Substrate effect on charging of electrified graphene/water interfaces

Graphene, a transparent two-dimensional (2D) conductive electrode, has brought extensive new perspectives and prospects to electrochemical systems, such as chemical sensors, energy storage, and energy conversion devices. In many of these applications, graphene, supported on a substrate, is in contact with an aqueous solution. An increasing number of studies indicate that the substrate, rather than graphene, determines the organization of water in contact with graphene, i.e., the electric double layer (EDL) structure near the electrified graphene, and the wetting behavior of the graphene: the graphene sheet is transparent in terms of its supporting substrate. By applying surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy to the silicon dioxide (SiO₂)-supported graphene electrode/aqueous electrolyte interface and comparing the data with those for the calcium fluoride (CaF₂)-supported graphene [Y. Wang et al., Angew. Chem., Int. Ed., 2023, 62, e202216604], we discuss the impact of the different substrates on the charging of both the graphene and the substrate upon applying potentials. The SiO₂-supported graphene shows pseudocapacitive behavior, consistent with the CaF₂-supported graphene case, although the surface charges on SiO₂ and CaF₂ differ substantially. The SiO₂ surface is already negatively charged at +0.57 V (vs. Pd/H₂), and the negative surface charge is doubled when negative potentials are applied, in contrast with the CaF₂ case, where the positive charge is reduced when negative potentials are applied. Interestingly, the charging of the graphene sheet is almost identical between the negatively charged SiO₂ surface and positively charged CaF₂ surface, demonstrating that the graphene charging is decoupled from the charging of the substrates.