Although high-entropy alloys have been intensively studied in the past decade, there are still many requirements for manufacturing processes and application directions to be proposed and developed, but most techniques are focused on high-entropy bulk materials and surface coatings. We fabricated high-entropy ceramic (HEC) nanomaterials using simple pulsed laser irradiation scanning on mixed salt solutions (PLMS method) under low-vacuum conditions. This method, allowing simple operation, rapid manufacturing, and low cost, is capable of using various metal salts as precursors and is also suitable for both flat and complicated 3D substrates. In this work, we engineered this PLMS method to fabricate high-entropy ceramic oxides containing four to seven elements. To address the catalytic performance of these HEC nanomaterials, we focused on CoCrFeNiAl high-entropy oxides applied to the oxygen-evolution reaction (OER), which is considered a sluggish process in water. We performed systematic material characterization to solve the complicated structure of the CoCrFeNiAl HEC as a spinel structure, AB2O4 (A, B = Co, Cr, Fe, Ni, or Al). Atoms in A and B sites in the spinel structure can be replaced with other elements; either divalent or trivalent metals can occupy the spinel lattice using this PLMS process. We applied this PLMS method to manufacture electrocatalytic CoCrFeNiAl HEC electrodes for the OER reaction, which displayed state-of-the-art activity and stability.

The structure of the etch film on aluminum plates etched in nitric acid was systematically studied by varying the ratio of positive to negative charges provided by a rectangular current waveform. When etched using a symmetrical square current, the plates were dotted with the etch pits inside which a layered etch film formed via the stacking of dark-contrast vs. light-contrast layers. The former was presumably aluminum hydroxide, while the latter was the channels that were filled with the electrolyte during etching. The thickness of the aluminum hydroxide layer increased with increasing the charge passed during the cathodic half-cycle. Hydrogen discharge during the cathodic half-cycle led to the formation of the hydroxide layer. Hydrogen evolution at the interface formed microvoids and might induce defects in the hydroxide layer. Based on dissolution-passivation of the same pit, a possible mechanism was proposed to explain the formation of the layered etch film on aluminum etched in nitric acid using the alternating current with emphasis on the effects of hydrogen discharge.

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