Covalent carbon surface modification with iron porphyrin : application to oxygen reduction reaction.

Abstract

This thesis describes the strategies used to covalently immobilize iron porphyrin on carbon surfaces and attempts to improve its oxygen reduction reaction (ORR) catalytic performance in aqueous acid media. In order to develop a sustainable future for energy, ORR is one of the redox reactions that gain high interest for fuel cell development. Designing Pt-free materials as electrocatalysts with high efficiency, selectivity for H₂O as the production (rather than H₂O₂) and high stability for ORR, especially in acid media, is the main goal. Immobilizing molecular metal complexes on the surface for ORR is one of the approaches. A few studies have been reported to immobilize iron porphyrin derivatives on surfaces for ORR catalysis, but there is lack of systematic study of their catalytic performance in acid medium. Not much work has focused on ORR enhancement by environment tailoring, and the degradation mechanism of metal complexes under ORR conditions remains unclear. To help address these issues, this thesis work identifies strategies to enhance the ORR performance in acid media catalysed by iron porphyrin immobilized on carbon surface.

In this work, meso-tetraphenylporphyrin iron (III) chloride (FeTPPCl), a classic electrocatalyst, was chosen for ORR catalysis in acid medium. [FeTPP]+ was immobilized on glassy carbon and pyrolyzed photoresist film via imidazole (Im-Ar) axial ligand using post-coordination and pre-coordination approaches based on aryldiazonium ions grafting to form [FeTPPIm-Ar]⁺ film. [FeTPP]⁺ was also covalently bound to the surface directly via the porphyrin ligand (rather than via axial ligands) to form [FeTPP]⁺ film.

For [FeTPPIm-Ar]⁺ film, among different strategies used in this thesis, the pre-coordination method was identified as the promising approach for [FeTPP]+ immobilization, as it provided a straightforward, simple and controllable way. The successful immobilization was confirmed by electrochemistry, spectroelectrochemistry, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). A wide range of surface concentrations (from 2×10-¹¹ to 3×10-⁹ mol/cm²) of electro-active [FeTPPIm-Ar]⁺ were obtained. By investigating the grafting mechanism, redox grafting phenomenon was observed. It showed that the [FeTPPIm-Ar]⁺ film composition can be easily controlled by grafting potentials.

During ORR catalysis in acid media, 70% of O₂ was reduced to H₂O₂ for [FeTPPIm-Ar]⁺ film. Mixed layers of [FeTPPIm-Ar]⁺ with a second modifier were prepared to decrease the %H₂O₂ produced by 10% under ORR condition, which improved the stability of [FeTPPIm-Ar]⁺ as confirmed by spectroelectrochemistry.

Compared to [FeTPPIm-Ar]⁺ film, [FeTPP]⁺ film showed better ORR catalytic performance in terms of onset potential, peak current and %H2O2 produced. However, degradation was observed for both [FeTPPIm-Ar]⁺ film and [FeTPP]⁺ film. It was demonstrated that demetallation and reactive oxygen species produced from Fenton reaction are the two origins for degradation of immobilized [FeTPP]⁺ under ORR condition.

Overall, this thesis work shows the robust and simple strategy for [FeTPP]⁺ surface immobilization, which can be applied to other metal porphyrin or phthalocyanine. The potential degradation mechanism proposed for immobilized [FeTPP]⁺ for ORR catalysis in acid media offers an insight into understanding other Pt-free metal complexes used for ORR catalysis. Importantly, it demonstrates the concept that using mixed layer with functional groups to tailor the environment of metal complex is a promising approach for ORR enhancement.