Pseudocapacitive properties of high-entropy oxides through structural modulation (Mg/Al/Ti-O framework) and transition metal redox couples (Ni/Co, Ni/Fe, and Ni/Cr)

This study reports the synthesis, characterization, and evaluation of the electrochemical properties of MgAlTiNiCoO, MgAlTiNiFeO, and MgAlTiNiCrO HEOx. For the design of the HEOx, a redox-inactive structural matrix of Mg 2+ , Al 3+ and Ti 4+ (Mg/Al/Ti – framework) was used as a potential structural modulator to prevent structural degradation. The HEOx were synthesized by mechanical alloying for 8 h and oxidation thermal treatment (1073–1623 K) for 5 h. The HEOx-Co system formed a biphasic mixture dominated by a spinel-type structure (90.6%) and a rock-salt-type phase (9.4%). HEOx-Fe and HEOx-Cr exhibited a mixture of a spinel phase (> 62%) and a secondary pseudobrookite-type phase (< 38%). Mg, Al, and Ti lack redox activity to stabilize the crystal lattice. Furthermore, the HEOx exhibited oxygen vacancies in all three materials, resulting from the high lattice distortion. HEOx-Co, HEOx-Cr and HEOx-Fe achieved a gravimetric capacitance of 105, 23 and 12 F/g and an energy density of 3.77, 0.83 and 0.48 Wh/kg, respectively. Additionally, the galvanostatic capacitance was of 176, 83 and 34 F/g at 2 A/g, respectively. Meanwhile the capability rate was of 90.9%, 75.9%, and 47.7% at 2–4 A/g. The retention capacity of HEOx-Co, HEOx-Fe, and HEOx-Cr modified electrodes was 97%, 100%, and 100% after 5000 cycles, respectively. The capacitive performance of the modified electrodes is the result of several factors, such as the conductive material, the predominant HEOx phase content, HEOx electrical conductivity and the divalent ions that facilitate transitions between different oxidation states (Mn + to Mm + ).