Synergistic effect of intercalation and EDLC electrosorption of 2D/3D interconnected architectures to boost capacitive deionization for water desalination via MoSe2/mesoporous carbon hollow spheres

Spatially Confined Alloying of Pt Accelerates Mass Transport for Fuel Cell Oxygen Reduction

Pt-based alloy with high mass activity and durability is highly desired for proton exchange membrane fuel cells, yet a great challenge remains due to the high mass transport resistance near catalysts with lowering Pt loading. Herein, an extensible approach employing atomic layer deposition to accurately introduce a gas-phase metal precursor into platinum nanoparticles (NPs) pre-filled mesoporous channels is reported, achieved by controlling both the deposition site and quantity. Following the spatially confined alloying treatment, the prepared PtSn alloy catalyst within mesopores demonstrates a small size and homogeneous distribution (2.10 ± 0.53 nm). The membrane electrode assembly with mesoporous carbon-supported PtSn alloy catalyst achieves a high initial mass activity of 0.85 Amg Pt - 1 ${\mathrm{mg}}_{\mathrm{Pt}}^{-1}$ at 0.9 V, which is attributed to the smallest local oxygen transport resistance (3.68 S m-1) ever reported. The mass activity of the catalyst only decreases by 11% after 30000 cycles of accelerated durability test, representing superior full-cell durability among the reported Pt-based alloy catalysts. The enhanced activity and durability are attributed to the decreased adsorption energy of oxygen intermediates on Pt surface and the strong electronic interaction between Pt and Sn inhibiting Pt dissolution.

Enhanced Capacitive Deionization of Hollow Mesoporous Carbon Spheres/MOFs Derived Nanocomposites by Interface-Coating and Space-Encapsulating Design

Exploring new carbon-based electrode materials is quite necessary for enhancing capacitive deionization (CDI). Here, hollow mesoporous carbon spheres (HMCSs)/metal-organic frameworks (MOFs) derived carbon materials (NC(M)/HMCSs and NC(M)@HMCSs) are successfully prepared by interface-coating and space-encapsulating design, respectively. The obtained NC(M)/HMCSs and NC(M)@HMCSs possess a hierarchical hollow nanoarchitecture with abundant nitrogen doping, high specific surface area, and abundant meso-/microporous pores. These merits are conducive to rapid ion diffusion and charge transfer during the adsorption process. Compared to NC(M)/HMCSs, NC(M)@HMCSs exhibit superior electrochemical performance due to their better utilization of the internal space of hollow carbon, forming an interconnected 3D framework. In addition, the introduction of Ni ions is more conducive to the synergistic effect between ZIF(M)-derived carbon and N-doped carbon shell compared with other ions (Mn, Co, Cu ions). The resultant Ni-1-800-based CDI device exhibits excellent salt adsorption capacity (SAC, 37.82 mg g-1) and good recyclability. This will provide a new direction for the MOF nanoparticle-driven assembly strategy and the application of hierarchical hollow carbon nanoarchitecture to CDI.

Engineering Faradaic Electrode Materials for High-Efficiency Water Desalination

Water desalination technologies play a key role in addressing the global water scarcity crisis and ensuring a sustainable supply of freshwater. In contrast to conventional capacitive deionization, which suffers from limitations such as low desalination capacity, carbon anode oxidation, and co-ion expulsion effects of carbon materials, the emerging faradaic electrochemical deionization (FDI) presents a promising avenue for enhancing water desalination performance. These electrode materials employed faradaic charge-transfer processes for ion removal, achieving higher desalination capacity and energy-efficient desalination for high salinity streams. The past decade has witnessed a surge in the advancement of faradaic electrode materials and considerable efforts have been made to explore optimization strategies for improving their desalination performance. This review summarizes the recent progress on the optimization strategies and underlying mechanisms of faradaic electrode materials in pursuit of high-efficiency water desalination, including phase, doping and vacancy engineering, nanocarbon incorporation, heterostructures construction, interlayer spacing engineering, and morphology engineering. The key points of each strategy in design principle, modification method, structural analysis, and optimization mechanism of faradaic materials are discussed in detail. Finally, this work highlights the remaining challenges of faradaic electrode materials and present perspectives for future research.

Enhanced redox kinetics of Prussian blue analogues for superior electrochemical deionization performance

Prussian blue analogues (PBAs), representing the typical faradaic electrode materials for efficient capacitive deionization (CDI) due to their open architecture and high capacity, have been plagued by kinetics issues, leading to insufficient utilization of active sites and poor structure stability. Herein, to address the conflict issue between desalination capacity and stability due to mismatched ionic and electronic kinetics for the PBA-based electrodes, a rational design, including Mn substitution and polypyrrole (ppy) connection, has been proposed for the nickel hexacyanoferrate (Mn-NiHCF/ppy), serving as a model case. Particularly, the theoretical calculation manifests the reduced bandgap and energy barrier for ionic diffusion after Mn substitution, combined with the increased electronic conductivity and integrity through ppy connecting, resulting in enhanced redox kinetics and boosted desalination performance. Specifically, the optimized Mn-NiHCF/ppy demonstrates a remarkable desalination capacity of 51.8 mg g-1 at 1.2 V, accompanied by a high charge efficiency of 81%, and excellent cycling stability without obvious degradation up to 50 cycles, outperforming other related materials. Overall, our concept shown herein provides insights into the design of advanced faradaic electrode materials for high-performance CDI.

High-performance electrosorption of lanthanum ion by Mn3O4-loaded phosphorus-doped porous carbon electrodes via capacitive deionization

Transition-metal-oxide@heteroatom doped porous carbon composites have attracted considerable research interest because of their large theoretical adsorption capacity, excellent electrical conductivity and well-developed pore structure. Herein, Mn3O4-loaded phosphorus-doped porous carbon composites (Mn3O4@PC-900) were designed and fabricated for the electrosorption of La3+ in aqueous solutions. Due to the synergistic effect between Mn3O4 and PC-900, and the active sites provided by Mn-O-Mn, C/PO, C-P-O and Mn-OH, Mn3O4@PC-900 exhibits high electrosorption performance. The electrosorption value of Mn3O4@PC-900 was 45.34% higher than that of PC-900, reaching 93.02 mg g-1. Moreover, the adsorption selectivity reached 87.93% and 89.27% in La3+/Ca2+ and La3+/Na+ coexistence system, respectively. After 15 adsorption-desorption cycles, its adsorption capacity and retention rate were 50.34 mg g-1 and 54.12%, respectively. The electrosorption process is that La3+ first accesses the pores of Mn3O4@PC-900 to generate an electric double layer (EDL), and then undergoes further Faradaic reaction with Mn3O4 and phosphorus-containing functional groups through intercalation, surface adsorption and complexation. This work is hoped to offer a new idea for exploring transition-metal-oxide @ heteroatom doped porous carbon composites for separation and recovery of rare earth elements (REEs) by capacitive deionization.

Ultrathin nitrogen-doped carbon Ti3C2Tx-TiN heterostructure derived from ZIF-8 nanoparticles sandwiched MXene for high-performance capacitive deionization

Rational design of high-performance electrode materials is crucial for enhancing desalination performance of capacitive deionization (CDI). Here, ultrathin nitrogen-doped carbon/Ti3C2Tx-TiN (NC/MX-TiN) heterostructure was developed by pyrolyzing zeolite imidazolate framework-8 (ZIF-8) nanoparticles sandwiched MXene (ZSM), which were formed by assembling ultrafine ZIF-8 nanoparticles with size of 20 nm on both sides of MXene nanosheets. The introduction of ultrasmall ZIF-8 particles allowed for in situ nitridation of the MXene during pyrolysis, forming consecutive TiN layers tightly connected to the internal MXene. The two-dimensional (2D) heterostructure exhibited remarkable properties, including high specific surface area and excellent conductivity. Additionally, the resulting TiN demonstrated exceptional redox capability, which significantly enhanced the performance of CDI and ensured cycling stability. Benefiting from these advantages, the NC/MX-TiN exhibited a maximum adsorption capacity of 45.6 mg g-1 and a steady cycling performance in oxygenated saline water over 50 cycles. This work explores the rational design and construction of MXene-based 2D heterostructure and broadens new horizons for the development of novel CDI electrode materials.

Intercalation chemistry engineering strategy enabled high mass loading and ultrastable electrodes for High-Performance aqueous electrochemical energy storage devices

Aqueous electrochemical energy storage devices (AEESDs) are considered one of the most promising candidates for large-scale energy storage infrastructure due to their high affordability and safety. Developing electrodes with the merits of high energy density and long lifespan remains a challenging issue toward the practical application of AEESDs. Research attempts at electrode materials, nanostructure configuration, and electronic engineering show the limitations due to the inherent contradictions associated with thicker electrodes and ion-accessible kinetics. Herein, we propose an intercalation chemistry engineering strategy to enhance the electrolyte ion (de)intercalation behaviors during the electrochemical charge-discharge. To validate this strategy, the prototypical model of a high-mass-loading MnO2-based electrode is used with controlled intercalation of Na+ and H2O. Theoretical and experimental results reveal that an optimal content of Na+ and H2O on the MnO2-based electrode exhibits superior electrochemical performance. Typically, the resultant electrode exhibits an impressive areal capacitance of 1551 mF/cm2 with a mass loading of 9.7 mg/cm2 (at 1 mA/cm2). Furthermore, the assembled full-cell with obtained MnO2-based electrode delivers a high energy density of 0.12 mWh/cm2 (at 20.02 mW/cm2) and ultra-high cycling stability with a capacitance retention percentage of 89.63 % (345 mF/cm2) even after 100,000 cycles (tested over 72 days).

MOF-derived Fe/Ni@C marigold-like nanosheets as heterogeneous electro-Fenton cathode for efficient antibiotic oxytetracycline degradation

The widespread occurrence of organic antibiotic pollution in the environment and the associated harmful effects necessitate effective treatment method. Heterogeneous electro-Fenton (hetero-EF) has been regarded as one of the most promising techniques towards organic pollutant removal. However, the preparation of efficient cathode still remains challenging. Herein, a novel metal-organic framework (MOF)-derived Fe/Ni@C marigold-like nanosheets were fabricated successfully for the degradation of oxytetracycline (OTC) by serving as the hetero-EF cathode. The FeNi3@C (Fe/Ni molar ratio of 1:3) based hetero-EF system exhibited 8.2 times faster OTC removal rate than that of anodic oxidation and possessed many advantages such as excellent OTC degradation efficiency (95.4% within 90 min), broad environmental adaptability (satisfactory treatment performance for multiple antibiotics under various actual water matrixes), good stability and reusability, and significant toxicity reduction. The superior hetero-EF catalytic performance was mainly attributed to: 1) porous carbon and Ni existence were both conducive to the in-situ generation of H2O2 from dissolved O2; 2) the synergistic effects of bimetals together with electron transfer from the cathode promoted the regeneration of ≡ FeII/NiII, thereby accelerating the production of reactive oxygen species; 3) the unique nanosheet structure derived from the precursor two-dimensional Fe-Ni MOFs enhanced the accessibility of active sites. This work presented a promising hetero-EF cathode for the electrocatalytic treatment of antibiotic-containing wastewaters.

Well-designed lamellar reduced graphene oxide-based foam for high-performance solar-driven water purification

Although solar steam generation is promising for seawater desalination, it is less effective in purifying wastewater with both salt/heavy metal ions and organic contaminants. It is thus imperative to develop multifunctional integrated solar-driven water purification systems with high solar-thermal evaporation and photocatalytic degradation efficiencies. Herein, a lamellar reduced graphene oxide (L-RGO) foam with the vertical lamellar structure is fabricated by bidirectional-freezing, lyophilization, and slight chemical reduction for water purification. The unique vertical lamellar structure not only accelerates upward transport of water for facilitating water evaporation but also endows the L-RGO foam with superb high elasticity for tuning the interlayer distance and varying interactions between the oxygen-containing groups and water molecules to adjust water energy state. As a result, the L-RGO foam achieves a superb water evaporation rate of 2.40 kg m-2 h-1 along with an energy efficiency of 95.3 % under the compressive strain of 44.7 % under 1-sun irradiation. Equally importantly, the decoration of L-RGO foam with polypyrrole is capable of efficiently degrading organic pollutants while retaining high solar steam generation performances, exhibiting great potential in the comprehensive treatment of various water sources for relieving freshwater crisis.

Design and construction of heterostructured Zn2V2O7 cubes and hexagons as an electrode material for high-performance asymmetric supercapacitor applications

Hierarchical nanostructures have harvested noteworthy attention lately owing to their remarkable capabilities in the fields of energy storing and transformation, catalysis, and electrical devices. We established an effort less and template-free synthetic method to create hierarchical hetero nanostructures of Zn2V2O7, taking into account the benefits of hierarchical nanostructures, we investigated the performance of HNs (Hierarchical Nanostructures) as electrochemical supercapacitors. Electrochemical tests were tested in a 6 M KOH solution to assess their capabilities. The Zn2V2O7 electrode's measured specific capacitance was 750F/g at 1 A/g, with outstanding stability and an excellent retention capacity of 85 % later 5000 cycles in three- electrode electrochemical cells. Asymmetric device such as Zn2V2O7//AC provides a specific capacitance of 76.8F/g at 1 A/g with energy and power densities of 27.3 Wh kg-1 and 800 W kg-1 respectively. The device withstands 85 % of its initial capacity after 5000 continuous GCD cycles at 10 A/g. The outstanding performance observed clearly demonstrates the significant potential and practical utility of Zn2V2O7 in the realm of more efficient energy storage applications.

Copper hexacyanoferrate/carbon sheet combination with high selectivity and capacity for copper removal by pseudocapacitance

The efficient capture of copper ions (Cu2+) in wastewater has dual significance in pollution control and resource recovery. Prussian blue analog (PBA)-based pseudocapacitive materials with open frameworks and abundant metal sites have attracted considerable attention as capacitive deionization (CDI) electrodes for copper removal. In this study, the efficiency of copper hexacyanoferrate (CuHCF) as CDI electrode for Cu2+ treating was evaluated for the first time upon the successful synthesis of copper hexacyanoferrate/carbon sheet combination (CuHCF/C) by introducing carbon sheet as conductive substrate. CuHCF/C exhibited significant pseudocapacitance and high specific capacitance (52.92 F g-1) through the intercalation, deintercalation, and coupling of Cu+/Cu2+ and Fe2+/Fe3+ redox pairs. At 0.8 an applied voltage and CuSO4 feed liquid concentration of 100 mg L-1, the salt adsorption capacity was 134.47 mg g-1 higher than those of most reported electrodes. Moreover, CuHCF/C demonstrated excellent Cu2+ selectivity in multi-ion coexisting solutions and in actual wastewater experiments. Density functional theory (DFT) calculations were employed to elucidate the mechanism. This study not only reveals the essence of Cu2+ deionization by PBAs pseudocapacitance with promising potential applications but also provides a new strategy for selecting efficient CDI electrodes for Cu2+ removal.

Aggregation behavior of polystyrene nanoplastics: Role of surface functional groups and protein and electrolyte variation

Aggregation kinetics of plastics are affected by the surface functional groups and exposure orders (electrolyte and protein) with kinds of mechanisms in aquatic environment. This study investigates the aggregation of polystyrene nanoplastics (PSNPs) with varying surface functional groups in the presence of common electrolytes (NaCl, CaCl2, Na2SO4) and bovine serum albumin (BSA). It also examines the impact of different exposure orders, namely BSA + NaCl (adding them together), BSA → NaCl (adding BSA firstly and then NaCl), and NaCl → BSA (adding NaCl firstly and then BSA), on PSNPs aggregation. The presence of BSA decreased the critical coagulation concentration in NaCl (CCCNa+) of the non-modified PS-Bare from 222.17 to 142.81 mM (35.72%), but increased that of the carboxyl-modified PS-COOH from 157.34 to 160.03 mM (1.71%). This might be ascribed to the thicker absorbed layer of BSA onto the PS-Bare surface, known from Ohshima's soft particle theory. Their aggregation in CaCl2 was both increased because of Ca2+ bridging. Different from the monotonous effects of BSA on PS-Bare and PS-COOH, BSA initially facilitated PS-NH2 aggregation via patch-charge attraction, then inhibited it at higher salt levels through steric repulsion. Furthermore, exposure orders had no significant effect on PS-Bare and PS-COOH, but had a NaCl concentration-dependent impact on PS-NH2. At the low NaCl concentrations (10 and 100 mM), no obvious influence could be observed. While, at 300 mM NaCl, the high concentrations of BSA could not totally stabilize the salt-induced aggregates in NaCl → BSA, but could achieve it in the other two orders. These might be attributed to the electrical double layer compression by NaCl, "patch-charge" force and steric hindrance by BSA. These experimental findings shed light on the potential fate and transport of nanoparticles in aquatic environments.

Perspective into ion storage of pristine metal-organic frameworks in capacitive deionization

Metal-organic frameworks (MOFs), featuring tunable conductivity, tailored pore/structure and high surface area, have emerged as promising electrode nanomaterials for ion storage in capacitive deionization (CDI) and garnered tremendous attention in recent years. Despite the many advantages, the perspective from which MOFs should be designed and prepared for use as CDI electrode materials still faces various challenges that hinder their practical application. This summary proposes design principles for the pore size, pore environment, structure and dimensions of MOFs to precisely tailor the surface area, selectivity, conductivity, and Faradaic activity of electrode materials based on the ion storage mechanism in the CDI process. The account provides a new perspective to deepen the understanding of the fundamental issues of MOFs electrode materials to further meet the practical applications of CDI.

Induced-charge membrane capacitive deionization enables high-efficient desalination with polarized porous electrodes

Exposure of a conducting porous material to an electric field in electrolytes induces an electric dipole, which results in capacitive charging of cations and anions at opposite poles. In this letter, we investigate a novel desalination method using this induced-charge capacitive deionization (ICCDI). To do this, we devise a microscale ICCDI platform that can visualize in situ ion concentrations, pH shifts, and fluid flows, and study ion transport dynamics and desalination performances compared to conventional CDI with unipolar / bipolar connections. Similar ion concentration and fluid flow characteristics were observed in Ohmic, limiting, and over-limiting regimes, but variations in desalination performance trends were noted based on the number of stacks. In a single cell, ICCDI generates a higher electric field at the opposite poles of porous electrodes than simple conducted electrodes in CDIs with unipolar/bipolar connections, leading to superior salt removal and/or lower ionic current at a given applied voltage. This marks a clear contrast from CDI with bipolar connection, which lacks any advantage over CDI with unipolar connection in a single cell. These metrics of ICCDI however deteriorated as the stack number increased, likely due to short-circuiting between the dipoles. As a result, ICCDI in current form shows higher desalination efficient than conventional CDIs with low stack numbers (< 6), so we offer the scale-up module by repeating 4-stack ICCDI units. Our study enhances comprehension of ion transport dynamics and desalination performance in ICCDI, and the results could aid in the development of ICCDI for energy/cost-efficient desalination.

Strong interface coupling boosting hierarchical bismuth embedded carbon hybrid for high-performance capacitive deionization

Capacitive deionization (CDI) is regarded as a promising desalination technology owing to its low cost and environmental friendliness. However, the lack of high-performance electrode materials remains a challenge in CDI. Herein, the hierarchical bismuth-embedded carbon (Bi@C) hybrid with strong interface coupling was prepared through facile solvothermal and annealing strategy. The hierarchical structure with strong interface coupling between the bismuth and carbon matrix afforded abundant active sites for chloridion (Cl^-) capture, improved electrons/ions transfer and the stability of the Bi@C hybrid. As a result of these advantages, the Bi@C hybrid showed a high salt adsorption capacity (75.3 mg/g under 1.2 V), salt adsorption rate and good stability, making it a promising electrode material for CDI. Furthermore, the desalination mechanism of the Bi@C hybrid was elucidated through various characterizations. Therefore, this work provides valuable insights for the design of high-performance bismuth-based electrode materials for CDI.