Understanding natural convection in low-concentration electrochemical system: Validation of the convection modes

Natural convection could arise under density gradients of solutions caused by the electrochemical reaction occurring on the electrode surface. This density-driven convection decays rapidly with the decreasing redox concentration, and becomes negligible in dilute solutions (1–2 mM). However, steady-state voltammetry is still observed in these dilute solutions due to the presence of a convection–diffusion layer. Here we studied natural convection effects in low-concentration (5–30 mM) redox solutions and clarified the convection modes in these transient-concentration solutions using cyclic voltammetry and chronoamperometry. The combined theoretical–experimental work demonstrated the coexistence of convection–diffusion layer and density-driven convection. With the use of a 150-μm-radius Pt microelectrode, the convection–diffusion layer convection dominated natural convection in dilute solution (1–5 mM), reinforced mass transport with density-driven convection in low redox concentrations (5–11 mM), and suppressed the density-driven convection at high redox concentrations (>15 mM). The zone diagrams delineating the transition from diffusion to convection were then established to reveal the effects of electrode radius, thickness of the convection–diffusion layer, time scale, and redox concentration on natural convection. Consequently, the use of microelectrodes (∼25 μm radius) could greatly inhibit natural convection effects in 30 mM redox solution.