Study On Mechanism Of Defects Controlling Capacitive Desalination Of Carbon-based Materials

As the development of China has entered a new stage,the strict requirements have been put forward to solve problems such as water pollution,contradiction between supply and demand of water resources and shortages,for the purpose of achieving high-quality and sustainable development of country’s economy and society.China has abundant seawater and brackish water resources.Using desalination technology to convert seawater and brackish water into freshwater is an effective means to solve the water crisis.Capacitive deionization(CDI),as an important water treatment technology,has drawn particular attention due to its advantages of simple operation,low cost,lower energy input,no secondary pollution,and environmentally friendly features.However,in the current research,there is still a lack of efficient carbon-based electrode materials.And the defect engineering has been blindly demonstrated to modification of carbon-based materials in pursuit of high-level desalination performance,ignoring the effect mechanism of defect sites in carbon materials in capacitive deionization technology,and the internal mechanism has not been fully understood.For CDI,based on carbon-based material electrodes,this study explores the regulation mechanism of carbon-based defects on electrode desalination performance from a microscopic perspective,providing the new ideas for enhancing CDI performance.The content of this work is to reveal the mechanism of different defects of carbons in CDI technology,and to achieve the optimal desalination performance,rate capability and charge utilization efficiency with the most reasonable design of carbon electrodes.Specifically,the influence and mechanism of intrinsic defects,external defects of Co-N sites and the interaction between intrinsic and external defects on the performance of CDI,as well as a thorough discussion on how to realize the novel preparation of fully exposed defect-rich carbon-based materials suitable for high-performance CDI applications.The main conclusions obtained in this study are as follows:(1)Carbon-based intrinsic defects have an important impact on the capacitance desalination performance.Different degrees of intrinsic defect density have different storage activities for salt ions,resulting in differences in capacitance and desalination performance.In this work,porous carbons with different degrees of intrinsic defects were first designed via an effective dual-templating approach.The experimental results demonstrated that the abundant intrinsic carbon defects play crucial roles in the salt-adsorption capacity and rate capability,and can enable an exceptional electrical double-layer capacitance and facilitate the adsorption behavior of ions.Additionally,from density functional theory simulation results,we observed that abundant intrinsic defects in carbon can greatly promote the charge density redistribution,thereby enhancing the ion-adsorption ability.This work presents deep insights into defective carbon-based materials for further understanding the effects of intrinsic defects on the capacitive desalination performance.(2)The atomically dispersed Co-N active sites of carbon-based external defects have a positive effect on the capacitance and salt ion adsorption properties of carbon-based materials,and are the source of capacitive activity.Herein,porous carbons with highly efficient atomically dispersed Co-N active sites are designed and prepared by a one-step scalable pyrolysis process from a binary metal-organic framework.The experimental analysis reveals that the presence of atomically dispersed Co-N bonds as effective active sites is confirmed to play a crucial role in the CDI performance,as they are beneficial for the ion adsorption behavior and maximize the utilization of the porous structure.Furthermore,theoretical simulations demonstrate that the highly efficient Co-N moieties as the active centers can remarkably facilitate charge density redistribution,thereby improving the intrinsic affinity of ions and thus fundamentally enhancing the CDI performance.This work affords a deep insight for further understanding the critical role of Co-N sites in carbon-based CDI performance.(3)The interaction between heteroatom-doped external defects and carbon-based intrinsic defects has a positive effect on the improvement of the capacitance and salt ion adsorption performance of carbon-based materials.Herein,a series of porous carbons with different degrees of dopants and vacancy defects have been precisely designed and prepared from the pyrolysis of resorcinol formaldehyde(RF)-silicate composites to further explore their capacitive properties.Density functional theory simulations combined with the electrochemical results elucidate that the doping-defect underlying can further optimize the distribution of charge density and structural characteristics to enhance the capability of ion adsorption and diffusion,thereby maximizing pore and heteroatom utilization.For another,N dopants can be transformed into the effective N configurations as active sites to remarkably facilitate the intrinsic affinity of ions,benefiting to the ion-adsorption ability.This work not only provides a deep new insight to the critical role of the doping-defect interplay on carbon-based CDI applications,but also will inspire further work to reasonably design of carbons for prominent CDI performance.(4)In this work,we developed a sustainable molten salt-assisted acid exfoliation strategy for the production of highly accessible defect-rich carbon nanosheets with hierarchical porosity.Notably,the proposed method has successfully overcome the necessary alkali and template use,avoiding environmental pollution and energy consumption.The acetic acid can effectively exfoliate the aggregative polymer to the apparent 2D layered structure by preferentially hydrolyzing lotus and break numerous C-O groups.The subsequent molten salt-assisted pyrolysis can further crush the carbon framework,resulting in ultrathin nanosheets with abundant edges and porous defects.This favorable ultrathin layered and hierarchical structure guarantees accessible rich-defect features,which can not only benefit ion storage,but also provide sufficient diffusion pathways for ion penetration,contributing to excellent performance for CDI.Additionally,the capacitive contribution analysis and density functional theory also clearly demonstrate that the ultrathin 2D structure can remarkably expose more defect sites and enhance their accessibility.This work not only supplies an efficient and sustainable strategy to improve the ion-adsorption capability of carbons,but also realizes the green recycling of biomass waste in environmental remediation.