3. Air Electrode Electrochemistry
O2 Reduction (Discharge):
O2 + 2 H2O +4e‾ 4OH‾ (+ 0.3033V*)
or
O2 + H2O +2e‾ HO2- +OH‾ (-0.1737V*)
w/Ag catalyst (perhydroxyl ion
decomposition)
HO2- 1/2O2 +OH‾ or
or
HO2- + H2O +2e‾ 3OH‾
O2 Evolution (Charge): ↑
OH‾ O2 + 2H2O +4e‾ (-0.98V*)
*- vs Hg/HgO
4. Two-Ply Air Electrode
Active Material:
Shawnigan Black/Ag (3.3%) 7g
Ketjenblack/Ag (3.3%) 3g
WC-12% Co 3g
NiS 2g
FeWO4 2g
Triton X-100 0.18g
Water 60 cc
Hydrophobic Layer:
Shawnigan Black 30g
Teflon 30B 9g
Water 150-175cc
5. Air Electrode Processing
Carbon Powder Preparation:
a) carbon silverizing
b) wet slurry mixing
c) material drying
d) oven baking
e) final blending
Electrode Fabrication:
a) wet pasting
b) preform molding
c) dry powder technique
Electrode Processing
a) cold rolling
b) hot pressing
c) hot rolling
- optional
10. - Mechanistic studies
Goal: obtain a clearer understanding of both oxygen reduction
and oxygen evolution processes in bifunctional mode
- Processing variations
Goal: correlate various phases of electrode fabrication with
baseline performance to establish manufacturing
process leading to reproducible, satisfactory performance
-Operational Studies
Goal: examine performance uniformity under altered cycling
conditions to determine optimal operating conditions
Air Electrode Interest Areas
11. Electrical Resistivity
Experimental Setup
10 wt% Teflon
50 wt% Teflon30 wt% Teflon
30 wt% Teflon
= RA/t x units conversion factor
= resistivity (ohm-cm)
R = resistance (ohms)
A = plaque area (10cm2)
t = plaque thickness (mm)
Reduced particle contact
→ p ↑
12. Electrical Resistivity
Thin sample Thick sample
Plaque Under Pressure
↑
e increases &
more change in shape
upon release
less change in shape
← upon release
↑
adhesion
← as unbaked
Restoring force
on major faces
→ t↑ upon release
Restoring force
on all faces
→ less t ↑ upon release
13. Results –C/Teflon Plaque Resistivity
I. Resistivity ↓ with plaque thickness (up to 6.35 mm)
Resistivity ↔ with plaque thickness (up tp 10.16 mm)
II. Effects of Teflon Dispersion:
1) Teflon particle size effects above 30% loading
2) Dielectric effects are a function of loading
3) Teflon particle adhesion effects above 30% loading
4) Dispersion agent effects in 15-30% loading range
III. Resistivity small, in general
- Can use higher wt% Teflon in fabrication
14. Gas-fed Ring Disk Electrode
Ag2O Reduction →
No diffusion limitations of reactive species
- Obtain purely kinetic data
- Monitor products of reaction
- Eliminate dependence on
O2 solubility in electrolyte
AgO Reduction →
←O2 Evolution
Ag Oxidation →
O2 Reduction →
←no current
(peroxide
decomposed)
18. Electrolyte Penetration with Cycle
Potential Distribution depends on:
- IR (ohmic drop) from c.c.
- reaction site distribution
- local OH ion concentration in pores
- O2 diffusion limitations
Therefore:
-|Vd | → extent of penetration
- s → contour of “front”
Cycle Comments
22 Initial- Vd small. Active layer, Vc minimum (bulk
wetting), s minimum. (planar front), Vd low (3 phase
interface).
46 s maximum. (nonplanar front), penetration midway
into activelayer, Vd high (local jd high)
64 Penetration through active layer,
Vc maximum (planar front, little 3 phase
interface)
82 Flooded active layer, s maximum
96 Operation in hydrophobic layer,Vd
poor, sd large, Vc in normal range (many
reaction sites)
19. Summary –Potential Distribution
I. Electrode potential depends on electrolyte penetration
O2 Reduction Mode
1) potentials (magnitude) vary inversely with the
area of the electrolyte penetration “front”
2) Nonplanar penetration results in large potential
variation over electrode surface
O2 Evolution Mode
1) potentials (magnitude) vary inversely with the
“wetted” volume of the electrode
2) Nonplanar penetration has little effect on
potential variation over electrode surface
3) IR losses a major contributor toward the total
polarization of the electrode
II. Onset of electrode delamination signals increased
ohmic polarization in both modes
20. Teflon Loading Experiment
Air Electrode Operating Conditions:
25C, 25w/% KOH electrolyte
Unscrubbed ambient air
Cycling (vs flat Ni electrode)
4hr@ 25 mA/cm2 discharge
12.5 mA/cm2 charge
Potentials relative to Hg/HgO reference electrode.
Teflon is:
- a binder
- a hydrophobizing material.
- Contains dispersion (“wetting”) agent
(to form emulsion), which must be
thermally decomposed (“baked out”) .
- Can mask electrocatalytic effects
21. Teflon Experiment –
Loading & Agglomerate Size
← O2
Reduction
O2 →
EvolutionSmaller size
→ slightly better vd
↓
← Excessive wetting
(restricted O2 access)
- Enhanced wetting
(more active area for rxn.)
↓.
Little correlation
with Teflon loading
Little correlation
with Teflon loading
Little correlation
with Teflon loading
- ↑
- Enhanced wetting
(more active area for rxn.)
22. Teflon Loading Effects
O2 reduction: 1) unbaked samples – at extreme ends
(<10:1 & >10:8) , potentials poor due to:
a) lack of sufficient binder
b) excess of dispersion agent
2) baked samples – potentials nearly independent
of loading
O2 Evolution: 1) unbaked samples – little dependence on loading
2) baked samples – complicated behavior, possibly
related to wetting pattern
Open Circuit: 1) unbaked – higher potentials for high (>10:8) and
lower for low (<10:1) loadings
23. Summary-Teflon Experiment
O2 Reduction:
- Unbaked samples: initially improving Vd (enhanced electrolyte
penetration), then Vd degrades (uncontrolled wetting and flooding)
-Baked samples: more stable Vd overall, degradation as electrode delaminates
(flooding)
O2 Evolution:
- Unbaked samples: little correlation with Teflon loading
-Baked samples: poor at low loading(little electrolyte penetration),then little
correlation
Agglomerate size ↓ leads to slightly better [Vd] (more reaction area)
-less effect on O2 evolution mode
Therefore, lower Teflon loading and smaller agglomerate size gives better
oxygen reduction
-wetting agent leads to variability
24. Summary – Electrode Testing
Potential Distribution Study;
A. Charge Mode - Resistance higher in vertical direction, becoming
less significant with extended cycling
B. Discharge Mode – no definite trend
- both consistent with O2 evolution on Ni fibers and O2 reduction in
active layer
Teflon Binder Experiment:
A. Electrode flooding detrimental (beneficial) to O2 reduction (evolution)
B. Open circuit potential related to subsequent wetting patterns
Rotating Disk Test:
A. O2 reduction occurs via a 2e-, perhydroxyl ion production process
B. Ag catalyst improves peroxide elimination at least one order of
magnitude and perhydroxyl elimination
C. Anodic cycling ((+0.5 to +0.6 V vs Hg/HgO) causes carbon surface
modifications, which inhibit O2 reduction capability slightly
D. No evidence of Ag dissolution
26. Hot Pressing Effects
SiGe/Ge(111)
Vc Vd Rc Rd
Pressing Pressure,P:
300C, 10 min (5-10 Ton): ~P ~P ~1/P
(10-18 Ton): ~P ~P
(18-36 Ton): ~1/P ~1/P
300C, 20 min ~1/P ~1/P ~P ~1/P
Pressing Temperature, T:
36 Ton, 10 min (275-300 C): ~T ~T ~T
(275-325 C): ~T
(300-325 C): ~1/T
(300-350 C): ~1/T ~1/T
(325-350 C): T 1/T
36 Ton, 20 min ~1/T ~T ~T ~T
36 Ton, 30 min ~1/T ~1/T ~T ~1/T
Pressing Time, t:
275C, 36 Ton (10-20 min): :~1/t ~1/t ~1/t ~1/t
(20-30 min): ~t ~t ~t ~t
300C, 36 Ton (10-20 min): ~1/t ~1/t ~1/t ~1/t
(20-30 min): ~ t
300C, 18 Ton ~1/t ~1/t ~1/t ~1/t
Key: V=voltage; R=internal resistance; C=charge; D=discharge; strongly; slightly
27. Surfactant Variation
surfactant (0.1g/cc) Type Vc** (mV) Vd** (mV) Life Cycle
#39 FC-171 N 636 -204 252
#40 FC-430 N 640 -195 256
#41 X-100 N 649 -189 316
#42 LTA C N 662 -225 252
#47 FC-170 N 681 -293 256
#38 FC-95*** A 690 -229 252
#46 FC-129 A 680 -272 246
#48 FC-98 A 650 -231 159
#43 None 666 -250 332
Key: V=voltage; C=charge; D=discharge; **through life; *** 0.001g/cc;A=anionic;N=nonionic;
#41 Rohm & Haas Co., #42 ArmourServices, all others, 3M Commercial Solutions
Relative to the no surfactant case, nonionic surfactants generally give lower O2 reduction
potentials and slightly lower O2 evolution overpotentials, while slightly sacrificing cyclic life.
28. Active Layer Resistivity
SiGe/Ge(111)
s (m2/g) rel Vc* (mV) Vd* (mV)
#30 96 M 625 -227
#31 94 L 624 -230
#32 290 H 586 -284
#33 220 L 640 -254
#34 138 H 613 -339
#35 112 L 625 -284
Key: V=voltage; s =surface area; =resistivity; C=charge;
D=discharge; ,* through 120 cycles; M=medium; L=low, H=high
- Higher resistivity supports give rise to
more stable cyclic voltage performance.
- O2 reduction overpotentials 20-50 mV
lower on low resistivity supports.
- O2 evolution potentials 10-50 mV
higher on low resistivity supports,
as it depends on electrolyte penetration.
29. Other Processing Effects
SiGe/Ge(111)
Vd sd Vc sc Life
Active Material Drying:
Oven Dry (100C, 3h) 154 10.9 521 5.8 123
Vacuum Oven Dry (100C, 16400 N/m2,3h) 155 11.2 522 9.8 123
Air Dry (25C, 16h (std.) 163 7.9 517 8 121
-No particular advantage in deviation from standard air drying
Active Material Size Grading:
700-900 micron active layer 187 12.6 509 10.5 101
-Detrimental to discharge performance and lifetime, as agglomerates lose hydrophobic
character, permitting excessive electrolyte penetration
Hot Rolling:
375F, 25psia, 4dir 151 10.6 524 14.5 108
70F, 25psia, 4dir 193 37 528 7 127
375F, 25psia, 8dir 162 6 511 13.5 64
375F, 50psia, 4dir 136 11 514 6 64
300F, 25psia, 4dir 147 25 513 6 64
-Hot rolling can approximate hot pressing performance, albeit with more variable
performance and sometimes shortened lifetime
-Cold rolling exhibits “bulk” wetting patters, detrimental to discharge performance
30. Processing Effects - Rolling
Initial “Break-in”
↓
Stable Regime
↓
Electrode “Flooding”
↓
Room temperature rolling (CR-1) exhibits “bulk wetting” pattern
31. Other Processing Effects
SiGe/Ge(111)
Dispersion Agent:
1) higher levels lead to high open circuit and poor O2 reduction
potentials
2) moderate loading (10:2 to 10:4), dispersion agent initially leads
to better potentials, followed by rapid deterioration
Agglomerate Size:
Both O2 reduction and O2 evolution potentials improve slightly
with smaller agglomerates
Working Time:
1) ~ one hour optimal for open circuit and O2 reduction potentials
2) less working time leads to minimal O2 reduction overpotentials
Open circuit potential is related to subsequent wetting characteristics for
unbaked samples
32. Operating Variations - Cycling
Cycling Conditions Vd sd Vc sc Life*
4 hr. charge, 4 hr. discharge (CC1) -184 36 515 15 828
4 hr open ckt., 4 hr. discharge (CC2) -165 10 ---- ---- 2468
Continuous charge (CC3) ---- ---- 568 25 1920
4 hr. charge, 4 hr. open ckt. (CC4) ---- ---- 553 17 3000
Continuous discharge (CC5) -163 11 ---- ---- 2064
Notes:
Vd,Vc –discharge and charge potential (mV vs. Hg/HgO (avg. through test)
sd, sc – standard deviation, diacharge and charge mode (mV)
*- in hours (failure due to excessive leakage)
33. Summary - Cycling Variations
Effects of Operation mode (relative to std. 4h. Charge, 4h. Discharge)
Mode O2 Reduction O2 Evolution Life
Discharge Beneficial ---- Beneficial
Charge ---- Detrimental Beneficial
Open Circuit Beneficial No Effect Beneficial
- indicates minor effect
34. Summary – Tab Position
Motivation: electrolyte is drawn towards electrically operation areas of grid
Tab Position Vd sd Vc sc
Electrolyte side (inner) -190 13 535 12
Hydrophobic-hydrophilic interface (outer) -149 10 565 13
- Three-phase interface lies , on average, closer to hydrophilic-hydrophobic
boundary→ superior discharge performance detected here.
- Highest electrochemically active region for oxygen evolution occurs where
electrolyte exposure is maximized-i.e. the inner side, but potentials erratic
36. Summary of Operating Temperature
Effects
O2 Reduction Mode:
1) Vd ~ Operating T(1.1 mV/C)
-lower T better and more stable
2) higher T accelerates failure, due to suboptimal electrolyte penetration patterns
3) Rd ↑, then →, then ↓, with cycle, as reaction shifts from O2 reduction to H2
evolution (more rapidly with increasing operating T)
O2 Evolution Mode:
1) Vc – slight ↓ with time on test and slight ↑ increasing with operating T
-lower operating T gives better and more stable performance
2) Rc stable throughout
Open Circuit Mode:
1) Voc – ↑, then ↔, then ↓
with cycle, due to surface
oxide formation, electrolyte
penetration and loss of
catalytic activity
2) Little relation to operating T
37. Size-Graded Air Electrode
- Layer A – Hydrophillic (< 0.6mm):
- Shawnigan Black/Ag 30g
Teflon 30B 9g
Water 150-175cc
- Layer B(D) – Hydrophillic (0.6-1.18mm):
- Shawnigan Black*/Ag 30g
- (Ketjenblack** EC-330JMA)
Teflon 30B 12g
WC-12% Co 4.5g
NiS 4.5g
FeWO4 4.5g
Triton X-100 0.18g
Water 150-175cc
- Layer C – Hydrophobic (1.18-1.7mm) :
- Shawnigan Black 30g
Teflon 30B 9g
Water 150-175cc
*Chevron Phillips
**AzkoNobel
38. Size-Graded Air Electrode
Cycles 0-99 Cycles 100-199
Vd sd Vc sc Vd sd Vc sc
B60 192 53 561 17 165 11 581 33
B60* 169 19 567 12 178 6 559 9
/1g A3,1g B(D)3/
/2g A2,1.5g (B,D)3/
/2g A2, 1.5g B2/
/2.5g A1,1g B2/
/3g C
A = hydrophobic agglomerate
B = hydrophilic agglomerate (60 m2/g) – B60
D = hydrophilic agglomerate (1000 m2/g) – B60*
C = hydrophobic material
1 = 1270-1820 mm
2 = 660- 1270 mm
3 = < 660 mm
High surface area carbon introduced to reduce “break-in” period.
39. Size-Graded Air Electrode
B60
← distinct “break-in” B60*
← reduced “break-in”
- High surface area carbon reduces discharge “break-in”,
with no sacrifice in charge performance.
40. Alternative Processing Effects
B102 Oven bake (300C, natural convection); press 5 Ton, 10 min., 25 C
B103 N2 flowing gas furnace (300C, 10 min. ) for more uniform heat distribution;
cold roll 25 lb., 4 dir., 25 C to promote more uniform thickness.
B117 Active material baked 2hr. in flowing N2 oven (300 C); hot press 300C, 5 Ton., 10 min.
to increase wetting agent decomposition.
B119 Active material baked as per B117; otherwise as per B103
B123 Hot press 300C, 5 Ton., 10 min., cool to room T in press to reduce expansion upon
relaxation.
B60* Active material baked 2hr. in natural convection oven (300 C); hot press 300C, 5 Ton.,
10 min. (Standard Processing)
Vd sd Vc sc Life Comments
B102 185 36.5 573 34 536 (similar to std., but wetting
B103 171.5 24.5 529.5 14.5 189 less controlled).
B117 193 10.5 542.5 16.5 235 (less well-defined
B119 227 25 547 12.5 235 break-in required
B123 189 11 563 7 215 than std.).
B60* (std) 179 15.5 537 18.5
-Less P gives shorter “leak-free” life, can extend if oven bake
- Hot pressing not required for moderately leak-free life (~250 cycles)
- Cooling in press enhances reproducibility (relative to std.)
45. Tabbing Optimization
Position Rd (mW) Rc(mW)
“A” 22.9 13.9
“B” 21.7 13
“C” 19.9 10.4
“D” 21.0 11.6
“A” – tab position 1 “B” – tabs 1 and 2
“C” – tabs 2 and 4; “D” – all four tabs
Rd (mW) – mean polarization resistance,
oxygen reduction mode (25 - 125 mA/cm2)
Rc (mW) – mean polarization resistance,
oxygen evolution mode (12.5 - 62.5mA/cm2)
46. Tabbing Optimization Summary
Polarization Resistance (PR):
10 cm2:
5:2 lowest (50-200 mW in O2 evolution mode)
10:1: highest (100-250 mW in O2 reduction mode)
→ most loss along nickel tab length
100 cm2:
longest (17cm) highest (both modes)
↓with cycling (O2 evolution mode (less so in O2 reduction)
→ bulk electrolyte controlling
Ohmic Polarization:
No excessive losses in 10 to 100 cm2 scale-up
Fraction of total PR ↑ from <15% to 80% with cycling
Tab orientation mattered little.
47. Air vs Oxygen Operation
Nernst potential for half cell
reaction (O2 reduction):
for T1 = 45 C = 318 K; X1 =1
T2= 25 C = 298 K; X2 =0.21,
we obtain:, E1/E2 ~ 1.665
(observed: 1.25/0.75 = 1.66, initially).
O2 evolution overpotentials initially 30 mV higher
in pure O2 and elevated T.
Elevated T induced enhanced wetting
and eventual “flooding”, and deterioration of potentials.
48. References
Figures:
1). B. G. Demczyk and C. T. Liu, J. Electrochem. Soc. 129(6) 1159 1982.
2). B. G. Demczyk and C. T. Liu, J. Power Sources. 6 185 1981.
3). B. G. Demczyk and C. T. Liu , B. G. Demczyk and I. R. Rittko, United State Patent # 4,444,852.
General:
E. S. Buzzelli, B. G. Demczyk. A. Gibney. C. T. Liu, P. L. Ulerich and R. E. Grimble, Iron-Air
Battery Development Program, Final Report 1980 (U.S. Department of Energy Contract No.
7335709), Westinghouse R & D Document No. 83-9E62-MOBET-R2, July 1981.
E. S. Buzzelli, L. B. Berk, B. G. Demczyk. A. Gibney. C. T. Liu, and D. Zuckerbrod, Iron-Air Battery
Development Program, Interim Report 1981 (U.S. Department of Energy Contract No. 7335709),
Westinghouse R & D Document No. 82-9D12-MOBET-R2, June 1982
E. S. Buzzelli, B. G. Demczyk, , L. B. Berk, D. Zuckerbrod, A.Gibney. C. T. Liu, P. L. Ulerich and
R. E. Grimble, Iron-Air Battery Development Program, Final Report. March, 2. 1983 (U.S.
Department of Energy Contract No. 7335709), Westinghouse R & D Document No. 83-9012-
MOBET-R1, March 2, 1983.
49. Acknowledgements
Air Electrode Fabrication:
P. Gongaware, R. Egidio, I. Rittko
Air Electrode Testing:
G. Leap
This work was supported by
a U.S. Department of Energy contract EY-76-C-02-2949,*000
