  Fundamental mechanisms of energy storage, corrosion, sensing, and multiple
biological functionalities are directly coupled to electrical processes and
ionic dynamics at solid-liquid interfaces. In many cases, these processes are
spatially inhomogeneous taking place at grain boundaries, step edges, point
defects, ion channels, etc. and possess complex time and voltage dependent
dynamics. This necessitates time-resolved and real-space probing of these
phenomena. In this review, we discuss the applications of force-sensitive
voltage modulated scanning probe microscopy (SPM) for probing electrical
phenomena at solid-liquid interfaces. We first describe the working principles
behind electrostatic and Kelvin Probe Force microscopies (EFM & KPFM) at the
gas-solid interface, review the state of the art in advanced KPFM methods and
developments to (i) overcome limitations of classical KPFM, (i) expand the
information accessible from KPFM, and (iii) extend KPFM operation to liquid
environments. We briefly discuss the theoretical framework of the electrical
double layer (EDL) forces and dynamics, the implications and breakdown of
classical EDL models for highly charged interfaces, or under high ion
concentrations, and briefly describe recent modifications of the classical EDL
theory relevant for understanding nanoscale electrical measurements at the
solid-liquid interface. We further review the latest achievements in mapping
surface charge, dielectric constants, and electrodynamic and electrochemical
processes in liquids. Finally, we outline the key challenges and opportunities
that exist in the field of nanoscale electrical measurements in liquid as well
as provide a roadmap for the future development of liquid KPFM.
