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

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