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 Vapplied= −7 V vs Ag/AgCl to be applied without exceeding 200 μA/cm2of current density. At higher currents, bubble formation (i.e., hydrogen evolution reaction) prohibits reliable spectra from being obtained from the electrode surface. Using CaF2as 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 (ΔωG) and the applied potential, with blueshifts as high as ΔωG= 18 cm–1. These large Raman shifts indicate a large change in the Fermi level of ΔEF= −0.43 eV for graphene electrodes in contact with water, favoring reduction half-reactions. Based on the solution resistance measurement, there is a VIR= 3.1 V voltage drop across the solution for D2O (when the applied potential was Vapplied= −7 V vs Ag/AgCl) and the effective reducing potential on the working electrode is Veffective= −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 Vapplied= −2.7 V vs Ag/AgCl and a corresponding shift in the Fermi level ΔEF= −0.32 eV, indicating that pure water can provide a more robust electrolyte for reaching low reducing potentials than ionic liquids.