Structural Regulation Of Prussian Blue Analogues For Rocking-chair Capacitive Deionization

In the 21st century,when the world’s population is growing and socio-economic development is increasing,the pollution of the natural environment is intensifying,especially the pollution of water resources.At the same time,the earth’s freshwater reserves are limited,with some glaciers and deep groundwater difficult to extract.Therefore,the scarcity of fresh water resources has become a major issue that hinders the survival and development of human society.In order to improve the status of freshwater resources,desalination technology has attracted much attention,but the traditional desalination technology,such as multi-stage flash distillation,reverse osmosis,electrodialysis,etc.,generally have high cost,high energy consumption and secondary pollution and other disadvantages.Capacitive deionization（CDI）,a new desalination technology that has emerged in recent years,has received widespread attention from researchers because of its unique advantages such as energy efficiency and green environmental protection.As the main factor affecting the desalination performance of CDI technology,the electrode material is the most important research priority.However,it is difficult to achieve high desalination capacity and long cycle life with the carbon-based materials used in traditional CDI,and there remains an urgent need to develop high performance CDI electrode materials to meet actual production applications.Since 2014,ion-intercalation type faradaic electrode materials have been widely used as sodium ion adsorption electrodes in CDI technology,demonstrating extremely superior desalination capability by virtue of their excellent ion storage capacity.Prussian blue analogues（PBAs）has been widely used as a high-performance ion intercalation material in the field of CDI.Because faradaic electrode materials are generally used as negative electrodes to realize the embedding and de-embedding of sodium ions,they are often combined with carbon materials to build hybrid capacitive deionization（HCDI）device structures.The difference in capacity between positive and negative electrodes makes it difficult to fully exploit the performance of faradaic electrode materials.In this paper,we investigated the CDI system based on Prussian blue,and enhance its desalination capacity and cycling stability by doping metal ion,compounding carbon and combining with rocking-chair capacitive deionization（RCDI）device structure.The main research of this paper is as follows:1.In response to the capacity mismatching between positive and negative electrodes in the HCDI device structure,we synthesized a binary cobalt-iron PBA（Co Fe-PBA）by a simple co-precipitation method and applied it as an electrode material in RCDI.As a faradaic electrode material,Co Fe-PBA exhibits a high specific capacitance of 256 F g^-1 in electrochemical tests,which is superior to those of conventional carbon materials.In addition,the Co Fe-PBA-based RCDI system exhibits excellent desalination performance(maximum desalination capacity of 117.3 mg g^-1and maximum desalination rate of 33.6 mg g^-1 min^-1)by the desalination test.However,the long cycle life of this material is not ideal due to the repeated embedding and de-embedding of sodium ions during the cycling process,which causes structural collapse of the Prussian blue material and limits its further practical application.2.In order to effectively improve the cyclic stability of Co Fe-PBA,we synthesized a nickel-cobalt-iron PBA（Ni Co Fe-PBA）by doping metal ion,where the inactive Ni replaces part of the Co atoms in the original crystal structure.By optimizing the Ni:Co ratio,a specific capacitance of 267 F g^-1 is obtained for Ni Co Fe-PBA.In addition,the Ni Co Fe-PBA-based RCDI system manifests a maximum desalination capacity of 131.4 mg g^-1 and a maximum desalination rate of 27.6 mg g^-1 min^-1.Meanwhile,its cycle life is greatly improved compared with that of Co Fe-PBA due to the introduction of inactive Ni,which effectively mitigates the structural changes of the material during cycling.After desalination cycle test for 40 cycles,89.6%of the initial capacity is still maintained,which is much higher than that of Co Fe-PBA（68%）.However,the Prussian blue material generally suffers from poor electrical conductivity,which affects its desalination performance.3.To further enhance the desalination capacity,Ni Co Fe-PBA@CNT composites with different carbon nanotube（CNT）contents were successfully obtained by introducing CNT with one-dimensional structure and compounding Ni Co Fe-PBA.The addition of CNT not only shortens the electron transfer path and enhances the electrical conductivity of the Ni Co Fe-PBA.Moreover,the three-dimensional network structure of the composite also alleviates the volume change during cycling.By optimizing the content of CNT,the specific capacitance reaches 327.4 F g^-1,much higher than that of Ni Co Fe-PBA.In terms of desalination performance,the composite exhibits a high maximum desalination capacity of 140.3 mg g^-1 and a maximum desalination rate of30.7 mg g^-1 min^-1,which shows a further improvement over Ni Co Fe-PBA.Most importantly,the capacity retention arrives at 98.6%after 40 cycles.