6 research outputs found
Clarifying the Controversial Catalytic Performance of Co(OH)<sub>2</sub> and Co<sub>3</sub>O<sub>4</sub> for Oxygen Reduction/Evolution Reactions toward Efficient Zn–Air Batteries
Cobalt-based
nanomaterials have been widely studied as catalysts for the oxygen
reduction reaction (ORR) and oxygen evolution reaction (OER) due to
their remarkable bifunctional catalytic activity, low cost, and easy
availability. However, controversial results concerning OER/ORR performance
exist between different types of cobalt-based catalysts, especially
for CoÂ(OH)<sub>2</sub> and Co<sub>3</sub>O<sub>4</sub>. To address
this issue, we develop a facile electrochemical deposition method
to grow CoÂ(OH)<sub>2</sub> directly on the skeleton of carbon cloth,
and further Co<sub>3</sub>O<sub>4</sub> was obtained by post thermal
treatment. The entire synthesis strategy removes the use of any binders
and also avoids the additional preparation process (e.g., transfer
and slurry coating) of final electrodes. This leads to a true comparison
of the ORR/OER catalytic performance between CoÂ(OH)<sub>2</sub> and
Co<sub>3</sub>O<sub>4</sub>, eliminating uncertainties arising from
the electrode preparation procedures. The surface morphologies, microstructures,
and electrochemical behaviors of prepared CoÂ(OH)<sub>2</sub> and Co<sub>3</sub>O<sub>4</sub> catalysts were systemically investigated by
scanning electron microscopy, transmission electron microscopy, atomic
force microscopy, and electrochemical characterization methods. The
results revealed that the electrochemically deposited CoÂ(OH)<sub>2</sub> was in the form of vertically aligned nanosheets with average thickness
of about 4.5 nm. After the thermal treatment in an air atmosphere,
CoÂ(OH)<sub>2</sub> nanosheets were converted into mesoporous Co<sub>3</sub>O<sub>4</sub> nanosheets with remarkably increased electrochemical
active surface area (ECSA). Although the ORR/OER activity normalized
by the geometric surface area of mesoporous Co<sub>3</sub>O<sub>4</sub> nanosheets is higher than that of CoÂ(OH)<sub>2</sub> nanosheets,
the performance normalized by the ECSA of the former is lower than
that of the latter. Considering the superior apparent overall activity
and durability, the Co<sub>3</sub>O<sub>4</sub> catalyst has been
further evaluated by integrating it into a Zn–air battery prototype.
The Co<sub>3</sub>O<sub>4</sub> nanosheets <i>in situ</i> supported on carbon cloth cathode enable the assembled Zn–air
cells with large peak power density of 106.6 mW cm<sup>–2</sup>, low charge and discharge overpotentials (0.67 V), high discharge
rate capability (1.18 V at 20 mA cm<sup>–2</sup>), and long
cycling stability (400 cycles), which are comparable or even superior
to the mixture of state-of-the-art Pt/C and RuO<sub>2</sub> cathode
Self-Assembly of Graphene-Encapsulated Cu Composites for Nonenzymatic Glucose Sensing
Cu
has recently received great interest as a potential candidate for glucose sensing
to overcome the problems with noble metals. In this work, reduced
graphene oxide-encapsulated Cu nanoparticles (Cu@RGO) have been prepared
via an electrostatic self-assembly method. This core/shell composites
were found to be more stable than conventional Cu-decorated graphene
composites and bare copper nanoparticles in an air atmosphere because
the graphene shell can effectively protect the Cu nanoparticles from
oxidation. In addition, the obtained Cu@RGO composites also showed
an outstanding electrocatalytic activity toward glucose oxidation
with a wide linear detection range of 1 μM to 2 mM, low detection
limit of 0.34 μM (S/N = 3), and a sensitivity of 150 μA mM<sup>–1</sup> cm<sup>–2</sup>. Moreover, Cu@RGO composites exhibited a satisfactory reproducibility,
selectivity, and long effective performance. These excellent properties
indicated that Cu@RGO nanoparticles have great potential application
in glucose detection
Sub‑3 nm Co<sub>3</sub>O<sub>4</sub> Nanofilms with Enhanced Supercapacitor Properties
Two-dimensional materials often show a range of intriguing electronic, catalytic, and optical properties that differ greatly from conventional nanoparticles. Herein, we demonstrate the large-scale preparation of sub-3 nm atomic layers Co<sub>3</sub>O<sub>4</sub> nanofilms with a nonsurfactant and substrate-free hydrothermal method. This successful preparation of ultrathin nanofilms highlighted the reconstruction of cobalt–ammonia complexes and synergistic effect of free ammonia and nitrate on film growth control. Subsequent performance tests uncovered that these sub-3 nm atomic layer Co<sub>3</sub>O<sub>4</sub> nanofilms exhibited an ultrahigh specific capacitance of 1400 F/g in the first galvanostatic charge/discharge test. The specific capacitance of Co<sub>3</sub>O<sub>4</sub> nanofilms only slightly decayed less than 3% after 1500 cycling tests. With some parameter adjustments, similar Co(OH)<sub>2</sub> nanofilms with a thickness of 3.70 ± 0.10 nm were also prepared. The Co(OH)<sub>2</sub> nanofilms possessed maximum specific capacitance of 1076 F/g and peak performance attenuation of about 2% after a cycle stability test
Controllable Synthesis of Ni<sub><i>x</i></sub>Se (0.5 ≤ <i>x</i> ≤ 1) Nanocrystals for Efficient Rechargeable Zinc–Air Batteries and Water Splitting
The development of
earth-abundant, highly active, and corrosion-resistant
electrocatalysts to promote the oxygen reduction reaction (ORR) and
oxygen and hydrogen evolution reactions (OER/HER) for rechargeable
metal–air batteries and water-splitting devices is urgently
needed. In this work, Ni<sub><i>x</i></sub>Se (0.5 ≤ <i>x</i> ≤ 1) nanocrystals with different crystal structures
and compositions have been controllably synthesized and investigated
as potential electrocatalysts for multifunctional ORR, OER, and HER
in alkaline conditions. A novel hot-injection process at ambient pressure
was developed to control the phase and composition of a series of
Ni<sub><i>x</i></sub>Se by simply adjusting the added molar
ratio of the nickel resource to triethylenetetramine. Electrochemical
analysis reveals that Ni<sub>0.5</sub>Se nanocrystalline exhibits
superior OER activity compared to its counterparts and is comparable
to RuO<sub>2</sub> in terms of the low overpotential required to reach
a current density of 10 mA cm<sup>–2</sup> (330 mV), which
may benefit from the pyrite-type crystal structure and Se enrichment
in Ni<sub>0.5</sub>Se. For the ORR and HER, Ni<sub>0.75</sub>Se nanoparticles
achieve the best performance including lower overpotentials and larger
apparent current densities. Further investigations demonstrate that
Ni<sub>0.75</sub>Se could not only provide an enhanced electrochemical
active area but also facilitate electron transfer during the electrocatalytic
process, thus contributing to the remarkable catalytic activity. As
a practical application, the Ni<sub>0.75</sub>Se electrode enables
rechargeable Zn–air battery with a considerable performance
including a long cycling lifetime (200 cycles), high specific capacity
(609 mA h g<sup>–1</sup> based on the consumed Zn), and low
overpotential (0.75 V) at 10 mA cm<sup>–2</sup>. Meanwhile,
the water-splitting cell setup with an anode of Ni<sub>0.5</sub>Se
for the HER and a cathode of Ni<sub>0.75</sub>Se for the OER exhibits
a considerable performance with low decay in activity of 12.9% under
continuous polarization for 10 h. These results suggest the promising
potential of nickel selenide nanocrystals as earth-abundant and high-performance
electrocatalysts for metal–air batteries and alkaline water
splitting
Electrochemical Oxidation of Chlorine-Doped Co(OH)<sub>2</sub> Nanosheet Arrays on Carbon Cloth as a Bifunctional Oxygen Electrode
The
primary challenge of developing clean energy conversion/storage systems
is to exploit an efficient bifunctional electrocatalyst both for oxygen
reduction reaction (ORR) and oxygen evolution reaction (OER) with
low cost and good durability. Here, we synthesized chlorine-doped
CoÂ(OH)<sub>2</sub> in situ grown on carbon cloth (Cl-doped CoÂ(OH)<sub>2</sub>) as an integrated electrode by a facial electrodeposition
method. The anodic potential was then applied to the Cl-doped CoÂ(OH)<sub>2</sub> in an alkaline solution to remove chlorine atoms (electro-oxidation
(EO)/Cl-doped CoÂ(OH)<sub>2</sub>), which can further enhance the electrocatalytic
activity without any thermal treatment. EO/Cl-doped CoÂ(OH)<sub>2</sub> exhibits a better performance both for ORR and OER in terms of activity
and durability because of the formation of a defective structure with
a larger electrochemically active surface area after the electrochemical
oxidation. This approach provides a new idea for introducing defects
and developing active electrocatalyst