Mesoporous Single Atom-Cluster Fe–N/C Oxygen Evolution Electrocatalysts Synthesized with Bottlebrush Block Copolymer-Templated Rapid Thermal Annealing

Abstract

Current electrocatalysts for oxygen evolution reaction (OER) are either expensive (such as IrO 2 , RuO 2 ) 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−N 4 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/cm 2 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, earthabundant, tailorable, mechanically stable, mesoporous carbon-coordinated single-atom electrocatalysts that can be used for renewable energy production.

Probing Electrocatalytic Synergy in Graphene/MoS2/Nickel Networks for Water Splitting through a Combined Experimental and Theoretical Lens

“…The hollow site of the Ni hydroxide cluster was considered as the catalytically active center . Generally, the standard OER mechanism in alkaline media consists of four coupled hydroxide-electron transfer steps on surface metal sites, , involving four different surface intermediates: *OH, *O, *OOH, and *OO. The catalytic cycle starts with a hydroxide ion attacking the center Ni atom in the first oxidation step, generating *OH.…”

“…Electrolysis of water using solar power to generate hydrogen is one of the most appealing strategies to produce green and sustainable fuel. − Unfortunately, two main factors are currently preventing it from finding a widespread use: (a) current electrocatalysts are designed using unsustainable materials such as platinum, palladium, and iridium, which are expensive and too scarce for sustainable widespread commercial usage, and (b) kinetics of hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) for nonplatinum-group metal electrocatalysts are sluggish and exhibit a very high overpotential . Fortunately, recent progress in HER and OER reactions has led to the development of a wide range of earth-abundant electrocatalysts (e.g., two-dimensional materials including phosphides and oxyhydroxides), which are low in price and show similar or, sometimes, even higher efficiency compared to the noble metals. − …”