Andrew Park, Ryszard Wycisk, and Peter Pintauro

Department of Chemical and Biomolecular Engineering

Vanderbilt University

Nashville, TN 37235

1

2

Fabricate membranes where fibers of high IEC hydroxide ion-conducting polymer

are surrounded by uncharged reinforcing matrix for reduced swelling and

mechanical strength.

Incorporate a crosslinking step to prevent the solubility of high IEC polyelectrolyte

fibers (crosslinking with either an aliphatic diamine or diol)

Evaluate the following functional groups and backbones:

1. Functional groups: benzyl trimethylammonium and benzyl 2,3-

dimethylimidazolium

2. Backbones: poly(phenylene oxide) (PPO) and polysulfone (PS)

Inert Reinforcing Material Polyelectrolyte Fibers

Pathway for

OH-

conduction

Processing Variables:

1) Concentration of polymer solution

2) Applied voltage

3) Flow rate of polymer through needle

4) Spinneret-to-collector distance

5) Relative Humidity

Electrospinning Polymer Fibers

Source: nano.mtu.edu/documents/Electrospinning.swf

5 µm

Simultaneous electrospinning two

different fibers onto the same rotating

drum collector to produce a dual fiber

mat with a random dispersion of each

polymer

Polymers Selected for Composite Membranes

Uncharged PolymerRadel® Polyphenylsulfone (PPSU)

Polyelectrolyte Precursors

Brominated Poly(phenylene oxide) (BrPPO)

Chloromethylated Polysulfone (Chloromethyl-PS) Chloro/Iodomethylated Polysulfone (Iodomethyl-PS)

Electrospinning a precursor polymer for the polyelectrolyte allows for the examination of different functional group chemistries and the use of polymer crosslinking during mat conversion to a dense membrane.

Soak dual fiber mat in diamine solution at room temperature (20% diamine in 7:3

dimethylacetamide:water). Diamine: 1,6-hexamethylenediamine

Wash in DI water and then dry for 1 hour at 70ºC

Control the degree of crosslinking by soak time in solution; 4-25% of the

chloromethyl groups were crosslinked.

Crosslinking Chloromethylated or Brominated Polymer with an Aliphatic Diamine

Carried out as an alternative to diamine crosslinking

Requires partial iodization of chloromethyl groups in chloromethyl-PS

Diol was added to the electrospinning solution before spinning

Mat was heated (110ºC for 4 hours) between glass plates to create crosslinks

Degree of crosslinking was controlled by the concentration of diol in the

spinning solution.

Crosslinking Iodo/chloromethylated-PS with an Aliphatic Diol

Diamine-Crosslinked Polyelectrolytes

1. After electrospinning, soak the dual fiber mat in diamine solution.

2. Dry for 1 hour at 70ºC.3. Mechanical Compaction (5,000 psi at RT for

~20 sec).4. Expose the mat to chloroform vapor for 10

minutes at room temperature. PPSU softens and flows/fills interfiber void space.

5. Soak the dense film in trimethylamine or imidazole solution at 40ºC.

6. Soak the membrane in 1.0 M KOH7. Wash in degassed DI water.

Diol-Crosslinked Polyelectrolytes

Same as diamine procedure, but the aliphatic diolis electrospun with the polyelectrolyte precursor and the fiber mat is heated to create crosslinks.

Membranes had either 35 wt% or 50 wt% polyelectrolyte fibers.

Dual-Fiber Mat

Dense Membrane

5 µm

5 µm

Conductivity of Diol-Crosslinked Composite Membranes with Various Fixed Charge Cations

Conductivity increases with IEC.

Highest conductivity was achieved with benzyl trimethylammonium fixed charges (the lowest MW groups) at an IEC of 2.0, with 8% crosslinking (the lowest degree of crosslinking).

Will conductivity continue to rise above an IEC of 2.0? Will swelling affect conductivity?

Membrane samples contained

65 wt.% diol-crosslinked PS

with 35 wt.% PPSU

9

Membrane composition: 65 wt.% crosslinked polysulfone-based polyelectrolyte, 35 wt.%

polyphenylsulfone (for reinforcement) with benzyl trimethylammonium fixed charge groups and

diamine crosslinks

Conductivity peaks at an

effective membrane IEC of

2.0 mmol/g.

Water swelling adversely

affects conductivity.

Control swelling by

crosslinking and adding

uncharged polymer – both

reduce the effective

membrane IEC. So what is

the proper mix of the two, to

achieve the highest possible

conductivity?

At low crosslinking degree, fixed charges are diluted by high water swelling

For a crosslinking degree > 8%, the reduction in water swelling is balanced

by a reduction in membrane IEC, resulting in values of χ that are

independent of crosslinking degree.

Impact of Swelling on the Volumetric Concentration of Membrane Fixed Charges

χ (mmol/cm3) = 𝑀𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝐼𝐸𝐶

𝑆𝑤𝑒𝑙𝑙𝑖𝑛𝑔 / ρ𝐻2𝑂

χ is the volumetric concentration of

fixed charge groups in a wet

membrane.

IEC is concentration of fixed

charges per gram of dry polymer.

OHVsiteperwaters

2

1)(

Diamine-Crosslinked PS with QA fixed charges

For a crosslinking degree <8%, the low volumetric concentration of fixed charge cations (due to excessive swelling) causes the low OH- conductivity

For a crosslinking degree >8%, the decline in conductivity is attributed to an increase in the tortuosity of ion conducting pathways within the fibers with an increased number of crosslinks.

In-Plane Hydroxide Ion Conductivity at 23°C

Polysulfone fibers with diamine crosslinks and benzyl trimethylammoniumfixed charge sites

35 wt% PPSU

Membrane samples in the OH- form

In-plane AC impedance conductivity measurements in degassed liquid water

Membrane IEC: 2.0 mmol/g

χ = 1.4 mmol/cm3 H2O

Membrane IEC: 2.1 mmol/g

χ = 0.8 mmol/cm3 H2O Membrane IEC: 1.8 mmol/g

χ = 1.4 mmol/cm3 H2O

Electrospun Composite Membranes with PPO:Hydroxide Ion Conductivity at 23ºC

The results show:

Membranes with more PPSU produce the same maximum hydroxide ion conductivity, but at

a lower dry membrane IEC.

The fixed charge site chemistry does not affect OH- conductivity

Examining/comparing specific data points in the plot provides additional insights into the

inter-relationship between membrane structure and conductivity.

Membrane composition:

50 wt.% polyelectrolyte

50 wt.% PPSU

65 wt.% polyelectrolyte

35 wt.% PPSU

Electrospun Composite Membranes with PPO:-Based Polyelectrolyte Fibers: Gravimetric Swelling

The water swelling of membranes with 35 wt.% uncharged PPSU

was significant (>150%)

Increasing PPSU content to 50 wt.% produced a substantial decrease

in water uptake (50-110% swelling)

Membrane composition:

50 wt.% polyelectrolyte

50 wt.% PPSU

65 wt.% polyelectrolyte

35 wt.% PPSU

Electrospun Composite Membranes with PPO:Hydroxide Ion Conductivity at 23ºC

Hydroxide ion conductivity is controlled by three factors:

(1) χ, the volumetric concentration of fixed charges in the polyelectrolyte fibers

(2) the tortuosity for ion transport within the fibers, which increases with increasing crosslinking

(3) the tortuosity of the fiber network (which increases with increasing PPSU content) and the

decrease in cross-sectional area for ion transport (which decreases with increasing PPSU content).

Here, the low conductivity of a membrane with the same PPSU content and same χ is due to the

greater number of crosslinks within the 3.5 IEC fiber, which increases intra-fiber tortuosity.

Membrane composition:

50 wt.% polyelectrolyte

50 wt.% PPSU

65 wt.% polyelectrolyte

35 wt.% PPSU

Polyelectrolyte IEC: 4.2 mmol/g

χ = 1.4 mmol/cm3 H2O

Polyelectrolyte IEC: 3.5 mmol/g

χ = 1.4 mmol/cm3 H2O

Electrospun Composite Membranes with PPO:Hydroxide Ion Conductivity at 23ºC

The membrane with fibers of 4.3 IEC has fewer polyelectrolyte crosslinks than the

3.5 IEC sample (improves conductivity). Fibers swell less due to more PPSU (greater

χ improves conductivity), but the higher PPSU content should lower conductivity.

The results show that the first two effects dominate.

The results above suggest that crosslinking should only be used to prevent water

solubility (minimal crosslinking). Use PPSU to control water swelling and χ .

Membrane composition:

50 wt.% polyelectrolyte

50 wt.% PPSU

65 wt.% polyelectrolyte

35 wt.% PPSU

Polyelectrolyte IEC: 4.3 mmol/g

χ = 2.1 mmol/cm3 H2O

Polyelectrolyte IEC: 3.5 mmol/g

χ = 1.4 mmol/cm3 H2O

Electrospun Composite Membranes with PPO:Hydroxide Ion Conductivity at 23ºC

The two data points above have essentially the same polyelectrolyte fiber IEC (i.e., the same number of

crosslinks) and the same conductivity, even though χ is different

High χ of 2.1 is due to reduced water swelling (because of high PPSU content), but greater PPSU content

increases tortuosity of fibers (less interconnectivity of fiber network in PPSU) and decreases the cross-sectional

area available for OH- transport.

Low χ of 1.4 is due to high swelling (because of low PPSU content), but the fibers in the membrane are less

tortuous and there is a greater number of ion conducting fibers (due to less PPSU).

The results show: χ and PPSU effects can balance one another.

Membrane composition:

50 wt.% polyelectrolyte

50 wt.% PPSU

65 wt.% polyelectrolyte

35 wt.% PPSUPolyelectrolyte IEC: 4.2 mmol/g

χ = 1.4 mmol/cm3 H2O

Polyelectrolyte IEC: 4.3 mmol/g

χ = 2.1 mmol/cm3 H2O

In the wet state, the effect of a high PPSU content on membrane tensile properties is dramatic.

Membrane with 50% PPSU gives better mechanical properties at the same conductivity.

Conclusion:

The preferred strategy for highly conductive, mechanically robust membranes: A high uncharged polymer content to reduce swelling with just enough crosslinking to prevent water solubility.

Mechanical Testing of PPO-Based Ionomer Fiber Membranes

Equilibrated

in water, 23ºC Equilibrated in air,

23ºC and 20% RH

Each sample had benzyl trimethylammonium groups and RT OH- conductivity of 60 mS/cm

( ): 50 wt.% PPSU; (---------): 35 wt.% PPSU

PPO-based electrospun composite AEMs with benzyl trimethylammonium

groups demonstrate good chemical stability up to 60°C, but fail at 80ºC.

Using a C6 linker between backbone and cationic charge or alternative

ammonium chemistries may improve alkaline stability at ≥80oC

Chemical Stability of PPO-based Electrospun Composite AEMs – Immersion in 1.0 M KOH

All films: PPO with benzyl trimethylmmonium fixed charges; 50 wt% polyphenylsulfone; an effective membrane IEC of 2.0 mmol/g.

Summary Comparison of Three Electrospun Composite Anion Exchange Membranes

Diamine

Crosslinked

Polysulfone

Diol

Crosslinked

Polysulfone

Diamine

Crosslinked PPO

Effective membrane IEC (mmol/g) 2.01 (35% PPSU) 1.79 (35% PPSU) 2.15 (50% PPSU)

Cationic group Benzyl

Trimethylammonium

1-benzyl-2,3-

dimethylimidazolium

Benzyl

Trimethylammonium

Gravimetric swelling in water (%) 144% 96% 97%

OH- Conductivity in water at 23ºC

(mS/cm)

65 49 66

Ultimate tensile strength (MPa) 13 16 15

Membranes with 35 wt.% PPSU

♦: PS-QA (diol-crosslinked)

+: PS-Imid (diol-crosslinked)

▼: PS-QA (diamine-crosslinked)

▲: PPO-QA (diamine-crosslinked)

■: PPO-Imid (diamine crosslinked)

Membranes with 50 wt.% PPSU

∆: PPO-QA (diamine-crosslinked)

□: PPO-Imid (diamine-crosslinked)

• Power densities are high and the results are promising for an initial test

• More work is needed to minimize membrane/electrode contact resistance and to

identify the proper binder material and electrode composition for optimum power

output.

Fuel Cell Performance of PPO-Based Electrospun and Crosslinked Composite Membrane

Membrane Electrode Assembly

• Electrospun composite membrane with crosslinked QA-PPO polyelectrolyte

• 40 µm thickness

• 2.0 mmol/g membrane effective IEC

• PPO polyelectrolyte binder (2.2 mmol/g IEC)

• 40% Pt on Carbon catalyst

• 0.5 mg Pt/cm2 loading (anode and cathode)

Operating Conditions

• 60ºC Cell Temperature

• Fully humidified anode and cathode

• 0.125 L/min H2, 0.250 L/min O2

• 2 atm backpressure at anode & cathode

Electrospun composite anion exchange membranes were fabricated via a dual

fiber electrospinning process. All membranes comprised crosslinked

polyelectrolyte fibers surrounded by uncharged polyphenylsulfone.

The polyelectrolyte backbone was either a polysulfone or poly(phenylene oxide)

that was crosslinked with either hexamethylenediamine or 1,6-hexanediol

The fixed charge groups were either benzyl trimethylammonium or imidazolium

Membranes were prepared with 50 wt% or 65 wt% polyelectrolyte, where the

effective membrane IEC was 1.2-2.8 mmol/g.

The best membrane (with the best combination of high conductivity, mechanical

properties, and good chemical stability) had a poly(phenylene oxide) backbone

with diamine crosslinks and benzyl trimethylammonium ion exchange sites.

The amount of crosslinking was sufficient to prevent water dissolution; membrane

water swelling was controlled by the presence of uncharged polyphenylsulfone.

Conductivity in RT water was 66 mS/cm, with 97% gravimetric water uptake and an

ultimate tensile strength of 15 MPa.

Nanofiber composite membranes were successfully testing in an alkaline fuel

cell, where power output was high (max power of 320 mW/cm2)

Conclusions

Financial support was provided by:

Army Research Office (Contract No. W911NF-11-1-0454)

National Science Foundation (Grant CBET-1032948)

Dr. Xiaoming Ren at the Army Research Lab for assistance in preparing and

testing MEAs

Acknowledgements