  In electrochemical systems, an understanding of the underlying transport
processes is required to aid in their better design. This includes knowledge of
possible near-electrode convective mixing that can enhance measured currents.
Here, for a binary acidic electrolyte in contact with a platinum electrode, we
provide evidence of electroconvective instability during electrocatalytic
proton reduction. The current-voltage characteristics indicate that
electroconvection, visualized with a fluorescent dye, drives current densities
larger than the diffusion transport limit. The onset and transition times of
the instability do not follow the expected inverse-square dependence on the
current density, but, above a bulk-reaction-limited current density are delayed
by the water dissociation reaction. The dominant size of the electroconvective
patterns is also measured and found to vary as the diffusion length scale,
confirming previous predictions on the size of electroconvective vortices.
