Effects of Atomic-Layer-Deposition Alumina on Proton Transmission through Single-Layer Graphene in Electrochemical Hydrogen Pump Cells

Fifty atomic layer deposition (ALD) cycles of trimethylaluminum and water were applied to single-layer graphene on copper and graphene on Nafion membranes that result in alumina coatings that fully block photoelectron emission from the underlying substrate materials (copper and Nafion, respectively). This finding is consistent with relatively uninhibited alumina ALD nucleation and growth to a thickness of ∼5 nm and further suggests that the alumina layer is continuous with no uncoated regions exposed. The ALD-derived alumina coatings are good barriers to large ions, e.g., from aqueous ferric chloride, but they have a relatively modest effect on proton transmission through graphene as measured in electrochemical hydrogen pump cell experiments. Proton currents of ∼0.5 A cm–2 were obtained through Nafion/graphene/ALD alumina/Nafion at a modest bias voltage of 150 mV, which reflects a diminishment by less than half relative to the value for membranes without the 50 cycles of ALD alumina applied to the graphene. Proton transmission through the ALD alumina layer is thought to occur via hydrated alumina surfaces within the relatively porous ALD alumina layer on the graphene surface and perhaps also at grain boundaries formed in the alumina during the ALD process.