Reducing impedance to ionic flux in capacitive deionization with Bi-tortuous activated carbon electrodes coated with asymmetrically charged polyelectrolytes

Sulfonated GO coated carbon electrodes with cation-selective functions for enhanced capacitive deionization of saltwater

Deionization of salt, contaminated underground and inorganic waste waters for water recycling and reuse is of increasing importance mainly due to the shortage of freshwater worldwide. Membrane capacitive deionization (MCDI) possessing a high electrosorption capacity and energy efficiency has been considered a promising method for desalination. However, the MCDI reaction system has limited applications because of the high interfacial resistance during operation. In the present work, the novel sulfonated graphene oxide (SGO) serving as a hydrophilic cation exchange membrane that was coated directly on the activated carbon (AC) electrode was prepared to enhance capacitive deionization of saltwater. Experimentally, the electrosorption capacity and charge efficiency of the AC/SGO (negative)||AC (positive) electrode pair using the coated SGO thin film increased from 12.8 to 19.8 mg/g and 56.7 to 89.3%, respectively. The enhancements were associated with the reduction of the co-ion effect during electrosorption. The strong negative PhSO3- group grafted on the SGO thin film could selectively accelerate the transport rate of cations during CDI. The increase of the charge efficiency also led to lower implemented current. This work demonstrates a simple, low-cost and effective desalination method that will likely have many new applications especially in water recycling and reuse.

Boosted brackish water desalination and water softening by facilely designed MnO2/hierarchical porous carbon as capacitive deionization electrode

Capacitive deionization (CDI) is a promising technique for brackish water desalination. However, its salt electrosorption capacity is insufficient for practical application yet, and little information is available on hardness ion (Mg²⁺, Ca²⁺) removal in CDI. Herein, hierarchical porous carbon (HPC) was prepared from low-cost and renewable microalgae via a simple one-pot approach, and both MnO₂/HPC and polyaniline/HPC (PANI/HPC) composites were then synthesized using a facile, one-step hydrothermal method. Compared with the MnO₂ electrode, the MnO₂/HPC electrode presented an improved hydrophilicity, higher specific capacitance, and lower electrode resistance. The electrodes exhibited pseudocapacitive behaviors, and the maximum salt electrosorption capacities of MnO₂/HPC-PANI/HPC CDI cell was up to 0.65 mmol g^-1 NaCl, 0.71 mmol g^-1 MgCl₂, and 0.76 mmol g^-1 CaCl₂, respectively, which were comparable and even higher than those of the previously reported CDI cells. Additionally, the MnO₂/HPC electrode presented a selectivity order of Ca²⁺ ≥ Mg²⁺ > Na⁺, and the divalent cation selectivity was found to be attributed to their stronger binding strength in the cavity of MnO_2. Multiscale simulations further reveal that the MnO₂/HPC electrodes with the unique luminal configuration of MnO₂ and HPC as supportive framework could offer a great intercalation selectivity of the divalent cations and exhibit a great promise in hardness ion removal.

Low porosity, high areal-capacity Prussian blue analogue electrodes enhance salt removal and thermodynamic efficiency in symmetric Faradaic deionization with automated fluid control

Prussian blue analogues (PBAs) show great potential for low-energy Faradaic deionization (FDI) with reversible Na-ion capacity approaching 5 M in the solid-state. However, past continuous-flow demonstrations using PBAs in FDI were unable to desalinate brackish water to potable levels using single-pass architectures. Here, we show that recirculation of effluent from a symmetric cation intercalation desalination cell into brine/diluate reservoirs enables salt removal exceeding 80% at thermodynamic efficiency as high as 80% when cycled with 100 mM NaCl influent and when controlled by a low-volume, automated fluid circuit. This exceptional performance is achieved using a novel heated, alkaline wet phase inversion process that modulates colloidal forces to increase carbon black aggregation within electrode slurries to solidify crack-free, high areal-capacity PBA electrodes that are calendered to minimize cell impedance and electrode porosity. The results obtained demonstrate the need for co-design of auxiliary fluid-control systems together with electrode materials to advance FDI beyond brackish salinity.

Asymmetric Membrane Capacitive Deionization Using Anion-Exchange Membranes Based on Quaternized Polymer Blends

Membrane capacitive deionization (MCDI) for water desalination is an innovative technique that could help to solve the global water scarcity problem. However, the development of the MCDI field is hindered by the limited choice of ion-exchange membranes. Desalination by MCDI removes the salt (solute) from the water (solvent); this can drastically reduce energy consumption compared to traditional desalination practices such as distillation. Herein, we outline the fabrication and characterization of quaternized anion-exchange membranes (AEMs) based on polymer blends of polyethylenimine (PEI) and polybenzimidazole (PBI) that provides an efficient membrane for MCDI. Flat sheet polymer membranes were prepared by solution casting, heat treatment, and phase inversion, followed by modification to impart anion-exchange character. Scanning electron microscopy (SEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) spectroscopy were used to characterize the morphology and chemical composition of the membranes. The as-prepared membranes displayed high ion-exchange capacity (IEC), hydrophilicity, permselectivity and low area resistance. Due to the addition of PEI, the high density of quaternary ammonium groups increased the IEC and permselectivity of the membranes, while reducing the area resistance relative to pristine PBI AEMs. Our PEI/PBI membranes were successfully employed in asymmetric MCDI for brackish water desalination and exhibited an increase in both salt adsorption capacity (>3×) and charge efficiency (>2×) relative to membrane-free CDI. The use of quaternized polymer blend membranes could help to achieve greater realization of industrial scale MCDI.

Enhancing Performance of Capacitive Deionization with Polyelectrolyte-Infiltrated Electrodes: Theory and Experimental Validation

The energy efficiency of capacitive deionization (CDI) with porous carbon electrodes is limited by the high ionic resistance of the macropores in the electrodes. In this study, we demonstrate a facile approach to improve the energy efficiency by filling the macropores with ion-conductive polyelectrolytes, which is termed polyelectrolyte-infiltrated CDI (pie-CDI or πCDI). In πCDI, the filled polyelectrolyte effectively turns the macropores into a charged ion-selective layer and thus increases the conductivity of macropores. We show experimentally that πCDI can save up to half of the energy consumption compared to membrane CDI, achieving identical desalination during the charging step. The energy consumption can be even lower if the process is operated at a smaller average salt adsorption rate. Further energy breakdown analysis based on a theoretical model confirms that the improved energy efficiency is largely attributed to the increased conductivity in the macropores.

Significance of the micropores electro-sorption resistance in capacitive deionization systems

Capacitive Deionization (CDI) is an emerging technology representing a potential alternative to the common, energy-intensive desalination methods for low salinity water streams. In CDI an electrical field is applied to separate ionic species from aqueous solutions and electro-adsorb them into a highly porous material. CDI is a complex multi-scale system which requires robust mathematical models to closely describe its performance. Here, a dynamic two-dimensional model is developed coupling the diffusion and advection of the species in the bulk solution with their diffusion and electro-sorption in the porous electrodes. In this model, the adsorption/desorption resistance between the micropores and macropores along with variable non-electrostatic attractive forces in the micropores are also incorporated. The proposed theory is validated against experiments using a circular CDI cell operating under various conditions, where different transport mechanisms are limiting the total ion removal process. Performance of the CDI systems is also evaluated using inclusive figures of merit. The obtained results accentuate the significant effect of the rate-limited transfer of the ionic species from the macropores into the micropores, especially in systems subject to severe ion starvation, where neglecting this electro-sorption resistance leads to up to 50% and 210% overestimation of the energy efficiency and overall desalination performance, respectively. Furthermore, although the commonly used transport theory describing CDI fails to capture the dynamics of the systems at low initial concentration and high adsorption capacity by assuming fast electro-sorption without any resistance, the presented theory closely models the transport mechanisms in such systems. Moreover, we experimentally and numerically demonstrate a trade-off between the energetic and desalination performance in systems with low and high mass Péclet number.

Technoeconomic Analysis of Brackish Water Capacitive Deionization: Navigating Tradeoffs between Performance, Lifetime, and Material Costs

Capacitive deionization (CDI), a class of electrochemical separation technologies, has been proposed as an energy-efficient brackish water desalination method. Previous studies have focused on improving capacity and energy consumption through material (e.g., ion-selective membranes [IEMs], charged carbon) and operational modifications, but there has been no analysis that directly links lab-scale experimental performance to capital and operating costs of full-scale water production. In this study, we developed a parameterized process model and technoeconomic analysis framework to project capital and operating costs at the million gallon per day scale based on reported material and operational characteristics for constant current CDI with and without low ($20 m^-2)- and high-cost ($100 m^-2) IEMs. Using this framework, we conducted global sensitivity and uncertainty analyses for water price across the reported CDI design space. Our results show that the operating constraints of brackish water desalination lead to capital costs 2-14 times greater than operating costs (particularly for MCDI). While MCDI outperforms CDI, IEM prices dictate the threshold at which MCDI is more cost-effective. The high relative capital costs highlight the importance of achieving system lifetimes at 2 years or beyond. Last, we set performance and areal cost benchmarks for material-based CDI performance and lifetime improvements.

Effect of conductive additives on the transport properties of porous flow-through electrodes with insulative particles and their optimization for Faradaic deionization

Deionization devices that use intercalation reactions to reversibly store and release cations from solution show promise for energy-efficient desalination of alternative water resources. Intercalation materials often display low electronic conductivity that results in increased energy consumption during desalination. Accordingly, we performed experiments to quantify the impact of the size and mass fraction of conductive additives and insulative active particles on the effective electronic conductivity, ionic conductivity, and hydraulic permeability of porous electrodes. We find that Ketjen black conductive additives with nodules <50 nm in diameter produce superior electronic conductivity at lower mass fractions than the larger carbon blacks commonly used in capacitive deionization. Hydraulic permeability and effective ionic conductivity depend weakly on carbon black content and size, though smaller active particles decrease hydraulic permeability. Based on these results we analyzed the energy consumption and salt removal rate of different electrode formulations by constructing an electrochemical Ashby plot predicting the variation of desalination performance with electrode transport properties. Optimized electrodes containing insulative Prussian blue analogue (PBA) particles were then fabricated and used in an experimental cation intercalation desalination (CID) cell with symmetric electrodes. For 100 mM NaCl influent energy consumption varied from 7 to 33 kJ/mol when current density increased from 1 to 8 mA/cm², approaching ten-fold increased salt removal rate at similar energy consumption levels to past CID demonstrations. Complementary numerical and analytical modeling indicates that further improvements in energy consumption and salt removal rate are attainable by enhancing transport in solution and within PBA agglomerates.