Surface Modification Of Carbon Materials For Capacitive Deionization

The demand for freshwater is increasing at an unprecedented rate with the rapid growth of population and economy while deforestation,water contamination,etc.,greatly constrains freshwater supply.Consequently,the imbalance between the demand and supply of freshwater is becoming more and more prominent.Considering that saline water is abundant on the earth,increasing freshwater supply through desalination technologies has become the most effective way to resolve the conflict.Capacitive deionization(CDI)is an emerging desalination technology based on electrosorption and has many advantages such as low pressure,low cell voltage and ambient operation,environmental friendly and highly energy efficienty,etc.Surface properties and structure of CDI electrodes greatly affect the desalination performance.This dissertation aims to improve the salt adsorption capacity(SAC),charge efficient and cyclic stability,and reduce the energy consumption of CDI by tuning the surface charges of electrodes,preparing new electrode materials and combining new operational modes.The main contents of the dissertation are as follows:(1)An asymmetric CDI cell(N-CDI)with pristine activated carbon fiber(ACF)as the anode and nitric acid treated activated carbon fiber(ACF-HNO3)as the cathode was assembled.Nitric acid treatment introduced oxygen containing functional groups onto the surface of ACF and positively shifted its potential of zero charge(Epzc),which endowed ACF-HNO3 with good cation selectivity.Therefore,ACF-HNO3 can effectively avoid co-ion expulsion when it is used as the cathode of CDI.N-CDI exhibited a SAC of 12.8 mg/g at 1.2 V in 500 mg/L NaCl and a highest charge efficiency of 91.1%at 0.6 V,which are much higher than that of the CDI cell consisting of two pieces of pristine ACF.(2)Quaternized poly(4-vinyl pyridine)modified activated carbon(AC-QPVP)and nitric acid treated activated carbon(AC-HNO3)were respectively prepared.The obtained AC-QPVP and AC-HNO3 electrodes were assembled into an asymmetric CDI cell(Q-N).As there are a large quantity of positively and negatively charged functional groups on the surface of AC-QPVP and AC-HNO3 electrodes,respectively,the operating voltage window of Q-N under inverted CDI(i-CDI)operation can reach 1.4 V.Compared with the CDI cell composed of two AC electrodes(AC-AC),Q-N exhibited comparable SAC but much better cyclic stability.For extended voltage CDI operation(eV-CDI),the operating voltage window of Q-N can reach 2.4 V and its SAC can be as high as 20.6 mg/g at the cell voltage of 1.2/-1.2 V.(3)Pesuodocapacitive MnO2 was synthesized and the MnO2 electrode was assembled with an AC-QPVP electrode into a membrane-free hybrid CDI cell(MnO2/AC-QPVP),which exhibited inverted performance.MnO2/AC-QPVP exhibited an operating voltage window of 1.4 V and a SAC of 14.9 mg/g at 1.4/0 V in 500 mg/L NaCl.As it avoided the use of carbon anodes and the consequent irreversible carbon oxidation,MnO2/AC-QPVP exhibited much higher cyclic stability than AC-AC.After 350 adsorption-desorption cycles,the SAC retention rate of MnO2/AC-QPVP can be as high as 95.4%.(4)Porous carbon nanosheets with ultrahigh specific surface area were prepared by the direct carbonization of soluable starch followed by KOH activation for the electrode material of CDI.CDI was combined with commercial solar cells constituting a hybrid desalination system.The desalination performance,energy consumption and energy recovery of the system were systematically investigated.The ultrahigh specific surface area of PCNSs provided sufficient sites for ion adsorption while the nanosheet structure is in favour of fast ion transport,which leads to a high SAC of 15.6 mg/g at～1.1 V in 500 mg/L NaCl.Moreover,the hybrid system exhibited a minimum energy consumption of 96 kJ/mol and a maximum energy recovery of 55.6%.