Mechanism of Proton Conduction in Perfluorinated Sulfonic Acid Polymers with High Ionic Exchange Capacity at Water-Freezing Temperature Region

Abstract

Molecular-level understanding of dynamic behaviors of complex mass transports, especially proton and water- in polymer electrolytes is essential to design better membrane materials used for polymer electrolyte fuel cells (PEFCs). NMR is one of the suitable experimental techniques for analyzing such molecular dynamics, since both complex static structures and dynamical motions of molecules can be tracked by high-resolution NMR spectra and a pulsed field gradient NMR (PFG-NMR). This technique is widely used to understand proton dynamics however, it cannot clearly distinguish vehicle and grotthuss mechanisms. Recently, we have developed a combination method of 2H, 17O MAS NMR and 1H PFG NMR measurement to overcome the drawback. We applied the technique to an organic-inorganic composite membrane, Zr-SPP-SPES, which forms controlled configuration of proton functional groups [1,2]. In the paper we proposed enhanced grottuss mechanism in a water-freezing temperature region called "packed acid mechanism”. [6]In the present study we prepared perfluorinated sulfonic acid membranes with different equivalent weight (EW) 500, 600, 700 and 900, to understand relationship between value of ion exchange capacity (IEC = 1000/EW) and proton conduction mechanism in water freezing region. 17O labeled water was used in order to distinguish signals from non-exchangeable protons in the membranes, thus natural abundance 17O (0.04%) NMR signals of water in membrane cannot be detected. 1H and 17O MAS NMR measurements were carried out on Agilent NMR systems 400WB at 9.4 T. A 4 mm diameter air tight rotor was used for sample spin at 12 kHz. 1H and 17O 1D-NMR spectra were obtained by using a single pulse sequence. Sample temperature was calibrated by measuring the 79Br signal of KBr powder under the same measurement condisions as described elsewhere. [3] 1H PFG NMR measurements were carried out on JNM LA-400 at 9.4 T. NMR spectrometer equipped with maximum gradient strength of 12 T/m. The diffusion coefficient (D) was measured by using a pulsed-field-gradient stimulated-echo pulse sequence. [4, 5] 17O NMR spectrum in all the membrane show a sharp peak around 0ppm, from temperature 6 to 42℃. With lowering temperature from -20 to -45 ℃, the width of the spectra of all the membranes is drastically increased then finally the spectrum is vanished at T = -45 ℃. The behavior of 17O NMR in all samples were almost similar. Since the width of the NMR spectrum is inversely proportional to the movement of the observed nucleus, it indicates that absorbed water molecules are completely freezing.In contrast to the 17O NMR, solid 1H NMR spectra show a sharp peak even at the low temperature range, from -25 to -45 ℃. This result indicates that proton conductivity is still remaining, even while absorbed water molecules are freezing. 1H PFG-NMR also revealed that proton conduction is present at low temperature as well as the results of 1H NMR. The sample with highest IEC value ( IEC ~ 2 ) shows highest proton conductivity D = 7 x 10-7 (m2/s) at T = -25 ℃ and D = 0.5 x 10-7 (m2/s) at T = -60 ℃.These three results indicate that protons in the membranes with high IEC move independently like a "packed acid mechanism” in freezing temperature region. This conduction mechanism would happen efficiently because sulfonic acid is tightly-packed, which is a noteworthy property of the high IEC membranes.