Construction of self-supporting ultra-micropores lignin-based carbon nanofibers with high areal desalination capacity

Lignin is a renewable biomacromolecule that can be used as precursors for carbon materials. In this work, highly flexible lignin-based carbon nanofibers with abundant ultra-micropores are constructed via electrospinning, oxidative stabilization and carbonization. The results indicate that replacing PAN with 80 % lignin is feasible in regulating ultra-micropores. The synthesized L4P1-CNFs possess many attractive properties (e.g., pore size distribution, electrochemical and deionization property) compared with that produced from other non-renewable precursors or more-complexed processes. It shows excellent electrochemical double-layer capacitance in 6 M KOH (233 to 162 F g^-1 at 0.5 to 5 A g^-1) and 1 M NaCl (158 to 82 F g^-1 at 0.5 to 5 A g^-1) electrolytes. Upon assembling into CDI cells, the average salt adsorption rate could reach 1.79 mg g^-1 min^-1 at 1.2 V and 3.32 mg g^-1 min^-1 at 2 V in 500 mg L^-1. Benefiting from the excellent flexibility, we innovatively stack four layers of L4P1-CNFs to improve the areal electrosorption capacity to 0.0817 mg cm^-2 at 500 mg L^-1, significantly higher than that of traditional carbon-based electrodes. The good desalination property makes lignin-based carbon nanofibers ideal for practical, low-cost capacitive deionization applications.

Recent advances in lignin-based carbon materials and their applications: A review

As the most abundant natural aromatic polymer, tens of million of tons of lignin produced in paper-making or biorefinery industry are used as fuel annually, which is a low-value utilization. Moreover, burning lignin results in large amounts of carbon dioxide and pollutants in the air. The potential of lignin is far from being fully exploited and the search for high value-added application of lignin is highly pursued. Because of the high carbon content of lignin, converting lignin into advanced carbon-based structural or functional materials is regarded as one of the most promising solutions for both environmental protection and utilization of renewable resources. Significant progresses in lignin-based carbon materials (LCMs) including porous carbon, activated carbon, carbon fiber, carbon aerogel, nanostructured carbon, etc., for various valued applications have been witnessed in recent years. Here, this review summarized the recent advances in LCMs from the perspectives of preparation, structure, and applications. In particular, this review attempts to figure out the intrinsic relationship between the structure and functionalities of LCMs from their recent applications. Hopefully, some thoughts and discussions on the structure-property relationship of LCMs can inspire researchers to stride over the present barriers in the preparation and applications of LCMs.

Regulation, quantification and application of the effect of functional groups on anion selectivity in capacitive deionization

Capacitive deionization (CDI) has been widely studied as a highly efficient method for the removal of charged pollutants in sewage. However, the control of ion selectivity has always been challenging, limiting the application of this approach. In this article, the regulation of different acid/base functional group distributions on the selectivity of four anions are comprehensively discussed. The effects are quantified through simulations and statistical analysis. Finally, optimized CDI is used for the simultaneous denitrification and dephosphorization of municipal wastewater. The results show that carboxyl groups significantly promote the selectivity of dihydrogen phosphate and that amino groups promote the selectivity of sulfate and dihydrogen phosphate. Density functional theory is used to calculate the influence of the functional groups on the anion adsorption energy. Compared with other anions, the energy released is improved when carboxyl groups are included in the adsorption of dihydrogen phosphate. The increase in the released energy is highest when amino groups participate in the adsorption of sulfate and is second-highest when they participate in the adsorption of dihydrogen phosphate. Statistical analysis shows that the valence and hydration energy of the anion and the effect of the functional groups on anion adsorption are significantly related to anion adsorption (P < 0.05), and the correlation coefficient of the model is 0.7253. A CDI stack for the removal of phosphorus and nitrogen under high background ion concentrations is constructed and applied, and it is shown that the treated wastewater meets higher discharge standards. Moreover, the method reaches nearly 80% water production under optimized operating modes. This study reveals the importance of functional groups in ion-selective regulation and provides a potential method for high-standard wastewater treatment.

Cu-based MOF-derived architecture with Cu/Cu2O nanospheres anchored on porous carbon nanosheets for efficient capacitive deionization

The design of high-performance electrode materials with excellent desalination ability has always been a research goal for efficient capacitive deionization (CDI). Herein, a hybrid architecture with Cu/Cu₂O nanospheres anchored on porous carbon nanosheets (Cu/Cu₂O/C) was first synthesized by pyrolyzing a two-dimensional (2D) Cu-based metal-organic framework and then evaluated as a cathode for hybrid CDI. The as-prepared Cu/Cu₂O/C exhibits a hierarchically porous structure with a high specific surface area of 305 m² g^-1 and large pore volume of 0.55 cm³ g^-1, which is favorable to accelerating ion migration and diffusion. The porous carbon nanosheet matrix with enhanced conductivity will facilitate the Faradaic reactions of Cu/Cu₂O nanospheres during the desalination process. The Cu/Cu₂O/C hybrid architecture displays a high specific capacitance of 142.5 F g^-1 at a scan rate of 2 mV s^-1 in 1 M NaCl solution. The hybrid CDI constructed using the Cu/Cu₂O/C cathode and a commercial activated carbon anode exhibits a high desalination capacity of 16.4 mg g^-1 at an operation voltage of 1.2 V in 500 mg L^-1 NaCl solution. Additionally, the hybrid CDI exhibits a good cycling stability with 18.3% decay in the desalination capacity after 20 electrosorption-desorption cycles. Thus, the Cu/Cu₂O/C composite is expected to be a promising cathode material for hybrid CDI.

Recent progress in materials and architectures for capacitive deionization: A comprehensive review

Capacitive deionization is an emerging and rapidly developing electrochemical technique for water desalination across the globe with exponential growth in publications. There are various architectures and materials being explored to obtain utmost electrosorption performance. The symmetric architectures consist of the same material on both electrodes, while asymmetric architectures have electrodes loaded with different materials. Asymmetric architectures possess higher electrosorption performance as compared with that of symmetric architectures owing to the inclusion of either faradaic materials, redox-active electrolytes, or ion specific pre-intercalation material. With the materials perspective, faradaic materials have higher electrosorption performance than carbon-based materials owing to the occurrence of faradaic reactions for electrosorption. Moreover, the architecture and material may be tailored in order to obtain desired selectivity of the target component and heavy metal present in feed water. In this review, we describe recent developments in architectures and materials for capacitive deionization and summarize the characteristics and salt removal performances. Further, we discuss recently reported architectures and materials for the removal of heavy metals and radioactive materials. The factors that affect the electrosorption performance including the synthesis procedure for electrode materials, incorporation of additives, operational modes, and organic foulants are further illustrated. This review concludes with several perspectives to provide directions for further development in the subject of capacitive deionization. PRACTITIONER POINTS: Capacitive deionization (CDI) is a rapidly developing electrochemical water desalination technique with exponential growth in publications. Faradaic materials have higher salt removal capacity (SAC) because of reversible redox reactions or ion-intercalation processes. Combination of CDI with other techniques exhibits improved selectivity and removal of heavy metals. Operational parameters and materials properties affect SAC. In future, comprehensive experimentation is needed to have better understanding of the performance of CDI architectures and materials.

Fabrication of Hierarchical Lignin-Based Carbon through Direct High-Temperature Pyrolysis and Its Electrochemical Application

Green synthesis of lignin-based carbon materials can accelerate the development of energy storage and conversion in supercapacitors. In this work, hierarchical graphenelike carbon was prepared by alkali lignin (AL) pyrolysis at a high temperature of 1700 °C. Accompanied by metal salt catalysis and a nitrogen hybridization reaction, a unique nanostructure of graphitized ALC was obtained with both a well-ordered 2D sheet lamellar structure and a uniform bowl-like porous structure. ALC exhibited a graphenelike lattice structure, a BET specific surface area of 1190 m2·g-1, and excellent electrochemical performance (104 F·g-1/0.5 A·g-1). The study offers a prospective way to the high-value application of industrial lignin in supercapacitor electrode materials.

The mechanism and regulation of the electrosorption selectivity of inorganic anions during capacitive deionization

Explore and verify the influence of hydration energy and valence on selectivity of anions, and the role of anion's shape

Agricultural waste buckwheat husk derived bifunctional nitrogen, sulfur and oxygen-co-doped porous carbon for symmetric supercapacitors and capacitive deionization

A practical strategy for the value-added utilization of agricultural waste was provide. The buckwheat husk derived N, S, O-co-doped porous carbon was used as bifunctional electrode materials for symmetric supercapacitor and capacitive deionization.

Controllable synthesis of spherical carbon particles transition from dense to hollow structure derived from Kraft lignin

The tailored synthesis of carbon particles with controllable shapes and structures from biomass as a raw material would be highly beneficial to meet the demands of various applications of carbon materials from the viewpoint of sustainable development goals. In this work, the spherical carbon particles were successfully synthesized through a spray drying method followed by the carbonization process, using Kraft lignin as the carbon source and potassium hydroxide (KOH) as the activation agent. As the results, the proposed method successfully controlled the shape and structure of the carbon particles from dense to hollow by adjusting the KOH concentration. Especially, this study represents the first demonstration that KOH plays a crucial role in the formation of particles with good sphericity and dense structures. In addition, to obtain an in-depth understanding of the particle formation of carbon particles, a possible mechanism is also investigated in this article. The resulting spherical carbon particles exhibited dense structures with a specific surface area (1233 m²g^-1) and tap density (1.46 g cm^-3) superior to those of irregular shape carbon particles.