In Situ Imaging of Ion Transport in Electrochemical Cells for Predictive Solar Fuel Generation

Ion transport underpins the functionality of many modern electrochemical devices involved in energy generation and storage. Despite its importance, label-free spatiotemporal imaging of ion transport in the solution phase in-operando has not been widely explored. Here we have performed interferometric reflection microscopy (IRM) measurements of voltage-induced ion transport in aqueous solution, spatiotemporally mapping ion concentration gradients and extracting transport parameters. We developed an electrochemical cell composed of a narrow channel etched into a microscope coverslip with vertical electrodes running along its sides to drive a reaction in a ferricyanide-based aqueous redox electrolyte. Applying a voltage while imaging the cell, we measure the evolution of concentration profiles through their changes to the local refractive index. Analysis of the evolving ion concentration gradients allows us to extract a diffusion coefficient consistent with that of aqueous ferricyanide, demonstrating the ability to do label-free optical imaging of ion transport in an aqueous electrolyte. This work serves as a basis to proceed to study CO2 reduction for solar fuel generation. We anticipate imaging in situ mass transport to characterize the involved reactions and to compare local and global efficiency and durability across photoelectrode systems for improved device performance.