Low\nReducing Potentials Enabled by CaF<sub>2</sub>‑Supported Graphene\nElectrodes in High Impedance Solutions

We report electrochemical measurements\nusing in situ Raman spectroscopy\nat graphene/D<sub>2</sub>O interfaces under extremely low applied\npotentials. Here, the hydrophobic and catalytically inert nature of\ngraphene and the insulating nature of the deionized (DI) water enables\npotentials as low as <i>V</i><sub>applied</sub> = −7\nV vs Ag/AgCl to be applied without exceeding 200 μA/cm<sup>2</sup> of current density. At higher currents, bubble formation (i.e.,\nhydrogen evolution reaction) prohibits reliable spectra from being\nobtained from the electrode surface. Using CaF<sub>2</sub> as the\nsupporting substrate enables significantly lower reducing potentials\nto be reached compared to glass substrates, likely due to trapped\ncharge and impurities in the glass substrate. G band Raman spectra\ntaken under various applied electrochemical potentials exhibit a linear\nrelationship between the G band shift (Δω<sub>G</sub>)\nand the applied potential, with blueshifts as high as Δω<sub>G</sub> = 18 cm<sup>–1</sup>. These large Raman shifts indicate\na large change in the Fermi level of Δ<i>E</i><sub>F</sub> = −0.43 eV for graphene electrodes in contact with\nwater, favoring reduction half-reactions. Based on the solution resistance\nmeasurement, there is a <i>V</i><sub>IR</sub> = 3.1 V voltage\ndrop across the solution for D<sub>2</sub>O (when the applied potential\nwas <i>V</i><sub>applied</sub> = −7 V vs Ag/AgCl)\nand the effective reducing potential on the working electrode is <i>V</i><sub>effective</sub> = −3.9 V vs Ag/AgCl. We have\nalso tested these graphene electrodes in ionic liquids [DEME][TFSI],\nwhich are limited to applied potentials above <i>V</i><sub>applied</sub> = −2.7 V vs Ag/AgCl and a corresponding shift\nin the Fermi level Δ<i>E</i><sub>F</sub> = −0.32\neV, indicating that pure water can provide a more robust electrolyte\nfor reaching low reducing potentials than ionic liquids.