Summary of major research accomplishments during PhD. Emphasizes my most recent work: the development of a new method for the determination of Peukert's constant, and the demonstration of the utility of Peukert's equation for the analysis of supercapacitor performance.
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Characterization of the electrical properties of interfaces by impedance spectroscopy for clean energy devices
1. Characterization of the electrical
properties of interfaces by impedance
spectroscopy for clean energy devices
Edmund Mills
Major Professors: Sangtae Kim and
YayoiTakamura
2. Projects
• YSZ thin films with minimized grain boundary
resistivity
• Evaluating the rate-dependence of supercapacitor
performance using Peukert’s equation
• Direct Determination of Field Emission across the
Heterojunctions in a ZnO/GrapheneThin-Film
Barristor
1
3. Interfacial engineering
for solid oxide fuel cell electrolytes
2
Ionic conductivity is enhanced in yttria-stabiled zirconia thin films
Jun Jiang, Joshua L. Hertz ,J Electroceram (2014) 32,37–46
4. Investigation ofYSZ thin films
MgO
YSZ
20 nm
Film growth
Pulsed laser deposition
ofYSZ thin films on
MgO substrates
Structural Characterization
X-ray diffraction
X-ray reflectivity
HRTEM
ElectricalCharacterization
Impedance spectroscopy
12. Rate-dependent capacitance
11
Zhi et al. “Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high
energy and power density supercapacitors” Journal of Power Sources 208 (2012) 345–353
13. Rate-dependent capacitance
12
Zhi et al. “Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high
energy and power density supercapacitors” Journal of Power Sources 208 (2012) 345–353
21. Peukert’s equation
20Supercapacitor rate-dependence
• Used for batteries
• Empirical
• k = 1: rate independent capacity
• k > 1: capacity decays at higher rates
• Novel frequency dependent form
for Impedance spectroscopy
31. Selected paper 1:
Influence of carbon nanotube addition
• Add CNTs to activated carbon electrodes
• CNTs increase pore accessibility
30Supercapacitor rate-dependence
Portet et al. “Influence of carbon nanotubes addition on carbon–carbon supercapacitor
performances in organic electrolyte” Journal of Power Sources 139 (2005) 371–378
33. Selected paper 2: MnO2
32Supercapacitor rate-dependence
Zhi et al. “Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high
energy and power density supercapacitors” Journal of Power Sources 208 (2012) 345–353
AAI-CNF CNF
34. Selected paper 2: MnO2
33Supercapacitor rate-dependence
Zhi et al. “Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high
energy and power density supercapacitors” Journal of Power Sources 208 (2012) 345–353
0.05
0.5
1
1.05
1.1
1.15
1.2
1.25
0% 10% 20% 30% 40% 50% 60% 70%
ElectrodeResistivity(Ohm*cm)
Peukert'sConstant
MnO2 Loading
0.7
7
1
10
100
0% 10% 20% 30% 40% 50% 60% 70%
ElectrodeResistivity(Ohm*cm)
Peukert'sConstant MnO2 Loading
AAI-CNF CNF
35. Selected paper 2: MnO2
34Supercapacitor rate-dependence
Zhi et al. “Highly conductive electrospun carbon nanofiber/MnO2 coaxial nano-cables for high
energy and power density supercapacitors” Journal of Power Sources 208 (2012) 345–353
1
10
0 0.5 1 1.5 2 2.5
Peukert'sconstant
Electrode Resistiviy (Ohm*cm)
37. Summary
Supercapacitor rate-dependence 36
• Developed novel method for determination of
Peukert’s constant using impedance spectroscopy
– Consistent with dc method of determination
– Simple and precise
• Demonstrated utility of Peukert’s constant for
evaluation of supercapacitors
– SCs follow Peukert’s law
– Facilitates interpretation of rate-dependent capacitance
38. Direct Determination of Field
Emission across the Heterojunctions
in a ZnO/GrapheneThin-Film
Barristor
43. Bias and temperature dependence
42
0 25 50
0
25
50
0.0 0.5 1.0 1.5
10
-5
10
-4
10
-3
10
-2
Vo
(mV)
Vth
(mV)
FE
TFE (forward)
TFE (reverse)
TE
CB
-80
o
C
-60
o
C
-40
o
C
-20
o
C
0
o
C
20
o
C
40
o
C
Current(A)
Voltage (V)
15 20 25 30
0
250
500
Vo
(mV)
Vth
(mV)
A
• Temperature-independent electrical characteristics
• Indicates electron tunneling mechanism at the
graphene-ZnO interface
Only 3 data points for N enriched porous carbon, maybe leading to innacuracy in k from dc method
Utility for SCs: porosity, pseudocapacitance
Equivalent circuit modeling, analysis of literature papers
papers
Measure reduced surface area with additional CNTs
Iron acetylacetonate
No Rct
Reduced capacitance in more mno2 loaded samples: This was due to the fact that too thick oxide coating caused part of material to be electrochemically inactive, and jeopardized the charge transport.
From the Nyquist plots, the capacitance retardation in the CNF@MnO2 samples was due to two problems: (i) high series resistance attributed to insufficient con- ductivity of the electrode, especially at a high MnO2 loading, and (ii) large charge transfer resistance at the carbon/MnO2 interface that inhibits the Na+ insertion/adsorption process in MnO2, leading to capacitance degradation at a high operation rate. The two problems were mitigated by the addition of AAI into CNF. The conductivity of the AAI-CNF (1.35 S cm−1 ) was almost 27 times higher than CNF
(0.05 S cm−1 ) even at high MnO2 loading (59% of MnO2 loading), which minimized the series resistance of the electrode, reducing the IR loss. In addition, the AAI-CNF@MnO2 samples showed an almost linear shape in the Nyquist plot in the absence of an evident semicircle, which indicated a negligible charge transfer resistance. Benefited from the higher surface area of the AAI-CNF backbone, the C/MnO2 contact area can be enlarged and the interfacial resistance between carbon and MnO2 can be reduced, therefore the electron and ions can be more easily transferred.