Abstract:

Oxygen reduction reaction (ORR) largely governs the overall performance of fuel cells. Commercial Pt/C has long been employed as the state-of-the-art electrocatalyst for ORR. The scarcity and high price of Pt, however, have restrained the broad application of fuel cells. Thus, it is crucial to substitute commercial Pt/C with non-precious metal or metal-free electrocatalysts. Among them, heteroatom-doped metal-free electrocatalysts (DMFEs) are promising candidates. Heteroatom doping can modify the electron distribution of carbon materials, generating active sites suitable for the adsorption and reduction of oxygen. Despite significant progress in recent years, high-performance DMFEs remain rare. It is possible to obtain improved ORR activity by the introduction of more active sites to DMFEs, in combination with a large specific surface area. Since Jasinski reported cobalt phthalocyanine is active for ORR more than half a century ago (Nature 1964, 201, 1212), tremendous investigations on metallomacrocycles as ORR electrocatalysts have been carried out. Nevertheless, few studies have further enriched the active sites of DMFEs by adding metallomacrocycles. Herein, we attempt to introduce metallomacrocycles to DMFEs, and to use templates to fabricate porous nanostructures with high specific surface areas. By controlling pH, positively charged aniline monomers can be adsorbed on the negatively charged surface of SiO₂ nanospheres via electrostatic interactions. After in situ polymerization of aniline monomers, a polyaniline (PANI) coated SiO₂ (SiO₂@PANI) composite was formed. To introduce more active components, Fe^Ⅲ tetrakis(4-methoxyphenyl) porphyrin (FeP) was deposited on the surface of SiO₂@PANI by rotary evaporation method. After pyrolysis and removal of the template, FeP-modified porous PANI-based electrocatalysts were synthesized. Remarkably, the resultant 40%FeP/PANI-18-700 electrocatalysts demonstrate a high ORR activity, in terms of a half-wave potential (E_1/2) of 0.843 V (vs. reversible hydrogen electrode (RHE)) in 0.1 mol·L^-1 KOH aqueous solution, which is better than that of most DMFEs, and comparable to that of commercial Pt/C. The improvement of the ORR activity likely originates from the abundant pore structure (18 nm average pore diameter, pore volume of 1.1 cm³·g^-1), large surface area (687.5 m²·g^-1), and high N content (6.4%). Only 25 mV degradation of E_1/2 was observed for 40%FeP/PANI-18-700 during the accelerated durability test, in contrast to a 74 mV negative shift of E_1/2 for commercial Pt/C. Additionally, a hydroxide exchange membrane fuel cell (HEMFC) fabricated with 40%FeP/PANI-18-700 as the cathode approaches a peak power density of 42 mW·cm^-2. The results exhibit 40%FeP/PANI-18-700 may have potential applications in HEMFCs. Our strategy highlights a new avenue for the design and synthesis of non-precious metal electrocatalysts toward ORR in alkaline media.

Key words: Metalloporphyrin, Polyaniline, Template, Porous material, Oxygen reduction reaction