2E09
Theoretical Investigation of Proton Conductivity
in Proton Exchange Membranes for Fuel Cells(Fuel Cell Nanomaterials Centre, University of Yamanashi) Raman K. Singh, Takao Tsuneda,Kenji Miyatake, and Masahiro Watanabe
Introduction: Nafion (DuPont) is the most used proton exchange membrane in polymericelectrolyte membrane fuel cells (PEMFCs) because of its highly proton conductive properties
and excellent chemical and physical stabilities. However, some potential drawbacks such as
high production cost, high methanol permeability and poor mechanical stability at hightemperatures of these polymers have prompted for development of novel membranes.1 Inquest of alternatives to perfluorinated polymers membranes for automotive applications,multiblock sulfonated poly (arylene ether sulfone)s (SPE) (Fig. 1) containing highly
sulfonated hydrophilic component seem to be very
promising.1 The multiblock SPE membranes have shown
more developed phase separation between hydrophobicand hydrophilic blocks than that of the random
copolymer equivalents. It has been observed that theproton conductivity of SPE-bl-2 membrane is higher thanNafion (NRE212) over a wide range of humidity and athigh temperature (>100 oC) but fails at low humidity. The
reasons behind the poor proton conductivities of these
SPE membranes at low humidity are still unknown andunexplored either experimentally or theoretically, and Figure 1: The optimized structure
deserve utmost attention for the future membrane of SPE membrane, in whichdevelopment. This study intends to figure out why Nafion green and blue dotted circles
indicate the separated and paired
shows higher proton conductivity than those of othersulfonic acid groups.
hydrocarbon membranes at low humidity.
Computational Details: Geometry optimizations and frequency calculations have been
carried out by the generalized Kohn-Sham method using long-range corrected Becke 1988
exchange+ Lee-Yang-Parr correlation (LC-BLYP) functional in conjunction with cc-pVDZ
basis set without imposing any symmetry restriction and all structures have positive realfrequencies. The computations have been performed using GAUSSIAN 09 program.
Results and Discussion: To discuss the state of protons in the hydrated sulfonic acid groups(–SO3H), we firstly optimized the geometries of Nafion and SPE models with various numberof water molecules, i.e. –SO3H-(H2O)n for n = 0, 1, 2, …, 10. It was found that the optimized
distance between the sulfonic acid group and proton increases sharply from 1.032 to 3.084 Åand 1.037 to 2.084 Å for one to three hydration water molecules in Nafion and SPE,respectively, while the distance between the sulfonic acid group and the closest watermolecule are nearly constant up to
eight hydration water molecules forboth the membranes. This clearly
indicates that minimum three watermolecules are needed to detachproton from the sulfonic acid of
both the membranes. For SPE,
however, we found that the sulfonicacid groups hydrated in pairs(“paired” in Fig. 1) detach no proton
Figure 2: The optimized hydrated structures of Nafion
even with 12 hydration water (left) and SPE (right) membrane models, for which themolecules (i.e. 6 molecules per detachment of proton (red circle) requires threegroup). The presence of such hydration water molecules in vacuo.
inactive sulfonic acid groups issupported by an IR spectrum study. In addition, we also found that the proton is always
located at the edge of the hydration water cluster after the dissociation as supported by a study
on ab initio molecular dynamics simulations and isotopic-exchange spectroscopy.2
Furthermore, we analysed the molecular orbitals of hydrated Nafion and SPE membranes todiscuss the proton detachments from the viewpoint of electronic structure. As results, it wasnoticed that the electron distribution of HOMO drastically changes from the backbone of thechain to the sulfonic acid groups in Nafion as the hydration level increases. This seems tomake it easy to dissociate protons and protonated water clusters from the sulfonic acid groups.
On the other hand, the HOMO and LUMO of SPE are hardly affected by the hydration.
To investigate the proton conductivityof Nafion and SPE at low humidity infurther detail, we calculated thedissociation energies of protonatedwater clusters from the sulfonic acidgroups. As a result, we found that theserequire more than 100 kcal mol-1 invacua. This indicates that protonatedwater clusters hardly detach from the Figure 3: The optimized structure of double Nafion
sulfonic acid groups at low humidity. models hydrated in pairs with 5 hydration waterWe, therefore, considered that the molecules. The distance of the sulphur atoms isproton conductance proceeds through fixed at the averaged distance given in a previousthe relay. It was found that the MD calculation.
dissociation energies drastically decrease to around 40 kcal mol-1 through the relay. We arenow exploring the feasibility of this relay.
References:
1. Miyatake, K.; Bae, B.; Watanabe, M. Polym. Chem. 2011, 2, 1919-1929.
2. Buch, V. et al., Proc.. Nat. Acad. Sci. 2007, 104, 7342-7347.