Enhanced Capacitive Deionization Performance And Mechanism Of Binary Co-doped Carbon Nanomaterials Electrode

The high concentration of salt in industrial wastewater will cause corrosion to pipelines and processing equipment,shorten the service life of equipment,and directly affect the production process and efficiency.Capacitive Deionization（CDI）technology is a low-cost,no secondary pollution and efficient wastewater purification technology,in which the electrode material is one of the core components of CDI technology.Currently,the commonly used carbon electrode materials suffer from poor electronic conductivity,few structural defects and disordered pore structure,resulting in low salt adsorption capacity,slow deionization rate and insufficient cycle stability in the deionization process,limiting the development and application of CDI technology in the field of deionization.Based on the electronegativity difference between heteroatoms with carbon atoms and the synergistic effect of various heteroatoms,a strategy of binary co-doping to control the charge distribution,surface structural defects and pore structure of carbon structures was proposed.The prepared nitrogen-sulfur co-doped carbon nanosheets,cobalt-nitrogen co-doped porous carbon and Co and Co₃O₄ nanoparticles co-incorporating nitrogen-doped carbon nanotubes were used to investigate the electrochemical behavior and deionization performance of these co-doped carbon materials.In addition,the mechanism of the synergistic effect of the binary co-doping on the enhancement of capacitive deionization was also explored.The main research efforts are as follows:（1）Three nitrogen sulfur co-doped porous carbon nanosheets（N,S-CN-x,and x as the pyrolysis temperature）were prepared by self-assembly and pyrolysis using lithium nitrate as nitrogen source and sodium thiosulfate as sulfur source:N,S-CN-500,N,S-CN-600,and N,S-CN-700.Due to the higher electronegativity of nitrogen and sulfur,the formation of pyridine nitrogen,pyrrolic nitrogen,C-S-C and S-O/S=O by nitrogen and sulfur co-doping in the carbon structure can synergistically improve the hydrophilicity,degree of structural defects,and electrical conductivity of the material.The synergistic effect of the high content of pyridine nitrogen,pyrrolic nitrogen,C-S-C and S-O/S=O in N,S co-doped carbon nanosheets promotes the electrochemical performance and capacitive deionization capacities of carbon-based materials compared with undoped,single-doped N or S.As a consequence,the N,S-CN-600 electrode exhibited a salt adsorption performance of 17.73 mg g^-1,which was much higher than that of the undoped and single-doped materials.In addition,the N,S-CN-600electrode also exhibited good reversibility and cyclability over 20consecutive charge-discharge cycles.（2）Three co-doped porous carbons with cobalt and nitrogen were prepared by molten salt-pyrolysis using lithium nitrate as nitrogen source and cobalt nitrate as cobalt source:Co-N/CN-1:20,Co-N/CN-1:30 and Co-N/CN-1:50（1:x is the mass ratio of cobalt nitrate to glucose）.Taking advantage of the easy coordination between Co（Lewis acid site）and N（Lewis base site）in the carbon structure,the co-doping of Co and N not only makes carbon materials rich in pyridine nitrogen and pyrrolic nitrogen active sites,but also forms highly unsaturated Co-N active centres to promote electron transfer,thus improving its electrochemical and capacitive deionization performance.In particular,compared with the single nitrogen-doped samples,the Co-N/CN samples have higher specific capacitance,lower ion/electron transport resistance,and larger deionization capacity.The Co-N/CN-1:20 electrode exhibited a high salt adsorption performance of 51.71 mg g^-1,which was significantly better than that of the monodoped carbon material.Meanwhile,the Co-N/CN-1:20 electrode exhibited no obvious degradation in charge storage over 100charge/discharge cycles.（3）Using urea and cobalt nitrate as nitrogen cobalt sources,respectively,cobalt was uniformly encapsulated in-situ in nitrogen-doped carbon nanotubes by means of high temperature pyrolysis,and different forms of cobalt-nitrogen-doped carbon nanotubes（Co/N-CNTs,Co₃O₄/N-CNTs and Co-Co₃O₄/N-CNTs）were prepared.Notably,the cobalt nitrogen bond as the active site under the enhancement of nano-confinement effect can not only enhance ion-adsorption ability with facilitated electron transfer but also inhibit metal leaching,thus greatly leading to the deionization capacity and stability of the electrodes.Meanwhile,the interaction between multiple cobalt species can facilitate electron transfer and possess more Co-N bonds.The Co-Co₃O₄/N-CNTs electrode exhibited an excellent salt adsorption capacity of 59.03 mg g^-1,which was higher than that of the monomorphic cobalt-nitrogen co-doped material.In addition,the Co-Co₃O₄/N-CNTs electrode maintained 85.6%performance after 20 adsorption/desorption cycles,demonstrating its good cycling stability.（4）Based on the selective permeability property of ion exchange membrane,it was introduced into the CDI device to construct a membrane up capacitive deionization（MCDI）device,and the optimal operating parameters of three material electrodes,N,S-CN-600,Co-N/CN-1:20 and Co-Co₃O₄/N-CNTs,in the dynamic MCDI device were investigated.The results showed that under suitable operating parameters(operating voltage of 1.2 V,p H value of 6.0,influent concentration of 824 mg L^-1,solution temperature of 20℃and electrode spacing of 2.0 mm),the deionization capacity and deionization rate of the M-Co-Co₃O₄/N-CNTs electrode reached 66.58 mg g^-1 and 2.66 mg g^-1 min^-1,respectively.Meanwhile,the kinetic behavior of salt ion adsorption at MCDI electrodes is pseudo-first-order kinetic and intraparticle diffusion modeled,with surface diffusion and intraparticle diffusion as the main ion adsorption rate control steps.Further fitting with a dynamic ion transport kinetic model revealed the salt storage and transport pathways present in the micropores:co-ions are stored in the micropores during ion adsorption.The co-ions were stored in the micropores during adsorption and were depleted as counter-charges during desorption,thereby enhancing the deionization capacity and rate.79 figures,35 tables and 259 references...