Metal-Ion Depletion Impacts the Stability and Performance of Battery Electrode Deionization over Multiple Cycles

Adsorption selectivity of nickel hexacyanoferrate foam electrodes and influencing factors: extraction of a 98 % potassium fraction solution from pseudo-seawater

Metal hexacyanoferrates are promising adsorbents for desalination and concentration of seawater and wastewater, because of a high capacity for selective cation intercalation into their three-dimensional lattice through redox reactions. The ratio between fractional quantities of cations removed from a solution is a metric commonly used to evaluate adsorption selectivity. However, this metric also depends directly on cation concentrations in the adsorption solution, thus on the electrode potential through the reaction yield. Here, we analyzed the adsorption selectivity of nickel hexacyanoferrate (NiHCF) foam electrodes, characterized by high porosity and excellent ion diffusivity, using only the selectivity coefficient for ion exchange, i.e., independently of the electrode potential. We conducted a potassium extraction experiment with three consecutive stages, each comprising first an adsorption then a desorption process. From pseudo-seawater (K+ = 10 mmol/L, Na+ = 495 mmol/L) as first adsorption solution, we obtained a final desorption solution with a high potassium fraction (98 %; K+ = 123 mmol/L, Na+ = 3 mmol/L). Temporal concentration variations illustrated the close agreement between measurements and values calculated using only the selectivity coefficient for ion exchange, demonstrating that the adsorption selectivity of NiHCF foam electrodes was primarily influenced by ion exchange reactions, and did not depend on the electrode potential. We also demonstrated the usefulness of our foam electrodes for industrial application through a cyclability assessment and a detailed K+ adsorption selectivity evaluation in synthetic seawater, a more realistic seawater analogue containing not only Na+ and K+, but also divalent cations (Mg2+, Ca2+).

Enhanced hybrid capacitive performance for efficient and selective potassium extraction from wastewater: Insights from regulating electrode potential

Prussian blue analogues hold great promise for directly extracting potassium resource from wastewater via hybrid capacitive deionization (HCDI). However, there remain unresolved scientific issues regarding low efficiency and selectivity arising from asymmetric potential distribution induced by spontaneous charge matching. This work systematically investigated the underlying mechanisms for enhancing the storage capacity and specific affinity of representative Berlin Green towards K+ through precise regulation of insertion potential during HCDI operation. Empowered by controlling electrochemical intercalation behaviors, the compatibility between ionic and electronic kinetics was significantly enhanced. Impressive values of 160.12 mg/g, 61.27 %, and 0.07 kWh/mol were achieved under potentiostatic mode (0.1 V vs. Ag/AgCl) for insertion capacity, charge efficiency, and energy consumption, respectively. These results significantly outperformed the optimal levels obtained under constant cell voltage (0.9 V), which were 128.52 mg/g, 47.50 %, and 0.12 kWh/mol, respectively. In both aqueous solution with binary components and urine, the results emphasized the potential of the synergy effect between lattice hindrance and insertion chemistry in promoting intercalation selectivity, with the highest selectivity coefficients of 28.35 (K+/Na+), 76.22 (K+/Ca2+) and 175.12 (K+/Mg2+), respectively. The presented concept-to-proof offers a versatile approach for the advancement of high-performance HCDI and paves the way towards its sustainable application in nutrient recycling from natural waters or wastewaters.

Ostwald-Ripening Induced Interfacial Protection Layer Boosts 1,000,000-Cycled Hydronium-Ion Battery

Collapsing and degradation of active materials caused by the electrode/electrolyte interface instability in aqueous batteries are one of the main obstacles that mitigate the capacity. Herein by reversing the notorious side reactions include the loss and dissolution of electrode materials, as we applied Ostwald ripening (OR) in the electrochemical cycling of a copper hexacyanoferrate electrode in a hydronium-ion batteries, the dissolved Cu and Fe ions undergo a crystallization process that creates a stable interface layer of cross-linked cubes on the electrode surface. The layer exposed the low-index crystal planes (100) and (110) through OR-induced electrode particle growth, supplemented by vacancy-ordered (100) superlattices that facilitated ion migration. Our design stabilized the electrode-electrolyte interface considerably, achieving a cycle life of one million cycles with capacity retention of 91.6 %, and a capacity retention of 91.7 % after 3000 cycles for a full battery.

Enhanced Desalination Performance Using Phosphate Buffer-Mediated Redox Reactions of Manganese Oxide Electrodes in a Multichannel System

Water desalination mediated by electrochemical reactions to directly capture and release salt at electrode materials offers a low-voltage method for producing freshwater. Developing new system designs has allowed electrode materials to maximize their capacity for salt separation, especially when a multichannel system is used to introduce a separate electrode rinse solution. Here, we show that the use of an additive can provide a new strategy for improving electrode capacity and, hence desalination performance, which so far has been limited to increasing the electrolyte concentration. A custom-built, 2/2-channel flow cell divided by two cation exchange membranes and an anion exchange membrane was fed with 50 mM NaCl as the feed (two inner channels) and 0.5 M NaCl containing up to 0.1 M phosphate as the electrode rinse (two outer channels). Using manganese oxide electrodes with phosphate buffer-mediated redox reactions exhibited an improved desalination capacity of 68.0 ± 5.2 mg g-1 (0.55 mA cm-2) and a rate of 5.6 ± 1.3 mg g-1 min-1 (0.96 mA cm-2). The improvement was attributed to the buffer that served as a proton donor for promoting the H+ insertion reaction of amorphous or poorly crystalline MnO2. Additionally, the buffering capacity against acidification and the creation of insoluble manganese phosphate on the electrode surface prevented the dissolution of Mn2+, which could otherwise occur at the anode due to a decrease in the local pH upon H+ deinsertion. Thus, the use of manganese oxide electrodes coupled with phosphate provides a new strategy of increasing electrode capacity for water desalination.

Structural/Compositional-Tailoring of Nickel Hexacyanoferrate Electrodes for Highly Efficient Capacitive Deionization

Prussian blue analogs (PBAs) represent a crucial class of intercalation electrode materials for electrochemical water desalination. It is shown here that structural/compositional tailoring of PBAs, the nickel hexacyanoferrate (NiHCF) electrodes in particular, can efficiently modulate their capacitive deionization (CDI) performance (e.g., desalination capacity, cyclability, selectivity, etc.). Both the desalination capacity and the cyclability of NiHCF electrodes are highly dependent on their structural/compositional features such as crystallinity, morphology, hierarchy, and coatings. It is demonstrated that the CDI cell with hierarchically structured NiHCF nanoframe (NiHCF-NF) electrode exhibits a superior desalination capacity of 121.38 mg g-1 , a high charge efficiency of up to 82%, and a large capacity retention of 88% after 40 cycles intercalation/deintercalation. In addition, it is discovered that coating of carbon (C) film over NiHCF can lower its desalination capacity owing to the partial blockage of diffusion openings by the coated C film. Moreover, the hierarchical NiHCF-NF electrode also demonstrates a superior selectivity toward monovalent sodium ions (Na+ ) over divalent calcium (Ca2+ ) and magnesim (Mg2+ ) ions, allowing it to be a promising platform for preferential capturing Na+ ions from brines. Overall, the structural/compositional tailoring strategies would offer a viable option for the rational design of other intercalation electrode materials applied in CDI techniques.

Ultrafast Nucleation Reverses Dissolution of Transition Metal Ions for Robust Aqueous Batteries

The dissolution of transition metal ions causes the notorious peeling of active substances and attenuates electrochemical capacity. Frustrated by the ceaseless task of pushing a boulder up a mountain, Sisyphus of the Greek myth yearned for a treasure to be unearthed that could bolster his efforts. Inspirationally, by using ferricyanide ions (Fe(CN)₆^3-) in an electrolyte as a driving force and taking advantage of the fast nucleation rate of copper hexacyanoferrate (CuHCF), we successfully reversed the dissolution of Fe and Cu ions that typically occurs during cycling. The capacity retention increased from 5.7% to 99.4% at 0.5 A g^-1 after 10,000 cycles, and extreme stability of 99.8% at 1 A g^-1 after 40,000 cycles was achieved. Fe(CN)₆^3- enables atom-by-atom substitution during the electrochemical process, enhancing conductivity and reducing volume change. Moreover, we demonstrate that this approach is applicable to various aqueous batteries (i.e., NH₄⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺).

Investigations of Nanoparticle Suspensions of Prussian Blue and Its Copper Analogue: Amine Functionalization and Electrochemical Studies

Prussian blue (PB) and its analogues are promising materials for electrochemical energy storage, yet their use in flow-type devices is limited by their lack of redox responsiveness as colloidal suspensions. We have investigated the redox chemistry amine functionalization of PB along with its Cu analogue (CuPBA). No redox response of colloidal PB was observed and suspensions of CuPBA formed films on electrode surfaces with and without applied potentials; the films were redox-active but the material that remained suspended in solution did not participate in redox chemistry. Propylamine (pa), ethylenediamine (en), or tetramethylethylenediamine (TMEDA) were added in an attempt to maintain well dispersed suspensions through nanoparticle surface functionalization. Propylamine modifications resulted in a loss of the CuPBA network and subsequent precipitation of insoluble materials. Coordination of ethylenediamine prompted the formation of Cu and Fe monomers ([Cu(en)₂]^m+/[Fe(CN)₆]^n-]) that remained soluble in aqueous electrolytes. In the absence of supporting electrolytes, these monomers formed a one-dimensional (1D) polymeric structure (Cu₂Fe-1D). TMEDA modification preserved the CuPBA extended structure with only modest precipitate formation over 30 min. The redox responsiveness of these suspensions depended on conditions; in 1 M KCl, no redox chemistry was observed for the CuPBA. In pH 4 potassium hydrogen phthalate buffer, a signal was observed that was attributed to the Fe centers of CuPBA. Under these conditions, the material precipitated in ∼15 min and the signal was lost. Although the Fe centers in these networks are redox-active, additional work is needed to realize longer-term redox activity and stability. Ligand modifications can alter the properties of these networks but within a given ligand class, e.g., amines, the effects can vary greatly from the deconstruction of the framework to preventing film formation.

Ti3 C2 -MXene Partially Derived Hierarchical 1D/2D TiO2 /Ti3 C2 Heterostructure Electrode for High-Performance Capacitive Deionization

Constructing faradaic electrode with superior desalination performance is important for expanding the applications of capacitive deionization (CDI). Herein, a simple one-step alkalized treatment for in situ synthesis of 1D TiO₂ nanowires on the surface of 2D Ti₃ C₂ nanosheets, forming a Ti₃ C₂ -MXene partially derived hierarchical 1D/2D TiO₂ /Ti₃ C₂ heterostructure as the cathode electrode is reported. Cross-linked TiO₂ nanowires on the surface help avoid layer stacking while acting as the protective layer against contact of internal Ti₃ C₂ with dissolved oxygen in water. The inner Ti₃ C₂ MXene nanosheets cross over the TiO₂ nanowires can provide abundant active adsorption sites and short ion/electron diffusion pathways. . Density functional theory calculations demonstrated that Ti₃ C₂ can consecutively inject electrons into TiO₂ , indicating the high electrochemical activity of the TiO₂ /Ti₃ C₂ . Benefiting from the 1D/2D hierarchical structure and synergistic effect of TiO₂ and Ti₃ C₂ , TiO₂ /Ti₃ C₂ heterostructure presents a favorable hybrid CDI performance, with a superior desalination capacity (75.62 mg g^-1 ), fast salt adsorption rate (1.3 mg g^-1 min^-1 ), and satisfactory cycling stability, which is better than that of most published MXene-based electrodes. This study provides a feasible partial derivative strategy for construction of a hierarchical 1D/2D heterostructure to overcome the restrictions of 2D MXene nanosheets in CDI.

Pairing of Aqueous and Nonaqueous Electrosynthetic Reactions Enabled by a Redox Reservoir Electrode

Paired electrolysis methods are appealing for chemical synthesis because they generate valuable products at both electrodes; however, development of such reactions is complicated by the need for both half-reactions to proceed under mutually compatible conditions. Here, a modular electrochemical synthesis (ModES) strategy bypasses these constraints using a "redox reservoir" (RR) to pair electrochemical half-reactions across aqueous and nonaqueous solvents. Electrochemical oxidation reactions in organic solvents, the conversion of 4-t-butyltoluene to benzylic dimethyl acetal and aldehyde in methanol or the oxidative C-H amination of naphthalene in acetonitrile, and the reduction of oxygen to hydrogen peroxide in water were paired using nickel hexacyanoferrate as an RR that can selectively store and release protons (and electrons) while serving as the counter electrode for these reactions. Selective proton transport through the RR is optimized and confirmed to enable the ion balance, and thus the successful pairing, between redox half-reactions that proceed with different rates, on different scales, and in different solvents (methanol, acetonitrile, and water).

Selective capture of ammonium ions from municipal wastewater treatment plant effluent with a nickel hexacyanoferrate electrode

Currently, intercalation materials such as Prussian blue analogs have attracted considerable attention in water treatment applications due to their excellent size-based selectivity toward cations. This study aimed to explore the feasibility of using a nickel hexacyanoferrate (NiHCF) electrode for selective NH₄⁺ capture from effluent from a municipal wastewater treatment plant. To assess the competitive intercalation between NH₄⁺ and other common cations (Na⁺, Ca²⁺), a NiHCF//activated carbon (AC) hybrid capacitive deionization (CDI) cell was established to treat mixed-salt solutions. The results of cyclic voltammetry (CV) analysis showed a higher current response of the NiHCF electrode toward NH₄⁺ ions than toward Na⁺ and Ca²⁺ ions. In a single-salt solution with NH₄⁺, the optimized operating voltage of the hybrid CDI cell was 0.8 V, with a higher salt adsorption capacity (51.2 mg/g) than those obtained at other voltages (0.1, 0.4, 1.2 V). In a multisalt solution containing NH₄⁺, Na⁺, and Ca²⁺ ions, the selectivity coefficients of NH₄⁺/Ca²⁺ and NH₄⁺/Na⁺ were 9.5 and 4.9, respectively. The feasibility of selective NH₄⁺ capture using the NiHCF electrode in a hybrid CDI cell was demonstrated by treating the effluent from a municipal wastewater treatment plant (WWTP). The intercalation preference of the NiHCF electrode with the WWTP effluent was NH₄⁺>K⁺>Na⁺>Ca²⁺>Mg²⁺, and NH₄⁺ showed the highest salt adsorption capacity among the cations during consecutive cycles. Our results revealed that cations with smaller hydrated radii and lower (de)hydration energies were more favorably intercalated by the NiHCF electrode. The results provide important knowledge regarding the use of intercalation-type electrodes for selective nutrient removal and recovery from wastewater.

Thermodynamic and Kinetic Analyses of Ion Intercalation/Deintercalation Using Different Temperatures on NiHCF Electrodes for Battery Electrode Deionization

Prussian blue analogues are used in electrochemical deionization due to their cation sorption capabilities and ion selectivity properties. Elucidating the fundamental mechanisms underlying intercalation/deintercalation is important for the development of ion-selective electrodes. We examined the thermodynamic and kinetic properties of nickel hexacyanoferrate electrodes by studying different temperatures effects on intercalation/deintercalation with monovalent ions (Li⁺, Na⁺, K⁺, and NH₄⁺) relevant to battery electrode deionization applications. Higher temperatures reduced the interfacial charge transfer resistance and increased the diffusion coefficient of cations in the solid material. Ion transport in the solid material, rather than interfacial charge transfer, was found to be the rate-controlling step, as shown by higher activation energies for ion transport (e.g., 31 ± 3 kJ/mol for K⁺) than for interfacial charge transfer (5 ± 1 kJ/mol for K⁺). The largest increase in cation adsorption capacity with temperature was observed for NH₄⁺ (28.1% from 15 to 75 °C) due to its smallest activation energy. These results indicate that ion hydration energy determines the intercalation potential and activation energies of ion transport in solid material control intercalation/deintercalation rate. Together with the endothermic behavior of deintercalation and exothermic behavior of intercalation, the higher operating temperature results in improvement of ion adsorption capacity depending on specific cations.