Ion Diffusion-Reaction Property In Porous Film Electrodes For Electrochemically Controlled Ion Separation Process

Electrochemically controlled ion separation (ECIS) is a separation technology as an alternative to conventional ion exchange for removing metal ions from wastewater. In ECIS, Ion loading and unloading can be easily controlled by modulating the redox states of ion exchange thin films, which formed on conductive substrates to separate ions from mixed solutions and regenerate the matrix. Because the main driving force is the electrode potential in the ECIS process, chemical regeneration of the ion exchange substrate is not necessary. Secondary waste created by chemical regenerants is eliminated. So it makes this new ion exchange technology more environmentally benign.For the purpose of improving ion exchange capacity of Nickel Hexacyanoferrate (NiHCF) film, the three-dimensional electrode, which has superior specific surface area becomes the first selected. In porous ECIS process, film electrode reaction associated with ion diffusion is occurring simultaneously. And it is a complex charge transfer - reaction process. Ion diffusion in porous film electrode has an important effect on the efficiency of separation of this novel ion separation technology.Multi-row graphite core (MRGC) substrate has simple structure, been easy to apply voltage and amplification easily. In this paper, NiHCF thin films were synthesized on graphite core substrates by cathodically deposition, and then they were assembled as MRGC film electrodes. The cyclic voltammetry (CV) of the MGRC film electrodes was determined in 1 mol·L-1 KNO3 solution. The separation (ΔEp) between the anodic and cathodic peak potentials in the CV curves of MRGC film electrodes with different thickness and spacing was investigated, and the change of CV curves at different scan rates was also measured. Experimental results show that, the excursion of the peak potential of CV curves is caused by ion diffusion in three-dimensional MRGC film electrodes;ΔEp decreases with the increase of spacing between graphite cores, because of the decrease of the ion diffusion resistance in electrode, but increases with the thickness of graphite matrix. With the increase of the scan rate,ΔEp is also increased because of the inner diffusion in the film electrodes. Therefore, the separation degree between the anodic and cathodic peak potentials in the CV curves can be used to analyze the ion diffusion in three dimensional film electrodes.The active surface area and catalytic layer thickness of porous electrode are not only important structure parameters that used to characterize catalytic performance of electrode, but also engineering parameters, which used to establish electrode reaction macrokinetics model, to design catalyst or reactor. In this study, a new method for measuring the active area of porous three-dimensional electrode or porous three-dimensional film electrode using cyclic voltammetry (CV) is first proposed. And it was used to investigate CV curves at different scan rates for porous three-dimensional electrode in aqueous K3Fe(CN)6 solution. Then Nickel Hexacyanoferrate (NiHCF) thin films were synthesized on this porous three-dimensional substrate by cathodically deposition. Cyclic voltammograms of the porous NiHCF film electrode at various potential scan rates were observed in the 1 mol·L-1 KNO3 solution. Experimental results show that, the active area of porous three-dimensional film electrode can be characterized by CV behavior for porous electrode in K3Fe(CN)6 solution, porous film electrode in KNO3 solution, and the dependence of the peak current on the square root of the scan rate. Combining with chronocoulometry could obtain the active volume of NiHCF film, and then the average thickness of film can also be calculated.In ECIS process, ions loading into or unloading from the porous microstructure of the film electrode were drived by concentration and potential. Thus, a larger separation (ΔEp) between the anodic and the cathodic peak potentials in the CV are attributed to diffusion limitations inside the porous matrix during the ion-exchange process. According to the property of ion diffusion inside the porous film electrode and effects of feature sizes, combining with theories of chemical reaction engineering and electrode kinetics, we have established ion transfer - reaction theoretical model for ECIS peocess of porous film electrode. And it provides theoretical foundation for designing electrode sizes and industrial application.