Counter-ion transport number and membrane potential in working membrane systems

Effects of Current on the Membrane and Boundary Layer Selectivity in Electrochemical Systems Designed for Nutrient Recovery

During electrochemical nutrient recovery, current and ion exchange membranes (IEM) are used to extract an ionic species of interest (e.g., ion) from a mixture of multiple ions. The species of interest (ion 1) has an opposing charge to the IEM. When ion 1 is extracted from the solution, the species fractions at the membrane and the adjunct boundary layers are affected. Hence, the species transport through the electrochemical system (ES) can no longer be described as electrodialysis-like. A dynamic state is observed in the compartments, where the ionic species are recovered. When the boundary layer-membrane interface is depleted, the IEM is at maximum current. If the ES is operated at a current higher than the maximum current, the fluxes of both ion 1 and other competing ions, with the same charge (ion 2), occur. This means, for example, ion 1 will be recovered, and the concentration of ion 2 will build up in time. Therefore, a steady state is never reached. Ideally, to prevent the effect of limiting current at the boundary layer-membrane interface, ES for nutrient recovery should be operated at low currents.

Osmotic Pressure and Diffusion of Ions in Charged Nanopores

The transport of ions and water in nanopores is of interest for a number of natural and technological processes. Due to their practically identical long straight cylindrical pores, nanoporous track-etched membranes are suitable materials for investigation of its mechanisms. This communication reports on simultaneous measurements of osmotic pressure and salt diffusion with a 24 nm pore track-etched membrane. Due to the use of dilute electrolyte solutions (1-4 mM KCl and LiCl), this pore size was commensurate with the Debye screening length. Advanced interpretation of experimental results using a full version of the space-charge model has revealed that osmotic pressure and salt diffusion can be quantitatively correlated with electrostatic interactions of ions with charged nanopore walls. The surface-charge density is shown to increase with electrolyte concentration in agreement with the mechanism of deprotonation of weakly acidic surface groups. Moreover, a lack of significant surface-charge dependence on the kind of cation (K+ or Li+) demonstrates that binding of salt counterions does not play a major role in this system.

Nanopore-Based Power Generation from Salinity Gradient: Why It Is Not Viable

In recent years, the development of nanopore-based membranes has revitalized the prospect of harvesting salinity gradient (blue) energy. In this study, we systematically analyze the energetic performance of nanopore-based power generation (NPG) at various process scales, beginning with a single nanopore, followed by a multipore membrane coupon, and ending with a full-scale system. We confirm the high power densities attainable by a single nanopore and demonstrate that, at the coupon scale and above, concentration polarization severely hinders the power density of NPG, revealing the common, yet significant, error in linearly extrapolating single-pore performance to multipore membranes. Through our consideration of concentration polarization, we also importantly show that the development of materials with exceptional nanopore properties provides limited enhancement of practical process performance. For a full-scale NPG membrane module, we find an inherent tradeoff between power density and thermodynamic energy efficiency, whereby achieving a high power density sacrifices the energy efficiency. Furthermore, we derive a simple expression for the theoretical maximum energy efficiency of NPG, showing it is solely related to the membrane selectivity (i.e., S²/2). Through this relation, it is apparent that the energy efficiency of NPG is limited to only 50% (for a completely selective membrane, i.e., S = 1), reinforcing our optimistic full-scale simulations which result in a (practical) maximum energy efficiency of 42%. Finally, we assess the net extractable energy of a full-scale NPG system which mixes river water and seawater by including the energy losses from pretreatment and pumping, revealing that the NPG process-both in its current state of development and in the case of highly optimistic performance with minimized external energy losses-is not viable for power generation.

Steady State and Dynamic Response of Voltage-Operated Membrane Gates

An electrochemical flow cell with Nafion 212, aqueous LiI/I2 redox solution, and carbon paper electrode was operated as an electro-osmotic gate based on the Electrokinetic Energy Conversion (EKEC) principle. The gate was operated in different modes. (i) In normal DC pump operation it is shown to follow the predictions from the phenomenological transport equations. (ii) Furthermore, it was also demonstrated to operate as a normally open, voltage-gated valve for microfluidic purposes. For both pump and valve operations low energy requirements (mW range) were estimated for precise control of small flows (μL range). (iii) Finally, the dynamic response of the pump was investigated by using alternating currents at a range of different frequencies.