The replacement of platinum with nonprecious metal electrocatalysts
for hydrogen evolution reaction (HER) remains an important challenge.
We report facile synthesis of precious-metal-free HER electrocatalysts
that are made up of metal–organic framework-derived cobalt/carbon
nanostructures and semicrystalline ultrathin MoS2 or WS2 nanosheets. The as-synthesized catalysts MoS2/Co@NC
and WS2/Co@NC delivered an electrochemical HER current
density of 25 mA cm–2 at overpotentials of 0.23
and 0.28 V, respectively. Both the catalysts were found to be highly
stable in 0.5 M H2SO4 with small Tafel slope
values. The high-performance HER activity can be related to (i) covalent
cobalt doping into MoS2 and WS2 layers confirmed
by X-ray photoelectron spectroscopy, (ii) the presence of cobalt nanoparticles
in close vicinity of MoS2 and WS2 layers, (iii)
the presence of bridging disulfide S2
2– into MoS2 and WS2 layers, and (iv) synergistic
cooperation among multicomponents present in the catalyst structure.
Density functional theory calculations suggested that Co doping at
Mo sites in MoS2 has a favorable Gibb’s free energy
(ΔG) value for HER. Interestingly, the interface
between the Co nanocluster and MoS2 is found to be a favorable
HER active site with localization of electrons. To the best of our
knowledge, the simultaneous effect of single metal substitution and
metal clusters in/on MoS2 and WS2 layers has
not been studied for HER. Moreover, we have also demonstrated a durable
acid–base water electrolyzer using MoS2/Co@NC and
WS2/Co@NC as cathodes, generating 10 mA cm–2 current density at a cell voltage of ∼0.89 V.
Developing nonprecious metal-based oxygen reduction reaction
(ORR)
electrocatalysts with superior activity and durability is crucial
for commercializing proton-exchange membrane (PEM) fuel cells. Herein,
we report a metal–organic framework (MOF)-derived unique N-doped
hollow carbon structure (NiCo/hNC), comprising of atomically dispersed
single-Ni-atom (NiN4) and small NiCo alloy nanoparticles
(NPs), for highly efficient and durable ORR catalysis in both alkaline
and acidic electrolytes. Density functional theory (DFT) calculations
reveal the strong coupling between NiN4 and NiCo NPs, favoring
the direct 4e– transfer ORR process by lengthening
the adsorbed O–O bond. Moreover, NiCo/hNC as a cathode electrode
in PEM fuel cells delivered a stable performance. Our findings not
only furnish the fundamental understanding of the structure–activity
relationship but also shed light on designing advanced ORR catalysts.
Herein, for the first time, a metal-organic framework (MOF) is reported as catalyst for α-alkylation of ketones with alcohols. Using an encapsulation strategy via nano-confinement of homogeneous Mn-phenanthroline complex into...
A unique synthetic approach has been introduced where
nanostructurally
grown zinc layered double hydroxide on graphitic carbon felt (CF)
is converted into a zeolitic imidazolate framework (ZIF-8), and then,
subsequent carbonization resulted in a N/O-functionalized porous carbon
electrode (N,O/CF). Because of the presence of N/O-containing functional
groups and deposition of ZIF-8-derived nanoporous carbon on the CF,
the N,O/CF is found to be highly hydrophilic in nature with a large
surface area. Cyclic voltammetry measurements with N,O/CF suggest
the fast electrochemical kinetics of V(IV) ↔ V(V) reactions.
Polarization curves and electrochemical impedance spectroscopy measurements
of the vanadium redox flow battery (VRFB) assembly illustrate the
significant decrease in charge transfer resistance at electrode surfaces.
At 50 mL/min electrolyte flow rate, N,O/CF delivers energy efficiencies
of 83.11 and 76.66% at current densities of 40 and 80 mA/cm2, respectively. The values are 82.59 and 76.39%, respectively, at
100 mL/min, showing the negligible effect of the flow rate. The power
density of VRFBs at various electrolyte flow rates is also presented,
which increases with increasing flow rates and is higher for N,O/CF
(∼821 mW/cm2) than for bare CF (606 mW/cm2). The stability test confirms the retention of energy, voltage,
and coulombic efficiencies after recycling of the electrode. The above-mentioned
findings of improved performance of VRFBs with employing the N,O/CF
electrode are a cumulative effect of enhanced nanoporosity, an increased
number of catalytic active sites, high wettability, and low charge
transfer resistance.
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