ABSTRACT
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.
ABSTRACT
We report the self-assembly of amphiphilic polystyrene-block-poly(ethylene oxide) (PS-b-PEO) brush block copolymers (BBCPs) into spherical micelles in an ethanol/water mixture as an efficient templating approach to fabricate mesoporous carbon spheres using polydopamine as a carbon source. Mesopore sizes of up to 25 nm are well controlled and are dependent on the molecular weight (Mw) of the BBCP. Such large pores are difficult to obtain using traditional linear block copolymers templates. Furthermore, bimodal mesoporous carbon spheres with two populations of pore sizes (24.5 and 6.5 nm) are obtained using a BBCP coassembled with a small molecule surfactant (Pluronic F127). An oxygen reduction reaction is used to demonstrate that electrocatalytic performance can be tuned by controlling the carbon sphere morphologies. This work provides a novel and versatile method to fabricate carbon spheres with broadly tunable bimodal pore sizes for potential applications in catalysis, separations, and energy storage.