Single-metal
site catalysts have exhibited highly efficient electrocatalytic
properties due to their unique coordination environments and adjustable
local structures for reactant adsorption and electron transfer. They
have been widely studied for many electrochemical reactions, including
oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).
However, it remains a significant challenge to realize high-efficiency
bifunctional catalysis (ORR/OER) with single-metal-type active sites.
Herein, we report atomically dispersed Fe–Co dual metal sites
(FeCo–NC) derived from Fe and Co co-doped zeolitic imidazolate
frameworks (ZIF-8s), aiming to build up multiple active sites for
bifunctional ORR/OER catalysts. The atomically dispersed FeCo–NC
catalyst shows excellent bifunctional catalytic activity in alkaline
media for the ORR (E
1/2 = 0.877 V) and
the OER (E
j=10 = 1.579
V). Moreover, its outstanding stability during the ORR and the OER
is comparable to noble-metal catalysts (Pt/C and RuO2).
The atomic dispersion state, coordination structure, and the charge
density difference of the dual metal site FeCo–NC were characterized
and determined using advanced physical characterization and density
functional theory (DFT) calculations. The FeCo–N6 moieties are likely the main active sites simultaneously for the
ORR and the OER with improved performance relative to the traditional
single Fe and Co site catalysts. We further incorporated the FeCo–NC
catalyst into an air electrode for fabricating rechargeable and flexible
Zn–air batteries, generating a superior power density (372
mW cm–2) and long-cycle (over 190 h) stability.
This work would provide a method to design and synthesize atomically
dispersed multi-metal site catalysts for advanced electrocatalysis.
KFeII[FeIII(CN)6] with a symmetric cubic structure exhibits exceptional electrochemical performance based on a solid solution mechanism, and its high structural stability and electrochemical reversibility.
Single
metal site catalysts are the most promising candidates to
replace platinum-group-metal (PGM) catalysts for the oxygen reduction
reaction (ORR), yet insufficient performance and scalable preparation
approaches remain great challenges. Here, we report a nitrogen (N)/sulfur
(S) codoped single Fe site catalyst (Fe–N/S–C) through
a chemical vapor deposition (CVD) strategy. Using the cyclopentadiene-shielded
Fe atom ferrocene (Fc) as the precursor, atomically dispersed single
Fe sites were successfully embedded into the N, S codoped 2D carbon
nanosheets. The superior catalytic activity for the ORR in alkaline
media is stemmed from the N, S codoping, tuning the optimal charge
distribution of Fe sites. In addition, the CVD approach could surpress
the formation of iron-carbide-containing iron clusters (“Fe
x
C/Fe”), thereby leading to high surface
areas and porosity. Furthermore, the Fe–N/S–C catalyst
was further studied as a cathode catalyst in direct methanol fuel
cells showing encouraging performance.
Sluggish
kinetics of the methanol oxidation reaction (MOR) at the anode of
direct methanol fuel cells (DMFCs) is primarily due to adsorbed CO
poisoning of precious metal catalysts. CeO2 is known to
provide oxygen containing species to adjacent precious metal sites
for facilitating CO removal during the MOR. In this work, highly dispersed
Pd nanoparticles surrounded by CeO2 dots were deposited
on a core–shell structured and nitrogen-doped mesoporous carbon
sphere (NMCS) support, which exhibited encouraging electrocatalytic
activity, CO tolerance, and stability for the MOR in alkaline media.
The ratios of Pd to CeO2 were found crucial for overall
catalytic performance enhancement. When compared to a commercial PtRu/C
catalyst, an optimized Pd(20%)-CeO2(20%)/NMCS
catalyst presented a comparable CO stripping onset potential, ∼6
times higher peak current density, and enhanced cyclic stability.
The unique mesoporous carbon with nitrogen doping also benefits for
uniform dispersion of Pd nanoparticles and CeO2 dots. In
good agreement with experimental spectroscopy analysis, density functional
theory calculations suggest that the strong electronic interactions
between Pd and surrounding CeO2, as well as nitrogen dopants
in supports, dramatically reduce the adsorption energy of CO at the
Pd surface, therefore enhancing CO tolerance of the Pd-CeO2/NMCS catalyst and further improving MOR activity. Using a polymer
fiber membrane-based alkaline DMFC, the Pd(20%)-CeO2 (20%)/NMCS anode catalyst further demonstrated encouraging
performance when a NiCo2O4 catalyst was used
for the oxygen cathode.
Excessive CO emission due to a large amount of fossil fuel utilization has become a widespread concern, which causes both environmental and energy problems. To solve these issues, electrocatalytic and photocatalytic reduction of CO to produce value-added chemicals have gained immense attention. Recently, metal-organic frameworks (MOFs) and their derived materials with high specific surface areas, controllable pore structures, and tunable chemical properties exhibit promising performance among the reported catalytic materials for CO conversion. This review describes the recent advances on the rational design and synthesis of MOF-based electrocatalysts and photocatalysts for CO reduction. The importance of the catalytic processes is highlighted, followed by systematic understanding of MOF-based catalysts for CO reduction through electrochemical and photochemical approaches. Special emphasis of this review is to introduce basic catalyst design strategies and synthesis methods as well as their resulting electrocatalysts and photocatalysts. One of the major goals is to elucidate the structures and properties that link to their catalytic activity, selectivity, and stability towards to CO reduction. We also outline the challenges in this research area and propose the potential strategies for the rational design and synthesis of high-performance catalysts.
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