- Email: [email protected]
Photosensitive carbon supercapacitor: cavitated nanoporous carbon from iodine doped β–cyclodextryn
Accepted Manuscript Photosensitive carbon supercapacitor: Cavitated nanoporous carbon from iodine doped β–cyclodextryn
I. Grygorchak, R. Shvets, I.V. Kityk, A.V. Kityk, R. Wielgosz, O. Hryhorchak, I. Shchur PII:
S1386-9477(18)31237-2
DOI:
10.1016/j.physe.2018.12.009
Reference:
PHYSE 13397
To appear in:
Physica E: Low-dimensional Systems and Nanostructures
Received Date:
15 August 2018
Accepted Date:
07 December 2018
Please cite this article as: I. Grygorchak, R. Shvets, I.V. Kityk, A.V. Kityk, R. Wielgosz, O. Hryhorchak, I. Shchur, Photosensitive carbon supercapacitor: Cavitated nanoporous carbon from iodine doped β–cyclodextryn, Physica E: Low-dimensional Systems and Nanostructures (2018), doi: 10.1016/j.physe.2018.12.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Photosensitive carbon supercapacitor: Cavitated nanoporous carbon from iodine doped β–cyclodextryn I.Grygorchak1 , R.Shvets1, I.V.Kityk2*, A.V.Kityk2, R.Wielgosz3, O. Hryhorchak4, I. Shchur5 1Lviv
Politechnic National University, Bandery 1, Lviv, Ukraine, e-mail: [email protected]. 2Faculty of Electrical engineering, Czestochowa University of Technology, Poland E-mail: [email protected] 3Company
ENERGIA OZE Ltd, Konopiska, Poland e-mail: [email protected] 4Ivan
Franko National University of Lviv (12, Dragomanov Str., Lviv 79005, Ukraine; email: [email protected]) 5 Lviv Politechnic National University, Bandery 1, Lviv, Ukraine, e-mail: [email protected]
We have discovered a giant enhancement of the supercapacity (up to 4 times) under sun illumination of electrodes fabricated from cavitated nanoporous carbon being carbonized from a iodine doped β–cyclodexryn. This effect was achieved even without a frequently used KOH treatment and is caused by enhancement of effective activated surfaces. The synthesized cavitated nanoporous carbon exhibits principally different accumulative features at the carbon/electrolyte interface. Relationship between the porous structure, electronic properties of supramolecular nanoporous carbon and enhanced supercapacitance is explored. The impedance spectroscopy measurements (Nyquist diagram) apparently suggest a capacitance mechanism for the energy accumulation. Enhancement of the supercapacity is explained by light-generated carriers in low-dimensional nano-interfaces. In relevant mechanism an increasing concentration of delocalized electrons results in unlocking of the Helmholtz layer capacity.
Keywords: Porous structures, nanoporous carbon, pseudocapacity, intercalation, Nyquist diagrams. *Corresponding author: I.V.Kityk, e-mail: [email protected]
1. INTRODUCTION Recent progress in electric vehicles and alternative energetic likewise impressive advances in nanoelectronics and spintronics have exposed the underlying fundamental contradictions between the growing demand on cutting-edge solutions and traditional technologies being still applicable for generation and accumulation of the electric energy [1,2]. Accordingly,
ACCEPTED MANUSCRIPT
increasing of the electrical storage capacity appears nowadays as a priority issue in modern electrical transport and energetic systems as well as related to it electrochemical engineering. Despite the progress achieved in this field one can still observe a huge demand on novel efficient materials for energy storage devices.
First electrochemical systems have been
designed more than 100 years ago. Over the years, major breakthroughs in research and development are evidently related with the technology of supercapacitors (1953) and batteries with Li anodes (1959) [3]. However, the performance of such devices appears to be not sufficient for a wide number of modern industrial applications [4].
Actually achieved
capacity of lithium-ionic batteries is equal about to 1/20 of the theoretically predicted one, as being estimated with respect to the reference lithium electrochemical potential, whereas their power remains relatively low. This problem may be solved applying supercapacitors (SC) [5] with capacity or pseudocapactiy mechanisms for energy storage. Successful design and development of relevant devices are based on advanced nanoporous carbon technologies [7], which combine optimized nanoporous morphology with appropriate electronic structure of carbon materials enabling thus a release of the Helmholtz capacity by the space charge capacity in the solid state. It is necessary to emphasize that there exists some critical sizes of the nanopores corresponding to a pretty fitting to the penetration of the electrolyte into the pores. The highest performing materials can be targeted by optimizing of the pore sizes. This approach to the characterization of porous carbon materials could open a new path for the design of carbon materials for supercapacitors, as testing is a relatively quick process[8].
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It should be emphasized that recently different material have been explored for this aims.In the ref. 9 was proposed all-carbon supercapacitors using an organic electrolyte. By using ZIF-derived nanoporous carbon electrodes, using 2 M NEt4BF4/PC as the electrolyte. Some promising approach was proposed for using of morphological map for monodisperse ZIF-8 crystals with five distinctive shapes in terms of the synthetic conditions [10]. ]. Very promising approach of synthesis of all-carbon layer-by-layer motif architectures by introducing 2D ordered mesoporous carbons (OMC) within the interlayer space of 2D nanomaterials was proposed in the ref. 11. Sepaarate c;lass of supercapacitors is based on metal organic framework [12] and graphene-based supercapcatiros [ 13]. . A principally new approach was demonstrated using synergy between the physicochemical properties of nanoporous carbon and conducting polyaniline polymer (carbon–PANI), leading to some new interesting electrochemical properties [ 14].
Generally electrochemical devices the energy storage may be realized by the two different pathways: (i) via capacity mechanism related with a double layer formed on the electrode/ electrolyte interface and
(ii) due to
reversible Faraday processes like ionic
adsorption originated from oxide-redox reaction processes resulting in pseudocapacity which differs from electrical double layer (EDL) capacity. Both mechanisms are important. Faraday pseudocapacity is most likely the consequence of
discrepancy between the values of the
specific capacity determined experimentally and EDL capacity evaluated theoretically, which even beyond the BDM (Bockris, Devanathan and Müller) model should not exceed the value of specific capacity ~140÷150 F/g taking into account that the specific surface area (SSA) in carbon nanomaterials never exceeds value of 3000 m2/g. For certain classes of nanoporous
ACCEPTED MANUSCRIPT
carbons with largest values of SSA and relevant electrolyte systems the differential capacity, due to increasing of the Thomas-Fermi screening length, never exceeds 15÷17 μF/cm2, whereas the electrochemical surface accessibility is about 30%. Challenging issues mentioned above stimulate a search for principally new approaches that could provide further improvement of supercapacitors characteristics, particularly increase their energy storage capacity. We present here supercapacitor fabricated on the base of cavitated nanoporous carbon being carbonized from iodine doped β–cyclodextryn. Relevant carbon electrodes exhibit the specific capacitance which may be considerably enhanced under light illumination.
2. PRINCIPAL CONCEPTION AND EXPERIMENTAL METHODS Following the reasons presented above the main way for enhancement
of the capacity
characteristic is a release of Helmholtz capacity (СН) which is limited by the capacitance of the depleted region of carbon charge (CSC) due to a growing of electron density states for delocalized charge carriers at the Fermi level, D (EF). According to [1] the capacitance of the depleted region may be presented as : 𝐶𝑆𝐶 = 𝑒0[𝜀𝑆𝐶𝜀0𝐷(𝐸𝐹)]
1/2
,
(1)
One of promising way here may be caused by a formation of the charge accumulation is formation of
architecture with the use of supramolecular layer due to its low-dimensional feature and high efficient surface . This one may leads not only to a new types of charge transfer but it can also favors a charge capacity accumulation optimized by influence of low-dimensional singularities.
The principal restraining
factor here is a fact that used today row materials applied for carbon synthesis have a structure which
can not
accumulate in the microstructure some dopants which potentially may manage the electronic features in the desirable directions.
Following these reasons
and
considering costs and ecological factors we have chosen as Fig.1 a. Molecular structure of β –cyclodextrin
dopants
β – cyclodextryn [16] (see molecular structure
shown in Fig. 1 a) . There exist intra-molecular voids able
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to implement guest components by recognition of guest chromophore
using a principle
„lock-key”. For this reason the cyclodextryn was doped by iodine atoms and three atoms have been situated into the voids.. The formation of the cavitant β–cyclodextryn with iodine has been carried out for saturated solution of iodine at ambient temperature during carbonized and activated at temperature equal to about
8 hours, which after was
840ºС during 1 hour.
The Porous morphology of the synthesized carbon structures was determined by porous-metry using porous meter
ASAP 2000 M.
Electrochemical measurements have been done using two- and three-electrode schema using Ag electrodes as reference.
The studied materials with bonding agent
(5 %
polivinylidene fluoride (PVF)) was deposited at the pressure on the Ni grid with surface about
0,5 cm2. The active materials mass did not exceed 3 mg. As an electrolyte was used
30% solvant of КОН. The electrode potentials
Е have been evaluated with respect to
reference hydrogen electrode. The impedance spectroscopy measurements have been carried out applying ac field along the crysatallographic C direction (perpendicularly to nanopowders planes) in the frequency range 10-3 ÷ 106 Hz using measurement set-ups “AUTOLAB” and “ECO CHEMIE” (The Netherlands), supplied by computer programs
FRA-2 and GPES. The
amplitude of the measured signals was equal to about 5*10-3 V. Elimination of the unreliable points has been done using Dirichle filter. The spectra dependences of the complex Z have been analyzed using graphical analytical method by program package ZView 2.3 (Scribner Associates). The errors of the fitting did not exceed 4 %. It was demonstrated a high stability of the super capacitors with respect to laser treatment which changed the capacity not more than 2 %. Additionally the titled supercapacitors have shown a small deviation of the capacity in time, and a huge stability to external environment.
The correctness of the impedance
models based on the experimental data was confirmed by the completely random nature of the frequency dependences of the residual first-order differences. RESULTS and DISCUSSION The Fig.1 b. presents electronic SEM microscopy morphology pictures of nanoporous carbon which was synthesized by activation carbonization of β–cyclodextryn.
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Fig. 1 b. Electron microscopy picture of studied samples, synthesized by activation carbonization of Jodie moped β–cyclodextryn Following the presented results one can see almost completely pure carbon phase of the obtained product. The optometry data treated by ВЕТ and DFT methods indicate about the bimodal porous structure of carbonisat with the maxima of porous distribution corresponding to diameter equal to about 1,3 nm and 4,3 nm. The total active surface is equal to about 72 m2/g. After
КОН- modification this value is increased at least
one order. This value
increases more one order during enhancement of mesoporous volume. Very crucial here is also a fact that ultrasound treatment favors significant (up to 4 orders) enhancement of the efficient surfaces. So it is more efficient with respect to КОН- extended porous efficient surfaces. It was interesting that the role of the carbon in supramolecular form
(Fig.2) is
prevailingly caused by porous morphology. The presence of the intrinsic defects for the such morphology may leads to a striking asymmetric maximum in the electronic DOS near the Fermi energy level. It is very crucial that the value and spectral shift of the DOS maximum should be very huge. Following the performed experimental data it was established that for cyclodextryn the optimal technological regimes correspond to the to about 60 min. at temperatures 840 ± 50С. The achieved carbon is maximal and its value is equal to
treatment duration equal
specific capacity for pure
50 F/g for symmetric system. After KOH
modification it is enhanced for this architecture up to from
β –
158 F/g, and totally it is enhanced
101 up to 203 F/g. The total active surface determined by DFT method is equal to
ACCEPTED MANUSCRIPT
532 m2/g. Comparing with experimental specific capacity we have established that the differential capacity of the synthesized nanoporous carbon is equal to about
~ 38 μF/cm2.
Following the valuable references [18] this magnitudes are at least two times higher with respect to other types of activated carbon applied in supercapacitors (SC). The main physical reasons here is determined by high density of states near the Fermi level which favors screening of adsorbed charges forming the mentioned above doubled electrical layer. In the Fig. 3 are presented galvanostatic cycles”charge-discharge” of the synthesized cavitant nanoporous carbon doped by β–cyclodextryn +iodine after КОН – modification. 1,0
80
0,8
-Z'', Ohm
60
U, V
0,6 0,4 0,2 0,0
40 20 0
0
2000
4000
t, s
6000
8000
0
10000
5 10 15 20 25 30 35 40 45 50 Z', Ohm
а b Fig. 3. Cyclovoltammetry measurements for nanoporous carbon with β–cyclodextryn +iodine and Nyquist diagram (b) One can see a huge (almost one hundred percent) Coulomb efficiency at specific capacity equal to about from
162F/g, which is equal at least
60% with respect to commercial material
«Norit». Following the Nyquiste diagram one can see a prevailing capacity
mechanisms for energy accumulation , which may be a good modeled by equivalent electrical circuit de Levi, which is modified in accordance with Vojt model by sequential joining of parallel
RSCCSC – links. Here RSC and CSC correspond to resistance and capacity. The results
of computer parametrical identifications allow to monitor the variation of the DOS energy positions near the Fermi level, caused by nanoporous modification
(see insert to Fig. 3b).
However, the behavior of the cavitant nanocarbons have been appeared to be unexpected without KOH modification.
For this case the system carbon nanocomposite
CNC||30%КОН(Н2О)||CNC becomes strongly photosensitive: the specific capacity under the light treatment becomes more than 4 times stronger.
(see Fig. (4).
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It is well known [3] that one of the principal parameters characterizing the super capacitors is Columbic efficiency determining as a ratio of charge during the discharging to the charging. Following the cycles presented in the Fig. 4 this parameter exceed 90 % which is excellent parameters during the exploitation. Moreover, it was established that after 10000 cycles of “charge-discharge” in dark the decrease of the capacities was changed within the 4 %...7%. which confirms a high stability of the cavitant carbon both in dark as well during the illumination. It confirms its high purity.
1,0
6000
1
5000
-Z'', Ohm
0,8
U, V
0,6 0,4
2
1
4000 3000 2000
2
1000
0,2
0
0,0 2200
t, s
2400
2600
0
1000
2000
3000
4000
5000
6000
Z', Ohm
а b Fig. 4. Cyclovoltametry for the synthesized cavitant nanoporous carbon β–cyclodextryn (а) and Nyquist diagram (b), for the dark measurements (1) and after sun light illumination(2) This specific sun stimulated supercapacitor is determined by photo carriers
kinetics (see Fig.
4 b) with the proper diffusion control. The principal mechanisms of the observed giant enhancement of the capacity during illumination may be explained by an oversimplified simple model. It is well-known that the capacity of the SC is determined by a capacity of released Helmholtz layer. However, it is very important also that on the border separating carbon-electrolyte there occurs some space deformation of the electronic structure for low-dimensional (mainly 2D) carbon network. It is similar to the flattering of the sub-bands for the low-dimensional structures [17] For the two-dimensional structures there occurs possibility of occurrence of the sharp DOS peaks similar to van Hove singularities. A relation between electric field on the carbon surface and
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energy shift of the band energy gap for the case of absence the electrical current may be described by an equation :
(1) where T – means temperature ; k – Boltzmann constant, ε – dielectric permittivity of the given carbon grid, u – the energy Shift of the bottom of conduction band
,
-
concentration of conductive electrons intra the forbidden energy gap where the carbon is neutral. Lets introduce some indication:
(2)
In this case Afterwards the electric strength El may be presented as a capacity , created by efficient capacity :
(3) Briefly one can write: (4) One can see that the capacity
Cv following the equations (3)-(4) may be introduced
phenomenologically and its microscopical strong quantum calculations require exact information about the electronic structure of nanoporous carbon, its structure impurities etc. Particularly during the doping this mechanism is caused by the localization of the charges on
ACCEPTED MANUSCRIPT
the dopants and formation of the dc-internal field as well as the occurrence of additional surface level:
(5) Finally one can write:
(6) It is evident that the potential of the nanoporous carbon will be determined by an energy shift of the conduction band bottom. The efficiency of this process
will be
proportional to the magnitude of conduction band energy shift, which is determined by a specific surface. It is caused by a fact that the localized surface states will be more important with respect to the
bulk state contribution.
proportional to the ratio
As a consequence the capacity will be
q/u.
Important here is the case of the relatively low values of accumulated charge
q and
we restrict our consideration to the first two terms.i.e. :
(7)
For the case
, we deal with completely sequential contact of
capacitors Cl and Cv. An influence of this shunting of Helmholtz capacity can be taken into account using following the equation:
(8) One can see that following the eq. (8) the first term completely reproduce the previous result . At the same time at lower values Cv may contribute significantly more. That describes the
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first term. However, for this reason it is necessary
to fit the corresponding parameters.
These parameters are determined by the charge because for the higher amount of accumulated charge the shunting ability of Cv is decreased assuming that the capacity Cv is slightly dependent on the charge. This factor is in a pretty good confirmed by photoillumination of the nanoporous carbons with different amount of the accumulated charge. Generally, it means that the illumination generates free carriers which in turn define concentrations of the delocalized electrons and released Helmholtz layer capacities.
CONCLUSIONS We have shown a giant enhancement of the capacity during illumination of the β– cyclodextryn doped cavitated nanoporous carbon under sun light up to 4 times. This was achieved even without KOH treatment and may be caused even by enhancement
of solid
state contribution. The synthesized cavitated carbon possesses principally
different
accumulative features on the border with 30 % aqueous KOH solution.
The optimal
carbonization has been durated 60 min. at fixed temperature 840 ± 50С. These parameters allow to achieve the specific capacities equal to 50 F/g. It is caused by an enhancement of partial concentration of mesoporous area and in turn it favors an increase of the porous materials volume . The doping by β–cyclodextryn favors additional formation of the cavitant after the activation of the carbonization. AKNOWLEDGEMENT The work was supported by a Project WND RPSL.01.02.00-24-0670/16-002
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of
ACCEPTED MANUSCRIPT >Four times increase of supercapacity cavitated nanoporous carbon electrodes is discovered. >The effect is achieved after sun illumination. > This is explained by explained by light-generated carriers in low-dimensional nano-interfaces > The doping β–cyclodextryn favors additional formation of the cavitant after the activation
I. Grygorchak, R. Shvets, I.V. Kityk, A.V. Kityk, R. Wielgosz, O. Hryhorchak, I. Shchur PII:
S1386-9477(18)31237-2
DOI:
10.1016/j.physe.2018.12.009
Reference:
PHYSE 13397
To appear in:
Physica E: Low-dimensional Systems and Nanostructures
Received Date:
15 August 2018
Accepted Date:
07 December 2018
Please cite this article as: I. Grygorchak, R. Shvets, I.V. Kityk, A.V. Kityk, R. Wielgosz, O. Hryhorchak, I. Shchur, Photosensitive carbon supercapacitor: Cavitated nanoporous carbon from iodine doped β–cyclodextryn, Physica E: Low-dimensional Systems and Nanostructures (2018), doi: 10.1016/j.physe.2018.12.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Photosensitive carbon supercapacitor: Cavitated nanoporous carbon from iodine doped β–cyclodextryn I.Grygorchak1 , R.Shvets1, I.V.Kityk2*, A.V.Kityk2, R.Wielgosz3, O. Hryhorchak4, I. Shchur5 1Lviv
Politechnic National University, Bandery 1, Lviv, Ukraine, e-mail: [email protected]. 2Faculty of Electrical engineering, Czestochowa University of Technology, Poland E-mail: [email protected] 3Company
ENERGIA OZE Ltd, Konopiska, Poland e-mail: [email protected] 4Ivan
Franko National University of Lviv (12, Dragomanov Str., Lviv 79005, Ukraine; email: [email protected]) 5 Lviv Politechnic National University, Bandery 1, Lviv, Ukraine, e-mail: [email protected]
We have discovered a giant enhancement of the supercapacity (up to 4 times) under sun illumination of electrodes fabricated from cavitated nanoporous carbon being carbonized from a iodine doped β–cyclodexryn. This effect was achieved even without a frequently used KOH treatment and is caused by enhancement of effective activated surfaces. The synthesized cavitated nanoporous carbon exhibits principally different accumulative features at the carbon/electrolyte interface. Relationship between the porous structure, electronic properties of supramolecular nanoporous carbon and enhanced supercapacitance is explored. The impedance spectroscopy measurements (Nyquist diagram) apparently suggest a capacitance mechanism for the energy accumulation. Enhancement of the supercapacity is explained by light-generated carriers in low-dimensional nano-interfaces. In relevant mechanism an increasing concentration of delocalized electrons results in unlocking of the Helmholtz layer capacity.
Keywords: Porous structures, nanoporous carbon, pseudocapacity, intercalation, Nyquist diagrams. *Corresponding author: I.V.Kityk, e-mail: [email protected]
1. INTRODUCTION Recent progress in electric vehicles and alternative energetic likewise impressive advances in nanoelectronics and spintronics have exposed the underlying fundamental contradictions between the growing demand on cutting-edge solutions and traditional technologies being still applicable for generation and accumulation of the electric energy [1,2]. Accordingly,
ACCEPTED MANUSCRIPT
increasing of the electrical storage capacity appears nowadays as a priority issue in modern electrical transport and energetic systems as well as related to it electrochemical engineering. Despite the progress achieved in this field one can still observe a huge demand on novel efficient materials for energy storage devices.
First electrochemical systems have been
designed more than 100 years ago. Over the years, major breakthroughs in research and development are evidently related with the technology of supercapacitors (1953) and batteries with Li anodes (1959) [3]. However, the performance of such devices appears to be not sufficient for a wide number of modern industrial applications [4].
Actually achieved
capacity of lithium-ionic batteries is equal about to 1/20 of the theoretically predicted one, as being estimated with respect to the reference lithium electrochemical potential, whereas their power remains relatively low. This problem may be solved applying supercapacitors (SC) [5] with capacity or pseudocapactiy mechanisms for energy storage. Successful design and development of relevant devices are based on advanced nanoporous carbon technologies [7], which combine optimized nanoporous morphology with appropriate electronic structure of carbon materials enabling thus a release of the Helmholtz capacity by the space charge capacity in the solid state. It is necessary to emphasize that there exists some critical sizes of the nanopores corresponding to a pretty fitting to the penetration of the electrolyte into the pores. The highest performing materials can be targeted by optimizing of the pore sizes. This approach to the characterization of porous carbon materials could open a new path for the design of carbon materials for supercapacitors, as testing is a relatively quick process[8].
ACCEPTED MANUSCRIPT
It should be emphasized that recently different material have been explored for this aims.In the ref. 9 was proposed all-carbon supercapacitors using an organic electrolyte. By using ZIF-derived nanoporous carbon electrodes, using 2 M NEt4BF4/PC as the electrolyte. Some promising approach was proposed for using of morphological map for monodisperse ZIF-8 crystals with five distinctive shapes in terms of the synthetic conditions [10]. ]. Very promising approach of synthesis of all-carbon layer-by-layer motif architectures by introducing 2D ordered mesoporous carbons (OMC) within the interlayer space of 2D nanomaterials was proposed in the ref. 11. Sepaarate c;lass of supercapacitors is based on metal organic framework [12] and graphene-based supercapcatiros [ 13]. . A principally new approach was demonstrated using synergy between the physicochemical properties of nanoporous carbon and conducting polyaniline polymer (carbon–PANI), leading to some new interesting electrochemical properties [ 14].
Generally electrochemical devices the energy storage may be realized by the two different pathways: (i) via capacity mechanism related with a double layer formed on the electrode/ electrolyte interface and
(ii) due to
reversible Faraday processes like ionic
adsorption originated from oxide-redox reaction processes resulting in pseudocapacity which differs from electrical double layer (EDL) capacity. Both mechanisms are important. Faraday pseudocapacity is most likely the consequence of
discrepancy between the values of the
specific capacity determined experimentally and EDL capacity evaluated theoretically, which even beyond the BDM (Bockris, Devanathan and Müller) model should not exceed the value of specific capacity ~140÷150 F/g taking into account that the specific surface area (SSA) in carbon nanomaterials never exceeds value of 3000 m2/g. For certain classes of nanoporous
ACCEPTED MANUSCRIPT
carbons with largest values of SSA and relevant electrolyte systems the differential capacity, due to increasing of the Thomas-Fermi screening length, never exceeds 15÷17 μF/cm2, whereas the electrochemical surface accessibility is about 30%. Challenging issues mentioned above stimulate a search for principally new approaches that could provide further improvement of supercapacitors characteristics, particularly increase their energy storage capacity. We present here supercapacitor fabricated on the base of cavitated nanoporous carbon being carbonized from iodine doped β–cyclodextryn. Relevant carbon electrodes exhibit the specific capacitance which may be considerably enhanced under light illumination.
2. PRINCIPAL CONCEPTION AND EXPERIMENTAL METHODS Following the reasons presented above the main way for enhancement
of the capacity
characteristic is a release of Helmholtz capacity (СН) which is limited by the capacitance of the depleted region of carbon charge (CSC) due to a growing of electron density states for delocalized charge carriers at the Fermi level, D (EF). According to [1] the capacitance of the depleted region may be presented as : 𝐶𝑆𝐶 = 𝑒0[𝜀𝑆𝐶𝜀0𝐷(𝐸𝐹)]
1/2
,
(1)
One of promising way here may be caused by a formation of the charge accumulation is formation of
architecture with the use of supramolecular layer due to its low-dimensional feature and high efficient surface . This one may leads not only to a new types of charge transfer but it can also favors a charge capacity accumulation optimized by influence of low-dimensional singularities.
The principal restraining
factor here is a fact that used today row materials applied for carbon synthesis have a structure which
can not
accumulate in the microstructure some dopants which potentially may manage the electronic features in the desirable directions.
Following these reasons
and
considering costs and ecological factors we have chosen as Fig.1 a. Molecular structure of β –cyclodextrin
dopants
β – cyclodextryn [16] (see molecular structure
shown in Fig. 1 a) . There exist intra-molecular voids able
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to implement guest components by recognition of guest chromophore
using a principle
„lock-key”. For this reason the cyclodextryn was doped by iodine atoms and three atoms have been situated into the voids.. The formation of the cavitant β–cyclodextryn with iodine has been carried out for saturated solution of iodine at ambient temperature during carbonized and activated at temperature equal to about
8 hours, which after was
840ºС during 1 hour.
The Porous morphology of the synthesized carbon structures was determined by porous-metry using porous meter
ASAP 2000 M.
Electrochemical measurements have been done using two- and three-electrode schema using Ag electrodes as reference.
The studied materials with bonding agent
(5 %
polivinylidene fluoride (PVF)) was deposited at the pressure on the Ni grid with surface about
0,5 cm2. The active materials mass did not exceed 3 mg. As an electrolyte was used
30% solvant of КОН. The electrode potentials
Е have been evaluated with respect to
reference hydrogen electrode. The impedance spectroscopy measurements have been carried out applying ac field along the crysatallographic C direction (perpendicularly to nanopowders planes) in the frequency range 10-3 ÷ 106 Hz using measurement set-ups “AUTOLAB” and “ECO CHEMIE” (The Netherlands), supplied by computer programs
FRA-2 and GPES. The
amplitude of the measured signals was equal to about 5*10-3 V. Elimination of the unreliable points has been done using Dirichle filter. The spectra dependences of the complex Z have been analyzed using graphical analytical method by program package ZView 2.3 (Scribner Associates). The errors of the fitting did not exceed 4 %. It was demonstrated a high stability of the super capacitors with respect to laser treatment which changed the capacity not more than 2 %. Additionally the titled supercapacitors have shown a small deviation of the capacity in time, and a huge stability to external environment.
The correctness of the impedance
models based on the experimental data was confirmed by the completely random nature of the frequency dependences of the residual first-order differences. RESULTS and DISCUSSION The Fig.1 b. presents electronic SEM microscopy morphology pictures of nanoporous carbon which was synthesized by activation carbonization of β–cyclodextryn.
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Fig. 1 b. Electron microscopy picture of studied samples, synthesized by activation carbonization of Jodie moped β–cyclodextryn Following the presented results one can see almost completely pure carbon phase of the obtained product. The optometry data treated by ВЕТ and DFT methods indicate about the bimodal porous structure of carbonisat with the maxima of porous distribution corresponding to diameter equal to about 1,3 nm and 4,3 nm. The total active surface is equal to about 72 m2/g. After
КОН- modification this value is increased at least
one order. This value
increases more one order during enhancement of mesoporous volume. Very crucial here is also a fact that ultrasound treatment favors significant (up to 4 orders) enhancement of the efficient surfaces. So it is more efficient with respect to КОН- extended porous efficient surfaces. It was interesting that the role of the carbon in supramolecular form
(Fig.2) is
prevailingly caused by porous morphology. The presence of the intrinsic defects for the such morphology may leads to a striking asymmetric maximum in the electronic DOS near the Fermi energy level. It is very crucial that the value and spectral shift of the DOS maximum should be very huge. Following the performed experimental data it was established that for cyclodextryn the optimal technological regimes correspond to the to about 60 min. at temperatures 840 ± 50С. The achieved carbon is maximal and its value is equal to
treatment duration equal
specific capacity for pure
50 F/g for symmetric system. After KOH
modification it is enhanced for this architecture up to from
β –
158 F/g, and totally it is enhanced
101 up to 203 F/g. The total active surface determined by DFT method is equal to
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532 m2/g. Comparing with experimental specific capacity we have established that the differential capacity of the synthesized nanoporous carbon is equal to about
~ 38 μF/cm2.
Following the valuable references [18] this magnitudes are at least two times higher with respect to other types of activated carbon applied in supercapacitors (SC). The main physical reasons here is determined by high density of states near the Fermi level which favors screening of adsorbed charges forming the mentioned above doubled electrical layer. In the Fig. 3 are presented galvanostatic cycles”charge-discharge” of the synthesized cavitant nanoporous carbon doped by β–cyclodextryn +iodine after КОН – modification. 1,0
80
0,8
-Z'', Ohm
60
U, V
0,6 0,4 0,2 0,0
40 20 0
0
2000
4000
t, s
6000
8000
0
10000
5 10 15 20 25 30 35 40 45 50 Z', Ohm
а b Fig. 3. Cyclovoltammetry measurements for nanoporous carbon with β–cyclodextryn +iodine and Nyquist diagram (b) One can see a huge (almost one hundred percent) Coulomb efficiency at specific capacity equal to about from
162F/g, which is equal at least
60% with respect to commercial material
«Norit». Following the Nyquiste diagram one can see a prevailing capacity
mechanisms for energy accumulation , which may be a good modeled by equivalent electrical circuit de Levi, which is modified in accordance with Vojt model by sequential joining of parallel
RSCCSC – links. Here RSC and CSC correspond to resistance and capacity. The results
of computer parametrical identifications allow to monitor the variation of the DOS energy positions near the Fermi level, caused by nanoporous modification
(see insert to Fig. 3b).
However, the behavior of the cavitant nanocarbons have been appeared to be unexpected without KOH modification.
For this case the system carbon nanocomposite
CNC||30%КОН(Н2О)||CNC becomes strongly photosensitive: the specific capacity under the light treatment becomes more than 4 times stronger.
(see Fig. (4).
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It is well known [3] that one of the principal parameters characterizing the super capacitors is Columbic efficiency determining as a ratio of charge during the discharging to the charging. Following the cycles presented in the Fig. 4 this parameter exceed 90 % which is excellent parameters during the exploitation. Moreover, it was established that after 10000 cycles of “charge-discharge” in dark the decrease of the capacities was changed within the 4 %...7%. which confirms a high stability of the cavitant carbon both in dark as well during the illumination. It confirms its high purity.
1,0
6000
1
5000
-Z'', Ohm
0,8
U, V
0,6 0,4
2
1
4000 3000 2000
2
1000
0,2
0
0,0 2200
t, s
2400
2600
0
1000
2000
3000
4000
5000
6000
Z', Ohm
а b Fig. 4. Cyclovoltametry for the synthesized cavitant nanoporous carbon β–cyclodextryn
kinetics (see Fig.
4 b) with the proper diffusion control. The principal mechanisms of the observed giant enhancement of the capacity during illumination may be explained by an oversimplified simple model. It is well-known that the capacity of the SC is determined by a capacity of released Helmholtz layer. However, it is very important also that on the border separating carbon-electrolyte there occurs some space deformation of the electronic structure for low-dimensional (mainly 2D) carbon network. It is similar to the flattering of the sub-bands for the low-dimensional structures [17] For the two-dimensional structures there occurs possibility of occurrence of the sharp DOS peaks similar to van Hove singularities. A relation between electric field on the carbon surface and
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energy shift of the band energy gap for the case of absence the electrical current may be described by an equation :
(1) where T – means temperature ; k – Boltzmann constant, ε – dielectric permittivity of the given carbon grid, u – the energy Shift of the bottom of conduction band
,
-
concentration of conductive electrons intra the forbidden energy gap where the carbon is neutral. Lets introduce some indication:
(2)
In this case Afterwards the electric strength El may be presented as a capacity , created by efficient capacity :
(3) Briefly one can write: (4) One can see that the capacity
Cv following the equations (3)-(4) may be introduced
phenomenologically and its microscopical strong quantum calculations require exact information about the electronic structure of nanoporous carbon, its structure impurities etc. Particularly during the doping this mechanism is caused by the localization of the charges on
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the dopants and formation of the dc-internal field as well as the occurrence of additional surface level:
(5) Finally one can write:
(6) It is evident that the potential of the nanoporous carbon will be determined by an energy shift of the conduction band bottom. The efficiency of this process
will be
proportional to the magnitude of conduction band energy shift, which is determined by a specific surface. It is caused by a fact that the localized surface states will be more important with respect to the
bulk state contribution.
proportional to the ratio
As a consequence the capacity will be
q/u.
Important here is the case of the relatively low values of accumulated charge
q and
we restrict our consideration to the first two terms.i.e. :
(7)
For the case
, we deal with completely sequential contact of
capacitors Cl and Cv. An influence of this shunting of Helmholtz capacity can be taken into account using following the equation:
(8) One can see that following the eq. (8) the first term completely reproduce the previous result . At the same time at lower values Cv may contribute significantly more. That describes the
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first term. However, for this reason it is necessary
to fit the corresponding parameters.
These parameters are determined by the charge because for the higher amount of accumulated charge the shunting ability of Cv is decreased assuming that the capacity Cv is slightly dependent on the charge. This factor is in a pretty good confirmed by photoillumination of the nanoporous carbons with different amount of the accumulated charge. Generally, it means that the illumination generates free carriers which in turn define concentrations of the delocalized electrons and released Helmholtz layer capacities.
CONCLUSIONS We have shown a giant enhancement of the capacity during illumination of the β– cyclodextryn doped cavitated nanoporous carbon under sun light up to 4 times. This was achieved even without KOH treatment and may be caused even by enhancement
of solid
state contribution. The synthesized cavitated carbon possesses principally
different
accumulative features on the border with 30 % aqueous KOH solution.
The optimal
carbonization has been durated 60 min. at fixed temperature 840 ± 50С. These parameters allow to achieve the specific capacities equal to 50 F/g. It is caused by an enhancement of partial concentration of mesoporous area and in turn it favors an increase of the porous materials volume . The doping by β–cyclodextryn favors additional formation of the cavitant after the activation of the carbonization. AKNOWLEDGEMENT The work was supported by a Project WND RPSL.01.02.00-24-0670/16-002
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ACCEPTED MANUSCRIPT >Four times increase of supercapacity cavitated nanoporous carbon electrodes is discovered. >The effect is achieved after sun illumination. > This is explained by explained by light-generated carriers in low-dimensional nano-interfaces > The doping β–cyclodextryn favors additional formation of the cavitant after the activation