To optimize the productivity of ion exchange membranes used in electric field-driven ion separation processes, an understanding of the relationship between membrane structure–property metrics and a measure of ion separation is necessary. The membrane separation factor is a commonly used indicator of ion separation efficacy, and as outlined in this review, it can be related to the intrinsic sorption and diffusion selectivity properties of the membrane. Doing so connects the separation factor to key theories that describe ion transport, and this connection facilitates an analysis of the implications of these theories on electric field-driven ion separations. The process of electrodialysis and ion exchange membranes can be applied for both desalination and ion separation applications, and this review discusses relationships between commonly used metrics for electric field-driven transport (e.g., transport number) and properties commonly used in desalination contexts (e.g., sorption and diffusivity selectivity). These relationships provide context for commonly observed experimental trends. Additionally, some common assumptions (and their implications for describing membrane transport properties related to a multicomponent ED system) are discussed. This review also links fundamental membrane properties (such as sorption and diffusivity selectivity) to ion separation-critical properties (such as the ion exchange affinity). While the diffusivity selectivity may be more important at lower current density values, the sorption selectivity is expected to be important across a wider range of current density values. This review further highlights the interconnected manner by which ion exchange membrane properties and external process conditions couple to influence ion separation performance.

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The major hurdle limiting the widespread use of intermittent renewable solar energy is the lack of efficient and cost-effective energy storage. Photoelectrochemical (PEC) water splitting offers one solution towards producing storable solar fuels like hydrogen. However, commercialization of PEC technology is inhibited by challenges to identify and develop photoelectrodes that are stable, efficient, and made from low-cost materials. Unique to this project is the metal-insulator-semiconductor (MIS) architecture used to decouple the efficiency and stability tradeoff that typically limits conventional photoelectrodes (Fig. 1).(1,2,3,4)  Within this design, a narrow band gap semiconductor efficiently absorbs light and a thin insulating layer protects against corrosion. Meanwhile, metal particles deposited on the insulating layer collect photogenerated electrons and catalyze the water splitting reaction. If the metal deposited is discontinuous and smaller than the wavelength of incident photons, the particles will be effectively optically transparent and will not significantly affect the light absorption properties of the semiconductor.(5) In this work, we perform a systematic study of platinum electrodeposition on p-Si-based photocathodes to better understand electrodeposition on the MIS platform and to optimize nanoparticle size, density, and spacing to improve hydrogen evolution reaction (HER) performance. In order to fabricate well-defined, uniform MIS geometries the electrodeposition process must be finely tuned to control structure properties. Important to this goal is a deep understanding of the nucleation and growth behavior of the catalytic metal particles, and balancing the two processes through pulsed electrodeposition to control particle density and size. In order to nucleate on bare SiO2, a critical voltage or energy must be reached to electrodeposit platinum on the oxide surface. Once nucleated, particles deposited in this regime will continue to grow while additional particles are simultaneously nucleated nearby. By contrast, electrodeposition at a more positive potential will lead to preferential growth of existing particles rather than nucleation of additional particles. To elucidate these regimes, two consecutive linear sweep voltammograms were performed in the electrodeposition electrolyte. As the scans sweep the potential more negative than E0 [PtCl4]2-/Pt = 0.755 V, an onset potential for the reduction reaction/deposition can be seen. In Figure 2, the first scan in the electrodeposition solution reveals the potential required to nucleate Pt species onto bare SiO2 (1. nucleation), whereas the second scan defines a growth regime where Pt species preferentially reduce onto Pt particles (2. growth).  Our goal is to nucleate then grow platinum nanoparticles to control density and size, respectively. Figure 3 verifies this control over particle density/size through a two-step deposition. The controlled pulsed deposition (Fig 3a top) shows a lower density and smaller particle size than the deposition at a constant potential in the nucleation regime (Fig 3b bottom). Additionally, the electrode with the controlled deposition demonstrated an improved photoelectrochemical performance in comparison with the higher density electrode. This is a result of enhanced light absorption. Using this knowledge of nucleation and growth, we can create MIS photoelectrodes with optimal catalyst particle spacing, surface area, and optical properties for enhanced HER performance. References 1. H.J. Lewerenz, et al., Electrochem. Acta, 56, 10726 (2011). 2. A.G. Munoz and H.J. Lewerenz, ChemPhysChem, 11, 1603 (2010). 3. Y. Chen, P. McIntyre, et al., Nature Materials, 10, 539 (2011). 4. D.V. Esposito, I. Levin, T.P. Moffat, A.A. Talin, Nature Materials, 12,562 (2013). 5. A. Heller, D. Aspnes, J. Porter, Journal of Physical Chemistry, 89, 4444 (1985). Figure 1

Polyamide thin-film composite (PA-TFC) membranes make large-scale desalination effective. Interfacial polymerization (IP) is used to make PA-TFC membranes, but it may limit the range of monomers that can be used, which hinders progress toward advanced membranes. Layer-by-layer (LbL) sequential deposition could circumvent kinetic and thermodynamic limitations of the conventional IP process to facilitate incorporation of different co-monomers into the membrane. The selective layer needs to be deposited onto a microporous support, but depositing LbL coatings on microporous supports often results in defective membranes. Using a poly(vinyl alcohol) (PVA) primer between the support and the LbL polyamide layer may prevent defect formation. The water permeance and salt rejection of a three layer, PVA-primed, LbL-based PA-TFC membrane are discussed and compared to a membrane made without the PVA primer and a commercially available membrane. Mass transfer resistances are analyzed using a series resistance model and appear to be small or even negligible compared to that of the polyamide layer. Incorporation of a sulfonated co-monomer into the polyamide via LbL is reported. The combination of a PVA primer layer and LbL sequential deposition may expand the range of co-monomers that could be used relative to polyamide membranes prepared by the conventional IP process.