Nanoscale mapping of the double layer potential at the graphene-electrolyte interface
Evgheni Strelcov, Christopher M. Arble, Hongxuan Guo, Brian D. Hoskins, Alexander Yulaev, Ivan Vlassiouk, Nikolai B. Zhitenev, Alexander Tselev, Andrei A. Kolmakov
The structure and potential drop across the electrical double layer (EDL) govern the operation of multiple electrochemical devices, determine reaction potentials and condition ion transport through the cellular membranes in living organisms. Despite more than a century of research, very few methods of direct measurement of the composition and potential of the EDL exist. These methods often suffer from complexity and low spatial resolution, usually only providing spatially-averaged information. On the other hand, 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 potential drop across EDL in aqueous electrolytes of any concentration 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 proved to be especially useful for imaging spatially inhomogeneous systems with 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 nano-catalysts and biological cells in equilibrium with liquid solutions.