The Electrocatalytic Oxygen Reduction Performance And Application Of The Polypyrrole Film Electrode Doped By Anthraquinone

Polypyrrole (PPy), as a typical conducting polymer, has many advantages such as harmlessness, simple and convenient preparation, high conductivity, electrocatalytic activity and high environmental stability. In seeking to functionalize films with the desired electrochemical properties such as electrocatalysis and electroanalysis, a wide variety of anionic functional groups were incorporated into PPy matrix during the polymerization process of pyrrole (Py) monomer. The electrochemical reduction of oxygen has been received much attention because of the wide use of oxygen reduction electrodes in fuel cells and metal-air batteries. Moreover, the electrochemical reduction of oxygen has also been widely studied on quinones and their various derivatives modified electrodes, since the surface modification of electrodes with quinones greatly increase the rate of oxygen reduction to hydrogen peroxide (H₂O₂). However, quinones and their various derivatives which are grafted as spontaneously adsorbed or covalently bound monolayer on the electrode surface, tend to desorb from the surface during the long-term operation, leading to a loss of electrocatalytic ability and stability of the working electrodes. Therefore, aimed at improving the stability and the electrocatalytic activity of electrocatalyst, and taking advantage of both PPy film and quinonoid compounds, anthraquinonedisulphonate (AQDS) was incorporated into PPy matrix using the electrochemical method during the preparation of the modified electrode.The electrochemical behaviors of AQDS dissolved in solution and incorporated into PPy matrix as doping species at PPy film modified glassy carbon (GC) electrode were investigated in various pH buffered solutions, also examined was the stability of the AQDS/PPy composite film (AQDS in doping phase). It was found that the redox process of AQDS in solution and doping phase both exhibit quasi-reversible behavior and pH dependence. For AQDS in solution phase, the reduction of AQDS is diffusion-controlled adsorption process, while for AQDS in doping phase the reduction of AQDS is non-diffusion-controlled process. The value of ionization constant pKa for the H₂AQ/HAQ^- couple increases obviously from 7.6 in solution phase to 9.5 when AQDS is incorporated into PPy matrix. The immobilized and diffusive AQDS both proceed a single two-electron reduction step in aqueous solution. The PPy matrix not only increases the specific surface area of electrode and lowers the redox peak potential separation of AQDS, but also increases the redox peak currents. In addition, the AQDS/PPy composite film modified electrode exhibited a good electrochemical stability.In view of the earlier works which have been done on the electrocatalytic effect of quinonoid compounds on O₂/O₂^2- redox reaction, the electrocatalytic activity of the immobilized and diffusive AQDS towards the electroreduction of oxygen was investigated using cyclic voltammetry (CV), rotating disc electrode (RDE), chronoamperometric/chronocoulometric and Tafel polarization technique. The diffusive and reactive kinetic parameters were also determined using these electrochemical approaches and the probable mechanism for oxygen reduction catalyzed by AQDS operating in various pH buffered solutions were proposed. It was found that the pH range of 5.5-7.0 buffered solution is a more suitable medium for oxygen reduction catalyzed by the immobilized and diffusive AQDS. In various pH buffered solutions, the electrocatalytic reduction of oxygen mediated by AQDS establishes a pathway of irreversible two-electron reduction to form H₂O₂. The dihydroanthraquinone (H2AQ) and semiquinone radical anion (AQ^-) are responsible for the extraordinary catalytic activity to the oxygen reduction reaction in acidic and alkaline media, respectively. The catalytic reaction occurred in the presence of AQDS and O₂ is in agreement with an electrochemical-chemical (EC) mechanism, and the electrocatalytic current is under mixed diffusion-reaction kinetic control. The AQDS/PPy composite film exhibits reproducible characteristic. After performing repetitive CVs in an O₂-saturated buffer solution, there was a slightly decrease (< 15%) in the corresponding voltammograms in the same N₂-saturated solution.The degradation of amaranth azo dye was investigated by in-situ electrogenerated Fenton's reagent using the bare graphite and the AQDS/PPy composite film modified graphite (AQDS/PPy/Graphite) as cathodes and Fe²⁺ or Fe³⁺ as catalyst. The H₂O₂ was produced by electrochemical reduction of oxygen on cathode and the Fe²⁺ was also regenerated by cathodic reduction of Fe³⁺. The influence of various operational parameters such as solution pH, cathode potential, Fe³⁺ and Fe²⁺ concentration, and AQDS doping concentration on amaranth azo dye degradation efficiency (color fading and total organic carbon (TOC) removal) were studied and the best experiment conditions for dye degradation was established. In addition, the electrocatalytic activities of the bare graphite and the AQDS/PPy/graphite cathodes towards oxygen reduction and Fe²⁺ regeneration were studied using CV and cathodic polarization technologies, and the effects of various operational parameters such as solution pH, cathodic potential, oxygen flow rate and AQDS doping concentration on H₂O₂ accumulation and current efficiency were investigated systematically. The results show that H₂O₂ generation and Fe²⁺ regeneration mainly depend on the cathode materials utilized. The AQDS/PPy/Graphite composite film electrode exhibits the characteristic of gas diffusion cathode and is highly efficient for H₂O₂ electrogeneration with high generation rate and current efficiency, while the bare graphite electrode exhibits better electrocatalytic activity for Fe²⁺ regeneration. The suitable Fe²⁺ and Fe³⁺ concentration is an important and crucial factor for electro-Fenton oxidation ability and efficiency. The CV and fourier transfer infrared (FT-IR) analyses confirm that the bulk AQDS strongly interacts with PPy matrix and cannot be expelled from the polymer after use in electro-Fenton process, but there is a slight decrease in the electrocatalytic activity of AQDS/PPy composite film.