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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)
22
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
33
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.
44
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
55
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
66
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
77
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
88
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
99
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:
1010
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
1111
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
