The electrochemical N2 reduction reaction (NRR) under
ambient conditions is attractive in replacing the current Haber-Bosch
process toward sustainable ammonia production. Metal-heteroatom-doped
carbon-rich materials have emerged as the most promising NRR electrocatalysts.
However, simultaneously boosting their NRR activity and selectivity
remains a grand challenge, while the principle for precisely tailoring
the active sites has been elusive. Herein, we report the first case
of crystalline two-dimensional conjugated covalent organic frameworks
(2D c-COFs) incorporated with M–N4–C centers as novel,
defined, and effective catalysts, achieving simultaneously enhanced
activity and selectivity of electrocatalytic NRR to ammonia. Such
2D c-COFs are synthesized based on metal-phthalocyanine
(M = Fe, Co, Ni, Mn, Zn, and Cu) and pyrene units bonded by pyrazine
linkages. Significantly, the 2D c-COFs with Fe–N4–C center exhibit higher ammonia yield rate (33.6 μg
h–1 mgcat
–1) and Faradaic efficiency (FE, 31.9%)
at −0.1 V vs reversible hydrogen electrode than those with
other M–N4–C centers, making them among the
best NRR electrocatalysts (yield rate >30 μg h–1 mgcat
–1 and FE > 30%). In situ X-ray absorption spectroscopy, Raman spectroelectrochemistry,
and theoretical calculations unveil that Fe–N4–C
centers act as catalytic sites. They show a unique electronic structure
with localized electronic states at Fermi level, allowing for stronger
interaction with N2 and thus faster N2 activation
and NRR kinetics than other M–N4–C centers.
Our work opens the possibility of developing metal–nitrogen-doped
carbon-rich 2D c-COFs as superior NRR electrocatalyst
and provides an atomic understanding of the NRR process on M–N
x
–C based electrocatalysts for designing
high-performance NRR catalysts.
Oxygen electrocatalysis is vital for advanced energy technologies, but inordinate challenges remain due to the lack of highly active earth-abundant catalysts.
Exfoliation of atomically thin layers from non-van der Waals bulk solids gave rise to the emergence of a new class of two-dimensional (2D) materials, such as hematene (Hm), a structure just a few atoms thick obtained from hematite. Due to a large number of unsaturated sites, the Hm surface can be passivated under ambient conditions. Using density functional theory calculations, we investigate the effects of surface passivation with H and OH groups on Hm properties and demonstrate that the passivated surfaces are energetically favorable under oxygenrich conditions. Although the bare sheet is antiferromagnetic and possesses an indirect band gap of 0.93 eV, the hydrogenated sheets are half-metallic with a ferromagnetic ground state, and the fully hydroxylated sheets are antiferromagnetic with a larger band gap as compared to the bare system. The electronic structure of Hm can be further tuned by mechanical deformations. The band gap of fully passivated Hm increases monotonically with biaxial strain, hinting at the potential applications of Hm in electromechanical devices.
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