Organic cations based on the imidazolium core constitute a promising class of covalently bonded cations for use in anion exchange membranes (AEM). The durability of AEM in alkaline fuel cells is largely determined by the stability of the cationic moiety. Despite tremendous research efforts, the commonly employed cations still have limited alkaline stability. A hydroxide-mediated C-H deprotonation is a possible initial step of degradation of organic cations. Another possible degradation mechanism of imidazolium derivative is the ring opening of the imidazole. In this study, we have developed computational methodologies based on density functional theory (DFT) to identify organic cations capable of withstanding extreme alkaline conditions under high temperatures for prolonged periods of time. We look at several aspects of stability, including pKa
calculations and prediction of the activation energies for hydroxide nucleophilic attack on the imidazolium central carbon (carbon atom between the two nitrogen atoms, (C-2)). Eighteen organic cations from different families were selected and used in this study; these include imidazolium, guanidinium, phosphonium, ammonium, oxygen, and sulfur containing organic cations. We performed DFT calculations using three different functionals: viz. B3LYP, M06, and M06-2X, and solvation models: viz. SMD, CPCM and IEFPCM, in conjunction with the 6-311++G** basis set. The results from these computations were gauged against high-level MP2 and coupled-cluster calculations as well as the available experimental data. Altogether, we considered 21 computational protocols for predicting the pKa
values of organic cations. The most accurate protocols, as compared to the experimental data, employ CCSD(T) and M06-2X methods with solvent effects computed using the SMD solvation model. The combination of B3LYP functional and the SMD solvation model also gave results of comparable accuracy. Also, we used B3LYP/6-31+G* level of theory and SMD implicit solvation model to benchmark the experimental alkaline stability of a series imidazolium cations against nucleophilic attack by hydroxide. It is observed that the calculated activation energies can correctly predict the relative stability for a series of imidazolium cations. In agreement with previous experimental and theoretical studies, we find that a replacement of the C-2 hydrogen by a methyl group substantially increases the barrier height for both the nucleophilic addition and the ring opening steps. These results provide intrinsic details into the design of stable imidazolium based cations for anion exchange membranes that are currently being investigated in our group.
This work was funded by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231