(308d) Electrokinetics in Simple and Complex Fluids

Electrodeposition is a widely practiced method for creating metal, colloidal, and polymer coatings on conductive substrates. In Newtonian liquid electrolytes typically employed, the process is stable up to voltages one order of magnitude higher than the thermal voltage but thereafter gives way to hydrodynamic instabilities. The instabilities have been linked to failure of microcircuits, dendrite formation on battery electrodes, and overlimiting conductance in ion-selective membranes, which has motivated interest in understanding their origins. This talk considers electrokinetics and electrodeposition of metals in simple Newtonian and complex fluid electrolytes and the relationship between the ion distribution at ion selective interfaces and the hydrodynamic instability termed electroconvection. By means of experiment and theoretical analysis it is shown that viscoelastic electrolytes composed of semidilute solutions of very high molecular weight neutral polymers suppress these instabilities by multiple processes. The voltage window in which a liquid electrolyte can operate free of electroconvective instabilities is also shown to be markedly extended and is a power-law function of electrolyte viscosity. This power-law relation is replicated in the resistance to ion transport at liquid/solid interfaces, but is not captured by continuum models for electrodeposition in simple liquids. I will discuss consequences of our observations and show that viscoelastic electrolytes enable stable electrodeposition of many metals, with the most profound effects observed for soft, reactive metals, such as sodium and lithium of contemporary interest for high-energy electrochemical energy storage.