Activated Carbon: Hybrid Capacitor

Apr 29, 2013 - The aqueous hybrid capacitor, maricite (as the cathode), and AC (as the anode) studied by galvanostatic (charge−discharge) cycling in...
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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, Western Australia 6150, Australia Australian Synchrotron Company Limited, Blackburn Road, Clayton, Victoria 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 behavior involving a non-faradaic process. The aqueous hybrid capacitor, maricite (as the cathode), and AC (as the anode) studied by galvanostatic (charge−discharge) cycling in the range of 0−1.6 V at 0.5 A g−1 exhibited a specific discharge capacitance of 45 F g−1 stable over 1000 cycles.

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 or non-aqueous or 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., nonfaradaic) 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 leveling. 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; therefore, 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 a host compound has been synthesized by sol−gel and combustion approaches. In the mid-1990s, Goodenough and co-workers5 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 oxide family, such as MnO2 and LiCoO2. The large polyanions are able to stabilize the phosphate structure and, in the case of materials containing transition metals, “tune” the © XXXX American Chemical Society

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 media,8,9 but we have shown in recent work10 that this material is electrochemically active when using an 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. Because 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 a cathode coupled with activated carbon (AC) as an anode for nonportable electrical or renewable energy storage applications using an aqueous NaOH electrolyte. Maricite is chosen for its electrochemical and host properties. AC is often chosen for supercapacitor applications,11 because of 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 (pseudo-capacitance, achieved by a faradaic electron charge transfer) synthesized by non-ceramic methods coupled with AC (double-layer capacitance, electrostatic charge storage) has been reported. Through combining the chemistries Received: February 25, 2013 Revised: April 29, 2013

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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 an 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 to 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 counteranions from the electrolyte. Hence, the physicochemical properties of synthesized maricite powder strongly depend upon 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 XRD patterns of maricite, NaMn1/3Ni1/3Co1/3PO4, prepared through combustion and sol−gel syntheses as described are shown in Figure 1. Both of the products are

of the battery and capacitor, the electrochemical behavior of AC|NaMn1/3Ni1/3Co1/3PO4 has been characterized and discussed.

2. EXPERIMENTAL SECTION 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 amounts 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 homogeneous solution. An oxidant/fuel ratio of 1:1 was maintained. The pH of the precursor solution was adjusted to 8 by dropwise 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 furnaceheated at 300 °C for 8 h and subsequently at 600 °C for 5 h with intermediate grinding. For the sol−gel method, metal (Na, Ni, Mn, and Co) acetate precursors were used as starting materials. The required stoichiometric amount of each of the metal acetate precursors and ammonium dihydrogen phosphate was dissolved homogeneously in doubledistilled water. Polyvinylpyrrolidone (PVP), as a chelating agent, was added to the mixed metal ion solution with a metal/PVP weight ratio of 1:1. The pH was adjusted to ∼3.5 by the 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 NaMn1/3Co1/3Ni1/3PO4 powders were subject to systematic physical and electrochemical studies. Powder X-ray diffraction (XRD) analyses were made on a Siemens D500 X-ray diffractometer 5635 with a Cu Kα source at a scan speed of 1°/min with voltage at 30 kV and current at 28 mA. Infrared (IR) spectra were collected on a Bruker IFS 125/HR Fourier transform spectrometer. Cyclic voltammetry (CV) 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 AC (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. A total of 10 μL of slurry was coated on the 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 an 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 a graphite sheet and NaMn1/3Co1/3Ni1/3PO4 active electrodes. Galvanostatic charge/discharge cycles of the cell were performed using an eight channel battery analyzer from MTI Corporation, Richmond, CA, operated by a battery testing system at a current density of 5 mA cm−2.

Figure 1. XRD patterns of as-synthesized maricite prepared by the (a) combustion method using urea as fuel and (b) sol−gel method using PVP as a chelating agent.

indexed in the orthorhombic structure, while only the combustion-synthesized product is highly crystalline. The calculated unit cell parameters for maricite, NaMn1/3Co1/3Ni1/3PO4, are as follows: a, 9.097 Å; b, 6.903 Å; and c, 5.119 Å. The XRD data of the sol−gel product showed that the peaks at 30.3° and 35.7° (marked with an oval shape) are not well-resolved in comparison to those for the combustion-synthesized product, indicating the key role of the high-temperature environment for the formation of the phosphate compound (Figure 1a). In the case of the sol−gel process with PVP as a chelating agent, the pyrrolidone ring in PVP with a nitrogen atom decomposes at 450 °C, forming a trapped organic layer over maricite. This organic layer improves

3. RESULTS AND DISCUSSION 3.1. Physical Characteristics of Maricite, NaMn1/3Ni1/3Co1/3PO4. In one of our earlier studies,10 materials synthesized for cathodes by the conventional solidstate reaction method did not perform well in battery/ supercapacitor studies because of poor crystallinity and nonuniform 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) Pechini process, have been developed worldwide12−14 for synthesizing the cathode material, but the extent of B

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composite. Consistently, these bands were not observed for the combusted product. This further suggests the presence of an organic polymer on the electrode surface for a PVPsynthesized sample. 3.2. Electrochemical Performance of Maricite, NaMn1/3Ni1/3Co1/3PO4: Combustion versus Sol−Gel. Cyclic voltammetric measurements were performed on NaMn1/3Ni1/3Co1/3PO4-synthesized electrodes to identify the redox behavior in hybrid aqueous cells. The high conductivity of aqueous electrolytes relative to conventional organic (nonaqueous) solvents provides an opportunity to analyze 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.

the particle−particle contact (and hence conductivity) within maricite, but there is a suggestion of an adverse effect on crystallinity (Figure 1b). The XRD pattern of the as-prepared powders obtained from the sol−gel method shows less intense peaks, indicating that the particles are less crystalline than their combustion counterparts (Figure 1). The sol−gel product thus requires post-synthesis treatment at a high temperature that will result in agglomerated particles.20 The suitability of the combustion approach to synthesis is further evidenced by IR spectroscopy (Figure 2). Far-IR

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

Figure 2. (a) Far- and (b) mid-IR spectra (using a synchrotron source) of maricite electrodes synthesized through combustion and sol−gel methods.

spectral bands (Figure 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 synthesize 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

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

Figure 3 shows a comparison of the first cyclic voltammog r a m s , o b t a i n e d u n d e r id e n t i c a l c o n d i t i o n s , o f 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 mV s−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 C

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concentrations (1 and 2 M NaOH) (curves a and b of Figure 4). 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, curve c of Figure 4), the peak separation decreases but the current response is lower. This could be due to the variation in the conductivity in going from 1 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 a surface-controlled process rather than 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 have selected 2 M concentration for use in further investigations. 3.4. Effect of the Scan Rate. Figure 5 shows the CV 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 (curves a and b of Figure 5), the voltammograms are nearly symmetrical and characterized by a well-defined redox peak. At the higher scan rate (curves c and d of Figure 5), 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 the 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 mV s−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, a sweep rate of 5 mV s−1 was selected for the hybrid capacitor studies. Continuous cycling test (100 cycles) at a 1 e− charge transfer of maricite (Figure 6) revealed that the current response and peak potential were 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. 3.5. 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 mA cm−2 is shown in Figure 7. The energy storage mechanism at the cathode uses a faradaic process (electron transfer) of reversible sodium extraction (during charge) and insertion (during discharge), as 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 oxidation/reduction of transition-metal centers (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 the voltage profile or deficit in charge/discharge times over 100 cycles (Figure 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

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

Figure 6. Cyclic voltammogram of a maricite-combustion-synthesized electrode for 100 repetitive cycles. Data have been shown for 1, 50, and 100 cycles.

the lower intensity implying lesser electrochemical reactivity. The peak separations (ideally, ΔEp = C1 − A1 is 0.059 V) are large for both samples, indicating that the process is quasireversible.24 The area under the curve for the sol−gel sample (curve b of Figure 3) is smaller than that for the combustionsynthesized product, and the reduction peak is at a more negative value, indicating that the intercalation process occurs with more difficulty for the maricite sample synthesized via the sol−gel method. The enhanced electrochemical process illustrated in curve a of Figure 3 for the combustion product can be attributed to the role of the fuel in producing homogeneous product particles of desirable size.10,18 On the basis of these CV results, the combustion product with relatively enhanced electrochemical performance was selected for further studies. 3.3. Effect of the 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 Because conductivity is an important parameter, we have optimized 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 mV s−1 with electrolyte solution of different molar concentrations (1, 2, and 5 M). All of 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 D

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electrochemical properties. Hence, the NaOH(aq) concentration was maintained at 2 M throughout the studies with a constant sweep rate of 5 mV s−1. 3.7. Galvanostatic Charge−Discharge Studies of AC. The galvanostatic tests were carried out at a constant current of 5 mA cm−2 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 Figure 9. The

Figure 7. Galvanostatic cycling of a maricite-combustion-synthesized electrode for 100 repetitive cycles at 5 mA cm−2. Cycle numbers are indicated in the figure.

specific capacitance of 405 F g−1, with the initial discharge capacitance of 405 F g−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. 3.6. Preliminary Studies on the Anode AC. Electric double-layer capacitors (EDLCs) use 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 AC, are preferred as EDLC electrodes, owing to their low cost, chemical stability in aqueous solutions with concentrated NaOH electrolytes, and high surface area.31,32 EDLCs are complementary to batteries because they deliver high power density and low energy density. The effects of physical and electrochemical properties of AC on the behavior 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 Figure 8. CV tests have been performed to evaluate

Figure 9. Galvanostatic cycling of an AC electrode for 100 repetitive cycles at 5 mA/cm2. Cycle numbers are indicated in the figure.

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 behavior.37 The AC delivers a discharge capacitance of 105 F g−1. 3.8. Hybrid Capacitor Cell AC∥NaMn1/3Ni1/3Co1/3PO4. In our hybrid charge storage device, an electrochemical capacitor with AC as the anode is interfaced with a battery material, NaMn1/3Ni1/3Co1/3PO4. Having the same rate capabilities of maricite and AC (shown earlier in Figures 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 as 0.25 times that of the cathode material. This hybrid cell is expected to have a longer cycle life than a battery with a maricite cathode and possess higher energy density relative to conventional capacitors. The new cell was tested, and its galvanostatic cycling performance is shown in Figures 10 and

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

the effect of a NaOH electrolyte on the EDLC behavior 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. When the NaOH(aq) concentration was increased from 2 to 5 M, the curve became less rectangular and the current response was lower, implying a sluggish ion transfer rate and, therefore, poor

Figure 10. Galvanostatic first charge−discharge curves of hybrid capacitor AC∥maricite. E

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W kg−1, while at 600 W kg−1, the energy density is stabilized at 10 W h kg−1. However, the given energy density for maricite is rather low relative to that of reported metal oxide materials in aqueous solutions.38,39 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.

11. The cathode uses a faradaic reaction at the maricite cathode, and the anode uses a non-faradaic process (electric double layer

4. CONCLUSION The proposed maricite, NaMn1/3Ni1/3Co1/3PO4, in a hybrid capacitor develops an important new family of energy storage devices based on an affordable, globally available element, sodium (in a 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 F g−1 over 1000 cycles. The hybrid charge storage device is found to have a longer cycle life with a higher energy density of 15 W h kg−1 and power density of 400 W kg−1. The observed value compared reasonably well to the reported values in the literature for aqueous hybrid capacitors. The innovative science in this study involves reversible aqueous sodium electrochemistry through a faradaic process at a low temperature (against the available relatively hightemperature 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.

Figure 11. Specific capacitance versus cycle life of the hybrid capacitor AC∥maricite cycled 1000 times. Data for the 10th and 1000th cycles of the hybrid cell are shown in the inset of the figure.

with the OH− anion present in the electrolyte). The voltage window of the cell is 1.6 V, delivering a specific capacitance of 45 F g−1 and power density of 400 W kg−1 at a constant current of 0.5 A g−1. The specific capacitance obtained from the discharge data (in Figure 10) was calculated using the equation C = It/(ΔEm), where I is the discharge current, t isthe discharge time (s), ΔE is the 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 = 1/2C(ΔE)2 (W h kg−1), where C is the capacitance of the cell and PSP = ESP/t × 3600 (W kg−1). The plot of specific discharge capacitance versus cycle number of the hybrid cell is given in Figure 11. The maricite electrode is able to retain its specific capacitance during long-term cycling. The voltage profiles (shown in the inset in Figure 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 nonconventional energy sources. The Ragone plot of the hybrid cell is shown in Figure 12, calculated from the data of Figure 10. It is seen that an energy density of 15 W h kg−1 was obtained at a power density of 400

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and/or lithiumbattery@ hotmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Manickam Minakshi acknowledges the Australian Research Council (ARC). This research was supported under the 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 Grant AS123/HRIR 5428A.

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REFERENCES

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Figure 12. Ragone plot of the hybrid capacitor AC∥maricite. F

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dx.doi.org/10.1021/ef400333s | Energy Fuels XXXX, XXX, XXX−XXX