Conductive Polyaniline Preparation And Electrochemical Properties Of Polyaniline Composite Anode Materials

The anode of lead-based alloy is always used in industrial production of zinc owing to high concentration of H2SO4 in the electrolyte. However, there are several problems on the lead-based anode such as high oxygen evolution overpotential which can increase the energy consumption about 30% of the total energy consumption, low corrosion resistance and decrease of zinc quality due to lead dissolution in the electrolyte. Although titanium-based electro-catalytic coating anode can effectively reduce the oxygen evolution potential, it still cannot replace the lead-based anode due to its short life and high cost. Therefore, it still is an important subject to develop new energy-saving anode in the zinc hydrometallurgy.The concept of polyaniline (PANI)-based anode was put forward by the author based on the knowledge of organic chemistry, materials, non-ferrous metal and electrochemistry. PANI-based anode can reduce the anodic overvoltage, lower the anode corrosion rate and improve the product Zn quality while maintaining the advantages of Pb-Ag alloy anode. In this paper, the equipment and technics of PANI and PANI composite materials were developed, the polymerizing mechanism, conducting mechanism and doping mechanism of PANI and compositing mechanism of PANI/TiO2 and PANI/WC composite materials were studied systematically through exploring their synthesis conditions, and the optimal preparation condition under which the resultant PANI/WC had the highest conductivity was obtained. The equipment and technics of PANI-based anodes were developed, the electrochemical properties and anti-oxidation nature of PANI-base anodes were evaluated systematically。The cell voltage, current efficiency, anode life and zinc quality were discussed using PANI-based anodes in ZnSO4-Mn2+-H2SO4 system. Based on that, further with the industrial Pb-Ag (1.0%) compared with PANI-based anode.The microstructure, crystal phase, morphology, oxidation degree, doping level surface composition, particles size and thermal stability of synthesized materials were characterized by means of FTIR,UV-vis, Raman, XRD, SEM, TEM,XPS,EDS,laser grain size analysis and thermal analysis and so on. The electrochemical properties of anodes had also been studied by linear sweep voltammetry, A.C. impedance, Tafel plot and cyclic voltammetry. The main results are as following: (1) The equipment and technology of conductive PANI and PANI/composite materials were designed and developed. The forming process of PANI-based anodes was established and PANI-based anodes with dimension of 70mm×50mm×5mm are prepared.(2) Aniline polymerization was head-to-tail structure. Even intermediate products were higher oxidation states, and then turned to be emeraldine base gradually. Quinine-imine (-N=) was priorly protonated in doping reaction process. Pronton acid was ionized form H+, and H+ transfer in polyaniline molecule chain, and then the positive charge was distributed periodically along the molecular chain according to the inner-chain charge transfer. After protonated reaction, valence band of the doping segment polyaniline appeared hole (p model doping). In conductive polyaniline molecule, no matter among the chains or among the molecule, variable-range hopping and tunneling conductive existed at the same time and competed with each other.(3) WC or TiO2 particles and PANI molecular chains were not simply blended or mixed up, and a strong interaction existed at the interface of WC or TiO2 and PANI. Addition of TiO2 or WC could improve the conductivity composites. The environment and thermal stability of the composites were higher than that of the pure PANI. At a high temperature oxidation, the main structure of materials didn't change, so they displayed a better anti-oxidation nature. Anyway, the anti-oxidation nature of PANI/WC anodes was best.(4) In the PANI/WC composites, chemical bonds were formed the WC particles and the PANI. The conductivity of the PANI/WC composites was the collaboration effect of the electronic conduction of the PANI and the conduction of free electrons. While in the PANI/TiO2 composites, the adjacent anion produced by the radical cation on the PANI molecule chain were replaced the 0 ions in the TiO2, its conductivity was the result of the participation of the hole and electron by the form of pn junction in the conduction process。(5) The electrochemical behavior of PANI-based anodes was studied systematically and electrochemical characteristics of PANI-based anodes were revealed. Results showed that the corrosion resistance of PANI-based anodes was best in sulfuric acid system and was worst in hydrochloric acid system. In sulfuric acid system, the corrosion resistance and catalytic activity of PANI/WC were best. In a word, PANI/WC anode has a good application prospects in sulfuric electrowinning zinc in the industry.(6) The PANI-based anodes with dimension of 70mm×50mm×5mm were tested in simulation electrolysis experiment and the anodes were found to perform well in industrial electrolysis conditions. The results showed that the anodic potential was decreased about 200-475mV at the current density of 500A/m2 compared with the Pb-Ag(1%) anode. The specific energy consumption was smaller than that of Pb-Ag (1%) anode and the lower material costs. Zinc deposits obtained had not Pb pollution. For the most prominent PANI/WC anode, the cell voltage was about 2.87V, current efficiency was about 92.4%, energy savings of electrowinning 1 ton zinc was about 16.4%. The current efficiency for PANI-based anodes was almost same as that of Pb-Ag (1%) anode which was 91%-93% while the PANI-based anodes were beneficial for reducing anode slime because the phenomenon of Mn2+ dilution was less obvious.(7) In order to illustrate effect of current density on PANI/WC anode, cell potential, current efficiency and the quality of cathode zinc were studied systematically at different current densities. Current density had great effect on the orientation of zinc crystals in electrowinning zinc. When the current density was 250A/m2, the zinc was superior to develop on the crystal face of (101), while to develop on the crystal face of (002) when the current density turned to 2000A/m. At the same time, current density also had effect on the surface morphology of deposited zin.(8) In the process of electrolysis, the PANI covered on the surface of the WC was peeled off gradually, which resulted in the decline of the conductivity of the PANI/WC anode. In addition, the cracks and even the crevices were emerged on the surface of the PANI/WC anode, appeared gradually on anodes made the cutting-out of the entire electrode conduct system, and finally lost conductivity.