/2 and Emax and Emin are, respectively, the voltage at the

start of discharge and at the end of discharge, t is the timetaken for charge/discharge, I is the applied current and m isthe weight of active material in the electrode.54

Electrochemical impedance spectroscopy (EIS) was per-formed on the assembled hybrid device at the open circuitvoltage and after multiple galvanostatic cycles. EIS is a powerfultool for investigating the electrochemical behaviour at the bulkand interface of the device and to determine the degree towhich ions access the surface at specific frequencies.55,56 Fig. 14shows Nyquist plots of the NaMn1/3Co1/3Ni1/3PO4 versus com-mercial carbon device, in the frequency range from 100 kHz to100 mHz. It can be observed that for the open circuit potential,the plot is composed of a small diameter semicircle in the high-frequency range, corresponding to the charge transfer processesoccurring at the electrode/electrolyte interface,57,58 and aninclined line at 90° corresponding to the impedance of the Na+

diffusion process. The line close to 90° was attributed to theideal capacitive behaviour.59 The observed low ohm resistance(the high frequency intersect with the real axis) corresponds tothe electrolyte resistance with slope of 90° portion indicates thatthe Warburg resistance is negligible, reflecting that the syn-thesized maricite is a highly porous electrode and apt forhybrid devices. Comparing the plots after multiple cycles, it isapparent that a greater charge transfer resistance is observed(Fig. 14, inset) and the slope changes from 90° towards a lowerangle with a minor variation in the ohm resistance. Thesechanges are due to the faradaic and non-faradaic reactions atthe interface during the charge/discharge process and the iontransport from the electrolytes into the electrode.60 Hence, theobtained EIS spectrum is in accordance with the other electro-chemical results.

V. Conclusions

In summary, our theoretical and experimental studies of amixed sodium metal phosphate reveal its improved electronic

Fig. 13 Voltage vs. time plot of the as-synthesized m-NaMn1/3Co1/3Ni1/3PO4

electrode versus carbon in 1 M NaPF6 dissolved in ethylene carbonate,diethyl carbonate and dimethyl carbonate (2 : 1 : 2 v/v) non-aqueouselectrolyte at a constant current of 0.12 A g−1. (a) and (b) show voltageprofiles with consistent capacitance values for the first 100 and last100 cycles (before 1000 cycles) respectively, and (c) shows the cellperformance during multiple cycling.

Fig. 14 Nyquist plots of the as-synthesized m-NaMn1/3Co1/3Ni1/3PO4

electrode versus carbon (black line) during open circuit voltage and (redline) after 1000 cycles.

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conductivity and lower band gap than the parent NaFePO4.The density of states spectrum for the metal phosphate is con-sistent with the experimental results, exhibiting improved per-formance. An asymmetric hybrid device, NaMn1/3Co1/3Ni1/3PO4

versus commercial carbon, showed a discharge capacitance of40 F g−1 in non-aqueous electrolyte. The charge–dischargecurve maintained a triangular bent curve at a current of0.25 mA and the electrode exhibited a low ohm resistance withexcellent reversibility. The maricite electrode shows dominantNa+ insertion/extraction (Faradaic) behavior at a low sweep ratewith dominant adsorption/desorption (non-Faradaic) behaviorat the high sweep rate voltage window of 0–3.0 V. The capacityretention and coulombic efficiency of the hybrid device ismaintained at 95% and 92% respectively during multiplecycling, indicating a high electrochemical reversibility. Thepresence of mesoporous, carbon coated maricite enhanced theelectrochemical performance in terms of excellent stabilityover the cycling period. The sodium derived electrode materialis found to be suitable for energy device applications.

Acknowledgements

The author (MMS) wishes to acknowledge the funding bodiesAustralia-India Strategic Research Fund (AISRF) and AustralianResearch Council (ARC). This research was supported underthe Australian Research Council (ARC) Discovery Projectfunding scheme DP1092543 and Australia-India Early CareerResearch Fellowship. Authors TW, SC and RA would like toacknowledge the Carl Tryggers Stiftelse for Vetenskaplig For-skning (CTS), Swedish Research Council (VR), Swedish EnergyAgency and Development and Promotion of Science and Tech-nology Talents Project (DPST) promoted by the Royal Thai Gov-ernment. SNIC, HPC2N and UPPMAX are also acknowledgedfor the computing time.

References

1 M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245.2 Z.-L. Wang, D. Xu, H.-G. Wang, Z. Wu and X.-B. Zhang, ACS

Nano, 2013, 7, 2422.3 L. Li, Z. Wu, S. Yuan and X.-B. Zhang, Energy Environ. Sci.,

2014, 7, 2101.4 B. Fang, A. Bonakdarpour, M.-S. Kim, J. H. Kim,

D. P. Wilkinson and J.-S. Yu, Microporous MesoporousMater., 2013, 182, 1.

5 M. Nakamura, M. Nakanishi and K. Yamamoto, J. PowerSources, 1996, 60, 225.

6 A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld andJ. P. Ferraris, J. Power Sources, 1994, 47, 89.

7 F. Fusalba, N. El. Mehdi, L. Breau and D. Belanger, Chem.Mater., 1999, 11, 2743.

8 E. Naudin, H. A. Ho, S. Branchaud, L. Breau andD. Belanger, J. Phys. Chem. B, 2002, 106, 10585.

9 H. Kim and B. N. Popov, J. Power Sources, 2002, 104, 52.

10 K.-H. Chang and C.-C. Hu, J. Electrochem. Soc., 2004, 151,A958.

11 P. Soudan, J. Gaudet, D. Guay, D. Belanger and R. Schulz,Chem. Mater., 2002, 14, 1210.

12 H. Y. Lee and J. B. Goodenough, J. Solid State Chem., 1999,144, 220.

13 A. Brandt and A. Balducci, Electrochim. Acta, 2013, 108, 219.14 S. R. Sivakkumar and A. G. Pandolfo, Electrochim. Acta,

2012, 65, 280.15 A. Abouimrane, W. Weng, H. Eltayeb, Y. J. Cui, J. Niklas,

O. Plouektov and K. Amine, Energy Environ. Sci., 2012, 5,9632.

16 M. Toupin, T. Brousse and D. Belanger, Chem. Mater., 2004,16, 3184.

17 T. Brousse and D. Belanger, Electrochem. Solid-State Lett.,2003, 6, A244.

18 K. R. Prasad and N. Miura, Electrochem. Commun., 2004, 6,849.

19 R. Ramkumar and M. Minakshi, Dalton Trans., 2015, 44,6158.

20 H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Solid State,1976, 13, 5188.

21 P. E. Blochl, Phys. Rev. B: Condens. Matter, 1994, 50, 17953.22 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter,

1999, 59, 1758.23 S. Grimme, J. Comput. Chem., 2006, 27, 1787.24 S. L. Dudarev, G. A. Botton, S. Y. Savrasov, C. J. Humphreys

and A. P. Sutton, Phys. Rev. B: Condens. Matter, 1998, 57,1505.

25 F. Zhou, M. Cococcioni, K. Kang and G. Ceder, Electrochem.Commun., 2004, 6, 1144.

26 M. Avdeev, Z. Mohamed, C. D. Ling, J. Lu, M. Tamru,A. Yamada and P. Barpanda, Inorg. Chem., 2013, 52, 8685.

27 M. Minakshi, S. Kandhasamy and D. Meyrick, J. AlloysCompd., 2012, 544, 62.

28 B. L. Ellis, W. R. M. Makahnouk, Y. Makimura, K. Toghilland L. F. Nazar, Nat. Mater., 2007, 6, 749.

29 J. P. Huang, D. D. Yuan, H. Z. Zhang, Y. L. Cao, G. R. Li,H. X. Yang and X. P. Gao, RSC Adv., 2013, 3, 12593.

30 S.-Y. Chung, J. T. Bloking and Y.-M. Chiang, Nat. Mater.,2002, 1, 123.

31 J. Qu, L. Shi, C. He, G. Gao, B. Li, Q. Zhou, H. Hu, G. Shao,X. Wang and J. Qiu, Carbon, 2014, 66, 485.

32 V. Aravindan, M. V. Reddy, S. Madhavi, S. G. Mhaisalkar,G. V. Subba Rao and B. V. R. Chowdari, J. Power Sources,2011, 196, 8850.

33 B. Senthilkumar, K. Vijaya Sankar, R. Kalai Selvan,D. Meyrick and M. Minakshi, RSC Adv., 2013, 3, 352.

34 D. Ghosh, S. Giri, Md. Moniruzzaman, T. Basu, M. Mandaland C. K. Das, Dalton Trans., 2014, 43, 11067.

35 A. Langrock, Y. Xu, Y. Liu, S. Ehrman, A. Manivannan andC. Wang, J. Power Sources, 2013, 223, 62.

36 M. Minakshi, S. Duraisamy, P. Tirupathi Rao,S. Kandhasamy and N. Munichandraiah, Int. J. Electrochem.Sci., 2014, 9, 5974.

37 K. Bhattacharya and P. Deb, Dalton Trans., 2015, 44, 9221.

Dalton Transactions Paper

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Publ

ishe

d on

22

Oct

ober

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ER

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AR

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n 07

/12/

2015

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9.

View Article Online

38 J. Huang, B. G. Sumpter and V. Meunier, Angew. Chem., Int.Ed., 2008, 47, 520.

39 J. W. Lee, M. S. Park, B. Anass, J. H. Park, M. S. Palika andS. G. Doo, Electrochim. Acta, 2010, 55, 4162.

40 K. Sathiyaraj, G. Babu, D. Bhuvaneswari and N. Kalaiselvi,Ionics, 2011, 17, 49.

41 Handbook of X-ray Photoelectron Spectroscopy, Physical Elec-tronics Division, Perkin Elmer Corporation, USA, 1992.

42 Y. Xia, M. Wei and Y. Lu, Synth. Met., 2009, 159, 372.43 S. Lim and J. Cho, J. Electrochem. Commun., 2008, 10, 1478.44 G. G. Amatucci, F. Badway, A. Du Pasquier and T. Zheng,

J. Electrochem. Soc., 2001, 148, A930.45 B. E. Conway, J. Electrochem. Soc., 1991, 138, 539.46 M. Minakshi, S. Baskar and M. Ionescu (Unpublished report).47 M. Q. Wu, G. A. Snook, G. Z. Chen and D. J. Fray, Electro-

chem. Commun., 2004, 6, 499.48 Q. Liang, L. Ye, Z.-H. Huang, Q. Xu, Y. Bai, F. Kang and

Q.-H. Yang, Nanoscale, 2014, 6, 13831.49 X. Fan, C. Yu, Z. Ling, J. Yang and J. Qiu, ACS Appl. Mater.

Interfaces, 2013, 5, 2104.50 J. Yin, L. Qi and H. Wang, ACS Appl. Mater. Interfaces, 2012,

4, 2762.

51 S.-M. Oh, S.-T. Myung, J. Hassoun, B. Scrosati andY.-K. Sun, Electrochem. Commun., 2012, 22, 149.

52 Y.-K. Sun, S.-T. Myung, M.-H. Kim, J. Prakash andK. Amine, J. Am. Chem. Soc., 2005, 127, 13411.

53 H. Kim, G. Yoon, I. Park, K.-Y. Park, B. Lee,J. Kim, Y.-U. Park, S.-K. Jung, H.-D. Lim, D. Ahn,S. Lee and K. Kang, Energy Environ. Sci., 2015, 8,3325.

54 T. Cottineau, M. Toupin, T. Delahaye, T. Brousse andD. Belanger, Appl. Phys. A, 2006, 82, 599.

55 P. L. Taberna, P. Simon and J. F. Fauvarque, J. Electrochem.Soc., 2003, 150, A292.

56 B. G. Choi, J. Hong, W. H. Hong, P. T. Hammond andH. S. Park, ACS Nano, 2011, 5, 7205.

57 M. W. Wu, D. D. Zhao, S. J. Bao and H. L. Li, J. Solid StateElectrochem., 2007, 11, 1101.

58 M. Ghaemi, F. Ataherian, A. Zolfaghari and S. M. Jafari,Electrochim. Acta, 2008, 53, 4607.

59 R. Ding, L. Qi and M. Jia, Electrochim. Acta, 2013, 107,494.

60 M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An and R. S. Ruoff,Nano Lett., 2008, 8, 3498.

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