Welcome to Chem 179 Nanomaterials for Supercapacitors Week #7 Supercapacitor Devices Tim/Tianyu
E-mail:
[email protected] Office Hour: By Appointment Lab: PSB 198
Outline Basic structure Electrolytes
Working mechanism Energy density & power density
Types of supercapacitors Flexible supercapacitors
Symmetric and asymmetric supercapacitors
Basic Structure
Basic Structure Electrodes Conductive High surface area
Separator Filter paper Special separators
Electrolyte
(ionic conductor)
Aqueous electrolyte Organic electrolyte Ionic liquid electrolyte Gel electrolyte Redox electrolyte
Electrolytes Aqueous electrolytes
Advantages
Facile preparation Fast ion diffusion rate (diffusion
J. Mater. Chem. C 2009, 113, (31), 14020-14027 coefficient D~10-5 cm2/s)
Low solution resistance (~ several ohms) Safe to handle
Disadvantages Small potential window due to decomposition of water ___________________.
Usually, < 1.2 V
Pourbaix diagram of H2O
chemwiki.ucdavis.edu
Electrolytes Organic electrolytes
Advantages Large potential window (~2.5 – 4 V)
Acetonitrile (AN)
Propylene Carbonate (PC)
Disadvantages Safety issues (e.g. combustion) Limited ion diffusion rate (D~10-10 cm2/s)
Difficult to handle (prevent moisture) Glove box en.wikipedia.org
Béguin, F.; Presser, V.; Balducci, A.; Frackowiak, E., Adv. Mater. 2014,
Electrolytes Ionic liquids Salt(s) in liquid phase en.wikipedia.org Liquid phase composed of ion(s) only, no solvent (solvent-free)
NaCl
Ionic Bond van der Waals Force
Ionic Bond
van der Waals Force
(melting point: 801 oC)
EMIM:Cl (melting point: -21 oC) EMIM: 1-Ethyl-3-methylimidazolium
Electrolytes Ionic liquids Large cations and small anions
r r
Electrolytes Ionic liquids
Advantages Non-flammable Non-volatile (ZERO vapor pressure)
Large potential window (~4.5 V)
Disadvantages Hard to handle (prevent moisture) High viscosity (sluggish ion diffusion) MRS Bulletin 2013, 38, (07), 554-559.
Electrolytes Gel electrolytes A polymer (usually PVA)-based electrolyte Inorganic salt serves as solute (e.g. LiCl, H2SO4, LiOH)
Polyvinyl Alcohol A crucial part of flexible supercapacitors
Wikipedia
SEM Images
After dipping in PVA electrolyte
Electrolytes Redox electrolytes Dissolve redox pair(s) into electrolyte
Chen, K.; Liu, F.; Xue, D.; Komarneni, S. Nanoscale 2014
Electrolytes Redox electrolytes Enhance capacitance
Mai, L et al. Nature Communications 2013, 4.
Working Mechanism
Working Mechanism
Charging Positive electrode Potential increases
Workstation
Electrons inject in e
+ -
Negative electrode Potential decreases Electrons move out
Ions in electrolyte
Working Mechanism
Discharging Positive electrode
Device
Potential decreases Electrons move out
Negative electrode
e
Potential increases
+ -
Electrons inject in
Ions in electrolyte
Theoretical Specific Capacitance of Devices The overall specific capacitance of the device composed of two identical electrodes equals one-forth of the specific capacitance of the single electrode. The overall capacitance:
1 Ctotal
1 1 2 C Ctotal C C C 2
Convert to specific capacitance:
Cs ,total
Ctotal Ctotal 1 mtotal 2m 2m
C 1 C 1 Cs 2 4m 4
C C Ctotal
Energy Density and Power Density Recall from Lecture #1
Ragone Plot Energy Density (Wh•kg) How much energy per kg material can store
Power Density (kW•kg) How fast per kg material can charge/discharge
Energy Density and Power Density Calculate energy density and power density Energy Density (E)
1 2 E C U 2 Power Density (P)
P
E tdischarge
Energy Density and Power Density Energy Density (A) Power Density (B) Fill in the following blanks using A or B:
1 2 E C U 2 E P tdischarge
Supercapacitors using aqueous electrolytes usually have low A but high ________. B _______
Supercapacitors using organic/ionic liquids electrolytes A B but high ________. usually have low _______
Types of Supercapacitors
Common Classifications Based on the charge storage mechanism Electrical double layer supercapacitors Pseudo-capacitive supercapacitors (a.k.a. pseudocapacitors) Hybrid supercapacitors
wikipedia
Symmetric and Asymmetric Supercapacitors Based on the configuration Symmetric
Asymmetric
A
A
A
B
Charge balance QA QB C A U A CB U B
Flexible Supercapacitors Flexible substrates (e.g. Carbon cloth)
Gel electrolyte
+ Carbon cloth electrodes H2SO4/PVA gel electrolyte
Wang et al. Adv. Mater. 2014, 26, (17), 2676
Energy & Environmental Science, 2011, 4, 5060
Flexible Supercapacitors Flexibility
Wang et al. Adv. Mater. 2014, 26, (17), 2676
Flexible Supercapacitors Fabrication PVA Powder
+
H2O
+
Salt
→
PVA Gel Electrolyte
separator
Flexible Supercapacitors Practical applications
Lu, X.; et al. Energy & Environmental Science 2014, 7, 2160.
Practical Applications
Power a red LED
Power a timer
(nominal potential: 1.6 V)
(nominal potential: 1.5 V)
Extracurricular Readings Papers are available on eCommons.
[1] Brandt, A.; Pohlmann, S.; Varzi, A.; Balducci, A.; Passerini, S., Ionic liquids in supercapacitors. MRS Bulletin 2013, 38, (07), 554-559. [2] Chen, K.; Liu, F.; Xue, D.; Komarneni, S., Carbon with ultrahigh capacitance when graphene paper meets K3Fe(CN)6. Nanoscale 2015, 7, (2), 432-439 [3] Benedetti, T. M.; Gonçales, V. R.; Córdoba De Torresi, S. I.; Torresi, R. M., In search of an appropriate ionic liquid as electrolyte for macroporous manganese oxide film electrochemistry. Journal of Power Sources 2013, 239, 1-8.
[4] Park, J.; Kim, B.; Yoo, Y.; Chung, H.; Kim, W., Energy-Density Enhancement of Carbon-Nanotube-Based Supercapacitors with Redox Couple in Organic Electrolyte. ACS Applied Materials & Interfaces 2014, 6, (22), 19499-19503
End of Lecture #7 Thank You! See you in the lab Lab report #2 due THIS Friday (by 5 pm) NO EXTENSION
