Surface-Fluorinated Proton-Exchange Membrane with High Electrochemical Durability for Direct Methanol Fuel Cells

Abstract

Random disulfonated poly(arylene ether sulfone)-silica nanocomposite (FSPAES-SiO2) membranes were physicochemically tuned via surface fluorination. Surface fluorination for 30 min converted about 20% of the C-H bonds on the membrane surface into C-F bonds showing hydrophobicity and electronegativity at the same time. The membranes with hydrophobic surface properties showed high dimensional stability and low methanol permeability when hydrated for direct methanol fuel cell applications. In particular, the surface enrichment of fluorine atoms led to anisotropic swelling behavior, associated with a stable electrode interface formation. Interestingly, in spite of the use of a random copolymer as a polymer matrix, the low surface free energy of the C-F bonds induced a well-defined continuous ionic channel structure, similar to those of multiblock copolymers. In addition to the morphological transition, fluorine atoms with high electron-withdrawing capability promoted the dissociation of sulfonic acid (-SO3H) groups. Consequently, FSPAES-SiO2 membranes exhibited improved proton conductivity. Thus, FSPAES-SiO2 membranes exhibited significantly improved single-cell performances (about 200%) at a constant voltage of 0.4 V in comparison with those of Nafion 117 and nonfluorinated membranes. Surprisingly, their good electrochemical performances were maintained with very low nonrecovery loss over the time period of 1400 h and interfacial resistances 380% times lower than those of conventional membrane-electrode assemblies comprising the control hydrocarbon membrane and a Nafion binder for the electrodes.

Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

“…Under the same fluorination conditions, the fluorine content was much higher in an aromatic polymer matrix (atomic F content: ∼15%) [183] than in an aliphatic matrix (atomic F content: ∼5.4%) [184] because the substitution reaction of a C-H bond with a C-F bond in the aromatic rings is the most thermodynamically favorable route. The C-F substitution reaction in BPSH-40 during surface fluorination was expected to occur randomly in its aromatic backbone since such F 2 gas reactions have been reported to be governed by a complicated radical chain mechanism [185].…”

“…Surface fluorination of sulfonated hydrocarbon membranes [BPSH-40 [183] and PVA-poly(styrene sulfonic acid-co-maleic acid) (PSSA-MA) [184]] has been achieved using a dilute F 2 gas (e.g., 500 ppm) in an F 2 /N 2 mixture under ambient pressure and temperature for a short period of time (5-60 min). Such a process minimizes undesirable side reactions (e.g., chemical degradation) that may arise due to the use of highly reactive F 2 gas.…”

“…The use of amphiphilic surfactants in previous research was helpful in demonstrating this strategy [109,183,193,201,216,[225][226][227]. A representative amphiphilic surfactant is a sort of non-ionic triblock copolymer (ABA block, A = poly(ethylene oxide) (PEO) and B = poly(propylene oxide) (PPO), e.g., Pluronics ® ) that has a good balance of hydrophilicity and hydrophobicity so as to distribute the surface-functionalized inorganic additives.…”

“…BPSH [183,216,227]). In the case of SPI-SiO 2 nanocomposites, well-dispersed SiO 2 nanoparticles greatly extended the lifetime of SPI matrices which are easily decomposed under hydrolysis conditions [193].…”

Development of Nafion/Tin Oxide Composite MEA for DMFC Applications

“…Despite its high proton conductivity and excellent chemical stability, Nafion performance, especially at high temperatures, is impaired by methanol crossover through the polymer membrane and by conductivity decay above 90 °C [8]. These drawbacks have been associated with the anisotropic swelling that Nafion undergoes at high temperature which damages the membrane dimensional stability [9] and also gives rise to interfacial complications, including high resistance and poor adhesion between membrane and electrodes [10].…”