278097 Hydroxyl Functionalized Graphene: A Promising Anhydrous Proton Exchange Membrane

Thursday, November 1, 2012: 1:01 PM
402 (Convention Center )
Pabitra Choudhury and J. Karl Johnson, Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

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consisting of graphane (fully hydrogenated graphene) where a one-dimensional (1-D) chain of the hydrogen atoms has been replaced by OH groups (hydroxyl graphane), as shown in Figure 1, and (2) a membrane consisting of graphane completely saturated with OH groups on one side (i.e., every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.

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