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Developing highly active catalyst for oxygen evolution reaction (OER) with low noble metal loading is crucial for the application of proton exchange membrane (PEM) water electrolysis. Ir-transition metal (TM) binary catalyst with chemical leaching was reported to perform excellent activity with TM leaching. However, most of the studies on the activation-decay focused on the final status, and the variation process remains unclear. With cysteamine·HCl, a modified facile molten salt method was applied to prepared highly active Ir–Ni mixed oxide (c-xNiIrMO), which perform 5.5 times higher mass activity than commercial IrO 2. The variation of active surface area, intrinsic activity, conductivity of prepared catalyst layer during the activation process are studied to unveil the key factors for the activity enhancement. With further stability test, c-xNiIrMO with Ni content lower than 67% performs decrease intrinsic activity and improved kinetic performance in the first hundred cycles. After obtaining a stable Ir 0.95 Ni 0.05 O x structure, further cycling results in typical activity decay with dissolution of Ir. • Cysteamine·HCl modified c-NiIrMO performs a highest 2.51 times of Ir mass activity compared to nc-NiIrMO. • Cysteamine·HCl not only plays a role in particle fining but also contributes to the formation of active Ir–OH. • For activation with Ni leaching, surface enlargement plays a key role for c-IrNiMO with Ni content lower than 40%. • Formation of Ir–OH is the dominant factor for the activation of sample with higher Ni content. • A three–step variation process dominated by Ni and Ir dissolution can be observed in the activation-decay process. [ABSTRACT FROM AUTHOR]