Low Reducing Potentials Enabled by CaF₂-Supported Graphene Electrodes in High Impedance Solutions

We report electrochemical measurements using in situ Raman spectroscopy at graphene/D₂O interfaces under extremely low applied potentials. Here, the hydrophobic and catalytically inert nature of graphene and the insulating nature of the deionized (DI) water enables potentials as low as V_applied = -7 V vs Ag/AgCl to be applied without exceeding 200 μA/cm² of current density. At higher currents, bubble formation (i.e., hydrogen evolution reaction) prohibits reliable spectra from being obtained from the electrode surface. Using CaF₂ as the supporting substrate enables significantly lower reducing potentials to be reached compared to glass substrates, likely due to trapped charge and impurities in the glass substrate. G band Raman spectra taken under various applied electrochemical potentials exhibit a linear relationship between the G band shift (Δω_G) and the applied potential, with blueshifts as high as Δω_G = 18 cm^-1. These large Raman shifts indicate a large change in the Fermi level of ΔE_F = -0.43 eV for graphene electrodes in contact with water, favoring reduction half-reactions. Based on the solution resistance measurement, there is a V_IR = 3.1 V voltage drop across the solution for D₂O (when the applied potential was V_applied = -7 V vs Ag/AgCl) and the effective reducing potential on the working electrode is V_effective = -3.9 V vs Ag/AgCl. We have also tested these graphene electrodes in ionic liquids [DEME][TFSI], which are limited to applied potentials above V_applied = -2.7 V vs Ag/AgCl and a corresponding shift in the Fermi level ΔE_F = -0.32 eV, indicating that pure water can provide a more robust electrolyte for reaching low reducing potentials than ionic liquids.