Preparation And Electrochemical Properties Of Conductive Polyaniline Composite Anode Materials

Currently, the electrowinning system in cobalt electrowinning industry is mainly chloride system or sulfuric acid system. The anode materials are mainly graphite anode and titanium-based dimensionally stable anode (DSA) in chloride system, and lead-based alloy anode in sulfuric acid system. The graphite anode is cheap, easy to preparation, but it is short-lived and it has high cell voltage as well as high carbon content in cathode cobalt. As for lead-silver alloy anode, cheapness, easy formability and self-healing are its strengths, but it also has weakness, such as low mechanical strength, high cell voltage. In addition, the cathode cobalt will be affected by lead contamination. Though titanium-based dimensionally stable anodes (DSA) can avoid cathodic product contamination issue, its easy passivation of the substrate and costly coating layer are detrimental to their large-scale industrial applications. So, it has practical urgency and potential applications to find a novel inert anode.In view of the various shortcomings of the traditional anodes, the polyaniline nanofiber was prepared via a template in the present paper. By using of the polyaniline nanofiber, the PVC/MWCNT(20%)-PANI(25%) anode material was synthesized by blending; through thermal decomposition, the Ti/SnO2-Sb2O5-PANI(9%) anode was made. The coating layer was synthesized by SnO2-Sb2O5 and the thermal decomposition product of polyaniline. In this paper, the preparation and electro-catalytic activity of oxygen evolution, chlorine evolution of polyaniline nanofiber, PVC/MWCNT(20%)-PANI(25%) anode and Ti/SnO2-Sb2O5-PANI(9%) anode were systematic investigated. At the same time, we also explored the essential reason of electro-catalytic activity of the new anode materials. In laboratory conditions, we had examined the electrodepositing of cobalt in chloride system experiments with the Ti/SnO2-Sb2O5-PANI(9%) anode, and analyzed their superiority in the cell voltage and the current efficiency. The main results obtained are as follows:1. The preparation and electrochemical properties of conducting polyaniline nanofiber(1) Through chemical oxidation, conducting polyaniline nanofiber was prepared via ethyl cellulose(EC) template, and its optimal preparation conditions were as follows:the concentration of aniline was 4%; the molar ratio of (NH4)2S2O8): aniline was 1.20:1 ～1.30:1; the initial concentration of hydrochloric acid in the reaction was 1 mol/L, the volume ratio Valcohol/Vwater was 0.5, and 1 g EC (dissolving in 100～150ml alcohol). The polymerization temperature was 0～5 ℃ and the reaction time is 6 h. The polyaniline nanofiber had uniform structure, such as diameter was about 20～50 nm, fiber length was 2～5 μm, the conductivity was 13.5 S/cm.(2) Synthesis mechanism of polyaniline nanofiber was:at the effect of static electricity, aniline molecules started polymerization reaction on ethyl cellulose surface, forming polyaniline nanofiber aligned along the ethyl cellulose surface. The polyaniline nanofiber material was stable when the processable temperature was less than 150 ℃. It was confirmed that the conductive way of polyaniline nanofiber was variable range hopping model. Polyaniline nanofiber has high oxygen evolution catalytic activity, and lower chlorine evolution electro-catalytic activity than that of the Ti/RuO2 anode.2. The preparation and electrochemical properties of PVC/MWCNT(20%)-PANI(25%) anode(1) The PVC/MWCNT(20%)-PANI(25%) anode was prepared by blending method. The optimum preparation was the component proportion:PVC (55wt%)、 WMCNT(20wt%)、PANI(25wt%) and the solvent was tetrahydrofuran. The fabrication processes were:the PANI and MWCNT were dispersed fully in tetrahydrofuran and mixed with the PVC solution, then evaporating the solvent and solidified to the anode materials.(2) Environmental pH strongly effected on PVC/MWCNT(20%)-PANI(25%) anode. In the acidic environment (pH<6), PVC/MWCNT(20%)-PANI(25%) anode could operate stably. Polyaniline would occur dedoping reaction at pH>7 and resulting in inactivation of anode. In addition, the anode material had a negative temperature resistance effect, and its main conductive model was variable range hopping.(3) In comparison with Pb-Ag(0.8wt%) anode, PVC/MWCNT(20%)-PANI(25%) anode, at the current density below 720 A · m-2, had better oxygen evolution catalytic activity. At 500 A·m-2 industrial current density, the overpotential of oxygen evolution of PVC/MWCNT(20%)-PANI(25%) anode was 170mV lower than that of Pb-Ag(0.8wt%) anode. When the PVC/MWCNT(20%)-PANI(25%) anode compared with Ti/RuO2 anode in the chloride system, it showed better electro-catalytic activity of chlorine evolution at less than 445 A · m-2 current density.3. The preparation and electrochemical properties of Ti/SnO2-Sb2O5-PANI(9%) anode(1) The Ti/SnO2-Sb2O5-PANI(9%) anode was made by thermal decomposition. The optimum preparation conditions was thermal decomposition at 500 ℃, the polyaniline percentage of the weight of the solid reactant was 9%. The porous coating layer (SnO2-Sb2O5-PANI(9%)) were formed by the nitrogen-doped carbon materials that degraded from polyaniline combined with SnO2-Sb2O5 solid solution. In the pyrolysis processing, the nitrogen in polyaniline and titanium substrate form chemical bonds TiN and TiNxOy, which bonded the electrode coating layer and the substrate closely.(2) In comparison with the traditional anode, Ti/SnO2-Sb2O5-PANI(9%) anode showed a better electro-catalytic activity of chlorine evolution, and its chlorine evolution overpotential was 210mV lower than that of Ti/RuO2 anode at 500 A·m-2 current density. The electro-catalytic activity for oxygen evolution of Ti/SnO2-Sb2O5-PANI(9%) anode was lower than that of Pb-Ag(0.8wt%) anodes. At 500 A·m-2 current density, its oxygen evolution overpotential was 420mV higher than that of Pb-Ag(0.8wt%) anode.4. In the chloride system, at 0.5 A·cm-2 current density, the Ti/SnO2-Sb2O5-PANI (9%) anode could operate stably for; 186 hours. The fundamental reason for the invalidation was the shedding of coating layer by brushing and corrosion of the chlorine that generated on the surface of the Ti/SnO2-Sb2O5-PANI (9%) anode. The PVC/WMCNT(20%)-PANI(25%) anodes, at same current density, worked 26 hours in the sulfuric acid system, whose failure was attributed to the shedding of electrically active material carbon nanotubes and polyaniline materials caused by oxygen generated on the anode. Furthermore, polyaniline on electrode surface reacted with oxygen generated on anode, causing dedoping reaction and resulting in failure of the electrode.5. In the chloride system, compared with traditional Ti/RuO2 anode, Ti/SnO2-Sb2O5-PANI (9%) anode showed better electro-catalytic activity, lower cell voltage,which dropped by 181mV, and higher current efficiency rose by 1.26%.