Supercapacitor - Revision history

Cmditradmin: /* Pseudocapacitance */

2016-11-09T22:39:12Z

Pseudocapacitance

← Older revision		Revision as of 15:39, 9 November 2016
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	In certain electrode / electrolyte systems at certain applied potentials, some ions may actually participate in reversible redox reactions at or near the electrode surface. Unlike lithium intercalation in lithium-ion batteries, these redox processes are inherently fast and do not result in phase transformation of the electrode material. This represents a second mechanism for capacitive energy storage, known as pseudocapacitance. In many cases, pseudocapacitance offers much higher energy density compared to double-layer capacitance. Two major types of pseudocapacitance are used in supercapacitor technology:		In certain electrode / electrolyte systems at certain applied potentials, some ions may actually participate in reversible redox reactions at or near the electrode surface. Unlike lithium intercalation in lithium-ion batteries, these redox processes are inherently fast and do not result in phase transformation of the electrode material. This represents a second mechanism for capacitive energy storage, known as pseudocapacitance. In many cases, pseudocapacitance offers much higher energy density compared to double-layer capacitance. Two major types of pseudocapacitance are used in supercapacitor technology:

	[[file:Redox_pseudocap.png\|thumb\|Figure 1: Illustration of redox pseudocapacitance.]] ~~Source: [1]~~		[[file:Redox_pseudocap.png\|thumb\|Figure 1: Illustration of redox pseudocapacitance.]]

	Redox pseudocapacitance: This occurs when ions adsorb on or near the surface of the electrode with an accompanying transfer of electron(s). One example of a system that displays such behavior is hydrous ruthenium dioxide (RuO<sub>2</sub>*nH2O) in acidic solution. In this example, the charge storage mechanism is explained by the simultaneous insertion of electrons and protons into the structure, causing the oxidation state of ruthenium to shift between Ru<sup>4+</sup>, Ru<sup>3+</sup>, and Ru<sup>2+</sup>.[2]		Redox pseudocapacitance: This occurs when ions adsorb on or near the surface of the electrode with an accompanying transfer of electron(s). One example of a system that displays such behavior is hydrous ruthenium dioxide (RuO<sub>2</sub>*nH2O) in acidic solution. In this example, the charge storage mechanism is explained by the simultaneous insertion of electrons and protons into the structure, causing the oxidation state of ruthenium to shift between Ru<sup>4+</sup>, Ru<sup>3+</sup>, and Ru<sup>2+</sup>.[2]

Cmditradmin: /* Galvanostatic Cycling */

2016-11-09T22:37:59Z

Galvanostatic Cycling

← Older revision		Revision as of 15:37, 9 November 2016
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	Specific capacitance can be calculated by:		Specific capacitance can be calculated by:

	<math>C_s = \frac{I \Delta V}{m \Delta V} ~~(4)~~</math>		<math>C_s = \frac{I \Delta V}{m \Delta V} </math> (4)

	where C<sub>s</sub> is the specific capacitance, I is the constant discharge current, m is the mass of active material, ΔV is the potential window, and Δt is the time elapsed in the range of ΔV.		where C<sub>s</sub> is the specific capacitance, I is the constant discharge current, m is the mass of active material, ΔV is the potential window, and Δt is the time elapsed in the range of ΔV.

Cmditradmin: /* Galvanostatic Cycling */

2016-11-09T22:36:00Z

Galvanostatic Cycling

← Older revision		Revision as of 15:36, 9 November 2016
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	Specific capacitance can be calculated by:		Specific capacitance can be calculated by:

	<math>C_s = \frac{I \Delta V}{m \Delta V}</math>		<math>C_s = \frac{I \Delta V}{m \Delta V} (4)</math>

	where Cs is the specific capacitance, I is the constant discharge current, m is the mass of active material, ΔV is the potential window, and Δt is the time elapsed in the range of ΔV.		where C<sub>s</sub> is the specific capacitance, I is the constant discharge current, m is the mass of active material, ΔV is the potential window, and Δt is the time elapsed in the range of ΔV.

	Analogous to cyclic voltammetry, galvanostatic cycling can be conducted over a range of current densities to gauge rate capability.		Analogous to cyclic voltammetry, galvanostatic cycling can be conducted over a range of current densities to gauge rate capability.

Cmditradmin: /* Galvanostatic Cycling */

2016-11-09T22:35:17Z

Galvanostatic Cycling

← Older revision		Revision as of 15:35, 9 November 2016
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	Specific capacitance can be calculated by:		Specific capacitance can be calculated by:

			<math>C_s = \frac{I \Delta V}{m \Delta V}</math>

	where Cs is the specific capacitance, I is the constant discharge current, m is the mass of active material, ΔV is the potential window, and Δt is the time elapsed in the range of ΔV.		where Cs is the specific capacitance, I is the constant discharge current, m is the mass of active material, ΔV is the potential window, and Δt is the time elapsed in the range of ΔV.

Cmditradmin: /* Cyclic Voltammetry */

2016-11-09T22:32:20Z

Cyclic Voltammetry

← Older revision		Revision as of 15:32, 9 November 2016
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	Another nonideality commonly observed in supercapacitors is electrolyte degradation, which can occur at the extrema of the voltage sweep. For example, if an aqueous electrolyte is used, applying a voltage above ~1.2 V in magnitude will oxidize water to form oxygen gas and H+:		Another nonideality commonly observed in supercapacitors is electrolyte degradation, which can occur at the extrema of the voltage sweep. For example, if an aqueous electrolyte is used, applying a voltage above ~1.2 V in magnitude will oxidize water to form oxygen gas and H+:

			<math>2H_2O \leftrightarrow O_{2(G)} + 4h^+ + 4E^- </math>

	Such redox reaction cause an increase in current above that which is needed to charge the interfacial double layer, but they do not contribute to capacitance. This shows up in the voltammogram as a “sharpening” of two of the corners (Figure 1C).		Such redox reaction cause an increase in current above that which is needed to charge the interfacial double layer, but they do not contribute to capacitance. This shows up in the voltammogram as a “sharpening” of two of the corners (Figure 1C).

			<math>C_s = \frac{\int I(V)dV}{\mu m \Delta V}</math>

	Specific capacitance can be obtained by integration of the voltammogram over a specified voltage range:		Specific capacitance can be obtained by integration of the voltammogram over a specified voltage range:

Cmditradmin: /* Current Supercapacitor Research at UW / CEI */

2016-11-09T22:26:11Z

Current Supercapacitor Research at UW / CEI

← Older revision		Revision as of 15:26, 9 November 2016
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	=== Current Supercapacitor Research at UW / CEI ===		=== Current Supercapacitor Research at UW / CEI ===
			Guozhong Cao (MSE) is studying modified porous carbon for supercapacitor electrodes. These materials are obtained by freeze-drying organic polymeric gels, followed by high-temperature pyrolysis in an inert atmosphere. One recent paper presents on the co-doping of such carbon materials with sulfur and nitrogen by mixing phosphorus pentasulfide with the gel precursors.[24] The hypothesis is that the foreign elements can lead to higher surface charge density, inducing pseudocapacitive reactions in addition to double-layer capacitance. A specific capacitance of 103.5 F/g was achieved for the doped aerogel, 43.5% higher than that of pristine porous carbon. The sulfur and nitrogen dopants were also found to improve the electrocatalytic activity of the host carbon for the oxygen reduction reaction (ORR), the key reaction for fuel cells.

	~~Guozhong Cao~~ (MSE) is ~~studying modified~~ porous ~~carbon for supercapacitor electrodes. These materials are obtained by~~ freeze-drying ~~organic polymeric~~ gels~~, followed by high~~-~~temperature pyrolysis~~ in ~~an inert atmosphere~~. ~~One recent paper presents on the co-doping~~ of ~~such carbon materials with sulfur and nitrogen by mixing phosphorus pentasulfide with~~ the gel precursors~~.[24] The hypothesis is that the foreign elements can lead~~ to ~~higher surface charge density, inducing pseudocapacitive reactions in addition to double~~-~~layer capacitance. A specific capacitance~~ of ~~103.5 F/g was achieved for the doped aerogel~~, ~~43.5% higher than that~~ of ~~pristine porous~~ carbon. ~~The sulfur and nitrogen dopants were also found to improve~~ the ~~electrocatalytic activity~~ of ~~the host carbon for the oxygen reduction reaction (ORR)~~, the ~~key reaction for fuel cells~~.		Peter Pauzauskie (MSE) is also investigating porous carbons derived from organic gels, but through the process of supercritical drying as opposed to freeze drying. In particular, the gels are made from polymerizing resorcinol and formaldehyde using a novel acid-catalyzed technique in acetonitrile that accelerates the gelation by a factor of >10. Graphene oxide (GO), an oxidized and exfoliated derivative of graphite, can be introduced to the gel precursors to create (following pyrolysis) a covalently cross-linked assembly of graphene, a two-dimensional structure of carbon atoms bonded in a hexagonal lattice. This is a promising supercapacitor material considering the extraordinary electron transport properties of graphene, which could minimize impedance in the device and enhance its rate capability. These hypotheses have been confirmed by constant-current discharge tests and impedance spectroscopy.[25]

	Peter Pauzauskie (MSE) is also investigating porous carbons derived from organic gels, but through the process of supercritical drying as opposed to freeze drying. In particular, the gels are made from polymerizing resorcinol and formaldehyde using a novel acid-catalyzed technique in acetonitrile that accelerates the gelation by a factor of >10. Graphene oxide (GO), an oxidized and exfoliated derivative of graphite, can be introduced to the gel precursors to create (following pyrolysis) a covalently cross-linked assembly of graphene, a two-dimensional structure of carbon atoms bonded in a hexagonal lattice. This is a promising supercapacitor material considering the extraordinary electron transport properties of graphene, which could minimize impedance in the device and enhance its rate capability. These hypotheses have been confirmed by constant-current discharge tests and impedance spectroscopy.[25]		Christine Luscombe (MSE / Chemistry) is studying conductive hyperbranched polymers for negative electrode materials for asymmetric supercapacitors (i.e., where the two electrodes are made of different materials). Conductive polymers are redox-active, offering pseudocapacitance, and can be synthesized at lower cost than pseudocapacitive metal oxides such as RuO2 owing to the relative abundance of the precursor materials. In a recent study, supercapacitor electrodes based on polymers with triphenylamine cores and naphthalene diimide terminal units were limited to modest capacitance (< 22 F/g), but exhibited less than 10% capacity fade over 500 cycles—on par with existing battery technologies.[26]

	Christine Luscombe (MSE / Chemistry) is studying conductive hyperbranched polymers for negative electrode materials for asymmetric supercapacitors (i.e., where the two electrodes are made of different materials). Conductive polymers are redox-active, offering pseudocapacitance, and can be synthesized at lower cost than pseudocapacitive metal oxides such as RuO2 owing to the relative abundance of the precursor materials. In a recent study, supercapacitor electrodes based on polymers with triphenylamine cores and naphthalene diimide terminal units were limited to modest capacitance (< 22 F/g), but exhibited less than 10% capacity fade over 500 cycles—on par with existing battery technologies.[26]

	References		References

Cmditradmin: /* Electrochemical Impedance Spectroscopy */

2016-11-09T22:25:06Z

Electrochemical Impedance Spectroscopy

← Older revision		Revision as of 15:25, 9 November 2016
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	==== Electrochemical Impedance Spectroscopy ====		==== Electrochemical Impedance Spectroscopy ====

	[[File:Nyquist.jpg\|thumb\| Figure 3: Example Nyquist plot for a double-layer capacitor based on activated carbon aerogel. Plot covers the frequency range 106–10‑3 Hz.]] Source: [2]		[[File:Nyquist.jpg\|thumb\| Figure 3: Example Nyquist plot for a double-layer capacitor based on activated carbon aerogel. Plot covers the frequency range 106–10‑3 Hz. Source: [2]]]

	Electrochemical Impedance Spectroscopy (EIS) is perhaps the most comprehensive electrochemical analysis technique. This involves applying an alternating voltage to the device over a wide range of frequencies and measuring the amplitude and phase shift of the resulting current. From these measurements at each frequency, the impedance is calculated, and is presented in the form of a Nyquist plot (which plots the imaginary component of impedance against the real component) or a Bode plot (which shows the magnitude of impedance and phase shift versus frequency). The interpretation of this data is beyond the scope of this website, but it can be fit to different equivalent-circuit models of supercapacitors (of which there are many). From the best fit, specific parameters such as the double-layer capacitance, series resistance, charge transfer resistance (resistance associated with redox reactions at the electrode–electrolyte interface, if pseudocapacitance is significant), and diffusion-related resistance can be extracted. In addition, the use of many different frequencies in the applied voltage allows for studying the rate capability of the device—i.e., how quickly it can be cycled without significant capacity loss.		Electrochemical Impedance Spectroscopy (EIS) is perhaps the most comprehensive electrochemical analysis technique. This involves applying an alternating voltage to the device over a wide range of frequencies and measuring the amplitude and phase shift of the resulting current. From these measurements at each frequency, the impedance is calculated, and is presented in the form of a Nyquist plot (which plots the imaginary component of impedance against the real component) or a Bode plot (which shows the magnitude of impedance and phase shift versus frequency). The interpretation of this data is beyond the scope of this website, but it can be fit to different equivalent-circuit models of supercapacitors (of which there are many). From the best fit, specific parameters such as the double-layer capacitance, series resistance, charge transfer resistance (resistance associated with redox reactions at the electrode–electrolyte interface, if pseudocapacitance is significant), and diffusion-related resistance can be extracted. In addition, the use of many different frequencies in the applied voltage allows for studying the rate capability of the device—i.e., how quickly it can be cycled without significant capacity loss.

Cmditradmin: /* Electrochemical Impedance Spectroscopy */

2016-11-09T22:24:11Z

Electrochemical Impedance Spectroscopy

← Older revision		Revision as of 15:24, 9 November 2016
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	==== Electrochemical Impedance Spectroscopy ====		==== Electrochemical Impedance Spectroscopy ====

	Figure 3: Example Nyquist plot for a double-layer capacitor based on activated carbon aerogel. Plot covers the frequency range 106–10‑3 Hz. Source: [2]		[[File:Nyquist.jpg\|thumb\| Figure 3: Example Nyquist plot for a double-layer capacitor based on activated carbon aerogel. Plot covers the frequency range 106–10‑3 Hz.]] Source: [2]

	Electrochemical Impedance Spectroscopy (EIS) is perhaps the most comprehensive electrochemical analysis technique. This involves applying an alternating voltage to the device over a wide range of frequencies and measuring the amplitude and phase shift of the resulting current. From these measurements at each frequency, the impedance is calculated, and is presented in the form of a Nyquist plot (which plots the imaginary component of impedance against the real component) or a Bode plot (which shows the magnitude of impedance and phase shift versus frequency). The interpretation of this data is beyond the scope of this website, but it can be fit to different equivalent-circuit models of supercapacitors (of which there are many). From the best fit, specific parameters such as the double-layer capacitance, series resistance, charge transfer resistance (resistance associated with redox reactions at the electrode–electrolyte interface, if pseudocapacitance is significant), and diffusion-related resistance can be extracted. In addition, the use of many different frequencies in the applied voltage allows for studying the rate capability of the device—i.e., how quickly it can be cycled without significant capacity loss.		Electrochemical Impedance Spectroscopy (EIS) is perhaps the most comprehensive electrochemical analysis technique. This involves applying an alternating voltage to the device over a wide range of frequencies and measuring the amplitude and phase shift of the resulting current. From these measurements at each frequency, the impedance is calculated, and is presented in the form of a Nyquist plot (which plots the imaginary component of impedance against the real component) or a Bode plot (which shows the magnitude of impedance and phase shift versus frequency). The interpretation of this data is beyond the scope of this website, but it can be fit to different equivalent-circuit models of supercapacitors (of which there are many). From the best fit, specific parameters such as the double-layer capacitance, series resistance, charge transfer resistance (resistance associated with redox reactions at the electrode–electrolyte interface, if pseudocapacitance is significant), and diffusion-related resistance can be extracted. In addition, the use of many different frequencies in the applied voltage allows for studying the rate capability of the device—i.e., how quickly it can be cycled without significant capacity loss.
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	[2] Y.-Z. Wei, B. Fang, S. Iwasa, M. Kumagai, A novel electrode material for electric double-layer capacitors, Journal of Power Sources. 141 (2005) 386–391. doi:10.1016/j.jpowsour.2004.10.001.		[2] Y.-Z. Wei, B. Fang, S. Iwasa, M. Kumagai, A novel electrode material for electric double-layer capacitors, Journal of Power Sources. 141 (2005) 386–391. doi:10.1016/j.jpowsour.2004.10.001.



	=== Current Supercapacitor Research at UW / CEI ===		=== Current Supercapacitor Research at UW / CEI ===

Cmditradmin: /* Galvanostatic Cycling */

2016-11-09T22:22:45Z

Galvanostatic Cycling

← Older revision		Revision as of 15:22, 9 November 2016
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	Furthermore, all real supercapacitors exhibit self-discharge, where a small amount of current is conducted when the device is in a charged state but not connected to any external load. If the cell is shorted, such as by inadvertent direct contact of the electrodes, the parasitic current may be significant. This causes the cell voltage to drop faster than expected during discharge and is more noticeable at low applied current densities due to the higher relative magnitude of the parasitic current.		Furthermore, all real supercapacitors exhibit self-discharge, where a small amount of current is conducted when the device is in a charged state but not connected to any external load. If the cell is shorted, such as by inadvertent direct contact of the electrodes, the parasitic current may be significant. This causes the cell voltage to drop faster than expected during discharge and is more noticeable at low applied current densities due to the higher relative magnitude of the parasitic current.

	Figure 2: Examples of ideal (A) and non-ideal (B, C) galvanostatic charge-discharge curves. Source: [1]		[[File:Charge-discharge.png\|thumb\|Figure 2: Examples of ideal (A) and non-ideal (B, C) galvanostatic charge-discharge curves.]] Source: [1]

	Specific capacitance can be calculated by:		Specific capacitance can be calculated by:
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	Analogous to cyclic voltammetry, galvanostatic cycling can be conducted over a range of current densities to gauge rate capability.		Analogous to cyclic voltammetry, galvanostatic cycling can be conducted over a range of current densities to gauge rate capability.

	==== Electrochemical Impedance Spectroscopy ====		==== Electrochemical Impedance Spectroscopy ====

Cmditradmin: /* Cyclic Voltammetry */

2016-11-09T22:21:04Z

Cyclic Voltammetry

← Older revision		Revision as of 15:21, 9 November 2016
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	Where I = current, Q = charge, V = voltage, C = capacitance, and t = time. This means that if the voltage is swept linearly over time, then the corresponding current will be of constant magnitude, positive when the voltage is increasing and negative when the voltage is decreasing. In reality, deviations from rectangularity will occur due to series resistance in the system, which may be associated with electron transport in the electrode material, contacts between the electrode and terminals of the device, and/or diffusion of ions in the electrolyte. Such resistances mean that the current cannot respond instantaneously to changes in the direction of the voltage sweep, causing two of the corners in the voltammogram to be rounded-off (Figure 1B), with the rounding becoming more pronounced as the sweep rate is increased. At fast enough sweep rates, the interfacial double layer will not have sufficient time to form at all, resulting in negligible capacitance.		Where I = current, Q = charge, V = voltage, C = capacitance, and t = time. This means that if the voltage is swept linearly over time, then the corresponding current will be of constant magnitude, positive when the voltage is increasing and negative when the voltage is decreasing. In reality, deviations from rectangularity will occur due to series resistance in the system, which may be associated with electron transport in the electrode material, contacts between the electrode and terminals of the device, and/or diffusion of ions in the electrolyte. Such resistances mean that the current cannot respond instantaneously to changes in the direction of the voltage sweep, causing two of the corners in the voltammogram to be rounded-off (Figure 1B), with the rounding becoming more pronounced as the sweep rate is increased. At fast enough sweep rates, the interfacial double layer will not have sufficient time to form at all, resulting in negligible capacitance.

	[[file:~~CVcurve~~.png\|thumb\|Examples of ideal (A) and non-ideal (B, C) CV curves.]] Source: [1]		[[file:CVcurves.png\|thumb\|Examples of ideal (A) and non-ideal (B, C) CV curves.]] Source: [1]

	Another nonideality commonly observed in supercapacitors is electrolyte degradation, which can occur at the extrema of the voltage sweep. For example, if an aqueous electrolyte is used, applying a voltage above ~1.2 V in magnitude will oxidize water to form oxygen gas and H+:		Another nonideality commonly observed in supercapacitors is electrolyte degradation, which can occur at the extrema of the voltage sweep. For example, if an aqueous electrolyte is used, applying a voltage above ~1.2 V in magnitude will oxidize water to form oxygen gas and H+: