Controllable Modification Of Carbon Nanotubes And Its Performance In Capacitive Deionization

The capacitive deionization is an efficient,energy-saving,economical and promising desalination technology.The electrode materials are the core of this technology.Chemical modification of materials' surface to increase its surface hydrophilicity,electrochemical performance and desalination performance has attracted extensive attention.In this paper,the pristine multi-walled carbon nanotubes(p-MWCNTs)were oxidized by single acid(HNO₃)and mixed acid(1:1 HNO₃: H₂SO₄)to remove impurities.And the oxidation effects of a1-MWCNTs and a2-MWCNTs were analyzed.The results showed that the content of Si,N and Fe in acidified MWCNTs decreased,while the impurity in a2-MWCNTs decreased more,indicating that acid can effectively remove impurities,and the purification effect of mixed acid was better.Secondly,the quaternary ammonium polyelectrolyte(QAP)was grafted onto the surface of the mixed acidified MWCNTs by chemical modification using 1,4-butanediol diglycidyl ether(BDDE)as a monomer and methylamine(MA)as a modification reagent.The effects of QAP on the hydrophilicity and electrochemical properties of MWCNTs materials were investigated.And the saturation adsorption performance of the electrode pair assembled with p-MWCNTs as the negative electrode and the quaternized carbon nanotubes with one layer(Q-MWCNTs-1)as the positive electrode was compared with that of the cell with two pMWCNTs electrodes.The results showed that the contact angle of Q-MWCNTs-1 electrode was dramatically reduced from 111.9° to 60.4°,indicating that the hydrophilicity of the QMWCNTs-1 electrode was greatly improved.The specific capacitance of the Q-MWCNTs-1 electrode was 1.70 times that of the p-MWCNTs electrode(37.27 F/g).In addition,the saturated adsorption amount of Cell Q1-C was 22.33 mg/g,which was 2.60 times that of Cell C-C(8.59 mg/g).This indicated that QAP was successfully grafted on the surface of MWCNTs,and greatly improved the electrode capacitance of Q-MWCNTs-1 and the desalination performance of the Cell Q1-C module.This new modification method provides a new approach and method for improving the hydrophilicity of the MWCNTs electrode surface and its electrochemical performance and desalination performance.The 1,2 and 3 layers of QAP were grafted onto the surface of the mixed acidified MWCNTs by cyclic operation of the epoxy-amine reaction between the BDDE(monomer)and MA reagent.The effects of QAP layers on the electrochemical and the desalination performance of MWCNTs were investigated.The results showed that the specific capacitances of Q-MWCNTs-1,Q-MWCNTs-2 and Q-MWCNTs-3 electrodes were 1.71,2.30 and 2.21 times of that of the p-MWCNTs electrode(37.27 F/g),respectively.In addition,the saturated adsorption capacities of the electrode pairs Cell Q1-C,Cell Q2-C and Cell Q3-C were 2.60,3.37 and 3.15 times of that of the Cell C-C(8.59 mg/g),respectively.This indicated that Q-MWCNTs-2 was most advantageous for improving the electrode capacitance performance and desalination performance.After modification,the different numbers of QAP layers are beneficial to improve the CDI performance of MWCNTs.In general,when the two layers of QAP are grafted,Q-MWCNTs-2 has the best effect on the electrochemical performance and desalination performance of the electrode.