Single-atom-catalysts (SACs) have recently gained significant attention in energy conversion/storage application, while the low-loading amount due to their easy-to-migrate tendency poses a major bottleneck. For energy-saving H2 generation, replacing sluggish...
The scope of any metal oxide as a catalyst for driving electrocatalytic reactions depends on its electronic structure, which is correlated to its oxygen‐defect density. Likewise, to transform a spinel oxide, such as cobalt ferrite (CoFe2O4), into a worthy universal‐pH, bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions (HER and OER, respectively), oxygen defects need to be regulated. Prepared by coprecipitation and inert calcination at 650 °C, CoFe2O4 nanoparticles (NPs) require 253 and 300 mV OER overpotentials to reach current densities of 10 and 100 mA cm−2, respectively, if nickel foam is used as a substrate. With cost‐effective carbon fiber paper, the OER overpotential increases to 372 mV at 10 mA cm−2 at pH 14. The NPs prepared at 550 °C require HER overpotentials of 218, 245, and 314 mV at −10 mA cm−2 in alkaline, acidic, and neutral pH, respectively. The intrinsic activity is reflected from turnover frequencies of >3 O2 s−1 and >5 H2 s−1 at overpotentials of 398 and 259 mV, respectively. If coupled for overall water splitting, the extremely durable two‐electrode electrolyzer requires a cell potential of only 1.63 V to reach 10 mA cm−2 at pH 14. The homologous couple also splits seawater at impressively low cell voltages of 1.72 and 1.47 V at room temperature and 80 °C, respectively.
Exploring single-atom catalysts (SACs) for the nitrate
reduction
reaction (NO3
–; NitRR) to value-added
ammonia (NH3) offers a sustainable alternative to both
the Haber–Bosch process and NO3
–-rich wastewater treatment. However, due to the insufficient electron
deficiency and unfavorable electronic structure of SACs, resulting
in poor NO3
–-adsorption, sluggish proton
(H*) transfer kinetics, and preferred hydrogen evolution, their NO3
–-to-NH3 selectivity and yield
rate are far from satisfactory. Herein, a systematic theoretical prediction
reveals that the local electron deficiency of an f-block Gd single atom (GdSA) can be significantly regulated
upon coordination with oxygen-defect-rich NiO (GdSA-D-NiO400) support. Thus, facilitating stronger NO3
– adsorption via strong Gd5d–O2p orbital coupling and further improving the
protonation kinetics of adsorption intermediates by rapid H* capture
from water dissociation catalyzed by the adjacent oxygen vacancy site
along with suppressed H* dimerization synergistically boosts the NH3 selectivity/yield rate. Motivated by DFT prediction, we delicately
stabilized electron-deficient (strongly electrophilic) GdSA on D-NiO400 (∼84% strong electrophilic sites),
which exhibited excellent alkaline NitRR activity (NH3 Faradaic
efficiency ∼97% and yield rate ∼628 μg/(mgcat h)) along with superior structural stability, as revealed
by in situ Raman spectroscopy, significantly outperforming
weakly electrophilic Gd nanoparticles, defect-free GdSA-P-NiO400, and reported state-of-the-art catalysts.
A bimagnetic nanostructure was designed
where the antiferromagnetic
(AFM) NiO nanoparticles (NPs) are confined within the pores of a mesoporous
ferrimagnetic (FiM) CoFe2O4 matrix. An amount
of 3.4 wt % of 9 ± 1 nm NiO NPs was inserted into pores of 35
± 5 nm clustered CoFe2O4 NPs when the −NH3
+ groups of cysteamine on the NiO NP surface electrostatically
bind to the −OSO3
– of sodium dodecyl
sulfate (SDS) attached to CoFe2O4 NPs. The role
of in situ embedded NiO NPs is 3-fold: (i) to nearly
double the saturation magnetization (M
S) and coercivity (H
C) by suppressing
the frozen disordered spins on the surface of CoFe2O4 NPs surrounding the NiO NPs inside the pores at the cost
of enhanced FiM ordering, (ii) to introduce AFM/FiM exchange coupling
by breaking the spin glass surface layer to provide exchange bias
(EB) of 233.0 ± 0.2 Oe at 5 K with a cooling field of 2 T, and
(iii) to provide symmetry to the asymmetric nature of the hysteresis
loop of CoFe2O4. In the absence of cooling field,
the pristine CoFe2O4 NP porous matrix shows
hysteresis loop shifts of >1000 Oe and asymmetric magnetization
reversal
which are uncommon in spinel oxides.
While altering physical properties by self-assembly is a common phenomenon, controlled inclusion of a secondary phase that in turn enhances the properties of the ensemble is a rare occurrence. Herein monodisperse Mn 3 O 4 spherical nanoparticles were self-assembled into hierarchical flakes and cubes by regulating the surfactant−metal precursor molar ratio, reaction atmosphere, and time. The secondary phase of Mn 2 O 3 was incorporated differently, depending on the type of self-assembly as 2, 3.5, and 6.5 wt % in the flake, spherical, and cubic morphologies, respectively. The highest percentage of Mn 2 O 3 in the cubes boosts its multifunctionality in terms of enhanced magnetic exchange coupling and oxygen evolution reaction (OER) activity. With a 2 T cooling field, the hysteresis loop shift corresponding to coupling between antiferromagnetic Mn 2 O 3 and ferrimagnetic Mn 3 O 4 reached 3813 ± 2 Oe for the cubes, which is a record high for any reported Mn 3 O 4 −Mn 2 O 3 system. The presence of a e g 1 electron due to a higher Mn 2 O 3 fraction in the cubes facilitated high structural flexibility for optimum strength of interaction between the catalyst and intermediate ions during OER. Likewise, a current density 10 mA cm −2 was reached at an overpotential of 0.946 ± 0.02 V for the cubes, which is to superior those of the spherical morphology and flakes.
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