Nanoscale Mapping of the Double Layer Potential at the Graphene–Electrolyte Interface

Abstract

The electrical double layer (EDL) governs the operation of multiple electrochemical devices, determines reaction potentials, and conditions ion transport through cellular membranes in living organisms. The few existing methods of EDL probing have low spatial resolution, usually only providing spatially averaged information. On the other hand, traditional Kelvin probe force microscopy (KPFM) is capable of mapping potential with nanoscale lateral resolution but cannot be used in electrolytes with concentrations higher than several mmol/L. Here, we resolve this experimental impediment by combining KPFM with graphene-capped electrolytic cells to quantitatively measure the potential drop across the EDL in aqueous electrolytes of decimolar and molar concentrations with a high lateral resolution. The surface potential of graphene in contact with deionized water and 0.1 mol/L solutions of CuSO4 and MgSO4 as a function of counter electrode voltage is reported. The measurements are supported by numerical modeling to reveal the role of the graphene membrane in potential screening and to determine the EDL potential drop. The proposed approach proves to be especially useful for imaging spatially inhomogeneous systems, such as nanoparticles submerged in an electrolyte solution. It could be suitable for in operando and in vivo measurements of the potential drop in the EDL on the surfaces of nanocatalysts and biological cells in equilibrium with liquid solutions.

Graphene-Capped Liquid Thin Films for Electrochemical Operando X-ray Spectroscopy and Scanning Electron Microscopy

“…The variation of the peak position in Figure 3 b is <±0.2 eV and is probably composed of an uncertainty in the excitation energy, which is ±0.1 eV (see the Experimental Section ), the work function of the sample, and incomplete screening of the surface potential of graphene. 34 The latter is caused by quantum capacity effects.…”

“…The latter procedure therefore includes changes in the work function of the sample. All potentials were corrected for pH and iR , but we neglected possible deviations caused by incomplete screening of the surface charge, 34 whose magnitude was not measured. For reasons of the latter, the significant figures of the calculated potentials against the reversible hydrogen electrode (RHE) might not be adequate.…”

Probing Heterogeneity in Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) Response with Synchrotron Infrared Microspectroscopy

“…8 The surface enhancement effect in ATR-SEIRAS is present in non-coinage metals making the technique applicable to highly electrocatalytically active metals such as Pt, 9 Rh, 10 and Pd, 11,12 as well as first row transition metals. 13 However, in general, the spatial resolution of surface sensitive infrared spectroscopy is diffraction limited unless coupled with near-field techniques that, with few exceptions, 14,15 are incompatible with in situ/operando electrochemical conditions. This has greatly limited the use of infrared-based techniques for spectroelectrochemical mapping experiments.…”

Observing Electrochemical Reactions on Suspended Graphene: An Operando Kelvin Probe Force Microscopy Approach

“…Recently, reactor‐cells developed with this graphene‐membrane approach were shown to be feasible for characterizing materials in their reaction environment via electron microscopy and X‐ray spectroscopy techniques. [ 3–10 ] However, graphene's high specific surface area makes it particularly susceptible to doping by its operating environment, [ 11 ] which may be influenced by its electromechanical behavior, [ 12,13 ] electrochemical reactions, [ 11 ] functionalization, as well as contamination during preparation of the reactor‐cell, [ 14 ] including exposure to air. [ 15,16 ] Furthermore, the width of the electrical double layer (EDL) formed in liquid environments can have a major influence on doping.…”

“…The description we use here, which is doping of graphene, is essentially equivalent to the description used in ref. [ 14 ] , which is electrolyte potential being altered by the dipole layer created at the EDL and unscreened potential being detected from above the surface. According to our model, ≈125 meV shift in Fermi‐level ( E F ) equivalent to roughly 10 12 cm −2 doping can be estimated for a 10 –3 m NaOH solution at ∓1 V using this model and assuming the EDL thickness to be equal to the Debye length (Figure S1, Supporting Information).…”