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High-entropy oxides (HEOs) exhibit excellent structural stability, and the synergistic effects among their constituent elements contribute to performance enhancement, providing vast opportunities for designing novel and efficient electrocatalysts. Utilizing four common transition metals such as Fe, Co, Ni, and Cr as the base and introducing a fifth metal, M (M = Mg, Mn, Zn, and Cu), with different outer electron shells, relevant high-entropy oxides with varying sizes were successfully synthesized via the hydrothermal method. Among all considered samples, FCNCMg, with the smallest particle size, possessed a much larger specific surface area, leading to the exposure of more active sites and thus favoring an improvement of the OER performance. The introduction of the fifth metal induced a redistribution of charges among Fe, Co, Ni, and Cr within the catalyst, resulting in different oxidation states for these metals. Mg addition led the XPS peaks of Fe/Ni and Cr/Co elements to shift towards lower and higher binding energies, respectively. This dynamic compensation effect between metal ions synergistically promoted their OER performance. In alkaline environments, FCNCMg exhibited the best OER performance, with an overpotential of 278 mV at 10 mA cm−2 and a Tafel slope of 49.90 mV dec−1. Moreover, it demonstrated excellent stability, achieving 50 h at 10 mA cm−2 and 20 h at 50 mA cm−2. This study, by controlling the composition of HEOs, explored the impact of particle size and synergistic effects on their performance, offering new insights for design and optimization of relevant high-performance HEOs. • A series of spinel (Fe 0.2 Co 0.2 Ni 0.2 Cr 0.2 M 0.2) 3 O 4 materials were successfully synthesized. • The introduction of M (M = Mg, Mn, Zn, and Cu) leads to tunable oxidation states of other elements. • The dynamic compensation effect among metal ions promots the OER activity of the catalysts. • FCNCMg exhibits an overpotential of 278 mV at 10 mA cm−2 and a Tafel slope of 49.90 mV dec−1. • FCNCMg can sustain stably for 50 h at 10 mA cm−2 and 20 h at 50 mA cm−2. [ABSTRACT FROM AUTHOR]

• Branched Ni 3 C nanoparticles in situ grown on the reduced graphene oxide (rGO). • Ni 3 C/rGO composite exhibited a minimum reflection loss of − 42.8 dB. • More interfacial polarization active sites improved reflection loss. Carbide/carbon composites are new types of electromagnetic (EM) wave absorbing materials because of their good stability, light weight, easy synthesis and excellent performance. In this work, monodisperse branched nickel carbide (Ni 3 C) nanoparticles with controlled morphology were fabricated through high-temperature pyrolysis of nickel acetylacetonate under anhydrous and oxygen-free conditions. Then Ni 3 C nanoparticles were uniformly in situ grown on the surface of reduced graphene oxide (rGO) to form nickel carbide/reduced graphene oxide (Ni 3 C/rGO) nanocomposite. Series of characterization and performance tests were conducted, and the results showed that the EM wave absorption performance of Ni 3 C/rGO composite was greatly enhanced compared with that of pure phase Ni 3 C nanoparticles. Due to the combined effects of interfacial polarization loss, conductivity loss and surface defects, Ni 3 C/rGO nanocomposite exhibited strong reflection loss in 2–18 GHz band, which could attenuate the EM waves incident to the internal loss to a greater extent. When the Ni 3 C/rGO composite is at 3.5 mm thickness, the maximum effective absorption bandwidth can reach 3.6 GHz, and at 3.0 mm thickness, the lowest reflection loss value can reach − 42.8 dB. This high-performance nanocomposite can be a new candidate material for EM wave absorption. [ABSTRACT FROM AUTHOR]