Synthesis of High-Entropy Alloys with a Tailored Composition and Phase Structure Using a Single Configurable Target

Previously, refractory high-entropy alloys (HEAs) with high crystallinity were synthesized using a configurable target without heat treatment. This study builds upon prior investigations to develop nonrefractory elemental HEAs with low crystallinity using a novel target system. Different targets with various elemental compositions, i.e., Co20Cr20Ni20Mn20Mo20 (target 1), Co30Cr15Ni25Mn15Mo15 (target 2), and Co15Cr25Cu20Mn20Ni20 (target 3), are designed to modify the phase structure. The elemental composition is varied to ensure face-centered cubic (FCC) or body-centered cubic (BCC) phase stabilization. In target 1, the FCC and BCC phases coexist, whereas targets 2 and 3 are characterized by a single FCC phase. Thin films based on targets 1 and 2 exhibit crystalline phases followed by annealing, as indicated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. In contrast, target 3 yields crystalline thin films without any heat treatment. The thin-film coatings are classified based on the atomic size difference (δ). The δ value for the target with the elemental composition CoCrMoMnNi is 9.7, i.e., ≥6.6, corresponding to an HEA with an amorphous phase. However, the annealed thin film is considered a multiprincipal elemental alloy. In contrast, δ for the CoCrCuMnNi HEA is 5, i.e., ≤6.6, upon the substitution of Mo with Cu, and a solid solution phase is formed without any heat treatment. Thus, the degree of crystallinity can be controlled through heat treatment and the manipulation of δ in the absence of heat treatment. The XRD results clarify the crystallinity and phase structure, indicating the presence of FCC or a combination of FCC and BCC phases. The outcomes are consistent with those obtained through the analysis of the valence electron concentration based on X-ray photoelectron spectroscopy. Furthermore, a selected area electron diffraction analysis confirms the presence of both amorphous and crystalline structures in the HEA thin films. Additionally, phase evolution and segregation are observed at 500 °C.

Electrocatalytic Properties of Co3O4 Prepared on Carbon Fibers by Thermal Metal-Organic Deposition for the Oxygen Evolution Reaction in Alkaline Water Electrolysis

Cobalt oxide (Co₃O₄) serves as a promising electrocatalyst for oxygen evolution reactions (OER) in water-electrolytic hydrogen production. For more practical applications, advances in dry-deposition processes for the high-throughput fabrication of such Co₃O₄ electrocatalysts are needed. In this work, a thermal metal-organic deposition (MOD) technique is developed to form Co₃O₄ deposits on microscale-diameter carbon fibers constituting a carbon fiber paper (CFP) substrate for high-efficiency OER electrocatalyst applications. The Co₃O₄ electrocatalysts are deposited while uniformly covering the surface of individual carbon fibers in the reaction temperature range from 400 to 800 °C under an ambient Ar atmosphere. It is found that the microstructure of deposits is dependent on the reaction temperature. The Co₃O₄ electrocatalysts prepared at 500 °C and over exhibit values of 355-384 mV in overpotential (η₁₀) required to reach a current density of 10 mA cm^-2 and 70-79 mV dec^-1 in Tafel slope, measured in 1 M KOH aqueous solution. As a result, it is highlighted that the improved crystallinity of the Co₃O₄ electrocatalyst with the increased reaction temperature leads to an enhancement in electrode-level OER activity with the high electrochemically active surface area (ECSA), low charge transfer resistance (R_ct), and low η₁₀, due to the enhanced electrical conductivity. On the other hand, it is found that the inherent catalytic activity of the surface sites of the Co₃O₄, represented by the turnover frequency (TOF), decreases with reaction temperature due to the high-temperature sintering effect. This work provides the groundwork for the high-throughput fabrication and rational design of high-performance electrocatalysts.