(697b) Hydroxyl Functionalized Graphene: A Promising Anhydrous Proton Exchange Membrane

We predict that graphene functionalized with OH groups can be used as an
efficient proton exchange membrane capable of functioning in the complete
absence of water.  Fast anhydrous proton transport takes place on the surface
of functionalized graphene as a result of the nanostructured spacing of the
functional groups. We have considered two model membranes to test our
hypothesis: (1) a membrane consisting of graphane (fully hydrogenated graphene)
where a one-dimensional (1-D) chain of the hydrogen atoms has been replaced by
OH groups (hydroxyl graphane), as shown in Figure 1, and (2) a membrane consisting
of graphane completely saturated with OH groups on one side (i.e., every other
C atom) of the graphane. This membrane forms a two dimensional (2-D) hydrogen
bonded network, as shown in Figure 2. The fully hydroxylated structure has the
advantage of providing redundant pathways for proton diffusion. However, the
saturated OH coverage on the 2-D surface could reduce the proton diffusivity. We
have used first principles density functional theory to study the structural,
energetic, and transport properties of model membranes. Our calculations
predict that the proton mobility on the surface of a 1-D hydrogen bonded
graphane membrane will be close to the diffusivity of H⁺ in bulk
water and about 10 times faster than through Nafion at room temperature. In
contrast, the proton diffusivity along the surface of a 2-D hydrogen bonded graphane
membrane is about two orders of magnitude slower than the estimated diffusivity
of proton across the 1-D hydrogen bonded chain of graphane membrane. We predict
that functionalized graphene proton exchange membranes could operate over a
wide temperature range, making them more efficient than membranes requiring
hydration.

Acknowledgement:
We gratefully acknowledge the support by DTRA under Contract No.
HDTRA1-09-1-0008. We also gratefully acknowledge NSF XSEDE (TeraGrid) resources
under allocation numbers TG-DMR100097, TG-DMR110091 and TG-SEE090006. We thank the
Center for Simulation and Modeling at the University of Pittsburgh for
providing computational support.

Figure 1. Functionalized 1-D hydrogen
bonded hydroxyl graphene (sp³ like structure). The cyan color
represent C, white color represent hydrogen atom, and red-green combination
represent O and H in OH groups, respectively.

Figure 2. OH saturated 2-D hydrogen
bonded graphane membrane. The cyan color represent C, white color represent
hydrogen atom, and red-green combination represent O and H in OH groups,
respectively.