3 research outputs found
Effect of High Cobalt Concentration on Hopping Motion in Cobalt Manganese Spinel Oxide (Co<sub><i>x</i></sub>Mn<sub>3–<i>x</i></sub>O<sub>4</sub>, <i>x</i> ≥ 2.3)
Hopping motions in
cobalt manganese spinel oxides with high cobalt
concentration (Co<sub><i>x</i></sub>Mn<sub>3–<i>x</i></sub>O<sub>4</sub>, 2.3 ≤ <i>x</i> ≤
2.7) are investigated in order to clarify the origin of unusual electrical
behaviors as negative temperature coefficient (NTC) thermistors. Based
on the resistance versus temperature (R–T) characteristics,
hopping conduction mechanisms in MCO compounds (<i>x</i> = 2.3 and 2.5) are attributed to variable range hopping (VRH) motion
with a parabolic distribution of the density of states (DOS) near
the Fermi level. However, when Co content increases up to 2.7, transition
in the hopping motion occurs from VRH to the nearest neighboring hopping
(NNH) motion, which can be responsible for a huge increase of the
resistance accompanied by decrease of the factor of thermal sensitivity
(B value) in MCO compounds (<i>x</i> = 2.7). Also, hopping
distance and activation energies for MCO (<i>x</i> = 2.3
and 2.5) compounds following VRH conduction are calculated as a function
of temperature, indicating that higher B value observed in MCO (<i>x</i> = 2.5) compound is due to the larger hopping distance
compared to that of MCO (<i>x</i> = 2.3) compound
Surface-Modified Carbon Nanotubes with Ultrathin Co<sub>3</sub>O<sub>4</sub> Layer for Enhanced Oxygen Evolution Reaction
Alkaline
water electrolysis is a vital technology for sustainable
and efficient hydrogen production. However, the oxygen evolution reaction
(OER) at the anode suffers from sluggish kinetics, requiring overpotential.
Precious metal-based electrocatalysts are commonly used but face limitations
in cost and availability. Carbon nanostructures, such as carbon nanotubes
(CNTs), offer promising alternatives due to their abundant active
sites and efficient charge-transfer properties. Surface modification
of CNTs through techniques such as pulsed laser ablation in liquid
media (PLAL) can enhance their catalytic performance. In this study,
we investigate the role of surface-modified carbon (SMC) as a support
to increase the active sites of transition metal-based electrocatalysts
and its impact on electrocatalytic performance for the OER. We focus
on Co3O4@SMC heterostructures, where an ultrathin
layer of Co3O4 is deposited onto SMCs using
a combination of PLAL and atomic layer deposition. A comparative analysis
with aggregated Co3O4 and Co3O4@pristine CNTs reveals the superior OER performance of Co3O4@SMC. The optimized Co3O4@SMC exhibits a 25.6% reduction in overpotential, a lower Tafel slope,
and a significantly higher turnover frequency (TOF) in alkaline water
splitting. The experimental results, combined with density functional
theory (DFT) calculations, indicate that these improvements can be
attributed to the high electrocatalytic activity of Co3O4 as active sites achieved through the homogeneous distribution
on SMCs. The experimental methodology, morphology, composition, and
their correlation with activity and stability of Co3O4@SMC for the OER in alkaline media are discussed in detail.
This study contributes to the understanding of SMC-based heterostructures
and their potential for enhancing electrocatalytic performance in
alkaline water electrolysis
Parallelized Reaction Pathway and Stronger Internal Band Bending by Partial Oxidation of Metal Sulfide–Graphene Composites: Important Factors of Synergistic Oxygen Evolution Reaction Enhancement
The electrocatalytic performance
of transition metal sulfide (TMS)–graphene composites has been
simply regarded as the results of high conductivity and the large
surface/volume ratio. However, unavoidable factors such as degree
of oxidation of TMSs have been hardly considered for the origin of
this catalytic activity of TMS–graphene composites. To accomplish
the reliable application of TMS-based electrocatalytic materials,
a clear understanding of the thermodynamic stability of TMS and effects
of oxidation on catalytic activity is necessary. In addition, the
mechanism of charge transfer at the TMS–graphene interface
must be studied in depth to properly design composite materials. Herein,
we report a comprehensive study of the physical chemistry at the junction
of a Co<sub>1–<i>x</i></sub>Ni<sub><i>x</i></sub>S<sub>2</sub>–graphene composite, which is a prototype
designed to unravel the mechanisms of charge transfer between TMS
and graphene. Specifically, the thermodynamic stability and the effects
of oxidation of TMSs during the oxygen evolution reaction (OER) on
the reaction mechanism are systematically investigated using density
functional theory (DFT) calculations and experimental observations.
Cobalt atoms anchored on pyridinic N sites in the graphene support
form metal–semiconductor (SC) junctions, and the internal band
bending at these junctions facilitates electron transfer from TMSs
to graphene. The junction enables fast sinking of the excess electron
from OH<sup>–</sup> adsorbate. Partially oxidized amorphous
TMS layers formed during the OER can facilitate adsorption and desorption
of OH and H atoms, boosting the OER performance of TMS–graphene
nanocomposites. From the DFT calculations, the enhanced electrocatalytic
activity of TMS–graphene nanocomposites originates from two
important factors: (i) increased internal band bending and (ii) parallelized
OER pathways at the interface of pristine and oxidized TMSs