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.
Label-free spectromicroscopy methods offer the capability to examine complex cellular phenomena. Electron and X-ray-based spectromicroscopy methods, though powerful, have been hard to implement with hydrated objects due to the vacuum...
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