Abstract
Platinum electrodes are commonly used electrocatalysts for oxygen reduction reactions (ORR) in fuel cells. However, this material is not economical due to its high cost and scarcity. We prepared an Mn(III) catalyst supported on graphene and further coated with polydopamine, resulting in superior ORR activity compared to the uncoated PDA structures. During ORR, a peak potential at 0.433 V was recorded, which is a significant shift compared to the uncoated material’s −0.303 V (both versus SHE). All the materials reduced oxygen in a wide pH range via a four-electron pathway. Rotating disk electrode and rotating ring disk electrode studies of the polydopamine-coated material revealed ORR occurring via 4.14 and 4.00 electrons, respectively. A rate constant of 6.33 × 106 mol−1s−1 was observed for the polydopamine-coated material–over 4.5 times greater than the uncoated nanocomposite and superior to those reported for similar carbon-supported metal catalysts. Simply integrating an inexpensive bioinspired polymer coating onto the Mn-graphene nanocomposite increased ORR performance significantly, with a peak potential shift of over +730 mV. This indicates that the material can reduce oxygen at a higher rate but with lower energy usage, revealing its excellent potential as an ORR electrocatalyst in fuel cells.
Highlights
The electrochemical reduction of oxygen to water is an essential cathodic half reaction in proton exchange membrane fuel cells
Metal complexes supported on carbonaceous nanomaterials, such as a Co-porphyrin supported on MWCNT15, a Mn tetrakis(4-hydroxyphenyl)porphyrin catalyst immobilized on poly(sodium-p-styrenesulfonate) modified reduced graphene oxide[16], and an Fe-phthalocyanine anchored on single-walled carbon nanotubes[17], have demonstrated excellent activities as oxygen reduction reactions (ORR) electrocatalysts
Supporting metal complexes on carbon nanomaterials has proven to be efficacious in ORR, improvements in stability and activity can still be made in order to further reduce the electrochemical overpotential that is observed in these materials
Summary
The electrochemical reduction of oxygen to water is an essential cathodic half reaction in proton exchange membrane fuel cells. Platinum-based catalysts are the most efficient materials for oxygen reduction reactions (ORR) and are currently used in commercial fuel cells[1,2] This metal is expensive, scarce, and suffers from sluggish reaction kinetics and carbon monoxide poisoning, which diminishes its catalytic activity and longevity over time[3,4]. The π-πinteractions between the lone pair of electrons on the nitrogen and oxygen atoms in the ligand and the carbon nanomaterial may reduce their nucleophilicity, which may contribute to the overall activity and stability of the metal complex[21]. Supporting metal complexes on carbon nanomaterials has proven to be efficacious in ORR, improvements in stability and activity can still be made in order to further reduce the electrochemical overpotential that is observed in these materials. Apart from increasing activity, PDA-coated structures of this nature can potentially benefit in overall stability, because PDA forms a protective barrier against harsh pH environments, which can help improve their widespread application in fuel cells
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