Increasing the operating temperature of proton exchange membrane (PEM) fuel cells is desirable for many reasons, including increased tolerance to CO poisoning of the catalyst and improved waste heat rejection. The most common PEM material, Nafion, has a practical upper operating temperature of about 80 °C, because above this temperature Nafion will dehydrate, resulting in a loss of proton conductivity. A PEM material that does not require hydration could potentially tolerate much higher operating temperatures. We therefore propose that hydroxyl functionalized graphene can be used as a water-free solid state proton transport membrane. We have used both quantum mechanical and classical modeling to study the thermodynamics and kinetics of proton transport across functionalized hydrogen bonded hydroxyl graphene. Our calculations indicate that the proton mobility along functionalized hydrogen bonded hydroxyl graphene in the complete absence of water is on the same order of magnitude as proton mobility in bulk water and is about 10 times faster than in Nafion. We have computed the activation energy barrier for proton transport, including quantum effects, and found a value of 15.4 kJ/mol. This low activation energy indicates that hydroxyl functionalized graphene may be a viable PEM material for low humidity applications.
Acknowledgement: We gratefully acknowledge the support by DTRA under Contract No. HDTRA1-09-1-0008. We also gratefully acknowledge NSF TeraGrid resources under allocation numbers [TG-DMR100097] and [TG-SEE090006]. We thank Center for Simulation and Modeling at the University of Pittsburgh for providing computational support.
Figure 1. Functionalized hydrogen bonded hydroxyl graphene (sp3 like structure). Left figure is for top view and right figure is for side view. The gray color represent C, white color represent hydrogen atom, and red-green combination represent O and H in OH groups, respectively.
Figure 2. Snapshots of an excess proton (marked as green) through the 1D periodic functionalized graphene from our ab initio molecular dynamic (AIMD) simulations. C, gray; H, white; O, red.