Synthesis – oxygen evolution performance relationships in “Co-free” high-entropy oxide electrocatalysts for alkaline water electrolysis

The development of efficient oxygen evolution reaction (OER) electrocatalysts free from platinum-group metals (PGMs) and cobalt is essential for advancing sustainable hydrogen production by alkaline water electrolysis. Multimetallic high-entropy oxide (HEO)-based systems offer unique opportunities to tune electrocatalytic per- formance through compositional complexity, phase engineering, and defect chemistry. In this study, a series of multiphasic HEO-based electrocatalysts containing Cr, Mn, Fe, Ni and Sn is synthesized, and the effects of calcination temperature (400 – 900 ◦ C), calcination duration (2 – 9 h), and post-calcination cooling rate on their physicochemical properties and OER activity are systematically investigated. The synthesis conditions control the partitioning of Sn, Ni, and Fe among spinel HEO, rock-salt HEO, cassiterite, and NiFe carbonate/hydroxide phases, thereby affecting crystallinity, morphology, surface chemistry, defect-related features, and electro- chemical performance. Several of the developed electrocatalysts outperform commercial IrOx in “ ex-situ ” OER measurements. Single-cell anion-exchange membrane water electrolysis tests further confirmed the superior performance of the best two electrocatalysts, which exhibited cell voltages 66 and 108 mV lower than that of the IrOx-benchmark cell at a current density of 1 A cm 2. The corresponding cells also show practically unchanged polarization behavior during 24 h of operation, indicating short-term operational stability under the investigated AEMWE conditions. The enhanced activity is attributed to synthesis-controlled phase partitioning, minimized Sn segregation into cassiterite, the presence of reconstruction-prone HEO and NiFe carbonate/hydroxide (NiFe-CH) phases, and the small crystallite size of the reconstruction-prone components. Post-operation TEM/HRTEM and STEM-EDX analyses further reveal pronounced morphological and compositional reorganization of HEO-400/3 and HEO-500/2 after 24 h of AEMWE operation, consistent with the reconstruction of the electrocatalysts. These findings demonstrate the potential of Co- and PGM-free multiphasic HEO-based systems as high-performance OER electrocatalysts and provide design guidelines for advanced materials for alkaline water electrolysis.