Pyrolysis of Iron‐Containing Polyanilines under Micropore Generation Control: Electrocatalytic Performance in the Oxygen Reduction Reaction

Abstract

Pyrolyzed iron-containing polyaniline (CÀ Fe-PANI) is one of the most promising candidates as a non-precious metal based electrocatalyst for oxygen reduction reaction (ORR). Although the ORR activity depends on the surface area arisen from pyrolysis-generated micropores on CÀ Fe-PANI particles, the micropore generation is hindered by pyrolysis-formed iron nanoparticles (Fe NPs) embedded inside CÀ Fe-PANI particles. Here, we demonstrate the pyrolysis of iron-containing PANIs under suppression of micropore-generation hindrance by blocking the Fe NPs formation. The higher-molecular-weight (MW: 100,000) PANI was dispersed in an FeCl 3 solution before pyrolysis for preventing FeCl 3 penetration inside PANI particles. As a result, as compared to the case of lower-MW (5,000) PANI, the Fe NPs formation was more suppressed inside catalyst particles to give 1.9 (1.8) times micropore volume (specific surface area), leading to a 11 % higher current density in ORR electrocatalytic performance test in acidic media. Polymer electrolyte fuel cells (PEFCs) attract considerable attentions as potential power sources for both stationary and mobile applications. [1] To increase energy conversion efficiency, Pt-based materials are currently employed as the catalysts for both hydrogen oxidation reactions (HORs) at the anode and oxygen reduction reactions (ORRs) at the cathode. [1c] However, due to kinetically sluggish ORRs, as compared to HOR, a relatively large amount of Pt would need to be used at the cathode, making PEFCs too expensive to be applied as practical power sources. For the widespread application of PEFCs, therefore, a low-cost ORR catalyst with high performance must be developed. From such a view point, low-Pt or non-precious metal based electrocatalysts have been developed for ORR. Among them, non-precious metal based catalysts attract attentions because of their low cost. This seems to evolve from the report on ORR catalytic activity of Cophthalocyanine, [2] one of the N 4-chelate complexes. After that, the stability of N 4-chelates in acidic media was enhanced through the thermal pretreatment on carbon supports. [3] Since then, a huge number of researchers have reported a wide variety of analogous carbonaceous materials containing heteroatoms (e. g. N, B, P, S) and/or transition metal species (e. g. Fe, Co, Cu, Mn, Ni) with or without carbon supports. [4] On the other hand, polyaniline (PANI) is well known as one of the conducting polymers possessing the redox properties. Thus far, we have reported complexation of PANI with various transition metals [5] and catalytic reactions [6] mediated by the complexes. In this case, d-π conjugated systems are considered to be formed. Meanwhile, PANI is one of the most promising candidates as a precursor for ORR electrocatalysts. The pyrolyzed products of such complexes or mixtures comprising transition metals and PANIs

Hierarchically Porous Three-Dimensional (3D) Carbon Nanorod Networks with a High Content of FeNx Sites for Efficient Oxygen Reduction Reaction

“…Generally, the most prevailing procedure to produce Fe–N–C catalysts with dense FeNx active sites was the pyrolysis of precursors, which contained Fe, N, and C at high temperatures. − However, due to the nonuniform dispersion/fixation of the Fe species, some Fe atoms inevitably tended to agglomerate into Fe-containing compound particles during pyrolysis, leading to the aggregation with a high Fe concentration. In this case, the formation of active atomic FeNx sites was seriously hindered, and the content of Fe was decreased at the same time (<1 wt %). , Thus, numerous works have been devoted to developing single-atom catalysts with a high metal content.…”