Electrochemical Conversion in Solvent Free Salt Electrolytes
Don Gervasio
Dept of Chemical & Environmental Energy
Dept.
University of Arizona
Email: gervasio@email.arizona.edu
Office Phone: (520) 361 – 4879
NGC2011
Nano and Giga Challenges in Electronics, Photonics and Renewable Energy
Moscow Zelenograd, Russia
Moscow-Zelenograd,
September 12 – 16, 2011
11:00 to 11:30 Thursday 15 September2011
1
OUTLINE
A. Motivation for Salt Electrolytes
B. Properties of Salt Electrolytes
C. Electroplated silicon from non protic ionic liquid (IL)electrolytes
advanced processing for forming photovoltaic
D. Fuel Cell performance in proton-hopping salt electrolytes
li id salt
liquid
lt electrolytes,
l t l t
protic
ti iionic
i li
liquid
id (pIL)
( IL)
solid salt polymer ionic membranes (pIM)
F. Conclusions
2
Motivation
Problems with water
• System that require solvent water need to be humidified, a expensive and
inefficient complication
• Water limits the temperature of operation to below 100oC
• Below 100oC there are 2 phase fluidics which are difficult to handle
• Water interferes with electrocatalysis even when using an “unreactive”
noble metal, platinum
Solution…don’t use aqueous electrolytes use non-aqueous salts
• Ionic liquids - prototypical salt electrolytes
• salt membranes - ion conducting electrode separators
3
Properties of Ionic Liquids
• Ionic Liquids (ILs) are molten salts
with
ith melting
lti points
i t below
b l
100°C
100°C.
• Room temperature ionic liquids
(RTILS) melt below 25oC.
• Composed
C
d off over 99.99%
99 99% ions
i
• Low vapor pressures
• Highly conductive
• Excellent solvating properties
• Thermally stable, for both high and
low temperature uses.
• Large voltage window for electrohttp://www.nature.com/nature/journal/v439/n7078/images/nature04451-f2.2.jpg
chemical reactions (Vcell up to 5 volt)
• Low water activity
- important to activity of Pt
Pt-catalyzed
catalyzed cathode
- may permit use of non-Pt catalysts
4
Typical room temperature ionic liquids (RTILs)
 Room temperature ionic liquids (RTILs) consist of:
 Bulky and asymmetric organic positive ions (cations), such as:
• 1-alkyl-3-methylimidazolium,
• 1-alkylpyridinium,
• N-methyl-N-alkylpyrrolidinium
• ammonium ions.
 Negative
N
ti iions (anions)
( i
) include:
i l d
• halides (Cl-, F-, Br-, I-) which generally give high melting salts,
• Inorganic anions such
• tetrafluoroborate
Tailorable
• hexafluorophosphate
Properties, such as:
• large organic anions
• melting point,
• bistriflimide
• viscosity,
viscosity
• triflate
• solubility
• tosylate
• stability
• formate
are determined by the identity and
• alkylsulfate
geometry of substituents on the
• alkylphosphate
cation and anion.
• glycolate
5
Simple preparation of ILs
Amine + Acid
Stirring in acetone/dry-ice bath
Example: ammonium
hydrogen sulfate salts
HSO4-
Solution
Heated at 120oC in Silicon bath
H2O Removal
Bu2NH2+
EtNH3+
Me2NH2+
Me3NH+
Dried in Vacuum Oven at 90oC
Mass analysis, Karl-Fischer titration
Samples : H2O = 100 : <1 (mol%)
2 RT ILs
6
Conductivity of Ionic Liquids
[NH4]+[HF2]-
7.7M aqueous LiCl solution
1 1M aqueous LiCl
1.1M
4:6 DMAN-MAN
EAN
IL electrolytes have
ionic conductivities
rivaling aqueous solutions
W. Xu and C.A. Angell, Science, (302), 422, 2003
7
Advanced Silicon Processing
In stable ionic liquids
8
Requirements of Electrolytes For Plating Active Metals
CONDUCTIVITY
• Conductor for ions, Ionic Conductivity > 10-2 Siemen/cm
• Insulator for electrons, Electron Conductivity < 10-9 Siemen/cm
COMPATIBILITY WITH ELECTROPLATING OF ACTIVE METALS
• No
N water,
t no passive
i layer
l
( t l oxide,
(metal
id “tar”)
“t ”) formation
f
ti
STABILITY
• Stable to electric field; no reaction with products
All electrolyte properties accessible by tailoring the ionic liquid
Abedin, S. Z. et al., “Electrodeposition of Metals and Semiconductors in
Air- and Water-Stable Ionic Liquids”, Chemphyschem [2006] 7 , 58-61
9
Silicon Plating from Room Temperature Ionic Liquid (RTIL)
like Osteryoung’s
Osteryoung s aluminum plating process
• Ionic liquid electrolyte: R+ X- (e.g.,
R+ = n-butylpyridinium, X- = Cl-) and
source of Si (SiCl4, NaSiF6, SiHCl3, or Si2Cl6).
• Reactions for silicon electroplating:
IL Rxn with Si:
2 R+ Cl- +
Cathode Rxn:
Anode Rxn:
Net Rxn:
Si2Cl6 →
2 R+ SiCl4-
R+ SiCl4- + 3 e- → Si + R+ Cl- + 3 ClSi + R+ Cl- + 3 Cl- → R+ SiCl4- + 3 eTransfer of Si from anode to cathode
i
Power supply
Experimental setup
- 3 electrode configuation cell
i) cathode, ii) anode, iii) reference electrode (e.g., Ag/AgCl)
- operating temperature: 120oC to exclude water
- inert gas purge (N2 or Argon) to exclude water
Borisenko et al., “In Situ STM Investigation of Gold Reconstruction and of Silicon Electrodeposition on Au(111) in the Room
Temperature Ionic Liquid 1-Butyl-1-methylpyrrolidinium+ Bis(trifluoromethylsulfonyl) imide-”, J. Phys. Chem. B, 2006, 110, 6250-6256.
Abedin et al., “Electrodeposition of Metals and Semiconductors in Air- and Water-Stable Ionic Liquids”, Chemphyschem., 2006, 7 ,
58-61.
+
-
Ref:
Cathode
Ag/AgCl
Si
Cathode
Si
A d
Anode
Si
Anode
: Si
10
Si Electroplating vs. conventional Si processing
Motivation for electroplating Silicon
it’s a cleaner, lower energy and more versatile alternative to conventional Si processing

Electroplating Si from Ionic Liquid
•
•
•
•
•

Power Supply
Si
Simple
l process tto make
k complex
l structures
t
t
Can plate doped Si
Relatively low T process ( < 120ºC)
y friendly
y ((no VOCs))
Environmentally
Relatively low cost
WE:
Si cathode
RE:
Ag/AgCl
CE:
Pt
Silicon production from Si-wafer processing
• High temperature, Energy Intensive process
– React sand w/ Zn metal (950ºC) OR
– CVD on pure Si rod (1150ºC)
– Purified material melted down (1414ºC)
(1414 C), Czochralski process
• Complex processes to make structures in wafer
– Si wafer prep.: Wafer sliced, lapped, polished, masked, etched, etc.
– High temp process, Doping, annealing, oxide growth, etc.
• Hazardous waste and process fluids (HF, SiH4)
• High cost
11
First step: Silicon plating on Metal
Electrodeposition
El
t d
iti off silicon
ili
on titanium
tit i
in
i a room temperature
t
t
ionic
i i
liquid: butyl, 3-methylimidazolium chloride , [Bmim] BF4
Cyclic voltammogram of titanium in
[Bmim]BF4 with ferrocencene but no SiCl4
under N2 atmosphere.
atmosphere Scan rate: 10mV/s.
10mV/s
Temperature: 25C.
Cyclic voltammogram of titanium in
[Bmim]BF4 saturated with SiCl4 under
N2 atmosphere. Scan rate: 10mV/s.
Temperature: 25C.
12
Electroplating of Si on titanium metal at constant potential
Chronoamperometry of titanium metal in [Bmim]BF4 electrolyte saturated with SiCl4 at
constant potential of ‐2.0 V vs. Fc/Fc+ under a N2 atmosphere at a temperature of 25C.
13
Scanning Electron Microscopy (SEM)
Si
bare Ti
Ti
SEM image of silicon on titanium and bare titanium substrate
14
Energy Dispersive Absorption of X
X-rays
rays (EDAX)
Si
EDX for the Ti sample
b f
before
Si electrodeposition
l t d
iti
EDX for the Ti sample with Si deposited
at -2.0V
2 0V for 10 minutes
15
Thick versus thin silicon photovoltaics
Efficiency  1/2
Large Grain
Small Grain
Polycrystalline
Solids
Single
Crystal Si
El t l t d Si
Electroplated Si
d
For high efficiency
With long d
Requires high large grain)
High Cost
d
With shorter d
Allows lower small grain)
Low Cost
 (time for recapture of photoelectron by Si) decreases as grain size (and cost) decreases
“RADIAL PN JUNCTION, WIRE ARRAY SOLAR CELLS”, B. M. Kayes, M. A. Filler, M. D. Henry, J. R. Maiolo III, M. D. Kelzenberg, M. C. Putnam, J. M. Spurgeon, K. E. Plass,A. Scherer, N. S. Lewis, H. A. Atwater California Institute of Technology, Pasadena, CA 91125
16
Versatility of electroplating Si : Photovoltaics
ELECTROPLATING allows
ll
fforming
i
complex
l Si structures
t
t
att llow temperatures
t
t
This is almost IMPOSSIBLE to make using CONVENTIONAL Si PROCESSING
e.g., Vertically oriented nano-structured Schottky diodes
Brews and Palusinski
17
Decoupling
ecoup g
LiAlCl4
loog (equivqlent conduuctivity)
Advanced “Ion Hopping” Salt Electrolytes
Superionic
glasses
Good Ionic
Liquids
1M KCl (standard)
Poor Ionic
Liquids
Superionic
Liquids
q
Ion Association
Acetate
Formate
high vapor pressure
Id l line
Ideal
li
Non-ionic Liquids
Log
(Fluidity)
log (fluidity)
Summary: ion conduction mechanisms • on Ideal line (KCl standard) is by translation only
• in Superionic Liquid region is by hopping
• in Poor Ionic Liquid
i P
I i Li id region is due to frictional drag
i i d t f i ti
ld
Conduction mechanism: the Walden Plot
P. Walden, Bull. Acad. Imper. Sci., 1800 (1914).
18
H2 / O2 Fuel Cell Characteristics
- ion conducting electrolyte surrounded by two electrodes
- dissimilar feeds at the identical electrodes ((hydrogen
y
g at anode,, oxygen
yg at cathode)) generate
g
0
0
dissimilar electrode potentials whose difference E C - E A is the fuel cell voltage V0FC
Battery
‐ Different electrodes Different electrodes
* Nickel
* Cadmium
‐ in closed package
Cathode Reaction:
Fuel Cell Reaction:
_
Anode
+
H2
Air
(H2  2H+ + 2 e-)
Fuel Cell
‐ Same electrodes ‐ fed different feeds * hydrogen
* oxygen
‐ open package
Anode Reaction:
Ion-conducting
Ion conductorElectrolyte
Pt-catalyzed
Pt
catalyzed
(1/2 O2 + 2 e- + 2H+  H20)
H+
Cathode
2e
Pt-catalyzed
Load
H2
 2 H + + 2 e-
½ O2 + 2 e- + 2 H+ 
H2 + ½ O2

External Circuit
Eoa = 0 V
H2O
Eoc = 1.23 V
H2O + G0
V0FC = E0c – E0a = 1.23 V
ΔG0 = -nFV0FC = -230 kJ
19
Role of the Electrolyte
General Electrolyte considerations
1. Conductor for protons, Conductivity for H+ > 10-2 Siemen/cm
2. Insulator for electrons, Conductivity for e- < 10-9 Siemen/cm
3. Operate at temperatures from - 50 to + 230oC
4 Stable
4.
St bl to
t acid
id (anode)
(
d ) and
d alkaline
lk li (cathode)
( th d ) environments
i
t
5. Stable in presence of H2 and catalyst (Pt) under up to 230oC
6. Stable in presence of O2 (from air) and catalyst (Pt) to 230oC
Special considerations with non-aqueous protic salt electrolyte
 more efficient fuel cells, because salt electrolytes
o have no water
water, expect no Pt
Pt-oxide
oxide hindering O2 cathode
 more robust fuel cells, because salt electrolyte
o stable over large range of operating temperature
o less corrosive
o conducts only proton with no water, so no humidification needed.
20
Electrocatalysis in Non-aqueous vs Aqueous Electrolytes
Compound Cyclic voltammogram of Pt surface in:
• aqueous sulfuric acid electrolyte (solid black line)
• versus protic salt (dotted blue line).
Heat of Formation at T 298 K
Heat of Formation at T = 298 K
kJ/mole
cal/g
PtO2 A
‐80 ‐84
PtH2.76O3.89 B
‐520 ‐478
PtO2.52 C
‐101 ‐104
____________________________________
A: MW = 227 B: MW = 260 C: MW=235
“Standard enthalpy of formation of platinum hydrous oxide”,
Yatsuhisa
Y
t hi Nagano,
N
JJ. Th
Thermall Analysis
A l i andd Calorimetry,
C l i t
69
69,
831-839 (2002).
With protic salt electrolyte
There is
•
no
o Pt-oxide
t o de formation
o at o
•
nor reduction
Pt →Pt-oxide
With aqueous electrolyte
Pt oxide formation
Pt-oxide
Pt + H2O → Pt-OH + H+ + e-
no Pt-oxide forms
no impeding of O2 reduction.
Pt-oxide reduction
Pt ← Pt
Pt-oxide
oxide
Pt-OH + H+ + e- → Pt + H2O
Pt-oxide impedes O2
reduction.
H20
H20 H20
H20
molecular oxygen
Pt atom
Voltammetry of Pt in aqueous and ionic liquids.
liquids
Initial potential was 0.5 V vs RHE. Scan rate: 50
mV/s. Ar atmosphere. Temp. : 30 °C, A= 1cm2.
H20
H20
21
Ion Hopping in aqueous electrolytes

Electrons are readily transported in conduction band in metals and semiconductors.


IIon conduction
d ti is
i nott as simple.
i l Ions
I
are conducted
d t d in
i electrolytes.
l t l t
Most electrolytes contain water, which plays 2 roles:
1.water ionizes salts into positive and negative ions; ionic conductivity results from the translational diffusion of
ions, a vehicular mechanism;
2.water p
provides a “proton
p
hopping
pp g p
path” (p
(proton transport
p via hydrogen
y g bonding
g and rotations in molecules that
are not translating) resulting in higher proton mobility than is possible by vehicular diffusion of ions alone.
H+ hopping in water
from coordinated rotations and vibrations
In water solution at 298 K, sodium ion (Na+) is approximately the same size as hydronium ion (H+), yet
Na+ mobility is 5.19 x 10-4 cm2s-1V-1
H+ mobility is 36.23 x 10-4 cm2s-1V-1
Greater H+ mobility in water because H+ moves by both “hopping” and “vehicular” motion.
Lower Na+ mobility because Na+ can only move by slower “vehicular” translational diffusion.
Few natural materials besides water show H+ Hopping
22
Proton-hopping in phos. acid &salt electrolytes
 Phosphoric
p
acid ((H3PO4) conducts H+ with no free water.
H
H
H
|
|
|
O
O
O
|
|
|
O = P - OH … O = P - OH … H - O -- P = O
|
|
|
O
O
O
|
|
|
H
H
H
Initial State
Proton transfers between phosphoric acid, H3PO4, and
phosphonium ion, H4PO4+, by rotational and vibration motions,
because these 2 species have:
• suitable energy separation (ΔpK =5 ~ 0.3eV)
0 3eV)
• high symmetry (tetrahedra).
 Salt electrolytes
A new class of proton-conducting salt electrolyte was
conceived called :
• protic ionic liquid (pIL) when in liquid form,
protic ionic membrane (p
(pIM)) in p
polymeric
y
form.
• p
in which a neat protic salt electrolyte forms by transfer
of proton from a Bronsted acid to a Bronsted base.
Salt electrolyte has high proton-conductivity with no water
when the constituent acid and base have :
• optimal difference in pKa (5-14) and
• highly symmetrical ions (rotationally free).
H+
Proton IN
H
H
H
|
|
|
O
O
O
|
|
|
O = P - OH … O = P - OH … H - O -- P = O
|
|
|
O
O
O
|
|
|
H
H
H
1) vibrate H+ on
H+ …
H
+ H
H
|
|
|
O 2) vibrate O
O
+
| H transfer|
|
H …O - P - O - H … O = P - O- H … O = P - O-H
|
|
|
O
O
O
|
|
|
H
H
H
H
H
+ H
|
| 3) vibrate |
O
O
O
+
|
| H transfer|
H-O - P = O … H - O - P - O - H …O = P - O-H
|
|
|
O
O
O
|
|
|
H
H
H
H
H
|
|
O
O 4) vibrate
|
|
H+ over
H-O - P = O … H-O - P = O … H - O |
|
O
O
|
|
H
H
H
|
O
|
P-O-H
|
O
|
H
H
H
H
|
|
| 5)
O
O
O
|
|
|
HO - P = O … HO P = O … H O -- P = O
|
|
|
O
O
O
|
|
|
H
H
H
+
vibrate H+ off
…. H+
6) 3 rotations
H
H
H
|
|
|
O
O
O
|
|
|
O = P - OH … O = P - OH … H - O -- P = O
|
|
|
O
O
O
|
|
|
H
H
H
Proton OUT
H+
23State
Final
Proton energy levels and pILs
A protic ionic liquid (pIL) is
made by
y transferring
gap
proton
from an acid to a base.
Tf - H3O+
E
EtNH3+… NO3-
HNO 3 … EtNH2
EAN
Δpk
p = 14
Proton Coordinate
Energy Diagram for the EAN
(ethyl ammonium nitrate) pIL
showing :
• proton transferred (Left)
• not transferred (Right)
EAN, P. Walden, Bull. Acad. Imper. Sci. St.
Pétersbourg, 6 8: pp. 405-422 (1914) .
Gurney
yp
proton energy
gy level diagram.
g
For any pair of levels, the stable entities are
upper right and lower left.
24
R. W. Gurney, Ionic Processes in Solutions, Dover publications, New York (1953).
Preparation of protic ionic liquids (pILs)
Example: ammonium
hydrogen sulfate salts
A neat protic salt electrolyte
forms by transfer of proton
from a Bronsted acid to a
Bronsted base.
HSO4-
• Use Gurney diagram as a
guide for energetics ( pK)
Bu2NH2+
EtNH3+
Me2NH2+
Me3NH+
• Use size mismatch
drive liquid formation.
to
• Use symmetry as a guide
to proton conductivity
2 RT pILs
25
First pIL fuel cell electrolyte tested, EAN
“Binary Inorganic salt mixtures as high conductivity electrolytes for >100ºC fuel cells”, J.-P. Belieres, D. Gervasio, C. A.
Angell, Chem. Commun., 4799-4801 (2006).
“Brønsted acid–base ionic liquids and their use as new materials for anhydrous proton conductors”, Md. A. B. H. Susan, Akihiro Noda,
Shigenori Mitsushima and Masayoshi Watanabe, Chem. Commun., 2003, 938 - 939.
load
O2 in
H2 in
Pt wire
Anode
Cathode
Bubbler
Electrolyte
Oil
Bath
Stir Bar
Simple set up for evaluating performance of
a fuel cell with a liquid electrolyte.
Schematic diagram of the set up for evaluating liquid fuel cell
electrolytes with Pt-catalyzed porous gas-fed electrodes.
Cell V
Voltage (volts)
1.3
1.2
Pt wire electrodes
1.1
active
ti area ~ 1cm
1
2
1.0
0.9
0.8
[EtNH3][NO3] - 100oC
[Me2NH2][HF2] - 25oC
0.7
0.6
0.5
0.4
0.3
0.2
H3PO4 85% - 100oC
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Current (milliamps)
Polarization curves for fuel cell with Pt wires fed
hydrogen and oxygen in EAN (red star), Me2NH2:HF2
(green triangle) and 85% H3PO4 (black circle).
Polarization curves H2/O2 fuel cell with EAN, 99% EAN1%EAP and 85% H3PO4 electrolytes on Pt-catalyzed porous
gas-fed ETEK V3 electrodes. Pt loading =0.5 mg Pt/cm2.
26
Electrode wetting flooding greatly attenuated when 1% ethyl ammonium phosphate (EAP) was added to EAN
V lt
Voltammetry
t off Pt in
i EAN equilibrated
ilib t d with
ith oxygen gas
-0.20.2
0.1
EAN- 5 scans
Diminishing O2 reduction
i (m
mA)
I (m
mA)
00.0
-0.1
5
-0.2-0.2
3
WE: Pt 2cm2
RE:WE:
SCE
Platinum 2cm
RHE
CE:RE:
Graphite
CE: Carbon Graphite
SR:100mV/s
100mV/s
o
Atmosphere:
Oxygen
C
T: 25
25 C
At
Atmosphere:
h
O2
2
-0.3
1
0.4
-0.4
-0.5
-0.5
-0.5
o
0.0
0E vs. RHE (V)
0.5
0.5
E (volts) vs. SCE
Voltammetry of Pt wire in EAN equilibrated with
oxygen over electrolyte. Pt wire area = 2 cm2.
EAN has limited oxidative stability on Pt at high (oxidizing) potentials.
27
A stable inorganic pIL: Hydronium Triflate
CF3SO3H
(acid)
+
H2O
F3C
C-SO
SO2 -
O-
T ifl t = F3CSO3Triflate
H
O - H+
H
H d i
Hydronium
= H3O+
(base)
Fully optimized (B3LYP/6-31G) conformations of hydronium triflate
“The modeling of molecular structure and ion transport in sulfonic acid based ionomer membranes”, S. J.
Paddison, Journal of New Materials for Materials for Electrochemical Systems 4, 197-207 (2001)
28
NMR characterization of H+ diffusivity and Ea for hydronium triflate , model pIL
1H Diffusivity (m2/s)
19F Diffusivity (m2/s)
ln(1H Diffusivity (m2/s))
ln(19F Diffusivity (m2/s))
-10
-21.5
4 10
y = -14.285 - 2812.8x R= 0.99909
-10
3.5 10
y = -13.823 - 3129x R= 0.9996
ln( H
H, F diffusivity (m /s))
-10
2
3 10
1
-100
2 5 10
2.5
Hydronium Triflate
-10
2 10
-10
-23
19
1.5 10
-22.5
Ea for transport properties
1
19
2
H, F Diffusivity (m /s)
H
-22
-10
1 10
-11
5 10
0
30
40
50
60
70
80
90
Temperature (°C)
100
110
120
-23.5
H+ diffusion: 23.4 +/- 1.2 kJ/mol
F diffusion: 26.0 +/- 1.3 kJ/mol
Ion Conduction: 22.1 +/- 1.1 kJ/mol
Viscosity: 26.9 +/- 1.3 kJ/mol
-24
0 0026
0.0026
0 002
0.0027
0 0028
0.0028
0 0029
0.0029
0 003
0.003
0 0031
0.0031
0 0032
0.0032
1/Temperature (1/K)
•That
That proton has 1.5x
1 5x’s
s higher diffusivity and lower Ea than F over the
range of temperatures indicates H+ hopping can occur in a pIL.
29 triflate.
•Similar studies underway for another pIL, fluoropyridinium
29
Conduction mechanism: the Walden Plot
Decoupling
LiAlCl4
logg (equivqleent conducctivity)
P. Walden, Bull. Acad. Imper. Sci., 1800 (1914).
Superionic
glasses
Good Ionic
Liquids
q
1M KCl (standard)
Poor Ionic
Liquids
Superionic
Li id
Liquids
Ion Association
Acetate
Formate
high vapor pressure
Ideal line
Non-ionic Liquids
Log
log
ogg (fluidity)
(((Fluidity)
u d ty) y)
Summary: ion conduction mechanisms • on Ideal line (KCl standard) is by translation only
• in Superionic
in Superionic Liquid region is by hopping
region is by hopping
• in Poor Ionic Liquid region is due to frictional drag
30
Fluorinated Ionic Liquid vs Aqueous Fluorinated Acid
2-FPTf Neat, Platinum Cyclic Voltammetry
Currrent Density (m
mA/cm^2)
0.5
12V
1.2
09V
0.9
0.0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-0.5
-1.0
-1 5
-1.5
WE: Platinum 0.1 cm2
CE: Platinum
RE: RHE
Scan Rate: 100mV/s
Temp: 80°C
1 atm Oxygen
WE: Platinum 0.1 cm2
CE: Platinum
RE: RHE
Scan Rate: 100mV/s
Temp: 80°C
1 atm Oxygen
-2.0
Potential (V) vs. RHE
Voltammetry of 2
2--Fluoropyridinium triflate shows that it is stable after 100 cycles.
yg reduction starts near the thermodynamic
y
limit for ORR [[1.18V at 80
80°°C]]
Oxygen
31
Fluoropyridinium
py
triflate (2-FPf)
(
) Electrolyte
y
- the first synthetic liquid electrolyte that out performs phosphoric acid
-
+
+
H
Trifluoromethane
sulfonic acid
(TFMSA)
2-Fluoropyridine
15% more efficient
2-Fluoropyridinium Triflate
(2-FPTf) Protic Ionic Liquid
m.p. 85
85°C
C
2-FPTf is made by mixing
1 part triflic acid (TFMSA) and
1 part 2-fluoropyridine
2 fluoropyridine
Why does 2-FPTf work better?
2-FPTf
is
a
“water-free”
electrolyte so platinum-oxide
platinum oxide
does NOT form and the oxygen
electrode is NOT polarized.
I/V curves for H2 and O2 fed to Pt‐catalyzed porous electrodes in 2‐
FPTf electrolyte at 80oC and 120C and 85% phosphoric acid
electrolyte at 80oC. σ(2‐FPTf) = 4x10‐3 Scm‐1 , A = 0.5 cm2,
thicknesselectrolyte = 0.3 cm.
The cell voltage and fuel cell
efficiency increase. “A Fluorinated Ionic Liquid as a High-Performance Fuel Cell Electrolyte”, Jeffery Thomson, Patrick Dunn,
Lisa Holmes, Jean-Phillipe Belieres, C. Austen Angell, Don Gervasio, Electrochemical Transactions (2008)
32
The need for solid polymer electrolytes
Three major issues make liquid electrolytes in fuel cells
unacceptable ….
1. Liquids can seep out of cells leaking harmful corrosive material
2. Leaked liquid can cause electrical shorts between cells stacked in
series
i leading
l di
to
t voltage
lt
and
d power density
d
it drops.
d
3. Liquids soften structural materials, which accelerates failure of a fuel
cell power source.
All of these issues can be corrected by using solid electrolytes.
33
Liquid
q
vs. Polymer
y
Electrolyte
y Membrane ((PEM)) Fuel Cell
Liquid Fuel Cell
Teflon block
Holds electrodes
Contains liquid acid
Schematic representation of
PTFE micro fuel cell with Gas Fed
Electrodes and liquid electrolyte,
like phosphoric acid.
Solid PEM Fuel Cell
MEA
With electrodes
on membrane
PEM Preferred
No leaks
No shorts
Less Corrosion
Schematic representation of
a micro fuel cell with Gas Fed
Electrodes and PEM electrolyte,
like Nafion.
34
Electrolyte requirements
with solid polymer electrolyte membranes (PEMs)
1.
2.
3.
4
4.
5.
6.
Low permeability to H2 and O2 gas
No electro-osmosis (transfer of water with proton)
Dimensionally stable with change of hydration state
Di
Dimensionally
i
ll stable
t bl with
ith temperature
t
t
(l
(low
expansion
i coefficient)
ffi i t)
Good cohesion/adhesion to electrode
Pinhole free (low reactants crossing over, no electrode shorting)
Polysiloxane
R2
R1
R1 and R2 are pendant acid or base groups
Cellulose
Useful
Polymers
[
]x
4-Polyvinylpyridine
Hydrocarbon and perfluorocarbon (Nafion)
N
]x
[
N
N
P
P
R1
Polysulfone
N
P
Polyphosphazene
R1 and
d R2 are pendant
d t acid
id or b
base groups
R2
N
N
N
N
H
H
n
•Polybenzimidazole (PBI)
•
Typically doped with 3 to 6 H3PO4 per polymer repeat unit
35
State of the Art low temperature PEM, Nafion, uses water
Too little water, no proton conduction,
• pendant sulfonic acid groups neither all ionized nor bridged by water, Figure (a).
With 3 (or more) waters per acid group, the membrane conducts proton.
• water bridges completely‐ionized acid groups, as shown in Figure (b).
a) Non-conducting
Low water form
-S
SO3H
H 3O +
b) Conducting
High water form
bulk water
-
-S
SO 3
H 2O
 But..3 waters per acid group means bulk‐like water is in the membrane, consequently:
oC or lower at atmospheric pressure;
• the fuel‐cell operation temperature must be 80
p
p
p
p
;
o humidification of feed gases is required to retain solvent water;  so system efficiency suffers from parasitic power for humidifier
o large radiators are needed to reject waste heat from 80oC to room temperature
 so system power‐density drops due to bulk of radiator • performance of platinum catalyzed cathodes is poor in presence of water so Pt catalysis suffers, because at performance of platinum catalyzed cathodes is poor in presence of water so Pt catalysis suffers because at
high potentials, platinum oxides form…No air‐cathode activity until Pt‐oxide is reduced at 0.85V or lower, so fuel‐cell efficiency is no higher than ~60%, • Parasitic losses from radiator and humidifier drive SYSTEM efficiency ever lower (<40%).
36
State of Art HT PEM Fuel Cell: phosphoric acid loaded PBI,
uses NO WATER
Polymer
y
modified Electrodes in Elevated Temperature
p
Fuel Cell
T = 160oC, P = 1atm, O2 , H2
0.9
350
0.8
300
250
0.6
0.5
200
0.4
150
0.3
100
Power out (mW/cm2)
Cell Volta
age (volts)
0.7
0.2
50
0.1
0
1
10
0
1000
100
Current Density (mA/cm2)
cell V polymer-E >
cell V polymer-E <
Power out
Power out <
1. Good performance under load with simplified BOP, small radiator and no need for humidification.
2 However there are issues :
2.
However there are issues :
• operation limited to above 140oC (water washed out phos. acid) but no more than 190oC (dehydrates) • low cell efficiency (fuel cell voltage) due to phosphoric acid adsorption on Pt 37
Solid Protic Ionic Membrane (pIM)
Polyvinylpyridinium phosphate (PVPP) salt electrolyte
PVPP, a solid “proton wire” that uses no water and leaches no ions
]x
H3PO4
18
x
Voltage
g ((V))
10
1.0
o
second IV test at 162 C
16
14
N
-2
0.8
12
H2PO4‐
The cell was run overnight under
constant load of 30 mA/cm2. After this
overnight test, the polarization (I/V test)
did not change.
g strong
g evidence that
the proton is hopping through this
solid membrane that has no leachable
ions or solvents
solvents..
10
0.6
E/V
PVPP made by reacting each
pyridine
in
polyvinylpyridine
polymer with 1 phosphoric acid
8
0.4
mW cm
Power / m
[
6
4
0.2
2
0
0.0
0
20
40
60
80
100
-2
I / mA cm
I/V curve for H2/O2 fuel‐cell with poly vinyl
pyridine fully neutralized with phosphoric
acid Temp.
acid.
Temp = 162oC; σ =0.005
=0 005 S/cm.
S/cm
This kind of advanced salt PEM in a fuel cell will be very reliable in practical use.
38
Cell voltage in time at constant load
for a H2/O2 fuel-cell with a PVPP membrane
1.1
Volts
-2
o
I = 30mA cm
at 162 C
1.0
09
0.9
The cell was run overnight under
constant load of 30 mA/cm2. After
this overnight test, the polarization
(I/V test)
t t) did nott change.
h
0.8
0.7
E/V
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-2
0
2
4
6
8
10
12
14
16
18
time / hour
Cell voltage in time at constant load for a
H2/O2 fuel‐cell with a membrane made of
polyvinylpyridine in which each pyridine is
phosphoric
p
acid. Pt loadingg
reacted with p
2
=0.5 mg/cm E‐Tek ELAT electrodes. Load is
30 mA/cm2. Temperature = 162oC.
Overnight fuel cell stability while
passing H+ current is strong
evidence that the proton is
hopping through this solid
membrane that has no leachable
ions or solvents.
39
E-chem H-Pump with PVPP membrane
00
0.0
o
P4VP H3PO4 (2:1)
P4VP-H
(2 1) H
Hydrogen
d
pump ttestt att 160 C
-0.2
-0.4
-0.6
E/V
08
-0.8
-1.0
-1.2
-1.4
I/V test
-1.6
-1.8
0
10
20
30
40
50
I / mA cm
60
70
80
90
100
-2
o
P4VP-H3PO4 (2:1) Hydrogen pump test at 175 C
0.0
I = 20mA cm
-0.2
-2
-0.4
E/V
-0.6
V = V0 + RT/ 2F ln [ pII H2 / pI H2 ]:
-0.8
-1.0
-1.2
Electrochemical H2 pressurization
Constant load test
-1.4
-1.6
Pt loading =0.5 mg/cm2 E-Tek ELAT electrodes.
-1.8
0
2
4
6
8
10
12
Time / Min
14
16
18
20
40
Galvanostatic electrolysis of water on Pt in PVPP PEM cell
PVPP membrane requires no water for H+ conduction yet allows water electrolysis
Cell Voltaage (volts)
0
‐0.5
Voltage of Electrolysis Cell ‐1
‐1.5
‐2
‐2.5
0
60
120
180
240
300
360
Time (seconds)
Cell voltage in time for the galvanostatic electrolysis of water on E‐tek ELAT electrode with Pt
loading of 0.5 mg/cm2 used for anode and cathode with a solid electrolyte membrane (t=35 mil)
of polyvinyl pyridinium phosphate (PV P+ : H2PO4‐ ; 1:1). Constant cell load current = 11 mA/cm2,
Cell Temperature
p
= 150oC, Argon
g flow humidified at 80oC.
41
Comparison of liquid (pIL) and solid (pIM) conductivity
0
-1
Log  (S cm )
-1
-2
-3
membrane: PVP-H3PO4
-4
pIL: Py-H3PO4
pIL: 2F-PyTf
-5
1
2
3
4
-1
1000/T (K )
Conductivity for 3 electrolyte samples as a function of temperature. Electrodes: E‐Tek Pt/C
(0.5mg/cm2) fed dry H2 gas. Temperature range =25 to 150oC.
Solid triangle: Solid poly vinyl pyridinium phosphate (PVP‐H3PO4) membrane,
Open diamond: Liquid vinyl pyridinium phosphate pIL (P‐H3PO4),
Open square: Liquid 2‐fluoro pyridinium triflate (2‐FPyTf).
42
Proton conductivity for 3 solid water-free pIMs
- as a function of temperature
0
-1
Log  (S cm )
-1
-2
-3
pure ITP
-4
blend of 70% ITP-30% PVPP
membrane: PVP-H3PO4
-5
1
2
3
4
-1
1000/T (K )
Conductivity determined by impedance spectroscopy for 3 electrolyte samples as a function
of temperature. Electrodes E-Tek Pt/C (0.5mg/cm2) fed dry: H2 gas. T=25 to 275oC. Solid
triangle: Solid membrane of poly vinyl pyridinium phosphate (PVP-H3PO4), Solid
diamond: solid ceramic membrane of pure 10%indium 90%tin phosphate (ITP). Solid
square: a solid membrane of 70% ITP blended with 30wt% PVP-H3PO4.
43
Fuel Cell Polarization with pure ITP ceramic pIM
Fuel cell with pure ITP (10%In 90%Sn) electrolyte,
t=1.0mm, area= 0.5cm2, no Pt sputter interface, electrodes: E-Tek Pt/C (0.6mg/cm2),
fed dry H2 and O2, temp: 25 to 250C.
1
Cell Voltage
e (volts)
25C
80C
120C
150C
200C
250C
0.8
0.6
0.4
0.2
0
0
100
200
Current density (mA/cm2)
300
1/3 performance PBI with: no water, no leachable ions, greater T range. 1/3
performance PBI with: no water, no leachable ions, greater T range.
Issues are:
• the ceramic membrane is brittle with little tolerance to shock and vibration
• the ceramic membrane is porous (gas crossover leading to low OCV)
44
Fuel Cell Polarization Curves in inorganic/organic composite pIM
first composite with “dry” inorganic and “dry” organic proton conductors, ITP/PVPP “Protic Salt Polymer Membranes: High-Temperature Water-Free Proton Conducting Membranes”, 2009 DOE Hydrogen Program
Review, Washington DC, May, 2009
Fuel cell with electrolyte of 70%ITP-30%PVPP,
t=1mm, active area: 0.5cm2,
Pt sputter: ~22nm, E-Tek Pt/C, dry gases: H2/O2, IR free
cell potential (V)
12
1.2
25C
50C
100C
150C
200C
250C
275C
1
08
0.8
0.6
04
0.4
0.2
0
0
10
20
30
40
50
current density (mA/cm2)
Adding PVPP increased OCV, membrane flexibility, and decreased porosity and crossover. Issues are:
• Loss of power
• Need to optimize composite composition, membrane/electrode interphase
45
High fuel cell performance using a composite membrane (b)
Photographs of membrane of (a) 10 wt% organics
and
d 90% ITP composite
it and
d (b) a pure ITP pellet.
ll t
(b)
(a)
(b)
(a)
Powerr density (mW
W/cm2)
(a)
Cell voltage (v
volts)
with “dry”
with dry inorganic and inorganic and “humidity
humidity sensitive
sensitive” organic proton conductor
organic proton conductor
Current density (mA/cm2)
Cell voltage and power density versus current density of fuel cells with
the composite membrane of 90wt% of Sn0.9In0.1P2O7 and 10wt% of an
equimolar mixture of
TES-Oct and (THS)Pro-SO3H at 150°C.
Unhumidified H2 was supplied to the anode and air to the cathode at a
flow rate of 30 mL min−1. The electrolyte thickness was 60 m. The light
lines (a) have no intermediate layer between electrodes and
membrane. The lines with circle markers (b) have an intermediate layer
of 20 p
parts ITP: 1 p
part organic
g
between electrodes and membrane. Pt
loading= 0.6mg cm−2.
Like state of Art phosphoric acid in PBI …
But NOW NO LEACHING of phosphoric Acid
“Sn0.9In0.1P2O7 –Based Organic/Inorganic Composite Membranes Application to Intermediate Temperature Fuel Cells”, P. Heo, M. Nagao,
T. Kamiya, M. Sano, A. Tomita and T. Hibino, J. Electrochem. Soc., 154, B63-B67 (2007).
46
C
Conclusions
l i
 El
Electrochemistry
t
h i t in
i neatt salt
lt electrolytes
l t l t has
h been
b
overlooked
l k d but
b t
interest is growing
 Salt electrolytes (pILs and pIMs) need no water for high ion conductivity
 Electroplating of silicon from a non-protic ionic liquid electrolytes gives
• a low-cost, energy-efficient and clean manufacturing process
• a relatively simple way for forming complex Si structures
 Solid neat salt electrolytes (pIL and pIMs) give a promising route to
• More efficient electrode reactions
• More efficient and simpler fuel-cell power-source systems, water
electrolyzers, and electrochemical hydrogen compressors.
47
Acknowledgements
D. Gervasio is grateful to
the U. S. Department of Energy, Greg Kleen, manager
and the U. S. Army Research Office, Rob Mantz, manager
for support of the Fuel Cell electrolyte work
Jeffery Thomson, Greg Tucker, John Gustafson and D. Gervasio (advisor)
are grateful to the Edson Foundation for funding
“Solar Paint”,
a student start up company,
for developing the electroplating of
semiconducting silicon
"To
To be a great champion you must believe you are the best
best. If you're
you re not
not, pretend you are
are."
- Muhammad Ali
Questions?
48
49
50
51
52