Fe/N/C catalysts systhesized using graphene aerogel for electrocatalytic oxygen reduction reaction in an acidic condition

Impact of Aerogel Modification for Fe-N-C Activity and Stability towards Oxygen Reduction Reaction in Phosphoric Acid Electrolyte

Resorcinol-formaldehyde based carbon aerogel (CA) has been tailored to meet the requirements as a Fe-N-C carbon support, aiming to provide sufficient, inexpensive cathode catalysts for high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Therefore, different treatments of the aerogel are explored for optimal pore structure and incorporation of surface functionalities, which are crucial for Fe-N-C synthesis and electrochemical performance. Fe-N-Cs of differently modified aerogel are investigated in phosphoric acid electrolyte. The results show that HNO3 treatment for 5 h yields the Fe-N-C with highest mass activity and selectivity, attributed to the highest amount of nitrogen functionalities revealed by energy dispersive X-ray spectroscopy (XPS) and proper Fe-Nx site formation. HNO3 oxidation for 2 h leads to Fe-N-C with slightly lower oxygen reduction reaction (ORR) activity and selectivity. In contrast, the Fe-N-C synthesized from CA with H3PO4 treatment shows negligible ORR activity. The feasibility of one-step activation and carbonization treatment with K2CO3 and, for the first time, with K2CO3 and melamine is proven as the obtained Fe-N-Cs exhibit promising ORR activity. The results are compared with the commercial Fe-N-C PMF-014401. This study contributes to the advancement of cost-efficient HT-PEMFCs by optimizing Fe-N-C catalyst properties.

Pore Modification and Phosphorus Doping Effect on Phosphoric Acid-Activated Fe-N-C for Alkaline Oxygen Reduction Reaction

The price and scarcity of platinum has driven up the demand for non-precious metal catalysts such as Fe-N-C. In this study, the effects of phosphoric acid (PA) activation and phosphorus doping were investigated using Fe-N-C catalysts prepared using SBA-15 as a sacrificial template. The physical and structural changes caused by the addition of PA were analyzed by nitrogen adsorption/desorption and X-ray diffraction. Analysis of the electronic states of Fe, N, and P were conducted by X-ray photoelectron spectroscopy. The amount and size of micropores varied depending on the PA content, with changes in pore structure observed using 0.066 g of PA. The electronic states of Fe and N did not change significantly after treatment with PA, and P was mainly found in states bonded to oxygen or carbon. When 0.135 g of PA was introduced per 1 g of silica, a catalytic activity which was increased slightly by 10 mV at −3 mA/cm² was observed. A change in Fe-N-C stability was also observed through the introduction of PA.

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

Pyrolyzed iron-containing polyaniline (C-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 C-Fe-PANI particles, the micropore generation is hindered by pyrolysis-formed iron nanoparticles (Fe NPs) embedded inside C-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 FeCl₃ solution before pyrolysis for preventing FeCl₃ 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.