Preparation,Characterization And Applications Of High Capacity And Stability NiHCF Films On Graphite Substrate For Electrochemically Controlled Ion Separation

Electrochemically controlled ion separation(ECIS)is an environmentally benign method to separate ions via reversible electrochemical modulation of the matrix charge density.Ion loading and unloading can be easily controlled by modulating the redox states of ion exchange thin films formed on conductive substrates to separate ions from mixed solutions and regenerate the matrix. Since electrochemical rather than chemical potential modulation is the main driving force for exchange,the secondary waste created by chemical regeneration and associated rinse water is eliminated.Thus,ECIS has the potential to replace traditional ion exchange and so has recently garnered coinsiderable interest.A successful procedure for the synthesis of NiHCF within the porous conductive composite material is presented in this work.Ni mesh,graphite particle,ethanol are used for preparing porous graphite substrate,carbon paper/graphite composite substrate,carbon fibre/graphite composite substrate, Ni mesh/graphite substrate.NiHCF was deposited on the substrates by capillary chemical deposition.The optimum process conditions for porous NiHCF film electrodes on graphite substrate are obtained:the diameter of graphite particle is 50-300μm.2.5 ml ethanol and 0.3 g conductive colloid are needed in a gram graphite.The concentration of K₃Fe(CN)₆ and NiSO₄ is 0.3 mol·L^-1.And for porous NiHCF film electrodes on carbon paper/graphite composite substrate, carbon fiber/graphite substrate and Ni mesh/graphite substrate are obtained too: the diameter of graphite particle is 80μm.5 ml ethanol and 0.05g PTFE are needed in a gram graphite.The concentration of K₃Fe(CN)₆ and NiSO₄ is 0.1 mol·L^-1.In 1 mol·L^-1KNO₃ solution cyclic voltammetry(CV)was used to reversibly intercalate and deintercalate K^| from the matrix and to investigate the electroactivity,ion-exchange capacity and the regeneration capability of NiHCF films.The morphology,composition and application of the film were characterized by scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),infrared spectroscopy(IR)and X-ray photoelectron spectroscopy(XPS).Experimental results show that the electrodes have a higher surface area;NiHCF thin film electrodes with carbon paper/graphite substrate have high separation capacity,low diffusion resistance,good stability and regenerability.The film-electrode systems are suitable for electrochemically controlled ion separation(ECIS)process.Nickel hexacyanoferrate(NiHCF)thin films were prepared on graphite substrates by pulse electrodeposition and investigated for use as electrochemically controlled ion separation(ECIS)materials.The pulse waveforms investigated consisted of an applied potential during the on-period and zero current during the off-period.In 1 mol·L^-1KNO₃ solution,cyclic voltammetry(CV)was used to investigate the effects of the pulsed deposition parameters such as pulse potential,duty cycle and deposition time(ton/toff)on the ion-exchange capacity of NiHCF film electrodes.The composition and morphology of the film were characterized by X-ray photoelectron spectroscopy (XPS)and scanning electron microscopy(SEM).The cycle life and regenerability of the film electrodes were also investigated.Experimental results show that the optimum pulse potential,ON/OFF period(ton/toff)and duty cycle are 0.3 V(vs.SCE),300 ms/300 ms and 50%,respectively.The deposited NiHCF thin films are very adherent and uniformly distributed over the surface of the graphite substrate,while the NiHCF thin film electrodes have high ion exchange capacity for K⁺,good stability and regenerability.The electroactive NiHCF thin films were synthesized as electrochemically controlled ion-seperation on the Pt,graphite,three-dimensional graphite substrates by a cathodical deposition.Cyclic Voltammetry were used to determine the diffusion coefficient of K⁺.Chronocoulometry was performed to determine the concentrations(6.64×10^-3mol/cm³)of oxidized and reduced centers of the NiHCF film electrode.According to the maximum electric charge O the total number n_totalof redox centers was calculated to be 0.651×10^-8mol. The ECIS is an electrochemical reversible process controlled by the diffusion process.With cyclic voltammetry a diffusion coefficient of K⁺ was determined. They are D_Pt=1.4×10¹⁰cm² s^-1,D_graphite=1.6×10^-11cm² s^-1,D_3D=2.8×10^-13cm² s^-1. In ECIS,alkaline cations loading and unloading the porous microstructure of the composite electrode with driving force of concentration and potential.Thus, a larger separation(â–³E_p)between the anodic and the cathodic peak potentials in the CV are attributed to diffusion limitations inside the composite matrix during the cation-exchange process.