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-mu m-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 (similar to 25 mu m radius) could greatly inhibit natural convection effects in 30 mM redox solution.