Structural Stability Modulation And Application Of Bi@C Composites For Selective Removal Of Cl~- In EDI

Chlorinated wastewater is a common industrial wastewater,which is highly corrosive,widely sourced,high in volume,and complex in composition,posing a great threat to the ecological environment,human health,and production safety.The traditional treatment methods such as chemical precipitation and reverse osmosis generate a large amount of hazardous waste and mixed salts,resulting in problems such as secondary pollution and loss of resources.Electrochemical deionization（EDI）technology based on Bi-based electrodes can achieve selective removal of Cl^-from the solution system,providing a viable way to fundamentally solve the pollution of chlorinated wastewater and realise resource utilization.However,Bi-based electrodes are subject to phase transition-induced volume expansion（158%）during the EDI cycles,which leads to structural instability of the electrode and affects its performance.Based on the destabilisation characteristics of Bi nanoparticles during the electrochemical transformation process,this paper proposes the idea of carbon physical confinement and synergistic alloying reinforcement,and investigates the effects and mechanisms of in situ confinement encapsulation of carbon nanocages,as well as Tibonding interface reinforcement,Ag intercalation synergistic reinforcement,and double-layer carbon encapsulation reinforcement strategies on the structural stability of Bi@C materials,and constructs a series of highly stable Bi@C electrode materials,solving the problem that traditional Bi-based electrode materials are prone to structural destabilization and difficult to be applied in practice.This study provides a new theoretical basis and technical method for the resourceful disposal of chlorinated wastewater,as well as a new means and new ideas for the preparation of stable structures for metal/carbon composites.The main research contents and results are as follows:（1）The idea of in situ confinement encapsulation of carbon nanocages was proposed,and Bi@NC composites with N-doped carbon nanocage were obtained by one-step pyrolysis using commercially available ammonium bismuth citrate as precursor.Based on the complexation effect of organic ligands,uniform dispersion of Bi nanoparticles in the carbon matrix was induced.The characteristics of the bismuth carbon content,Bi nanoparticle distribution,and pore structure in the Bi@NC composites were determined.Cl^-adsorption capacities of up to 96.3 mg g^-1 at 1.4 V and initial Cl^-concentration of 500 mg L^-1 were achieved and the capacity retention was 86.5%after 100 cycles,which was much better than that of pure Bi（32.8%）.The unique core-shell structure formed in situ provides an excellent conductive network and void space,which can effectively buffer the volume change of Bi nanoparticles during the phase transition.Combined with density functional theory calculations,it is found that Bi³⁺in the precursor is converted to Bi₂O₃ in situ starting at around 300°C during the pyrolysis process,and subsequently Bi₂O₃ is reduced to metallic Bi above 450°C,while the organic ligands in it are converted to the carbon substrate,and the conversion of the Bi centre is preferred over that of the organic ligands.（2）Based on the idea of metal-carbon alloying bonding for enhanced carbon confinement encapsulation,a class of Bi-Tialloyed carbon-encapsulated Bi-Ti@C composites was prepared using Bi-Tibimetallic organic framework as the precursor.At 1.2 V and initial Cl^-concentration of 1000 mg L^-1,the Bi-Ti@C electrode exhibited a Cl^-adsorption capacity of 106.5 mg g^-1 and a capacity retention of 80%after 100 cycles,which was better than the unalloyed Bi@C-TiO_x electrode（46%）.The presence of Tiand carbon nanocages enhances the Bi Ti-C interfacial bonding and provides void space,which can effectively buffer the volume change during the phase transition of Bi nanoparticles.（3）The idea of Ag intercalation synergistically enhancing the carbon confinement encapsulation was proposed,and an N-doped porous carbon Bi Ag@NC composite was prepared by sol-gel method with in-situ pyrolysis.The obtained bimetallic-hydrogel network derived a unique graphene-like porous structure,and the ultrafine Bi-Ag nanoparticles were highly uniformly dispersed in the porous carbon matrix,which was beneficial to enhance the structural stability of Bi-Ag nanoparticles as well as the ion accessibility during the electrosorption process.During the EDI treatment,the best Cl^-adsorption capacity of 111.6 mg g^-1 was obtained at1.6 V and initial Cl^-concentration of 500 mg L^-1 for the Bi Ag@NC electrode.The capacity retention was up to 100%over 100 cycles for the Bi Ag@NC electrode,while the Bi@NC and Ag@NC electrodes containing only single metal nanoparticles decayed by 49.5%and 18.5%,respectively.（4）The idea of double-layer carbon encapsulation reinforced carbon confinement encapsulation was proposed,and a Bi@NC/G composite with a dual carbon layer encapsulation structure was constructed by graphene two-dimensional flexible cladding.The effect of polymerization time on the structural characteristics and performance of the core-shell Bi@NC composite was investigated,and the stable loading of Bi@NC in the graphene network was achieved.As an EDI Cl-storage electrode,the Bi@NC/G electrode achieved a Cl^-adsorption capacity of 102.2 mg g^-1 at a current density of 20 m A g^-1 and an initial Cl^-concentration of 500 mg L^-1,with a capacity retention of 100%over 100 cycles,demonstrating an efficient and stable Cl^-storage performance in EDI.The performance enhancement effect is mainly derived from the large-scale flexible graphene network coating,which can promote the synergistic interconnection of porous carbon layers.This not only provides an optimized continuous pathway for electron transfer and ion transport,but also better buffer the mechanical stress caused by the volume change of Bi nanoparticles and inhibit Bi nanoparticle exfoliation.（5）The relationship between the structural properties and the performance of Bi@C electrodes was explored;the higher the Bi content,the better the pore distribution,and the larger the specific surface area,the more favourable the ion adsorption and storage;it was confirmed that the carbon confinement encapsulation with Tibonding interface reinforcement,Ag intercalation reinforcement,and double-layer carbon encapsulation reinforcement strategies can effectively enhance the stability of Bi@C electrodes.Compared with other strategies,the graphene-enhanced double carbon layer encapsulated structure exhibits the best electrochemical dechlorination performance.In addition,a synergistic purification-concentration treatment plant with Bi@NC/G electrodes for selective Cl^-removal was built to solve the difficulties of continuous operation of the conventional two-electrode system and to achieve simultaneous operation and efficient linkage of the adsorption and desorption processes.For a high concentration of simulated chlorinated wastewater(1500 mg L^-1 Cl^-),the EDI operates continuously for 30 h.The Cl^-concentration in the purification solution is reduced to 293.8 mg L^-1 and the Cl^-concentration in the concentration solution is 13,467.2 mg L^-1,with an enrichment of up to 900%;the energy consumption is as low as 0.32 k Wh m^-3,which is better than other technologies such as reverse osmosis(1.5～7.5 k Wh m^-3),and the treatment cost of electrode material is only 1.67 Yuan m^-3.Moreover,it can achieve highly efficient selective treatment of chlorinated wastewater and has the potential for industrial application.