Direct Preparation of Carbon Nanofiber Electrodes via Pyrolysis of Iron(II) Phthalocyanine:  Electrocatalytic Aspects for Oxygen Reduction

A facile method for the direct preparation of carbon nanofiber (CNF) electrodes by pyrolysis of iron(II) phthalocyanine on nickel substrates is reported. Uniform, large area coverage is observed with aligned bundles of CNFs exhibiting bamboo-like, hollow fibril morphology possessing diameters of 40−60 nm and lengths of ∼10 μm. The electrochemical behavior and stability of CNF electrodes as oxygen reduction catalysts were investigated by electrochemical methods. Without necessitation for extensive electrode pretreatment or surface activation, these electrodes demonstrate significant electrocatalytic activity in aqueous KNO3 solutions at neutral to basic pH for the reduction of dioxygen to hydrogen peroxide, O2 + H2O + 2e- ⇌ H + OH-. As determined from chronocoulometry, slopes of Anson plots indicate that the overall electrochemical reaction proceeds by the peroxide pathway via two successive two-electron reductions. pH-dependent cyclic voltammetry studies indicate that the CNF electrodes are very active toward adsorption. At pH < 10 the one-electron reduction of O2 to superoxide is rate limiting, whereas at more alkaline pH the reduction process is limited by the protonation of adsorbed superoxide. This is reflected by a change in measured apparent charge-transfer coefficient (αobs) from αobs = 0.5 to αobs = 1 at neutral and high pH values, respectively. XPS, Raman, and TEM measurements suggest that the disorder in the graphite fibers and the presence of exposed edge plane defects and nitrogen functionalities are important factors for influencing adsorption of reactive intermediates and enhancing electrocatalysis for O2 reduction.