An
effective modulation of the active sites in a bifunctional electrocatalyst
is essentially desired, and it is a challenge to outperform the state-of-the-art
catalysts toward oxygen electrocatalysis. Herein, we report the development
of a bifunctional electrocatalyst having target-specific Fe–N4/C and Co–N4/C isolated active sites, exhibiting
a symbiotic effect on overall oxygen electrocatalysis performances.
The dualism of N-dopants and binary metals lower the d-band centers
of both Fe and Co in the Fe,Co,N–C catalyst, improving the
overpotential of the overall electrocatalytic processes (ΔE
ORR‑OER = 0.74 ± 0.02 V vs RHE).
Finally, the Fe,Co,N–C showed a high areal power density of
198.4 mW cm–2 and 158 mW cm–2 in
the respective liquid and solid-state Zn–air batteries (ZABs),
demonstrating suitable candidature of the active material as air cathode
material in ZABs.
Tuning
the electronic structure of perovskite oxides via aliovalent
substitution is a promising strategy to attain inexpensive and efficient
electrocatalysts for energy conversion and storage devices. Herein,
following the d-band center positions and using a simple sol–gel
method followed by a pyrolysis step, LaNi1–x
Co0.5x
Fe0.5x
O3 (LNFCO-x; x = 0.0, 0.4, 0.5, and 0.6) electrocatalysts are designed and synthesized
for oxygen redox reactions in 1 M KOH. Among them, LNFCO-0.5 has exhibited
the lowest overpotential and the highest charge transfer kinetics
in oxygen redox reactions. Overall, a 90 mV lower overpotential was
observed in oxygen redox activity of LNFCO-0.5 compared to that of
pristine LaNiO3. The mass activity of LNFCO-0.5 in the
oxygen reduction reaction (at 0.7 V vs RHE) and oxygen evolution reaction
(1.60 V vs RHE) was calculated to be 2.5 and 2.13 times higher than
that of LaNiO3, respectively. The bifunctionality index
(potential difference between the oxygen evolution at a current density
of 10 mA cm–2 and the oxygen reduction at a current
density of −1 mA cm–2) of LNFCO-0.5 was found
to be 0.98. The substitution of Fe and Co for the Ni-site shifted
the d-band center close to the Fermi level, which can increase the
binding strength of the *OH intermediate in the rate-determining step.
Also, the surface was enriched with Fe3+Δ, Co3+, and partially oxidized Ni3+ states, which is
susceptible to tune the eg-orbital filling for superior
oxygen redox activity.
Despite numerous advantages over the traditional light absorbing materials, colloidal cesium lead halide (CsPbX 3 , X = Cl, Br, or I) perovskite nanocrystals (NCs) suffer from enormous defect density, leading to shorter lifetime of charge carriers and material instability. A large number of positively and negatively charged ionic defects are inevitably formed from crystallization via high temperature. Herein, we have studied a simple post-synthesis defect passivation of blue emitting CsPbCl 3 NCs using monovalent metal ion LiCl as a dual-passivating agent. The observed effect (on optical properties) went up by leaps and bounds. Photoluminescence (PL) quantum yield increases from 2.8 to 47.6%, while PL life time increases from 0.56 to 20.79 ns. Various other chloride salts (CaCl 2 , NH 2 Cl, KCl, and NaCl) and Li salts (LiBr and LiI) with different cation and anion combinations, respectively, did not give this effect. All these together with the enhanced overall stability of NCs suggest the synergistic effect of dual passivation and deep defect passivation that leads to significant suppression of non-radiative recombination. An X-ray photoelectron spectroscopy study also reveals that this simple strategy promotes simultaneous passivation of both defects (vacancies) formed from negatively (chlorine) and positively charged ions (lead) of CsPbCl 3. Theoretical study and experimental analysis in this work, together delivers a perceptive understanding of cationic and anionic vacancy healing by LiCl in CsPbCl 3 NCs, thus enhancing its utilization as efficient blue light emitters.
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