  Ion transport is a fundamental process in many physical, chemical, and
biological phenomena, and especially in electrochemical energy conversion and
storage. Despite its immense importance, demonstrations of label-free,
spatially and temporally resolved ion imaging in the solution phase under
operando conditions are not widespread. Here we spatiotemporally map ion
concentration gradient evolution in solution and yield ion transport parameters
by refining interferometric reflection microscopy, obviating the need for
absorptive or fluorescent labels. As an example, we use an electrochemical cell
with planar electrodes to drive concentration gradients in a ferricyanide-based
aqueous redox electrolyte, and we observe the lateral spatiotemporal evolution
of ions via concentration-dependent changes to the refractive index. Analysis
of an evolving spatiotemporal ion distribution directly yields the diffusivity
of the redox-active species. The simplicity of this approach makes it amenable
to probing local ion transport behavior in a wide range of electrochemical,
bioelectronic, and electrophysiological systems.
