(528b) Proton Transport Across a Hydroxyl Functionalized Graphene Membrane

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.