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Article

Maricite NaMn1/3Ni1/3Co1/3PO4 / activated carbon: Hybrid CapacitorManickam Minakshi, Danielle Meyrick, and Dominique Appadoo

Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400333s • Publication Date (Web): 29 Apr 2013

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Maricite NaMn1/3Ni1/3Co1/3PO4 / activated carbon: Hybrid Capacitor

Manickam Minakshi*, Danielle Meyrick and Dominique Appadoo#

Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, Australia

#Australian Synchrotron Company Ltd., Blackburn Road, Clayton, VIC 3168, Australia

Abstract

A hybrid capacitor comprising mixed transition metal sodium phosphate / activated carbon

(AC) with sodium hydroxide electrolyte is reported for the first time. The sodium phosphate

(maricite, NaMn1/3Ni1/3Co1/3PO4) positive material was prepared by both urea-assisted

combustion and polyvinyl pyrrolidone (PVP) assisted sol-gel syntheses. The electrochemical

behavior of maricite and AC was characterized by cyclic voltammetry (CV) and charge-

discharge methods. The reaction mechanism at the maricite electrode in NaOH(aq)

electrolyte appears to be reversible, involving a faradaic process, while the AC shows

capacitive behaviour involving a non-faradaic process. The aqueous hybrid capacitor,

maricite (as cathode) and AC (as anode) studied by galvanostatic (charge-discharge) cycling

in the range 0 – 1.6 V at 0.5 A g−1 exhibited a specific discharge capacitance of 45 F g−1

stable over 1000 cycles.

Keywords: hybrid; aqueous; sodium; capacitor; maricite

* E-mail: minakshi@murdoch.edu.au; lithiumbattery@hotmail.com

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1. Introduction

Electrochemical energy storage possesses a number of desirable features, including, in

many cases, environmentally benign operation, low maintenance, excellent efficiency and

cycling stability. Regardless of their chemistry - aqueous, non-aqueous, Li- or Na-based –

batteries store energy within the electrode structure through faradaic processes.

Supercapacitors, on the other hand, offer a storage mechanism via capacitive (i.e., non-

faradaic) processes arising from an electrochemical double layer at the electrode-electrolyte

interface.1 Each of the mechanisms has its own merits, and each can be exploited to improve

the power and energy densities of the energy storage approach. The common symmetric

supercapacitor faces challenges in terms of high capacitance with high operating voltage,

while batteries face cycle life limitations. An asymmetric (hybrid) capacitor may overcome

the difficulties and limitations of both capacitors and batteries.

Storage solutions based on Li-ion technologies are high cost and are not easily able to

meet the distribution and peak load demands of end users.2 Aqueous sodium batteries show

promise as large-scale, stationary storage devices for electricity grid stabilization and load

levelling. Sodium is abundant in nature, inexpensive and environmentally benign, with

obvious advantages over, for example, its lithium counterpart, in terms of cost and safety.3

However, the ionic volume of sodium is 2.5 times larger than that of lithium, so host

compounds must have larger sites to accommodate Na+ in the matrix. The success of MnO2

as a cathode material for an aqueous sodium battery4 inspired the present investigation on the

sodium phosphate compound in a hybrid capacitor. In this work, the new mixed transition

metal sodium phosphate, NaMn1/3Ni1/3Co1/3PO4 as host compound has been synthesized by

sol-gel and combustion approaches.

In the mid 1990s Goodenough5 proposed that materials based on the tetrahedral

polyanion unit (XO4)n– (X= P, S, As, Mo or W) are structurally more stable than those of the

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oxide family, such as MnO2 and LiCoO2. The large polyanions are able to stabilise the

phosphate structure and, in the case of materials containing transition metals, ‘tune’ the redox

potential of the 3d metal cations such that they yield higher voltages and energy densities.6

Recently the olivine, LiFePO4, has been considered an attractive cathode material in lithium

ion batteries.7 Substitution for lithium in olivine by sodium produces the analogue, NaFePO4,

maricite.8 The maricite structure is similar to the olivine structure, except that the alkali (Na)

occupies the M(2) sites, and the transition metal occupies the M(1) sites. To date, it has been

claimed that NaFePO4 is not viable as a cathode material for battery applications in non-

aqueous media8-9, but we have shown in recent work10 that this material is electrochemically

active when using aqueous NaOH electrolyte. Na+ ions shuttle between the maricite cathode

and aqueous electrolyte hosts during the reduction and oxidation processes. During oxidation,

Na+ ions are electrochemically extracted from the cathode, and in reverse, Na+ ions are

intercalated into the cathode. As the entire mechanism is based on the diffusion of Na+ ions

into the host maricite, the working current density of this maricite-based battery is limited; it

is not able to deliver high current, and requires charging over a long period of time (> 5 h).

Therefore, a sodium battery alone cannot meet the requirements of storage applications.

In this paper, we report the potential applicability of a novel hybrid capacitor having

maricite as cathode coupled with activated carbon as anode for non-portable electrical or

renewable energy storage applications using aqueous NaOH electrolyte. Maricite is chosen

for its electrochemical and host properties. Activated carbon (AC) is often chosen for

supercapacitor applications11, due to its wide potential window, high specific capacitance,

low cost and environmental compatibility. To the best of our knowledge, this is the first time

the hybrid capacitor having a maricite host (pseudocapacitance – achieved by a faradaic

electron charge-transfer) synthesized by non-ceramic methods coupled with activated carbon

(double layer capacitance – electrostatic charge storage) has been reported. Through

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combining the chemistries of battery and capacitor, the electrochemical behaviour of the AC |

NaMn1/3Ni1/3Co1/3PO4 has been characterised and discussed.

2. Experimental

Maricite (NaMn1/3Co1/3Ni1/3PO4) powders were synthesized by two different approaches: (1)

solution combustion synthesis (SCS) and (2) sol-gel technique. In the case of SCS,

stoichiometric amount of sodium nitrate, cobalt nitrate, manganese nitrate, nickel nitrate and

ammonium dihydrogen phosphate were dissolved homogeneously in double distilled water

with effective stirring at 80°C. Urea was added as a fuel to the mixed homogenous solution.

An oxidant-to-fuel ratio of one-to-one was maintained. The pH of the precursor solution was

adjusted to 8 by drop wise addition of ammonia solution. Continuous stirring and heating

resulted in complete evaporation of water and initiation of the combustion reaction. Complete

ignition resulted in the formation of a foam, which was dried in an oven at 110°C for 12 h

and furnace heated at 300°C for 8 h, subsequently at 600°C for 5 h with an ntermediate

grinding.

For the sol-gel method, metal (Na, Ni, Mn, Co) acetate precursors were used as starting

materials. The required stoichiometric amount of each of the metal acetate precursors and

ammonium dihydrogen phosphate were dissolved homogeneously in double distilled water.

Polyvinylpyrrolidone (PVP), as a chelating agent, was added to the mixed metal ion solution

with a metal-to-PVP weight ratio of one-to-one. The pH was adjusted to ~ 3.5 by addition of

nitric acid. The solution was continuously stirred and heated until the formation of a thick

transparent gel, which was then subject to the same drying and heating protocol as the

combustion foam.

The furnace-cooled NaMn1/3Co1/3Ni1/3PO4 product obtained by each of the methods

was ground to achieve homogeneity using a mortar and pestle. The synthesized

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NaMn1/3Co1/3Ni1/3PO4 powders were subject to systematic physical and electrochemical

studies. Powder XRD analyses were made on a Siemens D500 X-ray diffractometer 5635

with Cu-Kα source at a scan speed of 1 degree per minute with voltage 30 kV and current 28

mA. Infrared (IR) spectra were collected on a Bruker IFS 125/HR Fourier Transform

spectrometer.

Cyclic voltammetry of the samples was carried out using an EG&G Princeton Applied

Research Versa Stat III model. To prepare active electrode material, NaMn1/3Co1/3Ni1/3PO4 or

activated carbon (85 wt %), carbon black (10 wt %) and polyvinlidene fluoride (PVDF) (5 wt

%) were suspended in 0.4 mL of N-methyl-2-pyrrolidinone (NMP) to form a slurry. Ten µL

of slurry was coated on graphite sheet (area of coating, 1 cm2). The mass of the cathode and

anode electrode material was 2 and 8 mg respectively. An aqueous solution of 2 M NaOH

(standard) was used as electrolyte. For the three electrode tests a platinum wire electrode and

a saturated Hg/HgO electrode were used as counter and reference electrodes, respectively.

The asymmetric cell was constructed with AC coated on graphite sheet and

NaMn1/3Co1/3Ni1/3PO4 active electrodes. Galvanostatic charge/discharge cycles of the cell

were performed using an 8 channel battery analyser from MTI Corp., USA, operated by a

battery testing system at a current density of 5 mAcm−2.

3. Results and Discussion

Physical characteristics of maricite NaMn1/3Ni1/3Co1/3PO4

In one of our earlier studies10, materials synthesized for cathodes by the conventional solid-

state reaction method did not perform well in battery/supercapacitor studies due to poor

crystallinity and non-uniform particle size (>10 µm). Generally, the ceramic method is

controlled by the diffusion of atoms and ionic species through reactants and products. To

overcome this limitation, several techniques, such as (a) sol-gel (b) melt-impregnation and (c)

the Pechini process have been developed world wide 12-14 for synthesizing the cathode

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material, but the extent of improvement in battery performance is still limited.15 SCS has

been found to yield products with suitable electrochemical and other characteristics without

further treatment.16 This rapidly emerging technique involves an aqueous solution of nitrate

ions of the metal precursor, acting as oxidizer, and urea, acting as a fuel.17 The reactant

solutions are mixed and heated to 600°C. The self-propagating reaction is exothermic and

provides the energy for the formation of, in this case, maricite phosphate. It is thought that

the fuel (urea) forms stable complexes with the metal ions, thus facilitating homogeneous

mixing of cations.18 The combination of these features makes SCS an attractive technique.

The combustion synthesized NaMn1/3Ni1/3Co1/3PO4 of the maricite type prepared in

this work is compared with the product of a polymer assisted sol-gel technique. One of the

unique properties of PVP as a chelating agent is its decomposition during synthesis and

formation of a polymer composite over the material. This increases the conductivity of the

phosphate family when prepared using this technique.19 The electrochemically active

polymer permits penetration of the NaOH electrolyte into the bulk maricite. This new porous

layer on the electrode surface is involved in the redox reaction while inserting Na+ ions or

counter anions from the electrolyte. Hence, the physicochemical properties of synthesized

maricite powder strongly depend on the chelating agent used in the sol-gel method. This

prompted us to investigate and determine the influence of PVP on the performance of a

hybrid capacitor.

Powder x-ray diffraction (XRD) patterns of maricite, NaMn1/3Ni1/3Co1/3PO4 prepared

through combustion and sol-gel syntheses as described are shown in Fig. 1. Both the products

are indexed in the orthorhombic structure, while only the combustion synthesized product is

highly crystalline. The calculated unit cell parameters for the maricite NaMn1/3Co1/3Ni1/3PO4

are 9.097 a (Å), 6.903 b (Å) and 5.119 c (Å). The XRD data of the sol-gel product showed the

peaks at 30.3° and 35.7° (marked with an oval shape) are not well resolved as compared to

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those for the combustion synthesized product, indicating the key role of the high temperature

environment for the formation of the phosphate compound (Fig. 1a). In the case of the sol-gel

process with PVP as chelating agent, the pyrrolidone ring in PVP with nitrogen atom

decomposes at 450°C, forming a trapped organic layer over the maricite. This organic layer

improves the particle-to-particle contact (and hence conductivity) within maricite, but there is

a suggestion of an adverse effect on crystallinity (Fig. 1b). The XRD pattern of the as-

prepared powders obtained from the sol-gel method shows less intense peaks, indicating the

particles are less crystalline than their combustion counterparts (Fig. 1). The sol-gel product

thus requires post-synthesis treatment at high temperature that will result in agglomerated

particles.20

The suitability of the combustion approach to synthesis is further evidenced by IR

spectroscopy (Fig. 2). Far IR spectral bands (Fig. 2a) observed at wavelengths around 560,

590 and 625 cm–1 are assigned to the intramolecular PO43– bending modes (ν4) for both

products.21 The differences observed in the ν4 band variations may be due to the synthetic

method. Figure 2b shows the mid IR spectra of two samples prepared by combustion and sol-

gel process. It is apparent that relative intensities of the spectral bands are different. There is a

broadening in the region 1000 cm−1 for the sol-gel sample, while three bands appeared

corresponding to C-O stretching for the combusted sample. The C-O stretch indicates the

presence of carbonaceous particles composed of primarily carbon and oxygen. Thus, the SCS

method offers a versatile means to synthesise technologically important phosphate materials

for battery/capacitor applications. The absence of a prominent peak in the region 1650 cm−1

corresponding to the carbonyl band of PVP22 suggests that PVP is decomposed during

synthesis. However, bands at other regions (1390, 1425 and 1550 cm−1) support the existence

of an interaction between metal precursors and some kind of polymer composite.

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Consistently, these bands were not observed for the combusted product. This further suggests

the presence of organic polymer on the electrode surface for PVP synthesized sample.

Electrochemical performance of maricite NaMn1/3Ni1/3Co1/3PO4: Combustion vs. Sol-gel

Cyclic voltammetric measurements were performed on NaMn1/3Ni1/3Co1/3PO4 synthesised

electrodes to identify the redox behaviour in hybrid aqueous cells. The high conductivity of

aqueous electrolytes relative to conventional organic (non-aqueous) solvents provides an

opportunity to analyse the voltammetric response of phosphate electrodes at very high scan

rates. Figures 3-6 show the stable voltammetric profiles of maricite in NaOH(aq) solutions.

Figure 3 shows a comparison of the first cyclic voltammograms, obtained under

identical conditions, of NaMn1/3Ni1/3Co1/3PO4 prepared by both the combustion technique and

the sol-gel technique. The scan was initiated at -0.2 V moving in the anodic direction until

0.7 V and then reversed to the initial potential at a rate of 5 mVs−1. The CV profile shows an

oxidation peak, A1, and a corresponding reduction peak, C1, indicating that the redox pair are

involved in the loss and gain of electrons in the maricite crystal structure during the sodium

extraction and insertion processes.23 The CVs of these two materials show differences in

terms of peak intensities and peak positions. The C1 and A1 peaks for the combustion

product occur at 0.26 and 0.51 V, while those for the sol-gel product are at 0.16 and 0.45 V

respectively, with the lower intensity implying lesser electrochemical reactivity. The peak

separations (ideally, ∆ Ep = C1 – A1 is 0.059 V) are large for both samples, indicating that

the process is quasi-reversible.24 The area under the curve for the sol-gel sample (Fig. 3b) is

smaller than that for the combustion synthesised product, and the reduction peak is at a more

negative value, indicating the intercalation process occurs with more difficulty for the

maricite sample synthesized via sol-gel method. The enhanced electrochemical process

illustrated in Fig. 3a for the combustion product can be attributed to the role of the fuel in

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producing homogenous product particles of desirable size.10, 18 Based on these CV results, the

combustion product with relatively enhanced electrochemical performance was selected for

further studies.

Effect of aqueous NaOH electrolyte concentration

Aqueous electrolytes present many advantages over non-aqueous solvents. Water is the most

natural of all electrolyte solvents; it is inexpensive and has high ionic conductivity.25 As

conductivity is an important parameter, we have optimised the molar concentration of Na+ in

the electrolyte. CV tests were performed to monitor the effect of the NaOH concentration on

the electrochemical properties of the maricite electrode. Figure 4 shows the CV curves

recorded at a scan rate of 5 mVs−1 with electrolyte solution of different molar concentrations

(1, 2 and 5 M). All the curves exhibit a pair of redox peaks and reversible redox reactions,

indicating that the faradaic reaction involving the sodium extraction and insertion is taking

place. The maximum electrochemical activity was obtained for the systems with the

electrolyte at the lower concentrations (1 and 2 M NaOH) (Figs. 4a-b). In the CV curves

produced in dilute NaOH, the distance between the C1 and A1 peaks is greater than that

produced in the 5 M solution. For a higher concentration (i.e., 5 M NaOH, Fig. 4c), the peak

separation decreases but the current response is lower. This could be due to the variation in

the conductivity in going from 1 M and 2 M NaOH(aq) to 5 M NaOH(aq) or may suggest a

semi-infinite diffusion control of the intercalation process. At the higher NaOH(aq)

concentration, the bulk Na+ concentration is not considerably changed during the

electrochemical process.26 Moreover, in the concentrated NaOH solution, electrolyte ions can

be adsorbed on the electrode surface mostly as ion pairs and could be surface controlled

process rather diffusion.27 On the basis of this preliminary study, we conclude that a lower

concentration of NaOH(aq) is most suitable in this context and we have selected 2 M

concentration for use in further investigations.

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Effect of scan rate Figure 5 shows the CVs of NaMn1/3Ni1/3Co1/3PO4 at various sweep rates from 1 to 20 mV/s

between 0.7 and -0.2 V. At a lower scan rate (Figs. 5 a–b) the voltammograms are nearly

symmetrical and characterised by a well-defined redox peak. At the higher scan rate (Figs. 5

c–d) the oxidation peaks are ill defined and the corresponding reduction peaks are slightly

broad. The peak potential difference between the two (A1 and C1) peaks increased

dramatically at the higher scan rate (short duration of electrochemical process) implying a

slow electron transfer.26-27 Hence the sodium extraction and insertion process changes from

being kinetically semi-reversible to irreversible when the sweep rate increases from 1 to 20

mVs−1. From these results, it is established that a higher sweep rate may lead to faster kinetics

of the redox process than the sodium ion diffusion into the bulk maricite, resulting in

decreased specific capacitance. Hence, sweep rate of 5 mVs−1 was selected for the hybrid

capacitor studies.

Continuous cycling test (100 cycles) at a 1e– charge transfer of maricite (Fig. 6)

revealed that the current response and peak potential was unchanged. The maricite electrode

was successfully cycled within the redox potential window of 0.6 V and shows promise as a

cathode in the hybrid supercapacitor. This is discussed in the following sections.

Galvanostatic charge-discharge cycling of cathode NaMn1/3Ni1/3Co1/3PO4

The galvanostatic cycling performance of the maricite cathode between 0 and 0.6 V at a

current of 5 mAcm−2 is shown in Fig. 7. The energy storage mechanism at the cathode utilises

faradaic process (electron transfer) of reversible sodium extraction (during charge) and

insertion (during discharge), as is clearly seen from the charge and discharge curves. The

charge accumulation or energy storage is linked to the insertion of Na+ ions into an

unchanged maricite host matrix. The charge carried by Na+ ions is compensated by

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oxidation/reduction of transition metal centres (M2+/3+) in the host NaMn1/3Ni1/3Co1/3PO4

compound during charge and discharge processes.

The maricite electrode was cycled 100 times at the same rate in the flooded cell,

revealing no change in voltage profile or deficit in charge/discharge times over 100 cycles

(Fig. 7). This indicates that bonds in the host maricite are stable during the electrochemical

processes, making this maricite suitable as a reversible electrode material. The host lattice

will ideally revert to its pristine state when the guest Na+ ions are inserted in the recharge

process. The cell was successfully cycled, delivering a specific capacitance of 405 Fg−1, with

the initial discharge capacitance of 405 Fg−1 stable for greater than 100 cycles. This suggests

that the chosen potential window is safe and the battery-type intercalation mechanism28 is

fully reversible.

Preliminary studies on the anode activated carbon (AC)

Electric double-layer capacitors (EDLCs) utilise an electrochemical double layer capacitance

at the electrode/electrolyte interface through non-faradaic processes where electric charges

are accumulated on the electrode surface and ions of opposite charge are arranged on the

electrolyte side.29-30 Carbon based materials, such as activated carbon (AC), are preferred as

EDLC electrodes owing to their low cost, chemical stability in aqueous solutions with

concentrated NaOH electrolytes and their high surface area.31-32 EDLCs are complimentary to

batteries as they deliver high power density and low energy density. The effects of physical

and electrochemical properties of activated carbon on the behaviour of EDLCs have been

widely reported.31, 33-36 However, a preliminary voltammetric study of AC using NaOH (aq)

electrolyte at 2 and 5 M has been performed. Results are shown in Fig. 8. CV tests have been

performed to evaluate the effect of NaOH electrolyte on the EDLC behaviour of the AC

electrode. The curve recorded at a molar concentration of 2 M is approximately rectangular in

shape, indicating that AC mainly exhibits electric double-layer capacitance. The curve

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became less rectangular, and the current response was lower, when the NaOH (aq)

concentration was increased from 2 to 5 M, implying a sluggish ion transfer rate and

therefore poor electrochemical properties. Hence, the NaOH(aq) concentration was

maintained at 2 M throughout the studies with a constant sweep rate of 5 mVs−1.

Galvanostatic charge-discharge studies of activated carbon

The galvanostatic tests were carried out at a constant current of 5 mAcm−2 in order to

investigate the charge storage mechanism and the capacitance value of AC in 2 M

concentration of NaOH. The first charge-discharge curves are shown in Fig. 9. The

mechanism is based on interfacial charge transfer at the electrode/electrolyte interface i.e.,

adsorption and desorption of sodium ions from the electrolyte at the maricite surface. The

initial portions of the charge curve exhibit the ‘IR drop’, while the final portions of the charge

curve show high symmetry within the applied potential window, suggesting purely capacitive

behaviour.37 The activated carbon delivers a discharge capacitance of 105 Fg−1.

Hybrid capacitor cell AC || NaMn1/3Ni1/3Co1/3PO4

In our hybrid charge storage device, electrochemical capacitor with activated carbon as anode

is interfaced with a battery material, NaMn1/3Ni1/3Co1/3PO4. Having the same rate capabilities

of maricite and activated carbon (shown earlier in Figs. 4 and 8), and taking into account that

these two electrodes have the most negative and most positive redox potentials, we assembled

a full cell of 1.6 V, taking the amount of anode material 0.25 times that of the cathode

material. This hybrid cell is expected to have a longer cycle life than a battery with maricite

cathode, and possess higher energy density relative to conventional capacitors. The new cell

was tested and its galvanostatic cycling performance is shown in Figs. 10 and 11. The

cathode utilizes a faradaic reaction at the maricite cathode and the anode utilizes a non-

faradaic process (electric double layer with the OH– anion present in the electrolyte). The

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voltage window of the cell is 1.6 V, delivering a specific capacitance of 45 Fg−1 and power

density of 400 Wkg−1 at a constant current of 0.5 Ag−1. The specific capacitance obtained

from the discharge data (in Fig. 10) was calculated using the equation C = It/(∆Em) where I

is discharge current, t is discharge time (s), ∆E is potential window (i.e., 1.6 V) and m is the

weight of the electroactive material.11 The specific energy density (ESP) and specific power

density (PSP) of the AC||NaCo1/3Mn1/3Ni1/3PO4 were calculated using the relation ESP = ½ C

(∆E)2 (Wh/Kg), where C is the capacitance of the cell and PSP = ESP/t*3600 (W/Kg). The plot

of specific discharge capacitance versus cycle number of the hybrid cell is given in Fig. 11.

The maricite electrode is able to retain its specific capacitance during long term cycling. The

voltage profiles (shown in the inset in Fig. 11) showed no change between the 10th and 1000th

cycles while delivering a constant specific discharge capacitance. Thus, it can be concluded

that the newly constructed hybrid cell has excellent long term stability at relatively low cost.

The aqueous hybrid capacitor with maricite as an electrode could find potential applications

to store energy generated from non-conventional energy sources.

The Ragone plot of the hybrid cell is shown in Fig. 12, calculated from the data of Fig. 10. It

is seen that an energy density of 15 WhKg−1 was obtained at a power density of 400 WKg−1,

while at 600 WKg−1, the energy density is stabilised at 10 WhKg−1. However the given

energy density for the maricite is rather low relative to that of reported metal oxide materials

in aqueous solutions. 39-40 This marginal performance may be improved upon with changes to

the cation substitution ratio in the parent compound. This will be reported in our next

publication.

4. Conclusions

The proposed maricite NaMn1/3Ni1/3Co1/3PO4 in hybrid capacitor develops an important new

family of energy storage devices based on an affordable, globally available element: sodium

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(in NaOH electrolyte). Urea assisted SCS employed for the preparation of maricite produced

a material with better performance than the sol-gel derived counterpart. A typical hybrid

capacitor employing AC || NaMn1/3Ni1/3Co1/3PO4 delivers a constant discharge specific

capacitance of 45 Fg−1 over 1000 cycles. The hybrid charge storage device is found to have

longer cycle life with a higher energy density 15 WhKg−1 and power density of 400 WKg−1.

The observed value compared reasonably well with the reported values in the literature for

aqueous hybrid aqueous capacitors.

The innovative science in this study involves reversible aqueous sodium

electrochemistry through faradaic process at low temperature (against the available relatively

high temperature battery technology at which Na is molten). The sodium energy storage

technology will offer immediate advantages over existing primary battery or capacitor

technologies in terms of high energy and power densities, cycle life, cost, safety and

environmental considerations.

Acknowledgements

The author (M.M.) wishes to acknowledge the Australian Research Council (ARC). This

research was supported under Australian Research Council (ARC) Discovery Project funding

scheme (DP1092543). The views expressed herein are those of the authors and are not

necessarily those of the ARC. The Infrared analysis was undertaken in the FRIR beamline at

the Australian Synchrotron, Victoria, Australia through the grant no. AS123/HRIR 5428A.

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Figure Captions

Fig. 1 X-ray diffraction patterns of as-synthesized maricite synthesized by (a) combustion method using urea as a fuel and sol-gel method using (a) polyvinyl pyrrolidine; PVP as chelating agent.

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Fig. 2 a) Far and (b) mid-infrared spectra (using synchrotron source) of maricite synthesized electrodes through combustion and sol-gel method.

Fig. 3 Cyclic voltammogram of maricite synthesized electrodes (a) combustion and sol-gel method, in aqueous 2 M NaOH electrolyte at a scan rate of 5 mV/s.

Fig. 4 Cyclic voltammogram of maricite combustion synthesized electrode tested in aqueous NaOH electrolyte containing various molar concentrations (a) 1 (b) 2 and (c) 5 M at a scan rate of 5 mV/s.

Fig. 5 Cyclic voltammogram of maricite combustion synthesized electrode tested in aqueous 2 M NaOH electrolyte at various sweep rates (a) 1, (b) 5, (c) 10 and (d) 20 mV/s.

Fig. 6 Cyclic voltammogram of maricite combustion synthesized electrode for 100 repetitive cycles. Data has been shown for 1, 50 and 100 cycles.

Fig. 7 Galvanostatic cycling of maricite combustion synthesized electrode for 100 repetitive cycles at 5 mA/cm2. Cycle numbers are indicated in the figure.

Fig. 8 Cyclic voltammogram of the activated carbon tested in aqueous NaOH electrolyte containing (a) 2 and (b) 5 M (molar concentrations) at a scan rate of 5 mV/s.

Fig. 9 Galvanostatic cycling of activated carbon electrode for 100 repetitive cycles at 5 mA/cm2. Cycle numbers are indicated in the figure.

Fig. 10 Galvanostatic first charge-discharge curves of hybrid capacitor activated carbon|| maricite.

Fig. 11 Specific Capacitance (SC) vs. Cycle life of the hybrid capacitor activated carbon|| maricite cycled 1000 times. Data for 10th and 1000th cycle of the hybrid cell is shown in the inset of the figure.

Fig. 12 Ragone plot of the hybrid capacitor activated carbon|| maricite.

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20 30 40 50 60 70

•

(031)

•

(02

2)

•

(402

)

(251)

(40

0)

(222)

(122)

(221)(0

12

)(0

31)

(211)

•

(122

)

(230)

(200

)(111)

•• ••••

••

•

•

•(020

)

••••

(213

)

(013)

(402)

(25

1)

(400)

(22

2)

(02

2)

(122)

(221

)(0

12)(0

02)

(031

)

(211)

(122)

(23

0)

(20

0)

•(1

11

)(0

11)

(020

)

•

(b)

• ••••

••••

•

•

••

Inte

nsit

y /

a.u

.

2θθθθ / degrees (CuKαααα))))

(a)

Fig. 1 X-ray diffraction patterns of as-synthesized maricite prepared by (a) combustion

method using urea as fuel and (b) sol-gel method using polyvinyl pyrrolidine (PVP) as

chelating agent.

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750 700 650 600 550 500 450 400

56

059

0

Sol-gel Tra

nsm

itta

nce /

%

Wavenumber / Cm-1

625

v4 antisymmetric bending mode

Combustion

(a)

1800 1600 1400 1200 1000 800

combustion

C-O stretch

C-C stretch10

25

94

5

10

00

10

70

155

0

14

25

139

0

Wavenumber / Cm-1

Transmittance / %

sol-gel

(b)

Fig. 2 (a) Far and (b) mid-infrared spectra (using synchrotron source) of maricite electrodes

synthesized through combustion and sol-gel methods.

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-0.2 0.0 0.2 0.4 0.6 0.8-0.010

-0.005

0.000

0.005

0.010A1

(b)

Cu

rre

nt

/ A

Potential vs. Hg/HgO / V

(a)

C1

Fig. 3 Cyclic voltammogram of maricite synthesized electrodes (a) combustion and sol-gel

method, in aqueous 2 M NaOH electrolyte at a scan rate of 5 mV/s.

-0.2 0.0 0.2 0.4 0.6 0.8-0.010

-0.005

0.000

0.005

0.010A1

C1

(c)

(b)

Cu

rren

t /

A

Potential vs. Hg/HgO / V

(a)

Fig. 4 Cyclic voltammogram of maricite combustion synthesized electrode tested in aqueous

NaOH electrolyte containing various molar concentrations (a) 1 (b) 2 and (c) 5 M at a scan

rate of 5 mV/s.

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-0.2 0.0 0.2 0.4 0.6 0.8

-0.02

-0.01

0.00

0.01

0.02

0.03 A1

(a)

(b)

(c)

Cu

rre

nt

/ A

Potential vs. Hg/HgO / V

(d)C1

Fig. 5 Cyclic voltammogram of maricite combustion synthesized electrode tested in aqueous

2 M NaOH electrolyte at various sweep rates (a) 1, (b) 5, (c) 10 and (d) 20 mV/s.

-0.2 0.0 0.2 0.4 0.6 0.8-0.010

-0.005

0.000

0.005

0.010A1

Cu

rre

nt

/ A

Potential vs. Hg/HgO / V

C1

Fig. 6 Cyclic voltammogram of maricite combustion synthesized electrode for 100 repetitive

cycles. Data has been shown for 1, 50 and 100 cycles.

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0 100 200 300 400

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Char

ge

Discharge

Discharge

Time / s

Po

ten

tial

vs

. H

g/H

gO

/ V

Char

ge

1 100

Fig. 7 Galvanostatic cycling of maricite combustion synthesized electrode for 100 repetitive

cycles at 5 mA/cm2. Cycle numbers are indicated in the figure.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.009

-0.006

-0.003

0.000

0.003

0.006

(a)Cu

rren

t /

A

Potential vs. Hg/HgO / V

(b)

Fig. 8 Cyclic voltammogram of the activated carbon tested in aqueous NaOH electrolyte

containing (a) 2 and (b) 5 M (molar concentrations) at a scan rate of 5 mV/s.

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0 100 200 300 400

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Po

ten

tia

l v

s.

Hg

/Hg

O /

V

Time / s

DischargeCharge

Fig. 9 Galvanostatic cycling of activated carbon electrode for 100 repetitive cycles at 5

mA/cm2. Cycle numbers are indicated in the figure.

0 50 100 150 200 250 300 3500.0

0.4

0.8

1.2

1.6

2.0

Discharge

Ce

ll P

ote

nti

al

/ V

Time / s

Charge

Fig. 10 Galvanostatic first charge-discharge curves of hybrid capacitor activated carbon||

maricite.

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0 200 400 600 800 10000

10

20

30

40

50

274400 274500 274600 274700

0.4

0.8

1.2

1.6

2.0

Cell V

olt

ag

e / V

Time / s

1000th cycle

3200 3300 3400 3500

0.4

0.8

1.2

1.6

2.0 10th Cycle

Ce

ll V

olt

ag

e / V

Time / s

Sp

ecif

ic c

ap

acit

an

ce

/ F

g-1

Cycle number

Fig. 11 Specific Capacitance (SC) vs. Cycle life of the hybrid capacitor activated carbon||

maricite cycled 1000 times. Data for 10th and 1000th cycle of the hybrid cell is shown in the

inset of the figure.

400 450 500 550 6000

5

10

15

20

25

En

erg

y d

en

sit

y /

Wh

Kg

-1

Power density / W Kg-1

Fig. 12 Ragone plot of the hybrid capacitor activated carbon || maricite.

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