Surfactant-templated syntheses of nanostructured high-entropy spinel oxide electrocatalysts and effects on water-splitting

The production of clean and affordable hydrogen energy relies on the development of affordable, efficient, and durable bifunctional electrocatalysts for overall water-splitting. In this research, a quinary high-entropy inverse spinel oxide, (CoCuFeMnNi)₃O₄, was prepared via the reverse co-precipitation method, employing different surfactants as templates, namely, PVP, SDS, and CTAB. XRD, SEM, XPS, and HRTEM were employed to analyze the structure, morphology, and other properties of (CoCuFeMnNi)₃O₄. The effects of the different surfactants on the properties and electrocatalytic performance of (CoCuFeMnNi)₃O₄ in water-splitting reactions (HER/OER) were discussed. The results show that different surfactants influence the dispersion, surface area, and other properties of the synthesized (CoCuFeMnNi)₃O₄ particles. The electrochemical test results of the HER and OER were compared. Due to the superior performance of the PVP-assisted (CoCuFeMnNi)₃O₄ in the HER and OER, PVP was selected as the surfactant of choice. From the metal salt-to-PVP ratio studies, HESOx with 1 wt% PVP as a synthesis template (HESOx_{PVP 1%}) exhibited the best performance of all, requiring an overpotential of 16 mV to reach 10 mA cm^-2 in HER and 381 mV to reach 10 mA cm^-2 in the OER. The effects of the presence and absence of Cu on the HER and OER are also investigated. Quaternary MESOx ((CoFeMnNi)₃O₄) is compared to HESOx. Cu is observed to increase the SSA and ECSA of HESOx, giving it superior initial HER/OER performance. However, it suffers active site losses due to the ready formation of cuprous oxide (Cu₂O), leading to sluggish electrocatalysis at higher overpotentials. In the two-electrode test, HESOx reaches 10 mA cm^-2 at an overpotential of ca. 0.267 V, compared to the 0.43 V required by (MESOx). However, MESOx shows a remarkably superior performance than HESOx, improving by 130 mV after 100 h of activity.