Doctor of Philosophy
Intelligent Polymer Research Institute, Department of Chemistry
Almoiqli, Mohammed Suliman, Nanostructured ICP-CNT Electrodes for Capacitive Deionisation and Water Clean Up, Doctor of Philosophy thesis, Intelligent Polymer Research Institute, Department of Chemistry, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4295
The development of electrodes and flow-through cells in a CDI system are considered in this thesis. Novel 3D nanostructured electrodes and improved designs of flow-through cells in a CDI system are described. The main objectives were: (1) to develop a fundamental knowledge and understanding of reticulated vitreous carbon (RVC), conducting polymers such as Poly(3,4-ethylenedioxythiophene) (PEDOT), single-walled carbon nanotubes (SWCNT) and graphene; (2) to use a RVC electrode structure to build 3D PEDOT microstructure electrodes, 3D nanoweb structure SWCNT and 3D nanoweb hierarchical graphene/SWCNT composite electrodes; (3) to explore possible applications of these electrodes in a CDI system; and (4) to study the effect of increasing the amount of materials in terms of unit geometric volume and geometric area on the electrosorption capacity.
PEDOT/RVC composite electrodes with varying amounts of PEDOT loadings were considered for application as novel 3D microstructure electrodes in Chapter 3. PEDOT was successfully deposited by electropolymerization on RVC and used for the first time as materials and electrodes in CDI technology. The aim of this chapter was achieved as demonstrated by the improved performance of the CDI electrode in terms of unit geometric volume and geometric area. The electrosorption capacity in terms of unit geometric volume and geometric area of electrodes increase with increasing amounts of PEDOT in the electrode, and the highest electrosorption capacity obtained was 0.37 mg/cm3 or 0.12 mg/cm2 or 6.52 mg/g of PEDOT in the PEDOT-120/RVC electrode (240 mg coated 4.2 cm3 RVC electrode) at 75 mg/L NaCl solution, at 0.8 V electrode voltage, and 80 ml/min flow-rate. This result is a better desalting performance than carbon materials, and the adsorption/ regeneration of PEDOT/RVC electrodes was facile with high efficiency achieved. The water production by 1m3 of PEDOT-120min/RVC electrode from 75 mg/L NaCl feed solution was 129,176 L/day to produce water less than 1 mg/L NaCl concentration. It has been shown that the capacitance of PEDOT-120min/RVC electrode compared to a bare RVC electrode had increased by a factor of 2230, and the electrochemical properties were ideal.
The successful use of 3D PEDOT/RVC in a CDI system led to the use of a RVC electrode again in Chapter 4 to build huge 3D functionalized SWCNT (a- SWCNT) nanoweb structures by filling the RVC pores using a dip coating method. A unique 3D electrode was constructed and explored as a novel CDI electrode. The electrical voltage of 1.5 V and flow-rate of 50 ml/min were the optimum conditions and they were the key factors that affected the NaCl ion removal performance at the sites of acid treated SWCNT (a-SWCNT). The maximum electrosorption capacity result for 23.58%wt of a-SWCNT/RVC composite electrode (50mg a-SWCNT coated 2.16 cm3 RVC electrode) was 3.23 mg/g of a-SWCNT or 0.08 mg/cm3 at 75 mg/L feed concentration. After that, an improved CDI system was designed to accept solution flowing through the electrodes and its effect on desalination cycle time was studied. It is clear that for one desalination cycle, 42 minutes was required for the flow-between (FB) electrodes configuration and 18 minutes for the flow-through (FT) electrodes configuration. This encouraged efforts to design a new CDI cell with a flow-through electrode system. The electrosorption capacity of all electrodes in new cell was increased and the time required for one desalination cycle decreased as well. For example, the electrosorption capacity for 3.63 %wt a-SWCNT was increased from 8.39 mg/g to 10.40 mg/g and the time of one desalination cycle decreased from 30 min to 10 min using flow feed between (FB) electrodes and flow feed through (FT) electrodes, respectively. This means that electrosorption capacity increased 23.96% and the time required for one desalination cycle decreased around three times. In addition, the effect of distance between electrodes, the electrosorption dynamic and isotherm were studied using new cell. The ion removal characteristics were affected by various distances between electrodes. As the distance increased, the ion removal amount was not affected, but the adsorption time required increased when the distance was increased in all cases. The energy output of the CDI system was affected by an increase in the space between electrodes. It was found that NaCl adsorption obeyed pseudo first -order kinetics rather than pseudo second-order kinetics and that NaCl ions were not adsorbed onto the a-SWCNT surface via chemical interaction. Furthermore, the electrosorption for this electrode obeys both the Langmuir isotherm and the Freundlich isotherm models. This phenomenon suggests that monolayer adsorption was the primary adsorption mechanism during the electrosorption process. The maximum electrosorption capacity result for 23.58 %wt a-SWCNT electrode was 8.89 mg/g at 500 mg/L feed concentration, as compared with a theoretical maximum value of 13.08 mg/g calculated using the Langmuir isotherm model.
The goals of Chapter 5 are to increase the electrosorption capacity of 3D a- SWCNT/RVC electrodes from Chapter 4 and reduce the duration of electrosorption– desorption cycles of Chapter 4 by improving the ease of ions adsorption to and ions desorption from the electrode surfaces. This was achieved by use of composite microwave irradiated graphene oxide (mwGO) with a-SWCNT. The a-SWCNT materials were contained sandwiched between the graphene sheets to build a 3D highly porous architecture inside the electrodes and increase the electrodes conductivity as well as afford rapid ions diffusion. The results led to a conclusion that the best performing electrode, with a specific capacitance of 179.39 F/g was the 9-CNT/mwGO/RVC (ie a-SWCNT:mwGO ratio was 9:1) electrode, which represents a 29 % increase in specific capacitance compared with the a- SWCNT/RVC electrode. This 9-CNT/mwGO/RVC electrode also had very high CV curve stability, maintaining 99% current stability after 200 cycles. Moreover, the time saving of one electrosorption–desorption cycle with the 9-CNT/mwGO/RVC electrode was 27.78 %; compared with the CNT/RVC electrode which required 18 min. In addition, the electrosorption removal of NaCl by the 9-CNT/mwGO/RVC electrode in terms of mass of the electrode (3.82 mg/g) increased 18.27 % compared with that of the CNT/RVC electrode (3.23 mg/g) using 1.5V applied voltage and 50 ml/min flow-rate as the optimum conditions. The full desalination process to produce water of less than 1 mg/L NaCl concentration in the CDI system using 9- CNT/mwGO/RVC composite electrode increased desalinated water production by 67.78% per day compared with the same CDI system using a-SWCNT/RVC composite electrode. The maximum water produced per day is 29,958 L using 1m3 of 9-CNT/mwGO/RVC electrode and the maximum electrosorption capacity result for the same electrode was 10.84 mg/g at 500 mg/L feed concentration, as compared with a theoretical maximum value of 16.59 mg/g calculated using the Langmuir isotherm model. Also, the performance of electrodes adsorptions was evaluated by dynamics study and it was shown to follow the Pseudo-first-order model.