A LaTeX Format for Theses and Dissertations

I am very grateful for the support of my advisor, Dr. Peter N. Pintauro, for all the help and guidance he gave me during my time at Vanderbilt. Finally, I would like to thank all the members of my family who have supported me tremendously throughout my graduate career. 64 4.26 Relationship between price of composite membranes and fuel cell stack (system) annually. production rate, from an economic study conducted by Strategic Analysis, Inc. All . membranes are 14 μm thick with 95% Nafion PFSA ionomer Surface and cross-sectional SEM images of the 3M825-cPPSA Blend20 membrane sample... 71 5.6 a) water absorption and b) swelling through the plane of the 3M825-cPPSA blend membranes And.

INTRODUCTION

34;Properties and fuel cell performance of a 660 equivalent weight perfluorosulfonic acid nanofiber composite membrane." Journal of The Electrochemical Society F429. 34;Approaches and technical challenges for high-temperature operation of proton exchange membrane fuel cells." Power Sources Journal. 34; Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes." Journal of Membrane Science.

BACKGROUND

Background on Fuel Cells
Requirements of FC PEMs
Prior Research on Proton Exchange Membrane

Perfluorosulfonic Acid Membrane
Partially Fluorinated Membrane
Non-fluorinated Membrane

Polymer Nanofiber Electrospinning
Nanofiber-Reinforced Composite Proton-Exchange Membranes
ePTFE-Reinforced Composite Proton-Exchange Membranes
PFSA-hydrocarbon Ionomer Blend Membranes
References

34;Impact of polyvinylidene fluoride on nanofiber cathode structure and durability in proton exchange membrane fuel cells." Journal of The Electrochemical Society. 34;Elektrospun Nafion nanofiber for application in proton exchange membrane fuel cells." Journal of fuel cell science and technology 6.3 (2009). 34; Sulfonated polysulfone/POSS nanofiber composite membranes for PEM fuel cells." Journal of The Electrochemical Society B914.

Figure 2.2: Chemical structures of (a) Nafion, (b) 3M, and (c) Aquivion ionomers [17]

ELECTROSPUN FIBER REINFORCED POLY(PHENYLENE SULFONIC ACID)S-

Background of Crosslinkable Poly(Phenylene Sulfonic Acid)s

Experimental

Reagents Used
Polyphenylene Ionomer Synthesis
Nanofiber Electrospinning

Reinforcing Polymer Electrospinning
cPPSA Electrospinning

Fabrication of Pore-filled Composite Membrane
Fabrication of Casted cPPSA Film
Characterization of Proton-Exchange Membranes

Proton Conductivity
Morphology
Mechanical Properties
MEA Fabrication and Fuel Cell Testing

The fabrication of a dual-fiber composite membrane required simultaneous electrospinning of cPPSA fibers and reinforcing polymer fibers. The dual-fiber electrospinning conditions were the same as those found for individual cPPSA and reinforcing (PPSU or PBI) polymer fibers. Usually, 0.03 mL of reinforcing polymer solution was electrospun first to prevent the cPPSA fibers from adhering strongly to the aluminum foil on the collector.

The resultant dual-fiber composite membranes had a cPPSA matrix with electrospun-embedded reinforcing polymer fibers. The usual method of creating a composite membrane filled with pores (i.e., immersing a layer of reinforcing polymer fiber in an ionomer solution [12]) cannot be used due to the low viscosity of the cPPSA solution MeOH and limited amount of synthesized cPPSA ionomers. at Vanderbilt University. Initially during the fabrication of pore-filled composite membranes, the cPPSA solution was cast onto the surface of a layer of electrospun reinforcing polymer fibers and a casting knife was used to coat the ionomer onto the fiber layer.

This solution caused the reinforcing polymer fiber mat to wrinkle and left solution residue on the casting knife. To increase interfacial compatibility, a reinforcing polymer fiber mat was soaked in a Poloxamer 407 (surfactant) solution in 1:1 ethanol (EtOH): H2O and then placed in a fume hood to evaporate solvent. After this treatment, surfactant remained on the reinforcing polymer fibers, making the reinforcing polymer fiber mat hydrophilic.

The cast cPPSA film was placed on top of the surfactant-treated reinforcing polymer fiber mat, and the two layers were exposed to MeOH vapor to soften/melt the cPPSA film.

Figure 3.2: Copolymerization of DBPDSA-Li and DBBPDSA-Li salts to form the precursor ionomer P x B y (x

Results and Discussion

NMR Spectrum
Morphology

Morphology of Reinforcing Polymer Fibers
Morphology of cPPSA-PEO Fibers
Morphology of cPPSA-PAA Fibers

Mechanical Property
Fuel Cell Performance

As shown in the table, the left and right tails of the three radius distributions accounted for less than 10% of the fibers, which indicated that the milestone criterion was met. As can be seen from Figures 3.12 to 3.17, the obtained cPPSA-PAA fibers, except for those shown in Figure 3.12a, were relatively uniform without beads or droplets. A summary of the fiber radius distribution for the cPPSA-PAA mats is shown in Table 3.4.

However, both values were still >40% lower than those of cross-linked pristine cPPSA. Electrospun PPSU and PBI fiber-reinforced composite membranes can maintain more than 70–80% proton conductivity compared to that of pure cPPSA bonded film under the same conditions. The proton conductivity of PBI-reinforced membranes was slightly lower than that of PPSU-reinforced membranes over all relative humidities examined.

The stress-strain curves of the crosslinked pristine cPPSA films varied greatly with crosslinking conditions. Since results in Figure 3.23 showed that the addition of PEO as a carrier would dramatically reduce the proton conductivity of double fiber membrane, I focused on making tensile strength measurements only on double fiber membranes with PAA carrier and PPSU reinforcing fibers. Here the mechanical properties of these membranes are better than those of the double fiber membranes.

This difference may be the result of an almost 3 times larger diameter of the PPSU fibers.

Figure 3.7 and 3.8 show SEM images of these fiber mats along with plots of their fiber diameter distribution

Conclusion

EXPANDED PTFE REINFORCED POLY(PHENYLENE SULFONIC ACID)S-BASED

Introduction

Experimental

Polyphenylene Ionomer Synthesis for cPPSA-BDM and cPPSA-ePTFE-A

Synthesis of 1,4-dibromobenzene-2,5-disulfonic Acid Dilithium Salt (DBPDSA-Li)
Synthesis of 4,4’-dibromobiphenyl-3,3’-disulfonic Acid Dilithium Salt (DBBPDSA-Li)
Synthesis of PxBy Copolymer
Grafting Biphenyl Groups (BP) on Copolymer

Polyphenylene Ionomer Synthesis for cPPSA-ePTFE-B
Membrane Fabrication

cPPSA-BDM cast membranes
Development of cPPSA-ePTFE-A Pore Filled Composite Membranes
Development of cPPSA-ePTFE-B Pore Filled Composite Membranes

Morphology
Mechanical Properties
Gravimetric Water Uptake
Ion Exchange Capacity
MEA Fabrication and Fuel Cell Testing

The structure of the copolymer was confirmed by 1H NMR; the copolymer is designated as P75B25 (yield 94%). In addition to the importance of using proper Cu powder, for the copolymer synthesis additional critical parameter is the molecular weight (MW) of the copolymer as it determines the "graftability" of the copolymer. The solubility of the copolymer in polyphosphoric acid (PPA) decreases with the increase in MW.

Since the MW of the copolymer depends on the ratio of the two monomers (DBPDSA and DBBPDSA), the MW of the copolymer can be easily controlled by varying the monomer ratio. It was found that, if the DBBPDSA monomer content in the copolymer is less than 19% (for example, copolymer P82B18), it shows good biphenyl grafting ability. However, above this biphenyl monomer content (e.g. copolymer P75B25), the solubility of the copolymer in PPA is very low and thus does not show good biphenyl grafting ability.

Based on preliminary experiments, the ratio of the two comonomers was adjusted to obtain a biphenylenedisulfonic acid comonomer fraction of 0.25. Samples were sputter coated with a gold layer to prevent charging of the sample and to reduce thermal damage while enhancing the secondary electron emission. The IEC of the sample was calculated by dividing the average number of mmol of NaOH titrant used at the equivalence point by the dry weight of the sample in acid form (in grams) and multiplying the result by four.

An anodic sweep was applied between 0.01 V and 0.4 V at a rate of 10 mV/s using a potentiostat (Reference 3000, Gamry Instruments), which resulted in the oxidation of H2 permeating through the membrane.

Figure 4.1: Sulfonation of dibromophenylene (DBP) and dibromobiphenylene (DBBP) with fuming sulfuric acid and conversion to the lithium salts

Results and Discussion

NMR Spectrum
Ex-situ Experimental Results

Improvement of Grafting with 1,4-Benzenedimethanol Additive

PEM Fabrication Material Cost Analysis

The highest conductivity was recorded for cPPSA-ePTFE-B (crosslinked at 210 °C for 1 h) and was more than twice that of Nafion XL above 40. The conductivity of the cPPSA-ePTFE-A was also significantly greater than that of Nafion XL, although somewhat lower than cPPSA-ePTFE-B in the 60-90%. The reduction in the maximum power density of Nafion XL was almost twice that of cPPSA-ePTFE-B.

The fuel cell performance of cPPSA-ePTFE-based MEA was clearly higher than that of Nafion XL MEA at 100% RH, with back pressure. With a back pressure of 200 kPa, the maximum power densities at 100 and 30% RH of cPPSA-ePTFE-based MEAs are almost the same (less than 2% difference). Both cPPSA-ePTFE-based MEA and Pemion-based MEA showed higher activation loss compared to PFSA-based MEA.

But the activation loss of cPPSA-ePTFE-based MEA is less than that of Pemion-based MEA. Due to its better water retention ability, the proton conductivity of cPPSA-ePTFE-B only decreased by 15%. The reduction in the maximum power density for Nafion XL was almost twice that of cPPSA-ePTFE-B.

Nevertheless, the decrease in power with the cPPSA-ePTFE-based MEA was still lower than with.

Figure 4.5: 1 H NMR spectra in CD 3 OD of: (a) P75B25 (PPSA), and (b) P75B25-g-BP17% (cPPSA)

Conclusion

POLY(PHENYLENE SULFONIC ACID)S-PFSA BLEND MEMBRANES

Introduction
Experimental

Membrane Fabrication

Results and Discussion

Ex-situ Testing Results

Conclusion
References
Introduction
Experimental

Synthesis of cPPSA
Gas Diffusion Electrode Fabrication and Fuel Cell Testing

Results and Discussion

NMR Spectrum
Fuel Cell Performance and Morphology of GDEs

Conclusion
References

03S membrane with about 2-3 MPa; however, the tensile stress of Aquivion membrane at 14-25% strain was approximately 25% lower than that of the blend membranes. This was the result of the cPPSA's excellent water retention ability which effectively helped the blend membrane to provide higher proton conductivity, thereby improving fuel cell power output at low humidity. As can be seen, the HFR values are lower for both mixture membranes compared to HFR of the reference Aquivion membrane.

Increasing back pressure resulted in a decrease in power output at 100% RH and 30% RH, and for EA with Blend20 the difference was negligible compared to 135 mW/cm2 with Aquivion membrane. Their proton conductivity and water uptake were higher than that of pristine PFSA over the entire RH range tested. The integration of AEI powders in the catalyst layer (CL, Fig. 6.1) of both electrodes resulted in a maximum H2/O2 fuel cell power density of 240 mW/cm2 at 50 °C.

Figure 6.3 shows the 1H NMR spectrum of one of the newly synthesized water-soluble cPPSA ionomers. When the mass ratio of catalyst to ionomer was 52:48, the very poor performance of the fuel cell with an OCV of only 0.76 V and a peak power density of 10 mW/cm2 was attributed to the poor distribution and size of the cPPSA particles in the gas diffusion electrodes. In the next experiment, the amount of catalyst was increased to obtain a catalyst:cPPSA ratio of 4:1.

SEM images of one of the grafted cPPSA particles obtained after brief grinding with a mortar and pestle are shown in Figure 6.7.

Figure 5.2: Photograph of the 3M825-cPPSA Blend20 membrane sample.

SUMMARY AND CONCLUSION

The 20 wt.% cPPSA blend membrane had similar output power as cPPSA-ePTFE-B, although the 3M825-cPPSA blend membrane was thicker than cPPSA-ePTFE-B. Progressively improved fuel cell performance was observed when the binder content of the gas diffusion electrodes and cPPSA particle size were reduced. When the weight ratio of ionomer to catalyst in gas diffusion electrodes was decreased from 52:48 to 80:20, the power output of the fuel cells increased significantly.

When the average size of the cPPSA particles decreased from 20 μm to 1 μm, the power of the fuel cell continued to increase and the problem of membrane penetration caused by large cPPSA particles was also avoided. A thinner proton exchange membrane can be combined with smaller cPPSA particles to achieve better fuel cell performance without membrane penetration. The H2/air fuel cell performance of gas diffusion electrodes using cPPSA as a binder was still not as good as that of gas diffusion electrodes using PFSA as a binder, indicating that a smaller particle size was needed to improve catalyst utilization.