RESUMO
Current electrocatalysts for oxygen evolution reaction (OER) are either expensive (such as IrO2, RuO2) or/and exhibit high overpotential as well as sluggish kinetics. This article reports mesoporous earth-abundant iron (Fe)-nitrogen (N) doped carbon electrocatalysts with iron clusters and closely surrounding Fe-N4 active sites. Unique to this work is that the mechanically stable mesoporous carbon-matrix structure (79 nm in pore size) with well-dispersed nitrogen-coordinated Fe single atom-cluster is synthesized via rapid thermal annealing (RTA) within only minutes using a self-assembled bottlebrush block copolymer (BBCP) melamine-formaldehyde resin composite template. The resulting porous structure and domain size can be tuned with the degree of polymerization of the BBCP backbone, which increases the electrochemically active surface area and improves electron transfer and mass transport for an effective OER process. The optimized electrocatalyst shows a required potential of 1.48 V (versus RHE) to obtain the current density of 10 mA/cm2 in 1 M KOH aqueous electrolyte and a small Tafel slope of 55 mV/decade at a given overpotential of 250 mV, which is significantly lower than recently reported earth-abundant electrocatalysts. Importantly, the Fe single-atom nitrogen coordination environment facilitates the surface reconstruction into a highly active oxyhydroxide under OER conditions, as revealed by X-ray photoelectron spectroscopy and in situ Raman spectroscopy, while the atomic clusters boost the single atoms reactive sites to prevent demetalation during the OER process. Density functional theory (DFT) calculations support that the iron nitrogen environment and reconstructed oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced. This work has opened a new avenue for simple, rapid synthesis of inexpensive, earth-abundant, tailorable, mechanically stable, mesoporous carbon-coordinated single-atom electrocatalysts that can be used for renewable energy production.
RESUMO
Overcoming throughput challenges in current graphene defect healing processes, such as conventional thermal annealing, is crucial for realizing post-silicon device fabrication. Herein, a new time- and energy-efficient method for defect healing in graphene is reported, utilizing polymer-assisted rapid thermal annealing (RTA). In this method, a nitrogen-rich, polymeric "nanobandage" is coated directly onto graphene and processed via RTA at 800 °C for 15 s. During this process, the polymer matrix is cleanly degraded, while nitrogen released from the nanobandage can diffuse into graphene, forming nitrogen-doped healed graphene. To study the influence of pre-existing defects on graphene healing, lattice defects are purposefully introduced via electron beam irradiation and investigated by Raman microscopy. X-ray photoelectron spectroscopy reveals successful healing of graphene, observing a maximum doping level of 3 atomic nitrogen % in nanobandage-treated samples from a baseline of 0-1 atomic % in non-nanobandage treated samples. Electrical transport measurements further indicate that the nanobandage treatment recovers the conductivity of scanning electron microscope-treated defective graphene at ≈85%. The reported polymer-assisted RTA defect healing method shows promise for healing other 2D materials with other dopants by simply changing the chemistry of the polymeric nanobandage.
