Study of Porous Carbon-based Electrode Materials for Capacitive Deionization and Energy Storage

Author:Kong Wei Qing

Supervisor:hu jia wen


Degree Year:2018





Currently,water scarcity is a serious global problem.Although global water supplies exceed 1.38 billion km3,freshwater comprises only 2.5%of this.Furthermore,less than 1.2%of total freshwater is available for human needs because a significant proportion is inaccessible,locked away as ice and underground water.The demand for freshwater will increase with population growth,industrial development,and increasingcontamination.Seawaterdesalinationtechniquehasreceived ever-increasing attention because it allows sustainable obtaining of clean water from seawater or brackish water.Amongst diverse desalination techniques,capacitive deionization(CDI),which is more environmentally friendly,energy efficient,and cost effective in comparison with other conventional desalination techniques,has drawn great attention from the researchers worldwide in recent years.CDI relies on the use of electric double-layer(EDL)of electrodes for desalination:When saline water flows over two porous electrodes applied with a voltage(typically less than 1.2 V),cation ions and anion ions are,respectively,adsorbed by the negatively charged electrode and positively charged electrode,forming an EDL at the interface.After desalination,the electrode can be easily regenerated by reversing the applied voltage or removing the voltage.Moreover,the electrode regeneration process is a discharging process so that the energy released can be recovered using,for example,an electrochemical supercapacitor,and avoiding wasting energy.According to the principle of the CDI process,the electrode material is crucial to the CDI performance.Ideally,the CDI electrode materials should possess high specific surface area,suitably sized pores,high conductivity,and good wettability.Moreover,their preparation should be low-cost,simple,and scalable.To date,activated carbon,carbon aerogels,carbon nanotubes,mesoporous carbon,and carbon nanofibers have been used as the CDI electrode materials.Owing to the shortcomings such as low specific surface area and non-ideal porous structure,their specific capacitances,however,are relatively low,which largely limits their electrosorption capacities.On constrast,graphene hydrogel and mesoporous carbon show great potential for practical CDI application and because of their higher specific surface area and plenty of micropore structure,which delivers enhanced specific capacitance.This thesis prepared a series of carbon materials such as graphene hydrogel and hierarchically mesoporous cabon bowls,and systematically investigated their CDI and energy storage behavours.The main contents and results in this thesis are listed as follows:1.Design and construction of a three-dimensional(3D)holey graphene hydrogel with in-plane pores(HGH)for high performance CDI.The HGH was prepared by a simple one-step hydrothermal reaction using graphene sheets with in-plane pores as the precursor.In this material,the abundant in-plane pores offer efficient ion transport pathways,its highly interconnected network of graphene sheets provides efficient electron transport pathways,and its 3D hierarchical porous structure can provide a large specific surface area for the adsorption and storage of ions.Meanwhile,HGH can be directly pressed on the current collector,forming CDI electrode without the need of any electrode binder,and thereby not deteriorating the excellent conductivity of the HGH.With these unique chateristics,the optimized HGH can delivers a specific capacitance of 358.4 F·g-1in 6 M KOH solution and of 148 F·g-1in 0.5 M NaCl solution.As a result of its high capacitance,the HGH exhibits a desalination capacity of as high as 26.8 mg·g-1(applied potential:1.2 V;initial NaCl concentration:5,000mg·L-1).2.Synthis of bowl-like porous carbon materials with rough surface(PCB)for high performance CDI.The PCB was fabricated by combing a hard templating process and a H2O2 etching strategy.This material contains abundant mesopores,offering efficient ion transport pathway.The rough surface and bowl-like structure can provide a large specific surface area for the adsorption and storage of ions.With these characteristics,PCB exhibits a large specific surface area of 1119.5 m2·g-1,a high specific capacitance of 144.1 F·g-1(at 5 mV·s-1)and excellent cycling stability.Furthermore,the desalination capacity and charge efficiency are,respectively,as high as 6.5 mg·g-11 and12.6%(applied potential:1.2 V;initial NaCl concentration:50 mg·L-1;desalination time:60 min).3.Modified commercial carbon paper current collent for CDI with enhanced areal electrosorption capacity.Current collector constitutes a large gravimetric portion of the capacitivedeionization(CDI)electrode.However,itusuallyhaslimited electrosorption capacity,thereby greatly plaguing the overall electrosorption capacity of the current CDI device.In the third capter,this thesis reports the use of villiform carbon fiber paper(v-CFP)as current collector for CDI device with high areal electrosorption capacity.The v-CFP was prepared by thermal annealing and subsequent O2 plasma etching of the commercial,pristine carbon fiber paper(p-CFP).With greatly improved surface area and extreme hydrophilicity,the v-CFP itself showed an areal capacitance of as high as 0.70 F·cm-22 in 0.5 M NaCl solution.Consequently,its areal electrosorption capacity reaches 122 and 400 mg·m-22 when feeding a NaCl solution with initial concentration of 50 and 3500 mg·L-1,respectively.When loading with additional electrode material(AEM),e.g.,graphene hydrogel,activated carbon,carbon nanotubes,and porous carbon spheres,the v-CFP/AEM electrode exhibited a further improved areal electrosorption capacity,generally outperforming the corresponding p-CFP/AEM electrode by more than 2 times.4.Construction of a 3D holey N-and S-codoped graphene hydrogel with abundant in-plane pores(NS-HGH)and its application in supercapacitors.Its highly interconnected network structure(with intimate graphene sheet-sheet contacts)and abundant in-plane pores offers efficient pathways both for electron transport and for ion diffusion within NS-HGH.The N-and S-codoping further improves its porousity and conductivity.With these characteristics,NS-HGH exhibits a large specific surface area of 317 m2·g-1,high specific capacitance of 320.0 F·g-11 at 1 A·g-1,and excellent cycling stability.Furthermore,the supercapacitors made from the NS-HGH electrodes delivers an impressive energy density of 24.7 Wh·kg-1at 1 A·g-1.By performing first-principle simulations,this thesis further show that the density of states(DOS)near the Fermi energy level of the NS-HGH can be largely increased as a result of abundant polarized sites produced by heteroatom doping,thereby offering additional capacitance to the NS-HGH.These results suggest that NS-HGH has great promising for practical application in supercapacitors.