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Charge Transport and Storage in a Supercapacitor Structure
Kuparowitz, Tomáš ; Zedníček, Tomáš (oponent) ; Hájek, Karel (oponent) ; Sedlák, Petr (vedoucí práce)
Supercapacitor (SC), or electric double-layer capacitor, represents electrical energy device, which offers high power density, short charging time, high number of charging cycles, and long-life duration. This device is of particular interest in fast energy-storage applications, where highly dynamic charging and discharging profiles are required. Detailed study and modeling of the electrical charge transport and its storage is the output of this thesis. Processes, which occur during charging and discharging, are studied and their correlation to fading of SC's parameters is assessed. The noticeable differences between measured results and the simple SC model are: 1) the nonlinear rise and fall of SC’s voltage, and 2) the change in voltage after the charging and discharging stops. Charge redistribution during SC charging and relaxation are important. New model of SC is proposed. Electric charge stored in SC appears to be divided into two sections. One could be attributed to the Helmholtz capacitance and the other to the diffuse capacitance. The equivalent circuit model contains time dependent resistance RD(t) between Helmholtz and diffuse capacitances. While the SC ages, all parameters of equivalent circuit model change. The change of Helmholtz capacitance may be described most accurately by a pure exponential function. Total capacitance in relation to the number of energy cycling aging cycles n or the time of cycling t follows an exponential stretched law. From experiments it follows, that the greatest influence on SC’s degradation has the amount of transferred energy. The degradation due to induced Joule’s heat only has impact after 2000 hours of continuous energy cycling. The time constant D of the diffusion process is responsible for the speed of diffusion capacitance filling. The decrease of diffusion process time constant is faster for 75% discontinuous energy cycling than it is for 75% continuous energy cycling method. The dependence of aging on SC’s equivalent series resistance parameter remains the same for both continuous and discontinuous energy cycling. It obeys the quadratic relation. The difference is that the quadratic component is of the order of magnitude smaller for 75% discontinuous energy cycling than it is for 75% continuous energy cycling. The dependence of equivalent circuit model parameters on ambient temperature before and after aging by discontinuous energy cycling is explored. The value of total capacitance CT increases linearly with temperature. This is also true for aged samples. The slope for new samples is about 3 times higher than for aged one. Helmholtz capacitance of SC CapXX 2.75V/2.4F is constant in temperature range 22 °C to 65 °C, with the value of Helmholtz capacitance CH = 2.78 F for new samples and 2.07 F for aged ones. At temperature range below 22 °C Helmholtz capacitance decreases about 0.5 F due to the aging. The calendar life tests are devised to simulate SC under light work load at differing ambient temperatures. Temperatures used are from -35 °C up to 65 °C. This experiment is devised to prove, that the increased temperature accelerates the electrochemical reactions, which are responsible for SC’s total capacitance degradation.
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Charge Transport and Storage in a Supercapacitor Structure
Kuparowitz, Tomáš ; Zedníček, Tomáš (oponent) ; Hájek, Karel (oponent) ; Sedlák, Petr (vedoucí práce)
Supercapacitor (SC), or electric double-layer capacitor, represents electrical energy device, which offers high power density, short charging time, high number of charging cycles, and long-life duration. This device is of particular interest in fast energy-storage applications, where highly dynamic charging and discharging profiles are required. Detailed study and modeling of the electrical charge transport and its storage is the output of this thesis. Processes, which occur during charging and discharging, are studied and their correlation to fading of SC's parameters is assessed. The noticeable differences between measured results and the simple SC model are: 1) the nonlinear rise and fall of SC’s voltage, and 2) the change in voltage after the charging and discharging stops. Charge redistribution during SC charging and relaxation are important. New model of SC is proposed. Electric charge stored in SC appears to be divided into two sections. One could be attributed to the Helmholtz capacitance and the other to the diffuse capacitance. The equivalent circuit model contains time dependent resistance RD(t) between Helmholtz and diffuse capacitances. While the SC ages, all parameters of equivalent circuit model change. The change of Helmholtz capacitance may be described most accurately by a pure exponential function. Total capacitance in relation to the number of energy cycling aging cycles n or the time of cycling t follows an exponential stretched law. From experiments it follows, that the greatest influence on SC’s degradation has the amount of transferred energy. The degradation due to induced Joule’s heat only has impact after 2000 hours of continuous energy cycling. The time constant D of the diffusion process is responsible for the speed of diffusion capacitance filling. The decrease of diffusion process time constant is faster for 75% discontinuous energy cycling than it is for 75% continuous energy cycling method. The dependence of aging on SC’s equivalent series resistance parameter remains the same for both continuous and discontinuous energy cycling. It obeys the quadratic relation. The difference is that the quadratic component is of the order of magnitude smaller for 75% discontinuous energy cycling than it is for 75% continuous energy cycling. The dependence of equivalent circuit model parameters on ambient temperature before and after aging by discontinuous energy cycling is explored. The value of total capacitance CT increases linearly with temperature. This is also true for aged samples. The slope for new samples is about 3 times higher than for aged one. Helmholtz capacitance of SC CapXX 2.75V/2.4F is constant in temperature range 22 °C to 65 °C, with the value of Helmholtz capacitance CH = 2.78 F for new samples and 2.07 F for aged ones. At temperature range below 22 °C Helmholtz capacitance decreases about 0.5 F due to the aging. The calendar life tests are devised to simulate SC under light work load at differing ambient temperatures. Temperatures used are from -35 °C up to 65 °C. This experiment is devised to prove, that the increased temperature accelerates the electrochemical reactions, which are responsible for SC’s total capacitance degradation.
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