Determination of Iron Active Sites in Pyrolyzed Iron-Based Catalysts for the Oxygen Reduction Reaction

Fe-based oxygen reduction reaction (ORR) catalyst materials are considered promising nonprecious alternatives to traditional platinum-based catalysts. These catalyst materials are generally produced by high-temperature pyrolysis treatments of readily available carbon, nitrogen, and iron sources. Adequate control of the structure and active site formation during pyrolysis methods is nearly impossible. Thus, the chemical nature, structure, and ORR mechanism of catalytically active sites in these materials is a subject of significant debate. We have proposed a method, utilizing CN^– ions as ORR inhibitors on Fe-based catalysts, to provide insight into the exact nature and chemistry of the catalytically active sites. Moreover, we propose two possible catalytically active site formation mechanisms occurring during high-temperature pyrolysis treatments, dependent on the specific type of precursor and synthesis methods utilized. We have further provided direct evidence of our proposed active site formations using ToF-SIMS negative and positive ion imaging. This knowledge will be beneficial to future work directed at the development of Fe-based catalysts with improved ORR activity and operational stabilities for fuel cell and battery applications

Oxygen Reduction on Graphene–Carbon Nanotube Composites Doped Sequentially with Nitrogen and Sulfur

The development of unique, reliable, and scalable synthesis strategies for producing heteroatom-doped nanostructured carbon materials with improved activity toward the electrochemical oxygen reduction reaction (ORR) occurring in metal–air batteries and fuel cells presents an intriguing technological challenge in the field of catalysis. Herein, we prepare unique graphene–carbon nanotube composites (GC) doped sequentially with both nitrogen and sulfur (GC-NLS) and subject them to extensive physicochemical characterization and electrochemical evaluation toward the ORR in an alkaline electrolyte. GC-NLS provides ORR onset potential increases of 50 and 70 mV in comparison to those of dual-doped individual graphene and carbon nanotubes, respectively. This highlights the significant synergistic effects that arise because of the nanocomposite arrangement, consisting of highly graphitized carbon nanotubes assembled on the surface of graphene sheets. The addition of sulfur as a co-dopant is also highly beneficial, providing an 80 mV increase in the ORR onset potential in comparison to that of GC nanocomposites doped with only nitrogen. Excellent electrochemical stability of GC-NLS is also observed through 5000 electrode potential cycles, indicating the promising potential of this new class of dual-doped GC nanocomposites as ORR catalysts

Co–N Decorated Hierarchically Porous Graphene Aerogel for Efficient Oxygen Reduction Reaction in Acid

Nitrogen-functionalized graphene materials have been demonstrated as promising electrocatalyst for the oxygen reduction reaction (ORR), owning to their respectable activity and excellent stability in alkaline electrolyte. However, they exhibit unacceptable catalytic activity in acid medium. Here, a hierarchically porous Co–N functionalized graphene aerogel is prepared as an efficient catalyst for the ORR in acid electrolyte. In the preparation process, polyaniline (PANI) is introduced as a pore-forming agent to aid in the self-assembly of graphene species into a porous aerogel networks, and a nitrogen precursor to induce in situ nitrogen doping. Therefore, a Co–N decorated graphene aerogel framework with a large surface area (485 m² g^–1) and an abundance of meso/macropores is effectively formed after heat treatment. Such highly desired structures can not only expose sufficient active sites for the ORR but also guarantee the fast mass transfer in the catalytic process, which provides significant catalytic activity with positive onset and half wave potentials, low hydrogen peroxide yield, high resistance to methanol crossover, and remarkable stability that is comparable to commercial Pt/C in acid medium

Web-like 3D Architecture of Pt Nanowires and Sulfur-Doped Carbon Nanotube with Superior Electrocatalytic Performance

Development of highly durable electrocatalysts for oxygen reduction reaction (ORR) is critical for proton exchange membrane fuel cells. Herein, we report the synthesis, characterization, and electrochemical performance of 1D sulfur-doped carbon nanotubes (S-CNT) supported 1D Pt nanowires (PtNW/S-CNT). PtNW/S-CNT synthesized by a modified solvothermal method possesses a unique web-like 3D architecture that is beneficial for oxygen reduction. We demonstrate that PtNW/S-CNT exhibits impressive activity retention under potential cycling between 0.05 and 1.5 V vs RHE over 3000 cycles. The reductions in electrochemically active surface area (ECSA, 7% loss) and mass activity (19% loss) of PtNW/S-CNT after accelerated durability testing (ADT) are found to be much lower than the dramatic losses observed with commercial Pt/C (>99% loss in ECSA and mass activity) under identical conditions. The PtNW/S-CNT catalyst also shows very high specific activity (1.61 mA cm^–2) in comparison to Pt/C (0.24 mA cm^–2)