Developing readily available and cost-effective electrocatalysts as an alternative to the noble metals is a critical issue for renewable hydrogen energy generation. Mesoporous high-entropy oxides (HEOs) are among the most noteworthy candidates in the kinetically challenged process of oxygen evolution reaction (OER); The current study notifies the successful fabrication, and characterization of a series of meso-structured HEOs (5 elements) and MEOs (4 elements) nanopowders with remarkable electrocatalytic performance. Obtained results reveal that the synthesized HEOs and MEOs nanocompounds are meso-structured (with pore size of ⁓43–67 Å) with particle size of ⁓20–30 nm. Combusted HEOs and MEOs powders have high crystallinity degree (derived from XRD analysis) and homogenous elemental distributions. The electrocatalytic activity of the prepared HEOs and MEOs nanopowders were studied using linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) analyses. It was specified that HEO nanoparticles (HEO 0.95 sample) exhibit superior electrocatalytic performance compared to MEOs particles with respect to their overpotential of 310 mV at 10 mA cm−2 and Tafel slope of 53 mV.dec−1. The drop of the charge transfer resistance (RCT) and increase of the conductivity in HEO 0.95 sample contributes to the improved electrocatalytic OER activity of the HEOs nanopowders. Taking ECSA into account, the improved electrocatalytic activity of meso-structured HEO 0.95 sample relates to the large number of active sites in this sample. The XRD, TEM, and FE-SEM results of HEO 0.95 sample after OER analysis indicated no alteration in phase composition and morphology revealing the reusability of the synthesized nanoparticles. This study also provides an insight into the behavior of Co, Ni, Cu elements as typical electrocatalysts as well as Zn and Mg elements as obscure electrocatalysts for efficient OER electrocatalysis. Furthermore, the impact of adiabatic temperature, crystallinity degree and physicochemical features of meso-structured particles on electrocatalytic performance are explored.

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