(431a) Stability Of Cationic Headgroups In Alkaline Anion-Exchange Membrane Fuel Cells

Alkaline anion-exchange membranes (AAEM) can be used as the polymer electrolyte in fuel cells which operate under alkaline conditions [1]. These membranes have cationic headgroups (e.g., [N(CH3)4]+) whereas the commonly used acidic membranes have an anionic headgroup (e.g., HSO3- in Nafion). Alkaline conditions allows the use of catalysts without platinum and may provide better water management properties. However, currently available AAEM materials are chemically less stable compared to their acidic counterparts such as Nafion. The headgroups of AAEMs are chemically attacked by OH- and undergoes degradation leading to loss of conductivity under the operating conditions in a fuel cell. Density functional theory (DFT) calculations reveal the critical role played by water in these membrane degradation reactions.

Electronic structure calculations with a polarizable continuum model (Gaussian 03 software,B3LYP/6-311++g(2d,p) level theory) of aqueous tetramethylammonium ([N(CH3)4]+) cations revealed two mechanisms of degradation. In the first mechanism, the OH- attacks a methyl group on the [N(CH3)4]+ in SN2 fashion and forms CH3OH and trimethylamine (N(CH3)3). In the second mechanism the OH- extracts a proton from a CH3 group and leads to the formation of water and a ylide - trimethyl ammonium methylide ((CH3)3 N+ (CH2)-). This ylide complex then undergoes additional degradation and forms methanol and trimethylamine. Reaction paths for both these mechanisms were obtained with the growing string method [2]. The highest saddle point in both mechanisms are equivalent and the activation barrier is +13.5 kcal/mol. Experimental measurements confirms (Mass Spectroscopy and Thermogravimetric Analysis) that CH3OH and trimethylamine are formed during the degradation of the model compound tetramethylammonium hydroxide pentahydrate (N(CH3)4OH-(H2O)5) . Mass spectrometry during the degradation of the deuterated version ((CH3)4N][OD](D2O)5) shows the scrambling of D and H on the methyl groups and thus confirms the presence of the ylide mechanism.

DFT calculations were repeated by changing the di-electric constant of the polarizable continuum medium. It was found that OH- becomes more reactive (i.e., smaller activation barrier) when the dielectric constant is reduced. This is expected because OH- will be poorly solvated at low dielectric constants.

Ab-initio MD simulations (VASP software, PW91 exchange) with one tetramethylammonium, one OH-, and n water molecules (n = 0 .. 4) showed that the head group is attacked by hydroxide when n=0 or n=1. However when four water molecules are present in the system the OH- is well solvated by the water molecules and no reaction is observed. Ab-initio MD simulations in a periodic cell with 48 water molecules also show no reaction. Radial distribution functions obtained from such simulations show that hydroxide is not tightly bound near the TMA+ cation. This is because the +1 charge of tetramethylammonium is spread uniformly around the bulky cation.

These results imply that a OH- anion that is not well hydrated will attack the cationic head groups in the AAEM. An alkaline membrane operating under relatively dry conditions will degrade much faster compared to a well hydrated membrane.

REFERENCES

[1] Prospects for alkaline anion-exchange membranes in low temperature fuel cells.; Varcoe, JR and Slade, RCT; Fuel Cells; 2005 (5), pp 187-200.

[2] A growing string method for determining transition states: comparison to the nudged elastic band and string methods.; Peters B, Heyden A, Bell AT, and Chakraborty A; J. Chem. Phys.; 2004 (120), pp 7877-7886.