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MANAGING FUEL CROSSOVER IN DIRECT METHANOL FUEL CELLS BY OPTIMIZING THE ANODE GAS DIFFUSION LAYER
Prepared by: Nathan [email protected] Author: Dr. Xianglin LiUniversity of [email protected] Date: 10/11/2021
Submitted for the ECS 240th biannual meeting
Presentation #I01A-1039
Background
Unreacted methanol crosses from anode to cathode and limits performance
• Chemical to electrical energy conversion
• Utilizes Platinum Group Metals (PGM)
as catalysts for reaction
• Pt/C – Cathode
• PtRu/C – Anode
• Methanol as a fuel
• Inexpensive
• Environmentally sustainable
• Considerable energy density
compared to most fuels (22 MJ/kg)
• Major Limitations
• Sluggish reaction kinetics
• Fuel crossover leads to
performance and efficiency
reductions
Figure 1: A Direct Methanol Fuel Cell (DMFC) under ideal fuel consumption conditions
Proton Exchange Membrane (PEM)
Background• Membrane electrode assembly (MEA)
• Gas diffusion layer
• Carbon support; typically, carbon
paper with or without a
microporous layer
• PEM
• Nafion® polymer membranes
• Acts as a separator between
cathode and anode that allows
hydrogen ions to pass through
• Structure is mirrored on both sides of
the PEM
• Catalyst varies on both sides
• Ionomer/Catalyst ratio also varies
(0.4 vs 0.2)
Figure 2: Membrane electrode assembly (MEA) schematic (Metzger, Li et al, 2021)
Pore-Scale Model ResultsKey Takeaways
• Microporous Layer (MPL) on anode gas diffusion layer
(GDL) plays a large role in fuel crossover
• Pore size distribution is the most significant
contributing factor to reducing crossover
3 Design Proposals
1. Add a hydrophilic layer between the GDL and MPL
1. Capillary pressure increase of 1.2x105 Pa
2. Add a layer of large pores between the MPL and
catalyst layer (CL)
1. Capillary pressure increase of 5.1x 102 Pa
3. Design the MPL to be strongly hydrophilic
1. Capillary pressure increase of 1.2x105 Pa
Figure 3: Design proposals based on pore-scale model results (Metzger, Li et al, 2021)
1.
2.
3.
Design Proposal 3MEA Anode Cathode Anode GDL/MPL MPL Fabrication
Details
1
TKK PtRu 50%Approximately 4.5
mgcm-2
JM PtC 60%Approximately 1.5
mgcm-2
Toray Carbon Paper (60um)/Custom Hydrophilic MPL
Spray coated to 1.28 mgcm-2; dehydrated
at 165°F for one hour
2 Toray Carbon Paper (60um)/Custom Hydrophilic MPL
Spray coated to 0.98 mgcm-2; Air dried in
atmospheric conditions
3 Sigracet 29BC with Commercial
Hydrophobic MPL(Baseline
Comparison)
N/A
4
TKK PtRu 77%Approximately 4.5
mgcm-2
Toray Carbon Paper (60um)/Custom Hydrophilic MPL
Spray coated to 1.52 mgcm-2; Air dried in
atmospheric condition
5 Toray Carbon Paper (60um)/Custom Hydrophilic MPL
Spray coated to 1.52 mgcm-2; Dehydrated
at 165°F for one hour
6 Sigracet 29BC with Commercial
Hydrophobic MPL(Baseline
Comparison)
N/A
• Hydrophilic Solution Details• Carbon Support: Vulcan
XC-72R
• Ionomer: Nafion® 10%
• Ionomer to Carbon Ratio: 0.8
Table 1: MEA Fabrication Details
• Sigracet 29BC Details• PTFE Treatment: 5% wt
• Thickness: 235 um
• Porosity: 80%
Study 1: MEA 1 & 2
Successes
• Peak power density achieved using 3M methanol
• Similar peak power and current density to base comparison
Figure 4: MEA 1 (dehydrated) compared to MEA 3 (baseline) (a) and MEA 2 (Air dried) compared to MEA 3 (baseline) (b)0.1 l/min air; 50 kPa backpressure
(a) (b)
0
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0
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0 100 200 300 400
Po
we
r D
en
sity
(m
W/c
m^
2)
Stac
k V
olt
age
(V
)
Current Density (mA/cm^2)
80 C 50 kPa - 1M MEA 180 C 50 kPa - 3M MEA 180 C 50 kPa - 1M MEA 380 C 50 kPa - 3M MEA 3
0
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0 100 200 300
Po
we
r D
en
sity
(m
W/c
m^2
)
Stac
k V
olt
age
(V
)
Current Density (mA/cm^2)
80 C 50 kPa - 1M MEA 280 C 50 kPa - 3M MEA 280 C 50 kPa - 1M MEA 380 C 50 kPa - 3M MEA 3
Failures
• Did not yet have a way to quantify fuel crossover
• Only indirect observations made
Study 2: MEA 4 & 5
Figure 5: MEA 4 (Air dried) compared to MEA 6 (baseline) (a) and MEA 5 (Dehydrated) compared to MEA 6 (baseline) (b)0.1 l/min air; 50 kPa backpressure
(a) (b)
0
20
40
60
80
100
120
140
160
0
0.1
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0 200 400 600
Po
we
r D
en
sity
(m
W/c
m^
2)
Stac
k V
olt
age
(V
)
Current Density (mA/cm^2)
80 C 50 kPa - 1M MEA 480 C 50 kPa - 3M MEA 480 C 50 kPa - 1M MEA 680 C 50 kPa - 3M MEA 6 0
20
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0 200 400 600
Po
we
r D
en
sity
(m
W/c
m^2
)
Stac
k V
olt
age
(V
)
Current Density (mA/cm^2)
80 C 50 kPa - 1M MEA 580 C 50 kPa - 3M MEA 580 C 50 kPa - 1M MEA 680 C 50 kPa - 3M MEA 6
Successes
• Fuel crossover quantified
• Pore size distribution measured
Failures
• Low tolerance to increased concentrations
• MEA 5 showed reduced overall peak power density
Fuel Crossover Quantification
Figure 6: MEA 4 and 5 crossover current (a) and MeOH flux (b)Experimental reference: (Hikita et al, 2001)
(a) (b)
0
1
2
3
4
0 0.5 1
Cro
sso
ver
Cu
rre
nt
(A)
Stack Voltage (V)
0.25M - MEA 4 80C
1M - MEA 4 80C
3M - MEA 4 80C
7.5M - MEA 4 80C
0.25M - MEA 5 80C
1M - MEA 5 80C
3M - MEA 5 80C
7.5M - MEA 5 80C
y = 7E-07x + 2E-06R² = 0.7693
y = 7E-07x + 2E-06R² = 0.8497
0.00E+00
1.00E-06
2.00E-06
3.00E-06
4.00E-06
5.00E-06
6.00E-06
7.00E-06
8.00E-06
0 2 4 6 8
Me
OH
Flu
x (m
ol/
cm^2
/s)
Molarity (M)
MeOH Flux - MEA 4 80C
MeOH Flux - MEA 5 80C
Key takeaways
• MEA 5 exhibited the best tolerance to fuel crossover at each concentration
Hydrogen Performance
Figure 7: MEA 1 (dehydrated), 2 (Air dried), and 3 (baseline) hydrogen (a) MEA 4 (Air dried), 5 (dehydrated), and 6 (baseline) hydrogen performance (b) 0.1 l/min air; No backpressure
(a) (b)
Key takeaways
• Custom hydrophilic MPLs improve hydrogen performance in all cases
0
50
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400
00.10.20.30.40.50.60.70.80.9
0 200 400 600 800 1000
Po
we
r D
en
sity
(m
W/c
m^2
)
Stac
k V
olt
ag (
V)
Current Density (mA/cm^2)
80 C - MEA 4
80 C - MEA 5
80 C MEA 6
0
50
100
150
200
250
300
00.10.20.30.40.50.60.70.80.9
1
0 200 400 600 800
Po
we
r D
en
sity
(m
W/c
m^2
)
Stac
k V
olt
age
(V
)
Current Density (mA/cm^2)
80 C - MEA 1
80 C - MEA 2
80 C - MEA 3
Failure CausesLoading
1. Further literature review indicates the loading of 1 mg/cm2 is too low by a factor of 10
Pore size distribution (PSD)
1. Low loading of MPL coating resulted in a large PSD
2. Large pores allow for gas pressure to be reduced leading to higher crossover
Fabrication
1. Experimental results indicate that hot pressing the MPL improves PSD, but causes damage to carbon support
Figure 8: MEA 4 (Air dried) (a) MEA 5 (dehydrated) (b) hot pressed hydrophilic GDL (c) and pore size distribution (d)
(a)
(b)
(c)
(d)
0
0.01
0.02
0.03
0.04
0.05
0 1000 2000
Po
re S
ize
Dis
trib
uti
on
Pore Radius (um)
MEA 4 PSD
MEA 5 PSD
Hot Pressed MPL PSD
Potential Challenges and Future WorkCurrent Challenges
1. Difficulty in reproducing results with hydrophilic GDL
1. New catalyst may impact the effectiveness of the hydrophilic layer
2. MPL loading must be investigated further
2. Need to develop a new method of hot pressing to improve pore-size distribution without damaging the carbon support layer
Figure 9: Gasketed fuel management layer (a) and hot-pressed layer showing an improved PSD (b)
Future Work
1. Modify hot press procedure to reduce damage to GDL
2. Increase MPL loading to uniformly fill the pores on the carbon substrate
3. Add a fuel management layer of large pores on the anode
(a)
(b)
THANK YOU
QUESTIONS?
Special thanks to our collaborators:
• University of Buffalo
• Dr. Qiurong Shi
• Dr. Gang Wu
• Kansas State University
• Archana Sekar
• Dr. Jun Li
• Carnegie Melon University
• Dr. Shawn Lister
• Mohamed Abdelrahman
• University of Kansas
• Andre Adams (SEM image analysis)
• Dr. Gibum Kwon (various coatings)
This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Hydrogen and Fuel Cell Technologies Office, Award Number DE-EE0008440
MANAGING FUEL CROSSOVER IN DIRECT METHANOL FUEL CELLS BY OPTIMIZING THE ANODE GAS DIFFUSION LAYER
Prepared by: Nathan [email protected] Author: Dr. Xianglin LiUniversity of [email protected] Date: 10/11/2021
Submitted for the ECS 240th biannual meeting
Presentation #I01A-1039
REFERENCES
Crossover measurement procedure:
2. Hikita, S. “Measurement of Methanol Crossover in Direct Methanol Fuel Cell.” JSAE Review, vol. 22, no. 2, 2001, pp. 151–156., https://doi.org/10.1016/s0389-4304(01)00086-8.
Pore scale model:
1. Metzger, Nathaniel, et al. “Understanding Carbon Dioxide Transfer in Direct Methanol Fuel Cells Using a Pore-Scale Model.” Journal of Electrochemical Energy Conversion and Storage, vol. 19, no. 1, 2021, https://doi.org/10.1115/1.4050369.