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Theoretical and Practical Approach of Soft Template Synthesis for the Preparation of MnO Supercapacitor Electrode 2

Chau Cam Hoang Tran, Jesus Santos-Peña, and Christine Damas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07131 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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The Journal of Physical Chemistry

Theoretical and Practical Approach of Soft Template Synthesis for the Preparation of MnO2 Supercapacitor Electrode Chau Cam Hoang Tran, Jesùs Santos-Peña, Christine Damas * Laboratoire de Physico-Chimie des Matériaux et des Electrolytes pour l’Energie (EA 6299), Université François Rabelais, Parc de Grandmont, F-37200, France *Corresponding authors: Christine Damas/ Chau Cam Hoang Tran [email protected], [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT.

Mesoporous manganese dioxide was successfully synthesized by alkene oxidation with permanganate in soft template medium. Four linear and three cyclic alkenes have been used as reactants to show their impact on the MnO2 particles microstructure (specially the pore volume), on their agglomeration and hence on their electrochemical performances as positive electrode materials in supercapacitors. The reaction proceeds via an intermediate product step, a manganate (V) cyclic diester, capable of producing aggregates. Aggregate macromolecular characteristics, such as intrinsic viscosity [η], molecular weight M, aggregation number N and radius R, depend on the alkene employed. The role of CTAP as phase transfer agent and as templating agent for MnO2 synthesis when associated to alkene is also confirmed. This first study allowed the proposition of a structure for the aggregates (called model B in the text). The electrochemical performances of the manganese oxides were subsequently determined with aqueous, friendly towards environment, K2SO4 electrolyte. Suitable performances, in terms of high specific capacitances (> 150 F.g-1) are provided by oxides prepared by employing bulky cyclic or long chain linear alkenes. Asymmetric devices AC||MnO2 with one of the mesoporous manganese oxide as the positive electrode, obtained from the oxidation of 1-octadecene, and activated carbon (AC) as the negative electrode yield suitable energy density of 18.2 Wh.kg-1 and a power density of 0.2 kW.kg-1 for electrochemical capacitor purposes. Finally, this particular device, cycled under galvanostatic regimes show a high capacitance retention (81.2% after 10000 cycles). Therefore, this particular soft template method is suitable for the preparation of supercapacitor electrodes.


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1. INTRODUCTION Faced with the depletion of fossil fuels and the dangers of nuclear energy and ozone layer decomposition, the need for alternative, non-polluting and risk-free energies is mandatory. In this context, many researches are moving towards electrochemical energy storage systems such as supercapacitors and batteries, both technologies associated to the development of electric vehicles. There are generally two types of supercapacitors depending on the charge storage mechanisms involved: electrochemical double layer supercapacitors (EDLCs) and pseudo-supercapacitors. EDLCs store energy electrostatically by adsorption/desorption of electrolyte ions at the electrode/electrolyte interfaces, while the operation of pseudosupercapacitors is based on rapid and reversible faradic reactions between the electrolyte ions and the surface or quasi-surface of the electrode. 1, 2 Supercapacitors can deliver 1-2 orders of magnitude higher power density than rechargeable batteries3 and store much more energy than conventional dielectric capacitors.4 In combination with a long cycle life, low maintenance cost, and safe operation, supercapacitors offer versatile solutions to various applications. Currently, they have been widely used in consumer electronics, memory back-up systems, electrical vehicles and industrial power/energy management.5 One of the major advantages is their ability to provide sufficient energy to accelerate a vehicle at its startup. Transition metal oxides and conducting polymers are typically employed as electroactive electrode materials for pseudocapacitors.6 Pseudocapacitive materials based on transition metal oxides such as RuO2, MnO2, Co3O4, NiO, ZnO, SnO2 7-12 have attracted much attention. Manganese oxides appear to be an alternative to RuO2 thanks to their relatively low cost, low toxicity and environmental safety, and theoretical high capacitances going up to 1100-1300 F.g−1.13



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Several strategies are adopted to improve the energy storage capacity of MnO2 by modifying the crystallinity

14-16

or by preparing nanostructured materials and composites

17

or hybrid

materials 12. The principle is to improve the specific surface area, the electronic conductivity, and/or the ionic conductivity of the electrode materials, as these parameters influence the charge storage process. For example, a coaxial hybrid structure formed by carbon nanotubes (CNT) core, MnO2 shell and topped with Au showed excellent electrochemical performance (68 F.g-1 of specific capacitance, 33 kW.kg-1 of power density and 4.5 Wh.kg-1 of specific energy). The CNT core ensures an efficient electronic transport to the MnO2 shell and Au top decrease the contact resistance. Other hybrid systems are nanowires consisting of MnCo2O4 (>97%) mixed with highly conducting CoO and electrode material MnO2.

12

Very large

specific capacitance of 1650 F.g−1 (at 1 A.g−1) is furnished by these nanowires. Many researches demonstrate that the supercapacitor of MnO2 strongly depends on its crystallographic structure.14-16 For instance, birnessite, cryptomellane and todorovskite polymorphs provide capacitances of 110 F.g−1,15 241 F.g−1,14 and 42 F.g−1,16 respectively. Mesoporous materials are candidates for many applications such as gas sensors, catalysts, base materials for transparent conductive oxides and optoelectronic materials. A material prepared as a mesoporous structure possesses a large surface area, which can positively affect its performance.

18, 19

Mesoporous transition metal oxides can be obtained by soft template

method. The soft agent approach involves the use of molecules capable of forming organized structures such as diblock copolymers (Polystyrene-block-poly(4-vinylpyridine),20 tri-blocks (poly (ethylene oxide)–poly (propylene oxide)–poly (ethylene oxide)),21 or surfactants (sodium

bis(2-ethylhexyl)

sulfosuccinate

AOT,22

sodium

dodecyl

sulfate

SDS,23

cetyltrimethylammonium bromide CTAB 24). This approach is much applied in the synthesis of pseudocapasitive materials such as RuOx, 25 NiO, 26 Co3O4 27 and MnO2. 24


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In the present work, mesoporous manganese dioxides MnO2 are synthesized by using a novel approach based on redox reaction between cetyltrimethylammonium permanganate (CTAP) and alkenes, which can be written as:

= + + 2 → − + +

(1)

where R=alkyl or aryl group. It proceeds through an intermediate state whose chemical structure has been previously proposed.

28, 29

Thus one of the major aims of this work is to

investigate the physico-chemical behavior of the intermediate state in the reaction medium in order to understand its impact on the characteristics of the resulting MnO2 materials. The aggregation of the intermediate is studied by viscometry and its dimensions are shown on one hand to depend on the alkene reactant, and on the other hand to impact on the resulting MnO2 morphology (pore size, specific surface area, pore volume). The latter has a dramatic effect on its electrochemical performances as electrode material in supercapacitor application. Our work provides original information of the intermediate state aggregation that implies a model where the core is filled by hydrocarbon chains of soft template or alkene, surrounded by a first layer composed of polar heads from manganate(V) cyclic intermediate and N+(CH3)3 from CTA. There also exists a second layer (or shell) containing the hydrocarbon chains of CTAP or alkene. The dimensions of this intermediate state clearly influence the pore volume of the manganese oxide obtained after its posterior hydrolysis. Among the various MnO2 materials prepared, which exhibit large specific surface area and pore diameter in the 3.1-5.5 nm range, one of them behaves as a very interesting electrode for supercapacitor purposes. Thus, it provides a capacitance of 156 F.g-1 in a three electrodes configuration, and an energy density of 13.82 Wh.kg-1 at a power of 0.27 kW.kg-1 in an activated carbon/MnO2 device for at least 10000 cycles.



2. EXPERIMENTAL AND FORMULAE SECTION 2.1. Synthesis of MnO2 Cetyltrimethylammonium permanganate CTAP was chosen as soft template precursor for preparing MnO2. CTAP was synthesized by adding KMnO4 (Sigma-Aldrich, 97%) into deionized water (0.4M) and subsequently adding it into a cetyltrimethylammonium bromide (CTAB) (Sigma-Aldrich, 95%) aqueous solution (0.4M).30 The solution thus obtained was well mixed by means of ultra-sonication. Purple gel was rapidly formed after mixing. The gel was aged overnight to enable complete reaction between KMnO4 and CTAB, and was then filtrated and washed with distilled water several times until the filtrate became colorless. CTAP was obtained after drying the solid in vacuum at 30°C for 6h. MnO2 was prepared by redox reaction between CTAP and alkene in a two-step method which was first originally described for perfluoroalkylated diols synthesis.31 Alkenes were: 1-octene (Sigma-Aldrich, 98%, so called C8), 1-undecene (Sigma-Aldrich, 97%, so called C11), 1tetradecene (Sigma-Aldrich, 97%, so called C14), 1-octadecene (Sigma-Aldrich, 95%, so called C18), allylbenzen (Sigma-Aldrich, 98%, so called AllylB), allylcyclohexane (SigmaAldrich, 96%, so called AllylC) and cyclododecene (Sigma-Aldrich, 96%, so called C12C). The reactor was a flask surmounted by a condenser and a dropping funnel to introduce 7.5 mmoles alkene dissolved in 15 mL of CH2Cl2 (Sigma-Aldrich, 99%). The flask was cooled using an ice bath. A CTAP solution is prepared by dissolving 5 mmoles CTAP in 50 mL of CH2Cl2. It is dropped through an addition funnel. The stirring is maintained at 0°C for 2 hours and then at r.t. for 14 hours. 120 mL of ethyl ether are added afterwards. The reaction mixture is finally hydrolyzed with 10 mL of water. The black precipitate MnO2 which formed is filtered, washed with water and dried at 60°C under vacuum for 5h.


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2.2. Study of the intermediate state during the synthesis of MnO2 The solutions of CTAP and alkene in dichloromethane before hydrolysis have been analyzed at 20°C by capillary viscometry using Ubbelohde type viscometers. Flow times t, are measured at least three times until repeatability. The uncertainty on flow time is ± 0.14 s. The corresponding reduced viscosities, ηred, are determined through the equation [1]: η =

=(

)

[1]

where ηsp is the specific viscosity, t and t0 are respectively the flow times of the solution and of the solvent, and C is the weight concentration of the solute. The intrinsic viscosity, [η], which corresponds to the limiting reduced viscosity at infinite dilution of the system, can be determined by application of the Huggins law (equation [2]): η = η +

! . η

.

[2]

where kH is the Huggins constant and C is the solute weight concentration. The intrinsic viscosity is related to the molecular weight of the system, M, by the empirical Mark-Houwink equation (equation [3]),

[η ] = KM a

[3]

where K and a are constants depending on temperature, solvent and solute. The radius (R) of equivalent spherical colloidal structures has been calculated from equation [4]: = #

$ /

%&'(

)

[4]



That derives from Einstein’s law of viscosity, equation [5], originally applied to dilute dispersions of spherical particles:

= 1 + 2.5 - [5]

where φ is the total volume fraction of the dispersed phase.

2.3. MnO2 materials characterization. X-ray diffraction patterns were obtained with a Phillips X’PERT MPD diffractometer equipped with a Cu Kα radiation. (λ =1.5405 Å). Diffractograms were collected over a 2θ range of 10-70° with a step size of 0.02° and a step time 0.4s. The Scherrer equation was employed to determine the crystallite domain size. The microstructures of the materials were examined by scanning electron microscopy (SEM) with a Zeiss® ULTRA Plus field effect microscope working at 5 kV. Textural properties were established from nitrogen adsorption/desorption isotherms on a MicromeriticASAP 2020 instrument using nitrogen gas as adsorbate. BET (Brunauer, Emmett, Teller) method was applied to the specific surface area calculation. Pore volume, pore size distribution and average pore size, , were obtained by applying the Non-Local Density Functional Theory (NLDFT) method to the adsorption branch of the isotherms. When micropores volume was in the technique error, we prioritized the use of the Barrett, Joyner and Halenda (BJH) method to the desorption branch of the isotherms. The size distribution of MnO2 was measured using a Zetasizer (ZSP, Malvern Instrument Ltd, Worcestershire, UK) at 25°C based on laser Doppler velocimetry and dynamic light scattering techniques.


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2.4. Preparation of MnO2 electrodes and electrochemical measurements. Electrode slurries are prepared by dispersing a mixture of MnO2 and carbon black (conductive additive) and a solution of polyvinylidene difluoride PVDF (binder) in ethanol, under stirring for 6h. Active material, conductive additive and binder are mixed in a 75/12.5/12.5 weight ratio. The slurries are rolled on a flat glass surface to obtain a homogeneous film. A thick film (about 500 µm) is obtained after drying at r.t. for 2 hours, prior to store in a desiccator. Prior to the electrochemical test, part of the film is cut for obtaining 4 mg of electrode composite, pressed between two sheets of stainless steel (area 0.35 cm2). Cyclic voltammetry (CV) was conducted on a three-electrode cell containing a MnO2-based composite electrode as working electrode, an Ag/AgCl(saturated) as reference electrode and an activated carbon electrode as counter electrode. Electrolyte consists of aqueous 0.5M K2SO4 (Sigma-Aldrich, 99%) solution. The electrolyte was bubbled by argon gas for 10 minutes to eliminate oxygen before assembly. Cyclic voltammetry is carried out with a scan rate of 5, 10, 20, 50, 100 and 200 mV.s-1 in the voltage range 0V-1V. Capacitance values obtained in the three electrodes configuration is calculated according to the formula [6]:

=

/ 01

. 23 4

[6]

where i is the intensity developed in the cell, ∆U, the potential explored, and m, the manganese oxide mass. Asymmetric devices were constructed with charge-equilibrated MnO2-based composite and activated carbon as positive and negative electrodes, respectively, after charge balancing. The cell balance was adjusted based on the capacitance of each electrode, previously determined in a three electrodes configuration. Galvanostatic cycling between 0 V and 1.7 V was performed using activated carbon as the negative and MnO2 as the positive electrodes



respectively. A current density of 0.5 A.g-1 was applied. Capacitance values were obtained by employing the formula 32: C = 4Ccell/m [7]

where Ccell is the capacitance provided by the cell and m the total mass of the electrodes. In a galvanostatic configuration, the capacitance provided by the cell is obtained by employing the formula Ccell = I ∆t/∆V [8] where I is the current applied, ∆t the time of charge crossing and ∆V, the voltage window of cycling after correcting the iR drop.

3. RESULTS AND DISCUSSION 3.1. Study of the intermediate state during the alkenes oxidation 3.1.1. Mechanism of the reaction Progress of the synthesis protocol is illustrated in two steps by Scheme 1.

Insert Scheme 1. where Q=CTA and R corresponds to the fluoroalkyl or the alkyl chain. CTAP was easily dissolved in the organic medium reaction. In fact, permanganate ion can be easily solubilized in nonaqueous solvents by the use of a variety of phase transfer agents including quaternary ammonium and phosphonium ions,33 crown ethers,34 and linear polyethers.35 Phase-transfer agents involve the transport of an anion to an organic layer by the use of an onium counterion.36 Surfactants such as tetraalkylammonium bromides, when combined with potassium permanganate, produce tetraalkylammonium permanganates, which


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act as excellent phase-transfer oxidants for organic substrates in completely non-polar organic solvents, such as benzene, CH2Cl2, CHCl3, CCl4, toluene, etc..36 Thus, it is supposed that CTA cation helps to dissolve CTAP. The 1H RMN spectra of CTAB and CTAP (not shown therein) have been realized with deuterated CH2Cl2 as solvent. Compared with CTAB, the resolution of CTAP spectrum is dramatically reduced, with broad singlets and multiplets. Assuming that the lack of resolution in the organic solvents is due to close association of the quaternary ammonium ion with the permanganate ion, it would appear that CTAP exists as a tight ion pair in CH2Cl2.37 Since the ion-pairing stability is inversely dependent upon the interionic distance between the centers of positive and negative charge, the transition state is expected to form a tighter ion pair than the ground state.36 Hence the transition state should exhibit more stability from close association with the cation than the ground state. Under phase transfer conditions, where the oxidations are carried out in nonpolar solvents, the nature of the intermediate state depends on the manner their action is developed. Alkenes are oxidized by permanganate under nonaqueous conditions to yield either cis-diols or cleavage products. The nature of the intermediate inorganic product obtained when alkenes are oxidized by quaternary ammonium permanganates, has been shown to be the manganite (V) cyclic diester (1) (see Scheme 1). 28, 29 Moreover, Lee and Perez-Benito detected autocatalysis during the reaction of methyltributylammonium permanganate with 1-tetradecene in CH2Cl2 solutions.38 However, there is some controversy about this intermediate. Some authors claim that the cyclic manganate (V) are in fact one of the various forms of colloidal manganese oxide. Other authors consider that this phase is actually formed after the decomposition of the manganate(V) intermediate.28,

39

Mainly colloidal MnO2 is observed in some cases such as

alkyne in CH2Cl2 whereas manganate (V) compound is observed in the permanganate oxidation of alcohols in CH2Cl2 for example.42 In fact, manganate(V) diesters can be reduced ACS Paragon Plus Environment


to Mn (IV), possibly by abstraction of a hydrogen atom from the solvent. Moreover the organic product (diol) may be complexed to a manganese (IV) compound (probably MnO2) and released by hydrolysis.36 Oxidation of various alkenes in different organic solvents by using cetyltrimethylammonium permanganate CTAP was earlier studied by Dash and Mishra.40,41 They found that as CTAP exhibits both hydrophobic interaction and electrostatic effect, the reactant and the substrate are brought to vicinity and the alkene oxidation is enhanced.36 Furthermore, the same authors detected a self-oxidation process of CTAP in a CHCl3 medium where CTAP exists as a tight ion pair.42 In such conditions, the permanganate ion easily abstracts a proton from the βcarbon atom of the cetyl chain, producing pentadecanal. Insert Table I. 3.1.2. Aggregation of the intermediate state In this work, we employed seven alkenes such as 1-octene (C8), 1-undecene (C11), 1tetradecene (C14), 1-octadecene (C18), allylbenzen (AllylB), allylcyclohexane (AllylC) and cyclodocecene (C12C). Table 1 collects their empirical and developed formula, as well their respective molar volumes. The side group of C8, C11, C14 or C18 is a linear alkyl chain with six to sixteen carbon atoms, whereas the side groups of AllylB and AllylC are respectively aliphatic and aromatic six-membered carbon rings linked to the vinyl group through a methylene group. The cyclodocecene structure is a twelve-membered carbon ring. From the data collected in Table 1, similar sizes (for instance 1-octene and allylcyclohexane), seem to correspond to similar molar volumes. Unlike, allylbenzene has a lower space requirement (Vm = 132.5 mL.mol-1). The different alkene group, volumes and spatial disposition should certainly influence the intermediate state nature and structure, potentially impacting on the morphology of the final manganese oxide formed.


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In a first time we have examined the aggregation of such intermediate state, whose structure is analogous to that of ionic surfactants. It is known that aggregation behavior of ionic surfactants can be quite different in aqueous and in nonaqueous media.43 Some studies reveal their ability to associate into reverse micellar structures in apolar or low polar solvents,44 depending on several factors such as the force between ion pairs, steric factors and solvent polarity. When aggregation is evidenced, it seems that the aggregation numbers are rather lower than in water, a medium where stronger cooperativity in micellization is observed. General tendencies are however difficult to establish and each surfactant/solvent system has to be examined in order to see if aggregation takes place or not. In the present work the formation of CTAP or CTAP-alkene aggregates in CH2Cl2 is examined through viscometric studies. The molecular weight of CTAP and manganite (V) intermediates (1) in CH2Cl2 before hydrolysis is estimated through intrinsic viscosities [η] determined by capillary viscometry. Intrinsic viscosity, [η], was determined by application of the Huggins law [equation 2], considering structural analogy between the systems CTAP or CTAP-alkene and polyelectrolytes in terms of macrostructural character and presence of ionic groups. Insert Figure 1. Figure 1 shows the variations of η vs. of CTAP-n-alkene (CTAP-C8 to CTAP-C18), CTAP-AllylC, CTAP-AllylB and CTAP-C12C in CH2Cl2 at 20°C. Application of the Huggins law yields values of intrinsic viscosity, [η], collected in Table 2. Such values are comprised between 4.8 mL.g-1 and 14.0 mL.g-1, which are greater than that obtained in the case of CTAP alone in CH2Cl2 (2.1±0.1 mL.g-1). Such results reveal the occurrence of macrostructures from CTAP-alkene in agreement with previous studies undertaken on permanganate oxidation of organic substrates in non or low polar solvents.43



Insert Table 2. The intrinsic viscosity is also linked with the macromolecular weight, M, by the empirical Mark-Houwink equation [equation 3]. As no previous viscometric work has been done on our systems in non-aqueous solution, an estimation of (K, a) parameters was obtained from macromolecular systems with similar characteristics. Thus, polyelectrolytes carrying ammonium groups and associated to surfactants were chosen. In particular, polyelectrolyte complexes

of

the

ultrahigh-molecular

weight

poly-(N,N-dimethyl-N-benzyl-N-

methacryloyloxyethyl)ammonium chloride (PDMBMAC) with oppositely charged surfactants (SDS) in chloroform45 and dodecylammonium 2-acrylamido-2-methylpropane sulfonate (DDA-AMPS).46 The data are gathered in Table 2, and expressed as variations of log [η] vs. log M in Figure 2. The linear relation found for log M vs log [η] validates the Mark-Houwink equation and allows us to determine K (1.64.10-3 for [η] expressed in mL.g-1) and a (0.56). On the other hand, it allows estimating the respective aggregate molecular weight and aggregation numbers of CTAP-n-alkenes and CTAP-cyclic alkenes structures in CH2Cl2. As issued from Table 2, the values of M range from 10090 g.mol-1 (N=18.1) to 23920 g.mol-1 (N=36.5) for the CTAP-Cn series, and from 8440 g.mol-1 (N=16) to 67840 g.mol-1 (N=119.1) for the CTAP-cyclic alkene series. The equivalent spherical colloidal structures radius (R) was calculated from equation [4] for the seven different systems prepared in this work. The values are collected in Table 2, as nm, and are in the 1.8-5.3 nm range. Such sizes are far lower than those reported in previous studies: around 54 -250 nm attributed to colloidal MnO2.47,48 By comparing the R and N values corresponding to cyclic alkenes with those from C8 and C11, one can see that the volumetric characteristics are quite similar. The sole exception corresponds to the system CTAP-C12C, showing a three times larger value of R and six times larger value of N.


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Results of this study reveal that the macrostructural size of intermediate CTAP-alkene species in CH2Cl2 depends on the alkene employed for the synthesis, and gives a better understanding of the CTAP role not only as phase transfer agent but also as templating agent for MnO2 synthesis when associated to alkene. In order to understand its ultimate effect on the manganese oxide properties, next section will examine their textural properties. 3.1.3. Study of the micelles disposition and influence on the textural properties of manganese oxide product of the alkenes oxidation reaction According to the scheme 1, hydrolysis of the intermediate state leads to the formation of manganese oxide. Insert Figure 3.

Figure 3a shows XRD patterns for manganese oxide synthesized by oxidation of the linear alkenes (C8, C11, C14, and C18). The weak diffraction peaks are mainly attributed to poor crystalline character. Nevertheless, peaks at 2θ = 29.69°, 36.42°, 43.7°, 55.04°, 66.04° corresponding, respectively, to the (110), (101), (111), (211) and (002) planes of the pyrolusite -type manganese oxide structure can be noticed.15, 16 For the syntheses involving the cyclic alkenes, only the most intense peaks ((101) and (102)) are present (Figure 3b), indicating a more amorphous nature for these materials. By application of the Scherrer equation, it was demonstrated that length chain of linear alkenes has no effect on the microdomains size associated to (101) and (102) planes. Instead, average values of 4.05 nm and 3.95 nm are found respectively for both planes. Unlike, planar distances are directly proportional to the size of the cyclic alkene. (101) and (102) spacings range in the interval 2.68-3.07 nm and 3.57-4.27 nm, respectively. Thus, as a first conclusion, the use of huge volume alkenes decreases the crystallite size.



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The different solids issued from the respective alkenes were examined by SEM (Figure 4). Regarding the series obtained with linear alkenes (Figure 4a-d), the particle morphology and aggregation was found to depend on the alkene used for synthesis. Thus, whereas solids MOCn (n=8, 14, 18) are solely constituted of nanoparticles with a rounded shape, MOC11 also contains stick-shaped particles, that can correspond to pyrolusite.12 There is a general trend of agglomeration of the nanoparticles, building porous structures, containing mesopores ( between 2 and 50 nm) for MOC8 and MOC11 and/or macropores ( > 50 nm) for MOC14 and MOC18. Regarding the manganese oxides obtained for cyclic alkenes (Figure 4e-g), the particles are homogeneous in shape and presumably of smaller size than those obtained in the linear alkene series. The general trend of particles agglomeration is enhanced for the cyclic alkene series, keeping mainly mesopores. However, in the MOC12C solid, micropores ( 98%). The results are consistent with what is commonly found in the literature for supercapacitor systems. 63 The best device is based on the MOC18 material that retains about 81.2% of the capacitance after 10000 cycles. The others electrodes in the MOCn series maintain only 70% of their maximum capacity. For the MOCc series, the maximum device capacitance corresponds to the MOC12C oxide which retains 73.8%. This result is unexpected since MOC12C showed larger capacitance than MOC18 in a three electrodes configuration. Further investigation needs to be carried out to understand this feature. Galvanostatic charge/discharge (GCD) curves at different current densities of AC//MnO2 devices based on MOC18 and MOC12C oxides are shown in Figure 9 (a,b). The curves exhibit a triangular and symmetric shape revealing a typical capacitive behavior

64

with high

columbic efficiency (98%-99%). As expected, specific capacitances values decrease when current density applied on the device increases from 0.05 to 0.5A.g-1. Energy and power densities extracted from galvanostatic cycling are indicated in Table 5. One can see that for a power density between 0.22 to 0.41 kW.kg-1, any device delivers an energy density in the order of 5.42 – 13.82 Wh.kg-1 after 10000 cycles. The performance of MOC12C and MOC18 are comparable or better than that of other asymmetric systems, such as graphene // MnO2/ graphene mesoporous in Na2SO4 (69% of energy retention after 10000 cycles with a potential range of 1.7 V ),

65

graphene // MnO2/ graphene in Na2SO4 (80.5%

retention after 5000 cycles with a 2V potential range). 66


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Ragone plot (energy density of the device as a function of its power density) for MOC18 and MOC12C-based devices are shown in Figure 9c. Any mesoporous MnO2 electrode prepared in this work delivers an energy density close to 20 Wh.kg-1 at a power density of 0.2 kW.kg-1. The maximum energy density is obtained for AC||MOC18 and AC||MOC12C asymmetric supercapacitor (18.2 and 19.9 Wh.kg-1 respectively). Such performance is impressive related to that of symmetrical AC||AC supercapacitors (< 10 Wh.Kg-1),67 CNTs||CNTs supercapacitors (< 10 Wh.Kg-1),68 and MnO2||MnO2 supercapacitors (< 3.3 Wh.Kg-1).69,

70

Furthermore, this value is comparable with some asymmetric MnO2-based devices found in the literature with aqueous electrolyte solutions such as AC||MnO2 (17.3 Wh.Kg-1 at 0.6 kW.kg-1),70 AC||NaMnO2 (17.5 Wh.Kg-1 at 0.25 kW.kg-1), Wh.Kg-1 at 0.28 kW.kg-1).

72

71

and AC||MnO2 microsphere (17

However, the electrochemical performance is limited to larger

powers. It is noteworthy that in the literature, only composite devices containing MnO2 in a lower content (relative to that in our electrode), and more electronic percolation components (such as graphene G, graphene oxide RGO, carbon nanofiber CNF, porous carbons PC) deliver superior performance to our devices.

73-76

However, the use of low density (0.022 -

1.91 g.cm-3) carbons additives results in decreasing the total electrode volumetric energy and power.

4. CONCLUSION The present work attempted to explore the physico-chemical characteristics of MnO2 materials and their electrochemical performances as materials for supercapacitors in relation with their structure. In recent years, MnO2 materials have attracted tremendous interest in a ACS Paragon Plus Environment


wide range of potential applications (catalysis, energy storage, sensors and biomedical area) and their preparation by various methods (co-precipitation, sol-gel, hydrothermal treatments, microwave, solid state reactions, templating method) is largely described. Despite the oxidation of alkene by permanganate is a quite easy way particularly for the synthesis of diols, none has been undertaken on the so-called by-product MnO2. Many studies focused on the reaction mechanism showed a two-step synthesis with the formation of an alkene-CTAP intermediate product. In the present study the viscometry analysis of the alkene-CTAP solutions in CH2Cl2 clearly evidenced the colloidal nature of the solutions and their templating effect on the resulting mesoporous and mainly amorphous MnO2 materials as the dimension of the alkene-CTAP aggregates is strongly linked to the pore diameter of MnO2. The use of the long chain alkene (such as C18) or bulky chain alkene (such as C12C) offers MnO2 a favorable porosity for pseudocapacitive property. Indeed, they show large pore volume and wide pore size and in the case of MOC12C, high specific surface area and micropore volume. Asymmetric ECs based on MnO2 as positive electrode and activated carbon as negative electrode were assembled and investigated in an aqueous K2SO4 solution. These ECs can be cycled reversibly in the high voltage region of 0 V - 1.7 V and exhibit an energy density up to 20 Wh.kg-1 at 0.2 kW.kg-1. Moreover, one of such asymmetric supercapacitors preserves 81.2% of its initial capacitance after 10000 cycles of galvanostatic charge–discharge. Nevertheless, our devices energy fades upon increasing powers due to their high content in manganese oxide and the lack of expensive and low density conductor carbonaceous additives. The excellent capacitive performance at low power makes the mesoporous MnO2 obtained by this technique a promising electrode material for electrochemical energy storage, the limitation at high power presumably cancelled by formulating the electrode.


Page 24 of 49

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ACKNOWLEDGEMENT CCT is indebted to the Région Centre for granting a Ph.D. doctoral fellowship. The authors acknowledge Dr. Rocio Otero Izquierdo (Instituto Universitario de Quimica Fina, University of Córdoba, Spain) for recording the Zetasizer experiments and Dr. Manuel Cruz Yusta (Departamento de Química Inorgánica, University of. Córdoba, Spain) for the XRD and BET studies. Dr. Pierre-Yvan Reinal (CHU Bretonneau, Tours) is also acknowledged for the SEM observations. This work was partially funded by an APR Initiative Académique Région Centre (France) project (Albattros Contract No. 2014-00094567).



CAPTIONS Scheme 1. Progress of the synthesis protocol Scheme 2. Cationic CTA+ micelles disposition according to model A (a), model B with alkene core (b) and model B with cetyl core (c).

Figure 1. Variation of the reduced viscosity as a function of concentration in the systems CTAP-C8, CTAP-C11, CTAP-C14, CTAP-C18, CTAP-C12C, CTAP-AllylB and CTAPAllylC. Figure 2. Relation between intrinsic viscosity [η] and molecular weight, M, found for polyelectrolyte/surfactant systems in non-polar solvents 45,46 and DDAC and CTAB in aqueous medium.77,78 Figure 3. XRD patterns collected for manganese oxides prepared in this work. Figure 4. SEM images of the manganese oxides (a) MOC8, (b) MOC11, (c) MOC14, (d) MOC18, (e) MOAllyl),(f) MOAllylB and (g) MOC12C. Figure 5. Nitrogen adsorption and desorption isotherms and pore size distributions for MOC18 and MOC12C. Figure 6. Plot of mean (alkene or cetyl core) diameter values of B model (daggr) as a function of the MnO2 material pore diameter dmax. Figure 7. CV curves of the MnO2-based electrodes in a three electrodes configuration: (a) MOC8, MOC11, MOC14 and MOC18 and (b) MOC12C, MOAllylB and MOAllylC under 5 mVs-1, (c) MOC18 and (d) MOC12C under different voltage steps. Figure 8. Long-term cycling stability of AC//MnO2 asymmetric devices for manganese oxides obtained from (a) linear alkenes, (b) cyclic alkenes, measured at 0.5 A.g-1 in the 01.7V range (the AC//MOAllylB finished only 9000 cycles). (c) Theirs coulombic efficiencies. Figure 9. Charge-discharge curves at different current densities for AC//MnO2 asymmetric device based on (a) MOC18, (b) MOC12C in the 0-1.7V range. (c) Ragone plot of asymmetric devices based on MOC18, MOC12C and other asymmetric devices in aqueous electrolyte from the literature.


Page 26 of 49

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Table 1. Chemical parameters of the alkenes employed in this work, empirical formula, expanded formula, molecular weight, density. Table 2. Molecular characteristics ([η], M, aggregation number N and aggregate radius R) of ionic macromolecular systems in nonaqueous solutions. (n.a. not available) Table 3. Textural properties of manganese oxide samples obtained in this work and CTAPalkene aggregate diameter, d, in CH2Cl2. a Mean pore diameter determined at the desorption stage.b Between brackets, pore diameter corresponding to the maximum in the dV/dD vs. dp distribution curve.c Values obtained by NLDFT method Table 4. Estimated dimensions of the intermediate CTAP-alkene aggregates in the models A and B (scheme 2), and pore diameter values of the resulting MnO2 materials. Table 5. Energetic parameters of devices with the manganese oxides prepared in this work.



Scheme 1.


Page 28 of 49

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Scheme 2.



Figure 1.

50

45 CTAP-C8

40

CTAP-C11 CTAP-C14 CTAP-C18

35 CTAP-C12C

10

30

CTAP-AllylB

-1

η red / (mL.g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 49

CTAP-AllylC

25

20 5 15 0,00

0,03

0,06

10

5 0,00

0,01

0,02

0,03

0,04

0,05

0,06 -1

Weight concentration C / (g.mL )


0,07

Page 31 of 49

Figure 2.

3,0 2,5 2,0

Log [η]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1,5 1,0 0,5 0,0 3

4

5

6

Log M


7

8


Figure 3.

(a)

(b) 110

110

101

101

002

111

002

211

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 32 of 49

10

20

30

40

50

60

2θ / ° MOC8 MOC11

70

10

20

30

40

50

60

2θ / °

MOC14 MOC18

MOC12C MOAllylB


MOAllylC

70

Page 33 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4.



Figure 5. 300

300

MOC18

MOC12C Volume / cm .g

200

3

3

Volume / cm .g

-1

250

-1

250

150 100

Adsorption Desorption

50 0 0.0

0.2

0.4

0.6

0.8

200 150 100

Adsorption Desorption

50 0 0.0

1.0

Relative Pressure (P/P0)

0.2

0.4

0.6

0.8

Pore Diameter/nm

0.08 0.14 -1

dV/dD / cm .g nm

-1

MOC12C

0.04

3

3

-1

0.06

-1

dV/dD / cm .g nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 34 of 49

0.02

MOC18

0.12 0.10 0.08 0.06 0.04 0.02

0.00 0.00 10

10

Pore Diameter/nm

Pore Diameter/nm


1.0

Page 35 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 6.



Figure 7.

200

MOC12C MOAllylB MOAllylC

-1

300

Specific capacitance / F.g

-1

Specific capacitance / F.g

(a)

MOC18 MOC14 MOC11 MOC8

300

100

0

-100

-200

200

100

0

-100

0.2

0.4

0.6

0.8

1.0

0.0

0.2

Potential vs. Ag/AgCl / V

MOC18

(c)

300

Specific capacitance / F.g-1

200

5mV.s -1 10mV.s -1 50mV.s -1 200mV.s

0.4

0.6

0.8

1.0

Potential vs. Ag/AgCl / V

400

-1

300

(b)

-200 0.0

Specific Capacitance / F.g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 36 of 49

100

0

-100

200

-1

5mV.s -1 10mV.s -1 50mV.s -1 200mV.s

(d)

MOC12C

100 0 -100 -200

-200 -300

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

Potential / V vs. Ag/AgCl

Potential / V vs. Ag/AgCl


0.8

1.0

Page 37 of 49

(a) 20

(b) Specific capacitance / F.g-1

Figure 8.


20

10 5 0

AC | MOC18 AC | MOC14 AC | MOC11 AC | MOC8

-5 -10 -15

24 20 16 12 8 0

-20 1000

2000

3000

4000

50

5000

100 150 200 250 300 350 400

6000

7000

8000

10

5 AC | MOC12C AC | MOAllylB AC | MOAllylC

0

-5

20

0

-10

9000 10000

0

1000

2000

3000

4000

5000

100

200

6000

Cycle number

Cycle number

(c) Coulombic efficiency %

0

15


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


105

AC | MOC18 AC | MOC14 AC | MOC11 AC | MOC8

98

91 105 AC | MOC12C AC | MOAllylB AC | MOAllylC

98

91 0

2000

4000

6000

8000

10000

Cycle number


300

7000

400

500

8000

600

700

9000 10000


Figure 9.

(a) 1.8

(b)1.8

-1

0.05 A.g -1 0.1 A.g -1 0.25 A.g -1 0.5 A.g

1.6 1.4

-1

0.05 A.g -1 0.1 A.g -1 0.25 A.g -1 0.5 A.g

1.6 1.4 1.2

Potentiel / V

1.2 1.0 0.8 0.6

MOC18

0.4

1.0 0.8 0.6

MOC12C

0.4

0.2

0.2

0.0

0.0 25

50

75

100

125

150

175

200

225

250

0

25

50

75

Time / mn

(c)

100

125

Time / mn 35

MnO2/ PC

MnO2 / CNT / G

MOC18

30 -1

0

Energy density /Wh.kg

Potential / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 38 of 49

MnO2/ G

25

MOC12C 20

MnO2/ RGO NaMnO2

15

MnO2 microsphere

MnO2/ CNF

10

MnO2 amorphous αMnO2/ G

5 10

100

1000

10000 -1

Power density / W.kg


150

175

200

225

Page 39 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1.

Alkene name (empirical formula)

Molecular Weight g/mol

Density g/mL

112.21

0.715

156.9

1-Undecene (C11H22)

154.29

0.75

205.8

1-Tetradecene (C14H28)

196.37

0.775

253.4

1-Octadecene (C18H36)

252.48

0.789

320.0

Allylbenzene (C9H10)

118.18

0.892

132.5

Allylcyclohexane (C9H16)

124.22

0.803

154.7

Cyclododecene (C12H22)

166.3

0.863

198.9

Expanded formula

1-Octene

Molar volume mL/mol

(C8H16)



Page 40 of 49

Table 2.

System PDMBMACSDS

Solvent / T ( °C) [η] /mL.g-1

M /kg.mol1

N

R/nm

Reference

CHCl3 / 20°C

390

26700

n.a.

n.a.

41

CHCl3 / 20°C

441

27100

n.a.

n.a.

41

DDA-AMPS

dioxane / 60°C

72

950

n.a.

n.a.

42

DDA-AMPS

CCl4 / 70°C

30

300

n.a.

n.a.

42

DDA-AMPS

p-xylene / 60°C

40

440

n.a.

n.a.

42

DDA-AMPS

CHCl3 / 70°C

60

750

n.a.

n.a.

42

DDA-AMPS

THF / 60°C

27

260

n.a.

n.a.

42

DDACl

water / 25°C

5.6

13.9

44

CTAB

water / 25°C

11.2

34

45

CTAP-AllylB

CH2Cl2 / 20°C

4.8

10.05

19.3

2.0

This work

CTAP-AllylC

CH2Cl2 / 20°C

4.35

8.44

16.0

1.8

This work

CTAP-C12C

CH2Cl2 / 20°C

14.0

67.84

119.1

5.3

This work

CTAP-C8

CH2Cl2 / 20°C

5.0

10.66

20.7

2.0

This work

CTAP-C11

CH2Cl2 / 20°C

4.8

10.09

18.1

2.0

This work

CTAP-C14

CH2Cl2 / 20°C

6.45

17.04

28.4

2.6

This work

CTAP-C18

CH2Cl2 / 20°C

7.8

23.92

36.5

3.1

This work


Page 41 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 3.

Sample MO-alkene

Side carbon number in alkene

Specific surface area / m2·g-1

Total pore volume / cm3·g-1

Micropore volume /cm3·g-1

Pore diameter averagea / nm

MOC8

6

44.0

0.084

~0

3.2 (2.5)b

d (CTAP-alkene) aggregate / nm 4.1

MOC11

9

86.0

0.150

~0

3.1 (2.9)

3.9

MOC14

12

187.0

0.242

~0

3.2 (2.7)

5.2

MOC18

16

175.0

0.384

~0

5.9 (3.5; 5.5)

6.2

MOAllylB

7

135.3

0.326

~0

5.5 (2.5)

3.9

c

3.6

c

10.6

MOAllylC MOC12C

7 10

398.1 273.9

0.384

c

0.435

c

0.041

c

0.033

c


3.7 (3.3) 5.4 (5.5)

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 49

Table 4.

Pore diameter average(a)/ nm

Model A : Polar core diameter / nm

Model B : Alkene core diameter / nm

Model B : Cetyl core diameter /nm

MnO2-C8

3.2 (2.5)b

2.0

2.3 ± 0.1

2.90 ± 0.03

MnO2-C11

3.1 (2.9)

2.0

2.4 ± 0.1

2.77 ± 0.03

MnO2-C14

3.2 (2.7)

2.3

2.9 ± 0.1

3.22 ± 0.03

MnO2-C18

5.9 (3.5; 5.5)

2.5

3.4 ± 0.1

3.50 ± 0.03

MnO2AllylB

5.5 (2.5)

2.0

2.1 ± 0.1

2.83 ± 0.03

MnO2AllylC

c

1.9

2.1 ± 0.1

2.66 ± 0.03

c

3.7

4.4 ± 0.1

5.20 ± 0.05

Sample

MnO2-C12C

3.7 (3.3) 5.4 (5.5)


Page 43 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5.

MnO2-based electrode

Capacitance (F.g-1) at 5mV.s-1

MOC8 MOC11 MOC14 MOC18 MOAllylB MOAllylC MOC12C

87.8

130.8

138.8

156.0

148.0

169.3

172.9

5.42

6.26

8.77

13.82

10.90

7.55

11.03

Power density (kW.kg-1)

0.41

0.30

0.24

0.27

0.24

0.22

0.23

Pmax (kW.kg-1)

2.12

1.54

1.22

3.63

1.43

0.91

2.10

Capacitance retention (%)

63.9

73.9

66.3

81.2

65.9

50.7

73.8

Cycle Energy density 10000 (Wh.kg-1)


43


Page 44 of 49

REFERENCES

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research 1..14 - American Chemical Society

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