1. Forrest Hosea, Marek Danilczuk, and Shulamith Schlick
Department of Chemistry and Biochemistry, University of Detroit Mercy
INTRODUCTION
EXPERIMENTAL
RESULTS and DISCUSSION
Proton exchange membrane fuel cells (PEMFC) operating at moderate temperature are a pop-
ular topic of study because of their potential uses for transportation, stationary, and portable
applications.[1] A key component of the hydrogen fuel cell is the proton exchange membrane
(PEM): an ionomer altered to include ions, such as sulfonic groups. The most commonly used
PEMs for fuel cells are perfluorinated polymers, which have a Teflon-like backbone chain
with perfluorinated side chains containing sulfonic groups (i.e. Nafion, Aquivion, Aciplex,
etc.). Because of their high cost, however, much research has been devoted to the develop-
ment of hydrocarbon membranes, for example the sulfonated poly(ether ether ketone)
(SPEEK) ionomer.[2] Its proton conductivity is comparable to that of Nafion, and has high
chemical and thermal stability, as previously studied in our group.[3]
We present a detailed study on specific regions of the microphase-separation in SPEEK iono-
mer with degree of sulfonation (DS) 68%, based on a series of doxyl-stearic acid (DSA) spin
probes with different polarities, using electron spin resonance (ESR) spectroscopy.
ESR spectra in the range 110 - 350 K were recorded in order to obtain information on the dy-
namical properties and distribution of the spin probes in the various regions of the SPEEK
ionomer as a function of temperature. The ESR spectra of three DSA spin probes (7DSA,
12DSA, and 16DSA (Chart 1) were analyzed in terms of their dynamical and magnetic prop-
erties.
The SPEEK ionomer was prepared and characterized using the procedure previously de-
scribed.[3] The SPEEK membranes used in this study were prepared using the solvent casting
method from N,N-dimethyl formamide (DMF). Although several studies on SPEEK as a PEM
in both hydrogen and direct methanol fuel cells have been conducted, discrepancies have been
reported between data for proton conductivity, water uptake, cell performance, and morpholo-
gy of SPEEK ionomers obtained in different laboratories.[4]
Samples weighing ~ 20 mg each were cut from the original membrane and left to soak in
the aqueous spin probe solutions for 24-48 h at ambient temperature. Before ESR measure-
ments, the prepared membrane was removed from the spin probe solution, patted dry, and left
to equilibrate at ambient temperature for about 10 min.
ESR spectra were simulated and fitted to experimental data using the EasySpin 5.0.11
package.[5] The software was run in MATLAB 8 version R2014a. ESR spectra measured at
350 K were simulated using “chili” function. An interactive least squares fitting method
(Nelder/Mead Simplex) was used.[3]
Dynamics and Location of Doxyl Stearic Acid Spin Probes in Sulfonated Poly(Ether Ether Ketone) Ionomer
Deduced from Electron Spin Resonance Studies
The ESR method using spin probes was found useful for detecting the phase separation
and local polarity in hydrated SPEEK membranes. Experimental and simulated ESR
spectra of the DSA spin probes in SPEEK are shown in Figure 1. The hyperfine split-
tings deduced from the spectra simulations are listed in the Table 1.
The extreme separation (ES = 2Azz) of outer peaks in the slow spectral components re-
flect the immobilization of the probes in the SPEEK ionomer, and can be used as a
quantitative indicator of the molecular motion. Small differences in 2Azz values suggest
that those probes are located at similar locations in the ionomer.
Figure 1. Experimental ESR spectra of 7DSA, 12DSA and 16DSA spin probes in
SPEEK. The extreme separation (ES) and the fast spectral component for each spin
probe are indicated.
Table 1. Hyperfine splitting constants deduced from spectra simulation.
Figure 2. Suggested locations of the spin probes in the phase-separated SPEEK system based on
the analysis of ESR spectra of the spin probes.
As shown in Figure 2, we expect that all spin probes are located in similar locations in the hy-
drophobic domain of the ionomer, with heads located near water pockets. When the probe mole-
cules rotate and the motion is sufficiently fast compared to the time scale of the ESR measure-
ments, the spectrum is motionally averaged into a triplet pattern with the isotropic Aiso values. As
seen in Figure 1, the weak signal of the fast spectral component is also observed, as indicated by
the red arrows, suggesting that at 350K a small fraction of the probes is located in hydrophilic
environment. The lower ES for 16DSA suggests that the probe is located deeper into the hydro-
phobic regions.
CONCLUSIONS
The ESR spectra and magnetic parameters deduced from the simulations suggest that the three
spin probes are located in hydrophobic environment, near the ionomer backbone chain. However
a small fraction is also present in the hydrophilic pockets in the ionomer. Small differences in the
extreme separation (ES) may suggest a similar location for all probes.
REFERENCES
1. Gittleman, C. S.; Coms, F. D.; Lai, Y. H. In Polymer Electrolyte Fuel Cell Degradation,
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2. Kaliaguine, S.; Mikhailenko, S. D.; Wang, K. P.; Xing, P.; Robertson, G.; Guiver, M.
Properties of SPEEK-Based PEMs for Fuel Cell Applications, Catal.Today 2003, 82, 213-
222.
3. Brush, D.; Danilczuk, M.; Schlick, S. Phase Separation in Sulfonated Poly(ether ether
ketone) (SPEEK) Ionomers by Spin Probe ESR: Effect of the Degree of Sulfonation and
Water Content. Macromolecules 2015, 48, 637-644.
4. Xing, P. X.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K. P.; Kaliaguine, S.
Synthesis and Characterization of Sulfonated Poly(ether ether ketone) for Proton Exchange
Membranes J. Membr. Sci. 2004, 229, 95-106.
5. www.easyspin.org.
6. Kutsumizu, S.; Schlick, S. Structure and Dynamics of Ionic Aggregates in Ethylene Ionomer
Membranes: Recent Electron Spin Resonance (ESR) Studies. J. Molecular Structure 2005,
739, 191-198.
Acknowledgements
This study was supported by the Polymers Program of the National Science Foundation and by an
unrestricted grant from the University Research Program of Ford Motor Company.