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

Area of Science:

• Electrochemistry
• Materials Science
• Spectroscopy

Background:

• Achieving extremely low electrochemical potentials is crucial for studying reduction half-reactions and developing advanced electrochemical systems.
• Graphene's unique properties, including hydrophobicity and catalytic inertness, offer potential for stable electrochemical interfaces.
• Conventional electrolytes often face limitations in achieving very low potentials due to side reactions like bubble formation.

Purpose of the Study:

• To investigate electrochemical measurements at graphene/D2O interfaces using in situ Raman spectroscopy under extremely low applied potentials.
• To compare the performance of D2O as an electrolyte with ionic liquids for achieving low reducing potentials.
• To understand the relationship between applied potential, Fermi level shifts, and Raman spectra of graphene.

Main Methods:

• Electrochemical measurements combined with in situ Raman spectroscopy were performed on graphene electrodes in D2O.
• Applied potentials ranged down to -7 V vs Ag/AgCl, with current densities monitored to avoid hydrogen evolution.
• Calcium fluoride (CaF2) substrates were used to facilitate lower potentials compared to glass substrates.

Main Results:

• Graphene electrodes in D2O allowed potentials as low as -7 V applied, with an effective reducing potential of -3.9 V vs Ag/AgCl.
• A linear relationship was observed between the G band Raman shift (ΔωG) and applied potential, indicating Fermi level changes up to ΔEF = -0.43 eV.
• D2O demonstrated superior performance over ionic liquids ([DEME][TFSI]), enabling lower potentials (-3.9 V vs -2.7 V effective) and larger Fermi level shifts.

Conclusions:

• Pure water (D2O) serves as a more robust electrolyte than ionic liquids for achieving significantly low reducing potentials at graphene electrodes.
• The observed large Fermi level shifts in graphene suggest its suitability for applications favoring reduction half-reactions.
• CaF2 substrates are advantageous over glass for reaching lower potentials due to reduced substrate interference.