Langmuir–Blodgett Deposition of Graphite Oxide Nanosheets as Catalysts for Bipolar Membrane Electrochemistry

Graphite oxide (GO) has been widely studied as an interfacial layer in bipolar membranes because of its activity as a catalyst for the water dissociation and acid–base neutralization reactions. However, the roles of GO nanosheet size, orientation, and surface coverage in controlling its catalytic activity are not well understood. GO nanosheets with lateral dimensions of several microns were prepared by oxidative exfoliation of natural graphite crystals, enabling their orientation and coverage to be observed directly by optical microscopy. The coverage and orientation of the GO nanosheets were measured as a function of different deposition methods, which included solution adsorption, Langmuir–Blodgett transfer from an air–water interface, and spray-coating. Highly oriented GO films made by the Langmuir–Blodgett method gave the best performance metrics. Interestingly, full coverage of the bipolar interface by GO nanosheets resulted in lower performance than partial coverage. This effect could be rationalized in terms of the differential permeability of H⁺ and OH^– ions through GO, which leads to concentration polarization of OH^– on one side of the sheets.