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Na3V2(PO4)3@C as Faradaic Electrodes in Capacitive Deionization for High Performance Desalination
Abstract
Among various desalination technologies, capacitive deionization (CDI) has rapidly developed due to its low energy consumption and environmental compatibility, among other factors. Traditional CDI stores ions within the electric double layers (EDLs) in the nanopores of the carbon electrode, but carbon anode oxidation, the co-ion expulsion effect, and a low salt adsorption capacity (SAC) block its further application. Herein, the faradaic-based electrode is proposed to overcome the above limitations, offering an ultrahigh adsorption capacity and a rapid removal rate. In this paper, the open framework structure Na3V2(PO4)3@C is applied for the first time as a novel faradaic electrode in the hybrid capacitive deionization (HCDI) system. During the adsorption and desorption process, sodium ions are intercalated/de-intercalated through the crystal structure of Na3V2(PO4)3@C while chloride ions are physically trapped or released by the AC electrode. Different concentrations of feed water were investigated, and a high SAC of 137. 20 mg NaCl g-1 NVP@C and low energy consumption of 2.157 kg NaCl kWh-1 were observed at a constant voltage of 1.0 V, a concentration of 100 mM and flow rate of 15 mL·min-1. The outstanding performance of the Na3V2(PO4)3@C faradaic electrode demonstrates that it is a promising material for desalination and that HCDI offers great future potential.
... Carbonaceous materials, via the electrical double layer (EDL) mechanism [4][5][6], had hit a roadblock due to their low adsorption capacity (usually < 25 mg g −1 ) and potential for side reactions [7]. The non-carbon electrode materials mainly included transition metal oxides (TMOs) [8][9][10][11][12][13], Prussian blue analogs [14][15][16], polyanionic phosphates [17,18], MXene [19][20][21], and layered double hydroxide (LDH) [22,23], and can achieve high capacity desalination by redox reaction [24]. Among these materials, TMOs comprising the advantages of easy preparation, element diversity, facile morphology control, excellent reversible intercalation pseudocapacity and promising sodium storage theoretical capacitance, have shown great potential for application in CDI [11,12,[25][26][27][28][29][30][31][32][33]. ...
... Zn 0.2 Ni 0.8 O@CF electrode showed lower internal resistance (R int ) value (5.10 Ω) compared to NiO@CF (5.46 Ω), indicating a lower proportion of internal resistance to charge consumption and higher charge efficiency. The charge transfer resistance (R ct ) value of the Zn 0.2 Ni 0.8 O@CF electrode (0.44 Ω) was also lower than NiO@CF (2.33 Ω), which meant Zn 0.2 Ni 0.8 O@CF had a better conductivity and superior electrochemical kinetics [17]. ...
... SEC was a crucial metric in engineering practice due to the global energy crisis and the economic benefits of desalination. Compared with recently reported CDI electrodes (including carbonaceous and faradaic materials) (Fig. 3g) [15,17,18,56,[70][71][72][73][74][75][76][77], it was clearly observed that Zn 0.2 Ni 0.8 O@ CF displayed the highest SAC and lowest SEC among the cutting-edge CDI electrodes. The excellent CDI performance of Zn 0.2 Ni 0.8 O@CF originated from kinetic promotion through a hierarchical nanosheet interconnection network structure with high active sites, enhanced intrinsic electron transfer and adsorbed activation of Zn-doping. ...
Despite the promising potential of transition metal oxides (TMOs) as capacitive deionization (CDI) electrodes, the actual capacity of TMOs electrodes for sodium storage is significantly lower than the theoretical capacity, posing a major obstacle. Herein, we prepared the kinetically favorable Zn x Ni 1 − x O electrode in situ growth on carbon felt (Zn x Ni 1 − x O@CF) through constraining the rate of OH ⁻ generation in the hydrothermal method. Zn x Ni 1 − x O@CF exhibited a high-density hierarchical nanosheet structure with three-dimensional open pores, benefitting the ion transport/electron transfer. And tuning the moderate amount of redox-inert Zn-doping can enhance surface electroactive sites, actual activity of redox-active Ni species, and lower adsorption energy, promoting the adsorption kinetic and thermodynamic of the Zn 0.2 Ni 0.8 O@CF. Benefitting from the kinetic-thermodynamic facilitation mechanism, Zn 0.2 Ni 0.8 O@CF achieved ultrahigh desalination capacity (128.9 mg NaCl g ⁻¹ ), ultra-low energy consumption (0.164 kW h kg NaCl ⁻¹ ), high salt removal rate (1.21 mg NaCl g ⁻¹ min ⁻¹ ), and good cyclability. The thermodynamic facilitation and Na ⁺ intercalation mechanism of Zn 0.2 Ni 0.8 O@CF are identified by the density functional theory calculations and electrochemical quartz crystal microbalance with dissipation monitoring, respectively. This research provides new insights into controlling electrochemically favorable morphology and demonstrates that Zn-doping, which is redox-inert, is essential for enhancing the electrochemical performance of CDI electrodes.
... Fig. 1(a) show the structure of NASICON material. The NASICON materials have been used as cathode to complement the capacity of carbon based electrode materials in water desalination [36,50,51]. This is because NASICON materials have several competitive advantages such as high adsorption capacity, mitigation of co-ion expulsion and high stability since they do not undergo faradaic reduction when in service. ...
... In NASICONs the whole bulk material is utilized for salt removal unlike in the carbon based materials in which regardless of the size of the material, salt removal depends on the available pores and therefore when the material is having only few pores, the salt removal is hindered and the large part of it becomes useless. For instance, Cao et al. [50] used Na 3 V 2 (PO 4 ) 3 @C a NASICON material to capture Na ion and activated carbon as anode to capture Cl ion which result to the removal of 137.20 mg/g at 1 V. Also, about 2.157 kg-NaCl kWh − 1 was consumed for desalination compared to 0.90 kg-NaCl kWh − 1 of activated carbon based electrode material. The low energy consumed by the Na 3 V 2 (PO 4 ) 3 @C may be attributed to the mitigation of co ion expulsion, since the potential supplied (1.0 V) is whole used for the removal of the counterions unlike in carbon based materials in which the applied potential is used for both the adsorption of counterions as well as repulsion of co-ions especially when the pore size >0.6 nm. ...
... In detail, compared to CoNiP@CF (3.704 Ω) and CoNiOH@CF (4.722 Ω), CoNiPS@CF showed a lower internal resistance (R int ) value (3.082 Ω), indicating a lower percentage of charge consumption and higher charging efficiency. The charge transfer resistance (R ct ) value of the CoNiPS@CF electrode (3.386 Ω) was also lower than that of CoNiP@CF (3.882 Ω) and CoN-iOH@CF (7.666 Ω), implying that CoNiPS@CF had better conductivity and superior electrochemical kinetics 48 . After the introduction of sulfur into CoNiP@CF, the ratio of P to P-O increases, resulting in a decrease in the surface passivation layer of CoNiP and an increase in the metallicity of the electrode, thereby increasing the conductivity of the electrode. ...
Developing stable, high-performance chloride-ion storage electrodes is essential for energy storage and water purification application. Herein, a P, S co-doped porous hollow nanotube array, with a free ion diffusion pathway and highly active adsorption sites, on carbon felt electrodes (CoNiPS@CF) is reported. Due to the porous hollow nanotube structure and synergistic effect of P, S co-doped, the CoNiPS@CF based capacitive deionization (CDI) system exhibits high desalination capacity (76.1 mgCl– g–1), fast desalination rate (6.33 mgCl– g–1 min–1) and good cycling stability (capacity retention rate of > 90%), which compares favorably to the state-of-the-art electrodes. The porous hollow nanotube structure enables fast ion diffusion kinetics due to the swift ion transport inside the electrode and the presence of a large number of reactive sites. The introduction of S element also reduces the passivation layer on the surface of CoNiP and lowers the adsorption energy for Cl– capture, thereby improving the electrode conductivity and surface electrochemical activity, and further accelerating the adsorption kinetics. Our results offer a powerful strategy to improve the reactivity and stability of transition metal phosphides for chloride capture, and to improve the efficiency of electrochemical dechlorination technologies.
... The higher peak currents after Mn substitution and ppy connection at the same scan rate indicate the improved charge transfer and increased electrochemical activity. 63,64 Regarding the linear fitting results of peak current versus square root of scan rate (Fig. 5g), the high index (R 2 ＞0.99) for the fitting reveals that all the five samples undergo the same ionic diffusion mechanism, all of which is controlled by surface diffusion. 65 Specifically, the exact ionic diffusion coefficient can be obtained based on the following equation: where, Ip (A), n, S (cm 2 ), v (mV s -1 ), and C0 (mol L -1 ) represent the peak current, the number of electrons involved in the redox reaction, the load area of active material, the scan rate, and the concentration of Na + . ...
Prussian Blue analogues (PBAs), representing the typical Faradic electrode materials for efficient capacitive deionization (CDI) due to their open architecture and high capacity, have been plagued by kinetics issues, leading...
... Bi@C-2 h electrode: (a1) before reaction; (b1) After the first storage of chlorine; (c1) After the first chlorination; (d1) After the fifth chlorination; (e1) After the 50th chlorination; Bare bismuth electrode: (a2) before reaction; (b2) After the first storage of chlorine; (c2) after the first discharge of chlorine; (d2) After the fifth chlorination; (e2) After the 50th chlorination; Figure S4: Impedance changes during the reaction of (a) bare bismuth electrode, (b) bismuth-carbon black electrode; (c) Bi@C electrode; Table S1: Comparison of desalination capacity of Bi@C based DB with various carbon, pseudocapacitor and Faraday electrode materials reported in the literature. References [29][30][31][32][33][34][35][36][37] are cited in the supplementary materials. ...
Cost-effective bismuth (Bi) boasts a high theoretical capacity and exceptional selectivity towards Cl- ion storage, making it a promising material for desalination batteries (DBs). However, the substantial volume expansion and low conductivity severely hinder the cycling performance of Bi-based DBs. In this study, a carbon-layer-coated Bi nanocomposite (Bi@C) was synthesized by pyrolyzing a metal–organic framework (Bi-MOF) containing Bi using a straightforward method. The results show that the Bi@C synthesized under the condition of annealing at 700 °C for 2 h has the optimum properties. The Bi@C has good multiplication performance, and the desalination capacity is 106.1 mg/g at a high current density of 1000 mA/g. And the material exhibited a high desalination capacity of 141.9 mg/g at a current density of 500 mA/g and retained 66.9% of its capacity after 200 cycles. In addition, the Bi@C can operate at a wide range of NaCl concentrations from 0.05 to 2 mol/L. The desalination mechanism analysis of the Bi@C revealed that the carbon coating provides space for Bi particles to expand in volume, thereby mitigating the issues of electrode material powdering and shedding. Meanwhile, the porous carbon skeleton establishes electron and ion channels to enhance the electrode material’s conductivity. This research offers a promising strategy for the application of chloride-storage electrode materials in electrochemical desalination systems.
... 18,20,21 Nevertheless, a number of issues primarily related to the stability of electrode/electrolyte materials in aqueous environments still need to be solved before their full potential could be realized. 18,[20][21][22][23] In addition to the electrolytic decomposition of water, there is also transition metal dissolution (leaching) which is typically more pronounced in aqueous systems. [24][25][26] Vanadium dissolution into the electrolyte is widely assumed to be the main degradation process of most V-based electrode materials. ...
Na 3 V 2 (PO 4 ) 3 and Na 3 V 2 (PO 4 ) 2 F 3 with their relatively high capacities and redox potentials are among the most studied and applied positive electrode materials in non-aqueous sodium-ion batteries. However, their stability in aqueous environments is relatively low limiting the application of these materials in aqueous batteries or deionization cells. In this study, we provide a comprehensive analysis of Na 3 V 2 (PO 4 ) 3 and Na 3 V 2 (PO 4 ) 2 F 3 degradation in aqueous media using a number of techniques such as standard electrochemical methods, elemental analysis, powder X-ray diffractometry, and rotating ring-disc electrode method. The latter allows for real time in situ/operando degradation analysis. The results show that Na 3 V 2 (PO 4 ) 3 suffers from chemical vanadium dissolution in neutral pH, whereas it is negligible in Na 3 V 2 (PO 4 ) 2 F 3 . The results obtained in unbuffered electrolytes by the ring-disc electrode technique explicitly show that Na 3 V 2 (PO 4 ) 3 and Na 3 V 2 (PO 4 ) 2 F 3 generate most of the soluble V (V) species during the charging process. Whereas at lower pH, there is an additional electrochemically-induced generation of soluble V (IV) species during discharging as well. The overall results suggest that fluoride ions significantly increase the structural stability of phosphate materials in aqueous environments and a careful electrolyte design with controlled proton and water activity could enable the use of Na 3 V 2 (PO 4 ) 2 F 3 in water-based electrochemical devices.
... Three-dimensional faradaic materials for CDI are mainly represented by NASICON structural type. Such electrodes are characterized by fantastic SAC values up to 130 mg/g [110,111], which are unattainable for carbon materials. The main problems of NASICONs are the high selectivity (intercalation of ions other than sodium is practically impossible) and the significant volume expansion during the operation. ...
The availability of clean water at affordable prices is one of the key technological, social, and economic challenges of the 21st century. The increased extraction of groundwater worldwide is leading to the gradual intrusion of salty water into sources and water horizons. In order to use this water for industrial and agricultural purposes, or as drinking water, it needs to be purified and desalinated. Thus, methods of desalinating water of different salinity levels, ranging from brackish to seawater, are becoming more prominent. The overall goal of current research is to make water desalination technologies more energyefficient and cost-effective. One promising technology that meets these requirements is capacitive deionization (CDI) of water. This technology has been widely known for over 30 years, but significant progress in CDI research has only been achieved in recent years. In this review, we examine the currently developed architectures of CDI cells, advancements in carbon materials, and discuss the prospects and challenges of commercializing this technology.
... The first type consists of two-dimensional materials such as MXene and molybdenum disulfide (MoS 2 ), where ions are inserted into the interlayer space, mainly driven by electrostatic interactions and only partially by redox reactions. The second type involves materials such as sodium-superionicconductor (NASICON) type electrodes, [67][68][69][70][71] Prussian blue (PB), [72][73][74][75] and metal phosphates. In these materials, ions are inserted into specific interstitial sites of solid bulk material completely based on redox reactions. ...
Electrochemically responsive materials (ERMs) that respond to external electrical stimuli offer advanced control over physio-chemical processes with a high degree of tunability and flexibility. Recently, the use of ERMs in environmental remediation processes has increased to address the grand sustainability challenges associated with water scarcity and climate change. Here, we provide a timely review on the applications of ERMs to electrochemically mediated water treatment (EMWT) and electrochemically mediated carbon capture (EMCC). We first examine the working principles of ERMs-based systems for water treatment and carbon capture, followed by a detailed summary of key figures of merit that quantify the overall performance. Second, we present an in-depth discussion of the multiscale design principles of EMWT and EMCC systems, ranging from materials-level engineering to electrode-level considerations to device configuration optimization. Next, we discuss the development and application of in situ and operando characterization methods, with a particular emphasis on imaging tools, which uncover ubiquitous static and dynamic heterogeneities in ERMs and critically inform rational materials design. Finally, we point out future opportunities and challenges in the emerging field of electrochemically mediated environmental remediation, including developing new tools to monitor complex multiphase transport and reactions, repurposing existing energy nanomaterials for environmental technologies, and scaling and combining EMWT and EMCC systems.
... To enhance the electrochemical energy storage performance, intercalation-type pseudocapacitive electrode materials are incorporated to increase the surface area and provide additional pseudocapacitance, such as manganese dioxide (MnO 2 ) [19][20][21], Prussian blue and its analogs [22][23][24], Na 3 V 2 (PO 4 ) 3 [25], Co 3 O 4 [26], Fe 3 O 4 [27], and NaTi 2 (PO 4 ) 3 (NTP) [28]. One of these materials, MnO 2 , is particularly interesting for research because it is widely available, inexpensive, and has good cycle stability [29]. ...
MXene has drawn widespread attention as a potential material for electrode use in capacitive deionization (CDI). However, the applications of MXene are limited by its property of low electrical capacity. Herein, a MnO2/MXene composite was firstly evaluated in a capacitive deionization system, in which the MnO2 acts as intercalation-type pseudocapacitive electrodes to enhance the electrical capacity, and MXene provides an electron conduction highway network that improves the charge transfer of the MnO2. The result showed that the low-crystallinity MnO2 with irregular particles was well-distributed on the surface of the MXene. The desalination capacity of 30.5 mg·g−1 is achieved at a voltage window of 1.2 V, which was higher than that of the reported pure MXene and MnO2. The electrical double-layer (EDL) capacitive and the diffusion-controlled processes are the main charge storage mechanisms, and the EDL contribution provides 50.3% to the total capacitance. This result suggests a promising direction for further applying a MnO2/MXene composite in CDI.
... It is reasonable that the responded current and the curve area increases with the ion concentration. The curves in ZnSO 4 with different concentrations still maintained the pseudocapacitive feature, manifesting the superior electrochemical performance towards Zn 2+ removal [36]. ...
Selective separation of zinc(II) ions is vital not only for the removal of toxic heavy metal pollution in drinking water, but also the recovery of resource in sustainable supply. In this work, nitrogen-doped porous carbon (NPC) derived from silk cocoon was developed for symmetric capacitive deionization towards Zn²⁺ separation. The high specific surface area, hierarchical porous structure with rich micro-meso-pores and the considerable N-doping content, endow the NPC with the outstanding pseudocapacitive capacity towards Zn²⁺ ions capture as 33.1 mg g⁻¹ at optimized voltage of 1.0 V. Quantitative analysis of electrochemical kinetics demonstrated that the Zn-ion removal was dominated by surface-dominated charge storage process. Moreover, DFT calculations illustrated the benefits of nitrogen doping and the ascendency of the pyrrolic and pyridinic nitrogen over the quaternary nitrogen dopant for selectively adsorption of Zn²⁺ ion. The highest selectivity coefficient (& = 90.5) of Zn²⁺ removal was obtained in Zn-Na ions mixed solution with a molar ratio of 1:100. Moreover, the related electrochemical measurements revealed the supportive explanation for intrinsic pseudocapacitive selective behavior of Zn²⁺ ion. Our work would present new insights in effects of N-doping and selective separation towards Zn²⁺ ion for carbon materials.
... Now more and more electroactive materials with the faradaic desalination capability, including Na-ion and Cl-ion storage materials, have been reported. The Na-ion storage materials investigated for the desalination batteries/ pseudocapacitors including transition metal oxides (TMOs) [20,[23][24][25], NASICON [26][27][28], iron phosphate [29][30][31], Prussian blue analogs (PBAs) [32][33][34][35][36], Mexene [37][38][39], and so on. The Cl-ion storage materials include Ag-based materials [19,24,40,41], Bi-based materials [42][43][44], and conducting polymers [45][46][47]. ...
A membrane-free hybrid system employing the battery-type copper hexacyano-ferrate (CuHCF) and the pseudocapacitive polypyrrole (PPy) as the positive and negative electrodes are designed for faradaic desalination. Both materials exhibit a memory effect, a unique characteristic of the faradaic reaction, with the dual function of ion removing and concentrating. The appropriate mass ratio of two materials is adjusted according to the positive/negative charge balance to ensure that both electrodes work in their respectively suitable potential windows. Through the little inverted voltage (−0.2 V) during the discharge process in order to keep the two electrodes in the desirable potential windows in comparison with the case discharged at 0 V, the salt removal capacity (SRC) of this hybrid system is significantly increased from 21.17 mg g⁻¹ to 26.62 mg g⁻¹ in 8 mM NaCl. Under this operation condition, the cell keeps >90 % of the original SRC in a 100-cycle stability test. The SRC obtained in 8 mM NaCl solution can increase to 38.64 mg g⁻¹ after employing the acid-treated CNT/CuHCF composite. The system also exhibits superior performance on removing other cations including K⁺, Mg²⁺, and Ca²⁺. All results reveal the promising application potential of this system to the faradaic desalination and metal-ion concentrating and recovering.
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.
Biochar materials have shown great potential for broad catalytic application. However, using these materials in the capacitive deionization technology (CDI) system for heavy metal removal still faces a significant challenge due to their low specific capacity and removal capability. Here, a comprehensive regulation on the interfacial/bulk electrochemistry of biochar by Zn doping is reported, which suggests a high renewable capacity (20 mg g ⁻¹ ) and outstanding selective capacitive removal ability (SCR) of Pb ²⁺ from leachate. The SCR efficiency of Pb ²⁺ is as high as 99% compared to K ⁺ (8%), Na ⁺ (13%), and Cd ²⁺ (37%). This work proves that the doped Zn on the biochar can combine with OH ⁻ generated by water splitting to form M─OH bonds, which is beneficial for improving the specific capacity. Significantly, the relationship between double‐layer capacitance and pseudo‐capacitance can also be optimized by regulating the content of Zn, leading to different removal abilities of heavy metals. Therefore, this work offers insights into charge‐storage kinetics, which provide valuable guidelines for designing and optimizing the biochar electrode for broader environmental applications.
As the cardinal part of capacitive deionization apparatus, the performance of electrode directly determines the capacitive deionization's adsorption rate, adsorption capacity and selectivity. However, the adsorptivity and reusability of electrode material for special adsorbate need to be developed urgently for the application of capacitive deionization in wastewater purification. In this research, we successfully enhanced the Cu(II) adsorptivity of activated carbon fiber felt (ACFF) to 23 mg/g (4.47 times of ACFF0) by the sonication‐assisted chemical reactivation process with the solution of 20 wt % NaOH used as activator (ACFF‐N20). As ACFF‐N20 used as cathode, the Cu(II) electro‐adsorptivity of ACFF‐N20 under a current of 0.15 A is 84 mg/g for 130 mg/L Cu(II) standard working fluid. For the Cu(II) wastewater with a concentration of 50 mg/L, it can be purified to 0.14 mg/L by one step of capacitive deionization, which below the drinking standard of World Health Organization. By the technology of low‐temperature pickling regeneration, the regeneration rates of ACFF‐N20 are 102 %–106 % for five recycling times of Cu(II) electrosorption process. Based on fitting analysis of adsorption processes using different kinetic models, it was found that Langmuir model is more convergent than Freundlich model to the adsorption isotherms. Additionally, Bangham adsorption kinetic model and pseudo‐first order adsorption kinetic model have the best fitting correlation for the Cu(II) static adsorption and electrosorption of ACFFs.
The two‐phase reaction of Na3V2(PO4)3 – Na1V2(PO4)3 in Na3V2(PO4)3 (NVP) is hindered by low electronic and ionic conductivity. To address this problem, a surface‐N‐doped NVP encapsulating by N‐doped carbon nanocage (N‐NVP/N‐CN) is rationally constructed, wherein the nitrogen is doped in both the surface crystal structure of NVP and carbon layer. The surface crystal modification decreases the energy barrier of Na⁺ diffusion from bulk to electrolyte, enhances intrinsic electronic conductivity, and releases lattice stress. Meanwhile, the porous architecture provides more active sites for redox reactions and shortens the diffusion path of ion. Furthermore, the new interphase of Na2V2(PO4)3 is detected by in situ XRD and clarified by density functional theory (DFT) calculation with a lower energy barrier during the fast reversible electrochemical three‐phase reaction of Na3V2(PO4)3 – Na2V2(PO4)3 – Na1V2(PO4)3. Therefore, as cathode of sodium‐ion battery, the N‐NVP/N‐CN exhibited specific capacities of 119.7 and 75.3 mAh g⁻¹ at 1 C and even 200 C. Amazingly, high capacities of 89.0, 86.2, and 84.6 mAh g⁻¹ are achieved after overlong 10000 cycles at 20, 40, and 50 C, respectively. This approach provides a new idea for surface crystal modification to cast intermediate Na2V2(PO4)3 phase for achieving excellent cycling stability and rate capability.
Capacitive deionization is an emerging water desalination technology for industrial applications. Recent advancements in electrode design and system development have led to the reporting of ultra-high salt adsorption performance, benefiting its potential application in agricultural water treatment at a potentially low cost. In this study, we provide a comprehensive summary of the porous electrode design strategy to achieve ultra-high ion adsorption performance, considering factors such as experimental parameters, chemically tuned material properties, redox chemistry and smart nanoarchitecture for future electrode design. Furthermore, we endeavor to establish a correlation between capacitive deionization (CDI) technology and its applicability in the agricultural sector, specifically concentrating on water treatment with an emphasis on undesirable ions associated with salinity, hardness, and heavy metals, to achieve harmless irrigation. Additionally, to ensure the efficient and cost-effective application of CDI systems in agriculture, a thorough overview of the literature on CDI cost analysis is presented. By addressing these aspects, we anticipate that ultra-high salt adsorption CDI systems hold great promise in future agricultural applications.
Recently, all‐solid‐state sodium batteries (Na‐ASSBs) have received increased interest owing to their high safety and potential of high energy density. The potential of Na‐ASSBs based on sodium superionic conductor (NASICON)‐structured Na3V2(PO4)3(Na3VP) cathodes have been proven by their high capacity and a long cycling stability closely related to the microstructural evolution. However, the detailed kinetics of the electrochemical processes in the cathodes is still unclear. In this work, the sodiation/desodiation process of Na3VP is first investigated using in situ high‐resolution transmission electron microscopy (HRTEM). The intermediate Na2V2(PO4)3 (Na2VP) phase with the P21/c space group, which would be inhibited by constant electron beam irradiation, is observed at the atomic scale. With the calculated volume change and the electrode–electrolyte interface after cycling, it can be concluded that the Na2VP phase reduces the lattice mismatch between Na3VP and NaV2(PO4)3 (NaVP), preventing structural collapse. Based on the density functional theory calculation (DFT), the Na⁺ ion migrates more rapidly in the Na2VP structure, which facilitates the desodiation and sodiation processes. The formation of Na2VP phase lowers the formation energy of NaVP. This study demonstrates the dynamic evolution of the Na3VP structure, paving the way for an in‐depth understanding of electrode materials for energy‐storage applications.
Capacitive deionization (CDI) has been emerged as potential energy-efficient desalination technique to provide fresh water from brine, and the rational design of deionization electrode remains a challenge. Cation insertion materials...
Faradaic deionization (FDI) is a promising technology for energy-efficient water desalination using porous electrodes containing redox-active materials. Herein, we demonstrate for the first time the capability of a symmetric FDI flow cell to produce freshwater (<17.1 mM NaCl) from concentrated brackish water (118mM), to produce effluent near freshwater salinity (19.1 mM) from influent with seawater-level salinity (496 mM), and to reduce the salinity of hypersaline brine from 781 mM to 227 mM. These remarkable salt-removal levels were enabled by using flow-through electrodes with high areal-loading of nickel hexacyanoferrate (NiHCF) Prussian Blue analogue intercalation material. The pumping energy consumption due to flow-through electrodes was mitigated by embedding an interdigitated array of <100 $\mu$m wide channels in the electrodes using laser micromachining. The micron-scale dimensions of the resulting embedded, micro-interdigitated flow fields (e$\mu$-IDFFs) facilitate flow-through electrodes with high apparent permeability while minimizing active-material loss. Our modeling shows that these e$\mu$-IDFFs are more suitable for our intercalation electrodes because they have >100X lower permeability compared to common redox-flow battery electrodes, for which millimetric flow-channel widths were used exclusively in the past. Total desalination thermodynamic energy efficiency (TEE) was improved by more than ten-fold relative to unpatterned electrodes: 40.0% TEE for brackish water, 11.7% TEE for hypersaline brine, and 7.4% TEE for seawater-salinity feeds. Water transport between diluate and brine streams and charge efficiency losses resulting from (electro)chemical effects are implicated as limiting energy efficiency and water recovery, motivating their investigation for enhancing future FDI performance.
Due to substantial consumption and widespread contamination of the available freshwater resources, green, economical, and sustainable water recycling technologies are urgently needed. Recently, Faradic capacitive deionization (CDI), an emerging desalination technology, has shown great desalination potential due to its high salt removal ability, low consumption, and hardly any co‐ion exclusion effect. However, the ion removal mechanisms and structure–property relationships of Faradic CDI are still unclear. Therefore, it is necessary to summarize the current research progress and challenges of Faradic CDI. In this review, the recent progress of Faradic CDI from five aspects is systematically reviewed: cell architectures, desalination mechanisms, evaluation indicators, operation modes, and electrode materials. The working mechanisms of Faradic CDI are classified as insertion reaction, conversion reaction, ion‐redox species interaction, and ion‐redox couple interaction in the electrolytes. The intrinsic and desalination properties of a series of Na⁺ and Cl⁻ capturing materials are described in detail in terms of design concepts, structural analysis, and synthesis modulation. In addition, the effects of different cell architectures, operation modes, and electrode materials on the desalination performance of Faradic CDI are also investigated. Finally, the work summarizes the challenges remaining in Faradic CDI and provides the prospects and directions for future development.
The poor cycling stability of faradaic materials owing to volume expansion and stress concentration during faradaic processes limits their use in large-scale electrochemical deionization (ECDI) applications. Herein, we developed a "soft-hard" interface by introducing conducting polymer hydrogels (CPHs), that is, polyvinyl alcohol/polypyrrole (PVA/PPy), to support the uniform distribution of Prussian blue analogues (e.g., copper hexacyanoferrate (CuHCF)). In this design, the soft buffer layer of the hydrogel effectively alleviates the stress concentration of CuHCF during the ion-intercalation process, and the conductive skeleton of the hydrogel provides charge-transfer pathways for the electrochemical process. Notably, the engineered CuHCF@PVA/PPy demonstrates an excellent salt-adsorption capacity of 22.7 mg g-1 at 10 mA g-1, fast salt-removal rate of 1.68 mg g-1 min-1 at 100 mA g-1, and low energy consumption of 0.49 kW h kg-1. More importantly, the material could maintain cycling stability with 90% capacity retention after 100 cycles, which is in good agreement with in situ X-ray diffraction tests and finite element simulations. This study provides a simple strategy to construct three-dimensional conductive polymer hydrogel structures to improve the desalination capacity and cycling stability of faradaic materials with universality and scalability, which promotes the development of high-performance electrodes for ECDI.
Pyrolysis-free metal–organic frameworks (MOFs) with optimized particle sizes were used as capacitive deionization (CDI) materials in oxygenated saline water. Upon decreasing the particle size of the MOFs, excellent cycling stability...
Capacitive deionization (CDI) is an emerging water desalination technology for removing different ionic species from water, which is based on electric charge compensation by these charged species. CDI is becoming popular because it is more energy‐efficient and cost‐effective than other technologies, such as reverse osmosis and distillation, specifically in dealing with brackish water having low or moderate salt concentrations. Over the past decade, the CDI research field has witnessed significant advances in the used electrode materials, cell architectures, and associated mechanisms for desalination applications. This review article first discusses ion storage/removal mechanisms in carbon and Faradaic materials aided by advanced in situ analysis techniques and computations. It then summarizes research progress toward electrode materials in terms of structure, surface chemistry, and composition. More still, it discusses CDI cell architectures by highlighting their different cell design concepts. Finally, current challenges and future research directions are summarized to provide guidelines for future CDI research.
Capacitive deionization (CDI) is a newly developed desalination technology with low energy consumption and environmental friendliness. The surface area restricts the desalination capacities of traditional carbon-based CDI electrodes while battery materials emerge as CDI electrodes with high performances due to the larger electrochemical capacities, but suffer limited production of materials. LiMn2O4 is a massively-produced lithium-ion battery material with a stable spinel structure and a high theoretical specific capacity of 148 mAh·g−1, revealing a promising candidate for CDI electrode. Herein, we employed spinel LiMn2O4 as the cathode and activated carbon as the anode in the CDI cell with an anion exchange membrane to limit the movement of cations, thus, the lithium ions released from LiMn2O4 would attract the chloride ions and trigger the desalination process of the other side of the membrane. An ultrahigh deionization capacity of 159.49 mg·g−1 was obtained at 1.0 V with an initial salinity of 20 mM. The desalination capacity of the CDI cell at 1.0 V with 10 mM initial NaCl concentration was 91.04 mg·g−1, higher than that of the system with only carbon electrodes with and without the ion exchange membrane (39.88 mg·g−1 and 7.84 mg·g−1, respectively). In addition, the desalination results and mechanisms were further verified with the simulation of COMSOL Multiphysics.
Capacitive Deionization (CDI) is an emerging technology with great potential applications. Most researchers view it as a viable water treatment alternative to reverse osmosis. This research reports the preparation and application of a carbon aerogel polypyrrole (CA-PPy) composite for the desalination of NaCl solution by the hybrid CDI method. The carbon aerogel (CA) was prepared from a Resorcinol / Formaldehyde precursor by the sol-gel method. The aerogel obtained from the sol-gel was then pyrolysed in a tube furnace to form CA. Polypyrrole (PPy) was prepared by the Oxidative chemical polymerisation of pyrrole, ferric chloride hexahydrate (oxidant), and sodium dodecyl sulfate (dopant). A composite of CA and PPy was then prepared and used to modify carbon electrodes. The CA-PPy composite was characterised to verify its composition, morphology, thermal properties, and functional groups. The electrochemical properties of the material were determined by Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) tests. The electrochemical tests were done using a GAMRY potentiostat electrochemical workstation, a 1.0 M KCl was used as the electrolyte, and the applied potential window was (-0.2 to +0.6) V for the CV test. The EIS test was done with the same concentration of KCl electrolyte at an applied potential of 0.22V and at a frequency range of (0.1 – 100, 000) Hz. The optimal specific capacitance of the CA is 115 F/g, and that of the composite is 360.1 F/g, they were both obtained at a scan rate of 5 mV/s. The CDI desalination study of the CA-PPy composite showed a salt adsorption capacity (SAC) of 10.10 mg/g (300mg/L NaCl solution) – 15.7 mg/g (800 mg/L NaCl solution) at 1.2V applied voltage. The salt recovery efficiency of the electrode material in the 300mg/L solution is 27%, in the 500 mg/L solution, it is 20.12%, and in the 800 mg/L solution, it is 15.41%. The electrode material also showed good electrochemical stability after nine cycles of ion adsorption/desorption study.
Although widely used as hybrid capacitive deionization (HCDI) electrode material, the low intrinsic conductivity of metal hexacyanometalate (MHCF) severely hinders the fast insertion/extraction of Na⁺ in/from its 3D framework structure, damaging its desalination performance. Herein, we design a carbon nanotube (CNT) bridged nickel hexacyanoferrate architecture (NiHCF). The highly conductive CNT not only acts as the skeleton for the uniform growth of NiHCF to provide more ion-accessible surface and active sites but also serves as the conductive bridge to connect the NiHCF particles, which prevents the agglomeration of NiHCF particles and facilitates the charge transfer and ion diffusion during the desalination process. Therefore, the HCDI cell assembled by NiHCF/CNT cathode and AC anode exhibits an excellent desalination performance with a high desalination capacity of 29.1 mg g⁻¹ and a superior desalination rate of 7.2 mg g⁻¹ min⁻¹ in 500 mg L⁻¹ NaCl solution. This work provides a facile method for preparing high-performance MHCF-based electrodes for desalination application.
Here we report the electrochemical performances of a Na3V2(PO4)3/C nanocomposite as a cathode material for aqueous sodium-ion batteries (SIBs). Compared to a previously reported Na3V2(PO4)3 microparticle, this nanocomposite demonstrated much improved cycling stability. While the improvement mainly attributed to the right pH and the carbon matrixmediated protection against the electrolyte, the capacity fade was mainly due to the deterioration of crystallinity and structure of the nanocomposite caused by various interactions between the nanocomposite and electrolyte. This work not only help to understand the degradation of Na3V2(PO4)3 in aqueous SIBs, but also shed light on the design and fabrication of electrode materials with high cycling stability for aqueous SIBs.
A novel method to determine reverse draw and forward feed solute fluxes in forward osmosis (FO) membrane was developed to analyze FO performance more accurately. Specifically, apparent draw solute permeability (B-d) and feed solute permeability (B-f) were proposed, instead of relying on single solute permeability (B). Our results clearly demonstrated that both draw and feed fluxes were not well predicted with the solute permeability (B) measured by RO mode experiment, typically employed in FO membrane characterization. In this study, the draw and feed solute permeabilities were evaluated independently by the experimental protocols which simulated actual FO operation more closely. Much better agreement between experimental observations and theoretical predictions was obtained when both Bd and Bf were applied for the analysis of draw and feed solute fluxes, respectively. Thus, the utilization of apparent draw and feed solute permeabilities provides more precise assessment of draw solute loss and permeate water quality, which are very important for FO membrane process design and operation.
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Matthew Suss obtained his PhD in Mechanical Engineering in 2013 from Stanford University and was a Postdoctoral Associate in the Department of Chemical Engineering at MIT from 2013 to 2014. Matthew was also a Lawrence Scholar at Lawrence Livermore National Laboratory in Livermore, California, from 2010 to 2013. Since 2014, Matthew has been an Assistant Professor at Technion – Israel Institute of Technology where he also holds an Alon Fellowship. At Technion, Matthew leads the Energy & Environmental Innovations Laboratory, which focuses on development of next-generation electrochemical systems for energy storage and water desalination applications.
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Volker Presser obtained his PhD in Applied Mineralogy in 2006 from the Eberhard Karls University in Tübingen, Germany. As a Humboldt Research Fellow and Research Assistant Professor, he worked between 2010 and 2012 at the A.J. Drexel Nanotechnology Institute in the team of Yury Gogotsi at Drexel University, Philadelphia, USA. Since 2015, he has been Full Professor for Energy Materials at Saarland University and Program Division Leader at the INM – Leibniz Institute for New Materials in Saarbrücken, Germany. His research activities encompass nanocarbon and hybrid nanomaterials for electrochemical energy storage, harvesting, and water desalination.
Hybrid capacitive deionization (HCDI), which combines a capacitive carbon electrode and a redox active electrode in a single device, has emerged as a promising method for water desalination, enabling higher ion removal capacity than devices containing two carbon electrodes. However, to date, the desalination performance of few redox active materials has been reported. For the first time, we present the electrochemical behavior of manganese oxide nanowires with four different tunnel crystal structures as faradaic electrodes in HCDI cells. Two of these phases are square tunnel structured manganese oxides, α-MnO2 and todorokite-MnO2. The other two phases have novel structures that cross-sectional scanning transmission electron microscopy analysis revealed to have ordered and disordered combinations of structural tunnels with different dimensions. The ion removal performance of the nanowires was evaluated not only in NaCl solution, which is traditionally used in laboratory experiments, but also in KCl and MgCl2 solutions, providing better understanding of the behavior of these materials for desalination of brackish water that contains multiple cation species. High ion removal capacities (as large as 27.8 mg g⁻¹, 44.4 mg g⁻¹, and 43.1 mg g⁻¹ in NaCl, KCl, and MgCl2 solutions, respectively) and high ion removal rates (as large as 0.112 mg g⁻¹ s⁻¹, 0.165 mg g⁻¹ s⁻¹, and 0.164 mg g⁻¹ s⁻¹ in NaCl, KCl, and MgCl2 solutions, respectively) were achieved. By comparing ion removal capacity to structural tunnel size, it was found that smaller tunnels do not favor the removal of cations with larger hydrated radii, and more efficient removal of larger hydrated cations can be achieved by utilizing manganese oxides with larger structural tunnels. Extended HCDI cycling and ex situ X-ray diffraction analysis revealed the excellent stability of the manganese oxide electrodes in repeated ion removal/ion release cycles, and compositional analysis of the electrodes indicated that ion removal is achieved through both surface redox reactions and intercalation of ions into the structural tunnels. This work contributes to the understanding of the behavior of faradaic materials in electrochemical water desalination and elucidates the relationship between the electrode material crystal structure and the ion removal capacity/ion removal rate in various salt solutions.
Capacitive deionization (CDI) is considered to be one of the most promising technologies for the desalination of brackish water with low to medium salinity. In practical applications, Faradaic redox reactions occurring in CDI may have both negative and positive effects on CDI performance. In this review, we present an overview of the types and mechanisms of Faradaic reactions in CDI systems including anodic oxidation of carbon electrodes, cathodic reduction of oxygen and Faradaic ion storage and identify their apparent negative and positive effects on water desalination. A variety of strategies including development of novel electrode materials and use of alternative configurations and/or operational modes are proposed for the purpose of mitigation or elimination of the deterioration of electrodes and the formation of byproducts caused by undesired side Faradaic reactions. It is also recognized that Faradaic reactions facilitate a variety of exciting new applications including i) the incorporation of intercalation electrodes to enhance water desalination or to selectively separate certain ions through reversible Faradaic reactions and ii) the use of particular anodic oxidation and cathodic reduction reactions to realize functions such as water disinfection and contaminant removal.
Capacitive deionization (CDI) is a promising desalination process, but conventional static electrode CDI operates by sequentially cycling through charging and discharging to produce fresh and concentrated water, respectively. However, an effective continuous operation is desirable for optimized system operation. Here, we report a semi-continuous desalination process with a novel modified CDI cell architecture using a multi-channel flow stream and ion exchange membranes (MC-MCDI). This MC-MCDI consists of two channels including side and middle channels with a pair of cation and anion ion exchange membranes where the feed streams can be separately distributed without mixing. The MC-MCDI design allows semi-continuous production of clean water since the separated middle and side channels are alternately desalinated and regenerated: one channel is being desalinated while the other channel is regenerated. Therefore, the cell can produce clean water during both charging and discharging, enabling semi-continuous operation. In addition, with the benefit from similar cell configuration with membrane CDI, the MC-MCDI design exhibits a high salt adsorption capacity (SAC) of 22 ± 2 mg/g and charge efficiency of 90 ± 2% at middle and side channels during charging and discharging with reverse voltage operation (cell voltage of + 1.2 V vs. − 1.2 V).
Capacitive deionization (CDI) using capacitive electrodes is highlighted as an alternative desalination technology because of its advantages of low cost and high energy-efficiency. However, the deionization capacity of CDI is somewhat limited because its capacity relies on the double layer capacitance of a carbon electrode. Thus, improving the deionization capacity of a CDI system is one of the most urgent issues in CDI technology. Herein, Ag coated carbon composite electrode employed hybrid CDI system (Ag coated HCDI) was investigated to enhance the deionization performances. The Ag coated carbon composite electrode was made by coating a small amount of Ag onto a carbon capacitive electrode, exhibiting the characteristics of a battery and a capacitor together. As major results, the CDI deionization capacity (88% more), rate (39% more), and charge efficiency (76% ➔ 92%) was dramatically enhanced due to the Ag coating. The significant improvement in deionization performance is explained by the enhanced specific capacity combining the capacitance in the carbon electrode with the Ag mediated charge transfer reaction. In addition, the Ag coated HCDI (73.3 kJ mole− 1) is superior to membrane assisted CDI (136.7 kJ mole− 1) in terms of energy consumption for deionization due to its low voltage feasible operation.
Desalination is a sustainable technology that removes sodium and chloride ions from seawater. Herein, we demonstrate a faradic mechanism to promote the capacity of capacitive deionization in high concertation salty water via an electrochemical deionization device. In this system, ions removal is achieved by the faradic mechanism via a constant current operation mode, which is improved based on the constant voltage operation mode used in the conventional CDI operation. Benefiting from the high capacity and excellent rate performance of Prussian blue as an active electrochemical reaction material, the designed unit has revealed a superior removal capacity with ultrafast ion removal rate. A high removal capacity of 101.7 mg g-1 has been obtained with proper flow rate and current density. To further improve the performance of the EDI, a reduced graphene oxide with nanopores and Prussian blue composite has been synthesized. The PB@NPG have demonstrated a high salt removal capacity of 120.0 mg g-1 at 1 C with an energy consumption of 6.76 kT per ion removed, which is much lower than most CDI methods. A particularly high rate performance of 0.5430 mg g-1 s-1 has been achieved at 40 C. The faradic mechanism promoted EDI has provided a new insight into the design and selection of host materials for highly salty water desalination.
Sodium-ion batteries (SIBs) have attracted increasing attention in the past decades, because of high overall abundance of precursors, their even geographical distribution, and low cost. Apart from inherent thermodynamic disadvantages, SIBs have to overcome multiple kinetic problems, such as fast capacity decay, low rate capacities and low Coulombic efficiencies. A special case is sodium super ion conductor (NASICON)-based electrode materials as they exhibit - besides pronounced structural stability - exceptionally high ion conductivity, rendering them most promising for sodium storage. Owing to the limiting, comparatively low electronic conductivity, nano-structuring is a prerequisite for achieving satisfactory rate-capability. In this review, we analyze advantages and disadvantages of NASICON-type electrode materials and highlight electrode structure design principles for obtaining the desired electrochemical performance. Moreover, we give an overview of recent approaches to enhance electrical conductivity and structural stability of cathode and anode materials based on NASICON structure. We believe that this review provides a pertinent insight into relevant design principles and inspires further research in this respect.
Capacitive deionization (CDI) which removes ionic species from solution by applying electric energy to carbon based electrodes is one of excellent convergence technologies combined with energy storage technology and environmental systems. In recent decades, the technology for synthesizing new carbon materials has advanced, the electrical double layer capacitance in porous structures has become better understood, and novel deionization systems have been developed. Nevertheless, achieving a higher deionization performance is required for CDI to compete with reverse osmosis for deionization. The recently introduced Hybrid CDI (HCDI), which utilizes sodium manganese oxide (Na4Mn9O18) and carbon material, successfully demonstrated its superior deionization performance over conventional CDI systems. Despite the great promise of the HCDI system, the limited availability of aqueous based intercalation materials and the lack of information regarding the characteristics of the HCDI system operations are obstacles to the advancement of HCDI. Thus, we report a new HCDI system with sodium iron pyrophosphate (Na2FeP2O7), which is a promising material for sodium ion batteries due to its high capacity, low cost and environmentally benign nature. The major results of the HCDI system with Na2FeP2O7 showed a superior maximum deionization rate performance (0.081 mg g-1 s-1) with a comparable deionization capacity (30.2 mg g-1) compared to the previous HCDI system with Na4Mn9O18. Furthermore, the analysis of the CDI Ragone plot revealed the hybrid behavior characteristics of this HCDI system and that the high deionization capacity originated from the high capacity of Na2FeP2O7 at a low current density, whereas the fast deionization rate originated from the supercapacitor system at a high current density. Consequently, this study on a new HCDI system with Na2FeP2O7 contributes to expanding the understanding of the kinetic properties of the HCDI system with respect to its diverse operations.
A superior Na3 V2 (PO4 )3 -based nanocomposite (NVP/C/rGO) has been successfully developed by a facile carbothermal reduction method using one most-common chelator, disodium ethylenediamintetraacetate [Na2 (C10 H16 N2 O8 )], as both sodium and nitrogen-doped carbon sources for the first time. 2D-reduced graphene oxide (rGO) nanosheets are also employed as highly conductive additives to facilitate the electrical conductivity and limit the growth of NVP nanoparticles. When used as the cathode material for sodium-ion batteries, the NVP/C/rGO nanocomposite exhibits the highest discharge capacity, the best high-rate capabilities and prolonged cycling life compared to the pristine NVP and single-carbon-modified NVP/C. Specifically, the 0.1 C discharge capacity delivered by the NVP/C/rGO is 116.8 mAh g(-1) , which is obviously higher than 106 and 112.3 mAh g(-1) for the NVP/C and pristine NVP respectively; it can still deliver a specific capacity of about 80 mAh g(-1) even at a high rate up to 30 C; and its capacity decay is as low as 0.0355 % per cycle when cycled at 0.2 C. Furthermore, the electrochemical impedance spectroscopy was also implemented to compare the electrode kinetics of all three NVP-based cathodes including the apparent Na diffusion coefficients and charge-transfer resistances.
Aqueous rechargeable sodium ion batteries has attracted a lot of interests because of its low cost, huge abundance of sodium resources and promising application for large-scale electric energy storage. Herein, we proposed the carbon-coated Na3V2(PO4)3 nanocomposite (Na3V2(PO4)3/C) as a cathode material, which was prepared using a simple sol–gel method. The structure and morphology analyses showed that the highly crystalline Na3V2(PO4)3 nanoparticle with an average size of 350 nm is well coated by a carbon layer with a thickness of 3 nm. Electrochemical tests showed that at high current rates, the Na3V2(PO4)3/C cathode exhibited excellent electrochemical performance. Impressively, it delivered a discharge specific capacity of 94.5 mAh/g at 10C (1176 mA/g), 90.5 mAh/g at 15C (1764 mA/g) and 71.7 mAh/g at 20C (2352 mA/g). To the best of our knowledge, the notable rate capability has never been reported before for aqueous sodium ion batteries. The enhanced electrochemical behavior could be attributed to the combined advantages of Na3V2(PO4)3 nanoparticles and carbon layer in the unique core–shell structure, which improved the intrinsic poor electronic conductivity of Na3V2(PO4)3 greatly. Our results confirmed the prepared Na3V2(PO4)3/C nanocomposite should be a promising cathode candidate for aqueous sodium ion batteries.
The NASICON-type Na3V2(PO4)3 (NVP) cathode material is investigated in an aqueous sodium-ion battery, which is explored by using a three-electrode system. The battery behaviors and capacitive properties of this electrode system are critically investigated by using 1 m Li2SO4, Na2SO4, and K2SO4 electrolytes, with an optimal performance found to arise in Na+-based electrolyte, which exhibits a capacitance of 209 Fg−1 at 8.5 C as well as enhanced ion diffusion. Larger, hydrated Li+ is less able to diffuse into the network of NVP, and the low conductivity and mobility leads to near noncapacitive behavior. In the case of K+-based electrolyte, NVP presents asymmetric cyclic voltammograms, owing to weak solvation and the high conductivity of K+, making the ions more easily able to form electric double-layer capacitance on the surface or pores of NVP, rather than insert into the network. The equivalent circuit based on electrochemical impedance spectroscopy result is analyzed to account for the electrochemical insertion behavior of Na+ into NVP, involving ion transfer in electrolyte solution, ion diffusion from the electrolyte to the electrode surface, as well as charge transfer and ion diffusion in the electrode solid.
Based on a porous carbon electrode, capacitive deionization (CDI) is a promising desalination technology in which ions are harvested and stored in an electrical double layer. However, the ion removal capacity of CDI systems is not sufficient for desalting high-concentration saline water. Here, we report a novel desalination technique referred to as "hybrid capacitive deionization (HCDI)", which combines CDI with a battery system. HCDI consists of a sodium manganese oxide (Na4Mn9O18) electrode, an anion exchange membrane, and a porous carbon electrode. In this system, sodium ions are captured by the chemical reaction in the Na4Mn9O18 electrode, whereas chloride ions are adsorbed on the surface of the activated carbon electrode during the desalination process. HCDI exhibited more than double the ion removal sorption capacity (31.2 mg/g) than a typical CDI system (13.5 mg/g). Moreover, it was found that the system has a rapid ion removal rate and excellent stability in an aqueous sodium chloride solution. These results thus suggest that the HCDI system could be a feasible method for desalting a highly concentrated sodium chloride solution in capacitive techniques.
Na3V2(PO4)3 is one of the most important cathode materials for sodium-ion batteries, delivering about two Na extraction/insertion from/into the unit structure. To understand the mechanism of sodium storage, a detailed structure of rhombohedral Na3V2(PO4)3 and its sodium extracted phase of NaV2(PO4)3 are investigated at the atomic scale using a variety of advanced techniques. It is found that two different Na sites (6b, M1 and 18e, M2) with different coordination environments co-exist in Na3V2(PO4)3, whereas only one Na site (6b, M1) exists in NaV2(PO4)3. When Na is extracted from Na3V2(PO4)3 to form NaV2(PO4)3, Na+ occupying the M2 site (CN = 8) is extracted and the rest of the Na remains at M1 site (CN = 6). In addition, the Na atoms are not randomly distributed, possibly with an ordered arrangement in M2 sites locally for Na3V2(PO4)3. Na+ ions at the M1 sites in Na3V2(PO4)3 tend to remain immobilized, suggesting a direct M2-to-M2 conduction pathway. Only Na occupying the M2 sites can be extracted, suggesting about two Na atoms able to be extracted from the Na3V2(PO4)3 structure.
In this Article, the consequence of continuous electrochemical oxidation at the positive electrode in an initially symmetrical capacitive deionization (CDI) cell, comprising two identical activated carbon electrodes, is examined and discussed. Extensive and intensive parameters of the CDI cell are defined, and the deviations occurring among them as a result of continuous electrochemical oxidation processes at the positive electrode during prolonged charge–discharge cycling are discussed. A special flow-through CDI cell containing activated carbon fiber (ACF) electrodes was developed for this purpose. Ex situ XPS measurements were conducted to prove the presence of oxidized surface groups on the positive electrode of these cells due to cycling. A surprising phenomenon that looks like an inversion functionality of the carbon electrodes occurring after numerous charge–discharge cycles is observed and explained.
Water desalination is an important approach to provide fresh water around the world, although its high energy consumption, and thus high cost, call for new, efficient technology. Here, we demonstrate the novel concept of a "desalination battery", which operates by performing cycles in reverse on our previously reported mixing entropy battery. Rather than generating electricity from salinity differences, as in mixing entropy batteries, desalination batteries use an electrical energy input to extract sodium and chloride ions from seawater and to generate fresh water. The desalination battery is comprised by a Na(2-x)Mn(5)O(10) nanorod positive electrode and Ag/AgCl negative electrode. Here, we demonstrate an energy consumption of 0.29 Wh l(-1) for the removal of 25% salt using this novel desalination battery, which is promising when compared to reverse osmosis (~ 0.2 Wh l(-1)), the most efficient technique presently available.
Capacitive deionization (CDI) with carbon-aerogel electrodes represents a novel process in desalination of brackish water and has merit due to its low fouling/scaling potential, ambient operational conditions, electrostatic regeneration, and low voltage requirements. The objective of this study was to investigate the viability of CDI in treating brackish produced water and recovering iodide from the water. Laboratory- and pilot-scale experiments were conducted to identify ion selectivity, key operational parameters, evaluate desalination performance, and assess the challenges for its practical applications. The performance of the CDI technology (CDT) system tested was consistent throughout the laboratory- and field-scale experiments. Deterioration of the carbon-aerogel electrodes was not observed during testing. The degree of ions adsorbed to the carbon aerogel (in mol/g aerogel) during treatment of brackish water was dependent upon initial ion concentrations in the feed water with the following selectivity I>Br>Ca>alkalinity>Mg>Na>Cl. The preferential sorption of iodide revealed merit to efficiently recover iodide from brackish water even in the presence of dominant co-ions. The research findings derived from this study identified parameters that merit further improvements regarding design and operation, including modification of pore-size distribution of aerogel, development of high capacitance and low-cost electrode materials, reducing the dead volume after regeneration and rinsing, minimizing energy consumption, and maximizing system recovery.
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