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AEq

  1. 1. Bifunctional Air Electrode Studies B. G. Demczyk- Westinghouse Science & Technology Center Collaborator: C. T Liu
  2. 2. - Significance: - In-principle, infinite lifetime - Of interest for electric vehicle propulsion
  3. 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. 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. 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
  6. 6. Goals & Actions - Goals: - Duplicate optimal performance - (-100mV @25ma/cm2, -200 @100 mA/cm2) - Attain extended stable cycle life - (300 cycles @ + 10 mV) - Issues: - Delamination of carbon layers - Electrolyte leakage through hydrophobic layer - Directions: - Processing variations - Accelerated life testing
  7. 7. Baseline Air Electrode Performance Ag Catalyst Effect ↓ O2 evolution: -530 + 20 mV 22-54 mW O2 reduction: -175 + 21 mV 22-80 mW Discharge @ 25mA/cm2 Charge @ 12.5 mA/cm2 25wt% KOH @25C Hg/HgO reference
  8. 8. Average Behavior with Cycling O2 Evolution O2 Reduction
  9. 9. Performance: 2-Ply, Non Size-Graded O2 ← Evolution → O2 ← Reduction →
  10. 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. 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. 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. 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. 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)
  15. 15. Potential Distribution Measurement Setup Measurement Points
  16. 16. Potential Distribution – O2 Reduction
  17. 17. Potential Distribution – O2 Evolution
  18. 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. 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. 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. 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. 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. 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. 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
  25. 25. Processing Variations-Pressing T (C) P (tons) t (min) Vd(-mV) Rd (mW) Vc(mV) Rc(mW) 275 36 10 176 36 529 33 20 153 21 518 26 30 170 28 530 26 300 36 10 178 52 552 165 20 166 32 513 35 30 153 25 517 ---- 325 36 10 180* ---- 529* 33 20 ---- ---- ---- 30 ---- ---- ---- ---- 350 36 10 165 24 521 30 375 36 10 ---- ---- ---- ---- 300 18 10 191 71 542 68 20 170 43 528 33 30 ---- ---- ---- ---- 300 5 10 161 36 522 18 10 10 160 34 532 22 30 170 28 530 26 Notes: Vd,Vc –discharge and charge potential vs Hg/HgO (avg.through 60 cycles), (* @ 28 cycles)) Rd, Rc-polarization resistance, discharge 25-125 mA/cm2, charge 12.5-62.5 mA/cm2 No data indicates structural failure.
  26. 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. 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. 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. 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. 30. Processing Effects - Rolling Initial “Break-in” ↓ Stable Regime ↓ Electrode “Flooding” ↓ Room temperature rolling (CR-1) exhibits “bulk wetting” pattern
  31. 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. 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. 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. 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
  35. 35. Operating Temperature Effects Open Circuit Mode O2 Reduction Mode O2 Evolution Mode
  36. 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. 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. 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. 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. 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.)
  41. 41. Alternative Processing Effects SiGe/Ge(111) ↓ “Break-in” ↓ Stable with cycling ↓ Slight decrease with cycling ↓ No “Break-in”
  42. 42. Other Processing Effects SiGe/Ge(111) Vd sd Vc sc Life Catalyst Substitution: R.V.C (#104) 219 27.5 611 17.6 169 R.V.C, WC-12% Co (#105) 173 20.7 602 12.0 148 R.V.C, NiS, WC-12% Co (#109) 197 18.0 621 34.9 115 Ni Hydrate (#113) 233 60.6 672 40.1 352 Fe Powder (#114) 175 26.4 583 15.0 352 Fe Powder, Ni Hydrate (#115) 178 25.5 557 22.2 352 WC-12% Co, NiS, FeWO4 (std.) 230 15.3 560 11.9 397 -Fe compounds reduce oxygen evolution overpotentials. - Fe-Ni synergistic effect in both modes. - Standard combination gives most stable performance. 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
  43. 43. Catalyst Substitution SiGe/Ge(111) ↓ No “Break-in” ↓ “Break-in” Variable, but no increase with cycling Less variable, but increases with cycling ↓
  44. 44. Tabbing Optimization 10 cm2 L (cm) D (cm) Rd (mW) Rc(mW) TO-1 1 10 960 460 TO-2 10 1 1010 600 TO-3 3.16 3.16 910 430 TO-4 5 2 800 330 100 cm2 T0-12 10 10 44 (33) 35.5 TO-13 17 6 74 (33) 62 TO-14 6 17 40 (27) 51 Rd (mW) – mean polarization resistance, oxygen reduction mode (1 – 10 mA/cm2) () ohmic component at 5 A level) Rc (mW) – mean polarization resistance, oxygen evolution mode (1 – 10 mA/cm2)
  45. 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. 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. 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. 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. 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

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