Effect of High Cobalt Concentration on Hopping Motion in Cobalt Manganese Spinel Oxide (Co_<i>x</i>Mn_3–<i>x</i>O₄, <i>x</i> ≥ 2.3)

Hopping motions in cobalt manganese spinel oxides with high cobalt concentration (Co_<i>x</i>Mn_3–<i>x</i>O₄, 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 Co₃O₄ 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_1–<i>x</i>Ni_<i>x</i>S₂–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^– 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