Investigation of an aqueous lithium iodide/triiodide electrolyte for dual-chamber electrochemical actuators

Electrochemical pumping, the electromigration-driven flow of ions and their associated solvent molecules across a permselective membrane, is investigated for the construction of dual-chamber electrochemical actuators. Important features include large volumetric strain, significant pressure generation, and minimal pressure-driven backflow. Aqueous electrolytes have a number of advantages over organic electrolytes such as dimethylformamide; four concentrations of a lithium iodide/triiodide electrolyte are investigated here. Fluid transport decreases as the ionic strength increases, with the waters associated with each cation decreasing from 16 to 6 as [Li+] increases from 0.5 to 3.5 M. As a result, the maximum volumetric strain which might be achieved in a symmetric dual-chamber actuator, about 18%, is for an electrolyte of intermediate concentration, 2 M LiI + 0.5 M I2. Pressure generation experiments using this electrolyte reached 295 psig (∼20 atm) in 10 min, with about 50% of the available charge consumed. For this pressure, losses measured at open circuit, ca. 13 psi/min, are lower than previously measured losses using a dimethylformamide electrolyte. Simultaneous measurement of pressure generation and fluid transport provides a measure of the pressure-driven backflow, 0.13 μL/min, which compares favorably with those estimated for the porous separators used for electroosmotic-driven flow.