Effects of Processing Time, Mixing Speed, and Mixer on ...
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Effects of Processing Time, Mixing Speed, and Mixer on Agglomerates in Fuel Cell Cathode Inks
Erin B. Creel (ORNL)
Carlos Baez-Cotto (NREL), James Young (NREL), Scott A. Mauger (NREL), Michael Ulsh (NREL), David L. Wood III (ORNL), and Alexey Serov (ORNL)
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Large Agglomerates Reduce PEM Fuel Cell Performance
ACS Appl. Energy Mater. 2019, 2 (9), 6417–6427.
Aggregates (smaller)
Agglomerates (larger)
1 min. bath sonication:
• More agglomerates
• Worse performance
10 s tip + 20 min. bath sonication:
• Fewer agglomerates
• Improved performanceBath Sonication Tip Sonication
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Large Agglomerates Reduce PEM Fuel Cell PerformanceAggregates
(smaller)Agglomerates
(larger)
1 min. bath sonication:
• More agglomerates
• Worse performance
10 s tip + 20 min. bath sonication:
• Fewer agglomerates
• Improved performance
ACS Appl. Energy Mater. 2019, 2 (9), 6417–6427.
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Pt/C catalyst, Nafion ionomer,
water, and alcohol shear
mixed
Shear viscosity of mixed ink measured
Ink coated on GDL with wire-
wound rod and dried at 80 °C
Agglomerates found, filtered,
and highlighted in micrographs
of GDE
Methods for understanding mixing of fuel cell inks
Mixing RheologyGDE
FabricationParticle Analysis
Continue mixing
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Optical images at 500X magnification over mixing time 5 minutes 10 minutes 15 minutes
20 minutes 25 minutes 30 minutes
IKA 18G, 10,000 rpm
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5 minutes 10 minutes 15 minutes
20 minutes 25 minutes 30 minutes
Optical images at 500X magnification over mixing time IKA 18G, 10,000 rpm
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Effect of rotor-stator mixer speed on large agglomerates
• Large agglomerates in micrograph area counted
• Number of large agglomerates decays exponentially with mixing time
• All speeds achieve an “ultimate fineness”
• Higher speeds break upmore agglomerates
18G
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Effect of rotor-stator geometry on particle sizes
• Rotor-stator geometry has a greater impact on number of large agglomerates than rotational speed
Mixer
Model
Dispersing
Element
Rotor/Stator
Gap# Rotor Teeth
Rotor
DiameterShear Number
Rotor Tip
Speed
18G 0.3 μm 2 12.7 mm3 × 108
@ 20,000 rpm
0.013 m/s
@ 20,000 rpm
25F 0.5 μm 8 18.0 mm1 × 1014
@ 7,000 rpm
0.007 m/s
@16,600 rpm
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Effect of rotor-stator geometry on particle sizes
• Shear number
– = shear rate × shear frequency
– Not sufficient in predicting the number of large agglomerates
• Rotor tip speed is a better predictor of number of agglomerates
Mixer
Model
Dispersing
Element
Rotor/Stator
Gap# Rotor Teeth
Rotor
DiameterShear Number
Rotor Tip
Speed
18G 0.3 μm 2 12.7 mm3 × 108
@ 20,000 rpm
0.013 m/s
@ 20,000 rpm
25F 0.5 μm 8 18.0 mm1 × 1014
@ 7,000 rpm
0.007 m/s
@7,000 rpm
0.013 m/s0.016 m/s
0.007 m/s0.007 m/sRotor tip speeds:
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Ink property comparison between mixer types
Mixing time (h)
Shear
Vis
cosity (
Pa.s
.)
Mixing time (min)
Shear
Vis
cosity (
Pa.s
.)Mini-Homogenizer (6k rpm)
Ball Mill (50 rpm)
Viscosity at a fixed 1 /s shear rate
Quickly achieves ultimate fineness
• Long mixing timesneeded
• Slowest motor speed but best agglomerate break up
Ultimate shear viscosity = 1 Pa.s
Ultimate shear viscosity = 1 Pa.s
Mixing time (min)
Shear
Vis
cosity (
Pa.s
.)
IKA 18G (10k rpm)
• Highest motor speed but worst agglomerate break up
Ultimate shear viscosity = 1 Pa.s
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Future Work
• Fuel cell cathode performance evaluation and correlation with agglomerates
– Does fewer larger agglomerates result in improved performance?
– Does excessive mixing damage the ink and decrease performance?
• Explore other models to better predict number of large agglomerates per cathode layer area
Questions? Attend the live Q&A or email