Jonghyun Choi

Nanofiber composite membranes with low equivalent weight perfluorosulfonic acid polymers

Two low equivalent weight perfluorosulfonic acid (PFSA) polymers (825 EW and 733 EW) were successfully electrospun into nanofibers by adding as little as 0.3 wt% of high molecular weight poly(ethylene oxide) as a carrier polymer. The electrospun fiber morphology transitioned from cylindrical filaments to flat ribbons as the total concentration of PFSA + carrier in solution increased from 5 wt% to 30 wt%. PFSA nanofiber mats were transformed into defect-free dense membranes using a four-step procedure: (i) annealing the PFSA polymer during which time intersecting fibers were welded to one another at cross points (ii) mechanically compacting the mats to increase the volume fraction of nanofibers to ∼75%, (iii) imbibing an inert polymer, Norland Optical Adhesive (NOA) 63, into the mats (to fill entirely the void space between nanofibers) and then crosslinking the NOA with UV light, and (iv) removing the poly(ethylene oxide) carrier polymer by boiling the membrane in 1.0 M H2SO4 and then in deionized water. The resulting membranes exhibited higher proton conductivities than that of commercial Nafion 212 membrane (0.16 S/cm at 80 °C and 80% relative humidity and 0.048 S/cm at 80 °C and 50% relative humidity for a membrane with 733 EW nanofibers), with low water swelling (liquid water swelling of 18% for membrane with high conductivity). The proton conductivity of both EW nanofiber composite membranes increased linearly with the PFSA nanofiber volume fraction, whereas gravimetric water swelling was less than expected, based on the volume fraction of ionomer. There was a significantly improvement in the mechanical properties of the nanofiber composite membranes, as compared to recast homogeneous PFSA films.

Sulfonated Polysulfone/POSS Nanofiber Composite Membranes for PEM Fuel Cells

In the present study, an implementation of the polymer/particle composite strategy is demonstrated, where fuel cell membranes are fabricated by electrospinning nanofibers composed of a proton conducting polymer with proton conducting, water-retaining inorganic nanoparticles. Specifically, a nanofiber mat was electrospun from sulfonated polyarylene ether sulfonesPAES to which sulfonated polyhedral oligomeric silsesquioxane sPOSS nanoparticles were added to improve water retention and proton conductivity. The interfiber voids were filled with an acid- and temperature-resistant hydrophobic photocurable polyurethane. This uncharged polymer surrounds each nanofiber, restricts fiber swelling in water, and provides mechanical strength to the membrane, thus permitting a higher loading of inorganic particles and the use of fibers with a fixedcharge concentration much greater than that which is practical in a homogeneous ion-exchange membrane. The resulting phaseseparated nanofiber/matrix-polymer morphology is similar to the desired/ideal bicontinuous structure in a block copolymer system, but there is more flexibility in our fabrication approach. Thus, the ionomer, fiber diameter, fiber volume fraction, and inert polymer can be selected independently. Additionally, the present approach might represent an effective way to fabricate a fluorocarbon-free “green” PEM with good mechanical properties and a conductivity exceeding that of Nafion at RHs less than 100%. Experimental

High Conductivity Perfluorosulfonic Acid Nanofiber Composite Fuel‐Cell Membranes

There is a need for polymeric hydrogen/air fuel-cell membranes that can efficiently conduct protons at moderate to high temperatures for wet and dry gas feeds. The US Department of Energy (DOE), for example, set an exceedingly stringent preliminary target for membrane conductivity, 0.10 S cm 1 at 1208C and 50 % relative humidity (RH). [1] Herein, we describe the fabrication and basic properties of one membrane that exhibits outstanding proton conductivity over a wide range of humidity conditions at temperatures of 808C and 1208C. The membrane is based on a nanofiber network composite design [2] with precise topological separation of the proton transporting and mechanically reinforcing polymer components. This desirable morphology is created via electrospinning, an electrostatic fiber processing technique that has been known for more than one hundred years and underwent a renaissance in the early 1990s, mainly due to the work of Reneker et al. [3, 4]

Nafion Nanofiber Membranes

A new type of proton conducting fuel cell membrane has been fabricated and evaluated via preliminary physical property determinations. The membrane is composed of a 3-D interconnected network of Nafion (perfluorosulfonic acid) nanofibers that have been embedded in an uncharged and inert polymer matrix. Nafion was successfully electrospun into fibers by adding to the spinning solution a low concentration (1 wt% relative to Nafion) of high molecular weight poly(ethylene oxide). Membranes were made with an average fiber diameter in the range of 161-730 nm, where the nanofiber mat occupied 60-80% of the membrane volume. The inert matrix polymer was UV-crosslinked Norland Optical Adhesive 63. The resulting membranes exhibited a high proton conductivity (0.06-0.08 S/cm at 25oC in water), low water swelling (12-23 wt% at 25oC), and improved mechanical properties, as compared to a recast homogeneous Nafion film.

Composite Nanofiber Network Membranes for PEM Fuel Cells

A new class of proton-exchange membranes has been developed and characterized, where an interconnected network of proton conducting polymer nanofibers is embedded in an inert (nonconducting) polymer matrix. In the present study the nanofiber network membrane contained electrospun fibers from sulfonated poly(arylene ether sulfone) solutions (with/without sulfonated polyhedral oligomeric silsesquioxane, sPOSS). The membranes were characterized in terms of morphology, proton conductivity, and water uptake. In-plane proton conductivity was measured using a BekkTech cell that was placed in a controlled temperature/humidity chamber. The data show: (1) There was a modest increase in nanofiber > 50%.membrane conductivity when the IEC of the fibers was increased from 2.1 to 2.6 mmol/g (in the absence of sPOSS), (2) There was a substantial increase in membrane conductivity when sPOSS was added to the fibers, and (3) The proton conductivity of the sPOSS-loaded membranes was significantly higher than that for a commercial Nafion 212 sample at 30C for a relative humidity