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Electrochemical and ion transport characterisation of a nanoporous carbon derived from SiC
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In this doctoral project, a relatively new form of carbon material, with unique narrow pore size distribution around 7 Å and with uniform structure, has been electrochemically characterised using the single particle microelectrode technique. The carbon has been used as electrode material for supercapacitors. This type of capacitors is used as high power energy buffers in hybrid vehicles and for stationary power backup. The principle for the microelectrode technique consists of connecting a carbon particle with a carbon fibre by means of a micromanipulator. The single particle and carbon fibre together form a microelectrode. Combination of this technique with electroanalytical methods such as cyclic voltammetry and potential step measurements allows for the survey of electrochemical phenomena and for the determination of ion transport parameters inside the nanopores.

A mathematical model based on Fick’s second law, for diffusion of ions inside the nanopores at non steady state, was used for the determination of effective diffusion coefficients (Deff). The coefficients were calculated from an asymptotic solution of Fick’s equation, applied for a thin layer adjacent to the external surface of the carbon particles and valid for the current response in a short time region. Another asymptotic solution was obtained, using spherical geometry and valid for the current response in a long time region.

In this doctoral work, the carbon particles have been exposed to potential cycling, which mimics that of large electrodes during operation of a double layer capacitor. The potential-current response, E-I, for the nanoporous carbon, shows a pure capacitive behaviour between –0.5 V and 0.1 V vs. the Hg|HgO reference electrode. The detection of the faradaic processes beyond these potentials was possible by lowering of the voltammometric sweep rate. The electrochemical processes occurring at positive and at negative potential were investigated separately.

Cyclic voltammometric measurements showed that the chemisorption of hydroxyl groups, occurring between 0.1 and 0.3 V, leads to a mild oxidation of the carbon structure, resulting in surface groups containing an oxygen atom at a specific carbon site (e.g., phenolic or quinine type). These oxygen-containing surface groups caused an increase of the specific capacitance, which remained constant throughout a number of voltammometric cycles. The Deff decreased on the other hand with the number of cycles. The Deff decreases also with the positive potential. The evaluation of Deff indicates adsorption of hydroxyl groups and an increase of the effective tortuosity of the pore system.

The oxidation of the carbon particles, between 0 and 0.5 V, leads to more extensive oxidation and to surface groups containing two oxygen atoms at a single carbon site, followed by formation of carbonate ions. The oxygen-containing surface groups and carbonate ions formed at these potentials do not contribute to the specific capacitance and drastically retard or obstruct the ion transport inside the nanopores.

At negative potentials the carbon particles show a dominantly capacitive behaviour. The faradaic processes taking place below –0.5 V vs. Hg|HgO reference electrode are generation and adsorption of hydrogen. These processes do not perturb significantly the electrochemical and ion transport properties of the nanoporous carbon particles. It was found that hydrogen generation occurs at –0.5 V vs. Hg|HgO and that two hydrogen oxidation processes take place at positive potentials. The results indicate that the weakly adsorbed hydrogen undergoes oxidation between 0 and 0.1 V and that the strongly adsorbed hydrogen is oxidised at more positive potentials.

The single particle technique was adapted for the determination of diffusion coefficients of an organic electrolyte. The different size of the anions and cations caused different transport characteristics at negative and positive potentials. Slow cycling was found important for ion penetration inside the nanopores and for the evaluation of the effective diffusion coefficients.

The effective diffusion coefficients for the nanoporous carbon using aqueous 6M KOH and 0.1M TEABF4 in acetonitrile were estimated to 1.4 (±0.8).10-9 cm2 s-1 and 1.3 (±0.4) 10-8 cm2 s-1, respectively.

Place, publisher, year, edition, pages
2005.
Series
Trita-KET, ISSN 1104-3466 ; 212
Keyword [en]
Chemical engineering, Cottrell equation, radial diffusion, chronoamperometry, potential step measurements, microelectrode technique, nanoporous carbon
Keyword [sv]
Kemiteknik
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-173ISBN: 91-7178-010-6 (print)OAI: oai:DiVA.org:kth-173DiVA: diva2:7694
Public defence
2005-04-22, Kollegiesalen, KTH, Valhallavägen 79, Stockholm, 10:00
Opponent
Supervisors
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2012-03-22
List of papers
1. Characterization of ion transport in a nanoporous carbon for double layer capacitors
Open this publication in new window or tab >>Characterization of ion transport in a nanoporous carbon for double layer capacitors
2000 (English)In: Proceedings of Advances in Science and technology 29: (Mass and Charge Transport in Inorganic Materials: Fundamentals to devices, part A), 2000, 439-446 p.Conference paper, Published paper (Refereed)
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5039 (URN)
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2011-09-20Bibliographically approved
2. Determination of the effective diffusion coefficient of nanoporous carbon by means of a single particle microelectrode technique
Open this publication in new window or tab >>Determination of the effective diffusion coefficient of nanoporous carbon by means of a single particle microelectrode technique
2003 (English)In: Journal of Electroanalytical Chemistry, ISSN 0022-0728, E-ISSN 1873-2569, Vol. 549, 101-108 p.Article in journal (Refereed) Published
Abstract [en]

A single particle microelectrode technique with a micromanipulator was applied and adapted for characterisation of mass transport properties of ionic species in a high surface area nanoporous carbon, with uniform pore size of 8 Angstrom. The effective diffusivity of 6 M KOH in this material was determined by means of potential step experiments on nanoporous carbon particles of different sizes. The results were analysed for short times (Cottrell model) and for long times (spherical diffusion model). The average effective diffusion coefficient for short and long times was 1.5x10(-9) and 1.2x10(-9) cm(2) s(-1), respectively. The relatively small diffusivity values are discussed in terms of interaction between the ion hydration shell and water molecules adsorbed on the pore wall.

Keyword
cottrell equation, chronoamperometry, microelectrode, double layer, nanoporous carbon, diffusion coefficient
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-5040 (URN)10.1016/S0022-0728(03)00246-8 (DOI)000183792100010 ()
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
3. Effects of pore surface oxidation on electrochemical and mass-transport properties of nanoporous carbon
Open this publication in new window or tab >>Effects of pore surface oxidation on electrochemical and mass-transport properties of nanoporous carbon
2005 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 152, no 2, A270-A276 p.Article in journal (Refereed) Published
Abstract [en]

A new nanoporous (NP) carbon material with a high surface area and a narrow pore size distribution, around 8 A, has been used to investigate the effects that electrochemical oxidation at positive potentials exerts on the capacitance values and effective diffusion coefficients of ions inside the nanopores. An electroanalytical method, based on the single-particle microelectrode technique with micromanipulator, was applied to calculate the diffusion coefficients of 6 M KOH ions in NP carbon. The results were analyzed for short times using the Cottrell model and for long times using the spherical diffusion model. Using cyclic voltammetry, was found that different stages of oxidation took place between 0 and 0.5 V vs. Hg\HgO. After repeated cycling in the first region of oxidation (0-0.3 V), an activation leading to higher capacitance was observed, but the diffusion coefficients decreased from approximately 2 x 10(-9) to 0.5 x 10(-10) cm(2) s(-1). In the second region of oxidation (0.3-0.5 V), where CO2 and 02 evolution can occur, both the capacitance and the diffusion coefficients decreased more dramatically. The effective diffusion coefficients of ions of an activated carbon particle were dependent on the operation potential; decreasing by an order of magnitude when going from -0.3 to +0.3 V. The results are discussed in terms of chemisorption of small oxygen functional groups (-OH or C=O) and ionic interaction with the pore wall.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-5041 (URN)10.1149/1.1843772 (DOI)000227142400002 ()2-s2.0-14744281250 (Scopus ID)
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
4. Characterization of the electrochemical and ion-transport properties of a nanoporous carbon at negative polarization by the single-particle method
Open this publication in new window or tab >>Characterization of the electrochemical and ion-transport properties of a nanoporous carbon at negative polarization by the single-particle method
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 153, no 1, A48-A57 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the electrochemical processes occurring in a nanoporous carbon, obtained from silicon carbide and used as negative electrode material for supercapacitors, have been investigated by means of the single-particle microelectrode method. The processes studied deal with hydrogen adsorption, evolution, and oxidation using 6 M KOH as electrolyte. It was found that adsorption of hydrogen started at -0.5 V, hydrogen evolution at -1.4 V vs Hg vertical bar HgO, and that hydrogen oxidation occurs in two steps. The first oxidation process takes place between 0 and 0.1 V, shown by a well-defined current peak on the voltammograms. The second oxidation stage occurs between 0.1 and 0.5 V, indicated by a successive increase in current with the number of cycles. It was also found that after the first oxidation process, subsequent cycling between -0.5 and -1 V leads to a larger accumulation of hydrogen inside the nanopores and to a decrease of the effective diffusion coefficient (D-eff) of potassium ions. Subsequent oxidation, in a second process, leads to a total consumption of hydrogen and to an increase of D-eff.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5042 (URN)10.1149/1.2129670 (DOI)000234142400008 ()2-s2.0-33645511421 (Scopus ID)
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved
5. Determination of diffusion coefficients of BF4- inside carbon nanopores using the single particle microelectrode technique
Open this publication in new window or tab >>Determination of diffusion coefficients of BF4- inside carbon nanopores using the single particle microelectrode technique
Show others...
2006 (English)In: Journal of Electroanalytical Chemistry, ISSN 0022-0728, E-ISSN 1873-2569, Vol. 586, no 2, 247-259 p.Article in journal (Refereed) Published
Abstract [en]

The electrochemical and mass transport properties of TEABF(4) in a nanoporous (NP) carbon material, obtained from silicon carbide, was studied using single particles and a microelectrode technique. The carbon particles of size 100-200 mu m were studied by cyclic voltammetry and potential step measurements. The effective diffusion coefficients (D-eff) were calculated starting from the asymptotic solutions of Fick's second law for short and long time regions. The results show that cycling at low sweep rates was needed in order for the electrolyte to penetrate the inner porosity of the particles. The carbon material showed different electrochemical and mass transport properties depending on the applied potential. At negative polarisation, the results suggest that TEA(+) was adsorbed on the pore wall, however, being transported very slowly inside the pores. The average D-eff after cycling at both positive and negative potentials was 1.1(+/- 0.4) x 10(-8) cm(2) s(-1), using the Cottrell relation and 1.5(+/- 0.6) x 10(-8) cm(2) s(-1), using the radial diffusion solution. The average value of D-eff after cycling at negative potentials was 1.7(+/- 0.6) x 10(-8) cm(2) s(-1) using both mathematical solutions.

Keyword
single particle, Cottrell, double layer, nanoporous carbon, diffusion coefficient, supercapacitor
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-5043 (URN)10.1016/j.jelechem.2005.09.026 (DOI)000233828800013 ()2-s2.0-28144449940 (Scopus ID)
Available from: 2005-04-18 Created: 2005-04-18 Last updated: 2017-12-05Bibliographically approved

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