7 research outputs found
Co<sub>7</sub>Se<sub>8</sub> Nanostructures as Catalysts for Oxygen Reduction Reaction with High Methanol Tolerance
Co<sub>7</sub>Se<sub>8</sub> nanostructures
electrodeposited on
glassy carbon (GC) electrodes show high efficiency for oxygen reduction
reaction (ORR) with high methanol tolerance as compared to Pt electrocatalysts.
In the presence of methanol, the onset potential for the ORR at Pt/GC
is shifted from 0.931 V (vs reversible hydrogen electrode (RHE))
to 0.801 V (vs RHE), whereas it remains the same (0.811 V vs RHE)
at Co<sub>7</sub>Se<sub>8</sub>/GC in the presence and absence of
methanol in 0.5 M H<sub>2</sub>SO<sub>4</sub> solution. The Co<sub>7</sub>Se<sub>8</sub>/GC electrodes also showed high cyclability
in the presence of methanol, with no degradation of catalytic performance.
It is also noteworthy that the Co<sub>7</sub>Se<sub>8</sub>/GC exhibited
exclusively a four-electron reduction pathway for ORR and very low
H<sub>2</sub>O<sub>2</sub> yield in acidic electrolyte. The admirable
performance of Co<sub>7</sub>Se<sub>8</sub>/GC catalyst along with
its cost-effective nature holds great potential for application in
direct methanol fuel cells
Synthesis of Superconducting Nanocables of FeSe Encapsulated in Carbonaceous Shell
The recent discovery of superconductivity in iron selenide has attracted considerable attention due to the simplicity of composition, unconventional nature of superconductivity, and ease of synthesis. We have synthesized superconducting FeSe nanowires with a simple catalyst-aided vapor transport reaction at 800 °C in an inert atmosphere. The precursors were chosen to be elemental Se and iron acetylacetonate [Fe<sup>III</sup>(C<sub>5</sub>H<sub>8</sub>O<sub>2</sub>)<sub>3</sub>]. These vaporized very easily, thereby facilitating transport, and also contributed to the formation of a carbonaceous shell encapsulating the FeSe nanowires. The superconductivity of these nanocables was confirmed through magnetic measurements and a <i>T</i><sub>c</sub> of ≈8 K was obtained for an ensemble of nanocables. The length of FeSe filling inside the carbon nanofibers could be varied by controlling the reaction conditions while the diameter of nanowires was dependent on the thickness of Au–Pd coating used as a catalyst. Extensive analysis through high-resolution microscopy revealed that there was considerable lattice contraction of FeSe in the nanocable up to about 3.6% along the <i>c</i>-direction leading to a reduced spacing between the (001) lattice planes. Interestingly, this compression was more pronounced near the catalyst-FeSe interface and was reduced further along the length of the nanocable. The presence of carbon nanofibers as a shell around the FeSe protected the FeSe nanowires from both atmospheric O<sub>2</sub> and moisture attack, as was evident from the very long ambient condition shelf life of these nanocables, and also makes them more stable under e-beam irradiation
Enhancement of Superconducting <i>T</i><sub>c</sub> (33 K) by Entrapment of FeSe in Carbon Coated Au–Pd<sub>17</sub>Se<sub>15</sub> Nanoparticles
FeSe has been an interesting member of the Fe-based superconductor family ever since the discovery of superconductivity in this simple binary chalcogenide. Simplicity of composition and ease of synthesis has made FeSe, in particular, very lucrative as a test system to understand the unconventional nature of superconductivity, especially in low-dimensional models. In this article we report the synthesis of composite nanoparticles containing FeSe nanoislands entrapped within an <i>ent</i>-FeSe-Pd<sub>16</sub>Se<sub>15</sub>–Au nanoparticle and sharing an interface with Pd<sub>17</sub>Se<sub>15</sub>. This assembly exhibits a significant enhancement in the superconducting <i>T</i><sub><i>c</i></sub> (onset at 33 K) accompanied by a noticeable lattice compression of FeSe along the ⟨001⟩ and ⟨101⟩ directions. The <i>T</i><sub><i>c</i></sub> in FeSe is very sensitive to application of pressure and it has been shown that with increasing external pressure <i>T</i><sub><i>c</i></sub> can be increased almost 4-fold. In these composite nanoparticles reported here, immobilization of FeSe on the Pd<sub>17</sub>Se<sub>15</sub> surface contributes to increasing the effect of interfacial pressure, thereby enhancing the <i>T</i><sub><i>c</i></sub>. The effect of interfacial pressure is also manifested in the contraction of the FeSe lattice (up to 3.8% in ⟨001⟩ direction) as observed through extensive high-resolution TEM imaging. The confined FeSe in these nanoparticles occupied a region of approximately 15–25 nm, where lattice compression was uniform over the entire FeSe region, thereby maximizing its effect in enhancing the <i>T</i><sub><i>c</i></sub>. The nanoparticles have been synthesized by a simple catalyst-aided vapor transport reaction at 800 °C where iron acetylacetonate and Se were used as precursors. Morphology and composition of these nanoparticles have been studied in details through extensive electron microscopy
Cobalt Selenide Nanostructures: An Efficient Bifunctional Catalyst with High Current Density at Low Coverage
Electrodeposited Co<sub>7</sub>Se<sub>8</sub> nanostructures exhibiting flake-like morphology show bifunctional
catalytic activity for oxygen evolution and hydrogen evolution reaction
(OER and HER, respectively) in alkaline medium with long-term durability
(>12 h) and high Faradaic efficiency (99.62%). In addition to low
Tafel slope (32.6 mV per decade), the Co<sub>7</sub>Se<sub>8</sub> OER electrocatalyst also exhibited very low overpotential to achieve
10 mA cm<sup>–2</sup> (0.26 V) which is lower than other transition
metal chalcogenide based OER electrocatalysts reported in the literature
and significantly lower than the state-of-the-art precious metal oxides.
A low Tafel slope (59.1 mV per decade) was also obtained for the HER
catalytic activity in alkaline electrolyte. The OER catalytic activity
could be further improved by creating arrays of 3-dimensional rod-like
and tubular structures of Co<sub>7</sub>Se<sub>8</sub> through confined
electrodeposition on lithographically patterned nanoelectrodes. Such
arrays of patterned nanostructures produced exceptionally high mass
activity and gravimetric current density (∼68 000 A
g<sup>–1</sup>) compared to the planar thin films (∼220
A g<sup>–1</sup>). Such high mass activity of the catalysts
underlines reduction in usage of the active material without compromising
efficiency and their practical applicability. The catalyst layer could
be electrodeposited on different substrates, and an effect of the
substrate surface on the catalytic activity was also investigated.
The Co<sub>7</sub>Se<sub>8</sub> bifunctional catalyst enabled water
electrolysis in alkaline solution at a cell voltage of 1.6 V. The
electrodeposition works with exceptional reproducibility on any conducting
substrate and shows unprecedented catalytic performance especially
with the patterned growth of catalyst rods and tubes
Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A<sub>1/5</sub>)Co<sub>2</sub>O<sub>4</sub> and Their Tunability
Hastening the progress
of rechargeable metal–air batteries
and hydrogen fuel cells necessitates the advancement of economically
feasible, earth-abundant, inexpensive, and efficient electrocatalysts
facilitating both the oxygen evolution reaction (OER) and oxygen reduction
reaction (ORR). Herein, a recently reported family of nano (5A1/5)Co2O4 (A = combinations of transition
metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides
(CCOs) [Wang et al., Chemistry of Materials, 2023, 35 (17), 7283–7291.] are studied
as bifunctional OER and ORR electrocatalysts. Among the different
low-temperature soft-templating samples, those subjected to 600 °C
postannealing heat treatment exhibit superior performance in alkaline
media. One specific composition (Mn0.2Fe0.2Ni0.2Cu0.2Zn0.2)Co2O4 exhibited an exceptional overpotential (260 mV at 10 mA cm–2) for the OER, a favorable Tafel slope of 68 mV dec–1, excellent onset potential (0.9 V) for the ORR, and lower than 6%
H2O2 yields over a potential range of 0.2 to
0.8 V vs the reversible hydrogen electrode. Furthermore, this catalyst
displayed stability over a 22 h chronoamperometry measurement, as
confirmed by X-ray photoelectron spectroscopy analysis. Considering
the outstanding performance, the low cost and scalability of the synthesis
method, and the demonstrated tunability through chemical substitutions
and processing variables, CCO ACo2O4 spinel
oxides are highly promising candidates for future sustainable electrocatalytic
applications
Soft-Chemical Synthetic Route to Superparamagnetic FeAs@C Core–Shell Nanoparticles Exhibiting High Blocking Temperature
Superparamagnetic FeAs nanoparticles
with a fairly high blocking
temperature (<i>T</i><sub>B</sub>) have been synthesized
through a hot injection precipitation technique. The synthesis involved
usage of triphenylarsine (TPA) as the As precursor, which reacts with
FeÂ(CO)<sub>5</sub> by ligand displacement at moderate temperatures
(300 °C). Addition of a surfactant, hexadecylamine (HDA), assists
in the formation of the nanoparticles, due to its coordinating ability
and low melting point which provides a molten flux like condition
making this synthesis a solventless method. Decomposition of the carbonaceous
precursors, HDA, TPA and FeÂ(CO)<sub>5</sub>, also produces the carbonaceous
shell coating the FeAs nanoparticles. Magnetic characterization of
these nanoparticles revealed the superparamagnetic nature of these
nanoparticles with a perfect anhysteretic nature of the isothermal
magnetization above <i>T</i><sub>B</sub>. The <i>T</i><sub>B</sub> observed in this system was indeed high (240 K) when
compared with other superparamagnetic systems conventionally utilized
for magnetic storage devices. It could be further increased by decreasing
the strength of the applied magnetic field. The narrow hysteresis
with low magnitude of coercivity at 5 K suggested soft ferromagnetic
ordering in these nanoparticle ensembles. Mössbauer and XPS
studies indicated that the Fe was present in +3 oxidation state and
there was no signature of Fe(0) that could have been responsible for
the increased magnetic moment and superparamagnetism. Typically for
superparamagnetic nanoparticle ensemble, the need for isolation of
the superparamagnetic domains (thereby inhibiting particle aggregation
and enhancing the <i>T</i><sub>B</sub>) has been in constant
limelight. Carbonaceous coating on these as-synthesized nanoparticles
formed <i>in situ</i> provided the physical nonmagnetic
barrier needed for such isolation. The high <i>T</i><sub>B</sub> and room temperature magnetic moment of these FeAs@C nanoparticles
also make them potentially useful for applications in ferrofluids
and magnetic refrigeration. In principle this method can be used as
a general route toward synthesis of other arsenide nanostructures
including the transition metal arsenide which show interesting magnetic
and electronic properties (e.g., CoAs, MnAs) with finer control over
morphology, composition and structure
Mesoporous RE<sub>0.5</sub>Ce<sub>0.5</sub>O<sub>2–<i>x</i></sub> Fluorite Electrocatalysts for the Oxygen Evolution Reaction
Developing highly active and stable
electrocatalysts
for the oxygen
evolution reaction (OER) is key to improving the efficiency and practical
application of various sustainable energy technologies including water
electrolysis, CO2 reduction, and metal air batteries. Here,
we use evaporation-induced self-assembly (EISA) to synthesize highly
porous fluorite nanocatalysts with a high surface area. In this study,
we demonstrate that a 50% rare-earth cation substitution for Ce in
the CeO2 fluorite lattice improves the OER activity and
stability by introducing oxygen vacancies into the host lattice, which
results in a decrease in the adsorption energy of the OH* intermediate
in the OER. Among the binary fluorite compositions investigated, Nd2Ce2O7 is shown to display the lowest
OER overpotential of 243 mV, achieved at a current density of 10 mA
cm–2, and excellent cycling stability in an alkaline
medium. Importantly, we demonstrate that rare-earth oxide OER electrocatalysts
with high activity and stability can be achieved using the EISA synthesis
route without the incorporation of transition and noble metals