Studies of selectivity of oxygen reduction reaction in acidic electrolyte on electrodes modified by products of pyrolysis of polyacrylonitrile and metalloporphyrins

Resumo

The rotating disk electrode technique was used to study in 0.5 M H₂SO₄ catalytic properties of products of pyrolysis of the metal-free polyacrylonitrile/carbon black composite, polyacrylonitrile/iron/carbon black composite, and also supported pyropolymers of Co(II) tetramethoxyphenyl porphyrine (CoTMPP) and Fe(III) tetramethoxyphenyl porphyrin chloride (FeTMPPCl). It is shown that the metal-free polyacrylonitrile/carbon black composite catalyzes the oxygen reduction reaction via the parallel path. Addition of up to 2% of Fe into the composite results in abrupt growth of the catalytic activity and share of the four-electron reaction, which provides the parallel–serial reaction path. The parallel reaction with no further catalytic conversion of H₂O₂ occurs on catalysts of the CoTMPP/Vulcan XC72 and FeTMPPCl/Vulcan XC72 series. The chemical composition is one of the key factors affecting activity and selectivity of CoTMPP/Vulcan XC72 catalysts. An increase in the precursor content from 5 to 30% is accompanied by an increase in selectivity k₁/k₂ from 0.14–0.30 to 0.5–1.7, where k₁ is the rate constant of the reaction of O₂ reduction to H₂O, k₂ is the rate constant of the reaction of O₂ reduction to H₂O₂.