Creating 3D Hierarchical Carbon Architectures with Micro-, Meso-, and Macropores via a Simple Self-Blowing Strategy for a Flow-through Deionization Capacitor

Integrated hierarchical porous lignin-based carbon electrode for boosting membrane-free capacitive deionization areal adsorption capacity

The development of high value-added lignin-based functional porous carbon electrodes with excellent properties from sustainable industry lignin powder remains a challenge. This work aims to create robust, binder-free, conductive additives-free, and current collector-free monolithic porous carbon electrodes using industrial lignin powder for membrane-free capacitive deionization (CDI). The material exhibits high mechanical strength, hierarchical porosity structure, large uniform size, and thickness of just a few millimetres (<2.6 mm). In a three-electrode supercapacitor system, the areal specific capacitance of CLCA300-3-1.0 reaches 5.03-1.02 F cm-2 when the scan rate between 1 and 20 mV s-1 in 1 M NaCl solution. As CDI electrodes, the charge efficiency of CLCA300-3-1.0 at different voltages of 1.2 V, 1.4 V and 1.6 V is 0.53, 0.72 and 0.71, respectively. The energy consumption of CLCA280-3-1.0, CLCA300-3-1.0 and CLCA320-3-1.0 tested at 1.2 V are 3.27, 3.40 and 3.25 Wh m-3, respectively. In addition, with thickness increasing to 1.5 mm, the developed CLCA300-3-1.5 electrode exhibits an areal adsorption capacity of 0.46 mg cm-2, and relative highly capacity retention of 84.78 % after 70 cycles. The impressive desalination performance is attributed to the well-designed hierarchical porosity, superhydrophilicity and robust monolithic structure.

Activated Carbon Aerogel as an Electrode with High Specific Capacitance for Capacitive Deionization

In this study, carbon aerogels (CAs) were synthesized by the sol-gel method, using environmentally friendly glucose as a precursor, and then they were further activated with potassium hydroxide (KOH) to obtain activated carbon aerogels (ACAs). After the activation, the electrochemical performance of the ACAs was significantly improved, and the specific capacitance increased from 19.70 F·g−1 to 111.89 F·g−1. Moreover, the ACAs showed a stronger hydrophilicity with the contact angle of 118.54° compared with CAs (69.31°). When used as an electrode for capacitive deionization (CDI), the ACAs had not only a better diffuse electric double layer behavior, but also a lower charge transfer resistance and intrinsic resistance. Thus, the ACA electrode had a faster CDI desalination rate and a higher desalination capacity. The unit adsorption capacity is three times larger than that of the CA electrode. In the desalination experiment of 100 mg·L−1 sodium chloride (NaCl) solution using a CDI device based on the ACA electrode, the optimal electrode spacing was 2 mm, the voltage was 1.4 V, and the flow rate was 30 mL·min−1. When the NaCl concentration was 500 mg·L−1, the unit adsorption capacity of the ACA electrode reached 26.12 mg·g−1, much higher than that which has been reported in many literatures. The desalination process followed the Langmuir model, and the electro-sorption of the NaCl was a single layer adsorption process. In addition, the ACA electrode exhibited a good regeneration performance and cycle stability.

Bimetallic FeNi nanoparticles immobilized by biomass-derived hierarchically porous carbon for efficient removal of Cr(VI) from aqueous solution

Nano zero-valent iron (nZVI) is an effective material for Cr(VI) treatment, however excessive agglomeration and surface oxidation limit its application. Herein, straw derived hierarchically porous carbon supported FeNi bimetallic nanoparticles (FeNi@HPC) was prepared for effective removal of Cr(VI) from water. FeNi nanoparticles were successfully loaded onto HPC with good dispersibility, and HPC caused an increase in specific surface area of FeNi nanoparticles. FeNi@HPC exhibited a significantly enhanced removal efficiency for Cr(VI) in comparison to Fe@HPC and FeNi NPs. The Ni doping content was further optimized, and the best Ni content in bimetallic NPs was estimated as 10 wt%. The conditions optimal for the activity of FeNi@HPC were assessed, and the highest removal efficiency equivalent to 30 mg L^-1 of Cr(VI) was achieved at pH= 4.0 in 360 min with a dosage of 0.5 g L^-1. Higher temperatures favored the removal of Cr(VI) and FeNi@HPC manifested the lowest activation energy as compared to Fe@HPC and FeNi NPs. The action mechanisms of FeNi@HPC presumably involved electron transfer from Fe⁰, Fe²⁺and atomic hydrogen. This work not only provide a cost-effective and available HPC material to stabilize nZVI but also revealed that using FeNi@HPC is a promising approach for the remediation of water pollution.

Novel Ag3PO4/boron-carbon-nitrogen photocatalyst for highly efficient degradation of organic pollutants under visible-light irradiation

Ag₃PO₄ is an indirect bandgap semiconductor with excellent photocatalytic activity. However, it has not been widely used so far for the treatment of polluted wastewaters. This scarce use in wastewater treatment can be mainly attributed to its large crystallite size, which would be due to rapid agglomeration during the synthesis process, as well as to the photo-corrosion problem affecting this material. Hence, it would be crucial to develop a photocatalytic system involving Ag₃PO₄ nanoparticles with enhanced properties, such as higher specific surface area and excellent photocatalytic stability. To meet this demand, a novel Ag₃PO₄/boron carbon nitrogen (Ag₃PO₄/BCN) composite photocatalyst was successfully prepared in the present study via electrostatically driven self-assembly and ion exchange processes. After characterization and assessment, it was shown that the as-prepared Ag₃PO₄/BCN nanocomposite photocatalyst not only contains smaller Ag₃PO₄ nanoparticles, but also exhibits an enhanced visible-light photocatalytic activity for Rhodamine B (RhB) Methyl Orange (MO) and Tetracycline (TC) and improved stability, without decrease after 5 cycles, compared with pure Ag₃PO₄ nanoparticles. Positive synergy between Ag₃PO₄ nanoparticles and BCN nanosheets, including the increase in the number of active adsorption sites, and the restriction of the formation of Ag due to the recombination of photogenerated electron-hole pairs in Ag₃PO₄ nanoparticles, are mainly responsible for the enhanced properties of the prepared catalyst. This study shows that Ag₃PO₄/BCN composite photocatalyst would be promising for wastewater treatment, which would be of clearly environmental and public health relevance.

Boosting Catalytic Purification of Soot Particles over Double Perovskite-Type La2-xKxNiCoO6 Catalysts with an Ordered Macroporous Structure

The catalytic performances for soot purification over the perovskite-type ABO3 oxides, as one of the most potential non-noble metal catalysts, are closely correlated with the substitution of A-site and B-site ions. Herein, three-dimensional ordered macroporous (3DOM) structural catalysts of double perovskite-type La2-xKxNiCoO6 were prepared by a method of colloidal crystal template. The contact efficiency between the catalyst and soot particles is significantly promoted by the 3DOM structure, and the partial substitution of A-site (La) with low-valence potassium (K) ions in La2-xKxNiCoO6 catalysts boosts the increasing surface density of coordinatively unsaturated active B-sites (Co and Ni) and active oxygen. 3DOM La2-xKxNiCoO6 catalysts exhibited superior performance during the purification of soot particles, and the 3DOM La1.80K0.20NiCoO6 catalyst exhibited the highest activity, that is, the values of T50, SCO2, and turnover frequency are 346 °C, 99.3%, and 0.204 h-1 (at 300 °C), respectively. According to the results of multiple experimental characterizations and density functional theory calculations, the mechanism of the samples during soot removal is proposed: the increase in surface oxygen density induced by the doping of K ions significantly promotes the critical step of the oxidation from NO to NO2 in catalyzing soot purification. It is one new strategy to develop the high-efficient non-noble metal catalysts for soot purification in practical application.

A Molecular Foaming and Activation Strategy to Porous N-Doped Carbon Foams for Supercapacitors and CO2 Capture

Hierarchically porous carbon materials are promising for energy storage, separation and catalysis. It is desirable but fairly challenging to simultaneously create ultrahigh surface areas, large pore volumes and high N contents in these materials. Herein, we demonstrate a facile acid-base enabled in situ molecular foaming and activation strategy for the synthesis of hierarchically macro-/meso-/microporous N-doped carbon foams (HPNCFs). The key design for the synthesis is the selection of histidine (His) and potassium bicarbonate (PBC) to allow the formation of 3D foam structures by in situ foaming, the PBC/His acid-base reaction to enable a molecular mixing and subsequent a uniform chemical activation, and the stable imidazole moiety in His to sustain high N contents after carbonization. The formation mechanism of the HPNCFs is studied in detail. The prepared HPNCFs possess 3D macroporous frameworks with thin well-graphitized carbon walls, ultrahigh surface areas (up to 3200 m² g^-1), large pore volumes (up to 2.0 cm³ g^-1), high micropore volumes (up to 0.67 cm³ g^-1), narrowly distributed micropores and mesopores and high N contents (up to 14.6 wt%) with pyrrolic N as the predominant N site. The HPNCFs are promising for supercapacitors with high specific capacitances (185-240 F g^-1), good rate capability and excellent stability. They are also excellent for CO₂ capture with a high adsorption capacity (~ 4.13 mmol g^-1), a large isosteric heat of adsorption (26.5 kJ mol^-1) and an excellent CO₂/N₂ selectivity (~ 24).

Frontiers of carbon materials as capacitive deionization electrodes

Carbon materials are widely used as capacitive deionization (CDI) electrodes due to their high specific surface area (SSA), superior conductivity, and better stability, including activated carbon, carbon aerogels, carbon nanotubes and graphene.

Pine cone mold: a toolbox for fabricating unique metal/carbon nanohybrid electrocatalysts

Nature presents delicate and complex materials systems beyond those fathomable by humans, and therefore, extensive effort has been made to utilize or mimic bio-materials and bio-systems in various fields. Biomass, an inexhaustible natural materials source, can also present good opportunities for the development of unprecedented, advanced materials and processing systems. Herein, we demonstrate the use of pine cones as a biomass mold for creating new and useful metal/carbon nanohybrids (MCNHs). The inherent water-induced folding actuation of the cone scales allows the casting of an aqueous solution of a single metal precursor or a binary metal mixture into the cone mold by simply immersing the cone in the solution. The cone actively absorbs aqueous-phase metal precursors through the bract scales and the precursor ions introduced into the cone are anchored to the functional groups of the interior tissues of the cone. Subsequent heat treatment successfully led to the formation of unique MCNHs. Iron, manganese, and cobalt were employed as model metals, binary mixtures of which were also cast into the cone mold to create further versatile MCNHs. Nanoparticulate metals were formed on the carbon supports, where the size, size distribution, and crystallinity of the nanoparticles were highly dependent on the identity of the single-component precursor and the combination of precursors. Consequently, the electrochemical activity of the MCNHs also depended on which metal precursors were cast into the cone mold. The MCNH prepared from the mixture of iron and manganese precursors (M_FeMnCNH) showed the best electrochemical activity. As model applications, M_FeMnCNH was applied to electrode materials for electrochemical charge storage and the oxygen evolution reaction. An electrochemical capacitor cell based on the M_FeMnCNH electrodes showed excellent performance with energy densities of 38.7-54.2 W h kg^-1 at power densities of 16 000-160 kW kg^-1. In addition, M_FeMnCNH demonstrated a low overpotential of 464 mV and fast kinetics with a Tafel slope of 64.6 mV dec^-1 as an electrocatalyst for the oxygen evolution reaction in 1.0 M KOH. These results substantiate that pine cones as a biomass mold show great promise for creating versatile MCNHs through further combination of various precursors.

3D printed electrodes for efficient membrane capacitive deionization

There is increasing interests in cost-effective and energy-efficient technologies for the desalination of salt water. However, the challenge in the scalability of the suitable compositions of electrodes has significantly hindered the development of capacitive deionization (CDI) as a promising technology for the desalination of brackish water. Herein, we introduced a 3D printing technology as a new route to fabricate electrodes with adjustable composition, which exhibited large-scale applications as free-standing, binder-free, and robust electrodes. The 3D printed electrodes were designed with ordered macro-channels that facilitated effective ion diffusion. The high salt removal capacity of 75 mg g^-1 was achieved for membrane capacitive deionization (MCDI) using 3D printed nitrogen-doped graphene oxide/carbon nanotube electrodes with the total electrode mass of 20 mg. The improved mechanical stability and strong bonding of the chemical components in the electrodes allowed a long cycle lifetime for the MCDI devices. The adjusted operational mode (current density) enabled a low energy consumption of 0.331 W h g^-1 and high energy recovery of ∼27%. Furthermore, the results obtained from the finite element simulations of the ion diffusion behavior quantified the structure-function relationships of the MCDI electrodes.

Fabrication of polyvinylidene fluoride-derived porous carbon heterostructure with inserted carbon nanotube via phase-inversion coupled with annealing for capacitive deionization application

As a promising desalination technology, capacitive deionization (CDI) has great potential to guarantee freshwater supply. It is urgently needed to explore novel electrode materials with excellent desalination performance. Herein, the PVDF-derived porous carbon heterostructure with inserted carbon nanotube (PPC/CNT) was prepared via phase-inversion coupled with annealing strategy and applied as electrode material for CDI desalination. The resultant PPC/CNT possesses the combined structural advantages of PPC and CNT, such as high specific surface, mesoporous structure and improved conductivity. By virtue of these remarkable properties, PPC/CNT exhibites an excellent electrosorption capacity of 15.1 mg/g in 500 mg/L NaCl, while that of PPC electrode is 10.3 mg/g. Specially, the charge efficiency of PPC/CNT electrode is 1.39 times higher as compared to PPC, which is largely responsible for the improvement of electrosorption capacity. Besides, PPC/CNT electrode demonstrated good cycle stability over 10 electrosorption-desorption cycles. Thus, PPC/CNT electrode presents promising prospects as CDI electrode for water desalination. This work may shed new light on the rational design of porous carbon heterostructures with suitable host matrix and improved conductivity, subsequently developing the CDI performance.

N-Doped Hollow Carbon Spheres/Sheets Composite for Electrochemical Capacitor

Functional carbon materials with a combination of 0 dimension and 2 dimension are particularly interesting in the electrochemical field owing to the low density, high surface area, and strong ion-bearing capacity. Especially, hollow mesoporous carbon spheres (0 dimension) and nanosheet (2 dimension) composite hollow porous carbon spheres and nanosheet (HMCS/S) have received much attention as electrochemical capacitor electrode materials. However, it is challenging for effective preparation of this complex composite structure. In this work, a novel and simple procedure to prepare N-doping HMCS/S (N-HMCS/S) is presented. This approach adopted silica spheres as the core and employed [C₁₈Mim]Br and tetraethyl orthosilicate as the structural directing agent for the formation of the flaky/spherical hybrid structure. The unique structure of nanosheets embedded by hollow carbon spheres and N-doping characteristics endow N-HMCS/S with good performance in electrochemical capacitor with high specific capacity (196.5 F g^-1) in the three-electrode system and excellent high-rate capability with retention of 61.5% in the two-electrode system.

Hierarchically Porous Zirconia Monolith Fabricated from Bacterial Cellulose and Preceramic Polymer

A hierarchically porous zirconia (ZrO₂) monolith was successfully fabricated by using bacterial cellulose (BC) as a biotemplate and preceramic polymer as a zirconium resource, via freeze-drying and two-step calcination process. Images of scanning electron microscopy showed that the ZrO₂ monolith well-replicated a three-dimensional reticulated structure of pristine BC and possessed good morphology stability till 1100 °C in air. Results of N₂ adsorption/desorption and mercury porosimetry analysis revealed the hierarchically porous structure and large specific area (9.7 m²·g^-1) of the ZrO₂ monolith, respectively. Patterns of X-ray powder diffraction indicated that the monoclinic phase and tetragonal phase coexisted in the ZrO₂ monolith with the former as the main phase. In addition, the ZrO₂ monolith possessed low bulk density (0.13 g·cm^-3) and good mechanical strength. These properties suggest that the as-prepared ZrO₂ monolith has a great potential to serve as an ideal catalyst or catalyst support.

Nanoparticulate Dielectric Overlayer for Enhanced Electric Fields in a Capacitive Deionization Device

The magnitude and distribution of the electric field between two conducting electrodes of a capacitive deionization (CDI) device plays an important role in governing the desalting capacity. A dielectric coating on these electrodes can polarize under an applied potential to modulate the net electric field and hence the salt adsorption capacity of the device. Using finite element models, we show the extent and nature of electric field modulation, associated with changes in the size, thickness, and permittivity of commonly used nanostructured dielectric coatings such as zinc oxide (ZnO) and titanium dioxide (TiO₂). Experimental data pertaining to the simulation are obtained by coating activated carbon cloth (ACC) with nanoparticles of ZnO and TiO₂ and using them as electrodes in a CDI device. The dielectric-coated electrodes displayed faster desalting kinetics of 1.7 and 1.55 mg g^-1 min^-1 and higher unsaturated specific salt adsorption capacities of 5.72 and 5.3 mg g^-1 for ZnO and TiO₂, respectively. In contrast, uncoated ACC had a salt adsorption rate and capacity of 1.05 mg g^-1 min^-1 and 3.95 mg g^-1, respectively. The desalting data is analyzed with respect to the electrical parameters of the electrodes extracted from cyclic voltammetry and impedance measurements. Additionally, the obtained results are correlated with the simulation data to ascertain the governing principles for the changes observed and advances that can be achieved through dielectric-based electrode modifications for enhancing the CDI device performance.

Intrinsic tradeoff between kinetic and energetic efficiencies in membrane capacitive deionization

Significant progress has been made over recent years in capacitive deionization (CDI) to develop novel system configurations, predictive theoretical models, and high-performance electrode materials. To bring CDI to large scale practical applications, it is important to quantitatively understand the intrinsic tradeoff between kinetic and energetic efficiencies, or the relationship between energy consumption and the mass transfer rate. In this study, we employed both experimental and modeling approaches to systematically investigate the tradeoff between kinetic and energetic efficiencies in membrane CDI (MCDI). Specifically, we assessed the relationship between the average salt adsorption rate and specific energy consumptions from MCDI experiments with different applied current densities but a constant effluent salinity. We investigated the impacts of feed salinity, diluted water salinity, diluted water volume per charging cycle, and electrode materials on the kinetics-energetics tradeoff. We also demonstrate how this tradeoff can be employed to optimize the design and operation of CDI systems and compare the performance of different electrode materials and CDI systems.

Facile synthesis of TiO2/ZrO2 nanofibers/nitrogen co-doped activated carbon to enhance the desalination and bacterial inactivation via capacitive deionization

Capacitive deionization, as a second generation electrosorption technique to obtain water, is one of the most promising water desalination technologies. Yet; in order to achieve high CDI performance, a well-designed structure of the electrode materials is needed, and is in high demand. Here, a novel composite nitrogen-TiO₂/ZrO₂ nanofibers incorporated activated carbon (NACTZ) is synthesized for the first time with enhanced desalination efficiency as well as disinfection performance towards brackish water. Nitrogen and TiO₂/ZrO₂ nanofibers are used as the support of activated carbon to improve its low capacitance and hydrophobicity, which had dramatically limited its adequacy during the CDI process. Importantly, the as-fabricated NACTZ nanocomposite demonstrates enhanced electrochemical performance with significant specific capacitance of 691.78 F g⁻¹, low internal resistance and good cycling stability. In addition, it offers a high capacitive deionization performance of NACTZ yield with electrosorptive capacity of 3.98 mg g⁻¹, and, good antibacterial effects as well. This work will provide an effective solution for developing highly performance and low-cost design for CDI electrode materials.

Ion-selective asymmetric carbon electrodes for enhanced capacitive deionization

With the development of capacitive deionization technology, charge efficiency and electrosorption capacity have become some of the biggest technical bottlenecks. Asymmetric activated carbon electrodes with ion-selective functional groups inspired by membrane capacitive deionization were developed to conquer these issues. The deionization capacity increased from 11.0 mg g-1 to 23.2 mg g-1, and the charge efficiency increased from 0.54 to 0.84, due to ion-selective functional groups minimizing the co-ion effect. The charge efficiency and electrosorption capacity resulting from better wettability of these electrodes are effectively enhanced by grafting ion-selective functional groups, which are propitious to ion movement. In addition, asymmetric deionization capacitors show better cycling stability and higher desalination rates. These experimental results have demonstrated that the modification of the ion-selective (oxygen-containing) functional groups on the surfaces of activated carbon could greatly minimize the co-ion effects and increase the salt removal from the solution. These results have indicated that the ion-selective asymmetric carbon electrodes can promote well the development of deionization capacitors for practical desalination.

Ultrahigh performance of novel energy-efficient capacitive deionization electrodes based on 3D nanotubular composites

TiO₂/CNT composites are energy-efficient capacitive deionization platforms with exceptional electrosorption capacity.

Three-dimensional honeycomb-like porous carbon derived from corncob for the removal of heavy metals from water by capacitive deionization

In this study, porous carbon (3DHPC) with a 3D honeycomb-like structure was synthesized from waste biomass corncob via hydrothermal carbonization coupled with KOH activation and investigated as a capacitive deionization (CDI) electrode material. The obtained 3DHPC possesses a hierarchal macroporous and mesoporous structure, and a large accessible specific surface area (952 m² g⁻¹). Electrochemical tests showed that the 3DHPC electrode exhibited a specific capacitance of 452 F g⁻¹ and good electric conductivity. Moreover, the feasibility of electrosorptive removal of chromium(vi) from an aqueous solution using the 3DHPC electrode was demonstrated. When 1.0 V was applied to a solution containing 30 mg L⁻¹ chromium(vi), the 3DHPC electrode exhibited a higher removal efficiency of 91.58% compared with that in the open circuit condition. This enhanced adsorption results from the improved affinity between chromium(vi) and the electrode under electrochemical assistance involving a non-faradic process. Consequently, the 3DHPC electrode with typical double-layer capacitor behavior is demonstrated to be a favorable electrode material for capacitive deionization.

A porous carbon electrode with a 3D honeycomb-like structure demonstrates a high removal efficiency for the removal of chromium(vi) from water.

Hierarchically Porous Carbon Derived from PolyHIPE for Supercapacitor and Deionization Applications

Hierarchically porous carbon (HPC) materials with interconnected porous texture are produced from a porous poly(divinylbenzene) precursor, which is synthesized by polymerizing high-internal-phase emulsions. After carbonation, the macroporous structures of the poly(divinylbenzene) precursor are preserved and enormous micro-/mesopores via carbonation with KOH are produced, resulting in an interconnected hierarchically porous network. The prepared HPC has a maximum specific surface area of 2189 m² g^-1. The electrode materials for supercapacitors and capacitive deionization devices employing the formed HPC exhibit a high specific capacity of 88 mA h g^-1 through a voltage range of 1 V (319 F g^-1 at 1 A g^-1) and a superior electrosorption capacity of 21.3 mg g^-1 in 500 mg L^-1 NaCl solution. The excellent capacitive performance could be ascribed to the combination of high specific surface area and favorable hierarchically porous structure.

Removal of NaCl from saltwater solutions using micro/mesoporous carbon sheets derived from watermelon peel via deionization capacitors

Micro/mesoporous carbon sheets derived from watermelon peel were demonstrated as highly efficient electrodes for flow-through deionization capacitors.

Cadmium Removal from Aqueous Solution by a Deionization Supercapacitor with a Birnessite Electrode

Birnessite is widely used as an excellent adsorbent for heavy metal ions and as active electrode materials for supercapacitors. The occurrence of redox reactions of manganese oxides is usually accompanied by the intercalation-deintercalation of cations during the charge-discharge processes of supercapacitors. In this study, based on the charge-discharge principle of the supercapacitor and excellent adsorption properties of birnessite, a birnessite-based electrode was used to remove Cd²⁺ from aqueous solutions. The Cd²⁺ removal mechanism and the influences of birnessite loading and pH on the removal performance were investigated. The results showed that Cd²⁺ was adsorbed on the surfaces and interlayers of birnessite, and the maximum electrosorption capacity of birnessite for Cd²⁺ was about 900.7 mg g^-1 (8.01 mmol g^-1), which was significantly higher than the adsorption isotherm capacity of birnessite (125.8 mg g^-1). The electrosorption specific capacity of birnessite for Cd²⁺ increased with an increase in initial Cd²⁺ concentration and decreased with an increase in the loading of active birnessite. In the pH range of 3.0-6.0, the electrosorption capacity increased at first with an increase in pH and then reached equilibrium above pH 4.0. This work provides a new method for the highly efficient adsorption of Cd²⁺ from polluted wastewater.