Nanoscale Materials: Exploring
the Energy Frontier
Prashant N. Kumta
Department of Materials Science and Engineering,
Biomedical Engineering
Carnegie Mellon University,
Pittsburgh, PA 15213
Need for Energy Storage Systems
www.maxwell.com
„ Hybrid Electric Vehicle
„ Backup Power Source
„ Telecommunication
www.nissandiesel.co.jp
„ Electronic Devices
„ Laser
Nissan Diesel
Sizuki Electric
Power Systems
„ Military Use
„ Particle Accelerator
www.hondacorporate.com
www.nissandiesel.co.jp
www.maxwell.com
Potential Energy Storage Systems
Li-ion Batteries*
*
‹ Super-capacitors
*
‹ Direct Methanol Fuel Cells
‹ Hydrogen Storage
‹
*Focus of the current presentation
Lithium-Ion Systems: Anodes and Cathodes
Comparison of Battery Energy Density
(gravimetric and volumetric)
Considerable improvement in area of cathodes with identification of new
layered compounds, LiFePO4 and LiNi0.5Mn0.5O2, LiMn1/3Ni1/3Co1/3O2.
Improvements in the anode area are still warranted.
Introduction and Background
°
Li-alloy (LixM) : Anodes for Li ion cells
- Intermetallic phases containing lithium
LixM
xLi+ + xe- + M
(M = Mg, Ca, Al, Si, Ge, Sn, Sb, Bi, Zn, etc)1
- Higher theoretical capacity than graphite
(LiC6 : 372 mAh/g,
832 Ah/L,
Li22Sn5 : 990 mAh/g, 14780 Ah/L
Li22Si5 : 4000 mAh/g, 10997 Ah/L)1, 2
°
Major problems of Li-alloys for use as anodes
- Poor reversibility caused by large volume changes
(Sn: 593%, Si: 412% volume change)
- Cracking or crumbling during cycling
1. M. Winter & J. O. Besenhard, Electrochimica Acta, 1999, 45, 31.
2. B. A. Boukamp & R. A. Huggins, J. Electrochemical Soc., 1981, 128, 729.
3. L.Y. Beaulieu, K.W. Eberman, R.L. Turner, L.J. Krause and J.R. Dahn, Elect. Sol. St. Lett. 2001,
A137.
3
Amorphous active phase
- No secondary phase
observed at the
interface
Æ Si/Transition metal
non-oxide system is
stable during HEMM.
Nanoscale structure
is the key to attaining
the high capacity
2 nm
20nm
Bright Field
20nm
Dark Field
Electrochemical Response(Sn:C=1:1)
(Tetraethyl tin+Ps-resin powder heat-treated at 600 oC for 5h in UHP-Ar,
current rate = 100µA/cm2 )
800
3000
2500
2000
2000
500
1500
400
1000
300
Graphite
200
500
100
Cycle between 0.02 and 1.2V
0
0
0
5
10
15
20
25
30
1000
d Q /d V ( m A h /g .V )
600
V o l.C ap acity (A h /l)
S p .C ap a city (m A h /g )
700
0
0
0.5
1
1.5
-1000
-2000
-3000
-4000
-5000
Cycle Number
- Theoretical Capacity = 636 mAh/g
- Higher capacity than graphite (~480mAh/g or 1450Ah/l)
with stability (0.15%loss/cycle).
Cell Potential (V)
2
2.5
3
HEMM derived Si-C nanocomposites
Nanostructured inactive matrix can endure
large volumetric stresses of the active material due to possible
superplastic deformation that can provide
compressive stresses to accommodate large strains*.
•
ε = σ n d -p D o exp( −Q/RT ) * M.J. Mayo, Nanostructured Materials, Vol. 9 (1997) 717
XTEM of sputter deposited Si film
(250 nm as-deposited film)
Si
100 nm
Conventional XTEM
Cu
2 nm
High Resolution XTEM
Galvanostatic Cycling of 250 nm Si Thin Film (C/2.5 and 2C rates)
High Capacity and Excellent Rate Capability of 250 nm Silicon Thin Film Anode (Area = 1 cm2)
Also note low irreversible
loss ~14% and 0.1 % loss
per cycle.
4500
4199 mAh/g : Theoretical Capacity of Pure Silicon Anode Material
4000
Increase of capacity with cycle # for
the 2C sample may be due to kinetic
limitation of the amorphous Si sample
decreasing the effective diffusion
distance for the lithium ions from the
electrolyte.
Specific Capacity (mAh/g)
in the initial cycles, which is mitigated
gradually in later cycles by
morphological changes in the
electrode,
3500
C/2.5 rate (100uA/cm2)
3000
2C rate (500uA/cm2)
2500
C/2.5 Charge
C/2.5 Discharge
2C Charge
2C Discharge
2000
1500
1000
500
372 mAh/g : Theoretical Capacity of Conventional Graphite Anode Material Used in Commercial Lithium-Ion Batteries
0
0
5
10
15
20
25
30
35
Cycle Number
2 µm
As-Deposited
2 µm
SEM : C/2.5 rate – 30 cycles
2 µm
Si EDAX : C/2.5 rate – 30 cycles
Multilayer Film Schematic
Silicon (250 nm)
Carbon (50nm)
Copper Foil (25 µm)
Note: Drawing not to scale
Note: Ecarbon ~ 50 – 500 GPa
Cr (10 nm)
5000
Cycling of Multilayer Film,
C/2.5 rate
4500
4000
3000
40000
20000
2500
discharge
charge
0
0
0.2
0.4
0.6
0.8
1
1.2
-20000
2000
dQ/dV (mAh/gV)
Specific Capacity (mAh/g-Si)
3500
1500
-40000
-60000
-80000
-100000
1000
-120000
-140000
500
-160000
Cell Potential (V)
0
0
5
10
15
20
25
30
35
40
45
50
Cycle Number
SEM image of a 500°C-4h annealed 250 nm
Si/50nm C/10 nm Cr/Cu multilayer thin film
cycled at 0.4 C after 75 cycles, 5000x.
Interface dynamics control of the nanoscale
films is the key to improved electrochemical
performance
Supercapacitors: Potential Energy Storage Systems
„ Electrochemical Capacitor
10000
4s
“Ultracapacitor”
“Supercapacitor”
40s
Power Density (W/kg)
6 min
„ Primary Battery
1h
1000 Supercapacitor
• Leclanche
• Primary Li
• Alkaline
10 h
100
Zinc-Air
Pb/Acid
NiMH
„ Secondary Battery
100 h
Li-ion
Alkaline
Ni-Cd
10
Fuel Cell
Primary Li
1000 h
Leclanche
1
10
100
1000
Energy Density (Wh/kg)
Ragone Plot
10000
•
•
•
•
•
Pb/Acid
Zinc-Air
Ni-Cd
NiMH
Li-ion
„ Fuel Cell
-
+
-
H2O (Solvent)
Gouy-Chapman
Diffuse Layer
-
1/k
+
_
Cation
2k B T  1 + γ exp(−κx) 

ψ =
ln
ze
 1 − γ exp(−κx) 
 zeϕ o 

 4 k BT 
Anion
γ = tanh
Neutral Molecule
+
-
_
-
Stern Layer
-
σ = (8k BTc ε ε )
∗
1/ 2
o o r
Outer Helmholtz Layer
 zeϕ o 

sinh 
 2 k BT 
Inner Helmholtz Layer
ψ0
ψ d (≈ ζ )
Non-Faradaic : No electron transfer
Ex) Dielectric Capacitor
Electric Double Layer Capacitor
KB :
ϕ0 :
z :
x :
T :
εo :
εr :
Co :
Boltzmann constant
Potential at (x=0)
Ion valency
Distance from the surface
Temperature
Permittivity of free space
Solvent dielectric constant
Electrolyte concentration
θB= f (V)
θA= f (V)
θ2
∆V0.5
Cφ
∆V0.5
Cφ
Ox + ze ↔ Re d
Faradaic : electron transfer
Ex) Battery, Pseudocapacitor
θ : Coverage
V : Voltage
K : Reaction rate constant
g : Adsorption isotherm constant
F : Faraday constant
θ
 VF 
= K exp 
exp( ± gθ )

1−θ
 RT 
Cφ ≈
 VF 
K exp 
 RT 
dθ
F
=
⋅
2
dV RT 
 VF  
1 + K exp 


 RT  

Cφ (= q·dθ/dV)
(g > 0)
θ
θ1
(g = 0)
Competitive Systems
Gravimetric
Capacitance
(F/g)
Electrolyte
Potential
Window
(V)
Scan rate
(mV/s)
Reference
MultiMulti-walled carbon
nanonano-tubes
113
38wt% H2SO4
0
-
Niu et al.
al.
Carbon nanonano-tubes
15 ~ 25
38wt% H2SO4
0 − 0.9
10mA
Ma et al.
Mesoporous Carbon
95
1M LiPF6
0.75 – 3.75
5
Zhou et al.
RuO2
350
0.5M H2SO4
0−1
100
Raistrick et al.
RuO2·xH2O
720 ~ 760
250
0.5M H2SO4
0−1
2
50
Zheng et al.
al.
RuO2 on Carbon Tape
900
0.5M H2SO4
0.2 − 0.7
2
Long et al.
Amorphous MnO2⋅H2O
153
1M KCl
0.1 − 0.85
10
Lee et al.
al.
NiOx
260
1M KOH
-0.4 – 0.5
10
Liu et al.
al.
Co(OH)2 Xerogel
291
1M KOH
-1.1 − 0.8
50
Lin et al.
Fe3O4
38
1M Na2SO4
-1.2 − 1.2
2
Wu et al.
Poly3Poly3(3,5(3,5-difluorophenylthiophene)
23 ~ 30mAh/g
Poly(acrylonitrile)
-2 − 0
25
Searson et al.
Poly(dithienothiophene)Poly(dithienothiophene)-pDTT
15
Poly(ethylene oxide) – PEO
0 − 0.3
50
Arbizzani et al.
Polyaniline (PANI)
25
Poly(methylmethacrylate) – PMMA
0 − 1.2
50
Clement et al.
Polyaniline (PANI)
on Stainless Steel
650~1300
1M LiClO4 in PC
0 − 0.75
200
Prasad et al.
Material
Carbon
Transition Metal Oxide
Polymer
Transition Metal Nitrides: Novel Capacitor Materials
„ Good Electrical Conductivity
• High rate electrochemical response of the capacitor depends on
electrical conductivity of the electrode and the ionic conductivity of
the electrolyte
„ Chemical Stability
• The electrolyte widely used are corrosive (H2SO4 or KOH)
„ Cost
„ High Mass Density (g/cm3)
• To increase volumetric capacitance (F/cc)
„ High Specific Surface Area (m2/g )
• To increase the electrode-electrolyte interface area for charge
storage
Density
Electronic Conductivity
(g/cm3)
σ×10-2Ω-1m-1
Tetragonal
(P42/mnm)
6.97
2×106
133.07
Carbon
-
<1
< 4×104
12.00
TiN
Cubic (Fm3m)
5.43
2.5×104
61.91
VN
Cubic (Fm3m)
6.04
1.67×104
64.95
ZrN
Cubic (Fm3m)
7.09
5.55×104
105.23
NbN
Cubic (Fm3m)
8.3
1.28×104
106.91
Mo2N
Cubic (Pm3m)
8.4
5.05×104
205.88
TaN
Cubic (Fm3m)
14.1
5.55×104
194.96
Ta3N5
Orthorhombic
(Cmcm)

< 10-4
612.87
WN
Cubic (Pn3m)
12.12

197.89
Material
Structure
RuO2
M.W.
Ref: 1) J. P. Zheng, P. J. Cygan, and T. R. Jow, J. Electrochem. Soc., 142(8), (1995) p.2699
2) M. Wixom, L. Owens, J. Parker, J. Lee, I. Song, and L. Thompson, Electrochem. Soc. Proc., 96 (25) (1999)
p. 63
3) P. T. Shaffer, Handbook of High-Temperature Materials, Vol.1, pp.292-92. Plenum Press, New York, (1964)
TEM Analysis: Morphology of the Nanocrystalline Nitirdes
VN500o C
VN-400oC
45
40
35
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
100mV/s
1400
50mV/s
Gravimetric Capacitance (F/g)
Gravimetric Current (I/g)
Electrochemical Response of the Nanocrystalline Nitrides
25mV/s
10mV/s
2mV/s
3
0.25mg/cm
3
0.28mg/cm
1200
3
0.44mg/cm
1000
3
0.67mg/cm
3
0.99mg/cm
800
600
400
200
0
-1.2
-1.0
-0.8
-0.6
-0.4
Potential V (vs. Hg/HgO)
-0.2
0.0
1
10
100
Scan Rate (mV/s)
Figure 2 The (a) cyclic voltammogram of nanocrystallites synthesized at 400oC and (b) gravimetric
capacitance (F/g) versus VN nanocrystallites loading scanned at various rates (2~100mV/s) in 1M
KOH electrolyte.
x 10
3
x 10
3
22
O 1s
1
O 1s
20
1
25
18
20
V 2p3
V 2p3
16
Counts
V 2p1
V 2p1
12
10
10
8
6
(a)
5
540
535
(b)
4
530
525
520
Binding Ene rgy (e V)
515
510
540
505
XPS: Presence of V2O5
before cycling
(a)
535
530
525
520
Binding Ene rgy (e V)
510
505
The FTIR spectra of (a) VN powder, immersed
in 1M KOH solution for 24h and cycled 200
times and XPS spectra of VN powder (b)
before and (c) after electrochemically cycled
for 200 times.
o
o
VN 400 C in 1M KOH for 24h
(c)
Controlled oxidation is the key
minimal reduction in electronic
conductivity
o
970
3047
3535
798
VN 400 C cycled 10times at 2mV/s
3423
4000
515
XPS: Formation of V2O3 after 200 cycles
VN 400 C
(b)
Transmittance (%)
Counts
14
15
3500
V-O
V=O
-OH stretching
3000
2500
2000
1500
-1
Wavenumber (cm )
1000
500
Major Benefits of Direct Methanol Fuel Cells
‹
High Efficiency of Energy Conversion
• Enhanced utilization of fuel
• Lower carbon dioxide emissions
‹
Direct Generation of Electrical Energy
• Losses in mechanical to electrical energy
conversion is avoided
‹
Low temperature operation
• Low emissions of nitrogen oxides, carbon
monoxide and particulates
Potential solution for portable applications
‹
‹
‹
Near Ambient temperature operation
High energy density required
Easy to handle fuel
2W
20W
50-100W
DIRECT METHANOL FUEL CELL SYSTEM
METHANOL
FUEL CELL
500W
1-5kW
CO2 Emissions and Efficiency for Traditional ICE
and methanol or hydrogen fed Fuel Cells
G. Cacciola et al. J. Power Sources 100 (2001) 67-79
Drawbacks
‹
‹
‹
‹
‹
Expensive materials
• Platinum catalysts, fluoropolymers
Components that are expensive to
manufacture
Lack of fuel flexibility
Requires clean fuel; e.g. sulfur free
Electrochemical reactions are sensitive to
poisons in the environment
Direct Methanol Fuel Cell Concept
Anode (Pt-Ru)
CH3OH + H2O
CO2 + 6H+ + 6e-
- LOAD +
6eCO2
H+
H+
+
H+
H+
6H+
H+
H+
H+
H+
H+
H+
H+
H+
CO2
<3% Methanol / water
FUEL
6e-
H2O
+
CH3OH
3% Methanol / water
- Electrode (Anode)
3H2O
6H+
+
3/2 O2
H20, N2, O2
OXIDANT
Air (O2)
+ Electrode (Cathode)
Cathode (Pt)
PROTON EXCHANGE MEMBRANE (PEM)
6H+ + 1.5 O2 + 6e-
3H2O
Synthesis of Catalysts
•
Most synthetic approaches in the literature are based on the reduction
of Halide precursors of Pt and Ru
•
Limitations:
—Need for several washing steps to eliminate unwanted by-products
—Tedious process leading to loss of active material
— Poisoning of the catalyst
•
Need for improved catalyst using non-halogen containing precursors
•
Objective: Develop novel sol-gel based methods using non-halogen
precursors for generating Pt-Ru catalysts
— High surface area
— Pt to Ru ratio close to unity
— High electrochemical activity
— (low polarization of the electrode)
Morphology Characterization Transmission Electron Microscopy (TEM)
•TEM micrographs of the powder possessing the highest surface area (120-160 m2/g)
with the best electrochemical activity (optimized sample)
—Diffraction pattern indicates single phase Pt-Ru composition
—HRTEM images show 2-4 nm sized crystallites
Electrochemical Characterization
Anode polarization data
•Results of electrochemical tests conducted on the powders derived using a large excess of
tetramethylammonium hydroxide [TMAH : (Pt-acac+Ru-acac) = 1.75:1 and Pt:Ru = 1:1]
SSA = 97.9 m2/g
0.8
0.8
o
E (25C, measured)
E' (25oC, corrected)
0.7
E' (60oC, corrected)
0.6
0.5
y = 0.28913 + 0.014178x R= 0.95483
0.4
o
E (25C, measured)
E' (25oC, corrected)
0.7
o
E (60C, measured)
Potential vs H2/H+
Potential vs H2/H+
SSA
= 139.6 m2/g
1214.10
0.3
y = 0.19655 + 0.01602x R= 0.95062
0.2
o
E (60C, measured)
E' (60oC, corrected)
0.6
0.5
0.4
y = 0.26396 + 0.014772x R= 0.97508
0.3
y = 0.18419 + 0.013872x R= 0.95196
0.2
0.1
0.1
0
0
0
(a)
2
4
6
Ln(mA/g)
8
Two step heat treatment of
10
0
2
4
6
Ln(mA/g)
8
(b) Two step heat treatment of
400/3h/1%O2 (3 g) Æ (0.5g)300/3h/1%O2 300/2h/1%O2 (3 g) Æ (0/5g)300/4h/1%O2
IV-1-1
IV-1-2
10
Comparison of catalytic activity
Specific Intercept on y axis
surface area in polarization
curve
(m2 /g)
25°C
60°C
JPL
49.5
0.305
0.182
Giner
94.2
0.269
0.177
Johnson71.3
0.298
0.173
Matthey
1208.12
94.2
0.214
0.155
Sample
Slope in
Cell potential (V)
polarization
at ln(mA/g)=8
curve
(60°C)
25°C 60°C
0.0216 0.0193
0.336
0.0273 0.0306
0.422
0.0127 0.0163
0.304
0.0284 0.0231
0.340
1212.03
95.9
0.322
0.217
0.0169 0.0194
0.372
1214.03
97.9
0.289
0.197
0.0142 0.0160
0.325
1214.10
139.6
0.264
0.184
0.0148 0.0139
0.295
TM AH:acac=1.25:1
Pt:Ru = 1:1
TM AH:acac=1.25:1
Pt:Ru = 1:1.25
TM AH:acac=1.25:1
Pt:Ru = 1:1.5
TMAH:acac=1.75:1
Pt:Ru = 1:1
Sample
identification
Surface
area(m2/g)
Phase/phases
present
Lattice
parameter
(nm)
Carbon
content
(%)
JM catalyst
70
Pt(Ru)
0.3866
0
JM catalyst(600oC/6h in Ar)
-
Pt(Ru) +Ru
0.3858
0
Sample A
(5000C/6h in Ar)
1
Pt(Ru) + C
0.3861
63
Sample B
(Sample A:
3000C/2h in Ar1% O2)
158
Pt(Ru) + C
0.3859
3
Sample C
(Sample B:
3000C/2h in Ar1% O2)
120
Pt(Ru)
0.3859
0
Prospects for Future
Nanomaterials show potential for energy
storage
‹ Nanoscale phenomena in batteries, fuel
cells and supercapacitors still need to be
studied
‹ Considerable potential for integration,
theoretical and experimental studies
‹